Photoelectrocatalytic selective oxidation of 4-methoxybenzyl alcohol in water by TiO₂ supported on titanium anodes

The photoelectrocatalytic partial oxidation of 4-methoxybenzyl alcohol in aqueous solution irradiated by near-UV light was carried out in a three-electrode batch reactor. TiO₂ films were either deposited by dip-coating of a TiO₂ sol onto a Ti foil and subsequent calcination or generated on Ti plates by thermal oxidation in air at 400-700°C. The effects of the anode preparation method and bias potential values on conversion and selectivity to the corresponding aldehyde were investigated. The photoelectrocatalytic results were compared with the photocatalytic and electrocatalytic ones. The results indicated that no reaction occurred during the electrocatalytic experiments, whereas the photocatalytic reactivity was positively influenced by the application of a small bias (0.75V vs. saturated calomel electrode). By applying the previous bias the highest initial degradation rates, r₀, and selectivities to 4-methoxybenzaldehyde, S, were obtained in photoelectrocatalytic experiments by using Ti plate calcined at 500°C (r₀=0.1507mMh^-1, S=85%) or TiO₂ film prepared by dip-coating and calcined at 700°C (r₀=0.1339mMh^-1, S=90%). In order to investigate the influence of the substituent groups on reactivity and selectivity, photoelectrocatalytic runs with benzyl alcohol, 2-methoxybenzyl alcohol, 3-methoxybenzyl alcohol, 2,4-dimethoxybenzyl alcohol, 2,3,4-trimethoxybenzyl alcohol, 4-nitrobenzyl alcohol and 4-hydroxybenzyl alcohol have been performed. The selectivity and reactivity values increased by increasing the electron-donor properties of the substituent groups. This positive effect, however, was obtained only if the electron-donor group is in para or ortho position with respect to the alcoholic group.