Mechanistic pathways in the photocatalytic CO2 reduction on halide double perovskite Cs2NaBiX6 (X = Cl/Br/I)

Metal halide double perovskites have recently gained significant attention for their potential applications in optoelectronics, perovskite solar cells, and artificial photocatalysis owing to their tunable bandgaps, abundant activation sites, and favorable photocatalytic properties. Herein, the proton-assisted electron transfer CO₂ photoreduction reaction mechanism on Cs₂NaBiX₆ (X = Cl/Br/I) (100) and (110) surfaces with the aim to elucidate the photocatalytic activity of Cs₂NaBiX₆ catalysts is investigated by using first-principles calculations. The Cs₂NaBiX₆ perovskites demonstrate a strong correlation between their band energy levels and the redox potentials of CO₂ reduction products (e.g., HCOOH, HCOH, CO, and CH₄), underscoring their potential as efficient photocatalysts. Complete reduction pathways reveal that these double perovskites possess abundant active adsorption sites, with CO₂ reduction occurring successfully on Bi, Na, X and Cs sites. Notably, Bi sites emerge as the most effective activation centers on both surfaces, as they exhibit the lowest energy barriers for the rate-determining steps across the reduction pathway. For the proton-assisted two electron transfer process, CO₂ can be reduced to CO and HCOOH. However, HCOOH is more likely to undergo further reduction to CH₃OH or CH₄, rather than being released as a single product, due to the endothermic nature of the HCOOH* → HCOOH desorption step. CH₄ formation is thermodynamically favorable on both surfaces, but can only desorb from the (110) surface, while it remains strongly adsorbed on the (100) surface. The calculated pathways with the lowest energy barriers indicate that the (110) surfaces exhibit superior catalytic activity compared to the (100) surfaces. Intriguingly, the Cs₂NaBiX₆ perovskites display distinct scaling relations between key intermediates for the photocatalytic reduction to CO and CH₄. Among the studied systems, Cs₂NaBiBr₆ demonstrates the highest activity and selectivity for CO₂ reduction, with an activation barrier of 0.67 eV, outperforming Cs₂NaBiI₆ (0.74 eV) and Cs₂NaBiCl₆ (0.99 eV). These findings provide critical insights into the rational design of double perovskite photocatalysts for CO₂ reduction and highlight their potential for sustainable energy applications.