The First Principle Study Of The Structures Of Au_xNi_y?x+y?4? Bimetallic Clusters Adsorbed On The CeO₂?111? Surface

Rare earth oxides featuring fluorite-type structures?CaF₂?have been widely used in the fields of industrial catalysis,energy materials and biomedicine because of their excellent structure and catalytic properties.Ceria and ceria containing materials were firstly found that it could be applied to an automotive exhaust purification,and were applied successfully to water gas shift reaction?WGS?,CO oxidation,propylene epoxidation,reforming of hydrocarbons and other catalytic reactions.The reason for this widely used of CeO₂ is mainly because of the presence of empty and strongly localized 4f?Ce?states close to the Fermi level,which can play a role in mediate the charge and oxygen exchange by facile O vacancy formation.The structural characteristics and catalytic properties of CeO₂ supported single metal nanoparticles have been extensively studied theoretically and experimentally in the early stage.Some experimental studies have also been carried out on some bimetallic nanoparticles deposited on CeO₂ surface.Adsorption and catalysis of metal nanoclusters for typical gas molecules such as CO,CO₂ and NO has gradually become a hot topic in theoretical research.Recently,it was discovered by experiment that bimetallic nanoparticles supported on the CeO₂ surface are a very promising heterogeneous catalyst which exhibits superior catalytic performance different than that of pure metals for CO oxidation.The Au-Ni nanoparticles catalyst have a great potential for extensive application due to it not only reduces the cost of the catalyst,but also has high thermal stability,easy sintering and easy preparation.Based on the generalized gradient approximation methods?GGA?under the framework of Density Functional Theory?DFT?.And the on-site Coulomb interactionbetween the electrons in the 4f?Ce?state were resolved by introducing a effective U parameter?GGA+U?.We have systematically studied the adsorption sites,structural morphology,and stability of the bimetallic clusters Au_xNi_y?x+y ?4?adsorbed on the stoichiometric CeO₂?111?surface with increasing Ni concentration and the electronic properties and density of states were discussed.Finally,We calculated the adsorption properties of CO molecules on the three specified structures.?1?The adsorption energy calcultions indicate that single Au atom is prefer to be loaded to the bridge sites between two surface O atoms.While Ni atom tends to be adsorbed on the top site of the subsurface O atom coordinated to three oxygen and three cerium atoms.The Au atoms have full shell 3d electrons,and only 4s?Au?of electrons are involved in bonding.While the Ni atoms have the unpaired 3d?Ni?electrons,So its contribution is derived from the 3d?Ni?and 4s?Ni?electrons.Thus the Au atoms in the AuNi clusters tend to bond with less O atoms of the substrate While the Ni atoms bond with the O atoms of the substrate as much as possible.Bader charge analysis shows that the Ce atoms which located far away from the adsorbed metal atom were reduced due to the weaker Coulomb repulsion.?2?We found a steady trends that favors the growth of bimetallic clusters when it supported on CeO₂?111?Surface.Which is the bimetallic clusters are all tetrahedral structures which center is located on the subsurface O atoms,and the Ni atom prefers to occupy higher coordination sites.Through further analysis we can speculate that the adsorption system tends to be in the most stable state for the larger size metal?alloy?clusters adsorbed on the CeO₂?111?surface when the number M-M bonds and the number of M-O bonds reach the maximum.?3?After adsorption of CO molecules on the Au₂Ni₂ and Ni₄ clusters deposited on the surface,the adsorption energy increased by 0.52 eV and 0.65 eV compared to the Au₄ clusters.While the corresponding differences in adsorption energy were-1.09 eV and-1.97 eV when these three clusters supported on CeO₂ surface.This result shows CO molecule reduce the adsorption stability of the system.And the significant electrons transfer was found from Ni₄/CeO₂ and Au₂Ni₂/CeO₂ to CO molecule after adsorption of CO while it was not found on Au₄/CeO₂ structure by further analyzing the charge density differences.