Article pubs.acs.org/JPCC
Cooperative H2 Activation at Ag Cluster/θ-Al2O3(110) Dual Perimeter Sites: A Density Functional Theory Study Pussana Hirunsit,*,† Ken-ichi Shimizu,*,‡,§ Ryoichi Fukuda,§,⊥ Supawadee Namuangruk,† Yoshitada Morikawa,§,∥ and Masahiro Ehara*,§,⊥ †
National Nanotechnology Center (NANOTEC), Thailand Science Park, Patumthani 12120, Thailand Catalysis Research Center, Hokkaido University, Kita 21 Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan § Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto 615-8520, Japan ∥ Department of Precision Science and Technology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita Osaka 565-0871, Japan ⊥ Institute for Molecular Science, Nishigo-naka 38, Myodai-ji, Okazaki, Aichi 444-8585, Japan ‡
S Supporting Information *
ABSTRACT: H2 dissociation by Ag clusters supported on the θ-Al2O3(110) surface has been investigated using density functional theory calculations. The crucial role of the dual perimeter site of Ag cluster and the surface oxygen (O) site of the alumina support is demonstrated with three theoretical models: anchored cluster, isolated cluster, and anchored cluster on hydroxylated alumina. The heterolytic cleavage of H2 at the silver−alumina interface, yielding Ag−Hδ− and O−Hδ+, is thermodynamically and kinetically preferred compared with H2 cleavage at two Ag atomic sites on top of the Al2O3-supported Ag cluster and the homolytic cleavage of H2 on the isolated Ag cluster. The hydroxylation at the O site of the alumina reduces the H2 dissociation activity, which indicates that the interfacial bare O site is indispensible. It is concluded that the interfacial cooperative mechanism between the Ag cluster and Lewis acid−base pair site (bare Al−O site) is essentially relevant for the H2 activation over Ag-loaded Al2O3 catalysts.
1. INTRODUCTION The catalytic activity of supported metal nanoparticles depends on the metal−support interaction and metal size. Size- and support-specific catalysis of gold1,2 is the most well-known example. Supported silver nanoparticles3 have also been the focus of research because of their characteristic catalytic activity. A number of fundamental studies discussed the reasons why TiO2-supported gold with particle size below a few nanometers show high activity, and there are two hypotheses: (1) the small nanoparticles have unique electronic and structural properties4 and (2) the decrease in the size increases the number of the metal−support perimeter sites which are catalytically important sites.5−13 Recent experimental results have shown that the water−gas shift reaction and the H2 or CO oxidation over Au/ TiO2 are driven by the H2 dissociation at the dual perimeter of the Au−TiO2 interface,5−9 which suggests that electronic and structural properties of small gold clusters, such as the quantum size effect and low coordination (edge and step sites), are secondarily important factors. On the basis of the H2−D2 exchange reaction over Au cluster-loaded TiO2(110) surface and TiO2 cluster-loaded Au(111) surface,5,10 Fujitani and coworkers conclusively demonstrated that the H2 dissociation is not driven by the quantum size effect of Au or edge and step sites of Au but by Au−O−Ti sites at the perimeter sites. It was also © 2014 American Chemical Society
shown that the dual perimeter site of Au−TiO2 seemingly assists the H2 dissociation by lowering the activation energy to the range of 0.13−0.25 eV.7 Similarly, the enhanced CO oxidation over Au/TiO2 is influenced by the active dual perimeter site where CO is favored to adsorb at the Au edge and O2 adsorption is stable on TiO2, which leads to a lower activation energy barrier.14,15 Yang et al.16 reported density functional theory (DFT) study of heterolytic and homolytic H2 dissociation by Au/TiO2(110) and concluded that heterolytic dissociation of H2 to Ti−O−H and Au−H at the perimeter sites is favored. Compared to the abundance of the research on the gold catalysts, fewer attempts have been made for the catalysts of the supported silver nanoparticles. However, the supported silver nanoparticle catalyst recently has been the focus of research because of its high catalytic activity in many types of reactions.17−36 Although the single-crystal silver surface is inert toward H2 dissociation both thermodynamically and kinetically,37 the supported silver nanoparticles can selectively catalyze hydrogenation reactions. For examples, the silver nanoparticles on TiO2 or SiO2 support perform highly selective catalytic Received: January 4, 2014 Revised: March 23, 2014 Published: March 24, 2014 7996
dx.doi.org/10.1021/jp5000792 | J. Phys. Chem. C 2014, 118, 7996−8006
The Journal of Physical Chemistry C
Article
4 Monkhorst−Pack k-point mesh provide the cell parameters where a = 11.919 Å, b = 2.939 Å, c = 5.666 Å, and β = 104.03° (The experimental values are a = 11.813 Å, b = 2.906 Å, c = 5.625 Å, and β = 104.10°).42 The calculated parameters are in excellent agreement with the previous report43 and the experimental values; the deviation is on the order of ∼1%. These parameters are also sufficient for the total energy to converge within 0.001 eV. The θ-Al2O3(110) surface is represented by a slab model. The (110) surface of the face-centered cubic (fcc) oxygen sublattice corresponds to (010) of the surface index of a monoclinic unit cell structure of θ-Al2O3.44 The previous studies showed that the preferential exposed surface of θ-Al2O3 is the (110) surface of the fcc oxygen sublattice.45,46 We employed the charge-neutral 3 × 1 supercell with a three θ-Al2O3 cell thickness slab containing 180 atoms and a vacuum region of ∼15 Å in the z-direction. It was previously found that the θ-Al2O3 charge-neutral slab was achieved with a three to four θ-Al2O3 cell thickness.44 The θAl2O3(110) surface is shown in Figure 2.
