Low-energy electron-diffraction profile analysis of reaction-induced substrate changes on Pt(110) during catalytic CO oxidation.

Abstract

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 \ensuremath{\sim}2000 \AA{} transfer width. The influence of different reaction conditions on the substrate changes was investigated by varying ${\mathit{p}}_{\mathrm{CO}}$, while ${\mathit{p}}_{\mathrm{O}2}$=2.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ 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 ${\mathit{p}}_{\mathrm{CO}}$ approaches the position of the rate maximum in the functional dependence of the reaction rate on ${\mathit{p}}_{\mathrm{CO}}$. 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\ifmmode\times\else\texttimes\fi{}1\ensuremath{\leftrightarrows}1\ifmmode\times\else\texttimes\fi{}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 \AA{}.