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Chemisorbed Oxygen on the Au(321) Surface Alloyed With Silver: A First-Principles Investigation Lyudmila V. Moskaleva, Theodor Weiss, Thorsten Kluener, and Marcus Bäumer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511884k • Publication Date (Web): 06 Apr 2015 Downloaded from http://pubs.acs.org on April 7, 2015

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The Journal of Physical Chemistry

Chemisorbed Oxygen on the Au(321) Surface Alloyed with Silver: A First-Principles Investigation

Lyudmila V. Moskaleva,1,* Theodor Weiss,1 Thorsten Klüner,2 Marcus Bäumer1 1 Institute of Applied and Physical Chemistry and Center for Environmental Research and Sustainable Technology, Universität Bremen, 28359 Bremen, Germany 2 Institut für Chemie, Carl von Ossietzky Universität Oldenburg, 26129 Oldenburg, Germany

Abstract The adsorption of oxygen on kinked Au(321) slabs is investigated theoretically on the basis of density functional theory. On-surface, subsurface, and surface-oxide forms of O are analyzed and compared on pure gold and on the surfaces containing silver atoms. At low O coverage (0.1 ML) subsurface O species are shown to be unstable both thermodynamically and kinetically due to a low barrier for conversion to stronger bound on-surface chemisorbed oxygen. The presence of Ag in the near-surface region was shown to increase the binding strength of on-surface as well as subsurface O but the activation barrier for releasing subsurface O to the surface remains essentially unaffected by the presence of Ag. At oxygen coverage 0.2 ML or higher, the most stable surface arrangements of O atoms are chain-like structures consisting of linear –O–Au–O– fragments. Subsurface O atoms being a part of such chains are significantly stabilized. We examine phase transitions between the clean surface and possible stable oxidized surface structures as a function of temperature and O2 partial pressure. Ag atoms replacing Au on the Au(321) surface are shown to stabilize the Ocovered surface with respect to the clean surface. Pre-existent chemisorbed atomic oxygen is predicted to facilitate the dissociation of molecular oxygen on the pure and alloyed gold surfaces.

Keywords Density functional theory, Au-Ag alloy surfaces, oxygen chemisorption, subsurface oxygen, surface gold oxide

*

Corresponding author. E-mail address: [email protected], Tel.: +49-421-21863187, Fax. +49-421-21863188

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1

Introduction

With the advent of catalysis by nanostructured gold and recent rapid development of this emerging field, gold has firmly established itself as a promising material for new unconventional catalytic solutions. Gold-based catalysts could be a major step toward increasing the efficiency and sustainability of the chemical industry because they lead to more environmentally friendly reactions under milder conditions. Examples of potential applications of gold include the usage in automotive exhaust control,1 for selective oxidation of hydrocarbons and alcohols toward commodity chemicals,2-4 for vinyl chloride monomer production5 or the water-gas shift reaction.6 Along with the great promise for many useful applications, the catalytic activity of gold has attracted an enormous interest from a fundamental scientific perspective because gold offers such a paradigm of chemistry on the nanoscale while being inactive as a bulk material. Meanwhile, studies on the catalytic activity of gold have been extended to gold alloys with less noble metals, such as Ag or Cu. Particularly interesting are Au-Ag binary alloys because these two metals have similar atomic radii and can form continuous solid solutions for all compositions.7 In view of intrinsically different predisposition of Ag and Au toward oxygen activation, Au-Ag alloy nanocatalysts can be used for tuning catalytic properties by varying particle size (or ligament size in case of nanoporous materials8) and chemical composition.9 Several recent studies reported high catalytic activity of Au-Ag NPs for alkene epoxidation,10 low-temperature CO oxidation,11-15 aerobic oxidation of p-hydroxybenzyl alcohol,16 and others17-18 in many cases exceeding that of monometallic Au or Ag catalysts.11-15, gold”,

