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Electrochemical Oxidation of Ethanol Using Nafion Electrodes Modified with Heterobimetallic Catalysts

Permanent Link: http://ufdc.ufl.edu/UFE0041294/00001

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Title: Electrochemical Oxidation of Ethanol Using Nafion Electrodes Modified with Heterobimetallic Catalysts
Physical Description: 1 online resource (59 p.)
Language: english
Creator: Moghieb, Ahmed
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

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Subjects / Keywords: Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

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Abstract: ELECTROCHEMICAL OXIDATION OF ETHANOL USING NAFION ELECTRODES MODIFIED WITH HETEROBIMETALLIC CATALYSTS This work describes the preparation and the use of chemically modified electrodes with a Nafion /catalyst film on the electrode surface. The electrochemical oxidation of ethanol has been studied at a Nafion/ Cp(PPh3)Ru(Mu-I)(Mu-dppm)PdCl2 coated glassy carbon electrode using cyclic voltammetry. The Nafion/ heterobimetallic complex modified glassy carbon electrode displays significant improvement of the catalytic activity for electrochemical oxidation of ethanol compared to that of the homogeneous process. The results obtained affirm that the immobilization of the heterobimetallic complex on the surface of the glassy carbon electrode is connected with higher catalytic activity. The electrolyte-solvent combination, electrolyte concentration, scan rate, ethanol concentration and lifetime of the electrode were optimized for the cyclic voltammetry measurements. Other previously prepared Ru/Pd, Ru/Pt, and Fe/Pd heterobimetallic complexes have been examined for electrochemical oxidation of ethanol. The alcohol oxidation products (acetaldehyde, acetic acid, diethoxyethane, and ethyl acetate) were analyzed by gas chromatography (GC).
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Ahmed Moghieb.
Thesis: Thesis (M.S.)--University of Florida, 2009.
Local: Adviser: McElwee-White, Lisa A.

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Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0041294:00001

Permanent Link: http://ufdc.ufl.edu/UFE0041294/00001

Material Information

Title: Electrochemical Oxidation of Ethanol Using Nafion Electrodes Modified with Heterobimetallic Catalysts
Physical Description: 1 online resource (59 p.)
Language: english
Creator: Moghieb, Ahmed
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: ELECTROCHEMICAL OXIDATION OF ETHANOL USING NAFION ELECTRODES MODIFIED WITH HETEROBIMETALLIC CATALYSTS This work describes the preparation and the use of chemically modified electrodes with a Nafion /catalyst film on the electrode surface. The electrochemical oxidation of ethanol has been studied at a Nafion/ Cp(PPh3)Ru(Mu-I)(Mu-dppm)PdCl2 coated glassy carbon electrode using cyclic voltammetry. The Nafion/ heterobimetallic complex modified glassy carbon electrode displays significant improvement of the catalytic activity for electrochemical oxidation of ethanol compared to that of the homogeneous process. The results obtained affirm that the immobilization of the heterobimetallic complex on the surface of the glassy carbon electrode is connected with higher catalytic activity. The electrolyte-solvent combination, electrolyte concentration, scan rate, ethanol concentration and lifetime of the electrode were optimized for the cyclic voltammetry measurements. Other previously prepared Ru/Pd, Ru/Pt, and Fe/Pd heterobimetallic complexes have been examined for electrochemical oxidation of ethanol. The alcohol oxidation products (acetaldehyde, acetic acid, diethoxyethane, and ethyl acetate) were analyzed by gas chromatography (GC).
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Ahmed Moghieb.
Thesis: Thesis (M.S.)--University of Florida, 2009.
Local: Adviser: McElwee-White, Lisa A.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0041294:00001


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1 ELECTROCHEMICAL OXIDATION OF ETHANOL USING NAFION ELECTRODES MODIFIED WITH HETEROBIMETALLIC CATALYSTS By AHMED MOGHIEB A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE R EQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2009

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2 2009 Ahmed Moghieb

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3 To my God who create d us for the be t terment of the entire world

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4 ACKNOWLEDGMENTS First of all, I thank my GOD for all his blessings without which nothing of my work would have been done. I would like to take the opportunity to express my deep appreciation to my research advisor Prof. Lisa McElwee White for her s trong support and encouragement. I always ad mire and r espect her as a research mentor, u nder her supervision, I have learned many scientific and precious aspects of research and this will be a good guideline for my future career. I would also like to thank the Institute of International Education (I IE) for the award of the Ford Foundation Fellowship, which has supported me during my two years of research. I cannot end without thanking my family for their constant encouragement and love that I have relied on throughout my time at the University. Fin ally, I am especially thankful for the patience and support of my wife, Marwa

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ ............... 4 LIST OF TABLES ................................ ................................ ................................ ........................... 7 LIST OF FIGURES ................................ ................................ ................................ ......................... 8 ABSTRACT ................................ ................................ ................................ ................................ ... 10 CHAPTER 1 LITERATURE REVIEW ................................ ................................ ................................ ....... 11 Electro Oxidation of Alcohols ................................ ................................ ................................ 11 Anode Catalysts ................................ ................................ ................................ ............... 13 Mechanism of Ethanol Electro Oxida tion Using Heterogeneous Catalysts .................... 14 Transition Metal Complex Catalysts ................................ ................................ ...................... 16 General Principle of Electrocatalysis ................................ ................................ .............. 16 Ruthenium Oxo Complexes as Electrochemical Catalysts for Alcohol Oxidation ......... 16 Heterobimetallic Catalysts for the Electrooxidation of Alcoho ls ................................ ........... 18 Synthesis ................................ ................................ ................................ .......................... 19 Cyclic Voltammetry ................................ ................................ ................................ ........ 21 Product Detection ................................ ................................ ................................ ............ 23 Bulk Electrolysis with Complexes 1, 2, and 6 ................................ ................................ 24 2 CHEMICALLY MODIFIED ELECTRODES CONTAINING IMMOBILIZED HETEROBIMETALLIC COMPLEXES ................................ ................................ ................ 26 Introduction ................................ ................................ ................................ ............................. 26 Chemically Modified Electrodes ................................ ................................ ............................ 26 Electrocat alysis Using Chemically Modified Electrodes (CMEs) ................................ .. 27 Polymer Modified Electrodes ................................ ................................ .......................... 27 Electrodes Modified with Heterobimetallic C omplexes ................................ ......................... 30 Preparation of Polymer Modified Electrodes ................................ ................................ .. 30 Cyclic Voltammetry of Nafion Modified Electrodes ................................ ...................... 31 Effect of Electrolyte and Solvent ................................ ................................ .................... 33 Electrolyte solvent concentration ................................ ................................ .................... 37 Effect o f Ethanol Concentration ................................ ................................ ...................... 39 Effect of Scan Rate ................................ ................................ ................................ .......... 39 Lifetime of the Electrode ................................ ................................ ................................ 40 Applications ................................ ................................ ................................ ..................... 41 Bulk Electrolysis ................................ ................................ ................................ ............. 47 3 EXPERIMENTAL SECTION ................................ ................................ ................................ 51

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6 General Methods ................................ ................................ ................................ ..................... 51 Synthesis ................................ ................................ ................................ .......................... 51 Preparation of the Nafion Modified Electrode ................................ ................................ 51 Electrochemistry ................................ ................................ ................................ .............. 51 Product Analysis ................................ ................................ ................................ .............. 52 LIST OF REFERENCES ................................ ................................ ................................ ............... 53 BIOGRAPHICAL SKETCH ................................ ................................ ................................ ......... 59

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7 LIST OF TABLES Table page 1 1 Formal Potentials of Complexes 1 7 ................................ ................................ ................. 21 1 2 Product Distributions and Current Efficiencies for the Electrochemical Oxidation of Methanol by 1 2 and 6 ................................ ................................ ................................ .... 25

