The influence of chromium on photocatalysis of propan-2-ol and octadecanoic acid oxidation by rutile TiO2

This paper compares the effect of chromium on photo-oxidation of propan-2-ol, for which hydroxyl radicals are the catalytically active intermediates, and stearic acid oxidation which proceeds mainly via direct hole transfer. The Cr/TiO2 samples were prepared by co-precipitation from aqueous TiCl4 solutions. The photo-oxidation of liquid propan-2-ol oxidation to acetone was followed by gas chromatography. The oxidation of octadecanoic acid (hereafter designated stearic acid) to carbon dioxide on irradiation of a pressed disc of stearic acid/TiO2 was followed by using FTIR spectrometry of the product CO2. This is a novel adaptation of a method previously used to following photo-oxidation of polymer films, and preliminary measurements were necessary in order to identify conditions under which the oxidation rates were insensitive to small variations in acid loadings and disc thicknesses. Photocatalytic oxidation of propan-2-ol was little affected for Cr ≤ 0.01% but decreased significantly for Cr ≥ 0.1%. By contrast, photocatalytic oxidation of stearic acid decreased monotonically with increasing concentration of chromium. It is suggested that the initial increase in propan-2-ol oxidation activity is associated with surface Cr with an oxidation state greater than 3 but that at higher chromium concentrations the activity for both stearic acid and propan-2-ol is decreased because of increased recombination in induced by Cr3+ ions that substitute for Ti4+ in the rutile lattice.