Role of steps and kinks in catalytic activity

In the literature, there is the idea that steps and kinks are the active sites fbr chemical reactions, but the experimental data are far from convincing. In this article we see if there is a correlation between step atom density, van Hardeveld and Hartog coordination numbers, or the electronic coordination number and reactivity for a number of simple decomposition and hydrogenolysis reactions on platinum as measured by temperature-programmed desorption. We have examined reactions of ethylene, nitric oxide, and methanol on (111); (110)-(1X1), (110)-(2X1), (100)-hex, (100)-(1X1), (210), (511), and (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 general, stepped surfaces have different reactivity than close-packed planes, but some stepped surfaces are more active than Pt(111) while others are less active than Pt(111). There are negligible correlations between step atom density and catalytic activity for our reactions. Similarly, there are negligible correlations between reactivity and the lowest or average van Hardeveld and Hartog coordination numbers. Calculations were done to understand the variations. There are correlations between the lowest electronic coordination number and reactivity for only the reaction of methanol to carbon dioxide, and between the average electronic coordination number and reactivity for the reaction of methanol to methane. The first correlation is not surprising considering the special nature of the rate-limiting step of this reaction. The second correlation raises many yet-unanswered questions. We find that, in general, stepped surfaces relax to more stable geometriest The relaxation process raises the electronic coordination number of the surface atoms. After relaxation, there is not a large difference between the coordination of atoms of platinum stepped surfaces and Pt(111). (C) 1999 American Vacuum Society. [S0734-2101(99)08404-3].