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8 LIST OF FIGURES Figure page 1 1 A schematic representation of Direct Ethanol/Oxyge n Fuel Cell ................................ .... 12 1 2 Mechanism of ethanol electro oxidation at a platinum surface in acid medium. .............. 15 1 3 Cyclic voltammograms of 1 under nitrogen in 2.5 mL of DCE/0.7 M TBAT (tetrabutylammonium triflate); glassy carbon working electrode; Ag/Ag + reference electrode. ................................ ................................ ................................ ............................ 22 1 4 Cyclic voltammograms of 2 under nitrogen in 2.5 mL of DCE/0.7 M TBAT (tetrabutylammonium triflate); glassy carbon working electrode; Ag/Ag + reference electrode. ................................ ................................ ................................ ............................ 22 1 5 Cyclic voltammograms of 6 under nitrogen in 3.5 mL of DCE/0.1 MTBAT (tetrabutylammonium triflate); glassy carbon working electrode; Ag/Ag + reference electrode. ................................ ................................ ................................ ............................ 23 2 1 Generic illustration of a modified electrode ................................ ................................ .... 26 2 2 An ion exchange polymer filled with free redox molecules (adapted from ref 88). ......... 28 2 3 (a): Structure o f Nafion ................................ ................................ ................................ ...... 29 2 3 (b): Three phase model of Nafion ................................ ................................ ................... 29 2 4 Heterobimetallic Ru/Pd complex 8 ................................ ................................ ................... 31 2 5 Cyclic voltammograms in 0.1 M TBACF 3 SO 3 / DCE, Ag/Ag + reference electrode; 245 mM Ethanol; 50 mV/s a) Nafion blank b) Nafion modified electrode with 10 mM of complex 8 and c) The difference between the voltammogram in the pres ence and absence of ethanol for both Nafion blank and Nafion modified. ................................ 32 2 6 Cyclic voltammograms of Nafion blank and Nafion modified electrodes with 10 mM of complex 8 in 0.1 M TBACF 3 SO 3 ,TBAP F 6 ,and TBABF 4 / DCE, DCM, and AN, 245 mM of ethanol, 50 mV/s and Ag/Ag + reference electrode ................................ ....... 34 2 6 Continued. ................................ ................................ ................................ .......................... 35 2 7 a) Cyclic volt ammograms and b) plot of oxidation peak currents against electrolyte concentration of Nafion modified electrode with 10 mM of complex 8 using 0.05 0.5M TBACF 3 SO 3 /DCE, 245 mM of ethanol, 50 mV/s and Ag/Ag + reference electrode. ................................ ................................ ................................ ............................ 37 2 8 a) Cyclic voltammograms of Nafion modified electrode with 10 mM of complex 8 b) plot of oxidation peak currents against ethanol concentrations (98 489 mM), and

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9 c) Cyclic voltammograms of Nafion blank electrode usin g 0.1M TBACF 3 SO 3 /DCE, 50 mV/s and Ag/Ag+ reference electrode. ................................ ................................ ...... 38 2 9 a) Cyclic voltammograms, b) plot of oxidation peak currents against the square root of the scan rates of Nafion modified el ectrode with 10 mM of complex 8 using 0.1M TBACF 3 SO 3 /DCE, 245 mM of ethanol, and Ag/Ag + reference electrode. ....................... 40 2 10 a) Cyclic voltammograms, b) plot of oxidation peak currents against the numbe r of times using the same Nafion modified electrode with 10 mM of complex 8 using 0.1M TBACF 3 SO 3 /DCE, 245 mM of ethanol, 50 mV/s ,and Ag/Ag + reference electrode. ................................ ................................ ................................ ............................ 41 2 11 Structures of compound s 8 11 ................................ ................................ ............................ 42 2 12 Cyclic voltammograms with 10 mM of complex 8 a) glassy carbon working electrode in 0.7 M TBACF 3 SO 3 /DCE; 50l methanol b) Nafion modified electrode of the complex in 0.1M TBACF 3 SO 3 /DCE, 245 mM (50l) ethanol, 50 mV/s ,and Ag/Ag + reference electrode. ................................ ................................ ............................... 44 2 13 Cyclic voltammograms with 10 mM of complex 9 a) 5 mM, glassy carbon working electrode; 50l methanol b) 10 mM, Nafi on modified electrode, 245 mM (50l) ethanol, in 0.1M TBACF 3 SO 3 /DCE, 50 mV/s ,and Ag/Ag + reference electrode. ............. 45 2 14 Cyclic voltammograms with 10 mM of complex 10 a) 5 mM, glassy carbon working electrode; 50l methanol b) 10 mM, Nafion modified electrode, 245 mM (50l) ethanol, in 0.1M TBACF 3 SO 3 /DCE, 50 mV/s ,and Ag/Ag + reference electrode. ............. 46 2 15 Cyclic voltammograms with 10 mM of c omplex 11 a) 5 mM, glassy carbon working electrode; 50l methanol b) 10 mM, Nafion modified electrode, 245 mM (50l) ethanol, in 0.1M TBACF 3 SO 3 /DCE, 50 mV/s ,and Ag/Ag + reference electrode. ............. 47 2 16 Product distribution for the electrolysis of methanol using 10 mM of Nafion/complex 8 vitreous carbon working electrode in 3.5 mL of 0.1 M TBACF 3 SO 3 /DCE, 350 mM methanol, and Ag/Ag + reference electrode. ................................ ................................ ....... 48 2 17 Product distribution for the electrolysis of ethanol using 10 mM of Nafion/complex 8 vitreous carbon working electrode in 3.5 mL of 0.1 M TBACF 3 SO 3 /Ethanol, 245 mM ethanol, and Ag/Ag + reference electrode. ................................ ................................ .. 49 2 18 Product distribution for the electrolysis of ethanol using 10 mM of Nafion/complex 9 vitreous carbon working electrode in 3.5 mL of 0.1 M TBACF 3 SO 3 /Ethanol, 245 mM ethanol, and Ag/Ag + reference electrode. ................................ ................................ .. 49 2 19 Product distribution for the electrolysis of ethanol using 10 mM of Nafion/complex 11 vitreous carbon working electrode in 3.5 mL of 0.1 M TBACF 3 SO 3 /Ethanol, 245 mM ethanol, and Ag/Ag + reference el ectrode. ................................ ................................ .. 50

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10 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science ELECTROCHEMICAL OXIDATION OF ETHANO L USING NAFION ELECTRODES MODIFIED WITH HETEROBIMETALLIC CATALYSTS By Ahmed Moghieb December 2009 Chair: Lisa McElwee White Major: Chemistry This work describes the preparation and the use of chemically modified electrodes with a Nafion /catalyst film o n the electrode surface. The electrochemical oxidation of ethanol has been studied at a Nafion/ Cp(PPh 3 dppm)PdCl 2 coated glassy carbon electrode using cyclic voltammetry. The Nafion/ heterobimetallic complex modi f ied glassy carbon electrode di splay s significant improvement of the catalytic activity for electrochemical oxidation of ethanol compared to that of the homogeneous process. The results obtained affirm that the immobilization of the heterobimetallic complex on the surface of the glassy carbon electrode is connected with higher catalytic activity The electrolyte solvent combination, electrolyte concentration, scan rate, ethanol concentration and life time o f the electrode were optimized for the cyclic voltammetry measurements. Other pre viously prepared Ru/Pd, Ru/Pt, and Fe/ Pd heterobimetallic complexes ha ve been examined for electrochemical oxidation of ethanol. The alcohol oxidation products (acetaldehyde, acetic acid, diethoxyethane and ethyl acetate) were analyzed by gas chromatogra phy (GC).

