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

Photocatalytic CO2 reduction (CO2R) in & SIM;0mM CO2(aq) concentration is challenging but is relevantfor capturing CO2 and achieving a circular carbon economy.Despite recent advances, the interplay between the CO2 catalyticreduction and the oxidative redox processes that are arranged on photocatalystsurfaces with nanometer-scale distances is less studied. Specifically,mechanistic investigation on interdependent processes, including CO2 adsorption, charge separation, long-range chemical transport(& SIM;100 nm distance), and bicarbonate buffer speciation, involvedin photocatalysis is urgently needed. Photocatalytic CO2R in & SIM;0 mM CO2(aq), which has important applicationsin integrated carbon capture and utilization (CCU), has rarely beenstudied. Using 0.1 M KHCO3 (aq) of pH 7 but without continuouslybubbling CO2, we achieved & SIM;0.1% solar-to-fuel conversionefficiency for CO production using Ag@CrOx nanoparticlesthat are supported on a coating-protected GaInP2 photocatalyticpanel. CO is produced at & SIM;100% selectivity with no detectableH(2), even with copious protons co-generated nearby. CO2 flux to the Ag@CrOx CO2R sites enhancesCO(2) adsorption, probed by in situ Raman spectroscopy. COis produced with local protonation of dissolved inorganic carbon speciesin a pH as high as 11.5 when using fast electron donors such as ethanol.Isotopic labeling using (KHCO3)-C-13 was used toconfirm the origin of CO from the bicarbonate solution. We then employedCOMSOL Multiphysics modeling to simulate the spatial and temporalpH 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 importantfor further understanding and manipulating CO2R activityand selectivity. This study enables direct bicarbonate utilizationas the source of CO2, thereby achieving CO2 captureand conversion without purifying and feeding gaseous CO2.