Distinctions between Supported Au and Pt Catalysts for CO Oxidation

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Distinctions between Supported Au and Pt catalysts for CO oxidation: Insights from DFT Study qiuxia cai, Xinde Wang, and Jian-guo Wang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 26 Sep 2013 Downloaded from http://pubs.acs.org on October 1, 2013

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The Journal of Physical Chemistry

Distinctions between Supported Au and Pt catalysts for CO oxidation: Insights from DFT Study Qiuxia Cai, Xinde Wang, Jian-guo Wang* College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou, 310014, P.R.China

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +86-571-88871037. Tel: +86-571-88871037.

ACS Paragon Plus Environment

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Abstract Distinctions between supported Au and Pt catalysts on TiO2(110) for CO oxidation have been investigated by means of density functional theory calculations in this study. Our study shows that the following factors determine the obvious differences between two kinds of catalysts for CO oxidation: (1) the adsorption strength of Au11 is much weaker than Pt11 on TiO2(110), but which are both strongly dependent on the surface properties of TiO2. The addition of Pt increases the interaction between alloyed cluster and TiO2 support. (2) O2 can only adsorb on the interfacial site between Au and TiO2(110), while O2 can adsorb on both the interfacial and metal sites of supported Pt nanoparticles. (3) CO is directly activated by the adsorbed molecular oxygen on the interfacial site of Au11/TiO2(110)_OH. While on Pt11/TiO2(110)_OH, the main reaction pathway is the dissociated oxygen reacts with CO. Once Pt ensemble is formed on Au clusters (such as Au8Pt3/TiO2(110)_OH), both of the reaction mechanisms work. Keywords: Interface; Surface; Atomic Oxygen; Molecular Oxygen; Bimetallic


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1. Introduction Supported gold nanoparticles over metal oxide have stimulated a great number of experimental and theoretical studies since the discovery of their superior catalytic CO oxidation activity by Haruta.1-11 Compared with Pt catalyst, supported Au catalyst not only shows extremely higher CO oxidation catalytic activity at low temperature, but also the CO oxidation activity of Au catalyst strongly depends on the size, shape of gold nanoparticles,12-14 the type of support, 12,15-16 the moisture17 and reactant gas.18, 19 Among these factors, the size, interfacial structure between gold nanoparticles and support are the most important ones for supported Au catalysts. However, CO oxidation activity over supported Pt catalysts shows no strong size or structure dependence.20-22 The different properties of CO oxidation on supported Au and Pt catalysts are partly determined by the different adsorption characteristics of reactant gas on them, including the adsorption coverage, site and strength. On Pt catalyst, the steady-state CO coverage is near saturation under reaction condition.23 While on Au catalyst, the maximum value of CO coverage is about 0.2, which is significantly below saturation.23 Especially, the sintering and morphology of Au particles are influenced by the reactant gas. The in situ STM studied by Goodman et al.24 showed that the presence of reactant gases, especially oxidants, often accelerates the sintering of supported metal catalysts, which indicated a reactant-induced mechanism of the sintering of supported metal catalysts. The Au-Pt bimetallic system has received increasing attention25-28 since it has unusual properties compared to pure Au and Pt. Recent studies about supported AuPt clusters over metal oxide showed better performance for CO oxidation. The STM studied by Chen et al.29 showed that the sintering of Au-containing clusters on TiO2(110) is inhibited by the


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presence of Pt in Au-Pt bimetallic clusters. The study of Au-Pt nanoparticles supported over Ta2O5/Ta showed the catalyst is more reactive than pure Pt nanoparticles and more stable than pure Au nanoparticles on Ta2O5/Ta.30 Several different kinds of reaction mechanisms3, 31-40 have been put forward to explain the unique CO oxidation activity on supported gold catalysts, which include quantum size effects of two-layer Au islands,31 negative charging of the Au clusters,3,

