THE ROLE OF STEP ATOM DENSITY ON THE BINDING AND REACTION OF SURFACE SPECIES

Temperature-programmed desorption was used to examine the adsorption a nd reaction of CO, H-2, O-2, NO, CH3OH, and C2H4 on several surfaces o f platinum to see if there is a correlation between the atom density o f step atoms and reactivity. Small variations in the desorption activa tion energy of hydrogen, oxygen, carbon monoxide, ethylene, and methan ol were found the changing crystal face. However, on platinum surfaces , there was no correlation between the desorption activation energy an d 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 meth ane, for the decomposition of methanol to carbon monoxide, for the hyd rogenolysis of methanol to methane, for the oxidation of methanol to c arbon dioxide, for the oxidation of methanol to formaldehyde, or for t he 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 corre lation did not carry through to steady-state experiments. From our stu dy, 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 r eactivity. (C) 1998 Academic Press