Energetic Stabilities and Phase Diagrams

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Equilibrium Au-Pd(100) Surface Structures Under CO Pressure: Energetic Stabilities and Phase Diagrams Ismail Can Oguz, Tzonka Mineva, Jerome Creuze, and Hazar Guesmi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04116 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 29, 2018

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The Journal of Physical Chemistry

Equilibrium Au-Pd(100) Surface Structures Under CO Pressure: Energetic Stabilities and Phase Diagrams Ismail Can OĞUZa, Tzonka MINEVAa, Jérôme CREUZEb*, and Hazar GUESMIa* a

Institut Charles Gerhardt Montpellier, CNRS/ENSCM/UM, 240, Avenue du Professeur Emile

Jeanbrau,34090 Montpellier, France. b

ICMMO/SP2M, UMR8182, Univ Paris Sud, CNRS, Université Paris-Saclay, Batiments 410/420/430,

rue du doyen Georges Poitou, 91405 Orsay cedex, France * Corresponding authors: [email protected], [email protected]

Abstract In this work first principles density-functional-theory (DFT) calculations and Monte Carlo (MC) simulations are used to study the stability of the new surface phases formed under CO gas pressure on Au-Pd (100). The segregation isotherms reveal the apparition at θCO=0.5 monolayer (ML) of two exotic ordered phase structures over the (100) facet, depending on the CO adsorption configurations. When CO is adsorbed on bridge sites, the ordered phase is formed by Pd-CO ensembles organized in linear chains, which are separated by Au atoms forming also linear chains free of adsorbed CO molecules. When CO is adsorbed on-top site, a checkerboard-like stable structure appears with CO adsorbed on Pd. The phase diagrams (temperature and CO pressure) of these large surface Pd ensembles for different bulk Pd concentration [Pdbulk] are calculated. Тhe effect of local density approximation (LDA) and generalized gradient corrected (GGA) exchange-correlation (XC) functionals are compared. The phase diagram of the configuration with linear chains, predicted by GGA calculations, shows excellent agreement with experimental results. In order to understand the origin of the formation and the stability of the two ordered phases, the energetic stability and the electronic structure properties of different large Pd ensembles are compared and deeply analyzed.

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I. Introduction

Bimetallic Gold–Palladium catalyst has gained considerable interest in the field of heterogeneous catalysis regarding the structure-catalytic properties relationship exhibited by this material.1 For instance, in a recent work, Ma et al.2 have reported superior performance for the selective hydrogenation of acetylene of agglomerates of Au–Pd octahedral samples organized as nanoflowers. Similarly, several core–shell designed Au–Pd structures have been reported to enhance catalytic reactions such as electrocatalytic activity for oxygen reduction reaction (ORR) at room temperature3 or the oxidation of formic acid4. High performing and stable supported Au-Pd nano-alloys were also reported for the catalytic hydrogenation of Levulinic acid, which is a key platform molecule in many biorefinery schemes.5 The beneficial effect of gold-palladium nano-alloying was found to result from the electronic modification of Pd. On Au–Pd(100) single crystal model catalysts, Goodman et al. have reported the importance of surface segregated Pd ensembles on gold surface for CO oxidation reaction at room temperature6,7. More precisely, CO oxidation reaction monitored using both PM-IRAS and reaction kinetic measurements have been shown to occur only on Au surfaces with contiguous Pd sites that were induced by the segregation of Pd during the CO exposure. On these contiguous Pd sites, which were evidenced by the formation of bridging CO species, the O2 dissociation could be possible6-9 and then the reaction of CO oxidation might occur. The understanding and the control of Pd segregation induced by the presence of reactive gas have been investigated in several experimental6,7,10-12 and theoretical works13-21. Using DFT total energies, equilibrium thermodynamics, and a simplified lattice–gas model, SotoVerdugo et al.22 have examined the equilibrium composition of Au-Pd(100) alloy surfaces as a function of temperature, CO pressure and the Pd/Au ratio. Surprisingly, DFT calculations have predicted negligible interaction between Pd-CO complexes formed when CO adsorbs on Pd atoms and by consequence the model has suggested random distribution of Pd on the surface where the formation of contiguous Pd sites was excluded. Indeed, these authors have considered the adsorption of CO on-top of Pd atoms instead of bridge site between two neighboring Pd atoms assuming that because of energy relaxation, the fraction of Pd2-CO may be very small compared to the fraction of Pd-CO. As reported by experimental works6,7 the fraction of CO bridging Pd atoms in Au-Pd surface was found to increase during CO time exposure, signature of the formation of contiguous Pd sites. These experimental observations


