Synergistic experimental and DFT investigation of photoreduction of CO₂ via gas-phase adsorption over Mn- and Cr-doped NiAl₂O₄@Ti₃C₂T_x heterojunction

The growing demand for carbon-neutral energy has accelerated interest in converting CO₂ into carbon-based fuels using solar energy. Energy consumption, fossil fuel depletion, and climate change highlight the urgent need for renewable energy. Designing efficient heterostructured photocatalysts with enhanced charge separation and controlled reaction pathways is key to enhancing CO₂ photoreduction. Here, a well-defined Mn- and Cr-bimetallic-doped NiAl₂O₄@Ti₃C₂T_x nanosheet (NMCAO@TCTS) hybrid photocatalyst was synthesized via etching followed by hydrothermal methods, thereby enhancing the transfer of photogenerated electrons from NMCAO to TCTS and creating a much greater excited reductive electron charge on the surface of TCTS. The comprehensive crystallographic, electron-microscopic, photoemission spectroscopic, electrochemical, and optical spectroscopic analyses of as-synthesized composites provided profound insights into the underlying mechanisms governing their photocatalytic performance towards CO₂ photoreduction to CO and CH₄. The optimized 15% NMCAO@TCTS photocatalyst exhibited excellent performance, delivering a CO and CH₄ production rate of 12.25 and 1.76 (Formula presented), respectively, thereby demonstrating its high efficiency using solar irradiation. Furthermore, DFT analysis demonstrated that the Mn site in the 15% NMCAO@TCTS heterojunction is the most active center for enhanced catalytic performance, exhibiting the lowest energy barrier and the least negative limiting potential in CO₂ reduction pathways. Furthermore, DOS/PDOS, d-band center, ICOHP, Bader charge, and CDD analyses demonstrate significant charge redistribution at the 15% NMCAO@TCTS interface, enhancing efficient electron transfer and CO₂ polarization. The synergistic interaction between the Mn active sites of NMCAO and conductive TCTS assists in facilitating interfacial charge transport, intermediate stabilization, and accelerating proton-coupled electron-transfer steps for high-performance photocatalytic CO₂ reduction. Overall, this work demonstrates a robust strategy for designing bimetallic-doped metal oxide/MXene heterostructures for efficient solar-driven CO₂ photoreduction into fuel products.