HAN Shixing, FANG Zhonghai, LI Xiaoxiong, LIN Feifei, XIAO Zhaohui
(School of Materials Science and Engineering, Hainan University, Haikou 570228, Hainan, China)

Extended Abstract: [Background and purpose] At present, the synthesis of FDCA from HMF mainly relies on the traditional liquid - phase catalytic reaction system. This reaction process requires the use of not only noble metal catalysts, such as platinum (Pt), gold (Au) and palladium (Pd), but also toxic strong oxidants, such as potassium dichromate (K₂Cr₂O₇) or potassium permanganate (KMnO₄). The reaction conditions are rather harsh, usually requiring relatively high temperatures (100–200 ℃) and high pressures (>5 bar) to effectively achieve the conversion of HMF. Therefore, efforts have been constantly made to seek more environmentally friendly and economical methods. Electrocatalysis, as an emerging technology, can be used for the conversion of HMF under milder conditions, while avoiding the use of toxic oxidants and noble metal catalysts. Noble metal catalysts exhibit certain performance in the electrocatalytic conversion of HMF, but their high cost and scarce raw materials are significant drawbacks. Therefore, the development of highly efficient and inexpensive non-noble metal catalysts has become a research hotspot in the field of biomass conversion.[Methods] Electrodeposition method was used to synthesize the catalysts. In an electrolyte with a total metal salt concentration of 50 mmol·L⁻¹, hydroxides of nickle-cobalt and other elements (Ni-Co-OH) were formed on titanium mesh substrate. After electrodeposition for 10 min, a blue-cyan NiCo hydroxide sample was obtained. Subsequently, the sample was calcined at 250 ℃ for 2 h in air, at a heating rate of 1 ℃·min⁻¹, converting the bimetallic cobalt-based hydroxide into Ni_xCo_yO₄. Electrocatalytic oxidation activity of HMF of the Ni_1.5Co_1.5O₄ catalyst could be optimized by adjusting the ratio of nickel. The regulation of Ni content could be used to enhance the conductivity of the catalyst, increase the number of active sites and adjust the electron density and energy-level distribution in the spinel structure, thereby improving the HMFOR activity. In situ Raman and in-situ EIS were combined to explore the mechanisms and roles of Ni and Co in Ni_1.5Co_1.5O₄ during the catalysis of HMFOR.[Results] Benefiting from the flexible metal-ion substitution mechanism and structural stability of the Co-based spinel, Ni_1.5Co_1.5O₄ exhibited nanosheet morphology and large specific surface area. There is an inter-ionic charge transfer between Ni and Co. In Ni_1.5Co_1.5O₄, electrons transfer from Ni to Co, making Co an electron-rich center and Ni an electron-deficient center. Secondly, the introduction of Ni increased the electrochemical active surface area of Ni_1.5Co_1.5O₄ and the charge-transfer efficiency inside the electrode, thus improving the performance and efficiency of HMFOR. Eventually, a HMF conversion rate of 99.77%, a FDCA yield of 98.10%, and a Faradaic efficiency of 97.86% were achieved.[Conclusions] For Ni_1.5Co_1.5O₄, on one hand, the Co-based spinel structure inherently possessed high structural stability. With the substitution of Ni ions, no significant lattice distortion or structural collapse occurred. On the other hand, Ni ions and Co ions are similar in size and charge, Ni ions can flexibly replace Co ions at different sites without disrupting the overall lattice structure. As a result, the nanosheet structure of the Co-based spinel is well-maintained, thus leading to increase in electrochemical specific surface area and number of active sites. There is an inter - ionic charge transfer between Ni and Co, rendering Co an electron - rich center and Ni an electron - deficient center. At applied voltages, Co³⁺ is more likely to be oxidized to generate highly active Co⁴⁺ for HMF. This active species key to the -OH—-CHO reaction. The results of in-situ Raman spectra indicated that the surface of Ni_1.5Co_1.5O₄ underwent reconstruction at applied voltages. The formation of Ni^3+/Ni⁴⁺ at relatively high applied voltages led to a competitive reaction between the oxygen evolution reaction (OER) and HMF. In the synergistic effect of Ni and Co, apart from contributing catalytic activity, Co also played a role in maintaining the structural stability of the catalyst.
Key words: cobalt-based spinel; 5-hydroxymethylfurfural; electrocatalysis; reaction mechanism; in situ characterization