LETTER pubs.acs.org/JPCL
Interaction of Gold Clusters with a Hydroxylated Surface De-en Jiang,*,† Steven H. Overbury,†,‡ and Sheng Dai†,‡,§ †
Chemical Sciences Division and ‡Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States
bS Supporting Information ABSTRACT: We explore the interaction between gold nanoclusters and a fully hydroxylated surface, Mg(OH)2’s basal plane, by using a density functional theory-enabled local basin-hopping technique for global-minimum search. We find strong interaction of gold nanoclusters with the surface hydroxyls via a short bond between edge Au atoms and O atoms of the OH groups. We expect that this strong interaction is ubiquitous on hydroxylated support surfaces and helps the gold nanoclusters against sintering, thereby contributing to their CO-oxidation activity at low temperatures.
SECTION: Surfaces, Interfaces, Catalysis
C
atalysis by supported gold nanoparticles has attracted many researchers’ interest.16 Although particle size has been a central issue in affecting gold nanoparticles’ catalytic activity, other factors also play important roles. For example, in CO oxidation, the most studied reaction by gold catalysis, support type, preparation method, and water vapor in the reaction stream all have been found to affect gold nanoparticles’ activity and stability.711 Despite being much less studied than oxides, hydroxides provide a unique support for gold nanoparticles. Haruta and co-workers found that gold nanoparticles supported on Mg(OH)2 are very active in low-temperature CO oxidation.12,13 Recently, Takei et al. found that La(OH)3-supported gold nanoparticles also catalyze CO oxidation at very low temperatures.14 By X-ray absorption spectroscopy, Haruta and co-workers attributed Mg(OH)2-supported gold’s activity to subnanometer icosahedral gold clusters on the surface.12 The high catalytic activity of gold nanoparticles on hydroxides is expected to be closely related to the hydroxyl groups dominating the surface of the hydroxide support. Recent characterization of small gold clusters deposited on hydroxylated MgO surface by CO temperature-programmed desorption and infrared spectroscopy indicates higher stability of gold clusters against sintering after hydroxylation of the oxide surface.15 It was also found that SiO2 and TiO2 treated at high pH can stabilize gold nanoparticles at small sizes.16,17 The interesting role of hydroxyls on the support surface prompted us to ask how the gold cluster interacts with the hydroxyl group. To address this question, we chose the Mg(OH)2 support for its simple structure: Mg(OH)2, found in nature as brucite, has a layered structure of the Mg2þ layer sandwiched by two OH layers. In the basal plane, the surface is terminated with all hydroxyl groups. This surface provides an ideal platform to examine the r 2011 American Chemical Society
interaction of gold clusters with the hydroxyl group. The next question we ask is what the most stable interfacial structure is when a gold cluster is deposited on the Mg(OH)2 surface. This question is important in that a local minimum from a geometry optimization of an initial guess may not truly reflect the interfacial structure. To address this question, we employed a density functional theory (DFT)-enabled local basin-hopping18 technique to search for the global minimum of the gold cluster on a support surface. In this method, during each Monte Carlo (MC) step the atoms of the cluster are randomly moved while those of the support are kept fixed. After the trial move, then all the atoms are geometry-optimized by DFT. This local basin-hopping technique was adapted from our previous work on a general version of DFT-based basin hopping for clusters.19 See the Computational Methods section for details. We chose Au10 as a subnanometer gold cluster to examine its interaction with Mg(OH)2’s surface. It has been shown by DFT computation that the Au10 cluster alone can catalyze CO oxidation below room temperature.20 Au10 was also used in a recent DFT study as a model catalyst on TiO2.21 Gas-phase Au10 has a flat geometry (Figure 1);22 this global minimum was confirmed by our DFT-based basin hopping. We placed this cluster parallel to the Mg(OH)2 surface in the initial state (Figure 2a); then after ∼400 MC moves, we found a putative global minimum (Figure 2b). In this structure, the cluster is still flat in geometry, but tilted with respect to the surface. More interestingly, we found two short AuOH bonds at the interface, which can be viewed more clearly in Figure 3. In this state, Au10 has an adsorption energy of 1.62 eV Received: March 28, 2011 Accepted: April 28, 2011 Published: May 03, 2011 1211
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Figure 1. The most stable structure of Au10 in the gas phase.
