Role of steps and kinks in catalytic activity

In the literature, there is the idea that steps and kinks are the active si tes fbr chemical reactions, but the experimental data are far from convinci ng. In this article we see if there is a correlation between step atom dens ity, van Hardeveld and Hartog coordination numbers, or the electronic coord ination number and reactivity for a number of simple decomposition and hydr ogenolysis reactions on platinum as measured by temperature-programmed deso rption. We have examined reactions of ethylene, nitric oxide, and methanol on (111); (110)-(1X1), (110)-(2X1), (100)-hex, (100)-(1X1), (210), (511), a nd (331) platinum surfaces. We have done a statistical analysis of our data to see if any of the correlations are non-negligible. We find that, in gen eral, stepped surfaces have different reactivity than close-packed planes, but some stepped surfaces are more active than Pt(111) while others are les s active than Pt(111). There are negligible correlations between step atom density and catalytic activity for our reactions. Similarly, there are negl igible correlations between reactivity and the lowest or average van Hardev eld and Hartog coordination numbers. Calculations were done to understand t he variations. There are correlations between the lowest electronic coordin ation number and reactivity for only the reaction of methanol to carbon dio xide, and between the average electronic coordination number and reactivity for the reaction of methanol to methane. The first correlation is not surp rising considering the special nature of the rate-limiting step of this rea ction. The second correlation raises many yet-unanswered questions. We find that, in general, stepped surfaces relax to more stable geometriest The re laxation process raises the electronic coordination number of the surface a toms. After relaxation, there is not a large difference between the coordin ation of atoms of platinum stepped surfaces and Pt(111). (C) 1999 American Vacuum Society. [S0734-2101(99)08404-3].