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11 CHAPTER 1 LITERATURE REVIEW Electro O xidation of Alcohols Whereas higher electric efficiency and performance can be obtained by the use of pure hydrogen or a hydrogen rich gas as a fuel rather than alcohols for polymer electrolyte membrane fuel cell s (PEMFC), there are some limitations for the development of such techniques like the clean production, storage and distribution of hydrogen 1 3 The use of the low molecular weight alcohols (methanol, ethanol, etc.), in Direct Alcohol Fuel C ells ( DAFC s) has advantages compared to pure hydrogen because they are liquids (eas ily handled, store d and transported). Moreover, their theoretical mass energy densities (6.1 and 8.0 kWh kg 1 corresponding to 6 and 12 el ectrons per molecule for the total oxidation of for methanol and ethanol, respectively to carbon dioxide) are comparable to that of gasoline (10 11 kWh kg 1 ). 4 6 DAFC s are attractive as power sources for mobile, st ationary, and portable applications due to their high energy conversion efficiency and low operating temperature 7 10 Discovered in England in 1839 by Sir William Grove, the fuel cell is an electrochemical device, which converts the heat of combustion of a fuel (hydrogen, natural gas, methanol, ethanol, hydrocarbons, etc.) directly into electricity The electrochemical cell consists of two electrodes, anode and cathode, which are electronic conductors, separated b y an electrolyte, protonic conductor. At the anode t he fuel is electrochemically oxidized without producing any pollutants (only water and/or carbon dioxide for complete oxidation ), while at the cathode the oxidant ( oxygen from the air) is reduced A sc hematic representation of a Direct Ethanol/Oxygen F uel C ell (DEFC) is shown in Figure 1 1 as an example for fuel cells.

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12 Figure 1 1. A schematic representation of Direct Ethanol/Oxygen F uel C ell (a dapted from ref 4 ) The direct oxidation of methanol in fuel cells has been widely investigated, as m ethanol has been considered the most promising fuel because it is more efficiently oxidized than other alcohols. However, there are stil l some development challenges, such as it is relatively toxic, inflammable, and it is not a primary fuel, nor a renewable fuel 5 Therefore, other alcohols have beg a n to be considered as alternative fuels. Among organic small molecule alcohols, ethanol has been subjected to electro oxida tion in numerous studies. 11 13 Ethanol offers an attractive alternative as a fuel in low temperature fuel cells because it can be produced in large quantities from agricultural products and it is the major renewab le biofuel from the fermentation of biomass. Also, after evaluation of ethanol, 1 propanol, and 2 propanol as alternative fuels for direct alcohol/oxygen fuel cells, Wang et al. 14 found that ethanol is a promising alternative to methanol by using on line mass spectrometry for determination of the relative product distribution for the electro oxidation of these a lcohols under fuel cell operating conditions, since ethanol has a higher electrochemical activity to oxidize to CO 2 However, the complete oxidation of ethanol to CO 2 is more difficult than that of methanol due to the difficulty of breaking the C C bond at low temperatures High yields of partial

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13 oxidation products, CH 3 CHO and CH 3 COOH are produced at Pt catalysts 15 and decrease the performance of the fuel cell. So, t here is a need to improve the activity of anode electrocat alysts for alcohol electro oxidation Anode Catalysts Platinum electrocatalysts are well known to be the best for the electrochemical oxidation of organic molecules in acid media 16 However, the formation of strongly adsorbed CO intermediates as the result of the dissociative chemisorption of methanol/ethanol poisons the plat inum anode catalysts and causes high overpotentials that drastically reduce the activity of pure platinum as shown by infrared reflectance spectroscopy. 17,18 Considerable efforts have been devoted towards the impro vement of the activity and endurance of platinum electrodes by decreasing the CO poisoning. A ddition of a sec ond element to f orm a bimetallic catalyst promotes the electrocatalytic activity of pure platinum by lowering the overpotential for alcohol electr o oxidation significantly. Most of the literature data agree that the onset of the oxidation reaction of alcohols on PtRu is about 0.2 V less positive than on the single Pt catalyst. 19 This improveme nt is attributed to two mechanisms: 1) T he bifunctional mechanism, 20 25 in which the Pt anode sites adsorb and dehydrogenate alcohol (methanol, ethanol) and produce carbon containing species, while the second metal sites which are more oxophilic, adsorb the OH species and supply the oxygen atom to the adjacent metal at lower potential than pure Pt surface. 2) The intrinsic mechanism, 26 which is based on the modification of Pt electronic structure by the presence of the second metal (Ru) which makes the Pt atoms more capable of the adsorption of oxyge n containing species at lower potential relative to pure Pt. Several binary catalysts were proposed for methanol and ethanol oxidation, most of them based on modification of Pt with some other metal. Examples of these catalysts are Pt Rh 15 Pt

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14 Ru 27,28 Pt Sn 28 30 Pt Mo 31 Pt Ni 32 Pt Co 33 Pt Os 34 and Pt Fe 35 which were reported as alcohol electro oxidation catalysts. The Pt Ru binary catalysts are known to be the most effective anode m aterial for methanol and ethanol oxidation. 36 However, it has been reported that some ternary and quaternary alloy catalysts based on the Pt/Ru have a higher catalytic ac tivity than Pt/Ru. Examples of these materials are Pt Ru W and Pt Ru Mo 37 Pt Ru Rh 38 Pt Ru Co 39 Pt Ru Ir/C 40 Pt Ru Ni/C 41 Pt Ru Fe/C 42,43 Pt Co Cr/C 44 Pt Ru Sn WO 3 45 Pt Ru Os Ir 46 and Pt Pd Ru Os 47 Mechanism of E thanol E lectro O xidation U sing H eterogeneous C atalysts Numerous studies on the electro oxidation of ethanol ha ve been carried out to identify the adsorbed intermediates on the electrode and to examine the reaction mechanism. Identification of the adsorbed intermediates and the reaction products for the electro oxida tion of ethanol were provided using various tech niques, such as chromatography (HPLC,GC), 6 48,49 differential electrochemical mass spectrometry (DEM S) 12,18,50 and in situ Fourier transform infrared spectroscopy (FTIRS) 51 53 For the electrocatalytic oxidation of isotopically labeled ethanol on Pt electrode s in acid medium, DEMS studies identified acetaldehyde and CO 2 as primary reaction products. It showed that by cleavage of the C H bond o n the carbon and the O H bond in the hydroxyl group the acetaldehyde is formed whereas CO 2 is formed via a multistep pathway involving strongly bonded intermediates Also, at low potentials methyl radical is formed by C C bond cleavage, and methane and etha ne are detected as desorption products. FTIRS studies showed that adsorbed carbon monoxide is a strongly adsorbed intermediate at the Pt surface. Based on previous studies (electrochemical and spectro electrochemical), the reaction mechanism of electro oxidation of ethanol on platinum electrode can be postulated as follows.

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15 Generally, it is known that ethanol has two reactive sites which can interact with the metal catalyst during the adsorption processes, the carbon atom and the OH group The ethanol can be adsorbed through any of these groups after the break ing of a particular C H or O H bond as shown from the previous work of Iwasita and Pastor 18 The ethanol can then adsorb on Pt following two modes: Figure 1 2 Mechanism of ethanol electro oxidation at a platinum surface in acid mediu m According to one of the generally accep ted reaction mechanisms given in F ig. 1 2 54 at a potential less than 0.8 V/ N HE, the ethanol is adsorbed b y the carbon at the surface of platinum. This adsorption involves a C H cleavage with the transfer of one electron ( 1 ), and then

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16 the adsorbed intermediate desorbs to produce acetaldehyde with the transfer of another electron ( 2 ). The second hydrogen of the acetaldehyde can be eliminated by the nucleophilic attack of H 2 O oxygen, and leads to adsorbed acetyl which completes the oxidation into acetic acid ( 3 ). The adsorbed acetyl also leads to adsorbed methyl, which is a reactive intermediate, and will des orb as CH 4 at E < 0.2 V/ N HE ( 4 ) or oxidize to carbon dioxide via the surface reaction of adsorbed CO species and adsorbed OH (coming from the dissociation of H 2 O) at E > 0.5 V/ N HE ( 5 ) Transition M etal C omplex C atalysts General P rinciple of E lectrocatalysi s For electrochemical reactions, it is required to find an electrocatalyst that lowers the overpotential and increases the reaction rate. Transition metal complexes are used to catalyze many redox reactions, working as electrocatalysts. In the electroche mical cell, the electrode oxidizes the transition metal complexes to generate an active species, which then catalyzes the oxidation/reduction of the substrate. There are two basic different types of electrocatalysis : heterogeneous and homogeneous The he terogeneous electrocatalytic process includes the immobilization of the electrocatalyst on the electrode surface, so that the substrate transfers from the bulk solution and exchanges the electron with the electrode. In homogeneous electrocatalysts, the el ectron transfer and chemical reactions occur in the bulk solution, and the substrate diffuses to the electrode surface. 55 Ruthenium Oxo C omplexes as E lectrochemical Catalysts for A lcohol O xidation The redox chemistry of ruth enium oxo complexes has attracted particular interest, due to the wide range and the stabilization of the high oxidation states. Among different types of the metal complexes poly(pyridyl)oxo ruthe nium complexes have been applied with success in electroca talytic oxidation of different organic compounds 56 58

