Effects of Adsorbates (CO, COH, and HCO) on the Arrangement of Pd

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Effect of Adsorption Structures of Adsorbates (CO, COH, HCO) in Adsorbate-Induced Migration of Pd Atoms in PdCu(111) Allan Abraham Bustria Padama, Anna Patricia S. Cristobal, Joey D. Ocon, Wilson Agerico Diño, and Hideaki Kasai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02794 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 24, 2017

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The Journal of Physical Chemistry

Effects of Adsorbates (CO, COH, HCO) on the Arrangement of Pd Atoms in PdCu(111) Allan Abraham B. PadamaȚ*, Anna Patricia S. CristobalȚ, Joey D. OconΠ, Wilson Agerico DiñoΓ,Υ,Λ, Hideaki KasaiΨ,Ξ,Φ Ț

Institute of Mathematical Sciences and Physics, College of Arts and Sciences, University of the Philippines Los Baños, College, Laguna 4031, Philippines Π

Laboratory of Electrochemical Engineering (LEE), Department of Chemical Engineering, University of the Philippines Diliman, Quezon City 1101, Philippines Γ

Department of Applied Physics, Osaka University, Suita, Osaka 565-0871, Japan

Υ

Center for Atomic and Molecular Technologies, Osaka University, Suita, Osaka 565-0871, Japan

Λ

Joining and Welding Research Institute, Osaka University, Ibaraki, Osaka 567-0047, Japan Ψ

National Institute of Technology, Akashi, Hyogo 674-8501, Japan

Ξ

Center for International Affairs, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan 

Φ

Institute of Industrial Science, The University of Tokyo, Meguro-ku, Tokyo 153-8505, Japan

ACS Paragon Plus Environment

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ABSTRACT: We investigated the arrangement of Pd atoms in PdCu(111) when CO, COH, and HCO are introduced as adsorbates, by performing density functional theory (DFT) based calculations. We modeled several Pd alloyed Cu(111) surfaces, i.e., PdCu(111), by substituting small numbers of Cu atoms with Pd atoms in the topmost and subsurface layers of Cu(111). The arrangement of Pd atoms in the presence of adsorbates is evaluated by comparing the energy profiles of adsorbate–PdCu configurations with aggregated and non-aggregated surface and subsurface Pd atoms. In clean PdCu(111) surface, the Pd atoms prefer the non-aggregated arrangement. In the presence of the adsorbed molecules, however, we found that the Pd atoms will favor the aggregated configuration. CO and HCO adsorption structures are determined by the coordination of Pd atoms in the topmost layer. Their adsorption energies do not depend on the number of Pd atoms in the topmost layer alone, but is also influenced by subsurface Pd atoms. On the other hand, COH is always stable on the fcc hollow site but will prefer higher number of Pd atoms in the topmost layer. We therefore conclude that the adsorption structure of the molecules influences the arrangement of Pd atoms in PdCu surfaces.


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INTRODUCTION PdCu surface is widely studied because of its use as catalyst for hydrocarbon formation1-3 and as membrane for hydrogen permeation and purification.4-7 PdCu allows hydrogen permeation, exhibits good hydrogen selectivity8, and resists contamination,9 which makes it a good material for hydrogen separation membranes. Also, its good selectivity and reactivity towards CO and CO2 hydrogenation facilitates its application in methanol synthesis.3,10 The importance of PdCu surfaces in catalytic reactions and membrane applications motivates researchers to further study its properties and behavior. Previous works related to this bimetallic surface focused on its stability and properties depending on the compositions of Pd and Cu, and on its performance towards the adsorption and synthesis of different molecules.7,11-19 One of the fundamental processes happening in the adsorbate–PdCu surface systems is the induced segregation of the surface atoms due to the presence of adsorbate. Segregation is the enrichment of the surface by one component of the alloy. In PdCu alloy, it is expected that Cu will segregate to the surface region due to the lower surface energy of Cu as compared to Pd.20 Studies reported however that Pd atoms can still exist in the surface region due to the strong Cu– Pd interaction.21-23 Interestingly, a recent experimental study reported that Pd atoms segregate to the surface region of the alloy surface when CO is adsorbed on the surface.24 In addition, it was concluded that the segregated Pd atoms improve the catalytic properties of the bimetallic surface for acetylene production.24 Likewise, other works have reported the adsorbate-induced Pd segregation in CuPd.1,25 In a previous study, we investigated the CO-induced segregation of Pd in Cu3Pd(111) by performing density functional theory (DFT) calculations.23 We analyzed the electronic properties of the surface atoms and verified the strong Cu–Pd interaction in clean Cu3Pd(111). We also observed that the presence of CO promotes strong Pd–Pd interaction and


