Synergetic effect of oxygen vacancies and morphology of ZnO photocatalyst prepared by non-hydrolytic sol-gel route for the photo-oxidation of 2-propanol in a gas-solid system

Abstract

The effects of the nature of some carboxylic acids (oxalic, malic, citric, and tartaric acid) and of the annealing temperature in the preparation of the ZnO photocatalyst by a non-hydrolytic sol-gel route (using zinc acetate as the precursor) have been studied for oxidation of 2-propanol in air as probe reaction in a gas-solid regime. The physico-chemical characterization by XRD, SEM/EDX and UV–vis-DRS revealed a synergetic effect of the presence of oxygen vacancies and morphology of ZnO on the reaction performance. Oxygen vacancies were found to increase with decreasing annealing temperature, and this result indicates the possibility of their production in situ without the addition of a doping agent.XRD results showed that ZnO samples present a pure wurtzite structure with an average crystal size ranging from 21 nm to 42 nm. The specific surface area of samples ranged from 3.0 m2·g–1 to 18 m2·g–1and SEM observation indicated the presence of different agglomeration of both faceted and non-faceted particles. The lowest band gap energy of ZnO was 3.2 eV. The photocatalytic oxidation of 2-propanol in gas-solid regime was carried out in oxygen atmosphere. An increase of acid concentration during the ZnO preparation caused a decrease on the activity of the photocatalyts. An increasing amount of zinc acetate with respect to acid during the preparation of the photocatalysts enhanced the oxygen vacancies in the solid, which induced an increase of the 2-propanol oxidation rate constant. The increasing of the annealing temperature decreased the oxygen vacancies and hence the photocatalytic activity of ZnO. The highest 2-propanol oxidation rate (kapp = 4.00·10–2 min–1) was obtained by using the ZnO sample prepared with a 1:2 molar ratio of zinc acetate:oxalic acid, and annealed at 400 °C. This photocatalyst showed an oxidation rate 80 times higher than that of the least active photocatalyst (kapp = 5.00·10–4 min–1) obtained using zinc acetate:tartaric acid at 1:1 molar ratio and annealed at 500 °C.