| Generally, there are three possible structures for metal atoms supported on CeO2:monatomic dispersion,2D clusters wetting or 3D aggregation on ceria support. For different kinds of metal elements and coverages, which kind of structure is more possible? This is dependent of thefactors of interactionbetweenmetal-metal and metal-CeO2,CeO2index surfaces and metal coverage, etc. Using density functional theory method, the possibly stable structures of Mnx/Ce02, Cun/CeO2 and the factors influenced the stability of modeling catalysts were systematically investigated. In addition, the adsorption behaviors of H/H2O on CeO2 were also studied in the present work.For Mnx/CeO2 modeling catalyst, our theoretical results indicate that, Mn atoms tend to uniformly disperse on the hollow site of CeO2(111) at the low coverage(1≤x≤4); and the Mn-O interaction is stronger than that of Mn-Mn. For Mnx cluster with 4<x<17, Mnxtended to form 2D structures wetting on ceria instead of aggregating to 3D clusters. When CeO2(111) surface is partially reduced, oxygen vacancy slightly enhances the metal adsorption energy of Mnx(x=1-3) on the top and hollow sites of CeO2(111). When x is 4, the stability of 3D clusters on the top site slightly increases, while the adsorption energy of Mn4 decreases on the hollow sites. This might be attributed to the reduction of theMn-O bond number due to the existence of oxygen vacancy, which weakens the interaction between Mn with slab.On the basis of the systematical investigation of the structures of Cun(1≤n≤10) on CeO2(111) surface, it is found thatCu atoms tended to be scattered on the hollow site of CeO2(111) for 1≤n≤2. Cu3 formsa triangle and lies on CeO2(111).For Cun clusters while3<n<10, theyaggregate into 3D structures due to the synergisticinteraction between Cu-Cu and Cu-CeO2. According to the systematically theoretical work above, it is concluded that the stability of Cun/CeO2(111) is related with the size andthe arrangement of Cun. When the concentration of oxygen vacancy increases, its hindering effects on the stability of Cun/CeO2(111)enhances. This might be resulted by the decrease of Cu-CeO2 interaction. According to the adsorption behaviors of Cun on CeO2-δ(111), there is little difference for Cun on the ceria substrate with 3 linear or triangle oxygen vacancy. Similar to Cun/CeO2(111), Cu clusters incline to form 3D cluster on CeO2-δ(111) surface.When Cunclusters deposit on CeO2(110) surface,Cun(n=1～3) prefers to monatomically adsorbedatthe shortO-O bridge site.For Cun clustes with n>4, the most stable Cun/CeO2(110) structures transform into 3D configurations. When Cun clusters adsorb onCeO2(100) surface, they preferentially monatomically scatter on O-O bridge site along x axis at the low coverage(n<8).If n is larger than 9, the most stable Cun clusters transformed into 3D configurations due to the strong Cu-Cu interactions.According to the theoretical study of the H/H2O-CeO2(111) system,H atom preferentially adsorbed at O top site to form OH species on the stoichiometric and partially reduced CeO2(111) surface. The most stable structure of H2O is on the Ce top siteof CeO2(111).When one oxygen vacancy forms on CeO2(111), H2O filled into oxygen vacancy. When linear or triangle oxygen vacancy forms, H2O stands at the edge of oxygen vacancy and H atom turns toward surface oxygen and tends to form weak hydrogen bond. Therefore, the adsorption behaviors of water on CeO2-δ(111) are determined by the type and distance of oxygen vacancy, the position of water, as well as its orientation. While H2Ostanded upright in the edge of three linear or angular oxygen vacancies, with one H belonged to H2O facing surface O,which illustrated H maybebond to surface O.Generally, the structural stability of H2O determined by the distance from the oxygen vacancies, H2O spatial location and orientation of H in the surface with oxygen vacancy. |
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