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abundance of low Au

coordination sites,33,34 and the presence of Au-support periphery sites.35-40 However, to our knowledge, very few systematic and comparative theoretical studies of CO oxidation on the supported Au and Pt clusters have been reported, especially on Au-Pt alloyed clusters over metal oxide support. 41 In this study, we investigate (1) the adhesion properties of Au11, Pt11, and Au11-xPtx(x=1-3) clusters on TiO2(110) surfaces under different reaction environment (oxidative, reductive). (2) O2 adsorption and dissociation on the interfacial and pure metal sites of Au11, Pt11, and Au8Pt3 clusters over TiO2(110). (3) CO oxidation on Au11, Pt11, and Au8Pt3 clusters over TiO2(110). Therefore, our study provides molecular understanding of the distinctions between supported Au and Pt catalysts for CO oxidation.

2. Computational Details The first-principles Density Functional Theory (DFT) calculations were performed using the DACAPO package with a plane-wave basis set (Ecut =25 Ry) and ultrasoft pseudopotentials. The generalized gradient approximation (GGA) with the revised Perdew-Burke-Ernzerhof


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(RPBE)42 functional was used to describe the exchange-correlation effects. Transition states were located by the Nudged Elastic Band method (NEB).43 In this study, the TiO2(110) surface was modelled using periodic slabs of two trilayer. A (5×2) TiO2(110) surface unit cell, a 1x2 k-point sampling set were used. Reduced slabs were realized by H adsorption on protruding Obr sites. Likewise, oxidized slabs were realized by O adsorption on 5fold Ti sites in the surface troughs (in addition to H adsorption on the Obr for alkaline slab). The study was limited to metal cluster containing 11 atoms. These cluster sizes were chosen because they allow for modeling of adsorbed clusters with shapes that appear hemispherical as sketched in Fig. 1.13 The clusters were composed of pure Pt, Au or Au11-xPtx(x=1-3). The orientation and registry of the clusters with respect to the support were varied and a local ionic relaxation was performed in the search for stable binding sites on the support. We note that this approach is adopted to lead to models of supported hemispherical clusters. A more complete search for cluster-support configurations would possibly lead to the identification of even more stable systems, but that is not the aim of the present work. The adhesion potential energy of the clusters on the TiO2(110) surface was calculated according to Eadh = Etot (Metal + support) – Etot (Metal) – Etot (support) with Etot (Metal + support), Etot (Metal) and Etot (support) being the total energy of the combined system, the most stable gas phase metal cluster, and the TiO2(110) surface in a certain state, respectively.

3. Results and Discussions


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Figure 1. (a) The adhesion potential energy of Au11, Pt11 and Au11-xPtx(x=1-3); (b) geometric structures of supported Au11, Pt11 on various TiO2(110) surfaces (golden, Au; blue, Pt; grey, Ti; white, H; pink and red, O.). We start our investigation by considering the adhesion of bare Pt and Au clusters to TiO2(110) substrate. The configurations found by this procedure are displayed in Fig. 1. They are seen to be rather similar. In all cases, the metal clusters bind over the Ti-trough forming bonds to the bridging oxygen atoms and to the on-top oxygen in the trough. The adhesion potential energies of Pt11 and Au11 on TiO2(110) are shown in Fig. 1a. It is seen that the adhesion potential energies of Pt11 and Au11 are strongly dependent on the surface properties of TiO2(110). The stronger alkaline/oxidized of TiO2(110), the stronger adhesion strength, which has been confirmed by different metal nanoparticles on various metal oxide supports.44-48 In addition, on the same TiO2(110) model, the adhesion of Pt11 is much stronger than Au11 about 1.50 eV, for example, Au11 cannot, while Pt11 weakly binds to the hydrated (1.07 and -0.23 eV) and stoichiometric (0.92 and -0.53 eV) TiO2(110). With the introduction of hydroxyl, the adhesion energies of both Au11 and Pt11 increase dramatically to -1.39 and -2.67 eV on the TiO2(110)_OH surface, respectively. Of course, on the purely oxidized TiO2(110), the adhesion of Au11 and Pt11 are very strong with -3.28 and -4.35 eV, respectively. From the above analysis, Pt and Au show different


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adhesion properties on TiO2(110) support, in which Pt is bound to TiO2 much stronger than Au. In other words, suitable surface modification/treatment of support is more vital for supported Au than Pt clusters or nanoparticles. In the following section, TiO2(110)_OH is chosen as TiO2 substrate due to its suitable adhesion properties for Au and Pt clusters and its easily experimental availability.

