Nucleation and Growth of Palladium Clusters on Anatase TiO2(101

Nov 14, 2008 - the perfect TiO2(101) surface is the bridge site formed by two two-coordinated .... Adsorption energy (Eads) was employed to describe t...
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J. Phys. Chem. C 2008, 112, 19506–19515

Nucleation and Growth of Palladium Clusters on Anatase TiO2(101) Surface: A First Principle Study Jinli Zhang,† Ming Zhang,† You Han,† Wei Li,† Xiangkun Meng,‡ and Baoning Zong*,‡ School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, China, and Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China ReceiVed: April 25, 2008; ReVised Manuscript ReceiVed: October 6, 2008

The nucleation and growth of palladium clusters supported on anatase TiO2(101) surface has been studied using periodic supercell models and density functional theory. The most active site for single Pd adatom on the perfect TiO2(101) surface is the bridge site formed by two two-coordinated oxygen (2cO) atoms at the step edge with the highest adsorption energy of 2.18 eV. On the defective surface, the effect of oxygen vacancy on Pd nucleation is not as strong as that in the Pt or Au deposition case. The shift of active sites for the Pd dimer growing to trimer on the perfect anatase TiO2(101) surface is observed. On both perfect and defective anatase surfaces, the adsorbed Pd3 clusters prefer to form planar triangles and the adsorbed Pd4 and Pd5 clusters tend to three-dimensional structures. The nucleation and growth of Pd clusters at the anatase TiO2(101) surface is mainly driven by the interaction between Pd and surface atoms when the cluster size is less than four. The strength of Pd-Pd interaction turns out to control dominantly the Pd deposition process as the Pd cluster gets larger. 1. Introduction Titanium dioxide is a versatile oxide material that has been widely used as a photocatalyst and catalytic support in heterogeneous catalysis.1-4 Especially titania-supported transition metal catalysts have attracted a great deal of attention over the past decades due to their good catalytic performance in many important processes such as CO oxidization,5-7 hydrogenation,8-11 water splitting,12-14 and so on. Exploring the nucleation and growth mechanism of metal species on TiO2 is fundamental to design and improve the catalytic activity of the transition metal/ TiO2 systems.15 It is well-known that TiO2 exists in three main crystalline forms, that is, rutile, anatase, and brookite. Rutile and anatase TiO2 are widely used because of their excellent physical and chemical properties.1 Plenty of experimental and theoretical investigations suggested that the nucleation and growth patterns of metal clusters involving gold16-26 and platinum27-32 were greatly dependent on the crystal structure of TiO2. For instance, a scanning tunneling microscopy (STM) study found that Au nucleated preferentially at the step edges on the rutile TiO2(110),20 which is different from some theoretical results. Both Nørskov’s group21 and Sanz’s group22 reported that the most stable adsorption site for a single Au was on top of a twocoordinated protruded oxygen (2cO) atom at the perfect rutile TiO2(110) from the DFT-GGA (RPBE) slab calculations and the DFT-GGA (B3LYP) cluster calculations, respectively. Being adsorbed on the rutile TiO2(110) surface, the growth of Au adatoms exhibits a tendency from quasi-2D (two-dimensional) to 3D (three-dimensional) clusters structures,23 which is observed by Goodman and co-workers using STM technology. When Au deposited at the anatase TiO2(101) surface, Selloni’s group33 reported that the single Au preferred to locate at the top site of * To whom correspondence should be addressed. E-mail: zongbn@ ripp-sinopec.com. † Tianjin University. ‡ SINOPEC.

a five-coordinated Ti(5cTi) atom at the perfect surface and it was easily captured by the oxygen vacancy at the defective surface using periodic DFT calculations, which is further confirmed by the Diebold group’s STM study.31 A strong influence of surface oxygen vacancy on Au cluster nucleation and distribution has also been observed by Besenbacher and co-workers using the STM method.24 Different from Au, the noble metal Pt deposited at TiO2 is well-known as a classic strong metal-support interaction (SMSI) system.34 X-ray photoemission spectroscopy (XPS) and ion backscattering spectroscopy (ISS) characterizations suggested that the 5cTi atom was the most active site for the nucleation of Pt clusters on rutile TiO2(110) surface,28 whereas a recent STM study showed that a single Pt atom preferred to adsorb at the rows of 2cO atoms on the anatase TiO2(101) surface.31 This result is consistent with the previous theoretical calculations, in which the authors suggested that 2cO bridge site was nucleation centers for the growth of metal clusters on the perfect surface,30 whereas the oxygen vacancy site also accelerated the aggregation of Pt adatoms on the defective surface.29 Crystal structure of the TiO2 support also plays an important role in adjusting the reaction activity of catalysts. For example, Pd-supported anatase TiO2 catalyst show higher catalytic performance than that supported on rutile TiO2 for the reactions involving selective hydrogenation of acetylene35-37 and long chain alkadienes.38,39 No report was found so far on the nucleation and growth mechanism of Pd clusters on the anatase TiO2(101) surface, although there are few reports23,40-42 on Pd deposition at the rutile TiO2 surface. Goodman and co-workers studied the adsorption of Pd on rutile TiO2(110) surface using STM,42 and they observed Pd dimers and tetramers at the rutile surface rather than single Pd adatoms in the nucleation stage. They suggested that only monomers are mobile, whereas dimers are stable nuclei, which was later demonstrated by Sanz and co-workers’ theoretical study using a periodic supercell and GGA approach within density functional theory.40 Sanz et al. investigated the single Pd and the Pd dimer adsorbed at the

