Kinetic and Catalytic Behavior

To determine what conclusions might be reached about the behavior of a catalyst, let us return to a uniform surface and again consider the two-step reaction described by steps 8.1 and 8.2 in the previous section. The activity per site, N, i.e., the TOP, is given by equation 8.9. If it is now assumed that a Bronsted relation exists for each step, then 

In other words, the optimum catalyst with Nmax is one with half the sites filled and half empty. This is a quantitative verification of Sabatier s Principle, which states that the best catalyst is one that forms an unstable intermediate compound at the surface which is neither too weakly nor too strongly adsorbed [2,3]. 

There is another important conclusion provided by these results, i.e., the best catalyst for a reaction in the forward direction should not be expected to be the best catalyst in the reverse direction. For the former situation far from equilibrium, [Bi] = [B2] = 0, and for Nmax 

However, for the latter situation, again far from equilibrium where 

The probability that equations 8.54 and 8.55 will be satisfied simultaneously is clearly extremely low, thus verifying this second conclusion. 

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Until recently, surprisingly little work had been done experimentally on the important aspect of coverage by adsorbed H in the kinetic and catalytic behavior of the cathodic H2 evolution reaction. Theoretically, the relation between potential dependence of coverage, 0 , of the H intermediate [see Eqs. (65) and (81)] and the mechanism and kinetics of the HER had been treated extensively, but experimentally evaluated On data to which kinetic behavior could be related remained mostly lacking until recently. It is obviously a very important aspect of electrocatalysis behavior that should be experimentally determined. 

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