CO oxidation over Ce1âxPdxO2âÎ´ takes place via vacancy hopping

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CO oxidation over Ce PdO takes place via vacancy hopping Phanikumar Pentyala, and Parag Arvind Deshpande Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00734 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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Industrial & Engineering Chemistry Research

CO oxidation over Ce1−xPdxO2−δ takes place via vacancy hopping Phanikumar Pentyala and Parag A. Deshpande∗ Quantum and Molecular Engineering Laboratory, Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, India. E-mail: [email protected] Phone: (+91) 3222 283916

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Mechanism of CO oxidation over ceria-based catalysts has been widely accepted to follow Mars-van Krevelen (MvK) mechanism involving the lattice oxygen exchange. Adsorption of CO over the metallic sites, lattice oxygen exchange and replenishment of the support by stream oxygen are considered to be the surface processes taking place during the MvK mechanism. This study reveals an alternative MvK-type mechanism for the reaction taking place over metal ion substituted ceria-based reduced catalysts involving hopping of vacancies. The mechanism was proposed on the basis of adsorption energetics of CO and O2 , probed with the help of density functional theory calculations. The computational analysis of different molecular systems (CeO2 , CeO2−δ and Ce1−x Pdx O2−δ ) under study showed that adsorption of O2 takes place over the reduced catalyst and non-sequential adsorption of CO and O2 take place over Ce1−x Pdx O2−δ in contrast to sequential adsorption of O2 followed by CO over CeO2−δ . Differences in the elementary surface processes in the proposed mechanism with the established MvK mechanism are highlighted with the major observation being the differences in the lattice oxygen abstraction and vacancy hopping observed with newly proposed alternative MvK mechanism.


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1

Introduction

Carbon monoxide, an odorless poisonous gas which causes respiratory and neural problems in human beings, is released into the atmosphere due to incomplete combustion of carbonaceous compounds. 1–3 Discharge of CO from mobile (internal combustion engines) and point (power plants) sources can be checked either by its conversion to other useful and less toxic chemicals or by the use of alternative energy systems. The former method is cost effective if one considers the cost of resources and processing. 4 CO can be converted to CO2 by CO oxidation in the presence of transition (Co, Ni, Cu, Ru, Rh, Pd, Ir, Pt) metals. 5,6 The bare metal catalyst activity is limited when compared to the activities of metals along with metal oxide supports. 7–9 Uniform dispersion of metals and thermal stability are the main reasons for choosing supported catalysts for CO oxidation. 10–12 Both impregnated as well metal ion substituted metal oxides have been employed for catalyzing CO oxidation. In substituted catalysts, the area of contact and metal-support interactions with the reactants are more as compared to those in metal impregnated systems. The surface covered under metal clusters in metal impregnated systems are not useful for reactions and reaction temperatures often promote the formation of aggregates of metals thus reducing the catalyst activities. 13 Metal ion substituted catalysts have thus emerged as a promising class of catalysts for gas-phase heterogeneous catalytic reactions. 14–16 This study accordingly focuses on developing molecular insights into surface processes during CO oxidation over Pd2+ ion substituted ceria (Ce1−x Pdx O2−δ ) using density functional theory (DFT) calculations.

As noted by Zhou and co-workers, 17 Pd, Pt, Ru and Au have been extensivly reported for CO oxidation. While the use of Au has demonstrated excellent low-temperature activity, Pd and Pt catalysts have exhibited high turnover numbers at moderate temperatures. Due to practical applications in automobile catalysis, one can afford moderate temperatures to avoid the use of expensive Au, and from economic persepective, Pd looks advantageous over

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Pt for similar temperature ranges of operation. Cheaper alternatives of non-noble transition metals like Cu, Co and Mn have clear limitations when compared to Pd in terms of moisture tolerance and inferior activities. Hence, Pd makes the best case for its analysis in the current study concerning the details of surface CO oxidation. Ce1−x Pdx O2−δ considered in this study is not a model compound that we built computationally for the analysis. Rather, such an ionically substituted catalyst has been synthesized by Hegde and co-workers and applied for CO oxidation. Such catalysts differ from the conventional metal nanoparticle impregnated catalysts in a manner that metal atoms are not present over the surface. Rather, metal ions are dispersed in CeO2 matrix in Ce lattice positions. A completely different set of surface species and mechanism may be expected over such compounds, hence this study.

