A DFT Study of CO Oxidation at the PdâCeO2(110) - ACS Publications

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A DFT Study of CO Oxidation at the Pd-CeO(110) Interface Weiyu Song, Ya-Qiong Su, and Emiel J. M. Hensen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09293 • Publication Date (Web): 16 Nov 2015 Downloaded from http://pubs.acs.org on November 17, 2015

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The Journal of Physical Chemistry

A DFT Study of CO Oxidation at the Pd-CeO2(110) Interface

Weiyu Song1,2, Yaqiong Su2 and Emiel J. M. Hensen*2

1

State Key Laboratory of Heavy Oil, College of Science, China University of

Petroleum-Beijing, Beijing 102249, P. R. China 2

Inorganic Materials Chemistry, Eindhoven University of Technology, P.O.Box 513,

5600 MB Eindhoven, The Netherlands

Corresponding author :

E-mail: [email protected] Tel : +31-40-247 5178

Abstract Ceria-supported Pd is one of the main components in modern three-way catalysts in automotive applications to facilite CO oxidation. The exact form in which Pd displays its high activity remains not well understood. We present a DFT+U study of CO oxidation for single Pd atoms located on or in the ceria surface as well as a Pdn nanorod model on the CeO2(110) surface. The oxidation of Pd to the 2+ state by ceria weakens the Pd-CO bond for the single Pd models and, in this way, facilitates CO2 formation. After CO oxidation by O of the ceria surface, Pd relocates to a position below the surface for the Pd-doped model; in this state, CO adsorption is not possible anymore. With Pd on the surface, O2 will adsorb and dissociate leading to PdO, which can be easily reduced to Pd. The reactivity of the Pd nanorod is low because of the strong bonds of the metallic Pd phase with CO and the O atom derived from O2 dissociation. These findings show that highly dispersed Pd is the most likely candidate for CO oxidation in the Pd-CeO2 system.

Key worlds: CO oxidation, Pd, ceria, active site, DFT+U

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1. Introduction Palladium supported on ceria is one of the main components of the three-way catalyst used to clean the exhaust gas of gasoline engines.1 Due to strong metal-support interactions, Pd atoms tends to be present on the ceria support in highly dispersed forms. The nature of the catalytically active Pd phase - as ions in the ceria, isolated atoms and clusters or larger particles - is strongly influenced by ageing at high temperature under reducing and/or oxidizing conditions. It has not been unequivocally resolved yet how the rate of CO oxidation depends on the active phase form and which phase is the most active. Kumatowaska et al.2 reported that as-prepared nanocrystalline Ce0.89Pd0.11O2−y demonstrates moderate activity in CO oxidation. Reduction at 400 °C in H2 strongly improved the catalytic activity, suggesting that Pd atoms in positions where they substitute Ce cations are less active than Pd on the ceria surface. Fernandez-Garcia et al. studied room-temperature CO oxidation of Pd/CeO2 prepared by wet-impregnation and concluded that ceria promotes the formation of metallic Pd clusters as the active centers for CO oxidation.3 Hinokuma et al. found that Pd/CeO2 displayed increased the room-temperature CO oxidation activity after thermal aging at 900°C in air due to increased metal-support interactions and phase transformations from Pd oxide into metallic Pd particles.4 In a recent contribution of the same group,5 an arc-plasma process to deposit Pd nanoparticles on CeO2 was found to lead to more active catalysts than conventional preparation methods. Thermal ageing at 600°C in air lowered the catalytic activity, which was attributed to the oxidation of metallic Pd to Pd-oxide. Ageing at 900 °C in 10% H2O/air was observed to enhance the performance again due to the reduction to metallic Pd. Other studies provide evidence for the opposite conclusion. Gulyaev et al.6 employed the same arc-plasma method as Hinokuma et al. to synthesize Pd/CeO2 with high activity at room temperature and argued that the activity must derive from a PdCeOx solid solution and highly dispersed PdOx nanoparticles at the ceria surface. Room temperature CO oxidation over Pd/CeO2 was also reported by Boronin et al.,7 who