10.1021/jp8036523 CCC: $40.75  2008 American Chemical Society Published on Web 11/14/2008

Nucleation and Growth of Palladium Clusters perfect rutile TiO2(110) and found that they preferentially adsorbed on the surface channels near the 5cTi atoms but tilted toward the 2cO atoms. When Pd dimer was adsorbed at the perfect rutile(110) surface, noticeable Pd-Pd interaction occurred. For the defective rutile TiO2(110) surface, Sanz et al. found that the adsorption of a single Pd at the surface oxygen vacancy was stronger than that on the perfect rutile surface. However, Bredow and Pacchioni41 reported opposite results according to their study on the Pd/perfect-rutile-TiO2(110) system using embedded cluster models as well as periodic supercell models within DFT calculations. They suggested that the Pd atoms were preferentially adsorbed at the top site of 2cO atoms using the cluster models, whereas at the top of 5cTi atoms using the periodic model. And when Pd dimers adsorbed on the perfect rutile TiO2(110) surface, the Pd-Pd interaction was weakened due to the occurrence of strong interaction between Pd and the substrate. In the present paper, therefore, we investigate the Pd deposition on perfect and defective anatase TiO2(101) surface using first principle DFT and periodic supercell calculations. The aim of this work is to disclose the mechanism of the Pd deposition process as well as the effect of oxygen vacancy at the anatase (101) surface on the nucleation and growth of Pd clusters. The results obtained here can shed light on designing high-performance TiO2-based catalysts, but also can promote exploring the growth mechanism of metal clusters at interfaces.

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Figure 1. Side (a) and top (b) views of the anatase TiO2(101) surfaces. Only the surface atoms were shown as balls. The red is an O atom and the argent is a Ti atom.

2. Methods The density functional theory (DFT) calculations were performed using the CASTEP module in Materials Studio package (Accelrys Inc.). Ultrasoft pseudopotentials were used to describe the ionic cores. The wave functions were expanded in a plane wave basis set with a cutoff energy of 300 eV. The generalized gradient approximation (GGA) was chosen here to describe the effects of exchange and correlation, which is better than the local density approximation (LDA) for Pd/metal oxide system.40 The Perdew-Wang (PW91) exchange-correlation function43,44 was used in all of the calculations. The geometries were optimized using a BFGs method.45 The atomic structures were relaxed until the forces on all unconstrained atoms were less than 0.05 eV/Å. Similar setups have been employed in the study of a wide range of systems including metal and metaloxide surfaces.46-48 The calculated bulk anatase TiO2 structure (a ) 3.80 Å, c ) 9.60 Å, c/a ) 2.53), which is in agreement with the experiments,49 identified the validity of parameters setting in this work. A periodic slab model with a dimension of 10.33 × 11.41 × 17.89 Å was used in this work, which included 1 × 3 sized TiO2 (101) substrate of twelve layers (atomic layer) and a vacuum region of ∼12 Å. This model has also been used to study the Pt and Au cluster adsorption on the anatase TiO2(101) surface in some published works.29,30,33 To evaluate the effect of the thickness of TiO2 substrate on the adsorption energy of Pd on the TiO2 surface, the adsorption of a single Pd atom on a thicker TiO2 substrate (18 atomic layers) was calculated. The results were given in Table S2 of the Supporting Information. Comparing the adsorption energy of a single Pd adatom on the TiO2 surface using 12 and 18 atomic layers, the difference is within 0.02 eV, which can be neglected. To save the computing time, the slab model of TiO2 with 12 atomic layers was adopted in this work. Top and side views of the anatase TiO2(101) surface were shown in Figure 1. The bottom six layer atoms were fixed in the calculations.

Adsorption energy (Eads) was employed to describe the interaction between the palladium clusters and titania surface here,33

Eads)-[E(Pdn ⁄ TiO2) - E(TiO2) - E(Pdn)]

(1)

where E(Pdn/TiO2), E(TiO2), and E(Pdn) are the total energies of the TiO2 substrate with Pdn cluster, the bare TiO2 substrate, and the free Pdn cluster in gas phase, respectively. The test calculations with different k-point (1 × 1 × 1, 2 × 2 × 1, 3 × 3 × 1) for a single Pd adatom on both perfect and defective anatase TiO2(101) surface were carried out. The difference of the adsorption energy is in the range of ∼1%. We therefore used Gamma-point sampling in the present study to save the calculation time. The results of test calculations for different cutoff energy and k-point were given in Tables S1 and S3 in the Supporting Information. 3. Results and Discussion 3.1. Bare Anatase TiO2(101) Surface. The anatase TiO2(101) surface has a stepped structure with the 2cO along the [010] direction forming the edge of the step, as shown in Figure 1a. In addition, 3-fold-coordinational oxygen atoms (3cO), 5cTi atoms, and 6-fold-coordinational titanium atoms (6cTi) were also exposed on the surface in which the 2cO and 5cTi atoms are unsaturated. These different kinds of atoms were labeled with blue arrows in the side (Figure 1a) and top (Figure 1b) views of the surface. Strong relaxation and surface reconstruction have been observed in the perfect anatase TiO2(101) surface. Compared with the bulk structure, the whole atoms in the surface exhibit outward relaxation along the [101] direction. The 2cO and 5cTi lifted by 0.09 and 0.13 Å, respectively, and the 3cO and 6cTi atoms were 0.41 and 0.23 Å, respectively. The relaxation also caused the decrease of angle ∠1 (2cO-6cTi-3cO) and ∠2 (5cTi-3cO-6cTi) from 105.94 and 101.96° in the bulk structure

