Boosting CO2 Reduction with Spinel CoAl2O4 Anchored on N-Doped Graphitic Carbon

Efficient charge transfer and effective separation of photo-generated charge carriers are pivotal to the photocatalytic process. In this study, a novel CoAl2O4@nitrogen-doped graphitic carbon (CoAl2O4@NGC) composite photocatalyst was fabricated via a stepwise hydrothermal method coupled with high-temperature calcination, and its photocatalytic performance for CO2 reduction was systematically investigated. X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and photoelectrochemical measurements were employed to characterize the phase structure, microstructure, surface chemical state and photoelectrochemical properties of the catalyst. Spinel-structured CoAl2O4 nanoparticles were uniformly anchored on the NGC substrate, forming a well-integrated composite interface. XPS analysis confirmed the coexistence of Co2+/Co3+ mixed valence states in CoAl2O4 which provides abundant redox sites for CO2 activation. Photocatalytic tests showed that CoAl2O4@NGC exhibits excellent catalytic activity and cycling stability, with CO and CH4 yields of 27.88 μmol·g−1·h−1 and 23.90 μmol·g−1·h−1, respectively. The narrow bandgap (1.54 eV) enhances visible light absorption, while efficient electron-hole separation and reduced charge transfer resistance improve photocatalytic efficiency. Theoretical calculations further reveal that CoAl2O4@NGC lowers the adsorption free energy of CO2 and the energy barrier for COOH formation, thus facilitating the photocatalytic CO2 reduction. This work provides insights for the design of efficient and stable photocatalysts for CO2 reduction and deepens the understanding of the synergistic catalytic mechanism in the spinel/nitrogen-doped carbon composite system.