Adsorption of CO on the surface is a crucial step in CO oxidation as it decides the further steps of the reaction. Three mechanisms of CO oxidation have been described previously: Langmuir-Hinshelwood mechanism (LH), Eley-Rideal (ER) mechanism and Mars-van Krevelen (MvK) mechanism. Following the LH mechanism, CO and O2 react after adsorption over the catalyst surface. The ER mechanism differs from the LH mechanism in that the reaction takes place by adsorption of only one of the reactants on the surface without any adsorption of the second species. 18,19 MvK has been the most widely accepted mechanism of CO oxidation over metal oxides which involves redox reactions. In the MvK mechanism, CO adsorbs on the active site and gets oxidized with the lattice oxygen by making a defect site; this defect site is refilled by O2 . 20 In previous reports, investigators have proposed several factors to be influencing CO adsorption over metal doped CeO2 including the adsorbent surface composition and vacancy defects. 21,22 Adsorption of CO has been reported to be strong over metal substituted CeO2 and weak over pure CeO2 along with the possibility of formation of carbonate species. 23 Interactions of these species with adsorbents have been previously investigated computationally and the results were in good agreement with experimental findings. 24 Detailed insights into the role of different species and surface states on

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(co-)adsorption are desired and this study provides the details of the same.

MvK mechanism of CO oxidation involves the removal of a lattice oxygen of CeO2 , and a distortion of surrounding local structure (metal-oxygen bond) around the dopant metal ion takes place which affects the adsorption of CO. 25–28 However, the formation and desorption of other species (carbonate and CO2 -like species) also affect the CO oxidation characteristics. 29–32 Yang et al., compared CO adsorption on pure and Zr-doped CeO2 with a conclusion of the dopant Zr to be improving the adsorption of CO and desorption of CO2 . 33 Bader charge analysis showed a reduction of charge on the substituted Fe during adsorption of CO because of changes in the electronic environment in the support. 34,35 In the present work, we have attempted to explain all possible adsorption sites for the three forms of the catalyst: CeO2 ; CeO2−δ and Ce1−x Pdx O2−δ , and their effects on CO oxidation. The effect of the presence of a co-reactant during adsorption was studied. Geometrical properties were compared with previous experimental and theoretical studies supporting the adsorption energetics.

Ceria has been reported to exhibit exceptional solid state ionic conductivities and vacancyassisted oxygen diffusion has been reported to be responsible for high conductivities. 36–38 Several studies have reported diffusion of oxygen in ceria-based compounds via vacancy hopping. 39–42 Vacancy formation and migration can prove to be vital in surface catalytic process also. Scanning tunneling microscopy studies by Schaub et al., on TiO2 systems have shown vacancy hopping and oxygen migration to take place. 43 In this study, we conclusively demonstrate that CO oxidation over Ce1−x Pdx O2−δ takes place via vacancy hopping. While vacancy hopping is an established phenomenon, this is the first study to provide mechanistic insights into CO oxidation and contribution of vacancy hopping to the elementary surface processes taking place over Ce1−x Pdx O2−δ during CO oxidation.

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2

Computational Details

Optimizations of different possible configurations of CO over the (1 1 1) planes of CeO2 , CeO2−δ and Ce1−x Pdx O2−δ were performed under DFT framework. The calculations were implemented on Quantum-espresso package. 44 The generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional and plane wave basis sets were used for the description of crystalline systems. Kinetic energy cutoff for plane wave expansion was 400 eV. Strong on-site interactions among the electrons of 4f orbitals of Ce exist. 45 Usual DFT techniques fail to describe these interactions correctly. Localization of these electrons in Ce atoms and the interactions were accounted by Hubbard model (DFT+U ). 46 Anderson et al., have observed that value of the Hubbard parameter (U ) varies based on the exchange-correlation functional. 47 So for computing the total energy under GGA+U, we have used U as 3.125 eV following previous reports. 48 The core electrons were described by ultrasoft pseudopotentials. 49 The number of electrons treated as valence electrons in O, Pd and Ce were 6, 10 and 12, respectively. Monkhorst-Pack mesh grid of 2×2×1 was used for Brillouin zone sampling during optimizations. 50

The surface model used for CeO2 (1 1 1) plane was the one proposed by Metiu and co-workers. 51 The model was a nine-layered 3×3 slab, containing a total of 81 atoms of which 54 were oxygen atoms, and the rest 27 were cerium atoms. The model used in the calculations was periodic in all three directions. A vacuum of 15 Å in a direction perpendicular to the adsorbent surface was applied to prevent the interactions between the adsorbate and the periodic image of the slab. In Ce1−x Pdx O2−δ , one of the Ce atoms in the first Ce-layer was replaced by a Pd atom. An oxygen vacancy was created near Pd by removing oxygen in the adjacent oxygen atomic layer towards the surface. 52 With the bottom six layers fixed at their crystallographic coordinates, the top three atomic layers, and the adsorbate CO/O2 were relaxed during optimizations until the residual forces on the atoms