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that subsurface Pd atoms enhance CO adsorption energy. From these results, we concluded in our previous work that the preference of CO to interact with Pd over Cu atoms drives the segregation of Pd. Several studies have shown that for a Cu surface alloyed with small amounts of Pd, Pd atoms are surrounded by Cu atoms in the topmost layer and in the subsurface region of the host Cu surface.7,26,27 Since Pd atoms are to segregate to the surface of PdCu in the presence of adsorbates, we see the need to further consider the influence of adsorbates on the arrangement of Pd atoms in PdCu surface. Interestingly, a recent work reported that the adjoining arrangement of surface and subsurface Pd atoms in PdCu can effectively reduce the activation barrier of H2 dissociation.28 Adsorption of molecules on the surface, which might affect the arrangement of Pd atoms in PdCu, however, is not addressed in previous works. This serves as our motivation to perform further investigations on this system. Furthermore, the adsorption of molecules on Cu surface doped with small amount of Pd will provide helpful information that can contribute to the goal of reducing the amount of precious metals in materials. In this work, we study the effect of CO, COH and HCO adsorption on the arrangement of Pd atoms in various PdCu(111) surfaces by performing DFT based calculations. These molecules are key species in the formation of hydrocarbons with the aid of catalyst.29 Specifically, we analyzed the electronic properties of the clean PdCu(111) and adsorbate–PdCu(111) systems to explain the energetically favored structures. The arrangement of Pd atoms is determined from the calculated energy profiles of different PdCu surfaces with and without adsorbates. We believe that the findings of our study will provide important insights on the behavior of PdCu atoms in the presence of adsorbates, which may be useful in different technological applications.


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COMPUTATIONAL DETAILS We performed first principles calculations based on density functional theory (DFT) as implemented in Quantum ESPRESSO.30 We utilized the generalized gradient approximation (GGA), using the Perdew–Burke–Ernzerhof (PBE) functional,31 to treat the exchange-correlation effect. The ionic cores were described by the projector augmented wave (PAW) method,32 while the Kohn-Sham one-electron valence states were expanded in a basis of plane waves with energy cutoff of 550 eV. Tierney et al. showed via scanning tunneling microscope (STM) image that in PdCu(111) with very small amounts of Pd (~0.01 ML), the Pd atoms are present in the surface and subsurface regions. 26 Concentration of Pd atoms in the surface and the subsurface depends on deposition temperature. Low temperature (350 K) depositions of Pd on Cu(111) result in more Pd atoms in the topmost layer than in subsurface. On the other hand, high temperature depositions (500 K) of Pd on Cu(111) yield more Pd atoms in the subsurface region than in topmost layer. We modeled PdCu(111) surfaces based on these experimental findings. First, we calculated the equilibrium lattice constant of bulk Cu. We obtained a value of 3.65 Å, which is in good agreement with previous computational33 and experimental studies.34 We then optimized the model Cu(111), which is composed of four atomic layers in a (3×3) supercell structure (nine atoms per layer) with 15.0 Å vacuum. To perform surface optimization, we allowed the two topmost atomic layers to relax while keeping the bottom layers fixed in their bulk parameters. Relaxation is performed until the forces on the unconstrained atoms are less than 0.01 eV/Å. Dipole correction is implemented to cancel the artificial electric field across the slabs imposed by the periodic boundary conditions.35 We considered Pd atom alloying of Cu(111) by substituting Cu atoms with Pd atoms. Three Pd atoms are introduced in the supercell giving two surface models: (1)


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one Pd atom in the topmost layer and two Pd atoms in the subsurface layer, and (2) two Pd atoms in the topmost layer and one Pd atom in the subsurface layer. We named these surfaces as Pd1,2Cu(111) and Pd2,1Cu(111) to represent models 1 and 2, respectively. Subscript 1,2 (2,1) denotes surface model with one (two) Pd atom(s) in the topmost layer and two (one) Pd in the subsurface layer. This means that the two bottom layers are composed of Cu atoms only. We further optimized these PdCu(111) surfaces to obtain their equilibrium structures. The surface Brillouin zone is sampled using 5×5×1 Monkhorst–Pack k-points.36 We performed convergence tests of CO adsorption energy on Cu surface to verify the accuracy of the parameters used in our study. Adsorption energy differences of ~0.020 and ~0.001 eV are obtained if higher k-points and energy cutoffs are used, respectively. These are with reference to the calculated adsorption energy using the parameters employed in our calculations. Also, convergence of the total energy calculations is set to 10−6 eV to ensure accuracy of the results. To investigate the arrangement of Pd atoms in PdCu in the presence of adsorbates, we constructed surfaces with non-aggregated Pd atoms (Pd atoms far from each other) and aggregated Pd atoms (Pd atoms close to each other) configurations. McCue et al. found that the segregated Pd atoms in CuPd due to CO adsorption enhanced the performance of the catalyst for acetylene hydrogenation.24 It is therefore crucial to investigate not only the adsorption of molecules in CuPd but also the favored ordering of Pd atoms in the presence of adsorbates because the performance of the catalyst is greatly influenced by the dynamic behavior of its surface atoms in the presence of adsorbates. In this work, we studied the adsorption of one molecule (CO, COH or HCO), corresponding to 0.11 ML coverage, on PdCu(111). The stable structures of the adsorbates on the surfaces are determined by performing optimization, in which, the adsorbed specie and the two topmost atomic layers of the surface are allowed to relax until