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were confirmed by DFT calculations on Au-Pd(100) surfaces14,20 and Au-Pd nanoparticles15 showing the high energetic stability of contiguous Pd sites in the presence of adsorbed CO. With the aim to understand CO adsorption induced Pd segregation phenomena in Au-Pd alloy surfaces, we have recently developed20 inter-atomic potential built from DFT-based Ising model that accurately describes the interactions between: (i) the metal atoms in the alloy, (ii) the metal atoms in the surface and the adsorbed CO molecules and (iii) the adsorbed CO-CO molecules. This inter-atomic potential, derived from LDA-DFT calculations, was used in the Monte Carlo simulations to obtain Pd segregation isotherms20. The results, although limited to the LDA-derived interatomic potential, evidenced the presence of contiguous Pd atoms and a stable phase structure with linear Pd chains covered by CO adsorbed on bridge sites. In the present study, we employed the same methodology to extensively study the Au-Pd(100) surface in equilibrium with the CO reactive gas and bulk Pd at two temperatures, T=300 and 600K, and CO pressure (PCO) ranges, considered experimentally6,7. In particular, attention is devoted to temperature and pressure conditions, at which the Pd chain structure appears. For this, the energy phase diagrams of both the bridge and the on-top CO adsorption modes were established and the effect of XC functional (LDA vs. GGA) was deeply analyzed. In addition to the Pd surface segregation in chains, the results show ordered Pd phase named as “checkerboard” structure for different (T,PCO) regime. In order to understand the stability of these ordered phases appearing under CO gas, energetic and electronic structure analyses were made. The present paper is organized as follows: in section 2 the technical details of the DFT calculations and the Ising model used in the Monte Carlo simulations are briefly outlined. Details on the DFT-based Ising model can be found elsewhere.20 The section 3 is devoted to the analysis of the effect of the LDA and GGA exchange correlation functionals on the prediction of the surface change and the Pd segregation that is caused by the interaction with CO. For this, DFT calculations of the energy evolution of CO adsorption on bridge and on-top sites as well as the CO−CO interactions are performed to define the effective Hamiltonian and the resulted segregation isotherms issued from Monte Carlo simulations are compared. In the last part of this section, the phase diagrams of the two identified ordered structures are established. Finally, in section 4, the energetic stability and the electronic structure through Bader charges and Local Density of States (LDOSs) calculations are analyzed to understand the origin of the stability of the identified large ordered Pd ensembles.



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II. Computational Details II.1. First-Principle Calculations DFT calculations within periodic approach were performed using the Vienna ab-initio simulation package (VASP)23-24. The exchange-correlation energy was calculated within both the local-density (LDA-PZ)25 and the Perdew, Burke and Ernzerhof generalized-gradient approximation (GGA-PBE)26. The valence electrons were treated explicitly, and their interactions with the ionic cores were described by the projector augmented-wave method (PAW)27-28, which allowed the use of a low energy cut off equal to 415 eV for the plane-wave basis. The positions of the atoms in the super cell were relaxed using the conjugate gradient algorithm until all the forces are below 0.01 eV Å−1. To model the bimetallic Au-Pd(100) surfaces as well as Au and Pd monometallic surfaces, slabs of 48 and 108 atoms were used, representing the 2x2 and 3x3 unit cell, respectively. The latter unit cell was used to ensure the calculation of large space of configurations on the surface of Au-Pd(100). The surface models are formed by six atomic layers separated by 15 Ǻ of vacuum space. All atoms in the top four layers of the slab and the C-O molecules were allowed to relax during the geometry optimization. The bottom two layers were constrained at the bulk geometry with optimized LDA and GGA lattice constants of 4.08 and 4.17 Å, respectively. The Brillouin zone (BZ) is sampled with a k-mesh of 3x3x1 and 5x5x1 within MonkhorstPack scheme for 3x3 and 2x2 unit cell, respectively. The adsorption energy of C-O on gold-palladium sites, was defined by equation 1, where E(surf.), E(CO(g)) and E(ads.) represent the energies of the optimized clean AuPd surface, C-O gas phase reactant and adsorbed phase, respectively.

∆Eads =E(ads.) − E(surf.) – E(CO(g))

(Eq. 1)

II.2. Monte Carlo simulations The Au and Pd distribution on the different sites of the simulation box was determined via Monte Carlo (MC) simulations where the chemical potential difference ∆µAuPd = µPd − µAu defines the nominal concentration of the PdcAu1−c alloy. The simulations were performed within the semi-grand canonical (sGC) ensemble in which the total number of metallic species (N= NAu + NPd), temperature (T), and pressure (P) were fixed. The partial number of each kind of atom (NAu, NPd) was changed by varying the 4 ACS Paragon Plus Environment

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value of chemical potential difference ∆µAuPd. The effect of the CO gas was considered by taking into account the chemical potential difference for the gas, ∆µCO. The CO chemical potential is related to the temperature and the pressure by assuming that the surface is in thermodynamic equilibrium with the gas phase. Thus, the CO is considered as an ideal gas reservoir and the temperature and pressure dependence of ∆ , is determined by ∆ , = , − 0°, − , + ln ⁄

(Eq. 2)

where the enthalpy H and the entropy S of CO are taken from the tabulated values of the JANAF thermodynamics tables29, P0 being the pressure of the reference state (1 bar). The simulated system consists of a 10×10×13 slab of 15 layers a fcc (100) crystal structure with a fixed lattice parameter of 4.07 Å and two adsorbate layers on the top/bottom of the slab for the CO adsorption. All details concerning the description of the energy part using Ising model for the interaction potential and DFT for the energetic quantities as well as simulated slab in Monte Carlo procedures are reported in ref.20.



