Surface and catalytic properties of TiO 2Al 2O 3

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Titania-aluminas of various compositions (atomic ratios: 19, 11, 91) were prepared from the sulfates by coprecipitation with aqueous ammonia or urea. The XPS study with sputteretching by Ar ions revealed that the composition of the surface layer was the same as that of the bulk for the sample coprecipitated with ammonia, whereas the Al atoms were enriched in the surface layer for the sample coprecipitated with urea. An acidity maximum was observed for TiO2Al2O3 (19) prepared with ammonia (TiO2Al2O3 (19)-A), while no acid sites were found for TiO2Al2O3 (19) prepared with urea (TiO2Al2O3 (19)-U). Basic property appeared only for TiO2Al2O3 (11)-A when it was exposed to water vapor. A maximum in oxidizing property was observed for TiO2Al2O3 (11)-U but not for TiO2Al2O3 (11)-A. On the other hand, reducing property decreased with the increase of TiO2 content for both TiO2Al2O3-A and TiO2Al2O3-U. The maximum activities were observed on TiO2Al2O3 (19,11)-A for the isomerization of 1-butene and on TiO2Al2O3 (19)-A for the dehydration of 2-butanol, respectively. A good correlation was found between the acidity and the activity for the dehydration. In the case of the isomerization, it has been concluded from the results of coisomerization of 1-butene-d0d8 that the reaction proceeds by the alkyl cation mechanism involving intermolecular hydrogen transfer on TiO2Al2O3 (19)-A, but the contribution of the carbanion mechanism caused by the action of basic sites becomes greater as the content of TiO2 becomes higher. The active sites for the alkyl cation mechanism have been inferred to be the protons supplied from butene molecules adsorbed on the surface.