Mesoscopic Hydrophobic Microenvironment Engineering on a Single-Atom Catalyst for Steering CO2 Photoreduction to CH4.

The selective photoreduction of CO2 to CH4 is notoriously challenged by competitive water adsorption and the facile desorption of the *CO intermediate. Herein, we tackle this dual challenge through the mesoscopic engineering of a single-atom catalyst with Cu-C sites, which transforms the surface into a hydrophobic environment. This engineered interface not only repels bulk water to ensure efficient CO2 access but also leverages confined water clusters to stabilize CO2 molecules. Crucially, finite element simulations reveal that the lattice distortion-induced surface roughness effectively hinders the diffusion of CO molecules, leading to a localized enrichment of the *CO intermediate around the active Cu sites. This physical confinement effect, synergizing with the electronic π-backdonation from Cu, suppresses premature desorption of the *CO intermediate and promotes its further reduction. Consequently, the catalyst achieves a high CH4 selectivity of 98.43% and a production rate of 16.43 μmol g-1 h-1, with notable activity preserved even under 700 nm irradiation. This work underscores that governing the mass transport of key intermediates at the mesoscale is a decisive design principle for steering catalytic selectivity.