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1 ACTIVITY AND CHARACTERIZATION ST UDIES IN METHANOL REFORMING CATALYSIS: Cu AND Cu-ZnO CATALY STS AND THE ROLE OF NANOMATERIALS By SAMUEL DAVID JONES A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2008
2 2008 Samuel David Jones
3 To Pop
4 There are some things which cannot be learned quickly, and time, which is all we have, must be paid heavily for their acquiring. They are the very simple st things and because it takes a man's life to know them the little new that each man gets from life is very costly and the only heritage he has to leave. E.M. Hemmingway. Death in the Afternoon 1932.
5 ACKNOWLEDGMENTS I was very fortunate while at Florida to have the opportunity to work with so many talented people. Helena Weaver has been a talented advisor and co lleague from the very beginning. She was always available to help wh en I felt I had reached a dead end, and allowed me the flexibility to pursue the research I fe lt was most promising and rewarding. I am very fortunate to have had such a good working relationship with my a dvisor who is so patient with my often highly non-linear style of working. My lab partner Luke Neal also proved to be extremely helpful in generating large amounts of the XPS data which have gone into this work and ultimately provided some of the most impor tant observations about the catalysts in this study. Luke was also instrumental in fabricating some of the first catalysts to go into our reactor and first helped me in making my own catalyst samples. I have been further blessed by the constant support of my family, especially my mom and my sister, who constantly listen to my tales of X-rays and catalysts very patiently and supportively, whether or not they had any idea what I was talking about. I would like to dedicate this dissertation to Dad, who unfortunately neve r got to see its completion but always enjoyed discussing its progress with me. Pop was the be tter craftsman and I doubt this dissertation would have been possible wi thout his guidance. I am also very thankful for the support of Rich and Nancy Kaminsky, who have always been ready to listen and offer advice, especially as this work and my tim e at Florida neared its completion. I am grateful for the relationship that I have with the Kaminskys. Of course none of this would have been possible without the cons tant and unconditional love and support from my future wife Chrissy. To my Chrissy: Thank you for you.
6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ............................................................................................................... 5 LIST OF TABLES ...........................................................................................................................9 LIST OF FIGURES .......................................................................................................................10 ABSTRACT ...................................................................................................................... .............12 CHAP TER 1 INTRODUCTION, LITERATURE REVI E W, EQUIPMENT AND PROCEDURES SUMMARY ....................................................................................................................... .....14 1.1 Introduction and Motivation for Research ....................................................................14 1.2 General Literature Review ............................................................................................15 1.3 Catalytic Reaction System Design and Construction....................................................21 1.3.1 Reactor System Design ..................................................................................... 21 1.3.2 Gas Chrom atograph Design and Operation ......................................................23 1.3.3 Gas Chrom atograph Calibration .......................................................................24 1.4 Water Gas Shift Equilib rium ......................................................................................... 28 1.5 Catalyst Characterization Techniques ...........................................................................30 1.5.1 Catalyst Activity Testing .................................................................................. 30 18.104.22.168 Catalyst pretreatm ent .......................................................................... 30 22.214.171.124 Tem perature dependent activity measurement ................................... 31 126.96.36.199 Contact time activity m easurement .................................................... 32 1.5.2 Brunauer-E mmett-Teller (BET) Surface Area Analysis ................................... 33 1.5.3 Temperature Programmed Reduction Experiments .......................................... 34 1.5.4 N2O Decomposition Experiments ..................................................................... 34 1.5.5 Catalyst Density Measurem ents ........................................................................ 35 1.5.6 X-ray Diffraction Analysis ................................................................................35 1.5.7 X-ray Photoelectron Spectroscopy A nalysis ..................................................... 36 2 STEAM REFORMING OF METHANOL USING Cu-ZnO CATALYSTS SUPPORTED ON NANOPARTICLE ALUMINA ............................................................... 37 2.1 Introduction and Literature Review ..............................................................................37 2.2 Experim ental Methods .................................................................................................. 39 2.2.1 Catalyst Preparation .......................................................................................... 39 2.2.2 Reactor Operation and Data Collectio n ............................................................ 41 2.2.3 BET Surfac e Area Analysis ..............................................................................41 2.2.4 Catalyst Density Measurem ents ........................................................................ 42 2.2.5 X-ray Photoelectron Spectroscopy A nalysis ..................................................... 42 2.2.6 Therm odynamic Calculations ........................................................................... 43
7 2.3 Results and Discussion. ................................................................................................. 43 2.3.1 Methanol Conversion. .......................................................................................43 2.3.2 CO Selectivity ...................................................................................................46 2.3.3 Blank Experim ents. ....................................................................................... 49 2.4 Catalyst Characterization Results. ................................................................................50 2.4.1 Surface Area Analysis ....................................................................................... 50 2.4.2 X-ray Diffraction Analysis ................................................................................53 2.4.3 Temperature Programmed Reduction Measurements ....................................... 58 2.4.4 X-ray Photoelectron Spectroscopy A nalysis ..................................................... 59 188.8.131.52 Fresh catalysts .....................................................................................60 184.108.40.206 Spent catalysts ....................................................................................62 2.5 Therm odynamic Calculations ....................................................................................... 66 2.6 Conclusions ...................................................................................................................68 2.7 Acknowledgem ents. ...................................................................................................... 70 3 INCORPORATION OF REDUCIBLE OXIDES: STEAM RE FORMING OF METHANOL OVER CeO2AND ZrO2-PROMOTED Cu-ZnO CATALYSTS SUPPORTED ON NANOPARTICLE Al2O3 ........................................................................71 3.1 Introduction and Literature Review. ............................................................................. 71 3.2 Experim ental Methods. .................................................................................................73 3.2.1 Catalyst Preparation. .........................................................................................73 3.2.2 Reactor and Gas Chrom a tograph Operation .....................................................76 3.2.3 Water Gas Shift Equilib rium ............................................................................. 76 3.3 Catalytic Activity Measurem ents .................................................................................. 77 3.3.1 Methanol Conversion ........................................................................................77 3.3.2 CO Selectivity ...................................................................................................81 3.4 Catalyst Characterization Results. ................................................................................87 3.4.1 Surface Area Analysis ....................................................................................... 87 3.4.2 Temperature Programmed Reduction Measurements ....................................... 89 3.4.3 X-ray Diffraction Analysis ................................................................................92 3.5 Breaking the W ater Gas Shift Equilibrium ................................................................... 95 3.6 Conclusions ...................................................................................................................97 3.7 Acknowledgem ents ....................................................................................................... 99 4 CHARACTERIZATION OF ZrO2-PROMOTED Cu/ZnO/NANO-Al2O3 METHANOL STEAM REFORMING CATALYSTS ................................................................................100 4.1 Introduction and Literature Review ............................................................................100 4.2 Experim ental Methods ................................................................................................ 101 4.3 Results and Discussion ................................................................................................ 102 4.3.1 Catalytic Activity and C O Selectivity ............................................................. 102 4.3.2 X-Ray Diffr action Analysis ............................................................................104 4.3.3 X-Ray Photoelectron S pectroscopy Analysis ................................................ 107
8 220.127.116.11 Peak area ratios ................................................................................. 107 18.104.22.168 Fresh catalysts ...................................................................................109 22.214.171.124 Spent catalysts ..................................................................................114 4.4 Conclusions .................................................................................................................116 5 DESIGN AND CONTROL OVER CuO PA RTI CLE SIZE AND Cu SURFACE AREA IN Cu/ZrO2 METHANOL REFORMING CATALYSTS FROM NANOPARTICLE AND MICROEMULSION PRECURSORS. ....................................................................... 118 5.1 Introduction and Literature Review ............................................................................118 5.2 Experim ental Methods ................................................................................................ 119 5.2.1 Catalyst Preparation ........................................................................................ 119 5.2.2 Reactor Exp eriments .......................................................................................121 5.3 Catalytic Activity Measurem ents. ............................................................................... 122 5.3.1 Methanol Conversion ......................................................................................122 5.3.2 CO Selectivity .................................................................................................125 5.4 Catalyst Characterization Results ...............................................................................128 5.4.1 Temperature Programmed Reduction ............................................................. 128 5.4.2 X-ray Diffraction Analysis ..............................................................................131 5.4.3 Surface Area Analysis ..................................................................................... 134 5.4 Water Gas Shift Equilib rium ....................................................................................... 137 5.5 Conclusions .................................................................................................................138 5.6 Acknowledgem ent ......................................................................................................139 6 SUMMARY AND CONCLUSIONS ...................................................................................140 REFERENCES .................................................................................................................... ........143 BIOGRAPHICAL SKETCH .......................................................................................................148
9 LIST OF TABLES Table page 1-1 Calibration curve fits: ................................................................................................... .....27 2-1 Comparison of CO selectivity values. ............................................................................... 48 2-2 Surface characteristics of reforming catalysts. .................................................................. 51 2-3 Particle sizes of the different com pounds on the various CuO-ZnO/Al2O3 catalysts in the current work. ............................................................................................................. ...55 2-4 Relative surface concentrations of differe nt elements calculated from the atomic concentrations at the surface.. ............................................................................................61 3-1 Catalyst preparation summary ...........................................................................................74 3-2 Surface characteristics and activ ities of steam reforming catalysts. .................................. 87 3-3 Particle sizes of the different com pounds on the various CuO/ZnO/ZrO2/Al2O3 catalysts in the current work. ............................................................................................. 94 4-1 Reactor effluent composition for all catalysts at s elected temperatures. ......................... 103 4-2 Peak area ratios for all catalysts calculated from the XPS data. ...................................... 108 5-1 Catalyst preparation details. .............................................................................................121 5-2 Particle sizes for CuO/ZrO2 catalysts. .............................................................................133 5-3 Surface characteristics of Cu/ZrO2 catalysts. ...................................................................134
10 LIST OF FIGURES Figure page 1-1 Catalytic Reactor system .................................................................................................. ..21 1-2 Calibration curves.. ...................................................................................................... ......26 2-1 Conversion of methanol as a function of tem perature at consta nt liquid feed flow rate......................................................................................................................................44 2-2 Conversion of methanol as a function of space time. ........................................................ 45 2-3 CO selectivity, CO/(CO2+CO) as a function of temperat ure at constant liquid feed flow rate. .................................................................................................................... ........47 2-4 CO selectivity, CO/(CO2+CO) as a function of space time. .............................................. 49 2-5 XRD spectra obtained from fresh catalysts. ...................................................................... 53 2-6 XRD spectra obtained spent catalysts. ...............................................................................57 2-7 Normalized Temperature Progr amm ed Reduction (TPR) Profiles.. .................................. 59 2-8 XPS spectra of the Cu 2p binding en ergy region obtained fr om catalysts:. ...................... 60 2-9 XPS spectra of the Cu Auger kinetic energy region obtained from catalysts. ................... 64 2-10 Comparison of Keq rWGS and ratio of m easured partial pressures.. ................................ 67 3-1 Methanol Conversion as a function of temperature. .......................................................... 79 3-2 Conversion as a function of contact tim e for the 10% ZrO2 catalysts, the 10% CeO2 catalyst, and the commerci al reference catalyst.. ............................................................... 81 3-3 CO selectivity as a function of tem perature for the 10% ZrO2 catalysts, the 10% CeO2 catalyst, and the commerci al reference catalyst. ......................................................83 3-4 CO selectivity as a function of contact tim e for the 10% ZrO2 catalysts, the 10% CeO2 catalyst, and the commerci al reference catalyst.. .....................................................85 3-5 Conversion and CO selectivity as a f unction of contact tim e for CuZnZrAl-10-NP catalyst. ..............................................................................................................................86 3-6 Temperature programmed reducti on profiles for all catalysts. .......................................... 89 3-7 XRD spectra obtained for all fresh and spent catalysts.. ...................................................93
11 3-8 Dimensionless water gas shift equilibrium constant, as a function of temperature three nanoparticle catalysts ................................................................................................ 96 4-1 CH3OH conversion and corresponding CO2, H2 and CO production rates for the CuZnZrAl-NP catalyst.. ................................................................................................... 102 4-2 H2 and CO production rates for all catalysts.. .................................................................. 103 4-3 XRD spectra obtained from fres h catalysts and spent catalysts ...................................... 106 4-4 Labeled XPS survey spectra for CuZnZrAl-CI catalyst. ................................................. 107 4-5 Zr 3d XPS spectra obtained from fr esh catalysts and spent catalysts.. ............................ 109 4-6 Cu 2p XPS spectra obtained fr om fresh and spent catalysts ............................................111 4-7. O 1s XPS spectra obtained fr om fresh and spent catalysts. ............................................. 113 5-1 Methanol conversion as a function of te m perature for catalysts fabricated from different microemulsion precursors. ................................................................................ 123 5-2 Methanol Conversion as a function of tem perature for catalysts of different calcination temperatures. ................................................................................................. 124 5-3 Methanol conversion for selected catalysts.. ................................................................... 125 5-4 CO selectivity for catalysts fabricated from different microemulsion precursors.. ......... 126 5-5 CO selectivity as a function of temperat ure for catalysts of different calcination tem peratures. ................................................................................................................. ...126 5-6 CO Selectivity for selected catalysts.. ..............................................................................127 5-7 TPR profiles for catalysts calcined at di fferent temperatures and from different microemulsion precursors. ............................................................................................... 129 5-8 XRD spectra collected from fr esh catalysts. spent catalysts. ...........................................132 5-9 Dimensionless water gas shift equilibrium constant, as a function of temperature for selected catalysts ........................................................................................................ 138
12 Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ACTIVITY AND CHARACTERIZATION ST UDIES IN METHANOL REFORMING CATALYSIS: Cu AND CuO-ZnO CATALY STS AND THE ROLE OF NANOMATERIALS By Samuel David Jones December 2008 Chair: Helena Hagelin-Weaver Major: Chemical Engineering Hydrogen storage is a major barrier to commercialization of proton exchange membrane (PEM) fuel cell powered automobiles. The prob lem can be circumvented by storing a liquid hydrogen source and then using a reforming reacti on to generate hydrogen onboard the vehicle. Methanol is the preferred li quid hydrogen source for onboard generation. Methanol steam reforming is a convenient way to generate large amounts of clean hydrogen at reasonable temperatures and atmospheric pressure. Cu and CuO-ZnO reforming catalysts on Al2O3, ZrO2/Al2O3, and CeO2/Al2O3 nanoparticle oxide supports were i nvestigated in this work. These systems ranged from traditional impregnated CuO-ZnO/Al2O3 to more complex CuO-ZnO on mixed nanoparticle ZrO2/Al2O3 supports. Finally, binary CuO/ZrO2 systems were constructed using a reverse microemulsion procedure. Detailed reaction studies were performed and kinetic reaction data was examined and compared to surface, structural and electronic ch aracterization data in order to determine both structural and valence state inform ation of the catalyst system befo re and after reaction. In all cases it was determined that a reasonable Cu su rface area is necessary to catalyze the reforming reaction but that high Cu surface area is not the critical criterion for highly active reforming
13 catalysts. It was shown th at using nanoparticle Al2O3 supports can greatly increase catalyst surface area but that Al2O3 has a retarding effect on catalytic activity which partially offsets any benefits. Therefore it was concluded that Al2O3 should only be used in relatively low concentrations or in conjuncti on with another oxide support. It was determined that an electron deficient Cu species formed due to an interaction with the nanoparticle ZrO2 support which was highly beneficial for catalyst performance. This electron deficient Cu species promoted the meth anol reforming reaction while also apparently suppressing CO production via the re verse water gas shift. This wo rk demonstrates that the CuZrO2 synergy can be exploited by using binary re forming catalysts and is increased by using calcination temperatures above 300 C, despite a slight loss of Cu surface area at high calcinations temperatures.
14 CHAPTER 1 INTRODUCTION, LITERATURE REVI E W, EQUIPMENT AND PROCEDURES SUMMARY 1.1 Introduction and Motivation for Research Fuel cells have been the s ubject of intense research ove r the past decade as an environm entally friendly alternative to the rath er dirty and inefficien t internal combustion engines. Fuel cells have severa l distinct advantages as mobile power sources when compared to combustion engines. First, since a fuel cell is an electrochemical power source and not a thermal power source, fuel cells of any t ype are not restricted by the Carnot efficiency, which states that the maximum theoretical efficiency for internal combustion engines is around 40%, with actual working efficiencies closer to 25% [ 1]. Freedom from the Carnot lim itation means that the maximum theoretical efficiency of a fuel cell is 100%. Nearly every car in existence today is powered by the combustion of hydrocarbons, and even with todays catalytic converters in cars and filters on diesel engines, this is still a relatively dirty business. Gasoline powered engines emit sulfur oxides, nitrogen oxides, and volatile organic carbons, and the emissions from their dies el counterparts also contain their share of sulfur oxides as well as fine particulates. PE M fuel cells, by contrast, emit only water. There are many types of fuel cells, and most ar e named by the type of electrolyte used to carry electric charge through some type of membrane or separa tion. The most common types are solid oxide, direct methanol, phosphoric aci d, and proton exchange membrane (or polymer electrolyte membrane) fuel cells. Direct methanol fuel cells are perhaps the most desirable since the fuel cell uses methanol itself as the fuel and thus there is no need for methanol reforming to produce hydrogen as is the case for proton excha nge membrane fuel cells. However, direct methanol fuel cells have notoriously poor anode kinetics and are not suitable for the high power requirements which would be necess ary in automotive applications [ 2].
15 Alternatively, PEM fuel cells have much better (faster) anode kinetics and can meet the power requirements of automotive applications [ 1,2] The first practical use for PEM fuel cells was in the 1960s in the Ge mini space missions [ 1]. One notable problem with PEM fuel cells is that onboard high pressure hydrogen storage is neither safe no r practical in a comm ercial application. An alternative is to store a hydrogen source in liquid fo rm and generate the hydrogen on board. One method of great interest in recent year s is hydrogen production from methanol via steam reforming over copper-based catal ysts. Methanol is the preferred fuel for the reforming process given that it has a high H:C ratio, can be reformed at reasonably low temperatures (200-300 C) and produces low amounts of CO [ 3]. One of the major challenges with the steam refor ming reaction is to avoid CO formation. Even small amounts (levels greater than 10 ppm) of CO present in the hydrogen fuel w ill result in lower fuel cell efficiencies due to CO poisoning of the PEM fuel cell anode [ 1]. The generally accepted limit on CO concentration which can be tolerated by the anode catalyst is below 100 ppm [1, 4]. Consequently, not only are high yields of hydrogen at low te m peratures important, a CO-fr ee or near CO-free hydrogen also has to be produced. This is a great challeng e to the heterogeneous catalyst community. 1.2 General Literature Review The first docum ented instance of hydrogen gene ration from steam reforming of methanol was in 1921 [ 5]. Reforming was achieved using pure coppe r m etal. Since that time, the trend in methanol reforming, and heterogeneous catalysis in general, has been towards more durable catalysts which generally incorporate some comb ination of an active me tal (copper), plus a support to increase surface area, as well as a stabilizing component, also called a promoter. The most common promoter for Cu-based me thanol steam reforming catalysts is ZnO [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22, 23]. The ZnO is added to improve the dispers ion of the Cu, increase the so-called spill over effect, and to improve the reducibility of
16 the CuO precursor [ 12,13,14,16,19,20, 24]. The adsorption of methanol is generally thought to occur on the copper m etal, and hydrogen atoms dissociate from the adsorbed methanol and move (spill over) from the copper onto the ZnO, where they are loosely bound [ 24]. Alum ina has long been a popular support for co pper-based steam reforming catalysts due to its high surface area [ 6,7,8,10,11,12,13,14,15,16,17,18,19,22]. It has also been shown that the alum ina can reduce the vulnerabil ity of copper to sintering [ 12,15,16,19]. Additionally, since m ethanol is a Lewis base, it should not be surprising that alumina, which is a strong Lewis acid, would support a strong electronic in teraction between the methanol reactant and the catalyst surface [ 11, 25]. Despite the common use of alumina as a support in these re actions, it has been shown that the H2 yield decreases with increasing alumina concentration and that Al2O3 actually hinders the reforming reaction [ 16]. It is therefore not surprising that recently work has been done m oving away from the traditional alumina support to alternative supports such as CeO2 and ZrO2 [14,16,18,26,27,28, 29]. Other references suggest that a m ixed oxide support would be better still [ 18,28, 30]. The main advantages of CeO2 and ZrO2 supports appear to be that the support oxides themselves are more easily reduced than the traditional Al2O3 supports, and furthermore, that increasing Al2O3 content tends to decrease the reducibility of the CuO species [15,26,31, 32]. The role of ZrO2, CeO2, and ZrO2 nanoparticles in metha nol reforming catalysis will be examined in Chapters 3 and 4. Not long after the Gemini space missions and with evidence that methanol reforming was potentially useful in power generation, effort s were made to model the catalytic reforming reaction. Most models assume that the steam reforming of methanol involves three main reactions, shown here in equations 1-1 to 1-3. Th ese include: 1-1) steam forming of methanol to
17 form hydrogen and carbon dioxide; 1-2) the wa ter-gas-shift (WGS) re action and 1-3) the decomposition of methanol. 2 2 2 33 COHOHOHCH 1-1 CO + H2O CO2 +2H2 1-2 COHOHCH 2 32 1-3 It has been demonstrated that the equilibrium of reaction 1 is over 99% in the presence of excess steam at 500 Kelvin and 1 atmosphere [ 33]. Among the earliest model was that of Santaces aria and Carra [ 34]. These researchers developed a Langmuir-Hinshelwood type expression that fit th eir reaction data but did not involve a spec ific surface mechanism. An Arrhenius-type kinetic equation for the rate of methanol conversion in the reforming reaction was later developed by Amphlett in 1985 [ 35]. This model is shown in equation 1-4 using the authors notation: COc HCO OHCH OHCHPk PkPPk r22 3 3' 1-4 In this equation PCH3OH, PCO and PH2 are the partial pressures of CH3OH, CO, and H2, respectively. The k terms are Arrhenius type rate constants. It is worth noting that the partial pressures of H2O and of CO2 do not appear anywhere in this equation. A more well-accepted model was developed in 1993 by Jiang, et al [ 36]. His kinetic expression is more com plex and demonstrates the nonlinear nature of the metha nol reforming reaction net work. This model is shown in equations 1-5 and 1-6. OH OHCH OHCHPPkr2 3 303.0 26.0' kPaPH72 1-5 2 2 3 32.0 03.0 26.0'H OH OHCH OHCHP PPk r kPaPH72 1-6
18 This model was fairly well accepted and is heavily cited in the methanol reforming literature. However, in this work no effort wa s made to develop an actual surface mechanism, only to empirically fit reaction data to an Arrhenius expression. In 1998 a very ambitious study by Peppley and coworkers developed a detailed surface mechanism for copper and zinc oxide on alum ina catalysts. Their goa l was to develop a comprehensive model for the entire reforming network [ 7, 8]. Many later models cite this work. Peppley et al. insisted that the m ethanol decomposition reacti on (equation 1-3 above) had to be considered for an accurate and comprehensive kinetic model [ 7, 8]. The authors based this conclusion, in no sm all part, on th e work of Vanderborough, et al. [ 6]. In their study Vanderborough et al. used a CuO-ZnO/Al2O3 catalyst which exclusivel y contained radio labeled oxygen atoms (18O). The authors were able to show that C18O18O and CO18O are immediately detectable after the start of th e reforming reaction, whereas C18O is never detected [ 6]. This is direct evidence that the carbon-oxygen bond in m ethanol was never broken and that carbon monoxide therefore is formed via methanol de composition and not via the reverse water-gasshift reaction. Peppley and coworkers performed the reformi ng reaction and studied the equilibrium of the water-gas-shift reaction. They showed that at very low meth anol conversions the CO partial pressure is higher than can be achieved through equilibrium of the water gas shift. Peppley states that the only explanation for this resu lt is that the rate of the decomposition reaction must be significant with respect to the rates of the steam reforming and water gas shift reactions [ 7]. W ith the information in the preceding three paragraphs, Peppley made two key conclusions: 1) the reaction pathway for CO prod uction must be different from the pathway for
19 CO2 production. This means that there must be one distinct type of activ e site for CO production and a separate type of active site for CO2 production, and 2) the direct decomposition of methanol must be included in a kinetic model. The latter conclusion is of greater relevance in this dissertation. The Peppley work has one distinct disadvantage in that is it extremely cumbersome. The model involves dozens of rate constants, active site a nd intermediate species concentrations, fitting parameters and equilibrium coefficients. When employed, it is almost exclusively used in an abbr eviated and simplified form. In contrast to the work by Peppley et al., Agrell and coworkers found that the CO formed in the steam reforming reaction over their catalysts was below the water gas shift equilibrium concentration at every temperat ure in the study and furthermore that CO production decreased to the point where it was a negligible product at very short contact times [ 9]. These observations do not support the assertion that CO is a prim ary product, as stated by Peppley. This is in agreement with a study by Purnama, et al. who stated that their data in dicates that CO is a secondary production produced exclusively through the reverse water gas shift [ 10]. It is interesting to note that T urco, et al. found that it is possible to a ttain decent methanol conversions through the direct decomposition of methanol, but only in the absence of both steam and O2 [ 11]. The authors reported that the activation energy fo r the r eforming reaction is considerably lower than that of the decomposition reaction and theref ore in the presence of steam methanol reacts via reaction (1-1) to form H2 and CO2 [ 11]. This is in agreement with results of Choi and Stenger [ 37]. Recent work by Mastalir, et al. agrees that the m odel which best fits the data includes the methanol decomposition [ 4]. Mastalir stated that the contribution of the decom position reaction was only sign ificant at low conversions [ 4], which agrees with the findings by Peppley [ 8]. However, Mastalir concluded th at CO was predominantly formed
20 through the reverse water gas shift, although methanol decomposition does play some role at low contact times [ 4]. Evidently the reaction is very sensit ive to the reaction c onditions and catalyst in use. The r ole of methanol decomposition and the equilibrium of the water gas shift will be covered in detail in this work. When reviewing th e literature it is eviden t that despite the number of articles on the copper-based steam reforming cat alysts, there is still some controversy in the literature on the actual re action mechanism and, furthermore, a detailed picture of the chemical and electronic characteristics of the active sites is to this date not available. Recent work has noted an increase in catalyt ic activity of Cu metal in the methanol reforming reaction when a re ducible oxide such as CeO2 or ZrO2 is used as the catalyst support [ 27,29,38,39,40,41,42, 43]. There has been a discussion of s ynergy in the C u/Zr catalysts that is credited for an increase in catalytic activity [ 29, 42]. The possible advantages of using nanoparticle ZrO2 in methanol reforming catalysts have not been investigated at the date this dissertation was published. The nature of the interaction between the ZrO2 support and the Cu metal in methanol reforming catalysts is inve stigated in Chapter 3. The Cu-Zr electronic interaction in the catalyst is co vered in detail in Chapter 4. In order to exploit the sy nergy of the Cu/Zr system, novel microemulsion catalyst preparation techniques have been used by Ritzkopf, et al. [ 44]. The utility of the microemulsion preparation procedure has previously been dem onstrated for Cu/ZnO [ 45] and Pd/ZnO systems [ 46] by other researchers. The apparent advantag e of the m icroemulsion preparation technique for Cu/ZrO2 reforming catalysts is that the resultant cat alysts have a lower CO selectivity than is attainable with catalysts from more traditional preparation techniques [ 44]. However, the possible advantages of using na noparticle supports in conjuncti on with m icroemulsion catalyst
21 preparation procedure have yet to be examined. This new area of methanol reforming research will be covered in Chapter 5. In the present work, we have investigated th e consequences of usi ng nanomaterials in the methanol reforming catalyst fabrication. Sp ecifically, this work employs nanoparticle Al2O3 and ZrO2 supports in the cataly sts that have not been investigat ed previously. A more detailed literature review will be included in the chapter where it is most rele vant to the topic at hand. 1.3 Catalytic Reaction System Design and Construction 1.3.1 Reactor System Design Since the initiation of this project, a complete reactor system for testing catalytic activity has been constructed. Figure 1-1 shows the reactor system di agram for the catalyst testing system Figure 1-1 Catalytic Reactor system
22 The system uses an internal (inert) standard of Argon as the calibration reference. Argon is introduced to the system by an MKS series M100 mass flow controller (MFC). The gas travels through a series of valves used to isolate different parts of the system depending on the type of experiment being performed. In the case of metha nol reforming, the argon flow is directed to a KDS series 101 syringe pump whic h dispenses the desired volume of methanol and water. The argon/methanol/water mixture trav els a short distance to the in house built evaporator, which is maintained at 220 degrees C by an Omega brand model CN616TC2 PID type controller. For the studies covered in Chapters 3 and 4, this in -house built evaporator was replaced by a more sophisticated Barnstead Thermolyne model F21125 tubular furnace. The effluent temperature from the evaporator is monitored via an Omega 1/8 type K thermocouple to make sure that there is single phase flow to th e reactor. The reactant mixture then travels a short distance in heated tubing to the catalytic reactor. The reac tor consists of a stainless steel tube which houses the catalyst bed. The catalysts used in this study were pressed into pellets in a diameter pellet die under two tons of force exerted by a Carver pre ss. For all reac tor experiments in this study, the catalyst pellet of the sample to be loaded into the reactor was crushed and sieved to obtain particle s with sizes between 500 m and 1 mm for activity testing. The reactor is loaded by simply disconnecting the compression fittings on either side of the reactor and removing the reactor tube from the furnace. Sieved catalyst particles are lo aded in the effluent (downstream) side of the reactor. The catalyst pa rticles are held in place by two plugs of quartz wool which take no part in the reac tion. The size of the bed (weight of catalyst) can be varied as desired. The downstream side of the catalyst bed is supported by an Omega type K thermocouple. From the reactor, the effluent gas again travels through a length of heated pipe into an ice bath condenser where unreacted CH3OH and H2O are removed. It was determined
23 early in this study that introduc tion of liquid species into the chromatograph was problematic. The different temperatures in different parts of th e instrument give rise to rapid condensation and evaporation of the liquid species, which likewise results in huge swings in the flow rate. This gives rise to problems in reproduc ibility. After the ice bath the non-condensable species travel a short distance to the GC for analysis. 1.3.2 Gas Chromatograph Design and Operation The main piece of analytical instrumentation fo r all of these tasks is a custom built Agilent 6890 gas chromatograph (GC). A ll reaction products can be meas ured in a single sample of product gas injected automatically to the GC. The GC is progr ammed to take a sample of a known volume of gas through an automatic sampli ng valve when prompted. Manual injections are also possible. From the sampling valve, the different components of the product gas are separated by two capillary columns in series inside the GC oven. The oven is set at 30C at the start of the run in order to separate the permanent gases, and ramps up to 65C to elute the remaining components. Permanent gases, except for CO2, first travel through an Agilent polar Plot Q column, and then onto a molecular sieve (mol. sieve) for further separation. Methanol vapor, CH2O and CO2 could potentially get hung up on th e mol. sieve, and are adequately separated by the Plot Q column. Theref ore, after the permanent gases besides CO2 pass onto the mol. sieve, it is isolated by means of a column isolation valve which pr events the other species from entering this column. Instead, any methanol vapor, CH2O, and CO2 are sent straight towards the detectors. Just before enteri ng the flame ionization detector, CO and CO2 pass over a nickel catalyst (methanizer) which reduces them to methane so that they can be detected on the flame ionization detector.
