Structure Sensitivity in CO Oxidation by a Single Au Atom Supported

Article pubs.acs.org/JPCC

Structure Sensitivity in CO Oxidation by a Single Au Atom Supported on Ceria Weiyu Song and Emiel J. M. Hensen* Laboratory of Inorganic Materials Chemistry, Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ABSTRACT: The mechanism of CO oxidation by a CeO2(110)-supported gold atom has been investigated by DFT calculations. A novel stable surface structure has been identified in which one surface O atom of ceria migrates toward the isolated Au atom, resulting in a surface Au−O species that can react with CO. After CO2 desorption, the oxidation state of Au changes from positive to negative. In contrast to earlier explored CO oxidation mechanisms for single Au atoms on the CeO2(111) surface, O2 can adsorb in the vacancy created in the CeO2(110) surface adjacent to Au. In essence, the difference in O2 adsorption originates from the geometries of the two ceria terminations, pointing to strong structure sensitivity in the CO oxidation reaction. Dissociation of adsorbed O2 heals the O vacancy and leaves an additional O atom on the surface. At this stage the Au atom is positively charged so that CO can adsorb followed by facile formation of CO2 with the bridging O atom, closing the catalytic cycle. This mechanism successfully explains the role of surface O of ceria in CO oxidation by highly dispersed Au catalysts. involving replacement of a Ce ion in the same surface by Au.6 The main conclusion was that supported and substitutional atomic Au can promote CO oxidation by O vacancy formation. These vacancies can also be healed by adsorption of molecular oxygen for the substitution case, completing the catalytic reaction cycle. However, the stability of a gold ion substituting for Ce remains questionable.8 For the supported case,6 the O vacancy formed during the CO oxidation reaction will be filled by the Au adatom. As a result, the Au atom will become negatively charged, which prevents CO adsorption and, consequently, results in catalyst deactivation. Camellone and Fabris suggested that the Au−Au cohesive energy in supported Au clusters might prevent the diffusion process of Au that gives rise to deactivation.6 This possibility was recently explored in detail by Kim et al.7 It was found that CO oxidation by a Au13 cluster supported on CeO2(111) is unfavorable because of the high desorption energy of CO2 formed by reaction of CO adsorbed on Au with a ceria lattice O atom. This relates to the high Ce−O bond strength in the CeO2(111) surface. Based on this result, the Henkelman group developed a strategy to lower the O vacancy formation energy by doping the ceria surface, which led to increased catalytic activity.9 Although this presents a relevant example of catalyst design, it leaves the mechanism of CO oxidation on nonpromoted Au/CeO2 unresolved. It is also known that the type of surface termination of ceria has a profound influence on its catalytic performance.10,11 Ceria

1. INTRODUCTION Gold supported on metal oxides support is an active catalyst for numerous reactions, the most thoroughly investigated being CO oxidation.1 Although it has been established that factors such as the particle size, the support, the synthesis, and pretreatment method significantly influence the reactivity of gold, the nature of the active site and the reaction mechanism for CO oxidation remain topics of intense scientific debate.2 Among the various oxides, CeO2 has been recognized as one of the best materials for supporting gold due to its high oxygen storage and release capacity. The nature of the active sites in Au/CeO2 and the mechanism of CO oxidation have been thoroughly investigated by experimental2−5 and theoretical6,7 methods. It is worthwhile to discuss several experimental observations, which confirm the involvement of O atoms of the ceria support in the CO oxidation mechanism. As usually observed for metals dispersed on ceria, CO2 formation also occurs when the catalyst is exposed to CO gas in the absence of O2.3,4 By use of in-situ Raman spectroscopy, Herman’s group monitored the oxygen vacancy concentration on ceria during CO oxidation in ceria supported gold.4 The intensity of the first-order CeO2 F2g peak near 460 cm−1 was observed to decrease during CO oxidation. This spectral change is explained by the lattice expansion resulting from replacement of Ce4+ by Ce3+ during oxygen vacancy formation. This elegant experiment provides direct evidence for the participation of surface oxygen atoms of the support in CO oxidation. Camellone and Fabris were the first to investigate this mechanism by theoretical modeling. Two models were considered: one involving a Au ion adsorbed on the CeO2(111) surface and the other one © 2013 American Chemical Society

Received: January 29, 2013 Revised: March 19, 2013 Published: March 25, 2013 7721

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nanorods, which mainly expose (110) facets, were shown to be more active in CO oxidation than ceria particles exposing predominantly (111) facets.11 This is usually explained by the lower Ce−O bond energy in the CeO2(110) surface. Thus, we asked ourselves the question whether the lower O vacancy formation energy of (110) facets could be at the basis of a novel proposal for a mechanism of CO oxidation which satisfactorily explains the role of lattice O atoms in Au/CeO2.