8, 21

19-20

Another remarkably active form of gold is the so-called “nanoporous

a nanostructured gold monolith with sponge-like morphology. Its high catalytic

activity has been attributed to a relatively high content of silver impurities in the near-surface region.9, 22-24 To understand the catalytic function of these systems, knowledge of oxidation properties of Au-Ag bimetallic surfaces is important. The nature of active O species on Au and Ag catalysts has been a matter of intensive research and debate. Molecular, subsurface, and various forms of surface atomic oxygen have been discussed in the literature.25-30 Whereas for monometallic Ag catalysts, both on-surface and subsurface atomic oxygen species were found to coexist and to exert an important influence to surface structure and catalytic properties,29, 31-32 the stability of subsurface oxygen and its possible involvement in catalytic reactions on Au catalysts has not been firmly established. Several studies implanting atomic oxygen on Au single-crystal surfaces under ultrahigh vacuum (UHV) conditions observed strongly bound oxygen species, which were attributed either to subsurface O or to surface oxide.33-35 Similarly, the study of catalytically


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active nanoporous gold foams with temperature programmed desorption (TPD) in UHV revealed the presence of several O species, some of which could not be removed even upon heating to 900 K and were tentatively interpreted as subsurface O.36 However, theoretical studies questioned the stability of subsurface oxygen on gold. Shi and Stampfl37 studied O adsorption on the Au(111) surface using density functional theory (DFT) and a periodic slab approach and predicted subsurface O to be energetically unstable with respect to on-surface oxygen at coverage below 0.25 ML. Above this coverage mixed on-surface/subsurface “surface oxide-like” structures on Au(111) were calculated to be more favorable than chemisorbed oxygen which occupies only on-surface sites.37 Another theoretical study using DFT and ab initio molecular dynamics, where random configurations of surface O species have been probed, found subsurface oxygen to co-exist with on-surface chemisorbed oxygen at a coverage of 0.33 ML and above.38 A recent combined experimental and theoretical study on the O-modified Au(100) surface also found subsurface O to be unstable at low coverage.39 At variance with mentioned theoretical studies, which dealt with the flat low-index Au(111) and Au(100) surfaces, in this paper we have chosen the stepped and kinked Au(321) surface as a realistic model for surfaces with rough morphology and we focused on the influence of surface alloying with Ag on the stability of various types of adsorbed O species. The surface topology of Au(321) combines several remarkable structural features: relatively narrow flat terraces of (111) orientation and zigzag-shaped steps giving rise to various surface sites with high and low coordination of Au. Such combination of structural features is expected to be representative of curved topologies typical for metal nanoparticles or foams, e.g., nanoporous gold.40 Hence, it could be considered as a particularly good model of nanostructured Au or Au-Ag catalysts. On-surface adsorption of oxygen on the pure Au(321) has been addressed in earlier theoretical studies.41-42 We hypothesized that Ag components present on surfaces of bimetallic catalysts might enhance the tendency for O penetration into the sub-surface region. In the following, we report on the kinetic and thermodynamic stability of on-surface and subsurface oxygen adsorbed on the Au(321) surface as a function of temperature and O2 pressure as well as the change of these properties due to the presence of Ag in the surface region.

2

Methods

2.1 Models and computational details The Au(321) surface has been represented by a conventional periodic slab model approach. Fig. 1 shows the top view of this surface, where the terraces of (111) type and kinked steps are clearly seen. The slab model has been constructed using the bulk lattice parameter of Au



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4.173 Å optimized by the density functional method employed throughout this work (see below). The slab model contains 14 atomic layers corresponding to the thickness of the resulting slab of ~7.2 Å and the vacuum spacing between periodically repeated slabs of ~8.5 Å. We use a (2 × 1) surface unit cell containing 28 metal atoms. The uppermost 14 metal atoms were allowed to fully relax, whereas the lower 14 atoms were kept fixed at their optimized bulk positions. For selected structures we checked the effect of Figure 1. Top view of the Au(321) surface. The (2 × 1) surface unit cell is indicated.

