First-principles Calculations Of Hydrazine Catalytic Decomposition Over Molybdenum Carbide And Transition Metal Catalysts

The development of efficient and safe hydrogen storage/transportation technology is a key step to promote the industrial application of hydrogen energy.Recently,the chemical hydrogen storage means which combine controlled hydrogen release high-capacity hydride and centralized hydride regeneration have gradually become an emerging hydrogen storage technology frontier.Among various candidates,hydrous hydrazine?N₂H₄·H₂O?is favored for its many advantages such as high hydrogen density?8 wt.%?,relatively low cost,good storage security,and free of solid byproduct in the decomposition reaction.Hydrazine?N₂H₄?is the effective hydrogen storage component in hydrous hydrazine?N₂H₄·H₂O?,and its decomposition reaction can occur according to two competitive paths to yield H₂+N₂ or NH₃+N₂.The synthesis of high activity and high selectivity catalysts is a central issue for development of controllable hydrogen production from hydrous hydrazine.In the past decade or so,scholars from various countries have made much positive progress in the rational design of catalysts for hydrogen production from hydrous hydrazine.However,due to the unclear catalytic mechanism and the absence of design principles for catalyst composition,the development of catalyst has been delayed.In view of this,we selected the representative molybdenum carbide??-Mo C?and various transition metal catalysts that have been proven by experiments as the research objects.First principles calculations were implemented focusing on the catalytic decomposition mechanism of hydrazine and the theoretical basis for catalyst selection,the main progress is as follows:?1?We studied the kinetic process and reaction mechanism of N₂H₄ adsorption and decomposition over the of Mo C?111?surface.Calculation results show that the N-N bond cleavage and the subsequent intermolecular reactions between NH₂ and N₂H_x?x=4-1?are the low-energy barrier paths which dominate the decomposition of N₂H₄.The reaction paths lead to the generation of ammonia,which is consistent with the experimental observation.?2?Based on the theoretical calculation results of the adsorption and decomposition behavior of N₂H₄ on the Mo C?111?and studied transition metals?Co,Ni,Mo,Ru,Rh,Ir?surfaces,we conclude the linear scaling between adsorption energy of N₂H₄ and activation energies for the N-H or N-N bond cleavage.Moreover,we explain the changing trend of adsorption energy of N₂H₄ on various catalyst surfaces by molecular orbital theory.For the first time,we identify a reliable descriptor,adsorption energy of N₂H₄,to properly address the catalytic activity,which provided a theoretical basis for rational design of N₂H₄decomposition catalysts.?3?We studied the effect of doping Ni,Ir,Rh atoms or depositing monolayer atoms on Mo C?111?surface on the adsorption configuration of N₂H₄.The calculation results show that the introduction of doped Ni,Ir,or Rh atoms on the Mo C?111?surface has no effect on the stable adsorption configuration of N₂H₄.While on the Mo C?111?surface modified by monolayer atoms,the most stable adsorption configuration is related to the supported atom type.?4?Combined with the experimental results,we studied the adsorption behavior of N₂H₄on Ni₁₇W₃?111?and Ni₄W?211?surfaces.According to the difference in adsorption energy and adsorption configuration of N₂H₄ on the two kinds of Ni-W alloys surfaces,we investigated the effect of Ni-W alloying on the performance of hydrazine catalytic decomposition,and we preliminarily explained the difference in catalytic performance between Ni₁₇W₃ and Ni₄W from theoretical perspective.