Synergy between Palladium Single Atoms and Nanoparticles via Hydrogen Spillover for Enhancing CO2 Photoreduction to CH4

Selective photoreduction of carbon dioxide (CO2) into carbon-neutral fuels such as methane (CH4) is extremely desirable but remains a challenge since sluggish multiple proton-electron coupling transfer and various C-1 intermediates are involved. Herein, a synergistic function between single Pd atoms (Pd-1) and Pd nanoparticles (Pd-NPs) on graphitic carbon nitride (C3N4) for photocatalytic CO2 methanation is presented. The catalyst achieves a high selectivity of 97.8% for CH4 production with a yield of 20.3 mu mol g(cat.)(-1) h(-1) in pure water. Mechanistic studies revealed that Pd-1 sites activated CO2, while Pd-NPs sites boosted water (H2O) dissociation for increased H* coverage. The H* produced by Pd-NPs migrate to the Pd-1 sites to promote multiple proton-electron coupling transfer via hydrogen spillover. Moreover, the adjacent Pd-1 and Pd-NPs effectively stabilized intermediates such as *CHO, thereby favoring the pathway for CH4 production. This work provides a new perspective into the development of selective photocatalytic CO2 conversion through the artful design of synergistic catalytic sites.