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17 Meyer and co workers reported the first example o f using the ruthenium oxo complexes for the oxidation of various substrates including alkenes The strong oxi dant Ru( IV) oxo complex ion, (bpy) 2 pyRuO 2+ (bpy = 2,2 bipyridine, py = pyridine), can be generated from the corresponding Ru(II) aqu o complex via proton couple d electron transfer as shown in Scheme 1. 56 Scheme 1 1. The s ystem involves a net 2e oxidation in aqueous solution and the resulting Ru IV =O complex in a buffered solution is rapidly reduced to [Ru II (bpy) 2 (py)(OH 2 )] 2+ when an unsaturated organic compound is added Both the Ru II /Ru III and Ru III /Ru IV couples are reversible thus the oxidati on reaction can be made for the substrates (organic compounds) catalytically by using an external source of oxidation ( electrode potential at which the redox reaction occurs ) sufficient to regenerate the [Ru IV (bpy) 2 (py)O] 2+ in aqueous solution One of the attractive applications of these complexes is as electrocatalysts in fuel cells that utilize organic fuels such as methanol or formaldehyde. 59 There are two ways to use the Ru oxo complex oxidation catalysts: in solution (homogeneous) or by i ncorporation of a metal catalyst into the electrode (h eterogenize d homogeneous) which could give a significant improvement The ruthenium aquo species [Ru(4,4 Me 2 bpy) 2 (AsPh 3 )(H 2 O)](ClO 4 ) 2 (4,4 Me 2 bpy = 4,4 dimethyl 2,2 bipyridine) was reported by De Giovani and co workers to undergo 2e /2H + transfer in the oxidation process at pH 7.0 in the homogeneous electro oxidation of benzyl alcohol, 1 phenylethanol and cyclohexene 60 Also, the electrocatalytic oxidation of benzyl alcohol, cyclohexanol, cyclohexene, 1 pentanol, 1,2 butanediol and 1,4 butanediol w as

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18 performed using t he ruthenium(II) [Ru( cis L)(totpy)(H 2 O)](PF 6 ) 2 and [Ru( trans L) 2 (totpy)(H 2 O)](PF 6 ) 2 (2) (L = 1,2 bis(diphenylphosphino)ethylene; totpy = 4 (4 tolyl) 2,2 :6 ,2 terpyridine) complexes in solution and immobilized in carbon paste electrodes. 61 The ruthenium aquo comp lexes containing phosphines and polypyridine as ligands [Ru(L)(totpy)(H 2 O)](PF 6 ) 2 (L = 1,2 bis(dimethylphosphino) ethane and 1,2 bis(dichlorophosphino)ethane; totpy = 4 (4 tolyl) 2,2 :6 ,2 terpyridine) were tested with benzyl alcohol, cyclohexanol, cy clohexene, ethylbenzene, 1,2 butanediol and 1,4 butanediol in solution and immobilized in carbon paste electrode and ITO/Nafion electrodes. 62 The proposed mechanism of using the Ru oxo complexes in the oxidation of organic compounds proposes the formation of an electron deficient carbon in the transition state, for which the substituent group induces the delocalization of the positive charge via the hyperconjugation, inductive, or resonance effect, which decreases the reaction activation energy. 61,63 The mechanistic details usually in volve a two electron hydride transfer, such as the oxidation of 2 propanol to acetone by a hydride transfer pathway (Scheme 2 ) 64 Scheme 1 2. Heterobimetallic Catalysts for the Electrooxidation of Alcohols Heterobimetallic catalysts, bimetallic complexes that have two different metals are of interest for catalysis. 65 68 Such mixed metal complex systems have attracted much attention because of their superior properties in organic synthesis. They are effective catalyst precursors for the aerob ic Heck coupling 69 the oxidation of secondary alcohols 70 e th ylene

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19 polymerization 71 and o lefin h ydroformylation 72 The presence of different types of metal centers close to each other within the same molecule in the heterobimetallic catalysts affects their re activity, give s significant properties different from their mononuclear analogues, and can lead to higher catalytic activity 73 Also, t he different electronegativities between metal ions in the mixed bi and polymetallic complexes are known to mediate the electrocatalytic reactions and lead to the cooperative effects between the two different metal centers. 74 Previous studies in the McElwee White research group that focused o n t he development of heterobimeta llic catalysts for electro oxidation of methanol resulted in the synthesis of the dppm ( bis (diphenylphosphino)methane) bridged Ru/Pd, Ru/Pt, and Ru/Au heterobimetallic complexes Cp(PPh 3 dppm)PtCl 2 ( 1 ) 75 Cp(PPh 3 )Ru( Cl)( dppm)PdCl 2 ( 2 ) 76 and Cp(PPh 3 dppm) AuCl ( 3 ) 76 Synthesis Complexes 1 3 were synthesized at room temperature by react ing CpRu(PPh 3 1 dppm)Cl ( 4 ) with Pt(COD)Cl 2 Pd(COD)Cl 2 and Au(PPh 3 )Cl, respectively (Scheme 1 3). Due to the poor solubility of complexes 1 3 in water and in order to compare the behavior of these compounds with ruthenium oxo catalysts that are soluble in water, s everal attempts have been carried out by the McElwee White research group to modify these complexes and synthesize water soluble heterobimetallic ammoni um substituted cyclopentadienyl complexes E xample s include the heterobimetallic Ru/Pd complexes [ 5 C 5 H 4 CH 2 CH 2 N(CH 3 ) 2 HI]Ru(PPh 3 )( I)( dppm)PtI 2 ( 6 ) 77 and [ 5 C 5 H 4 CH 2 CH 2 N(CH 3 ) 2 HI]Ru(PPh 3 )( I)( dppm)PdCl 2 ( 7 ) 77 Complexes 6 and 7 were synthesized by reaction of [ 5 C 5 H 4 CH 2 CH 2 N(CH 3 ) 2 HI]Ru(PPh 3 )(I)( 1 dppm) ( 5 ) with Pd(COD)Cl 2 in dichloromethane at room temperature (Scheme 1 4).

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20 Scheme 1 3 Scheme 1 4 Dppm was used as a bridging ligand due to the presence of the strong metal phosphorus bonds which give the ch ance for the two metal centers to be in close proximity and facilitate the electronic interaction between them The metal metal interactions, electron donation between