Figure 3. A closer view of the interfacial bonding between Au10 and Mg(OH)2. Au, green; O, red; H, black; Mg, blue.
Table 1. Geometry of the Interfacial AuOH Bonds at the Au10/Mg(OH)2 Interfacea
a
Figure 2. Initial (a) and final (b) structures of Au10 on Mg(OH)2 after DFT-based local basin hopping. Upper panel, side view; lower panel, top view. Au, green; O, red; H, black; Mg, blue.
(or roughly 0.8 eV per interfacial AuOH bond), indicating a rather strong interaction with the Mg(OH)2 surface. Both AuO bonds are about 2.16 Å with a AuOH angle of ∼100° (Table 1); the OH bonds are tilted toward a nearby O atom. Compared with the gas phase, the planar structure of Au10 is slightly rearranged on Mg(OH)2. If we remove Mg(OH)2, the Au10 configuration as in the final state (Figure 3) is about 0.24 eV higher in energy than the gas phase global minimum (Figure 1). So this small energetic penalty is readily overcome by forming the interfacial AuOH bonds. The short AuO distances of the interfacial AuOH bonds imply covalent bonding. We plot a slice of the charge density across one AuO bond (Figure 4). The isocontour of the charge density clearly shows some covalent bonding between Au and O. Recently, an N-heterocyclic carbene gold hydroxide complex was synthesized and characterized,23 where the AuOH distance was measured at 2.078 Å, only slightly shorter than what we found at the Au10/Mg(OH)2 interface. So the AuOH bond at the interface has some similarity to that in a complex, despite the fact that at the interface the OH group also interacts with the subsurface Mg2þ ions. To find out how the electron redistributes after formation the interfacial AuOH bonds, we plot the isosurfaces of charge density difference in Figure 5. One can clearly see that the two interfacial Au atoms lose electrons. More interestingly, the two O atoms directly bound to Au also lose electrons, despite some accumulation nearer to the O nuclei. Quantitative examination of the charge states of the atoms was done by Bader’s atom-inmolecule analysis. Table 2 shows the charge states before and after interfacial formation. As a result of the interfacial bonding, the two interfacial Au atoms become slightly positively charged, while the two O atoms become less negatively charged. Corner, low-coordinate Au atoms, not directly bound to the surface,
bond
distance (Å)
bond
angle (deg)
Au1O1
2.15
Au1O1H1
103.4
Au2O2
2.18
Au2O2H2
99.8
See Figure 3 for the atom labels.
become more negatively charged. Overall, the Au10 cluster in fact gains 0.45 e from the Mg(OH)2 surface, mainly out of the two O atoms of OH bound to Au. The negative charge on the Au10 cluster could help adsorb CO and catalyze its oxidation. Tsukuda and co-workers prepared small gold clusters in a polymer matrix and found that the clusters have a small negative charge, which helps its catalytic activity.24 Pacchioni et al. showed that an excess electron on Au4 can enhance its catalytic activity on a partially hydroxylated MgO surface.25 The results above indicate two roles of surface hydroxyl groups: stabilizing the gold cluster and providing electrons to the gold cluster. We expect that these two roles would be quite general on hydroxylated surfaces; that is, when hydroxyls are present on oxide supports such as TiO2, SiO2, and Al2O3, the gold cluster is expected to form interfacial AuOH bonds, which pull electrons from the hydroxyls to the gold cluster. For large, hemispherical gold nanoparticles of several nanometers, we expect that the AuOH bonds will be formed mainly at the perimeter of the interface. Our preliminary results with DFTbased local basin-hopping also showed formation of the AuOH bonding with the silanol group on SiO2. For nanoporous silicasupported gold nanoparticles,26 this result indicates that a concave-shaped internal surface with many silanols could better stabilize a gold nanoparticle through greater contact area and therefore formation of more AuOH bonds. Recently, Nasluzov et al. studied gold monomer, dimer, and trimer on a partially hydroxylated R-Al2O3(0001) surface with DFT and also found bonding between Au and OH. Wang and Hammer showed an indirect interaction between OH and Au7 on TiO2(110) from their DFT study,27 while Tong et al. showed with DFT that a single Au atom forms a stable HOAu complex on the TiO2 surface.28 We expect that a direct AuOH bond should also exist on the TiO2 surface for larger Au clusters. Global minimum is difficult to find and hard to confirm from first principles for medium to large systems (roughly, more than 100 atoms). So how can we be sure that the global minimum found for Au10/Mg(OH)2 is the true one? One the one hand, we cannot be absolutely sure; on the other hand, there are several 1212
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Table 2. Charge States of Atoms, as Labeled in Figure 3, before and after Formation of the Au10/Mg(OH)2 Interfacea atom
a
before
after
atom
before
after
Au1
0.07
þ0.08
Au2
0
þ0.15
Au3 Au5
þ0.06 þ0.04
0.08 0.03
Au4 Au6
0.09 þ0.04
0.20 þ0.01
Au7
þ0.13
þ0.04
Au8
0
0.13
Au9
0.06
0.14
Au10
0.06
0.16
O1
1.97
1.77
O2
1.97
1.76
See Figure 3 for the atom labels.
