Geometric effects in the control of catalytic selectivity by surface additives: Monte Carlo simulation studies

Abstract

The influence of lateral interactions between adspecies on a metal surface in the occurrence of “ensemble” and “electronic” effects in chemisorption and catalysis has been examined using Monte Carlo simulation method. The lateral forces are limited to pairwise interactions ( E ) of magnitide −5 (atractive) E ≤10 kJ mol −1 (repulsive) between poison species in adjacent sites. The variations in extent of chemisorption of “probe” molecules exhibiting critical ensemble sizes of three, seven and thirteen are evaluated as a function of E and poison coverage; the dependence on these variables of the concentration of electronically perturbed sites is also discussed. The general observation is that the results are rather sensitive to the value of E , suggesting that variations in lateral interactions may play a significant role in explaining the differing catalytic influences of particular surface additives. Multiple domain occupancy is shown to affect the standard procedure used to determine ensemble sizes from selective chemisorption measurements and phenomena relating to the fluid − 3 × 3 R 30 ° phase transition in the poison lattice are described.