Ethanol and Water Adsorption on Transition-Metal 13-Atom Clusters: A

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Ethanol and Water Adsorption on Transition-Metal 13-atom Clusters: A Density Functional Theory Investigation within van der Waals Corrections Larissa Zibordi-Besse, Polina Tereshchuk, Anderson Silva Chaves, and Juarez L. F. Da Silva J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b03467 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 13, 2016

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The Journal of Physical Chemistry

Ethanol and Water Adsorption on Transition-Metal 13-Atom Clusters: A Density Functional Theory Investigation within van der Waals Corrections Larissa Zibordi-Besse, Polina Tereshchuk, Anderson S. Chaves, and Juarez L. F. Da Silva∗ São Carlos Institute of Chemistry, University of São Paulo, PO Box 780, 13560-970, São Carlos, SP, Brazil E-mail: [email protected]

Phone: +55 16 3373 6641. Fax: +55 16 3373 9952

Abstract

by the addition of the vdW correction (i.e., from 4 % to 62 %), however, the trend is the same. We found that the magnitude of the adsorption energy increases by shifting the center of gravity of the dstates towards the highest occupied molecular orbital. Based on the Mulliken and Hirshfeld charge analysis, as well as electron density differences, we identified the location of the charge redistribution and a tiny charge transfer (from 0.04 e to 0.19 e) from the molecules to the TM13 clusters. Our vibrational analysis indicates the redshifts in the OH modes upon binding of both water and ethanol molecules to the TM13 clusters, suggesting a weakening of the O−H bonding.

Transition-metal (TM) particles supported on oxides or carbon black supports have attracted much attention as potential catalysts for ethanol steam reforming reaction for hydrogen production. To improve the performance of nanocatalysts, a fundamental understanding of the interaction mechanism between water and ethanol with finite TM particles is required. In this article, we employed first-principles density functional theory within van der Waals (vdW) corrections to investigate the interaction of ethanol and water with TM13 clusters (TM = Ni, Cu, Pd, Ag, Pt, and Au). We found that both water and ethanol bind via the anionic O atom to onefold TM sites, while at high energy configurations ethanol binds also via the H atom from the CH2 -group to the TM sites, which can play an important role at real catalysts. The putative global minimum TM13 configurations are only slightly affected upon the adsorption of water or ethanol, however, for few systems, the compact icosahedron structure (high-energy configuration) changes its structure upon ethanol or water adsorption, i.e., those configurations are only shallow local minimums in the phase space. We found similar trends for the magnitude of the adsorption energies of water and ethanol, i.e, Ni13 > Pt13 > Cu13 and Pd13 > Au13 > Ag13 , which is enhanced ∗

I

Introduction

The success of ethanol (CH3 CH2 OH) steam reforming reactions (SRR) for hydrogen (H2 ) production depends strongly on the operating conditions (temperature, pressure, etc) and on the catalyst (particle size, chemical composition, support, preparation method, etc), 1 which should have lower cost, high stability, and maximize ethanol conversion and selectivity towards H2 . 1–3 The overall ethanol SRR can be represented by the following ideal reaction, CH3 CH2 OH + 3 H2 O −−→ 2 CO2 + 6 H2 , with ◦ ∆H298 = 174 kJ mol−1 , 4,5 where the conversion of ethanol and the H2 selectivity is 100 %, 2 which

To whom correspondence should be addressed

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focused on the study of the interaction of water and ethanol with compact TM surfaces employing density functional theory 36,37 (DFT) and the slab geometry model. 28,34,38 They found that both water and ethanol bind via the anionic O atom to the onefold TM sites on the compact TM(111) surfaces, which includes all TM elements mentioned in this work. Water is nearly parallel to the surface for a wide range of surfaces, 25,26,39 while the C−C ethanol bond is nearly perpendicular to the surface, however, the lowest energy configurations with the C−C bond nearly parallel and perpendicular differ by less than 100 meV, i.e., both configurations exist in real conditions. Due to the high stability of compact TM surfaces, the adsorption energy is relatively weak, i.e., from 0.10 eV to 0.40 eV for ethanol/TM(111) employing semilocal DFT functionals, however, it increases substantially by adding van der Waals (vdW) corrections to the DFT total energy. 34,40 Thus, although several experimental and theoretical studies have been reported so far, our atomistic understanding of the interactions of water and ethanol with TM particles is limited, in particular, due to the lack of studies for the interaction of water and ethanol with finite size systems and to the complexity of adsorption on non-symmetric surfaces. To address this problem, we selected few TM particles with 13-atoms, which are subnanometer particles with a large number of nonequivalent adsorption sites and the role of the discrete nature of the electronic states on the adsorption properties is discussed. Our results are based on first-principles DFT calculations within vdW corrections for the adsorption of water and ethanol on the TM13 clusters (TM = Ni, Cu, Pd, Ag, Pt, and Au). We found that in the lowest energy configurations, water and ethanol bind via the anionic O atom to the onefold TM sites, which is similar to the adsorption on TM surfaces, however, we identified for few systems that at high energy configurations, ethanol binds also via the H atoms from the CH2 -group to the onefold TM sites. The vdW corrections enhances the adsorption energy, which is expected, however, the effects on the atomic structure are tiny beyond small changes on the O−TM distance. Based on the electron density difference, Hirshfeld, and Mulliken analysis, we found

