A density functional theory study of CO oxidation on Pd-Ni alloy with sandwich structure

Applied Catalysis A: General 451 (2013) 79–85

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A density functional theory study of CO oxidation on Pd-Ni alloy with sandwich structure Freda C.H. Lim a,∗ , Jia Zhang a , Hongmei Jin a , Michael B. Sullivan a , Ping Wu b a Institute of High Performance Computing, Agency for Science, Technology and Research, 1 Fusionopolis Way, #16-16 Connexis Singapore 138632, Singapore b Singapore University of Technology and Design, 20 Dover Drive 138682, Singapore

a r t i c l e

i n f o

Article history: Received 23 May 2012 Received in revised form 7 November 2012 Accepted 8 November 2012 Available online xxx Keywords: CO oxidation Bimetallic alloy Pd alloy Pd-Ni Sandwich structure Heterogeneous catalysis

a b s t r a c t Density functional theory calculations are performed to study CO oxidations on pure Pd(1 1 1) surface as well as on its Ni alloy nanostructures. Our calculations demonstrate a dependency of catalytic properties to the Ni occupancy site in the alloys. Furthermore, our results also show that optimal compressive strain (around 5%) of the alloy maybe beneficial to lower the reaction barrier and a compressive strain beyond 7% induces a distortion on the surface that eliminate the adsorption site for the reacting species. Based on our results, we propose that the Pd-Ni alloy with sandwich structure could be a potential candidate identified for lowering the cost of Pd alloys in the catalysis of the CO oxidation reaction. This new structure illustrates a potential lowering in the CO oxidation barrier. Furthermore, our results also indicate that the partial replacement of cheaper Ni into the Pd catalyst should not adversely affect the catalytic property. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The oxidation of carbon monoxide on metal or metallic alloy surfaces is a reaction that is worth probing both from an industrial and academic point of view. In the industry, the study of CO oxidation on transition metal catalysts, pure or alloyed, has several important applications. For instance, an understanding of this reaction is important for the design of an effective and efficient guard bed catalyst to prevent the CO poisoning of the proton exchange membrane fuel cell [1–3]. Other important applications of this reaction include those in the catalytic converters for automobile in order to control the release of CO into the environment as a pollutant as well as in the control of indoor air quality [4]. From an academic point of view, carbon monoxide is often used as a probe molecule to study the reactivity of various metallic and bimetallic catalytic systems due to its simplicity [5–9]. The major limitations faced in the application of pure metal catalyst for CO oxidation are the catalyst’s tolerance to CO poisoning as well as the thermal budget of the system during operation. One of the proposed methods to overcome these issues includes the alloying or co-depositing of the primary metal with a 2nd metal

∗ Corresponding author. Tel.: +65 6419 1224; fax: +65 6419 1111. E-mail address: [email protected] (F.C.H. Lim). 0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2012.11.015

[10–13]. It has been well established that alloy bimetallic systems have properties that can be significantly different from its pure metal counterparts. The effects that alloying can bring about to the reactivity of the pure metal are two-fold: firstly, the additional metal can induce a strain in the lattice and thus modify the electronic structure of the pure metal [14–17]; secondly, the additional metal is able to further modify the electronic structure of the host metal by inducing a chemical effect in the vicinity of the guest atom. The latter is well-known as the chemical effect [18]. In a previous work, we studied the CO oxidation pathway on the surface of pure Pd as well as on the surface of Pd-Au alloy [19]. In that work, we reported that an enhancement in the rate of the CO oxidation reaction on the Pd-Au alloy is due to the ligand effects of gold present in the surface. In the presence of gold, the d-band center is being pushed away from the Fermi level. As a result, the antibonding states are filled up and the surface–adsorbate bond is weakened. This weakening of surface–adsorbate bond is responsible for the enhancement of CO oxidation rates. The strain effects of gold, on the other hand, play only a limited role. In that work, we proposed a Pd-core-Au-shell catalyst structure as a potential catalyst for the reaction. However, since both gold and Pd are expensive metals, it would cost a lot even if a gold layer on Pd makes a good catalyst for this purpose. In order to reduce the cost of the catalyst for industrial scale catalytic purposes, we would need to alloy Pd with a cheaper metal. Thus, we have looked at the alloying of Pd with cheaper metals such as Ni, Cu and Co.

