METHANOL OXIDATION ON RHODIUM AS PROBED BY SURFACE-ENHANCED RAMAN ANDMASS SPECTROSCOPIES - ADSORBATE STABILITY, REACTIVITY, AND CATALYTIC RELEVANCE

The relationship between surface speciation and catalytic activity/sel ectivity during methanol oxidation on polycrystalline rhodium under am bient-pressure flow-reactor conditions was studied from 25 to 500 degr ees C by means of surface-enhanced Raman spectroscopy (SERS) along wit h parallel mass spectrometric (MS) measurements. By utilizing SERS-act ive Rh films formed by electrodeposition onto gold, the former techniq ue provides in situ surface vibrational spectra with unique sensitivit y under these demanding conditions, enabling adsorbed species to be pr obed in real time (approximate to 1 s) for comparison with the overall kinetics as evaluated by MS. Exposure of Rh to O-2-free methanol yiel ded no detectable vibrational bands between 25 and 500 degrees C, alth ough methanol decomposition to form CO and H-2 was evident from MS, Th e presence of even subunity molar ratios of oxygen, however, yielded r ich SER spectra, highlighted by bands indicative of CO(ads) (nu(Rh-CO) = 465 cm(-1), nu(Rh-CO) approximate to 2000 cm(-1)). The catalytic se lectivity toward CO2 (versus CO) gaseous product formation decreased m arkedly around the desorption temperature of CO(ads), approximate to 3 50 degrees C under these conditions. This is consistent with the facil itation of CO2 production by the presence of CO(ads). Complete selecti vity toward exhaustive methanol oxidation (i.e., CO2, H2O formation) w as observed in oxygen-rich methanol mixtures, adsorbed CO now being ab sent at all temperatures. The CO2 production occurs partly via methano lic C-O cleavage as deduced by O-18(2) substitution, The presence of r hodium oxide (Rh2O3) was diagnosed for such reactant mixtures above ca . 300 degrees C from the characteristic 500-580 cm(-1) nu(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 fo rmation rates following O-2 exposure are slowed markedly (>100-fold) b y the presence of even a small (5%) methanol mole fraction. Switching to pure methanol results in very rapid oxide reduction, so that, for e xample, removal is complete within ca. 1 s at 350 degrees C with 100 T orr of CH3OH. Examination of the transient oxide removal kinetics as a function of temperature and methanol pressure revealed a transition f rom strongly activated to essentially T-independent behavior at lower pressures and/or higher temperatures. This is indicative of a change f rom rate-determining removal of oxygen from the oxide lattice to a sub sequent step involving formation of and/or reaction with an adsorbed m ethanol scavenger. While such reactivity earmarks the oxide as a poten tial reaction intermediate, the overall catalytic turnover rates for m ethanol oxidation are nonetheless faster than can readily be accommoda ted on this basis.