is not the case in real conditions as ethanol SRR involves also water-gas-shift and methanation reactions. 1–3 The heat of formation at room temperature is positive and large, i.e., the ethanol SRR are highly endothermic, and hence, high operation temperatures are necessary. 1,2 Most of ethanol SRR catalysts are based on transition-metal (TM) particles supported on oxide compounds, namely, TM particles of Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au supported on oxides such as MgO, La2 O3 , ZnO, CeO2 , Al2 O3 , ZrO2 , V2 O5 , SnO2 , and SiO2 have been studied for ethanol SRR. 1–3,6–14 Among those TM/oxides systems, Co/ZnO, 14 Rh/Al2 O3 , Rh/CeO2 , 15 and Ni/La2 O3 −Al2 O3 , 13 have yielded excellent performance, 1–3,8 however, it depends also on the catalysts preparation method, which can affect ethanol conversion and H2 selectivity. The size of the TM particles has been reduced year by year, in particular, motivated by the experimental findings reported by Haruta et al. 16 for the oxidation of carbon monoxide at low temperatures (e.g., −70 ◦C) using Au nanoparticles (NPs) and by our increased ability to control particle size and shape at nano and subnano scale. 17–20 It has been reported that Au NPs with diameter from 1.0 nm to 5.0 nm exhibits surprisingly high activity and selectivity for several reactions. 16,19,21,22 Thus, there are large expectations that TM NPs supported on oxides can improve the breaking of the strong C−C σ bond and to reduce the number of undesirable intermediate compounds, 1–3,14 which present a great challenge for ethanol SRR catalysts. Platinum (Pt) NPs of about 3 nm supported on carbon black have been studied, 9,10,23 as well as combined with SnO2 , 10 however, acid acetic is one of the predominant undesirable products. Beyond of Pt NPs, Co NPs with size from 3 nm to 8 nm supported on SiO2 showed good activity, 24 in particular, the smaller particles, however, a solid explanation has not been suggested for the higher efficiency of Co NPs with about 3 nm so far. Although several experimental studies have been reported for ethanol SRR on TM NPs supported on oxides, 1–3,6–14,23,24 there is a lack of theoretical studies to obtain an atomistic understanding of the interaction of ethanol and water with TM particles with few atoms. For example, most of the experimental 25,26 and theoretical 26–35 studies have


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light-tier2, tight-tier1, and tight-tier2 levels, following FHI-aims terminology, 43 and the results are reported in the Supporting Information (SI). The main difference between the light and tight basis set is the length of the cutoff potential, respectively, 5.0 Å and 6.0 Å, while tier indicates the level of improvement from the minimal basis with the aim to improve the accuracy of the calculations. We found that the differences in the bond lengths are smaller than 0.01 Å among the light-tier1 and tight-tier2 basis sets, while there is an average difference of about 0.02 eV/atom in the binding energies. Thus, we selected the light-tier2 basis set to describe the TM atoms, namely, Ni, Pd, Pt, Cu, Ag, and Au, while the C, H, and O atoms were described by the light-tier3 basis-set to keep the same level of energy accuracy within the basis set. For all calculations, we employed a gaussian broadening parameter of 10 meV, while the electronic self-consistency is obtained once the total energy and forces reach the convergence of 10−6 eV and 10−3 eV/Å, respectively. The equilibrium configurations were obtained once the atomic forces on every atom are smaller than 10−2 eV/Å employing the modified Broyden–Fletcher–Goldfarb–Shanno (BFGS) algorithm with trusted region, as implemented in FHI-aims. 43 For the vibrational frequency calculations, we employed finite differences to calculate the Hessian matrix with an electronic selfconsistency convergence of 10−5 eV/Å, which is required to obtain high accurate forces and atomic displacements of 25 × 10−4 Å.

a charge transfer from the molecules to the TM13 clusters, which contributes to enhance the adsorption energy.

