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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 REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
2009
2
© 2009 Ahmed Moghieb
3
To my God who created us for the betterment of the entire world
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 strong support and encouragement. I always
admire and respect her as a research mentor, under 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 (IIE) 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. Finally, I am especially thankful for the patience and
support of my wife, Marwa.
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-Oxidation 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 Alcohols ...........................................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
Electrocatalysis Using Chemically Modified Electrodes (CMEs) ..................................27 Polymer Modified Electrodes ..........................................................................................27
Electrodes Modified with Heterobimetallic Complexes .........................................................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 of Ethanol Concentration ......................................................................................39 Effect of Scan Rate ..........................................................................................................39 Lifetime of the Electrode .................................................................................................40 Applications .....................................................................................................................41
Bulk Electrolysis .............................................................................................................47
3 EXPERIMENTAL SECTION ................................................................................................51
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
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
8
LIST OF FIGURES
Figure page
1-1 A schematic representation of Direct Ethanol/Oxygen 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 of 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 TBACF3SO3/ 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 Nafion modified. ................................32
2-6 Cyclic voltammograms of Nafion blank and Nafion modified electrodes with 10 mM
of complex 8, in 0.1 M TBACF3SO3,TBAPF6,and TBABF4/ 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 voltammograms 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 TBACF3SO3/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
9
c) Cyclic voltammograms of Nafion blank electrode using 0.1M TBACF3SO3/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 electrode with 10 mM of complex 8 using 0.1M
TBACF3SO3/DCE, 245 mM of ethanol, and Ag/Ag+ reference electrode. .......................40
2-10 a) Cyclic voltammograms, b) plot of oxidation peak currents against the number of
times using the same Nafion modified electrode with 10 mM of complex 8 using
0.1M TBACF3SO3/DCE, 245 mM of ethanol, 50 mV/s ,and Ag/Ag+ reference
electrode. ............................................................................................................................41
2-11 Structures of compounds 8-11. ............................................................................................42
2-12 Cyclic voltammograms with 10 mM of complex 8 a) glassy carbon working
electrode in 0.7 M TBACF3SO3/DCE; 50µl methanol b) Nafion modified electrode
of the complex in 0.1M TBACF3SO3/DCE, 245 mM (50µl) 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; 50µl methanol b) 10 mM, Nafion modified electrode, 245 mM (50µl)
ethanol, in 0.1M TBACF3SO3/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; 50µl methanol b) 10 mM, Nafion modified electrode, 245 mM (50µl)
ethanol, in 0.1M TBACF3SO3/DCE, 50 mV/s ,and Ag/Ag+ reference electrode. .............46
2-15 Cyclic voltammograms with 10 mM of complex 11 a) 5 mM, glassy carbon working
electrode; 50µl methanol b) 10 mM, Nafion modified electrode, 245 mM (50µl)
ethanol, in 0.1M TBACF3SO3/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 TBACF3SO3/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 TBACF3SO3/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 TBACF3SO3/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 TBACF3SO3/Ethanol, 245
mM ethanol, and Ag/Ag+ reference electrode. ..................................................................50
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 ETHANOL 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 on the electrode surface. The electrochemical oxidation of ethanol has been
studied at a Nafion/ Cp(PPh3)Ru(μ-I)(μ-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).
11
CHAPTER 1
LITERATURE REVIEW
Electro-Oxidation 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 cells (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 Cells (DAFCs), has advantages,
compared to pure hydrogen because they are liquids (easily handled, stored and transported).
Moreover, their theoretical mass energy densities (6.1 and 8.0 kWh kg-1
corresponding to 6 and
12 electrons 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
DAFCs are attractive as power sources for mobile, stationary, 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 by an electrolyte, protonic conductor. At the anode
the 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 schematic representation of a Direct Ethanol/Oxygen Fuel Cell (DEFC) is shown in
Figure 1-1 as an example for fuel cells.
12
Figure 1-1. A schematic representation of Direct Ethanol/Oxygen Fuel Cell (adapted from ref 4).
The direct oxidation of methanol in fuel cells has been widely investigated, as methanol
has been considered the most promising fuel because it is more efficiently oxidized than other
alcohols. However, there are still 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 began to be considered as alternative fuels. Among organic
small molecule alcohols, ethanol has been subjected to electro-oxidation 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 renewable 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 alcohols under fuel cell operating
conditions, since ethanol has a higher electrochemical activity to oxidize to CO2.
