Electrochemical electrocatalysis rate

Apart from the reaction steps involved in electrochemical and catalytic processes, there are other similarities. As we shall see in Chapter 3, electrochemical reaction rates can be increased by applying an electrical potential to the reacting surface. Indeed, a relatively modest electrode potential will be equivalent to raising the reaction temperature by several hundred degrees Celsius. The reason is that the applied potential reduces the height of the potential energy barrier that has to be overcome to make the reaction proceed. The analogous phenomenon in catalysis is the formulation of the catalyst to achieve the same objective. Further study of electrode processes indicates that intermediates adsorbed on an electrode surface will be able to modify a particular reaction route. We shall return to this phenomenon, called electrocatalysis, in Chapter 3. 

In the case of electrochemically promoted (NEMCA) catalysts we concentrate on the adsorption on the gas-exposed electrode surface and not at the three-phase-boundaries (tpb). The surface area, Ntpb, of the three-phase-boundaries is usually at least a factor of 100 smaller than the gas-exposed catalyst-electrode surface area Nq. Adsorption at the tpb plays an important role in the electrocatalysis at the tpb, which can affect indirectly the NEMCA behaviour of the electrode. But it contributes little directly to the measured catalytic rate and thus can be neglected. Its effect is built in UWr and [Pg.306]

The science of electrocatalysis provides the connection between the rates of electrochemical reactions and the bulk and surface properties of the electrodes on which these reactions proceed. 

In electrocatalysis, in contrast to electrochemical kinetics, the rate of an electrochemical reaction is examined at constant external control parameters so as to reveal the influence of the catalytic electrode (its nature, its surface state) on the rate constants in the kinetic equations. 

Electrocatalysis employing Co complexes as catalysts may have the complex in solution, adsorbed onto the electrode surface, or covalently bound to the electrode surface. This is exemplified with some selected examples. Cobalt(I) coordinatively unsaturated complexes of 2,2 -dipyridine promote the electrochemical oxidation of organic halides, the apparent rate constant showing a first order dependence on substrate concentration.1398,1399 Catalytic reduction of dioxygen has been observed on a glassy carbon electrode to which a cobalt(III) macrocycle tetraamine complex has been adsorbed.1400,1401... 

Electrocatalysis and the Rate of Electrochemical Reactions For a given electrochemical reaction A + ne B, which involves the transfer of n electrons at the electrode/ electrolyte interface, the equilibrium potential, called the electrode potential, is given by the Nernst law ... 

It has been known for a long time that the presence of specifically adsorbed anions affects the electrochemical reactivity of a metal electrode in deposition, etching, corrosion and electrocatalysis. For example, blocking of reaction sites through specific adsorption may result in a reduced reaction rate. 

The chemistry of electrochemical reaction mechanisms is the most hampered and therefore most in need of catalytic acceleration. Therefore, we understand that electrochemical catalysis does not, in principle, differ much fundamentally and mechanistically from chemical catalysis. In addition, apart from the fact that charge-transfer rates and electrosorption equilibria do depend exponentially on electrode potential—a fact that has no comparable counterpart in chemical heterogeneous catalysis—in many cases electrocatalysis and catalysis of electrochemical and chemical oxidation or reduction processes follow very similar if not the same pathways. For instance as electrochemical hydrogen oxidation and generation is coupled to the chemical splitting of the H2 molecule or its formation from adsorbed hydrogen atoms, respectively, electrocatalysts for cathodic hydrogen evolution—... 

Electrode Materials and Their Electrochemical Behavior. There is abundant evidence that the rate of electron transfer across an electrode-solution interface is dependent on the physical and chemical properties of the electrode material. The term electrocatalysis has been coined for this effect, and studies of the oxidation of hydrocarbons61 and the reduction of water and hydronium ion62 have provided ample evidence for its existence. 

A major application of eqn. (47) is to diagnose the presence of catalytic, presumably inner-sphere, electrochemical pathways. This utilizes the availability of a number of homogeneous redox couples, such as Ru(NH3)e+/2+ and Cr(bipyridine) +,2+ that must react via inner-sphere pathways since they lack the ability to coordinate to other species [5]. Provided that at least one of the electrochemical reactions also occurs via a well-defined outer-sphere pathway, the observation of markedly larger electrochemical rate constants for a reaction other than that expected from eqn. (47) indicates that the latter utilizes a more expeditious pathway. This procedure can be used not only to diagnose the presence of inner-sphere pathways, but also to evaluate the extent of inner-sphere electrocatalysis (Sect. 4.6) it enables reliable estimates to be made of the corresponding outer-sphere rate parameters [12a, 116, 120c]. 

The electrochemical techniques do not differ significantly with respect to time resolution. Pseudo first order rate constants ranging from about 0.1 to 10 S can be measured by techniques which monitor the response of the intermediate and LSV and electrocatalysis can give estimates of rate constants as high as 10 s . In the opinion of the author, the factors of most importance to be considered in selecting a measurement method of the first style are (i) the selectivity of the response, (//) the ease of obtaining reliable data, and (ill) the kinetic or thermodynamic information content of the data. Another factor of utmost importance to the non-specialist is (iv) the availability of instrumentation. 

Electrocatalysis is manifested when it is found that the electrochemical rate constant, for an electrode process, standardized with respect to some reference potential (often the thermodynamic reversible potential for the same process) depends on the chemical nature of the electrode metal, the physical state of the electrode surface, the crystal orientation of single-crystal surfaces, or, for example, alloying effects. Also, the reaction mechanism and selectivity 4) may be found to be dependent on the above factors in special cases, for a given reactant, even the reaction pathway [4), for instance, in electrochemical reduction of ketones or alkyl halides, or electrochemical oxidation of aliphatic acids (the Kolbe and Hofer-Moest reactions), may depend on those factors. 

Although catalysis in electrochemical reactions was probably first specifically recognized by Frumkin at a conference in Leningrad in 1939, a first and perceptive definition of electrocatalysis seems to have been by Busing and Kauzmann in 1952 (72) in terms of the ability of various electrode surfaces to promote the velocity of the rate-determining step of the reaction. In this respect, their definition preceded the common use of this term in North America in the 1960s by some years, when it was applied to the activities of fuel-cell electrodes by Liebhafsky (7i). 

When an electrocatalytic reaction involves a primary step of molecular dissociative chemisorption, for example, a c,e mechanism, then the electrocatalysis arises more directly, in the same way as for many regular catalytic processes that involve such a step of dissociative chemisorption. In this type of electrocatalytic reaction, the dissociated adsorbed fragments, for example, adsorbed H in H2 oxidation, become electrochemically ionized or oxidized in one or more charge-transfer steps following the initial dissociation. The rate... 

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