II A

Theoretical Approach and Computational Details Total Energy Calculations

Our total energy calculations are based on spinpolarized DFT 36,37 within the formulation proposed by Perdew–Burke–Ernzerhof (PBE) for the exchange-correlation energy functional. 41 To improve the description of the long range nonlocal correlation effects, we employed the vdW correction proposed by Tkatchenko–Scheffler (TS), 42 in vdW which a vdW energy correction, Eenergy , is added DFT+PBE to the DFT total energy, Etot , i.e., the total energy is given by, DFT-PBE vdW Etot = Etot + Eenergy ,

(1)

where vdW Eenergy =−

1X C6AB fdamp (RAB , R0A , R0B ) 6 . 2 A,B RAB

(2)

Here, fdamp is a damping function, which is required to obtain the correct vdW dependence at short distances between the atoms, while RAB , R0A and R0B indicates the interatomic distances and the vdW radii of atoms A and B, respectively. The C6AB parameters are the dispersion coefficients, which play a crucial role in the magnitude of the vdW energy correction. Here, those parameters are obtained by the self-consistent screening (SCS) TS approach, often called TS+SCS. 42 The Kohn-Sham (KS) equations were solved using the Fritz-Haber Institute Ab-Initio Molecular Simulations (FHI-aims) package, 43–45 which yields an all-electron solution for the KS equation using the scalar-relativistic framework within the zeroth-order approximation. 46 The KS orbitals are expanded in numerical atom-centered orbitals, 47,48 which were hierarchically constructed from minimal basis set on up to meV-level total energy convergence. 43 Total energy convergence test calculations were performed for water, ethanol, and 13atom clusters in gas-phase for basis-set light-tier1,

B Atomic Configurations The putative global minimum configurations (pGMC) for 13-atom clusters have been reported and discussed in details in several studies from our group. 49–54 Thus, with the aim to improve our atomistic understanding and to obtain possible new pGMC using the all-electron FHI-aims implementation, we optimized about 60 atomic configurations for each TM13 system, which includes well-known structures such as the doublesimple cubic (DSC), icosahedral (ICO) with Ih symmetry, cuboctahedral (CUB) with Oh symmetry, hexagonal bilayer (HBL) and buckled-biplanar (BBP) with C3v symmetry. Furthermore, it in-


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Figure 1: Putative global minimum configurations (pGMC) for water, ethanol (gauche), Ni13 , Pd13 , Pt13 , Cu13 , Ag13 , and Au13 (two-dimensional pGMC structure – pGMC2D) clusters. For ethanol and Au13 , the trans-ethanol and three-dimensional pGMC (pGMC3D) are also shown, respectively. For reference, the compact 13-atom icosahedron structure with Ih symmetry is also indicated. metrics, 58,59 every configuration is converted into a vector that contains the distances from every atom to the center of gravity of the particle, R = (x1 , x2 , x3 , · · · , xi , · · · , xN ), where N is the number of atoms in the particle. The normalized Euclidean distance between two vectors (two particles), S (α, β), is calculated by the equation below,

cludes also several configurations obtained from our implementation of the basin-hopping Monte Carlo (RBHMC) algorithm 55 using the SuttonChen empirical potential. 56 The adsorption of molecules such as water and ethanol on low-symmetry TM13 clusters is a challenge due to the large number of non-equivalent adsorption sites and the large number of possible orientations of the adsorbates. Thus, to minimize this problem, we employed the following strategy. (i) We selected the pGMC structure for a given TM13 cluster and one of the molecules, i.e., water or ethanol. (ii) We generated from 50 to 100 million random adsorbed configurations for the selected molecule on the TM13 cluster by defining a minimum and a maximum distance between both systems. The cluster atomic positions are frozen, while random three-dimensional rotations are applied for the molecule. (iii) Nowadays, it is very hard to calculate 50 to 100 million configurations using DFT, however, it is also not necessary. For example, there are a large number of similar configurations that can lead to the same local minimum structure upon the geometric optimization using BFGS or conjugated gradient as they belong to the same basin in the potential energy surface. 57 (iv) To reduce our structural databasis for a set of representative configurations, we employed a modified Euclidean metrics, 58 in which the distance between two vectors is normalized by the sum of the module of the two vectors, 59 and hence, it removes the dependence on the particle size. In our implementation of the modified Euclidean

S (α, β) =

N P

i=1

(xi,α − xi,β )2

N P

i=1

2 xi,α

+

.

(3)

2 xi,β

We selected only those structures for which S (α, β) > 5 × 10−4 for water/TM13 and 8 × 10−4 for ethanol/TM13 . Thus, from our initial set of 50 to 100 million configurations, we end up with about 100 to 150 configurations for each system, which represent different regions in the potential energy surface and were optimized using the modified BFGS algorithm as implemented in FHI-aims. From that, we identified the pGMC and higher energy configurations. Vibrational analysis was employed to confirm that the pGMC are true local minimums, i.e., all frequencies are positive.


