Optimized Nickel Phosphate Cocatalyst on Ge-Doped Hematite Photoanode for Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid

Abstract

The kinetics of the anode oxygen evolution reaction (OER) in photoelectrochemical (PEC) cells present challenges, motivating the exploration of alternative strategies such as biomass oxidation to yield high-value chemicals. In this study, we investigate the applications of nontoxic, stable, and earth-abundant α-Fe₂O₃ as a photoanode. In the first part of the research, a Ge-doped α-Fe₂O₃ thin film was synthesized via a hydrothermal method, with germanium oxide (GeO₂) introduced into the iron oxide precursor to prepare Ge-doped α-Fe₂O₃ thin films. We then identified a carbonate-bicarbonate solution as a suitable electrolyte for the α-Fe₂O₃ photoanode and for the selective oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-furandicarboxylic acid (FDCA), a precursor for polyethylene 2,5-furandicarboxylate (PEF) synthesis. To further enhance photoelectrochemical oxidation performance, nickel phosphate (Ni–P), a cocatalyst, was loaded on top of the Ge-doped α-Fe₂O₃ by varying electrodeposition time and precursor concentration. Furthermore, we introduced 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a mediator to facilitate selective HMF oxidation. Our results indicated that an electrodeposition time of 30 min and a molar ratio of nickel to phosphate in the precursor of Ni_0.075P (Ni:P = 15:200) yield optimal results for selective oxidation. Relative to the unloaded case, the selectivity of FDCA increases from 35.4% to 67.1% after 12 h of reaction, with a simultaneous increase in yield from 11.8% to 41.0% and HMF conversion reaching 61.1%. α-Fe₂O₃ shows promise as a photoanode for the selective oxidation of HMF, and the incorporation of Ni–P as a cocatalyst significantly contributes to FDCA formation. This research presents an environmentally sustainable approach to harnessing solar energy for the conversion of biomass into valuable chemical products.

In this study, we investigate the applications of nontoxic, stable, and earth-abundant α-Fe₂O₃ as a photoanode. In the first part of the research, a Ge-doped α-Fe₂O₃ thin film was synthesized via a hydrothermal method, with germanium oxide (GeO₂) introduced into the iron oxide precursor to prepare Ge-doped α-Fe₂O₃ thin films. We then identified a carbonate-bicarbonate solution as a suitable electrolyte for the α-Fe₂O₃ photoanode and for the selective oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-furandicarboxylic acid (FDCA), a precursor for polyethylene 2,5-furandicarboxylate (PEF) synthesis. To further enhance photoelectrochemical oxidation performance, nickel phosphate (Ni−P), a cocatalyst, was loaded on top of the Ge-doped α-Fe₂O₃ by varying electrodeposition time and precursor concentration.