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III. Results and discussion III.1. CO adsorption-induced segregation: effect of XC functional III.1.1. DFT energetics

In order to analyze the effect of the XC functional on the calculated CO surface interaction that induces Pd segregation on Au-Pd(100) we analyzed the evolution of CO adsorption energies as a function of the increasing number of CO surface Pd first neighbors (see figure 1) from LDA and GGA calculations. For both approximations, the most favorable adsorption site of CO on Au (100) and Au-Pd2(100) was found to be the bridge site (Figure 1a and 1c, respectively). On Pd diluted in Au(100) surface (Figures 1b and 1e), while GGA indicates the top site as the most favorable site, the LDA predicts the bridge adsorption site to be the most favorable. This result is related to the known artifact of LDA functional to overbinds molecules30. Note that, both LDA and GGA predict the 4-fold adsorption mode to be unfavorable on the latter considered Au Au-Pd configurations. Over pure Pd(100) surface the computed adsorption energy value of -1.87 eV over the 4-fold site is found to be 0.07 eV less favorable than the adsorption energy on the bridge site. In the following we analyze the evolution of CO interaction over Au-Pd(100) surface by considering the adsorption on both bridge and top sites with both types of XC approximations. Insert Figure 1 CO adsorption on bridge site On Au(100) surface, the most favorable site of adsorbed CO was found to be the bridge site with Eads= -1.29 eV and -0.52 eV for LDA and GGA, respectively. The latter value is in better agreement with the experimental heat of adsorption of 0.60 eV obtained using electron energy loss spectroscopy (EELS)31. Therefore, in line with previous calculations32-34 LDA functional is found to overestimate binding energy while GGA functional slightly underestimate it. In order to evaluate the effect of the presence of Pd on the adsorption of CO, we calculated the CO adsorption energy values for increasing Pd surface concentration. Figure 2 reports the evolution of computed DFT-LDA and DFT-GGA adsorption energies as a function of the number of Pd atoms in CO’s nearest m (0,1,2) and next nearest neighbor n (0,1,2,..,6) positions. The first point at (m=n=0) corresponds to the adsorption of CO on monometallic Au (100) surface. Then, we increased one by one the number of Pd in CO’s nearest neighbor 6 ACS Paragon Plus Environment

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m (1 and 2) and next nearest neighbor n (from 1 to 6) positions respectively. For more details we refer to our recent paper20. Concerning the effect of higher-order (e.g., 3-body) CO- metal interactions, our DFT calculations show that the CO adsorption energy doesn’t change when third nearest neighbors from the surface changes. This result is in line with our previously reported charge density perturbation analyzes of CO on Au-Cu35 and Au-Pd21 surfaces where the extremely local character of CO has been evidenced. In addition, the effect of subsurface Pd was evaluated. Our calculations show that subsurface Pd does not affect the adsorption of CO, except when the subsurface Pd bounds two Au atoms (m=0) adsorbing CO in bridge position. This energy decrease is 0.05 eV. It results from the non stability of Pd in the subsurface layer and its ability to segregate to the surface as soon as CO is adsorbed on the surface. Insert figure 2 Figure 2 shows similar behavior of the LDA (dashed lines) and GGA (full lines) results. The adsorption energies of CO are found to increase with increasing Pd atom number. Note that for all n values all possible configurations were taken into account. The computed adsorption energies were found to be within 0.03 eV for the varying structures corresponding to fixed m and n, which is within the DFT error bar. By consequence no deviation bars exist in the figures. The evolution of calculated adsorption energies follows three straight lines, indicating a linear relationship between the adsorption energy and m. The gap between the three curves is found to be similar from both LDA and GGA calculations (0.62±0.02 eV). These values correspond to the effect of first nearest substituted Pd atom on CO absorption energy. The contribution of second nearest modified Pd atom to CO adsorption energy is found to be constant in each three cases and in both calculation method. The effect of addition of a second nearest Pd atom, n, to the adsorption energy is found to be only of 0.04 eV. Based on these results the adsorption energy can be written as a function of the adsorption energy of CO on pure (100) Au surface ( &' () , = + !"#$% + !"#$%

(Eq. 3)

&' () where !"#$% and !"#$% are the energy contributions of additional *&' and *() . &' !"#$% can be derived by considering the difference between with fixed n, while () !"#$% is simply the slope of each curve in fig. 2. Thus according to our results eq.3 can be

written as: _,-., = −1.29 − 0.64 − 0.04

(Eq.4)



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_55., = −0.52 − 0.61 − 0.04

(Eq.5)

If we assume that the effect of next nearest Pd atom is negligible (0.04 eV), the adsorption 788

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energy can be described by two effective pair interactions (!"#$ and !"#$% ) between CO and its 1st neighboring Au and Pd atom, respectively. Considering that CO adsorbs in bridge 788

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site, the. !"#$ is given by half of and !"#$% is calculated as the sum of !"#$ and &' !"#$% .

