Identifying the Primary Bottleneck of Photocatalytic Direct Methane to Methanol Conversion over Anatase Titania in Aqueous Solution

Methane C–H bond activation is generally perceived as the key step for methane upgrading. Although photocatalysis employing H2O as a mild oxidizer could circumvent the competition between the O–O bond activation of molecular oxygen and the C–H bond activation of methane in thermocatalytic direct methane-to-methanol (DMTM), its activity remains limited without the incorporation of an additional oxidant. We employed first-principles-based microkinetic simulations to investigate photocatalytic DMTM at the water/anatase-TiO2(101) interface, aiming to identify the bottlenecks that hinder photocatalytic DMTM in the aqueous solution. Our calculations reveal that the methane C–H bond activation becomes energetically favorable once surface reactive oxygen species (ROS) are generated by the capture of photoexcited holes. This process is primarily constrained by the concentration of surface ROS, entropic effects, and the solubility of methane in the aqueous solution, rather than the intrinsic strength of the methane C–H bond. Our microkinetic analyses indicate that the concentration of surface holes is of paramount importance for photocatalytic DMTM. The low concentration and limited lifetime of the holes, coupled with the restricted diffusion coefficients of CH4 and ·CH3 in the aqueous solution, would contribute to a diffusion-limited photocatalytic DMTM process, whereas mass transfer factors exert a negligible impact on the surface reaction at high hole concentrations. The low concentration of surface holes, which not only results in sparse surface ROS but also exacerbates the diffusion-limited issue, constitutes the primary bottleneck for the activity of photocatalytic DMTM. Therefore, enhancing mass transfer and elevating methane solubility could improve the activity of DMTM when the concentration of surface holes is low. These mechanistic insights provide valuable guidance for the rational design of photocatalytic DMTM catalysts and the optimization of the liquid–solid interface.