O2 Adsorption and Oxidative Activity on Gold-Based Catalysts with

Article pubs.acs.org/JPCC

O2 Adsorption and Oxidative Activity on Gold-Based Catalysts with and without a Ceria Support Bo-Tao Teng,*,†,‡ Jia-Jian Lang,† Xiao-Dong Wen,§ Ce Zhang,† Maohong Fan,*,‡ and H. Gordon Harris‡ †

College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, China Department of Chemical & Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071, United States § Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ‡

ABSTRACT: Understanding the O2 adsorption and oxidative activity on goldbased catalysts is of great significance for gold catalysis. According to the adsorption behaviors of O2 on Au(111)+, 3Au/Au(111)+, Au19+, and Au9/ CeO2(111), the electronic nature of why O2 weakly interacts with free positively charged Au substrate while it strongly interacts with Au cluster supported on ceria is well-explained herein. The ceria support serves as an electronic repository, where it gains and stores electrons from the supported metal cluster and releases them when the metal cluster interacts with molecular O2. The possible oxygen species on gold-based catalysts have been systematically confirmed for the first time. On the Au9/CeO2 modeling catalyst, a peroxide species forms when O2 locates at the hollow site of Au9, while a superoxide forms for O2 at the top site of a Ce atom. It is very interesting to find that Ce3+ ion distributions in Aun/CeO2 catalysts have diverse possibilities. The superoxide close to Au9 has the highest oxidative activity. The interface between the Au cluster and ceria surface is the active site for Aun/CeO2 catalysts. The present work sheds light on understanding the oxidative mechanism of metal/support catalysts, as well as the development of new catalysts with high performance at relatively low temperature. Au3−, Au3, and Au3+ clusters, which firmly supports the above conclusions. Compared with the extensive theoretical studies of O2 adsorption and reaction on Au clusters,25−30 O2 adsorption behaviors on Aun/supports were relatively rarely reported. Boronat and Corma6 calculated the adsorption and dissociation of O2 on the isolated Au cluster with different sizes and Au13/ TiO2. They suggested that O2 activation is sensitive to the morphology of Au clusters and that dissociation occurs at the metal−support interface of Au13/TiO2. Molina and Hammer31 also indicated that the interface between the metal particle and oxide support is the most probable active site by calculating the adsorption of O2 on Aun/MgO. Rodrı ́guez et al.32 suggested that O2 chemically dissociated on the Aun/TiC(001) and that atomic oxygen takes part in CO oxidation. Kim et al.33 systematically investigated the CO reaction behaviors on a model catalyst of Au13 nanoparticles, supported on the stoichiometric and partially reduced CeO2 surface, in which three possible oxidation mechanisms were proposed. Despite of the extensive studies of O2 adsorption and reaction on gold-based catalysts, there are some questions that need further investigation. (1) Theoretical calculations all suggest that O2 weakly adsorbs on the positively charged Au clusters, while most experimental results indicate that the Aunδ+

1. INTRODUCTION In contrast to inert bulk Au, nanoparticles of gold on oxide supports exhibit unique catalytic properties.1−5 Since the high catalytic activity of CO oxidation at low temperature was reported by Haruta et al.,1 extensive applications in CO oxidation, odor decomposition, propylene epoxidation, and so forth have followed. For gold-based catalysts in oxidation reactions, the mechanism to activate molecular O2 is one of the most important key steps.6,7 Experimental and theoretical results all suggest that O2 activation is dependent on the morphology and particle sizes of gold nanoparticles, as well as the properties of supports.8−13 To interpret the different effects of gold nanoparticles and supports, the adsorption and reaction behaviors of O2 on gold clusters, oxide supports, and their interface have been studied in theory. Different from the dissociation adsorption on other metal surfaces,14 a consensus of O2 adsorption on gold surfaces or clusters has been reached that the adsorption and dissociation of molecular O2 on the flat terrace of bulk gold is energetically unfavorable;15−19 the orbital roughness instead of geometric roughness of Au clusters enhances their interaction with O2.11,13,15,20−22 The binding energies of O2 adsorption at differently charged Au clusters decrease as follows: Aun−, Aun, and Aun+.15,22,23 Liu and Hu12 systematically investigated O2 and CO adsorption and reaction on flat surfaces, defects, and small clusters; Wang et al.24 calculated O2 and CO oxidation on © XXXX American Chemical Society

