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Two types of Ti−β 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 CO 2 with H 2 O at 323 K to produce CH 4 and CH 3 OH. In situ photoluminescence, diffuse reflectance absorption, and XAFS (XANES and FT-EXAFS) investigations of these Ti−β zeolites indicate that the titanium oxide species are highly dispersed in their frameworks and exist in a tetrahedral coordination state. From the H 2 O adsorption isotherm on these Ti−β zeolites at 300 K, it was found that the Ti−β zeolites synthesized using OH - ions (Ti−β(OH)) exhibited hydrophilic properties and the Ti−β zeolites synthesized using F - ions showed hydrophobic properties. With the addition of H 2 O, Ti−β(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 H 2 O as compared with that of Ti−β(F) having hydrophobic properties. These results indicated that the H 2 O molecules added were easily able to gain access to the tetrahedrally coordinated titanium oxide species in the Ti−β(OH) zeolite. The differences in the H 2 O affinity to the zeolite surface led to a strong influence on the reactivity and selectivity for the photocatalytic reduction of CO 2 with H 2 O. Therefore, the properties of the zeolite cavities were important factors controlling the reactivity and selectivity in the photocatalytic reduction of CO 2 with H 2 O to produce CH 4 and CH 3 OH on these Ti−β zeolite catalysts.
Ikeue et al. (Wed,) studied this question.