slab thickness on the adsorption energies of O, see the SI, Table S1. Calculations with thicker slabs up to 17 layers showed

changes in adsorption energy of 0.09 eV at most and by 0.06 eV on average. Because the adsorption energies calculated with the current model are systematically lower (less endothermic) than for the 17 layer model, the relative order of stabilities is not changed. To model the presence of silver alloy components selected atoms in the uppermost layer and the layer directly below were replaced by silver. Generally, it is not known whether silver prefers certain locations on the surface. On the basis of its lower cohesion energy compared to gold, silver is expected to segregate to steps and kinks. Yet, a recent study on the atomic structure of nanoporous gold with high-resolution spherical-aberrationcorrected transmission electron microscopy40 has not observed any measurable Ag segregation. Therefore, we substituted silver at those locations, where we expected the effect of the substitution to be most pronounced, e.g. at the O binding sites. Because bulk gold and silver have very similar lattice constants (the optimized values of a0 are 4.1725 and 4.1563 Å, respectively), the effects of strain due to surface alloying are negligible. The differences in the adsorption energy of O calculated with the two values of lattice constants did not exceed 0.025 eV. Electronic structure calculations were performed using DFT and the PBE form of the generalized-gradient approximation (GGA) for the exchange-correlation functional43 as implemented in the plane-wave based Vienna ab initio simulation package (VASP).44-45 Systems which may possess net spin have been studied using spin-polarized DFT. The effect of the core electrons in the valence density has been taken into account by means of the projector augmented wave (PAW) method46-47 with an energy cutoff of 415 eV. Integration


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in the reciprocal space has been carried out using the k point sampling scheme of Monkhorst and Pack.48 5×5×1 and 15×5×1 k point meshes were used for geometry optimizations and for the density of states (DOS) calculations, respectively. All atomic coordinates were optimized until the force acting on each atom dropped below 2×10−2 eV/Å. Transition states (TSs) of the reactions were determined by applying the nudged elastic band (NEB) method49,50 (using six images along the path between fixed end points) and were further refined with the dimer method.51 The adsorption energies Eads of O and O2 were calculated as follows: Eads (O) =

1 1 ( Esubstrate − O − Esubstrate − N O EO 2 ) NO 2

Eads (O2 ) = Esubstrate − O 2 − Esubstrate − EO 2

(1) (2)

where NO is the number of O atoms in the surface unit cell, Esubstrate− O and Esubstrate− O 2 denote the total energy of the adsorbate-substrate system in the optimized geometry; Esubstrate and EO 2 are the total energies of the clean substrate or a substrate with pre-adsorbed co-

adsorbate in the case of O and O2 co-adsorption and of the O2 molecule in the gas phase, respectively. With this definition, negative values of binding energy indicate exothermic adsorption. 2.2 Thermodynamic models for evaluation of surface energies The effect of temperature and oxygen partial pressure has been studied using the approach of “ab initio atomistic thermodynamics”.52 The surface is considered at equilibrium with an oxygen atmosphere, which is described by an O2 partial pressure p and a temperature T. The relative stability of various surface compositions as a function of T and p is compared on the basis of their surface free energy defined with respect to the clean (pure or partially substituted with Ag) Au(321) surface as γ (T , p ) =

1 ∆Gads (T , p ) A

(3)

where ∆Gads is the free energy of oxygen adsorption and A is the surface area of the lateral unit cell. Due to the same size of the surface unit cell for all structures studied in this work, A is a constant and can be considered as a normalizing factor. The Gibbs free energy of adsorption is defined as ∆Gads (T , p ) = Gsubstrate - O − Gsubstrate − N O µ O (T , p )