Received: June 7, 2013 Revised: July 29, 2013

A

dx.doi.org/10.1021/jp4056279 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. O2 on the neutral and positively charged Au substrates.

method with a sigma value of 0.1 eV. The ionic cores were represented by the projector augmented wave (PAW) potentials.37 The Kohn−Sham equations were solved selfconsistently, and the convergence criterion for the energy calculation was set to 1.0 × 10−4 eV, while the maximum Hellmann−Feynman force tolerance of structure optimization was set to 0.03 eV/Å, which yielded total energy convergence down to the 1 meV level. In order to explore the difference between O2 adsorption behaviors on the gold-based catalysts with and without a support, a series of neutrally and positively charged Au(111) terrace, 3Au/Au(111) step surface, and Au19 cluster modeling catalysts, as well as a periodic Au9/CeO2(111) catalyst, were employed in the present work. For the Au(111) terrace, a three-layer Au(111) slab with a (3 × (3)1/2) repeated unit was used herein. Three Au atoms adsorbed on the periodic Au(111) terrace to form a step surface, which is denoted as 3Au/ Au(111). The corresponding k point was set as 2 × 3 × 1. Only the bottom atoms of the three-layer slabs were fixed during optimization. A typical pyramidal Au19 cluster in a periodic 15 × 15 × 15 Å3 cube was used to study the O2 adsorption behavior, in which all of the atoms were fully relaxed. For the positively charged Au(111)+ , 3Au/Au(111) +, and Au 19+ modeling catalysts, one electron was subtracted from the total valence electrons of the involved system during the SCF calculation. For the Au cluster supported on a ceria catalyst, a Au9/CeO2(111) modeling catalyst in which a pyramidal Au9 cluster adsorbs on a six-layer CeO2(111) slab with a 3× 2(3)1/2 superlattice was involved in this work.38 The three bottom Ce− O atoms were fixed, and the k point was set as 2 × 2 × 1. To eliminate the interactions between vertical slabs, the vacuum height was set as 12 Å. To correct the on-site Coulombic and exchange interaction of the strongly localized Ce-4f electrons for the partially reduced ceria surface, the DFT+U method was used for the Au9/CeO2(111) catalyst with a value of U = 5 eV.39−41

nanoparticles are the possible active sites for CO oxidation. What is the electronic nature for the experimental and theoretical contradiction? (2) It is well-known that the peroxide and superoxide are the two important oxygen species in oxidative reaction. Which kind of oxygen species is the O2 adsorbed on the Aun/oxide catalysts, superoxide, or peroxide? (3) Furthermore, what are their catalytic roles in the oxidative reaction? What are the differences between oxygen species on Aun/CeO2 and a pure ceria support? In the present work, we first calculated the O2 adsorption behaviors on gold terrace, step surface, and typical clusters without a support. Then, choosing a typical Au cluster supported on a CeO2 catalyst, we systematically investigated the molecular oxygen adsorption on the Au9/CeO2(111) modeling catalyst (Au9 cluster, ceria surface, and the interface between the cluster and substrate) to explore the differences of O2 adsorption on Au nanoparticles with and without a support. Correspondingly, the electronic properties and possible oxygen species of adsorbed O2 on the different positions of Au9/ CeO2(111) were systematically studied. Finally, their catalytic roles in oxidative reaction were explored by reacting with CO as a probe molecule. The present work will shed light on understanding the reaction mechanism of O2 on metal/support catalysts, as well as the development of gold-based catalysts with high performance.