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found that subnanometer Pd particles have increased interaction with the ceria surface. Recently, Colussi et al. used DFT calculations to propose a Pd-O-Ce surface structure.8 XPS combined with X-ray absorption spectroscopy studies of the oxidation state and local atomic structure of Pd species led Zhou et al. to propose that Pd ions interacted with the ceria support and the resulting PdOx species are the most active component for CO oxidation.9 Using a combustion-based method, Bera et al. synthesized ceria-supported Pd catalyst for CO oxidation and observed higher activity compared with alumina-supported Pd.10 The observed enhanced catalytic activity of Pd/CeO2 was attributed to the ionic forms of highly dispersed Pd on CeO2 leading to strong metal-ceria interactions, likely in the form of a solid solution. Despite the importance of these experimental efforts, relatively few theoretical studies have been carried out aimed at elucidating the active state of Pd in Pd/CeO2 catalysts for CO oxidation. Ab initio thermodynamic calculations were employed by Mayernick et al. to determine the stability of Pd atoms doped into the ceria lattice as compared with isolated Pd atoms and small Pd clusters as a function of the temperature and oxygen partial pressure.11 The activity of the various Pd-CeO2 models towards methane activation has also been investigated by the same group.12,13 In comparison, more work has been done on the topic of CO oxidation for the Au-CeO2 combination. Song et al. investigated the active state of gold in Au/CeO2 for CO oxidation14,15 and the water-gas shift reaction,16 comparing Au models and different reaction pathways. Here, we use the same approach to compare the performance of a single Pd atom adsorbed on the ceria surface, a single Pd atom incorporated in theceria lattice and a Pd nanorod adsorbed on ceria as catalysts for the CO oxidation reaction. The latter nanorod surface structure allows modelling Pd nanoparticles by DFT calculations.17 Experimental as well theoretical investigations14,15,18,19 suggest that CeO2(110) is a reactive surface termination of ceria, likely involved in catalytic reactions. The vacancy formation energy for the (110) surface is lower than for the (111) surface. Ceria nanorods that according to many authors expose predominantly (110) surface show increased performance in CO oxidation.20 The dominant surface termination of ceria nanorods is, however, under debate; Agarwal et al. have recently shown that



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ceria nanorods may be terminated by CeO2(111) surfaces.21 In our calculations, we adopt the CeO2(110) surface model as a point of reference for future calculations. 2. Computational details Density functional theory (DFT) with the PBE (Perdew-Burke-Ernzerhof) functional22 as implemented in the Vienna Ab Initio Simulation Package (VASP)23-25 was employed. A Hubbard U term was added to the PBE functional (DFT+U) employing the rotationally invariant formalism by Dudarev et al.,26 in which only the difference (Ueff = U – J) between the Coulomb U and exchange J parameters enters. Spin-polarized calculations were performed. The projector augmented wave method (PAW)27-29 was used to describe the interaction between the ions and the electrons with the frozen-core approximation. The Ce (4f, 5s, 5p, 5d, 6s), O (2s, 2p), and Pd (4p, 4d, 5s) electrons were treated as valence states using a plane-wave basis set with a kinetic energy cut off of 400 eV. For Ce, a value of Ueff = 4.5 eV was used, which was calculated self-consistently by Fabris et al.30 using the linear response approach of Cococcioni and de Gironcoli.31 This value is within the 3.0-5.5 eV range reported to provide localization of the electrons in the Ce 5f orbital left upon oxygen removal from CeO2.32 For all the surface calculations, the model was a periodic slab with a (33) surface unit cell and, for the Brillouin zone integration, a Monckhorst-Pack 111 mesh was used. The CeO2 (110) slab model is four layer thick and a vacuum gap was set to be 15 Å. The bulk equilibrium lattice constant (5.49 Å) previously calculated by PBE+U (Ueff = 4.5 eV) was used.33 A nanorod of 32 Pd atoms was placed on the (33) surface unit cell of CeO2(110) to serves as a model for supported Pd clusters and nanoparticles. The Pd adatom and the two top atomic layers of the ceria slab were allowed to relax, while the two bottom layers were 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.34


