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.