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In this work, we present our density functional theory calculations study of the alloy formed by Pd and Ni. While it has been reported that the presence of Ni in Pd-Ni alloy is able to enhance reaction rates and selectivity [4,20], the effects of Ni on Pd-Ni alloy for CO oxidation are not clear. In some studies, Ni is known to enhance the rate of reactions by encouraging Pd binding to the support materials [21]. In another, it is the geometric effects caused by the dilution of Pd with Ni that result in the rate enhancement [20]. Yet, there are also experimental studies showing that the presence of Ni neither enhance nor deteriorate reaction rate [22]. In our calculations, we have simplified models of the alloy so as to provide an understanding of how the two isolated factors, chemical and strain, might affect the reactivity of the otherwise pure metal. Calculations presented here are performed on pure metal slabs, strained slabs and pseudomorphic alloy slabs. 2. Methodology Density functional pseudo-potential plane wave calculations have been used in this study. All the calculations were performed using the Quantum-ESPRESSO package. [23] The electron exchange correlation is approximated with the Perdew–Burke–Ernzerhof generalized gradient approximation functional [24]. The ion cores are treated using ultrasoft pseudopotentials. The Kohn–Sham equations are solved using a plane wave basis set with a converged cutoff energy of 50 Ry and electronic states were sampled at 25 k points described by a (7 × 7 × 1) Monkhorst–Pack k-point grid [25]. The (pure metal and alloyed) surfaces are modeled using a slab containing five layers of metal atoms. The bottom two layers are fixed at ideal bulk-like positions, while the topmost three layers are fully relaxed without any constraints. Adsorption and oxidation are modeled on the top most surface of the slab. The vacuum ˚ A (2 × 2) super-cell is used. Geomthickness is approximately 10 A. etry optimization calculations were considered converged when ˚ the forces on all free atoms were less than 0.01 eV/A. The potential energy surface of CO and O on each slab was mapped out by allowing adsorbates to begin optimization in the top, bridge, fcc and hcp site on both the pure and alloyed surface. The adsorption energy, Eads , of the surface species and the reaction energy, Erxn , were calculated respectively as follows: Eads = Eadsorbed+surf − Eadsorbate − Esurf where Eadsorbed+surf refers to the energy of the system with the adsorbate on the surface. Eadsorbate refers to the energy of the stand-alone adsorbate species in vacuum. Esurf refers to the energy of the clean surface. Erxn = Efinal-product − Einitial-reactant where Efinal-product refers to the total energy of the system at its final state. Einitial-reactant refers to the total energy of the system at its initial state. The co-adsorption energy is calculated similarly as the Eads above. In the case of the co-adsorption energy, the term Eadsorbate+surface refers to the energy of the system with both adsorbate co-adsorbed on the surface and Eadsorbate refers to the sum of the energy of the individual adsorbate species in vacuum (in this

Fig. 1. An illustration of the various possible types of adsorption sites that adsorption could occur on a (1 1 1) surface. (a) HCP hollow site. (b) FCC hollow site. (c) Top site. (d) Bridged site.

work, it refers to the energy of O atom in vacuum and energy of CO molecule in vacuum). In trying to understand how our catalytic system affects the adsorption energy of the adsorbates and thereby the catalytic activity, we looked at how the value of the d-band center is modified by a change induced in the system either by adding a foreign atom or by changing the lattice constant of the system. We calculate our d-band center based on Friedel’s square d-band model [26]. In that model, the d-band is represented by a rectangular density of states plotted with respect to the energy width. The center of d-band is then defined as the energy value which exactly divides the density of states into half. In the same manner for a nonrectangular density function, we can also find a value of energy, Ed , which divides the area under the density plot into two equal halves. In the search of the minimum energy path and the transition states the constrained optimization scheme is used. In the constrained optimization scheme [27], the reaction coordinate constrained is the O CO separation up to a critical separation and in the exit valley of the reaction, we constrained the height of the CO2 molecule formed from the reaction. A series of geometries were constrained-optimized in a grid between the O CO distance and the height of the CO2 molecule to find the critical O CO distance. 3. Results and discussion 3.1. Adsorption, co-adsorption of CO and O on the pure metals surface It is now widely accepted that the CO oxidation reaction on Pd(1 1 1) surface follows the Langmuir–Hinshelwood mechanism [28–32]. Thus, in order to trace the pathway for this reaction, we investigate the adsorption and co-adsorption of CO and O species on the metal surfaces. We have calculated the adsorption energies, Eads , of O and CO at various adsorption site on pure Pd(1 1 1) and Ni(1 1 1) surfaces. The four possible adsorption sites that we calculated are illustrated in Fig. 1. The difference in the two hollow-sites is that the hcp-hollow site sits directly above the second layer atom while the fcc-hollow site sits above the third layer atom. Table 1 summarizes our calculated adsorption energies on the pure Pd and pure Ni surface. Our results agree well with calculations and experiments reported in literature [19,33–35]. On both Pd and Ni surfaces, the most stable adsorption site for an oxygen atom is clearly the fcc-hollow site while the most stable adsorption site

