Systematic study of TiO₂/ZnO mixed metal oxides for CO₂ photoreduction

A two component three degree simplex lattice experimental design was employed to evaluate the impact of different mixing fractions of TiO₂ and ZnO on an ordered mesoporous SBA-15 support for CO₂ photoreduction. It was anticipated that the combined advantages of TiO₂ and ZnO: low cost, non-toxicity and combined electronic properties would facilitate CO₂ photoreduction. The fraction of ZnO had a statistically dominant impact on maximum CO₂ adsorption (β ₂ = 22.65, p-value = 1.39 × 10^-4). The fraction of TiO₂ used had a statistically significant positive impact on CO (β ₁ = 9.71, p-value = 2.93 × 10^-4) and CH₄ (β ₁ = 1.43, p-value = 1.35 × 10^-3) cumulative production. A negative impact, from the interaction term between the fractions of TiO₂ and ZnO, was found for CH₄ cumulative production (β ₃ = -2.64, p-value = 2.30 × 10^-2). The systematic study provided evidence for the possible loss in CO₂ photoreduction activity from sulphate groups introduced during the synthesis of ZnO. The decrease in activity is attributed to the presence of sulphate species in the ZnO prepared, which may possibly act as charge carrier and/or radical intermediate scavengers.