DFT Study On Mechanisms Of Hydrogenation And Craking Reaction Over Metal-based Catalysts And The Effect Of Defects And Doping

Selective hydrogenation of acetylene to ethylene,ethylene cracking and ammonia synthesis are all important industrial reactions.Nowadays,the researches regarding their catalysts mainly focus on pure metal,but the effects of defects and doping on catalytic performance remain unclear.In this thesis,the density functional theory(DFT)calculations were performed to study the selective hydrogenation of acetylene on Ni(111)with point defects,the ethylene cracking on Ni(211)with step defects,and ammonia synthesis on the nitrogen-doped molybdenum carbide.The main research contents and conclusions are as follows:(1)DFT calculations were carried out to investigate the effect of point defects on acetylene hydrogenation reaction over Ni(111)surface,with the comparative study on perfect Ni(111)surface.The adsorptions of C2 species and H atoms,partial density of states(PDOS)of Ni electron and the mechanism of acetylene hydrogenation via ethylene pathway were systematically calculated.The results indicate that the adsorption energies of acetylene over defect surfaces are higher than those on perfect surface,and increase with the decrease of defect concentration(DC).This is supported by the PDOS analyses that electrons of Ni atoms over the surface with DC=0.0500 are easier to transfer to carbon atoms than the other surfaces.The first hydrogenation step is the rate-determining step on each Ni surfaces.The energy barriers of the first and the second hydrogenation steps(C2H2+H-*C2H3 and C2H3+H?C2H4)on the defective surfaces are both lower than that on the perfect Ni(111)surface,suggesting that the existence of point defects can improve the activity of acetylene hydrogenation.The defective surface with the lowest DC(=0.0500)shows the highest activity and the best selectivity of ethylene due to its lowest effective barrier and highest ?Ea(the difference between the hydrogenation and desorption barriers of ethylene).This work provides useful theoretical information on the defects effect on acetylene hydrogenation and is helpful for the design of Ni and related metal catalysts with defects.(2)The catalyst models of pure nickel Ni(211)and Ag-Ni(211)doped with silver at the top step site were constructed,and the optimal adsorption structure,adsorption energy of the ethylene cracking pathway about CxHy(C2H4,C2H3,CH2)and hydrogen atoms on the catalyst surface were calculated respectively.The results showed that the activation energy of ethylene carbon-carbon bond cracking on the step defect catalyst was lower than that on the Ni(111)without defect.The doping of Ag atom at the step site will reduce the adsorption energy of ethylene,and reduce the activation energy of carbon-hydrogen bond cracking of ethylene,but increase the activation energy of carbon-carbon bond cracking.Therefore,step defect is conducive to the cracking of carbon-carbon bond,while the cracking of ethylene carbon-carbon bond will be hindered to some extent after silver doping.(3)The adsorption structures of nitrogen-containing species and the energy barrier of the first hydrogenation are calculated.It was found that the reactants adsorb stably around the sites with high density of Mo atoms.on the surface of Mo2C(121),1N-Mo2C(121)and 6N-Mo2C(121).As the proportion of doping N increases,the adsorption capacity to N2 of the Mo2C surface gradually decreases,and the energy barrier of the first hydrogenation reaction of nitrogen(rate-determining step)gradually reduced.The surface of 6N-Mo2C(121)is most active for the first step of nitrogen hydrogenation,which matches the experimental results.This work provides theoretical understandings for mechanism of acetylene hydrogenation and nitrogen hydrogenation,which can give useful instruction for the design and preparation of metallic catalysts with defects and doping.