Figure 4. An isoplane of the charge density across one AuO interfacial bond. Red color is at zero value; purple the highest. The atom labels are the same as in Figure 3.
Figure 6. Au13 on Mg(OH)2: the initial icosahedral geometry (a) transforms to a disordered structure with two interfacial AuOH bonds (b), after 500 steps of DFT-based local basin-hopping (B.H.); the energy is lowered by ∼3 eV. Au, green; O, red; H, black; Mg, blue.
Figure 5. Isosurface of charge density difference after formation of the Au10/Mg(OH)2 interface. Pink, charge depletion; blue, charge accumulation. Isovalue is at 0.02 e/Å3. Atom color: Au, green; O, red; H, black; Mg, blue.
ways to make sure that this minimum is close to the global minimum. First, we ran 500 more MC steps after finding the minimum in Figure 3 and did not find any structure of lower energy. Second, we started with a three-dimensional initial configuration for Au10, different from the flat gas-phase global minimum in Figure 1, and after basin hopping we obtained a local minimum with a more two-dimensional (2D)-like geometry for Au10 (see Figure S1 in the Supporting Information), indicating that Au10 on Mg(OH)2 also tends to be flat as in the gas phase. This local minimum has one interfacial AuOH bond, and its energy is about 0.9 eV higher than the putative global minimum in Figure 3. Third, we manually created a third interfacial AuOH bond from the structure in Figure 3, hoping that more interfacial AuOH bonds will lead to a more stable structure. But we found that the resultant structure is in fact 0.4 eV higher in energy, partly due to the strain from a bent geometry (see Figure S2 in the Supporting Information); as we showed in Table 1 and Figure 3, the interfacial AuOH bonds follow certain angles. The final interfacial structure or the optimal number of the AuOH bonds is a balanced result between the formation of such bonds and minimizing the perturbation to the most stable form of the gold cluster. The Au10 cluster is probably too small to afford three AuOH
interfacial bonds. With increasing cluster size, we expect that more such bonds can be formed. In their experimental study of CO oxidation on Mg(OH)2supported gold, Haruta and co-workers found that subnanometer gold clusters are particularly active.12,13 By extended X-ray absorption fine structure spectroscopy and Debye function analysis, they concluded that the icosahedral geometry is more active than the face-centered cubic (fcc) one and used Au13 as a model for a subnanometer cluster that has the icosahedral geometry.12 We placed the icosahedral Au13 on Mg(OH)2 and then performed DFT-based local basin-hopping search for the global minimum (Figure 6). Interestingly, two interfacial AuOH bonds are also formed with ∼3 eV lowering in energy as the Au13 cluster transforms to a disordered and loose structure. We note that an isolated Au13 prefers a planar structure.22 It is likely that the structure in Figure 6b is not the true global minimum yet, but the interfacial bonding should be a reasonable representation of the interface. Hence, we conclude that the AuOH bonding should be a general feature of the interface between nanometer-sized gold clusters and Mg(OH)2. The interfacial structures we found for Au10/Mg(OH)2 and Au13/Mg(OH)2 show a common feature that the cluster is anchored by two AuOH bonds and standing up on the surface, instead of lying down or wetting the surface. The standing-up structure has an advantage of exposing more low-coordinate Au atoms and making them more accessible. This advantage may make the gold cluster more catalytically active, as it has been found that low-coordinate Au atoms play an important role in catalysis.20 Recently, oxidation of alcohol by gold clusters has attracted interest.29,30 The OH group on an alcohol should share some similarity with that on a support surface, so the interaction we see here between gold and OH of Mg(OH)2 indicates that alcohol molecules may chemisorb on a gold cluster surface via their 1213
