Surface States of Titania(Rutile and Anatase)-Supported Vanadium Oxide Catalysts as Prepared by a Column-Exchange Adsorption Method and Their Catalytic Features

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Abstract
Monolayer or less monolayer vanadium oxide catalysts supported on TiO2(rutile and anatase), V2O5/R-and A-TiO2, were prepared by a “column-exchange adsorption” method such as passing an oxovanadium ion solution, the pH of which was adjusted at 2, 4, and 7, through a column packed with titania (TiO2) for impregnation. V2O5/R-TiO2 pretreated at 450 °C showed an ESR spectrum with an unresoluved hfs for V(IV) ions at 77 K without depending upon the pH upon impregnation. V2O5/A-TiO2 petreated at room temperature showed quite small ESR signals due to Ti(III) and V(IV) ions. These signals tended to be intensive with increasing pH upon impregnation. After a heat treatment at 400 °C a huge and quite broad spectrum, which was ascribed to some reduced vanadia species(V2O5−δ), appeared in addition to the spectra for the Ti(III) and V(IV) ions, while such a change was never found in V2O5/R-TiO2. Based on a theoretical treatment for the g-values, V(IV) ions on V2O5/R-TiO2 have been confirmed to be replaced just into Ti ion sites on rutile(almost D2h symmetry). With V2O5/A-TiO2, V(IV) ions were shown to be placed so as to be partially subject to an anatase surface field(almost D2d symmetry); that is, to be placed only near the vacant Ti ion site on anatase. From a comparison of the hf-coupling constant the vanadia species on rutile was clearly shown to interact more strongly with the surface than that on anatase. The 2-propanol decomposition rates over the catalysts were measured in the temperature range 270–350 °C as a test reaction. The activity of V2O5/R-TiO2 was generally higher than that of V2O5/A-TiO2. The lowest activation energy was obtained upon the impregnation at pH=2 for V2O5/R-TiO2 and at pH=7 for V2O5/A-TiO2. The best catalyst was not necessarily prepared through conditions with pH=4, which had been suggested as giving an effective monolayer-vanadia species.