Influence of a ZrO2 Support and Its Surface Structures on the Stability

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The Influence of ZrO Support and Its Surface Structures on the Stability and Nucleation of Ptn (N = 1-5) Clusters: A Density Functional Theory Study Yanxin Wang, and Hongwei Gao J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b00017 • Publication Date (Web): 13 Feb 2017 Downloaded from http://pubs.acs.org on February 14, 2017

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The Journal of Physical Chemistry

The Influence of ZrO2 Support and Its Surface Structures on the Stability and Nucleation of Ptn (n = 1-5) Clusters: A Density Functional Theory Study Yanxin Wang, Hongwei Gao∗ Key Laboratory of Plant Resources and Chemistry in Arid Regions, Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urumqi 830011, China ____________________________________________________________________ ABSTRACT: Density functional theory (DFT) calculations together with periodic slab models are carried out to investigate the influence of ZrO2 support and its surface structures on the stability and nucleation of Ptn (n = 1-5) clusters. Three surfaces of ZrO2 include cubic ZrO2(c-ZrO2) (111), monoclinic ZrO2(m-ZrO2) (-111) and tetragonal ZrO2(t-ZrO2) (101) surfaces. Our results show that the stability of Ptn clusters on three surfaces and the isolated Ptn clusters follow the trend: m-ZrO2(-111) > t-ZrO2(101) > c-ZrO2(111) > isolated cluster, while the nucleation ability over three surfaces follow the opposite trend: isolated cluster > c-ZrO2(111) > t-ZrO2(101) > m-ZrO2(-111). Therefore, Ptn clusters can have a better stability and dispersion due to the effect of the support. _____________________________________________________________________ 1. Introduction

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Corresponding author: Tel.: +86-991-3858319; Fax : +86-991-3858319; E-mail: [email protected]

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Heterogeneous catalysts play an important role in industrial catalytic reactions.1 Transition metals supported on metal-oxide substrates, such as ZrO2,2 TiO2,3,4 γ-Al2O3, MgO,5,6 SiO2,7 CeO28 and so on, are attracting considerable attention because of their significant application in heterogeneous catalysis. A number of studies have indicated that the interaction between metal and metal-oxide support significantly affects the activity and selectivity of catalytic reactions.9-12 Therefore, understanding metal cluster stability and nucleation mechanisms on oxide support is fundamental to further explore the metal–support interaction and improve the catalytic activity of the systems. Among various mental oxides, zirconia (ZrO2) is widely applied in different fields including solid oxide fuel cells (SOFC),13-15 gas sensors16,17 and passivating coatings for metal anticorrosion.18,19 Besides, the perfect physical and chemical stability of ZrO2 make it be an appropriate candidate for both catalysts and support materials in heterogeneous catalysis.20,21 ZrO2 exists in three main crystal structures: monoclinic (m-ZrO2), tetragonal (t-ZrO2), and cubic (c-ZrO2) structures.22 Apparently, the crystalline forms of the ZrO2 support also affect the reactions activity of catalysts. It was reported by Jung et al.23 that Cu catalysts supported over m-ZrO2 are more active in methanol synthesis than catalysts with the same Cu surface density deposited on t-ZrO2. Zhang et al.24 found that the catalytic activity for CO methanation of Ni/m-ZrO2 catalyst is much higher than that of Ni/t-ZrO2 catalyst. Small clusters or particles often serve as metal active components deposited on the support surface as catalysts to catalyze chemical reactions in previous studies.25,26

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For one reason, small clusters or particles can be stably distributed on the substrate surface, and for another, they show special qualities thanks to the appearance of low-coordination atoms and electron confinement effects,27 which is different from those of bulk metals. Meanwhile, previous researches28-33 have indicated that the metal particles sizes strongly affect the catalytic performance of reactions. For examples, Isaifanet al.32 pointedoutthat the smallest Pt/yttria-stabilized zirconia particles show the highest catalytic activity for CO and C2H4 oxidation. Meier et al.33 also suggested that the metal cluster size played the vital role in determining the reactions activity of catalysts. Nowadays, Pt/ZrO2catalysts have been widely used for several catalytic reactions,34-37 such as carbon monoxide oxidation,34 reforming of methane with carbon dioxide35,36 and water-gas-shift (WGS) reactions.37 Therefore, it is necessary for us to explore Pt clusters stability and nucleation mechanisms on ZrO2 support which will make preparation for further study of the reaction activity. Unfortunately, it has certain limitation for experimental means to acquire the information about surface structure and metal-support interaction. While theoretical calculations based on density functional theory study can effectively complement experimental results. Yang et al.38 has examined the growth of Nin clusters and their interaction with ZrO2 surfaces with theoretical and experimental approaches. However, to our best knowledge, few reports have illustrated the stability and nucleation of Pt in a Pt/ZrO2 catalyst. In this paper, we in detail and systematically investigate the stability and

