Nowadays, semiconductor-based heterogeneous photocatalysis is receiving much more attention than ever before owing to the increasing challenge of environmental pollution and worldwide demand for clean energy. The stoichiometry of the photocatalytic reactions usually includes the simultaneous gain or loss of electrons and protons. However, not enough emphasis has been placed on the effect of proton transfer on the photocatalytic reactions, particularly on the kinetics of interfacial redox steps. In this article, we highlight the effects of proton on the interfacial electron transfer during the photocatalytic redox reactions. We try to emphasize that the proton transfer can largely determine the energetic profile and reaction pathway by participating in the interfacial redox reactions. Better understanding on the roles and the detailed mechanisms of proton transfer in the photocatalytic surface reactions is required for the further improvement of photocatalytic efficiency.

The hole-driving oxidation of titanium-coordinated water molecules on the surface of TiO₂ is both thermodynamically and kinetically unfavorable. By avoiding the direct coordinative adsorption of water molecules to the surface Ti sites, the water can be activated to realize its oxidation. When TiO₂ surface is covered by the H-bonding acceptor F, the first-layer water adsorption mode is switched from Ti coordination to a dual H-bonding adsorption on adjacent surface F sites. Detailed in situ IR spectroscopy and isotope-labeling studies reveal that the adsorbed water molecules by dual H-bonding can be oxidized to O₂ even in the absence of any electron scavengers. Combined with theoretical calculations, it is proposed that the formation of the dual H-bonding structure can not only enable the hole transfer to the water molecules thermodynamically, but also facilitate kinetically the cleavage of OH bonds by proton-coupled electron transfer process during water oxidation.

The hole-driving oxidation of titanium-coordinated water molecules on the surface of TiO₂ is both thermodynamically and kinetically unfavorable. By avoiding the direct coordinative adsorption of water molecules to the surface Ti sites, the water can be activated to realize its oxidation. When TiO₂ surface is covered by the H-bonding acceptor F, the first-layer water adsorption mode is switched from Ti coordination to a dual H-bonding adsorption on adjacent surface F sites. Detailed in situ IR spectroscopy and isotope-labeling studies reveal that the adsorbed water molecules by dual H-bonding can be oxidized to O₂ even in the absence of any electron scavengers. Combined with theoretical calculations, it is proposed that the formation of the dual H-bonding structure can not only enable the hole transfer to the water molecules thermodynamically, but also facilitate kinetically the cleavage of OH bonds by proton-coupled electron transfer process during water oxidation.

Epoxidation of olefins with H₂O₂ is one of the most important reactions in organic synthesis. We found that anatase TiO₂ can be a good catalyst for the epoxidation of cyclooctene with H₂O₂ at room temperature. However, the catalyst deactivated quickly in the presence of excess amount of H₂O₂ because of the formation of inactive side-on Ti-η²-peroxide species on the surface of TiO₂, the presence of which was confirmed by isotope-labelled resonance UV Raman spectroscopy and kinetics studies. Interestingly, the epoxidation reaction could be dramatically accelerated under irradiation of UV light with λ≥350 nm. This phenomenon is attributed to the photo-assisted removal of the inactive peroxide species, through which the active sites on the surface of anatase TiO₂ are regenerated and the catalytic epoxidation of cyclooctene with H₂O₂ is resumed. This finding provides an alternative for sustained epoxidation reactions on TiO₂ at room temperature. Moreover, it also has significant implications on the deactivation pathway and possible solutions in Ti-based heterogeneous catalysis or photocatalysis.

Titanium dioxide with surface-loaded palladium (Pd–TiO₂) was able to easily remove all ten bromine atoms from decabromodiphenyl ether (BDE209) within 1 h under the irradiation of sunlight or an artificial light source. By contrast, fewer than three bromine atoms were eliminated on the pristine TiO₂ even with prolonged irradiation (5 h). During the photocatalytic debromination, moreover, the formed BDE intermediates exhibited a significant difference between the Pd–TiO₂ and pristine TiO₂ systems, and much less position selectivity for the debromination on Pd–TiO₂ was observed than that on the pristine TiO₂ surface. For another polybrominated diphenyl ether (BDE15), pristine TiO₂ was incapable of its photocatalytic reduction, whereas the loading of Pd enabled its debromination to diphenyl ether within 20 min. In addition, an evident induction period appeared in the photocatalytic debromination of BDE15 on Pd–TiO₂. The experiments imply that the Pd-cocatalyzed effect changes significantly the photocatalytic reductive debromination pathways.

The hydroxylation process is the primary, and even the rate-determining step of the photocatalytic degradation of aromatic compounds. To make clear the hydroxylation pathway of aromatics, the TiO₂ photocatalytic hydroxylation of several model substrates, such as benzoic acid, benzene, nitrobenzene, and benzonitrile, has been studied by an oxygen-isotope-labeling method, which can definitively assign the origin of the O atoms (from oxidant O₂ or solvent H₂O) in the hydroxyl groups of the hydroxylated products. It is found that the oxygen source of the hydroxylated products depends markedly on the reaction conditions. The percentage of the products with O₂-derived hydroxyl O atoms increases with the irradiation time, while it decreases with the increase of substrate concentration. More intriguingly, when photogenerated valence-band holes (h_vb⁺) are removed, nearly all the O atoms (>97 %) in the hydroxyl groups of the hydroxylated products of benzoic acid come from O₂, whereas the scavenging of conduction-band electrons (e_cb⁻) makes almost all the hydroxyl O atoms (>95 %) originate from solvent H₂O. In the photocatalytic oxidation system with benzoic acid and benzene coexisting in the same dispersion, the percentage of O₂-derived hydroxyl O atoms in the hydroxylated products of strongly adsorbed benzoic acid (ca. 30 %) is much less than in that of weakly adsorbed benzene (phenol) (>60 %). Such dependences provide unique clues to uncover the photocatalytic hydroxylation pathway. Our experiments show that the main O₂-incorporation pathway involves the reduction of O₂ by e_cb⁻ and the subsequent formation of free ^.OH via H₂O₂, which was usually overlooked in the past photocatalytic studies. Moreover, in the hydroxylation initiated by h_vb⁺, unlike the conventional mechanism, the O atom in O₂ cannot incorporate into the product through the direct coupling between molecular O₂ and the substrate-based radicals.