(4)

where Gsubstrate−O and Gsubstrate are the Gibbs free energies of the O covered surface and the clean surface, respectively, and µ O (T , p ) is the chemical potential of oxygen. With this definition of the surface free energy, the clean substrate without adsorbed oxygen is taken as reference and therefore its surface free energy is zero. 5 Environment ACS Paragon Plus


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We assume that for the metal substrate, the dependence of the Gibbs free energy on the pV term is negligible. The contribution from vibrations is also small for the considered T range, as pointed out in earlier studies on similar systems;37, 52-53 therefore, Gsubstrate−O and Gsubstrate are approximated by total energies Esubstrate−O and E substrate , respectively, and the

dependence of ∆Gads on p and T comes solely from ∆µO (T , p ) : µO (T , p ) =

(

1 E + ∆µO (T , p ) 2 O2

)

(5)

Because of the known failure of standard DFT-GGA approaches to correctly predict the energy of the triplet ground state of O2 (the atomization energy of O2 is overestimated by ~1 eV at the PBE level43), in this part of the study we derived EO 2 indirectly using the experimental value for the heat of formation of water, the experimental vibrational frequencies of O2, H2, and H2O (the values and references are listed in the SI, Table S2) and the DFT energies of H2 and H2O, which are accurately described by the PBE functional.43 This approach has been previously successfully used to accurately predict formation energies of various metal oxides.54-55 EO 2 = 2( EH 2 O − EH 2 + ∆ZPE − ∆ f H 0o, H 2 O )

(6)

where EH 2O and EH 2 are the PBE total energies of H2O and H2, ∆ZPE is the zero-point vibrational energy correction, and ∆ f H 0o, H 2 O is the standard enthalpy of formation of gasphase H2O at 0 K. Hence, Eq. (5) rewrites as 1 2

µO (T , p ) = EH 2 O − EH 2 + ∆ZPE − ∆ f H 0o, H 2 O + ∆µO (T , p )

(7).

The T, p dependent part of the chemical potential of oxygen is calculated as follows:  pO ∆µO (T , p ) = µ~O 2 (T , p o ) + k BT ln  o2  p

  

(8)

where p° is the standard pressure (1 bar = 0.987 atm) and µ~O 2 (T , p o ) is the thermodynamic function known as [G°(T)–H°(0)]. Here, we use tabulated values of this function from thermochemical tables.56

3

Results and Discussion

3.1 On-surface and subsurface adsorption at low O coverage Because the dissociation probability of O2 on single-crystal gold surfaces is very low, other methods have been developed to implant chemisorbed atomic oxygen on Au surfaces under UHV conditions, such as oxygen ion sputtering, ozone treatment etc.57 Several TPD studies of oxygen-modified Au surfaces observed strongly bound O species, with desorption 6 Environment ACS Paragon Plus

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temperatures far above 400-550 K, which is the expected desorption range for on-surface chemisorbed O.34-35 Gottfried et al.34 studied surfaces formed after sputtering of Au(110)1×2 with O+/O2+ and assigned one of the high-temperature desorption states (γ3, at 620 K) to the subsurface oxygen. Recently Baber et al.39 investigated the oxygen-sputtered Au(100) surface with TPD and also identified two high-temperature desorption states (at 620 and 720 K). On the basis of the theoretically predicted instability of subsurface O,37, 39 these authors interpreted the stronger bound O species as bulk oxygen or oxygen residing at least four layers below the surface.39 In the following, we investigate the thermodynamic and kinetic stability of on-surface and subsurface O species on the clean and alloyed Au(321) surfaces. 3.1.1 The thermodynamic stability at 0 K. In this part of the study we compared the adsorption energies of on-surface and subsurface oxygen on the model Au(321) surface and examined how these adsorption energies are possibly affected by the Ag content. First, we considered the adsorption at low coverage, i.e., adsorbing only one O atom per (2 × 1) unit cell of Au(321), which corresponds to O coverage of 0.1 ML. Comparison of adsorption energies at the 0.1 ML and 0.2 ML coverage has shown very minor change in the calculated values of the adsorption energy suggesting that the interaction between O species at neighbor unit cells is minimal at this coverage. Various considered locations of on-surface and subsurface O species on the Au(321) surface are depicted in Fig. 2. On-surface oxygen generally prefers three-fold hollow sites. Exceptions may arise when oxygen binds at locations close to the step edge because binding to undercoordinated metal atoms is energetically

preferred.