In table 1 are depicted the calculated energy values of derived effective pair interactions on Au and Pd atoms. Insert Table 1 Finally, in order to evaluate the change in the CO adsorption energies induced by repulsive CO-CO interactions, different configurations of two adsorbed CO molecules on different AuPd ensembles were computed by DFT-LDA and DFT-GGA calculations. Results show no difference between the two functionals for treating the repulsion term (see more details in ref 20).

On-top CO adsorption Besides the bridge site, the adsorption of CO on-top sites of Au-Pd(100) surface is analyzed. As the top site is only predicted by GGA calculation, the analysis is limited to DFT-GGA. Figure 3 shows the evolution of CO adsorption energies as a function of the number of Pd atoms in CO’s

nearest m (0,1) and next nearest neighbor n (0,1,2,3,4) positions.

Insert Figure 3 Over Au(100) surface (m=0) CO binds with an adsorption energy of -0.46 eV. The effect of the

addition of n second nearest Pd atoms on the CO adsorption energy (blue line) is found to be very small (0.02 eV). In contrast, on Pd atom where the CO adsorption energy is calculated to be of -1.24 eV, the effect of the addition of second neighboring Pd is found to be more pronounced (0.05 eV). Because of the difference in the behavior of CO adsorbed on Au or on Pd, it is difficult to accurately formulate the effect of 2nd neighbor Pd of CO in terms of adsorption energy evolution. In addition, as the effect remains negligible (0.02-0.05 eV) compared to the value of adsorption energies, st

the Eads may be governed by the interaction with 1 nearest-neighboring Au and Pd atoms. By


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the consequence !"#$ = -0.46 eV and !"#$% = -1.24 eV are considered as the effective interaction energies of CO on-top of Au and Pd atoms, respectively. In order to evaluate the repulsive CO-CO lateral interaction when CO are adsorbed on-top positions, different Pd ensembles of CO on Au-Pd(100) surface was considered. The results show CO repulsive interaction ranging from 0.005 eV (for CO on Au) to 0.137 eV (for CO on Pd), and confirm the importance of including such interaction into the formalism. All calculation details are provided in the Supporting Information (see Figure S1 and Table S1). 3.1.2. Segregation isotherms

Now that all energetic parameters of the DFT-based Ising Hamiltonian are determined (MetalMetal and Metal-CO interactions either by DFT/LDA and GGA), we can treat the statistical thermodynamics of the underlying Ising model by means of Monte Carlo simulations in the semi-Grand Canonical ensemble, as described in Sec. II.2. We present firstly CO adsorption isotherms for the bridge sites obtained using either LDA or GGA energetic parameters and secondly for the top sites obtained only with the GGA energetic parameters. We also performed Monte Carlo simulations (based on DFT-GGA parameters) where on-top and bridge modes were considered simultaneously either on pure Au(100) and Pd(100) surfaces and on AuPd(100) one. The phase, including top site adsorption appears on pure Au(100) surface, but does not appear (limited to very low pressure/temperature range) on Au-Pd(100) and Pd(100) as follows from the resulted isotherms that are provided and discussed in the Supporting Information (See Figures S2, S3 and S4)”. This indicates that the more energetically preferred adsorption site has a leading role in the phase stability determination and it is not worthy to consider a simultaneous occupation of CO on both adsorption sites.

CO adsorbed on bridge site Figures 4a and 4b show the segregation isotherms of Pd in Au-Pd(100) surface under different partial CO pressures (or equivalently for different fixed value of ∆µCO) at T = 300 K and T = 600 K. They have been obtained by increasing and decreasing continuously ∆µ(Au-Pd) to verify their reversibility. The CO coverage, θ, is defined as the ratio of the number of CO adsorbed molecules to the number of surface atoms.



