Selective Solar CO2 Conversion into Ethanol Using Atomic-Scale Copper Clusters Anchored π-extended Poly(heptazine imide).

Although photocatalytic CO2 reduction to C1 products has been demonstrated, achieving selective photoconversion to C2 products remains a formidable challenge, as it requires sequential proton-coupled electron transfer and kinetic and thermodynamic control to promote C─C coupling over competing pathways. We address this challenge through the atomic-scale copper (Cu) clusters anchored on carbon-doped potassium poly(heptazine imide) (Cu/C-K-PHI). Carbon doping extends the π-conjugation within poly(heptazine imide) framework, narrowing optical bandgap to 1.89 eV, suppressing photogenerated charge-carrier recombination and enhancing interfacial charge transfer. The anchored Cu clusters lower the kinetic barrier for C─C coupling. Focused on the liquid-phase C─C coupling pathway, Cu/C-K-PHI selectively produces ethanol with ∼100% selectivity among liquid-phase products, exhibiting production rate of 18.98 µmol g-1 h-1, reaching 77.01 µmol g-1 after 4 h and retaining ∼98% of its activity over five cycles under 1-sun illumination without sacrificial agents. The solar-to-ethanol conversion efficiency is 0.175% with apparent quantum yield is 0.516%. In situ attenuated total reflectance-infrared spectra, together with density functional theory calculations, reveal O*C─CO-mediated *CO dimerization pathway, providing a mechanistic origin for CO2-to-C2H5OH selectivity. These results show that synergistic electronic-structure modulation and atomic-scale metal engineering can steer CO2 photoreduction selectively toward ethanol under mild conditions without sacrificial agents.