For

example, binding site f (Fig. 2) involves two Au atoms of the lower terrace and a kink Au atom of the upper terrace. Similarly, binding site g consists of one Au atom of the lower terrace and two Au atoms of the step edge. On Figure 2. (A) Three types of subsurface sites considered. (B) Various on-surface (lower terrace and step) and subsurface (upper terrace) sites on the Au(321) surface. Dashed lines help to visualize three-fold on-surface sites f and g. Yellow, green, and light blue indicate octahedral, tetrahedral “vertex down”, and tetrahedral “vertex up” sites, respectively.

pure Au, however, binding to just two Au atoms of the step edge in a bridge mode is more favorable than binding at site g. In our earlier

publication23

we

have

shown that atomic oxygen on the Au(321) surface prefers adsorption sites immediately adjacent to the step edge, i.e. where it binds to two undercoordinated Au atoms and one Au atom inside the terrace. We calculated the adsorption energy of O with respect to ½ O2



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molecule in the gas phase. This gives us the first idea about the thermodynamic stability of adsorbed oxygen at a respective pure or alloyed model gold surface. As already mentioned above, the ground state of O2 is notoriously poorly described in DFT-GGA calculations. Consequently, the adsorption energies reported herein are expected be too positive by about 0.18 eV. The latter value was estimated by deriving the corrected electronic energy of O2 from the experimental heat of formation of water, see the Computational Methods section. However, this correction would not change the relative order of stability of various surface sites; therefore, we refer to uncorrected adsorption energy in this part of the work for simplicity and to be consistent with our earlier work.23 On pure gold, adsorption energies are low or even positive (indicating an endothermic adsorption). Even at the most favorable binding sites the adsorption energy with respect to gas-phase O2 is at most by 0.12 eV exothermic.23 Generally the binding of on-surface O

Figure 3. O adsorption energy (relative to O2 and the clean surface) as a function of the number of Ag atoms at the binding site. Panels A and B compare on-surface site e with the corresponding subsurface site O3. Panels C and D compare on-surface site c with the corresponding subsurface site dT2. Positive values of Eads indicate endothermic adsorption. Positions of Ag substituents relative to O are schematically shown at the bottom of each panel.


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becomes stronger towards the step edge. Introducing silver substituents at O-binding sites increases the adsorption energy of O by 0.1-0.3 eV at those binding sites which involve at least one Ag atom.23 For subsurface oxygen adsorption, we considered 10 possible locations of three types (Fig. 2): octahedral sites, Oi, i = 1-3, tetrahedral vertex-down sites, dTi, i = 1-3, and tetrahedral vertex-up sites, uTi, i = 1-4. In each case, we considered structures where Au atoms at the O-binding site were partially or fully replaced by Ag. Our results obtained for the pure Au(321) surface agree qualitatively with the earlier theoretical studies for O adsorption on Au(111) and Au(100) surfaces, namely, subsurface oxygen at low coverage is found significantly less stable than on-surface oxygen, by 0.4-0.9 eV.37,39 Accordingly, the adsorption energy of subsurface O with respect to gas-phase oxygen is strongly endothermic by 0.5-1.3 eV. No stable minima were identified for the sites at the step edge, O1 and dT1. Attempts to converge such structures always led to more favorable on-surface O adsorption. For the adsorption on Au(321) substrates with Ag substituents the following trends should be mentioned. First, for all types of sites the stability of adsorbed oxygen increases with increasing the number of Ag substituents at the O-binding site. Second, we found that adsorption at subsurface sites is in all cases energetically less favorable than at the nearest on-surface sites. Third, for the same type of site (on-surface hcp or fcc, subsurface Oi, dTi, or uTi), stability decreases as the oxygen moves further away from the step edge. These trends are illustrated graphically in Figs. 3 and 4. Using O3 and e sites as an example, we show in Fig. 3 how the adsorption energies of on-surface and subsurface adsorption modes change with the number of Ag substituents at the O binding site. Recall that the adsorption energy is calculated relative to O2 and a clean surface; therefore, the dissociative adsorption of O2 is in most cases endothermic and hence the value Figure 4. Color plot of the O adsorption energy (eV) at various (A) subsurface and (B) on-surface sites of the pure Au(321) surface. Colored triangles mark O adsorption sites. In each adsorption structure only one O atom is considered per (2×1) unit cell. Adsorption energy is calculated with respect to gas-phase O2 and the clean Au(321) surface. Negative values indicate exothermic adsorption. An analogous diagram for Agmodified Au(321) is shown in the SI, Fig. S1.

