THE EFFECTS OF BULK TITANIA CRYSTAL-STRUCTURE ON THE ADSORPTION AND REACTION OF ALIPHATIC-ALCOHOLS
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Previous studies on TiO2 (rutile) single-crystal surfaces have suggested that product distributions in the reactions of primary alcohols and carboxylic acids are governed primarily by the coordination environment of individual surface cations. Since the cation coordination environment is the same in both the anatase and rutile bulk structures, this model would suggest that the reactivity of adsorbed alcohols should be insensitive to bulk structure. In order to test this hypothesis, methanol, ethanol, and 2-propanol were adsorbed at room temperature on TiO2 anatase and rutile powders. Temperature-programmed desorption spectra were obtained in a high-vacuum microbalance system. On rutile, the molecular coverage was equal for methanol and ethanol but decreased for 2-propanol, most likely due to steric effects observed in previous comparisons of the saturation coverages of primary and secondary alcohols on anatase. The alcohols were dissociatively adsorbed to form alkoxides and surface hydroxyls. The alkoxide species were removed via two channels, recombination with surface hydroxyls at approximately 400 K and decomposition at higher temperatures. The high-temperature decomposition products were identical on both powders, with some differences in the product selectivity. Dehydration and dehydrogenation pathways were observed for all of the alcohols, with only the primary alcohols yielding bimolecular reaction products. The similarities in product distribution and peak temperatures from the aliphatic alcohols on anatase and rutile, particularly with regard to the selectivity for diethyl ether formation from ethanol, indicate that the bulk crystal structure of the oxide does not have a significant influence on the reactions of these molecules. The small differences in selectivity observed between anatase and rutile may be attributed to different populations of coordinatively unsaturated Ti cations on the surfaces of the two powders. However, the reactivity of both high-surface-area anatase and rutile can be understood in terms of the surface site requirements deduced from experiments on rutile single crystals. 1995 Academic Press, Inc.