24 There are two detectors in series in this GC, a thermal conductivity detector (TCD) and a flame ionization detector (FID). The FID is used to measure the concentrations of any species which contains a carbon-hydrogen bond (methanol, methane, plus CO and CO2 after reduction). For all experiments in this study the FID temperature is set at 400C, and the H2, air, and N2 make-up gas flow rates to the FID are set to 40, 450, 6 sccm, respectively. The FID is operated in Constant Make-Up mode where the make-up N2 flow to the detector is held constant. These settings give a baseline signal of ~6.0 pA. The TCD performs the same task but on all chemical species. Although these measurements could be performed with the TCD alone, the additional detector provides higher sensitivity and a level of redundancy for the hydrocarbon species. The FID is also able to detect the presence of trace amounts of CO that may not be detectable with the TCD alone. Further details of how the detectors work can be found elsewhere [ 47]. For all experiments in this study the TCD tem perature is set at 250C. The reference He flow is set at 33.0 sccm and the make-up He flow is set at 4 sccm. The TCD is operated in Constant Make Up m ode for all experiments. These settings give a baseline signal of ~40 V. After exiting the de tectors, the product gas travels to the fume hood for ventilation. The direct line of helium to the GC serves as both the carrier gas and the reference gas for the TCD (see Figure 1-1). The direct air, hydrog en, and nitrogen lines to the GC serve as oxidant, fuel, and make up gas, respectively, for the FID. 1.3.3 Gas Chromatograph Calibration After the complete construction of the system, th e next step before catalytic activity testing was a complete calibration of the analytical equipment. This was accomplished by sending a
25 known flow rate of the gas to be used in the ca libration and a known flow of the internal (argon) standard to the GC. The signal area from the detector(s) was th en noted and the ratio of the signal areas of the calibration gas to argon was recorded. Si nce the concentration of the calibration gas sent to the GC is known, a plot can be constructed of the concentration (volume percent) of the gas in question as a function of the gas to argon signal area ratio. TCD and FID calibration plots for several of th e gases present in the reforming reactions are shown in Figure 12. The TCD response to hydrogen is obviously ve ry nonlinear towards th e upper end of the detection limit. The data are fit with a logarithmic expression and the measurement appears reproducible. A slightly more accura te calibration curve can also be used which is linear over the low H2/Ar ratio signal range. It is also worth noti ng that the detector doe s not have the exact same response for a sample containing 20% CO2 as it would for a sample containing 20% CH4. This calibration takes into account that the detector has a different response for each component. In Figure 1-2 B it is evident from the overlap of the CH4 and CO curves that essentially all of the CO sent to the methanizer is reduced to methane. Therefore the de tector sees all CO as CH4 and does not report a differen ce. However, the CO and CH4 peaks are easily identifiable since they have different elution times. The fact that the CO2 curve is further to the left on the plot reflects the fact that CO2 is more difficult to reduce th an CO and consequently the methanizer does not reduce the entire volume of CO2 in the sample to methane, as it does in the case of CO. The details from the calibration curv es generated from these plots are displayed in Table 1-1.
26 0 10 20 30 40 50 60 70 80 90 00.10.20.30.126.96.36.199.80.9Area ratioVolumetric % 0 5 10 15 20 25 30 35 024681 01 2Area ratioVolumetric % Figure 1-2 Calibration curves. A. Calibration Curves for the thermal conductivity detector (TCD) response to various gases. B. Ca libration curves for the flame ionization detector (FID) response to various gases. H2 CH4 CO2 CO For all calibrations the flow of th e argon internal standard was constant at 15 sccm. Some non-linearity in the detect or response is to be expected especially at the upper and lower limits of detection [ 48]. Sources of non-linearity in ch rom atographic systems result from
27 Table 1-1. Calibration curve fits: Species Detector Curve type fit (R2) CO TCD Linear 0.994 CO TCD quadratic 1.0 CO FID Linear 0.992 CO FID quadratic 0.999 CO2 TCD Linear 0.981 CO2 TCD quadratic 0.999 CO2 FID Linear 0.981 CO2 FID quadratic 0.995 H2 TCD logarithmic 0.979 CH4 TCD linear 0.993 CH4 TCD quadratic 0.999 CH4 FID Linear 0.993 CH4 FID quadratic 0.999 interactions with inlet linings, valve lubricants, or other com ponents in the system which can incur some sample losses [ 48]. The non linearity is not nece ssarily a problem as long as it is both reproducible and quantifiabl e. Developing a mathematical relationship betw een the signal response and the concentration takes this non-linearity into account. Manual curve fitting is also possible in regions where the mathematical relation differs notably from the data. Note that the linear fits for most gases are ad equate, but relationships which ta ke into account the non-liner (quadratic) contributions at the upper and lower end of the con centration range give the best fit. From this table it is evident which calib ration curve should be used for each species. CH2O is difficult to detect with a GC [ 49]. It is also difficult to p repare a calibration curve for this species. In this study, CH2O vapor calibration was performed using a saturation chamber, since the vapor pressure (and total sy stem pressure) is known at a set temperature. Condensate analysis was performe d via liquid injections into the same chromatograph. The condensate analysis showed th at even at the highest CH2O production rates, the water in the condenser had either no CH2O at all, or only negligible amounts of dissolved CH2O. In either
28 case the tiny amount of CH2O in the condensate does not cha nge the shape of the methanol conversion curves shown in the follo wing chapters since the amount of CH2O dissolved in the condensate is several orders of magnitude less than the amount of CO2 produced in the reactor. Calibrations for liquid CH2O in the condensate were performed using a saturated CH2O solution (Fisher Scientific) a nd diluting it to known concentrations of CH2O. There is also no evidence of coking of the catalysts (measured via carbon co ntent of the surface via XPS), which would further complicate calculating an accurate conve rsion via a carbon balance. All chromatography data is analyzed using the Cerity 3.0 software, which is the same software used to operate the GC. 1.4 Water Gas Shift Equilibrium The water gas shift, shown in equation 1-2, ha s been studied by genera tions of chemists. It is of particular intere st in this study to elucidate the sour ce of CO. To this end, equilibrium calculations were performed to compare the partial pressure of CO in the reactor effluent to the equilibrium partial pressure of CO for the re verse water gas shift reaction. The thermodynamic equilibrium constant for the water gas shif t was calculated accor ding to equation 1-7: ) (exp *2 2 2TR G OH eq CO eq H eq CO eq WGS eqWGSPP PP K 1-7 In this equation CO eqP 2CO eqP 2H eqP and OH eqP2 are the equilibrium partial pressures of the species involved in the water gas shift reaction. G WGS is the free energy change associated with the water gas shift reaction, R is the ideal gas constant and T is the temperature in Kelvin. The free energy change G WGS is itself a function of temperature The temperature dependence is given by the vant Hoff equation:
29 2)/( RT H dT RTGd 1-8 It is commonly assumed that the stan dard enthalpy change of reaction, H is independent of temperature. If equation 1-7 is solved for G and this result is substituted into equation 1-8, the result is: 2))(ln( RT H dT KdWGS eq 1-9 Thus, by integrating from the reference temperature T to the temperature of interest, T the result is: ) 11 (*) ln( TTR H K KWGS eq WGS eq 1-10 Using equations 1-7 and 1-10, the water gas shif t equilibrium constant can be calculated at any temperature given G and H Keq WGS can in turn be used to determine the source(s) of CO in the system at a given set of re action conditions by defining the function as shown in equation 1-11: OH CO HCO WGS eqPP PP K2 221 1-11 Equation 1-11 shows that is defined as the inverse of the water gas shift equilibrium constant at the temperature of in terest times the ratio of partial pressures of the products of the water gas shift reaction divided by the partial pressures of the reactants as measured in the reactor effluent. If the ratio of the partial pressures is equal to the equilibrium partial pressure ratio shown in equation (7) then the system is at equilibrium with respect to the water gas shift reaction and =1.0. If the CO concentr ation in the effluent is above the water gas shift
30 equilibrium concentration, then this is indica tive that there is CO production via the methanol decomposition reaction in addition to the reverse water gas shift. In this case PCO is relatively large (greater than Peq CO) and therefore is less than unity. The condition of being less than unity is therefore indicative of CO being pr oduced via the methanol decomposition reaction. This same procedure was originally used by Graa f, et al. in their study of the water gas shift reaction [ 50]. The procedure was later used by Pe ppley, who concluded that m ethanol decomposition, though undesirable, had to be include d in the model of the reaction network for a complete picture [ 7, 8]. 1.5 Catalyst Characterization Techniques 1.5.1 Catalyst Activity Testing The primary objective in the course of this i nvestigation is to fabricate and test highly active methanol reforming catalysts using nanomaterials. The fabrication techniques used in this study varied according to what property of th e catalyst was to be examined. Detailed information of each of the fabrication techniques used can be found in the appropriate chapter. This section contains a general overview on the methods used to examine catalytic activity and characterize the physical and electron ic properties of these catalysts. 188.8.131.52 Catalyst pretreatment In all reactor experiments in this study, the catalyst loaded in th e reactor was reduced by dilute H2 before being exposed to the H2O/CH3OH feed. This reduction was accomplished by flowing ~15% H2 in Ar over the catalyst bed at 300C for 3 hours. The flow rates used were 7.5 sccm of H2 and 40 sccm of Ar. After the reduction the catalyst was allowed to cool under a pure Ar flow before beginning an act ivity experiment. This is the pretreatment recommended by the catalyst manufacturer. It has also b een used by other research groups [ 9].
31 184.108.40.206 Temperature dependent activity measurement The primary method of determining the activity of a given catalyst in any reaction is to vary the temperature of the reaction and meas ure the corresponding activity of the catalyst. Catalytic activity is determined by measuri ng the composition of the reactor effluent and calculating methanol conversion and CO selectivity as described previously. In the present work this was accomplished by loading the reactor with a set mass of the catalyst to be tested as described in section 1.3.1. Generally this mass was approximately 340 mg unless otherwise noted. The catalyst was then reduced in situ as described in section 1. 5.1.1. After reduction the reactor was cooled to 200C and the experiment started. To begin an experiment, the syringe pump was loaded with a mixture of CH3OH and H2O. There is much discussion in the literature as to the proper H2O/CH3OH ratio for the methanol reforming r eaction. It has been shown that excess H2O suppresses the formation of undesirable sp ecies such dimethyl ether and methyl formate [ 37]. The H2O/CH3OH ratio used in each experiment is noted in the appropriate chapter, but was generally either 3.1 or 1. 4. At the start of the temperat ure sweep, the reac tor temperature was allowed to stabilize at 200C, the condenser was loaded with ice water, the temperature of the evaporator coil was set at 200C, the syringe pump was starte d, and all heating tapes are turned on at maximum output. The two detect ors in the GC were set at the appropriate temperatures as described in section 1.3.2. The system generally reaches equilibrium approximately one hour after th e initial power-up. Longer equilibration times were noted in Chapter 2 and 3 but changes in the power up procedure shortened this time in the later studies (Chapters 4 and 5). Equilibrium was said to be a ttained when the measured output of the GC was no longer changing with time at a given temper ature. Six gas samples are taken at each temperature, which are averaged to give the me thanol conversion, CO se lectivity and any other information from the reaction. If the data was s een to be stable and there are no changes in the
32 measured outputs then the temp erature is increased by 15-20C and the process is repeated. In order to ensure reproducibility, it was often necessary to repeat the high te mperature points in order to demonstrate that time on stream has not im pacted the catalyst performance. In order to do this, the same procedure was followed except that the temperature sweep was started at 300C or higher. This generates a second activity curve, which should overlap w ith the first curve if time on stream does not deactivate th e catalyst. Because unreacted CH3OH was not introduced into the GC, methanol convers ion was calculated by performing a carbon balance. That is, the moles of CO, CO2 and CH2O (if any) measured in the outlet are subtracted from the moles in CH3OH in the influent. 220.127.116.11 Contact time activity measurement In addition to temperature sweep, contact tim e sweeps were also performed on selected catalysts to determine what effect the residence ti me of the reactant had on catalytic activity. In homogeneous catalysis, space time, is generally used to measure the residence time of the reactants. Space time is defined as the volume of the reactor divided by the volumetric flow rate of the reactant feed to the reac tor, as shown in equation 1-12. feed reactorV V 1-12 The complication in heterogeneous catalysis is that the reactor volume is not necessarily the same as the packed bed volume, and to know the actually bed volum e, the density of the catalyst must be known. This is simple enough to measure but not always included in the literature. Hence, contact time (CT) is very often used instead of space time in methanol reforming studies [ 7,9,10,12,28, 38]. In order to facilitate comparison of the data in this research to the av ailable literature the common definition of contact time has been used:
33 feed catalystF W CT 1-13 In equation 1-13, W is the mass of the catalyst in kg and F is the feed rate of methanol in mmol/sec. This gives units of CT as kg*sec/mmol CH3OH. It is worth noting here that the most common definition for space time incl udes the total reactant flow (H2O + CH3OH) while the common definition of contact time is only in terms of CH3OH. In Chapter 2 the Figures are shown using space time as the i ndependent variable and the dens ity of each catalyst is also reported so that the reader can convert between the two. In order to perform a contact time sweep, the reactor is lo aded with a known mass of catalyst as described previously a nd the system is initialized as de scribed in section 18.104.22.168. The syringe pump is started at a known fl ow rate and six gas samples are taken at this flow rate. In order to probe other contact times, the flow ra te on the syringe pump is changed after data collection at one inlet flow is completed. The lo w flow rate (high contact time) data points are difficult to measure and the system often takes long periods of time to equilibrate since the system is very sensitive to perturbations at low flow rates. In any case, in order to ensure that the data is indeed reproducible, cont act times sweeps are repeated in a similar manner as temperature sweeps, with the catalyst bed reloaded and the sweep started at a different (generally higher) flow rate. 1.5.2 Brunauer-Emmett-Teller ( BET) Surface Area Analysis Brunauer-Emmett-Teller (BET) surface area measurements were performed on either a Quantachrome NOVA 1200 instrument or a Quantachrom ChemBET 3000 instrument. Further details are given in each chapter.
34 1.5.3 Temperature Programmed Reduction Experiments Temperature Programmed Reduction (TPR) measurements were performed in a Quantachrome ChemBET 3000 instrument. For these measurements, 55 milligrams of the catalyst was loaded in a quartz t ube reactor and secured with plugs of quartz wool on both ends. An Omega K-type thermocouple monitored the temp erature in the catalyst bed. Reduction was carried out at a heating rate of 5C per minute up to a temperature of 400C under a flow of 5% H2/N2 mixture at a total flow rate of 70 sccm until the catalyst was fully reduced and did not consume additional hydrogen. The hydrogen upta ke was continuously monitored using a TCD detector during the reduction. 1.5.4 N2O Decomposition Experiments The copper surface area was measured on a ChemBET 3000 instrument (QuantaChrome Corporation) using the N2O decomposition method, as is typically done for copper based reforming catalysts [ 14,26, 51]. The catalysts were loaded in a quartz tube reacto r and held in place by quartz wool. Prior to each measurement the sample s were reduced in 5% H2 at 300C (20 sccm) and then cooled overnight to room temperature under helium. During the measurements helium gas flowed over the samp les at a rate of 70 sccm, and the reactive adsorption of N2O was performed by titra ting known amounts of N2O over the catalyst surface and monitoring the N2 evolved using a thermal conductivity detector. W ith this instrument unreacted N2O is removed from the system through use of a liquid argon cold trap, which condenses out N2O but allows N2 to pass on to the TCD. In this way N2O is titrated over the sample until no signal is record ed on the TCD. At this poin t the surface is saturated and no further N2O reacts to N2. The total amount of N2 formed (or N2O consumed) was determined
35 and used to calculate the amount of copper su rface atoms according to the stoichiometry in Equation 1-14. OCuNONCu2 2 22 1-14 The copper surface area was calculated from these results assuming a copper surface density of 1.47x1019 copper atoms per square meter [ 26, 51]. The dispersion is defined as the ratio of copper atom s on the surface of the catalysts to the total number of copper atoms in the catalyst. The error in this measurement has been estimated by other researchers to be approximately 15% [ 26], which is reasonable in the current in v estigation as well. This procedure was used in each of the following chapters. 1.5.5 Catalyst Density Measurements For the calculations of space time (volumetric flow rate divided by the volume of catalyst), the catalyst densities were measured on a Quantachrome Ultrapyc 1000 Gas Pycnometer. 1.5.6 X-ray Diffraction Analysis The XRD data was gathered on a Philips powder X-ray diffractom eter using BraggBrentano geometry with Cu-K radiation ( = 1.54 ). The XRD spectrum was recorded in a 2 range of 20 to 80 at ambient conditions. Diffraction patte rns were obtained for all samples after calcinations and for selected samples after reductive pretreatment and after reaction. Average particle sizes were calculated from the line-broadening of the XRD peaks using the Scherrer equation, show here in equation1-15. )cos( FW K d 1-15
36 Instrument broadening effects were neglected. In this equation K is a constant generally taken as unity, is the wavelength of the incident radiation, FW is the full width at half maximum and is the peak position. 1.5.7 X-ray Photoelectron Spectroscopy Analysis The X-ray Photoelectron Spectroscopy (XPS) da ta presented in Chapter 2 was collected using a Kratos XSAM 800 XPS instrument equipped with a hemispherical analyzer. This instrument was located in the Ma jor Analytical Instrumentation Ce nter (MAIC) at the University of Florida. Incident radiation was from an aluminum anode with h = 1486.6 eV. Charge shift corrections were made by assuming a C 1s signa l of 284.5 eV unless otherwise noted. Some of the XPS data used in Chapter 2 and all of the XPS data in Chap ter 4 were taken using a second XPS instrument located in Gar Hoflunds lab in the Chemical Engineeri ng department at the University of Florida. This system ha s been described in detail elsewhere [ 52]. In either case the atom ic percentages of each element on the surf ace were calculated from the areas under the peaks taking into consideration the atomic sensit ivity factor for each element. Further details on the procedures used in collecting and analyz ing XPS data can be found in the appropriate chapter.
37 CHAPTER 2 STEAM REFORMING OF METHANOL USIN G Cu-ZnO CATALYSTS SUPPORTED ON NANOPAR TICLE ALUMINA 2.1 Introduction and Literature Review As mentioned in Chapter 1, the earliest catalysts used in methanol reforming were the simple binary Cu/ZnO systems similar to the one used in the study by Amphelett [ 35]. Since then use of Al2O3 and other oxide supports have become common. Al2O3 concentrations as low as 1% were used in the past [ 6], but later works generally included a higher Al2O3 concentration [ 53]. In these instances the alumina is included bot h as a bind ing agent a nd an oxide support. More recently, the trend in he terogeneous catalysis in genera l and reforming catalysts in particular has been to develop highly dispersed catalysts where the promoter or support/promoter complex may be 90% or more of the catalyst mass [ 12,26,31, 54]. In this chapter we explore the role of nanoparticle Al2O3 supported Cu/ZnO reforming cat alysts which have not been previously investigated in the literature. More specifically, it is of intere st to determine if the drawbacks of high Al2O3 content published by Matter, et al. [ 16] are inherent in the use of Al2O3 as a support or can be av oided if nanoparticle Al2O3 supports are used in place of traditional impregnated supports. While investigating the role of the nanoparticle support, a clos ely related area of interest which has received considerable attention in the l iterature is the nature of the active sites, in particular the oxidation state of th e copper in methanol steam reform ing catalysts. It is generally agreed that Cu2+ is inactive and Cu+ as well as Cu0 are active species in the steam reforming reaction [ 20,26,27, 28]. While some authors do not attribute any activity to Cu+[ 55], other groups have found that stabilizing Cu2O with reducible oxides, such as ZrO2 and CeO2, results in
38 highly active methanol reforming catalysts [ 20,26,27, 28]. Stabilization of Cu2O with an Al2O3 support has never been repor ted in the literature. Catalyst characterizations of supported Cu-ZnO steam reforming catalysts have indicated the formation of spinel-type compounds, either CuAl2O4 or ZnAl2O4 [ 11,21,23,31,32,56]. Several groups have identifie d th e spinel phase as CuAl2O4 [ 21, 31,32, 56], while other authors report tha t ZnAl2O4 forms in the bulk and that CuAl2O4 exists only as a surface species [ 11,23, 56]. Formation of surface CuAl2O4 results in the presence of inactive Cu2+ species and is undesirable given that the total number of active Cu0 or Cu+ atoms is decreased. On the other hand, ZnAl2O4 has been found to be a very efficient catalyst support in the methanol steam reforming reaction [ 57]. Therefore, determination of the t ype of spinel ph ase formed is critical. The above summary of previous results reveals the complexity of Cu-ZnO-based methanol steam reforming catalysts. These catalysts are further complicated by th e fact that a high Cu surface area is not always a sufficient requirement for a high catalytic activity. Recent studies indicate that characteristics other than the Cu surface area, such as introducing micro-strain in the active metal phase, can improve the catalytic activity [ 12, 13]. Consequently, engineering catalysts at the nanom eter scale to achieve high er catalytic activities is important in these reactions. It is our hypothesis that starting with nanoparticle alumin a as the support, it is possible to prepare highly active Cu particles on the catalyst s. Therefore, the goals of this study are to prepare catalysts using nanoparticle alumina supports and determine how they compare with a more traditionally prepared reforming catalyst. The present study will focus on the following: 1) Investigate if Al2O3 is a retardant in the reforming reactio n, or if it is possible to make high performance reforming catalysts with copper and zinc oxides deposit ed onto nanoparticle Al2O3. 2) Determine if there are advantages to us ing nanoparticle precursor materials in catalyst synthesis.
