Nanostructured TiO2/KIT-6 catalysts for improved photocatalytic reduction of CO2 to tunable energy products

Abstract

Nanostructured TiO2/KIT-6 catalysts with different concentrations of incorporated TiO2, have been synthesized, characterized, and examined in order to improve the photocatalytic reduction of carbon dioxide (CO2) feedstock with water vapor (H2O) to produce tunable value-added energy products. Nanostructured TiO2, dispersed on KIT-6 (three-dimensional mesoporous silica), was found to be present in both the silica framework and on the surface, where it produced large surface area photocatalysts with enhanced adsorption capability of the reactants to photocatalytically convert into CH4, CH3OH (hydrocarbons) and CO, H2 (similar to syngas). The formed products were influenced directly by the dispersed TiO2 concentration as well as by the calcination temperature. Hydrocarbon and CO formation as well as the reaction kinetics improved as the TiO2 concentration was increased from 1 to 20wt%. However, a further increase in TiO2 loadings (to 90%) decreased the hydrocarbon and CO, and increased H2 formation. The highest optimization toward hydrocabon selectivity was shown by 20wt% TiO2, while a 90wt% TiO2 loading was more selective for H2 formation. This was likely due to the uniform dispersion and stabilization of the anatase TiO2 with 20wt% on KIT-6, which in turn allowed more CO2 adsorption and a better light penetration than 90% TiO2/KIT-6 in which it showed a bulk phase and large agglomerates with light penetration limitations that were more favorable for H2O adsorption. A reaction mechanism, which has helped to understand these findings, has been proposed. Moreover, a 24h activity test with the optimized 20wt% TiO2/KIT-6 showed an increase in the product yield but only a minor, gradual decrease in the reaction rate, which points out that these photocatalysts could be promising for turning CO2 greenhouse gas feedstock into selective renewable energy products.