Identification of active sites and their redox strength to select products from photocatalysis in aqueous solutions

Abstract

Solar-driven photocatalytic H2 evolution from aqueous solutions is promising for sustainable energy production, but it typically requires sacrificial electron donors to expedite the evolution. The fate and reaction pathways of these donors, especially their selective oxidation driven by active sites with distinct redox strengths, are rarely studied. The present study demonstrates that the reforming of sacrificial methanol in photocatalytic H2 evolution can be directed toward selective production of value-added C2 chemicals by tuning the redox properties of co-catalyst-derived active sites. Model catalysts, potassium poly(heptazine imide) (KPHI) decorated with Pt and CoP co-catalysts, i.e., Pt@KPHI and CoP@KPHI, are used to elucidate the reaction pathways, where acetate and ethylene glycol are the predominant liquid-phase products, respectively, alongside the gaseous H2 evolution. In the Pt@KPHI system, Pt serves as a strong electron trap to effectively reduce water into H2 and methanol into •CH3 and induces hole-driven deep oxidation of methanol into •CO2− on KPHI. The C−C coupling of •CH3 and •CO2− radicals selectively forms acetate. In the CoP@KPHI system, CoP served as a mild hole trap to proceed with methanol dehydrogenation into •CH2OH radicals and then ethylene glycol, while mild reduction takes place over KPHI to produce H2, presenting parallel redox reactions without any interaction. Our work illustrates how co-catalyst-induced active sites govern charge transfer and interaction between the redox reactions, thus selectively producing valuable chemicals from H2-evolution photocatalysis.