39 3) Determine if CO is produced solely via the water-gas-shift, or if methanol decomposition also plays a role over the nanoparticle catalysts. 4) Determine if the surface and structural ch aracteristics of the nanopa rticle-supported catalysts differ from those reported in th e literature and how they affect the catalytic activity. 2.2 Experimental Methods Most of the experimental methods used in th is chapter are covered in detail in Chapter 1. However, the catalyst preparati on method used in this study is unique to this chapter and is described herein. Also, a detaile d overview of the reactor oper ation is included. The details provided for density measurements, BET, and X PS experiments also represent experimental procedures unique to this chapter. TPR, N2O titration experiments and XRD data collection were performed as described in Chapter 1. 2.2.1 Catalyst Preparation Catalysts in this study are named according to the concentrations of the active metal oxides, promoter, and support as mass percenta ges. Labels are assigned according to the concentrations of CuO, ZnO, and Al2O3, respectively, in the catalys t sample. For instance, the 35/35/30 sample is 35% CuO, 35% ZnO, and 30% Al2O3 by mass. The 35/35/30 and 42/47/11 catalysts were prepared via a sequential wet impregnation method with the goal of attaining 2.0 grams of the finished catalyst sample. For th ese catalysts, a mass of commercial nanoparticle Al2O3 (NanoScale, surface area ~650 m2/g) was dispersed in deioni zed water. A solution of Zn(NO3)2 (Alfa Aesar) was then added to the mixtur e. The total volume of water used was 50 ml, which resulted in dispersions with nitrate concentrations of 0.18 M for the 35/35/30 and 0.21 M for the 42/47/11 samples. The water was boiled off until a paste consistency was formed. The pastes were dried overnight at 105C and calcined at 350C for 3 hours. The resultant ZnO/Al2O3 samples were ground and redispersed in wa ter. The copper was then added as an
40 aqueous solution Cu(NO3)2 (Alfa Aesar) to the dispersion (w ater added = 50 ml and the molar concentration of the copper nitrate precursor: 0. 18 M copper nitrate for the 35/35/30 catalyst and 0.24 M for the 42/47/11 catalyst). Th e water was boiled off and the paste was dried in the same manner as in the previous step. The samp le was then calcined a second time at 350C for 3 hours. For the sequentially precip itated 5/5/90 catalyst 1.8 g of the nanoparticle Al2O3 support was dispersed into 50 ml of deionized water and a solution of Zn(NO3)2 added, which resulted in a 0.025 M zinc nitrate dispersion as described above. The mixt ure was titrated with a 50% stoichiometric excess (i.e. 50 ml of 0.0375 M) so lution of NaOH and then aged overnight before it was filtered and washed by stirring in deionized water again overnight. The catalyst dispersion mixture was filtered a second time and the re sulting catalyst dried in an oven at 105C over night and calcined at 450C for 3 hours. A calcination temperature of 450C was used instead of 350C for this catalyst since the lo wer calcination temperature resulted in higher CO selectivites. The lower calcination temperature would also give less reproducible results, in that the conversion and the selectivity w ould change with time on stream. The calcined 5/5/90 sample was then ground and again dispersed in water. The precipitation and calcination procedures were repeated exactly as describe d above with a solution of Cu(NO3)2. The commercially available Sd-Chemie catal yst is approximately 42% CuO, 47% ZnO and 11% Al2O3 by mass according to the manufacturer. This catalyst was designed as a low temperature shift catalyst, which is typical fo r commercially available reforming catalysts.
41 2.2.2 Reactor Operation and Data Collection The GC and the catalytic reactor were initializ ed and operated as described in Chapter 1. Catalyst samples were activated in situ in 15% H2 in Ar at 300C for 3 hours before being exposed to the reactant mixture. Gas samples from the reactor were taken af ter the system had reached steady state, approximately two hours after system star t-up. The concentrations of CO, CO2 and H2 were measured as a function of temperature by starting at 200C and increasing the temperature in increments of approximately 15C for each data point. At least six gas samples were taken for each data point, and all data was seen to be repr oducible. Points plotted in the figures are the average of these samples. An average standard deviation is % for the methanol conversion and % for the CO selectivities was calculated for the reference catalyst, and is typical compared to the other samples. Likewise, in cases when space time was the independent variable, the reactor feed was started at a low fl ow rate and the flow wa s gradually increased to examine decreasing space times. Th e curves were repeated by starting with a fresh sample and taking measurements at high flow rates and de creasing the flow to examine increasing space times. This procedure ensures the curves were reproducible and that catalyst aging could be ruled out in the analysis. The catalyst behavior, even the sharp ch anges in activity, was verified by repeated experiments in each ca se using fresh catalysts. In all cases catalyst deactivation due to time on stream for relevant time lengths (~40 hours) was not found to affect the analysis or conclusions in this study. 2.2.3 BET Surface Area Analysis Brunauer-Emmett-Teller (BET) surface area measur ements in this chapter were performed on a Quantachrome NOVA 1200 instrument. Samp les were outgassed under vacuum for 3 hours
42 at room temperature for fresh catalysts and at 105C for spent catalysts. The BET was performed over five isotherms, which gave roughly linear fits. 2.2.4 Catalyst Density Measurements For the calculations of space time (volumetric flow rate divided by the volume of catalyst), the catalyst densities were measured on a Qu antachrome Ultrapyc 1000 Gas Pycnometer as described in Chapter 1. 2.2.5 X-ray Photoelectron Spectroscopy Analysis The preliminary XPS data was collected using a Kratos XSAM 800 XPS instrument equipped with a hemispherical analyzer. The samples were supported on double-sided tape during the measurement. Incident radiat ion was from an aluminum anode with h = 1486.6 eV. The pass energy settings on the instrument were set to low for the survey spectra and medium for the scans of the individual element peaks. Pressure was maintained at 10-7 Torr or less during the entire measurement. Charge shif t corrections were first made by assuming a C 1s signal of 284.5 eV. The data collected from this instrument using the C 1s as the shift reference resulted in binding energies 2 eV above what is expected for the Cu 2p peaks. To test if the observed high binding energies were due to differential charging, i.e. different sections of the XPS spectrum experience a different charge, the fresh 35/35/30 catal yst was selected for analysis using a different XPS instrument. In the sec ond system, the catalysts are pressed into an aluminum cup before entry into the UHV chamber [ 52]. The binding energies for the Cu 2p peaks obtained from the fresh 35/35/30 catalyst in this system is centered at 933.6 eV as expected for CuO, which is also in agreement with the XRD data and the conclusions from the XPS data obtained from the other system. Cons equently, the XPS peaks, such as the Cu 2p peaks, obtained from the 35/35/30 catalyst using the Kratos system were shifted to match the
43 binding energies obtained from the same catalyst in the second system. The shift difference was then used to adjust the peak posit ions from the other catalysts (5/5 /90 and 42/47/11) in this study. The atomic percentages of each element on the surface were calcul ated from the areas under the peaks taking into consideration the atomic sensitiv ity factor for each element and specified peak. 2.2.6 Thermodynamic Calculations Water gas shift equilibrium calculations are also performed as described in Chapter 1 using equations 1-7 through 1-11. The main difference between the calculations done in this chapter and those of subsequent chapters is that in this chapter the thermodynamic equilibrium constant for the reverse water gas shift reaction, Keq rWGS, is plotted as a function of temperature and this is compared with the ratio of th e partial pressure of the products of the reverse water gas shift over the reactants. This is the same method th at was used in the publication this chapter was based on. In later chapters the function is used. 2.3 Results and Discussion. 2.3.1 Methanol Conversion. Figure 2-1 shows methanol convers ion as a function of temperat ure at constant liquid feed flow rate (0.80 ml/h) fo r each catalyst tested. It is eviden t from the conversion plot that the reference catalyst is more activ e at a lower temperature than any of the nanoparticle supported catalysts, although the activities of the reference and the 42/47/11 catalysts are fairly similar below 280C. It is also evident that no catalyst is even close to approa ching the equilibrium conversion (>95%) under these conditions (see Figur e 2-1). However, at higher temperatures, both the reference catalyst and th e 42/47/11 catalyst lose activity very rapidly. The 42/47/11 catalyst exhibits severe deactivation at only 280C. The reference catalyst does not exhibit significant deactivation until 20C higher. No reaction data for the reference catalyst could be
44 obtained above approximately 320C due to severe deactivation. Significant deactivation of the 5/5/90 catalyst does not occur until over 400C, whereas significant loss of activity for the 35/35/30 catalyst did not occur in the temperat ure range investigated in this study. The deactivation of the catalysts is irreversible. After a second reduction in dilute H2 none of the deactivated catalysts could be retu rned to their original activity levels. The fact that no catalyst achieved over 65% conversion at the space time us ed in Figure 2-1 regardless of temperature reflects a kinetic limitation (since the equilibriu m conversion of the methanol steam reforming reaction is 95% or higher in this temperature range). Higher c onversions can be achieved with these catalysts by increasing the space time of the reactants, which is discussed in the next paragraph. Two curves are di splayed for the 5/5/90 sample to differentiate between total CH3OH reacted to all products (CO2, CO and CH2O), and CH3OH reacted specifically to CO2 and CO. No CH2O was produced by any other catalyst. 200220240260280300320340360380400420440 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 Figure 2-1 Conversion of Methanol [%]Temperature [deg. C] Figure 2-1. Conversion of metha nol as a function of temperature at constant liquid feed flow rate (0.8 ml/h). : catalyst 5/5/90 total conversion, : catalyst 5/5/90 conversion to CO and CO2, : catalyst 35/35/30, : catalyst 42/47/11, : reference catalyst, : blank run (no catalyst or support) and : conversion to CO and CO2 over nanoparticle alumina.
45 Methanol conversion as a function of space time at 280C is presented in Figure 2-2. The curves for the nanoparticle suppor ted catalysts in Figur e 2-2 show the classi c sigmoidal growth shape asymptotically approaching a conversion of 100% at the highest space times. It is interesting to note that the curve for the reference catalyst does not display this shape, but rather goes through a very sharp change in the shape of the curve about half way through the range of space times investigated. It is not evident what is responsible for the differences between the shapes of these curves. Purnam a has also noted a change in c ontrolling kinetics at conversions higher than 70% in his space time study of a commercially available Cu-ZnO/Al2O3 catalyst at 250C . In the present experiment the distinct change in kinetics app ears to occur at conversions of approximately 60%, but the jump in the curve does suggest a change in reaction mechanism. Conversions above 90% can be achieve d rather easily at moderate temperatures for the 35/35/30 catalyst as well as the reference ca talyst given sufficiently long space times. High conversions are also evident for the 5/5/ 90 sample, but a large amount of undesired CH2O is produced by this catalyst with increasing space time. The other disa dvantage with increasing the 0.00.20.40.60.81.01.2 0 10 20 30 40 50 60 70 80 90 100 Figure 2-2 Conversion of Methanol [%]Space Time [seconds] Figure 2-2. Conversion of methanol as a function of space time, T=283C. : catalyst 5/5/90 total conversion, catalyst 5/5/90 conver sion to CO and CO2, : catalyst 35/35/30, : catalyst 42/47/11, and : reference catalyst.
46 temperature or the space time is that the CO selectivity increases dramatically for all catalysts (see Figures 2-3 and 2-4). 2.3.2 CO Selectivity Figure 2-3 shows the CO selectivity as a functio n of temperature (liquid feed flow rate = 0.80 ml/h). Perhaps the most interesting feature of this figure is that the CO selectivity for the nanoparticle supported catalysts actually decreases with increasing temperature up to approximately 300C (up to 260C for the 42/47/11 catalyst). In this temperature range (220300C) the conversion is increasing rapidly with temperature for all catalysts (Figure 2-1). The decrease in CO selectivity with increasing te mperature is unexpected given that both CO production routes (the methanol decomposition reac tion and reverse water-gas-shift reaction) are endothermic, which means that CO formation via these routes are favored both kinetically and thermodynamically with increasing temperature. However, the steam reforming reaction is also endothermic and the equilibrium conversion very hi gh in this temperature range. The fact that the CO selectivity decreases over this temperatur e range for all of the nanoparticle catalysts reveals that the rate of CO2 formation is increasing more ra pidly with increasing temperature than the rate of CO formation. Evidently, this is true for all nanoparticle catalysts, despite the significant differences in conversi on in this temperature range. This may indicate that the catalyst surface is being altered at th ese temperatures, for example the Cu+/Cu0 ratio could be changing (see discussion in secti on 2.4.4). Since the conversion and the CO selectivity are both very dependent on the temperature, it is useful to compare the CO selectivities of different catalysts at a specific co nversion, as well as at the maximu m conversion (Table 2-1). At 50% conversion the commercial cataly st and the 42/47/11 cat alyst exhibit similar CO selectivities, which are considerably lower th an those of the other nanopartic le supported catalysts. More
47 importantly, both the CO selectivity and the temperature at maximu m conversion for the 42/47/11 catalyst are lower than the corresponding values for the commercial catalyst. Even though the maximum conversion is slightly lower for the 42/47/11 catalyst compared with the commercial catalyst, the decreased CO selectivity at 50% conversion indicates that the catalysts in this study can be tuned to out perform the commercial catalyst. Th is is further demonstrated by altering the space times, as shown in Figure 2-4. 220240260280300320340360380400420 0 2 4 6 8 10 12 14 16 18 Figure 2-3 CO Selectivity [%]Temperature [deg. C] Figure 2-3. CO se lectivity, CO/(CO2+CO) as a function of temperat ure at constant liquid feed flow rate (0.8 ml/h). : catalyst 5/5/90, : catalyst 35/35/30, : catalyst 42/47/11, : reference catalyst. The CO selectivity as a function of space time at 280C is shown in Figure 2-4. A temperature of 280C was selected since this temper ature corresponds to the maximum conversion of the reference cat alyst at a space time of 0.15 seconds. The figure shows that increasing the space time to the le vel needed for maximum convers ion drastically increases the CO selectivity for all catalysts. Evidently, the re ference catalyst has a higher CO selectivity at all space times than either the 35/35/30 or the 42/47/11 catalysts at this temperature. Only the
48 Table 2-1. Comparison of CO selectivity values at 50% a nd maximum conversion and fixed liquid feed flow rate, and CO selectivity at 65% conversion and 280C and variable space times (liquid feed flow rates). Catalyst Conversion [%] Selectivity [%] a Temperature [C] ST [s] b 5/5/90 50 3.9 300 0.21 35/35/30 50 4.3 280 0.13 42/47/11 50 2.5 255 0.14 Commercial 50 2.8 230 0.16 5/5/90 65 (max) 7 341 0.19 35/35/30 60 (max) 4.6 300 0.13 42/47/11 58 (max) 2.5 263 0.14 Commercial 62 (max) 4.1 280 0.15 5/5/90 65 6.5 280 0.40 35/35/30 65 5.3 280 0.31 42/47/11 65 6.4 280 0.40 Commercial 65 8 280 0.39 a CO selectivity defined as in Equation 4. b Space time. 5/5/90 catalyst exhibits a slightly higher CO sel ectivity than the reference catalyst at space times below 0.15 seconds. The CO selectivities of bot h the reference catalyst and the 42/47/11 catalyst also display a large increase at space times around 0.5 seconds. The same discontinuous increase in conversion was evident for the reference catalyst in Figure 2-2. Repeated tests verified that this step increase was neither due to cataly st aging nor an experimental artifact. The discontinuous change in conversion indicates a change in mechanism or reaction pathway for this catalyst at this sp ecific value of the space time. The na noparticle supported catalysts show superior performance over the reference catalyst at high space times in that they can achieve
49 increased methanol conversions w ithout undergoing the jump in CO se lectivity that is evident in the reference catalyst. In fact at a conversion of 65% (T=280C), the CO selectivity of all catalysts in this study are below th at of the commercial catalyst (Table 2-1). The CO selectivity curves clearly display a zero in tercept with decreasi ng space time. This has been taken as an indication that CO is a secondary product in several articles [ 9, 10]. 0.00.20.40.60.81.01.2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Figure 2-4 CO Selectivity [%]Space Time [seconds] Figure 2-4. CO se lectivity, CO/(CO2+CO) as a function of space time at T=283C. : catalyst 5/5/90, : catalyst 35/35/30, : catalyst 42/47/11, : reference catalyst. 2.3.3 Blank Experiments. The 5/5/90 catalyst produced CH2O during the reforming reaction. No other catalyst showed detectable levels of CH2O at any temperature. The fact that CH2O is produced over the nanoparticle catalyst th at contained 90% Al2O3 but not over the 30% or 11% Al2O3 catalysts suggests that the formation of CH2O is a consequence of the high Al2O3 concentration. This was confirmed by performing a steam reforming experiment over bare nanoparticle alumina. The nanoparticle alumina produced relatively high levels of CH2O, but the levels of CO and CO2 were similar to those observed for the homogene ous reaction, i.e. a blank steam reforming
50 experiment with no support or catalyst in th e reactor (see Figure 2-1). Furthermore, the CH2O produced over the nanoparticle alumina was higher than over the 5/5/90 catalyst, which is likely an effect of a lower alumina surface area on the 5/5/90 catalyst due to coverage of the alumina by copper and zinc oxide. Thes e results indi cate that Al2O3 is active in CH2O production, but not in reactions 1-1 to 1-3. Other researchers have also reported formation of CH2O from CH3OH on SiO2/Al2O3 supports [ 58]. Of the catalysts in this st udy, the 35/35/30 catalyst dem ons trated the greatest thermal stability. The 35/35/30 catalyst is more stab le and does not suffer from the same degree of deactivation with temperature as the commercial reference and the 42/47/11 catalysts. The low CO selectivity coupled with the high dur ability of the catalyst means that high CH3OH conversions can be achieved while generating lowe r amounts of CO than w ould be possible with a commercially available catalyst. 2.4 Catalyst Characterization Results. 2.4.1 Surface Area Analysis The BET surface areas before and after reacti on, active copper surface areas, and turnover frequencies of the catalysts in th is study are shown in Table 2-2. Ev idently, the samples higher in alumina have a larger surface area. The nanopartic le supported catalysts rely almost entirely on the Al2O3 for the surface area of the catalyst. Therefore, when the concentration of Al2O3 is small in the sample, the surface area is relatively low. The refere nce catalyst has a larger surface area compared with the nanoparticle supported cata lyst of the same composition. For the 5/5/90 and 35/35/30 nanoparticle supported catalysts the su rface areas of the spent catalysts are notably lower than those of the fresh catalyst samples. It is interesting to note that as the copper and zinc content increases the difference in surface area before and after e xposure to the reaction
51 conditions decreases. The surface area for the 42/47/11 catalyst is nearly unaffected by the reaction. Table 2-2. Surface characteristics of reforming catalysts. Catalyst Nominal CuO concentration [weight %] Cu surface area [m2/g Cu] Dispersion [%] Fresh BET surface area (m2/g) Spent BET surface area [m2/g] Density [g/cm3] TOF at 280C [103sec -1]a Reference 42 20.2 3.1 68 82 5.2 34.6 42/47/11 42 3.5 0.5 23 24 5.8 160.1 35/35/30 35 5.3 0.8 46 40 5.0 128.8 5/5/90 5 56.2 8.6 218 170 3.5 59.4 a TOF = Turnover frequency, defined as the molecules of hydrogen produced per surface copper atom per second. The decrease in surface area of the 5/5/90 catalysts after e xposure to reaction conditions most likely corresponds to a sinter ing effect that causes some of the pores in the alumina support to be clogged due to particle growth of the c opper. The loss of surface area is not due to the sintering of the alumina itsel f, given that there is no si gnificant change in the Al2O3 particle size on this sample before and after the reaction (s ee XRD section, below). In contrast, on the reference catalyst, it appears that there is a slight increase in surface area after reaction. This effect was unique to the reference catalyst. Desp ite the fact that the re ference catalyst did not lose surface area during the reacti on, there was strong deactivation of this sample at elevated temperatures. Consequently, there is no corre lation between the overall surface area and the catalytic activity for these catalysts. The results of the N2O titrations data provide a quantitativ e view of the differences in the surface copper concentration of the catalysts. Table 2-2 clearly shows that very high copper surface areas can be attained by using a high concentration of nanoparticle Al2O3. The copper
52 surface area per gram of copper on the 5/5/90 sample is an order of magnitude higher than on the other nanoparticle catalysts, and mo re than double that of the refe rence catalyst. In comparing the data in Table 2-2 to the curves in Figure 2-1, another interesti ng trend is evident. Despite the fact that the copper surface area per gram of the 42/47/11 is a fr action of that in the reference catalysts, the two samples achieve similar maxi mum conversions. The di fference is that the turnover frequency (TOF) of the 42/47/11 catalyst is dramatically higher than that of the reference sample. In fact, a ll nanoparticle-suppo rted catalysts have higher TOFs than the reference catalyst. The surface copper of the nanopa rticle catalysts is clea rly more active than the copper on the surface of the reference catalyst. Ho wever, the activity does not correlate with the Cu surface area of the catalysts. In fact, the TOF appears to correlate with the nominal CuO concentration rather than the Cu surface area for the nanoparticle catalysts. Alternatively, it can be stated that the TOF decreas es linearly with increasing Al2O3 content (Table 2). The highest Al2O3 concentration (and highest co pper surface area) is the 5/5/90 catalyst, which also displays the lowest TOF. The catalyst with the lowest Al2O3 concentration (42/47 /11) has the highest TOF. Past researchers using copper based metha nol reforming catalysts have also noted that properties other than c opper surface area affect the catalytic activity [ 14]. Obviously this is the case in the present study as well. It has been proposed that favorable m orphology [ 59], increased reducib ility [ 60], or simply higher quality [ 14] of the copper species in m ethanol reforming catalysts increases catalytic activ ity. In the samples used in this study, it is evident that increasing the alumina content of the samples tends to make the surfaces both less reducible and less active in the reforming reaction. This could be due to strong Cu-Al2O3 interactions at high Al2O3 concentrations, which in turn can alter the electronic prope rties of the Cu on the surface and also influence the Cu+/Cu0 ratios on the catalyst surfaces (s ee below). Hence, the quality
53 of the copper decreases with increasing Al2O3 content despite the increase in Cu surface area, and high Al2O3 concentrations ultimately result in a less active catalyst. 2.4.2 X-ray Diffraction Analysis XRD spectra obtained from the fresh catalysts are shown in Figure 2-5. 304050607080 Figure 2-5 c) b) a) 2Intensity [arbitrary units] Figure 2-5. XRD spectra obtained from fresh catalys ts: a) 5:5:90 CuO-ZnO/Al2O3 catalyst, b) 35:35:30 CuO-ZnO/Al2O3 catalyst and c) 42:47:11 CuO-ZnO/Al2O3 catalyst. +: Al2O3, : CuO, o: ZnO. Arrows indicate peaks due to ZnAl2O4. The characteristic peaks of CuO are eviden t in all the spectra obtained from the fresh catalyst samples and a crystalline ZnO phase is present on the 35/35/30 and 42/47/11 catalysts, but not on the 5/5/90 catalyst. On the 5/5/90 the ZnO is below the detection limit or is present in amorphous form. No peaks due to alumina can be detected in the XRD spectrum obtained from the 35/35/30 or 42/47/11 catalysts. This is not surprising due to the low crystallinity and the lower alumina concentration of the catalysts co mpared to the 5/5/90 ca talyst. There are small broad peaks visible in the spectrum of the 35/35/30 catalyst at 2 = 31.3 37.1 44.6 55.6 59.2 65.1 which are not due to CuO or ZnO. The 2 values of the peaks are consistent with
54 those obtained from aluminate species such as CuAl2O4 [ 61] or ZnAl2O4 [ 62]. Since both the CuAl2O4 and ZnAl2O4 have nearly identical diffraction pa tterns is it difficult to distinguish between the two in the XRD spectrum obtained from the 35/35/30 catalyst [ 23,61, 62]. An experim ent was therefore designed to determine th e type of aluminate formed during the catalyst preparation procedures used in our study. Two sample catalysts composed of ZnO-Al2O3 and CuO-Al2O3 were prepared using the catalyst prepar ation method described above, keeping the CuO:Al2O3 and ZnO:Al2O3 ratio the same as in the 35/35/30 catalyst (i.e. at 1.17 by mass), and characterized with XRD measurem ents. The presence of an aluminate phase was visible with XRD only in the ZnO-Al2O3 sample (not shown). This resu lt suggests that the aluminate phase seen in the XRD spectra obtained fr om the 35/35/30 catalysts is ZnAl2O4 and not CuAl2O4 as has been assigned in the past [ 21]. However, due to the presence of copper in addition to zinc on these catalys ts, the presence of CuAl2O4 cannot be ruled out comple tely. Our assignment of a ZnAl2O4 phase agrees with those made by Turco and coworkers [ 11]. Table 2-3 sh ows average crystal sizes of all species visible in the XRD spectra calculated using equation 1-15. The sizes of the CuO particle s are slightly smaller on catalyst 42/47/11 (27 nm) compared with catalyst 35/ 35/30 (34 nm), despite the higher CuO concentration of the 42/47/11 catalyst. This contra dicts the lower dispersion obser ved on the 42/47/11 catalyst (smaller particle sizes give larg er surface areas and higher dispersions), and is likely due to more ZnO covering the CuO on catalyst 42/47/11 compared with catalyst 35/35 /30. This may be expected due to the higher ZnO content on catalyst 42/47/11 and, the ZnO particle sizes are also considerably larger on catalyst 42/47/11 (58 nm) compared with those on catalyst 35/35/30 (32 nm). It is interesting to note that no CuAl2O4 or ZnAl2O4 can be detected on the 42/47/11 catalyst. Consequently, if present, the alumin ates are either amorphous or below the detection
55 limit of the XRD instrument. Another interesting observation is that the particle sizes of the aluminate species are smaller than for the CuO and ZnO species. The Scherrer equation gives a value of 8 nm for the aluminate. The same beha vior of the aluminate phase was observed in the study by Murcia-Mascars et al. [ 21]. The small particle size observed is likely due to the strong m etal-support interactions that result in formation of this compound. Table 2-3. Particle sizes of the di fferent compounds on the various CuO-ZnO/Al2O3 catalysts in the current work. The partic le sizes have been determin ed from the XRD data using the Scherrer equation. Particle sizes [nm] Species Catalyst Cu CuO ZnO ZnAl2O4 Cu2O Al2O3 5:5:90 Cu/ZnO/Al2O3 Fresh -13.8 ---4.5 5:5:90 Cu/ZnO/Al2O3 Spent ----7.3 4.7 35:35:30 CuZnO/Al2O3 Fresh -33.6 32.1 7.8 --35:35:30 CuZnO/Al2O3 Spent 35 -33.4 6.9 --42:47:11 Cu/ZnO/Al2O3 Fresh -26.9 57.5 ---42:47:11 Cu/ZnO/Al2O3 Spent 22.5 -34.7 ---After reduction treatment and exposure to the reaction conditions no crystalline CuO is present on any of the catalysts (Figures 2-6A and B). The XR D spectra obtained from all catalysts exhibit features due to Cu metal after these treatments. In f act, the spectra obtained from the 35/35/30 and 42/47/11 catalysts af ter reductive treatment (not shown) are indistinguishable from those obtained after exposu re to the reactions cond itions. In addition to the peaks due to Cu metal, the XRD spectrum obtained from the spent 5/5/90 catalyst also exhibits peaks located at 2 = 36.8, 42.5, 61.7 and 73.9. These peak positions are consistent with Cu2O [ 63]. The fact that the Cu2O species can be detected with XRD is remarkable, since Cu2O has only been detected previously using XRD on catalysts with significantly higher copper
56 contents [ 27, 43]. Furthermore, in those cases it appears that the ZrO2 used as the support stabilizes the Cu2O [ 27, 43], and such support stabilization of Cu2O is unlikely on Al2O3. Figure 2-6B shows that only Cu metal is present on the 5/5/90 catalyst after the reductive treatment. This unequivocally demonstrates that the Cu2O is formed during the reforming reaction and is not a result of either the brief air exposure before analysis or incomplete reduction during the pretreatment. To our knowledge, Cu2O has never been observed on Al2O3-supported spent reforming catalysts. This result also indicates that increasing the concentration of nanoparticle Al2O3 alters the redox properties of the surface (as indicated above), which in turn affects the surface activity. The Al2O3 makes the surface more difficult to reduce (see below), which apparently translates to a decr ease in turnover frequency, as s een in the previous section. The reduction treatment and exposure to the r eaction conditions does not appear to affect the Al2O3 particle size on the 5/5/90 ca talyst (see Table 2-3). The pa rticle sizes of the Cu metal on the spent 35/35/30 catalyst are similar to those of the CuO particles on the fresh catalyst (~33 nm). This suggests that there is some sintering of the copper during th e reduction (or reaction) [i.e. there are more Cu atoms per particle afte r reduction and reaction]. On catalyst 42/47/11 the Cu metal particles are slightly smaller (23 nm) than the CuO particles on the fresh catalyst (27 nm). Although some sintering is expected, this indicat es that the sintering of copper is more severe on the 35/35/30 catalyst co mpared with the 42/47/11 cataly st. The ZnO appears to be unaltered after reduction and r eaction on the 35/35/30 catalyst, while on the 42/47/11 catalyst they appear to be smaller af ter reduction and reaction (35 nm) compared to the fresh 42/47/11 catalyst (58 nm). The particle size of ZnAl2O4 on the 35/35/30 catal yst is not altered significantly, as expected. Other research groups have also noted that aluminates can be very difficult to reduce [ 31,32, 64,]. Even at elevated temp eratures and under vacuum a CuAl2O4
57 spinel phase is fairly stable and will not readily reduce to Cu and Al2O3 [ 64]. Hence, it is not surprising th at under the reduction treatment and th e reaction conditions of these experiments the ZnAl2O4 spinel phase is also rather stable. 304050607080 Figure 2-6A+c) b) a)o o + + I I I I IoIo o o o o o o o o o o o o 2Intensity [arbitrary units]2030405060708090100110 Figure 2-6B c) b) a) 2Intensity [arbitrary units] Figure 2-6. XRD spectra obt ained spent catalysts. A: a) spent 5:5:90 CuO-ZnO/Al2O3 catalyst, b) spent 35:35:30 Cu-ZnO/Al2O3 catalyst, and c) sp ent 42:47:11 Cu-ZnO/Al2O3 catalyst. B: a) fr esh 5:5:90 Cu-ZnO/Al2O3 catalyst, b) reduced5:5:90 Cu-ZnO/Al2O3 catalyst, and c) spent 5:5:90 Cu-ZnO/Al2O3 catalyst. +: -Al2O3, : CuO, o: ZnO, I: Cu metal and : Cu2O.
