I

General introduction

"Try not to become a man of success but rather try to become a man of

value.”

~Albert Einstein

1

Chapter _ I

1.1. KINETIC STUDIES

iving organisms are all chemically reacting systems depending on the

continuance of oxidations and reductions for plants proliferate by using radiant

energy to reduce carbon dioxide whilst animals obtain their vital energy by the

oxidation of carbon compounds. Thus, the comprehension of mechanisms of

oxidation and reduction processes is essential to the understanding of the nature

of life and it is a task for the chemists to provide a clear picture of way in

which these chemical changes may take place with simple molecules, so that

biologists have a sure basis for their understanding of these behavior of

complex organisms.

Man knew chemical reactions long before chemistry had attained the

status of science and sensed that substances changed their properties under

certain external conditions, which is a characteristic of chemical reactions.

Kinetic studies are receiving much importance in the recent years. Since, it is

such a part of science that deals with the rate of chemical reaction, with all

factors, which influence the rate of reaction, and with the explanation of the

rate in terms of the reaction mechanism. To many chemists the real heart of

chemistry is the study of mechanisms. Thus, chemical kinetics can be defined

as that branch of chemistry concerned with the study and prediction of time

dependent systems.

Chemical kinetics with its dynamic viewpoint may be contrasted with

thermodynamics with its static viewpoint. Thermodynamics, a powerful tool

L

K

2

Chapter _ I

for investigating the conditions at equilibrium concerns mainly on the state of

equilibrium. Kinetics is concerned fundamentally with the details of the

process in which a system changes from state to other in a definite transition

state. The kinetic data will be the source of a great deal of detailed insight into

the mechanism of a reaction. Although, other types of experimental evidences

are also sought for purpose of formulating a reaction mechanism, the study of

reaction kinetics generally forms the backbone of a thorough mechanistic

investigation. Finally, as an area of pure science in itself, the study of rates and

mechanism is one of rich varieties, concerned with the chemistry of every

element and full of experimental challenges.

To understand the mechanism of any reaction we must know a reaction

as a function of time. The order of a reaction can be used to interpret the

reaction on molecular level. Chemists propose reaction mechanism by

predicting the sequence in which bonds break and atoms rearrange during the

reaction by considering the order of a reaction with respect to different reactive

species. Virtually all information regarding reaction mechanism comes by

inference of indirect evidence. Hence, it is the important job of chemists to

device the proper experiments to generate most conclusive confirmation. The

crucial steps in any kinetic investigation1 are: (1) product and intermediate

detection, (2) concentration determination of all species present, (3) deciding

on a method of following the rate, (4) the kinetic analysis and (5) determination

of the mechanism.

3

Chapter _ I

The award of Nobel Prize for the year 1992 to Prof. R. A. Marcus on the

“Electron Transfer Reactions”, 1999 to Prof. Ahmed Zewail for discovery in

“Femtochemistry”, Nobel Prize 2001 to Profs. William Knowles, K. Barry

Sharpless and Royji Noyori for their work on “Chirally Catalyzed

Hydrogenation Reactions” and Nobel Prize for the year 2005 to Profs. Robert

Grubbs, Richard Schrock, and Yves Chauvin on their research achievements on

“Metathesis Catalyst Technology” emphasize the importance of field of

reaction kinetics. Electron transfer reactions play a significant role in physical,

chemical and biological processes. Because of the ubiquity of electron transfer

processes, the study of electron transfer reactions, perhaps more so than that of

any other area of chemistry is characterized by a strong interplay of theory and

experiment2. The importance of electron transfer in transition metal redox

chemistry and in organic chemistry is well documented.3

The work of Henry Taube4 in redox systems unequivocally

demonstrated the transport of electron from reductant to oxidant. This

discovery certainly added many important features in the syntheses of

coordination complexes and organometallics. An oxidation process is always

accompanied by a reduction process;5 such reactions are called redox reactions.

Therefore, oxidation-reduction reaction needs at least two reactants, one

capable of gaining electrons (oxidant) and the other capable of losing electrons

(reductant), i.e., a reducing agent (reductant) by losing electrons, gets oxidized

and an oxidizing agent (oxidant) by gaining the electrons gets reduced. Redox

4

Chapter _ I

reactions are the basis for numerous biochemical pathways and cellular

chemistry, biosynthesis, and regulation.6

1.1.1. Oxidation-reduction in inorganic reactions

Two general classes of transition states emerge for redox reactions

involving metal complexes, the so called “outer-sphere and inner-sphere”

types.7 In outer sphere, electron transfer event occurs between chemical species

that remain separate intact before, during, and after the electron transfer

occurrence, whereas, in the inner sphere electron transfer the participating

redox sites undergoing electron transfer become connected by a chemical

bridge. Since in outer sphere electron transfer occurs between two non-

connected species, the electron is forced to move through space from

one redox center to the other. From Franck-Condon principle, it follows that

before electron transfer between two ions is possible, the energy of the electron

must be the same in the two sites. There must also be sufficient orbital overlap

between the two sites to provide for a reasonable probability of a transfer.

