Johannes Ehrmaier: The holy grail of solar fuel generation: A theoretically conceived pathway based on carbon nitride molecules

Recently, carbon nitride materials have attracted vast interest in the field of solar water-splitting and energy conversion, as they are photostable, consist of earth abundant materials and can split-water. The detailed mechanism, however, is not yet known. To understand the fundamental reaction steps we use pyridine, triazine and heptazine as molecular model catalysts in our calculations. Based on highly accurate ab initio calculations and non-adiabatic surface hopping trajectory dynamics simulations, we propose a two-step reaction mechanism for solar fuel generation: First, by the absorption of a photon an H-atom from water is transferred to the accepting N-atom of the catalyst in an electron driven proton transfer reaction, generating OH radicals and a hydrogenated catalyst. In the second step, the transferred H-atom can be detached forming molecular hydrogen and regenerating the catalyst. Alternatively, the transferred H-atom can be further transferred to a hydrogen bonded CO2 molecule by the absorption of a second photon, thus reducing CO2 to OCOH, which can be further reduced to methanol. These results strongly support that the underlying reactions for solar energy conversion using carbon-nitride materials are of molecular nature.