Domino Photoreduction of CO2 to CH4/C2H6 by Steering Catalyst Amounts

Abstract

The global imperative for carbon-neutral fuels starkly contrasts with the persistent failure to achieve solar-driven CH4/C2H6 synthesis from CO2/H2O with both high selectivity and yield, as the thermodynamic propensity for CH4/C2H6 formation collides with kinetic bottlenecks inherent in the multi-electron/proton-transfer cascade. The prevailing paradigm has long posited that CH4/C2H6 selectivity is predominantly governed by reaction kinetics rather than thermodynamics, consequently steering catalyst design toward precise modulation of reactant interfacial dynamics at catalytic surfaces. Contrary to previous assumptions, our study demonstrates for the first time that CO2 photoreduction exhibits a characteristic domino-like chain reaction mechanism: when the catalyst amounts in the reaction system reach the threshold, the reaction cascade preferentially proceeds toward thermodynamically favorable CH4 formation through low-barrier pathways, with even C2H6 generation emerging. The universality of this mechanistic paradigm has been rigorously validated across archetypal catalysts (TiO2, MoO3, CeO2, Ni/CeO2, etc.), revealing that low amounts (5.0 mg) selectively yield H2 and CO (＞60 %, 2e- transfer), whereas higher amounts (200.0 mg) drive product distribution toward thermodynamically favorable CH4 (＞50 %, 8e- transfer). To probe this mechanism, the widely utilized Ni/CeO2 was selected as the benchmark photocatalyst, where the primary constituent of the reduction product shifts from CO/H2 (＞80 %) to CH4/C2H6 (＞80 %) upon increasing Ni/CeO2 amounts. Employing quasi in situ XPS and D2O kinetic isotope effects experiments, we unveil that Ni/CeO2 suprathreshold amounts (>150 mg) create electron-enriched microenvironments at interfacial Niδ+ sites, which drive domino photoreduction of CO2 to CH4/C2H6, while kinetically quenching competing CO/H2 evolution via proton-transfer/CO-desorption impedance. Consequently, this study elucidates that the prerequisite for designing catalysts to resolve the persistent activity-selectivity trade-off is to ensure that the photogenerated carrier concentration at the active site reaches the minimum threshold required for CH4/C2+ generation.