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Table 1: Energetic, structural, and electronic properties of the Ni13 , Cu13 , Pd13 , Ag13 , Pt13 , and Au13 in the putative global minimum configurations (pGMC) and icosahedron (ICO) structure, Figure 1. Relative total energy with respect to the ICO structure, ∆Etot , binding energy per atom, Eb , average weighted bond length, dav , average effective coordination number, ECN, in number of nearest-neighbors (NNN), total magnetic moment, mT , and average center of gravity of the occupied d-states, ǫd . TM13

Cluster Config.

∆Etot (eV)

Eb (eV/atom)

dav (Å)

ECN (NNN)

mT (µB )

ǫd (eV)

Ni13

ICO pGMC ICO pGMC ICO pGMC ICO pGMC ICO pGMC ICO pGMC3D pGMC2D

0.00 −0.31 0.00 −1.00 0.00 −0.26 0.00 −1.29 0.00 −3.43 0.00 −2.56 −2.56

−3.13 −3.15 −2.27 −2.35 −2.32 −2.34 −1.60 −1.70 −3.65 −3.91 −1.93 −2.13 −2.13

2.40 2.36 2.50 2.46 2.74 2.68 2.88 2.83 2.72 2.59 2.87 2.78 2.69

6.38 5.67 6.40 5.70 6.36 5.66 6.40 5.65 6.37 4.28 6.40 4.94 3.80

8 10 5 1 8 8 5 1 2 2 5 1 1

−1.53 −1.62 −2.40 −2.20 −1.65 −1.78 −4.21 −3.94 −2.24 −2.46 −2.92 −2.90 −2.69

Cu13 Pd13 Ag13 Pt13 Au13

III A

Results

neighbors (NNN) and the weighted bond length, i dav , where the label i indicates every atom in the system. The relative pGMC total energies with respect to the compact ICO structure (∆Etot = pGMC ICO Etot − Etot ) are negative for all systems and the absolute ∆Etot values are smaller for Ni13 and Pd13 , while it is larger for Pt13 and Au13 . Thus, it indicates a stronger preference of the Ni13 and Pd13 systems for compact structures, however, an opposite behavior is expected for Pt13 and Au13 . In fact, our ECN results support this picture, e.g., average ECN = 4.28 NNN for Pt13 and 5.67 NNN for Ni13 . For Au13 , pGMC is a two-dimensional (pGMC2D) structure, ECN = 3.80 NNN, which is only 0.3 meV/atom lower in energy than the threedimensional (pGMC3D) structure with ECN = 4.94 NNN. These trends can be explained by the stronger localization of the 3d- and 4d-states, which favors compact structures. 51 As expected, the binding energies decrease (absolute value) by increasing the number of valence electrons, which can be explained by the increased occupation of the antibonding d-states. That is, an increasing in the occupation of the antibonding states located in high-energies contributes to

Water, Ethanol, and TM13 Clusters in Gas-Phase

For water, the O−H bond lengths (0.97 Å) and HOH angle (104.2◦ ) deviate by 1.3 % and 0.3 % compared with experimental results, respectively. 60,61 For ethanol, we calculated the gaucheand trans-ethanol structures, 62–64 Figure 1, which differ by the torsion angle of the hydroxyl group (OH) with respect to the C−C bond. The PBE gauche-ethanol is 0.12 meV/atom lower in energy than trans-ethanol, i.e., both trans- and gaucheethanol are degenerated in energy in the accuracy limit of our calculations. The bond lengths and angles deviate by about 0.6 % from experimental 64 and quantum-chemistry results. 62,63 The TMpGMC clusters are shown in Figure 1, 13 while the most important properties are summarized in Table 1. Furthermore, high-energy TM13 configurations are reported in the SI. For structural analysis, we employed the effective coordination concept, 65,66 which yields the effective coordination number, ECNi , in number of nearest-


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Figure 2: Lowest energy configurations for water and ethanol on the Ni13 , Pd13 , Pt13 , Cu13 , Ag13 , and Au13 clusters in their respective putative global minimum configurations (pGMC) and compact icosahedron (ICO) structure, while dICO indicated ICO structures distorted upon the water or ethanol adsorption. For the case of Au13 , we considered the two- and three-dimensional pGMC. The adsorption energy (Ead , in meV) is shown below each configuration. water orientation is nearly parallel to the cluster surface, and hence, the results are similar to the adsorption of water and ethanol on TM(111) surfaces. For ethanol, the hydrogen atom in the OH group approaches closer to the cluster, which is also similar to the results obtained for TM(111) surfaces. 34,40 With respect to the adsorption site, there is no substantial difference among the different structures, i.e., the onefold TM site preference does not change among the TM13 structures. Furthermore, we noticed that at high energy configurations ethanol binds also via the H atom from of the CH2 -group, which might play an important role in the breaking of the C−C bond. Those isomers are 6.86 meV/atom to 17.43 meV/atom higher in energy than the lowest energy structures, however, those configurations are true local minimum only for few TM13 systems, i.e., the vibrational frequencies are not real for few vibrational modes.

