Both XANES and XPS investigations point to the presence of Co + species in our Co-CN photocatalyst. DFT calculations predict an exoergonic adsorption (ΔG= -2.65 eV)of water on a Co 2+ that is originally coordinated to three pyridinic (pyr) N atoms (at distances of 2.15-2.17-2.18 Å, see Figure in supplementary Information), leading to the formation of H2O-Co 2+ -N(pyr) (Figure with O-Co distance of 2.02 Å), which is adopted as the initial state of the photocatalytic cycle.