Controlled Synthesis Of Molybdenum Carbides/Nitrides And Their Catalytic Properties

Transition metal carbide（TMC）and transition metal nitride（TMN）can be formed by inserting C and N into the interatomic space of transition metals,which have great catalytic performance in many reactions.In this study,different crystal forms of MoC_x and MoN_x were prepared through temperature programmed reaction（TPR）,which were applied to catalyze the reactions involving hydrogen.In addition,the strategies of morphology regulation and ad-metal loaded were used to improve the activity.The as-prepared catalysts show excellent catalytic performance in ammonia synthesis,ammonia decomposition and hydrogen evolution processes.The main research contents of this paper are as follows.（1）In ammonia synthesis reaction,the mass specific reaction rate ofβ-Mo₂C at 450℃was 7789.5μmol/g/h,higher than that ofα-MoC（5317.3μmol/g/h）.However,the specific surface area ofβ-Mo₂C was only 10 m²/g,much lower than that ofα-MoC（98 m²/g）.The kinetic study and the reaction order test show that the E_a ofβ-Mo₂C is lower thanα-MoC and the reactant molecular coverage onβ-Mo₂C is higher.To further improve the activity ofβ-Mo₂C,theβ-Mo₂C catalysts with specific morphology were synthesized.The mass specific reaction rate ofβ-Mo₂C nanoflowers andβ-Mo₂C nanoplates reached 10484μmol/g/h and10494μmol/g/h at 450℃,respectively,larger than that ofβ-Mo₂C（7789.5μmol/g/h）.It was found thatβ-Mo₂C with morphologies has more active sites and stronger H₂ adsorption capacity by NH₃-TPD and H₂-TPD.The activation ability to H₂ can be enhanced by loading Ru/Pt onα-MoC,through H₂-TPD result,the adsorption capacity of H₂ on catalysts followes:1%Ru/α-MoC>0.1%Pt/α-MoC>α-MoC,which was consistent with the activity.（2）In the ammonia decomposition reaction,it was found that molybdenum carbide used as the catalyst can transform to molybdenum nitride.Among this,different transformation processes have been observed,which show thatα-MoC（fcc）transforms into Mo₂N（fcc）andβ-Mo₂C（hcp）transforms into MoN（hcp）.The kinetic study shows that MoN exhibits higher intrinsic activity than that of Mo₂N.The structural stability of Mo₂N and MoN phases in the reaction process was investigated,and it was found that Mo₂N gradually transforms to MoN at high space velocity condition,which proved that MoN was the stable phase at 650℃.（3）α-MoC exhibits better catalytic activity thanβ-Mo₂C in Hydrogen Evolution Reaction,and the activity was further improved by synthesizing rod-likeα-MoC with higher specific surface area.The reaction mechanism results show that different rate-determining steps（RDS）were detected,with a Heyrovsky reaction overα-MoC and a Volmer reaction overβ-Mo₂C,respectively.In addition,the DFT results further reveal that the binding energy of hydrogen（ΔG_H*）overα-MoC is close to zero,which benefits to the desorption of H₂.It was found that MoN shows better performance when compared with Mo₂N,the activity of Mo₂N was improved when Fe/Co/Ni was loaded.