However, the complete oxidation of ethanol to CO2 is more difficult than that of methanol
due to the difficulty of breaking the C–C bond at low temperatures. High yields of partial
13
oxidation products, CH3CHO and CH3COOH are produced at Pt catalysts15
and decrease the
performance of the fuel cell. So, there is a need to improve the activity of anode electrocatalysts
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
platinum 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 improvement of the activity and
endurance of platinum electrodes by decreasing the CO poisoning. Addition of a second element
to form a bimetallic catalyst promotes the electrocatalytic activity of pure platinum by lowering
the overpotential for alcohol electro-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 improvement is attributed to two mechanisms:
1) The 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 oxygen-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-
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
material 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 activity 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-WO3,45
Pt-Ru-Os-Ir,46
and Pt-Pd-Ru-Os.47
Mechanism of Ethanol Electro-Oxidation Using Heterogeneous Catalysts
Numerous studies on the electro-oxidation of ethanol have 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-oxidation of ethanol were
provided using various techniques, such as chromatography (HPLC,GC),6,48,49
differential
electrochemical mass spectrometry (DEMS), 12,18,50
and in situ Fourier transform infrared
spectroscopy (FTIRS).51-53
For the electrocatalytic oxidation of isotopically labeled ethanol on
Pt electrodes in acid medium, DEMS studies identified acetaldehyde and CO2 as primary
reaction products. It showed that by cleavage of the C-H bond on the α-carbon and the O-H
bond in the hydroxyl group, the acetaldehyde is formed, whereas CO2 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 ethane 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.
15
Generally, it is known that ethanol has two reactive sites which can interact with the metal
catalyst during the adsorption processes, the α-carbon atom and the OH group. The ethanol can
be adsorbed through any of these groups after the breaking 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 medium.
According to one of the generally accepted reaction mechanisms given in Fig. 1-2,54
at a
potential less than 0.8 V/NHE, the ethanol is adsorbed by the α-carbon at the surface of
platinum. This adsorption involves a C-H cleavage with the transfer of one electron (1), and then
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
H2O 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
desorb as CH4 at E < 0.2 V/NHE (4) or oxidize to carbon dioxide via the surface reaction of
adsorbed CO species and adsorbed OH (coming from the dissociation of H2O) at E > 0.5 V/NHE
(5)
Transition Metal Complex Catalysts
General Principle of Electrocatalysis
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 electrochemical 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 heterogeneous 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 electron transfer and chemical reactions occur in the bulk solution, and the
substrate diffuses to the electrode surface.55
Ruthenium Oxo Complexes as Electrochemical Catalysts for Alcohol Oxidation
The redox chemistry of ruthenium 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 ruthenium complexes have been applied with success in
electrocatalytic oxidation of different organic compounds.56-58
17
Meyer and co-workers reported the first example of using the ruthenium-oxo complexes
for the oxidation of various substrates including alkenes. The strong oxidant Ru(IV) oxo
complex ion, (bpy)2pyRuO2+
, (bpy = 2,2-bipyridine, py = pyridine), can be generated from the
corresponding Ru(II) aquo complex via proton-coupled electron transfer as shown in Scheme
1.56
Scheme 1-1.
The system involves a net 2e− oxidation in aqueous solution and the resulting Ru
IV=O
complex in a buffered solution is rapidly reduced to [RuII(bpy)2(py)(OH2)]
2+ when an
unsaturated organic compound is added. Both the RuII/Ru
III and Ru
III/Ru
IV couples are reversible,
thus the oxidation 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 [RuIV
(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 incorporation of a metal catalyst
into the electrode (heterogenized homogeneous) which could give a significant improvement.