decrease the magnitude of the binding energy and increase the bond lengths, which is supported by our results. As expected, the average center of gravity of the d-states, ǫd , is closer to the highest occupied molecular orbital (HOMO) state for Ni13 , Pd13 , and Pt13 , while an increasing in the occupation of the d-states shifts the value of ǫd away from the HOMO state for the coinage systems. From our results, the structural changes affect ǫd by up to 0.3 eV, however, there is no clear correlation between ECN and the ǫd results as those results are average over a wide range of atoms.

B Water and Ethanol Adsorption on TM13 Clusters Adsorption Site Preferences: We obtained the lowest energy configurations for water and ethanol on Ni13 , Cu13 , Pd13 , Ag13 , Pt13 , and Au13 , Figure 2. We found that water and ethanol bind preferentially via the anionic oxygen atom on the onefold TM sites, which behave as cationic TM sites (in most cases) upon the water and ethanol adsorption (supported by charge analysis). The

Adsorption Energy: To obtain a better understanding of the (water, ethanol)–TM13 interactions, we calculated the adsorption energy, Ead , which is


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fold sites, 67 quantum-size effects (discrete nature of the electronic states), 68 and differences in the charge state of the onefold sites on clusters and surfaces. 69

given by, Mol/TM13

Ead = Etot

TM13

− Etot

Mol − Etot ,

(4)

TM

Mol where Etot and Etot 13 are the total energies of molecules and clusters in gas-phase, respectively, Mol/TM13 while Etot is the total energy of the adsorbed system. The Ead results are summarized in Table 2. As expected, we found that the adsorption energy of water and ethanol is larger on the group 10 (Ni, Pd, Pt) than on group 11 (Cu, Ag, Au), which can be explained by the nature of the d-states, e.g., the d-states are fully occupied in group 11, and hence, the d-states cannot be depopulated or populated easily in the formation of the O−TM bonding. Except for few cases, the PBE adsorption energies for water and ethanol follow the same trend, i.e., in absolute value, Ni > Pt > Cu > Pd > Au > Ag. Furthermore, the adsorption energy is larger on 3d than on 5d systems, which can be related with the localization of the d-states, which is larger for the 3d-states. Although the anionic O atom binds only on the onefold TM sites on different systems and structures, there is a clear dependence of the adsorption sites on the TM13 structures, i.e., differences from few meV up to about 150 meV. Thus, the present results indicate a strong dependence on the atomic environment beyond of the onefold TM site interaction. For adsorption on TM surfaces, the adsorption energy is larger for surfaces with higher surface energy, namely, the TM(110) and TM(100) surfaces. Thus, we can expect higher adsorption energies for water or ethanol on highenergy cluster configurations, however, it is not the case for all systems. For example, for water/TM13 , we found as expected that NiICO > NipGMC and 13 13 pGMC dICO Au13 > Au13 , however, it is the opposite for Cu13 , Pd13 , Ag13 , and Pt13 . For ethanol, we observed similar results, however, the order is not preserved, which indicates a dependence on the adsorbate specie. Compared with the compact TM(111) surfaces, 34 we noticed a large enhancement of the adsorption energy due to the adsorp-

Structural Parameters: The PBE equilibrium O−TM distances, dO−TM , which are related with the magnitude of the adsorption energy, are reported in the Table 2. The dO−TM bond lengths spread from 2.03 Å to 2.50 Å for both water and ethanol, which is expected based on the wide range of values obtained for the adsorption energy, i.e., the bond length is shorter (longer) for larger (smaller) adsorption energies. Based on the analysis of the coordination numbers and bond lengths, we found that the TMpGMC systems change their 13 structure only slightly, e.g., the average coordination number changes up to 0.04 %, which indicates the high stability of the pGMC, however, we found large structural changes in the high-energy ICO structure for particular systems upon the water or ethanol adsorption. For example, the ICO structure is preserved only for Ni13 , Cu13 , Pd13 , Ag13 , while there are large structural deformations for PtdICO and AudICO 13 13 . Our vibrational calculations confirmed that the ICO structure is a true local minimum for all studied TM13 systems, and hence, it indicates that the adsorption induces the structural change. The Role of van der Waals Corrections: We found that vdW corrections do not affect the structures of the TM13 clusters, while they contribute to change slightly the equilibrium dO−TM distance, e.g., changes smaller than 0.05 Å, Figure 2. As expected, the vdW correction enhances the adsorption energy, however, the enhancement is not large. For example, the adsorption energy increases from 4 % to 29 % for water/TM13 , and from 16 % to 62 % for ethanol/TM13 . As found previously, the adsorption energies for water and ethanol on the respective compact TM(111) surfaces 34 increase by more than 100 %, however, those values were obtained using the vdW correction proposed by S. Grimme, 70 and hence, it is hard to obtain a definitive conclusion as difference vdW corrections can yield different enhancements for the binding energy. 71 Based only on the trends, we can conclude that the vdW correction is not critical for ethanol