of Eads is positive. As the number of Ag substituents at the octahedral site increases, Eads decreases for both onsurface and subsurface O, Fig. 3, in other words, the binding between the O



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atom and the surface gets stronger but this occurs more rapidly for subsurface O than for onsurface O. Whereas for subsurface O Eads drops continuously, this is not the case for onsurface O adsorption. For the latter, only Ag substituents in the top surface layer have a pronounced effect on the oxygen binding energy as they form direct bonds with O, whereas Ag substituents in the second topmost layer do not change the binding energy. Also for subsurface O, the effect of the second-layer Ag substituents is somewhat smaller than of the top-layer substituents. This is evident from comparing the plots on the left of Fig. 3 (where the atoms in the top layer are substituted first) with the corresponding plots on the right (where the atoms of the second layer are substituted first). Substitution in the top layer leads to a stronger binding than substitution in the second layer for the same number of Ag atoms. This manifests a weaker interaction of O with the second metal layer than with the top layer, as reflected in longer O-M (M = Au, Ag) bonds, 2.36-3.37 Å, compared to 2.11-2.54 Å for O-M bonds with the top layer. The trends shown here for O3/e and dT2/c sites are typical for other on-surface/subsurface pairs. Clearly, for each subsurface site in question there is an on-surface site in its direct vicinity which is energetically more favorable (Fig. 4). Our results thus suggest that at low oxygen coverage subsurface O species are thermodynamically unfavorable and hence O cannot penetrate spontaneously to subsurface interstitial sites. 3.1.2 Kinetic stability. Next, we addressed the question whether subsurface O could be Table 1. Activation energies (eV) of the O diffusion from subsurface to on-surface sites of clean and Ag-modified Au(321). reactiona

pure Au

number of Ag atoms at the adsorption site

O1 → a O2 → d

-(unstable) 0.01

1 Ag -(unstable) -(unstable)

O3 → e

0.18b

0.00

0.07 c

dT1 → b

-(unstable)

-(unstable)

0.02

0.01

dT2 → c

0.03

0.00

0.05

0.03

dT3 → f uT1 → g

0.00 0.20

0.00 0.18

0.06 0.17

0.01 0.13

uT2 → h

0.18

0.19

0.11

0.11

uT3 → a

0.20

0.21

0.08

0.10

uT4 → d

0.18

0.14

0.16

0.14

a

3 Ag 0.01 0.02

4 Ag

6 Ag 0.15 0.14 0.30

See Fig. 2 for definition of adsorption sites. b Indirect path O3 → uT4 → e with the activation barriers 0.12 and 0.18 eV for the first and second step, respectively. c O3 → a.


