Adsorption of CO, NO, and H2 on the PdnAu55-n Nanoclusters: A DFT

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C: Physical Processes in Nanomaterials and Nanostructures 2

n

55-n

Adsorption of CO, NO, and H on the PdAu Nanoclusters: A DFT Investigation within the van der Waals D3 Corrections Krys E. A. Batista, Juarez L. F. Da Silva, and Maurício Jeomar Piotrowski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12219 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019

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The Journal of Physical Chemistry

Adsorption of CO, NO, and H2 on the Pdn Au55 − n Nanoclusters: A DFT Investigation within the van der Waals D3 Corrections Krys E. A. Batista,† Juarez L. F. Da Silva,‡ and Maur´ıcio J. Piotrowski∗,† Department of Physics, Federal University of Pelotas, PO Box 354, 96010-900, Pelotas, RS, Brazil, and São Carlos Institute of Chemistry, University of São Paulo, PO Box 780, 13560-970, São Carlos, SP, Brazil E-mail: [email protected]

Abstract

I Introduction

The PdAu nanoclusters have innumerable potential applications in catalysis, especially because of the changes of the physical and chemical properties as a function of the composition and geometric shape, however, the understanding of the adsorption process is far from satisfactory mainly because of the strong dependence of the adsorption properties on the size, shape, and composition of the nanoclusters. Here, we report a study, based on density functional theory calculations, of the CO, NO, and H2 molecules adsorbed on PdAu nanoclusters, where we performed a systematic study for bimetallic Pdn Au55 − n nanoclusters considering the main energetic, structural, and electronic properties in relation to the composition, and after, for specific compositions, we performed the adsorption of one molecule (CO, NO, and H2 ) on the nanoclusters. We have obtained the lowest energy structures for monometallic and bimetallic nanoclusters, based on the excess energy, where we identified the maximum stability, n = 20, with the Au atoms in the surface region (atoms directly exposed to the vacuum) and Pd atoms in the core region. For the molecular adsorption, we consider the Pd55 , Au55 , and Pd20 Au35 , for which we find that the adsorbed NO systems have higher adsorption energies (in modulus), due to highest position of the center of gravity of the d-states in relation to the Fermi level. In general, the molecules prefer the top site, with the Pdmolecule bonds being more intense than the Au-molecule bonds. The intermediate adsorption energy values obtained for Pd20 Au35 nanoclusters, for CO and NO adsorptions, show the promising Pd−Au combination to build catalysts that are adequate to avoid catalyst poisoning.

Transition-metal (TM) particles are widely used as catalysts for a large number of chemical reactions. 1 Among finitesize TM particles, 55-atom TM nanoclusters are particularly interesting due to the possibility to control size, geometry, and composition, which affects the adsorption properties of molecular systems. 1–3 To design TM bimetallic nanoclusters as good catalysts, it is fundamental to understand the interaction between the adsorbed molecules and the nanoclusters, since there is a strong dependence of the adsorption properties on the size, different configurations, and composition, which makes the problem non-trivial. In addition, the knowledge acquired and well-founded for the molecular adsorption on TM surfaces can not be transferred directly to the adsorption on nanoclusters. 4–10 In nanoclusters, the surface region plays an important role, since there is a larger number of atoms in the surface (atoms directly exposed to the environment) than in the core region, for example, for TM55 nanoclusters the surface coverage has a percentage about 81.1 %. Consequently, the nanoclusters present a larger number of active sites, to interact with molecules, than another bulk- or surface-like systems. The presence of a second TM species in nanocluster constitution allows to tune the main chemical and physical properties, in the same time, opening up the possibility to reduce the manufacturing expenses (substituting the expensive TM species). 3 There are several works that explore the molecular adsorption on small monometallic nanoclusters, where practically all the TM atoms are in the surface region, 5–11 while the number of studies that consider the molecular adsorption on bimetallic nanoclusters is very restricted. 12–15 Practically all these studies have focused on CO, NO, and H2 molecules, because of their involvement in important catalytic processes. 6,16,17 From all possible TM combinations to build bimetallic nanoclusters, the PdAu is particularly interesting, since these elements are largely employed in catalytic applications 18–21 and, specifically, in the understanding of the adsorption process. 22–24 Consequently, (PdAu)55 nanoclusters are promising candidates for study, since this particle size allows to

∗ To

whom correspondence should be addressed of Physics, Federal University of Pelotas, PO Box 354, 96010-900, Pelotas, RS, Brazil ‡ S˜ ao Carlos Institute of Chemistry, University of São Paulo, PO Box 780, 13560-970, São Carlos, SP, Brazil † Department

