Synergy between Palladium Single Atoms and Nanoparticles via Hydrogen Spillover for Enhancing CO 2 Photoreduction to CH 4 .

Selective photoreduction of carbon dioxide (CO 2 ) into carbon-neutral fuels such as methane (CH 4 ) 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 (C 3 N 4 ) for photocatalytic CO 2 methanation is presented. The catalyst achieves a high selectivity of 97.8% for CH 4 production with a yield of 20.3 µmol g cat. -1 h -1 in pure water. Mechanistic studies revealed that Pd 1 sites activated CO 2 , while Pd NPs sites boosted water (H 2 O) 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 CH 4 production. This work provides a new perspective into the development of selective photocatalytic CO 2 conversion through the artful design of synergistic catalytic sites.