A biomimetic photoelectrocatalyst of Co-porphyrin combined with a g-C3N4 nanosheet based on π-π supramolecular interaction for high-efficiency CO2 reduction in water medium

Aiming at high-efficiency biomimetic CO2 photoelectrochemical conversion, a photoelectrocatalyst with excellent CO2 catalytic activity was designed and prepared by immobilizing CoTPP (cobalt meso-tetraphenylporphyrin) onto g-C3N4 conveniently via self-assembly based on pi-pi supramolecular interaction. The quasi 3-D structure of CoTPP showed a suitable hole with a size of 3.8 angstrom x 9.6 angstrom which favoured CO2 adsorption. The pores formed by the pi-pi stacking of CoTPP and g-C3N4 also provided additional space for CO2 adsorption, which was confirmed by the appearance of a desorption peak at 250 degrees C in the temperature programmed desorption measurement for CoTPP/g-C3N4. As a normal efficient homogeneous catalyst in organic media, CoTPP commendably maintained outstanding CO2 photoelectrocatalytic activity in heterogeneous aqueous solution, even at a low overpotential of -0.6 V (vs. normal hydrogen electrode, NHE). Under 8 h PEC CO2 reduction, formic acid generation on CoTPP/g-C3N4 reached 154.4 mu mol with a TON of 137 and high selectivity of nearly 100% in liquid products. The formation of CoTPP-COO- and CoTPP-COOH intermediates by the Co(II) active site and CO2 was investigated by in situ UV-vis and Raman spectra. Moreover, an isotopic labelling experiment indicated that water supplied abundant protons for the production of formic acid.