Photocatalytic Isomerization of Butenes over TiO2. Photo-Formed Active Species and Characteristics of Reaction

Article abstract of DOI:10.1246/bcsj.59.259

It has been found that UV irradiation of powdered TiO2 catalysts in the presence of butene molecules leads to both cis-trans and double bond shift isomerization reactions with high efficiency.In the photocatalytic isomerization the reactivity of 2-butene is much higher than that of 1-butene, being in contrast to the feature in the thermal isomerization.The addition of O2 or NO molecules leads to the complete inhibition of photocatalytic isomerization of butenes.The effect of adsorbed water on the photocatalytic isomerization has been investigated.From these results together with the ESR measurements under UV irradiation of TiO2 at 77 K in the presence of butene as well as O2 or NO, the following conclusion emerges: The interaction of the photo-formed O^- species or 3+-O^-> pairs results in opening of the C=C bond of buthene, i.e. formation of a radical species, which acts as the intermediate in the photocatalytic isomerization.The O^- as well as surface OH^- species participates in the hydrogen shift, i.e. the double bond shift isomerization.Thus, the reaction mechanism which has been proposed for the photocatalytic isomerization of butene over ZnO has been confirmed over TiO2.