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21 the metals take s place through the halide bridge and can be detected by the shifts of the redox potentials Cyclic Voltammetry Cyclic voltammetry can be used to explore the metal metal interactions in the heterobimetallic complexes by comparing the redox potential of the heterobimetallic systems to that of monomeric ones. 78 For example, the cyclic voltammogram of Cp(PPh 3 dppm)PdCl 2 ( 2 ) shows a 730 mV positive shift for the Ru(II/III) waves compared to the monomeric compound CpRu(PPh 3 )( 1 dppm) Cl ( 4 ) as a result of electron donation from the Ru to Pd through the chloride bridge. However, the cyclic voltammogram for Cp(PPh 3 dppm) AuCl ( 3 ) shows only a 30 mV positive shift for the Ru(II/III) waves compared to CpRu(PPh 3 )( 1 dppm) Cl, indicating that the dppm bridge has no effect in the electronic interaction between the two metals in contrast to the halogen on e. 76 The cyclic voltammograms for the Ru/Pd compounds ( 6 ) and ( 7 ) each exhibit irreversible ox idation at 1.35 V and 1.00 V vs. NHE, respectively, while the mononuclear complex ( 5 ) is at 1.05 V T his potential shift i ndicated the electron donation through the iodide bridge. The CV data for complexes 1 7 are listed in Table 1 1 75 77 Table 1 1 Formal Potentials of Complexes 1 7 Complex Couple E 1/2 (V) a Couple E 1/2 (V) a Couple E 1/2 (V) a Ru/Pt ( 1 ) Ru(II/III) 1.13 b Pt(II/IV) 1.78 c Ru(III/IV) Ru/Pd ( 2 ) Ru(II/III) 1.29 Pd(II/IV) 1.45 c Ru(III/IV) Ru/Au ( 3 ) R u(II/III) 0.89 Au(II/IV) 1.42 c Ru(III/IV) Ru ( 4 ) Ru(II/III) 0.56 b Ru(III/IV) Ru ( 5 ) Ru(II/III) 1.05 Ru(III/IV) 1.95 Ru/Pd ( 6 ) Ru(II/III) 1.35 Pd(II/IV) 1.67 c Ru(III/IV) 2.04 Ru/Pd ( 7 ) Ru(II/III) 1.00 Pd(II/IV) 1.57 c Ru(III/IV) 2.09 a All potent ials obtained in DCE/TBAT (tetrabutylammonium triflate) and reported vs NHE and referenced to Ag/Ag + b Performed in CH 2 Cl 2 /TBAH (tetrabutylammonium hex a flu o ro phosphate ). c Irreversible wave, E pa reported.

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22 Cyclic voltammetry of complexes 1 2 and 6 in the p resence of methanol results in a significant increase in the oxidation current of the second metal redox waves Pt(II/IV) or Pd(II/IV ) that indicate the catalytic process of the complexes in electrocatalytic oxidation of methanol (Figures 1 3 1 4 and 1 5 ) Figure 1 3. Cyclic voltammograms of 1 75 under nitrogen in 2.5 mL of DCE/0.7 M TBAT (tetrabutylammonium triflate); glassy carbon working elect rode; Ag/Ag + reference electrode. Figure 1 4. Cyclic voltammograms of 2 76 under nitrogen in 2.5 mL of DCE/0.7 M TBAT (tetrabutylammonium triflate); glassy carbon working electrode; Ag/Ag + reference electrode.

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23 Figure 1 5. Cyclic voltammograms of 6 77 under nitrogen in 3.5 mL of DCE/0.1 M TBAT (tetrabutyla mmonium triflate); glassy carbon working electrode; Ag/Ag + reference electrode. Product Detection T he electro oxidation process of m ethanol produce s formaldehyde and formic acid, as detected by on line FTIR spectroscopy analysis of the product of on a plat inum ruthenium catalyst in a prototype direct methanol fuel cell 79 However neither was observed in the reaction mixtures D imethoxymethane (DMM) and meth yl formate (MF) were detected instead. It is reported that oxidation of methanol take s place via multistep mechanism. Methanol oxidizes to formaldehyde, then in excess methanol form s dimethoxymethane or is completely electrochemically oxidized to CO 2 (Eq 1.1 1.3)

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24 Also, methanol oxidation can go through a four electron electro oxidation to formic acid which in excess methanol form s methyl formate or is further electrochemically oxidized to CO 2 ( Eq. 1.4 1.6 ) Finally, methanol can go through a six electron electro oxidation to form CO 2 (Eq.1.7) Bulk Electrolysis with Complexes 1 2 and 6 The evolution of the product distributions as the reaction progresses is shown in Table 1.2. 75 77 For the complexes 1 and 2 a t low quantities of charge passed, they give a much higher amount of DMM. As the reaction progresses, water is formed in situ during the condensation of the formaldehyde and formic acid with excess of methanol. T he tendency toward production o f (MF) increases more than that for DMM because the presence of water shifts the product ratios toward the four electron oxidation product, MF For the complex 6 the behavior is different as throughout the reaction the two electron oxidation product D MM is favored The current efficiencies for the oxidation processes are the ratio of the charge necessary to produce the observed yield of DMM and MF to the total charge passed during the bulk electrolysis (Eq. 1. 8).

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25 Table 1 2. Product D istributions an d Current Efficiencies for the Electrochemical O xidation of M ethanol by 1 2 and 6 Charge / C 2 (OCH 3 ) 2 ( DMM )/ HCOOCH 3 ( MF )] c Ru/Pt ( 1 ) a Ru/Pd ( 2 ) a Ru/Pd ( 6 ) b 25 50 75 100 130 150 200 2.45 3.18 2.35 2.41 1.66 1.51 1.54 1.76 1.23 0.97 2.34 1.20 0.87 3.17 3.82 Current Efficiency (%) d after 130 C 1 8.6 24.6 Current Efficiency (%) d after 200 C 44 Electrolyses were performed at 1.7 a and 1.5 b V vs. NHE. A catalyst concentration of 10 mM was used. Methanol concentration was 0.35 M c Determination by GC with respect to n heptane as internal standa rd. Each ratio is reported as an average of 2 5 experiments. d A verage current efficiencies after 75 130 C of charge passed

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26 CHAPTER 2 CHEMICALLY MODIFIED ELECTRODES CONTAININ G IMMOBILIZED HETEROBIMETALLIC COMPLEXES Introduction Modification of the surface of the electrodes by coating with a thin film of selected chemical produces improvements of the electrochemical function over conventional electrodes. The electrodes with modified surfaces get a large usable potential window, increasing the sensitivity, selectivity and electrochemical stability. There is a rapid ly growing need for the improvement of the electrode performance chemically in many disciplines of science due to the possibility of transferring the desirable properties of a chemical site into t he electrode surface. 80 Chemically Modified Electrodes Chemically modified electrode (CME) 81 is: a conducting or semiconducting conventional electrode that has been coated with a monomolecular, multi molecular, ionic, or polymeric film (see Figure 2 1, the film or adlayer is anchored to a conventional elec trode to enhance performance or achieve a specialized function ) 80 to enhance the electrochemical properties of the interface Figure 2 1 Chemically Modified Electrodes (a dap ted from ref 80 ).

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27 Electrocatalysis U sing Chemically Modified Electrodes (CMEs) In electrocatalysis, immobilization of the electroactive molecule to the electrode surface produces an interface between the electrode surface and the analyte in solution that assist s its reduction or oxidation. 82 Using CMEs in the electrocatalysis lower s the overpotential of the heterogeneous electron transfer o f the target analyte between the electrode that is derivatized by the electroc atalyst and the solution. This amplif ies the detect ed signal with respect to that at a bare electrode. 81 The porous structure and high surface area of c onducting polymers provide the opportunit y for the immobilization of metal catalysts and development of new catalytic and ele ctrocatalytic materials. Hi gh electric al conductivity of some polymers gives these composite materials great chance to achieve electrocatalysis by electron transfer through the polymer chains between the electrode and the immobilized materials. 83 Considerable interest has been devoted in recent years to develop ment of suitable electrode materials for alcohol oxidation for possible application in fuel cells Conducting polymers possess both proto nic and electronic conductivity B. Rajesh 84 and co workers reported that the use of Pt nanoparticles loaded conducting hybrid material based on polyaniline and V 2 O 5 as an electrode exhibited high electrocatalytic activity and stability for e lectrooxidation of methanol. Also, Maiyalagan 85 reported that poly( o phenylenediamine) (PoPD ) nanotube electrodes give significant improvement for the catalytic activity of platinum nan oparticles for oxidation of methanol Polymer Modified Electrodes The network nature of the polymer provides the electrode surface with a high number of the active sites which give the possibilities for studying many interfacial phenomena f or example, cha rge transport or electron transfer mediation reactions. 86

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28 A t least three processes occur during the electrocatalytic conversion of the analyte at the modified polym er electrode : 83 1) the heterogeneous electron transfer between the electrode and the conducting polymer layer, and electron transfer within the polymer film. The rate of the heterogeneous electron transfer process depends on the electric al conductivity of the polymer. 2) The diffusion of the analyte through the conductive layer to the electrode. This process depends on the interaction between the analyte and the polymer film. 3) The heterogeneous reaction occurs between the conducting polymer and the sol ution. One of t he polymer modifier types is ion exchange polymer that has the ability to serve as the electron transfer site within and on the surface of the film on the modified electrode. By changing the counter ion for the charged electroactive ion th e r esult i s a layer with the ability to transport electron s 87 For example, Nafion can be immobilize d on the electrode surface through drop casting, spin coating, or dip coating. Th en the desired redox ion ( the charged electroactive ion ) is ion exchanged into the polymer (see Figure 2 2, Filled circles depict the reduced form o f the couple and open circles the oxidized form. Smooth arrows show electron transfer events between the two forms, and wavy arrows indicate diffusional processes) 88 Figure 2 2 An ion exchange polymer fille d with free redox molecules.