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Insert Figure 4

Whatever the temperature is, the only difference between the isotherms obtained with the GGA- and LDA-derived parameters, is into the range of partial CO pressures at which CO adsorption and instantaneous Pd segregation occurs on the Au-Pd(100) surface. The PCO range (1mbar < PCO < 10-4 mbar) predicted by GGA, agrees qualitatively with the experimentally reported pressure of 10-1 mbar6,7, whereas the results with the LDA-derived parameters deviate significantly (see PCO values in Fig. 4). Except the large PCO differences, the isotherms exhibit very similar profiles, which are described below following the analysis in ref.20. There are three main stages of CO adsorption-induced Pd surface segregation at T = 300 K that we recall hereafter: (i) at a low CO partial pressure (PCO(LDA) = 1 x 10-29 mbar or PCO(GGA) = 1 x 10-13 mbar), the surface Pd concentration is always smaller than the bulk one, indicating a strong Au segregation in agreement with previous theoretical works about Au-Pd alloys under vacuum conditions22,36-40; (ii) at a high CO partial pressures (PCO(LDA) = 10-4 mbar or PCO(GGA) = 1 mbar), there is a completely reversed Pd surface segregation, leading to the formation of a pure Pd (100) surface (concentration of Pd surface [Pdsurf] = 1) upon a pure Au bulk ([Pdbulk] = 0) and the surface saturation by CO gas occurs at the coverage of 1.0 ML, in agreement with experimental results about CO/Pd(100)41-43; (iii) at an intermediate CO partial pressure (PCO(LDA) = 1 x 10-19 mbar or PCO(GGA) = 1 x 10-4 mbar), a reversed Pd segregation is still observed ([Pdsurf] > [Pdbulk]) but both the surface Pd concentration and the CO coverage isotherms exhibit a plateau at [Pdsurf] = 0.5 and θ= 0.50 ML, indicating the existence of an equilibrium ordered phase. This ordered phase is characterized by a succession of linear chains of Pd atoms with adsorbed CO molecules separated by Au atoms free of CO molecules. Note that the value of PCO(GGA) (1 x 10-4 mbar), at which this ordered structure is formed, is close to the experimental pressure often used for this kind of studies (10-1 mbar)6,7. At T = 600 K, the ranges of CO partial pressures are shifted towards higher values as expected from Eq. 2 and the plateau disappears, indicating that the ordered phase with linear chains described above is no more stable at this temperature. However, a reversed Pd surface segregation is still observed for sufficiently high values of CO partial pressures.

CO adsorbed on-top site Figure 5 (a and b) show the segregation isotherms of Pd in Au-Pd(100) surface under different partial CO pressures (or equivalently for different fixed values of ∆µCO) at T = 300 K and T = 10 ACS Paragon Plus Environment

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600 K, together with snapshots of selected equilibrium configurations. They have been obtained by increasing and decreasing continuously ∆µ(Au-Pd) to verify their reversibility as for the previous case. At T = 300 K, as for CO adsorption on bridge sites, three main stages of CO adsorption-induced Pd surface segregation can still be defined as a function of PCO. At a low CO partial pressure (1 x 10-8 mbar), the system behaves like in vacuum conditions, the surface Pd concentration being always smaller than the bulk one, which indicates a strong Au surface segregation. However, for sufficiently high values of ∆µ(Au-Pd) corresponding to a mixed AuPd surface plane upon a rather Pd pure bulk, the few adsorbed CO molecules are systematically situated on-top of the surface Pd atoms, as can be seen on the corresponding snapshot in Fig. 5a obtained at [Pdbulk] = 0.30 and θ= 0.20 ML. This could be explained by the fact that the adsorption energy of a CO molecule on-top sites of Pd(100) and the one of a CO molecule on-top of a surface Pd atom surrounded by Au atoms are almost 3.2 and 2.6 times larger than the one of a CO molecule on-top sites of Au(100) respectively. At a high CO partial pressure (1 x 107 mbar), there is a completely reversed Pd surface segregation, leading to the formation of a pure Pd (100) surface ([Pdsurf] = 1) upon a pure Au bulk ([Pdbulk] = 0). Actually, this is a very high value of CO partial pressure which is out of the scope of experimental studies where the main goal is to avoid total segregation of Pd and to profit from the synergy of the two Au and Pd atoms present on the surface of the catalyst. Moreover, recalling that the most stable configuration in presence of two segregated Pd atoms in Au(100) is the CO molecule adsorbed on the bridge site between the two Pd atoms situated in nearest-neighbor position, complete saturation of the (100) surface with CO on-top of Pd is rather an artifact of having considered adsorption only on-top sites. At an intermediate CO partial pressure (1 x 103 mbar), a reversed Pd segregation is still observed ([Pdsurf] > [Pdbulk]) and both the surface Pd concentration and the CO coverage isotherms exhibit a plateau at [Pdsurf] = 0.5 and θ= 0.5 ML, indicating once again the existence of an ordered phase, which obviously cannot be the same as the one observed for CO adsorption on bridge sites. Actually, taking a look at the corresponding snapshot in Fig. 5a, the ordered phase corresponds to a checkerboard-like configuration with one half of the surface sites occupied by Pd atoms with a CO molecule adsorbed on-top of each one and the other half of the surface sites occupied by Au atoms free of adsorbed CO molecules. At T = 600 K, as for CO adsorption on bridge sites, the plateaus disappear, indicating that the checkerboard-like ordered phase described above is no more stable at this temperature, a reversed Pd surface segregation being still observed for sufficiently high values 11 ACS Paragon Plus Environment


