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-bearing quaterpyridine molecular species can be anchored to a light-absorbing, crystalline, carbon nitride, i.e., yielding a molecular-catalyst/material hybrid capable of facilitating selective CO2 reduction in aqueous solution. This hybrid material leverages the metal center’s ability to bind CO2 upon a one electron reduction transfer from the carbon nitride light-absorbing excitation for efficient cycling of substrate to product. 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. Transient absorption spectroscopy and computational studies outline loading impacts on excited electron lifetimes and evaluate the thermodynamics of the CO2 reduction mechanism on the molecular complex to help explain the observed selectivity (up to 97-98% for CO evolution). Lastly, it was also found that higher light intensities imposed on the hybrid provided initial increases in activity but negatively impacted photocatalytic performance over time. However, when keeping a lower power density in a climate-controlled environment, the hybrid material reached a sustained CO evolution rate of 608 µmol·g-1·h-1 and totaling 305 turnovers over the course of 8 h. This study outlines considerations for leveraging mechanistic impacts of molecular CO2 reduction catalysts on light absorbing surfaces and defines optimal conditions for this system in targeting sustained efficient rates in aqueous solution.