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nucleation of Ptn(n=1-5) on three different ZrO2 phases including cubic, monoclinic and tetragonal ZrO2. This paper aims to determine the most stable adsorption structures and preferred adsorption sites of Pt clusters on different ZrO2 surface and to disclose their nucleation mechanisms which will give helpful design and synthesis instructions of metal-support catalysts in heterogeneous catalysis.

2. Computational methods and models 2.1 Computational methods All calculations are carried out with full plane-wave DFT calculations implemented in the Vienna Ab initio Simulation Program (VASP).39,40 Generalized gradient approximation (GGA) using Perdewe Burkee Ernzerhof (PBE) functional41,42 and projector augmented wave (PAW) method43,44 are employed to represent non-valence core electrons. The plane-wave basis set with a cut-off energy of 450 eV is set. The surface Brillouin zone is sampled with a 2×2×1 Monkhorst–Pack mesh for both cubic (111) and monoclinic (-111) surface and a 2×3×1 Monkhorst–Pack mesh for tetragonal (101) surface to obtain converged results. The convergence criterion for electronic self-consistent iteration is set at 1.0×10-4eV, and a conjugate gradient algorithm is employed to optimize atomic positions until atomic forces were smaller than0.03eV/Å. Spin polarization is performed for all calculations.

2.2 Computational models Among three crystal forms of ZrO2, the tetragonal and cubic structures are the

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most thermodynamically stable. Zr atoms are eightfold coordinated in tetragonal and cubic phase, while sevenfold coordinated in monoclinic phase.45 We employ (111), (-111) and (101) surfaces of cubic, monoclinic, and tetragonal ZrO2, respectively, since they are the most stable.46,47 The c-ZrO2(111) surface (see Fig. 1a) is modeled using a (3x3) supercell with dimensions of 10.84Å×10.84Å×22.37Å，which contains three O-Zr-O trilayers (nine atomic layers). During optimization, the top six atomic layers including the adsorbed Pt clusters are allowed to fully relax, whereas the bottom three layers are remained restrained in their bulk positions. For the m-ZrO2(-111) surface (see Fig. 1b), a (2×2) unit cell slab with two layers and a 13.55 Å ×14.64 Å ×20.97 Å dimension are adopted in this work. In this case, the top layer together with the adsorbed Pt clusters are allowed to relax and the rest ones are frozen in the bulk positions. The t-ZrO2(101) surface (see Fig. 1c) is modeled with a dimension of 12.81Å×7.28Å×22.86Å，which included (2×2) supercell of layers. It contains six layers of O atoms and three layers of Zr atoms. At the same with the c-ZrO2(111) surface, the top six atomic layers including the adsorbed Pt clusters are allowed to fully relax, while the rest ones are frozen in the bulk positions. The vacuum region is set to 15 Å for these three surfaces to ensure the negligible interaction between two consecutive slabs.

3. Results and discussion 3.1 Ptn clusters in the gas phase We first explore the geometries and energies of Ptn clusters in the gas phase. The

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1D, 2D, and 3D geometries of the isolated Ptn are considered. Only the energetically preferred structures of Ptn(n = 2-5) clusters are considered, as listed in Table 1 To obtain the stability of Ptn(n = 2-5) clusters in the gas phase, the average binding energy of Ptn(n = 2-5) clusters in the gas phase, Ebind(Ptn), is calculated as follows: Ebind(Ptn) = [n × E(Pt) − E(Ptn)]/n

(1)

where E(Pt) and E(Ptn) are the total energies of a single Pt atom and an isolated Ptn cluster, respectively, and n is the number of Pt atoms in the Ptn clusters; herein, n = 2-5. The isolated Ptn(n = 2-5) clusters with larger Ebind(Pt) are more stable. Table 1 shows that the average Pt-Pt bond length of the clusters is smaller than that of the bulk structure (d (Pt-Pt) = 2.77 Å) and increases with cluster size, although approximating the bulk value is difficult. Moreover, Ebind(Ptn) increases with increasing atomic coordination in Ptn(n = 2-5) clusters.

