Methanol Oxidation on Palladium Compared to Rhodium at Ambient Pressures as Probed by Surface-Enhanced Raman and Mass Spectroscopies

Abstract

The influences of surface speciation upon the catalytic oxidation kinetics of methanol on palladium under ambient-pressure flow-reactor conditions was examined by surface-enhanced Raman spectroscopy (SERS) combined with mass spectrometry (MS) and compared with corresponding data on rhodium. The former technique provides uniquely sensitive surface vibrational information under real-time (≈1 s)in situconditions, by utilizing ultrathin catalyst films electrodeposited onto an inert SERS-active gold substrate. These transition-metal surfaces exhibit sufficiently robust SERS activity to enable temperature-dependent spectral measurements over the range 25–500°C. Parallel kinetic measurements undertaken with MS show the occurrence of methanol decomposition (to CO and H2) in the absence of O2on both Pd and Rh. While the presence of a molar deficiency of O2yields methanol oxidation (to form CO2and H2) in addition to methanol decomposition on Rh, only the latter occurred (at slower rates than Rh) on Pd. These dissimilar reaction selectivities are consistent with the absence of surface 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 arising from the greater ability of the latter to dissociatively chemisorb O2. Both catalysts induced exhaustive methanol oxidation (yielding CO2and H2O) in a heavily O2-rich reactant mixture, although Pd again yielded less facile reaction kinetics. In addition, a significant catalyst deactivation occurred upon heating Pd in this reactant mixture, which was entirely absent on Rh. The corresponding temperature-dependent SER spectra indicate the formation of palladium oxide (PdO) by 350°C, which was retained entirely upon subsequent cooling. While an oxide (Rh2O3) was also seen by SERS to form on Rh by 350°C under these conditions, this species was removed upon subsequent cooling. Transient SERS measurements following sudden exposure of such oxidized surfaces to a methanol gas stream revealed that PdO was entirely unreactive toward methanol even at 350°C, while, in contrast, Rh2O3was 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 reaction on Pd under O2-rich conditions, as suggested by the SERS data, is also discussed.