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Table 1 Consolidated values of adsorption energy for various adsorption sites. The data from our current work is compared to the experimental and calculated values reported in literature. Data source

O adsorption energy/eV

CO adsorption energy/eV

fcc

hcp

bri

top

fcc

hcp

bri

top

Pure Ni

Current Work Ref [31] (cal.) Ref [32] (MC expt)

−5.54 −5.27 −4.48

−5.37

−5.51

−3.55

−2.04 −1.88 −1.35

−2.06

−1.92

−1.62

Pure Pd

Current Work Ref [31] (cal.) Ref [18] (cal.) Ref [33] (TPD expt)

−4.08 −3.98 −4.63

−3.93

−4.08

−3.81

−1.81 −1.96 −1.85 −1.47

−1.82

−1.66

−1.23

for the carbon monoxide molecule can either be the fcc-site or the hcp-site as the difference in adsorption energy is of the same order of magnitude as the errors in these calculations [36,35,37–46]. Oxygen atom is adsorbed more strongly than the carbon-monoxide molecule on both surfaces. This potentially makes the oxygensurface adsorption a property we can use to gauge catalytic activity. It has been previously suggested that the reaction barrier is related to the adsorption strength of oxygen on the surface [31,47,48]. Thus for most part of the discussion that follows in the next section, we will be using the adsorption energy of oxygen atom as an indication of catalytic activity.

3.2. Adsorption of CO and O on Pd-Ni surfaces Based on the adsorption strength for oxygen on the pure metals surface, we would expect CO oxidation to be faster on Pd than alloy of Pd-Ni by chemical intuition, since adsorption on Ni is stronger than adsorption on Pd. However, this is not the case. The catalytic activity of the alloy depends on the position and content of Ni in the alloy as well. Thus, in order to have an understanding of the effects of Ni in the catalytic properties of Pd-Ni alloys, we have built our alloy models mainly to isolate the two main known effects affecting reaction rates–the chemical effect and strain effect of a foreign atom. We investigate the chemical effect of Ni by keeping the lattice constant of the slab to that of Pd’s and varying the concentration and positions of Ni in the slab. Ni atoms are placed on the surface as well as in the underlying layers. We have slabs that include 1, 3, 4, 8, 12 and 16 Ni atoms in a 20 atom slabs. The effects of strain on the catalytic performance of Pd were studied by varying the lattice constant in the range ±20% that of Pd lattice constant. Other than the fictitious 20% and 10% compressive strain imposed on the slab, there are also Pd slabs that has lattice constant with the compositions, Pd5 Ni15 , Pd10 Ni10 and Pd15 Ni5 according to Vegard’s law [49], an approximate empirical rule that states the linear relationship between the concentration of the alloy and the lattice parameter. Similarly, for tensile strain, we have created strained slab with the lattice constant enlarged from about 1% up to 20%.

