Cobalt-porphine catalyzed CO2 electro-reduction: a novel protonation mechanismElectronic supplementary information (ESI) available. See DOI: 10.1039/c7cp01881a

The urgent need for artificially fixing CO 2 calls for catalysts of high efficiency. The transition metal functionalized porphyrin (TMP) is one of the most important types of organic catalysts for CO 2 reduction. However, the catalytic mechanisms of TMP in CO 2 reduction still remain controversial. Starting from the previously neglected catalyst self-protonation model, we uncover a new CO 2 reduction mechanism on cobalt-porphine, which involves an indirect proton transfer step occurring at the beginning of the reduction cycle. Based on this protonation mechanism, we demonstrate the different correlations between producing rate and pH for the formation of CO and methane, in good agreement with available experimental observations. Our results reveal how pH and potential affect the CO 2 reduction process, providing important clues and insights for further optimization of TMP catalysts. The presence/absence of [CoP·H] + determines the different producing rate-pH correlations for the formation of CO and hydrocarbons.