Photocatalytic hydrogenation, decomposition and isomerization reactions of alkenes over TiO2-adsorbed water

Abstract

Photocatalytic reactions of alkenes over TiO₂-adsorbed water have been investigated. The products are hydrogenated molecules (C_nH_2n+2) formed by the addition of two H atoms to the π bond of the alkene (C_nH_2n, 2⩽n⩽ 4) and lower alkanes (CH₄, C_{n – 2}H_{2n – 2} and C_n–1H_2n) formed by the cooperative addition of an H atom and an OH radical to the π bond followed by decomposition. The photocatalytic isomerization of butenes by surface OH radicals is also observed. The generalized reaction mechanisms for these photocatalytic reactions have been postulated. The initial rate of formation of the hydrogenation product and the sum of the initial rates of formation of lower alkanes per amount of alkene adsorbed are proportional to k_H and (k_Hk_OH)^1/2, respectively. Here, k_H and k_OH are rate constants for addition of H atoms and OH radicals to alkene in the vapour phase. For photocatalytic isomerizations, it is found that the isomerization of (E)-but-2-ene to but-1-ene (trans form) which has the same geometrical form is about twice as effective as that between (Z)-but-2-ene and but-1-ene and that the addition of OH to the C(1) position of but-1-ene prevents the isomerization to but-2-enes.