pGMC

TM

TM(111) tion on clusters, e.g., the ratio Ead 13 /Ead increases by factors from 1.59 up to 2.68 for water and from 1.77 up to 4.31 for ethanol, which can be explained by the low-coordination of the one-


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Table 2: Adsorption energy, Ead , (in meV) and O−TM bond lengths, dO−TM (in Å), for water and ethanol on the TM13 in their respective putative global minimum configurations (pGMC) and icosahedron (ICO) structures employing the PBE and PBE+vdW functionals. dICO indicates distorted ICO structures upon the adsorption. TM13

Cluster config.

Ni13 Cu13 Pd13 Ag13 Pt13 Au13

ICO pGMC ICO pGMC ICO pGMC ICO pGMC dICO pGMC dICO pGMC3D pGMC2D

water/TM13 PBE Ead

−670 −627 −449 −461 −404 −410 −255 −270 −535 −576 −470 −333 −392

ethanol/TM13

PBE+vdW PBE PBE+vdW Ead dO−TM dO−TM

−858 −731 −525 −513 −462 −445 −300 −349 −598 −624 −490 −374 −411

2.03 2.05 2.12 2.10 2.31 2.32 2.50 2.53 2.29 2.26 2.37 2.46 2.39

2.02 2.05 2.10 2.10 2.32 2.32 2.51 2.56 2.30 2.26 2.37 2.46 2.40

PBE Ead

−761 −717 −526 −546 −471 −487 −300 −317 −837 −695 −322 −390 −471

PBE+vdW PBE PBE+vdW Ead dO−TM dO−TM

−1095 −985 −723 −721 −601 −596 −455 −515 −999 −840 −513 −498 −544

2.01 2.01 2.08 2.07 2.28 2.28 2.47 2.49 2.17 2.22 2.33 2.40 2.34

2.00 2.00 2.07 2.07 2.27 2.28 2.47 2.49 2.18 2.22 2.31 2.39 2.35

particular, the stretching modes reduce the magnitude of their frequencies by 42 cm−1 to 218 cm−1 , which is consistent with previous DFT results for water interacting with TM surfaces. 25 Thus, based on the concept of empirical Badger rules, 72,73 the redshifts indicate a weakening of the O−H bond upon adsorption. For ethanol in gas-phase, there are 21 distinct vibrational modes, and the vibrational frequencies spread in three distinct regions, namely, 3300 cm−1 to 3700 cm−1 , 2800 cm−1 to 3100 cm−1 , and 1000 cm−1 to 1300 cm−1 , which have been associated with the stretching of the OH, 74,75 CH, 76 and CO 75 parts, respectively. For ethanol/TM13 , we obtained redshifts from 21 cm−1 to 216 cm−1 in the OH stretch, which can be attributed to the direct interaction of the hydroxyl group with the TM13 clusters. For CH, we obtained blue-shifts from 29 cm−1 to 52 cm−1 , which is related with an increase in the strength of the C−H bonds. For CO, we observed a redshift from 24 cm−1 to 43 cm−1 , and hence, it indicates a weakening of the C−O. Furthermore, we obtained vibrational frequencies in the range from 319 cm−1 to 551 cm−1 , which are slightly higher than the vibrational frequencies of the TM13 clusters, and hence, it can be attributed

or water adsorption on TM13 clusters as it is for ethanol or water on the TM(111) surfaces, which can be related with the localized nature of the electronic states in finite-size systems. Vibrational Frequencies Analysis: The infrared active vibrational frequencies and their respective intensities for water and ethanol on TM13 are shown in Figure 3, while additional results are reported in the SI for all important configurations. We found that all the vibrational frequencies for the pGMC structures, Figures 1 and 2, are positive numbers, which implies that all those structures are true local minimums. Furthermore, ICO is also a local minimum configuration for all TM13 systems. All the vibrational frequencies of the TM13 clusters extend from about 50 cm−1 to 400 cm−1 , which is far from the vibrational frequencies of water and ethanol discussed below. For water in gasphase, we obtained the bending (HOH), symmetric and asymmetric stretch (O−H) vibrational modes with the 1594 cm−1 , 3700 cm−1 , and 3793 cm−1 frequencies, respectively, which deviate by about 1 % from the experimental results. 60 The interaction of water with the TM13 clusters induces a redshift in the water vibrational frequencies, Figure 3, in


