Photocatalytic reduction of CO2 with H2O on Ti-β zeolite photocatalysts:: Effect of the hydrophobic and hydrophilic properties

Two types of Ti-beta zeolites synthesized by a hydrothermal synthesis method under different conditions using OH- and F- ion as anions of the structure-directing agents (SDA) exhibited photocatalytic reactivity for the reduction Of CO2 with H2O at 323 K to produce CH4 and CH3OH. In situ photoluminescence, diffuse reflectance absorption, and XAFS (XANES and FT-EXAFS) investigations of these Ti-beta zeolites indicate that the titanium oxide species are highly dispersed in their frameworks and exist in a tetrahedral coordination state. From the H2O adsorption isotherm on these Ti-beta zeolites at 300 K, it was found that the Ti-beta zeolites synthesized using OH- ions (Ti-beta (OH)) exhibited hydrophilic properties and the Ti-beta zeolites synthesized using F--ions showed hydrophobic properties. With the addition of H2O, Ti-P(OH) exhibited a more, efficient quenching of the photoluminescence of the highly dispersed tetrahedrally coordinated titanium oxide species and a more remarkable decrease in the preedge intensity of the XANES spectra of the Ti K-edge by the addition of H2O as compared with that of Ti-beta (F) having hydrophobic properties. These results indicated that the H2O molecules added were easily able to gain access to the tetrahedrally coordinated titanium oxide species in the Ti-beta (OH) zeolite. The differences in the H2O affinity to the zeolite surface led to a strong influence on the reactivity and selectivity for the photocatalytic reduction of CO2 with H2O. Therefore, the properties of the zeolite cavities were important factors controlling the reactivity and selectivity in the photocatalytic reduction of CO2 with H2O to produce CH4 and CH3OH on these Ti-beta zeolite catalysts.