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3. Results and Discussions Pd1/CeO2(110) We identified three adsorption sites for a single Pd atom on the CeO2(110) surface (Fig. 1). The first two involve Pd coordinating in a bridging fashion between two surface O atoms. In the symmetric mode (Fig. 1a), the distance of the Pd atom to each of these O atoms is 2.02 Å; the adsorption energy of the Pd atom is -2.02 eV. A very similar adsorption site has been identified before,11 with a slightly lower adsorption energy of -1.78 eV and longer Pd-O distances. The difference most likely arises from the smaller supercell used in the calculations of Mayernick et al., leading to increased lateral repulsive interactions. The adsorption of the Pd atom results in the reduction of one of the Ce atoms in the surface to the +3 oxidation state. We also identified another less symmetric adsorption mode in which Pd bridges between two O atoms in the way shown in Fig. 1b. It is the result of the migration of one of the O atoms in the symmetric structure to a bridge site between Ce and Pd atoms with Ce-O and Pd-O bond distances 2.20 Å and 1.92 Å, respectively. This non-symmetrical adsorption site is more stable by 0.54 eV as compared with the symmetric adsorption model in Fig. 1a. The most stable adsorption site for a single Pd atom on the CeO2(110) surface is the four-fold hollow site with Pd connected to four surface O (Fig. 1c). The adsorption energy is -2.99 eV. The bond distance between the Pd atom and the four surface O atoms is close to 2.09Å. In this adsorption mode, two surface ceria atoms are reduced to 3+, implying a formal oxidation state of +2 for Pd. The replacement of a Ce atom for a Pd atom in the surface is also considered (Fig. 1d). In the next section, we will discuss the mechanism of CO oxidation for the Pd atom in the four-fold site of CeO2(110) denoted by Pd1(4f)/CeO2(110) and the Ce surface substitution model denoted by Pd1Ce1-xO2(110).

CO oxidation on Pd1(4f)/CeO2(110) As a first step in the catalytic cycle, CO adsorbs on Pd with an adsorption energy of 146 kJ/mol (state ii, Fig. 2). The bond distance of C to Pd is 1.84 Å. The C-O bond



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distance is slightly elongated to 1.17 Å as compared with the C-O distance of 1.14 Å in gas-phase CO. It points to electron back-donation from the Pd d orbitals to the anti-bonding orbitals of CO. Electron back-donation is also evident when the electron density differences following adsorption of CO on Pd1(4f)/CeO2(110) are visualized (Fig. 2). As a result of the electron redistribution, Ce3+ present before CO adsorption is oxidized to Ce4+. The adsorption of CO also leads to the migration of the Pd atom to the bridge site between two O atoms. In contrast to strong CO adsorption (-146 kJ/mol), O2 binds only weakly on Pd1/CeO2(110) with an adsorption energy of -16 kJ/mol. Consistently, the bond length of O2 is only slighty elongated to 1.25 Å (gas phase 1.23 Å). The result point out that Pd1/CeO2 will bind CO preferentially. The CO2 product is formed by tilting of the adsorbed CO molecule to the surface (state iii, Fig. 2). The activation barrier for this process is 82 kJ/mol and the reaction energy is -51 kJ/mol. Two Ce4+ ions in the surface are reduced to Ce3+. Desorption of CO2 into the gas phase costs 79 kJ/mol and it generates a surface oxygen vacancy (state iv, Fig. 2). The vacancy can be filled by adsorption of molecular oxygen (state v, Fig. 2). Adsorption of O2 is strongly exothermic (∆E = -223 kJ/mol). The O-O bond length is elongated to 1.46 Å (cf. gas-phase O-O bond length = 1.23 Å). The next CO molecule can absorb to the Pd atom with an adsorption energy of 106 kJ/mol (state vi, Fig. 2), slightly lower than the adsorption on the bare Pd1(4f)/CeO2(110) surface. Consistent with this, the bond distance between Pd and C is 1.88 Å, slightly longer than for CO bound to Pd1(4f)/CeO2(110) (1.84 Å, Fig. 2). All of these data indicate that electron back-donation from the Pd d orbitals to the anti-bonding orbital of CO molecule is weaker, because the Pd has been further oxidized by the adsorption of O2. The barrier for O2 dissociation is 51 kJ/mol. In the transition state, the O-O bond is elongated to 1.81 Å; the associated reaction energy is downhill by 137 kJ/mol. One of the O atoms fills the oxygen vacancy site, accompanied by re-oxidation of two Ce3+atoms back to Ce4+. The other O atom coordinates to Pd together with CO and forms CO2 with a relatively low barrier of 31 kJ/mol (∆E = -87 kJ/mol) (state viii, Fig. 3). Desorption of the second CO2 molecule costs 39 kJ/mol and is followed by the migration of the Pd atom back to the four-fold site, completing the reaction cycle (state i, Fig. 2).


