Factors of Electrocatalysis

Two are the main factors governing the activity of materials (i) electronic factors, related to chemical composition and structure of materials influencing primarily the M-H bond strength and the reaction mechanism, and (ii) geometric factors, related to the extension of the real surface area influencing primarily the reaction rate at constant electronic factors. Only the former result in true electrocatalytic effects, whereas the latter give rise to apparent electrocatalysis. 

The two factors are seldom completely independent most times they are interdependent. A typical example is the effect of particle size. A decrease in particle size produces an increase in surface area at constant amount of material. At the same time, as the particle size decreases the surface-to-volume ratio increases, which may lead to modifications of the electronic properties of surface atoms. 

From a practical point of view, which effect is responsible for the performance of an electrode material is in principle uninteresting in that both converge to improve the electrode behavior. However, from a fundamental point of view, such a distinction is essential to be able to improve and optimize the experimental situation. In terms of reaction rate (current density), only knowledge of the real surface area can allow one to separate experimentally the two factors. If a plot of j against S (real surface area) at constant potential gives a straight line, the effects are more likely to be geometric only. On the other hand, if the correlation deviates from linearity, electronic factors are most likely to operate. This approach assumes that the 

If we consider Ni as an active site for Ni-based materials, changing the environment in which the ion is immersed is expected to influence its electronic properties. This is in principle the reason for testing a series of alloys or intermetallic compounds of Ni. On the other hand, on changing the environment, bond lengths will also be modified and this will modify the actual concentration of active sites, in turn determining the active surface area. A few examples can better illustrate these concepts. 

A coating of NiS appears to activate smooth Ni considerably. This has led to intense investigations of sulfides as possible electrocatalysts for H2 evolution. In principle, Ni + in the presence of can possess different electronic properties. 

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As the particle size decreases, the ratio between the number of atoms at the surface to those in the bulk increases with a parallel decrease in the average coordination number for the metal atom, which is also expected to be a factor of electrocatalysis. It has been calculated for Pt that the minimum size of a crystallite (cluster) for all atoms to be on the surface is 4 nm, corresponding to a specific surface area of 280 m2g-1 [322] (note that this is larger than the critical particle size where absorption of H atoms disappears on Pd) [333]. It is also interesting that dispersed catalysts can in turn influence the electronic properties of the support so that an interesting combination of sites with varied properties can result [330]. At low catalyst loadings, spillover of intermediates is also possible. 

Guerrini E, Trasatti S. Recent developments in understanding factors of electrocatalysis. Russian Journal of Electrochemistry. 2006 42(10) 1017—1025. 

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]

Effective core potential, 269 Effective double layer characterization of, 189 isotherm, 306, 315 kinetic expressions, 316 observations of with STM, 259 stability of, 225, 351, 503 Effectiveness factor of promotion computation of, 505 definition of, 505 Electrocatalysis... 

Additional information about this Fc GO preparation has been reported elsewhere (112). The intramolecular electron transfer rate constant kmirn calculated using Eq. (36) equals 40 s-1 and is by a factor of 50 higher than that for the randomly modified GO (104). The distance separating the ferrocene unit and FAD in Fc GO is believed to be ca. 19 A, by 2 A shorter than in the most effective electrically contacted enzyme generated by the random modification of GO by ferrocene units. This information supports the hypothesis about the key locations of ferrocene groups that play the dominant role in the electrocatalysis (104). 

So much, then, for two essential cases in which the adsorptive bond between an atom of the substrate acts differently, depending on the nature of the rds. This bonding, and how it affects electrocatalysis, is called the electronic factor, in electrocatalysis. 

The search for new electrode materials is expected to be guided by the fundamental understanding of the factors governing the activity. In electrochemistry, this branch of the discipline is known by the name of electrocatalysis . Strictly speaking, electrocatalysis is the science devoted to the relationship between the properties of materials and the electrode reaction rate. The scope of electrocatalysis as a science is to establish a predictive basis for the design and the optimization of electrocatalysts. 

