Reduction kinetics of surface rhodium oxide by hydrogen and carbon monoxide at ambient gas pressures as probed by transient surface-enhanced Raman spectroscopy

The transient reduction kinetics of surface rhodium oxide (Rh2O3) by gaseous H-2 and CO have been probed in situ by surface-enhanced Raman spectroscopy (SERS). The Rh surfaces are ultrathin films electrodeposited onto SERS-active gold, enabling surface vibrational spectroscopic information tn be obtained with high temporal resolution (approximate to 1 s) at elevated temperatures (up to 500 degrees C) and under ambient-pressure flow-reactor conditions. Surface Rh2O3 is formed by heating Rh in flowing O-2 at 1 arm and fingerprinted by an intense 530 cm(-1) nu(Rh-O) feature. The reduction of such oxidized surfaces upon sudden exposure to either H-2 or CO over a range of partial pressures (1-760 Torr) was monitored from the decay kinetics of the nu(Rh-O) band intensity. Surface oxide is readily reduced by both reductants, although striking differences in the observed kinetic behavior indicate the occurrence of distinct reaction pathways. In the case of H-2, at low partial pressures (less than or equal to 7.6 Torr) below 200 degrees C a temperature-dependent induction period is observed prior to rapid first-order oxide reduction. Such "autocatalytic" behavior is indicative of a "nucleation/growth'' mechanism, where H-2 dissociatively adsorbs to form reaction centers, followed by facile reaction between adsorbed (or lattice) oxygen and (possibly subsurface) hydrogen atoms. Immeasurably fast H-2-induced oxide reduction, however, occurs at higher temperatures (greater than or equal to 200 degrees C) and pressures (greater than or equal to 76 Torr). In contrast, GO-induced oxide reduction was found to be substantially (at least 10-fold) slower than with H2 at similar pressures and temperatures, yet no induction period was detected. At lower temperatures (less than or equal to 250 degrees C), the oxide reduction kinetics by CO exhibit fast initial removal followed by a much more sluggish zero-order response. Such "autoinhibited" kinetic behavior, along with the lack of an appreciable CO pressure dependence, suggests that oxide removal is hindered by the extensive formation of adsorbed CO, which is observed to develop rapidly under these conditions from the characteristic Rh-CO and C-O stretching vibrations. This mechanism is further supported by the first-order kinetic response observed throughout oxide removal at temperatures (greater than or equal to 300 degrees C) where buildup of adsorbed CO is not detected. The transient kinetics for CO-induced oxide reduction are linked to the well-known poisoning effect that Rh surface oxidation exerts on catalytic CO oxidation by O-2. Comparisons are also made between the present results and those recently obtained for methanol-induced oxide removal in order to elucidate which chemisorbed alcohol fragment(s) constitute the reactive oxygen "scavenger".