Theoretical Study of Hydrogen Adsorption on Au@ Pd Icosahedral

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Theoretical Study of Hydrogen Adsorption on Au@Pd Icosahedral Nanoparticle Mario G. Sandoval, Carla Romina Luna, Graciela Petra Brizuela, Aline Olimpio Pereira, Caetano R. Miranda, and Paula V. Jasen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00286 • Publication Date (Web): 06 Apr 2017 Downloaded from http://pubs.acs.org on April 10, 2017

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The Journal of Physical Chemistry

Theoretical Study of Hydrogen Adsorption on Au@Pd Icosahedral Nanoparticle M. G. Sandoval1,2, R. Luna1,2, G. Brizuela1,2, A. O. Pereira3, C. Miranda4, and P. Jasen*1,2 1

2

Departamento de Física, UNS, Av. Alem 1253, 8000, Bahía Blanca, Argentina. Instituto Física del Sur, UNS-CONICET, Av. Alem 1253, 8000, Bahía Blanca,

Argentina. 3

Universidade Federal do ABC, Av. dos Estados 5001, 09210-580, Santo AndrÃl,

SP, Brazil. 4

Departamento de Física dos Materiais e Mecânica, Instituto de Física, Universidade

de São Paulo, 05508-090, São Paulo, SP, Brazil.

*

Corresponding author:

Dr. Paula V. Jasen Departamento de Física (UNS) and Instituto de Física del Sur (UNS-CONICET), Av. Alem 1253, 8000, Bahía Blanca, Argentina, e-mail:[email protected]

ACS Paragon Plus Environment


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Abstract First-principles calculations based on the density functional theory (DFT) were applied to study the H2 adsorption on Au@Pd NP (core@shell icosahedral bimetallic nanoparticle). The calculations indicate that, for almost all adsorption sites, there is no energy barrier for H2 dissociation at the surface of Au@Pd NP, and the H2 molecule spontaneously dissociate. The only exceptions are the case of atop from edge (AE) and atop from vertex (AV) sites, where there are no-dissociation at all. Looking at the adsorption energies, dissociated cases are 1.3 eV more stable than non-dissociated cases. The work function (WF) values associated to NP with H2 adsorbed are lower than the obtained in the case of Pd/Au(111) surface. When H2 is dissociated on the NP or surface the WF increases while in the non dissociated case decrease. We also considered the changes in hydrogen adsorption and dissociation in mixed shell NP structures. The atomic H penetration was also studied for Au@Pd NP. Hydrogen adsorption on both, Au@Pd NP and Pd/Au(111) surface, systems lead to slight shift of Pd's d states to lower energies, while the s and p states are almost unaffected. A higher hybridization between Pd and H is detected in the NP case. Each H atom of the H2 molecule adsorbed on the NP becomes negatively charged. Seem that the charge transference occurs toward the NP. The bond order on orbital population analysis indicates no bond for H-H and a decrease in the metal-metal bond while a Pd-H bond is formed.

Keywords DFT, Hydrogen Adsorption, Nanoparticle, Hydrogen Storage, Core-Shell nanomaterials


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1. Introduction

The consolidation of high efficient hydrogen technologies relies on the search and development of storage materials capable to provide a high hydrogen storage density and good H sorption properties 1-3. The scientist community has studied the detailed mechanisms of electrocatalysis on atomic scale with the aim of get a better control of chemical reactions and then improves catalysts 4. Particularly, palladium has been widely studied in H storage area due to its high selectivity and sensitivity as well as room-temperature operation capability 5. Nanoparticles (NPs) promise to be one of the greatest advances for hydrogen storage materials due to the imminent technology evolution of last years 6, 7. The size range of them can vary from 1 to 100 nm, seen as a bridge between the isolated atoms and massive materials respectively. The experimental measurements and the computational calculations indicate that NPs improve the catalytic activity respect to extended flat surfaces

6,8-13

. Moreover, alterations observed in NPs such as size

reduction have involved fundamental changes in a variety of chemical, physical, and electronic properties in the catalytic steps and even optical characteristics

14-26

.

