CONVERSION OF METHANOL, FORMALDEHYDE AND FORMIC-ACID ON THE POLAR FACES OF ZINC-OXIDE
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The decomposition reactions of methanol, formaldehyde, and formic acid on the (0001̄)-O and (0001 )-Zn polar faces of ZnO were examined by temperature programmed desorption and X-ray photoelectron spectroscopy. The O-polar crystal face exhibited no reactivity, and these reactants desorbed intact. In contrast, on the Zn-polar crystal face HCOOH and CH2O were oxidized primarily to CO2 via surface formates, while CH3OH was dehydrogenated to CO and CH2O via surface methoxy species and oxidized to CO2 via surface formates. The selectivity of oxidation and dehydrogenation reactions on the (0001)-Zn polar face appeared to be dependent on the extent of surface reduction in the vicinity of adsorbed intermediates. XPS spectra of adsorbed methoxy and formate intermediates were obtained, demonstrating the utility of XPS in monitoring the interconversion of intermediates on oxide surfaces. XPS spectra of adsorbed molecular species suggest that the lack of reactivity of the O-polar crystal face is due to the absence of accessible acid-base site pairs necessary for dissociative adsorption on this surface, and not due to differences in the interaction of molecularly chemisorbed species with the two polar surfaces. The evolution of oxidation products on the (0001)-Zn surface was accompanied by Zn metal desorption. The results of this investigation are consistent with previous studies on polycrystalline ZnO and clarify some of the discrepancies between these studies and previous investigations on single crystal surfaces. 1986.