3.2.1. The chemical effect of Ni on Pd Table 2 shows a tabulation of the adsorption energies and dband center shifts with respect to Fermi-level of various slabs that we have investigated. In this table, the d-band center shifts are calculated for a clean-surface Pd atom in an alloy slab. The adsorption energy on Pd slab systems with 0, 1, 3, 4, 8, 12 and 16 Ni embedded in the slab respectively is compared. For the slab with only 1 Ni atom, the Ni resides either on the surface or in the 2nd layer. For the slabs with more Ni atoms, the Ni atoms are positioned in the 2nd, 3rd or bottom 2 layers. Fig. 2 illustrates how our models look like layer by layer. These slabs use the lattice constant of pure Pd. On all these surfaces, the oxygen atom is adsorbed more strongly than the CO molecule. Compared to the pure Pd surface, the adsorption energy on the various Pd-Ni surfaces varies with the number of Ni and the proximity of Ni to the surface. When Ni is on the surface, both surface species are adsorbed more strongly than on pure Pd. This is due to the stronger O Ni interaction compared to O Pd interaction. However, when Ni is present in the subsurface layers, the adsorption of the surface species becomes progressively weakened. The strength of the adsorption decreases with increasing number of Ni atoms present in the subsurface layer immediately below the surface. Adsorption becomes the weakest when Ni fills up the entire second layer. On the other hand, if we fill up the subsurface layer that is 2 layers below the surface with Ni, as in Fig. 2(ii), with the rest of the layers being Pd, the adsorption energy of O and CO does not change as drastically as in the case when it was placed in the immediate subsurface layer. We try to understand the weakening of the O Pd surface adsorption from two perspectives. Firstly, from the variations of the electron (re)distribution induced by the Ni as demonstrated by the change in the d-electrons density of states and secondly, from the amount of charge transfer or electronic polarization induced on the surface–adsorbate bond in the presence of Ni in the substrate. The last column of Table 2 shows the d-band center values of the clean surface Pd-atoms with respect to the Fermi level as we increase the number of Ni in the subsurface layers. From our calculations and plots of the clean and chemisorbed surfaces, we can observe qualitatively that the width of the d-band does not vary as we add Ni, but the center of gravity of the d-band shifts away from

Table 2 A tabulation of the adsorption energies and d-band center value on various slabs with Ni in the underlying layers. d-band center shifts (Ed − Ef )

E(ads)

Pd-Ni (1 Ni on surface) Pure Pd Pd-Ni (1 Ni-2ndL) Pd-Ni (3 Ni-2ndL) Pd-Ni (4 Ni-2ndL) Pd-Ni (4 Ni-3rdL) Pd-Ni (8 Ni-2nd&3rdL) Pd-Ni (16 Ni-all bulk)

O(fcc)

CO(fcc)

−4.76 −4.08 −4.06 −4.03 −3.90 −4.02 −3.84 −3.86

−1.98 −1.81 −1.77 −1.68 −1.48 −1.78 −1.46 −1.49

−1.39 −1.72 −1.75 −1.87 −2.04 −1.85 −1.99 −1.87

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Fig. 2. This is an illustration of the slabs we used to test the effect of adding more layers of Ni in the underlying layers to the adsorption energy. All these slabs have lattice constant of pure Pd. (i) Pd-Ni (4 Ni-2ndL): refers to slabs with 4 Ni atoms in the 2nd underlying layer. (ii) Pd-Ni (4 Ni-3rdL): refers to slabs with 4 Ni atoms in the 3rd underlying layer. (iii) Pd-Ni (8 Ni-2nd&3rdL): refers to slabs with 8 atoms in the 2nd and 3rd underlying layer. (iv) Pd-Ni (12Ni Pd-3LNi-Pd sandwich): refers to slabs with 3 layers of Ni sandwiched between 2 Pd layers) (v)Pd-Ni (16Ni-all-bulk): refers to slabs with 16 atoms in the bulk. (Not shown in this figure, we have data to show that Ni prefers to be in the underlying layers on a bare surface; 1 Ni atom in the topmost surface layer is 0.34 eV less stable than 1 Ni atom embedded in the 2nd layer.).

