Domino photoreduction of CO2 to CH4/C2H6 by steering catalyst amounts

Abstract

The global imperative for carbon-neutral fuels stands in stark contrast to the persistent challenge of achieving 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 holds that CH₄/C₂H₆ selectivity is governed by reaction kinetics rather than thermodynamics; thus conventional catalyst design has focused on the precise modulation of reactant and intermediate dissociation–adsorption behavior. Challenging this long-held view, we demonstrate for the first time that under sufficient interfacial photogenerated charge supply, the CO₂ reduction pathway is controlled by thermodynamics instead of kinetics. This implies that generating CH₄ and C₂H₆ is intrinsically more facile than generating H₂ and CO. Given that the aggregation of photocatalyst particles improves the sufficient absorption of incident light and increases the total yield of interfacial photogenerated charges, Ni/CeO₂ was selected as the benchmark photocatalyst with its amount as the sole variable during activity evaluation, thereby ensuring that any observed change in the reaction pathway stemmed from the modulated photocharge density rather than from structural variations of the catalyst. Increasing the amount of Ni/CeO₂ shifts the primary reduction products from CO/H₂ (>80% molar selectivity, 2e⁻ transfer) to CH₄/C₂H₆ (>80% molar selectivity, >8e⁻ transfer), a shift that necessitates either a catalyst amount of ≥150 mg or extensive catalyst aggregation. Notably, the Ni/CeO₂ amount dictates product selectivity irrespective of temperature, whereas heat governs the production rates of CH₄/C₂H₆. Under optimized conditions, the system achieved sustained production rates of 20.8 µmol h⁻¹ (80.07%) for CH₄ and 1.83 µmol h⁻¹ (7.03%) for C₂H₆ over a 4-hour period, surpassing the performance of most reported systems. To establish the generality of this phenomenon and rule out interference from Ni species, the relationship between product distribution and catalyst amount was systematically examined across a series of archetypal catalysts (CeO₂, MoO₃, P25, Ag/P25, etc.). Collectively, a low catalyst amount (5.0 mg) selectively yields H₂ and CO (>60%), whereas a higher amount (200.0 mg) drives product distribution toward more thermodynamically favored CH₄ (>60%). In situ XPS/DRIFTS and D₂O KIE experiments reveal that Ni/CeO₂ aggregation elevates interfacial photogenerated charge density to favor CH₄/C₂H₆ production over CO/H₂. This selectivity switch originates from two synergistic mechanisms: (i) the conversion of weakly adsorbed monodentate carbonate (m-CO₃²⁻) into strongly adsorbed bidentate carbonate (b-CO₃²⁻), which promotes CO₂ activation, stabilizes carbon-containing intermediates, and unlocks a formaldehyde (HCHO) route for CH₄/C₂H₆ formation that bypasses *CO intermediates; and (ii) the reconfiguration of interfacial water into an asymmetric, low-connectivity network that facilitates proton supply kinetically restricting its transport to suppress H₂ generation. Notably, a domino redirection of the CO₂ reduction pathway is triggered when interfacial photogenerated charge density reaches a critical threshold sharply defined by the catalyst amount and aggregation state. Consequently, interfacial photogenerated charge density is a necessary and sufficient condition for switching the CO₂ reduction pathway, whereas heat primarily governs the production rates of CH₄/C₂₊. Furthermore, a reliable evaluation of intrinsic catalytic activity requires that the CO₂ reduction pathway be independent of the catalyst amount and, hence, of its aggregation state.