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of CO partial pressures, as can be seen on the snapshot of an equilibrium configuration obtained at [Pdsurf] = 0.55 and θ= 0.55 ML for PCO = 1x 109 mbar in Fig. 5b. Insert Figure 5

3.1.3. Phase stability of ordered Pd phases

In this section, we complete the previous results by performing Monte Carlo simulations at different temperatures and by exploring large domains of ∆µCO and ∆µ(Au-Pd) in order to draw the domains of stability for the two identified ordered phase as a function of Pd bulk concentration, CO partial pressure and temperature. For the sake of clarity, we only present the results obtained with GGA energetic parameters, since the main difference with the ones obtained with LDA parameters lies in a shift towards higher values of PCO as this has already been noticed in Sec. 3.1.2. Figure 6 shows the different domains of phase stability obtained for temperatures ranging from 300 K to 550 K with a step of 50 K as enclosed areas in the ([Pdbulk],PCO) plane. For both ordered phases, the higher the temperature, the smaller the domain of their phase stability. This means that these ordered phases can exist at high temperature only in a narrow range of Pd bulk concentration and for large range of CO partial pressures, which may not be realistic as already noticed before. This is mainly due to the fact that we neglect the vibrational part of entropy in our MC simulations, especially for Metal-CO interactions, leading to an overestimation of the critical temperature of order/disorder transition and thus to an overestimation of the CO partial pressures potentially accessible. However, at sufficiently low temperatures, the configurational part of entropy is the major contribution to the total entropy of the system and the present results becomes reliable again. At each studied temperature, the area of the domain of phase stability of the checkerboard-like configuration is larger than the one of the domain of phase stability of the configuration with linear chains. However, if the range in Pd bulk concentration is quite similar for both ordered phases, seven orders of magnitude in CO partial pressure separate them, the checkerboard-like configuration existing only at very high values of PCO, much higher than those usually encountered. The most important result here is the agreement obtained between the calculated phase diagram of the configuration with linear chains and the experimental results. Actually,


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Goodman et al. reported the presence of contiguous Pd ensembles with bridging CO molecules on AuPd(100) surface when the sample was prepared under CO partial pressures between 0.13 mbar and 13.3 mbar6,7 up to 450 K, the contiguous Pd ensembles disappearing at higher temperatures characterized by an attenuation of the signal of the pick associated with the bridging configuration on the PM-IRAS spectra. Moreover, the presence of contiguous Pd ensembles on a freshly ion-sputtered sample at a low CO partial pressure (1.3 10-6 mbar) and at room temperature is also reported.

Insert Figure 6

III.2. Energetic and electronic structure analysis of ordered Pd phases In order to understand the high stability of the formed Pd chains during CO gas exposure, we compared (see Figure 7) the DFT electronic energies of three different Au–Pd surface configurations with [Pdsurf] = 0.5 for CO coverage θ = 0 and 0.5 ML. The considered configurations are the two identified ordered phases, the checkerboard structure (A) and the Pd linear chains (B), and a modeled structure formed by Pd zigzag chains (C). This latter structure was selected to compare the geometric stability of the Pd chains. The energetic stabilities are calculated with respect to the lowest minimum energy structure for each CO coverage. Under vacuum (θ = 0 ML), the energetic stability of the configuration in which Pd is isolated by gold atoms (A) is larger than that of Pd linear or zigzag chains by +0.33 eV and +0.30 eV, respectively. This expected behavior results from the positive mixing energy between gold and palladium where ordered tendency of the alloy is favored.35 In presence of CO (θ = 0.5 ML), the calculated electronic energies of the three structures evidence the high stability of the linear chain configuration which becomes more stable than the checkerboard and the zigzag chain structures by +0.84 eV and +1.10 eV, respectively. By consequence, and in line with the computed phase diagram, the formation of the Pd linear chain phase as an equilibrium surface structure of the Au-Pd(100) under CO pressure is very likely to occur.

Insert Figure 7

Finally, in order to gain insight into the electronic structure of Pd chain phase and to analyze the origin of its high energetic stability, we compared the charge distribution and the local 13 ACS Paragon Plus Environment