the Fermi level. (See supplementary information for plots.) As the d-band center moves further away from the Fermi-level, we see a weakening of the surface–adsorbate bond both from the reduction of the Eads as well as in the elongation of the Pd O bond length. This observation is consistent with the trends reported in literature [50–52]. Our plots of the variations in the density of d-states about a surface Pd-atom as well as that of the interaction between the O p-orbital and the Pd d-orbital can be found in the supplementary information. On the other hand, the variations in the Lowdin charge density on the various atoms involved in the Pd O bonding can also be used to explain the reduction in adsorption energy between O and the surface Pd atom. In order to illustrate this, we need to assume the bonding between Pd and O to be that between the Pd 4d and 5s hybrid orbitals and that the O p orbitals. It is then clear that when a Ni atom, which has a lower electron workfunction than Pd, is added to the bulk, its charge spills over to the 5s orbitals of Pd and makes Pd less available for bonding with the adsorbate. Data showing the Löwdin charges on O and Pd and the adsorption energy of Pd on O are shown in Fig. 3(a) and (b). We can see from Fig. 3 that the two trends plotted against the number of Ni atom added to the subsurface layer of the slab corresponds well with each other. This indicates that the spilling of charge from Ni to Pd is another key factor affecting the adsorption strength of O on Pd. Beyond the immediate subsurface layer as we can see from Table 2, the adsorption energy does not decrease much with increasing the number of underlying Ni layers. That is to say, the effect of Ni is confined to the immediate layer just below the surface Pd. This is consistent with the fact that when we place 1 layer of Ni in the 3rd layer as in Fig. 2(ii), its effect on surface adsorption is not strong (see Fig. 2 Pd-Ni(4Ni-3rdL)). However, it is important to point out here that additional layers of Ni beneath the Pd would not be detrimental to the catalytic performance of the alloy. Since Ni is significantly cheaper than Pd, it is tempting to propose a pseudomorphic layer of Pd on Ni lattice as a possible candidate for catalysis of the CO oxidation. However, as we shall see in the later section, the optimum Ni content is limited by the lattice mismatch between Ni and Pd, and pseudomorphic Pd on Ni would not be feasible. Since Ni has a smaller lattice constant compared to Pd, the pseudomorphic Pd on a Ni lattice will be undergoing compressive strain. According to reports in the literature, it was shown that compressive strain is able to reduce the strength of the surface–adsorbate interaction. The result of our strained Pd investigation is shown in Fig. 4. 3.2.2. The effect of strain on Pd For the purpose of discussion, tensile strain will be denoted by a “+” sign and compressive strain will be denoted by a “−” sign.

Our results are consistent with what is reported in the literature [50–52], the d-band center shifts vary linearly with respect to the amount of strain applied to the slab. As can be seen in Fig. 4, the d-band center will shift further away from the fermi level under compressive strain, while the converse is true under tensile strain. These shifts in d-band center are expected to affect the strength of the surface–adsorbate bond. We have calculated the adsorption energies of O atom and CO molecule and plotted the former against the d-band center shifts as strain is being applied. From Fig. 4, we can see that there is a linear

Fig. 3. (a) A plot of Löwdin charge variation on a surface Pd atom induced by the presence of Ni in the underlying layer. (b) A plot of adsorption energy on of O on Pd with respect to the number of Ni added to the underlying layer. These two plots indicate that the amount of charges spilling over from Ni to Pd is strongly correlated to the weakening of the chemisorptions between O and Pd.

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Table 3 Consolidated values of co-adsorption energy for the 2 reacting surface species in two different configurations. Co-adsorption energy (GGA-PBE)/eV

O(fcc) + CO(fcc)/Pd(1 1 1) O(fcc) + CO(top)/Pd(1 1 1)

Fig. 4. A plot of d-band center shifts and adsorption energy change with respect to the amount of strain applied to the slab. 0% strain refers to a slab of Pd with Pd lattice constant while −10% strain refers to a slab of Pd with lattice constant reduced by 10% (i.e. compressively strained) and +10% strain refers to a slab whose lattice constant expanded by 10% (i.e. tensile strain). Compressively straining the system beyond 5% introduces distortions in the lattice which removes the fcc site for adsorption. It is clear from this plot that the adsorption of O onto the surface Pd gets weakened as the d-band center moves nearer to the Fermi level.