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the forces on these unconstrained atoms are less than 0.01 eV/Å. The interatomic distances of the adsorbates in the gas phase (listed in Table 1) are initially used in the optimization. We considered different adsorption configurations for each adsorbate on the surfaces in order to obtain the most stable configuration of the adsorbate–surface systems. The effect of adsorbate on the preferred arrangement of Pd atoms is then determined by calculating the energetics of the clean surfaces and the adsorbate–surface systems for various arrangements of Pd atoms in PdCu(111). We define the adsorption energy with reference to the sum of the total energies of the isolated molecule and the clean surface. Figure 1 shows the optimized geometric structures of different PdCu(111) configurations used in this study. The structure of Pd atoms is either nonaggregated (na) (Figure 1a and 1b) or aggregated (a) (Figure 1c and 1d) in the topmost and subsurface layers which are denoted as a-PdCu and na-PdCu in this manuscript, respectively. In this manuscript, clean surface refers to Pd alloyed Cu(111) surface. We referred to the system as adsorbate–surface system if adsorbate (CO, HCO, COH) is present on the surface.

Figure 1. Top view of the geometric structures of (a) na-Pd2,1Cu(111), (b) na-Pd1,2Cu(111), (c) a-Pd2,1Cu(111) and (d) a-Pd1,2Cu(111). a-PdCu(111) and na-PdCu(111) represent the surfaces with aggregated Pd atoms and non-aggregated Pd atoms arrangements, respectively. Subscript


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1,2 (2,1) denotes one (two) Pd atom(s) in the topmost layer and two (one) Pd in the subsurface layer. The supercells considered in this study are enclosed by the dashed boxes with dimensions of 7.743 Å × 7.743 Å. The height of the supercell, including the vacuum region, is 21.321 Å.

RESULTS AND DISCUSSION Clean PdCu(111) surfaces. We first discuss the geometric structures of the optimized PdCu(111) surfaces with reference to Cu(111). The interatomic distances of Cu atoms in Cu(111) are 2.581 Å. In PdCu, the interatomic distance between Pd and Cu atoms lying parallel the surface plane increased by an average of ~0.024 Å. This is attributed to the relaxation of Cu atoms in the presence of Pd which has slightly larger atomic radius than Cu. We note that for topmost layer atoms, the Pd atoms are at higher position than the Cu atoms, but their heights differ only by an average of ~0.035 Å. On the other hand, the interlayer spacing of the two topmost layers in the alloy surfaces increased by an average of ~0.083 Å with reference to pure Cu(111), which has an interlayer spacing of 2.066 Å. These parameters agree with previous experimental and theoretical works. Previous experimental studies estimated that the interatomic distances in Cu(111) alloyed with small amount of Pd are identical to pure Cu with value of ~2.50 Å ± 0.1 Å.26,27 Lopez and Nørskov performed DFT calculations on Pd-doped Cu(111) and reported that the interatomic distance of Pd and Cu is 2.60 Å.22 The group of Castegnaro, on the other hand, determined the interplanar distances in PdCu nanoalloys, using the peak positions of the (111) diffraction peaks obtained from X-ray diffraction (XRD) analysis.13 The reported interplanar distances for the different PdCu nanoalloys are approximately 2.18 Å.13 Due to the small number of Pd atoms in our models, the geometric properties of PdCu will remain comparable with pure Cu surface as had been shown in previous works. 26,27


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Figure 2 shows the energetics of the different clean PdCu surfaces. Here, we calculated the difference in total energies (∆E), in which, we used the total energy per supercell of the clean naPd2,1Cu as the reference energy. We note that the same approach has been made in previous works.23,37-39 Since the surfaces considered have the same supercell size and the same atom compositions and only the arrangement of atoms is modified, the method used can predict the stability of the surfaces by relying on the change in total energy per supercell with reference to a particular surface configuration. Among the clean PdCu surfaces, na-Pd2,1Cu has the lowest total energy per supercell; therefore, it is the most stable surface configuration. Thus, Figure 2 shows a comparison between the stability of surfaces with aggregated and non-aggregated Pd. It also shows a comparison in the stability of surfaces depending on the number of Pd atoms in the topmost and subsurface layers.

Figure 2. Relative energies of the different clean PdCu surfaces. The total energy per supercell of na-Pd2,1Cu(111) is the reference energy. a-PdCu(111) and na-PdCu(111) represent surfaces with aggregated Pd atoms and non-aggregated Pd atoms, respectively. Subscript 1,2 (2,1)


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denotes that there is (are) one (two) Pd atom(s) in the topmost layer and two (one) Pd in the subsurface layer.