Recently, important applications have been reported in the electronics’ area for Palladium (Pd) NPs 27-29. The bimetallic systems are also studied as an innovative type of catalysts, mainly due to their physicochemical properties that can be tuned by varying the composition and atomic arrangement as well as their sizes and shapes

30

. Among

various bimetallic systems, the Pd-Au is one of the most attractive systems in catalysis due to broad range of applications

31

. For example, bimetallic and

core@shell Au-Pd structures have been used as catalysts in the oxidation reaction of



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CO at low temperatures

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32

, the Ullmann reaction of aryl chlorides in water

33

,

acetylene to ethylene conversion 34, oxidation of alcohols to aldehydes and production of vinyl acetate monomers, selective hydrogenation of butadiene 35, among others

36-

38

. Pd-alloy metallic NPs dispersed on supports, such as titanium oxide,

aluminum oxide, carbon, among others, are becoming the modern electrocatalysts in fuel cell technologies and other applications

14,39-47

. Especially, bimetallic

nanomaterials have attracted much attention due to their potential technological applications6,39,48. Their number of applications has been demonstrated that increases when multiple phases are combined. Some of these properties can be often greater than the sum of their parts

49

. Varying their sizes and shapes as well as the

composition and atomic arrangement will be possible change their physicochemical properties 30. The advantage of controlling these properties enables the design ratios of catalysts with specific properties for a particular application. The Au-Pd systems have been one of the most attractive objects of study in catalysis

50

. Several

experimental studies have reported the Au@Pd and Pd@Au NPs, as well different methods to synthesize them 51-56. In addition, there are some theoretical studies that have described about Au/Pd systems 57-60, and more recently about Au@Pd NPs 8. Serpell et al., using the anion coordination protocol, have reported a supramolecular route for the synthesis of core@shell nanoparticles of less than 5nm of size. They reported for the case of the Au@Pd nanoparticles, that the mutual electronic influence of these metals and their structural relationship has a clear influence in the catalytic properties of the resultant core@shell nanoparticles 6. Based on this, the aim of this work is to characterize the bimetallic Au@Pd NPs and understand their effects on the molecular hydrogen adsorption and its


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dissociation. Using First-principles calculations based on the density functional theory (DFT), regular icosahedral nanoparticle structure is considered as core@shell system. Indeed, chemical and physical properties are studied and compared with the Pd/Au(111) system (coverage/surface).

2. Models and theoretical method 2.1. Computational Model The icosahedral Au@Pd NP of 1nm of size is composed of a central core of Au atoms (core), coated by a surface of Pd atoms (shell), as can be seen on Figure 1a. The regular icosahedron has 30 edges and 12 vertices (Figure 1b). Therefore the 1nm core@shell cluster structure is a 55-atom regular icosahedral of which 42 atoms (76.4%) are located on the surface. This choose of size is in good agreement with the experimental results reported by Spitale et al. They found that the Au@Pd core-shell nanoparticles - with a core of Au and a shell of a monolayer Pd - are thermodynamically more stable for diameters smaller than 2 nm 8. Considering the fact that theoretical calculations predict that the core atoms segregates to the shell do to tension stress.

61,62

We performed additional calculations that agree with these

previous studies. We get a decrease in the excess energy from the first central Au to the last Au extracted from the core to the vertex positions in the shell. The difference of energy of the whole process is about 0.5 eV (detailed simulation process is described in SI). Pritchard et al report that adjusting the relative metal concentrations and co-reducing the metal precursors, well-dispersed Au-Pd random alloy NPs with tunable composition can be prepared. By varying the addition sequence during the colloidal preparation, Aushell-Pdcore and Pdshell-Aucore NPs were obtained 63. Experimental data showed the robust stability of Aucore-Pdshell and



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Pdcore-Aushell when synthesized via chemical methods 64-68. Su et al have identified Pdshell-Aucore NPs supported on TiO2 to have extraordinarily high efficiencies for the photocatalytic H2 evolution over purified crude glycerol from biorefinery 69. Additional considerations favored the study of Au@Pd NP. Mixed shell structure is not suitable for Hydrogen adsorption and dissociation. We also computed the adsorption of H2 molecule on Pd@Au and found that is unstable for all sites and molecule disposition.

Figure 1: The icosahedral core@shell architecture representation of Au@Pd bimetallic nanoparticle, and its adsorption sites.

Figure 1 shows the different sites of Au@Pd NP considered for H2 adsorption. AV (atop vertex), AE (atop edge), BE (bridge edge), B (bridge), FCC and HCPv (the subscript V denotes the sites that are near the Vertex). Moreover, several different orientations of H2 for each adsorption site were taking into account. The molecular tilting set-ups are side-on vertex, side-on face, side-on edge, parallel to a face on vertex, parallel to a face on edge, tilted to face, tilted to edge, vertical to vertex, vertical to face and vertical to edge. Further, for the side-on and parallel to face molecular orientations, the parallel, diagonal and


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transverse to the bridge, or to the bridge on edge to the Pd-Pd connections are tested (for a detailed description of this configurations, see Supporting Information). The icosahedral core@shell NP is compared with coverage/surface(111) system. We selected the Pd/Au(111) surface due to the icosahedral nanoparticles are formed by (111) like facet. These make the comparison between nanoparticles and surface direct. The Pd/Au(111) system studied here has a fcc crystallographic structure, as shown on Figure 2. The coverage/surface(111) system consist of a top layer slab as coverage and 8-layers below it as (111) surface, both being of different atom types as can be seen in Figure 2.

