CO oxidation on ruthenium: The nature of the active catalytic surface

Abstract

CO oxidation over ruthenium has been extensively studied over the last several decades from ultrahigh (UHV) vacuum to ambient pressures. While Ru is an inferior catalyst for this reaction compared to Pt, Rh, and Pd under ultrahigh vacuum conditions, it is comparable to these metals at or near atmospheric reaction conditions. Recent studies have suggested that the transformation from an inactive to an active catalyst can be attributed to a structural transformation of Ru to RuO2 and that an epitaxial film of RuO2(110) on Ru(0001) is the active catalytic surface under stoichiometric CO/O2 reaction condition at or approaching one atmosphere. Furthermore recent experimental and theoretical studies have suggested that under such elevated reactant pressures, a strongly bound CO (120 kJ/mol) on RuO2 reacts with O atoms from the oxide surface to form the CO2 product. However, that the active surface on Ru(0001) is a multilayer RuO2(110) oxide and that the CO reactant is strongly bound is inconsistent with early elevated pressure studies of CO oxidation on Ru showing: (i) the formation of a single surface oxide layer under elevated pressure reactant conditions; (ii) a weakly bound CO (<40 kJ/mol) is the reactant under elevated pressure reactant conditions; and (iii) themore » formation of RuO2 deactivates Ru as a catalyst. This article reviews CO oxidation over Ru for the last several decades emphasizing those aspects that rationalize the connection between the vacuum and high pressure results.« less