Figure 2 generally reveals that surfaces with non-aggregated Pd atoms are energetically favored than surfaces with aggregated Pd atoms. For example, na-Pd2,1Cu is more stable than aPd2,1Cu while na-Pd1,2Cu is more stable than a-Pd1,2Cu. Due to the strong Cu–Pd interaction,21-23 the alloy surface will be more stable if Pd atoms are farther apart. Similarly, experimental investigations found that Pd atoms occur in PdCu as individual atoms, isolated from one another.7,26,27,40 However, the energetics of a-Pd2,1Cu is comparable with na-Pd1,2Cu. The former is more favored, but only by 6 meV per supercell. It indicates that there is a competition between the amount of Pd atoms and their arrangement in the topmost and subsurface layers. Nonetheless, the energetics suggests that the surface with a higher number of Pd atoms in the topmost layer is favored than the surface with a higher number of Pd in the subsurface, which we will explain in the next paragraph. The na-Pd2,1Cu is more favored than the na-Pd1,2Cu by ~0.207 eV. This finding agrees with experimental data,26 in which, a higher number of Pd exists in the topmost layer than in the subsurface layer of PdCu(111) at low temperature depositions. However, Tierney et al. suggested that the configuration with higher Pd composition in the topmost layer than in the subsurface layer is a metastable structure. Topmost layer Pd atoms will eventually reside in the subsurface due to the larger surface energy of Pd than Cu.26 We investigated the electronic properties of na-Pd2,1Cu surface to support our conclusion that it is energetically more stable if a higher number of Pd atoms are in the topmost layer than in the subsurface layer of the models considered in our study. Figure 3 (upper panel) shows the density of states (DOS) profiles of Pd


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and Cu atoms in the topmost and subsurface layers of na-Pd2,1Cu(111) and of surface Pd atom of Pd(111). The DOS of surface Cu atom of Cu(111) is also plotted for reference. In pure Pd(111), a surface Pd atom has DOS near the Fermi level. Therefore, the different DOS profiles of Pd and Cu in PdCu at -1.5 to -1.0 eV energy range indicates that Pd atom still retains some of its electronic properties. Noticeably, the DOS profiles of surface and subsurface Pd atoms are different at -5.0 to -3.5 eV energy range. In this energy range, hybridization is more evident for the topmost layer Pd atom and Cu atoms than the subsurface Pd atom and Cu atoms. The figures in the lower panel of Figure 3 show the local (s, p and d -bands) density of states of Cu and Pd of na-Pd2,1Cu(111) in the Fermi level region. It illustrates that the s, p and d-bands of Cu and Pd have contributions at the Fermi level. The s-bands of surface and subsurface Cu and Pd atoms have comparable DOS at the Fermi level. The p-bands of surface and subsurface Cu atoms have higher DOS than the Pd atoms. For the d-bands, we note the higher DOS of surface and subsurface Pd atoms than the Cu atoms at the Fermi level. States of Cu and Pd atoms overlap in PdCu surface in general, which implies that Pd adopts the electronic properties of the host Cu surface. The results also support experimental claim that the Pd atoms are electronically identical to the Cu atoms in PdCu(111).26 We note however that there remains variations in the contributions of Cu and Pd in the DOS profiles as suggested by our results. Our analysis further implies that the behavior of bimetallic PdCu is strongly dictated by the interaction between its component atoms which supports our findings that higher number of Pd atoms in topmost layer will be more favored. In the work of Tierney et al., they annealed the surface to reach higher temperature before they observed higher concentration of Pd in the subsurface26. This means that additional energy is needed in order to allow the migration of topmost layer Pd to the subsurface.


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It also implies that at lower temperatures, more Pd atoms exist in the topmost layer than in subsurface layer.

Figure 3. Density of states (DOS) projected on the surface Pd, subsurface Pd, surface Cu and subsurface Cu atoms of clean Pd2,1Cu(111) with non-aggregated Pd atoms (upper panel). The DOS of topmost layer Pd atom of Pd(111) and Cu atom of Cu(111) are also shown. Subscript 2,1 denotes that there are two Pd atoms in the topmost layer and one Pd in the subsurface layer. The local s-, p- and d-bands density of states (LDOS) at the region of the Fermi level are shown in the lower panel.

CO on PdCu(111) surfaces. In this section, we present CO adsorption and the preferred arrangement of Pd atoms in PdCu(111) in the presence of CO. Table 1 gives the adsorption energies, heights of C from the surface and the bond lengths of adsorbed CO on PdCu(111). CO adsorbs in an upright orientation with the C atom attached to surface atoms of PdCu,21-23 similar


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to CO adsorption on pure Cu and Pd.41,42 Adsorption sites of CO depend on the coordination of Pd in the topmost layer: CO adsorbs on the top site if isolated Pd atom is present in the topmost layer and on the bridge site if two Pd atoms exist in the topmost layer. CO bond length increases upon adsorption as a result of its interaction with surface atoms. The average bond length of adsorbed CO species is ~1.155 Å, while its gas phase bond length is 1.136 Å, in good agreement with previous works.11,43 The distance of C and the closest metal atom to it is ~1.946 Å. The obtained C–O bond lengths and C–metal distances agree with previous theoretical works.21,22,44