Figure 2. The geometric structures and adhesion energies of (a) Au10Pt1 and (b) Au8Pt3 clusters on TiO2(110)_OH surface. From the above analysis, it is seen that the adhesion of Au, Pt on TiO2 shows different properties, it is very interesting to investigate the modification of the adhesion of Au on TiO2 by the addition of Pt. Therefore, the optimized structures of Au10Pt1 on TiO2(110)_OH are shown in


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Fig. 2a. For the most stable configuration of Au10Pt1 on TiO2(110)_OH, one Pt is located at the center of Au clusters, in which the adhesion potential energy is -1.86eV. This structure is much more stable than other structures, for example, one Pt is directly bonding with terminated, bridging oxygen and the second layer Pt are -1.41, -1.38 and -1.39 eV, respectively. When Pt is situated at the apex and interfacial sites, the structure are even less stable about -1.08 and -1.00 eV, respectively. Based on the most stable Au10Pt1/TiO2(110)_OH, various structures of Au9Pt2 and Au8Pt3 supported on TiO2(110)_OH have been considered and optimized, in which the optimized geometries of Au8Pt3/TiO2(110)_OH are shown in Fig. 2b. The adhesion energies of the most stable Au9Pt2/TiO2(110)_OH and Au8Pt3/TiO2(110)_OH are -2.32 and -2.46 eV, respectively, which increase a lot compared with pure Au11 cluster on TiO2(110)_OH. For supported Au10Pt1, the adhesion energy of the most stable structure is much more stable about 0.40 eV than other structures. However, for supported Au8Pt3, several structures have very similar adhesion properties, in which the energy differences between them are smaller than 0.10 eV. The major structure characters of these stable structures are the second and/or third Pt being directly bound to terminated or bridging oxygen and the triangle structures being formed. When one or two Pt atoms are located at the second or third layer, or linear structure is formed, the adhesion strength is weaker than those structures mentioned above. Especially, the adhesion energy of Au8Pt3 in which one Pt being located on the apex is much weaker about 0.57 eV than that of the most stable Au8Pt3. It is seen that the favorable substituted site of Au by Pt is, firstly, the core of Au clusters. Secondly, the first layer which can directly bond with the support.


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Figure 3. Oxygen adsorption on different sites of supported (a) Au11 and (b) Pt11 on various TiO2(110) surfaces . O2 adsorption is a necessary step for most oxidation reactions. Therefore, we further investigate O2 adsorption on the interface and pure metal sites of Au11 and Pt11 clusters supported on TiO2(110) with different terminal groups. For Au11/TiO2(110), only the interfacial sites are identified as the stable adsorption sites, while the pure Au sites, even the low coordinated apex Au atoms, do not bind to molecular O2 by using the present calculations methods (Fig. 3a). The adsorption energies of O2 are also influenced by the TiO2 support. The stronger alkaline/oxidized of TiO2, the weaker adsorption strength of O2, which is contrast with the adhesion properties of metal clusters. The adsorption energies of O2 on the interfacial sites of Au11/TiO2_H, TiO2, TiO2_OH and TiO2_O are -1.31, -1.26, -1.04 and -0.20 eV, respectively. The strong repulsion between O2 and terminated oxygen on TiO2(110)_O leads to very weak adsorption. For Pt11/TiO2(110), it is found that O2 not only adsorbs on the interfacial sites but also on the pure Pt


