Influence Of Surface Defects On NH_x Adsorption On Pyrite FeS₂?100?:A Theoretical Study

Pyrite FeS2 is one of the most abundant metal sulfides in the world,and it has high reactivity and catalytic activity.It has been shown that iron sulfide nanoclusters exhibited the similar function as nitrogenase.Nitrogenase can complete ammonia synthesis without high temperature and pressure.Therefore,investigation of adsorption of NHx(x=1,2,3)on FeS2 surface is of great significance to understand the catalytic role of pyrite on ammonia synthesis.In this thesis,influence of surface defects on NHx(x=1,2,3)adsorption on pyrite FeS2(100)was investigated by use of first principle calculation method based on density functional theory.Firstly,the geometry and electronic structure properties of three kinds of FeS2(100)surfaces with different thickness and atomic layers were studied,and the stable FeS2(100)surface configuration was obtained.The results show that the relaxation vectors of all three kinds of FeS2(100)surface atoms are small as a whole,and there is no obvious surface reconstruction phenomenon.The relaxation vectors of the outermost layer atoms are relatively large,while those of the other layers are very small.When the surface atomic layer thickness reaches 9 or more layers,the surface energy tends to be stable,and the FeS2(100)surface atomic relaxation vector with S as the surface atomic layer is the smallest,with the lowest surface energy and the most stable structure.Secondly,the adsorption configuration and electronic structure properties of NHX molecules on clean surface and FeS2(100)surface with s vacancy defects are studied.The most stable adsorption positions of NHx molecules on the two surfaces are obtained.NHx molecules tend to be adsorbed near the top position of Fe atoms on the surface.When s vacancy defects occur on the surface,because the Fe atoms on the surface play a leading role in the adsorption process and play a leading role in the s vacancy defects The adsorption stability of FeS2(100)surface with defects is higher than that of clean FeS2(100)surface.Thirdly,five transition metals(Ru,Co,Ni,Zn,Mo)were used to completely replace the Fe atoms in the outermost layer of FeS2(100)surface.The adsorption properties of NHx molecules on the surface of FeS2(100)were studied theoretically.The optimal adsorption sites of NHx on five TM-FeS2(100)surfaces were obtained.The bond length of NH,NH2 and NH3 increased gradually,and the absolute value of adsorption energy decreased gradually.The relaxation of S atom in five kinds of TM-FeS2(100)adsorption configuration is the key factor that leads to the larger absolute value of adsorption energy compared with the normal surface;the adsorption stability of NH3 molecule on five kinds of TM-FeS2(100)surfaces is Mo>Ru>Co>Zn>Ni.When more than one NH3 molecule is adsorbed on Ru-FeS2(100)and Co-FeS2(100),the adsorption stability of NH3 molecule will decrease slightly with the increase of the number of NH3 molecules.