1.1.2. Oxidation-reduction in organic reactions

The oxidation-reduction concepts, however, are not so clearly applicable

in organic chemistry, when carbon compounds are oxidized their component

5

Chapter _ I

atoms are very seldom deprived of their surrounding complete electron shells.

Covalent bond fission is an essential feature of organic reactions and it can be

affected by two different pathways,7 viz., “Homolytic reactions” in which

electron pairs are symmetrically disrupted and “Heterolytic reactions” in

which electron pairs are transferred from one molecule to another as an

undivided entity. Electron removal by these two pathways has clearly

distinguishable characteristics.

1.1.3. Probable ways of electron transfer reactions

Oxidation-reduction reaction may involve one or more electron transfer.

Depending upon the number of electrons transferred between oxidant and

reductant, the reaction may proceed in one or more steps. Transition metals

such as iron and cobalt and several others usually exhibit stable oxidation states

differing by one electron and react with each other through one equivalent

steps. However, the stable oxidation states in post transition elements such as

arsenic, antimony etc., differ by two electrons. Thus, on the basis of their

pattern of reactivity, the reactions of these elements are classified into two

main categories,5,8

Two main types of electron-transfer reactions9,10

are

“Complementary reactions”11

in which the oxidant and reductant change their

oxidation state by an equal number of units, and “Non-complementary

reactions”12

in which the oxidant and the reductant change their oxidation state

by a different number of units.

6

Chapter _ I

Electron transfer reactions are governed by two classical principles:

(a) Michaelis principle of compulsory univalent oxidation steps13

(b) Shaffer’s principle of equivalent change14

Michaelis hypothesis states that an oxidation-reduction reaction takes

place in one or more successive single electron transfer steps, apart from the

reactions involving metal ions, many two equivalent redox reactions are now

known which proceed in one step through the transfer of hydride ion or an

oxygen atom.15

Example:

The second principle15

refers to the observation that non-complementary

reactions are often slow compared with complementary ones. Examples are the

slow reduction of thallium(III) by iron(II) or cerium(IV) by thallium(I) as

compared to the rapid reduction of thallium(III) by tin(II) and cerium(IV) by

iron(II).

1.1.4. Unstable oxidation states

The formation of unstable oxidation state during the course of non-

complementary reactions has been now anticipated in a number of such

reactions with sufficient proofs. For example, the reductions of thallium(III) by

iron(II),16

vanadium(III) or vanadium(IV)17,18

and chromium(VI) by

thallium(I)19

can only be explained through the formation of unstable

thallium(II) species. Similar unstable oxidation states have been observed in

NO3- + Cl

- NO2

- + OCl

-

7

Chapter _ I

other studies.20,21

The inter conversions between chromium(III) and

chromium(VI) always appear to involve the unstable states, chromium(IV) and

chromium(V). A number of studies of the catalysis by platinum metals of

oxidation reactions have been made.22

The catalysis by Ag(I),23

Cu(II),24

Mn(III)25

and Cr(III)26,27

in oxidation–reduction reactions are also found to

occur through formation of unstable oxidation states.

1.1.5. Active species

If a particular substance (oxidant, reductant or catalyst) is capable of

existence in several forms in aqueous solution, all the species existing may not

be active. Those species, which are involved in a slow step, will influence the

reaction. The reaction conditions will determine the nature of the active

species.

1.1.6. Catalysis

Any substance, other than reactants which influences the rate of

chemical reaction and itself remains unchanged chemically at the end, is called

a catalyst. The phenomenon of rate alteration is designated as catalysis.

Catalysts influence the reactions by changing the reaction path. Such catalytic

influences arise as consequences of lowering of the energy of activation.

In solution involving inorganic oxidations, the catalysts are ions having

unstable oxidation states. This case is a particular example of homogeneous

8

Chapter _ I

catalysis where the catalysts present in the same phase as a part of reactants.

Homogeneous catalysts are readily amenable to kinetic and mechanistic

studies, in particular using in situ spectroscopic studies. Homogeneous

catalysts are uniform, reproducible and easily modified in a controlled manner.