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sites. The adsorption strength of O2 on the interfacial sites of Pt11/TiO2(110) are also dependent on the surface properties of TiO2, but it is weaker than Au11/TiO2(110). The adsorption energies of O2 on the interfacial site of Pt11/TiO2_H, TiO2, TiO2_OH and TiO2_O are -1.68, -1.36, -1.50 and -1.11 eV, respectively. However, the adsorption energies of O2 on pure Pt sites are not strongly influenced by the support. The adsorption strength on the interfacial site between Pt and TiO2 is slightly stronger than that on the pure Pt site except on TiO2_O surface, which is caused by the strong oxygen-oxygen repulsion. For the pure Pt sites, the adsorption energies of O2 on the bridge sites of the first and second layer are much stronger than that of the second and third layer. In summary, although O2 adsorption on Au11/TiO2(110) and Pt11/TiO2(110) shows different properties, for example, different effects of the support and different adsorption properties on the pure Au and Pt metal sites, for both supported Au and Pt on TiO2(110), O2 adsorption on the interfacial sites are much stronger than those on pure metal sites, and O2 cannot adsorb on pure Au sites. It is anticipated that the interfacial sites between metal (such as Au, Pt) and metal oxide (such as TiO2, Al2O3 and so on) are the dominate reactions sites for most oxidation reactions, which is contrast with carbon supported noble metal clusters.49


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Figure 4. Oxygen adsorption on different sites of (a) Au10Pt1 and (b) Au8Pt3/TiO2 (110)_OH surfaces. As shown in Fig. 4a, on the most stable Au10Pt1/TiO2(110)_OH, molecular oxygen can be adsorbed only at the interfacial site with adsorption energy -0.61 eV, which is similar to Au11 cluster. Again, O2 cannot adsorb on the pure Au sites on the most stable Au10Pt1/TiO2(110)_OH. Even when one Pt is exposed on the surface, located at the terminal and bridging site, O2 adsorption is endothermic with 0.93 and 0.58 eV. O2 adsorption geometries and energies on both the interfacial and pure metal sites of Au8Pt3/TiO2(110)_OH are shown in Fig. 4b. O2 adsorption on two kinds of interfacial sites of Au8Pt3/TiO2(110)_OH are investigated. Firstly, O2 adsorption on the Pt interfacial site is -0.98 eV, which is much weaker than that of Pt11/TiO2(110)_OH and Au11/TiO2(110)_OH. Secondly, the adsorption energy of O2 on the Au interfacial sites of Au8Pt3/TiO2(110)_OH ranges from -0.46 to 0.07 eV depending on the structures of Au8Pt3. O2 does not adsorb on the pure Au sites of Au8Pt3/TiO2(110)_OH again, while on surface Pt dimers


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of Au8Pt3/TiO2(110)_OH, the adsorption energies of the most stable and second stable structures are -0.84 and -0.65 eV, respectively. On the surface Pt monomers of Au8Pt3/TiO2(110)_OH, the adsorption energies are -0.46, -0.35 and -0.28 eV, respectively. While on Au-Pt site, the adsorption energy is only -0.04 eV. After the detailed investigation of O2 adsorption on the interfacial sites of the most stable Au11/, Au8Pt3/ and Pt11/TiO2(110)_OH and pure Pt sites of Au8Pt3/ and Pt11/TiO2(110)_OH, we will investigate O2 dissociation on these different adsorption sites. The reaction barriers and transition states are shown in Fig. 5. On the interfacial sites between Au and TiO2(110) of Au11/TiO2(110)_OH and Au8Pt3/TiO2(110)_OH, the reaction barrier of O2 dissociation on these sites are 1.53 and 1.31 eV, which indicates the dissociation of molecular O2 into atomic oxygen is very difficult on these interfacial sites. However, O2 dissociation on the interfacial site of Pt11/TiO2(110)_OH is very facile, which only overcomes about 0.52 eV. The energy profile diagram and transition states for the dissociation of O2 on the pure Pt sites of Pt11/TiO2(110)_OH and Au8Pt3/TiO2(110)_OH are also shown in Fig. 5. The reaction barrier of O2 dissociation on Pt site of Pt11/TiO2(110)_OH and surface Pt dimer of Au8Pt3/TiO2(110)_OH are 0.56 and 0.72 eV, respectively. In summary, O2 dissociation on the Au-TiO2 interface is difficult, while on Pt-TiO2 interface and Pt ensemble (such as dimer) is quite easy.