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Figure 2. Relaxed structures of a single Pd adatom on a perfect anatase TiO2 (101) surface: (a) Pd1,(a); (b) Pd1,(b); (c) Pd1,(c); (d) Pd1,(d); (e) Pd1,(e); (f) Pd1,(f). The structures are ordered by their stabilities in this figure and also in the following figures.

to 97.92 and 96.04° in the relax surface, respectively. Further, the length of 2cO-5cTi and 2cO-6cTi decreased by 0.13 and 0.03 Å, respectively, which means that relaxation strengths the interaction between 2cO and 5cTi atoms on the surface. The oxygen vacancy (coverage ) 1/6) is obtained by removing one surface 2cO atom (marked with blue circle in Figure 1b) from the 1 × 3 sized TiO2(101) surface, which left two unpaired electrons in the slab. This defective surface shows distinguished reconstruction after optimization. The 3cO atom under the oxygen vacancy rises up about 0.3 Å and the angle ∠2 (6cTi-3cO-5cTi) increases from 101.96° in the relaxed ideal surface to 130.00° in this defective surface. 3.2. Single Pd Adsorption. Several possible active sites for a single Pd adatom on defect-free anatase TiO2(101) surface are considered here. The relaxed structures and their adsorption energies are shown in Figure 2. The most stable structure is the Pd1,(a) with the highest energy of 2.18 eV. In Pd1,(a), the Pd atom adsorbed at a bridge site of two edge 2cO atoms (2cO-bridge site) along the [010] direction, with the Pd-2cO bond lengths of ∼2.06 Å, respectively, which is similar with that of a single Pt adsorption at the perfect anatase TiO2(101) surface.30 Due to the inserting of Pd into the middle of two 2cO atoms, the distance between these two 2cO atoms elongates from 3.80 Å in the bare surface to 4.10 Å. The adsorbed energy of Pd1,(a) is strongly higher than that of the most stable structure of Au adsorption at the perfect anatase TiO2(101) surface (0.39 eV)33 but lower than that of Pt adatom (2.84 eV).30 Excluding

Zhang et al. the effect of the different calculation software, the comparison of the adsorption energies indicates that the order of the activity of the perfect anatase TiO2 surface for the three noble metal atoms is Pt > Pd > Au. Besides, the interaction of Pd with anatase TiO2(101) surface is stronger by ∼0.5 eV than that with the rutile TiO2(110) surface.40 The Pd atom exhibits a slight left displacement in the Pd1,(b) compared to Pd1,(a) structure, in which the distance between Pd and 2cO elongate slightly to 2.17 Å and the length of Pd-3cO (left) shorten about 0.2 Å compared with Pd1,(a). This indicates that the left 3cO atom exhibits a interaction with Pd in this structure than in Pd1,(a). Further, starting by placing a Pd atom either on the top of the left 3cO atom or on the bridge site of 2cO-3cO (left) led to same final structure Pd1,(b) after relaxation. In the Pd1,(c) configuration, the Pd adatom binds with the 2cO atom on the step edge, the 6cTi atom at the bottom of the step, and the 3cO atom on the lower terrace of the step with bond length of ∼2.09, ∼2.55, and ∼2.09 Å, respectively. This adsorption site is denoted as the 2cO-6cTi-3cO bridge site. The existence of Pd adatom causes the increase of 2cO-6cTi and 3cO-6cTi bond length from 1.93 and 2.01 Å in the bare surface to 2.08 and 2.23 Å, respectively, whereas they elongated to 2.49 and 2.59 Å when a single Pt adsorbed at the same site.30 The adsorption energies of the two configurations of Pd1,(d) and Pd1,(e) are close to each other, indicating they have similar stabilities. It should be noted that initial structures of Pd adatom on the top site of either 2cO atom or 5cTi atom all shifted to the bridge site of 2cO and 5cTi atoms (denoted as the 2cO-5cTi bridge site) after optimization (see Figure 1e). In Pd1,(f), which is the least stable structure among the six stable structures with an adsorption energy of 1.37 eV, the Pd adatom is located on the top of the 3cO atom with a distance of 2.03 Å. At variance with a defect-free surface, the Pd atom strongly prefers to adsorb in the vacancy site on the defective surface of anatase TiO2. The absorption structure of Pd on oxygen vacancy surface was shown in Figure 3. Different initial sites (marked by different number) of Pd adatom were investigated. It is distinguished that the Pd atoms at the locations of 1-8 all transfer to the same site (oxygen vacancy) after optimization, and this final structure was denoted as Pd1,v. The filling of the oxygen vacancy site by the Pd adatom causes the two Ti atoms neighboring vacancy oxygen relaxing toward each other. Consequently, the 5cTi(v)-3cO-4cTi(v) angle (∠2) decreases from 127.22° of the relaxed defective surface to 105.20° in Pd1,v, which is close to that in the clean perfect surface (96.04°). The bond lengths of Pd-4cTi(v) and Pd-5cTi(v) are 2.36 and 2.60 Å, respectively, which are longer than the distance of 2cO with the two kinds of surface Ti atoms. The adsorption energy of Pd1,v is 2.67 eV, which is higher than that of a single Pd on a defect-free surface (Pd1,(a), 2.18 eV). Herein, we summarized the adsorption energies of most stable structures for Pd, Pt, and Au adatom on perfect and defective TiO2 surfaces calculated by our and other groups in Table 1. Even though the adsorption energies of Pd, Pt, and Au adatom on anatase (101) surface and that on the rutile (110) surface listed in Table 1 are calculated by different computational approaches, the adsorption energies of Pd, Pt, and Au adatom on the anatase (101) surface are much larger than that on the rutile (110) surface, especially at the defective surface. Although the oxygen removal from the anatase (101) surface is more difficult than from the rutile (110),50 the oxygen vacancy at the anatase (101) surface might be more active than that at the rutile (110) surface for the noble metal deposition such as Pd, Pt, and Au. In addition, the oxygen vacancy at the TiO2

