Facile formation of CoN4 active sites onto a SiO2 support to achieve robust CO2 and proton reduction in a noble-metal-free photocatalytic system

Abstract

The conversion of CO₂ and protons to solar fuels (CO and H₂) is the basis of artificial photosynthesis. To date, most of the reported cobalt-based molecular catalysts or single-atom catalysts (SAC) feature a CoN₄ core structure. However, the preparation of these metal complex catalysts or SACs requires either a complicated organic ligand synthesis method or harsh material fabrication conditions. In this study, the facile formation of an active CoN₄ structure onto a SiO₂ support was achieved via self-coordination of Co²⁺ ions with aminated SiO₂ nanoparticles. The formation of the CoN₄ structure was identified by synchrotron-based X-ray absorption spectroscopy. The CoN₄-SiO₂ catalyst exhibited exceptional bifunctional activity of mediating the CO₂-to-CO and 2H⁺-to-H₂ conversions in a photochemical system containing g-C₃N₄ as a photosensitizer. The system robustly produced syngas (CO + H₂, CO/H₂ = 1 : 1–1 : 2) with a high activity (5053 μmol g⁻¹ and 36 μmol g⁻¹ h⁻¹ based on the catalyst) and remarkable stability (durability > 120 h). Mechanistic studies reveal that the Co(I)-species is the active species generated via a photoinduced electron transfer from g-C₃N₄ to CoN₄-SiO₂. The dissociation of Co²⁺ from the aminated SiO₂ support and the decomposition of g-C₃N₄ under irradiation are the main reasons for the inactivation of the system after long-time photocatalysis.