58 Despite the fact that sintering of the Cu metal appears to be th e highest on the 35/35/30 sample, this catalyst did not deactivate as rapidly as the 42/47/11 catalyst. Therefore the catalyst deactivation in the activity measurements is not simply due to sintering of copper in the samples (see section 2.4.4). 2.4.3 Temperature Programmed Reduction Measurements TPR experiments provide further in sight into these catalysts. Previous results indicate that the catalytic activity correlates with the reduc ibility of the copper species on the surface [ 29]. If this is tru e, then the commercial catalyst would be the easiest to reduce and the 5/5/90 the most difficult, which is indeed the case. The comm ercial catalyst starts to reduce below 200C, which is 30C lower than the nanoparticle cat alysts (Figure 2-7). While re duction of the 5/5/90 catalyst starts at the same temperature as th e other nanoparticle catalysts (i.e. 230C), hydrogen consumption does not end until well above 400C, long after the other catalysts are fully reduced. The high reduction temperature of the 5/5/ 90 catalyst is likely a result of the smaller copper particle size on this catalyst (14 nm versus 34 and 27 nm for the 35/35/30 and 42/47/11 catalysts, respectively) and stronge r copper-alumina intera ctions. It has been previously reported that reduction occurs at highe r temperatures, or more reluctantly, as CuO particle size decreases [ 65]. The TPR profiles of the 35/35/30 and 42/47/11 catalysts are sim ilar, with reduction beginning at approximately 225C and ending around 320C. However, two differences can be observed; 1) the hydrogen consumption of the 35/35/30 catalyst peaks at approximately 265C, while that of the 42/47/11 catalyst does not reach a maximum until 15C higher and 2) the reduction for the 35/35/30 cataly st is not completed until 15-20C higher than that for the 42/47/11 catalyst. C onsequently, at high alumina concentrations, the CuO precursor on the nanoparticle catalysts is more difficult to reduce and this results in a less active catalyst,
59 i.e. the TOF is higher for lower Al2O3 concentrations. The observed correlation between catalytic activity (TOF) and ease of CuO reduction on the catalysts is consistent with the literature [ 29]. 050100150200250300350400450500 Figure 2-7d) c) b) a)Normalized Response [Arbitrary Units]Temperature Figure 2-7. Normalized Temperature Programmed Reduction (TPR ) Profiles. a) 5/5/90, b) 35/35/30, c) 42/47/11, and d) reference catalysts. 2.4.4 X-ray Photoelectron Spectroscopy Analysis XPS measurements were performed to determine how the surface composition and the valence state of the surface copper vary between the different catal ysts and between the fresh and spent catalysts. Part of the goal with these measurements is to determine if Cu2O can be detected on the catalysts after exposure to the reaction conditions.
60 22.214.171.124 Fresh catalysts The XPS survey spectra reveal that despite the higher CuO c ontent, the Cu peaks obtained from the 42/47/11 catalyst have lower intensities compared to those obtained from the 35/35/30 catalyst, while the peaks due to ZnO are more inte nse. This is evident as a higher Cu/Al ratio on catalyst 35/35/30, while the Zn/Al ratio is hi gher on 42/47/11 (Table 24). Consequently, a higher fraction of CuO on the 42/ 47/11 catalyst is covered with ZnO compared with the 35/35/30 catalyst, which is in agreement with the conclu sions from the XRD particle sizes and the Cu surface areas. 960955950945940935930925 Figure 2-8Cu+,0Cu2+f) e) d) c) b) a) Cu 2pN(E) (arbitrary units)Binding Energy (eV) Figure 2-8. XPS spectra of the Cu 2p binding energy region obtai ned from catalysts: a) fresh 5:5:90 CuO-ZnO/Al2O3, b) spent 5:5:90 Cu-ZnO/Al2O3, c) fresh 35:35:30 CuOZnO/Al2O3 catalyst, d) spent 35:35:30 Cu-ZnO/Al2O3 catalyst, e) fresh 42:47:11 CuO-ZnO/Al2O3 and f) spent 42:47:11 Cu-ZnO/Al2O3. Vertical dashed lines mark the peak positions of the Cu 2p peaks in the current pape r and the dotted lines mark the CuO or Cu2O plus Cu metal peak positions.
61 Table 2-4. Relative surface concentrations of di fferent elements calculated from the atomic concentrations at the surface. The atomic concentrations are based on the peak areas of the peaks listed in the table with adjust ments made for the sensitivity factors of each peak. Catalyst 5/5/90 35/35/30 42/47/11 Ratio [Atomic %] Fresh Spent Fresh Spent Fresh Spent Zn 2p / Cu 2p 0.6 1.9 2.5 3.0 3.3 3.9 C 1s / Al 2p 1.0 1.0 1.2 1.2 1.2 1.1 Cu 2p / Al 2p 0.4 0.1 0.4 0.5 0.4 0.4 Zn 2p / Al 2p 0.2 0.3 1.0 1.4 1.2 1.7 The Cu 2p peaks obtained from the fresh and sp ent catalysts are shown in Figure 2-8. The presence of a Cu2+ species on the fresh catalysts is evident from the intense satellite peak at a binding energy of 940-945 eV in the XPS spectra. The presence of the satellite peak is attributed to the interaction of th e ejected photoelectron with another va lence band electron, and/or to the metal-to-ligand charge transfer which has been demonstrated to occur in Cu2+ [ 23, 66]. Satellite peaks are no t present in Cu0 or Cu+ spectra due to the full 3d bands [ 32, 66]. The presence of a Cu2+ species is in agreement with the XRD data, which indicates that CuO is the main Cu species before reduction and reaction. Af ter charge corrections (see secti on 2.3.5) the binding energy of the Cu 2p peaks on the fresh cat alysts are centered at 933.6 eV which is the reported binding energy for CuO [ 66,67,68, 69]. However, the peaks are broad w ith significant sh oulders at 935 eV. This high binding energy may indicate the presence of CuAl2O4. The presence of this compound is further supported by th e satellite-to-main peak intens ity ratios (0.55-0.58), which are higher than that expected for CuO (0.45) although not as high as that expected for CuAl2O4
62 (1.1). Only catalyst 5/5/90 has a slightly larg er ratio (0.65), which indicates that more CuAl2O4 is present on this catalyst compared to the 35/ 35/30 and the 42/47/11 cata lysts. This would be expected due to the higher dispersion on this catalyst (a larger degree of Cu-Al2O3 interface and thus stronger Cu-Al2O3 interactions). However, the main Cu species on all the calcined catalysts is CuO, which agrees with the XRD data. 126.96.36.199 Spent catalysts After reductive treatment and exposure to the reaction conditions, the Zn/Cu ratio increases on all catalysts (Table 2-4). This indicates that after reaction more ZnO covers the active Cu phase compared to before the reaction. This explains at least part of the catalyst deactivation since the activ e Cu surface area is reduced. In cont rast, the C/Al ratio is relatively constant before and after reacti on on all catalysts. The absenc e of carbon build-up at the surface suggests that coking is not a d eactivation pathway on these cataly sts. After exposure to the reaction conditions the bindi ng energy of the Cu 2p3/2 peak is shifted to a lower value revealing reduction of the CuO. The peaks are also narrower after exposure to the reaction conditions as is expected from reduction of CuO to Cu2O or Cu metal. The binding energies of the spent catalysts (after charge correction) are located at 932.4 eV, which is in agreement with Cu2O and Cu metal [ 70]. However, the peaks are broader th an what would be expected for pure Cu2O or pure Cu metal, which again suggests that more th an one Cu species are present on the surface. The absence of satellites in the spectra obtaine d from the spent catalysts indicates that no significant quantity of Cu2+ species is left on the surface. This supports our assignment of a ZnAl2O4 phase on the 35/35/30 catalyst, althoug h it does not exclude presence of CuAl2O4 in the bulk of the catalyst. Differentiating between the Cu+ and Cu0 valence states of copper compounds using the Cu 2p peaks is rather challenging due to the relatively large Cu radi us and thus electronic binding
63 energies are separated by only a small fraction of an eV [ 32, 66, 67,68,69,77774,]. Therefore, in order to determine the valence state of the Cu on the surface, the kinetic energy of the ejected Auger electrons was examined (Figure 2-9) [ 69]. This energy does not depend on either the Cu 2p3/2 electronic binding energy or the en ergy of the incident radiation [ 71, 74]. The Auger electron energy of Cu m etal (918.6 eV) is higher than that of CuO (917 .9 eV), while the Auger electron energy of Cu2O is lower (916.7 eV). The signal to noi se of the Auger electron energy region in the spectrum obtained from the fresh 5/5/90 cataly st is low and only a sm all feature at 916.2 eV can be observed. Although this electron energy would suggest the presence of CuAl2O4 (electron energy 916.8 eV) rather than CuO (917.9 eV), it is know n from the Cu 2p peaks that CuAl2O4 is not the major species on the surface. Consequently, the low electron energy observed could be due to charging. In contrast, both the fresh 35/35/ 30 and 42/47/11 catalysts exhibit pronounced Auger features at 918.1 eV, which is the elec tron energy expected for CuO. After reduction and exposure to the reaction co nditions, the Auger features shift to lower electron energies in all cases. This is indicative of Cu2O formation rather than Cu metal formation. Consequently, the Cu metal on the reduced catalysts is oxidized to Cu2O by steam during the reaction. Howeve r, the Auger peak is broad and the presence of Cu0 in the near surface region cannot be excluded, but the dominant species at the surface is not Cu metal. It is possible that both Cu metal and Cu2O are required for an active catalyst and that the specific Cu+/Cu0 ratio determines the catalytic activity. For example, an oxygen-deficient surface is required for H2O dissociation and formation of H2, while an oxidized surface is necessary for methanol di ssociation and oxidation [ 11, 75]. This could expl ain w hy the activities are not a simple function of the Cu metal surface areas. Furthermore, it is also known that CO
64 binds strongly on Cu+, while it is adsorbed only weakly on metallic Cu [ 11]. Consequently, in the pres ence of Cu2O the CO species will have time to react to CO2 (assuming sufficient oxygen 885890895900905910915920925 Figure 2-9f) e) d) c) b) a) Cu AugerN(E) (arbitrary units)Electron Energy (eV) Figure 2-9. XPS spectra of the Cu Auger kinetic energy region obtained from catalysts: a) fresh 5:5:90 CuO-ZnO/Al2O3, b) spent 5:5:90 Cu-ZnO/Al2O3, c) fresh 35:35:30 CuOZnO/Al2O3 catalyst, d) spent 35:35:30 Cu-ZnO/Al2O3 catalyst, e) fresh 42:47:11 CuO-ZnO/Al2O3 and f) spent 42:47:11 Cu-ZnO/Al2O3. The dashed line marks the CuO peak position, the dotted line marks th e Cu metal peak position and the dashdotted line marks the Cu2O peak position. present on the surface), while on a more reduced Cu surface the CO is not bound as strongly and can desorb before reacting. It ha s in fact been observed that th e CO concentrations increase with increasing extent of reduction on the surface of Cu-based methanol reforming catalysts . The above could explain the tre nd in CO selectivity with tem perature between 220 and 300C. The extent of Cu0 oxidation to Cu+ by the steam is likely to increase with the temperature in this
65 range, which would be expected to increase the CO2 over the CO product. At elevated temperatures the oxidation of Cu to Cu2O by steam is negated by the reductive properties of methanol that is reacting over th e catalyst. Consequently, at hi gher temperatures the surface is likely in a more reduced state (lower Cu+/Cu0 ratio) and the CO selectivity increases with temperature above 300C for all catalysts. The Zn 2p, Al 2p and O 1s binding energy regi ons were also examined for all catalysts before and after reaction (not s hown). However, these regions do not vary significantly between the catalysts and do not provide ne w information about the catalyst surface. For example, the Zn 2p peaks in ZnO and ZnAl2O4 are located at 1021.8 and 1021.7 eV, respectively and thus the XPS data alone cannot be used to di fferentiate between the two species [ 70]. The O 1s peaks are also not inform ative due to the presence of overl apping species. The O 1s peaks obtained from the 5/5/90 catalysts are dominate d by the contribution from Al2O3 at 531.5 eV. The other catalysts 35/35/30 and 42/47/11 exhi bit broad O 1s peaks due to Al2O3, CuO at 529.8 eV, Cu2O and ZnO at 530.5 eV and potentially also CuAl2O4 at 530.8 eV or ZnAl2O4 at 531.4 eV. In summary, all nano-alumina-supported catalysts contain Cu2O at the surface after exposure to the reaction c onditions. While some Cu+ likely is required for a high catalytic activity, it is possible that over-oxidati on, i.e. formation of CuO or bulk Cu2O, decreases the catalytic activity. This coul d explain the lower activity obser ved for the 5/5/90 catalyst. However, the strong Cu-Al2O3 interactions on the 5/5/90 also a ppear to result in formation of more inactive CuAl2O4, as well as a more difficult to reduce Cu2O phase, compared to the other catalysts and may be the main reasons for the lower TOF observed for this catalyst.
66 2.5 Thermodynamic Calculations The results in this study reveal that for the nanoparticle-supported catalysts the CO selectivity does not increase with increasing temperatures between 220C and 300C. The fact that the CO selectivity is not increasing over th is temperature range indicates that neither the reverse water gas shift nor the methanol decompos ition reaction rate is increasing faster than the rate of the reforming reaction (CO producti on is not increasing faster than the CO2 production rate). To elucidate the source of CO in this study, equilibriu m calculations (reverse water-gasshift reaction) were performed to compare the partial pressure of CO in the reactor effluent to the equilibrium partial pressure of CO for the reverse water-gas-shift reaction. The general procedure is the same as was described in greate r detail in Chapter 1. Briefly, the thermodynamic equilibrium constant for the reverse water-gas-s hift was calculated according to equation 2-7: ) (exp *2 2 2TR G H CO OH CO rWGS eqrWGSPP PP K 2-7 In this equation CO P 2CO P 2H P and OH P 2 are the partial pressures of the components exiting the reactor. GrWGS is the free energy change associated with the reverse water-gas-shift reaction, R is the ideal gas constant and T is the temperature in Kelv in. In this way the exiting concentrations of the reaction products can be co mpared with the thermodynamic equilibrium of the reverse water-gas-shift. The results for the 35 /35/30 catalyst are typical of the catalysts in this study and are shown in Figure 210. Error bars indicate that even if there is 25 % error in the measured gas pressures the conclusion that the CO concentrations are above levels which can be explained by the water-gas-shift equilibrium is still valid.
67 220240260280300320340360380400420 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 Figure 2-10 Keq rWGS or PCO*PH2O/PCO2*PH2 Temperature [deg. C] Figure 2-10. Comparison of Keq rWGS and ratio of measured partial pressures. Keq rWGS and 2 2 2/CO HOH COPPPP over the 35/35/30 catalyst. Reaction conditions are the same as for Figure 1. At all temperatures the partial pressure of CO in the reactor effluent is higher than the equilibrium CO partial pressure of the water-gas-shift reaction. Below 300C, the CO partial pressure approaches (decreases towards) the equi librium value, but then sharply increases away from equilibrium above 300C. CO levels higher than those at tainable via the water-gas-shift equilibrium indicate that the source of the CO is not solely the reverse water-gas-shift reaction. CO production at these levels must occur via th e decomposition reaction. This is reasonable particularly at higher temperatur es given that the activation en ergy of the decomposition reaction is 170 kJ/mol [ 8] compared to 108 kJ/mol [ 10] for the reverse water-gas-shift. However, it appears that for the nano particle catalysts in this study, the de composition reaction also must be taken into account at lower temperatures (and lo wer conversions) in agreement with the studies by Peppley and coworkers [ 7,8], Choi and Stenger [ 49] and Mastalir et al. [ 4].
68 2.6 Conclusions The above results demonstrate that nanoparticle alumina can be used effectively as support in the preparation of highly active Cu-ZnO steam reforming catalysts. Very high copper surface areas can be attained on nanopart icle alumina and activities similar to those of commercial catalysts with much high copper contents can also be achieved. However, the activities of the catalysts do not scale with th e copper surface area and high Al2O3 concentrations tend to retard the surface activity. Strong metal-support interacti ons between the small c opper particles and the nanoparticle Al2O3 support on catalysts with high Cu disp ersions, such as the 5/5/90 catalyst, appear to have a dramatic influence on the redox properties of the catalyst. The change in redox properties likewise appears to a ffect the methanol conversion, with less reducible nanoparticle catalysts having lower conversions. High Al2O3 concentrations also promote production of an undesired formaldehyde byproduct. The results in this study demonstrate that novel catalysts prepared using nanoparticle alumina have the advantage of lower CO selectivities compared to a commercially available low temperature shift catalyst. In fact, below reaction temperatures of 300C (260C for the 42/47/11 catalyst) the CO selectivity over the na noparticle catalysts decreases with increasing temperature. This is unexpected and observed only for the nanoparticle based catalysts, and must thus be due to changes in the surface comp osition of the nanoparticle systems, such as the Cu+ to Cu0 ratio, over this temperature range. It was also shown that use of a nanoparticle support results in a more active copper surface (evidenced by higher turnover frequencies) compared to a commercially available reference catalyst. XRD reveals that Cu metal is present in the bu lk of all the spent catalysts and XPS indicate that Cu2O and perhaps a small amount of CuAl2O4 are present at the surface. Only on the 5/5/90
69 could the Cu2O phase be detected with XRD. There is also evidence of a ZnAl2O4 spinel phase in the bulk of the 35/35/30 catalyst. The high stab ility of the 35/35/30 cataly st is likely due to a beneficial interaction between the zinc-aluminate species and the Cu species on the catalyst. Although it could also be due to th e fact that there is less ZnO available that can migrate and cover the active Cu phase. Permanent catalyst deactivation is likely due to ZnO covering part of the active Cu phase, although loss of activity, and incr easing CO selectivities, at higher temperature may also be due to decreasing Cu+/Cu0 ratios. The CO production levels are above what can be explained from the water-gas-shift equilibrium for the nanoparticle catalysts at all temperatures. Th is indicates that the methanol decomposition reaction is partly responsible for CO production. The results in this study suggest that it is the nature of the inte raction between the copper and prom oter or support, and not simply the copper surface area, which determines the catalytic activity. Our findings can be summarized as responses to the questions posed in the introduction: 1) Al2O3 is a retardant in the reforming reaction, but it is possible to make high performance reforming catalysts with copper and zinc oxides precipitated onto nanoparticle Al2O3 as long as the Al2O3 concentration is relatively low. 2) The apparent advantage to using nanopart icle precursor materials in reforming catalyst synthesis is that the surfaces of the nanoparticle catalysts are dramatically more active than more traditionally prepared reforming catalysts. 3) Methanol decomposition plays a role in the steam reforming reaction network over the nanoparticle catalysts. 4) The surface and structural characteristics of the nanoparticle-supported catalysts differ from those reported in the literature. More specifically, a high concentration of nanoparticle Al2O3 prevents CuO oxidation to Cu instead resulting in Cu2O formation in the 5/5/90 catalyst. This apparently decreases the activity of the surface in the reforming reaction.
70 The fact that Al2O3 was found to retard catalytic activity but that nanomateria ls were useful in making highly active reforming catalysts was th e basis for exploring al ternative nanoparticle supports. This work is cove red in the following chapter. 2.7 Acknowledgements. XRD and XPS measurements were performed at the Major Analytical Instrumentation Center at the University of Florida. The aut hors are especially tha nkful for the advice and instruction of Dr. Eric Lamber s and Dr. Valentin Craciun. The BET data were collected at the Particle En gineering Research Center at the University of Florida. The authors are grateful for the training on the machine provided by Mr. Gill Brubaker. We also thank Dr. Gar Hoflund for letting us use his XPS system and Dr. Michael Everett for technical assistance with the XPS measurements.
71 CHAPTER 3 INCORPORATION OF REDUCIBLE OXIDES: STEAM RE FORMING OF METHANOL OVER CeO2AND ZrO2-PROMOTED Cu-ZnO CATALYSTS SUPPORTED ON NANOPARTICLE Al2O3 3.1 Introduction and Literature Review. After investigating the eff ects of adding nanoparticle Al2O3 to the reforming catalyst matrix, the next study in this body of research wa s to investigate the e ffects of incorporating reducible oxides into the catalyst support. It ha s been shown in the preceding chapter that only small amounts of Al2O3 can be used in reforming catalysts, since higher concentrations can have a negative effect on the catalytic activity [ 16, 76]. Due to the negative effects of Al2O3 supports in methanol reforming catalysts, the trend in recent years is to add another oxide to the Cu/ZnO/Al2O3 catalysts or simply use supports other than alumina. Reducible metal oxides, such as ZrO2 and CeO2, appear to be attractive alte rnatives or additions to Al2O3 [ 4,14,18, 27, 28,29,43,56,60,77,78,79,80, 81]. For example, ZrO2 addition to Cu-based alumina-supported catalysts has been shown to increase meth anol conversion and reduce CO yields [ 14,18,56,80]. The effects of ZrO2 on Cu-based catalysts appear to be similar to the effects of ZnO in that ZrO2 improves Cu dispersion and leads to more reducible catalysts [ 14,29,56,80]. However, it has been noted that the m etal-s upport interactions in Cu/ZrO2 are different than in the more conventional Cu/ZnO catalysts [ 26]. The higher activity of Cu-ZrO2 catalysts has also been attributed to the stabilization of Cu2O on the surface of the redu ced catalysts or during the reaction [ 14,27,43]. It is believed that the formation of Cu2O can lead to more active and also more durable catalysts, since Cu2O is less susceptible to sintering compared with Cu metal [ 27,43]. Cu+ species have also been observed in CeO2-containing Cu catalysts [ 77, 82]. Addition of CeO2 to Cu/Al2O3 catalysts has also been shown to in crease methanol conversion, decrease CO selectivity and increase catalyst stability [ 80].