2. MODELS AND METHOD All calculations were performed based on plane wave periodic first-principles density functional theory using the Vienna ab initio simulation package (VASP) code.34 The electron exchange and correlation were treated within the generalized gradient approximation (GGA).35 The Kohn−Sham oneelectron wave functions were expanded in a plane wave basis with a cutoff energy of 400 eV. The Brillouin zone integration was approximated by a sum over special k points chosen using the Monkhorst−Pack (MP)36 grids and Gaussian smearing B



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consistent with the value reported in the literature.15 If the substrate is partially positively charged, the O−O bond length shortens to 1.377 Å, compared with that on the neutral substrate. Meanwhile, the Au−O bond lengths are slightly shorter than those on the neutral one. The corresponding adsorption energy decreases to −0.18 eV. Similar results are also observed when O2 adsorbs on the neutrally and positively charged Au19 clusters. When O2 is located at the hollow site of the Au19 cluster, three Au−O bonds form with a Au−O distance of 2.159−2.349 Å, while the O−O bond is elongated to 1.369 Å with an adsorption energy of −0.58 eV. If O2 adsorbs at the hollow site of the Au19+ cluster, the adsorption energy decreases to −0.29 eV, while the O−O bond length is shortened to 1.306 Å. It is well-known that the singly occupied molecular orbital (SOMO) and the unoccupied one of the O2 triplet ground state are both π2p* orbitals. When O2 interacts with Au atoms, electrons transfer from the substrate to the unoccupied π2p* orbitals. Hence, O2 is negatively charged, while the gold substrate has a positive charge. Different from the terrace, three Au atoms protruding into the vacuum results in the electronic density roughness. Therefore, more electrons can transfer from the 3Au/Au(111) step surface to O2. Correspondingly, the adsorption energy of the O2 surface is higher than that on the terrace. When the substrate is partially positively charged, few electrons can transfer to O2 compared with the neutral one. Therefore, the adsorption energies of O2 on the positively charged substrates are less than those on the neutral ones. This conclusion can be further verified by the charge analysis. To improve the understanding of the interactions between O2 and gold substrates with different charges, the charge distributions of O2 and its neighbor Au atoms were calculated using Bader analysis.45,46 The corresponding charge values of O2 and its neighboring Au atoms are listed in Table 2. As shown in Table 2, the charge values of the top three Au atoms of the gold substrates before O2 adsorption are in the range of −0.09−0.11. After O2 adsorption, there are more electrons of the neutral substrate (3Au/Au(111) and Au19) fed back to O2 than those of the 3Au/Au(111)+ and Au19+. The Bader charges of the neutral 3Au/Au(111) and Au19 substrates increase to 0.73 and 0.61, respectively, which are higher than the Bader charge increases of the corresponding positive substrates (0.61 and 0.35). Therefore, it is very clear that O2 adsorption on the partially positively charged gold models is weaker than those on the neutral ones. However, this conclusion is opposite to the experimental results, which suggest that the highly dispersed gold nanoparticles on a CeO2 support are partially positively charged, and the oxidative activities of the Aunδ+ are much higher than those of metallic Aun.47 What is the difference between the separate Au models with positive charge and the Aunδ+ cluster supported on metal oxide? It is necessary to compare the O2 adsorption behaviors on Au clusters with and without CeO2 support.

Molecular O2 adsorption at the different sites of the goldbased catalysts with and without a ceria support was systematically studied, and the corresponding adsorption energy was defined as Eads = E(O2/substrate) − [E(O2) + E(substrate)]

where E(O2/substrate) is the total energy of the substrate with adsorbed O2; E(O2) and E(substrate) are the energies of free molecular O2 and an isolated substrate, respectively. Therefore, a negative value means exothermic adsorption, while a positive one means endothermic adsorption. The more negative the adsorption energy, the stronger the adsorption. The climbing image nudged elastic band (CINEB) method developed by Jónsson and Mills42 was used to calculate the reaction barriers. The reaction path was discretized with eight intermediate images between the reactant and product, which was connected by elastic springs in order to prevent the images from sliding to the minimum during the optimization.43,44 The corresponding activation energies were calculated according to the energy difference between the initial and transition states. Frequency analysis was carried out to verify no imaginary frequency for the stable configurations and a single imaginary one for the transition state.

