Interfacial Microenvironment Effects on the Mechanism of Photocatalytic Methanol Conversion for Hydrogen Evolution.

The solid-liquid interfacial microenvironment is a critical, yet underexplored, determinant of efficiency in heterogeneous photocatalysis. Using methanol as the model hole scavenger over Cu/TiO2, we unveil how a methanol-concentration-regulated microenvironment governs hydrogen evolution activity in a nonmonotonic manner, exhibiting a volcano-shaped dependence with a maximum at 60 vol %. This macroscopic trend is found to stem from the methanol-modulated interfacial microenvironment, which controls a kinetic balance between surface preferential adsorption and proton transport. In-situ spectroscopic and isotopic evidence demonstrates a critical switch in the dominant oxidation pathway. Specifically, at low methanol content, a water-dominated interfacial microenvironment favors an indirect oxidation pathway, in which photogenerated holes oxidize water to hydroxyl radicals that subsequently oxidize methanol. Conversely, at high methanol content, a methanol-rich interfacial microenvironment enhances direct methanol oxidation via O-H bond cleavage. Meanwhile, Raman spectroscopy and ab initio molecular dynamics confirmed that severely depleted water content disrupts the hydrogen-bond network essential for proton transfer to the reduction site, making proton transport the new rate-limiting step. Therefore, the catalytic activity is governed by the surface microenvironment, which is jointly regulated by methanol surface coverage and the hydrogen-bond network, ultimately exhibiting a volcano-shaped dependence. This work underscores that deliberate tailoring of the interfacial microenvironment is a pivotal strategy for mechanism control and performance optimization in liquid-phase heterogeneous catalysis.