72 Comparing results between reforming catalyst studies referenced above is challenging since the reaction evidently is very sensitive to the catalysts used and large differences in Cu loadings and catalyst compositions have been reported. For exam ple, the copper concentrations on these types of catalysts have been varied from a few percent in some publications [ 77,81] up to 70% or above in others . Some catalysts also contain ZnO and Al2O3 in addition to Cu and ZrO2 [ 14,18,29,56]. Therefore, when comparing di fferent m ethanol steam reforming catalysts it is important to include reference catalysts for comparisons. In order to further facilitate the comparison of results between this chapter and the previous chapter, the same Sud Chemie catalyst will be used as the reference catalyst. Previous results from Chapter 2 have shown that using nanoparticle alumina as the support for Cu-ZnO-based methanol steam reforming cat alysts leads to catalysts with lower CO selectivity and similar conversions compared to commercially avai lable catalysts. The objective in the study presented in this chapter is to invest igate if the catalytic activity (methanol conversion) and the CO selectivity of the nano-Al2O3-supported catalysts can be further improved by the addition of CeO2 or ZrO2. Another goal is to determine if the specific ZrO2 precursor used (nitrate or nanoparticles) a nd the preparation method (co-impregnation and sequential impregnation) will affect the catalytic activity and the CO2 (or CO) selectivity. Furthermore, the CO levels in the reactor effl uent will be compared to the equilibrium CO concentrations from the water gas shift reaction to elucidate the most lik ely production pathway for CO (i.e. methanol decomposition or the reve rse water-gas-shift). This procedure will also allow a simple comparison to the CO levels from the catalysts examined in Chapter 2. As Amphlette et al. [ 33] and Ritzkopf et al. [ 44] stated in their respective papers, the equilibrium conversion in the m ethanol reforming reaction is well over 95%, although conversions that high
73 can be difficult to attain in pr actice. Therefore, clearly the thermodynamics offer the incentive for developing better catalysts [ 33]. The search for better catal ysts is continued here using nanoparticle reducible oxide supports. 3.2 Experimental Methods. As in Chapter 2, the catalyst preparation tech nique used to fabricate the catalyst samples used in this study is uniq ue to the data presented in Chapters 3 and 4. Also, details of the reactor operation are included for the readers reference. TPR, N2O titrations, and XRD measurements and catalyst pretreatment were performed precisely as described in Chapter 1. 3.2.1 Catalyst Preparation. Catalysts in this study are named according to the concentration of the active metal precursor, promoter, and support as a mass percenta ge. The concentrations of CuO and ZnO are kept constant at 14-15%. Thes e concentrations were selected after careful c onsideration of optimum literature values [ 29,77,78]. In addition to the ZnO and CuO phases, either ZrO2 or CeO2 were used as promoter or support and the remainder of the material is nanoparticle Al2O3. The catalyst labels list the species present in th e catalyst and the concen tration of either ZrO2 or CeO2, as well as the preparation method employed. For instance, the CuZnZrAl-10-CI sample consists of 15% CuO, 15% ZnO, and 10% ZrO2 by mass, with the balance (60 %) being Al2O3. The label CI indicates that the Cu, Zn, a nd Zr precursors were co-impregnated on the nanoparticle Al2O3, or Cu and Zn were co-impregnated on nanoparticle ZrO2 or a mixture of nanoparticle ZrO2 and nanoparticle Al2O3. All catalysts contain nanoparticle Al2O3 except the CuZnZr-70-NP, which is 15% CuO, 15% ZnO, and 70% nanoparticle ZrO2 by mass. The label NP indicated when nanoparticle ZrO2 is the zirconia precursor (as opposed to ZrO(NO3)2H2O). The label SQ indicates sequential impregnation where the ZrO2 was deposited onto the support
74 first. The CuO and ZnO were then co-impregnated on the ZrO2/Al2O3 (see below). This information is summarized in Table 3-1. Table 3-1. Catalyst Preparation Summary Catalyst Label a Composition Preparation Zr or Ce Precursor CuO ZnO ZrO2 CeO2 Al2O3 CuZnZrAl-10-CI 15% 15% 10% 0% 60% CoImpregnation ZrO(NO3)2H2O CuZnCeAl-10-CI 15% 15% 0% 10% 60% CoImpregnation Ce(NO3)2H2O CuZnZrAl-36-CI 14% 14% 36% 0% 36% CoImpregnation ZrO(NO3)2H2O CuZnZrAl-10-NP 15% 15% 10% 0% 60% CoImpregnation Nano-Particle ZrO2 CuZnZr-70-NP 15% 15% 70% 0% 0% CoImpregnation Nano-Particle ZrO2 CuZnZrAl-10-SQ 15% 15% 10% 0% 60% Sequential Impregnation ZrO(NO3)2H2O a Catalyst labels are based on the composition of CuO, ZnO, ZrO2 (or CeO2) and Al2O3 by weight. The CuO and ZnO concentrations are kept constant at 15% (or 14%). The concentration of ZrO2 or CeO2 is given in the label and the balance is Al2O3. The last part of the label indicates the preparation method, CI for co-impregnation, NP for coimpregnation on nanoparticle ZrO2 and SQ for sequential impregnation. All catalysts in this study were prepared vi a some variation of wet impregnation, either concurrently or sequentially. The catalyst s CuZnZrAl-10-CI and CuZnCeAl-10-CI were prepared by dissolving (in proper amounts) Zn(NO3)2H2O(Alfa Aesar), Cu(NO3)2H2O (Alfa Aesar), and ZrO(NO3)2H2O or Ce(NO3)2H2O in 50 ml of deionized water. The nanoparticle Al2O3 (NanoScale, surface area ~650 m2/g) was then dispersed in the solution, the excess water boiled off and the resulting paste dried at 105C overnight. The dried sample was calcined at 300C for 3 hours.
75 The catalyst CuZnZrAl-10-NP was prepared in a similar manner except that the zirconia was added as nanoparticle ZrO2 powder (Nanostructured and Am orphous Materials) along with the nanoparticle Al2O3 to the aqueous solu tion (50 ml) of Zn(NO3)2H2O and Cu(NO3)2H2O (Alfa Aesar). The water was evaporated and the resultant mixture was then dried and calcined in the same manner described above. The catalyst CuZnZr-70-NP was prepared as described above except that only ZrO2 nanoparticles were dispersed in the aqueous solution of Cu and Zn nitrates and no Al2O3 was used in this catalyst formulation. After impregnation of the ZrO2 nanopowder the resulting catalyst was dried and calcin ed as described above. Finally, the sequentially precipitated CuZnZrAl-10-SQ was prepared via two sequential impregnations. First, the ZrO(NO3)2H2O was dissolved in 50 ml water and then the Al2O3 nanoparticles were dispersed in the solution. The sample was then boiled down to a paste and dried overnight at 105C and calcined at 300 C for 3 hours. The calcined support was then ground to a fine powder. The Zn(NO3)2H2O and Cu(NO3)2H2O were then dissolved in 50 ml water and the impregnated ZrO2/Al2O3 support was dispersed in the n itrate solution. The sample was again boiled down to a paste, dried and calci ned in the same manner as the other samples. All catalysts were prepared in batches of 5 grams total weight after calcination. BET, N2O adsorption and temperature programmed reduction were performed on the calcined catalysts without further treatment. The reference catalyst used in this study is the commercially available Sd-Chemie catalyst that is approximately 42% CuO, 47% ZnO and 11% Al2O3 according to the manufacturers specifications. This is the same reference catalyst used in Chapter 2. Other related studies have used a similar reference catalytst [ 10,49].
76 3.2.2 Reactor and Gas Chromatograph Operation Except where otherwise noted for all catalyst experiments in this chapter the water/methanol ratio was constant at 1.4 volum e / volume (~3.1 mol/mol), as was the case in Chapter 2. A water/methanol ratio of 0.6 volume/volume (1.4 mol/mol) was used in two temperature sweeps to demonstr ate the insensitivity of the catalyst performance to the water/methanol ration in the f eed. The chromatograph used in this study is the same Agilent 6890 series online gas chromatogra ph described previously. Repetiti on of data points to ensure reproducibility was done in the same manner describe d in Chapter 2. The major difference in the operation of the reactor and GC in this study is that no signif icant levels of CH2O were detected at any temperature or any contact time. In all cases, reproducible results were obtained and there were no signs of catalyst deactivation due to aging with time on stream. 3.2.3 Water Gas Shift Equilibrium In this chapter the calculated equilibrium data is presented by plotting the ratio as defined in Equation 3-1. This is a similar pr ocedure to what was outlined in Chapter 1. OHCOWGS HCOPPK PP2 22 3-1 In this equation KWGS is the temperature dependent equi librium constant of the water gas shift reaction, calculated from the activation ener gy and pre-Arrhenius factor used by Peppley as was done in Chapter 2 [ 7]. This approach is used instead of the method used in Chapter 2 since in this study there were large fluctuations in the value of PCO that make plotting the pressure ratio shown in Equation 2-7 of Chapter 2 difficult. Since water is not measured on the gas chromatograph, the water level in the effluent gas is estimated from the CO2 level via the reforming reaction. Specifically, since there is a 1:1 molar
77 ratio of CO2 produced to H2O consumed, the moles of H2O reacted are assumed to be equal to the (measured) moles of CO2 produced. This number of moles of water is then subtracted from the (known) inlet c oncentration of H2O. The remainder is assumed to be in the effluent. The assumption used here is that the amount of wate r reacted in the steam reforming reaction is much greater than any water involved in the water gas shift. This is reasonable given the disparity in the levels of CO2 and CO in the effluent. Confidence levels are presented, and it is evident that even if the water level is off by 50%, the co nclusions are not effected. According to the definition of a value of 1 represents the watergas-shift reaction equilibrium. If <1 the CO concentration is higher than what can be expl ained by the water-gas shift equilibrium and is >1 the CO concentration is lower than the equilibrium CO water-gas-shift concentration. 3.3 Catalytic Activity Measurements 3.3.1 Methanol Conversion Figures 3-1A through 3-1C show the methanol conversion as a function of temperature for all catalysts in Table 3-1. For comparison, the data obtained from the reference catalyst is also included in the figures. It is ev ident from Figure 3-1A that ZrO2 is a much better promoter of the catalysts under these conditions compared with CeO2. The CeO2-containing catalyst (CuZnCeAl-10-CI) never at tains a conversion comparable to the commercial catalyst. In contrast, above a temperature of 280C the CuZnZrAl-10-CI catalys t exhibits a higher conversion than the reference catalyst. Increasing the ZrO2 content from 10% to 36% increases the low temperature conversion, but this cataly st experiences deactivation above 265C. To probe how sensitive the catalysts are to preparation method and ZrO2 precursor, two additional 10% ZrO2 catalysts were prepared (CuZnZrAl-10-SQ and CuZnZrAl-10-NP). Consequently, the nominal chemical composition of the CuZnZrAl-10-CI, CuZnZrAl-10-SQ and CuZnZrAl-10-NP
78 catalysts is the same. The differences are the or der of Zr deposition (versus Cu and Zn) and the precursor (ZrO(NO3)2H2O versus nanoparticle-ZrO2), as explained in sec tion 3.2.1. While the performance of the various CuZnZrAl-10 cataly sts is similar up to 240C, the preparation method and the ZrO2 precursor evidently have a large in fluence on the catalytic activity above this temperature. The CuZnZrAl-10-NP catalyst e xhibits the best performance of all catalysts in this series, and it is the only one to achieve ~8 0% conversion at the contact time used in the temperature sweep experiments. This conversion is significantly higher than the maximum of 62% obtained from the commercially available reforming catalyst. Only at the lowest temperature (225C) does the commercial catalyst have a slightly high er conversion than our CuZnZrAl-10-NP catalyst. The CuZnZrAl-10-SQ catalyst exhibits the poorest performance of these three CuZnZrAl-10 catalysts with a maxi mum methanol conversion of 66%, although this is still higher than the maximum conversion of the commercial reference catalyst. The sequential catalyst is limited by deactivation above 285C. The CuZnZrAl-10-CI and the CuZnZrAl-10-NP do not deactivate until temperat ures above 300C. A Cu-Zn catalyst supported on nano-particle ZrO2 was also prepared to determine if presence of alumina is necessary to assure a high catalytic activity. The CuZnZ r-70-CI catalyst exhibits the lowest maximum conversion of all catalysts in th is study. Only at temperat ures where the other catalysts experience significant deactivation, does the CuZnZ r-70-NP catalyst have a higher conversion. In order to probe the sensitivity of the system to the water/methanol ratio in the feed, two catalysts were also tested at a water/methanol fe ed ratio of 1.4 mol/mol. These results were then compared to the temperature sweep done at a wate r/methanol molar ratio of 3.1. This data is shown in Figure 3-1C. Evidently these catalysts ar e not sensitive to the water/methanol feed ratio in this range given there is no major difference in catalyst performance fo r either the CuZnZrAl-
79 10-NP or the CuZnZr-70-CI catalyst. This is the only experiment in this chapter which used the lower H2O/CH3OH feed ratio. 200220240260280300320340360380 0 10 20 30 40 50 60 70 80 90 Figure 3-1A Conversion of Methanol [%]Temperature [deg. C] 200220240260280300320340360380 0 10 20 30 40 50 60 70 80 90 Figure 3-1B Conversion of Methanol [%]Temperature [deg. C] Figure 3-1. Methanol Conversion as a function of temperature A) for selected catalysts. CuZnZrAl-10-CI, CuZnZrAl-36-CI, CuZnZr-70-NP, CuZnCeAl10-CI and Commercial Reference. Contact time for all curves is 0.15 kg cat*sec/mmol CH3OH. B) for additional catalysts. CuZnZrAl-10-CI, CuZnZrAl-10-SQ, CuZnZr-70-NP, CuZnZrAl-10-NP and Commercial Reference. C) at two H2O/CH3OH molar feed ratios for two catalysts. CuZnZr-70-NP feed H2O/CH3OH=3.1, CuZnZrAl-10-NP feed H2O/CH3OH=3.1 and CuZnZr-70-NP feed H2O/CH3OH=1.4, CuZnZrAl-10NP feed H2O/CH3OH=1.4.
80 200220240260280300320340360380 0 10 20 30 40 50 60 70 80 90 Figure 3-1C Conversion of Methanol [%]Temperature [deg. C] Figure 3-1. Continued Figure 3-2 displays the conversion as a functi on of contact time for selected catalysts, including the commercial reference. Only th e most active catalysts with reasonably low CO conversions were subjected to the contact time activity measur ements. The only exception to this is the CuZnCeAl-10-CI which was included for comparison. Again, the CuZnZrAl-10-NP catalyst exhibits the best perf ormance of all the catalysts. The other catalysts required considerably longer contact times to reach 100% c onversion. As with the conversion data shown in Figure 3-1, the CeO2 containing catalyst exhibited the lowest activity. In the absence of catalyst deactivation, such as sint ering, it is expected that all catalysts in the study will achieve 100% conversion given suffi cient contact time. However, as is well documented in the litera ture, longer contact times also increase the undesirable CO yields (see section 3.3.2) [ 4,9,10,28,60]. The CuZnZrAl-10-NP catalyst attains 100% conversion at shorter contact tim es than required for 100% conversion of the commercial catalyst. The shapes of the curves in Figure 3-2 are also worth noting. The CuZnZrAl-10-NP and CuZnZrAl-10-CI samples both exhibit the classic shape asymptotically approaching a
81 0.00.10.20.188.8.131.52.70.80.91.01.11.2 0 10 20 30 40 50 60 70 80 90 100 Figure 3-2 Conversion [%]Contact time [kg cat*sec/mmol] Figure 3-2. Conversion as a functio n of contact time for the 10% ZrO2 catalysts, the 10% CeO2 catalyst, and the commercial reference catalyst. CuZnZrAl-10-CI, CuZnZrAl-10-SQ, CuZnZrAl-10-NP, CuZnCeAl-10-CI and Commercial Reference. Temperature for a ll curves was held constant at 280C. conversion of 100% with increasing contact times. In contrast, the CuZnZrAl-10-SQ, CuZnCeAl-10-CI and the commercial reference catalysts display a rapid increase in conversion at intermediate contact times. This behavior is generally undesirable since the rapid increase in conversion is also followed by a significant increase in the CO concentrati on of the effluent gas (see Figure 3-4). 3.3.2 CO Selectivity The CO selectivity as defined in Chapter 1 is shown in Figure 3-3 for all catalysts as a function of temperature. The Cu ZnZrAl-10-NP catalyst maintains the lowest CO selectivity of any catalyst in this study over the entire temperature range. This is remarkable considering that it also exhibits the highest conversions at these temperatures. Normally the CO yields increase rapidly with increasing temperatur e and increasing methanol conve rsion. Even at the maximum
82 conversion (305C), the CuZnZrAl-10-NP catalyst exhibits a CO selectivity (3.6%) that is only slightly higher than the lowest value observed (2 .9%) for this catalyst. This CO selectivity corresponds to a CO concentration of 0.6% (dry) in the reactor effluent. Of the other 10% ZrO2 samples (CuZnZrAl-10-CI and CuZnZrAl-10-SQ), the sequentially impregna ted catalyst exhibits a higher CO selectivity than th e co-impregnated catalys t although the difference is not significant between 240 and 285C. The CuZnCeAl-10-CI a nd CuZnZrAl-10-CI cata lysts exhibit similar CO selectivities below 280C. However, above 280C there is a discontinuous increase in the CO selectivity for the CuZnCeAl-10-CI catalyst that is not evident in any of the other catalysts. The steep increase in the CO selectivity for this catalyst coincides with the loss of activity as seen in Figure 3-1A. Catalyst CuZnZrAl-36-CI exhibits the highest CO selectivity of the catalysts in the series. In fact, the CO selec tivity of this catalyst is higher than the CO selectivities observed on the Cu-ZnO/nano-Al2O3 catalysts shown in Chapter 2. Consequently, using an impregnation method to prepare Cu/ZnO/ZrO2/Al2O3 catalysts it is important to use ZrO2 concentrations below, perhap s well below, 36%. The 70% ZrO2 catalyst (CuZnZr-70-NP) exhibits a lower CO selectivity than the 36% ZrO2 sample, but it is still higher than the CO selectivity for the alumina containing samples at temperatures below 250C. The CO selectivity for this catalyst continues to decrease up to a te mperature of 295C. At this temperature only the 10% nano-ZrO2 catalyst (CuZnZrAl-10-NP) has a lowe r CO selectivity. The methanol conversion of the CuZnZr-70-NP ca talyst is only 45% at this te mperature, which indicates that this catalyst is not a competitive methanol steam reforming catalyst for PEM fuel cell applications.
83 200220240260280300320340 0 2 4 6 8 10 12 14 16 18 Figure 3-3 CO Selectivity [%]Temperature [deg. C] Figure 3-3. CO selectivity as a func tion of temperature for the 10% ZrO2 catalysts, the 10% CeO2 catalyst, and the commercial reference catalyst. CuZnZrAl-10-CI, CuZnZrAl-10-SQ, CuZnZrAl-10-NP, CuZnCeAl-10-CI and Commercial Reference. Temperature for a ll curves was held constant at 280C. It is worth noting that the CO selectivity for all nanoparticle catalysts in this study either decreases or stays constant with increasing temperature below approximately 250C. This is an important observation given that the only CO pr oduction routes, the revers e water gas shift and the methanol decomposition reaction, are both endothermic and thus CO production should be both kinetically and thermodynamica lly favored with increasing te mperature. Consequently, the CO selectivity curves demonstrate that the meth anol reforming reaction must be increasing at a faster rate than either the decomposition reacti on or the reverse water gas shift reaction (or a combination of the two) for all catalysts. A similar phenomenon was observed previously during the nano-Al2O3 supported catalysts in Chapter 2. The temperature at which CO selectivity
84 begins to increase is not consta nt for all the catalysts, but appears to be near 250C. The CuZnZrAl-10-NP catalyst is remarkable in that it does not exhibit a significant increase in CO selectivity until well above 300C. Figure 3-4 displays CO selec tivity as a function of contact time for the most active catalysts, as well as the CuZnCeAl -10-CI and commercial referen ce catalyst. In this figure the CO selectivity of the CuZnZrAl-10-SQ catalyst is nearly double that of the other samples at and above contact times of 0.28 mmol/(gsec). Sinc e the conversion of CuZn ZrAl-10-SQ catalyst at this contact time is only 59%, this catalyst is not appropriate for methanol steam reforming reactions where low CO concentrations are critical. The most interesting feature of this figure is the CO selectivity of the CuZnZrAl-10-NP. Fo r this sample the CO production increases only slightly and then remains fairly constant with increasing contact time up to a value of 0.3 kg*sec/mmol. Consequently, the CuZnZrAl-10-NP catalyst is superior to the others in this study in that the conversion is incr eased significantly, while the CO selectivity remains almost constant. An interesting hysteresis effect was also record ed in the contact time experiments. It was noted in Chapter 2 that catalysts exposed to te mperatures higher than the maximum conversion temperature suffered deactivation and could not achieve previous (i.e. lower temperature) activity levels. A different hysteresis effect was recorded by varying the flow rate (contact time) for the CuZnZrAl-10-NP catalyst. The data for th e CuZnZrAl-10-NP catalyst at different contact times is shown in Figure 3-5A and 3-5B. If the contact time sweep is star ted at longer contact times (lower flow rates)
85 0.00.10.20.184.108.40.206.70.80.91.01.11.2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Figure 3-4 CO Selectivity [%]Contact time [kg cat*sec/mmol] Figure 3-4. CO selectivity as a func tion of contact time for the 10% ZrO2 catalysts, the 10% CeO2 catalyst, and the commercial reference catalyst. CuZnZrAl-10-CI, CuZnZrAl-10-SQ, CuZnZrAl-10-NP, CuZnCeAl-10-CI and Commercial Reference. Temperature for a ll curves was held constant at 280C. and the feed flow rate is increased, the CO se lectivity curve does not have a zero intercept as described in Chapter 2 and shown in Figure 3-4. The zero intercept is observed for this catalyst if the contact time sweep is started at the highest flow rate (lowest contact time) and the flow rate is decreased (toward higher contact times). The e ffect of increasing the feed flow rate and then decreasing it again after exposure to high flow rates is shown in Fi gure 3-5. There is no apparent difference in CH3OH conversion (Figure 3-5B), but the CO se lectivity is affected (Figure 3-5A). Original CO selectivity levels cannot be repe ated once the CuZnZrAl10-NP catalyst has been exposed to the highest feed flow rates. The orig in of this phenomenon is not clear, but likely has to do with the oxidation state of the Cu phase, wh ich is covered in detail in the next chapter.
86 0.000.050.100.220.127.116.110.350.40 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 decreasing feed flow increasing feed flow Figure 3-5A CO Selectivity [%]Contact Time [kg cat*sec/mmol] 0.000.050.100.18.104.22.1680.35 0 10 20 30 40 50 60 70 80 90 100 decreasing feed flow increasing feed flow Figure 3-5B Conversion [%]Contact Time [kg cat*sec/mmol] Figure 3-5. A. Conversion and B. CO selectivit y as a function of cont act time for CuZnZrAl-10NP catalyst increasing feed flow and decreasing feed flow. Temperature for all curves was held constant at 280C.
