Faceting of Rhodium(553) in Realistic Reaction Mixtures of Carbon Monoxide and Oxygen

We have investigated the gas composition-dependent faceting of a Rh(553) surface during catalytic CO oxidation under semirealistic reaction conditions using surface X-ray diffraction. We find that under stoichiometric CO and O-2 conditions, the Rh(553) surface maintains its surface orientation without facet formation. In oxygen excess, the CO oxidation reaction becomes mass-transfer limited by the CO diffusion, and the surface is observed to expose (331) or (11 (1) over bar) facets in coexistence with larger (111) terraces. The observed facet formation has previously been observed for pure O-2 exposures of the Rh(553) surface, but at significantly lower O2 partial pressures. Surprisingly, in CO excess, which results in a CO-poisoned surface with low activity, we instead find coexisting (110) and (111) facets. The reasons for and possible implications of the observed facetings are discussed.