Photocatalytic CO₂ Reduction with Dissolved Carbonates and Near-Zero CO₂(aq) by Employing Long-Range Proton Transport

Photocatalytic CO₂ reduction (CO₂R) in ∼0 mM CO₂(aq) concentration is challenging but is relevant for capturing CO₂ and achieving a circular carbon economy. Despite recent advances, the interplay between the CO₂ 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 CO₂ adsorption, charge separation, long-range chemical transport (∼100 nm distance), and bicarbonate buffer speciation, involved in photocatalysis is urgently needed. Photocatalytic CO₂R in ∼0 mM CO₂(aq), which has important applications in integrated carbon capture and utilization (CCU), has rarely been studied. Using 0.1 M KHCO₃ (aq) of pH 7 but without continuously bubbling CO₂, we achieved ∼0.1% solar-to-fuel conversion efficiency for CO production using Ag@CrO_x nanoparticles that are supported on a coating-protected GaInP₂ photocatalytic panel. CO is produced at ∼100% selectivity with no detectable H₂, even with copious protons co-generated nearby. CO₂ flux to the Ag@CrO_x CO₂R sites enhances CO₂ 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 KH¹³CO₃ 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 CO₂(aq). We found that light-driven CO₂R and CO₂ reactive transport are mutually dependent, which is important for further understanding and manipulating CO₂R activity and selectivity. This study enables direct bicarbonate utilization as the source of CO₂, thereby achieving CO₂ capture and conversion without purifying and feeding gaseous CO₂.