Some predictions beyond the theory of electrocatalysis for pure metals seem indeed possible. It is, however, necessary to stress again that the applicability of a cathode depends on the impact of many factors, the most outstanding ones being the intrinsic stability and the resistance to poisoning. This is probably still the weak point of cathodes. Their life-time appears to be lower than for anodes, although the deactivation process for cathodes is slower and less abrupt than for anodes. 

In all experimental situations, some judgment must be made concerning the quality of the results.10 In this, it is important to understand the validity of the measurements, and the limits of error in the analytical determinations. In the case of electrocatalysis, the reaction rate on a catalyst surface is the most significant factor. Measurements of the reaction rate can easily be lower than the true reaction rate value but never higher. For this reason, the highest values must be given greater consideration than the lowest values. 

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. 

VII. Tafel Slope Factor in Electrocatalysis and Its Relation to Chemisorption of Intermediates... 

The bond strength measured or estimated at a constant temperature or at a specified facet of single-crystal planes are, of cause, important factors in the discussion of electrocatalysis. But, if they are obtained under different conditions from the practical ones, the conclusion might not be correct. The author believes that the general understanding on the size effect still needs further experimental and theoretical studies. Particularly careful attention should be paid to the operation temperature, the coagulation of nanoparticles during the experiment, or the territory. Of course, evaluation on the stability of such small particles is essential in the practical application. 

In the previous section, it was pointed out that electrocatalysis is akin to heterogeneous catalysis. The essential differences are the effect of the electric field on the reaction rate and the presence of nonreacting species (ions of electrolyte, solvent), which may also affect the reaction rate. The following sections are concerned with (i) elucidation of the effect of the electric field on the reaction rate (n) role of adsorption which is somewhat more complicated in electrocatalysis by the fact that the adsorbed species are not only reactants, intermediates, or products, but also the solvent or ions of the solution (in) conditions under which a comparison of the electrocatalytic activity of various substrates for a particular reaction should be made (iv) the role of electronic and geometric factors of the electrocatalyst. 

Understanding of factors affecting the activity of a catalyst requires a knowledge of the reaction mechanism. Otherwise the study would be only of an empirical nature. The same is true of electrocatalysis in which it is essential to elucidate the rate-controlling step and the reaction path, or at least the steps preceding the rate-determining step (rds). 

Electrocatalysis is closely related to chemical catalysis but with two major differences—the influence of the electric field and of the solvent on the rate of reaction. The additional variable of potential is in many ways an advantage in mechanism determination since the rate of the reaction may be varied over several decades by simply changing the potential at any one temperature. In chemical catalysis, it is necessary to increase the temperature by a considerable amoimt (for a reaction with activation energy 10 kcal mole, to increase the reaction rate by a factor of 10 it is necessary to increase the temperature from 25 to 1000°) so that the rate of the reaction may be varied over several decades. Another advantage is that the variation of reaction rate with potential is an additional criterion to determine the mechanism. 

In relation to item 6. mentioned above on electrocatalysis, the coexistence of a tyrosine residue model, jo-cresol (p-Cre), greatly enhanced the catalytic activity of Ru-red confined in a Nafion membrane coated on an electrode (Fig. 13-6) [20], which was interpreted as a lengthening of the charge-hopping distance by p-cresol by a factor of almost two (from 1.28 nm to 2.25 nm). 

A reaction exhibits electrochemical promotion when lAI > 1, while electrocatalysis is limited to I Al < 1. A reaction is termed electrophobic when A > 1 and electrophilic when A < —1. In the former case, the rate increases with catalyst potential, U, while in the latter case the rate decreases with catalyst potential. A values up to 3x10 and p values up to 150 have been found for several systems. For example, p values between 300 and 1,400 have been observed for C2H4 and C3Hg oxidation oti Pt/YSZ, respectively. In the experiment of Fig. 1, A = 74,000 and p = 26, i.e., the rate of C2H4 oxidation increases by a factor of 25, while the increase in the rate of O consumption is 74,000 times larger than the rate, 1/2 F, of supply to the catalyst [4]. 

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