3.2 Adsorption of the Ptn cluster on different ZrO2 surfaces All the adsorption configurations and the corresponding total energies of Ptn(n = 1-5) clusters adsorbed on different surfaces at all possible sites are explored and listed in Fig. S1–S3 in ESI,† respectively. We focus on studying the most stable adsorption structures of Ptn(n =1-5) clusters on surfaces as well as their corresponding calculated energies. The adsorption energy of Pt clusters, Eads, can be defined by the following equation: Eads = E(Ptn/ZrO2) - E(Ptn) - E(ZrO2)

(2)

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where E(ZrO2) and E(Ptn/ZrO2) are the total energies of the bare ZrO2 surfaceand the surface with Ptn clusters, respectively. To clearly express the chemisorption process, we decompose the adsorption energy into deformation energy and metal-support interaction energy. The surface or metal cluster deformation energy is defined as the energy required to be overcome during surface or metal cluster distortion toward geometry. The surface deformation energy is calculated as follows: Edef,surface = E(ZrO2') − E(ZrO2)

(3)

where E(ZrO2') is the total energy of the deformed surface after Ptn cluster adsorption. With this definition, lower Edef,surface leads to weaker surfaces. Similarly, the deformation energy of Ptn(n = 1-5) clusters is calculated by the following equation: Edef,Ptn = E(Ptn') − E(Ptn)

(4)

where E(Ptn') is the total energy of Ptn clusters supported on the ZrO2 surface. With this definition, lower Edef,Ptn results in weaker cluster deformation. The cluster-support interaction energy (Eint) represents the adsorption energy between the already deformed objects, without the energy contributions of configuration deformation in both parts of the system. It is calculated as follows: Eint = E(Ptn/ZrO2) − E(Ptn') − E(ZrO2')

(5)

Thus, the adsorption energy can also be calculated as follows: Eads = Eint − Edef(Ptn) − Edef(surface)

(6)

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3.2.1 Adsorption of Pt clusters on the c-ZrO2(111) surface The side and top views of c-ZrO2(111)surface are presented in Fig.1a. The structure of the surface has a high symmetry and only one kinds of Zr and O atoms are exposed on the surface, which are marked in Fig.1a. The adsorption locations of Ptn (n = 1-5) clusters are examined by two top sites for O and Zr atoms, one bridge site (Zr-O) and one hollow site on the surface. The most stable adsorption structures are presented in Fig. 2, and the corresponding energies and bond lengths are shown in Table 2. Fig . 2a shows that the single Pt atom tends to occupy the Zr-O bridge site with an adsorption energy of -3.91 eV which is in accordance with the adsorption of Ni,38 Pd and Rh atoms48 on c-ZrO2(111)surface. The bond lengths of Pt-Zr and Pt-O are 2.63 and 1.93 Å, respectively. The adsorption of the Pt atom can introduce a little surface deformation with the surface deformation energy of 0.88 eV. The Pt-support interaction energy is -3.84 eV, which greatly contributes to the adsorption energy. As shown in Fig. 2b, the most stable adsorption configuration differs from the corresponding structure of the Ni dimer.38 The bond length of Pt1-Pt2 is 2.52 Å, which is longer than that of the isolated Pt2 clusters (2.32 Å). The bond length of Pt1-Zr and Pt1-O elongates to 2.84 and 2.01 Å from 2.63 and 1.93 Å for the monomer. This difference is in agreement with the great increase about the surface deformation energy from 0.88 eV for the monomer to 2.08 eV for the dimer. The adsorption energy of Pt2 cluster is -3.34eV, which is smaller than that for the monomer owing to its sharp decrease of the Pt-support interaction energy from -0.88 eV for the monomer

