LOW-ENERGY ELECTRON-DIFFRACTION PROFILE ANALYSIS OF REACTION-INDUCED SUBSTRATE CHANGES ON PT(110) DURING CATALYTIC CO OXIDATION

The substrate changes that occur during catalytic CO oxidation on a Pt(110) surface have been investigated with a high-resolution low-energy electron-diffraction (LEED) instrument of approximately 2000 angstrom transfer width. The influence of different reaction conditions on the substrate changes was investigated by varying p(CO), while p(O2) = 2.0 x 10(-4) Torr and T = 480 K were kept constant. Spot-profile-analysis LEED was used to characterize the surface in terms of terrace-width distributions and the roughening depth. During catalytic CO oxidation, the Pt(110) surface is attacked by the reaction, but with the degree of roughening and the long-range order being strongly dependent on the reaction conditions. On the high-rate branch, the surface roughens such that the roughening depth increases as p(CO) approaches the position of the rate maximum in the functional dependence of the reaction rate on p(CO). In the transition region in between the high- and the low-rate branch, one observes the formation of regularly spaced (430) facets. The reaction-induced substrate changes can be traced back to the mass transport of 50% of the surface atoms that is associated with the CO-induced 1 x 1 <--> 1 x 2 phase transition. Upon annealing the faceted Pt(110) surface, a thermal-reordering process restores the flat (110) surface. One can distinguish between two stages in the restoration of the original flat surface. At relatively low temperature near 500 K, a reordering process in the vertical direction removes the surface roughness almost completely. In contrast, the lateral ordering is more strongly activated, and annealing above 800 K is required in order to produce an average terrace width above 100 angstrom.