Insight into the photoexcitation effect on the catalytic activation of H2 and C-H bonds on TiO2(110) surface

Semiconductor photocatalysis holds great promise for breaking the inert chemical bonds under mild condition; however, the photoexcitation-induced modulation mechanism has not been well understood at the atomic level. Herein, by performing the DFT+U calculations, we quantitatively compare H-2 activation on rutile TiO2(110) under thermo- versus photo-catalytic condition. It is found that H-2 dissociation prefers to occur via the heterolytic cleavage mode in thermocatalysis, but changes to the homolytic cleavage mode and gets evidently promoted in the presence of photoexcited hole (h(+)). The origin can be ascribed to the generation of highly oxidative lattice O-radical (O-br(center dot-)) with a localized unoccupied O-2p state. More importantly, we identify that this photo-induced promotion effect can be practicable to another kind of important chemical bond, i.e., C-H bond in light hydrocarbons including alkane, alkene and aromatics; an exception is the C(sp(1))-H in alkyne (HC CH), which encounters inhibition effect from photoexcitation. By quantitative analysis, the origins behind these results are attributed to the interplay between two factors: C-H bond energy (E-bond) and the acidity. Owing to the relatively high E-bond and acidity, it favors the C(sp(1))-H bond to proceed with the heterolytic cleavage mode in both thermo- and photo-catalysis, and the photoexcited O-br(center dot)- is adverse to receiving the transferred proton. By contrast, for the other hydrocarbons with moderate/low E-b(ond), the O-br(center dot-) would enable to change their activation mode to a more favored homolytic one and evidently decrease the C-H activation barrier. This work may provide a general picture for understanding the photocatalytic R-H (R = H, C) bond activation over the semiconductor catalyst. (C) 2022 Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.

Automated summary

Semiconductor photocatalysis shows promise in breaking inert chemical bonds, but the atomic-level mechanism of photoexcitation-induced modulation is still not well understood. This study compares the activation of H2 on rutile TiO2(110) under thermal and photo-catalytic conditions, and finds that photoexcitation promotes homolytic cleavage and activation of H2 and C-H bonds, except for C(sp(1))-H in alkynes, which is inhibited. These findings provide insights into photocatalytic bond activation over semiconductor catalysts.