CO Oxidation on Pt-Group Metals from Ultrahigh Vacuum to Near Atmospheric Pressures. 1. Rhodium

The CO oxidation reaction on Rh(111) was studied both at low pressures (<= 2 x 10(-4) Torr) under steady-state conditions and at high pressures (0.01-88 Torr) in a batch reactor at various gaseous reactant compositions. Surface CO and O coverages, were determined using polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and X-ray photoelectron spectroscopy (XPS). CO titration experiments were also carried out on surfaces with known oxygen coverages. Both CO and 0 inhibition were evident at low pressures so that only within a relatively narrow temperature range were the reaction conditions optimized such that the CO conversion reached similar to 20% of the CO flux to the surface. For high pressures and with stoichiometric or slightly oxidizing reactant ratios (O-2/CO <= 2), the reaction fell into three regimes: (i) a CO-inhibited low temperature regime where the reaction rate was determined by CO desorption; (ii) a mass transfer limited regime at high temperatures; and (iii) a transient, high-rate regime lying between regimes (i) and (ii) where the reaction was not completely controlled by mass transfer limitation. For all reaction conditions investigated (when O-2/CO <= 2), the surface oxygen coverage did not exceed similar to 0.5 monolayer. With very oxidizing reactants (O-2/CO >= 5), the reactivity of the Rh surface decreased dramatically at high temperatures due to oxidation. Furthermore, the so-called "superior oxide reactivity" for CO oxidation that has been proposed in several recent studies is not evident in this investigation.