ACS Paragon Plus Environment 1

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change the composition to obtain properties variability in the molecular adsorption process. 22–26 Several previous works have focused on the molecular adsorption study on (PdAu)55 nanoclusters, for example, Cheng et al. 22 have studied (PdAu)55 nanoclusters combining analytical potentials with the density functional theory (DFT) calculations, where they have studied PdAu nanoclusters in the oxidation of CO to CO2 . They verified that the CO adsorption energies are substantially dependent on the local environment and on the composition of the nanoclusters. Similarly, the CO oxidation process presented the energy barriers sensitive to the composition, where Au-rich nanoclusters have higher catalytic activity. 22 On the other hand, Sandoval et al. 26 considering DFT calculations have studied the H2 adsorption on (PdAu)55 nanoclusters. They have found that the hydrogen adsorption leads to a slight change in the d-states of Pd species, where each H atom adsorbed in the nanocluster becomes negatively charged. However, despite these previous studies, there are still several open questions about molecular adsorption on (PdAu)55 nanoclusters, especially, the main role that the synergy between Pd and Au species exerts on the molecular adsorption. In other words, how the formation of bimetallic nanoclusters can affect the adsorption properties? To address this question, we selected 55-atom nanoclusters composed by PdAu and CO, NO, and H2 molecules, which yield the possibility to investigate how the mechanisms of the bimetallic nanoclusters formation affect the adsorption process. For that, we performed ab initio calculations based on DFT to the Pdn Au55 − n nanoclusters (n = 0, 6, 13, 20, 28, 35, 42, 49, and 55) and, for selected systems, we have performed DFT calculations with van der Waals corrections for the CO, NO, and H2 adsorption on PdAu systems.

II Theoretical Approach and Computational Details A Total Energy Calculations Our total energy calculations are based on spin-polarized DFT within the Perdew–Burke–Ernzerhof 27 (PBE) formulation for the exchange-correlation energy functional. To improve the description of the molecules-nanoclusters interactions, we employed the van der Waals (vdW) correction proposed by S. Grimme, namely, the D3 framework, 28 which has been employed in several DFT studies. 29–32 The Kohn– Sham equations were solved using the all-electron projector augmented wave 33,34 (PAW) method, as implemented in the Vienna ab initio simulation package (VASP, version 5.4.1), 35,36 where the Kohn–Sham orbitals are expanded in plane-waves up to a given cutoff energy. We employed a cutoff energy of 450 eV for the adsorption of CO, NO, and H2 on Pdn Au55 − n , which is 12.5 % larger than the largest recommended cutoff energy by VASP for those chemical species, while for Pd55 , Au55 , and PdAu systems, a cutoff energy of 300 eV was employed as the H, C, N, and O atoms are not present. For all nanocluster calculations, we employed cubic boxes with side of 22 Å,

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which yields a minimum distance of about 12 Å among the particles and their periodic images for all systems. For the Brillouin zone integration, we employed the Γ-point as there is no dispersion in the electronic states within the Brillouin zone due to the large distance among the particles and their images. For all calculations, we obtained the equilibrium geometries once the atomic forces were smaller than 0.025 eV/Å on each atom and with a total energy convergence of 1.0 × 10−5 eV.

B Atomic Configurations To obtain the lowest energy configuration for the Pdn Au55 − n nanoalloys, we employed several steps, which were also employed in previous studies, 37,38 as follows: (i) In principle, we could consider all possible nanoalloy compositions, i.e., n = 0, 1, 2, · · · , 55, however, to reduce the computational cost, we considered only a limited set of compositions, i.e., n = 0, 6, 13, 20, 28, 35, 42, 49, and 55. (ii) The nanoalloys configurations were designed by atom substitutions using three structures, namely, the putative global minimum configurations (pGMCs) of Pd55 (bulk-like fragment of the face-centered cubic - FCCf), 32 Au55 (distorted reducedcore - DRC), 32 and the high symmetry icosahedron (ICO) structure. 39 Molecular dynamics or simulated annealing simulations was not considered as structure generator as for the Pt55 and Au55 systems, 40 because of the slow diffusion rate of surface atoms to the core region or vice versa, which plays a crucial role in the study of nanoalloys. 37,38,41 (iii) For the substitution of the TM atoms, we separate the 55-atom nanoalloy into two regions, namely, core and surface regions based on the hard-sphere model, 42 and a limited number of configurations (homogeneous, heterogeneous, segregation, dimers and string-like patterns, 43 etc) were designed to verify the energetic preference of the Pd and Au species within the nanoalloy configuration space, see Figure 1. (iv) Furthermore, we considered also several magnetic configurations. Thus, using all steps above, we designed about 200 nanoalloy configurations, which were optimized using DFTPBE+D3. For the adsorption studies, we selected only three systems, namely, Pd55 , Au55 , and the composition that yields the lowest excess energy among the studied compositions. To identify the lowest adsorbed energy configurations, we considered all the non-equivalent sites based on their coordination and chemical species, e.g., onefold (top), twofold (bridge), threefold (hollow), and the combination of different chemical species to compose the adsorption sites. In contrast with the low-Miller-index surfaces, where the number of high-symmetry sites are few, the number of sites on the 55atom nanoclusters are larger, especially for Au55 due to the lower symmetry of the DRC structures. Thus, the adsorption study considered approximately 100 configurations among monometallic and bimetallic adsorption sites.