Table 1. Calculated CO, HCO and COH adsorption energies and adsorption parameters on different PdCu(111) surfaces. a-PdCu(111) and na-PdCu(111) represent surfaces with aggregated Pd atoms and non-aggregated Pd atoms arrangements, respectively. Subscript 1,2 (2,1) denotes that there is (are) one (two) Pd atom(s) in the topmost layer and two (one) Pd in the subsurface layer.

surfaces

adsorption energy (eV)

height of C atom from surface (Å)

interatomic distances of adsorbates (Å)

CO na-Pd2,1Cu(111)

-0.974 (top)

1.929

C-O: 1.150

a-Pd2,1Cu(111)

-1.177 (bridge)

1.534

C-O: 1.168

na-Pd1,2Cu(111)

-0.972 (top)

1.924

C-O: 1.150

a-Pd1,2Cu(111)

-1.156 (top)

1.916

C-O: 1.150

CO (gas phase)

C:O: 1.136 HCO

na-Pd2,1Cu(111)

-1.864 (top)

2.020

H-C: 1.112; C-O: 1.200

a-Pd2,1Cu(111)

-2.006 (bridge)

1.581

H-C: 1.112; C-O: 1.254

na-Pd1,2Cu(111)

-1.901 (top)

2.021

H-C: 1.116; C-O: 1.200


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a-Pd1,2Cu(111)

-2.035 (top)

2.000

HCO (gas phase)

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H-C: 1.116; C-O: 1.200 H-C: 1.139; C-O: 1.182

COH na-Pd2,1Cu(111)

-3.330 (fcc)

1.298

C-O: 1.328; O-H: 0.981

a-Pd2,1Cu(111)

-3.619 (fcc)

1.396

C-O: 1.323; O-H: 0.981

na-Pd1,2Cu(111)

-3.402 (fcc)

1.240

C-O: 1.327; O-H: 0.981

a-Pd1,2Cu(111)

-3.424 (fcc)

1.808

C-O: 1.326; O-H: 0.981

COH (gas phase)

C-O: 1.254; O-H: 1.027

We note the comparable adsorption energies (~ -1.20 eV) of CO on a-Pd2,1Cu and a-Pd1,2Cu despite the former having two Pd atoms in the topmost layer, while the latter contains only one Pd atom. When CO is adsorbed on PdCu, subsurface Pd atoms also interact with CO and thus, contribute to the enhancement of the adsorption energy.23 Likewise, CO promotes Pd-Pd interaction in the surface.23 Therefore, the two Pd atoms in the subsurface layer of a-Pd1,2Cu are responsible for the comparable adsorption energies. In order to justify the influence of subsurface Pd atoms toward CO adsorption in PdCu, we calculated the d-band centers of topmost layer Pd atoms of the surfaces. According to the d-band model, the position of the d-band center with reference to the Fermi level can predict the strength of the adsorbate–metal surface interaction.45 The closer the d-band center is to the Fermi level, the greater the adsorption energy. The d-band centers of Pd atoms in a-Pd2,1Cu(111) and a-Pd1,2Cu(111) are -2.376 and -2.325 eV, respectively. The d-band centers are comparable, which explains the comparable CO adsorption energies on the surfaces. On the other hand, the d-band centers of topmost layer Pd atoms of naPd2,1Cu(111) and na-Pd1,2Cu(111) are -2.513 and -2.473 eV, respectively. This likewise explains the comparable adsorption energies of CO on these surfaces. Since the d-band centers of the topmost layer Pd atoms in the aggregated arrangement is closer to the Fermi level, the adsorption


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energies of CO on these surfaces are greater than on surfaces with non-aggregated arrangement of Pd atoms. In connection to this, the aggregated arrangement of Pd atoms in the subsurface explains the shift of d-band of the topmost layer Pd atom of Pd1,2Cu closer to the Fermi level and results to the strong adsorption despite of having only one Pd atom in the topmost layer. The effect of subsurface Pd in the electronic property of surface Pd of PdCu surface has also been found in a DFT study conducted by Fu and Luo.28 They demonstrated that subsurface Pd reduces the activation barrier of H2 dissociation on the surface. Their study supports our claim on the role of subsurface Pd resulting to the comparable CO adsorption energies on a-Pd2,1Cu and a-Pd1,2Cu. Adsorption energies of CO on PdCu(111) surfaces with aggregated Pd atoms are greater than on surfaces with non-aggregated Pd atoms: -1.177 and -1.156 eV for a-Pd2,1Cu and a-Pd1,2Cu, respectively; -0.974 and -0.972 eV for na-Pd2,1Cu and na-Pd1,2Cu respectively. The calculated adsorption energies lie between the adsorption energies of CO on pure Cu and Pd, -0.85 and 2.05 eV, respectively.21 We note that the large difference in the computed adsorption energies on a-PdCu(111) and na-PdCu(111) surfaces is due to the preference of CO to interact with Pd.23 Furthermore, this difference in adsorption energies suggests the preference of Pd atoms to take the aggregated Pd structure from the initially favored non-aggregated structure, which is driven by the preference of CO to interact with Pd over Cu.


