Dual-function nano zero-valent iron catalysts for synergistic photothermal CO₂-H₂O co-reduction: Uncovering competitive pathways through in-situ spectroscopy and DFT calculations

The catalytic reduction of CO₂ into valuable chemicals and fuels is a promising strategy for mitigating global carbon emissions. This study explores the synergistic co-reduction of CO₂ and H₂O using nanoscale zero-valent iron (NZVI) as a dual-function catalyst. Under light irradiation at 300 °C, the CO₂ reduction rate increased by more than 10-fold compared to dark conditions, demonstrating significant photothermal synergy. The study reveals that NZVI undergoes a phase transformation, where Fe₃O₄ functions as a photocatalyst while Fe⁰ serves as an electron donor, enabling a unique dual-functionality that facilitates efficient electron transfer. The addition of 10 vol% H₂ stabilizes the reaction, leading to a higher and more consistent CO yield. When investigating the simultaneous reduction of CO₂ and H₂O, we discovered competitive adsorption and reaction pathways that significantly influence product distribution. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations revealed three distinct mechanistic pathways for CO₂ conversion: direct C[sbnd]O bond cleavage, a carboxylate-mediated pathway and bicarbonate-mediated pathway, with the carboxylate and bicarbonate pathways being uniquely promoted by H and OH from H₂O dissociation. The addition of H₂O into the CO₂ and H₂ reaction significantly enhanced CH₄ generation due to the extra H source from H₂O dissociation. These findings offer valuable insights into the competitive mechanistic pathways of CO₂-H₂O co-reduction and contribute to the design of more efficient iron-based catalysts for sustainable CO₂ conversion and water splitting in energy and environmental applications.