Core: Au 13 Surface: Pd 20 Au 22 Distribution: Homogeneous

Au 55

Core: Pd 10 Surface: Pd 10 Au 35 Distribution: Segregated

Core: Pd 13 Surface: Pd 7 Au 35 Distribution: Segregated

Core: Au 10 Surface: Pd 20 Au 25 Distribution: Homogeneous

Core: Pd 13 Surface: Pd 7 Au 35 Distribution: Homogeneous

High Symmetry

III Results A PdAu Nanoclusters

1 Excess Energy The excess energy (or mixing energy), Eexc per atom, was employed to quantify the relative energetic stability of bimetallic nanoclusters, i.e., if the nanoalloy formation is energetically favorable in relation to the monometallic systems. 37,44,45 To perform this analysis, we employed the following equation, Eexc =

2.50

1.90

1.28

1.00

0.00

5.74

4.04

3.28

1.83

0.00

6.92

5.31

3.16

2.61

0.00

6.41

4.24

2.78

1.97

0.00

6.23

5.15

3.65

0.75

0.00

6.34

5.39

4.58

1.80

0.00

4.79

2.49

1.90

0.37

0.00

2.01

1.28

0.83

0.28

0.00

Pd 42 Au 13

The pGMCs and the most important high-energy configurations for the Pdn Au55 − n nanoclusters are shown in Figure 2, which will be discussed below.

Pdn Au55 − n

0.00

Pd 20 Au 35

Core: Au 10 Surface: Pd 20 Au 25 Ditribution: Segregated

Figure 1 Representative scheme of the chemical species distribution for a specific case of Pd20 Au35 nanoclusters, considering the FCCf (pGMC) and ICO (high symmetry) configurations.

Etot

1.10

Pd 28 Au 27

Core: Pd 7 Au 6 Surface: Pd 13 Au 29 Distribution: Homogeneous

1.60

Pd 13Au 42

Core: Au 13 Surface: Pd 20 Au 22 Distribution: Segregated

2.14

Pd 35 Au 20

pGMC Core: Pd 5 Au 5 Surface: Pd 15 Au 30 Distribution: Homogeneous

4.57

−

n Pdm m E tot

m

−

m−n Aum m E tot

,

(1)

where m = 55 and n = 0, 6, 13, 20, 28, 35, 42, 49, and Pdn Au Pd Au 55. The Etot 55 − n , Etot55 , and Etot 55 are the total energies of the Pdn Au55 − n , Pd55 , and Au55 systems, respectively. Therefore, when n = 0 and 55, Eexc = 0.0 eV, while for negative (positive) values the compositions are energetically favorable (unfavorable). From the excess energy results, Figure 3(A), we observed that the formation of bimetallic nanoclusters is energetically favorable for all compositions. Its curve has a quasiparabolic trend in relation to the Pd composition (as observed by the black curve in the Figure 3(A)). In addition, there are few deviations from the parabolic behavior, of which, we highlight the Pd20 Au35 composition (Eexc = −0.10 eV) as the Eexc minimum. For this specific composition, we have an ICO structure with a core-shell distribution. To complement the excess energy understanding and to verify the strength of the binding between Pd−Au bonds, we calculated the homo- and hetero-dimers. From our calculations, we obtained the following values for the binding energies: −0.67 (Pd2 ), −1.17 (Au2 ), and −0.97 eV/atom

Pd 49 Au 6

Core: Pd 10 Surface: Pd 10 Au 35 Distribution: Homogeneous

Pd 55

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


Pd 6 Au 49

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Figure 2 Lowest energy configurations and some selected higher energy configurations for the bimetallic Pdn Au55 − n nanoclusters, where n = 0, 6, 13, 20, 28, 35, 42, 49, and 55. The relative energies, ∆Etot in eV, are indicated below every structure and the atomic positions for all these configurations are provided in the Supporting Information material.

