Photocatalytic oxidation of 5-hydroxymethylfurfural to furandicarboxylic acid over the Au-Ag/TiO2 catalysts under visible light irradiation

In this work, plasmonic monometallic Ag or Au nanoparticles (NPs) and bimetallic Au-Ag NPs supported on TiO2 with tunable bandgap structure were prepared by incipient impregnation method. Visible-light-driven photocatalytic oxidation of 5-hydroxymethylfurfural (5-HMF) to furandicarboxylic acid (FDCA) was used to evaluate their catalytic properties of the bimetallic catalysts. The incorporation of Au NPs into Ag NPs was capable of tuning their d-band centers to upshift toward the Fermi Level of the bimetallic catalysts, as evidenced by density function theory calculations and valence band XPS measurements. The bimetallic catalysts have narrower bandgap energies and better efficiencies in photogenerated charge separation and transfer, thus resulting in higher catalytic activities in the photocatalytic oxidation of HMF compared with the monometallic Au or Ag NPs catalyst. More interestingly, the monometallic Ag NPs catalyst only gave rise to an oxidized intermediate, i.e. 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), whereas the monometalic Au NPs and the bimetallic catalysts could afford FDCA because of their d-band center differences. The bimetallic catalysts also showed an excellent stability at least five successive runs in the photocatalytic oxidation of HMF to FDCA. This work offers a novel strategy for designing bimetallic catalysts used for the oxidation of 5-HMF to FDCA under visible light irradiation by mediating the d-band centers to upshift toward the Fermi Level.