Synthesis Of Tungsten And Molybdenum Based Carbides And Their Catalytic Hydrodeoxygenation Properties

Transition metal carbides have been of great interest because of their noble-metal-like properties.Meanwhile,transition metal carbides have been proven to be a class of challenging materials to work with due to complications related to（i）phase purity,（ii）surface carbon,（iii）surface termination,（iv）surface oxides,（v）surface defects,and（vi）surface roughness/area.In this work,phase-pure W₂C nanorods with“clean”surface and different amounts of vacancy defects were prepared by pyrolysis of metatungstate and melamine hybrid nanorods with nanoscale periodic structure synthesized in the aqueous phase.The activity of different W₂C catalysts was evaluated by the hydrodeoxygenation of benzofuran at 320 to 350℃.The results show that the vacancy defect sites in W₂C are the keys to the high reactivity of W₂C.High pyrolysis temperature,lengthening pyrolysis time and introduction of proper amount of hydrogen,properly increasing temperature ramping rate are favorable to form more vacancy sites which lead to higher activity in hydrodeoxygenation of benzofuran.Rich vacancy sites in the tungsten carbides exhibit excellent oxophilicity and good scission effect on C_ar-O.C_ar-O bond is cleaved in the case of unsaturated aromatic ring,thereby reducing the consumption of hydrogen.It is found that the apparent activation energy of each chemical bond cleavage is linearly related to its bond dissociation energy.In view of the modification and synergistic effect to single metal carbides after the introduction of a second metal,the catalytic hydrodeoxygenation activity of the carbides was significantly improved by synthesizing nickel-tungsten bimetallic carbide nanoparticles with different W/Ni ratios.When W:Ni=6（the mass fraction of nickel is only 5%）,the catalytic deoxygenation activity and hydrogenation activity of the obtained nickel-tungsten bimetallic carbide were increased to 17 times and 11times that of W₂C,respectively.However,when the content of nickel was doubled,the reaction performance was not doubled,but slightly increased.And the activity of phase-pure Ni₆W₆C was weaker than the activity of bimetallic carbides containing a small amount of W₂C.It shows that W₂C and Ni₆W₆C have synergistic effect and the formation of a small amount of Ni₆W₆C will greatly improve the catalytic activity.However,we found that the same method can not be used to prepare cobalt-tungsten bimetallic carbides,so we have established a general low temperature route for the synthesis of nanostructured phase-pure bimetallic carbides of molybdenum and tungsten with group VIII first row transition metals via carbothermal hydrogen reduction of hydrothermal treated bimetallic oxides.As a representative to study,Ni₆Mo₆C exhibited good performance in the hydrogenation of naphthalene,which can be attributed to its porous nanostructure,“clean”surface and special electronic properties.