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CO oxidation on Pd1Ce1-xO2(110) After doping the CeO2(110) surface with Pd, the top layer is composed of Pd, Ce and O atoms (state i, Fig. 3). The Pd atom is coordinated to four surface O (Pd-O bond distance: 2.18 Å) and two subsurface O atoms at a slightly larger distance of 2.24 Å. CO adsorbs on the Pd atom with an adsorption energy of 96 kJ/mol (state ii, Fig. 4). The Pd-C bond distance is 1.85 Å. The relatively weak adsorption of CO on the Pd atom is clear from the C-O bond distance of 1.15 Å, which is almost the same value as for gas-phase CO. CO oxidation proceeds by tilting the adsorbed CO molecule to the closest surface O atom (state iii, Fig. 3). The reaction energy for this process is very strongly exothermic (∆E = -408 kJ/mol), which is due to the weak binding of the reacting O atom that bridges between the Pd and Ce surface atoms. This result is in keeping with literature findings.11 CO2 desorption is very facile with a desorption energy of only 22 kJ/mol. Due to the removal of a surface O atom and desorption of the CO2 product, Pd will migrate deeper into the surface by 0.73 Å; it is then coordinated to two surface O atoms (bond distance: 2.00 Å) and two subsurface O atoms (bond distance: 2.03 Å) (state iv, Fig. 3). For this stable state, we found that neither CO nor O2 can adsorb, which means that the catalyst is deactivated. The

Thus, for the Pd1Ce1-xO2(110) system CO adsorption is possible on the stoichiometric surface (state i, Fig. 3). After removal of the first O atom adjacent to the Pd atom, Pd moves to a sub-surface position and CO adsorption is not possible anymore. It is worthwhile to compare this case with a Pd-doped CeO2(111) surface. Then, the top layer of the surface contains only O atoms and the Pd and Ce atoms are located in the second layer and no stable CO adsorption state can be identified. We note that for a similar model weak CO adsorption has been reported using the PW91 functional.35 It is well known that the PW91 functional overestimates binding energies as compared with the PBE functional36 that was used in the present study. The use of more accurate functionals is thus needed to understand the weak CO adsorption on these Pd-doped



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ceria surfaces. It is clear, however, that CO adsorption will be strongly preferred on the other systems discussed in this work. These findings suggest that geometrical factors influence the adsorption of on the Pd atoms located on or in the surface. To illustrate that these differences are not due to the electronic state of Pd, Fig. 4 depicts the projected density of states (PDOS) of the Pd 4d-orbitals for a Pd atom in different configurations on CeO2(111) and CeO2(110) surfaces. Integration of the PDOS allows estimating the oxidation state of the Pd atom. The oxidation state of Pd doped in stoichiometric (111)- and (110)- terminated surfaces is about +2.35 e. On the former surface CO adsorption on Pd is not possible, while CO strongly adsorbs on Pd on the latter surface. When Pd is located below the top layer in Ce1-xO2-y, the oxidation state is only slightly lower (+2.1 e). CO adsorption is not possible on this Pd atom. When the Pd atom is located in the CeO2(110) surface, the oxidation state is almost the same (+2.07 e), but in this case CO adsorption is relatively strong. From these findings, we infer that the geometry around the Pd atom determines whether CO adsorption can take place: when the Pd atom is below the surface, the O atoms repel the CO molecule so that binding to the Pd atom is not possible.

CO oxidation on Pdn/CeO2(110) We also studied CO oxidation on a Pd nanorod placed on the CeO2(110) surface, which serves as a model for Pd clusters and nanoparticles. The reaction energy diagram and the key intermediate structures are shown in Fig. 5. CO adsorbs on a three-fold hollow site of the Pd nanorod with an adsorption energy of 222 kJ/mol (state ii, Fig. 6). The C-O bond distance of 1.20 Å is substantially longer than the gas-phase C-O bond distance. Comparing the three above models, we note that increased

metallic

character

(Pdn/CeO2(110)

>

Pd1Ce1-xO2(110)

>

Pd1(4f)/Ce1-xO2(110)) results in stronger Pd-CO bonds and weaker C-O bonds due to electron back-donation from Pd into CO anti-binding orbitals. The energy barrier for reaction of the adsorbed CO with a surface O atom is 122 kJ/mol and the reaction energy is exothermic by 20 kJ/mol. We compare these values to the same reaction on


