Sulfone-Functionalized Covalent Heptazine Polymeric Networks for Selective CO2 Photoreduction in Water: Mechanistic Insights into Sacrificial Donor-Dependent Selectivity

The urgent need to mitigate CO₂ emissions and develop sustainable energy alternatives has driven extensive research into photocatalytic CO₂ reduction. In this study, a rationally designed sulfone-functionalized covalent heptazine polymeric networks (HSF) that achieves highly efficient and selective CO₂ photoreduction in pure water is reported. The introduction of sulfone units enhances photogenerates charge separation, optimizes CO₂ adsorption, and improves protonation properties, leading to a remarkable CO production rate in pure water with >99% selectivity under visible light irradiation. An exceptionally high apparent quantum yield (AQY) of 32.13% at λ = 400 nm has been achieved- the highest ever reported to date for any metal-free photocatalyst. Intriguingly, by introducing triethylamine (TEA) as a sacrificial electron donor, the product selectivity shifts from CO to CH₄, achieving 80% CH₄ selectivity. Mechanistic insights from in situ transient absorption spectroscopy and density functional theory (DFT) calculations reveal that the sulfone moiety plays a pivotal role in facilitating charge carrier dynamics and modulating the adsorption configuration of CO₂. Furthermore, it shows that TEA not only acts as a hole scavenger but also functions as a proton transfer agent, enabling efficient protonation and the stepwise reduction of *CO to CH₄.