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29 Nafion 1 discovered in the late 1960s by Walther Grot of DuPont is a perfluorosulfonated polymer consisting of poly tetrafluoroethylene (PTFE) backbone and perfluo roether side chain with a sulfonate group Due to its high proton conductivity, thermal and mechanical stability it is commonly used as s olid p olymer e lectrolyte for f uel c e lls. 89 As seen in F igure 1.6 90 the three phase model is based on a three phase clustered system. The three region s consist of the hydrophobic fluorocarbon ( P TFE ) backbone (A) an interfacial zone containing some pendant side chains, some water, sulfate or carboxylic groups (B) and the hydrophilic ionic zone where th e ionic exchange sites, counter ions and ab sorbed water exist in majority (C) Figure 2 3. (a): Structure of Nafion Figure 2 3. (b ): Three phase model of Nafion (a dapted from ref 90 ) 1 Nafion is a registered trademark of E.I. DuPont de Nemou rs & Co

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30 Electrodes Modified with Heterobimetallic Comple xes Homogeneous electrocat alysts such as transition metal complexes have the ability to mediate electron transport when immobilized on the surface of the electrode. 82 The immobilization of a monolayer of a redox a ctive metal complex to an electrode surface offers the advantages of combining the homogeneous catalysis advantages (high activities and selectivities) and heterogeneous catalysis (easy separation of catalyst from the products) by fixing the homogeneous ca talysts in a heterogeneous matrix. 91 Preparation of Polymer M odified E lectrodes As classified by Drust 81 p olymer film coated electrodes may be subdivided into s even categories based on the process used to apply the film : ( 1 ) Dip coating Immersing the electrode material in the solution of the polymer for a certain time; th e film is formed by adsorption. ( 2 ) D roplet evaporation A pplying a droplet of the polymer solution to the surface of the electrode; the film is formed by evaporation of the solvent. ( 3 ) Spin coating A pplying a droplet of the polymer solution to the surface of a rotating electrode; the film is formed after excess solution is spun off the surface ( 4 ) Electrochemical deposition A pplying the electrochemical deposition ; the film is formed irreversibly when the polymer is oxidized or reduced to its less soluble state. ( 5 ) Elec trochemical polymerization The polymerization occurs on the surface of the electrode by using the solution of a monomer that is oxidized or reduced to an active form. ( 6 ) Radiofrequency polymerization A radiofrequency plasma discharge is exposed to the vapor of the monomer.

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31 ( 7 ) Cross linking A copolymerization of bifunctional and polyfunctional monomers to give a film on the electrode with desired properties. In this work, the droplet evaporation method was used to make Nafion film coated electrode s that ha ve t he advantage of known concentration and droplet volume. The compound that was used in this work to make Nafion supported heterobimetallic complexes is Cp(PPh 3 I dppm)PdCl 2 ( 8 ) 78 Figure 2 4 Heterobimetallic Ru/Pd complex 8 Cyclic Voltammetry of Nafion M odified Electrodes Cyclic voltammetry was used to study the ele ctrochemistry of the glassy carbon electrode (GCE) Nafion modified electrodes under different conditions. Various attempts were done to optimize the experimental conditions for the electro oxidation of ethanol using Nafion electrode s modified by the previ ously prepared heterobimetallic complex Ru/Pd 8 by changing the electrolyte solvent combination and concentration life time of the modified electrode, heat, ethanol concentration and scan rate C yclic voltammograms were obtained using 0.1 M electrolyte, 245 mM ethanol and 50 mV/s scan rate, and GCE coated with 10 mM Nafion catalyst layer as a working electrode. High catalyst concentration (15 or 20 mM) does n o t dissolve c ompletely in the Nafion solution and thus ha s a lower catalytic activity for elect rochemical oxidation of ethanol. Figure 2 5 shows the cyclic voltammograms (CVs) of ethanol oxid ation on Na f ion blank and Nafion modified with 10 mM complex 8 electrodes a ) and b) respectively. The ethanol

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32 Figure 2 5. Cyclic voltammograms in 0.1 M TBACF 3 SO 3 / DCE Ag/Ag + reference electrode; 245 mM Ethanol; 50 mV/s a) Nafion blank b) Nafion modified electrode with 10 mM of complex 8 and c) The difference between the voltammogram in the presence and absence of ethanol for both Nafion blank and Na fion modified.

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33 oxidation current peak is recorded at 1.65 9 V vs NHE for the modified electrode. However, this peak do es n o t appear when using the Nafion blank electrode which indicates the ethanol oxidation activity of the complex 8 To explore the activ ity of the heterobimetallic complex 8 for electro oxidation of ethanol, the cyclic voltammograms in presence and absence of ethanol for both Nafion blank and Nafion modified electrodes a) and b) were subtracted to give c). This method is applied for all t he CVs in this work to differentiate between the influencing factors, and to obtain the best conditions for the activity of the heterobimetallic compounds in electrochemical oxidation of alcohols and specifically ethanol. Effect of Electrolyte and Solvent Supporting electrolyte, a solvent containing dissolved mobile ions, is the medium that all electrochemical reactions occur in, and able to support the current flow between the electrodes in the electrochemical cell. The electrolyte maintains the charge ba lance and carries 97% of the current in the bulk solution by providing the pathway for the ions to flow in the cell instead of the ones produced or consumed at electrodes during the electrochemical reaction. Therefore, it eliminates the electroactive speci es migration and decreases the uncompensated resistance drops between the working and reference electrode and thus improve s the accuracy for the potential measurements at the electrode The main required properties of this medium are high solvating power to dissolve the reactants and products from the electrode reaction, high conductivity (the ability of ions to move in an electric field), low viscosit y for rapid transport, and low reactivity with the reactants and the products. 80

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34 Figure 2 6 Cyclic voltammograms of Nafion blank and Nafion modified electrodes with 10 mM of complex 8 in 0.1 M TBACF 3 SO 3 ,TBAPF 6 and TBABF 4 / DCE, DCM, and AN, 245 mM of ethanol, 50 mV/s, and Ag/Ag + reference electrode a c b d e f

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35 Figure 2 6 Continued g h i

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36 Figure 2 6 shows the cyclic voltammograms of the 10 mM Nafion complex 8 modified GCE using 0.1 M of different electrolyte solvent combinations, t etrabutylammonium trifluoromethan esulfonate (TBACF 3 SO 3 ), tetrabutylammonium tetrafluoroborate (TBABF 4 ), and tetrabutylammonium hexafluorophosphate (TBAPF 6 ) with 1,2 dichloroethane (DCE), dichloromethane (DCM), acetonitrile (AN). As shown in Figure 2 6a, d, and g, DCE is the best solvent for the electro oxidation of ethanol (245mM), because there is large increase in the oxidation current of ethanol displayed by the Nafion modified electrode indicating oxidation activity for ethanol, while the Nafion blank doe s not yield significant oxidat ion activity. The ethanol oxidation current peak using TBACF 3 SO 3 is located at 1.65V vs. NHE, while for both of TBABF 4 and TBAPF 6 oxidation occurs a t potentials more anodic than 2.0V vs. NHE. The higher potentials for oxidation of ethanol are not favore d; they signify higher overpotentials for the electro oxidation. For the CVs of Figure 2 6b, and e using DCM both Nafion blank and modified have oxidation peaks, so it is impossible to differentiate which one comes from the catalytic a ctivity of the comp lex, while Figure 2 6 h has a distinct oxidation peak for et hanol by the modified electrode but at high potential. Finally, in Figure 2 6 c, f, and i a cetonitrile has a very high dielectric constant (35.9) as compared to 1 2 d ichloroethane, (10.37) or d ic hloromethane (8.93) which make s the conductivity of the electrolyte high and does not give the complex the chance to show its catalytic activity for the electro oxidation of ethanol In acetonitrile the CVs for both the Nafion and Nafion modified electrod es are the same. Therefore, DCE is the best solvent that can be used f o r electrochemical oxidation of ethanol using complex 8 Due to the CV of TBACF 3 SO 3 /DCE exhibit ing the highest current peak at l ow potential, this electrolyte wa s selected for the elec trochemical studies for complex 8