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Figure 5. (a)The energy profile diagram and (b) the optimized transition state structures (side view) of O2 dissociation on the interfacial sites of Au11/, Au8Pt3/, Pt11/ and pure Pt sites of Au8Pt3/, Pt11/TiO2(110)_OH. The bond lengths are given in angstrom (Å). Finally, CO oxidation by the adsorbed molecular O2 and the dissociated atomic oxygen on different models has been investigated in detail (Fig. 6). The adsorption energies of CO on the interfacial sites of Au11/ and Pt11/TiO2(110)_OH are -0.94 and -1.39 eV, respectively. It indicates that the adsorption strength of CO on Au cluster is much weaker than Pt, which is consistent with previous study that the coverage of CO on Au catalysts is low.23 Due to the large dissociation barrier of O2 on the interfacial sites between Au and TiO2 of Au11/ and Au8Pt3/TiO2(110)_OH, we only consider CO oxidation by the adsorbed molecular O2 on these two reaction sites. It is found that a CO-OO complex is easily formed,37 the formation of CO2 only needs 0.23 and 0.19 eV, respectively. In order to make comparison with that on Au11/TiO2(110)_OH, we also


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investigate CO oxidation by adsorbed molecular O2 on the interfacial site of Pt11/TiO2(110)_OH, the reaction barrier is 0.98 eV, which is much larger than O2 dissociation on Pt11/TiO2(110)_OH (Fig. 5) and CO oxidation by the dissociated oxygen on Pt sites of Pt11/TiO2(110)_OH (not shown). While on Pt dimer of Au8Pt3/TiO2(110)_OH, the reaction barrier of CO oxidation by the adsorbed molecular O2 is 0.68 eV, which is comparable with that of O2 dissociation barrier. It indicates that CO can react with both dissociated atomic oxygen and adsorbed molecular oxygen on Pt dimers.

Figure 6. (a) The energy profile diagram and (b) the optimized transition state structures (side view) of CO oxidation on the interfacial sites of Au11/, Au8Pt3/, Pt11/ and pure Pt site of Au8Pt3/TiO2(110)_OH. The bond lengths are given in angstrom (Å).

4. Conclusion


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From our study, several important issues concerning CO oxidation on supported gold and platinum catalysts are clear: (1) Pt11 binds to TiO2(110) more strongly than Au11, suitable surface modification/treatment of support is more vital for supported Au than Pt clusters or nanoparticles. The interaction between alloyed cluster and TiO2 support is improved by the presence of Pt in Au-Pt bimetallic cluster. (2) O2 can adsorb at both the interface and metal sites on supported Pt nanoparticles, but O2 cannot adsorb on pure Au sites. O2 adsorption on the interfacial sites is stronger than that on pure metal sites. It indicates that the interfacial sites between metal and metal oxide are the dominate reactions sites for most oxidation reactions. (3) On Au11/TiO2(110)_OH, CO is directly activated by adsorbed molecular oxygen. While on Pt11/TiO2(110)_OH, the dissociated oxygen reacts with CO. On Au8Pt3/TiO2(110)_OH, both of the reaction mechanisms work.

ACKNOWLEDGMENT This work was supported by National Basic Research Program of China (973Program ) (2013CB733501), the National Natural Science Foundation of China (NSFC-21176221, 21136001, 21101137 and 21306169,), Zhejiang Provincial Natural Science Foundation of China (ZJNSF-R4110345) and the New Century Excellent Talents in University Program (NCET10-0979). Prof. Bjørk Hammer is greatly acknowledged for fruitful discussion.


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For TOC only


23

Distinctions between Supported Au and Pt Catalysts for CO Oxidation

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