Allotropes selection apropos of photocatalytic CO₂ reduction from first principles studies

Materials engineering of allotropism provides a possible strategy for controling CO₂ photoreduction into various C1 products. However, the understanding of the mechanism for tuning products selectivity through allotropism is still lacking. Herein, three structures of known phosphorus allotrope, i.e. black phosphorus (BP), fibrous red phosphorus (RP), and helical coil phosphorus (CP), were modelled as a CO₂ reduction photocatalyst and studied by first principles density functional theory calculations. Three adsorption sites (P_6C, P_2B, and P_T) of CO₂ on the phosphorus allotropes were investigated. The most stable adsorption configuration for each allotrope was then selected for further study. The quantum topological analysis revealed that CO₂ adsorption interaction on BP and RP exhibits physical binding of van der Waals interaction while CP shows physical binding of Keesom interaction. The CO₂ adsorption interaction is shown to be crucial in determining the activation barrier for the initial proton-coupled electron transfer of CO₂ → COOH. The pathways for different C1 products, including CO, HCOOH, C, CH₂O, CH₃OH, and CH₄, were examined by comparing the rate determining step (RDS) obtained via Gibbs free energy analysis. RP and CP show higher selectivity towards the two- and four-electron C1 products. CP evidences a lower activation barrier for the two- and four-electron products RDS while BP exhibits higher selectivity towards the six- and eight-electron reduction products. The difference in selectivity of each of the allotropes was attributed to its distinct p-band center. This study divulges an important understanding on the possible product selectivity modulation for CO₂ photoreduction based on allotropism.