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The Journal of Physical Chemistry Letters OH group as the initial step of their oxidation by a gold cluster. This point warrants further exploration. The role of hydroxyl groups in CO oxidation over oxidesupported Au has been extensively discussed previously.8,11,31,32 Kung and co-workers proposed a mechanism involving OH bound to Au atoms at the perimeter of the Au/oxide interface,8,11 similar to what Bond and Thompson had proposed earlier.33 In this mechanism, the OH group, bound to a Au atom but not directly interacting with the support, participates in the catalytic cycle by reacting with CO to form hydroxycarbonyl which subsequently reacts with oxygen to form bicarbonate; the surface-bound bicarbonate is then decomposed to OH (bound to Au) and CO2 (released to the gas phase). In the Au10/Mg(OH)2 interface, we find that the OH group forming the AuOH bond is still part of the support (directly interacting with the subsurface Mg2þ ions). We think that the major role of this interfacial bonding is to stabilize gold clusters at small sizes by anchoring them. Recent temperature-programmed desorption study of CO from subnanometer gold clusters deposited on hydroxylated MgO surfaces showed enhanced stability of gold clusters,15 supporting our conclusion. The extra partial charge on Au10 gained from the OH groups of Mg(OH)2 may help the cluster’s catalytic activity, but it is unclear whether the interfacial OH groups of Au10/Mg(OH)2 can participate in CO oxidation directly by the mechanism proposed by Kung and co-workers. Future exploration of CO adsorption, O2 activation, and CO2 formation on Au10/Mg(OH)2 is warranted, especially in the sense that the interfacial Au atoms adsorb and activate O2 while the corner and edge Au atoms with partial negative charges adsorb CO. In summary, we found interfacial AuOH bonding at the interface between subnanometer gold clusters (Au10 and Au13) and Mg(OH)2’s basal plane by employing a DFT-based local basin-hopping technique for global minimum search. The AuOH bond is short (∼2.16 Å) and has covalent character; it enhances the stability of the gold cluster on Mg(OH)2 by about 0.8 eV per AuOH bond. Atomic charge analysis indicates that the interfacial Au atoms are slightly positively charged, but the whole gold cluster has a partial negative charge mainly from depletion of electron from the interfacial OH groups. We expect that the AuOH interfacial bonding is ubiquitous on hydroxide and hydroxylated oxide surfaces. The interfacial structure found here will facilitate further understanding of the low-temperature CO oxidation activity of gold clusters supported on Mg(OH)2 and other hydroxides as well as on hydroxylated oxide surfaces.
’ COMPUTATIONAL METHODS The Vienna Ab Initio Simulation Package (VASP)34,35 was employed for DFT geometry optimization. VASP employs periodic boundary conditions and planewave bases. A kinetic energy cutoff of 400 eV was used for the planewaves, which was found sufficient to converge the total energy. The PerdewBurkeErzonhoff (PBE) form36 of the generalized-gradient approximation (GGA) was chosen for electron exchange and correlation,30 and the electroncore interaction was described by the projector-augmented wave (PAW) method within the frozen-core approximation.37 Mg(OH)2 or brucite has the hexagonal CdI2 structure (space group: P3m1). DFT-optimized lattice parameters (a = 3.142 Å; c = 4.766 Å) for the primitive cell of the bulk Mg(OH)2 structure agree well with the experiment (a = 3.145 Å; c = 4.769 Å38). A slab model was used for Au clusters on Mg(OH)2’s basal plane or the
LETTER