The ruthenium-aquo species [Ru(4,4’-Me2bpy)2(AsPh3)(H2O)](ClO4)2 (4,4’-Me2bpy =
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 was
18
performed using the ruthenium(II) [Ru(cis-L)(totpy)(H2O)](PF6)2 and [Ru(trans-
L)2(totpy)(H2O)](PF6)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 complexes containing phosphines and polypyridine as ligands
[Ru(L)(totpy)(H2O)](PF6)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, cyclohexene, 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 involve 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 aerobic Heck coupling,69
the oxidation of secondary alcohols,70
ethylene
19
polymerization,71
and olefin hydroformylation.72
The presence of different types of metal
centers close to each other within the same molecule in the heterobimetallic catalysts affects their
reactivity, gives significant properties different from their mononuclear analogues, and can lead
to higher catalytic activity.73 Also, the 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 on the development of
heterobimetallic 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(PPh3)Ru(μ-Cl)(μ-dppm)PtCl2 (1),75
Cp(PPh3)Ru(μ-Cl)(μ-dppm)PdCl2 (2),76
and
Cp(PPh3)RuCl(μ-dppm) AuCl (3).76
Synthesis
Complexes 1-3 were synthesized at room temperature by reacting CpRu(PPh3)(η1-dppm)Cl
(4) with Pt(COD)Cl2, Pd(COD)Cl2 and Au(PPh3)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, several attempts have been carried out by
the McElwee-White research group to modify these complexes and synthesize water soluble
heterobimetallic, ammonium substituted cyclopentadienyl complexes.
Examples, include the heterobimetallic Ru/Pd complexes [η5-C5H4CH2CH2N(CH3)2
HI]Ru(PPh3)(μ-I)(μ-dppm)PtI2 (6),77
and [η5-C5H4CH2CH2N(CH3)2 HI]Ru(PPh3)(μ-I)(μ-
dppm)PdCl2 (7).77
Complexes 6 and 7 were synthesized by reaction of [η5-
C5H4CH2CH2N(CH3)2 HI]Ru(PPh3)(I)(Κ1-dppm) (5) with Pd(COD)Cl2 in dichloromethane at
room temperature (Scheme 1-4).
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 chance for the two metal centers to be in close proximity and facilitate the
electronic interaction between them. The metal-metal interactions, electron donation between
21
the metals takes 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(PPh3)Ru(μ-Cl)(μ-
dppm)PdCl2 (2) shows a 730 mV positive shift for the Ru(II/III) waves compared to the
monomeric compound CpRu(PPh3)(Κ1-dppm)Cl (4) as a result of electron donation from the Ru
to Pd through the chloride bridge. However, the cyclic voltammogram for Cp(PPh3)RuCl(μ-
dppm)AuCl (3) shows only a 30 mV positive shift for the Ru(II/III) waves compared to
CpRu(PPh3)(Κ1-dppm)Cl, indicating that the dppm bridge has no effect in the electronic
interaction between the two metals in contrast to the halogen one.76
The cyclic voltammograms
for the Ru/Pd compounds (6) and (7) each exhibit irreversible oxidation at 1.35 V and 1.00 V vs.
NHE, respectively, while the mononuclear complex (5) is at 1.05 V. This potential shift
indicated 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 E1/2(V)
a Couple E1/2 (V)
a Couple E1/2(V)
a
Ru/Pt (1) Ru(II/III) 1.13b Pt(II/IV) 1.78
c Ru(III/IV)
Ru/Pd (2) Ru(II/III) 1.29 Pd(II/IV) 1.45c Ru(III/IV)
Ru/Au (3) Ru(II/III) 0.89 Au(II/IV) 1.42c 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) 1.67c Ru(III/IV) 2.04
Ru/Pd (7) Ru(II/III) 1.00 Pd(II/IV) 1.57c Ru(III/IV) 2.09
a All potentials obtained in DCE/TBAT (tetrabutylammonium triflate) and reported vs NHE and referenced to
Ag/Ag+.
bPerformed in CH2Cl2/TBAH (tetrabutylammonium hexafluoro phosphate).
cIrreversible wave, Epa
reported.
22
Cyclic voltammetry of complexes 1,2 and 6 in the presence 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 electrode; 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.
23
Figure 1-5. Cyclic voltammograms of 6
77 under nitrogen in 3.5 mL of DCE/0.1 M TBAT
(tetrabutylammonium triflate); glassy carbon working electrode; Ag/Ag+ reference
electrode.
Product Detection
The electro-oxidation process of methanol produces formaldehyde and formic acid, as
detected by on-line FTIR spectroscopy analysis of the product of on a platinum – ruthenium
catalyst in a prototype direct –methanol fuel cell.79
However, neither was observed in the
reaction mixtures. Dimethoxymethane (DMM) and methyl formate (MF) were detected instead.