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to -2.08 eV for the dimer, indicating that the Pt-support interaction energy mainly contributes to the adsorption energy. As presented in Fig. 2c, for Pt3 clusters, the most stable configuration isthe plane of triangular Pt trimer adsorption aslant on the surface with bond lengths of 2.54 Å (Pt1-Pt2), 2.53Å (Pt2-Pt3) and 2.58 Å (Pt1-Pt3). The adsorption energy of Pt3 cluster is -3.81 eV, which is smaller than that of Pt2cluster (-3.34 eV), whereas the interaction energy of Pt3 cluster (-5.48 eV) is larger than that of Pt2 cluster (-5.65 eV). The cluster and surface deformation energies for Pt3 reduce to 0.08 and 1.58 eV from 0.23 and 2.08 eV for Pt2 cluster, respectively. Therefore, the decrease of deformation energies mainly increase the adsorption ability of Pt3 cluster in contrast to that of Pt2 cluster. The most stable structure of the Pt4 cluster is shown in Fig. 2d. In this configuration, three Pt atoms directly interact with the surface via the Pt1-O (2.01 Å), Pt1-Zr (2.86 Å), Pt2-O (2.00 Å), Pt2-Zr (2.86 Å), Pt3-O(2.00 Å) and Pt3-Zr (2.87 Å) bonds, while Pt4 atom is located at the top vertex away from the support surface. Since the four atoms of Pt4 cluster locate distantly from the surface, the cluster deformation energy (0.03 eV) is negligible when compared with those for other four clusters. The adsorption energy and Pt4-support interaction energy are -4.76 and -6.84 eV, respectively, which are smaller than those for Pt3 cluster. This reduced interaction energy makes a major contribution to the decrease of the adsorption energy. For Pt5cluster, as shown in Fig. 2e, the most stable configuration is a trigonal bipyramid with Pt4-Pt5 bond cleavage. Pt5 cluster interacts with the surface via four Pt atoms with bond of Pt1-O (1.98 Å), Pt1-Zr (2.92 Å), Pt2-O (2.02 Å), Pt2-Zr (3.04 Å),

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Pt3-O (2.04 Å), Pt3-Zr (2.86 Å) and Pt5-O (2.05 Å), Pt5-Zr (2.89 Å), respectively. The cluster and surface deformation energies are 0.42 and 2.40 eV, respectively, which are higher than those for other clusters. The Pt5-support interaction energy (-8.20 eV) is very small, resulting in the decrease of adsorption energy (5.38 eV) in contrast with that for other clusters. From above analysis, we can conclude that on the c-ZrO2 surface, the adsorption energy of Ptn clusters has strong links with the Ptn-support interaction, Ptn cluster deformation, and surface deformation energies. With the increase of Ptn cluster size, both the adsorption energy of Ptn and Ptn-support interaction energy show a diminishing tendency, suggesting that the Pt-support interaction energy mainly contributes to the adsorption energy. In addition, the Ptn cluster deformation energy is much lower than other energies, indicating that the Ptn cluster deformation has little effect on the adsorption energy. The Pt4 cluster deformation energy can even be neglected because of the high stability of its tetrahedron structure. With clusters growth, the average distance of Pt-Pt bond of supported clusters increases, which is higher than that of the clusters in the gas phase.

3.2.2 Adsorption of Pt clusters on the m-ZrO2(-111) surface The top and side views of the m-ZrO2(-111)surface are shown in Fig. 1b, in which the fivefold coordinated zirconium atom (Zr2), and sixfold coordinated zirconium atoms (Zr1, Zr3 and Zr4) as well as threefold coordinated oxygen atoms (O1, O2, O3, and O4) are exposed. The adsorption of Ptn(n = 1-5) clusters are

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examined by placing them on numerous well-defined sites that are available on the surface. The most stable adsorption structures are presented in Fig. 3, and the corresponding energies and bond lengths are shown in Table 3. For the single Pt atom, it has been found that wherever we place it on the m-ZrO2(-111) surface, the final optimized result is that the Pt atom tend to adsorb on the bridge site of Zr2 and Zr4 atoms (Fig. 3a). The bond lengths of Pt-Zr2 and Pt-Zr4 are 2.49 and 2.68 Å, respectively. The interaction between the single Pt atom and surface is so weak that it can introduce a little surface deformation with the surface deformation energy of 0.57 eV. The adsorption energy (-6.14 eV) and interaction energy (-6.72 eV) of the Pt atom on the m-ZrO2 (-111) surface are both smaller than those on the c-ZrO2 (111) surface. The most stable adsorption configuration for Pt2 cluster is shown in Fig. 3b in which the Pt1 atom adsorb on the bridge site of Zr2 and Zr4 atoms and the Pt2 atom locates on the top site of the Zr3 atom. The bond lengths of Pt1-Zr2, Pt1-Zr4 and Pt2-Zr3 are 2.53, 2.76 and 2.51Å, respectively. The adsorption of Pt2 cluster induces a strong surface deformation with the surface deformation energy of 1.89 eV. The bond length of Pt1-Pt2 enlarges to 2.52 Å from 2.32 Å of the isolated Pt2 clusters. The deformation energy of Pt2 cluster (0.44 eV) is so small that it makes a little contribution to the adsorption energy. For Pt3clusters, as presented in Fig. 3c, the most stable configuration is the linear trimer binding to the surface with bond lengths of 2.57 Å (Pt1-Pt2) and 2.49Å (Pt2-Pt3). Due to powerful force by the surface, the cluster structure is strongly

