Ethylene production as a function of the illumination wavelength closely matches the Co-CN absorption spectrum (Figure ). Therefore, irradiation of H2O-Co 2+ -N(pyr)generates electron/hole pairs and TEOA quenches the photogenerated holes whereas reduction of Co 2+ to Co + via internal photoinduced electron transfer (PET) from the CN support occurs. Charge separation efficiency can be analyzed from the EPR spectrum during irradiation, while the recombination efficiency can be assessed by the decrease in EPR signals after switching off the light. As previously reported, the highest charge separation as well as the lowest recombination efficiency were observed for cobalt-free CN compared to Co-CN (Figure , Supplementary Information) suggesting that in Co-CN the photoinduced electrons are quickly transferred from the support to the cobalt species. Furthermore, the typical EPR signal of (2,2,6,6-tetramethylpiperidine-1-yl)oxyl (TEMPO) was suppressed upon illumination of the reaction mixture both under acetylene or Ar atmosphere (FigureS16, Supplementary Information). Indeed no ethylene was observed under the standard reaction condition in presence of TEMPO (FigureS17, Supplementary Information) pointing to the critical role of photogenerated electrons.