Figure 2: Schematic view of the adsorption sites on Au/Pd (111) surface. Left, top view and right, lateral view. For the sake of clarity only 5-layers are shown. The average of the nearest neighbor coordination of the Pd shell (or coverage) atoms defining the adsorption site is called coordination number of the adsorption site (Z). Table 1 presents the relevant adsorption sites as function of the Z.

Table 1: The location and coordination number (Z) for H2 adsorption sites in 1 nm core@shell NP and coverage/surface(111) systems (N/A means not available on that adsorption site).

Site

Location


Coord. Number (z) NP Surface


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B BE

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Bridge site on (111) facet

8

9

Bridge site at the edge of (111) facet

7

N/A

7.3

N/A

N/A

9

HCPv hcp site set up with 3 shell atoms at (111) facet closer to a vertex HCP hcp site around the center of (111) facet FCC

fcc around the center of (111) facet

8

9

AE

Atop site at the edge of (111) facet

8

N/A

AV

Atop site at vertex

6

N/A

N/A

9

A

Atop site at (111) facet

2.2 Theoretical method The calculations are performed using the DFT as implemented in the Vienna Abinitio Simulation Package (VASP)

70

. The projector augmented wave (PAW)

pseudopotential method is used to account for the electron-ion core interaction, using the PW91 functional in generalized gradient approximation (GGA) for the exchangecorrelation term. The Brillouin-zone is sampled at Gamma point. For the plane-wave basis set cut-off energy of 700 eV is considered. All the structures have been optimized until the forces acting in each atom are less than 10-4 eV/Å. Spin polarized calculations are taken into account. All Nanoparticles are placed in vacuum inside a cubic box with 20 Å edge length. In this way, the interactions between it and its periodic images are removed in each direction. Bader analysis as implemented by Tang et al. was used to calculate electronic charges on atoms 71, 72. The coverage/surface(111) interface is modeled using a 7-layer p(2x2) Au(111) surface slab covered with a 1-layer Pd thin film. A vacuum of 15 Å is placed on top of the studied surface slabs. The coverage film and the first four surface layers are relaxed while the bottom layers are fixed to the bulk positions. The optimized


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lattice constant is 4.18 Å which is characteristic for the Au(111) surface 73-75. The adsorption energy (Eads) is calculated following the equation: Eads = E(host+H2) - E(host) - E(H2),

(1)

where host can be Au@Pd NP or Pd/Au(111) surface. E(host+H2) and E(host) are the total energies of host-system with and without H2 adsorbed, respectively. The last term, E(H2), corresponds to energy of the isolated H2 molecule. In order to understand the hydrogen/metal and metal/metal interaction we explore the electronic properties such as density of states (DOS) and atom projected density of states (PDOS) of the most stable systems (lower adsorption energy). The last was calculated by projecting one-electron states onto spherical harmonic atomic orbitals centered on atomic sites. A qualitative study of the bonding between different atoms was also performed using the overlap population (OP) concept in extended structures (OPDOS) 76,77. Additional bonding analysis was calculated by the DDEC6 method 78-80.

3. Results and discussions 3.1 Optimized Geometry Table 2 lists the computed H2 adsorption energy and final bond distance between H atoms after adsorption for both host systems. For the sake of clarity only the most favorable sites are presented (for a complete list, see Table S1 in Supporting Information). H2 molecule is non dissociated only in two adsorption sites for Au@Pd NP. These sites are AE and AV and the final H-H bond distance presents an elongation, 13% and 14% respectively, respect to the isolated molecule (0.75 Å). In the other sites, H2 is dissociated and the H-H distance varies from 2.03 Å to 3.54 Å. Furthermore, Eads in these sites has similar values.



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Table 2: Sites and locations for H2 adsorption on Au@Pd NP and Pd/Au(111) surface. The Eads columns correspond to the adsorption energy of H2 for both host systems, and dH-H and dhost-H columns shown the distance between hydrogen atoms and the distance from H to the host, after relaxation respectively.

