On the Theory of Heterogeneous Catalysis

Hydrogenation of ethylene on metallic catalysts

“…The applicability must of course be decided on the basis of a general theory of reaction rates which does not rest upon the statistical independence of the system of the given step. Readers are referred, in this regard, to an article "On the Theory of Heterogeneous Catalysis" by Horiuti and Nakamura [134].…”

“…There exist two kinds of deviations from the statistical independence of a system of a step in case of heterogeneous steps [134]. First, a species is not admissible to a site of adsorption if the site is preoccupied by another species.…”

“…In the semiquantitative tFeatments presented in this monograph, we will be content to take account of the former kind of deviation only, ignoring the latter throughout. Under this limitation, the current treatments referred to in 10.1 are critically reviewed on the basis of the general theory [134]; conclusions only are cited, leaving out any detailed arguments.…”

“…Equations (10.12) are applicable when A and B are alternative occupants of one kind of sites, or when A and B occupy different kinds of sites exclusively, or when either one of them is the exclusive occupant of any sites. The necessary and sufficient condition for eqs (10.12.A) and (10.12.B) to be valid is that the critical complex occupies a single site, either of A or of B, respectively [134]; it is not necessary that either of A and B is adsorbed and collided with by the other from the gas phase. On the other hand, eqs (10.12.A) and (10.12.B) do not apply even if the one from the gas phase collides with the other one which is adsorbed, provided that the critical complex thus formed occupies combined sites of A and B; in the latter case eq (10.2.B) holds instead.…”

“…This product is and vi is the number of ith constituents of the initial system, I, in gas. The quantity q$ is by definition the Boltzmann factor of the work required to add a system of the step in question to a definite site on the catalyst as an activated complex, keeping the whole assembly involved in statistical mechanical equilibrium throughout the process [134,135].…”