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distorted and twisted to become the linear one from the initial triangular plane one in the gas phase. Therefore, the adsorption energy (-9.83 eV), interaction energy (-12.67 eV) of Pt3 on the surface are much lower than those of other clusters, while deformation energy (1.13 eV) are much higher. As shown in Fig. 3d, the most stable adsorption configuration for Pt4 cluster don't remain its tetrahedral structure in the gas phase. This structure can be seen as adding a Pt atom to the bridge site of Pt2 and Pt3 atoms in the linear trimer, in which the bond lengths of Pt1-Zr2, Pt1-Zr4, Pt2-Zr3, Pt3-Zr3, Pt3-Zr2 and Pt3-O4 are 2.53, 2.79, 2.73, 2.57 and 2.11 Å, respectively. The adsorption energy of Pt4 cluster and Pt4-support interaction energy are -8.52 and -11.4 eV, respectively. Compared with Pt3 clusters, the enhanced of the adsorption energy for Pt4 cluster is attributed to the increase of the Pt4-support interaction energy. The most stable adsorption configuration for Pt5 cluster is presented in Fig. 3e. Pt5 cluster interacts with the surface with the bond Pt1-Zr2 (2.55 Å), Pt1-Zr4 (2.76 Å), Pt2-Zr3 (2.56 Å), Pt2-O3 (2.18 Å), Pt3-Zr2 (2.65 Å), Pt3-O4 (2.13 Å), Pt3-Zr1 (2.85 Å), Pt5-Zr1 (2.71 Å) and Pt5-O2 (2.16 Å). Owing to the strong interaction between Pt5 and support, the deformation energy (1.22 eV) of Pt5 and the surface deformation energy(2.22 eV) are higher than other clusters. From what has been discussed above, we can conclude that on the m-ZrO2 surface, there is still strong links among the adsorption energy of Ptn clusters, the Ptn-support interaction, Ptn cluster deformation, and surface deformation energies. With the growth of cluster size, both the adsorption energy of Ptn and Ptn-support

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interaction energy basically present an decreasing trend except for Pt3 cluster, indicating that the decrease of the Pt-support interaction energy mainly contributes to the decrease of the adsorption energy. Unlike the case of Ptn cluster on the c-ZrO2 surface, the Ptn cluster deformation energy is slightly lower than the surface deformation energy on the m-ZrO2 surface, indicating that the Ptn cluster deformation has a certain effect on the adsorption energy.

3.2.3 Adsorption of Pt clusters on the t-ZrO2(101) surface The top and side views of the t-ZrO2(101) surface are shown in Fig. 1c, in which two kinds of Zr atoms(Zr1 and Zr2) and four kinds of O atoms (O1, O2, O3, and O4) are exposed. The adsorption of Ptn (n = 1-5) clusters are examined by placing them on numerous well-defined sites that are available on the surface. The most stable adsorption structures are presented in Fig. 4, and the corresponding energies and bond lengths are shown in Table 4. Fig. 4a shows that a single Pt atom tends to occupy the hollow site with Pt-O1 bond (2.10 Å), Pt-Zr1 bond (2.91Å) and Pt-Zr2 bond (2.54 Å). The interaction between the single Pt atom and support is relatively weak compared with other clusters (Pt2-Pt5) that it can introduce smaller surface deformation with the surface deformation energy of 1.05 eV. The adsorption energy (-3.59 eV) and interaction energy (-4.64 eV) of the Pt atom on the t-ZrO2 (101) surface are both higher than those on the m-ZrO2 (-111) surface but lower than that on the c-ZrO2 (111) surface. The most stable adsorption configuration for Pt2 cluster is presented in Fig. 4b in