correlation in the d-band center with an increasing tensile strain between about −4.75% and +10% strain. However when the strain goes beyond −4.75%, the structure of the slab becomes distorted by the adsorption of a surface species. This adsorption induced distortion is due to strain relaxation and cause the fcc site to become non-existent. Indeed when we compute the adsorption energy of O on a slab of pseudomorphic Pd on Ni lattice (∼9% compressive strain), we found that the slab became distorted after O adsorption Thus, pseudomorphic Pd is a bad choice as a candidate for CO oxidation due to the excessive strain. In agreement with the observation reported in the literature [53], the tensile and compressive strain that we applied to our system lead to a shift of the d-band center as well as a change in the d-band widths. In particular, the d-band width shrinks as we expand the lattice and conversely, the d-band width widens when we shrink the lattice. To keep the number of d electrons fixed, the dband center will therefore move toward the Fermi level as we apply a tensile strain and conversely, the d-band center moves away from the Fermi level as we apply a compressive strain. It is clear from Fig. 4 that the adsorption energy of the O species decreases in magnitude as the d-band center moves away from the Fermi level with compressive strain. For a heterogeneous catalytic reaction, the reaction rate is affected by the strength of the bonds between surface species and the substrate surface. For CO oxidation on the alloy, the reaction rate will be enhanced when the strength of the surface–adsorbate bond is weakened to a certain extent with respect to that on pure Pd. Thus, we would expect that by applying a certain degree of compressive strain to the alloy, we would be able to see an improvement in the catalytic property. One of the methods to compressively strain a metal is to pseudomorphically deposit it on another metal with a smaller lattice constant. For example, a Pd-Ni pseudomorphic alloy or a Pd-shell-Ni-core alloy structure. With these Pd-Ni alloys, we would expect an improved catalytic property over pure Pd alone. That is because, compressive strain will be induced by the smaller Ni lattice constant and at the same time there would not be surface Ni to push up the d-band center. However, we also found a limit to the beneficial effect of compressive strain. A synergy, however, can be achieved by making a surface sandwich structure where Ni is present in the immediate underlayer but not as the bulk for Pd to pseudomorphically adsorbed on. The

Ref [18]

Current work

−5.91 −5.79

−5.28 −5.16

amount of strain can be controlled to less than −5% if the composition is roughly 1:1 Pd-Ni. Hence, the surface-sandwich Pd-Ni-Pd structure is potentially a cheaper and better candidate for catalyzing the CO oxidation reaction. The idea of a sandwich structure has been previously proposed for Pt-Ni-Pt system by two independent molecular dynamics [54] and Monte Carlo simulations [55]. Such Pt-Ni-Pt sandwich systems have also been experimentally made [56]. Incidentally, recent molecular dynamics simulations found that such surface-sandwich structures are favorable over the Ni-Pd core-shell structure. In that work, they investigated the interdiffusion and structural transformations at 1000 K in an initial core-shell nanoparticle and they found that the Ni-Pd core-shell structure will eventually equilibrate to give such a surface-sandwich structure where the Ni atoms mostly accumulate in a layer just below the surface [57]. We have also calculated the energetic change of PdNi(monolayer)-Pd sandwich structures according to the position of the Ni layer in the slab. Our calculations show having 1 full layer of the Ni on the sub-surface layer of the slab (be it in the 2nd or 3rd layer) is about 1.5 eV more stable than having the 1 layer of Ni on the top-most surface of the slab. This results show that the sandwich structure is indeed energetically more favorable than the Pd-Ni(monolayer) counterpart. Based on our results, we propose a surface-sandwich structure of Pd-Ni-Pd as a potential candidate to catalyze the CO oxidation reaction. This is because the presence of Ni is beneficial to the catalytic property and can significantly lower the cost of the catalyst. Yet, a surface-sandwich structure would mean that the strain can be controlled to within the tolerable limit before distortion takes place. We calculated the reaction minimum energy path for such a structure and this is presented in the next section. 3.3. Co-adsorption of O and CO on the surface and the minimum energy path for the CO oxidation on Pd and Pd-Ni We calculated the co-adsorption of these two surface species and compared our energies to the other calculated values that are available. Table 3 shows the consolidated values of co-adsorption energies for the two most-reported configurations. Our results are qualitatively consistent with what we [19] and others [41] have reported in the literature, where the most stable co-adsorption configuration is when both O and CO are in the fcc position. Experimentally, it has been reported that the activation energy of the CO oxidation reaction is coverage dependent. At low coverages, when CO occupies only the hollow-sites, the barrier was reported to be 1.17 eV with a pre-exponent of 1011 ML−1 s−1 . However, at higher coverages, when CO starts to occupy the topsite, the reported barrier is about 0.6 eV with a pre-exponent of 106 ML−1 s−1 [58]. Calculations that are reported in literature show results that are qualitatively consistent with experiments. Calculated barrier for the reaction from the initial state of O-fcc co-adsorbed with CO-top has a lower barrier compared to that for the reaction from the initial state of O-fcc co-adsorbed with CO-fcc. This can be attributed to the lower adsorption energy for CO in the top site than in the fcc site. Fig. 5(a) shows the plot of the minimum energy path from the energetically favored Ofcc COfcc configuration, where both

