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 g aseous H-2 and CO have been probed in situ by surface-enhanced Raman s pectroscopy (SERS). The Rh surfaces are ultrathin films electrodeposit ed onto SERS-active gold, enabling surface vibrational spectroscopic i nformation tn be obtained with high temporal resolution (approximate t o 1 s) at elevated temperatures (up to 500 degrees C) and under ambien t-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 inte nsity. Surface oxide is readily reduced by both reductants, although s triking differences in the observed kinetic behavior indicate the occu rrence of distinct reaction pathways. In the case of H-2, at low parti al pressures (less than or equal to 7.6 Torr) below 200 degrees C a te mperature-dependent induction period is observed prior to rapid first- order oxide reduction. Such ''autocatalytic'' behavior is indicative o f 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. Immeasu rably fast H-2-induced oxide reduction, however, occurs at higher temp eratures (greater than or equal to 200 degrees C) and pressures (great er 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 a t similar pressures and temperatures, yet no induction period was dete cted. 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'' ki netic behavior, along with the lack of an appreciable CO pressure depe ndence, suggests that oxide removal is hindered by the extensive forma tion 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 respo nse 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 t he well-known poisoning effect that Rh surface oxidation exerts on cat alytic CO oxidation by O-2. Comparisons are also made between the pres ent results and those recently obtained for methanol-induced oxide rem oval in order to elucidate which chemisorbed alcohol fragment(s) const itute the reactive oxygen ''scavenger''.