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37 Electrolyte s olvent c oncentration Figure 2 7 a shows the influence of the concentration of the TBACF 3 SO 3 /DCE on the intensity of the anodic oxidation current using a 0.05 0.5 M concentration rang e at 1.659 mV vs. NHE. At the low concentration range, the anodic peak current increases as the concentration of the electrolyte increase s un til 0.2 M and then a decrease is observed Addition of excess supporting electrolyte does improve the conductivity of the electrolyte sol ution. However, at concentration s more than 0.2 M, the activity of the complex 8 for the electro oxidation of ethanol dec reased Figure 2 7 a) Cyclic voltammograms and b) plot of oxidation peak currents against electrolyte concentration of Nafion modified electrode with 10 mM of complex 8 usi n g 0. 05 0.5 M TBACF 3 SO 3 /DCE, 245 mM of ethanol, 50 mV/s and Ag/Ag + reference electrode. b a

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38 So, in the high ly conductive medium, the behavior of the complex 8 for the electrochemical oxidation of ethanol change s a nd decrease d currents are observed This is similar to the CVs using the electrolyte/ a cetonitrile combination that did n o t exhibit oxidation activity because of the high conductivity of a cetonitrile Figure 2 8 a) Cyclic voltammograms of Nafion m odified electrode with 10 mM of complex 8 b) plot of oxidation peak currents against ethanol concentrations (98 489 mM), and c) Cyclic voltammograms of Nafion blank electrode usi n g 0.1 M TBACF 3 SO 3 /DCE, 50 mV/s and Ag/Ag + reference electrode. a c b

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39 Effect of Eth anol Concentration For the quantitative evaluation of the electrocatalytic activity of the complex 8 for the electrochemical oxidation of ethanol, the dependence of the current peak height on the bulk concentration of ethanol was studied. The CVs of Nafio n modif ied and Nafion blank electrodes were performed using different concen trations of ethanol (Figure 2 8 a and c). In the potential range where the electrocatalytic oxidation of ethanol occurs, the anodic currents grow as the concentration of ethanol in creases. By plotting the plot of oxidation peak currents against different ethanol concentrations, a nearly linear dependence was obtained (correlation coefficient = 0.9596), indicating that the electrolysis process is pseudo first order in this ethanol c oncentration range. The bulk ethanol concentrations have no influence on the Nafion blank electrode as shown in Figure 2 8b. Effect of S can R ate The effect of scan rate on the electrocatalytic peak current was evaluated over the 20 70 mV/s rang e (Figure 2 9) to study the kinetic s of the oxidation rea ctions. T he anodic peak current increases non linearly upon changing the scan rate. It was found that t he resulting plot of oxidation peak currents against the square root of the scan rates d isplays linearity up to 40 mV/s and then a downward deviation at higher scan rates is observed. We can conclude that rapid diffusion controlled oxidation for the ethanol o ccurs only at slow scan rates, however at higher scan rates, the fast potential sweep do es not gave t he modified electrode time to oxidize the ethanol.

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40 Figure 2 9 a) Cyclic voltammograms b) plot of oxidation peak currents against the square root of the scan rates of Nafion modified electrode with 10 mM of complex 8 usi n g 0.1 M TBACF 3 SO 3 /DCE, 245 mM of ethanol, and Ag/Ag + reference electrode. Lifetime of the E lectrode Figure 2 10 demonstrates the decay of the Nafion modified electrode with the number of scans The lifetime of a Nafion modified electrode is depend on the binding of the heterobimeta llic complex to the electrode surface and the stability of Nafion modified layer in the electrolyte. The Nafion modified layer on the electrode has an effective life after which the activity for the electrocatalytic process will decrease. CVs were perfor med using Nafion modified electrode s and repeat ed for the same electrode. During the first five scans the Nafion was dissolving and while the electrode was still working performance was decaying b a

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41 Figure 2 10 a) Cyclic voltammograms b) plot of o xidation peak currents against the number of times using the same Nafion modified electrode with 10 mM of complex 8 usi n g 0.1 M TBACF 3 SO 3 /DCE, 245 mM of ethanol, 50 mV/s and Ag/Ag + reference electrode. Applications Generally, the cyclic voltammograms of th e heterobimetallic complexes Cp(PPh 3 ) M 1 ( dppm) M 2 X 2 ( M 1 = Ru or Fe, M 2 = Pd or Pt, X= Cl, or I) in homogeneous electrocatalytic oxidation of alcohols exhibit three redox waves within the solvent window. The first and the third are attributed to the first metal M 1 (II/III) and M 1 ( III/IV) coupl es, respectively, while the second wave is assigned to the redox couple of the second metal M 2 (II/IV) The first anodic potential M 1 (II/III) depend s on the extent of electron donation to the second metal through the halogen bridge, however, the M 1 (III/IV) potentials from one complex vary C urrent at the b a

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42 second metal wave M 2 (II/IV) is increased in presence of alcohols, indicating catalytic activity of the complex for electro oxidation of alcohols 76,77,92 Figure 2 11 Structures of compounds 8 11 The heterobimetallic Ru/Pd, Ru/Pt, and Fe/Pd complexes (Figure 2 11) Cp(PPh 3 dppm)PdCl 2 ( 8 ) 92 5 C 5 H 4 CH 2 CH 2 N(CH 3 ) 2 3 dppm)PtCl 2 ( 9 ) 77 CpFe(CO dppm)PdI 2 ( 10 ) 93 and dppm)PtI 2 ( 1 1 ) 93 were previously prepared as catalysts for the electrochemical oxidation of methanol. T hese complexes were s tud ied as examples of the main types of catalysts previous ly studie d in the McElwee White group. Figures 2 12 15 a show the cyclic voltammograms for the electrochemical oxidation of methanol by complexes 8 11 C omplexes 8 and 10 display t hree waves, the first and third ones for the oxidation of the first metal (Ru and Fe) while the second one for the oxidation of the second metal ( P d) in complex es 8 and 10 respectively Complex 9 has one more wave observed due to the oxidation of the am ino moiety and complex 11 displays only two waves because the

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43 second and third waves overlap. The formal potentials for the Ru and Fe (II/III) and Ru and Fe (III/IV) couples are in the range 1.07 1.36V and 1.76 2.25V, respectively, while those for the redox couple of the second metal Pd and Pt(II/IV) is 1.56 1.91V. Nafion modified electrodes exhibit different CVs than those in homogeneous solutions. T here is only one wave that is significantly increase d in the presence of alcohol, which is ethanol in this w ork. The cyclic voltammograms of complexes 8 11 using the Nafion modified electrode (Figures 2 12 15 b) show a catalytic anodic current in 1.5 2 .0 V range. Here the advantages of using the heterogeneous electrocatalysts in the electrochemical oxidation pro cess are evident as the heterobimetallic complex in the Nafion modified film is present in a concentration 350 times less than the homogeneous system A pproximately the same catalytic activity appear s in the anodic current density for the modified electr ode and the solution In the heterogeneous process, the substrate transfers from the bulk solution and exchanges the electron with the immobilized electrocatalyst on the electrode surface, resultin g high heterogeneous electron transfer rate, amplified det ection signal, and high electrocatalytic activity. However, in homogeneous electrocatalysts, the electron transfer and chemical reactions occur in the bulk solution, and the substrate diffuses to the electrode surface, which cost s more overpotential, and result s in less electrocatalytic activity.