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Figure 4. Electron density difference contour maps of a single Pd adatom on the 2cO-bridge site (a) and the oxygen vacancy site (b), respectively.

Figure 3. Side (a) and top (b) views of the adsorption structure of a single Pd adatom on the defective TiO2(101) surface. The numbers 1-8 indicate eight initial adsorption sites for Pd adsorption.

TABLE 1: Adsorption Energies (eV) of the Single Noble Metal Adatoms (Pd, Pt, and Au) on the Anatase TiO2(101) and the Rutile TiO2(110) Surfaces anatase (101)

Pd Pt Au

rutile (110)

Eads(perf)

Eads(def)

Eads(def) Eads(perf)

2.18 2.84a 0.39c

2.67 4.87b 3.15c

0.49 2.03 2.76

Eads(perf)

Eads(def)

Eads(def) Eads(perf)

1.93d 2.14e 0.60f

∼2.23 3.52e 2.78f

∼0.3d 1.38 2.18

a Value obtained from ref 30. b Value obtained from ref 29. Value obtained from ref 33. d Value obtained from ref 40. e Value obtained from ref 32. f Value obtained from ref 26. c

TABLE 2: Mulliken Charges of a Single Metal Adatom (Au, Pd, and Pt) on Both Perfect and Defective TiO2 Surfaces Mulliken charge (e) Pd1 Pt1 Au1

perfect

defective

0.09 0.10 0.01

-0.13 -0.22 -0.15

surface, whether it is anatase or rutile, has a strong ability to capture the Au adatom. The less one is Pt and the least one is the Pd adatom. To understand the different adsorption behavior of Au, Pd, and Pt on perfect and defective surfaces, the Mulliken charges of Au, Pd, and Pt adatoms in the most stable adsorption structures at the perfect and defective TiO2 surfaces are summarized in Table 2. The amount of Mulliken charge transferred from Au, Pd, and Pt adatoms to the perfect surface is 0.01, 0.09, and 0.10 e, respectively, and the amount of charge transferred from the TiO2 defective surface to Au, Pd, and Pt adatoms is -0.15, -0.13, and -0.22 e, respectively. The order of charge amount for Au, Pd, and Pt adatoms at the

perfect and defective TiO2 surfaces is consistent with that of their adsorption energy (see in Table 1). The results indicated that the amount of charge transfer between the metal adatom and the TiO2 surface reflects the stability of the TiO2 surface adsorbed by metals, but it did not show any direct relationship with the magnitude of the adsorption energy. The electron transfer is further demonstrated by electronic density difference (EDD) analysis (shown in Figure 4). The EDD of Pd1,(a) shows that the Pd-2cO bonds are polarized. The depletion of electron density from Pd adatom reduces the Pauli repulsion that the metal atom experienced and leads to a stronger attractive interaction at the Pd-TiO2 interface. The exchange of electron density between surface Ti atoms (5cTi(v) and 4cTi(v)) and Pd adatom in the EDD of Pd1,v implied that there may be an electron donation and back-donation process occurring between them. Although the advantage of oxygen vacancy for Pd adsorption is not as obvious as that for Pt and Au, there still are some differences in the electronic properties between Pd1,(a) and Pd1,v, which respond to the most stable structures for Pd adatom on perfect and defective anatase TiO2(101) surface, respectively. Figure 5 shows the total density of state (DOS) plots for the clean perfect and defective TiO2(101) as well as Pd1,(a) and Pd1,v. After Pd adsorbed at the perfect anatase (101) surface, the Fermi level moved from the top of the valence band to the middle of the band gap and the band gap was partly filled by the Pd states, which are almost Pd 4d-state contributions. The metal-induced gap states were also reported for Pt deposition at anatase TiO2(101)30 and Au adsorption on the rutile TiO2(110) surface.21 In contrast, the Fermi level is at the bottom of the conduction band in the defective anatase TiO2(101) surface and the position did not change after Pd adsorption at the oxygen vacancy. The Pd states induced not only in the band gap but also at the top of the valance band as well as the bottom of the conduction band. 3.3. Pd Dimer Adsorption. Six stable adsorption structures of the Pd dimer on the perfect TiO2(101) surface, denoted as Pd2,(a), Pd2,(b), Pd2,(c), Pd2,(d), Pd2,(e), and Pd2,(f), were investigated and shown in Figure 6. The adsorption energies are also given in this figure. In all of the six structures, the Pd-Pd bonds are

