Insight into the Role of TiO₂ Facets in Photocatalytic Selective Oxidation of p-Xylene

The crystal facets of semiconductors often have critical effects on photocatalytic reactions. The spatial charge separation between different facets in TiO2 reveals a preferential accumulation of photogenerated holes on the {001} facet; thereby, the activation of the C-H bond predominantly occurs on the {001} facet. However, the dissociative adsorption of initially generated p-methylbenzyl alcohol (p-MBY) forms alcoholate species, which impedes the interaction between p-xylene and the {001} facet, and thus blocks the photocatalytic reaction of p-xylene oxidation on {001}-72%. On the other hand, H3PO4 predominantly adsorbs on the {001} facet, with an adsorption energy higher than p-MBY (-5.15 vs -3.90 eV). Therefore, adding H3PO4 can prevent the dissociative adsorption of p-MBY on the {001} facet. The addition of H3PO4 also significantly improves the injection efficiency of photogenerated holes into p-xylene and suppresses the generation of O-2(2-), thereby enhancing the conversion and selectivity. Consequently, the addition of H3PO4 obtains a selectivity as high as 94.8% to the primary products at 15.2% conversion on {001}-72%. The results demonstrate that facet engineering for a semiconductor-based photocatalyst can regulate the charge separation, charge injection, and adsorption behavior of intermediates, which is an effective strategy to accomplish high performance of photocatalytic reactions.