Though, the mechanism of catalysis depends on the nature of the substrate,

oxidant and other experimental conditions, it has been shown that metal ion

acts as catalyst by different pathways.28

Catalysis plays a vital role in the production of fuels, commodity

chemicals, fine chemicals, and pharmaceuticals, as well as providing the means

for strengthening environmental safeguards all over the world. More than 60%

of all chemical products and 90% of chemical processes are based on catalysis.

Catalysis is the most effective and most rational of all means for accelerating

the chemical reactions. Most of the technological processes introduced recently

in chemical industry incorporate catalytic reactions. In presence of catalyst the

rates of reactions are accelerated thousands and millions of times and they take

place at low temperature, which is an economical advantage. Catalysis is

utilized in manufacturing some of the most important inorganic products such

9

Chapter _ I

as hydrogen, ammonia etc. Often, very small quantities of catalysts like

Os(VIII),29

Pd(II),30

Cr(III),31

Ru(III),32

V(V)33

etc., causes appreciable rates

accelerations of particular reactions.

1.1.7. Applications of kinetics

Kinetics has a wide variety of applications in the world of medicine. In

addition to the ways in which the human body undergoes processes such as

respiration and metabolism, kinetics also play a large part in the administration

of drugs. For example, the mechanisms for the sustained release of drugs are

based on the half-life of the substances used and sometimes the pH of the body

as well. Practically, this affects the way in which dosages are determined and

prescribed. The rate of reactions and the various conditions under which they

occur are crucial for determining certain aspects of environmental protection.

For example, the destructive effect of chloroflourocarbons (CFCs) on the ozone

layer is best understood through an analysis of catalyzed chemical reactions.

The mathematical models that describe chemical reaction kinetics provide

chemists and chemical engineers with tools to better understand and describe

chemical processes such as food decomposition, microorganism growth,

stratospheric ozone decomposition, and the complex chemistry of biological

systems. These models can also be used in the design or modification of

chemical reactors to optimize product yield, more efficiently separate products,

and eliminate environmentally harmful by-products.

10

Chapter _ I

1.2. VOLTAMMETRIC STUDIES

Electrochemistry is a powerful and sensitive analytical tool used for

both qualitative and quantitative analysis over a wide range of concentrations.

The efficacy of electrochemical methods stems not only from their sensitivity

to trace amounts and the simplicity of the instrumentation, but also because

these methods can be used for separation of ionic species in addition to their

detection. Voltammetric technique is concerned mainly with the measurement

of electrical quantities, such as current, potential, or charge, and their

relationship to the chemical parameters.

The beginning of voltammetry was facilitated by the discovery

of polarography in 1922 by the Nobel Prize winning Chemist Jaroslav

Heyrovsky (Nobel Prize in 1959). Early voltammetric techniques had many

problems, limiting their viability for everyday use in analytical chemistry. In

1942, Hickling built the first three electrodes potentiostat.34

The 1960s and

1970s saw many advances in the theory, instrumentation, and the introduction

of computer added and controlled systems. These advancements improved

sensitivity and created new analytical methods. The coincidence of these

advances with the advent of low cost operational amplifiers also facilitated the

rapid commercial development of relatively inexpensive instrumentation.

Electron transfer plays a fundamental role in governing the pathways of

chemical reactions. Measurement of speed of the electron transfer process and

the number of electrons involved are difficult in traditional experimental

11

Chapter _ I

methods like Spectroscopy. Consequently our knowledge of the driving force

for many reactions remains elusive. Electrochemical methods offer the

potential to investigate these processes directly by the determination of the

number of electrons involved.35

1.2.1. Instrumentation

Electrochemistry is the study of reactions in which charged particles

(ions or electrons) cross the interface between two phases of matter, typically a

metallic phase (the electrode) and a conductive solution, or electrolyte. A

voltammetric system consists of an electrolysis cell, a potentiostat, a current-to-

voltage converter, and a data acquisition system. The electrolysis cell consists

of a working (indicator) electrode, counter (auxiliary) electrode, reference

electrode, and electrolytic solution. The working electrode’s potential is varied

linearly with time, while the reference electrode maintains a constant potential.

The counter electrode conducts electricity from the signal source to the

working electrode. The purpose of the electrolytic solution is to provide ions to

the electrodes during oxidation and reduction. A potentiostat is an electronic

device which uses a dc power source to produce a potential which can be

maintained and accurately determined, while allowing small currents to be

drawn into the system without changing the voltage. The current-to-voltage

converter measures the resulting current, and the data acquisition system

produces the resulting voltammogram.

