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

Efficient photocatalysts to reduce CO2 into fuels is still lack. 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 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, Se) whose reduction products are COOH* and to Bader charge for those possessing strong metallicity (Al, Si, Ga, Ge) whose reduction products are 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 g-C3N4 modified by noble metal atoms. Si and Al doping also enhances significantly visible light absorption. In contrast, g-C3N4 doped by the elements possessing weak metallicity is inefficient for CO2RR due to the barrier higher than 1.36 eV. This work may provide a strategy for the design of efficient CO2RR catalysts based on metal-free materials.