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Table 4 A consolidated table of the oxidation barriers that are reported in literature. Oxidation barriers/eV GGA-RPBE

O(fcc) + CO(fcc)/Pd(111) O(fcc) + CO(top)/Pd(111) O(fcc) + CO(fcc)/Pd-Ni-Pd sandwich O(fcc) + CO(top)/Pd-Ni-Pd sandwich

GGA-PBE-NEB

GGA-PBE-CINEB

GGA-PBE-RIDGE

Ref [57]

Ref [58]

Ref [18]

Current work

1.4 0.78

1.36 1.06

0.87 0.75

0.7 0.6 0.55 0.55

reacting species are adsorbed in the fcc-hollow position. This initial configuration corresponds to a situation where coverage is low. We also looked at the Ofcc COtop configuration, where the O is adsorbed in the fcc-hollow position while the CO is adsorbed on the top position. This second initial configuration corresponds to a situation of high coverage where the fcc site is not readily available. Our result is consistent with those previously reported, showing that the CO oxidation barrier is greater for the initial configuration Ofcc COfcc where both O and CO coadsorbed in fcc site than the path starting from the Ofcc COtop configuration, where the O is adsorbed in the fcc hollow position and the CO is adsorbed on the top position. The former pathway has an activation barrier, Ea , of 0.7 eV (Erxn = −1.22 eV) while the latter has a barrier of 0.6 eV (Erxn = −1.34 eV). A consolidated table of oxidation barriers reported in the literatures can be found in Table 4. On the pure Pd surface, the former initial

configuration is energetically favored while on the alloy surface, both initial configurations are equally stable. We computed the minimum energy path for the CO oxidation reaction for both initial structures on the surface of a Pd-Ni-Pd surface-sandwich slab and compared it with the minimum energy path on pure Pd; this is shown in Table 4. The model of the slab is made up of 5 layers and each layer has only one type of metal (Pd-Ni-Ni-Pd-Pd). The first layer is fully Pd while the 2nd and 3rd layer is fully Ni. These 3 layers are allowed to relax. The bottom most 2 layers are fully Pd and are fixed at Pd bulk-like positions with lattice constants of Pd. Our calculation shows that with the incorporation of Ni into the subsurface layer of the slab forming the surface-sandwich structure the rate of CO oxidation will potentially be enhanced by a reduced activation barrier. While the role of Ni in the immediate subsurface layer improves the overall catalytic performance, the role of Ni in the third layer helps to lower the cost of the Pd catalyst. The combined effect makes Ni a good replacement metal for the Pd catalyst. More importantly, our results also indicate that the addition of cheaper metal Ni to replace expensive Pd does not adversely affect the performance of the catalyst for CO oxidation and could potentially reduce the oxidation barrier compared to the pure Pd case. And because the activation barrier (Ea = 0.55 eV) and reaction energy (Erxn = −1.70 eV) is the same for both initial configuration, the rate of the reaction is expected to be similar from both high coverage (with the O on the fcc site and the CO on the top site) and low coverage (with both O and CO on the fcc site) initial structure. 4. Conclusion

Fig. 5. (a) Minimum energy path for the CO oxidation reaction at high coverage sites (initial structure O-fcc CO-fcc). Our calculations show that by incorporating Ni in the bulk of Pd and forming a Pd-Ni-Pd surface-sandwich structure, the reaction rate is not adversely affected and we can potentially reduce the barrier by about 20%. (b) Structures of the initial state (I.S.), transition state (T.S.) and final state (F.S.) for the pure Pd slab (top row) and the Pd-Ni-Pd sandwich slab (bottom row).

