J. Phys. Chem. B 2006, 110, 7463-7472
7463
Interaction of Pt Clusters with the Anatase TiO2(101) Surface: A First Principles Study You Han,† Chang-jun Liu,*,† and Qingfeng Ge*,‡ Key Laboratory of Green Chemical Technology, School of Chemical Engineering, Tianjin UniVersity, Tianjin 300072, China, and Department of Chemistry and Biochemistry, Southern Illinois UniVersity, Carbondale, Illinois 62901 ReceiVed: February 9, 2006; In Final Form: March 2, 2006
The adsorption of Ptn (n ) 1-3) clusters on the defect-free anatase TiO2(101) surface has been studied using total energy pseudopotential calculations based on density functional theory. The defect-free anatase TiO2(101) surface has a stepped structure with a step width of two O-Ti bond distances in the (100) plane along the [101h] direction and the edge of the step is formed by 2-fold-coordinated oxygen atoms along the [010] direction. For a single Pt adatom, three adsorption sites were found to be stable. Energetically, the Pt adatom prefers the bridge site formed by 2 2-fold-coordinated oxygen atoms with an adsorption energy of 2.84 eV. Electronic structure analysis showed that the Pt-O bonds formed upon Pt adsorption are covalent. Among six stable Pt2 adsorption configurations examined, Pt2 was found to energetically favor the O-O bridge sites on the step edge along [010] with the Pt-Pt bond axis perpendicular to [010]. In these configurations, one of the Pt atoms occupies the same O-O bridge site as for a single Pt adatom and the other one either binds a different 2-fold-coordinated oxygen atom on the upper step or a 5-fold-coordinated Ti atom on the lower terrace. Three triangular and three open Pt3 structures were determined as minima for Pt3 adsorption on the surface. Platinum trimers adsorbed in triangular structures are more stable than in open structures. In the most stable configuration, Pt3 occupies the edge O-O site with the Pt3 plane being upright and almost perpendicular to the [001] terrace. The preference of Ptn to the coordinately unsaturated 2-fold-coordinated oxygen sites indicates that these sites may serve as nucleation centers for the growth of metal clusters on the oxide surface. The increase in clustering energy with increasing size of the adsorbed Pt clusters indicates that the growth of Pt on this surface will lead to the formation of three-dimensional particles.
1. Introduction Nanostructured metallic clusters supported on metal oxide substrates are of great interest due to their importance in heterogeneous catalysis.1-5 The nature and strength of the interaction between the metal particles and the support materials not only govern the growth and stability of the metal clusters but also control the fundamental processes that are critical to the catalytic activity of oxide-supported metal particles.6 Understanding the nature of the interaction is therefore important to tailor the oxide-metal cluster systems to achieve the desired reactivity and selectivity. Titanium dioxide is widely used as both different catalysts and support materials: TiO2 photocatalyst for water spliting7 and TiO2-supported Au catalyst for CO oxidation.8-10 TiO2 occurs in nature in three distinct crystallographic phases: anatase, brookite, and rutile. Among the polymorphs, TiO2 in the rutile structure is the most widely studied form both experimentally and theoretically as rutile is thermodynamically the most stable phase in bulk materials.11 On the other hand, TiO2 in the anatase structure has been found to be dominant in nanocrystalline phase12 and more active as a photocatalyst in solar energy conversion. Previously, we reported that the glow discharge plasma treatment of catalysts induced a remarkable change in the metal clusters and the interaction between the * Corresponding authors:
[email protected] (C.-j.L.); qge@ chem.siu.edu (Q.G.). † Tianjin University. ‡ Southern Illinois University.
