CO oxidation trends on Pt-group metals from ultrahigh vacuum to near atmospheric pressures: A combined in situ PM-IRAS and reaction kinetics study

The CO oxidation reaction on Pt-group metals (Pt, Rh, and Pd) has been investigated at low (⩽10−3 Torr) and near atmospheric (1-102 Torr) pressures in a batch reactor under steady-state conditions and at various gaseous reactant compositions using PM-IRAS and kinetic measurements. The results indicate that Langmuir-Hinshelwood kinetics adequately provides a general description of the kinetic trends over a wide range of pressures provided that mass transfer effects are considered. At high pressures, the reaction kinetics fall into three regimes: a CO-inhibited low temperature regime where the reaction rate is determined by CO desorption; a mass transfer limited regime at high temperatures; and a transient, high-rate regime which lies in between the other two regimes. The data show that the most reactive surface phase, at both low and high pressures, is a CO-uninhibited phase. This surface phase is not an oxide phase, but a surface phase that contains primarily chemisorbed atomic oxygen and a low coverage of CO.