It is reported that oxidation of methanol takes place via multistep mechanism. Methanol
oxidizes to formaldehyde, then in excess methanol forms dimethoxymethane or is completely
electrochemically oxidized to CO2 (Eq. 1.1- 1.3)
24
Also, methanol oxidation can go through a four – electron electro-oxidation to formic acid,
which in excess methanol forms methyl formate or is further electrochemically oxidized to CO2
(Eq. 1.4-1.6)
Finally, methanol can go through a six – electron electro-oxidation to form CO2 (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, at 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. The tendency toward production of
(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 DMM 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).
25
Table 1-2. Product Distributions and Current Efficiencies for the Electrochemical Oxidation of
Methanol by 1, 2, and 6.
Charge / C
Product ratio [μmoles of CH2(OCH3)2 (DMM)/ HCOOCH3(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
18.6 24.6
Current Efficiency (%)d
after 200 C
44
Electrolyses were performed at 1.7a
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 standard. Each ratio is
reported as an average of 2-5 experiments. dAverage current efficiencies after 75-130 C of charge passed.
26
CHAPTER 2
CHEMICALLY MODIFIED ELECTRODES CONTAINING 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 rapidly 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 the 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 electrode to enhance
performance or achieve a specialized function)80
to enhance the electrochemical properties of the
interface.
Figure 2-1. Chemically Modified Electrodes (adapted from ref 80).
27
Electrocatalysis Using 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 assists its
reduction or oxidation.82
Using CMEs in the electrocatalysis lowers the overpotential of the
heterogeneous electron transfer of the target analyte between the electrode that is derivatized by
the electrocatalyst and the solution. This amplifies the detected signal with respect to that at a
bare electrode.81
The porous structure and high surface area of conducting polymers provide the opportunity
for the immobilization of metal catalysts and development of new catalytic and electrocatalytic
materials. High electrical 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 development of suitable electrode
materials for alcohol oxidation for possible application in fuel cells. Conducting polymers
possess both protonic and electronic conductivity. B. Rajesh84
and co-workers reported that the
use of Pt nanoparticles-loaded conducting hybrid material based on polyaniline and V2O5 as an
electrode exhibited high electrocatalytic activity and stability for electrooxidation of methanol.
Also, Maiyalagan85
reported that poly(o-phenylenediamine) (PoPD) nanotube electrodes give
significant improvement for the catalytic activity of platinum nanoparticles 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, for
example, charge transport or electron transfer mediation reactions.86
28
At least three processes occur during the electrocatalytic conversion of the analyte at the
modified polymer 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 electrical 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 solution.
One of the 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, the result is a layer with the ability to
transport electrons.87
For example, Nafion® can be immobilized on the electrode surface through
drop-casting, spin-coating, or dip-coating. Then the desired redox ion (the charged electroactive
ion) is ion-exchanged into the polymer (see Figure 2-2, Filled circles depict the reduced form of
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 filled with free redox molecules.
29
Nafion®1
, discovered in the late 1960s by Walther Grot of DuPont, is a perfluorosulfonated
polymer consisting of poly-tetrafluoroethylene (PTFE) backbone and perfluoroether side chain
with a sulfonate group. Due to its high proton conductivity, thermal and mechanical stability, it
is commonly used as solid polymer electrolyte for fuel cells.89
As seen in Figure 1.6,90
the three phase model is based on a three phase clustered system.
The three regions consist of the hydrophobic fluorocarbon (PTFE) backbone (A), an interfacial
zone containing some pendant side chains, some water, sulfate or carboxylic groups (B) and the
hydrophilic ionic zone where the ionic exchange sites, counterions and absorbed water exist in
majority (C)
Figure 2-3. (a): Structure of Nafion
Figure 2-3. (b): Three-phase model of Nafion (adapted from ref 90).
1 Nafion
® is a registered trademark of E.I. DuPont de Nemours & Co
30
Electrodes Modified with Heterobimetallic Complexes
Homogeneous electrocatalysts 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 active 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 catalysts in a heterogeneous matrix.91
Preparation of Polymer Modified Electrodes
As classified by Drust,81
polymer film-coated electrodes may be subdivided into seven
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; the film is formed by adsorption.
(2) Droplet evaporation- Applying a droplet of the polymer solution to the surface of the
electrode; the film is formed by evaporation of the solvent.
(3) Spin coating- Applying 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–Applying the electrochemical deposition; the film is
formed irreversibly when the polymer is oxidized or reduced to its less soluble state.
(5) Electrochemical 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.
