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 C1 intermediates are involved. Herein, a synergistic function between single Pd atoms (Pd1) and Pd nanoparticles (PdNPs) 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 µmol gcat.−1 h−1 in pure water. Mechanistic studies revealed that Pd1 sites activated CO2, while PdNPs sites boosted water (H2O) dissociation for increased H* coverage. The H* produced by PdNPs migrate to the Pd1 sites to promote multiple proton–electron coupling transfer via hydrogen spillover. Moreover, the adjacent Pd1 and PdNPs 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. Synergistic Pd1 and PdNPs form a functional catalyst for enhancing photocatalytic CO2 methanation. Pd1 activate CO2, while PdNPs boost H2O dissociation. The H* produced by PdNPs migrates to Pd1 via hydrogen spillover to promote multiple proton–electron coupling transfer processes. Moreover, the adjacent Pd1 and PdNPs stabilize intermediates, such as *CHO. Therefore, efficient photocatalytic CO2 to CH4 is highly favored.