Sites

Au@Pd NP + H2 Eads (eV) dH-H (Å) dhost-H (Å)

Pd/Au(111) surface + H2 Eads (eV) dH-H (Å) dhost-H (Å)

B

-1.33

2.03

0.70

-1.20

2.00

0.36

BE

-1.44

3.28

0.76

-

-

-

HCP*

-1.45

3.54

0.80

-0.04

0.77

1.05

FCC

-1.39

2.93

0.76

-1.19

2.01

0.38

-

-

-

-0.35

0.85

1.02

AE

-0.60

0.88

1.66

-

-

-

AV

-0.48

0.85

1.70

-

-

-

A

*

Corresponds to HCPv site of NP (see Figure 1).

Finally the most energetically favorable sites for H2 adsorption on NP are the BE and the HCPv with the H2 in the side-on orientation, with similar adsorption energies, -1.44 eV and -1.45 eV, respectively. For the BE site, the H2 molecule is adsorbed in the diagonal orientation to the Pd-Pd bond, while for HCPv it is adsorbed along the Pd-Pd bond. The Figure 3 shows the H locations on Au@Pd NP before and after relaxation on HCPv. The Pd atoms nearest to H2 move outward. The Pd-Pd bond distance increases from 2.897 Å to 3.050 Å.


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Figure 3: The H2 adsorption at HCPV site, S-onF_B.P configuration on icosahedral Au@Pd NP, (a) before relaxation, and (b) after relaxation.

For Pd/Au(111) surface it can be seen that B and FCC sites are dissociative with the H2 in the horizontal orientation and with -1.20 eV and -1.19 eV of Eads, respectively (see Table 2). In addition, after adsorption each one of the hydrogen atoms is located on the nearest hollow sites. Indeed, both H-H distances are similar. Figure 4 shows the initial (Figure 4a) and final (Figure 4b) states of Pd/Au(111) surface + H2 system.

Figure 4: H2 adsorption on FCC site, horizontal location on Pd/Au(111) host system, (a) before relaxation, and (b) after relaxation.



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Figure 5 shows the effect of coordination number (Z) on H2 adsorption energy for different adsorption sites on Au@Pd NP and Pd/Au(111) surface. The figure is divided in two squares, one red that corresponds to sites where H2 is non dissociated and one blue that include all dissociated sites.

Figure 5: Adsorption energy (Eads) vs coordination number (Z) for all adsorption sites considered. All Z = 9 values correspond to surface sites (empty triangles) and the rest to the NP sites (filled triangles). It can be seen for nanoparticle that to smaller Z the Eads decreases when H2 is dissociated (blue square). There is an energy difference between dissociated and non dissociated H2 sites. This value is 0.72 eV for Au@Pd NP, whereas for Pd/Au(111) surface the energy difference is 0.85 eV . Similar results were reported by Han et al 14. Furthermore, it can be seem that in most of the cases, the H2 adsorption energy value is smaller for the nanoparticle than the surface. This indicates that the nanoparticle improve the hydrogen adsorption strength and also enhance the spontaneous dissociation. We additionally studied hydrogen (atomic) penetration into the subsurface. This process stars with hydrogen dissociated on the nanoparticle surface HCPv site and


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goes toward a tetrahedral sites (the more stables ones) to the inner of the nanoparticle. The first tetrahedral site is located at the Pd-Au interface and the final energy increase +0.2 eV. When H is located between an Au layer and the central Au atom the energy increase about +2 eV, in an unfavorable process.

(a)

(b)

Figure 6: The H2 adsorption at HCPV site on icosahedral Au@Pd NP with the Au central atom switched with a Pd vertex atom, (a) before relaxation, and (b) after relaxation. When one Au atom is located at the vertex and the H2 molecule is at HCPv site (which is the minimum in the Au@Pd NP) dissociation is not favorable and the adsorption energy is -0.53 eV (with a H-H distance of 0.98 Å), which is about three times lower than the value at pure Pd shell (see Figure 6). Even in the case when H2 is located in other HCPv sites away from Au position all sites suffer detrimental effects in their dissociative properties. The energy difference between molecular and dissociated states decrease from -1.45 eV to -1.22 eV when Au is extracted. In the case of mixed shell structures the extraction of Au to the NP surface vertex makes unfavorable the H2 dissociation process also affecting the properties of a Pd atoms that reduces its adsorption a dissociation capabilities. 3.3. Electronic structure



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3.3.1. Density of states The total DOS curves obtained for NP and surfaces are shown in Figure 6. We have plotted the projected DOS curves (PDOS) of d-bands for metals that conform host systems. Electronic behavior of Pd layer on Au strongly depends on system geometry. It can be seen that electronic states for the NP present a higher localization respect to surface (see Figure 7). Pd’s d-band has a sharper peak at 0.5 eV, which is particularly high. It can be seem in Figure 6 that for NP the major contribution is coming from Pd atoms d-band, whereas for surface case is due to Au atoms d-band.