2. COMPUTATIONAL DETAILS Density functional theory (DFT) with the PBE (Perdew− Burke−Ernzerhof) functional12 as implemented in the Vienna Ab Initio Simulation Package (VASP)13−15 was employed. A Hubbard U term was added to the PBE functional (DFT+U) employing the rotationally invariant formalism by Dudarev et al.,16 in which only the difference (Ueff = U − J) between the Coulomb U and exchange J parameters enters. The spinpolarized calculations were performed. The projector augmented wave method (PAW)17−19 was used to describe the interaction between the ions and the electrons with the frozencore approximation.18 The valence electrons were treated using a plane-wave basis set with a kinetic energy cutoff of 400 eV. For Ce, a value of Ueff = 4.5 eV was used, which was calculated self-consistently by Fabris et al.20 using the linear response approach of Cococcioni and de Gironcoli.21 This value is within the 3.0−5.5 eV range reported to provide localization of the electrons left upon oxygen removal from CeO2.22 A 2 × 3 cell was used for CeO2(110). Because of the large size of the surface cells (∼11 Å × 11 Å for each surface cell), only 1 × 1 × 1 kpoint mesh was used for the Brillouin zone integration. The use of higher 2 × 2 × 1 k-point mesh did lead to negligible energy differences. The bulk equilibrium lattice constant (5.49 Å) previously calculated by PBE+U (Ueff = 4.5 eV) was used.23 The Au adatom and the four top atomic layers of the ceria slab were allowed to relax, while the bottom layer was kept fixed to their bulk position. Atoms were relaxed until forces were smaller than 0.05 eV Å−1. The location and energy of transition states were calculated with the climbing-image nudged elastic band method.24

Figure 1. Structures of Au adsorbed on the CeO2(110) surface: (a) a symmetrical structure which has been reported earlier as the stable one and (b) a more stable structure involving migration of an O atom toward Au. Color scheme: red sphere (O), white sphere (Ce4+), yellow sphere (Au), black sphere (C), and blue sphere (Ce3+). The atoms in the subsurface layer are shown as small spheres.

structure is a more likely candidate for catalytic CO oxidationon the CeO2(110) surface. The calculated potential energy diagram and the corresponding structures are shown in Figures 2 and 3, respectively. It should be noted that CO

Figure 2. Reaction energy diagram.

3. RESULTS AND DISCUSSION The adsorption of Au on CeO2(110) has been extensively studied recently.25,26 In line with these studies, we identified a stable adsorption site for Au on the bridge site between two surface O atoms (structure a, Figure 1). The calculated structural parameters are very close to those reported earlier. The adsorption energy of the Au atom is 2.11 eV (2.07 eV;25 2.19 eV26); the bond distance to the O anions is 1.98 Å (1.98 Å;25 1.97 Å26) with the Ce3+ ion being located below the Au adatom. We found that CO adsorption to the Au atom is not possible. To explore the possibility of CO oxidation, we performed a nudged elastic band calculation for the reaction between CO close to the surface with one of the O atoms of the ceria surface connected to Au. Surprisingly, we identified another more stable structure for Au/CeO2(110) in this way (structure b, Figure 1). In this structure one of the surface O atoms initially bonded to Au has moved to a bridging position between Ce and Au (dCe−O = 2.14 Å; dAu−O = 1.98 Å, structure b, Figure 1). This less symmetric configuration is more stable by 41 kJ/mol than structure a. The location of the Ce3+ ion remains the same. The barrier for this migration process is negligible. Accordingly, we anticipate that the latter more stable

adsorption to the Au atom in structure b is also not possible. Accordingly, we computed the barrier for the reaction between gas-phase CO and an O atom of the ceria lattice and found a value of 45 kJ/mol (TS1, Figure 2). The reaction is exothermic by 28 kJ/mol. For the transition state of this Eley−Rideal-type mechanism, the C−O bond distance is 1.78 Å, while that of O to the support atoms are slightly elongated (dO−Ce from 2.13 to 2.31 Å; dO−Au from 1.97 to 2.01 Å). This result provides direct theoretical support for the speculation made in several experimental studies that the reactive oxygen species is located at the interface between gold and nanostructured CeO2.2−5,27 Earlier, Vayssilov et al. showed how oxygen spills over from a ceria support to a cluster of the more reactive metal Pt.10 Two possible paths exist to desorb the resulting CO2 molecule. The first one involves desorption of CO2, which is spontaneous and releases 72 kJ/mol energy. The energy gain is largely related to the transformation of the initially bend CO2 molecule into a linear one. The second process involves the migration of CO2 over the surface to state v (Figure 3). In this configuration, both O atoms are connected to a surface Ce cation (dCe−O = 2.58 and 2.45 Å). This migration step is also barrierless and releases 191 kJ/mol. It takes two consecutive 7722