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were less than 10−3 a.u. A convergence criterion of 10−6 a.u. was employed for structural optimizations using Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm. 53 Self-consistent field calculations were performed with a convergence criterion of 10−8 a.u. for the energy. A heterogeneous surface like Pd2+ -substituted CeO2 offers various energetically inequivalent sites for the adsorption of a gas, CO in the current case. We identified all such energetically inequivalent sites and tested the adosption of CO over them. These have been referred to as various configurations in the text. All the graphic images of configurations were generated using VMD. 54 The adsorption energies for various configurations were calculated using the formula, ∆E = E[Reactant − Catalyst] − E[Reactant] − E[Catalyst]

(1)

Reactant = CO/O2 Catalyst = CeO2 /CeO2−δ /Ce1−x Pdx O2−δ

3

Results and Discussion

Among the low-index crystal planes of CeO2 , we chose the (1 1 1) plane for the computational analysis in this study since this plane has higher stability when compared to (1 0 0) and (1 1 0) planes. 55,56 Cubic fluorite crystal structure of CeO2 was taken from the crystallographic database (COD ID: 9009008) 57 as the initial guess geometry for structural optimizations. The molecular models along with the optimized geometry are shown in Figure 1(a-c). The stoichiometric CeO2 had Ce-O distances of 2.34 Å and Ce-Ce distances of 3.83 Å in the crystal. The nearest distance of O-O, which was between the surface and the subsurface oxygen layers, was 2.74 Å in the crystal. After structural optimization, Ce-Ce distances were 3.8 Å and 3.85 Å with a marginal change of 0.05 Å and no changes were observed for surface and subsurface oxygen distances. CeO2−δ molecular model was created by the removal of surface oxygen from stoichiometric CeO2 . 58 Surface oxygen was removed because our results showed that the surface oxygen removal costed lesser energy than the 5



removal of subsurface oxygen and the two differed by nearly 2 kcal/mol. This was also in correspondence with previous reports. 55 Two Ce atoms, with a nearby oxygen vacancy, were observed to be displaced from their lattice positions and an increased distance of 4.14 Å was observed. All the oxygens coordinated to Ce in a CeO2 (111) plane are equivalent. Hence, it was possible to remove any one of the O atoms at random to obtain CeO2−δ .

Similar to the previous argument for locating the site for oxygen vacancies, the site for substitution of Pd in place of Ce was also random because all the Ce atoms in a CeO2 (111) plane are equivalent. However, it is the sites for Pd-vacancy combination which must be identified systematically without randomness. In Pd-substituted ceria (Ce1−x Pdx O2−δ ), the surface oxygen vacancy was positioned near Pd metal. Attention was paid during the creation of vacancy as the distance between the metal and the removed oxygen governed the energetics and vacancy formation far from the substituted metal demanded more vacancy formation energy. 52 Dutta et al., have also reported that the near oxygens were the reason for the higher reducibility of Ce1−x Mx O2 . 59 Vacancy formation energy for CeO2 was 70.6 kcal/mol whereas for Pd substituted ceria it was 15.3 kcal/mol. Yang et al., observed vacancy formation energy to be lesser for Pd substituted ceria than that for pure CeO2 . 60 All these observations showed the correctness of the molecular models shown in Figure 1 for the analysis of surface processes during CO oxidation.

Total density of states (TDOS) and projected density of states (PDOS) were determined for detailing the electronic structures of different models shown in Figure 1. CeO2−δ and Ce1−x Pdx O2−δ showed new electronic states when compared to CeO2 , as are highlighted in Figure 2. CeO2 has semiconducting behavior and exhibits a band gap between O (2p) and Ce (4f). In CeO2−δ , because of vacancy creation, reduction of Ce4+ to Ce3+ resulted in a new gap state (shown in Figure 2(b)) and this was because of the occupation of electron in Ce (4f) orbitals, as has also been observed in previous studies. 61 Observation of ccupation

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of the valance band of Pd (4d) in Ce1−x Pdx O2−δ led to the conclusion that substitution of Pd along with vacancy leaves Pd in 2+ oxidation state. Hence the catalyst under study was ionic catalyst with the nature of the surface different from neutral Pd nanoparticle impregnated CeO2 catalysts.

Free CO and O2 were simulated in a 15×15×15 Å3 box in which they had equilibrium bond distances of 1.14 Å and 1.23 Å , respectively. These matched with previous reports. 62 Initial CO adsorption sites were selected by analyzing their positions with respect to Ce and O atoms of CeO2 . With reference to these adsorption sites, structural relaxations of CO and O2 adsorbed over CeO2−δ and Ce1−x Pdx O2−δ were performed. O2 adsorption was done on vacancy sites of CeO2−δ and Ce1−x Pdx O2−δ . Apart from the preferred adsorption sites (most favorable), we found some other sites which were further categorized based on adsorption energies on different surfaces. These have been detailed in the following text.