(PdAu) and for the bond lengths: 2.47 (Pd2 ), 2.50 (Au2 ), and 2.48 Å (PdAu), which follow the trend obtained experimentally. 46–48 Considering the binding energy of the homoand hetero-dimers as a good estimation for the strength of the chemical bonds, we find that the binding energy obtained for the dimers helps to explain why the excess energy does not have a large magnitude. The energy gain due to the formation of the hetero-dimers, Emix , can be estimated by 1 using m = 2 and n = 1. The Emix measures the stability of PdAu dimer in relation to the homo-dimers (Pd2 and Au2 ) and is equal to −0.10 eV/atom, confirming the preference for the mixture (negative value). However, from



the dimer binding energies, the binding Au−Au is stronger, giving rise to a competition between the mixture and the partial segregation tendencies, obtained for the Pdn Au55 − n nanoclusters. 2 Chemical Order Parameter In order to characterize the distribution of the Pd and Au atoms in the nanoalloy, we calculated the chemical order parameter, σ, which is defined as follows, σ=

N Pd−Pd + N Au−Au − N Pd−Au , N Pd−Pd + N Au−Au + N Pd−Au

(2)

where, N Pd−Pd , N Au−Au , and N Pd−Au are the Pd−Pd, Au−Au, and Pd−Au bond numbers, respectively. These terms are defined by the number of nearest-neighbour bonds obtained from the effective coordination concept. 40,49,50 According to its definition, σ = +1 indicates complete segregation, i.e., there are no Pd−Au bonds (monometallic systems). However, σ = −1 indicates a complete homogeneous distribution without the formation of Pd−Pd and Au−Au bonds. Intermediate values between σ = +1 and −1, such as σ ∼ 0, indicate a possible homogeneous segregation and distribution, since the sum of the number of Pd−Pd and Au−Au bonds are almost the same as the number of Pd−Au bonds. From Figure 3(A), we observe a quasi-parabolic behavior of the σ in relation to the Pd composition. For n = 0 and 55 we observe σ = +1, because there are no Pd−Au bonds, with the substitution of Pd by Au, the value of σ decreases to n = 28. Furthermore, only this composition, which has ∼ 50 % of Pd and ∼ 50 % of Au, has negative σ value, i.e., there is a tendency to form hetero-bonds, Pd−Au. Hence, increasing the composition of Pd, σ increases until it reaches σ = 1 for n = 55, where we have the unary system (Pd55 ). Therefore, we find that minimizing the order parameter σ, which maximizes the number of heterobonds (Pd−Au), we can define the shape and magnitude of the surface energy. However, additional mechanisms are important to define some details of the excess energy in function of the composition. For example, the small change that occurs in excess energy for Pd-rich compositions, i.e., −0.08, −0.07, −0.06, and −0.02 eV for n = 28, 35, 42, and 49, respectively, can not be explained by the order parameter σ. The minimum value of σ (n = 28) is slightly less than zero, however, we observe smaller excess energy, consequently, there is a homo-bonds preference. 3 Radial Distribution Function Another analysis that contributes to the structural characterization is the radial distribution function, g(r ), which was calculated using the origin at the geometric center of each nanocluster (r = 0) and Mi = 1 for all atoms. The integral on g(r ) produces the number of atoms in the nanocluster and, therefore, the height of the g(r ) peaks is proportional to the number of atoms. The results are show in Figure 3(B). For DRC structure (Au55 ) we observe two broad peaks and no Au atoms near to the geometric center, i.e., the nearest Au

