Construction of Monoatomic-Modified Defective Ti4+αTi3+1-αO2−δ Nanofibers for Photocatalytic Oxidation of HMF to Valuable Chemicals

Efficiently upgrading 5-hydroxymethylfurfural (HMF) into high-value-added products, such as 2,5-diformylfuran (DFF) and 2,5-furan dicarboxylic acid (FDCA), through a photocatalytic process by using solar energy has been incessantly pursued worldwide. Herein, a series of transition-metal (TM = Ni, Fe, Co, Cu) single atoms were supported on Ti4+αTi3+1-αO2−δ nanofibers (NFs) with certain defects (Ov), denoted as TM SAC-Ti4+αTi3+1-αO2−δ NFs (TM = Ni, Fe, Co, Cu), aiming to enhance the photocatalytic conversion of HMF. A super HMF conversion rate of 57% and a total yield of 1718.66 μmol g–1 h–1 (DFF and FDCA) surpassing that of the Ti4+αTi3+1-αO2−δ NFs by 1.6 and 2.1 times, respectively, are realized when TM is Co (Co SAC-Ti4+αTi3+1-αO2−δ NFs). Experiments combined with density functional theory calculation (DFT) demonstrate that the TM single atoms occupy the Ti site of Ti4+αTi3+1-αO2−δ NFs, which plays a dominant role in the photo-oxidation of HMF. Raman, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) characterizations confirm the strong electron local exchange interaction in TM SAC-Ti4+αTi3+1-αO2−δ NFs and demonstrate the substitution of Ti by the TM SACs. The projected density of states and charge density difference reveal that the strong interaction between metal-3d and O-2p orbitals forms Ti–O–TM bonds. The bonds are identified as the adsorption site, where TM single atoms on the surface of Ti4+αTi3+1-αO2−δ NFs reduce HMF molecule adsorption energy (Eads). Furthermore, the TM single atom modulates the electronic structure of TM SAC-Ti4+αTi3+1-αO2−δ NFs through electron transfer, leading to narrow band gaps of the photocatalysts and enhancing their photocatalytic performance. This study has uncovered a newer strategy for enhancing the photocatalytic attributes of semiconducting materials.