hydrogenation of crotonaldehyde to the desired unsaturated alcohol products:17 a range of chloronitrobenzenes to their corresponding chloroanilines.22 Shimizu et al. reported a series of studies on the Al2O3-supported silver nanocluster catalyst which enables various types of reactions.28−33 It should be noted that the silver nanoparticles supported on θ-Al2O3 selectively catalyze the hydrogenation of the nitro group in the presence of CC, CO, or CN groups.33 This feature is of interest because the conventional platinum group metal-based catalysts are not selective in such a manner. The systematic study on the influence of the metal particle size and support oxides showed that the intrinsic activity increases with the decrease in the silver particle size and acid−base bifunctional supports such as Al2O3 give activity higher than acidic or basic supports.33 In the hydrogenation of nitrobenzene, H2 dissociation was found to be the rate-determining step by the kinetic isotope effect experimentally.33 Upon H2 activation, the conversion of -NO2 into -NH2 readily proceeds. The H2 dissociation is also the rate-limiting step for other substrates in the supported Ag cluster catalysts; therefore, it is essentially relevant in the hydrogenation on Ag cluster catalysis in general. The H2 dissociation is also an important step for hydrogenation of other substrates by supported Ag catalysts.20 It was suggested that the cooperative effects of Ag nanocluster and alumina support are responsible for the H2 dissociation. However, this effect in Ag/Al2O3 systems has not been fully understood or analyzed in detail theoretically. Recently, Grönbeck and Klacar38 studied the H2 dissociation over Ag/Al2O3 systems and elucidated that the activation barrier over Ag-ions and partially oxidized silver is lower than that on bare alumina, metallic Ag, and Ag/Al2O3. However, they examined the H2 dissociation at alumina sites and not at the dual perimeter site. Thus, the further examination of the H2 activation at the various sites is still required to elucidate the hydrogenation mechanism over Ag/Al2O3 systems. This work aims to theoretically elucidate the role of the interface perimeter site between the Ag cluster and alumina support for H2 dissociation by applying DFT calculations. For the alumina support, we examined the θ-Al2O3(110) surface because this surface was utilized in experimental work.33 The examined factors which potentially influence H2 activation at the dual perimeter site include the low coordination and the intrinsic reactivity of Ag atoms, the role of the alumina support, and the hydroxylation of the alumina support.
Figure 2. Top (left panel) and side (right panel) view of θ-Al2O3(110) supercell. The solid line outlines the cell boundary with ∼15 Å vacuum in z-direction.
Spin-polarized DFT calculations were performed with the generalized gradient approximation PW91 functional41 implemented with the projector augmented wave function (PAW)47,48 method for representing the nonvalence core electrons. Gaussian broadening49 was employed with a smearing width of 0.02 eV. Given the large size of the slab unit cell, the calculations were carried out at the Γ k-point of the Brillouin zone. The plane-wave cutoff energy was optimized at 400 eV. The results were checked for convergence with respect to energy cutoff. The convergence criteria for electronic self-consistent iteration were set to 1.5 × 10−7 eV, and the ionic relaxation loop was limited for all forces smaller than 0.03 eV/Å for free atoms. The adsorbates of molecular H2, two atomic H, the silver cluster, and all of the Al and O atoms were fully optimized except for the Al and O atoms on the bottom atomic layer of Al2O3 support. The resulting geometries were analyzed and used for adsorption energy calculations. The nudged elastic band calculations to locate the transition state and the Bader charge analysis were performed using VASP-VTST.50−52 All transition states were verified by the number of imaginary frequencies being equal to 1.
2. COMPUTATIONAL DETAILS The fully periodic plane-wave DFT calculations as implemented in the Vienna ab initio simulation program (VASP)39,40 were employed. In the bulk, θ-Al2O3 has a monoclinic structure (Figure 1). A unit cell consists of a total of 20 atoms with 8 Al atoms and 12 O atoms having four formula units per unit cell. The calculated bulk structures using GGA-PW9141 and a 6 × 6 ×
3. RESULTS AND DISCUSSION 3.1. Ag13/θ-Al2O3(110) Model Structure. Ag13 is selected as a model of the silver nanoparticle. The isomer of Ag13 can contain one (planar), two, or three atomic layers. Because the activity of silver atomic sites that are interacting and not
Figure 1. Monoclinic unit cell of θ-Al2O3 bulk structure. Purple atoms represent Al and red atoms represent O. 7997
dx.doi.org/10.1021/jp5000792 | J. Phys. Chem. C 2014, 118, 7996−8006
The Journal of Physical Chemistry C
Article
interacting with the alumina support is the focus of this work, the Ag13 cluster with two atomic layers has been chosen as the representative model of Ag nanoparticles. The two atomic layers of Ag13 were directly cut from bulk Ag and optimized as shown in Figure 3a. Taylor, Smalley, and co-workers53 showed that in
cluster (Figure 3b) energy is used in calculating the adsorption energy of Ag13 on the θ-Al2O3(110) surface. The strong interaction between the Ag13 and Al2O3 support stabilizes the Ag cluster, which is indicated by a strong Ag13 adsorption energy of −3.31 eV. The Ag13 adsorption energy Eads (Ag13) is calculated by Eads(Ag13) = E(Ag13/θ‐Al 2O3)opt − E(bare θ‐Al 2O3)opt distorted − E(Ag13 )opt
where E(Agdistorted )opt is the energy of the Ag13 structure in Figure 13 3b. The density of states (DOS) for the Ag13/θ-Al2O3(110) surface is shown in Figure 5. The d orbitals of Ag13 hybridize with
Figure 3. (a) Optimized isolated Ag13 structure for which the initial structure is cut from Ag bulk. (b) Optimized distorted isolated Ag13 for which the initial configuration is taken from optimized Ag13 on θ-Al2O3 (110).
small metal clusters (