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atom is about 1.6 Å from r = 0, and thus it imitates a hollow particle composed of two full shells. For Pd compositions, for example n = 13, where the lowest energy configuration is ICO, there is a Pd atom in the center of the particle, r = 0, characterized by the first peak, 12 Pd atoms at 2.67 Å (second peak), 30 Au atoms at 4.72 Å (third peak), and 12 Au atoms at 5.39 Å (equivalent Au atoms located at the edges) composing the fourth peak. For the composition with approximately 50 % of Pd (n = 28) the shell is Au-rich, this behaviour can be observed up to n = 49, although these compositions have a higher number of Pd atoms. We observe complex structural transitions for PdAu systems, since we have three structural models (DRC, ICO, and FCCf) as the lowest energy configurations for the studied compositions. For example, an ICO-like model was found for the compositions n = 6, 13, 20, 35, and 42, where in all cases Pd is in the core and for n = 42 the Au atoms are in the edges. These ICO structures are stabilized by the relief of the internal strain in the nanoclusters. In addition, the lowest energy configuration for n = 13 is a core-shell configuration, with Pd atoms in the core region and Au atoms in the surface region. 4 Average Bond Length and Effective Coordination Number In Figure 3(A), we have the structural results for the average bond length, dav , and the effective coordination number, ECN, (see Supporting Information material for technical details) as a function of the Pd composition. We observe a decrease, with small oscillations, in the dav values, associated with the structural transitions, which occurs with the increase of the Pd composition in the bimetallic nanoclusters. In general, the dav values as a function of the Pd composition follow at average the Vegard’s law, 51 i.e., its behavior can be explained by the size of the atomic radius of Pd and Au species (as can be seen in Figure 3(A) by the linear fitting, black line, in dav curve). From n = 6, the dav decreases almost linearly as the Pd composition increases. This behavior is related to the larger atomic radius of Au (1.47 Å) compared to Pd (1.39 Å). The structural transitions that occur as function of the composition are another important factor for the behavior of dav and ECN curves, since the deviations in linear behavior are directly associated with the structural transitions (DRC−ICO−FCCf−ICO−FCCf), however, it is important to note that these structural transitions are true only for the specific case of 55-atom PdAu nanoclusters, with the specific compositions and geometries obtained in the present study. In addition to dav , the ECN is also sensitive to structural transitions and can provide additional information on the structural patterns. The highest ECN values are found for the ICO structures, while the DRC has a lower value. Therefore, among the reduced core models, DRC shows a lower ECN value compared to the FCCf structure, which has the ECN value intermediate between the ICO and DRC structures. We found that the structural transitions for the lowest energy configurations reflect in ECN results and, therefore, we can conclude that ECN increases for ICO configurations,


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0.9

0.08

0.6

σ

0.16 0.00

0.0

-0.16

-0.3

2.88

8.4

ECN

2.82 2.76

εd (eV)

B

1.00 0.00

7.8 7.2

6.6 -1.5

2.70 2.00

Qeff (e)

0.3

-0.08

dav (Å)

Eexc (eV/atom)

(A)

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

cationic anionic

-1.00 -2.00 0

10

20

30

n

40

50

-2.0 -2.5 total core shell

-3.0 -3.5

0

10

20

30

n

40

50

Figure 3 (A) Excess energy, Eexc ; chemical order parameter, σ; average weighted bond length, dav ; effective coordination number, ECN; effective Bader charges; and the center of gravity of the occupied d-states, εd , in relation to the Pd composition, n, for the Pdn Au55 − n nanoclusters. The red solid lines connect the respective property values for the lowest energy configurations, while the open symbols are the values for the high energy configurations. For the excess energy, σ, and dav the continuous black lines were obtained using a linear (for dav ) and a quadratic (for excess energy and σ) fitting from the lowest energy curves. The effective Bader charges are average values for core and surface regions, where the summation of the charges for all atoms is zero. For εd the Fermi energy is in 0.0 eV. (B) Radial distribution function, g(r ), relative to the geometric center of the nanoclusters (r = 0) for the Pdn Au55 − n lowest energy configurations.

remaining constant in the intervals n = 6 − 20 and 35 − 42. In the DRC configuration (n = 0), the ECN has the lowest value and when in FCCf (n = 28, 49 − 55) the ECN values are intermediate between DRC and ICO. Thus, initially, the structures are compact (ICO) and for n ∼ 50 % of each metal, we have the FCCf formation, with smaller ECN values. Consequently, the reduction of the core region, directly related with the ECN decrease, is associated with the larger coordination of core atoms. 5 Bader Charge Analysis The Coulomb interaction among the chemical species within the nanoalloys can be enhanced by the magnitude of the