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Figure 5. Density of state for the (a) perfect TiO2(101) surface (top) and Pd1,(a) (bottom) and (b) defective TiO2(101) surface (top) and Pd1,v (bottom). The local DOS of Pd, which contributes to the total DOS in Pd1,(a) and Pd1,v, is represented in red color.

parallel to the TiO2(101) surface. In the first four structures (Figure 6a-d), the Pd1 atoms of the dimers are kept adsorbing at the most stable 2cO-bridge site, while Pd2 atoms are located at the 5cTi-3cO bridge site, 2cO-6cTi-3cO bridge site, 2cO-5cTi bridge site, and the top of 3cO, respectively. In these four structures, the bond lengths of Pd-Pd are 2.63, 2.82, 2.68, and 2.63 Å, respectively. The other two structures, Pd2,(e) and Pd2,(f), are the results of the relaxation by arranging the two Pd atoms at the 2cO-6cTi-3cO bridge site (like Pd1,(c)) and 2cO-5cTi bridge site (like Pd1,(e)). Clearly, the two adsorbed Pd atoms got close to each other after optimization and the Pd1-Pd2 distances became 2.86 and 2.72 Å, respectively. Comparing the adsorption energies of the six stable structures, even though the most stable dimer structure on the perfect TiO2(101) surface is Pd2,(a) with the highest Eads of 2.95 eV, the differences of adsorption energy between Pd2,(a) and the other five structures (Pd2,(b)-Pd2,(f)) are quite small. The result demonstrated that the different active sites for Pd dimer adsorption became competitive. Here, we defined the binding energy to measure the stability of the second Pd adsorption on Pd1/TiO2

Ebind)-[E(Pdn ⁄ TiO2) - E(Pdn-1 ⁄ TiO2) - E(Pd)] (2) where Ebind represents the adsorption energy of the nth Pd adatom on the Pdn-1/TiO2 system. After the calculation, we noticed that the “adsorption energies” of the second Pd in the Pd2,(a), Pd2,(b), Pd2,(c), and Pd2,d) are 1.81, 1.75, 1.74, and 1.71 eV, respectively, which are less than that in Pd2,(e) and Pd2,(f) (2.00 and 2.06 eV, respectively). The results further confirmed that, although the 2cO-bridge site is the most active for Pd monomer, the Pd dimer cannot simultaneously occupy two of them. When the

Zhang et al.

Figure 6. Adsorption structures of the Pd dimer on perfect anatase TiO2(101) surface. (a) Pd2,(a); (b) Pd2,(b); (c) Pd2,(c); (d) Pd2,(d); (e) Pd2,(e); (f) Pd2,(f).

Pd dimer adsorbed on the TiO2(101) surface, it will consider not only the activity of the adsorption sites but also the distance between the two sites. Therefore, both of the 2cO-6cTi-3cO bridge sites and the 2cO-5cTi bridge sites became more competitive during the Pd dimer adsorption than that in a single Pd adsorption. The shift of active sites during noble metal cluster growth on the anatase TiO2(101) surface was also observed by Diebold’s STM study.31 Three adsorbed structures of Pd dimer on defective TiO2 surface were obtained after relaxing five different initial configurations, as shown in Figure 7. These three structures are denoted as Pd2,(a)_v, Pd2,(b)_v, and Pd2,(c)_v, respectively. The corresponding adsorption energies were also given in the figure. In the configuration of Pd2,(a)_v (see Figure 7a), the two Pd atoms symmetrically distribute at the oxygen vacancy site and the Pd1-Pd2 bond axis is parallel with the [010] direction. The bond length of Pd1-Pd2 is 2.52 Å, which is much shorter than any of the Pd2 on a perfect surface, but this length is longer than the Pt-Pt bond (2.26 Å) reported in Han’s work,29 indicating that the binding interaction between two Pt atoms is stronger than two Pd atoms on the anatase TiO2 surface. Under the symmetrical interaction of Pd with the two-edged 2cO atoms, the two 2cO were dragged close to each other and their distance was reduced from 7.55 to 7.10 Å. In the two structures of Pd2,(b)_v and Pd2,(c)_v (see Figure 7b,c), the Pd-Pd bond axis is perpendicular with the 2cO edge. The bond lengths of Pd1-Pd2 are 2.66 and 2.52 Å, respectively. Comparing the adsorption energies of these three stable structures, the Pd2,(a)_v is the most stable structure with highest Eads of 4.60 eV, which is higher by about 2.0 eV than the Pd dimer on a perfect surface. It is