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Figure 3. Structures of intermediate and transition states for CO oxidation on Au/CeO2(110) (only the top two layers of the ceria model are shown). The calculated Bader charges for the selected structures are also specified. Color scheme: red sphere (O), white sphere (Ce4+), yellow sphere (Au), black sphere (C), and blue sphere (Ce3+). The atoms in the subsurface layer are shown as small spheres.

barriers were identified. After CO2 desorption, one O vacancy is present generated on the ceria support. Before proceeding, it is useful to discuss the issue of CO adsorption on Au. For a single Au atom supported on CeO2(111), it was found that CO can adsorb very strongly (Eads = −239 kJ/mol).6 However, for the intermediate Au/ CeO2−x state in which the Au atom is located on the vacancy site, CO did not adsorb anymore. It has been shown that the strong differences in CO adsorption energies are related to the oxidation state of gold.6,28 The more electrons on Au, the weaker CO adsorption becomes. To understand the influence of the oxidation state of Au, we performed a Bader charge analysis29 on some selected structures along the reaction pathway (states i, iv, vii, ix, and xiii, Figure 3). The Bader chargers of Au in these different states are +0.23 e (i), −0.65 e (iv), +0.25 e (vii), +0.63 e (ix), and −0.14 e (xiii). A study of CO adsorption on these states shows that only state ix with a

Table 1. Calculated Bader Charges for Selected States within the Reaction Patha

a

species

Bader charge (e)

Au/CeO2 (i) Au/CeO2−x (iv) O2*Au/CeO2−x (vii) O*Au/CeO2 (ix) Au/CeO2 (xiii)

+0.23 −0.65 +0.25 +0.63 −0.14

The number of the states is in line with that of Figure 3.

steps for CO2 to desorb: in the first step, an intermediate structure (state vi, Figure 3) is formed with the Ce−O bonds elongated to 2.98 and 3.73 Å, essentially representing weakly adsorbed CO2. The second step is the desorption of CO2 into the gas phase. The energy costs for these two steps are 91 and 28 kJ/mol, respectively. Also in these cases, no significant 7723

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Figure 4. (a) Electron density difference upon adsorption of an Au atom on CeO2−x(110) (yellow and blue colors represent increasing and decreasing electron densities, respectively). (b) Total DOS and atom-resolved projected DOS analysis (reference energy is the Fermi level marked by the vertical dashed line).

strongly positive charge of +0.63 e is able to absorb CO. None of the other states will adsorb CO. The next step in the reaction mechanism involves O2 adsorption at the interface between Au and the vacancy site. The adsorption energy is 194 kJ/mol. The oxidation state of the Au atom changes from −0.65 to +0.25 e, pointing to the donation of electrons from Au to adsorbed O2 (O2*). This charge transfer is consistent with the elongation of the O−O bond. The charge on Au in this state is close to that of state i (Figure 3). Accordingly, no adsorption mode of CO was found for O2*-Au/CeO2−x(110). The barrier for dissociation of adsorbed O2 is 88 kJ/mol, and the reaction energy is exothermic by 31 kJ/mol. One of the O atoms fills the earlier created vacancy, while the other one is finally located on the bridge site between a surface Ce atom and the Au adatom. Importantly, the Bader charge of Au in the resulting state ix is +0.63 e, which is much more positive than in all the other states. Indeed, CO will adsorb on this positively charged Au atom with an adsorption energy of 65 kJ/mol. The bond distance between Au and C is 1.90 Å. The adsorbed CO molecule then reacts with the O atom on the bridge site between Au and Ce with an energy barrier of 28 kJ/mol. This oxidation process is strongly exothermic (163 kJ/mol) (Figure 2). In the transition state (TS3, Figure 3), the C−O bond distance is 2.05 Å. Finally, it takes two steps for CO2 to desorb. First, the Au−C bond is elongated from 2.05 to 3.53 Å. The C−O−C angle changes from 133° to 180°, the final C−O bond distance becoming 1.18 Å. This process which decreases the interaction of CO2 with Au but results in a linear CO molecule costs 55 kJ/mol and does not have a significant activation barrier. Finally, it takes 21 kJ/mol to desorb CO2 in to gas phase. Note that these steps are very similar to what has been described above as the second route for desorption of CO2. The result of CO2 desorption is the formation of a metastable Au adsorption structure (state xiii, Figure 3) with two Au−O bonds of 2.18 and 2.94 Å. No Ce3+ was observed, consistent with the Bader charge of −0.14 e on Au. This structure will reconstruct to the more stable adsorption structure (state i, Figure 3). The reaction cycle is then closed.