effective charges on the chemical species, and hence, it is important to calculate the charge transfer between the Pd and Au atoms, which can takes place due to the Pauling electronegativity differences among the Pd (2.20) and Au (2.54) atoms. 52 Furthermore, different coordination environments can contribute to enhance the flux of electron density from one region to another. For this analysis, we employed the Bader concept, 53 which yields the charge confined within the Bader volume, being called Bader charge, QB , and it is based on the numerical analysis of the electron density topology. 54,55 Using QB , we calculated the effective Bader charge, QB , obtained from the difference in the number of eff B valence electrons, Zval , i.e., QB eff = Zval − Q . Thus, based B on the definition, Qeff > 0 indicates a cationic atom and QB eff < 0 an anionic atom. Our results, Figure 3(A), show a cationic core and an anionic surface for all PdAu nanoclusters, and hence, there is an attractive Coulomb interaction between the core and surface regions, which enhances the pressure of the surface atoms on the core ones. The effective Bader charge for PdAu nanoclusters can be affected by the simple fact of replacing Au atoms by Pd ones, since Au has higher electronegativity than Pd and, consequently, there is a force directed to the electron density flow from Pd to Au. Thus, the location of the Au atoms is crucial to define the signal and the magnitude of the effective Bader charge in the core or surface regions. For the PdAu nanoclusters, we obtained an anionic character of the surface, where the addition of Pd atoms increases this character until Pd13 Au42 and, then, it decreases. In all cases, we can observe an energetic preference of the Au atoms by the surface region, where the largest anionic charge is for the configuration Pd13 Au42 . 6 Center of Gravity of the Occupied d-states One of the best descriptors to correlate the density of states with adsorption properties is the center of gravity of the occupied d-states, 56 εd , which correlates almost linearly with the magnitude of the adsorption energy for many systems, e.g., water and ethanol on several Cu-, Pt-, and Au-based substrates, 57 water and ethanol on TM13 clusters (TM = Ni, Cu, Pd, Ag, Pt, and Au, 58 CO adsorbed on PtCO55 nanoalloys. 59 The εd for the Pdn Au55 − n nanoclusters are shown in the Figure 3(A), where the contributions for atoms of the core and shell regions are given separately (core and shell) and together (total). As the contributions of spin up and spin down are similar, we represent these values as an average of both contributions. In general, the εd presents a dependence on the Pd composition, i.e., it gets closer to Fermi’s energy with increasing Pd composition, from n = 0 (−3.15 eV) to n = 55 (−1.95 eV). Furthermore, the εd for total and surface atoms follows the same trend. For Au55 we have similar εd values (total, core, and shell), while for compositions n = 6, 13, 20, and 28, the εd for the core atoms are closer to the Fermi energy. For n = 6, the εd of the core atoms is −2.94 eV, however, for n = 13, 20, and 28, where the core region has Pd atoms, the εd decreases in magnitude −2.18, −2.20, and −2.13 eV, respectively, getting closer to


For the adsorption of CO, NO, and H2 on the PdAu nanoclusters, we selected the pGMCs for the Pd55 and Au55 nanoclusters, and the PdAu composition that minimizes the excess energy, Eexc , namely, Pd20 Au35 . Below, we show the main results, where we emphasize, when necessary, the improvements in the results with the addition of the vdW corrections.

−0.84

−0.58

−1.25

−1.65

−1.57

−2.11

−2.24

−0.40

−0.60

−0.92

−0.53

−1.01

−1.87

−1.85

−2.05

−2.60

−0.01

−0.02

−0.06

−0.02

−0.14

−0.54

−0.31

−0.05

−0.48

CO/Pd 55

CO/Pd 20 Au 35

−0.61

NO/Au 55

B CO, NO, and H2 Adsorption on PdAu Nanoclusters

−0.31

NO/Pd 20 Au 35

the Fermi energy than the surface atoms. For n = 35, 42, 49, and 55, the εd contribution is inverted and shell atoms have a larger contribution. In general, we can see that εd increases with the Pd composition. Hence, we verified that the εd depends on the variation of Pd, since, considering the surface atoms, the highest values are obtained for the compositions with the highest number of Pd atoms. In addition, from radial distribution function results, the shell region is dominated by Au atoms, thus, for Au-rich compositions (n = 0 − 13) the εd for shell atoms is around −3.15 eV, as Au atoms are replaced by Pd atoms, the Pd atoms begin to occupy the surface region and, consequently, the εd value for shell region begins to increase until reaching the Pd55 εd value (−1.95 eV).

2 Adsorption Energy To improve our understanding of the adsorption properties of CO, NO, and H2 on Pdn Au55 − n , we calculate the adsorption energy, Ead , obtained as follows, TM(mol) TM mol Ead = Etot − Etot − Etot , TM(mol) Etot ,

TM , Etot

mol Etot

(3)

are the total energies of and where the adsorbed systems, nanocluster, and isolated molecules,

H 2 /Au 55 H 2 /Pd 20 Au 35

The lowest energy configurations and two higher energy configurations are shown in Figure 4 and the adsorption energy, Ead , are given below the configurations. Considering the lowest energy configurations, the molecule is adsorbed on the top site for CO/Au55 (C−Au) and CO/Pd20 Au35 (C−Pd), while for CO/Pd55 the molecule is adsorbed on the bridge site. The NO/Au55 and NO/Pd20 Au35 systems have behavior similar to CO/nanoclusters, however, for NO/Pd55 , the molecule interacts on the hollow site. For H2 /Au55 , there is an energetic preference for top site, however, in H2 /Pd20 Au35 , each H atom binds with the Pd atom. Finally, for H2 /Pd55 , each H atom interacts with one or two Pd atoms. Thus, between Pd and Au species, the molecules prefer to interact with Pd, which is in agreement with the εd trend for shell atoms, i.e., the εd values for Pd systems are closer to the Fermi energy than Au systems. Consequently, systems with Pd species can be consider more reactive than Au ones, since Pd has an open d shell, while Au species presents a noble metal character, with a complete d shell (minimizing the interactions).

