Optimally Selecting Photo- and Electrocatalysis to Facilitate CH4 Activation on TiO2(110) Surface: Localized Photoexcitation versus Global Electric-Field Polarization

Abstract

Photo- and electrocatalytic technologies hold great promise for activating inert chemical bonds under mild conditions, but rationally selecting a more suitable method in between to maximize the performance remains an open issue, which requires a fundamental understanding of their different catalytic mechanisms. Herein, by first-principles calculations, we systematically compare the activation mechanisms for the C–H bond of the CH₄ molecule on TiO₂(110) under the photo- and electrocatalytic modes without or with water involved. It quantitatively reveals that the activation barrier of the C–H bond decreases dramatically with a surprising 74% scale by photoexcitation relative to that in thermocatalysis (1.12 eV), while the barrier varies with a maximum promotion of only 5% even under −1 V/Å external electric field (EEF). By detailed geometric/electronic analysis, the superior photocatalytic activity is traced to the highly oxidative lattice O_br^•– radical excited by a photohole (h⁺), which motivates the homolytic C–H bond scission. However, under EEF from −1 V/Å to 1 V/Å, it gives a relatively mild charge polarization on the TiO₂(110) surface region and thus a limited promotion for breaking the weakly polar C–H bond. By contrast, in the presence of water, we find that EEF can facilitate CH₄ activation indirectly assisted by the surface radical-like OH* species from the oxidative water cleavage at high oxidative potential (>1.85 V vs SHE), which explains the high energy cost to drive electrocatalytic CH₄ conversion in experiment. Alternatively, we demonstrate that more efficient CH₄ activation could be also achieved at much lower oxidative potential when integrating the light irradiation. In such a circumstance, EEF can not only promote the h⁺ accumulation at the catalyst surface but also help H₂O deprotonation to form hydroxide, which can serve as an efficient hole-trapper to generate OH^• radical (OH^– + h⁺ → OH^•), unveiling an interesting synergistic photoelectrocatalytic effect. This work could provide a fundamental insight into the different characteristics of photo- and electrocatalysis in modulating chemical bond cleavage.