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Figure 4. Relative energies of the different clean PdCu surfaces and the CO adsorption energies on (a) Pd2,1Cu(111) and (b) Pd1,2Cu(111) surface. The total energy per supercell of the naPd2,1Cu(111) (Pd1,2Cu(111)) and the adsorption energy of CO on this surface are the reference energies in Figure 4a (Figure 4b). a-PdCu(111) and na-PdCu(111) represent the surfaces with aggregated Pd atoms and non-aggregated Pd atoms, respectively. Subscript 1,2 (2,1) denotes that there is (are) one (two) Pd atom(s) in the topmost layer and two (one) Pd in the subsurface layer. The geometric structures are shown as insets in each figure.

We further clarified the effect of CO on the configuration of Pd atoms in PdCu(111). First, we focused on the case of Pd2,1Cu. We constructed additional surface in order to provide an intermediate structure that will connect the structures and energy profiles of the non-aggregated and aggregated Pd configurations. Figure 4a shows the plot of ∆E for the clean surfaces and for CO–surface systems. The change in energy (∆E) for the case of CO–surface systems corresponds


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to the difference in CO adsorption energies, with the adsorption energy of CO on na-Pd2,1Cu as the reference. CO–surface configurations are shown as insets. The energy profiles of the CO– surface systems show a downhill trend in contrast to the uphill trend for clean surfaces. In clean surfaces, non-aggregated Pd configuration is more preferred than aggregated Pd atoms arrangement by 0.199 eV. The downhill trend of energy in the presence of CO indicates that the arrangement with aggregated Pd atoms is energetically favored by 0.204 eV than the nonaggregated Pd atoms configuration. We conducted similar investigation for the case of Pd1,2Cu as shown in Figure 4b. The total energy per supercell of na-Pd1,2Cu and the adsorption energy of CO on the surface are used as reference energies for clean and CO–surface systems, respectively. Without CO, non-aggregated Pd arrangement is more stable by 0.140 eV compared to aggregated Pd case. Again, there is a reversal in the stability when CO is present. Aggregated Pd configuration becomes more stable by 0.184 eV with reference to the non-aggregated Pd arrangement. The energy profiles and the structures of Pd atoms in the non-aggregated, intermediate and aggregated configurations suggest that Pd atoms in the surface and subsurface layers will prefer the aggregated arrangement in the presence of CO. Since the adsorption energy of CO on a-Pd1,2Cu and a-Pd2,1Cu are comparable, we also conclude that with adsorbed CO, Pd atoms will favor both configurations (independent of the number of Pd atoms in the topmost layer) as long as there is an aggregation of surface and subsurface Pd atoms. We would like to stress the importance of the surface and subsurface Pd atoms of PdCu(111) in an aggregated arrangement. In our previous work, we found that the adsorption of CO is weak on Cu3Pd(111) that is mainly composed of Pd atoms in the topmost layer and without Pd in the subsurface.23 The group of McCue experimentally observed the segregation of Pd in PdCu catalyst in the presence of CO.24 Furthermore, they detected Pd-Pd dimers and isolated Pd after


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CO exposure. These Pd-Pd dimers support our claim on the preferred aggregated arrangement of Pd atoms. McCue et al. also hypothesized the possible presence of neighboring subsurface Pd atoms in the vicinity of isolated Pd.24 This agrees to our finding that subsurface Pd atoms in an aggregated arrangement with surface Pd atom influence CO adsorption. Thus, even with small amount of Pd, it is possible to modify the reactivity of the PdCu system to enhance CO adsorption by considering an adjacent arrangement of surface and subsurface Pd atoms. HCO and COH on PdCu(111) surfaces. We examined the adsorption of COH and HCO on the different PdCu surfaces to determine their effect on the arrangement of Pd atoms in PdCu. Table 1 also lists the adsorption energies of COH and HCO on non-aggregated and aggregated Pd2,1Cu and Pd1,2Cu. HCO adsorbs with its C atom directly interacting with metal atoms, similar to its adsorption structure on other metal surfaces.46-48 Adsorption sites of HCO also depend on the coordination of Pd atoms in the surface. Its most stable adsorption site is top site if nonaggregated Pd atom is present in the topmost layer. It is stable on bridge site if two Pd atoms exist in the topmost layer. HCO adsorption energies are -1.864 and -2.006 eV on na-Pd2,1Cu and a-Pd2,1Cu surfaces, respectively. The adsorption energies are -1.901 and -2.035 eV on na-Pd1,2Cu and a-Pd1,2Cu, respectively. Gu and Li reported that the adsorption energies of HCO on Cu(111) and Pd(111) are -1.41 and -2.51 eV, respectively.50 We note that similar to the case of CO, the obtained HCO adsorption energies on PdCu lie between the adsorption energies on pure Cu and Pd. Our results further show that there is comparable adsorption energies on a-Pd2,1Cu and aPd1,2Cu, independent of the number of Pd atoms in the topmost layer, as long as surface and subsurface Pd atoms are in an aggregated configuration. This observation is again attributed to the calculated d-band centers of the topmost layer Pd atom, comparable to the case of CO–PdCu surfaces.