8

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3.0 1.5 3.0 1.5 2

1.5

2

3.0 3.0

Intensity (D /Å )

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

1.5 3.0 1.5 3.0 1.5 3.0 1.5 0.0

0

3.0

water TM13 x 50

water/CupGMC 13

pGMC ethanol/Cu13

3.0

pGMC ethanol/Pd13

1.5 3.0

pGMC water/Ag13

pGMC ethanol/Ag 13

1.5 3.0

pGMC water/Pt13

pGMC

ethanol/Pt13

1.5 3.0

water/AupGMC3D 13

pGMC3D

ethanol/Au 13

1.5 3.0

water/AupGMC2D 13

ethanol/AupGMC2D 13

1.5 0.0 4000 0

pGMC

ethanol/Ni13

1.5

pGMC water/Pd13

1000 2000 3000 -1 Wavenumber (cm )

ethanol TM13 x 50

1.5 3.0

pGMC water/Ni13

1000 2000 3000 -1 Wavenumber (cm )

4000

Figure 3: Infra-red spectra for water and ethanol adsorbed on the TMpGMC clusters (red lines). The vibra13 pGMC tional spectra of gas-phase water (blue), ethanol (blue), and the TM13 (black) are shown. The infra-red intensities for the TM13 clusters were multiplied by 50 for visualization purpose. surfaces, 78 where the polarization of the adatoms induce similar changes in the d-states of the adsorption sites. In turn, there is charge accumulation mainly on C atoms of ethanol CH2 . A larger charge redistribution occurs at the region between the TM and O atoms and the results suggest that the O atoms of molecules might receive charge from H atoms and donate it to the TM13 clusters, or polarization effects might take place. In the case of ethanol, the process is similar, however, there is charge accumulation in the C atom region.

to the bonding between the molecules and clusters. Electron Density Difference: To improve our atomistic understanding of the mechanism that takes place on the Mol-TM13 interactions, we employed the electron density difference analysis, 77 namely, ∆ρ = ρMol/TM13 − ρTM13 − ρMol , which map the changes in the electron density of the molecule, ρMol , and TM13 cluster, ρTM13 , upon the adsorption of the molecule on TM13 . The results are reported in Figure 4 for Mol/TMpGMC , while 13 ICO/dICO the Mol/TM13 results are reported in the SI. From Figure 4, the changes are located mainly on the adsorption TM sites that binds to the water or ethanol, while the changes in the remaining TM atoms have smaller magnitude, in particular, to the coinage TM atoms due to the complete occupation of the d-states. For ethanol, the changes in the CH3 -group are negligible, i.e., the TM−O interaction affects mainly the CH2 −OH-group. From the 3D-plots, we can easily identify large changes of the TM dz2 -orbitals, while the diagonal TM dstates increase their electron density, which is similar to the adsorption of rare-gas atoms on TM

Mulliken and Hirshfeld Charges: The ∆ρ analysis provided hard proof for electron density depletion and accumulation, however, it cannot measure the charge transfer among the systems. Thus, to improve our understanding, we calculated the Hirshfeld, QH , and Mulliken, QM , charges on every atom, i. The effective charge on every atom (positive or negative) can be calculated as follows, = Z − QH/M , where Z is the total num∆QH/M i ber of electrons (all electron calculation). Thus, the sum of the effective charges over the molecule atoms, ∆QH/M Mol , yields the effective charge (posi-


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Page 10 of 16

Figure 4: Electron density difference for water and ethanol on the TM13 clusters in their respective putative global minimum configurations (pGMC). Blue (Purple) isosurfaces correspond to charge accumulation (depletion).

tive/negative) on water/ethanol. The results are summarized in Table 3. We found that both water and ethanol lost charge to the TM13 clusters, acting as Lewis basis, which is unexpected due to the higher electronegativity of the O atom compared with the TM atoms, however, a charge transfer from TM13 to water/ethanol would requires the population of the unoccupied states, which is unlike as those states are higher in energy. The magnitude of the effective charges, ∆QH/M Mol , is from 0.04 e to about 0.19 e in both analysis, and the magnitude increases for water/ethanol adsorbed on TMICO 13 . Thus, those minor charge changes can be explained by the broadening of the molecule states that extends above the HOMO state, while previously unoccupied TM states extend below the HOMO state, which is similar to the polarization effects that takes place on rare-gas adatoms on TM surfaces, 78 i.e., electrostatic effects.