In summary, density functional theory calculations are used to study the effects of Ni on the catalytic properties Pd-Ni alloys for the CO oxidation reaction. While pseudomorphic Pd on Ni is expected to be good due to the compressive strain that might be induced on the surface Pd, we have found that its performance is counterintuitive due to the distortion that the excessive strain can induce. Extensive strain and chemical effect study with d-band center results helped us understand why. Our results indicate that the chemical effects of Ni are twofolds. Firstly, the presence of Ni in the subsurface layer of the slab induces a shift in the d-band center. It causes the d-band center to move further away from the Fermi level and thereby weakening the O-surface interaction. Secondly, the presence of Ni in the subsurface layer also polarizes the Pd O bond by spilling charges over to the surface Pd atom such that it weakens the surface–adsorbate bond. We have found that by putting Ni in the underlying layers, the catalytic performance of the material can be enhanced via the weakening of the O-surface bond. On the other hand, we have also found that even as the presence of Ni in the underlayer is beneficial, excessive amount of Ni in the underlayer may induce excessive amount of strain that can induce distortions in the material. Instead of enhancing reaction rate by weakening bond, it can destroy surface symmetry and make the fcc-surface–adsorption site non-existent.

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Base on our results, we propose that there is a limit in using compressive strain to reduce the surface–adsorbate bond strength and that the limit lies somewhere between 5 and 7% strain. When the metal alloy is compressively strained beyond the critical limit, as is the case for pseudomorphic Pd on Ni (∼9% compressive strain), the surface is significantly distorted upon adsorption and as a result the catalytic performance of the material for CO oxidation can be adversely affected. In the same way, we can generalize the results and extend it to other alloys of Pd. For compressive strain to yield benefit for CO-oxidation reaction, other low cost alloys of Pd, e.g. Pd–Co and Pd–Cu, are expected to be good if compressive strain is kept within 5–7%. Acknowledgements This work was supported by the Science and Engineering Research Council of A*STAR (Agency for Science, Technology and Research), Singapore. We gratefully acknowledge the provision of computing facilities by the A*CRC (A*STAR Computing Resource Center). We would also like to thank Professor Bill Goddard, Dr. William Yim W. L. and Ms Emmeline Yeo Peishan for their time and useful discussions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata. 2012.11.015. References [1] M.M. Schubert, M.J. Kahlich, H.A. Gasteiger, R.J. Behm, J. Power Sources 84 (1999) 175–182. [2] D. Cameron, R. Holliday, D. Thompson, J. Power Sources 118 (2003) 298–303. [3] T.V. Choudhary, D.W. Goodman, Catal. Today 77 (2002) 65–78. [4] G. Srinivas, J. Wright, C.S. Bai, R. Cook, Stud. Surf. Sci. Catal. 101 (1996) 427–433. [5] X. Shen, D.J. Frankel, J.C. Hermanson, G.J. Lapeyre, R.J. Smith, Phys. Rev. B 32 (1985) 2120–2125. [6] P.J. Schmitz, H.C. Kang, W.-Y. Leung, P.A. Thiel, Surf. Sci. 248 (1991) 287–294. [7] M. Ruff, S. Frey, B. Gleich, R.J. Behm, Appl. Phys. A 66 (1998) S513–S517. [8] M.Ø. Pedersen, S. Helveg, A. Ruban, I. Stensgaard, E. Lægsgaard, J.K. Nørskov, F. Besenbacher, Surf. Sci. 426 (1999) 395–409. [9] W.-L. Yim, T. Klüner, J. Phys. Chem. C 114 (2010) 7141–7152. [10] H.A. Gasteiger, N. Markovic, P.N. Ross, E.J. Cairns, J. Phys. Chem. 98 (1994) 617–625. [11] M. Watanabe, H. Igarashi, T. Fujino, Electrochemistry 67 (1999) 1194–1196. [12] G. Avgouropoulos, T. Ioannides, Appl. Catal. B: Environ. 56 (2005) 77–86. [13] K. Wang, H.A. Gasteiger, N.M. Markovic, P.N. Ross, Electrochim. Acta 41 (1996) 2587–2593. [14] M. Mavrikakis, P. Stoltze, J.K. Nørskov, Catal. Lett. 64 (2000) 101–103. [15] M. Mavrikakis, B. Hammer, J.K. Nørskov, Phys. Rev. Lett. 81 (1998) 2819–2822.

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