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We plot the difference in the energetics of HCO–surface systems in Figure 5. HCO adsorption leads to a more stable aggregated Pd2,1Cu by 0.142 eV, as compared to the non-aggregated Pd configuration (Figure 5a). On the other hand, aggregated Pd1,2Cu is more stable than the nonaggregated Pd case by 0.133 eV when HCO is adsorbed on the surface (Figure 5b). There is a downhill trend in the energy profile of PdCu2,1 and PdCu1,2 when HCO is on the surface. These results indicate that HCO adsorption influences the preferred arrangement of Pd atoms: from non-aggregated to aggregated structure of Pd atoms in the surface and subsurface.

Figure 5. Relative energies of the different clean PdCu surfaces and the HCO adsorption energies on (a) Pd2,1Cu(111) and (b) Pd1,2Cu(111) surface. The total energy per supercell of the na-Pd2,1Cu(111) (Pd1,2Cu(111)) and the adsorption energy of HCO on this surface are the reference energies in Figure 5a (Figure 5b). a-PdCu(111) and na-PdCu(111) represent the surfaces with aggregated Pd atoms and non-aggregated Pd atoms, respectively. Subscript 1,2 (2,1) denotes that there is (are) one (two) Pd atom(s) in the topmost layer and two (one) Pd in the subsurface layer. The geometric structures are shown as insets in each figure.


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Figure 6. Relative energies of the different clean PdCu surfaces and the COH adsorption energies on (a) Pd2,1Cu(111) and (b) Pd1,2Cu(111) surface. The total energy per supercell of the na-Pd2,1Cu(111) (PdCu1,2(111)) and the adsorption energy of COH on this surface are the reference energies in Figure 6a (Figure 6b). a-PdCu(111) and na-PdCu(111) represent the surfaces with aggregated Pd atoms and non-aggregated Pd atoms, respectively. Subscript 1,2 (2,1) denotes that there is (are) one (two) Pd atom(s) in the topmost layer and two (one) Pd in the subsurface layer. The geometric structures are shown as insets in each figure.

Figure 6 shows the plot of ∆E of clean surfaces and COH–Pd2,1Cu (Figure 6a) and COH– Pd1,2Cu(111) (Figure 6b). COH adsorption energies and adsorption sites are also listed in Table 1. a-Pd2,1Cu becomes more favored than the na-Pd2,1Cu by 0.289 eV when COH is adsorbed. On the other hand, due to COH adsorption, a-Pd2,1Cu becomes more favored than the na-Pd1,2Cu but only by 0.022 eV. This behavior differs from CO and HCO cases, in which, adsorption energies are almost comparable on a-Pd2,1Cu and a-Pd1,2Cu. COH adsorption energies on a-Pd2,1Cu and


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a-Pd1,2Cu surfaces are -3.424 and -3.619 eV, respectively. This large difference in adsorption energies suggests a different adsorption mechanism for COH on PdCu surfaces. Nevertheless, the energy profiles imply that similar to CO and HCO, COH induces an aggregated arrangement of the Pd atoms that initially favor the non-aggregated configurations in clean PdCu surface. In order to explain the large difference in the adsorption energies of COH on a-Pd1,2Cu and aPd2,1Cu, we investigated the geometric structures and charge density difference distributions of the systems that are shown in Figure 7. In COH adsorption, C directly interacts with metal atoms. COH always relaxes to the fcc hollow site with at least one Pd atom in its neighbor, regardless if its initial position is on the bridge or top site of Pd atoms before optimization. It is also reported in previous works that COH favors adsorption on hollow sites of (111) metal surfaces.46,50 Therefore, COH deviates from the behavior of CO and HCO on PdCu. CO and HCO adsorptions strongly depend on the coordination of topmost layer Pd atoms. From the charge density distribution, there is a charge-accumulated region between Pd and the C atom of COH. Due to this, charge of O and H atoms redistribute which results to a charge-depleted region in the vicinity of H. This charge-depleted region in H interacts with the chargeaccumulated region between Pd and C which forces the adsorption of COH on hollow site rather than on symmetry site of the Pd atoms. This also clarifies the orientation of H atom, which is almost directed to the Pd atom in the surface as depicted in insets of Figure 6. Due to this adsorption mechanism of COH, it will prefer to have more Pd atoms in the topmost layer. This is evident from the stronger adsorption energy of COH on a-Pd2,1Cu than on a-Pd1,2Cu. From this observation, we conclude that in the presence of COH, an aggregated surface and subsurface Pd arrangement in PdCu will be favored. However, it will prefer to have a greater number of Pd atoms in the topmost layer than in subsurface due to its adsorption structure.