12

Chapter _ I

1.2.2. Voltammetric techniques and their theoretical aspects

The different voltammetric techniques that are used are distinguished

from each other primarily by the potential function that is applied to the

working electrode to drive the reaction and by the material used as the working

electrode. These can be described as follows;

Cyclic voltammetry (CV): is popular for its relative simplicity and its

high information content. It is used most often as a diagnostic tool for

elucidating electrode mechanisms. Although it was first practiced at a hanging

mercury drop electrode,36

it gained widespread use when solid electrodes like

Pt, Au and carbonaceous electrodes were used, particularly to study anodic

oxidations.37

It is a potential sweep technique. The potential of the working electrode

is measured against a reference electrode which maintains a constant potential,

and the resulting applied potential produces an excitation signal such as that of

Figure I (i) (p. 14).38

In the forward scan, the potential first scans negatively,

starting from a greater potential (a) and ending at a lower potential (d). The

potential extrema (d) is called the switching potential, and is the point where

the voltage is sufficient enough to have caused an oxidation or reduction of an

analyte. The reverse scan occurs from (d) to (g), and is where the potential

scans positively. It shows a typical reduction occurring from (a) to (d) and an

oxidation occurring from (d) to (g). It is important to note that some analytes

undergo oxidation first, in which case the potential would first scan positively.

13

Chapter _ I

This cycle can be repeated, and the scan rate can be varied. The slope of the

excitation signal gives the scan rate used. Cyclic voltammetry is generally used

to study the electrochemical properties of an analyte in solution. A cyclic

voltammogram is obtained by measuring the current at the working electrode

during the potential scans. Figure I (ii) (p. 14) shows a cyclic voltammogram

resulting from a single electron reduction and oxidation. Consider the

following reversible reaction:

M+ + e

- <=> M

In Figure I (ii) (p. 14), the reduction process occurs from (a) the initial

potential to (d) the switching potential. In this region the potential is scanned

negatively to cause a reduction. The resulting current is called cathodic current

(ipc). The corresponding peak potential occurs at (c), and is called the cathodic

peak potential (Epc). The Epc is reached when all of the substrate at the surface

of the electrode has been reduced. After the switching potential has been

reached (d), the potential scans positively from (d) to (g). This results in anodic

current (Ipa) and oxidation to occur. The peak potential at (f) is called the

anodic peak potential (Epa), and is reached when all of the substrate at the

surface of the electrode has been oxidized.

Cyclic voltammetry can be used to study qualitative information about

electrochemical processes under various conditions, such as the presence of

intermediates in oxidation-reduction reactions, the reversibility of a reaction.

CV can also be used to determine the electron stoichiometry of a system, the

14

Figure I (ii) (i)

Chapter _ I

diffusion coefficient of an analyte, and the formal reduction potential, which

can be used as an identification tool. In addition, because concentration is

proportional to current in a reversible, nernstian system, concentration of an

unknown solution can be determined by generating a calibration curve of

current versus concentration.

Differential pulse voltammetry (DPV): In DPV, a series of regular

voltage pulses superimposed on the potential linear sweep or stair steps. The

current is measured immediately before each potential change, and the current

difference is plotted as a function of potential. By sampling the current just

before the potential is changed, the effect of the charging current can be

decreased. In these measurements, the effect of the charging current can be

minimized, so high sensitivity is achieved.

Linear sweep voltammetry (LSV): is a voltammetric method where

the current at a working electrode is measured while the potential between the

15

Chapter _ I

working electrode and a reference electrode is swept linearly in time. Oxidation

or reduction of species is registered as a peak or trough in the current signal at

the potential at which the species begins to be oxidized or reduced.

Square wave voltammetry (SWV): In this technique, a square

wave of constant amplitude is superimposed on the potential staircase sweep.

In this voltammetry the potential sweep is a series of stair steps. Due to the

lesser contribution of capacitative charging current the detection limits for

SWV are on the order of nanomolar concentrations.

Normal pulse voltammetry (NPV): In NPV, each potential step

begins at the same value and the amplitude of each subsequent steps increases

in small increments. The resulting voltammogram displays the sampled current

on the vertical axis and the potential to which the pulse is stepped on the

horizontal axis.