3.1

Adsorption of CO over CeO2

We identified three sites on CeO2 (111) plane for adsorption of CO. The three sites were: Ce atom, lattice O and bridge between Ce and lattice O. Hollow sites over (111) plane were not possible due to large interatomic distances. Adsorption of CO was tested over all the three sites. It was observed that when adsorption was set up over lattice O, CO displaced to attain an equilibrium structure over the bridge between Ce and lattice oxygen. Therefore, only two energetically unique sites were possible. They were named as CO-like (Cco ) and CO2 -like (Cco2 ) sites (see Figure 3). The Cco site offered the adsorption of CO on top of Ce; C of CO interacted with Ce with O directed away from the catalyst surface. In the Cco2 site, C of CO interacted with the lattice oxygen. The distance between Ce and C of CO was 2.92 Å for Cco adsorption. The bond length of CO after adsorption was similar when compared with the equilibrium bond length of free CO of 1.14 Å. No change in CO bond length indicated physisorption. In the other adsorption site, CO displaced oxygen from the lattice forming 7



CO2 -like structure on the surface of CeO2 . In CO2 -like structure in Cco2 , the bond distances of C-O were 1.2 and 1.3 Å. C-O bond distance in CO and CO2 molecules were 1.12 and 1.16 Å. 63 There was no stable formation of CO3 -like structures on CeO2 (111) (where two of lattice oxygens should interact with CO) because of large the distance between surface O-O of 3.9 Å. Adsorption energies on the two sites followed the order Cco (-3.2 kcal/mol) > Cco2 (2.2 kcal/mol) (see Figure 3). Hence, Cco was the most favored adsorption site on bare CeO2 . On observing bond lengths of CO in different adsorption complexes and comparing with bond lengths and the adsorption energies, it was concluded that CO adsorption over CeO2 was favored when it interacted with Ce as in the case of Cco site adsorption (Figure 3). Physisorption of CO over CeO2 was further confirmed using DOS shown in the inset of Figure 3. Physical adsorption was confirmed from the electronic structure which was similar to CeO2 before CO adsorption.

3.2

Adsorption of CO over CeO2−δ

Adsorption sites for CeO2−δ were identified with the reference to the adsorption sites over CeO2 . We could identify two positions for the creation of a vacancy with reference to the Cco and Cco2 sites. The two vacancies with Cco and Cco2 sites were categorized as vacancies near and far from adsorbed CO (see Figure 4). The adsorption site on CeO2−δ where the vacancy was far from CO (CVco2 ) offered low adsorption energy (-1.67 kcal/mol). The other adsorption site with energetically favored adsorption was named as CVco and offered an adsorption energy of -8.5 kcal/mol. O of CO tilted towards Ce atom near the vacancy in the CVco site (C-O bond distance of 1.15 Å) whereas in the CVco2 site, it interacted with the lattice oxygen. In the CVco2 site, the vacancy was far from CO, and O of CO did not interact with any the Ce atoms. C of CO in CVco was at the vacancy but in the CVco2 , CO made CO2 -like structure with bond lengths of C-O as 1.21 and 1.31 Å. The distances of C from three near Ce atoms in case of CVco were in a range of 2.9-3.3 Å.

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As mentioned before, adsorption over CVco was superior with an adsorption energy of -8.5 kcal/mol while over CVco2 it was -1.67 kcal/mol, respectively. When compared with CeO2 , CO adsorption over CeO2−δ improved, as can be seen from a comparison of the highest adsorption energies over the two compounds (-3.2 kcal/mol, Cco vs -8.5 kcal/mol, CVco ). Further, vacancy induced CeO2 did not show the CO adsorption above the cerium atom. In adsorption sites of CO over CeO2−δ , the bond lengths of 1.15 Å (CVco ) and 1.211.31 Å (CVco2 ) were larger than equilibrium bond lengths of free CO and CO2 . Raman spectroscopy studies have revealed that adsorption over reduced ceria leads to CO dissociation. Increased C-O bond length observed in this study, thus, is in agreement with the previous experimental reports. 64

To corroborate the energetics findings TDOS and PDOS (shown in Figure 5) were analyzed. There was a reduced state (Ce3+ ) in CeO2−δ -CO due to localization of electrons from CO to Ce and lattice oxygen of surface. A decrease in HOMO contribution of CO molecule was observed. CO adsorbed on vacancy (CVco ) increased its bond length. COtype adsorption over CeO2−δ was more favorable than that over CeO2 because of electrons from vacancy occupied by CO molecule. CO adsorption far away from the vacancy (CVco2 ) exhibited similar electronic state as CeO2−δ . During adsorption of CO in Cco2 and CVco2 , the contribution of Ce (4f) occupation was more in CVco2 as compared to Cco2 because of vacancy creation. By interchange of electrons between CO and surface, one can conclude that adsorption with vacancy was more favorable than adsorption without vacancy, as is also seen from energetics.

