Molecular engineering of donor–acceptor heptazine-based porous organic polymers for selective photoreduction of CO2 to CO under non-sacrificial conditions in water

Abstract

Solar-driven photocatalytic CO₂ reduction in water often relies on sacrificial agents, photosensitizers, and metal or metal-based photocatalysts to boost the photocatalytic activity, and dependency on such auxiliary components can constrain its practical application. In this work, the successful synthesis of metal-free heptazine-based polymeric networks by incorporating diacetylene (–CC–CC–) and acetylene (–CC–) linkages is presented, and to the best of our knowledge, this is the first time that a metal-free photocatalyst was utilized for photocatalytic CO₂ reduction in water under non-sacrificial ambient conditions to synergistically produce CO and H₂O₂. Notably, the CO evolution rates were 8.83, 6.23, and 3.67 mmol g⁻¹ h⁻¹ for HEP-BTET, HEP-BDDA, and HEP-EDDA, respectively. Astonishingly, the apparent quantum yield (AQY) for CO production for HEP-BTET is about 14% at 500 nm wavelength. A sequence of control experiments along with ¹³CO₂ labeling experiments, density functional theory (DFT), and electron paramagnetic resonance (EPR) studies provided deep insight into the mechanism of CO₂ reduction using these polymeric networks. These findings highlight the significant potential of metal-free polymeric photocatalysts for sustainable and efficient CO₂ reduction under non-sacrificial conditions, offering an environmentally friendly alternative.