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a Aun/CeO2(110) model, for which the reaction barrier is negligible and the exothermic energy 146 kJ/mol.15 The difference relates to the much stronger Pd-CO bond. Formation of CO2 results in the reduction of two Ce4+ ions to Ce3+ (state iii, Fig. 5). Desorption of CO2 costs 50 kJ/mol and results in an oxygen vacancy at the interface between the Pdn rod and the ceria surface (state iv, Fig. 5). Molecular oxygen will adsorb at this site with (∆E = 108 kJ/mol) (state v, Fig. 5). O2 dissociates with a barrier of 55 kJ/mol, resulting in healing the surface vacancy and migration of the other O atom to the Pd nanorod (state vi, Fig. 5). In this way, the reduced Ce3+ ions are re-oxidized back to Ce4+. The reaction cycle is then completed by reaction of CO adsorbed in a three-fold hollow site with this O atom (state viii, Fig. 5). The barrier for this latter reaction of 133 kJ/mol is very close to the barrier for CO oxidation on the Pd(111) surface.37 Contrasting these findings with data for Aun/CeO2(110)15 and Aun/CeO2(111),38 one appreciates that the lower binding energy of O on gold surfaces leads to negligible barrier for CO2 formation. This implies that the synergy of ceria with Au nanoparticles in CO oxidation will be greater than with Pd nanoparticles. For Pd nanoparticles supported on ceria, CO oxidation by an O atom adsorbed on the metallic phase is the most difficult step and this step is as difficult as for an extended Pd surface. An alternative mechanism would involve O2 adsorption to Pd followed by dissociation or direct reaction with co-adsorbed CO as has been explored for Aun/CeO2(111).38 We investigated O2 adsorption on the Pd cluster and found an adsorption energy of 114 kJ/mol, much lower than that of CO adsorption on Pdn/CeO2(110) (222 kJ/mol). In addition to the lower adsorption energy, two other factors will contribute to unfavorable adsorption of O2 vs. CO. First, the adsorption of O2 needs two metal sites whereas, CO requires only one binding site. Second, O2 adsorption involves spin flip of O2 from the triplet gas phase state to the singlet state,39 which will significantly lower the sticking coefficient for adsorption. All of these factors contribute to predominant CO coverage under typical CO oxidation conditions. This is reasonable as typically these catalysts suffer from self-poisoning



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by CO.40 This discussion brings to the fore the important role of the Pd-ceria interface for O2 adsorption and activation.

Discussion We investigated here three models for Pd in interaction with the CeO2(110) surface for CO oxidation. When a single Pd atom is considered either on or in the surface, the Pd atom will be oxidized. This decreases the CO bond energy as compared with metallic Pd. The lower binding energy of CO with the single Pd atom benefits the formation of CO2. The single Pd atoms in these two models are also very close to the O atoms of the ceria surface, further facilitating CO2 formation. In contrast, when Pd is present in the form of a metallic nanorod, the binding energy of CO and the reaction barrier for CO2 are much higher. This holds for the reaction of adsorbed CO with an O atom of the ceria surface as well as an O adatom on the Pd nanorod surface. Another contributing factor is the much stronger adsorption of the O atom from O2 dissociation on the nanorod metallic surface (158 kJ/mol vs. gas-phase O2) than on the single Pd atom on the surface (77 kJ/mol). In case of Pd doping into the ceria surface, we show that the geometry of the surface strongly influences CO adsorption. Only when Pd is in the first surface layer, CO adsorption can take place; when Pd is in the second layer as occurs after one O vacancy is generated or when Pd is doped into the CeO2(111) surface, repulsion by the surface O atoms inhibits CO adsorption. The same holds for O2 molecule which is weak with an adsorption energy of 8 kJ/mol. With the formation of one oxygen vacancy generated by CO, the Pd will migrate in the subsurface layer and coordinate with four oxygen atoms. The oxidized Pd atom shows much decreased ability to adsorb O2. Also, the repulsive interactions between the large ceria surface O anions and an impinging O2 molecule decrease its possibility to bind to the surface. The combination of these two factors explain weak O2 adsorption. These considerations have also been discussed for oxygen adsorption in the Au-CeO2(111) system.41 We conclude that the oxygen vacancy site is a necessary but not a sufficient condition for O2 adsorption.