DOS Figure 7: DOS curves for the most stable H2 adsorption site on Au@Pd NP. TDOS (a), Au d-band (b), Pd d-band (c) and H s-band (d) PDOS. DOS curves for the most stable H2 adsorption site on Pd/Au(111) surface. TDOS (a), Au d-band (b), Pd d-band (c) and H s-band (d) PDOS. Where the dashed red line and the filled blue line correspond to the curves before and after adsorption respectively. For a better view some PDOS curves are magnified. In addition, the NP and surface do not present magnetic metallic behavior. All DOS curves are highly symmetric before and after adsorption (see figure S6 in SI). For this reason only up spin contribution are plotted. The interaction between H2 and Au@Pd


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NP or Pd/Au(111) surface is strongly related to the properties of d-states of metallic system. Hydrogen adsorption on surface leads to slight shift on d-states of Pd atoms to lower energies, while the electronic states of Au atoms are almost unaffected. Moreover, the hybridization occurs between s-state of H atoms and the d-state of Pd atoms (see Figure 7). The NP also presents a small shift in the Pd d-band, however the effect is quite difficult to see in the total DOS curves (compare Figure 7 c with g). For a better view, in Figure 8, we plotted the projected DOS curves on H atom and their metallic nearest neighbors. From this figure, is clear that the hybridization is between Pd and H atoms. Also it can be seen that the interaction is more important in the NP case, which is consistent with the lower adsorption energy values.

DOS Figure 8: PDOS curves for Au1 atom (a), Pd1 atom (b), for Pd2 atom (c), and for H1 atom (d) the most stable H2 adsorption site on Au@Pd NP. PDOS curves for Au1 atom (a), Pd1 atom (b), for Pd2 atom (c), and for H1 atom (d) the most stable H2 adsorption site on Au/Pd(111) surface. Where the dashed red line and the filled blue line correspond to the curves before and after adsorption respectively.

3.3.2. Electronic charge



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By analyzing the number of electrons carriers (e-) before and after H2 adsorption, we found that each H atom accepts 0.16 e-. The Pd atoms nearest neighbors to the H atoms have a lost about of 0.07 e-, except for the Pd located in vertex (Pd1 in Figure 3) that loses about 0.11 e-. For the Au closer to the H atoms, the lose is about 0.02 e-. Figure 9 shows the layer by layer net charge distribution and the distribution of charge onto the individual atoms in across section of Au@Pd NP before and after H2 adsorption. In this figure, the charges have been normalized by the total amount of charge transferred from Pd to Au, wich is particle independent. From this figure, is clear that the charge is not equally distributed onto the atoms within the same layer, and also different between layers.


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Figure 9: Effect of H2 adsorption on charge distribution in Au@Pd NP. Net charge distributions for each NP layer and H atoms (a) and (b). Cross sections of the nanoparticles where are identify some atoms (c) and (d). Distribution of charge onto individual atoms corresponding to the upper figures (e) and (f).

It can be seen from Figure 9a that almost all of the charge transfer occurs at the nanoparticle core-shell interface before adsorption (layer 1 and layer 2). This result is in good agreement with that reported by Holmberg et al 81. The charge transfer is distributed within the metal phases, where the total charge is retained directly at the contact interface. The charge transfer between the core and shell is almost unaffected after adsorption. Indeed, the Pd shell has an increase of the positive net charge, because charge transfer occurs from Pd atoms to H atoms, while the net charge for Au core is almost the same. This behavior is attributed to capacitive properties of its core 82

. Before H2 adsorption negative charge is accumulated on Pd atoms from the

vertices. After adsorption the charge is redistributed and the vertices decrease their charge with the exception of the Pd atom directly bonded to H-H in this case Pd becomes positively charged as it transfer electron density to the H-H bond and the H atoms become more negative. This flux of electron density towards the H-H is located in the π* antibonding orbital and facilitated H2 dissociation. This is consistent with an H-H adsorption distance of about 3 Å. After H adsorption, the Au atoms do not present changes in its charge states, that is to say the H adsorption do not affect the charge state of the NP core.