3. RESULTS 3.1. O2 Adsorption on the Gold Surface and Cluster. The optimized configurations of O2 adsorbed on the neutrally and positively charged Au(111) terrace, 3Au/Au(111) step surface, as well as Au19 cluster are shown in Figure 1; the corresponding geometrical parameters are listed in Table 1. Table 1. Adsorption Energies and Geometric Parameters of O2 on Au Models models

Eads/eV

dO−O/Å

free O2 Au(111) Au(111)+ 3Au/Au(111) 3Au/Au(111)+ Au19 cluster Au19+ cluster

−0.05 0.10 −0.23 −0.18 −0.58 −0.29

1.235 1.264 1.237 1.404 1.377 1.369 1.306

dAu−O/Å 2.796 3.122 2.103 2.105 2.349 2.515

3.523 3.622 2.245 2.262 2.346 2.502

3.631 3.583 2.232 2.243 2.159 2.216

Consistent with results reported in the literature,6,15,25 a very weak interaction between O2 and the Au(111) terrace is also observed herein. Its adsorption energy is −0.05 eV with a O−O bond length of 1.264 Å, which is elongated slightly compared with that of free molecular oxygen. O2 adsorption on the positively charged Au(111) terrace is energetically unfavorable, with an adsorption energy of 0.10 eV. When O2 adsorbs at the hollow site of the 3Au/Au(111) step surface, the O−O bond is elongated to 1.404 Å, and the Au−O bonds are about 2.103− 2.245 Å. The adsorption energy is −0.23 eV, which is very Table 2. Bader Charge of O2 and Its Neighboring Au Atoms substrate without O2 total 3Au/Au(111) 3Au/Au(111)+ Au19 Au19+

0 1 0 1

−0.06 0.01 −0.03 −0.02

0.05 0.11 −0.09 0.03

substrate with O2 top three Au

total

0.01 0.07 −0.01 0.06

0.73 1.61 0.61 1.35 C

O2 top three Au

0.22 0.21 0.17 0.15

0.18 0.26 0.16 0.18

0.19 0.25 0.12 0.10

−0.34 −0.34 −0.37 −0.13

−0.39 −0.27 −0.24 −0.23



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3.2. O2 Adsorption on Aun Clusters Supported on CeO2(111). For convenient comparison with O2 adsorption on the Au models above, the top two layers of the pyramidal Au19 cluster were put on the stoichiometric CeO2(111) surface to form a Au9/CeO2(111) model. The corresponding configuration, in which the six bottom gold atoms adsorb at the top site of oxygen and the center of the big gold triangle is located at the top site of cerium, proved to be the most stable structure of Au9/CeO2(111)38 and was adopted in the present work. To give a good understanding of the electronic properties for the Au9/CeO2(111) modeling catalyst, its corresponding charge density difference (CDD) and spin density are shown in Figure 2. The CDD of Aun/CeO2 is defined as Δρ = ρAun/substrate − ρAun

Figure 3. O2 adsorbed on Au9/CeO2(111). (a) O2 at the hollow site of Au9; (b) O2 at the top site of Ce next to the edge of Au9; (c) O2 at the top site of Ce next to the corner of Au9; (d) O2 at the top site of Ce far from Au9. (Pink balls are for adsorbed O2).

Figure 2. CDD (a) and spin density (b) of Au9 supported on CeO2(111). The isosurface value is set to 0.01 e/Å3. (The small red ball is for subsurface oxygen atom, the big red one is for the surface oxygen atom, the faint yellow is for the cerium atom, and the gold one is for the gold atom).

Table 3. Adsorption Energies and Geometrical Parameters of O2 on Aun/CeO2(111)

− ρsubstrate, where ρAun/substrate is the charge density of the Aun/ CeO2 system; ρAun and ρsubstrate are the charge densities of the noninteracting Aun cluster and CeO2 surface, respectively. The yellow and the gray−blue parts represent the accumulation and depletion electrons, respectively. It can be seen from Figure 2a that the Au9 cluster loses electrons while the support gains electrons. Therefore, the Au9 cluster is partially positively charged, and Bader charge calculation indicates that its total charge is 0.79. Simultaneously, three Ce4+ ions are reduced to corresponding Ce3+ ions. This conclusion can be further supported by the corresponding spin density of Au9/CeO2(111), which is defined as the charge difference between the spin-up and spin-down electrons. As shown in Figure 2b, three empty 4f orbitals of Ce atoms are filled due to the electron transfer between Au9 and the ceria support, indicating that three Ce3+ ions form in the Au9/ CeO2(111) modeling catalyst. We systematically studied the adsorption behaviors of O2 on the Au clusters, CeO2 surface, and the interface between the Au cluster and support. The optimized configurations are shown in Figure 3. The adsorption energies and geometric parameters are listed in Table 3. When O2 adsorbs at the hollow site of the Au9/CeO2(111) model, the O−O bond is elongated to 1.436 Å, which is much longer than that on the Au19+ cluster. The corresponding adsorption energy is −1.19 eV, which is much higher than those of the 3Au/Au(111)+ and Au19+ clusters. We further investigated the O2 adsorption behaviors at the interface of the Au9 cluster and CeO2(111) surface, as well as at the ceria support far from the Au cluster. When O2 adsorbs at the top site of the cerium atom next to the edge of the Au9 cluster (Figure 3b), the O−O bond is elongated to 1.409 Å, and the corresponding adsorption energy is −0.61 eV. If O2 adsorbs at the top site of the Ce atom next to the corner of the Au9 cluster (Figure 3c), the O−O bond length is 1.322 Å, with an

