CO Oxidation on Pt-Group Metals from Ultrahigh Vacuum to Near Atmospheric Pressures. 2. Palladium and Platinum

CO oxidation on Pd(100), -(111), -(110), and Pt(110) single crystals was studied at steady-state conditions at low (<= 2 x 10(-3) Torr) and high (2-88 Torr) pressures at various reactant compositions. At low pressures the reaction fell into two regimes, one with a CO-dominant surface where the CO2 formation rate is low, and a second with an O-dominant surface where the reaction rate is high. Within this second regime, the reaction is collision-limited with no oxygen inhibition. Under high-pressure reaction conditions, three reaction regimes are evident: (i) a CO-inhibited metallic regime displaying a low CO2 formation rate; (ii) an oxygen-rich metallic regime with a high CO2 formation rate; and (iii) a high-temperature regime where the CO2 formation rate is either mass transfer limited on a metallic surface or limited by the reduced reactivity of the oxidized surface. The superior activity of Pt group metal oxides compared to the reduced metal, as proposed recently, was not observed in this study.