3.3

Adsorption of CO over Ce1−x Pdx O2−δ

Palladium was substituted in place of a Ce atom in CeO2−δ for modeling Ce1−x Pdx O2−δ . CO adsorbed at two different sites consisting of C of CO on top of Pd atom (PCVt ) and C of CO at vacancy (PCVv ) (see Figure 6). Consistent with the previous studies, substitution of 9



Pd in vacancy induced CeO2 changed the bond lengths such that surrounding two Ce atoms moved farther and O atoms moved closer to Pd (Pd-O = 2.15-2.18 Å) when compared to the bond distances in CeO2 . 60 C of CO adsorbed at the atop site of Pd (PCVt ) with a distance of 1.89 Å. In case of PCVt , Pd moved to the subsurface from the above atomic layer. 65,66 In case of PCVt , two nearer oxygens from Pd atom moved away with a distance of Pd-O as 3.11 Å.

Adsorption energy of CO in PCVt site was -25.88 kcal/mol and for PCVv it was -12.97 kcal/mol. Hence, atop site PCVt offered the stronger adsorption (-25.88 kcal/mol) when compared to the other site. So CO adsorption at the vacancy was not feasible when compared to atop site adsorption in Ce1−x Pdx O2−δ . The Pd-O bonds changed to 3.11 Å in PCVt site, whereas in PCVv they were 2.18 Å. This large Pd-O bond distance was an indication of creation of loose O atoms of the lattice in Ce1−x Pdx O2−δ making it potentially a good redox catalyst for the reactions like CO oxidation.

Adsorption of CO over Ce1−x Pdx O2−δ improved when compared to that over stoichiometric CeO2 . Distance from the surface of CeO2 to C of CO in atop site Cet was 2.92 Å in CeO2 while in case of Ce1−x Pdx O2−δ Pd-O distance was 1.89 Å signifying stronger adsorption over PCVt site on Ce1−x Pdx O2−δ than Ct site on CeO2 . Further, surrounding lattice oxygen atoms changed their coordinates in PCVt but not in Cco . Previous studies have shown that bond lengths for metal doped CeO2 show a broad distribution. 52 The CO-like complex over CeO2−δ (CVco ) was less stable when compared with PCVv site. Introduction of Pd improved CO adsorption when compared to CeO2−δ when adsorption took place at vacancy sites. A comparison of energetics over all the adsorption sites showed that Ce1−x Pdx O2−δ offered stronger CO adsorption (∆E= -25.88 kcal/mol) than CeO2−δ (∆E= -8.5 kcal/mol). Among the different adsorption sites, adsorption of CO was concluded to be favored over the atop of Pd in Ce1−x Pdx O2−δ , i.e., the PCVt site. 4d orbitals of Pd in Ce1−x Pdx O2−δ were the

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major contributors for the occupied electrons because of vacancy creation. After adsorption of CO on top of Pd, transfer of electrons took place from to CO molecule to the surface. This could be correlated to reduced CO distance from surface. The major electron transfer took place to Pd (4d) and not Ce (4f) (see Figure 7).

3.4

Adsorption of O2 over CeO2−δ and Ce1−x Pdx O2−δ

Molecular adsorption of O2 was studied over CeO2−δ and Ce1−x Pdx O2−δ as O2 is the second species reacting during CO oxidation and the mechanism of the reaction can be influenced by the energetics of O2 adsorption. As shown in Figure 8, at the vacancy site, the energy of adsorption of O2 over CeO2−δ (-58.75 kcal/mol) was higher when compared to that over Ce1−x Pdx O2−δ (-24.83 kcal/mol). On CeO2−δ , O atom of molecular oxygen interacted with two Ce atoms in CVov . In case of Ce1−x Pdx O2−δ , there was a single adsorption site at vacancy with two different orientations of O2 (see in Figure 8). For a Ce-O interaction (PCVov1 ), the adsorption energy was -17.54 kcal/mol while for a Pd-O interaction (PCVov2 ) it was -24.83 kcal/mol. Clearly, the importance of Pd2+ substitution can be seen here with higher stability with of O2 over the surface with Pd-O interacts when compared to that with Ce-O interacts. Adsorbed molecular oxygen had a bond length of 1.44 Å in case of CeO2−δ which was larger than the equilibrium bond length of 1.23 Å in free O2 . In case of Ce1−x Pdx O2−δ bond length of O2 was 1.38 Å. Previous studies have shown that after adsorption at vacancy, transfer of electrons to O2 leads to the formation of peroxide species (O-O bond length ≈ 1.42 Å). 67 Our study hence shows the activation of O2 on adsorption over CeO2−δ and Ce1−x Pdx O2−δ surfaces.