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From the present findings, we infer that highly, preferably atomically, dispersed Pd on ceria is more active in CO oxidation than Pd atoms doped in the ceria or metallic Pd nanoparticles. Experimental studies have pointed out that CO can be oxidized effectively by Pd-doped ceria;6-9 we counter however that such works cannot exclude the presence of Pd adatoms on the surface. In this respect, it is worthwhile to mention the theoretical study of Kim et al.42 that shows CO adsorbed on a metallic adatom cluster can easily react with O atoms that neighbor to the metal atoms doped in the ceria surface. Our results also show that Pd clusters and nanoparticles do not benefit from the ceria surface despite the involvement of its surface O atoms: removal of the O adatom on the metallic Pd particle by CO has almost the same barrier as CO oxidation on Pd(111). 4. Conclusions The DFT+U approach was employed to study the interface of Pd with the CeO2(110) surface for CO oxidation. Three models were compared involving doping of the surface with a single Pd atom, placing a single Pd atom on the surface and a nanorod model placed on the surface. CO adsorption is possible on all three models. Because Pd is oxidized in the single atom models to a 2+ oxidation state, CO adsorbs weaker than on the metallic Pdn nanorod model. The lower CO adsorption strength explains the lower activation energy for the formation of CO2. When CO2 is removed, the single Pd atom that was doped into the ceria surface migrates to subsurface positions, which deactivates the catalyst because further CO or O2 adsorption is not possible. This is a geometric rather than an electronic effect. For the Pd atom on the surface, O2 adsorption and dissociation heals the oxygen vacancy in the surface and leads to PdO on the surface which is easily reduced to Pd with CO. The strong binding energy of CO on the metallic nanorod surface and the lower activation of the O atom of the ceria as compared with the single Pd models explain the higher barrier for CO2 formation. The most difficult step is the removal of the O atom from the surface after O2 dissociation at the interface. The barrier is close to the one computed for CO oxidation on the Pd(111) surface. These findings indicate that high dispersion,



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possibly atomic dispersion of Pd over the ceria support yields to intrinsically more active catalysts than metallic Pd particles. Doping of Pd in the surface cannot contribute to the catalytic activity as the catalytic cycle cannot be closed. Supporting Information Additional information (total energy of each state in Figure 2, 3 and 5; optimized structure file of selected structure) is presented in Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgement We acknowledge financial support for this research from China University of Petroleum-Beijing (no. 2462015YJRC005), the National Natural Science Foundation of China (no. 21503273). EJMH acknowledges financial support from the Netherlands

Organization

for Scientific

Research

through

a

VICI

grant.

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

References: 1. Trovarelli, A.; Fornasiero, P. Catalysis by Ceria and Related Materials, in: Catalytic Science Series, vol. 12, Imperial College Press, UK, 2013. 2. Kurnatowska, M.; Kepinski, L.; Mista, W. Structure Evolution of Nanocrystalline Ce1−xPdxO2−y Mixed Oxide in Oxidizing and Reducing Atmosphere: Reduction-Induced Activity in Low-Temperature CO Oxidation. Appl. Catl. B 2012, 117, 135-147. 3. Fernandez-Garcia, M.; Martinez-Arias, A.; Iglesias-Juez, A.; Hungria, A. B.; Anderson, J. A.; Conesa, J. C.; Soria, J. Influence of Ceria on Pd Activity for the CO+O2 Reaction. J. Catal. 1999, 187, 474-485. 4. Hinokuma, S.; Fujii, H.; Okamoto, M.; Ikeue, K.; Machida, M. Metallic Pd Nanoparticles

Formed

by

Pd−O−Ce

Interaction:


A

Reason

for

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Sintering-Induced Activation for CO Oxidation. Chem. Mater. 2010, 22, 6183-6190. 5. Hinokuma, S.; Fujii, H.; Katsuhara, Y.; Ikeue, K.; Machida, M. Effect of Thermal Ageing on the Structure and Catalytic Activity of Pd/CeO2 Prepared Using Arc-Plasma Process. Catal. Sci. Technol. 2014, 4, 2990-2996. 6. Gulyaev, R. V.; Slavinskaya, E. M.; Novopashin, S. A.; Smovzh, D. V.; Zaikovskii, A. V.; Osadchii, D. Yu.; Bulavchenko, O. A.; Korenev, S. V.; Boronin,

A.

