Ni-NC and Fe-NC Single-Atom Catalysts on Ti-doped Hematite for Selective Photoelectrochemical Glycerol Oxidation

Abstract

Glycerol, a major byproduct of biodiesel production, can be selectively oxidized into high-value chemicals; however, efficient and selective conversion remains challenging due to complex C-O bond cleavage pathways. Photoelectrochemical glycerol oxidation with α-Fe2O3 photoanodes could offer a sustainable solution, but α-Fe2O3 suffers from inadequate charge separation and surface catalytic activity. Herein, we address these challenges by constructing hybrid photoanodes through integration of Ti-doped α-Fe2O3 (Ti-Fe2O3) with single metal atom catalysts in N-containing carbon (M-NC, M = Fe or Ni). The structural and functional synergy of Ti-Fe2O3/M-NC photoanodes was investigated using physicochemical characterization, photoelectrochemical measurements, and density functional theory (DFT) calculations. Formation of a Schottky junction at the Ti-Fe2O3/M-NC interface generates a built-in electric field that enhances charge-carrier separation and directional hole transfer to M-NC active sites. Consequently, Ti-Fe2O3/Ni-NC and Ti-Fe2O3 /Fe-NC achieve photocurrent densities of 2.2 and 1.9 mA cm -2 at +1.23 V RHE , representing ~3-and ~2.4-fold improvements over bare Ti-Fe2O3 (0.8 mA cm-2 ). Moreover, M-NC incorporation enables tuning of the product selectivity: Ni-NC favors glyceric acid formation (~80% selectivity), whereas Fe-NC promotes glycolic acid production (~74% selectivity) at +0.8 V RHE . With increasing applied potential (from +0.8 to +1.2 VRHE ), Ni-NC induces pronounced changes in product distribution to formic acid, while Fe-NC maintains high glycolic acid selectivity with enhanced yields. DFT calculations attribute these metal-dependent selectivity trends to intrinsic differences in oxidation activity between Fe-NC and Ni-NC. Selectivity changes of Ni-NC may also arise from oxidized Ni clusters formation, as confirmed by post-reaction microscopy. This work highlights the potential of Ti-Fe2O3/M-NC photoanodes for selective biomass valorization and solar-driven photoelectrochemical reactions.