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which the Pt1 atom locates on the bridge site of O1 and Zr2 atoms and the Pt2 atom adsorbs on the hollow site of the Zr1, Zr2 and O4 atoms, forming Pt1-O1 bond (1.92 Å), Pt1-Zr2 bond (2.81Å), Pt2-O4 bond (2.03 Å), Pt2-Zr1 bond (2.85 Å) and Pt2-Zr2 bond (2.67 Å). The bond length of Pt1-Pt2 enlarges to 2.52 Å from 2.32 Å of the isolated Pt2 cluster due to the interaction between Pt2 cluster and surface. The adsorption energy and interaction energy of Pt2 cluster is -3.59 and -4.64 eV, which are larger than those for the single Pt atom indicating that the Pt-support interaction energy mainly contributes to the adsorption energy. As shown in Fig. 4c, the most stable configuration for Pt3 clusters is the plane of triangular Pt trimer adsorption aslant on the surface with one Pt-O1bond (2.00 Å), two Pt-Zr2 bonds (2.04 and 2.06 Å) and two Pt-O4 bonds (2.65 and 2.61 Å). The adsorption energy and interaction energy of Pt3 cluster are -4.38 and -6.26 eV, which are smaller than those of Pt2 cluster (-3.84 and -5.61 eV). Deformation energy for the Pt3 cluster and the surface are 0.15 and 1.73 eV, respectively, which are also larger than those for Pt2 cluster. This enhanced interaction energy makes a major contribution to the increase of the adsorption energy. As shown in Fig. 4d, for Pt4 cluster, the most stable configuration shows that Pt1, Pt2 and Pt3 atoms directly interact with the surface via Pt1-O1 (1.99 Å), Pt2-O4 (2.06 Å), Pt3-O1 (1.98 Å) and three Pt-Zr2 bonds (2.90, 2.83 and 2.94 Å), while Pt4 atom is located at the top vertex away from the support surface. Due to the phenomenon that the four atoms of Pt4 cluster locate at the place relatively distant from the surface, the cluster deformation energy (0.02 eV) is negligible when compared with those for

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other four clusters. The adsorption energy and Pt4-support interaction energy are -5.48 and -7.07eV, respectively, which are smaller than those for Pt3 cluster. Therefore, the decrease of the adsorption energy is attributed to the reduction of the Pt3-support interaction energy. As shown in Fig. 4e, the most stable configuration of Pt5 cluster is a trigonal bipyramid with Pt4-Pt5 bond cleavage. Pt5 cluster interacts with the surface with Pt1-O1 (2.02 Å), Pt2-O4 (2.05 Å), Pt2-Zr2 (2.83 Å), Pt3-O1 (2.06 Å) and Pt3-Zr2 (2.92 Å) bonds, respectively. Due to the strong interaction between Pt5 and support, the adsorption energy (-6.47 eV) of Pt5 cluster are smaller than that of other clusters and deformation energy (0.58 eV) as well as the surface deformation energy (1.52 eV) are higher than that of other clusters. The Pt5-support interaction energy (-8.57 eV) is very small, resulting in the decrease of adsorption energy in contrast with that for other clusters. As shown in Table 4, with the increase of Ptn(n = 1-5) size, both the adsorption energy of cluster and Ptn-support interaction energy decrease, indicating that the decrease of the Pt-support interaction energy mainly contributes to the decrease of the adsorption energy. Moreover, the deformation energy of Ptn cluster especially Pt4 cluster is much lower than other energies, indicating that the Ptn cluster deformation has a negligible effect on the adsorption energy. The average Pt–Pt bond length of supported clusters increases with clusters growth and is larger than that of the clusters in the gas phase.

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3.2.4 Summary about stability On the basis of above analysis, we can find that adsorption of Ptn(n = 2-5) clusters on the m-ZrO2(-111) surface is the most stable owing to its smallest energy in contrast with that on the other two ZrO2 surfaces. To better understand the influence of support and surface structures on the stability of Ptn(n = 2-5) cluster adsorption on these three different ZrO2 surfaces, we further calculated the average binding energy of supported Ptn clusters on three ZrO2surfaces, as listed in Table 5. The average binding energy, Ebind(Ptn/ZrO2) is given by the following equation: Ebind(Ptn/ZrO2) = [n × E(Pt) + E(ZrO2) − E(Ptn/ZrO2)]/n

(7)

where n = 2-5. Ebind(Ptn/ZrO2) represents the stability of Ptn cluster adsorption on the surface. The larger the value of Ebind(Ptn/ZrO2) is, the supported Ptn cluster is more stable. As listed in Table 5, the average binding energy of supported Ptn(n = 2-5) clusters is larger than that of isolated clusters, suggesting that the support plays an important role in stabilizing Ptn(n = 2-5) clusters. In addition, values of the average binding energy for Ptn clusters over three surfaces follow the trend: m-ZrO2(-111) > t-ZrO2(101) > c-ZrO2(111). 3.3 Nucleation of Ptn clusters on different ZrO2surfaces Based on the most stable adsorption configuration of Ptn (n = 1-5) cluster on the supports above, we further study the nucleation of Ptn clusters on the ZrO2surfaces. To better interpret the nucleation behavior of Ptn clusters on the support, the nucleation