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density of states (see Figure 8) before and after CO adsorption for the three considered configurations. Figure 8a reports the Bader charge distributions of structures A, B and C compared above from energetic point of view. Under vacuum, the comparison of charge distribution over the overall surfaces, show that checkerboard structure presents a homogeneous charge distribution, Au (-0.12 e) and Pd (+0.07 e), which is not the case for the two other Au-Pd alloy ensembles. This could partially explain the energetic stability of such ordered alloy distribution under vacuum. At CO coverage θ=0.5ML, the charge distribution become homogeneous for the three considered surfaces (all gold atoms being equivalent, and all Pd atoms being equivalent) but the more important point is that the positive charge of Pd significantly increases when CO is adsorbed on it (up to +0.3 e for bridge CO adsorption). These results provide evidence of back-donation from Pd to CO which is found to be more pronounced in the case of Pd chain structure. Indeed, the nature of the adsorption bond between CO and d-metal surfaces is conventionally understood in terms of the 5σ CO donation to the metal associated with a back-donation to the empty 2π* CO orbital44,45. Here, the strong increase in the positive charge of Pd when interacting with CO, particularly of the Pd chain configuration, indicates preponderance of the back-donation when Pd is organized in these particular ensembles on the alloy surface. This fact explains the strong CO adsorption energies calculated on such sites and therefore the high energetic stability of Pd chains in the presence of CO gas. Similar trend is found from the Local Density Of State (LDOS) of the d-bands of Pd atoms in the three considered Au-Pd(100) structures A, B and C interacting with CO. Indeed, as shown in the Figure 8b, the Pd d-band in the linear chain structure is highly shifted away from the Fermi level with a calculated band center at -2.36 eV compared to -2.07 eV and -1.98 eV for Pd d-bands in zigzag chain and checkerboard structures, respectively. Under vacuum, these values are calculated to be -1.00 eV, -0.99 eV and -0.98 eV, respectively. This shift towards more negative energy is exactly reflected by strong Bader charge transfer from Pd linear chains to CO. Furthermore, and according to the relation between the d-band center and the reactivity of transition metal surfaces46 the observed shifts in the d-band centers toward the Fermi level can be correlated with a strong reactivity of the Pd ordered linear chain structure toward CO adsorption reaction. Indeed, a strong CO-adsorption on Pd-chains/Au-Pd(100) was established47 to poison the Pd-reactive sites in the CO oxidation reaction over Pd-chains, whereas the checkerboard Pd-ensembles were highlighted the most reactive ensembles.

Insert Figure 8 14 ACS Paragon Plus Environment

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CONCLUSION Ordered phase structures of Au-Pd (100) surface exposed to reactive CO gas were studied by means of Monte-Carlo simulations. To reproduce the Pd segregation, induced by the presence of adsorbates, DFT-based Ising model was used to define the interatomic potential of the metal-metal, metal -CO and CO-CO interactions. At low Pd bulk concentration (~20%) and at room temperature, two stable phases with Pd ordered in linear chains or in checkerboard-like arrangements appear for 0.5 CO coverage, depending on the considered initial CO adsorption mode on Au-Pd(100). The on-top CO adsorption leads to the formation of checkerboard-like phase, whereas the CO in bridge mode leads to the formation of a linear chain phase. For the same range of Pd-bulk concentration, our study of the phase stabilities revealed that the checkerboard-like Pd segregation is stable under PCO pressure of 103 mbar and only 10-4 mbar CO pressure suffice to induce the Pd-surface segregation and the formation of the stable Pdchain phase. The latter result agrees reasonably well with the experimental evidences for the CO-bridge adsorption on contiguous Pd atoms at PCO = 10-1 mbar6,7. The energetic stability of these phases formed under CO adsorption is subsequently confirmed from the DFT calculations and compared to the energetic stability of the respective CO-free surfaces. The obtained stabilization of the surface segregated Pd in linear chains at low PCO is attributed to the enhanced π-back donation from CO to Pd in comparison to the checkerboard and other surface phases.

Supporting Information See supporting information for the following calculation details and results: i) Repulsive CO-CO lateral interaction with CO adsorbed on-top positions. The main possible configurations are depicted in Figure 1S. ii) Monte Carlo simulations (based on DFT-GGA parameters) where on-top and bridge modes are considered simultaneously on Au(100), Pd(100) and on AuPd(100) surfaces. iii) Details on the linear fits of Figures 2 and 3.



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Acknowledgements This work was granted access to the HPC resources of [CCRT/CINES/IDRIS] under the allocation 2017 [x2017087369] made by GENCI (Grand Equipement National de Calcul Intensif].

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Tables Table 1: Derived effective pair interactions on Au and Pd atoms (CO adsorbed at bridge site). LDA

GGA

VeffCO-Au

-0.65eV

-0.26 eV

VeffCO-Pd

-1.29 eV

-0.87 eV


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Figure captions Figure1: Optimized structures of CO on bridge (a, b, c) and on-top (d, e, f) sites of the monometallic Au(100) and in the presence of one and two Pd atoms. Computed DFT/GGA (LDA in parentheses) adsorption energies are indicated in eV.

Figure 2: Evolution of the adsorption energies of CO as a function of the number of Pd atoms in CO’s next-nearest-neighbor position n, ranging from 0 to 6, for each value of m: m = 0 (blue diamonds), m = 1 (red squares), and m= 2 (green triangles). The full and dashed lines represent GGA and LDA calculations, respectively. Details on the linear fit parameters are provided in Tables S2 and S3 (see Supporting Information).

Figure 3: Evolution of the on-top site adsorption energies of CO as a function of the number of Pd atoms in CO’s next-nearest-neighbor position n, ranging from 0 to 4, for each value of m: m = 0 (blue diamonds), m = 1 (red squares). Details on the linear fit parameters are provided in Table S4 (see Supporting Information).