I.

Highly

Active

PdCeOx Composite

Catalysts

for

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12. Mayernick, A. D.; Janik, M. J. Methane oxidation on Pd–Ceria: A DFT Study of the Mechanism over PdxCe1−xO2, Pd, and PdO. J. Catal. 2011, 278, 16-25. 13. Krcha, M. D.; Mayernick, A. D.; Janik, M. J. Periodic Trends of Oxygen Vacancy Formation and C–H Bond Activation over Transition Metal-Doped CeO2 (1 1 1) Surfaces. J. Catal. 2012, 293, 103-115. 14. Song, W.; Hensen, E. J. M. Structure Sensitivity in CO Oxidation by a Single Au Atom Supported on Ceria. J. Phys. Chem. C 2013, 117, 7721-7726. 15. Song, W.; Hensen, E. J. M. A Computational DFT Study of CO Oxidation on a Au Nanorod Supported on CeO2(110): on the Role of the Support Termination. Catal. Sci. Tech. 2013, 3, 3020-3029. 16. Song, W.; Hensen, E. J. M. Mechanistic Aspects of The Water−Gas Shift Reaction on Isolated and Clustered Au Atoms on CeO2(110): a Density Functional Theory Study. ACS Catal. 2014, 4, 1895-1892. 17. Green, I. X.; Tang, W.; Neurock, M.; Yates, J. T. Spectroscopic Observation of Dual Catalytic Sites During Oxidation of CO on a Au/TiO2 Catalyst. Science, 2011, 333, 736-739. 18. Si, R.; Flytzani-Stephanopoulos, 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. 2008, 47, 2884−2887. 19. Aneggi, E.; Wiater, D.; de Leitenburg, C.; Liorca, J.; Trovarelli, A. Shape-Dependent Activity of Ceria in Soot Combustion. ACS. Catal. 2014, 4, 172−181. 20. Zhou, K.; Wang, X.; Sun, X.; Peng, Q.; Li, Y. Enhanced Catalytic Activity of Ceria Nanorods from Well-Defined Reactive Crystal Planes. J. Catal. 2005, 229, 206-212. 21. Agarwal, S. A.; Lefferts, L.; Mojet, B. L.; Ligthart, D. A. J. M.; Hensen, E. J. M.; Mitchell, D. R. G.; Erasmus, W.; Anderson, B. G.; Olivier, E. J.; Neethling, J. H.; Datye, A. K. Exposed Surfaces on Shape-Controlled Ceria Nanoparticles Revealed Through AC-TEM and Water Gas Shift Reactivity, ChemSusChem. 2013, 6, 1898-1906.


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Caption to the figures: Figure 1:

Structure of Pd-CeO2(110) models (color scheme: red - O; white - Ce4+; cyan - Ce3+; green - Pd).

Figure 2:

Reaction mechanism of CO oxidation on the Pd1/CeO2(110) model. The electron density difference plot for CO adsorption on Pd1/CeO2(110) (color scheme: grey – C) is inserted next to the structure (state ii). The blue and yellow colors indicate, respectively, a decrease and increase in the electron density.

Figure 3:

Reaction mechanism of CO oxidation on the Pd1Ce1-xO2(110) model.

Figure 4:

Projected density of states (DOS) analysis for d-orbital of Pd on varies Pd-CeO2 model (reference energy is the Fermi level marked by the vertical dashed line).

Figure 5:

Reaction mechanism of CO oxidation on the Pdn/CeO2(110) model.


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Figure 1:



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Figure 2:


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Figure 3:



Figure 4:

4

Pd1Ce1-xO2(111): no adsorption

Pd1Ce1-xO2-y(110): no adsorption

Pd(+2.36)

Pd(+2.1)

0

Density (states/eV)

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-4

4

Pd1Ce1-xO2(110): adsorption

Pd1/CeO2(110): adsorption

Pd(+2.34)

Pd(+2.07)

0

-4

-4

-2

0

-4

-2

Energy (eV)


0

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Figure 5:



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TOC


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220x145mm (150 x 150 DPI)


A DFT Study of CO Oxidation at the PdâCeO2(110) - ACS Publications

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A DFT Study of CO Oxidation at the PdâCeO2(110) - ACS Publications