1.2.3. Electron transfer or charge transfer process

The electron transfer at the interface between the electrode and

electrolyte is central to an electrode reaction. Electro active species having

moved from the bulk of the solution by either diffusion or under forced

convection enters in the electrical double layer, which is under direct influence

of the electrode. On entering the double layer the species undergoes a structural

orientation so that it can take up or give up electrons from or to the electrode

16

Chapter _ I

surface respectively with the least activation energy when a suitable potential is

applied and macroscopically, we observe a current. This state of the reactant

species is known as transition state. Being unstable the species in transition

state converts itself to the final product by release of activation energy and gets

reduced or oxidized. This final product after undergoing suitable re-orientation

either gets deposited on the electrode surface or moves away from the electrode

surface into the bulk solution. The transfer of an electron to or from the

substrate is an activated process. The electron transfer process can be

“Reversible process” in which the electron transfer is much faster than the

mass transfer, “Irreversible process” in which electron transfer is much

slower than the mass transfer and “Quasi-reversible process” in which

electron transfer and mass transport has comparable rates.

1.2.4. Electrodes

The advent of modern electrochemistry created the need for new

electrodes and electrode set-ups. The most common arrangement today is the

electrochemical cell with three electrodes:

Working electrode (WE)

Reference electrode (RE)

Counter electrode (CE)

immersed in a solvent containing the analyte and also an excess of supporting

electrolyte. The choice of the solvent is dictated by the solubility of the analyte

17

Chapter _ I

and its redox activity and by solvent properties.39

Supporting electrolytes are

required in controlled-potential experiments to decrease the resistance of the

solution and also to maintain constant ionic strength.

1.2.4.1. Working electrode

The working electrode of an electrode system is the electrode, on the

surface of which, the analyte is oxidized or reduced (i.e. the electrode at which

the electrochemical reaction takes place). The active potential of the working

electrode is measured against a standard reference electrode. The working

electrode, where the reactions are studied, is usually made of noble metals,

carbon, or mercury. The most widely used metals are mercury, platinum, gold

and various forms of carbon. Numerous characteristics are required for this

kind of electrode: high signal-to-noise and reproducible response, no

interfering reactions in a certain range of potential, high electrical conductivity,

low surface capacitance, mechanical and chemical stability, low cost,

availability, low toxicity and long term stability.

1.2.4.2. Reference electrode

A reference electrode is an electrode which has a stable and well-

known electrode potential. The reference electrode is used as a half cell to build

an electrochemical cell. This allows the potential of the other half cell to be

18

Chapter _ I

determined. The calomel electrode and the silver/silver chloride electrode are

most commonly used reference electrodes.

1.2.4.3. Counter electrode

In voltammetric studies, the current flows between the working

electrode and counter electrode. Although the main interest is on the working

electrode, it must be ensured that the counter electrode does not complicate the

matters. It must not dissolve in the medium. The reaction product at the CE

must reach or react at the working electrode.

The counter electrode passes all the current needed to balance the

current observed at the working electrode. To achieve this current, the auxiliary

will often swing to extreme potentials at the edges of the solvent window,

where it oxidizes or reduces the solvent or supporting electrolyte. Platinum

electrodes fortunately meet most of these requirements and this is the most

widely used counter electrode in aqueous, non-aqueous as well as molten salt

media.

1.2.5. Applications of voltammetry

Voltammetry is the most effective and versatile electroanalytical

technique available for the mechanistic study of redox systems.40

It enables the

electrode potential to be rapidly scanned in search of redox couples once

19

Chapter _ I

located, a couple can then be characterized from the potential of peaks on the

cyclic voltammogram and from changes caused by variation of the scan rate.

The capability together with its variable time scale and good sensitivity

make this the most versatile electroanalytical technique. The determinations of

organic compounds in pharmaceuticals include many classes of drugs:

antibiotics, diuretics, antineoplastics, antidepressants, muscle relaxants,

neuroleptics, analgesics, vitamins, hormones, and others. Differential pulse

voltammetry/polarography (DPV/DPP) has been extremely useful for

determination of low amounts of electro-active compounds in pharmaceuticals.

A number of excellent voltammetric techniques are well described in the

literature.41,42

Recent advances in instrumental techniques, however, promise access to

molecular-level information about electrochemical systems that has been

unavailable. This exciting development opens up important new opportunities

in fundamental and applied science. In the future, it may become possible to

produce organized networks of molecules resembling, in their controlled

structure, biological systems, yet having properties different from those of any

material known today.

20

Chapter _ I

1.3. REFERENCES

1. M. R. Wright,

“An Introduction to Chemical Kinetics”, John Wiley and Sons, Inc.,

New York, (2004)

2. J. J. Zuckerman,

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3. Sir. G. Wilkinson,

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4. H. Taube,

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Chapter _ I

12. K. B. Wiberg,

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22

Chapter _ I

24. V. Sippola and O. Krause,

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Chapter _ I

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