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.