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Figure 7. Adsorption structures of Pd dimer on the defective anatase TiO2 surface. (a) Pd2,(a)_v; (b) Pd2,(b)_v; (c) Pd2,(c)_v.

interesting to remark that the most stable structure of the Pd dimer adsorbed on the defective TiO2(101) surface is different with that of adsorbed Pt dimer and Au dimer. In both of the most stable structures for Pt dimer and Au dimer adsorption on the defective TiO2(101) surface,29,33 one of the noble metal atoms (denoted as NM1) occupied the oxygen vacancy site and the other one sits at the NM1-4cTi(v) bridge site or at the NM1-2cO bridge site. The difference may be caused by the different interacting strength between noble metal dimer and the defective TiO2(101) surface. 3.4. Pd Trimer Adsorption. Two Pd trimer growth modes, line and triangle (or “2D”), were investigated in our present work. The stable structures of Pd trimers on perfect surface and their adsorption energies are shown in Figure 8. The Pd3,(a), Pd3,(b), and Pd3,(c) are triangular Pd adsorption configurations, while the Pd3,(d) and Pd3,(e) are linear Pd adsorption structures. In the structure of Pd3,(a), the adsorption sites for Pd1 and Pd2 atoms are similar to that in Pd2,(f) and the third Pd atom is at the 2cO-6cTi-3cO bridge site. The plane formed by these three Pd atoms is almost parallel with the TiO2 terrace. The distances of Pd1-Pd2, Pd2-Pd3, and Pd3-Pd1 are 2.68, 2.73, and 2.75 Å, respectively. The bond length of Pd1-Pd2 in this trimer structure is shorter than that in the dimer Pd2,(f), indicating the adding of Pd3 enhanced the Pd-Pd interaction. Similarly, the adsorption sites for Pd2 and Pd3 atoms in Pd3,(b) are as the same as in Pd2,(e), whereas the Pt1 atom sits at the 2cO-bridge site. The Pd3,(c) was also obtained by adding a third Pd atom to the Pd2,(f), but the Pt3 triangle in Pd3,(c) is almost perpendicular to the TiO2(101) terrace. Because there is no direct interaction between the Pd3 atom and the TiO2 surface atoms in Pd3,(c), its adsorption energy, 2.14 eV, is lower than that of Pd3,(a) (2.50

Figure 8. Adsorption structures of Pd trimer on the perfect anatase TiO2(101) surface. (a) Pd3,(a); (b) Pd3,(b); (c) Pd3,(c); (d) Pd3,(d); (e) Pd3,(e).

eV). In contrast with the triangular Pd3 cluster, the linear Pd3 adsorption structures have pretty low adsorption energies. The three Pd atoms of linear Pt3 in Pd3,(d) (see Figure 8d) are all located at the 2cO-6cTi-3cO bridge sites and they occupied the 2cO-5cTi bridge sites in Pd3,(e). In both of the two linear configurations, the Pd atoms at the two ends of the trimer, Pd1 and Pd3, were pulled toward the middle Pd2 atom to maximize the Pd-Pd bonding. The length of Pd1-Pd2 bond is equal to that of Pd2-Pd3 bond in both of Pd3,(d) and Pd3,(e) because of the symmetry. The Pd1-Pd2 bond length is 2.76 Å in Pd3,(d) and 2.70 Å in Pd3,(e), which is shorter than that of Pd dimers in Pd2,(e) and Pd2,(f). In general, the most stable structure for Pd trimer adsorption on TiO2(101) surface is Pd3,(a) and its adsorption energy is 2.50 eV, which is lower than that of the most stable structure for Pt trimer adsorption (3.08 eV)30 but is higher than that of the adsorbed Au trimer (1.87 eV).33 In addition, the Pb3 triangle in Pd3,(a) is parallel with the TiO2 terrace, whereas in both of the most stable configurations for Pt trimer and Au trimer30,33 adsorbed at TiO2(101) surface, the Pt3 triangle and Au3 triangle are almost vertical to the TiO2 terrace, indicating the growth trend for Pd cluster may be different with Pt and Au. Three Pd3 stable adsorption structures were determined on the defective anatase TiO2(101) surface and were shown in

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Figure 9. Adsorption structures of Pd trimer on the defective anatase TiO2(101) surface. (a) Pd3,(a)_v; (b) Pd3,(b)_v; (c) Pd3,(c)_v.