The crucial question to be answered is why this mechanism does not work for Au/CeO2(111).6 The key step in our proposed CO oxidation mechanism is the adsorption of O2 on Au/CeO2−x(110), which essentially prevents the deactivation that occurs in the case of Au/CeO2−x(111). Therefore, we compared the (electronic) structures of Au adsorbed close to the vacancy sites of CeO2(110) and CeO2(111). On the CeO2(110) surface, the Au atom slightly migrates toward the vacancy site. As discussed above, the oxidation state of Au is negative (−0.65 e) as determined by Bader charge analysis. The binding of the Au atom on the O vacancy site results in a strong rearrangement of the charges at the Au−CeO2 interface. Detailed analysis for this state (Figure 4a) shows that charge transfer occurs from the reduced ceria surface to the Au atom. The result is a partially reduced ceria surface with one Ce3+ ion. The transferred electron is localized in the Au 6s orbital (Figure 4b). This results in a negatively charged Au atom and formation of a directional Au−Ce covalent bond (Figure 4a). The electronic state of Au of Au/CeO2−x(111) is similar to that of Au/CeO2−x(110): the Bader charge of Au is −0.59 e, close to the value reported in a previous study (−0.60 e).30 Indeed, CO can also not adsorb on Au in Au/CeO2−x(111).5 This is in sharp contrast with the strong adsorption of O2 (194 kJ/mol) at the interface site in Au/CeO2−x(110). The elongation of the O−O bond from 1.21 Å in gas phase to 1.46 Å points to its strong activation. The two O atoms are coordinated to Ce (dO−Ce = 2.24 Å) and Au (dO−Au = 2.01 Å). On the basis of our new findings and in the light of those obtained for Au/ CeO2(111),6 we can now conclude that the geometry of the ceria surface has a strong influence on its ability to be involved in O2 adsorption at the interface between Au and Ce. The CeO2−x(110) surface is more open than the CeO2−x(111) surface. In the former the surface Ce and O atoms lie in the same plane, whereas in the latter the Ce atoms are in the second layer (Figure 5a,b). As a result, O2 can adsorb at the interface between Au and surface vacancy site by coordination to a Ce surface atom in CeO2−x(110). The charge density difference analysis shows that there is strong accumulation of electrons between O2* to Au and Ce3+ (Figure 5c), indicative of formation of a strong bond. This bonding is not possible on 7724

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Supercomputing facilities were funded by The Netherlands Organization for Scientific Research.

