Photocatalytic CO 2 Reduction with Dissolved Carbonates and Near-Zero CO 2 (aq) by Employing Long-Range Proton Transport.

Photocatalytic CO 2 reduction (CO 2 R) in ∼0 mM CO 2 (aq) concentration is challenging but is relevant for capturing CO 2 and achieving a circular carbon economy. Despite recent advances, the interplay between the CO 2 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 2 adsorption, charge separation, long-range chemical transport (∼100 nm distance), and bicarbonate buffer speciation, involved in photocatalysis is urgently needed. Photocatalytic CO 2 R in ∼0 mM CO 2 (aq), which has important applications in integrated carbon capture and utilization (CCU), has rarely been studied. Using 0.1 M KHCO 3 (aq) of pH 7 but without continuously bubbling CO 2 , we achieved ∼0.1% solar-to-fuel conversion efficiency for CO production using Ag@CrO x nanoparticles that are supported on a coating-protected GaInP 2 photocatalytic panel. CO is produced at ∼100% selectivity with no detectable H 2 , even with copious protons co-generated nearby. CO 2 flux to the Ag@CrO x CO 2 R sites enhances CO 2 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 KH 13 CO 3 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 2 (aq). We found that light-driven CO 2 R and CO 2 reactive transport are mutually dependent, which is important for further understanding and manipulating CO 2 R activity and selectivity. This study enables direct bicarbonate utilization as the source of CO 2 , thereby achieving CO 2 capture and conversion without purifying and feeding gaseous CO 2 .