Density Functional Theory Study On The Resistance To Sulfur Poisoning Of Pt_x（x=0,1,4and8）modified α-Mo₂C（0001） Surfaces

Sulfur is a common pollutant in the fuel,which often leads to toxic deactivation of the noble metal catalyst in the catalytic process.Because platinum group metal catalysts are expensive and non renewable.In the harsh chemical reaction environment,such catalysts still have many scientific problems,such as solubility,perishability,extremely susceptible to sulfur poisoning and poor durability and so on.Therefore,it has become a hot issue to find a good substitute material for noble metals.As the substrate material of the catalyst,transition metal carbides not only have the similar properties of precious metals,but also have good physical and chemical properties.The tolerance of sulfur poisoning of the Pt_x/Mo₂C（0001）（x=0,1,4 and 8）system was systematically studied by the means of the first-principles approaches based on the density functional theory（DFT）and the ab initio atomistic thermodynamic method combining the electronic structure results with the thermodynamics data.The sulfur tolerance ofα-Mo₂C（0001）based catalysts with different Pt coverages was investigated.Taking the Pt_x/Mo₂C system as the focus of our research,the mechanisms for the sulfur resistance of Pt_x/Mo₂C with different Pt coverage was studied.The main work and results reached are as follows:（1）The poisoning effects of S atom on the（0001）metal surfaces of pure Mo₂C and Mo₂C with single Pt adatom are systematically studied.It is found that the adsorption of S,HS and H₂S on the clean Mo₂C（0001）surface is very strong（with the adsorption energies of-6.46,-4.15,and-1.54 eV,respectively）.The introduction of Pt decreases the adsorption energies of S,HS and H₂S on the Mo₂C（0001）surface.When the sulfides are adsorbed on the Pt/Mo₂C（0001）surface around the Pt adatom,the adsorption energy of sulfide is decreased and the adsorption on the top position of Pt atom has the smallest adsorption energy.The adsorption energy increases when the sulfides are farther away from Pt.Hence,the introduction of Pt can play an important role in protecting the catalytically active sites around Pt from the sulfur poisoning.It can be anticipated that the effect of sulfur tolerance can be further enhanced by increasing the coverage of Pt on Mo₂C（0001）.（2）The tolerance of sulfur poisoning ofα-Mo₂C（0001）surfaces with different Pt coverages is investigated.It is found that on Mo₂C（0001）,Pt clusters tend to form two dimensional planar structures instead of aggregating.The clean Mo₂C（0001）surface interacts with sulfides very strongly and is susceptible to sulfur poisoning.With increasing the coverage of Pt on the Mo₂C surface,the interaction between sulfur and substrate is weakened.The sulfur tolerance ability increases in the order of Mo₂C≈Pt₁/Mo₂C<Pt₄/Mo₂C<Pt₈/Mo₂C,where the coverage of Pt on the Mo₂C plays a very effective role.The results provide theoretical guidance for designing Mo₂C based catalysts with high activity and high sulfur resistance.