metal and the support.13,14 In the case of Pt supported on anatase TiO2, the fringes of Pt(111) of the Pt clusters in the plasma treated catalysts were found to be almost perpendicular to those of anatase TiO2(101) at the interface. In contrast, the fringes of Pt(111) were nearly parallel to those of TiO2(101) in the catalyst without plasma treatment. The peculiar alignment in the plasma treated catalysts was believed to strengthen the interaction between Pt and TiO2 at the interface and lead to strong adhesion of Pt nanoparticles. The modification of Pt-TiO2 interfacial properties was believed to enhance the near-UV absorbance as a result of improved charge transfer and to improve the photocatalytic activity significantly for water splitting. On the other hand, the details of the interaction at the Pt-TiO2 interface have not been established. The present work is our first attempt to provide a better understanding of the interaction between Pt and anatase TiO2 using first principles based theory. Platinum supported on TiO2 has been a classic system that exhibits strong metal support interaction (SMSI).15,16 A small Pt loading can significantly modify the chemisorption and photocatalytic activity of TiO2.17-20 Takakusagi and co-workers investigated the growth of Pt nanoparticles from the metalorganic precursor on rutile-type TiO2(110) by using scanning tunneling microscopy (STM).21 They proposed a mechanism involving the self-limiting growth of Pt nanoparticles, where the competition between the decomposition of the Pt precursor at the periphery of Pt particles and the blocking of decomposition with TiOx species segregated from the interstitial of the TiO2 bulk determines the particle size. The initial adsorption of Pt occurs at the on-top site of the 5-fold-coordinated Ti atom
10.1021/jp0608574 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/23/2006
7464 J. Phys. Chem. B, Vol. 110, No. 14, 2006 and the Pt atom is bound with two Ti2O3 units from both sides. A recent scanning transmission electron microscope (STEM) study of Pt/TiO2 catalyst showed that the strong interaction between the Pt particles and the support was dependent on the Pt cluster size.22 For the small clusters with only a few Pt atoms, these authors also showed direct evidence of an epitaxial nucleation relationship between Pt and titania. In contrast to the abundant literature on Au/TiO2,23-27 computational studies on titania-supported Pt particles are scarce. Horsley carried out a molecular orbital calculation for Pt/TiO2 using the XR-SW-SCF method for (PtTiO6)8- and (PtTiO5)6cluster models and concluded that the later represents the metalsupport interaction in TiO2-supported SMSI catalysts.28 He went on predicting that the coordinately unsaturated Ti is the most favorable site for Pt adsorption on an ideal TiO2 surface although the Pt-Ti bond could not be formed due to the strong repulsive interactions between the O atoms and the Pt atom. In the Hartree-Fock calculations reported by Xu et al.,29 the rutile TiO2(110) surface was represented by (Ti4O16)16- and (Ti4O15)16cluster models. Strong interaction between Pt and TiO2(110) was reported and attributed to the contribution from Pt 5d-O 2p interactions.29 Jennison and co-workers used ultrathin TiO2 films on Pt(111) to study the interaction between Pt and rutile TiO2(110).30 These authors suggested that the O/Ti/Pt arrangement at the interface is energetically more favorable than the Ti/O/Pt structure and the stress relief at the interface drives the formation of the complex structure observed in STM measurements.31 In a very recent density functional theory (DFT) study of the potential energy profile for a single Pt adatom on rutile TiO2(110), Iddir et al. reported that a Pt adatom energetically prefers the hollow site and bridges the 2-fold-coordinated oxygen atom and the 5-fold-coordinated titanium atom.32 In the present paper, we report our results of first principles DFT supercell calculation of small Pt clusters supported on the defect-free anatase TiO2(101) surface. The (101) surface was chosen due to its thermodynamic stability.11,33 By analyzing the structural and electronic properties of the adsorption systems, we hope to understand the driving mechanism that determines the growth and morphology of anatase TiO2-supported Pt particles. This understanding will shed light on the physical origin of the unique catalytic properties observed in the plasma treated catalysts. 2. Methodology Calculations were carried out in the framework of density functional theory by using the Vienna ab-initio simulation package (VASP).34,35 Ultrasoft pseudopotentials36 were used to describe the ionic cores, and a plane-wave basis set with a cutoff energy of 400 eV was used to expand the valence electrons. The nonlocal exchange and correlation energies were calculated with the Perdew-Wang (PW91) functional37 of the generalized gradient approximation (GGA). Calculations including spinpolarization have been performed for all structures presented here. The results were compared with those from non-spinpolarized calculations in the case of a zero magnetic moment. The consistence of the results from both calculations ensures that the system is in a true nonmagnetic state. The geometries were relaxed using a conjugate-gradient or quasi-Newton scheme as implemented in VASP. The atomic structures were relaxed until the forces on all unconstrained atoms were less than 0.03 eV/Å. Similar setups have been employed in the study of a wide range of systems including metal and metal-oxide surfaces by the authors and many others.38-42 The calculated bulk anatase TiO2 lattice parameters (a ) b ) 3.803 Å, c ) 9.603 Å, c/a ) 2.525), which agree well with
Han et al.
Figure 1. Top (a) and side (b) view of the anatase TiO2(101) surface. Only the surface atoms were shown in part a. The surface unit cell and the coordinates of the surface atoms were labeled in parts a and b, respectively.
the experiments,43 were used to construct the periodic slabs for the (101) surface calculations. A supercell with a dimension of 10.33 × 11.41 × 17.89 Å includes a (1 × 3) unit in the surface plane and a vacuum region of ∼12 Å. Twenty-four TiO2 molecular units in the slab were distributed in four Ti layers and eight O layers. A top view of the surface unit cell was shown in Figure 1a. Only the atoms exposed on the surface were shown in the figure. The atoms in the lower layers were removed for clarity. In all calculations, the atoms in the two lower Ti layers and the associated O layers, i.e., the lower half of the slab, were fixed at their bulk positions. The atoms in top half of the slab together with the adsorbed Ptn clusters were allowed to relax according to Hellmann-Feynman forces calculated selfconsistently. The sampling of the surface Brillouin zone was restricted to the Γ point due to the large size of the unit cell. Test calculations with a (2 × 2) Monkhors--Pack grid in the surface plane showed that the relaxed structures and calculated adsorption energies were converged (