Figure 9. The adsorption energies for all these structures were also shown in this figure. The most stable structure corresponds to a triangular Pt3 occupying the oxygen vacancy site with the triangle plane almost perpendicular to the terrace of TiO2(101) surface (Figure 9a, Pd3,(a)_v). In this structure, the Pd1-Pd2 bond axis is almost parallel to the [010] direction which is similar to that of Pd2,(a)_v, and the Pd3 atom binds with surface 4cTi(v). This is in good agreement with recent calculations by Gong et al.31 The bond lengths of Pd1-Pd2, Pd2-Pd3 and Pd1-Pd3 are 2.70, 2.56, and 2.56 Å, respectively. The less stable configurations, Pd3,(b)_v, was relaxed from the initial structure based on Pd2,(a)_v in which a third Pd atom was added on the bridge site of 3cO and 5cTi(v). In Pd3,(b)_v, the Pd1-Pd2 bond is also parallel to the [010] direction, but the Pd3 triangle is almost parallel to the TiO2(101) terrace. The lengths of Pd1-Pd2, Pd1-Pd3 and Pd2-Pd3 are as equal as 2.62 Å. In the structure of Pd3,(c)_v, the Pd1 and Pd2 atoms are almost located at the top site of 5cTi(v) and 4cTi(v), respectively. The Pd3 atom bridges Pd1-Pd2 and the neighboring 2cO atom. The Pd-Pd bond lengths are in the range of 2.55-2.62 Å. The Pd3,(c)_v is the least stable among the three adsorbed Pd3 structures, which is in contrast against Pt3 adsorption at the defective TiO2(101) surface.29 The linear Pt3 trimer adsorption on the defective TiO2 surface was also considered in our calculation, but we found that the linear Pt3 trimer cannot be stably adsorbed at the defective surface. For example, the initial structure of three Pd atoms taking a linear distribution in the oxygen vacancy and parallel with [010] direction transformed into Pd3,(a)_v configuration after relaxation by pushing the middle Pd atom out of the line and then forming a Pd triangle. Similarly, the linear Pt3 structure is not as stable as Pt3 triangle at the

Zhang et al. defective anatase (101) surface.29 But the most stable structure for adsorbed Au trimer on this surface is linear Au3.31,33 3.5. Pd Tetramer and Pentamer Adsorption. Several possible structures of 2D and 3D Pd4 cluster adsorbed on both perfect and defective TiO2 surface were considered and only the most stable structures as well as their adsorption energies are shown in Figure 10. In both of the Pd4,(a) and Pd4,(c)_v, which are the most stable configurations for Pt4 3D cluster adsorption at the perfect and defective TiO2(101) surface, the four Pd atoms form a tetrahedral structure with the Pd-Pd length in the range of 2.55-2.70 Å. The planes formed by Pd1, Pd2 and Pd3 interact directly with the TiO2 surface atoms, while the Pd4 atom only binds to Pd1, Pd2, and Pd3 without direct Pt4-surface interaction. In Pd4,(b) and Pd4,(d)_v, the four Pd atoms construct a distorted diamond-shape in which all the four Pd atoms bind to the TiO2 surface atoms, such as 2cO, 3cO, and 5cTi. Comparing the adsorption energies of the four stable structures, we found that the adsorbed tetrahedral Pt4 clusters are more stable than their planar structures on both of the perfect and defective anatase TiO2(101) surface. The binding energies of the fourth Pd adatom on the adsorbed Pd3 clusters in Pd4,(a), Pd4,(b), Pd4,(c)_v and Pd4,(d)_v were also calculated using the formula (2) in section 3.3 and the values are 3.18, 2.95, 2.84, and 2.43 eV, respectively. The results conformed that the threedimensional (3D) growth for Pd cluster is favored on both of the perfect and defective anatase TiO2(101) surface, which is further proved by the results of Pd5 2D and 3D clusters adsorption on the anatase (101) surface (see Figure 10). 3.6. Nucleation and Growth Model of Pd Clusters on Anatase TiO2 (101) Surface. Our calculations showed that a single Pd atom prefers to adsorbed at the 2cO-bridge site on the perfect TiO2(101) surface, whereas the oxygen vacancy became the most active site for Pd deposition on the defective surface. The effect of oxygen vacancy on Pd nucleation is not as strong as that in the Pt or Au deposition case. With the increase of Pd adatoms, the growth of Pd cluster shows different trends from the other noble metals, such as Au and Pt. The adsorbed Pt and Au clusters on the TiO2(101) surface are 3D like.29,31,33 When Pd cluster grows on the TiO2 surface, it is 2D structure as the cluster size is less than four and then becomes three-dimensional structure. To know more about the growth of Pd clusters, we calculated the structure of the gas-phase Pdn clusters (n ) 1-5) and their anions (Pdn-). We found that the most stable structures of Pdn clusters and their anions were similar, that is, the linear, triangular, tetrahedronal, and trigonal bypiramidal configurations respectively. This result is similar to Deka group’s report.51 However, the average bond lengths of the Pdn and Pdn- cluster in gas phase are different from each other at the same cluster size as shown in Figure 11. The average bond lengths of the adsorbed Pdn cluster at the defective TiO2 surface are also summarized in Figure 11. It is clear that the average bond length of the gas-phase Pdn- cluster is larger than that of gas-phase Pdn, however, the transform trend with the cluster size is similar to each other. In addition, the average bond lengths of the Pdn- cluster are close to those of the adsorbed palladium cluster at the defective TiO2 surface. The reason is that the excess electrons in the defective TiO2 surface transfer to the Pdn cluster (see Table 2) during the Pd adsorption process so that the adsorbed Pd cluster turns to be anionic, which makes the structures of adsorbed Pd cluster on the defective TiO2 surface close to the Pdn- cluster rather than Pdn. To better understand the mechanism of the Pd cluster aggregation, the clustering energy is introduced in this work.

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Figure 11. Variation of average bond lengths (Å) with cluster size for the most stable palladium clusters in gas-phase and on the TiO2 surface.