models

Eads/eV

dO−O/Å

Au9/CeO2-hollow Au9/CeO2-edge Au9/CeO2-corner Au9/CeO2-ceria

−1.19 −0.61 −0.56 −0.40

1.436 1.409 1.322 1.354

dAu−O or dO−Ce/Å 2.250 2.406 2.416 2.295

2.256 2.347 2.486 2.304

2.105

adsorption energy of −0.56 eV. When O2 adsorbs at the top site of the Ce atom far from the Au9 cluster (Figure 3d), the O−O bond is slightly elongated to 1.354 Å. The corresponding adsorption energy is −0.40 eV, which is less than those of the two structures above and much less than that of peroxide. The Ce−O bonds for the three configurations are about 2.295− 2.486 Å. 3.3. CO Oxidative Reaction with Oxygen Species at Different Sites of Au9/CeO2(111). To explore the different catalytic effects of the oxygen species located at different sites of the Au9/CeO2 modeling catalyst, their reactivities were investigated using CO as a probe molecule. The corresponding carbonates, transition states, and CO2 products are shown in Figure 4. When CO reacts with O2 adsorbed at the hollow site of the Au9 cluster, a tridentate carbonate intermediate forms with the three Au−O bond lengths of 2.103−2.257 Å, while a bidentate carbonate generates when CO approaches O2 located at the edge of the Au9 cluster. The two Ce−O bond lengths are 2.413 and 2.611 Å, respectively. A similar bidentate carbonate is obtained by CO reacting with O2 adsorbed at the corner of the Au9 cluster with Ce−O bond lengths of 2.724 and 2.770 Å. If CO reacts with O2 far from the Au9 cluster, a bidentate carbonate also forms with Ce−O bond lengths of 2.227 and 2.297 Å. The correspondingly released energies for CO reaction with the different oxygen species above are −4.11, −3.87, −4.02, and −4.20 eV, respectively. As shown in Figure 4, the reactions in which the four carbonate intermediates desorb to form gas CO2 are all endothermic. The corresponding reaction energy differences D



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To explore the possible reason for the great increase of adsorption energy, we calculated the spin density of the typical configuration, O2 adsorbed at the hollow site of Au9/ CeO2(111), as shown in Figure 5a. It can be learned from

Figure 5. Spin density of O2/Au9/CeO2(111). The isosurface value is set as 0.01 e/Å3.