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
electrodes that have the advantage of known concentration and droplet volume. The compound
that was used in this work to make Nafion supported heterobimetallic complexes is
Cp(PPh3)Ru(μ-I)(μ-dppm)PdCl2 (8).78
Figure 2-4. Heterobimetallic Ru/Pd complex 8.
Cyclic Voltammetry of Nafion Modified Electrodes
Cyclic voltammetry was used to study the electrochemistry 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 electrodes
modified by the previously prepared heterobimetallic complex Ru/Pd 8, by changing the
electrolyte-solvent combination, and concentration, lifetime of the modified electrode, heat,
ethanol concentration and scan rate. Cyclic 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 not
dissolve completely in the Nafion solution, and thus has a lower catalytic activity for
electrochemical oxidation of ethanol.
Figure 2-5 shows the cyclic voltammograms (CVs) of ethanol oxidation on Nafion blank
and Nafion modified with 10 mM complex 8 electrodes a) and b) respectively. The ethanol
32
Figure 2-5. Cyclic voltammograms in 0.1 M TBACF3SO3/ 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 Nafion modified.
33
oxidation current peak is recorded at 1.659V vs NHE for the modified electrode. However, this
peak does not appear when using the Nafion blank electrode, which indicates the ethanol
oxidation activity of the complex 8. To explore the activity 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 the 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 balance 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 species migration and decreases the uncompensated resistance drops
between the working and reference electrode and thus improves 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 viscosity for rapid transport, and low reactivity
with the reactants and the products.80
34
Figure 2-6. Cyclic voltammograms of Nafion blank and Nafion modified electrodes with 10 mM of complex 8, in 0.1 M
TBACF3SO3,TBAPF6, and TBABF4/ DCE, DCM, and AN, 245 mM of ethanol, 50 mV/s, and Ag/Ag+ reference electrode
a c b
d e f
35
Figure 2-6. Continued.
.
g h i
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, tetrabutylammonium
trifluoromethanesulfonate (TBACF3SO3), tetrabutylammonium tetrafluoroborate (TBABF4), and
tetrabutylammonium hexafluorophosphate (TBAPF6) 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 does not yield significant oxidation activity. The
ethanol oxidation current peak using TBACF3SO3 is located at 1.65V vs. NHE, while for both of
TBABF4 and TBAPF6, oxidation occurs at potentials more anodic than 2.0V vs. NHE. The
higher potentials for oxidation of ethanol are not favored; 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 activity of the complex, while Figure 2-6h has a distinct oxidation peak for ethanol by
the modified electrode, but at high potential. Finally, in Figure 2-6c, f, and i, acetonitrile has a
very high dielectric constant (35.9) as compared to 1,2-dichloroethane, (10.37) or
dichloromethane (8.93) which makes 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 electrodes are the same.
Therefore, DCE is the best solvent that can be used for electrochemical oxidation of ethanol
using complex 8. Due to the CV of TBACF3SO3/DCE exhibiting the highest current peak at low
potential, this electrolyte was selected for the electrochemical studies for complex 8.
37
Electrolyte-solvent concentration
Figure 2-7a shows the influence of the concentration of the TBACF3SO3/DCE on the
intensity of the anodic oxidation current, using a 0.05 – 0.5 M concentration range at 1.659 mV
vs. NHE. At the low concentration range, the anodic peak current increases as the concentration
of the electrolyte increases until 0.2 M and then a decrease is observed. Addition of excess
supporting electrolyte does improve the conductivity of the electrolyte solution. However, at
concentrations more than 0.2 M, the activity of the complex 8 for the electro-oxidation of ethanol
decreased.
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 using 0.05-
0.5M TBACF3SO3/DCE, 245 mM of ethanol, 50 mV/s , and Ag/Ag+ reference
electrode.
b
a
38
So, in the highly conductive medium, the behavior of the complex 8 for the
electrochemical oxidation of ethanol changes and decreased currents are observed. This is
similar to the CVs using the electrolyte/acetonitrile combination that did not exhibit oxidation
activity because of the high conductivity of acetonitrile
Figure 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 c)
Cyclic voltammograms of Nafion blank electrode using 0.1M TBACF3SO3/DCE, 50
mV/s, and Ag/Ag+ reference electrode.
a
c
b
39
Effect of Ethanol 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 Nafion modified and Nafion blank electrodes
were performed using different concentrations of ethanol (Figure 2-8a and c). In the potential
range where the electrocatalytic oxidation of ethanol occurs, the anodic currents grow as the
concentration of ethanol increases. 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
concentration range. The bulk ethanol concentrations have no influence on the Nafion blank
electrode as shown in Figure 2-8b.