(001) surface, as shown in Figure 2. The lateral rectangular cell of the surface has the dimensions of 15.71 Å 16.33 Å and the sandwich structure of the Mg(OH)2 slab is separated from its images in the perpendicular direction by a 10-Å vacuum layer. DFTbased global-minimum search19 was performed by using a Python script to interface the basin-hopping algorithm21 with the VASP code.28,29 For each MC trial, Cartesian coordinates of the Au atoms were randomly moved while those of Mg(OH)2 were kept fixed. During geometry optimization, all the atoms were allowed to move. A low-precision setup was used for fast geometry optimization (PREC = L, force tolerance at 0.25 eV/Å, and Γ-point only for k-point sampling). Basin-hopping search was stopped when 500 more MC steps did not deliver a lowerenergy configuration; roughly 1000 MC steps in total were performed. The putative global minimum was finely optimized with a kinetic energy cutoff of 400 eV, a k-mesh of 2 2 1, and a force tolerance of 0.025 eV/Å. Atomic charges were analyzed by Bader’s atom-in-molecule method, as implemented by Henkelman et al.39
’ ASSOCIATED CONTENT
bS
Supporting Information. Two local minima of Au10/Mg(OH)2 and coordinates for structures in Figures 3 and 6b. This material is available free of charge via the Internet at http://pubs.acs. org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. ’ REFERENCES (1) Haruta, M. Gold as a Novel Catalyst in the 21st Century: Preparation, Working Mechanism and Applications. Gold Bull. 2004, 37, 27–36. (2) Christmann, K.; Schwede, S.; Schubert, S.; Kudernatsch, W. Model Studies on CO Oxidation Catalyst Systems: Titania and Gold Nanoparticles. ChemPhysChem 2010, 11, 1344–1363. (3) Garcia, P.; Malacria, M.; Aubert, C.; Gandon, V.; Fensterbank, L. Gold-Catalyzed Cross-Couplings: New Opportunities for CC Bond Formation. ChemCatChem 2010, 2, 493–497. (4) McEwan, L.; Julius, M.; Roberts, S.; Fletcher, J. C. Q. A Review of the Use of Gold Catalysts in Selective Hydrogenation Reactions. Gold Bull. 2010, 43, 298–306. (5) Ma, Z.; Dai, S. Development of Novel Supported Gold Catalysts: A Materials Perspective. Nano Res. 2011, 4, 3–32. (6) Rodriguez, J. A. Gold-Based Catalysts for the WaterGas Shift Reaction: Active Sites and Reaction Mechanism. Catal. Today 2011, 160, 3–10. (7) Haruta, M. Size- and Support-Dependency in the Catalysis of Gold. Catal. Today 1997, 36, 153–166. (8) Costello, C. K.; Yang, J. H.; Law, H. Y.; Wang, Y.; Lin, J. N.; Marks, L. D.; Kung, M. C.; Kung, H. H. On the Potential Role of 1214
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(29) Turner, M.; Golovko, V. B.; Vaughan, O. P. H.; Abdulkin, P.; Berenguer-Murcia, A.; Tikhov, M. S.; Johnson, B. F. G.; Lambert, R. M. Selective Oxidation with Dioxygen by Gold Nanoparticle Catalysts Derived from 55-atom Clusters. Nature 2008, 454, 981–983. (30) Zhu, Y.; Qian, H.; Zhu, M.; Jin, R. Thiolate-Protected Aun Nanoclusters as Catalysts for Selective Oxidation and Hydrogenation Processes. Adv. Mater. 2010, 22, 1915–1920. (31) Gao, F.; Wood, T. E.; Goodman, D. W. The Effects of Water on CO Oxidation over TiO2 Supported Au Catalysts. Catal. Lett. 2010, 134, 9–12. (32) Veith, G. M.; Lupini, A. R.; Pennycook, S. J.; Dudney, N. J. Influence of Support Hydroxides on the Catalytic Activity of Oxidized Gold Clusters. ChemCatChem 2010, 2, 281–286. (33) Bond, G. C.; Thompson, D. T. Gold-Catalysed Oxidation of Carbon Monoxide. Gold Bull. 2000, 33, 41–51. (34) Kresse, G.; Furthm€uller, J. Efficiency of Ab Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15–50. (35) Kresse, G.; Furthm€uller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169–11186. (36) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868. (37) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758–1775. (38) Duffy, T. S.; Shu, J. F.; Mao, H. K.; Hemley, R. J. Single-Crystal X-ray Diffraction of Brucite to 14 GPa. Phys. Chem. Miner. 1995, 22, 277–281. (39) Henkelman, G.; Arnaldsson, A.; Jonsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comput. Mater. Sci. 2006, 36, 354–360.
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