As compared to Ce1−x Pdx O2−δ , CeO2−δ shows adsorption because of electron transfer from the surface to O2 . Extra electrons in peroxide type species are from the reduced surface in case of Ce1−x Pdx O2−δ . Oxidation of the surface takes place due to O2 adsorption leads to transfer of electrons from the surface to O2 . Due to high electrons occupancy in Pd2+ when 11



compared to Ce3+ /Ce4+ , the configuration involving Pd-O interaction shows high stability.

3.5

Co-Adsorption of CO and O2

Another interesting factor influencing CO oxidation mechanism over ceria-based catalysts studied here was the influence of adsorption of a reactant on the adsorption energetics of the other reactant. Since vacancies were found to be necessary for the adsorption of O2 and best CO adsorption energies were observed in presence of Pd2+ , analysis of co-adsorption of CO and O2 was done only for CeO2−δ and Ce1−x Pdx O2−δ . Further, since the adsorption sites for CO and O2 were found to be distinct (CO adsorbed favorably over the metal while O2 adsorption required vacancy sites) (see in Figure 9), it was possible to study the influence of co-adsorption and simultaneous presence of the two species over the catalyst surface. Comparable energetics of CO adsorption (∆E= -25.88 kcal/mol) and O2 adsorption (∆E= -24.83 kcal/mol) over Ce1−x Pdx O2−δ indicated non-sequential adsorption of the two species i.e., the surface during the reaction may not be expected to be preferentially covered with one of the reactants only. Hence, co-adsorption of CO and O2 is expected to take place over Ce1−x Pdx O2−δ . In contrast, a large difference in adsorption energies of O2 adsorption (∆E= -58.75 kcal/mol) and CO adsorption (∆E=-8.5 kcal/mol) over CeO2−δ indicated sequential adsorption over CeO2−δ with O2 adsorption first followed by CO adsorption over O2 -laden CeO2−δ surface. The energy of CO adsorption over O2 -laden CeO2−δ surface was found to be only -0.72 kcal/mol (see in Figure 9) with CO at a double the distance as in Cco from the surface. This indicated that O2 adsorbed CeO2−δ surface was inert towards adsorption of CO. This is expected to heavily deteriorate the activity of the catalyst, as has also been experimentally reported with Ce1−x Pdx O2−δ being a superior catalyst when compared to CeO2−δ . 68 The superior catalytic activity of Ce1−x Pdx O2−δ can now ben explained on the basis of co-adsorption energetics. In Ce1−x Pdx O2−δ , CO adsorbed on lattice oxygen by displacing O2 over the vacancy (Figure 9). Co-adsorption energy of CO and O2 was -86.2 kcal/mol. This is the first study to the best of the knowledge of the authors to report that 12


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high oxidation capacity of solid solutions of ceria is not only because of metal and the vacancy sites but also because of the influence of the energetics of co-adsorption of the reactants.

3.6

Insights into CO Oxidation Mechanism

The MvK mechanism has been reported to be the most accepted mechanism for CO oxidation with metal oxides. After studying the energetics of adsorption of reactants over Ce1−x Pdx O2−δ , we found ourselves in a position to propose an alternative MvK-type mechanism for CO oxidation over metal ion substituted ceria catalysts which indicates vacancy hopping during the catalytic cycle. We compare and contrast the MvK mechanism and the proposed mechanism in detail in the text to follow.

The MvK mechanism, shown in Figure 10(a) assumes adsorption of CO over the oxidized surface followed by lattice oxygen abstraction resulting in reduced catalyst surface (Eq. 2.1). CO2 (L) in Eq. 2.1 emphasizes the fact that the resulting CO2 molecule has one of the oxygens coming from the lattice oxygen which is in correspondence with a wealth of isotope exchange reports. 69 Oxygen ion vacancies are created on the metal oxide surface which act as adsorption sites for O2 (Eq. 2.2). Replenishment of the lattice oxygen takes place by O2 from the stream re-oxidizing the catalyst surface. Active oxygen is present on the oxidized surface which reacts with CO to release another CO2 molecule. It is to be noted here that CO2 in this step has been denoted as CO2 and not as CO2 (L) to emphasize that both the oxygens in CO2 formed at this step come from the reactants and not from the lattice oxygen. Desorption of CO2 results in the completion of the catalytic cycle regaining the original catalytic oxide surface (Eq. 2.3). These processes have been depicted in Figure 10(a) and Eqs. 2.1-2.3.

