Photothermal CO₂ Hydrogenation over Ni/g-C₃N₄ Catalysts: Effect of Synthesis Methods on Structure, Activity and Mechanism

The intensification of global climate change by CO₂ emissions has led to increased research focus on reducing emissions and utilizing resources. Photothermal catalysis, combining photocatalytic and thermal catalytic advantages, holds promise for efficient CO₂ conversion. g-C₃N₄, with its superior band structure and visible light responsiveness, is commonly employed in catalytic material development. Nickel, a cost-effective transition metal, exhibits strong CO₂ adsorption and H₂ activation capabilities, making it highly active in photothermal catalytic CO₂ reduction. However, the structural properties of g-C₃N₄ are sensitive to metal loading, with different preparation methods significantly impacting Ni particle dispersion, the metal-support interface structure, and active site exposure, thereby influencing the overall catalytic performance of g-C₃N₄. Based on this, Ni/g-C₃N₄ catalysts were synthesized using co-deposition, solid-state impregnation, and the Co-pyrolysis method, where g-C₃N₄ served as the carrier and Ni as the active core component. The catalysts underwent comprehensive physical and chemical characterization through SEM-EDS, TEM, XRD, FT-IR, BET, CO₂-TPD, H₂-TPR, XPS, UV-vis DRS, and photoelectrochemical analyses. The catalytic activity was assessed in the photothermal catalytic CO₂ hydrogenation reaction, and the reaction mechanism was elucidated through in situ DRIFTS analysis. The results demonstrate that the preparation method significantly impacts the phase structure and surface properties of the catalysts, leading to performance variations. Notably, the Ni/g-C₃N₄-CD catalyst synthesized via the co-deposition method exhibits a higher specific surface area, a richer pore structure, a weaker metal-support interaction, a broader visible-light absorption range, and enhanced CO₂ adsorption and reduction capabilities. Photoelectrochemical tests confirmed its more efficient photogenerated carrier separation and electron transport abilities. Under photothermal coupling conditions at 300 °C, the catalyst achieved a CO yield of 15937.2 μmol·g^-1·h^-1 and a CO selectivity of 84%. Mechanistic investigations reveal that CO₂ is hydrogenated on the catalyst surface, forming the COOH* intermediate, which then decomposes into CO and H₂O, with a portion of the CO further hydrogenated to CH₄. This study provides valuable insights for optimizing the synthesis of g-C₃N₄-based supported metal catalysts and their application in photothermal catalytic CO₂ reduction.