16


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energy is defined for the process illustrated schematically using the formation of Pt4 cluster as an example in Fig. 5.Nucleation energy (Enuc) is defined as the energy gained or lost in adding an adsorbed isolated atom to an Ptn-1 cluster to form an Ptn cluster[49]: Enuc = E(Ptn/ZrO2) + E(ZrO2) − E(Ptn−1/ZrO2) − E(Pt1/ZrO2)

(8)

Negative values of Enuc indicate that the nucleation of Ptn clusters is exothermic and thermodynamically favorable, whereas positive values mean the opposite. Combining a single Pt atom with an existing Ptn cluster on the support results in both new Pt-Pt and Pt-substrate interactions, and weakens their previous interactions. The competition between these two effects generates the overall energy balance. The calculated nucleation energies of Ptn clusters on each surface as well as the isolated Ptn cluster are presented in Table 6 and Fig. 6. For c-ZrO2 and t-ZrO2 surfaces, the value of nucleation energy for Ptn(n =2-5) clusters

is

negative,

indicating

that

their

nucleation

is

exothermic

and

thermodynamically favorable, whereas the nucleation becomes thermodynamically unfavorable for Ptn(n = 2-5) clusters (except for Pt3 cluster) on the m-ZrO2 surface (see Table 6). Fig.6 shows that values of the nucleation energy for Ptn clusters over three different surfaces follow the trend: m-ZrO2(-111) > t-ZrO2(101) > c-ZrO2(111), suggesting that the nucleation of Ptn(n = 2-5) clusters on the c-ZrO2(111) surface is more favorable than two other surfaces. In addition, the nucleation of Ptn(n = 2-5) clusters on these various ZrO2 surfaces exhibits lower exothermicity than that of

17



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isolated clusters. This result indicates that the support does not favor the nucleation of Ptn(n = 2-5) clusters. Thus, the ZrO2 support can inhibit the aggregations of cluster and favor the formation of small clusters.

4. Conclusions In this study, the stability and nucleation of Ptn (n=1-5) clusters on cubic ZrO2(111), monoclinic ZrO2(-111) and tetragonal ZrO2(101) surfaces were systematically investigated by density functional theory method. Our results show that the support and its surface structure have an influence on the stability and nucleation of Ptn cluster. The stability of Ptn clusters on three surfaces and the isolated Ptn clusters follow the trend: m-ZrO2(-111) > t-ZrO2(101) > c-ZrO2(111) > isolated cluster. The support can strengthen the stability of Ptn clusters. On the other hand, Ptn clusters on different ZrO2 surfaces present different nucleation behaviors with increasing cluster size. The nucleation of Ptn(n =2-5) clusters c-ZrO2 and t-ZrO2surfaces is exothermic and thermodynamically favorable, whereas the nucleation becomes thermodynamically unfavorable for Ptn clusters (except for Pt3 cluster) on the m-ZrO2 surface. The nucleation ability of Ptn clusters over three surfaces follow the trend: isolated cluster > c-ZrO2(111) > t-ZrO2(101) > m-ZrO2(-111). The ZrO2 support can inhibit the aggregations of cluster and favor the formation of small clusters.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website. All the adsorption configurations and the corresponding total energy of Ptn(n=1-5) clusters adsorbed on the c-ZrO2(111) surface, m-ZrO2(-111) surface and t-ZrO2(101) surface, respectively, at all possible sites.

Acknowledgements This work was financially supported by Natural Science Foundation of Xinjiang, China, Grant No. 2016D01A073. This work was also financially supported by Recruitment Program of Global Experts, and the Director Foundation of XTIPC, CAS, Grant No. 2015RC011.