Mechanism A (Mars-van Krevelen)

Ce1−x P dx O2 + CO * ) Ce1−x P dx O2−δ + CO2 (L) 13


(2.1)


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Ce1−x P dx O2−δ + O2 * ) Ce1−x P dx O2 -[O]

(2.2)

Ce1−x P dx O2 -[O]-CO * ) Ce1−x P dx O2 + CO2

(2.3)

The scheme in Figure 10(a) clearly shows the lack of vacancy hopping. Our computational results indicate that sequential adsorption, a requirement of MvK mechanism does not take place over Ce1−x Pdx O2−δ surface. Hence, on the basis of adsorption energetics results cited in the previous sections of this study, we now make a case for an alternative MvK-type mechanism. It is clear from the adsorption energetics that the best catalytic surface for CO oxidation is the one offered by Ce1−x Pdx O2−δ . Further, co-adsorption of CO and O2 gives the maximum stability to the system. Therefore, we propose that CO oxidation is initiated by co-adsorption of CO and O2 over a reduced surface (Eq. 3.1). This step is a quick succession of the two steps of Eq. 2.1 (adsorption of CO) and Eq. 2.2 (replenishment of oxygen) of MvK mechanism because of non-sequential co-adsorption.

Mechanism B (Alternative MvK mechanism proposed in this study)

Ce1−x P dx O2−δ + CO + O2 * ) Ce1−x P dx O2 -[O]-[CO]

(3.1)

Ce1−x P dx O2 -[O]-CO * ) Ce1−x P dx O2−δ -[O] + CO2 (L)

(3.2)

Ce1−x P dx O2−δ -[O] + CO * ) Ce1−x P dx O2−δ + CO2

(3.3)

In the MvK mechanism, CO2 is formed using the loosely bound oxygen. An analysis of the bond distances of the adsorbed complex strongly suggests that CO2 released in this step is the CO2 (L)-type CO2 (Eq. 3.2). The C-O bond distance with the lattice oxygen was 1.27 Å while it was 2.8 Å with the loosely bound O from molecular oxygen. A comparison of C-O bond distances would clearly indicate that CO2 -like species has already been formed at this step with lattice oxygen on the surface. The release of CO2 (L) from the surface keeps the loosely bound O intact on the surface (Eq. 3.3). The last step is analogous to previous of 14


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MvK mechanism but differs in the final surface. It can be seen from the two configurations of catalyst surfaces shown in Figure 10(b) that a migration of oxygen vacancy takes place on completion of a reaction cycle. Hence, the vacancy has been said to be “hopped" between the two sites. MvK forms stochiometric surface whereas in hopping mechanism forms reduced surface with hopped vacancy. The new position of the vacancy creates an equivalent reduced surface same as the initial reduced surface configuration, However, the new reaction cycle may lead to further migration of vacancy by hopping which is similar to the phenomenon observed during ionic conductivity.

4

Conclusions

Considering an overwhelmingly large amount of literature on CO oxidation and Pd-CeO2 based catalysts, it is apparent that Pd as the transition metal and CeO2 as a support is an excellent combination to catalyze CO oxidation reaction. However, ionicity in Pd can altogether change not only the activity of the catalyst but also the surfaces processes and intermediates over the catalyst. Hence, with an intention of developing fundamental insights into the surfaces processes and establish an exact mechanism of CO oxidation over Ce1−x Pdx O2−δ , we carried out DFT analysis and conclusively observed vital differences in the surfaces processes taking place over the studied catalysts when compared to traditionally reported ones. While involvement of oxygen vacancies is an established phenomenon during CO oxidation over reducible oxides, ours is the first study to report vacancy hopping during the reaction. To the best knowledge of the authors, there have been no mechanistic studies reporting vacancy hopping in CeO2 -based systems during CO oxidation. The currently proposed mechanism can be considered to be a variation of the well-established Mars-van Krevelen mechanism. The observed vacancy hopping phenomenon is, however, a well reported phenomenon concerning ionic conductivies of CeO2 harnessed to develop efficient fuel cell electrodes. Our report on vacancy hopping during a surface reaction is

15



expected to contribute to the development of more accurate kinetic models for CO oxidation over CeO2 -based catalysts. The major insights developed from the present computational DFT+U analysis can be summarized as follows: • Introduction of a lattice oxygen vacancy improves CO adsorption energetics when compared to that over pure CeO2 . • Substitution of a Pd ion significantly improves CO adsorption energetics. • Substitution of Pd does not improve O2 adsorption when compared to CeO2−δ . • Sequential adsorption of O2 and CO takes place over CeO2 . However, the resulting system is stable only by -0.72 kcal/mol predicting the inferior activity of CeO2−δ . • Non-sequential co-adsorption of CO and O2 takes place over Ce1−x P dx O2−δ with a large stabilization of the system by -86.2 kcal/mol. • CO oxidation mechanism over Ce1−x P dx O2−δ is initiated by co-adsorption of CO and O2 . Alternative MvK-type mechanism correctly describes the surfaces process involving lattice oxygen exchange and vacancy hopping.

