Understanding the nature of vanadium species supported on activated carbon and its catalytic properties in the aerobic oxidation of aromatic alcohols

Vanadium oxide catalysts supported on activated carbon (V/AC) with V loadings ranging from 1 to 20 wt.% were prepared by a wet-impregnation method. Various physicochemical characterization techniques, including nitrogen physisorption, X-ray diffraction (XRD), Raman spectroscopy, X-ray absorption (XANES and EXAFS), X-ray photoelectron spectroscopy (XPS), and electron spin resonance (ESR), were employed to understand the nature of vanadium species on activated carbon. The results revealed that vanadium oxide mainly existed in a highly dispersed state for 10 wt.% or less vanadium loadings; a large amount of vanadium resulted in aggregated microcrystalline phase. Vanadium species on activated carbon surface showed a similar local coordination structure to that of NH₄VO₃ with a distorted tetrahedral symmetry at low vanadium loadings, whereas octahedral coordination was dominant at high vanadium loadings (>10 wt.%). All V/AC samples showed V⁵⁺ as the major oxidation state. Nevertheless, V⁴⁺ centered in a distorted tetrahedral symmetry could be detected at a vanadium loading greater than 4 wt.%. The catalytic activity for the benzyl alcohol oxidation largely depended on the dispersion, oxidation state, and local coordination of vanadium oxides on activated carbon. Highly dispersed vanadium (5+) species with a distorted tetrahedral coordination were postulated to account for the excellent catalytic performances of V/AC catalysts (TOF = 39.1 h^-1).