Theoretical Study On Desulfurization And Sulfur Transfer Mechanism Of Ni/ZnO Catalyst

As an important fossil energy,petroleum products such as gasoline and diesel promote the development of the world economy and the progress and prosperity of human beings.They are widely used in all aspects of social life.However,with the long-term and large-scale exploitation of crude oil,the quality of oil products becomes worse and worse.Crude oil contains a large amount of sulfur and nitrogen compounds,which reduces the utilization rate of oil and affects the utilization rate of industrial fuels.At the same time,high sulfur content in petroleum will lead to more and more automobile exhaust emissions to the surrounding environment,a lot of toxic substances,serious pollution of the environment on which humans rely for survival.All countries in the world are accelerating the research of deep desulfurization technology and the synthesis of new adsorbents to meet the needs of ultra-low sulfur oil production.Among various desulfurization technologies,the reaction adsorption desulfurization technology using Ni/ZnO as catalyst has attracted extensive attention.However,the desulfurization mechanism and sulfur transfer mechanism on Ni/ZnO have not been timely studied.Therefore,the clarification of the desulfurization mechanism and sulfur transfer mechanism on Ni/ZnO can provide good theoretical support for further understanding and application of reactive adsorption desulfurization technology in the future.In this paper,density functional theory(DFT)was used to calculate and analyze the C-S bond activation rules of thiophene molecules on the surface of the active center Ni,and sulfur transfer mechanism and ZnO sulfuration mechanism were described.The adsorption configuration of thiophene was most stable in the molecular state parallel to the surface or in the dissociation state on the Ni(111)and Ni₅₅ cluster.We found thiophene in Ni₅₅ cluster on the adsorption of superior to the adsorption on the surface of the Ni(111),on the one hand,this is because the Ni₅₅ cluster of d band center is more close to the Fermi level and coordination number with center caused by the reason of higher,on the other hand we can through the adsorption decomposition,the results of these two kinds of substrate surface adsorption energy difference is mainly of thiophene and Ni₅₅ interaction can be more strong.The hydrodesulfurization reaction was studied by taking the stable adsorption configuration of thiophene on Ni(111)and Ni₅₅ cluster as the research object.The results showed that the direct desulfurization energy barrier of thiophene on Ni(111)and Ni₅₅cluster was low,which indicated that the direct desulfurization of thiophene was an easy and rapid reaction.Through the calculation and study of hydrogenation reaction,it can be found that the hydrogenation energy barrier of thiophene on both bases is higher than the direct desulfurization energy barrier,which indicated that thiophene tends to proceed along the DDS path on both Ni(111)surface and Ni₅₅ clusters,and the desulfurization activity of both models is very high.The S transfer mechanism in the RADS process is controversial,and it is generally believed that H₂S can transfer from the active center Ni to the adsorbent ZnO by means of"gas phase transfer",that is,S*?SH*?H₂S*?H₂S(g)?ZnO.Two mechanisms,(1)gas phase transfer and(2)surface transfer,were investigated.The so-called"surface transfer"is achieved through the surface diffusion of S and/or SH.For the mechanism of"gas phase transfer",the energy barrier for the hydrogenation of S to SH on the Ni(111)surface and Ni₅₅cluster was 0.89 and 1.48 e V,respectively.The energy barrier for the further hydrogenation to H₂S was higher,reaching 1.44 and 2.06 e V,respectively.For the"surface transfer"mechanism,the diffusion barrier of S on the surface of Ni(111)and Ni₅₅ is 0.22 and 0.37 e V,while the diffusion barrier of SH is 0.39 and 0.72 e V,respectively.It can be seen that the surface diffusion of S atom on Ni(111)and Ni₅₅ clusters is easy,while the diffusion of SH group is relatively difficult.Therefore,on the surface of Ni(111)and Ni₅₅,the product S of RADS reaction tends to transfer directly to ZnO in the form of S atomic surface diffusion.At the same time,there is the possibility that S hydrogenates to generate SH and then transfers by SH surface diffusion on Ni(111).Since sulfur atoms removed by thiophene can be transferred in the form of SH group and S atom,we considered the sulfurization mechanism of these two forms on ZnO catalyst.The results showed that ZnO was sulfurated mainly by thiophene removing sulfur atoms on the surface of ZnO.By clarifying the desulfurization reaction on Ni/ZnO adsorbent,sulfur transfer reaction and ZnO vulcanization reaction process,theoretical basis was provided for the design and improvement of Ni/ZnO catalyst.