NO/Pd 55

1 Adsorption Site Preference

H 2 /Pd 55

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

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CO/Au 55


Figure 4 Lowest energy configurations and some higher energy configurations for CO, NO, and H2 adsorbed on Au55 , Pd20 Au35 , and Pd55 nanoclusters. The adsorption energies, Ead in eV, are indicated below every structure and the atomic positions for all these configurations are provided in the Supporting Information material.

respectively. The results are shown in Figure 5, where the same trend is obtained for the adsorption energy in relation to the Pd composition for calculations with and without vdW


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the trend obtained for the εd , i.e., the relative position of the center of gravity of the occupied d-states in relation to the Fermi level is determinant to drive the molecular interaction in the nanoclusters. We have obtained εd values of −3.15 eV (Au55 ) and −1.95 eV (Pd55 ), which reflects a noble (complete d shell) character for Au and a more reactive (open d shell) character for Pd systems. Thus, the CO, NO, and H2 interactions with Au55 occur on top sites, with the smallest adsorption energy values (in modulus), on the other hand, the molecular interaction with Pd55 is more intense (reactive), resulting in bridge and hollow adsorption sites. For the adsorption on Pd20 Au35 , we observe an intermediate behaviour for CO and NO adsorptions, with intermediate Ead values between Au55 and Pd55 . For the H2 adsorption on nanoclusters, we observe a different trend, with the smallest Ead values in relation to CO and NO. This behavior is associated with the complete filling of H s orbital, which presents a repulsive character, avoiding intense interactions with metals.

Ead (eV)

0.00 -0.80 -1.60 -2.40 -3.20 2.70

CO/PdnAu55-n - PBE

(Å)

CO/PdnAu55-n - PBE+vdW NO/PdnAu55-n - PBE

2.40

mol-TM

NO/PdnAu55-n - PBE+vdW H2/PdnAu55-n - PBE

2.10

d

H2/PdnAu55-n - PBE+vdW

1.80

d0 (Å)

1.20 CO - Exp. NO - Exp. H2 - Exp.

1.00

3 Structural Parameters

0.80 0.60 4000

-1

v (cm )

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


3000

2000

1000

0

10

20

n

30

40

50

Figure 5 Adsorption energy, Ead ; equilibrium distance between the molecules and the nanoclusters, dmol-TM ; molecular equilibrium bond length, d0 ; and vibrational frequencies, ν, for CO, NO, and H2 adsorbed on Au55 , Pd20 Au35 , and Pd55 nanoclusters, obtained by DFT-PBE and DFT-PBE+vdW calculations. The vibrational frequencies were obtained for molecules in gasphase and adsorbed on the nanoclusters. The dotted lines in figure indicate the experimental results.

corrections. The largest adsorption energies, in absolute values, are obtained for molecules adsorbed on Pd55 , i.e., −2.24 eV (CO) and −2.60 eV (NO). However, the largest Ead value for H2 molecule is obtained for Pd20 Au35 nanocluster, with the value −0.54 eV for DFT-PBE calculations. Similarly, for DFT-PBE+vdW we have obtained the largest adsorption energy values, in modulus, for CO adsorbed on Pd55 (−2.37 eV), NO adsorbed on Pd55 (−2.74 eV), and H2 adsorbed on Pd20 Au35 (−0.61 eV). Thus, we observe that the vdW corrections lead to higher Ead values, in modulus, which reflects in a correction for an enhanced nanoclustermolecule interaction. Furthermore, for both approaches, the smallest adsorption energy values (in modulus) are obtained for Au systems. The adsorption energy results are in direct agreement with

