High-Spin Cobalt Oxyhydroxide Enhancing Electrocatalytic Oxidation of 5-Hydroxymethylfurfural to FDCA

The oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is governed by the high-valence metal ion in the catalysts. Spin regulation can modulate the electronic structure of active metal centers and influence the formation of metal-oxo species. In this work, we synthesized cobalt oxyhydroxide enriched with high-spin Co by molybdenum dissolution from the CoMoO₄ precursor. We found that such a formed catalyst has a high content (28.3%) of high-spin cobalt (h-CoOOH). Compared to low-spin cobalt (l-CoOOH) synthesized by the electrodeposition method, h-CoOOH exhibits enhanced HMFOR performance (21.9 times higher current density at 1.2 V versus RHE) and excellent FDCA Faraday efficiency (> 98%). High-spin Co³⁺ facilitates cobalt-oxo formation, accelerating the kinetics of HMF nucleophilic oxidation. In situ Raman and X-ray absorption fine structure (XAFS) reveal that high-spin Co³⁺ induces octahedral distortion, promoting the formation of Co⁴⁺-oxo (Co⁴⁺═O) species. Based on the ¹⁸O labeling experiment, an HMFOR reaction pathway via h-CoOOH in HMFOR was proposed.