We limited our analysis in the present study only to the (111) plane of CeO2 . The prime reason behind this was that the (111) plane has been reported, experimentally as well as theoretically, to be the most stable low index plane. An overwhelmingly large amount of experimental literature reports CeO2 synthesis with (111) plane exposed. Hence, this particular plane was chose for the analysis. The effect of crystal plane on activity, mechanism and vacancy hopping can be studied further to provide experimentalist with a guide to synthesize CeO2 with specifically exposed crystal planes.

16


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List of Figures 1

Three molecular systems used in this study (a) CeO2 (b) CeO2−δ (c) Ce1−x Pdx O2−δ . The dotted box represents the top three layers of adsorbent used to indicate (b) and (c) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2

TDOS and PDOS of (a) CeO2 , (b) CeO2−δ and (c) Ce1−x Pdx O2−δ . The vertical dotted line corresponds to the Fermi level . . . . . . . . . . . . . . .

3

25

26

Adsorption sites (Cco and Cco2 ) and adsorption energies for CO adsorption over CeO2 . The inset shows the TDOS and PDOS of Cco configuration during CO adsorption over CeO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . .

27

4

Adsorption sites (CVco and CVco2 ) and adsorption energies for CO over CeO2−δ 28

5

TDOS and PDOS of (a) CVco and (b) CVco2 . The vertical dotted line corresponds to the Fermi level . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6

Adsorption sites (PCVt and PCVt ) and adsorption energies for CO over Ce1−x Pdx O2−δ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7

32

Co-adsorption sites and adsorption energies for O2 over CeO2−δ (CVc/o ) and Ce1−x Pdx O2−δ (PCVc/o ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10

31

Adsorption sites and adsorption energies for O2 over CeO2−δ (CVov ) and Ce1−x Pdx O2−δ (PCVov1 and PCVov2 ) . . . . . . . . . . . . . . . . . . . . . .

9

30

TDOS and PDOS of (a) PCVt , (b) PCVv , (c) CVov and (d) PCVov2 . The vertical dotted line corresponds to the Fermi level . . . . . . . . . . . . . . .

8

29

33

Comparison of two mechanisms for CO oxidation over Ce1−x Pdx O2−δ . (a) Mars-van Krevelen mechanism (b) alternative MvK-type mechanism proposed in this study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

24


34

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Figure 1: Three molecular systems used in this study (a) CeO2 (b) CeO2−δ (c) Ce1−x Pdx O2−δ . The dotted box represents the top three layers of adsorbent used to indicate (b) and (c)

25



Figure 2: TDOS and PDOS of (a) CeO2 , (b) CeO2−δ and (c) Ce1−x Pdx O2−δ . The vertical dotted line corresponds to the Fermi level

26


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Figure 3: Adsorption sites (Cco and Cco2 ) and adsorption energies for CO adsorption over CeO2 . The inset shows the TDOS and PDOS of Cco configuration during CO adsorption over CeO2

27



Figure 4: Adsorption sites (CVco and CVco2 ) and adsorption energies for CO over CeO2−δ

28


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Figure 5: TDOS and PDOS of (a) CVco and (b) CVco2 . The vertical dotted line corresponds to the Fermi level

29



Figure 6: Adsorption sites (PCVt and PCVt ) and adsorption energies for CO over Ce1−x Pdx O2−δ

30


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Figure 7: TDOS and PDOS of (a) PCVt , (b) PCVv , (c) CVov and (d) PCVov2 . The vertical dotted line corresponds to the Fermi level

31



Figure 8: Adsorption sites and adsorption energies for O2 over CeO2−δ (CVov ) and Ce1−x Pdx O2−δ (PCVov1 and PCVov2 )

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Figure 9: Co-adsorption sites and adsorption energies for O2 over CeO2−δ (CVc/o ) and Ce1−x Pdx O2−δ (PCVc/o )

33



Figure 10: Comparison of two mechanisms for CO oxidation over Ce1−x Pdx O2−δ . (a) Marsvan Krevelen mechanism (b) alternative MvK-type mechanism proposed in this study

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TOC Graphic

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CO oxidation over Ce1âxPdxO2âÎ´ takes place via vacancy hopping

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CO oxidation over Ce1âxPdxO2âÎ´ takes place via vacancy hopping