Determining the structure and properties of CO2 reduction photocatalysts: single atom cobalt atoms supported on various carbon nitrides†

Carbon nitride materials paired with Co atoms have been shown to be effective for CO₂ photoreduction. However, the structures of Co/carbon nitrides are uncertain, especially when the degree of polymerization of the carbon nitrides is unknown. Literature has focused predominantly on the fully condensed carbon nitride, g-C₃N₄, despite evidence of other carbon nitrides being present in carbon nitride catalysts. We therefore used density functional theory (DFT) to model Co binding with molecular, partially condensed polymeric, and fully condensed polymeric carbon nitrides. We found strong binding of Co to the carbon nitrides, and larger coordination tended to lead to stronger binding of Co, as well as more cationic Co atoms. Co had a significant effect on photocatalytic processes. Co lowered band gaps significantly, enabling greater photoexcitation yields. Activation of CO₂ into a bent, anionic state is an important initial step during CO₂ reduction. Our calculations show that all Co/carbon nitride systems, except two-layer carbon nitrides, activated CO₂. For bent CO₂, strong interactions occurred between Co/carbon nitride systems, as evidenced by density of states plots and calculated interaction energies. On the other hand, weak interactions occurred with adsorbed linear CO₂. Activation energies of CO₂ spanned a wide range of values (−2.33 to −0.38 eV), and intermediate values over partially condensed carbon nitrides may be more likely to enable CO₂ reduction. Our work provides insights and understanding of Co/carbon nitride catalysts, and motivates study of carbon nitrides beyond g-C₃N₄ as photocatalysts.