Molecular Dynamics Study On The Adsorption,Dissociation And Diffusion Behavior Of Wet H₂S On The Surface Of Iron And Iron-sulfide Compounds

There are often some corrosive media in the exploitation and transportation environment of oil and natural gas,such as H₂S.Carbon steel is prone to corrosion in wet H₂S environment,and due to the“poisoning effect”of H₂S,the corrosion is often accompanied by hydrogen embrittlement,seriously threatening the safe use of oil and gas equipment.Therefore,it is of great significance to explore the adsorption and dissociation mechanism of H₂S on the surface of carbon steel and its corrosion products in H₂S environment through theoretical calculations for clarifying the corrosion mechanism of carbon steel and the protection of corrosion products.However,the current theoretical calculation methods and models tend to be idealized,which is very different from the complex corrosive environment in actual working conditions.Therefore,this thesis has carried out in-depth theoretical calculation research on above problems through first-principles molecular dynamics.In the research process,the influence of various factors on the adsorption and dissociation process of H₂S has been considered.The main research contents and results are as follows:The adsorption and dissociation processes of H₂S molecules on different low-dimensional surfaces of bcc-Fe and the diffusion behavior of H atoms in bcc-Fe lattice were studied at 300 K.It was found that H₂S molecules can only undergo first-order dissociation on perfect（100）Fe surface,and the aqueous solution can effectively promote the dissociation of H₂S and change dissociation site.The presence of surface H₂O molecules would promote the dissociation of H₂S molecules,but did not affect the adsorption sites after dissociation.The presence of vacancy defects on the surface not only significantly promotes the dissociation of H₂S,but also affects site where the dissociation occurs.The study also found that H₂S coverage rate,that is,the H₂S concentration,also had an impact on the H₂S dissociation process,and the H₂S molecules would inhibit the dissociation of each other,but did not affect site where the dissociation occurred.The adsorption and dissociation studies of H₂S molecules on Fe（110）surface have proved that the most stable site for the adsorption of H₂S is bridge site,and the primary and secondary dissociation rates are faster than those at the（100）surface.In addition,the presence of aqueous solution will slow down the dissociation rates to a certain extent and change the site of dissociation.The effect of surface and internal vacancy defects on the dissociation mechanism of H₂S has proved that both surface and internal defects change the site where H₂S dissociation occurs.The difference is that surface defects slow down the secondary dissociation of H₂S while internal defects accelerate it.The diffusion behavior of H atoms in bcc-Fe lattice at different temperatures was also studied by ab initio molecular dynamics.The diffusion coefficients of H atoms at different temperatures were calculated and the energy barrier of diffusion was obtained by formula fitting as 0.064 e V.The study of interaction between mackinawite FeS and small molecules/atoms（H₂S,H₂O and H）in wet H₂S environment at 300 K found that H₂S could not adsorb stably on FeS（001）surface,but showed relatively stable chemisorption on（011）and（100）surface without dissociation.While on FeS（111）surface,H₂S could undergo first-order dissociation,and the dissociated H atoms tended to diffuse into the interlayer of mackinawite FeS.The study also proves that H₂S and H₂O can dissociate in the interlayer after entering the mackinawite layer.The degree of dissociation of H₂S is greater and the energy barrier is lower.When H atoms exist alone in the mackinawite layer,it can be formed H₂ molecules.The diffusion coefficients of H atoms between mackinawite layers at 300 K are obtained by calculation and formula fitting,which proves that the diffusion coefficient of H atoms between mackinawite layers is close to that in Fe lattice.The study also found that when mackinawite contains defects,H atoms can easily diffuse through the defect to another layer.The adsorption and dissociation process of H₂S molecules on pyrite-type FeS₂ low-dimensional surfaces with different terminals shows that H₂S molecules do not dissociate on Fe-terminated low-dimensional surfaces.Only on（211）and（311）surfaces,adsorption occurs and no adsorption even occurs on the（100）and（210）surface.After considering the surface aqueous solution,H₂S molecules can be adsorbed on the（100）,（210）and（211）planes,and first-order dissociation occurs on the（311）surface.On low-dimensional surfaces terminated by S atoms,H₂S cannot be adsorbed regardless of whether aqueous solutions are considered.The diffusion process of H atoms in the pyrite FeS₂ lattice is also studied,and the results show that pyrite has good barrier property for H atoms.The adsorption and dissociation process of H₂S molecules on two-dimensional FeS₂surface was explored,and the results showed that H₂S could only adsorb but not dissociate.The study also predicted a new two-dimensional pentagon FeS₂ by GGA+U method,which proved that it is full-planar structure and has the characteristics of Cairo pentagon,and its basic properties were explored.The phonon spectrum and elastic constant matrix were obtained by calculation to prove the dynamic and mechanical stability of two-dimensional pyrite,and the thermal stability was proved by molecular dynamics simulation.GGA+U calculations show that this all-planar 2D FeS₂ exhibits antiferromagnetic semiconductor properties with a band gap of 0.31 e V.Considering the spin-orbit coupling effect,the magnetocrystalline anisotropy energy（MAE）of different orientations is calculated,and the highest MAE value can reach 454.77μe V/Fe atom.The study also found that the electronic structure of 2D FeS₂ can be tuned and magnetically transformed by applying biaxial strain using strain engineering.