87 3.4 Catalyst Characterization Results. 3.4.1 Surface Area Analysis The BET surface area data, active copper surf ace area, copper dispersion, and turnover frequency (TOF) at approximately 250C and 300C are presented in Table 3-2 for all catalysts. Added to the table are also results for the comm ercial reference catalyst and the 35/35/30 catalyst from Chapter 2. Table 3-2. Surface characteristics and ac tivities of steam reforming catalysts. Catalyst BET Surface Area [m2] Cu Surface Area [m2/g] Cu Dispersion [%] TOF*103 [s-1] 250C 300C CuZnZrAl-10-CI 48.0 11.3 1.7 175 226 CuZnZrAl-10-NP 115.4 11.9 1.8 220 244 CuZnZrAl-10-SQ 119.6 13.8 2.1 134 122 CuZnZrAl-36-CI 93.4 13.7 2.1 197 177 CuZnCeAl-10-CI 39.5 14.7 2.3 91 74 CuZnZr-70-NP 19.0 3.4 0.53 216 342 CuZnAl-35/35/30-CI 46 5.3 0.8 89 132 Commercial 68 20.2 3.1 34 28 Evidently there is no correla tion between the overall surface area and the Cu surface area of the catalysts, except that the catalyst with the lowest total surface area (CuZnZr-70-NP) also
88 does have the smallest Cu surface area. As expect ed, the lowest Cu surface area is observed on the catalyst which does not cont ain any high surface area nanoparticle alumina. It is, however, surprising that the different prepar ation methods and the varying ZrO2 (or Al2O3) concentrations do not result in catalysts with a larger vari ation in Cu surface areas. All ZrO2or CeO2containing catalysts, except the CuZnZr-70-NP, have Cu surface areas in the range between 11 and 15 m2/g. Despite the similar copper surface areas the catalytic activities of the catalysts are considerably different. There is also no appare nt correlation between th e Cu surface area and the catalytic activity on these catalysts. A very in teresting observation is th at the copper phase on the nanoparticle ZrO2-supported catalysts is significantly more activ e than the copper on the CeO2-promoted catalyst, the catalyst without CeO2 or ZrO2, as well as the catalysts prepared via impregnation using the zirconium n itrate species. This is particularly evident on the CuZnZr-70NP catalyst, which has the lowest Cu surface area of the catalysts under investigation, but it has the highest TOF observed. Conseque ntly, the Cu on the surface of this catalyst is very active. It is also evident from Table 3-2 that co-impregna tion of the Cu, Zn and Zr precursors results in more active catalysts than if the Zr precursor is added before the Cu and Zn precursors (sequential impregnation). It is interesting to note that the cata lyst with the highest Cu surface area, the CuZnCeAl-10-CI catalyst, exhibits the lowest TOF. The results reveal that the nanoparticle alumina does provide a high surface area on which to deposit the active metal and promoters, which in turn is necessary to assure a reasonable Cu surface area. However, addition of ZrO2 (or CeO2) further improves the Cu surface area, as is evident when comparing the CuZnZrAl-10-series catalysts with the non-ZrO2 containing CuZnAl-35/35/30-CI catalyst. Perhaps ev en more importantly, addition of ZrO2 evidently results in a more active Cu species on the surface. This supports the notion of a synergy effect
89 between the copper phase and the zirconia s upport, which has been documented by other researchers [ 29,38]. 3.4.2 Temperature Programmed Reduction Measurements To further probe how the ZrO2 influence the Cu on the surf ace, temperature programmed reduction experiments were performed on the prepar ed catalysts. The reduction profiles of all catalysts are presente d in Figure 3-6. 0100200300400 Figure 3-6Commercial CuZnZrAl-36-CI CuZnZrAl-10-CI CuZnZrAl-10-SQ CuZnZrAl-10-NP CuZnCeAl-10-CI CuZnZr-70-NP Hydrogen Uptake [Arbitrary Units]Temperature Figure 3-6. Temperature programme d reduction profiles for all catalysts in this study. Curves are labeled appropriately in the Figure. Da shed line has been added for visualization and shows a temperature of 300C. Reduction was performed under 5% H2 in N2 at a temperature ramp of 5C/min. The TPR data obtained from the commercial re ference catalyst has also been added for comparison. It is evident that all ZrO2and CeO2-containing catalysts are reduced at higher temperatures compared to the reference commercial catalyst. This is surp rising, since addition of
90 ZrO2 and CeO2 usually results in catalysts that are easier to reduce than their ZrO2-free analogues [ 14,18,29,44]. Furthermore, the study on impre gnated and precipitated CuO, ZnO, ZrO2 and Al2O3 catalysts done by Breen indicated th at catalysts which reduce at lower temperatures were more active methanol steam reforming catalysts [ 60] Other authors have indicated sim ilar findings . Evidently, the catalysts in this study do not fo llow this trend. In fact, the m ost active catalyst, CuZnZrAl-10-NP, e xhibits the highest redu ction temperature and a reduction profile consisting of two reduction peaks at 328 and 348C. The reduction temperatures of the CuZnZrAl-10-SQ (320 a nd 338C) and CuZnZrAl-10-CI (324C) catalysts, which are among the more active catalysts, are also higher than the other catalysts with higher ZrO2 content. Of the ZrO2-containing catalysts, the CuZnZr-70NP exhibits the lowest reduction temperature (273 and 300C), which could be due to the lack of alumina. As noted previously, this catalyst also has the highest TOF of any sample in this st udy. Accordingly, the surface of this catalyst is highly active and this agrees with the trend in reducti on temperatures noted by Breen [ 29]. However, the sample is apparently crippled by the low Cu surface area and hence is not an effective catalyst. The CuZnZrAl-36-CI catalyst (316C) is only slightly m ore reducible than the CuZnZrAl-10-CI catalyst (324C) ev en though it has a signifi cantly higher ZrO2 content. It is evident from Figure 3-6 that the method of preparation, i.e. sequential versus coimpregnation, affects the reducti on properties of the re sulting catalysts. The CuZnZrAl-10-CI and CuZnZrAl-36-CI catalysts, in which the Cu, Zn and Zr precursors are impregnated onto the nano-Al2O3 at the same time, are the only ZrO2-containing catalysts which exhibit a single reduction peak. All the other ZrO2-containing catalysts exhibit two distinct reduction peaks. While it is possible that the two reduction peak s are due to a step-wis e reduction of CuO via
91 Cu2O to Cu metal [ 81], another explanation is that the two peaks are due to different types of Cu on the surface [ 14]. These two copper species could be a highly dispersed copper phase together with larger copper particles [ 18,29]. In addition, copper supported on monoclinic ZrO2 has been shown to give two reduction peak s due to copper interactions w ith two different types of oxygen on the ZrO2 surface [ 84]. It is not easy to explain why the CuZnZrAl-10-NP and CuZnZrAl-10SQ catalysts would result in a two-step reduction of CuO, wh ile the CuO on the CuZnZrAl-10CI catalyst would reduce to Cu m etal in one step. Therefore, it may be more likely that the two reduction peaks are due to the presence of two different Cu species on the surface. The ZrO2 nanoparticles, according to the manuf acturer, consist of 95% or more of the monoclinic phase. If this phase indeed results in two different coppe r-oxygen interactions at the surface, then it is expected that the two reducti on peaks would be more distinct on the CuZnZr-70-NP compared with the CuZnZrAl-10-NP catalyst, which is obser ved. However, this does not rule out the presence of widely varying Cu particle sizes on the surface, i.e. highly dispersed Cu and larger Cu particles. Evidently, th e copper phases on the CuZnZrAl -10-SQ and CuZnZrAl-10-NP catalysts exhibit similar reduction behavior, since they both display two distinct reduction peaks. In contrast, co-deposition of Cu, Zn and Zr apparently does not result in the same Cu-ZrO2 interactions as depos ition of Cu and Zn onto a nanoparticle-ZrO2/nanoparticle-Al2O3 mixture, since the CuZnZrAl-10-CI catalyst only exhibit a single reduction peak. The same is true for the CuZnZrAl-36-CI catalyst, although this catalyst has a shoulder at higher temperature which is probably due to the higher ZrO2 content. Of the catalysts with a 10% dopant level, the CuZnCeAl-10-CI exhibits the lowest reduction temperature (326C, with a shoulder at 308C) and reduction is complete at a lower temperature than any of the 10-36% ZrO2-containing catalysts. Desp ite the lower reduction
92 temperature and the higher Cu surface area, th e CuZnCeAl-10-CI has a lower catalytic activity than any of the ZrO2-containing catalysts. Therefore, under these conditions there is no correlation between the methanol conversion or TOF and the reduction temperature of the catalyst for this series of catalysts. In agreem ent with our results there are recent studies which indicate that the reducibility of the copper specie s does not play the decisive role in determining catalytic performance, and in fact in some cases the catalysts which are more difficult to reduce tend to be more active [ 59]. 3.4.3 X-ray Diffraction Analysis Figure 3-7A shows the XRD spectra collected from all catalysts after calcination. The characteristic peaks of CuO are evident on all sa mples, as expected. None of the impregnated zirconia samples exhibit a crystalline ZrO2 phase. The monoclinic ZrO2 phase of the nanoparticle zirconia is evid ent on catalysts where the ZrO2 nanoparticles are used in the catalyst preparation (NP catalysts). It is interesting that even for the case of the CuZnZrAl-36-CI sample no crystalline ZrO2 phase is present. In fact, the catalyst with 36% ZrO2 exhibits the poorest crystallinity of the catalysts in this investigation, with only the CuO phase and a poorly crystalline ZnAl2O4 spinel phase evident from the spectra. The CuZnZr-70-NP sample is highly crystalline with all thre e phases (CuO, ZnO, ZrO2) evident in the XRD spectra. There is a spinel ZnAl2O4 phase present on the CuZnZrAl-10-CI a nd CuZnCeAl-10-CI, and CuZnZrAl-36-CI catalysts. This is the same spinel phase docum ented in Chapter 2. This phase does not form during the sequential preparation method, when the ZrO2 is deposited first, and it also appears to be hindered by the presence of the nanoparticle ZrO2.
93 20304050607080 Figure 3-7Af) e) d) c) b) a) 2Intensity [arbitrary units]20304050607080 Figure 3-7Bf) e) d) c) b) a) 2Intensity [arbitrary units] Figure 3-7. XRD spectra obtained for all A. fresh and B. spent catalysts. a) CuZnZr-70-NP, b) CuZnZrAl-36-CI, c) CuZnZrAl -10-CI, d) CuZnZrAl-10-NP, e) CuZnZrAl-10-SQ and f) CuZnCeAl-10-CI. Cubic CeO2, ZnAl2O4, CuO, Cu, ZnO, and Monoclinic ZrO2. Particle size information presented in Table 3-3 was calculated using the (1 1 1) peak position and the full width at half maximum of CuO, ZrO2, CeO2 (when present) and ZnAl2O4.
94 Figure 3-7B shows the spectra from the spent catalysts after the temper ature sweep experiments shown in Figure 3-1. The large well defined peak at 43 is the (1 1 1) peak of Cu metal which is evident in all samples. This peak was used to calculate the Cu metal pa rticle size of the spent catalysts. The CeO2, ZrO2 and ZnAl2O4 phases are not significan tly altered by exposure to reaction conditions. In contrast, there is a significant increase in the Cu particle size after exposure to reaction conditions for all catalysts except the Cu ZnZrAl-10-CI and CuZnZrAl-36CI, compared to the particle size of the original CuO after calcination. This increase in Cu particle size is evidence of si gnificant sintering of the catalysts. This is the most likely explanation for the deactivation evident in Figure 3-1. Table 3-3. Particle size s of the different compounds on the various CuO/ZnO/ZrO2/Al2O3 catalysts in the current work. The particle sizes have been determined from the XRD data using the Sche rrer equation. Particle sizes [nm] Species Catalyst Cu CuO ZnO ZnAl2O4 ZrO2 CeO2 CuZnZrAl-10-CI Fresh -21.2 -5.4 --CuZnZrAl-10-CI Spent 15.5 --6.0 --CuZnZrAl-10-SQ Fresh -20.0 25.8 ---CuZnZrAl-10-SQ Spent 30.0 -28.0 ---CuZnZrAl-10-NP Fresh -22.2 21.3 -20.0 -CuZnZrAl-10-NP Spent 36.4 -38.9 -20.3 -CuZnZr-70-NP Fresh -13.1 --19.1 -
95 Table 3-3. Continued CuZnZr-70-NP Spent 30.6 ---19.0 -CuZnZrAl-36-CI Fresh -21.6 6.8 --CuZnZrAl-36-CI Spent 13.5 -5.2 --CuZnCeAl-10-CI Fresh -17.0 -5.8 -6.3 CuZnCeAl-10-CI Spent 30.5 --5.9 -7.4 3.5 Breaking the Water Gas Shift Equilibrium Figure 3-8 shows the ratio, as defined in equation 3-1, as a function of conversion for three catalysts. As can be seen in the figure, <1 for both the CuZnZrAl-10-SQ and CuZnZrAl36-CI catalysts, which means that the CO concen trations are higher than the water-gas-shift equilibrium CO concentration. Only the Cu ZnZrAl-10-NP catalyst gives values of is larger than 1.0 at methanol conversions of 65% and above (which corresponds to a temperature range of 300C to 345C). At higher temperatures, methanol conversion decreases (see Figure 3-1) and the ratio also decreases as the CO levels incr ease with increasing temperature. Since the CO concentration is higher than the water-gas-shi ft equilibrium concentration, this means that the source of the CO must be from another reacti on, i.e. the methanol decomposition. This is in agreement with the observation by Peppley et al. [ 7,8]. Evidently, the CuZnZrAl-10-NP catalyst appears to s uppress the decomposition reaction at re latively high methanol conversions. This is interesting, since the contribut ion of CO from the methanol decomposition reaction is usually greater at higher temperatures and higher meth anol conversions. Th e fact that the CO concentrations in the reactor effluent are belo w that of the water-gas-shift equilibrium at
96 methanol conversions of 65-80% for the CuZnZrAl10-NP reveals that this is a very promising methanol steam reforming catalyst fo r PEM fuel cell applications. 200210220230240250260270280290300310320330340350 0.0 0.5 1.0 1.5 2.0 Figure 3-8 [Dimensionless] Temperature [deg C] Figure 3-8. Dimensionless water gas shift equilib rium constant, as a function of temperature three nanoparticle catalysts. CuZnZrAl-36-CI, CuZnZrAl-10-SQ and CuZnZrAl-10-NP. Contact time for all curves is 0.15 kg catsec/mmol CH3OH which corresponds to 300mg catalyst and a total liquid inlet flow rate of 0.8 ml/hr. Dashed line reveals the equilibrium value ( = 1). Addition of the nanoparticle ZrO2 to the system may induce a certain morphology of the copper phase on the surface which could suppress CO formation via the methanol decomposition reaction. A favorable surface morphology has been cited by other researcher s in the fabrication of highly active methanol reforming catalysts [ 13]. However, the ZrO2 may also stabilize a Cu+ species on the surface [ 27], which could potentially suppress CO for mation. This is supported by the observation that CO is adsorbed more strongly on Cu2O compared with Cu metal [ 11], which m eans that CO desorption would not be as facile on a Cu2O covered surface. Furthermore, there would also be oxyge n present for oxidation of the CO to CO2, which is
97 important since the CO production has been sh own to increase with increasing extent of reduction of the catalyst [ 20]. Evidently, presence of nanoparticle ZrO2 also results in a highly active surface. This increased activity is not relate d to a decrease in reduction temperature or an increase in the Cu surface area of the catalyst. It has been shown by Bell that in the methanol synthesis reaction the system is highl y sensitive to the zirconia phase [ 85, 86]. For instance, m ethanol synthesis catalysts supported on monoclinic ZrO2 are more active than those supported on tetragonal ZrO2 [ 85,86]. It has further been demonstr ated that during calcination of impregnated zirconia catalysts, the ZrO2 first forms as an amorphous phase, and then the metastable tetragonal phase forms below 550C [39,42]. Monoclinic ZrO2 forms only above 550C, which is much higher than the calcination s used in this work. In the present case, however, the ZrO2 nanoparticles consist of mainly a monoclinic ZrO2 phase. Therefore, the CuZnZrAl-10-NP and the CuZnZr-70-NP catal ysts, which exhibit th e highest turnover frequencies in this study, ar e also the only two catalysts that contain a monoclinic ZrO2 phase, as seen in the XRD spectra. The differences in the electronic characteristics of these materials, which are likely responsible for th e differences in catalytic behavi or, will be the subject of the following chapter. 3.6 Conclusions Addition of ZrO2 to Cu-ZnO catalysts supported on nanoparticle alumina results in highly active methanol reforming catalysts. Under similar conditions, our ZrO2-promoted catalysts achieve higher methanol conversions and lower CO selectivities in the methanol steam reforming reaction compared to the commercial catalyst. Th is is an improvement over the data presented in Chapter 2. The catalysts are sensitiv e to both preparation method and ZrO2 precursor. Coimpregnation of Cu, Zn and Zr onto the nanoparticle alumina resu lts in a more active catalyst
98 compared to a catalyst that is prepared by addi ng Zr before the Cu and Zn using the sequential impregnation method. The best performing catal yst is the CuZnZrAl-10-NP, in which the ZrO2 is added in the form of nanoparticles. This catal yst outperforms all the ot her catalysts, including the commercial reference, since it attains the highest methanol convers ion and the lowest CO selectivity over a wide range of the conditions tested. Only th e CuZnZr-70-NP catalyst exhibits a higher TOF than the CuZnZrAl-10-NP catalyst. However, since the CuZnZr-70-NP catalyst does not contain any high surface area alumina, this catalyst is limited by its low Cu surface area. There is no correlation between the Cu surf ace area and the catalytic activity (methanol conversion and turnover frequency) for the ZrO2and CeO2-promoted catalysts in this study. Under the conditions used in the study, CeO2 is a much less efficient promoter compared with ZrO2. It is evident from the high turn over fre quencies of the CuZnZr-70-NP and CuZnZrAl-10NP that the use of nanoparticle ZrO2 results in a very active Cu phase on the surface. Furthermore, there is no correlation between the reducibility and the catalytic activity under the conditions used. This is surprising since data fr om the literature usually indicate that the more active catalysts contain copper oxide phases that are easy to reduce. In contrast, our best performing catalyst, CuZnZrAl-10NP, also exhibits the highest reduction temperature. Another interesting re sult is that the CO selectivities over our nanoparticle-supported catalysts decrease with temperat ure below 250-300C. Since both the CO forming reactions are exothermic and the rates thus increase with increasing temperatures, this indicates that the steam reforming reaction (equation 1-1) in creases faster than the reverse water-gas-shift (equation 1-2) and the methanol decomposition (equa tion 1-3) reactions. Despite this, the CO levels for most of our catalysts are above the watergas-shift equilibrium concentrati on, which indicate that the CO
99 decomposition reaction must be considered. Only the CuZnZrAl-10-NP catalyst results in CO levels well below the water-gas-shift CO concentra tion at temperatures in the range 275-325C. In summary, the results from this study clearl y demonstrate that there are advantages in the use of nanoparticle precursor materials. The use of ZrO2 nanoparticles results in highly active copper phases. Under the conditions used in this study, the reaction system is evidently sensitive to structural or electronic differences between th e different catalysts sinc e there is not a simple correlation between the Cu surf ace area and the catalytic activity. Furthermore, the ZrO2containing catalysts suppress CO formation via the methanol decomposition and reverse watergas-shift reactions. The combined effects result in unique, promising catalysts for the methanol steam reforming in fuel cell applications. The task remaining is to determine what chemical or electronic characteristic is responsible for the increase in catalytic activity observed for the nanoparticle ZrO2 catalyst. A more intensive investigation into the electronic characteri stics of the catalysts is the subject of Chapter 4. 3.7 Acknowledgements This work was supported by NASA Gle nn Research Center Grant NAG 3-2930, monitored by Mr. Tim Smith. The authors would also like to thank Oseas Ayerdi for help with catalyst preparations and data processing of the GC measurements.
100 CHAPTER 4 CHARACTERIZATION OF ZrO2-PROMOTED Cu/ZnO/NANO-Al2O3 METHANOL STEAM REFORMING CATALYSTS 4.1 Introduction and Literature Review As was mentioned in Chapter 3, a the comprehensive study of Al2O3 and ZrO2 supported reforming catalysts performed by Breen and Ross found that the most active reforming catalysts contained both Al2O3 and ZrO2 and that the catalysts are very sensitive to the fabrication techniques used [ 29]. For example, Breen et al. [ 29] found that catalyst s m ade by sequential precipitation (copper, then zinc) on a mixed ZrO2/Al2O3 support are more active in the reforming reaction than coprecipitated (coppe r and zinc together) catalysts. They also postulated that the role of the Al2O3 is to protect the amorphous nature of ZrO2 in the catalyst and that catalytic activity decreases signi ficantly if the ZrO2 crystallizes [ 29]. This is in agreement with an earlier study of CH3OH synthesis over ZrO2 supported catalysts [ 87]. In a recent study by Wang et al. [ 42], the catalytic activity of Cu/ZrO2 catalysts increases with the crystalliza tion of a monoclinicenriched zirconia in the surface region. Likewise there have been seve ral studies on methanol synthesis by Bell et al. [ 85,86] which demonstrate that monoclinic ZrO2 is considerably more active than tertiary ZrO2 toward methanol synthesi s. The problem with ZrO2 from impregnated or precipitated precursors is that the metastable tertiary ZrO2 phase appears to form first and that the monoclinic phase does not form below 550C [ 39,87]. The result is that the Cu particles are subject to severe sintering due to the elevated tem peratures needed to form monoclinic ZrO2 in the bulk of the catalyst [ 86,87]. This results in a decrease in b oth Cu surface area and also in catalytic activity in both the methanol synthesis [ 86] and steam reforming reactions [ 87]. The purpose of this work is to demonstrate that these difficulties in forming monoclinic ZrO2 can be avoided if the catal yst is fabricated using a phys ical mixture of nanoparticle
101 monoclinic zirconia and nanoparticle -alumina as the catalyst s upport. The copper and zinc phases can then be impregnated onto this mixed oxi de support as is typica lly done. In this way the high calcination temperatures, with concomitant Cu sintering, can be avoided resulting in a more active Cu/ZnO/ZrO2/Al2O3 methanol reforming catalyst. Extensive XPS studies will also be performed in an attempt to elucidate the cause for the superior performance of the CuZnZrAl10-NP catalysts as shown in the previous chapter. 4.2 Experimental Methods Most of the experimental procedures used in this chapter have al ready been covered in detail in the preceding chapters. The catalysts studied in this chapter are the same as those studied in Chapter 3, and therefore the catalyst preparation procedure, TPR, N2O titration, and reactor experiments are covered in Chapter 3. The naming used in this chapter has been simplified since all catalysts ha ve the formula CuZnZrAl-10, and only the preparation method is different. Therefore, the ZrO2 concentration is dropped from the la bel since it is constant in this chapter. Thus, CuZnZrAl-10-NP becomes CuZnZrAl-NP. Some additional reactor data is shown here to further explore the differences in pe rformance between three catalysts of the same nominal composition. XRD data s hown here is fundamentally the same as was shown in Chapter 3 except that only there three CuZnZrAl-10 catalyst s are investigated in th is chapter. The XPS data is unique to this chapter. For all XPS measurements, the catalyst powders were pressed into aluminum cups prior to inserti on into the ultra-high vacuum ( UHV) chamber (base pressure 11010 Torr). The XPS data were collected using a double pass cylindrical mirror analyzer (PHI model 25-270AR) with incide nt radiation from a Mg K X-ray source (PHI 04-151). Spectra were taken in the retarding mode with a pass energy of 50 eV for survey spectra and 25 eV for
102 high resolution spectra. Data were collected using a computer interface and then digitally smoothed [ 88]. Charge shift corrections were made by assuming a C 1s signal of 284.6 eV [ 89]. 4.3 Results and Discussion 4.3.1 Catalytic Activity and CO Selectivity The conversion of methanol over the Cu ZnZrAl-10-NP catalyst with increasing temperature is shown in Figure 4-1 for a cons tant feed rate and water/methanol ratio. 200220240260280300320 20 30 40 50 60 70 80 0 2 4 6 8 10 12 14 16 Figure 4-1 CH3OH Conversion [%]Temperature [deg. C] H2, CO2, CO production (sccm) Figure 4-1. CH3OH conversion and corresponding CO2, H2 and CO production rates for the CuZnZrAl-NP catalyst. CH3OH conversion, CO2 production, H2 production and CO production in sccm. The wate r/methanol ratio was ~3/1 and the total inlet liquid flow rate was 0.8 ml/hr over 340 mg catalyst. The system does not achieve thermodynamic equilibrium at this feed rate so higher conversions are possible at longer contact times [ 35,76]. The actual prod uction rates of H2, CO and CO2 are also displayed. Over the entire temperature range, H2 and CO2 are produced in approximately a 3:1 ratio, as expected for st eam reforming of methanol. The decrease in
103 methanol conversion at high temp eratures reflects the thermal deactivation of the catalyst at elevated temperatures as disc ussed in previous studies [ 29,76]. The m oles of H2 produced per surface Cu atom per ti me, expressed as turnover frequency (TOF), and CO production as a function of temp erature for all three catalysts are shown in Figure 4-2 (same reaction conditions as in Figur e 4-1) and the gas effluent compositions at different temperatures are given in Table 4-1. 220240260280300320 60 80 100 120 140 160 180 200 220 240 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 CO Production [sccm]Figure 4-2 TOF [sec-1]Temperature [deg. C] Figure 4-2. H2 (solid lines) and CO (dashed lines) production rates for all catalysts. CuZnZrAl NP, CuZnZrAl CI, and CuZnZrAl SQ. All reaction conditions were the same as those in Figure 4-1. Below 240C, there is no significant difference in the activity of the three catalysts. Between 240C and 270C the CuZnZrAl-NP and CuZnZrAl-CI catalysts perform similarly, but above 270C the CuZnZrAl-NP catalyst achieves higher H2 production levels than either of the impregnated ZrO2 catalysts.
104 Table 4-1. Reactor effluent composition fo r all catalysts at selected temperatures. Volume % in Effluent Catalyst Temp H2 CO CO2 CuZnZrAl-CI 305 78.6 1.27 20.2 CuZnZrAl-SQ 305 77.2 1.27 21.5 CuZnZrAl-NP 305 78.6 0.69 20.7 CuZnZrAl-CI 240 74.6 1.32 24.1 CuZnZrAl-SQ 235 75.3 1.08 23.7 CuZnZrAl-NP 235 71.7 0.84 27.4 At 305C the CuZnZrAl-NP reaches a H2 production rate that is 15% higher than that of the CuZnZrAl-CI catalyst and over 30% higher than that of the CuZnZrAl-SQ catalyst, demonstrating the benefit of the nanoparticle ZrO2 precursor in the reforming reaction. For all catalysts the CO production increases as a function of temperature. This is often attributed to an increase in the reaction rate of the reverse water gas shift an d/or the methanol decomposition reaction with increasing temperature [ 29,49,87,76, 80]. The CuZnZrAl-NP catalyst exhibits the lowest CO production rate over the tem perature range examined. 4.3.2 X-Ray Diffraction Analysis XRD spectra obtained from the fresh and used ca talysts are shown in Figure 4-3. This is a more detailed view of the XRD data presente d in Chapter 3. There are several interesting differences in the XRD spectra obtained from the th ree catalysts that will be covered in greater detail here than in the previous ch apter. There is no crystalline ZrO2 phase present in either the
105 CuZnZrAl-SQ or CuZnZrAl-CI catalysts, in ag reement with available literature data for impregnated and precipitated ZrO2 catalysts calcined below 550C [ 86,87]. The CuZnZrAl-NP catalyst d isplays a crystalline monoclinic ZrO2 phase. This result is expected since the nanoparticle ZrO2 precursor is in the monoclinic phase, as stated above. This was independently verified using XRD on the untreated ZrO2 nanoparticles (not shown). Another difference is the poorly crystalline ZnAl2O4 spinel phase present on the CuZn ZrAl-CI which was documented in Chapter 3. The figure clearly shows there is no change in the monoclinic ZrO2 phase as for the CuZnZrAl-NP catalyst. Similarly, no crystalline ZrO2 phase is formed during reaction for the other two catalysts. The partic le sizes of the crys talline components in the catalysts were calculated using the Scherrer equatio n and were presented in Chapte r 3. It was shown in Table 33 that Cu particle sizes confirm the tre nd in Cu surface area observed from the N2O decomposition measurements; i.e. that the CuZnZr Al-SQ catalyst has the smallest Cu particles (largest Cu surface area) and the CuZnZrAl-CI cata lyst has the largest Cu particles (smallest Cu surface area), although the diffe rence between the Cu partic les on the CuZnZrAl-CI and CuZnZrAl-NP is not significant. The ZnAl2O4 spinel phase is not significantly altered during reduction and reaction. An unexpected result is that the sintering of the Cu phase is more severe on the most active catalyst (CuZnZrAl-NP) compar ed to the other catalysts. These results further support the conclusion that there is not a simple correlation between the catalytic activity and the Cu surface area, which has been noted throughout this dissertation. Also, the coimpregnation catalyst preparation technique results in a catalyst more resistant to Cu sintering compared to the sequential method. Furthermore, the catalyst prepara tion does not appear to alter the particle
106 25303540455055 c b a Figure 4-3A 2Intensity [arbitrary units] 25303540455055 c b a Figure 4-3B 2Intensity [arbitrary units] Figure 4-3. XRD spectra obtained from A) fresh catalysts and B) spent catalysts, a) CuZnZrAlNP, b)CuZnZrAl-CI and b) CuZnZrAl-SQ. monoclinic ZrO2, CuO, ZnAl2O4, Cu
107 sizes of the nanoparticle ZrO2, although there is a slight increase in ZrO2 particle size during the reaction (used CuZnZrAl-NP). Ho wever, sintering of the ZrO2 particles is less severe than the Cu particle agglomeration. 4.3.3 X-Ray Photoelectron Spectroscopy Analysis 22.214.171.124 Peak area ratios A labeled survey spectrum obtained from th e fresh CuZnZrAl-CI catalyst is shown in Figure 4-4. The survey spectra obtained from al l the fresh and used catalysts are similar except for differences in the relative peak sizes so only one survey spectrum is shown. 110010009008007006005004003002001000 Figure 4-4Zr 4s Zr 3d Cu 2p O KLL Zn 2p O 1s Cu LMM Zn LMM Zn 3p Al 2s Al 2p Zn 3s C 1s N(E) (arbitrary units)Binding Energy (eV) Figure 4-4. Labeled XPS survey sp ectra for CuZnZrAl-CI catalyst. Peak area ratios obtained from these spectra are given in Table 4-2. These peak area ratios are determined by two factors: (1) the relative amounts of the elements in the outermost 6 nm and (2) the spatial arrangement of the elements in this region (the matrix effect). Since these catalysts all have the same support and simila r compositions, the differences observed between the catalysts in Table 4-2 are mostly due to the matrix effect.
