With the proposed mechanism, described above, as the most plausible from our experimental results and DFT calculations, we also considered alternative pathways. We could not rule out proton transfer from proximal protonated pyridinic N with a proton that could be transferred from the water molecule coordinated to the Co site (Figure , Supplementary Information) or from a non-coordinate water molecule (Figure , Supplementary Information) to an uncoordinated proximal pyridinic nitrogen of the CN, thus forming a N-H bond. We note that, addition of pyrrole under the standard reaction conditions led to reduced ethylene production by half, which is ascribed to the possible role of the acidic pyrrole in blocking basic sites such as the pyridinic nitrogen (Figure , Supplementary Information), in agreement with a previous report. The formation of a cobalt hydride species could be ruled out, as it is predicted to be an endoergonic process (ΔG = +1.47 eV) and therefore unlikely to form. Furthermore, illumination of the reaction mixture both under acetylene or Ar atmosphere did not evolve any detectable H2 (Figure ) thus excluding that, for our system, under the adopted working conditions, acetylene semi-reduction and H2 evolution proceed via the same catalytic intermediate with contribution from a cobalt hydride species.