Phase effect of TiO₂ on surface hydrogen adsorption/desorption in controlling photocatalytic methane conversion

Identifying the rate-determining step is crucial for designing an effective photocatalytic system. The surface adsorption/desorption behaviour of reactants has received much less attention in photocatalyst design because the charge separation and transfer in the bulk is commonly regarded as a more sluggish process. In this work, we investigate photocatalytic methane (CH₄) conversion (PMC) on various titanium oxide (TiO₂) surfaces, including rutile and anatase, and reveal that the influence of surface CH₄ adsorption can outweigh the photogenerated charge separation and transfer. Specifically, the rutile TiO₂ surface is totally inert for CH₄ activation. Further theoretical calculations reveal the significance of the hydrogen-adsorption/desorption process during the initial C-H bond cleavage on the TiO₂ surface. A reversible hydrogen adsorption/desorption process with a small Gibbs free energy not only enables the activation of the first C-H bond in CH₄ but also ensures a timely clearance of surface-adsorbed species, leading to a continuous PMC process. The findings of the phase effect study on the interaction between the photocatalyst surface and hydrogen atoms provide new insights into the rational design of efficient photocatalysts towards PMC. It also highlights the gap in transferring the knowledge of photocatalytic water splitting into PMC.