Effect of Scan Rate
The effect of scan rate on the electrocatalytic peak current was evaluated over the 20-70
mV/s range (Figure 2-9) to study the kinetics of the oxidation reactions. The anodic peak current
increases non-linearly upon changing the scan rate. It was found that the resulting plot of
oxidation peak currents against the square root of the scan rates displays 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 occurs only at slow scan rates, however at higher
scan rates, the fast potential sweep does not gave the modified electrode time to oxidize the
ethanol.
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 using 0.1M
TBACF3SO3/DCE, 245 mM of ethanol, and Ag/Ag+ reference electrode.
Lifetime of the Electrode
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
heterobimetallic 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 performed using Nafion
modified electrodes and repeated 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
41
Figure 2-10. a) Cyclic voltammograms, b) plot of oxidation peak currents against the number of
times using the same Nafion modified electrode with 10 mM of complex 8 using
0.1M TBACF3SO3/DCE, 245 mM of ethanol, 50 mV/s, and Ag/Ag+ reference
electrode.
Applications
Generally, the cyclic voltammograms of the heterobimetallic complexes Cp(PPh3)M1(μ-
I)(μ-dppm)M2X2 (M1= Ru or Fe, M2= 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 M1(II/III) and M1(III/IV) couples, respectively, while the
second wave is assigned to the redox couple of the second metal M2(II/IV). The first anodic
potential M1(II/III) depends on the extent of electron donation to the second metal through the
halogen bridge, however, the M1(III/IV) potentials from one complex vary. Current at the
b
a
42
second metal wave M2(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(PPh3)Ru(μ-
I)(μ-dppm)PdCl2 (8),92
[η5-C5H4CH2CH2N(CH3)2•HI]Ru(PPh3)(μ-I)(μ-dppm)PtCl2 (9).
77
CpFe(CO)(μ-I)(μ-dppm)PdI2 (10)93
and CpRu(CO)(μ-I)(μ-dppm)PtI2 (11)93
were previously
prepared as catalysts for the electrochemical oxidation of methanol. These complexes were
studied as examples of the main types of catalysts previously studied in the McElwee-White
group.
Figures 2-12-15a show the cyclic voltammograms for the electrochemical oxidation of
methanol by complexes 8-11. Complexes 8 and 10 display three 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 (Pd) in complexes 8 and 10, respectively. Complex 9 has one more wave observed
due to the oxidation of the amino moiety and complex 11 displays only two waves because the
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.
There is only one wave that is significantly increased in the presence of alcohol, which is ethanol
in this work. The cyclic voltammograms of complexes 8-11 using the Nafion modified electrode
(Figures 2-12-15b) show a catalytic anodic current in 1.5-2.0V range. Here the advantages of
using the heterogeneous electrocatalysts in the electrochemical oxidation process are evident, as
the heterobimetallic complex in the Nafion modified film is present in a concentration 350 times
less than the homogeneous system. Approximately the same catalytic activity appears in the
anodic current density for the modified electrode 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, resulting high heterogeneous electron transfer rate,
amplified detection 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 costs more overpotential, and results in less
electrocatalytic activity.
44
Figure 2-12. Cyclic voltammograms with 10 mM of complex 8 a)92
glassy carbon working
electrode in 0.7 M TBACF3SO3/DCE; 50µl methanol b) Nafion modified electrode of
the complex in 0.1M TBACF3SO3/DCE, 245 mM (50µl) ethanol, 50 mV/s ,and
Ag/Ag+ reference electrode.
a
b
45
Figure 2-13. Cyclic voltammograms with 10 mM of complex 9 a)77
5 mM, glassy carbon
working electrode; 50µl methanol b) 10 mM, Nafion modified electrode, 245 mM
(50µl) ethanol, in 0.1M TBACF3SO3/DCE, 50 mV/s ,and Ag/Ag+ reference electrode.
b
a
46
Figure 2-14. Cyclic voltammograms with 10 mM of complex 10 a)93
5 mM, glassy carbon
working electrode; 50µl methanol b) 10 mM, Nafion modified electrode, 245 mM
(50µl) ethanol, in 0.1M TBACF3SO3/DCE, 50 mV/s ,and Ag/Ag+ reference electrode.
b
a
47
Figure 2-15. Cyclic voltammograms with 10 mM of complex 11 a)93
5 mM, glassy carbon
working electrode; 50µl methanol b) 10 mM, Nafion modified electrode, 245 mM
(50µl) ethanol, in 0.1M TBACF3SO3/DCE, 50 mV/s ,and Ag/Ag+ reference electrode.