Center of Gravity of the Occupied d-states: Several analysis were discussed above, however, we could not identify a direct relation to explain the most important adsorption energy trends. Thus, to improve our understanding, we analyzed the relation between the center of gravity of the occupied d-states, ǫd , of the TM13 clusters in gasphase and the adsorption energy. The ǫd results for the TM atoms that bind directly to the anionic O atoms and the adsorption energy are shown in Figure 5. Although, there are deviations, we obtained an almost linear relation between the adsorption energy versus ǫd , where the magnitude of the adsorption energy increases as the center of gravity of the d-states approaches the HOMO state, which was obtained using both PBE and PBE+vdW functionals. Thus, the following adsorption energy trends can be explained, i.e., Ni13 > Pt13 > Cu13 and Pd13 > Au13 > Ag13 , while while the devia-


10

-200

Table 3: The charge in the molecule/TM13 systems presented by Mulliken, ∆QM , and Hirshfeld, ∆QH , charges given in e.

Ni13 Cu13 Pd13 Ag13 Pt13 Au13

-400

∆Qethanol

Config.

∆QM

∆QH

∆QM

∆QH

ICO pGMC ICO pGMC ICO pGMC ICO pGMC dICO pGMC dICO pGMC3D pGMC2D

0.07 0.09 0.10 0.11 0.10 0.11 0.04 0.05 0.14 0.15 0.13 0.04 0.11

0.19 0.19 0.13 0.14 0.13 0.14 0.07 0.06 0.15 0.16 0.16 0.04 0.14

0.06 0.09 0.11 0.13 0.10 0.12 0.03 0.01 0.17 0.16 0.11 0.08 0.15

0.17 0.18 0.11 0.13 0.11 0.12 0.06 0.05 0.15 0.15 0.08 0.06 0.15

-800

-1000 -200

PB

E+v

dW

Ni13 Cu13 Pd13 Ag13 Pt13 Au13 ethanol/TM13

PB

E

-400 -600 -800 -1000

tions are expected due to the weak nature of the adsorbate-cluster interactions. Therefore, we can conclude that the center of gravity of the d-states play an important contribution to the adsorption energy, and hence, the d-band model proposed by Hammer 79 provide useful insights to understand the interaction of water and ethanol with the studied TM13 clusters.

IV

-600

PBE+vdW PBE

E

Ead (meV)

∆Qwater

water/TM13

PB

Ead (meV)

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


PBE+vdW PBE

Page 11 of 16

-1200 -5

PB

E+v

dW

Ni13 Cu13 Pd13 Ag13 Pt13 Au13 -4

-3 εd (eV)

-2

-1

Figure 5: Adsorption energy, Ead , versus the center of gravity of the occupied d-states, ǫd , where filled and open symbols indicate PBE and PBE+vdW results, respectively. The continuous lines were obtained by a linear fitting of the adsorption energy versus the center of gravity of the occupied d-states.

Summary

In this work, we reported a Ab-initio theoretical investigation of the adsorption properties of water and ethanol on the TM13 clusters (TM = Ni, Cu, Pd, Ag, Pt, Au) based on DFT-PBE within vdW corrections as implemented in the FHI-aims package. In the lowest energy configurations, water and ethanol bind preferentially via the anionic O atom to the onefold TM sites, which is similar to the adsorption of water and ethanol on compact TM(111) surfaces. For particular systems, at high energy configurations, ethanol binds to the TM13 cluster via one of the H atoms located in the CH2 -group, which might play a crucial role at real ethanol SRR. The direct OH interaction with the TM13 clusters yields a significant redshift for the OH vibrational mode for both molecules, which can be correlated

with a weakening of the O−H bond due to the interaction with the TM13 clusters. From the Hirshfeld and Mulliken analysis, we found a charge transfer from the molecules to the TM13 clusters, which indicates a Coulomb contribution to the adsorption energy. Although, there are deviations, we found an almost linear relation between the adsorption energy versus the center of gravity of the occupied d-states, i.e., except the deviations, the magnitude of the adsorption energy increases by moving the center of gravity of the occupied d-states closer to the HOMO state. Acknowledgement The authors are thankful to the Coordination for Improvement of Higher Level Education (CAPES), the São Paulo Research


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Foundation (FAPESP), National Council for Scientific and Technological Development (CNPq), and the Brazilian National Program of PosDocs (PNPD/CAPES) for the financial support. Authors also thank the Department of Information Technology - Campus São Carlos for the infrastructure provided to our computer cluster. Supporting Information Available: The relative total energies for the gas-phase TM13 clusters and further analysis employed to support our conclusions are summarized in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Ethanol and Water Adsorption on Transition-Metal 13-Atom Clusters: A

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