Figure 4: Isotherms (with CO at bridge site) at T = 300 K (a) and T=600 (b) representing the ? evolutions of the CO coverage (θCO, up), surface Pd concentration (c:; , down) as function of @=AB bulk Pd concentration (c:; ) at three given ∆µCO (-1.0, -1.9, -2.5) parameterized considering

LDA results and (-0.75, -1.0, -1.5) parameterized considering GGA results and compared with vacuum condition.

Figure 5: Isotherms (with CO at on-top site) at T = 300 K (a) and T=600 (b) representing the ? evolutions of the CO coverage (θCO, up), surface Pd concentration (c:; , down) as function of @=AB bulk Pd concentration (c:; ) at three given ∆µCO (-0.3, -0.55, -1.2) and compared with

vacuum condition.

Figure 6: Energy phase diagrams of (a) “linear Pd chains” and (b) “checkerboard” ordered structures for different temperatures as a function of bulk concentration of Pd and CO pressure.



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Figure 7: Evolution of energetic stability of different configurations of Pd before (left) and after (right) CO adsorption. The bold values represent energy difference (eV) with respect to the most stable structure (0.00 eV) of each calculated case: checkerboard (A) linear Pd chain (B) and zigzag chain (C) structures.

Figure 8: a) Bader charge distributions on the configurations (A), (B) and (c) before and after CO adsorption. b) Projected Density of state of Pd d-bands and d-band centers in the three computed configurations after CO adsorption.


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Figures Figure 1:

a)

d)

-0.52 (-1.29)

-0.46 (-1.03)

CO on bridge site of Au(100)

CO on-top site of Au(100)

b)

e)

-1.14 (-1.93)

-1.24 (-1.84)

CO on bridge site of Pd1Au(100)

CO on-top site of Pd1Au(100)

c)

f)

-1.74 (-2.57)

-1.30 (-2.57)

CO on bridge site of Pd2Au(100)

CO on-top site of Pd2Au(100)

Figure1: Optimized structures of CO on bridge (a, b, c) and on-top (d, e, f) sites of the monometallic Au(100) and in the presence of one and two Pd atoms. Computed DFT/GGA (LDA in parentheses) adsorption energies are indicated in eV.



Figure 2 0.0

(0,n) (1,n) (2,n)

-0.5

GGA GGA GGA

LDA LDA LDA

-1.0

Eads(eV)

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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-1.5

-2.0

-2.5

-3.0 0

1

2

3

4

5

6

n Figure 2: Evolution of the adsorption energies of CO as a function of the number of Pd atoms in CO’s next-nearest-neighbor position n, ranging from 0 to 6, for each value of m: m = 0 (blue diamonds), m = 1 (red squares), and m= 2 (green triangles). The full and dashed lines represent GGA and LDA calculations, respectively. Details on the linear fit parameters are provided in Tables S2 and S3 (see Supporting Information).


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Figure 3

Figure 3: Evolution of the on-top site adsorption energies of CO as a function of the number of Pd atoms in CO’s next-nearest-neighbor position n, ranging from 0 to 4, for each value of m: m = 0 (blue diamonds), m = 1 (red squares). Details on the linear fit parameters are provided in Table S4 (see Supporting Information).



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Figure 4 a)

b)

Figure 4: Isotherms (with CO at bridge site) at T = 300 K (a) and T=600 (b) representing the ? evolutions of the CO coverage (θCO, up), surface Pd concentration (c:; , down) as function of @=AB bulk Pd concentration (c:; ) at three given ∆µCO (-1.0, -1.9, -2.5) parameterized considering

LDA results and (-0.75, -1.0, -1.5) parameterized considering GGA results and compared with vacuum condition.


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Figure 5 a)

b)

Figure 5: Isotherms (with CO at on-top site) at T = 300 K (a) and T=600 (b) representing the ? evolutions of the CO coverage (θCO, up), surface Pd concentration (c:; , down) as function of @=AB bulk Pd concentration (c:; ) at three given ∆µCO (-0.3, -0.55, -1.2) and compared with

vacuum condition.



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Figure 6 a)

b)

Figure 6: Energy phase diagrams of (a) “linear Pd chains” and (b) “checkerboard” ordered structures for different temperatures as a function of bulk concentration of Pd and CO pressure.


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Figure 7

Figure 7: Evolution of energetic stability of different configurations of Pd before (left) and after (right) CO adsorption. The bold values represent energy difference (eV) with respect to the most stable structure (0.00 eV) of each calculated case: checkerboard (A) linear Pd chain (B) and zigzag chain (C) structures.



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Figure 8

a)

b)

Figure 8: a) Bader charge distributions on the configurations (A), (B) and (c) before and after CO adsorption. b) Projected Density of State of Pd d-bands and d-band centers in the three computed configurations after CO adsorption.


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TOC Graphic


Energetic Stabilities and Phase Diagrams

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