108 Table 4-2. Peak area ratio s for all catalysts calculated from the XPS data. Peak SQ CI NP Ratio Fresh Spent Fresh Spent Fresh Spent C 1s / Al 2p 0.77 0.78 0.59 0.55 0.72 0.66 O 1s / Al 2p 2.83 1.83 1.61 2.05 2.94 2.37 Zr 3d / Al 2p 0.51 0.39 0.07 0.08 0.12 0.11 Cu 2p / Al 2p 0.09 0.05 0.08 0.04 0.01 0.03 Zn 2p / Al 2p 0.17 0.12 0.09 0.08 0.09 0.06 Use of the homogeneous assumption to obtain compositions of supporte d catalysts leads to large errors so peak area ratios are used in this study. The differences between the three types of fr esh catalysts are quite large. The fresh CuZnZrAl-SQ catalyst has much larger Zr/Al, Cu /Al and Zn/Al peak area ratios than the other two types of catalysts. The fresh ChZnZrAl-CI ca talyst has a smaller O/Al peak area ratio than the other two fresh catalysts. A ll three fresh catalysts have similar C/Al peak area ratios. Significant changes occur in these peak area ra tios during the reacti on. The Zr/Al peak area ratio of the CuZnZrAl-SQ catalyst decreases by about 25% while those of the other two types of catalysts do not cha nge significantly. The fresh CuZnZrAl-CI and CuZnZrAl-SQ catalysts have similar Cu/Al peak area ratios which decrease by about 50% during reaction. Contrary to this the fresh CuZnZrAl-NP has a lo w Cu/Al peak area ratio of 0.48 which more than doubles to 1.14 during reaction, but this is still lower than the corresponding values of the other two catalysts. The Zn/Al peak area ratios decreas e significantly (20-35%) for all these catalysts. The O/Al peak area ratio of the fresh CuZnZrAl-C I is much lower than that of the other two
109 catalysts. However, during reaction the O/Al peak area ratio increases for the CuZnZrAl-CI catalyst and decreases for the other two catalysts. After reaction the O/Al peak area ratios are similar for the CuZnZrAl-CI and CuZnZrAl-SQ cat alysts but markedly higher for the CuZnZrAlNP catalyst. The C/Al ratios are similar for a ll of the catalysts and do not change very much during reaction. 126.96.36.199 Fresh catalysts Zr 3d XPS spectra obtained from the calcined catalysts and th e nanoparticle ZrO2 support are shown in Figure 4-5. 195190185180175170 Figure 4-5ANP ZrO2CuZnZrAl_10_NP CuZnZrAl_10_SQ CuZnZrAl_10_CI Zr 3dN(E) (arbitrary units)Binding Energy (eV) Figure 4-5. Zr 3d XPS spectra obtained from A) fresh catalysts and B) spent catalysts. Nanoparticle ZrO2 is included as a reference. Curves are appropriately labeled in the figure.
110 190 185 180 175 Figure 4-5BNP ZrO2CuZnZrAl_10_NP CuZnZrAl_10_SQ CuZnZrAl_10_CI Zr 3dN(E) (arbitrary units)Binding Energy (eV) Figure 4-5. Continued The Zr 3d5/2 peak obtained from the CuZnZrAl-CI cat alyst is centered at a binding energy of 181.5 eV, and that obtained from the CuZnZrAl-SQ catalyst is centered at a BE of 181.4 eV. This is lower than the Zr 3d5/2 binding energy for the nanoparticle ZrO2 at 181.8 eV. The presence of a more reduced Zr species has b een associated with a high population of oxygen vacancies in the ZrO2 lattice [ 40,87], and Zr species with high electron densities have been observed previously for both Cu on ZrO2 methanol reforming catalysts and also on Fe-promoted ZrO2 catalysts [ 90, 91]. The low binding energies of the Zr 3d5/2 peak suggest the possibility of strong CuO-ZrO2 interactions at the su rface of these catalysts [ 90]. Compared to the Zr 3d peaks obtained from the nanoparticle Z rO2, the Zr 3d peaks obtained from the CuZnZrAl-CI and CuZnZrAl-SQ catalysts are broader due to contributions from additional states and/or differing
111 amounts of charge transfer. Also a significant shoulder at 181.9 eV is ap parent on the CuZnZrAl-CI and CuZnZrAl-SQ spectra, which is due to the unperturbed ZrO2 state. In contrast, the Zr 3d5/2 obtained from the CuZnZrAl-NP catalys t is shifted to 181.0 eV, indicating an even stronger interacti on between the CuO and ZrO2 at the surface of th is catalyst; i.e. a higher contribution from the electron-rich Zr 3d state [ 90]. The peak shapes of the Zr 3d peaks obtained from the the CuZnZrAl-NP catalyst and the nanoparticle ZrO2 are similar, indicating one predominant Zr species. For the CuZnZrAl-N P catalyst the dominant species is the electronrich Zr species with a 3d5/2 binding energy of 181.0 eV and th ere is only a minor contribution from the unperturbed ZrO2 state at 181.9 eV. The Cu 2p peaks obtained from the calcined and spent catalysts are show n in Figure 4-6. 965960955950945940935930925 Figure 4-6ACuO Cu2O, Cu0c b a Cu 2pN(E) (arbitrary units)Binding Energy (eV) Figure 4-6. Cu 2p XPS spectra obt ained from A) fresh and B) spent catalysts. a) CuZnZrAl-NP, b) CuZnZrAl-CI c) CuZnZrAl-SQ
112 965960955950945940935930925 Figure 4-6BCuO Cu2O, Cu0c b a Cu 2pN(E) (arbitrary units)Binding Energy (eV) Figure 4-6. Continued The Cu 2p3/2 feature is located near 933.6 eV, and the Cu 2p feature is located near 953.3 eV for all catalysts in Figure 4-6A. These bi nding energies and the in tense satellite bands located at approximately 10 eV higher than the photoelectron lines are typical of Cu2+ [ 39,90]. This is a lso typical for Cu/ZnO/ZrO2/Al2O3 reforming catalysts [ 39,90]. The Cu 2p peaks obtained from the CuZnZrAl-CI an d CuZnZrAl-SQ are similar. Both display the same peak position, and both exhibit a small low binding energy shoulder. The peak position of the Cu 2p3/2 main line is the same for the CuZnZrAl-NP catalys t, but the low binding energy shoulder is more pronounced than on the other two catalysts, resulting in a highly asymmetric Cu 2p3/2 peak for the CuZnZrAl-NP catalyst. The low binding energy shoulder of the Cu 2p3/2 electron is located at 932.4 eV, compared to the main peak at 933.6 eV. A state at a low binding energy of 932 eV
113 has been observed by Wang et al. , and was attributed to co pper oxide located near oxygen vacancies o n the surface of the zirconia. The oxygen vacancies in the matrix give the copper species a higher electron density similar to the Zr species. This reveals a very close Cu-ZrO2 interaction. The larger cont ribution from the state at low binding energy obtained from the CuZnZrAl-NP catalyst further suggests that the Cu-ZrO2 interactions in this catalyst are more developed than in the other two catalysts. The O1s spectra of all fresh and used catalysts and the nanoparticle ZrO2 precursor are shown in Figure 4-7. 540535530525520 Figure 4-7CuZnZrAl-10-CI CuZnZrAl-10-SQ CuZnZrAl-10-NPO 1sN(E) (arbitrary units)Binding Energy (eV) Figure 4-7. O 1s XPS sp ectra obtained from fresh (solid lines ) and spent (dashed lines) catalysts. Curves are labeled appropriately in the figure. The O1s spectra for the catalysts are not si gnificantly affected by exposure to reaction conditions. Therefore the spectra from fresh a nd spent catalysts are co mbined into a single figure. The minimal difference between the fresh a nd spent O1s spectra indicates that the spectra
114 are dominated by the contribution of the O present in Al2O3, which is not reduced during the reaction. There is a high BE shoulder present on the O1s spect ra of the nanoparticle ZrO2 precursor located at 532.3 eV. This is indi cative of hydroxyl groups present at the surface. 188.8.131.52 Spent catalysts After reaction the ZrO2 peaks obtained from all catalysts are shifted to slightly higher binding energy (Figure 4-5B). A higher Zr 3d5/2 binding energy due to the presence of Zr(OH)4 after reduction and exposure to the reaction conditions have been observed previously [ 16]. Although the Zr 3d5/2 binding energies obtained from the used catalysts are not as high as those obtained from Zr(OH)4 [ 16], the presence of hydroxyl groups in the ZrO2 precursor indicates that there is likely a cont ribution from a Zr(OH)x species present on the catalyst surfaces after reaction. Hydroxyl groups near oxygen vacancies could potentially result in binding energies similar to ZrO2. The increase in binding energy suggest s that the electron densities on the Zr species after reaction are slightly lower than before reduction and exposure to the reaction conditions, but they are still higher th an that of the pure nanoparticle ZrO2. This suggests that there are oxygen vacancies on the surface after reaction, but comparing the catalysts before and after reaction is complicated by the potential presence of hydr oxyl groups on the surface. Reduction of the Cu2+ to Cu1+ or Cu0 is evident in the Cu 2p peaks obtained from the used catalysts (Figure 4-6B). The p eaks are narrower and the Cu 2p3/2 main line is located at 931.6 eV for all catalysts after reaction, compared to the 933. 6 eV before reaction. In all cases the satellite structures, indicative of Cu2+, are no longer present. The bindi ng energy is lower than expected for Cu or Cu2O (932.4-932.6 eV), which again may indicat e that the Cu is in teracting with the oxygen vacancies on the ZrO2 support, although there may not be as many oxygen vacancies as on the fresh catalysts. In additi on to the feature at 931.6 eV, all catalysts also exhibit shoulders of varying intensity at 933.6 eV. The CuZnZrAl -SQ catalyst exhibits the smallest contribution
115 from the state at 933.6 eV, while the largest shou lder at this binding energy is observed on the CuZnZrAl-NP catalyst. As on the fresh catalysts this suggests that the CuZnZrAl-NP catalyst experiences the strongest metal-supp ort interactions of the catalysts in this study. It is possible that the Cu-ZrO2 interactions on these catalysts le ad to O-transfer from the ZrO2 to the Cu during the reaction. This would expl ain the shoulder at 933.6 eV. Th e binding energy of this peak suggests the presence of CuO. However, there are no CuO satellites present so this may be due to charge-transfer interactions between the Cu and ZrO2 support, as noted for other Cu/ZrO2 catalysts [ 92]. This can be taken as anothe r indication of very strong Cu-ZrO2 interactions on these catalysts, and most prominently on the CuZnZrAl-NP catalyst. The analysis of the Zr 3d5/2 XPS peaks is complicated by th e presence of oxygen vacancies and also hydroxyl groups on the ZrO2 surface. Apparently, the crystalline phase of ZrO2 influences the catalytic reaction. There is a decrease in Zr 3d5/2 binding energy with increasing calcination temperature for Cu/ZrO2 catalysts in the literature wh ich coincides with the formation of a monoclinic ZrO2 phase [ 39,42]. The catalytic activity also increases up to a certain tem perature, above which th e activity declines again [ 39,42]. This is consistent with Cu-based ZrO2-containing methanol synthesis catal ysts, in which a monoclinic ZrO2 support results in catalysts with significantly higher activities than catalysts supported on tetragonal ZrO2 [ 85,86]. These facts are cons istent with a complex interaction between the ZrO2 and the Cu at the catalyst surface. Despite this complexity, there are strong meta l-support interactions on these catalysts. These interactions appear to re sult in electron-rich Cu and Zr species on both fresh and used catalysts, as well as oxygen transfer on the used catalysts. The more oxidized Cu species observed on the used catalysts may have a higher affinity for CO, and can promote the activity
116 for methanol synthesis [ 39]. This partially oxidized Cu surface species m ay have a higher affinity for the oxidation of CO via the water gas shift or, conversely, is less active in the reduction of CO2 via the reverse water gas shift. Th e CuZnZrAl-NP catalyst, which has the lowest CO production rate, has the largest Cu oxida tion feature in the Cu 2p spectra of the used catalysts. Similarly, the CuZnZrAl-SQ has the highest CO production rate and the Cu 2p spectra of the used catalyst show little or no evidence of a partially oxidized Cu species. The CuZnZrAlCI is intermediate in both CO production and intensity of the Cu higher binding energy shoulder. A similar argument can be made that the partia lly oxidized Cu sites ha ve a stronger affinity towards methanol in the reforming reaction. Th is may explain why the CO production levels and selectivity are consistently lower for the Cu ZnZrAl-NP catalyst compared to the other two catalysts; i.e. the oxidized surface Cu species is bo th more active in the reforming reaction and at the same time inhibits CO2 reduction via the revers e water gas shift. Th e data presented here suggests that strong metal-support (i.e. Cu-ZrO2) interactions are importa nt for a high catalytic activity in the methanol steam reforming reaction and a high selectivity to CO2. 4.4 Conclusions This study has shown that it is possible to fo rm highly active methanol reforming catalysts using nanoparticle ZrO2 supports. The use of nanoparticle ZrO2 supports is preferable to the use of a more traditional ZrO2 phase impregnated from ZrO(NO3)2H2O. The nanoparticle ZrO2 catalyst is more active in the reforming reaction and also exhi bits lower CO production levels, which is consistent with a suppression of th e reverse water gas shif t. The nanoparticle ZrO2 precursor is monoclinic, allowing a monoclinic ZrO2 phase to be introduced into the catalyst matrix without the use of high calcination temper atures, which are necessary when zirconia is impregnated from a nitrate precurso r. The less active tetragonal ZrO2 phase is avoided by using
117 the monoclinic ZrO2 nanparticles. The impregna ted catalysts calcined at 300C result in an amorphous ZrO2 phase which is not apparent with XRD. The higher activity of the monoclinic ZrO2 catalyst is in agreement with the available literature in both methanol synthesis and methanol reforming studies. The advantage of the monoclinic ZrO2 phase appears to be that it has a relatively higher population of oxygen vacancies. This resu lts in a partially oxidized Cu surface species which is more ac tive in the methanol reforming reaction than copper without the oxidation features.
118 CHAPTER 5 DESIGN AND CONTROL OVER CuO PARTI CLE SIZE AND Cu SURFACE AREA IN Cu/ZrO2 METHANOL REFORMING CATA LYSTS FROM NANOPARTICLE AND MICROEMULSION PRECURSORS. 5.1 Introduction and Literature Review As noted, traditional methanol steam reformi ng catalysts often consist of Cu and ZnO and sometimes a small amount of Al2O3 [ 6,35]. Chapters 3 and 4 focused on the use of reducible m etal oxides, such as ZrO2 and CeO2 as alternatives or additions to Al2O3. It was shown that ZrO2 addition to Cu-based alumina-supported ca talysts increased methanol conversion and reduced CO yields. This is c onsistent with the literature [ 14,18,27,56]. It was further dem onstrated that there is a distinct advantage to using nanoparticle ZrO2 compared to more traditional impregnated ZrO2. There is strong evidence of an electronic inte raction (charge transfer) between Cu and Zr in the Cu and nanoparticle ZrO2 reforming catalysts [ 39,42,90]. To further com plicate the picture, Bell, et al. [ 86] showed that the ZrO2 crystal phase present in the catalyst is of critical importance. Wu et al. [ 39] further demonstrated th at there is an advantage to increasing the calcination temperature of Cu/ZrO2 catalysts. It was concluded that the increase in calcination temper ature to 550C or above encourag es the crystallization of a monoclinic ZrO2 phase [ 39]. This agrees well with the results of Bell. However it was also dem onstrated that there are potential problems a ssociated with calcination temperatures above 550C, including a severe loss of Cu surface are due to sintering of the Cu phase [ 42]. The role of m onoclinic ZrO2 in quaternary CuO/ZnO/ZrO2/Al2O3 systems was demonstrated in Chapter 3 and further explained in Chapter 4. These resu lts demonstrated that potential problems with increased calcination temperatur es could be avoided entirely if monoclinic nanoparticle ZrO2 was used as the ZrO2 precursor in the catalyst.
119 In addition to the advantages of the use of monoclinic ZrO2 in methanol reforming, a recent study by Ritzkopf, et al. demonstrated that there is also an advantage to using Cu/ZrO2 methanol reforming catalysts that are prepared via a reverse (i.e. water in oil) microemulsion technique. Ritzkopf showed that this microemulsion catalyst had a dramatically lower CO selectivity at similar methanol conversions when compared to a commercial catalyst [ 44]. It was m entioned in Chapter 1 that Agrell showed that a similar microemulsion technique also gave very promising results for Cu/ZnO [ 45] and Pd/ZnO [ 46] catalysts. W ith the results from the Bell, Agrell and Ritzkopf studies, in addition to our previous results, the authors were interested in determinin g if there was a further advantage in the use of nanoparticle precursors in conjunction with the microemulsion fabricatio n techniques mentioned above. This study incorporates these recent adva nces in methanol reforming catalysts and the use of nanoparticle catalyst materials. 5.2 Experimental Methods As in the previous chapters, the catalyst preparation methods used herein are unique to this chapter and are covered in detail here. TPR, N2O titrations, XRD and BET surface area analysis were performed as described in Chapter 1. 5.2.1 Catalyst Preparation The concentrations of CuO and ZrO2 were held constant at 15% and 85%, respectively, for all catalysts in this study except the commercial reference catalyst. The 15% CuO catalysts were used to facilitate comparison with the catalysts in the Ritzkopf study (4-16% Cu) mentioned in the introduction. The microemulsion catalysts were fabricated by adding an aqueous solution of Cu(NO3)2H2O (Alpha Aesar) of varying Cu molarity to an organic solution of Tween 80 (Fisher) in toluene (Acros organics). The componen ts were added such that the concentrations of
120 each component of the emulsion were held cons tant at 10% water, 15% surfactant and 75% toluene by weight for all samples. After dissolv ing the Tween 80 in toluene, the aqueous phase was prepared by dissolving the Cu(NO3)2H2O precursor in H2O to the desired concentration of metal (see Table 5.1). After the Cu(NO3)2H2O was completely dissolved, the appropriate mass of ZrO2 nanoparticles (Nanostructured and Amorphous Materials) was dispersed into the aqueous solution. The aqueous phase was then added drop-wise to the organic solution containing the surfactant under intense stirring. The molarity of the aqueous phase was altered to determine the effect of the Cu(NO3)2H2O concentration in the emulsion on the ca talyst performance. Three different molarities were used in this study. Cu45Z r samples was made by adding 2.8 grams of Cu(NO3)2H2O to 21 ml H2O for a molarity of 0.45 M Cu. The molarity of the Cu22Zr samples was altered by doubling the amount of H2O to 42 ml while keeping mass of Cu(NO3)2H2O constant at 2.8 grams, for a Cu molarity of 0.22 M. The toluene and surfactant volumes were scaled up (or down, as appr opriate) to keep the relative concentrations of toluene, surfactant, and aqueous phase the same in the emulsion. A similar procedure was followed for the Cu73Zr sample by decreasing the amount of water used to make the emulsion while again holding the Cu(NO3)2H2O mass constant. After the emulsion had stabilized, Cu(NO3)2H2O is reduced in solution to Cu metal by addition of 50 % excess hydrazine in the form of hydrated N2H2 (85% N2H2 to 15% H2O weight/weight, Fisher). This is a simila r procedure to what was done in the Pd/ZnO microemulsion catalyst study done by Agrell, et al. [ 46]. After the reduction was complete, the resultant m ixture was centrifuged at 3000 rpm for one hour. The re sultant solids were recovered and sonicated in CH3OH for one hour. This step was n eeded to break up the micelles and
121 remove any residual surfactant from the solids. After sonication, the washed solids were again centrifuged at 3000 rpm for one hour to separate th e solids from the alcohol solution. Finally, the solids were dried overnight at 105C. The catalysts were then calcined at 300C, 500C, or 700C for 3 hours. Any Cu in the sample is oxidized to CuO during the calcination. In this way three different Cu molarities were each calcined at three different temperatures for a total of 9 catalysts used in this study. In all cases the cata lysts were prepared in 5 gram batches of calcined CuO/ZrO2 catalyst. Table 5-1 summarizes the differences in the catalyst samples used in this study. Table 5-1. Catalyst Preparation Details Name Cu Molarity (mol/L) Calcination Temperature (C) Cu22ZrA 0.22 300 Cu22ZrB 0.22 500 Cu22ZrC 0.22 700 Cu45ZrA 0.45 300 Cu45ZrB 0.45 500 Cu45ZrC 0.45 700 Cu73ZrA 0.73 300 Cu73ZrB 0.73 500 Cu73ZrC 0.73 700 The reference catalyst used in this study is the same commercially available Sd-Chemie catalyst used throughout this dissertation. 5.2.2 Reactor Experiments The only difference in reactor operation in this chapter compared to previous chapters is that the H2O/CH3OH ratio in the feed was decreased to1.4 mol/mol. The reference catalyst was also tested at this decreased feed ratio. No CH4, CH2O, CH3OCH3, or any other carbon
122 containing by-product were ever detected in this chapter, as in Chapters 3 and 4. Catalyst samples were activated in situ in 10% H2 in Ar (45 sccm total flow rate) at 300C for 3 hours before being exposed to the reac tant mixture. The catalyst beha vior was verified by repeated experiments in each case using fr esh catalysts. In all cases catalyst deactivation due to time on stream for relevant time lengths (~40 hours) was not found to affect the analysis or conclusions in this study. 5.3 Catalytic Activity Measurements. 5.3.1 Methanol Conversion Figure 5-1 shows methanol conve rsion as a function of temper ature for the three different Cu molarity preparations used in this study and the reference material tested under the same conditions. All microemulsion catalyst s in Figure 1 were calcined at 300C. It is evident in the figure that the Cu45ZrA catalyst ex hibits the highest conversion of the prepared catalysts below 320C and that the Cu22ZrA catalyst reaches the highest activity of the three microemulsion catalysts and the commercial reference. The conversion maximum of the Cu22ZrA is shif ted to a higher temperature compared to the other two microemulsion samples, but the cat alyst retains 100% conversion of methanol over the temperature range of 320C to 350C. The Cu45ZrA catalyst exhi bits a steady increase up to a maximum activity at 300C above which the catalytic activity declines steadily. The catalyst prepared from the highest Cu molarity micr oemulsion used in this study, the Cu73ZrA, obviously does not achieve nearly the activity leve ls of the catalysts prepared from a lower Cu concentration in the microemulsion precursor. Reasons for this decreas e in activity will be
123 200220240260280300320340360380 10 20 30 40 50 60 70 80 90 100 110 Figure 5-1 Methanol Conversion (%)Temperature Figure 5-1. Methanol conversion as a function of temperature fo r catalysts fabricated from different microemulsion precursors. Cu73ZrA, Cu45ZrA, Cu22ZrA, Commercial Reference. H2O/CH3OH ratio 1.4mol/mol and total liquid inlet flow 0.8 ml/hr. Contact time for all cu rves is 0.1 kg cat*sec/mmol CH3OH. explored later in this study. The dramatic decr ease in activity in the Cu73ZrA catalyst was also reflected in the Cu73ZrB and Cu73ZrC catalysts. Therefore, additional figures with the Cu73Zr samples are omitted, although turn over frequency data is shown in section 5.4.3. Figure 5-2 shows methanol conversion for three different calcination temperatures for the 0.45 M Cu microemulsion catalysts. It is evident in th e figure that the Cu45ZrA achieves the highest conversion of the three calcinations temperatures. It is also ev ident that the calcination at 700C severely damages the catalytic activity, and the Cu45ZrC is the only sample that does not out perform the reference catalyst. Again, this trend was consistent for all C catalysts. It is also evident that there is not a large difference in performance of the Cu45ZrA and Cu45ZrB catalysts except for the spike in ac tivity for Cu45ZrA at approximately 300C. Since the Cu22ZrA catalyst had the highest activity of the catalysts shown in Figure 5-1, a comparison of the conversion versus temperature curves for all three Cu22Zr catalysts ar e shown in Figure 5-3.
124 200220240260280300320340360380 0 10 20 30 40 50 60 70 80 90 100 Figure 5-2 Methanol Conversion (%)Temperature Figure 5-2. Methanol Conversi on as a function of temperatur e for catalysts of different calcination temperatures. Cu45ZrA, Cu45ZrB, Cu45ZrC. Reactor conditions as in Figure 5-1. Cu45ZrB catalyst is also s hown to ease comparison between the two catalysts from different precursors both calcined at 500C. For the decreased Cu mola rity preparation method, it is evident that the calcination temperature of 500C is actually preferred. The conversion curve is shifted to both higher conversio ns and lower temperatures for th e Cu45ZrB sample. In fact, the Cu22ZrB attains the highes t conversion at the lowest temperat ure of any catalyst in this study, reaching 100% conversion at 275C and remaining at that conversion until 315C. Likewise, the Cu73ZrB catalyst achieved higher conversion than the Cu73ZrA cat alysts (not shown). In both cases the higher calcination temperature did not deactivate the catalyst but apparently has some beneficial consequence. It is evident that the 500C calcination does not have the same beneficial effect for the 0.45 M Cu microemulsion precursor catalysts given that the activity of Cu45ZrA is higher than Cu45ZrB.