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(1) (a) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Novel Gold Catalysts for the Oxidation of Carbon Monoxide at Temperature far below 0 °C. Chem. Lett. 1987, 2, 405. (b) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. Low-Temperature Oxidation of CO over Gold Supported on TiO2, α-Fe2O3, and Co3O4. J. Catal. 1989, 115, 301. (2) Flytzani-Stephanopoulous, M.; Gates, B. C. Atomically Dispersed Supported Metal Catalysts. Annu. Rev. Chem. Biomol. Eng. 2012, 3, 545−574. (3) Guzman, J.; Carrettin, S.; Corma, A. Spectroscopy Evidence for the Supply of Reactive Oxygen during CO oxidation Catalyzed by Gold supported on Nanocrystalline CeO2. J. Am. Chem. Soc. 2005, 127, 3286−3287. (4) Lee, Y.; He, G.; Akey, A. J.; Si, R.; Flytzani-Stephanopoulous, M.; Herman, I. P. Raman Analysis of Mode Softening in Nanoparticle CeO2−δ and Au-CeO2−δ during CO Oxidation. J. Am. Chem. Soc. 2011, 133, 12952−12955. (5) Yoshida, H.; Kuwauchi, Y.; Jinschek, J. R.; Sun, K.; Tanaka, S.; Kohyama, M.; Shimada, S.; Haruta, M.; Takeda, S. Visualizing Gas Molecules Interacting with Supported Nanoparticulate Catalysts at Reaction Conditions. Science 2012, 335, 317−319. (6) Camellone, M. F.; Fabris, S. Reaction Mechanism for CO Oxidation on Au/CeO2 Catalysts: Activity of Substational Au3+/Au+ Cations and Deactivation of Supported Au+ Adatoms. J. Am. Chem. Soc. 2009, 131, 10473−10483. (7) Kim, H. Y.; Lee, H. M.; Henkelman, G. CO Oxidation Mechanism on CeO2-Supported Au Nanocluster. J. Am. Chem. Soc. 2012, 134, 1560−1570. (8) (a) Zhang, C. J.; Michaelides, A.; King, D. A.; Jenkins, S. J. Structure of Gold Atoms on Stoichiometric and Defective Ceria Surfaces. J. Chem. Phys. 2008, 129, 194708. (b) Zhang, C. J.; Michaelides, A.; King, D. A.; Jenkins, S. J. Anchoring Sites for Initial Au Nucleation on CeO2{111}: O Vacancy versus Ce Vacancy. J. Phys. Chem. C 2009, 113, 6411−6417. (9) Kim, H. Y.; Henkelman, G. CO Oxidation at the Interface between Doped-CeO2 and Supported Au Nanoparticles. J. Phys. Chem. Lett. 2012, 3, 2194−2199. (10) Vayssilov, G. N.; Lykhach, Y.; Migani, A.; Staudt, T.; Petrova, G. P.; Tsud, N.; Skála, T.; Bruix, A.; Illas, F.; Prince, K. C.; et al. Support Nanostructure Boosts Oxygen Transfer to Catalytically Active Platinum Nanoparticles. Nat. Mater. 2011, 10, 310−315. (11) (a) Wu, Z.; Li, M.; Overbury, S. H. On the Structure Dependence of CO Oxidation over CeO2 Nanocrystals with WellDefined Surface Planes. J. Catal. 2011, 285, 61−73. (b) Si, R.; FlytzaniStephanopoulous, M. Shape and Crystal-Plane Effects of Nanoscale Ceria on the Activity of Au-CeO2 Catalysts for the Water−Gas Shift Reaction. Angew. Chem., Int. Ed. 2006, 47, 2884−2887. (c) Chen, F.; Liu, D.; Zhang, J.; Hu, P.; Gong, X. Q.; Lu, G. A DFT+U Study of the Lattice Oxygen Reactivity toward Direct CO Oxidation on the CeO2(111) and (110) Surfaces. Phys. Chem. Phys. Chem. 2012, 14, 16573−16580. (12) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (13) Kresse, G.; Furthmuller, J. Efficiency of ab-initio Total Energy Calculation for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (14) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for ab initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (15) http://cms.mpi.univie.ac.at/vasp. (16) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Electron-Energy-Loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+U Study. Phys. Rev. B 1998, 57, 1505− 1509. (17) Blöchl, P. E. Projector Augmented Wave-Method. Phys. Rev. B 1994, 50, 17953−17979.

Figure 5. Structure of Au adsorbed on ceria surfaces containing one O vacancy for (a) CeO2−x(110) and (b) CeO2−x(111) (only the top two layers of the ceria model are shown; see caption of Figure 3 for color scheme); (c) electron density difference after and before adsorption of O2 on Au/CeO2−x(110) (yellow and blue colors represent increasing and decreasing electron densities, respectively).

Au/CeO2−x(111), essentially because of the repulsive force between O2 and the surface O in the top layer. The result is that O2 adsorption is inhibited on Au/CeO2−x(111).

4. CONCLUSIONS In conclusion, we propose a new reaction mechanism by which a single Au adatom supported on the CeO2(110) surface promotes the oxidation of CO involving O atoms of the ceria surface. An important step is the migration of the O atom toward the Au atom, resulting in a very facile reaction with gaseous CO. In this way, CO oxidation takes advantage of the oxygen atoms, which can be removed from the ceria surface. Different from the Au/CeO2(111) system, the Au atom at the vacancy site of CeO2(110) can absorb and activate O2. This difference between the two ceria surfaces has a geometric explanation. The more open CeO2(110) surface allows for the coordination of O2 between Au and the vacancy site. This configuration is not possible for CeO2(111) because the Ce cations are located in the second layer. For the Au/ CeO2−x(110), the catalytic cycle is closed by CO adsorption and reaction with one of the O atoms following dissociation of O2. The proposed mechanism is the first to provide a theoretical explanation for the catalytic activity of Au/CeO2 and specifically the involvement of ceria O atoms.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel +31-40-2475178, e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledged the financial support from the Advanced Dutch Energy Materials Innovation Lab (ADEM). 7725

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