Illuminating the Mechanistic Impacts of an Fe-Quaterpyridine Functionalized Crystalline Poly(triazine imide) Semiconductor for Photocatalytic CO2 Reduction

The strategy of incorporating earth-abundant catalytic centers into light-absorbing architectures is desirable from the viewpoint of low cost, low toxicity, and versatility at activating small molecules to produce solar-based fuels. Herein, we show that an Fe-quaterpyridine molecular catalyst can be anchored to a light-absorbing, crystalline, carbon nitride (PTI), to yield a molecular-catalyst/material hybrid, Fe-qpy-PTI, capable of facilitating CO2 reduction to CO selectively (up to ~97-98%) in aqueous solution under low-intensity light irradiation. This hybrid material leverages the ability of the Fe-qpy catalyst to bind CO2 upon a one-electron reduction, as achieved by transfer of excited electrons from the carbon-nitride semiconductor. The catalytic activity of the hybrid material was measured across a range of catalyst loadings (from 0.1-3.8 wt %) at a low incident power density of 50 mW·cm-2, resulting in rates of CO evolution up to 596 µmol·g-1·h-1 at a 3.8 wt% loading for a 3 h experiment. The hybrid material attained a CO evolution rate of 608 µmol·g-1·h-1, or 305 turnovers over the course of 8 h for a TOF of ~38. Apparent quantum yields reached up to ~2.4%, but which decreased by ~25% at a higher power density of 150 mW·cm-2. Higher light intensities imposed on the hybrid material provided an initial increase in activity but negatively impacted photocatalytic rates over time. Transient absorption spectroscopy results showed electron survival probabilities that were consistent with the observed product rates. Computational modeling was also used to evaluate and understand the mechanistic pathway of the high product selectivity for CO versus H2. These results have helped unveil key factors to leverage the mechanistic understanding of molecular catalysts for CO2 reduction on light absorbing semiconductors and to establish optimal conditions to attain maximal rates in aqueous solution.