Figure 5a that two 4f electrons of Au9/CeO2(111) transfer to the π2p* orbitals of the adsorbed O2. Therefore, the corresponding oxygen species are diamagnetic, that is, there is no spin electron on the adsorbed O2. Obviously, the adsorbed O2 at the Au9 cluster supported on CeO2(111) is a peroxide species, O22−. Its stronger interaction with the positively charged Au9 cluster supported on ceria than those on the 3Au/Au(111)+ and Au19+ substrates is attributed to the fact that the electrons of ceria support feedback to O2, which strengths its interaction with O2. However, when O2 adsorbs on the free Au terrace or clusters, this kind of electronic transfer does not occur. This is the dominating difference between the Au cluster with and without a ceria support. Therefore, we can reasonably deduce that ceria serves as an electronic repository, in which it gains and stores electrons of the metal clusters supported on itself while releasing electrons when it interacts with molecular O2. This conclusion might be appropriate for other metal clusters supported on other reducible supports. For example, using resonant photoelectron spectroscopy, Vayssilov et al.48 found that Ce4+ ions were partially reduced to Ce3+ ones due to the strong interactions between Pt nanoparticles between ceria supports. 4.2. Oxygen Species on Ceria with and without a Au Cluster. It has been experimentally reported that peroxide and superoxide species exist on ceria49,50 and ceria−zirconia51,52 supports, and the superoxide has a higher CO oxidative activity than peroxide at low temperature. Using DFT, Zhao et al.53 proved the two oxygen species and compared their oxidative roles in theory. Does superoxide also exist on the catalyst of a Au nanoparticle supported on ceria? Furthermore, what is the difference in CO oxidation between the peroxide and superoxide? It is well-known that O2 adsorption on the stoichiometric CeO2(111) surface is energetically unfavorable.54 Why does relatively strong adsorption of O2 occur when the Aun cluster adsorbs on the CeO2(111) surface?

Figure 4. Potential energy surfaces of different carbonate desorption at the different sites of Au9/CeO2(111). (The purple ball is for the oxygen atom of CO, and the gray one is for the carbon atom).

between different reactants (carbonate) and products (CO2) are 0.49, 0.10, 0.55, and 0.76 eV. For the tridentate carbonate desorption, the C−O bond is elongated, then CO2 desorbs from the Au9 cluster, and the remaining oxygen atom moves to the hollow site of Au9, as seen in Figure 4a. The corresponding activation energy is 1.08 eV. It can be seen from Figure 4b that the remaining oxygen atom combines with the corner Au atom to form a Au−O bond with a length of 2.030 Å, after CO2 desorbs from the bidentate carbonate next to the edge of the Au9 cluster. The energy barrier is 0.43 eV. For the desorption of bidentate carbonate next to the corner of the Au9 cluster to form CO2, the C−O bond is also elongated to 2.144 Å in the transition-state structure. This leaves an isolated oxygen atom hung at the top site of the Ce atom in the product, as seen in Figure 4c. The energy barrier is 0.77 eV, which is higher than that of the former one. As shown in Figure 4d, if the bidentate carbonate intermediate far from the Au9 cluster desorbs to the gas CO2, it must overcome a high energy barrier of 1.03 eV, leaving an atomic oxygen atom at the top site of the Ce atom.

4. DISCUSSION 4.1. O2 Adsorption on the Positively Charged Au Cluster with and without Ceria. According to the calculations above, it is obvious that the adsorption of O2 on the partially positively charged Au9 cluster supported on CeO2(111) is much stronger than that on the free 3Au/ Au(111)+ and Au19+ clusters. What is the dominating reason for this interesting result? What is the role of ceria in O2 adsorption? E



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for the superoxide at the edge and corner of Au9 and 3.85 eV for the superoxide far from Au9. Furthermore, at the partially reduced CeO2(111), the remaining oxygen atom fills into the oxygen vacancy, which leads to the great energy decrease for CO reaction with the superoxide. However, at the Au9/ CeO2(111) modeling catalyst, only the remaining oxygen atom binds to Au and forms a Au−O bond for O2− at the edge of Au9, while the remaining oxygen atom hangs on the ceria surface after CO reacts with O2− at the corner or far from the Au9 cluster. This is the main difference for superoxide at the ceria with and without a Au cluster.

