Methanol Oxidation on Rhodium As Probed by Surface-Enhanced Raman and Mass Spectroscopies:  Adsorbate Stability, Reactivity, and Catalytic Relevance

The relationship between surface speciation and catalytic activity/selectivity during methanol oxidation on polycrystalline rhodium under ambient-pressure flow-reactor conditions was studied from 25 to 500 °C by means of surface-enhanced Raman spectroscopy (SERS) along with parallel mass spectrometric (MS) measurements. By utilizing SERS-active Rh films formed by electrodeposition onto gold, the former technique provides in situ surface vibrational spectra with unique sensitivity under these demanding conditions, enabling adsorbed species to be probed in real time (≈1 s) for comparison with the overall kinetics as evaluated by MS. Exposure of Rh to O 2 -free methanol yielded no detectable vibrational bands between 25 and 500 °C, although methanol decomposition to form CO and H 2 was evident from MS. The presence of even subunity molar ratios of oxygen, however, yielded rich SER spectra, highlighted by bands indicative of CO (ads) (ν Rh - CO = 465 cm -1, ν Rh - CO ≈ 2000 cm -1 ). The catalytic selectivity toward CO 2 (versus CO) gaseous product formation decreased markedly around the desorption temperature of CO (ads), ≈ 350 °C under these conditions. This is consistent with the facilitation of CO 2 production by the presence of CO (ads) . Complete selectivity toward exhaustive methanol oxidation (i.e., CO 2, H 2 O formation) was observed in oxygen-rich methanol mixtures, adsorbed CO now being absent at all temperatures. The CO 2 production occurs partly via methanolic C−O cleavage as deduced by 18 O 2 substitution. The presence of rhodium oxide (Rh 2 O 3 ) was diagnosed for such reactant mixtures above ca. 300 °C from the characteristic 500−580 cm -1 ν Rh - O bands. The kinetics of formation and removal of the oxide were probed by gas flow-switching coupled with transient SERS measurements. The oxide formation rates following O 2 exposure are slowed markedly (>100-fold) by the presence of even a small (5%) methanol mole fraction. Switching to pure methanol results in very rapid oxide reduction, so that, for example, removal is complete within ca. 1 s at 350 °C with 100 Torr of CH 3 OH. Examination of the transient oxide removal kinetics as a function of temperature and methanol pressure revealed a transition from strongly activated to essentially T -independent behavior at lower pressures and/or higher temperatures. This is indicative of a change from rate-determining removal of oxygen from the oxide lattice to a subsequent step involving formation of and/or reaction with an adsorbed methanol scavenger. While such reactivity earmarks the oxide as a potential reaction intermediate, the overall catalytic turnover rates for methanol oxidation are nonetheless faster than can readily be accommodated on this basis.