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Table 1. Geometry, coordination numbers (CN), binding energy (Eb/atom) and average bond length (Rave) of low energy gas phase Ptn(n = 2-5) clusters.

n

Geometry

CN

Eb/atom (eV)

Rave (Å)

2

1

1.92

2.32

3

1.33

2.47

2.48

4

2

2.74

2.58

5

3.6

2.98

2.56

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Table 2. Adsorption energy, interaction energy, Ptn deformation energy, surface deformation, and average Pt-Pt bond lengths of Ptn clusters supported on the cubic ZrO2(111) surface n

Eads (eV)

Eint (eV)

Edef,surface (eV) Edef,Ptn (eV)

1

-2.96

-3.84

0.88

-

-

2

-3.34

-5.65

2.08

0.23

2.52

3

-3.81

-5.48

1.58

0.08

2.55

4

-4.76

-6.84

2.05

0.03

2.59

5

-5.38

-8.20

2.40

0.42

2.62

27


d(Pt-Pt) (Å)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 3. Adsorption energy, interaction energy, Ptn deformation energy, surface deformation, and average Pt–Pt bond lengths of Ptn clusters supported on the monoclinic ZrO2(-111) surface n

Eads (eV)

Eint (eV)


1

-6.14

-6.72

0.57

-

-

2

-6.72

-9.05

1.89

0.44

2.57

3

-9.83

-12.67

1.72

1.13

2.53

4

-8.52

-11.41

2.11

0.78

2.55

5

-9.41

-12.85

2.22

1.22

2.63

28


d(Pt-Pt) (Å)

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Table 4. Adsorption energy, interaction energy, Ptn deformation energy, surface deformation, and average Pt–Pt bond lengths of Ptn clusters supported on the tetragonal ZrO2(101) surface n

Eads (eV)

Eint (eV)


1

-3.59

-4.64

1.05

-

-

2

-3.84

-5.61

1.47

0.30

2.52

3

-4.38

-6.26

1.73

0.15

2.57

4

-5.48

-7.07

1.56

0.02

2.60

5

-6.47

-8.57

1.52

0.58

2.65

29


d(Pt-Pt) (Å)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 38

Table 5. The binding energy of Ptn(n = 2-5) clusters supported on the ZrO2 surfaces per metallic atoms (eV), and the binding energy of isolated Ptn(n = 2-5) clusters, Ebind(Ptn) (eV). Isolated

c-ZrO2

m-ZrO2

t-ZrO2

2

1.92

3.59

5.28

3.84

3

2.47

3.92

5.75

3.93

4

2.74

3.93

4.87

4.11

5

2.98

4.06

4.86

4.28

30


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Table 6. Nucleation energy Enuc (eV) of Ptn cluster supported on the ZrO2 surfaces, and the isolated Ptn cluster. n

c-ZrO2

m-ZrO2

t-ZrO2

Isolated

2

-1.25

1.73

-0.49

-3.84

3

-1.09

-0.54

-0.53

-3.58

4

-1.52

3.92

-1.05

-3.53

5

-1.63

1.28

-1.36

-3.97

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Figure Captions Figure 1. The top and side views of the ZrO2 surface for (a) cubic (111), (b) monoclinic (-111), and (c) tetragonal (101) surfaces. Red and white-blue balls stand for O and Zr atoms, respectively. Figure 2. The most stable adsorption configurations of Ptn(n = 1-5) clusters on the cubic ZrO2(111) surface for (a) Pt1, (b) Pt2, (c) Pt3, (d) Pt4 and (e) Pt5. Bond lengths are in Å. Each cluster is shown with a top view (up) and side view (down). Blue balls stand for Pt atoms, and others are the same as in Fig.1. Figure 3. The most stable adsorption configurations of Ptn(n =1-5) clusters on the monoclinic ZrO2(-111) surface for (a) Pt1,(b) Pt2,(c) Pt3, (d) Pt4 and (e) Pt5. Bond lengths are in Å. Each cluster is shown with a top view (up) and side view (down). See Figs.1 and 2 for color coding. Figure 4. The most stable adsorption configurations of Ptn(n = 1-5) clusters on the tetragonal ZrO2(101) surface for (a) Pt1, (b) Pt2, (c) Pt3, (d) Pt4 and (e) Pt5. Bond lengths are in Å. Each cluster is shown with a top view (up) and side view (down). See Figs.1 and 2 for color coding. Figure 5. Schematic illustration of the nucleation process considered in the definition of Enuc for Ptn clusters on three different surfaces. Figure 6. Nucleation energies Enuc of gas phase Ptn cluster, Ptn cluster on the cubic ZrO2(111) surface, monoclinic ZrO2(-111) surface and tetragonal ZrO2(101) surface at different cluster sizes.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

37



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Figure 6

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Influence of a ZrO2 Support and Its Surface Structures on the Stability

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