Mechanistic Insights into CO2 Photoreduction to HCOOH on BH- and NH-Modified g-C3N4-Based Photocatalysts

Abstract Graphitic carbon nitride (g-C3N4) is an attractive metal-free platform for the photocatalytic conversion of CO2 into value-added products, but its activity is limited by poor visible-light absorption, fast electron–hole recombination, and the intrinsic inertness of the CO2 molecule. This computational study employs density functional theory (DFT) and its time-dependent formulation (TD-DFT), complemented by multistate CASPT2 calculations, to map a complete CO2 photoreduction pathway toward formic acid (HCOOH) on a triheptazine (THZ) model bearing BH and NH substitutions. CO2 activation is found to occur spontaneously in the electronic ground state (S0), but only under cooperative adsorption, in which the boron-containing fragment binds H2O while the NH-substituted fragment accepts an electron from the photocatalyst, producing a bent CO2– radical anion; no activation is observed on the unsubstituted THZ. Along the reaction pathway, three energy barriers are too high to be surmounted thermally, namely C–C bond cleavage (2.05 eV), C–O bond cleavage (0.69 eV), and water reformation at the boron site (1.18 eV). These barriers are surmounted by light excitation to the S1 state. For each of them, a conical intersection between the S0 and S1 potential energy surfaces was located in the vicinity of the preceding intermediate. These crossings provide efficient nonradiative funnels driving the system over each barrier. The results show that controlled BH and NH substitution couples visible-light absorption, charge separation, and excited-state energy funneling within a single metal-free model, while preserving the substitution pattern responsible for absorption throughout the catalytic cycle.