125 200220240260280300320340360380 10 20 30 40 50 60 70 80 90 100 Figure 5-3 Methanol Conversion (%)Temperature Figure 5-3. Methanol conversi on for selected catalysts. Cu45ZrB, Cu22ZrB, Cu22ZrA. Reaction conditions as in Figure 5-1. 5.3.2 CO Selectivity As mentioned in the introduction, the other main parameter of inte rest is CO selectivity. The CO selectivity over the three catalysts from different Cu molarity microemulsions and all calcinated at 300C is shown in Figure 5-4. Despite relatively large differences in metha nol conversion shown in Figure 1, the catalysts all have similar CO selectivity levels below 320C. The Cu45ZrA does show a slightly lower CO selectivity compared to the other two microemulsion samples, and has a lower CO selectivity than the reference catalyst at all temperatures above 220C. However, it is the Cu22ZrA which maintains the lowest CO selec tivity over the high end of the temperature range. Cu73ZrA is consistently, if only slightly, higher in CO sel ectivity than the other two microemulsion catalysts. This data represents the lowest CO selectivit y curves in any chapter of this dissertation.
126 200220240260280300320340360380 Fi g ure 5-4 CO Selectivity (%)Temperature Figure 5-4. CO selectivity for catalysts fabricated from diff erent microemulsion precursors. Cu73ZrA, Cu45ZrA, Cu22ZrA, Commercial Reference. Reactor conditions as in Figure 5-1. 200220240260280300320340360 0 5 10 15 Figure 5-5 CO Selectivity (%)Temperature Figure 5-5. CO selectivity as a function of temperature for cata lysts of different calcination temperatures. Cu45ZrA, Cu45ZrB, Cu45ZrC. Reactor conditions as in Figure 5-1.
127 Figure 5-5 compares the effect of calcination temperature on CO selectivity by showing the results from the Cu45ZrA, C u45ZrB and Cu45ZrC catalysts. Again, there is only a small difference in CO selectivity betw een the three different calcination temperatures in that Cu45ZrC deviates from the other two catalysts and has a notably higher CO selectivity above 280C. Surprisingly, Figure 6 shows that there is drama tic change in performance when the catalyst from the lower Cu concentration in the microemulsion preparation is calcined at 500C. Catalyst Cu22ZrB has a much lower CO se lectivity than the Cu22ZrA catal yst at the lower end of the temperature range, although this trend then reverses above 270C. Consequently, there is a large variation in CO selectivity betw een the two Cu22Zr A and B catalys ts, while the CO selectivities are similar for the Cu45ZrA and Cu45ZrB catalys ts as shown in Figure 5. The commercial reference material has a consistently higher CO selectivity than any of the microemulsion samples above 260C. 200220240260280300320340360380 0 5 10 Figure 5-6 CO Selectivity (%)Temperature Figure 5-6. CO Selectivity for selected catalysts. Cu45ZrB, Cu22ZrB, Cu22ZrA, Commercial Reference. Reacti on conditions as in Figure 5-1.
128 5.4 Catalyst Characterization Results 5.4.1 Temperature Programmed Reduction TPR was used to provide some insight into the reducibility of the copper species on the various catalysts. Figure 7a, 7b and 7c show TPR spectra collected from different catalysts used in this study. Figure 7a shows the effect of the Cu molarity in the microemulsion precursor on the catalyst TPR profile. As is shown in the figure, decreasing the Cu molarity shifts the TPR profiles to lower temperatures. Since it has b een noted in a related study that more active methanol reforming catalysts tend to have lower reduction temperatures [ 29], this trend is consistent w ith our data, i.e. Cu22ZrA is both more active and has a lo wer reduction temperature than Cu45ZrA (at least at reaction temperatures of 320C and above). It is also noted that decreasing the Cu molarity form s smaller CuO crystals in the catalysts (See Section 3.4). Intuitively, it is expected that smaller Cu pa rticles would be easier to reduce, unless there are strong metal-support interacti ons that inhibit reduction. Evidently, any metal-support interactions in the microemulsion catalysts do no t prevent reduction, and the catalysts follow the trend of smaller CuO particle sizes having a lower reduction temperat ure (Figure 5-7A and Section 5.3.4). It has been noted in a rela ted study of the reduction of unsupported CuO nanoparticles that decreasing CuO particle size does not necessarily facili tate reduction of CuO to Cu [ 65] although this trend is not observed in the present study. The trend of increasing H2 peak consumption temperature with increasing calc inations temperature is also observed for the Cu22Zr and Cu73Zr catalysts (not shown). As is shown in Figure 5-7B, higher calcin ations temperatures shift the maximum H2 consumption towards higher reduction temperatures If these catalysts follow the trend observed in the literature [ 29], then catalyst Cu45ZrA should have the lowest reduction tem perature and
129 100 200 300 400 c b a Figure 5-7A Normalized response (Arb.)Temperature 100 200 300 400 Figure 5-7B c b a Normalized response (Arb.)Temperature Figure 5-7. A) TPR profiles for Cu45Zr catalysts calcined at different temperatures. a. Cu45ZrA, b. Cu45ZrB, c. Cu45ZrC. B) TP R profiles for catalysts from different microemulsion precursors. a. Cu73ZrA, b. Cu45ZrA, c. Cu22ZrA. C) TPR profiles for Cu22Zr catalysts calcined at different temperatures. a. Cu22ZrA, b. Cu22ZrB, c. Cu22ZrC. TPR is performed with 100mg catalyst with 50 sccm of 5% H2 in N2 with a temperature ramp of 5C/min. The dashed line marks the maximum H2 consumption temperature for the bottom curve.
130 100 200 300 400 c b a Figure 5-7C Normalized response (Arb.)Temperature Figure 5-7. Continued Cu45ZrC the highest, which is indeed observed. H2 consumption for Cu45ZrA catalyst peaks at roughly 280C, while that of Cu45ZrB peaks at 290C and Cu45ZrC peaks at 305C. Higher calcinations temperatures apparen tly form a CuO phase on the catalys t that is more resistant to reduction than CuO formed at lower temperatures. This could also be a particle size effect, i.e. the smaller particles at the lower calcination temperatures are easier to reduce than larger particles at higher calcination temperatures. The trend of higher reduction temperatures due to higher calcinati ons temperatures is again evident for the Cu22Zr catalysts in Fi gure 5-7C. The only difference between the TPR profiles in of the Cu45Zr catalysts (Figure 5-7B) and those of the Cu22Zr catalysts (Figure 5-7C) is that the profiles of the Cu22Zr catalysts appear slightly narrow er. This is likely because the smaller CuO particles in the Cu22Zr catalysts reduce more readily. Otherwise the trends in reduction temperature are very similar.
131 5.4.2 X-ray Diffraction Analysis XRD spectra were collected for all catalysts both before and afte r exposure to reaction conditions. This data provides insi ght into the crystalline phases apparent in the sample and also to the particle sizes of each phase. Figure 58a shows the XRD spectra for the Cu45Zr catalysts after calcination from 2 between 25 to 47.5. The spectra shown in Figure 5-8b are for the same catalysts after exposure to reaction conditions over the same 2 range. Additional XRD data was collect ed for the other catalysts in the study to compute the particle sizes of the Cu and Zr phases. Through use of the Scherrer equation as described previously, particle sizes for the CuO and ZrO2 phases can be easily computed. This information is summarized in Table 5-2. As can be clearly seen in the table, the ZrO2 particle size is nearly constant at 20nm. This is consistent with the particle size indicated by the manufacturer of the raw ZrO2 nanoparticles and indicates that the nanoparticle s are not significantly affected by the catalyst preparation and calcination processes. The CuO particle size, on the other hand, depends on both the Cu molarity in the microemulsion and also on the calcination te mperature of the catalyst. CuO particle size steadily increases as a function of Cu concentra tion in the microemulsion for samples calcined at 300C: CuO particle sizes are 12.1 nm for the cat alyst prepared from the 0.22 M Cu aqueous phase precursor, 13.1 nm for the 0.45 M Cu precursor, and 13.6 nm for the 0.73 M Cu precursor. This trend is not evident at calcinations of 500C or 700C, with CuO particle size constant at just under 15 nm for 500C calcination and 18 nm-19 nm for 700C calcination regardless of the Cu molarity of the precursor. Increasing the calc ination temperature of the samples also tends to increase the CuO particle size.
132 25 30 35 40 45 c b a Figure 5-8A2 Intensity [Arbitrary units] 25 30 35 40 45 c b a Figure 5-8B2 Intensity [Arbitrary units] Figure 5-8. XRD spectra collected from a) Fresh Ca talysts. b). Spent Catalysts. A. Cu45ZrA, B. Cu45ZrB, C. Cu45ZrC monoclinic ZrO2, CuO, Cu metal
133 Table 5-2. Particle sizes for CuO/ZrO2 catalysts. Catalyst Name Cu Molarity in Aq phase Calcination Temp CuO Particle Size Cu particle size ZrO2 particle size Cu22ZrA 0.22 300 12.1 28.0 18.6 Cu22ZrB 0.22 500 14.8 31.2 21.7 Cu22ZrC 0.22 700 17.6 30.7 21.1 Cu45ZrA 0.45 300 13.1 22.0 20.6 Cu45ZrB 0.45 500 14.9 20.5 21.3 Cu45ZrC 0.45 700 19.3 18.1 21.4 Cu73ZrA 0.73 300 13.6 28.0 19.8 Cu73ZrB 0.73 500 14.8 32.9 20.6 Cu73ZrC 0.73 700 19.1 30.2 19.1 This is seen in catalysts from all of the thre e Cu concentration precurs ors used. Increase in CuO size with increasing temperature is cons istent with a sintering of CuO at higher temperatures. Interestingly, this trend is not consistent for the spent samples. In the case of the Cu45Zr samples, the trend actually reverses. Samples calcined at higher temperatures (B and C catalysts) have lower Cu partic le sizes than the A samples. The fresh Cu45ZrC catalyst goes from a CuO particle size of 19.3 nm to a Cu pa rticle size in the spent catalyst of 18.1 nm, a change of -7%. The fresh Cu45Z rA catalyst, on the other hand, has a CuO particle size of 13.1 nm but the spent catalyst has a Cu particle si ze of 22.0 nm, a change of +40%. Evidently the higher calcination temperatures, which inhibit Cu O reduction to Cu also inhibit Cu particle growth through sintering. This trend is consiste nt in all samples: the Cu73ZrA particle size increases by 231% while that of Cu73ZrC increa ses by 58%. Another intere sting feature of Table 5-2 is that despite similar CuO particle sizes for all three B catalysts, the resultant Cu particle size of the Cu45ZrB is drastically different from the other two samples. The catalyst with the intermediate Cu molarity in the precursor evidently undergoe s a very different reduction
134 mechanism than the other two samples. It is also evident from Table 5-2 that altering the Cu molarity of the aqueous phase provides some de gree of control over the CuO particle size in these catalysts. 5.4.3 Surface Area Analysis The BET surface area data, active copper surface area and copper dispersion, are presented for all catalysts in Table 5-3. Additionally, Turnover Frequency (TOF) data at approximately 250C and 300C are presented in Table 5-3 for a ll catalysts. Table 5-3. Surface Characteristics of Cu/ZrO2 catalysts. Name BET surface area (m2/g) Cu surface area (m2/g) Cu dispersion (%) TOFa 250C (sec-1) TOF 300C (sec-1) Cu22ZrA 28.7 16.7 2.5 0.2 1.2 Cu22ZrB 25.1 10.4 1.6 0.6 0.6 Cu22ZrC 19.2 1.2 0.18 49.2 46.1 Cu45ZrA 26.2 11.6 1.8 0.7 1.2 Cu45ZrB 24.5 4.0 0.61 2.3 2.1 Cu45ZrC 21.3 0.4 0.05 29.8 19.6 Cu73ZrA 22.1 10.2 1.6 0.2 0.5 Cu73ZrB 15.7 9.9 1.8 0.7 0.8 Cu73ZrC 17.6 5.9 0.90 7.8 8.7 Reference 68.0 20.2 3.1 0.03 0.03 a Turnover Frequency is defi ned as the molecules of H2 produced per surface Cu atom per second. The information contained in Table 5-3 show s that the Cu surface area is dramatically affected by both calcination temperature as we ll as the Cu concentration used in the microemulsion precursor. A lower Cu concentrat ion in the aqueous phase of the microemulsion clearly results in higher Cu surface area. This co rrelates well with the CuO particle sizes from Section 5.3.4. It is also eviden t that the increase in calcination temperature adversely affects the
135 Cu surface area. The decrease in Cu surface area with increasing calcination temperature evident for the Cu45Zr catalysts in Table 5-3 is interestin g given the decrease of the Cu particle size at the same calcination temperatures evident in Tabl e 5-2 (smaller crystalline Cu particles but also smaller Cu surface area at higher calcinations temp eratures). These results indicate the presence of a poorly crystalline or amorphous Cu phase on th e catalysts after reaction that is not detected with XRD. This observation is further supporte d by the fact that the CuO particle size does correlate well with the Cu surface area data shown in Table 5-3 (smaller CuO particle sizes result in larger Cu surface ar eas after reduction of the CuO phase, as expected). The growth of an amorphous Cu phase for Cu/ZrO2 catalysts during reaction has been proposed previously [ 42]. This related study by W ang, et al. [ 42] proposed a dominant morphol ogy change of the Cu phase at elevated calcin ations temperatures, and that th ere is a spreading effect which increases the Cu surface area despite a growth in Cu particle size which occurred simultaneously. That study attributed the effect to the formation of a monoclinic ZrO2 phase at calci nations above 500C. This is not a likely explanation in the present work given that the ZrO2 precursor used in the catalyst fabrication is monoclinic ZrO2 nanoparticles and there is no evidence that the ZrO2 is altered during the course of the reaction. However, with the m onoclinic phase already present, the higher temperatures may increase metal-sup port interactions. In any case, the slight difference in the Cu particle size shown be tween the spent Cu45ZrA and Cu45ZrB samples cannot account for the more than 50% change in the Cu surface area between the same two samples. There must be a Cu phase which form s over the course of the reaction which is not visible on the XRD spectra and accounts for the differences in Cu surface area. Likewise, the ~2% change in Cu particle si ze between Cu22ZrB (31.2 nm) to Cu22ZrC (30.7 nm) catalysts is difficult to correlate with the 850% decrease in Cu surface area between the same two catalysts
136 after reaction. Therefore in all cases there is not a simple co rrelation between Cu particle sizes determined via XRD and Cu surface area determined via N2O titration. This difficulty has been noted in a closely related study [ 42]. The BET surface areas, on the other hand, follo w the expected trend of decreas ing surface area with increasing calcination temperature. Th is observation is more easily explained, as the growth of CuO particles seen in Table 5-2 likely results both a decrease in CuO surface area as well as clogging of pores in the ZrO2 and thereby decreasing the total catalyst surface area as well. The turnover frequency (TOF) data clearly indicates that the mo st active Cu species in the study are on the Cu22ZrC and Cu45ZrC. However, it is worth noting that all of the microemulsion catalysts fabricated for this study have a TOF highe r (generally by more than an order of magnitude) than the commercial referen ce catalyst. TOF is also generally higher than has been attained in the preceding chapters. Mo re active surfaces can clearly be made by using nanoparticles and the microemulsion fabrication t echnique. The peak in activity for the Cu45ZrA catalyst shown in Figure 5-1 is also clearly evident in the jump in TOF between 250C and 300C. Perhaps the most interesting TOF data is that of the Cu22ZrC and Cu45ZrC catalysts. The Cu surface area of these cataly sts is very small compared to the A and B versions but what Cu remains at the surface is extremely activ e in the reforming reaction. The same trend is evident to a lesser extent in the Cu73Zr catalysts. A similar phenomenon of increasing TOF with a corresponding loss of Cu surface area ha s been documented by Gnter, et al. [ 59]. The TOF num bers are an order of magnitude higher than the next most active catalyst, and a full three orders of magnitude more active than the reference catalyst. However, as shown in Figure 5-2, the methanol conversion over the Cu45ZrC catalys t peaks at a value 20% lower than conversion
137 of the Cu45ZrA and Cu45ZrB catalysts. A similar finding is evident in th e Cu22Zr catalysts (not shown). This data indicates that a reasonable Cu surface area is necessary for catalytic activity, but the Cu surface area alone does not determine the activity of a catalyst. This is consistent with the results from Chapters 2 and 3. There is appare ntly a strong interaction between the Cu and Zr phases which is most evident at high calcination temperatures. This phenomenon has been noted by other researchers [ 38] who stated that the powerful synergy between copper and zirconia m ust be taken into account when determining catalyst activity. This synergy is not simply due to a phase change from tetragonal to monoclinic ZrO2 as has been proposed in the past [ 38,42] since the ZrO2 in this study is monoclinic ZrO2 in all catalysts. 5.4 Water Gas Shift Equilibrium Figure 5-9 shows the ratio shown in previous chapters plotted on a semi-log plot as a function of temperature for the Cu22ZrA, C u45ZrA, Cu73ZrA, and Cu22ZrB catalysts. There are dramatic differences in performance between the different catalysts. The Cu73ZrA shows CO levels that are similar to those seen in the cat alysts used in Chapters 2 and 3. The CO levels are above the WGS equilibrium level ( <1.0) at all temperatures. This indicates further that this catalyst holds little promise in PEM app lications. The Cu22ZrA a nd Cu22ZrB on the other hand show CO levels that are much lower ( >>1.0) than what can be explained by WGS equilibrium. Additionally, the system is the furthest from WGS equilibrium that has been documented in any part of this study. This demonstrates the superior performance of these catalysts.
138 200220240260280300320340360380 0.01 0.1 1 10 100 1000 Figure 5-9 Temperature Figure 5-9. Dimensionless water gas shift equilib rium constant, as a function of temperature for selected catalysts. Cu73ZrA, Cu45ZrA, Cu22ZrA Cu22ZrB 5.5 Conclusions This study has demonstrated that manipula tion of the catalyst preparation procedure using microemulsion catalyst precursors afford s some degree of cont rol over both the CuO particle size and the Cu surface ar ea of the resulting catalysts. Ca talysts fabricated using this microemulsion technique exhibit an amorphous Cu phase which develops over the course of the reaction, leading to Cu surface area measurem ents which do not correl ate well with the XRD particle size data. Conversions at tained using the microemulsion fa brication techni que are higher than what can be attained using a commonly available commercial reference. Additionally, CO selectivity levels are dramatically below those achieved using the same commercial reference. This leads to the conclu sion that the combination of using nanoparticle ZrO2 and a microemulsion catalyst prepara tion technique results in supe rior catalyst performance over commercially available alternatives There is apparently some a dvantage to usi ng a calcination temperature of 500C in these CuO/ZrO2 systems, since catalysts calcined at 500C demonstrate
139 either similar, if not superior, methanol conversion but lower CO se lectivity over the same temperature range. This increase in catalytic activity coincides with a decrease in Cu surface area of the catalysts, which indicates that Cu surface area alone is not responsible for catalytic activity. The change in catalytic activity at 500C calcination is not due to a phase change in the ZrO2 support since the ZrO2 is monoclinic in all cases. Calcinations of 700C result in extremely reactive Cu surfaces, although these catalysts are apparently crippl ed by the sintering of the CuO phase during calcination. The results of this study further demons trate that the microemulsion preparation technique is preferable to the simpler impregna tion and precipitation techni ques used in Chapters 2 and 3. It is also apparent that there is little need for tertiary or quate rnary systems and that the binary Cu/ZrO2 catalysts are promising in the methanol reforming reaction if the microemulsion preparation procedure is used. 5.6 Acknowledgement Special Thanks is owed to Miss Corene Cano and Miss Sara DiBiase for their help in the preparation of the microemulison cat alysts used in this chapter.
140 CHAPTER 6 SUMMARY AND CONCLUSIONS This study covered sev eral different methanol reforming catalyst formulations, multiple preparation techniques, and many characterization procedures. Se veral conclusions can be made drawn from the four complete catal yst studies in this work. Chap ter 2 clearly demonstrated that there was an advantage to using nanoparticle Al2O3. The most revealing piece of data from this study was that the Turnover Frequency (TOF) of the nanoparticle c ontaining catalysts was dramatically higher than that of the commercial reference material. Another observation that was made became a reoccurring theme through the diff erent studies contained herein. Namely, even thought the 5/5/90 CuO/ZnO-Al2O3 catalyst had a Cu surface area approaching 60 m2/g, it still did not surpass the catalytic activity of the other samples, which had much lower Cu surface areas. This demonstrated that simply having a hi gh Cu surface area is not sufficient to make a highly active reforming catalyst. This observation was confirmed repeatedly in the later studies. Chapter 2 also demonstrated that Al2O3 supports do ultimately retard ca talyst activity and if they are to be used in methanol reforming catalysts they must be used either in relatively low concentrations or in conjunction with an alternat ive oxide support. This lead naturally to the study done involving ZrO2 and CeO2 documented in Chapter 3. The work done with CeO2 and ZrO2 supports quickly demonstrated that ZrO2 is the preferable promoter. The CeO2 containing catalyst showed little promise. The most important observation in Chapter 3 is that there is certa inly an advantage to using nanoparticle ZrO2 in quaternary CuO/ZnO/ZrO2/Al2O3 catalyst, but that using simple impregnation of ZrO2 nanoparticles is insufficient for making highly ac tive reforming catalysts. This was evidenced by the relatively low activity of the CuZnZ r-70-NP catalyst where nanoparticle ZrO2 was used as the support. Hence the need for the nanoparticle Al2O3 to ensure a sufficien t catalyst surface area
141 in these systems. At the end of the study of the ZrO2 containing catalysts in Chapter 3 it was unclear whether the high activity of the 10% nanoparticle ZrO2 catalyst was due to the presence of the nanoparticles or the fact that this was the only catal yst to contain a monoclinic ZrO2 phase. Further information on the electron ic structure of the CuO/ZnO/ZrO2/Al2O3 catalyst was needed. Detailed XPS studies of the three 10% ZrO2 containing catalysts were subsequently performed. This study was shown in Chapter 4. XPS data showed very clearly that there is a strong charge transfer between the Cu and ZrO2 species that is most pronounced in the 10% nanoparticle ZrO2 catalyst. This charge transfer interac tion results in an electron deficient Cu species (and electron rich ZrO2) species which is responsible for th e increase in catalytic activity. The partially oxidized Cu species also appears us eful in the suppression of the reverse water gas shift reaction: the partially oxidized Cu species tends to promote the oxidation of CO to CO2. The next step was to attempt to exploit the Cu-Zr interaction w ithout the need of Al2O3 supports. In order to exploit th e synergy documented in Chapte r 4 while avoiding the problems with impregnation of ZrO2 supports seen in Chapter 3, new catalyst preparation techniques had to be employed. The microemulsion preparation procedure is inherently more difficult than the relatively simple impregnation and precipitation procedures used in the earlier studies. However, as seen in Chapter 5 the resultant catalysts are dramatically superior to the other tertiary and quaternary systems. The catalytic activity was fu rther promoted through an increased calcination temperature. This was true despite the fact that the Cu surface area decr eased with the increasing calcination temperatures. It was further shown th at a degree of control over the CuO particle size could be gained by manipulating the characteristics of the microemulsion precursor. The superior performace of the binary Cu/ZrO2 catalysts is especially evident in Figure 59. The water gas shift equilibrium plot show s very clearly that the Cu22ZrA, Cu22ZrB and
142 Cu45ZrA catalysts are able to effectively suppr ess the reverse water ga s shift reaction to an extent not seen in the previous studies. Certainly, the decrease in the excess of H2O used in the feed in this study contributes to the shifting of the equilibrium of the system. However, in any case the CO selectivity curves seen in Figure 5 of Chapter 5 are superior to any of the CO selectivity data seen in earlier studies. Given the promising results seen in Chapter 5, it is logical that future studies of this work would seek to continue to exploit the Cu-Zr inte raction seen in Chapters 4 and 5. Specifically, detailed XPS studies of the catalysts used in Ch apter 5 have at the time of this publication not been performed. It would be useful to examin e the electronic structure of the binary Cu/ZrO2 systems to determine if the charge transfer seen in the quaternary systems is further enhanced through the microemulsion preparation. Additionally, very recent results in the literature suggest that there may well be a benefit to using yttria stabilized zirconia (YSZ) supports in methanol reforming. At the time of this publication no work has been done involving either nanoparticle YSZ or YSZ catalyst prepared using the microe mulsion procedure detailed in Chapter 5. In summary, the key to fabricating highl y active Cu and Cu-ZnO based methanol reforming catalysts appears to be not simply making high surface area na nomaterials but rather using nanomaterials to promot e strong interactions between the catalyst components which results in highly active catalyst surfaces. Charge transfer relationships between the active phase and support seem to be the most important of thes e interactions, which resu lts in catalyst activity levels not previously attainable using more traditional catalyst materials and preparation methods.
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148 BIOGRAPHICAL SKETCH Sa m Jones was born in Columbus, Ohio in 1981. His family moved to Waco, Texas, in 1985; Austin, Texas, in 1988, and finally to Tampa, Florida, in 1994. Sam attended high school in Tampa and got his first job working on a fishin g boat near his home. He graduated second in a class of 560 graduating seniors wi th a perfect 4.0. He was admitted to Rice University in Houston, Texas, in August 2000. He finished his bachelors degree in chemical engineering with a focus in environmental engineering in May 2004 and graduated cum laude. During his time at Rice, Sam worked as a teaching assistant for th e Organic Chemistry Department as well as the Communications Department. Sam entered the University of Florida Depart ment of Chemical Engineering Ph.D. program in August 2004. He joined Dr. Helena Weavers research group and, along with Luke Neal, was one of her first two students. His research focused on development of methanol reforming catalysts for on-board hydrogen production in passenger vehicl es. Other research interests include reaction engineering, surfa ce chemistry, and fuel cells. Sam served as the social chair for the Graduate Association of Chemical E ngineers (GRACE) from 2005-2006 and as GRACE president from 2006-2007. In 2007 he completed a semester as a research assistant at the National Aeronautics and Space Administration (NASA) where he received a New Product Development award for his work in polymeric smart coatings. After graduation, he plans to join Intel Corporation in Portland, Oregon, as a process engineer.