To explain the significant differences of O2 adsorption behaviors on ceria with and without a Au cluster from an electronic point of view, we also calculated the corresponding spin density of the three configurations, as shown in Figure 5b−d. Because three Ce4+ ions are reduced to Ce3+ ions upon Au9 adsorption on a CeO2 support, one 4f electron feeds back to one of the π2p* orbitals of adsorbed O2 when O2 adsorbs at the top site of the Ce atom, as seen in Figure 5b−d. Simultaneously, one of the three Ce3+ ions again is oxidized to Ce4+. Therefore, the adsorbed O2 forms a paramagnetic superoxide species, which is very similar to those on the partially reduced ceria53 and that on the Au3/CeO2(110).55 However, there is no Ce3+ ion on the stoichiometric CeO2(111) surface. Hence, no electron transfer occurs between O2 and the ceria support. Correspondingly, their interactions are much weaker than that between O2 and Au9/CeO2(111). This is the significant difference between the ceria support with and without a Au cluster on it. 4.3. Diverse Distribution of Ce3+ Ions on Aun/CeO2. As shown in Figure 5b−d, we also find a very interesting phenomena, that is, the diverse distributions of the two Ce3+ ions in the Au9/CeO2(111) modeling catalyst. Although one Ce3+ ion is under the Au9 cluster for the three configurations, the other one is located at the position that is apart from it by about 6.764 Å, as seen in Figure 5b, while in Figure 5c and d, the other Ce3+ ion is located at the corner and edge of the periodic slab, respectively. Therefore, the distributions of Ce3+ ions in the ceria-based catalysts might have diverse possibilities and change according to external conditions, or they might coexist in practically complex catalysts. This deduction is very similar to the multiple configurations of the two excess 4f electrons on the partially reduced CeO2(111) reported by Li et al.41 and Ganduglia-Pirovano et al.56 4.4. Oxidative Activity of Oxygen Species on Ceria with and without a Au Cluster. According to the comparison of the energy barriers for CO2 formation, we learn that the decomposition of the tridentate carbonate produced by the reaction of CO and peroxide species has the highest energy barrier, while the bidentate carbonate generated by the reaction of CO and superoxide next to the edge of Au9 has the lowest energy barrier. When the superoxide species is located at the corner of Au9, the energy barrier of the bidentate carbonate decomposition increases to 0.77 eV. Furthermore, if the O2− species is located far from Au9 on the ceria, the corresponding activation energy of carbonate decomposition is up to 1.03 eV, indicating its low CO oxidative activity. Therefore, the above results indicate that the superoxide close to Au9 has the highest oxidative activity, and the interface between the Au cluster and ceria surface is the active site for Au nanoparticle supported on a ceria carrier. A similar conclusion was obtained in experiments by studying the dimethyl ether and ethanol decomposition at the Au/CeO2 interface,57,58 as well as the Au/TiO2 nanoparticle catalyst, in which the dual perimeter sites at the interface of Au and TiO2 are the active sites for H2 and O2 reaction.59 It was reported by Zhao et al.53 that superoxide at the partially reduced CeO2 surface can react with CO to form gas CO2 without an energy barrier. Why can CO not be directly oxidized to CO2 by superoxide on Au9/CeO2? It can be learned from the literature53 that the energy released after CO reaction with superoxide at the partially reduced CeO2(111) surface is about 6.0 eV, which is much higher than that from CO reacting with O2− at the different sites of Au9/CeO2, 4.38 and 4.04 eV

5. CONCLUSION Adsorption of molecular O2 on gold-based catalysts, with and without a ceria support, has been theoretically investigated by DFT methods. The electronic nature of why O2 weakly interacts with a free positively charged Au substrate while strongly interacting with a Au cluster supported on ceria was well-explained in the present work. The reducible ceria support serves as an electronic repository, in which it gains and stores electrons from the supported Au cluster and releases them when the Au cluster interacts with molecular O2. The spin density of O2/Au9/CeO2(111) suggests that O2 adsorbed at the hollow site of Au9 is peroxide, while a superoxide species forms when O2 is adsorbed at the top site of the Ce atom near the edge and corner of Au9, as well as far from Au9. It is very interesting to find that the distributions of Ce3+ ions in the Aun/CeO2 catalysts exhibit diverse possibilities and change according to the external conditions. Using CO as a probe molecule, the catalytic activity of peroxide and superoxide at different positions of Au9/CeO2 catalysts was systematically compared. Our calculations indicate that superoxide close to the Au cluster has the highest oxidative activity, and the interface between the Au cluster and ceria surface is the active site for the Au nanoparticle supported on a ceria support.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.-T.T.); [email protected] (M.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 21373187, 20903081), the Natural Foundation of Zhejiang Province, China (Grant No.Y407163), and the Opening Foundation of the State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University (Grant No.201308). X.-D.W. gratefully acknowledges a Seaborg Institute Fellowship (the LDRD program at LANL). The Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under Contract DE-AC5206NA25396.

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REFERENCES

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