3.3.3. Bonding analysis The bond order (OB), the bond order percentage change referred to the pure system (∆BO%), the overlap population (OP), the overlap population percentage change referred to the pure system (∆OP%) and the distances before and after H2 adsorption



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on Au@Pd NP and Pd/Au(111) surface are summarize in Table 3. A null BO and OP for H-H on the NP indicate clearly a dissociative states. The metal-metal OP and BO decrease when H2 is adsorbed. Table 3: Bond order (BO), ∆BO% (BO percentage change computing referring to pure system), Overlap Populations (OP), ∆OP % (OP percentage change computing referring to pure system) and distances before and after H2 adsorption. The labels of atoms are shown in Figures 3 and 4.

before

BO after ∆(BO%)

before

OP after ∆(OP%)

d (Å) before after

H2-H1

1.000

0.000

-100

0.551

0.000

-100

0.750

3.544

H1-Pd1

-

0.384

-

-

0.310

-

-

1.864

H1-Pd2

-

0.400

-

-

0.317

-

-

1.868

H1-Pd3

-

0.471

-

-

0.361

-

-

1.791

H1-Au1

-

0.019

-

-

0.019

-

-

3.007

Au1-Pd1

0.421

0.398

-5

0.350

0.332

-5

2.661

2.668

Au1-Pd2

0.339

0.307

-9

0.296

0.274

-7

2.760

2.792

Au1-Pd3

0.339

0.295

-13

0.296

0.265

-10

2.759

2.887

Pd1-Pd2

0.298

0.219

-26

0.265

0.208

-21

2.843

2.857

Pd1-Pd3

0.298

0.201

-32

0.264

0.195

-26

2.843

2.882

Pd2-Pd3

0.251

0.203

-19

0.232

0.197

-15

2.897

2.867

H2-H1

1.000

0.031

-97

0.551

0.020

-96

0.750

2.008

H1-Pd1

-

0.497

-

-

0.380

-

-

1.741

H1-Pd2

-

0.345

-

-

0.284

-

-

1.871

H1-Pd3

-

0.346

-

-

0.284

-

-

1.870

H1-Au1

-

0.025

-

-

0.025

-

-

2.925

NP

Surface


Page 19 of 37

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Au1-Pd1

0.302

0.266

-12

0.269

0.243

-10

2.830

2.863

Au1-Pd2

0. 302 0.245

-19

0. 269 0.228

-15

2.830

2.887

Au1-Pd3

0. 302 0.246

-18

0. 269 0.228

-15

2.830

2.886

Pd1-Pd2

0.227

0.149

-34

0.212

0.145

-32

2.965

3.004

Pd1-Pd3

0.227

0.150

-34

0.212

0.146

-31

2.965

3.001

Pd2-Pd3

0.227

0.153

-32

0.212

0.150

-29

2.965

2.920

In dissociative cases the H-H BO is almost zero. For H2, all values are computed in the same box as used for NP or surface.

The most important decrease is found in H-H bond while a Pd-H bond is developed at expense of metal-metal bonding with a bond length of about 1.8 Å. Before H2 adsorption on the NP, the total BO of each Au atom for layers 0 and 1 (as labeled in Figure 9) are 4.16 and 3.89 respectively. Whereas the BO of the Pd atoms in the shell are 1.92 for each Pd atom located in the vertex and 2.31 for the rest. After H2 adsorption on the NP, there are no noticeable changes on the total BO for Au atom. For the Pd atom located in NP vertex between the two adsorbed hydrogen atoms (Pd1 from Figure 9), the total BO increases around of 29% (from 1:92 to 2:48), while the rest of each Pd atom remain with almost the same BO value. In addition, the total BO of each H atom increases about 29% with respect to the H atom in the isolated H2 molecule. This is consistent with a negative charge developed on both H atoms (-0.16 e-) after adsorption. Considering specific atoms on the NP, H bonds with Pd i (i = 1, 2 or 3) at distances of about 1.8 Å with a BO of 1.255. On the surface, the H-H bond is elongated (2.00 Å) while in the NP the H-H distance is about 3 Å indicating a net dissociative state. The BO of H atom with Pdi (i = 1, 2 or 3) is 1.188 that is lower when compared with the BO in the NP. The metal-metal bonds also change. The Au1-



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Pd1 BO decreases from 0.421 to 0.398 on the NP. There is also a decrease for Au1-Pd2 and Au1-Pd3. Pd1-Pd2, Pd1-Pd3 and Pd2-Pd3 also decrease on the NP after H2 adsorption. The BO percentage change is more noticeable in the surface showing a decrease on the metal-metal bond while H-Pd bond is developed (see Table 3). The mean metal-metal BO percentage change is higher on the surface if compared with NP. Before H2 adsorption on surface, the total BO values of each Au atom increase with the layers (from the inner to the outer Au layers). As a result of that, the total BO of the two last Au atoms layers are 3.13 and 3.27. Whereas the BO values of Pd layer atoms are around 2.31, that are similar to the BO of the Pd layer atoms that are non located in vertex of NP. When H2 is adsorbed, there are no noticeable changes in BO values for the Au atoms, only the Au atom in the outer layer that is below one of the H atom adsorbed, present a slight decreases of its value of about 3%. Moreover, the BO values of Pd atoms increase about 14% (from 2.31 to 2.63 approximately). Similar consideration about bonding can be seen from changes in the OP. Figure 10 shows the electron density distribution around NP atoms after H2 adsorption, on where the blue and red colors indicate a negative and positive electronic charge density respectively.


