METHANOL OXIDATION ON PALLADIUM COMPARED TO RHODIUM AT AMBIENT PRESSURES AS PROBED BY SURFACE-ENHANCED RAMAN AND MASS SPECTROSCOPIES

The influences of surface speciation upon the catalytic oxidation kine tics of methanol on palladium under ambient-pressure flow-reactor cond itions was examined by surface-enhanced Raman spectroscopy (SERS) comb ined with mass spectrometry (MS) and compared with corresponding data on rhodium. The former technique provides uniquely sensitive surface v ibrational information under real-time (approximate to 1 s) in situ co nditions, by utilizing ultrathin catalyst films electrodeposited onto an inert SERS-active gold substrate. These transition-metal surfaces e xhibit sufficiently robust SERS activity to enable temperature-depende nt spectral measurements over the range 25-500 degrees C. Parallel kin etic measurements undertaken with MS show the occurrence of methanol d ecomposition (to CO and H-2) in the absence of O-2 on both Pd and Rh. While the presence of a molar deficiency of O-2 yields methanol oxidat ion (to form CO2 and H-2) in addition to methanol decomposition on Rh, only the latter occurred (at slower rates than Ph) on Pd. These dissi milar reaction selectivities are consistent with the absence of surfac e vibrational features on the latter surface and the observed presence of adsorbed CO on the former. The behavior can be rationalized by the paucity of adsorbed atomic oxygen, O-(ad), On Pd compared with Rh ari sing from the greater ability of the latter to dissociatively chemisor b O-2. Both catalysts induced exhaustive methanol oxidation (yielding CO2 and H2O) in a heavily O-2-rich reactant mixture, although Pd again yielded less facile reaction kinetics. In addition, a significant cat alyst deactivation occurred upon heating Pd in this reactant mixture, which was entirely absent on Rh. The corresponding temperature-depende nt SER spectra indicate the formation of palladium oxide (PdO) by 350 degrees C, which was retained entirely upon subsequent cooling. While an oxide (Rh2O3) was also seen by SERS to form on Rh by 350 degrees C under these conditions, this species was removed upon subsequent cooli ng. Transient SERS measurements following sudden exposure of such oxid ized surfaces to a methanol gas stream revealed that PdO was entirely unreactive toward methanol even at 350 degrees C, while, in contrast, Rh2O3 was removed entirely within ca. 5 s. This remarkable difference in oxide reactivity, which accounts for the Pd catalyst deactivation, was deduced to be due primarily to the inability of methanol to yield a suitable adsorbed ''oxygen scavenger'' by dissociative chemisorption on Pd. The possible involvement of a methoxy intermediate in the reac tion on Pd under O-2-rich conditions, as suggested by the SERS data, i s also discussed. (C) 1998 Academic Press.