Catalytic oxidation of lignin and model compounds over nano europium oxide

The heterogeneous catalyst for depolymerization of lignin has a good application prospect because the catalyst is easy to recover. Although heterogeneous noble metal catalysts exhibit excellent activity under mild conditions, high prices restrict their wide use. In recent reports, nano rare earth oxide catalysts have shown good performance in the catalytic oxidation of lignin, and their prices are relatively low, which has good research value. Our study found that rare earth oxide nanocatalysts have obvious structure-activity relationship in the catalytic oxidation of lignin. To investigate theses structure-dependent behaviour and the reaction mechanism, a series of shape-specific Eu2O3 nanocrystal catalysts (spheres, spindles, sheets and rods) with well-defined crystal facets were synthesized and investigated. The four catalysts show increasing conversions from 54.6% to 91.2% in the sequence of nanospindles, nanospheres, nanosheets and nanorods, under condition of 1 MPa O2, 493 K and 8 h. These results demonstrate that the Eu2O3 nanocrystal is structure-sensitive. It is found that the surface crystal facets of Eu2O3 nanocrystals play crucial roles in the catalytic properties. Among the nanocrystals, Eu2O3 nanorods with well-defined (220) crystal facets displayed superior catalytic activity. It is deduced that the high reactivity of Eu2O3 nanorods is related to the abundant lattice oxygen which is benefit to active oxygen species. A plausible reaction pathway that involves the Mars-van Krevelen mechanism was proposed.