Effect of palladium chemical states on CO2 photocatalytic reduction over g-C3N4: Distinct role of single-atomic state in boosting CH4 production

Cocatalyst decoration has been recognized as an effective strategy in photocatalysis, yet the critical role of sing-atomic state on CO2 photocatalytic reduction, distinguished from the oxide and elemental states, remains a mystery hitherto. Herein, single-atom Pd (Pd-SA), Pd oxides (PdOx), and Pd nanoparticles (Pd-NP), were homogeneously anchored on g-C3N4 (CN) to investigate their CO2 reduction behaviors under visible-light irradiation. Performance tests showed Pd species decoration improved the CH4 production of CN, with Pd/CN-SA exhibiting the optimum yields (2.25 μmol g–1), markedly higher than that of PdOx/CN (1.08 μmol g–1) and Pd/CN-NP (0.44 μmol g–1). After comprehensive mechanism analysis with various characterization techniques, in-situ DRIFT spectra and DFT calculations, it was found that the conducive activation of CO2, negative conduction band potentials, and excellent •H utilization efficiency, collaboratively contributed to the superior CH4 production of Pd/CN-SA. Despite the larger electron density of Pd/CN-NP and PdOx/CN, the moderate reduction ability of their photogenerated electrons restricted the further reduction of adsorbed CO2 species and CO intermediate, limiting the enhancement of CO2 reduction activity. Furthermore, the CH4 evolutions of Pd/CN-NP and PdOx/CN were also limited by the poor •H supply and inferior •H utilization efficiency, respectively. It is expected that the effect of chemical states, especially the critical role of single-atomic state, revealed in this work can inspire the rational design of more advanced photocatalysts for CO2 reduction.