Bulk Electrolysis
As previously mentioned, the oxidation of methanol (Eq. 1.1- 1.6) results in two and four –
electrons oxidation products, formaldehyde and formic acid. In excess methanol, these products
condense and form dimethoxymethane (DMM), and methyl formate (MF), respectively. In the
presence of water, the product ratio shifts more forward (MF) than (DMM) as shown in Figure 2-
b
a
48
16. The bulk 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 vitreous carbon working electrode in 3.5 mL of 0.1 M
TBACF3SO3/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 CO2. To obtain methane and CO2, 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-diethoxyethane (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, respectively. However, the retention
time in Gas Chromatography (GC) is the same, so I could not know the ratio of acetal to ethyl
acetate.
49
Figure 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 TBACF3SO3/ethanol, 245
mM ethanol, and Ag/Ag+ reference electrode.
.
Figure 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 TBACF3SO3/ethanol, 245
mM ethanol, and Ag/Ag+ reference electrode.
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 TBACF3SO3/ethanol, 245
mM ethanol, and Ag/Ag+ reference electrode.
51
CHAPTER 3
EXPERIMENTAL SECTION
General Methods
Synthesis
Cp(PPh3)Ru(μ-I)(μ-dppm)PdCl2 (8) was synthesized according to literature procedure.78
Other catalyst compounds were obtained from Daniel Serra and Marie Correia.
Preparation of the Nafion Modified Electrode
The glassy carbon electrode (GCE), 3 mm diameter, was polished with 0.05 µm Al2O3
paste and washed ultrasonically in distilled water. The working electrode was coated with the
catalyst layer from a 10 mM Nafion/catalyst solution with 5% wt Nafion ionomer, and 10µl of
catalyst solution were pipetted onto the glassy carbon electrode and the solvent was allowed to
evaporate.
Electrochemistry
Electrochemical experiments were performed using an EG&G PAR model 263A
potentiostat/galvanostat (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 all potentials are referenced 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 AgNO3 and 0.1 M TBACF3SO3.
The Ag+ solution and silver wire were contained in a 75 mm glass tube fitted at the bottom with
a Vycor tip.
The reference electrode was standardized against a 10 mM solution of ferrocene. The
FeCp2 +/FeCp2 potential is 0.558 V vs. NHE in 0.1M (n-Bu)4NClO4 in 1,2-dichloroethane, with a
52
scan rate of 100 mV/s.94
Cyclic voltammograms were recorded in 3.5 mL of 0.1 M
TBACF3SO3/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
TBACF3SO3/DCE, for electrolysis of 350 mM methanol, and in 3.5 mL of 0.1 M
TBACF3SO3/ethanol, for electrolysis of 245 mM M ethanol inside a glove box at room
temperature under nitrogen. Nafion modified vitreous carbon (0.5mL of 10 mM Nafion/catalyst
solution with 5% wt Nafion ionomer was pipetted onto the vitreous carbon electrode and the
solvent was allowed to evaporate) was used as a working electrode. Platinum 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 reached, the 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,
0.5 μm film) on fused silica which was attached to the injection port with a neutral 5 m × 0.32
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, 1.0 μm film) attached to
the injection port with a universal guard column. Dodecane was used as an internal standard.
Product identification was confirmed by comparison of the retention times of the oxidation
product with authentic samples.
53
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BIOGRAPHICAL SKETCH
Ahmed Moghieb was born in Kuwait in 1977 but was raised in Egypt, where his family is
from. In 2000, he received Bachelor of Science in chemistry from Cairo University, Egypt.
Then, he took master courses in inorganic chemistry at Beni-sueif University, Egypt. These
courses have broadened his understanding of inorganic chemistry. As a reward from the
Egyptian government for graduating 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 research assistant in
the Electro-Analytical Research Lab in 2003. In 2007, he was granted a Ford Foundation
Fellowship for two years to pursue his master’s degree in University of Florida, Florida, USA.
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