Characterization of Zirconium−Silicon Binary Oxide Catalysts Prepared by the Sol−Gel Method and Their Photocatalytic Activity for the Isomerization of 2-Butene

Zirconium−silicon binary oxide catalysts having different Zr concentrations were prepared by the sol−gel method and used as photocatalysts. The photocatalytic activity of these catalysts was investigated for the isomerization of cis -2-butene and was found to be enhanced in regions of lower Zr content. In situ photoluminescence, UV−vis reflectance, XRD, EXAFS, and XPS spectroscopic investigations of these zirconium−silicon oxide catalysts indicated that the Zr ions in the oxides which have a low Zr concentration exist separately as extremely small zirconium oxide species in the SiO 2 matrix while the radiative decay from the charge-transfer excited state of these species results in a characteristic photoluminescence. These results and the quenching of the photoluminescence by the added butene molecules showed that the higher photocatalytic activity of the zirconium−silicon binary oxides having a low Zr content is linked to the catalytic activity of the charge-transfer excited state of the zirconium oxide species, [Zr 3+ −O - ]*, in the catalysts.