Graphitic carbon nitride with the pyridinic N substituted by Al and Si as efficient photocatalysts for CO2 reduction

Abstract

Efficient photocatalysts to reduce CO₂ into fuels are still scarce. Graphitic carbon nitride (g-C₃N₄) is a promising metal-free photocatalyst, and replacing its lattice atoms by other elements is one common strategy to improve its activity in the CO₂ reduction reaction (CO₂RR). Using ten kinds of elements in groups IIIA to VIA as dopants to replace the pyridinic N in g-C₃N₄, our theoretical calculations uncover a strong correlation between electronegativity and Bader charges of the dopants and the reaction barrier in the initial step of the CO₂RR, which reaches 1.72 eV on the pristine g-C₃N₄. We find that the barrier is inversely proportional to electronegativity for the elements possessing weak metallicity (B, O, P, S, As, and Se) whose reduction product is COOH*, and to Bader charge for those possessing strong metallicity (Al, Si, Ga, and Ge) whose reduction product is HCOO*. Si-doped g-C₃N₄ exhibits excellent activity and selectivity in producing HCOOH with a rate-determining barrier of just 0.57 eV, while Al-doped g-C₃N₄ is a promising catalyst to produce CH₄ with all the barriers smaller than 0.81 eV except a 1.21 eV barrier in its rate-determining step. Activities of these two systems are comparable to that of g-C₃N₄ modified by noble metal atoms. Si and Al doping also significantly enhances visible light absorption. In contrast, g-C₃N₄ doped by the elements possessing weak metallicity is inefficient for the CO₂RR due to the barrier being higher than 1.36 eV. This work may provide a strategy for the design of efficient CO₂RR catalysts based on metal-free materials.