The role of step atom density on the binding and reaction of surface species

Temperature-programmed desorption was used to examine the adsorption and reaction of CO, H-2, O-2, NO, CH3OH, and C2H4 on several surfaces of platinum to see if there is a correlation between the atom density of step atoms and reactivity. Small variations in the desorption activation energy of hydrogen, oxygen, carbon monoxide, ethylene, and methanol were found the changing crystal face. However, on platinum surfaces, there was no correlation between the desorption activation energy and the step atom density. Reactivity was found to vary greatly with the crystal face. However, no correlation was found between the step atom density and the reactivity for the hydrogenolysis of ethylene to methane, for the decomposition of methanol to carbon monoxide, for the hydrogenolysis of methanol to methane, for the oxidation of methanol to carbon dioxide, for the oxidation of methanol to formaldehyde, or for the decomposition of nitric oxide to nitrogen and oxygen. Only for the self-hydrogenation of ethylene to ethane on platinum surfaces did the reactivity have any correlation with step atom density, and this correlation did not carry through to steady-state experiments. From our study, it appears that the active site for reaction is often not simply a step site. Rather, the active site consists of a special arrangement of step and terrace atoms that are aligned correctly to produce high reactivity. (C) 1998 Academic Press