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Figure 10: Electronic charge distribution around NP atoms after H2 adsorption. The blue (red) color indicates a negative (positive) electronic charge density.

The charge density difference (∆ρ) isosurface is calculated using the following equation: ∆ρ = ρ(NP+H2dis) - ρ(NP) - ρ(H2dis). Where ρ(NP+H2dis) is the charge density of the NP after H2 is adsorbed dissociative, ρ(NP) is the charge density of the relaxed NP and ρ(H2dis) is the charge density of the dissociated H atoms in their final configuration after adsorption. From this figure it can be seen that the transferred charge between H and Pd atoms is consistent with charge transfer in Figure 9 and previous BO analysis. In addition, the concentration of charge around H and Pd atoms is consistent with the hybridization shown in Figure 8.

3.3.4. Work functions



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Page 22 of 37

As shown by Holmberg et al. 81, electrons can freely exchange between the metalmetal contacts. When the initial Fermi levels (EF) of the metals are different, a net flow of electrons takes place from the material with the lower work function (WF) into the other until the EF are equal in both phases. This leads to an outer potential difference between Au and Pd metals, that is directly proportional to the WF difference between the two metals, i.e., some of the charge transferred during the equilibration is distributed on the surfaces, resulting in an electric field, although most of the charge is retained at the metal contact interface, as a surface dipole (as can be observe in Figure 9a and b). The WF was computed as the energetic difference between the vacuum level and the EF. Particularly, in metal systems the WF and EF values provided information about the ability of the metal to gain or lose electrons. In Table 4 are listed the WF and EF obtained for NP and surface, in both cases with different composition, Au or Pd pure and bimetallic composition. These values are similar to those reported in literature 8385

. The Fermi level of Au NP is higher than Pd NP, this fact indicate that Au may

transfer electrons to Pd when they are in contact. This is in agreement with the experimental results reported by Zhang et al. and with our results about charge transfer (see Figure 9a) 86. Nevertheless, this behavior is different that the shown by the surface. The WF of Pd(111) surface is 5.28 eV and the EF of Pd is lower than Au, thus suggesting that Au should transfer electrons to Pd, so that the Pd surface will be in a rich electronic state. We found that there is a slight transfer from Au atoms to Pd atoms. Similar results were obtained by Zhang et al.

86

. For Au@Pd NP and

Pd/Au(111) surface the WF obtained are 4.90 eV and 5.23 eV respectively. These values are similar to those reported in literature 83-85. Table 4: Work function (WF) and Fermi level (EF) for Au, Pd and Au@Pd NP and Au, Pd and Au/Pd(111) surfaces.


Page 23 of 37

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System

WF (eV)

EF (eV)

Au NP

5.07

-3.82

Pd NP

4.88

-3.84

Au@Pd NP

4.90

-3.80

Au(111) surface

5.20

0.58

Pd(111) surface

5.28

0.28

Pd/Au(111) surface

5.23

0.40

From Table 5 it can be seen that when H2 is adsorbed on Au@Pd NP (Pd/Au(111) surface), the WF presents a slight change respect to isolated Au@Pd NP (Pd/Au(111) surface). This change depends on if the H2 molecule is dissociated or not after adsorption. When H2 is dissociated on the nanoparticle the WF increase in the range of 0.20% to 0.61%, while in the surface increase 0.57% for both dissociative cases. In the non dissociated cases of H2 on Au@Pd NP the WF decrease around of 0.41%, whereas for Pd/Au(111) surface WF decrease about 2.29%.

Table 5: Work function (WF) values by sites, after H2 adsorption on both host systems, Au@Pd NP and Pd/Au(111) surface. Before H2 adsorption the WF values are 4.90 eV and 5.23 eV respectively. The symbol % indicates the percentage change of WF respect to each of the isolated host systems. Au@Pd NP Sites

Au/Pd(111) surface

WF (eV)

%

WF (eV)

%

B

4.92

+0.41

5.26

+0.57

BE

4.92

+0.41

-

-



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Page 24 of 37

HCP*

4.91

+0.20

5.14

-1.72

FCC

4.93

+0.61

5.26

+0.57

-

-

5.11

-2.29

AE

4.88

-0.41

-

-

AV

4.88

-0.41

-

-

A

*

Corresponds to HCPv site of NP (see Figure 1).

