Influence of structure on reaction efficiency in surface catalysis. 2. Reactivity at terraces, ledges, and kinks

Development of a lattice-based theory of reaction efficiency on catalytic surfaces is continual in this paper. The specific problem dealt with is the role of lattice imperfections in influencing the efficiency of diffusion-controlled reactions on such surfaces. A set of reaction centers distributed on a surface on which there are terrace, ledge, and kink sites are considered and calculated numerically using a Markovian (implicit function) approach the changes in the efficiency of the process (as monitored by calculating the average walk length (n)) in three distinct situations: (1) a single reaction center imbedded on a surface (of up to 121 sites) and located first at a terrace site, then at a ledge site, and finally at a kink site of the lattice, (2) a single reaction center (at a terrace vs. ledge vs. kink site) in competition with a whole set of reaction centers distributed uniformly over the surface of the support, and (3) a single reaction center (again at a terrace/ledge/kink site) in competition with a set of competing reaction centers but where the latter are positioned at the ledge sites of the lattice, with the remaining sites of support assumed to be neutral (or nontrapping) sites. Simple arguments are introduced (in which the trapping (or reaction) probability is correlated with the ligation number) to allow direct comparison of the interplay between entropic and energetic factors in influencing the overall efficiency of the reaction-diffusion process. The possible relevance of these calculations to the experimental studies of Somorjai and co-workers in which turnover number was studied as a function of step density and kink density for hydrogenation and hydrogenolysis reactions of hydrocarbons on clean platinum surfaces is brought out and discussed.