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44 Figure 2 12 Cyclic voltammograms with 10 mM of complex 8 a) 92 glassy carbon working electrode in 0.7 M TBACF 3 SO 3 /DCE ; 50l methanol b) Nafion modified electrode of the complex in 0.1 M TBACF 3 SO 3 /DCE, 245 mM (50l) ethanol, 50 mV/s ,and Ag/Ag + reference electrode. a b

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45 Figure 2 13 Cyclic voltammograms with 10 mM of complex 9 a) 77 5 mM, glassy carbon working electrode; 50l methanol b) 10 mM, Nafion modified electrode, 245 mM (50l) ethanol, in 0.1 M TBACF 3 SO 3 /DCE, 50 mV/s ,and Ag/Ag + r eference electrode. b a

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46 Figure 2 14 Cyclic voltammograms with 10 mM of complex 10 a) 93 5 mM, glassy carbon working electrode; 50l methanol b) 10 mM, Nafion modified electrode, 245 mM (50l) ethanol, in 0.1 M TBACF 3 SO 3 /DCE, 50 mV/s ,and Ag/Ag + reference electrode. b a

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47 Figure 2 15 Cyclic voltammograms with 10 mM of comp lex 11 a) 93 5 mM, glassy carbon working electrode; 50l methanol b) 10 mM, Na fion modified electrode, 245 mM (50l) ethanol, in 0.1 M TBACF 3 SO 3 /DCE, 50 mV/s ,and Ag/Ag + reference electrode. Bulk E lectrolysis As previously mentioned, the oxidation of methanol (Eq. 1.1 1.6) results in two and four electrons oxidation products form aldehyde and formic acid In excess methanol the se products condense and form dimethoxymethane ( DMM), and methyl formate ( MF), respectively. In the presence of water the product ratio shift s more forward (MF ) than (DMM) as shown in Figure 2 b a

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48 16. Th e bu lk electrolysis for methanol is carried out outside the glove box, then the humidity is enough to shift the electrolysis products to MF. Figure 2 16 Product distribution for the electrolysis of methanol using 10 mM of Nafion/complex 8 vitreou s carbon w orking electrode in 3.5 mL of 0.1 M TBACF 3 SO 3 /DCE 350 mM methanol, and Ag/Ag + reference electrode. For electrolysis of ethanol, ( Figure 1 2 ) the oxidation products are acetaldehyde, acetic acid, methane, and CO 2 To obtain methane and CO 2 the C C bond has to break which requires higher activation energy than C H bond cleavage. Also, in excess ethanol, the two and four electron oxidation products, acetaldehyde and acetic acid, will form 1,1 di ethoxyethane ( acetal, and ethyl acetate, respectively (Eq. 2.1 2.2) By using the complexes 8 11 for the electrolysis of ethanol, (Figure 2 17, 18, and 20), acetaldehyde and acetic acid are low concentration products because they condense in the presence of excess ethanol to the acetal and ethyl acetate, resp ectively. However, the retention time in Gas Chromatography (GC) is the same, so I could not know the ratio of acetal to ethyl acetate

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49 Figure 2 17 Product distribution for the electrolysis of ethanol using 10 mM of Nafion/complex 8 vitreou s carbon w orking electrode in 3.5 mL of 0.1 M TBACF 3 SO 3 / e thanol, 245 mM ethanol, and Ag/Ag + reference electrode. Figure 2 1 8 Product distribution for the electrolysis of e thanol using 10 mM of Nafion/complex 9 vitreous carbon working electrode in 3.5 mL of 0. 1 M TBACF 3 SO 3 / e thanol, 245 mM ethanol, and Ag/Ag + reference electrode.

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50 Figure 2 19 Product distribution for the electrolysis of ethanol using 10 mM of Nafion/complex 11 vitreous carbon working electrode in 3.5 mL of 0.1 M TBACF 3 SO 3 / e thanol, 245 mM eth anol, and Ag/Ag + reference electrode.

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51 CHAPTER 3 EXPERIMENTAL SECTION General Methods Synthesis Cp(PPh 3 I dppm)PdCl 2 ( 8 ) was synthesized according to literature procedure 78 Other catalyst compounds were obtained from Daniel Serra and Marie Correia Preparation of the Nafion Modified E lectrode The glassy carbon electrode (GCE), 3 mm diameter, was polished with 0.05 m Al 2 O 3 paste and washed ultrasonically in distilled water. The working electrode was coated with the catalyst layer from a 10 mM Nafion/catalyst solut ion with 5% wt Nafion ionomer, and 10 l of catalyst solution were pipetted onto t he glassy carbon electrode and the solvent was allowed to evaporate Electrochemistry Electrochemical experiments were performed using an EG&G PAR model 263A potentiostat/galv anostat (Princeton Applied Research) and a three compartment H cell separated by a medium porosity sintered glass frit. The Nafion modified electrode was the working electrode, a platinum flag was used as the counter electrode, and a ll potentials are refe renced to Ag/Ag + and reported versus NHE. The reference electrode consisted of a silver wire immersed in an acetonitrile solution containing freshly prepared 0.01 M AgNO 3 and 0.1 M TBA CF 3 SO 3 The Ag + solution and silver wire were contained in a 75 mm gl ass tube fitted at the bottom with a Vycor tip. The reference electrode was standardized against a 10 mM solution of ferrocene T he FeCp 2 + /FeCp 2 potential is 0. 558 V vs. NHE in 0.1 M ( n Bu) 4 NClO 4 in 1,2 dichloroethane, with a

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52 scan rate of 1 00 mV/s 94 C yclic voltammograms were recorded in 3.5 mL of 0.1 M T BACF 3 SO 3 / DCE at ambient temperature over the potential range 0 2 .5 V vs. Ag/Ag + Product Analysis Bulk electrolysis was performed using a three compartment H cell in 3.5 mL of 0.1 M TBACF 3 SO 3 /DCE for electrolysis of 350 mM methanol, and in 3.5 mL of 0.1 M TBACF 3 SO 3 / e thanol, for electrolysis of 245 mM M ethanol inside a glove box at room temperature under nitrogen. Nafion modified vitreous carbon (0.5m L of 10 mM Nafion/catalyst solution with 5% wt Nafion ionomer w as pipetted onto the vitreous carbon electr ode and the solvent was allowed to evaporate ) was used as a working electrode P latinum foil was used as the counter electrode, and an Ag/Ag + electrode was the reference Electrolyses were allowed to continue until the desired number of coulombs was reac hed, t he alcohol oxidation products from the bulk electrolysis were analyzed by gas chromatography on a Shimadzu GC 17A chromatograph. For methanol, the column was a 15 m 0.32 mm column of AT WAX (Alltech, mm AT Wax deactivated guard column and n heptane was used as an internal standard For ethanol, the column was a 15 m 0.45 mm of ECTM WAX ( Alltech ) attache d to the injection port with a u niversal guard column D odecane was used as an internal standard Product identification was confirmed by comparison of the retention times of the oxidation product with authentic samples

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59 BIOGRAPHICAL SKETCH Ah med Moghieb was born in Kuwait i n 1977 but was raised in Egypt, where his family is from. In 2000, he received Bachelor of Sc ience in c hemistry from Cairo University, Egypt. Then he took master courses in in organic chemistry at Beni sueif University Egypt These courses have broadened his understanding of in organic chemistry. As a reward from the Egyptian government for gra duating at the top of his class, Ahmed was nominated to work at the National Research Centre (NRC) Egypt as a chemical specialist in 2001. Then, due to his hard work and the recommendations of his supervisors, he was promoted to be a r esearch a ssistant in the E lectro A nalytical R esearch L ab in 2003 In 2007, he was granted a Ford Foundation Fellowship for two years to pursue his m