W2C nanorods with various amounts of vacancy defects: determination of catalytic active sites in the hydrodeoxygenation of benzofuran

Abstract

Transition metal carbides have been of great interest because of their noble-metal-like properties. Because of the complexity of their structures, it is crucial to design an experiment that can eliminate the influence of supports, surface carbon contamination and particle sizes when finding the exact catalytic active sites. In this work, phase-pure W₂C nanorods (lengths of 2–4 μm and diameters of 100–600 nm) with different amounts of crystal defects were prepared by the pyrolysis of metatungstate and melamine hybrid nanorods with nanoscale periodic structure synthesized in the aqueous phase. The nanoscale alternating structure between tungsten oxide and melamine effectively promotes the reduction of tungsten oxide and the formation of tungsten carbide, avoiding the deposition of carbon on the surface. At the same time, vacancy defects are generated due to the deficiency of carbon. High pyrolysis temperature (900 °C), prolonged pyrolysis time (4 h), introduction of hydrogen and the proper increase of the temperature ramping rate (5 °C min⁻¹) are favorable to the formation of vacancy defects. The activities of different catalysts were evaluated by the hydrodeoxygenation of benzofuran at 320 to 350 °C. The results show that the vacancy defect sites in W₂C are the key to the high reactivity of W₂C. The vacancy defect sites have favorable properties for the cleavage of carbon–oxygen bonds. The C_aromatic–O bond is cleaved in the case of unsaturated aromatic rings, thereby reducing the consumption of hydrogen. In addition, it is found that the apparent activation energy of each chemical bond is linear-like and positively related to its bond dissociation energy.