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Figure 7. Charge density difference distributions (top) of adsorbed COH on (a) Pd2,1Cu(111) and (b) Pd1,2Cu(111) surfaces with aggregated Pd atoms in the surfaces. Subscript 1,2 (2,1) denotes that there is (are) one (two) Pd atom(s) in the topmost layer and two (one) Pd in the subsurface layer. Isosurface levels were set at 0.001 e / a03. The side views of the distributions are shown in the insets in each figure. Yellow and blue regions indicate gain and loss of charge, respectively.

Given these results, we recommend that the arrangement of Pd atoms in PdCu surfaces should be considered in gas–PdCu investigations. These materials are used in energy related applications (i.e., hydrogen separation membranes and catalyst for hydrocarbon production) and the aggregated configurations of Pd atoms in the presence of adsorbates could possibly explain the performance of the material (i.e. selectivity, reactivity, permeation, adsorption). Considering the works of Tierney et al.,26 we infer that if adsorbate is deposited on PdCu surface at room temperature, Pd migration may possibly be observed at temperature lower than 500 K. This is supported by our predicted reversal of stability (based from the calculated energetics) between clean PdCu and adsorbate-PdCu systems. It means that smaller amount of energy input is needed in order for the Pd atoms to migrate in the presence of adsorbates than the clean surfaces. Nonetheless, we recommend that the effect of temperature in adsorbate-PdCu systems and the adsorbate-induced migration of Pd atoms in PdCu be investigated in future studies in order to


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determine the activation barrier for Pd atom migration with and without the presence of adsorbates. Lastly, it is possible to tune the reactivity of the PdCu surface with small amount of Pd by exploiting the aggregated arrangement of surface and subsurface Pd atoms in the presence of adsorbates and by considering the adsorption property of the adsorbates on the surface.

CONCLUSION We investigated the effects of adsorbates on the arrangements of Pd atoms in PdCu(111) by performing first principles calculations based on density functional theory. We modeled PdCu(111) surfaces by substituting small numbers of Cu atoms with Pd atoms in the topmost and in the subsurface layers of Cu(111). Various arrangements and number of Pd atoms in the topmost and subsurface layers are considered in this study. We found that the PdCu surface is energetically favored if the Pd atoms are non-aggregated and if higher number of Pd atoms is present in the topmost layer than in the subsurface layer. This is due to the more pronounced hybridization of states between the topmost layer Pd atom and Cu atoms than the subsurface Pd atom and Cu atoms as depicted by the density of states of the system. In the presence of adsorbates, Pd atoms in the surface and subsurface layers will prefer the aggregated configuration from their initially non-aggregated structure as determined from the calculated energy profiles. Adsorptions of CO and HCO depend on the coordination of Pd atoms in the topmost layer. Both molecules prefer the aggregated arrangement of surface and subsurface Pd atoms and their adsorption energies do not depend on the number of Pd atoms in topmost layer. On the other hand, COH will favor adsorption on fcc hollow site with higher number of Pd atoms in the topmost layer than in subsurface layer. While it has been shown by previous studies that the tendency of Pd to segregate in the presence of adsorbates is due to the stronger interaction of


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the adsorbate with Pd, we conclude from our findings that the adsorption structure of the adsorbate also influences the arrangement of Pd atoms in the surface and subsurface regions of the surface.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Tel/Fax: +63-49-536-6610

ACKNOWLEDGMENT This work is funded by the University of the Philippines Balik PhD Research Grant (OVPAABPhD-2015-07). It is also supported in part by: the GREEN POWER PROGRAM (Project 2: Energy Production using Polymer Exchange Membrane Fuel Cells) of The Commission on Higher Education (CHED); MEXT Grant-in-Aid for Scientific Research (15H05736, 24246013, 15KT0062, 26248006); NEDO Project ‘R&D Towards Realizing an Innovative Energy Saving Hydrogen Society based on Quantum Dynamics Applications’; and the Osaka University Joining and Welding Research Institute Cooperative. Some of the numerical calculations presented here are done using the computer facilities at the following institutes: CMC (Osaka University); ISSP; KEK; NIFS; YITP; Institute of Mathematical Sciences and Physics, University of the Philippines Los Baños.


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(50) Cahyanto, W.T.; Widanarto, W.; Shukri, G.; Kasai, H. Theoretical Studies of the Adsorption of Hydroxymethylidyne (COH) on Pt-alloy Surfaces using Density Functional Theory. Phys. Scr. 2016, 91, 025803.


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TABLE OF CONTENTS (TOC) IMAGE


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Effects of Adsorbates (CO, COH, and HCO) on the Arrangement of Pd

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