Furthermore, it can be seen that the WF values associated to NP with H2 adsorbed are lower than the obtained in the case of Pd/Au(111) surface. The WF changes are consistent with more favorable dissociative chemisorption of H2 on the NP compared to layered surface.

4. Conclusions In the present work, first principle calculation based on DFT were used to study H2 adsorption on Au@Pd NP (core@shell nanoparticle) and compare with Pd/Au(111) surface. From the obtained results it can be concluded that: •

The NP dissociates the H-H bond more easily that in the Pd/Au(111) surface. On the surface the H-H bond is elongate and not totally broken.

•

Atomic H penetration is unfavorable in the Au@Pd NP.

•

Even the mixed shell NP structures are slightly more stable, its hydrogen adsorption and dissociation properties are much lower than the ideal Au@Pd NP.

•

The metal-metal bonding is more weakened in the surface and the effect of H2 adsorption is local. In the NP, the metal-metal bond is less affected and the effect of the interaction with the adsorbed H atoms is more global. The Pd-H


Page 25 of 37

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bond is more developed in the NP than in the surface. All calculations for DOS, BO, OP and charge density agree with this analysis. •

The charge transfer shows that NP has a capacitive behavior and that after H2 adsorption the transference is from Pd atom to H atom.

•

The WF increases in the dissociative case on the NP, while on the non dissociative sites (AE and AB) decrease. The surface shows the same behavior with less intensity. The WF changes are consistent with a more favorable dissociative chemisorption of H2 on the NP compared with the layered surface.

Summarizing, is important to remark that our results indicate that the NP could adsorbed more dissociate H than the surface, without suffering an important weakening in its structural metal-metal cohesion. These results made NP suitable candidate for H storage.

Acknowledgments The simulations were performed at the National Center for High Performance Computing in São Paulo (CENAPAD-SP). MGS acknowledge the project Programa de Associação para Fortalecimento da Pós-Graduação, Setor Educacional do MERCOSUL, CAPES, PFPG 011/2011, and a fellowship from CONICET RL, GB on PJ are member of CONICET. AOP on CM acknowledge. We acknowledge financial support from SGCyT-UNS, ANPCyT-PICT 2014-1351 and CONICET - PIP 20142016 GI 11220130100436CO.

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Caption for figures Figure 1: The icosahedral core@shell architecture representation of Au@Pd bimetallic nanoparticle, and its adsorption sites. Figure 2: Schematic view of the adsorption sites on Au/Pd (111) surface. Left, top view and right, lateral view. For the sake of clarity only 5-layers are shown. Figure 3: The H2 adsorption at HCPV site, S-onF_B.P configuration on icosahedral Au@Pd NP, (a) before relaxation, and (b) after relaxation. Figure 4: H2 adsorption on FCC site, horizontal location on Pd/Au(111) host system, (a) before relaxation, and (b) after relaxation. Figure 5: Adsorption energy (Eads) vs coordination number (Z) for all adsorption sites considered. All Z = 9 values correspond to surface sites (empty triangles) and the rest to the NP sites (filled triangles).



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Figure 6: DOS curves for the most stable H2 adsorption site on Au@Pd NP. TDOS (a), Au d-band (b), Pd d-band (c) and H s-band (d) PDOS. DOS curves for the most stable H2 adsorption site on Pd/Au(111) surface. TDOS (a), Au d-band (b), Pd d-band (c) and H s-band (d) PDOS. Where the dashed red line and the filled blue line correspond to the curves before and after adsorption respectively. For a better view some PDOS curves are magnified. Figure 7: PDOS curves for Au1 atom (a), Pd1 atom (b), for Pd2 atom (c), and for H1 atom (d) the most stable H2 adsorption site on Au@Pd NP. PDOS curves for Au1 atom (a), Pd1 atom (b), for Pd2 atom (c), and for H1 atom (d) the most stable H2 adsorption site on Au/Pd(111) surface. Where the dashed red line and the filled blue line correspond to the curves before and after adsorption respectively. Figure 8: Effect of H2 adsorption on charge distribution in Au@Pd NP. Net charge distributions for each NP layer and H atoms (a) and (b). Cross sections of the nanoparticles where are identify some atoms (c) and (d). Distribution of charge onto individual atoms corresponding to the upper figures (e) and (f). Figure 9: Electronic charge distribution around NP atoms after H2 adsorption. The blue (red) color indicates a negative (positive) electronic charge density.


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TOC


Theoretical Study of Hydrogen Adsorption on Au@ Pd Icosahedral

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