Preparation Of Molybdenum Carbide With Resin And New Insights Into Their Catalytic Mechanisms For Dry Reforming Of Methane

The molybdenum carbide system has several phases,and among them theβ-Mo₂C andα-MoC_1-x were usually highly studied as catalysts.Meanwhile,theα-MoC_1-x-x showed higher activity and stability than theβ-Mo₂C in the dry reforming of methane（DRM）reaction.The molybdenum carbide was usually synthesized by temperature programmed reduction（TPR）of MoO₃ precursor under mixtures of hydrocarbons and hydrogen.Unfortunately,the pyrolysis of gas-state cabon sources would generate polymetric carbon.One strategy was to develop a cabothermal reduction with the solid-state carbon souce for the synthesis of molybdenum carbide,which can avoid the unnecessary carbonaceous deposition.However,these common solid-state carbon souces were limited to the preparation ofβ-Mo₂C.Therefore,it was attractive to develop a novel solid-state carbon souce for the synthesis of bothβ-Mo₂C andα-MoC_1-x.In this paper,the anion resin（D201）and（NH₄）₆Mo₇O₂₄H₂O were used as carbon source and molybdenum source,respectively.Three types of precursors were prepared,named as precursor-IE,precursor-IWI and precursor-MM.Theβ-Mo₂C andα-MoC_1-x-x were obtained by heating of these precursors under different temperatures and different atmospheres（Ar or H₂）.The products had been characterized by XRD,XPS,TEM,BET and TG-MS measurements in order to investigate their structures,morphologies and formation mechanisms.We compared the DRM performances between theβ-Mo₂C andα-MoC_1-x and studied the reason for deactivation and catalytic mechanisms.The main results are as follows:Highly dispersedβ-Mo₂C andα-MoC_1-x nanoparticles with average diameters of 3.88±1.21 and 1.79±0.72 nm,respectively,were synthesized by a simple resin-based carbothermal reduction route.It was interesting to note that the nature of the Mo carbide phases obtained considerably depended on precursor preparation method and flowing gas composition.In Ar flowing gas,the precursors prepared by ion-exchange,impregnation and mechanical mixture methods were transformed intoα-MoC_1-x,α-MoC_1-x/β-Mo₂C mixture andβ-Mo₂C,respectively.However,in H₂ flowing gas all the precursors were converted toβ-Mo₂C,regardless of the precursor preparation method.The complete synthesis processes involved reduction-carburization of Mo oxide precursors via the pathways of nontopotactic MoO₂→β-Mo₂C or MoO_x(Mo⁴⁺or/and Mo⁵⁺)→MoO_xC_y→β-Mo₂C,and topotactic MoO_x(Mo⁵⁺)→MoO_xC_y→α-MoC_1-x.It was generally accepted that there were two possible mechanisms of dry reforming of methane（DRM）onβ-Mo₂C,redox mechanism（involvingβ-Mo₂C（35）MoO₂）and noble metal type mechanism,with the former much more dominant than the latter.In this work,we addressed for the first time the differences in the two mechanisms betweenβ-Mo₂C andα-MoC_1-x.Over theα-phase,the redox mechanism involved the transformation ofα-MoC_1-x→MoO₂（35）β-Mo₂C and this pathway was minor in comparison to noble metal type mechanism.Clearly,the contribution of noble metal type mechanism to DRM activity on theα-phase was more than that on theβ-phase,which was proposed to account for the better stability of the former than the latter.It was therefore suggested that the noble metal type mechanism should be preferable for DRM over carbide catalysts.