Figure 12. Plots of the clustering energies for adsorbed Pd, Pt, and Au clusters with different sizes on the perfect and defective anatase TiO2(101) surface. The values of the clustering energies for absorbed Pt and Au clusters are obtained from the refs 29, 30, and 33.

Figure 13. Varied values of EPd-TiO2 and EPd-Pd with the cluster size of Pd on the perfect and defective anatase TiO2(101) surface. The blue curves are corresponding to EPd-Pd and the red are corresponding to EPd-TiO2. Figure 10. Adsorption structures of Pd4 and Pd5 clusters on the perfect (a, b, e) and defective (c, d, f) anatase TiO2(101) surface. (a) Pd4,(a); (b) Pd4,(b); (c) Pd4,(c)_v; (d) Pd4,(d)_v; (e) Pd5; (f) Pd5_v.

The definition of the clustering energy of adsorbed Pdn clusters is following,33

Eclus)-[E(Pdn ⁄ TiO2) - E(TiO2) - nE(Pd)] ⁄ n

(3)

where E(Pd) is the total energy of a single Pd atom in gas phase and n is the number of Pd adatoms. The value of clustering

energy reflects the clustering ability of Pd atoms on TiO2(101) surface. In Figure 12, the clustering energy for Pdn cluster (n ) 1-5) as well as Ptn (n ) 1-5)29 and Aun (n ) 1-3)33 are shown. It is clear that the change of clustering energy for Pd cluster is different from that for Au and Pt with the increase of cluster size. At the perfect TiO2 surface, the clustering energies of Ptn and Aun increase as the size of metal cluster gets bigger while that of Pdn almost keeps constant. Similarly, the clustering energies of Ptn and Aun at the defective TiO2 surface decrease as the size of metal cluster gets bigger, whereas that of Pdn still keeps constant. Our results indicate that the Pt and Au atoms

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are easy to form big clusters at the perfect TiO2(101) surface, whereas the advantage of oxygen vacancy as the nucleation center for Pt and Au clustering is reduced dramatically when the metal cluster gets larger,29,52 and the Pd clustering as well as Pd adsorption separately might be two competitive paths during the Pd deposition process. Generally, the formation of supported metal cluster is mainly driven by two factors. One is the interaction between metal adatoms and the supported surface, and the other is the metal-metal interaction within the cluster. In the case of adsorbed Pd cluster on anatase surface, which mainly control the Pd growth process? In order to answer this question, we divided the clustering energy (Eclu) of adsorbed Pd cluster at the perfect and defective TiO2(101) surface into two parts: one is EPd-TiO2 and the other one is EPd-Pd, which represent the contribution of Pd-TiO2 surface interaction and the Pd-Pd interaction, respectively

EPd-TiO2)-[E(Pdn ⁄ TiO2) - E(TiO2) - E(Pdn * )] ⁄ n (4) EPd-Pd)Eclus-EPd-TiO2

(5)

where the E(Pdn*) represent the total energy of adsorbed Pdn cluster, not in the gas phase. The variation of EPd-TiO2 and EPd-Pd with the cluster size was shown in Figure 13. As the Pd cluster gets larger, the EPd-TiO2 decreases, whereas the EPd-Pd increases, indicating the interaction between each Pd adatom and TiO2 surface decreases while the Pd-Pd interaction becomes gradually stronger. In other words, the Pd-TiO2 interaction is the main driving force at the initial stage of Pd nucleation, whereas the Pd-Pd interaction began to control the growth process of Pd cluster as the Pd cluster got larger. The conclusion is consistent with the experimental results reported by Xu et al.,42 even though the system they investigated is the nucleation of Pd on rutile TiO2(101). The size of adsorbed Pd cluster corresponding to the dividing point is four. That is why the adsorbed Pd cluster is 2D structure as the cluster size is less than four and then becomes 3D. It should be noted that the strength of Pd-Pd interaction at the perfect and defective surface are nearly equal when the size of the adsorbed Pd cluster is kept the same, indicating the strength of Pd-Pd interaction would not be affected by the property of the support surface. However, the Pd-TiO2 interaction at defective surface is stronger than that at the perfect surface by ∼0.5 eV under the same cluster size. 4. Conclusions First principles DFT calculations and periodic supercell models were employed to investigate the nucleation and growth of palladium cluster on perfect and defective anatase TiO2(101) surface. Structures and the energetics of Pd monomer, dimer, trimer, tetramer, and pentamer at both perfect and defective anatase TiO2(101) surface sites and oxygen vacancy sites were systematically investigated in the present work. The most active site for single Pd adatom on the perfect TiO2(101) surface is the bridge site formed by two two-coordinated oxygen (2cO) atoms at the step edge with the highest adsorption energy of 2.18 eV. On the defective surface, the effect of oxygen vacancy on Pd nucleation is not as strong as that in the Pt or Au deposition case. At the initial deposition stage, the shift of active sites for Pd growth on the perfect anatase TiO2(101) surface is observed. On both perfect and defective anatase surfaces, the adsorbed Pd3 clusters prefer to form planar triangles and the adsorbed Pd4 and Pd5 clusters tend to 3D structures. The nucleation and

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