From Figure 5, we found a similar behaviour for PdAu interacting with CO and NO, which does not occur in systems with H2 , where the dmol-TM for Au55 is larger than for Pd55 and Pd20 Au35 values, since it is the only case with top site adsorption for the H2 molecule (the another cases are bridge sites, with smaller dmol-TM values). Considering the vdW corrections, we have obtained the dmol-TM values of 1.95 (CO/Au55 ), 1.89 (CO/Pd20 Au35 ), and 1.98 Å (CO/Pd55 ), 2.01 (NO/Au55 ), 1.84 (NO/Pd20 Au35 ), and 2.02 Å (NO/Pd55 ), and 2.47 (H2 /Au55 ), 1.77 (H2 /Pd20 Au35 ), and 1.75 Å (H2 /Pd55 ). It is important to highlight that the H2 molecule adsorbs on top site for Au55 , binding 1 H atom, while on Pd20 Au35 the top-site adsorption occurs for both H atoms on 1 Pd atom, and for Pd55 case, we have similar behavior, however the another H atom adsorbs on bridge site, consequently, the dmol-TM is reduced in comparison with H2 on Au55 . To complete the structural analysis, we show in Figure 5 the equilibrium bond length, d0 , for the molecules before and after the adsorption. Basically, we observe small changes, i.e., after the adsorption the molecules preserve their d0 values from gas-phase isolated cases, with exceptions for the H2 molecule on Pd atoms (for Pd20 Au35 and Pd55 ), where d0 is increased because of two H atoms bind to Pd, decreasing the dmol-TM and, consequently, increasing the d0 . The vdW corrections cause imperceptible changes in the d0 values. 4 Molecular Vibrational Frequencies We have calculated the vibrational frequencies, ν, for the CO, NO, and H2 molecules isolated in gas-phase and adsorbed on the nanoclusters (Au55 , Pd20 Au35 , and Pd55 ), Figure 5. In general, the vibrational frequencies for the isolated molecules show a good agreement with the experimental values, 60 differing by 6.6, 2.7, and 7.4 %, and are larger than vibrational frequencies for the adsorbed molecules. From previous works, the vibrational frequency of the CO



molecule on the Au and Pd surfaces are 2042 cm−1 and 1740 cm−1 , respectively. 61,62 Considering NO on the same surfaces, we found 1740 cm−1 for Au55 and, 1812 cm−1 and 1574 cm−1 for Pd55 , at the top and hollow sites, respectively. 4,63 These values for CO and NO corroborate with our results, since Pd20 Au35 interacts with NO at the top site and Pd55 at the hollow site. In addition, it is interesting to observe the reduction in the values of vibrational frequency, which occurs because the intensity of the binding energy of the molecules becomes weakened, since d0 increases.

IV

Conclusions

We performed a DFT-PBE study within the vdW D3 corrections to investigate the molecular adsorption of CO, NO, and H2 on the Pdn Au55 − n nanoclusters. We obtained the pGMCs for the unary and binary PdAu nanoclusters for different compositions, and the excess energy analysis indicated that the bimetallic Pdn Au55 − n systems are favorable to be formed. From the excess energy, we found a maximum stability for n = 20. A combination of factors helps to explain the formation of bimetallic nanoclusters, such as surface energy and atomic radius size, where the results show that Au prefers the surface region. Also, the Bader charge showed a Coulomb interaction between core and surface regions, with a cationic core and an anionic surface due to the presence of Pd atoms in the core region and Au atoms in the surface region, which is related to the higher Au electronegativity than Pd. Considering the adsorption process, we have obtained that the adsorption energy is larger (more negative) for the NO adsorption on nanoclusters and less intense (less negative) for the H2 adsorbed on the nanoclusters. The molecular adsorption does not induce significant structural changes, and the preferential site is the top, except for Pd55 with the CO (bridge) and NO (hollow) molecules. The interaction with molecules is more pronounced for Pd than Au nanoclusters, due to the smallest position about the center of gravity of the occupied d-states. Therefore, Pd is more reactive than Au; consequently, the preferential interaction tends to benefit Pd atoms in the surface to maximize the molecular adsorption process. From the adsorption energy, molecular distance between molecule and nanocluster, and vibrational frequencies results, we have obtained an intermediate behaviour for the adsorption on Pd20 Au35 nanoclusters, for CO and NO adsorptions, which is interesting for the design of the good catalysts, since the obtained adsorption energies are adequate to avoid catalyst poisoning.

V

Supporting Information

Extra data and analyses are provided in the Supporting Information material.

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VI Acknowledgement Authors thank the Rio Grande do Sul Research Foundation − FAPERGS and National Council for Scientific and Technological Development − CNPq (Grant number 305161/2015-6 and 306297/2018-3) for the financial support. This study was financed in part by the Coordenaça˜ o de Aperfeiçoamento de Pessoal de N´ıvel Superior − Brasil (CAPES) − Finance Code 001. JLFDS gratefully acknowledges support from FAPESP (São Paulo Research Foundation, Grant Number 2017/11631-2), Shell and the strategic importance of the support given by ANP (Brazils National Oil, Natural Gas and Biofuels Agency) through the R&D levy regulation. Authors thank also the infrastructure provided to our computer cluster by the Department of Information Technology − Campus São Carlos. The authors acknowledge the National Laboratory for Scientific Computing (LNCC/MCTI, Brazil) for providing HPC resources of the Santos Dumont supercomputer, which have contributed to the research results reported within this paper. URL: http://sdumont.lncc.br.

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