Photocatalytic CO Reduction with Dissolved Carbonates and Near-Zero CO(aq) by Employing Long-Range Proton Transport.

Photocatalytic CO reduction (COR) in ∼0 mM CO(aq) concentration is challenging but is relevant for capturing CO and achieving a circular carbon economy. Despite recent advances, the interplay between the CO catalytic reduction and the oxidative redox processes that are arranged on photocatalyst surfaces with nanometer-scale distances is less studied. Specifically, mechanistic investigation on interdependent processes, including CO adsorption, charge separation, long-range chemical transport (∼100 nm distance), and bicarbonate buffer speciation, involved in photocatalysis is urgently needed. Photocatalytic COR in ∼0 mM CO(aq), which has important applications in integrated carbon capture and utilization (CCU), has rarely been studied. Using 0.1 M KHCO (aq) of pH 7 but without continuously bubbling CO, we achieved ∼0.1% solar-to-fuel conversion efficiency for CO production using Ag@CrO nanoparticles that are supported on a coating-protected GaInP photocatalytic panel. CO is produced at ∼100% selectivity with no detectable H, even with copious protons co-generated nearby. CO flux to the Ag@CrO COR sites enhances CO adsorption, probed by in situ Raman spectroscopy. CO is produced with local protonation of dissolved inorganic carbon species in a pH as high as 11.5 when using fast electron donors such as ethanol. Isotopic labeling using KHCO was used to confirm the origin of CO from the bicarbonate solution. We then employed COMSOL Multiphysics modeling to simulate the spatial and temporal pH variation and the local concentrations of bicarbonates and CO(aq). We found that light-driven COR and CO reactive transport are mutually dependent, which is important for further understanding and manipulating COR activity and selectivity. This study enables direct bicarbonate utilization as the source of CO, thereby achieving CO capture and conversion without purifying and feeding gaseous CO.