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stabilized by a high diffusion barrier from a subsurface layer to the surface. For all identified subsurface O positions we have calculated the diffusion barrier from subsurface to onsurface sites. In most cases, the calculated transition states correspond to the diffusion to the nearest most stable on-surface site, Table 1. The calculated activation energies for O transfer to the surface are rather low, not exceeding 0.3 eV. For those subsurface sites where no metal atom is located directly above oxygen (dTi, Oi) the calculated barrier heights are close to zero suggesting that subsurface O would prefer to come to the surface.

The only

exception is the site O3, where in Figure 5. Most stable mixed configurations containing subsurface O for a given O coverage. The coverage as well as the average adsorption energy (per O atom) are indicated. Large and small spheres represent Au and O atoms, respectively. Arrows point to the location of subsurface O.

certain cases of substitution by Ag barriers of 0.2-0.3 eV were computed. The higher barriers calculated for O diffusion from this site can be explained by the fact that metal atoms

forming site O3 have high coordination numbers and are not as flexible as those located close to the step edge. Diffusion barriers of 0.1-0.2 eV are calculated for vertex-up tetrahedral subsurface sites uTi capped at the top by a metal atom. Substitution of Au by Ag at the oxygen-binding site does not have any strongly pronounced effect on the activation energies of O transfer. Only for the diffusion from octahedral subsurface sites Oi to the surface, Ag substituents at the binding site seem to notably raise the activation barrier. On the contrary, for uTi subsurface sites, Ag substituents slightly decrease the diffusion barrier. Nevertheless, even the highest barrier found for the diffusion from the subsurface site O3 to the on-surface site e in the presence of six Ag substituents, 0.3 eV, is still too low to stabilize subsurface O at elevated temperatures. 3.2 On-surface/subsurface mixed structures at high O coverage Earlier theoretical studies of oxygen adsorption on the flat Au(111) surface37-38 indicated that the relative stability of subsurface O with respect to on-surface O increases with increasing O coverage. It is interesting that two theoretical studies using different approaches: the static equilibrium DFT thermodynamics study of Shi et al.37 and the ab initio MD study of Baker et al.,38 came to the same conclusion that at coverage above ~0.3 ML mixed on-surface and subsurface O structures become energetically stabilized. Importantly, 11 Environment ACS Paragon Plus


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in the identified mixed structures a subsurface O is always a part of a linear O–Au–O unit, see below, where the second O atom binding to gold is on-surface adsorbed O. Our results for the Au(321) surface concur with these findings. We considered O coverage of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.8 ML. For each value of the surface coverage considered, we examined a range of configurations with either only on-surface adsorbed or mixed on-surface/subsurface O atoms. Already at coverage 0.2 ML the most stable arrangements identified contain chain-like structural motifs consisting of O–Au–O linear units, such as those shown in Fig. 5 and also in Figs. S2 and S3 of the SI. Some of these most stable configurations contain subsurface O within a linear O–Au–O unit. The most stable configurations containing subsurface O for selected values of O coverage and the corresponding average O adsorption energies per O atom are given in Fig. 5. For coverage 0.2 and 0.4 ML we found even more stable chain-like structures containing only on-surface O, Fig. 7. For coverage 0.3 ML and higher the surface geometry is significantly distorted and the steps are flattened. Inspection of the adsorption energies, Fig. 7, reveals an interesting fact: for coverage 0.2-0.4 ML the average adsorption energies (per O atom) of the most stable surface oxide structures, -0.24 eV, -0.28 eV, and -0.29 eV, are a factor of 2 to 2.5 larger in magnitude than the value calculated for individually adsorbed on-surface O at the most favorable site a (Fig. 4) at a low coverage, 0.1 ML, -0.12 eV, indicating that there is some additional stabilization in these particularly favorable configurations (typically high coverage leads to weaker binding of adsorbates due to increased repulsion between neighbor adsorbates). Remarkably, even for a rather high O coverage of 0.8 ML (Fig. 5), we found a low-energy structure, where the adsorption of oxygen is still slightly exothermic (average adsorption energy per O atom is -0.03 eV). From the adsorption energies of structures at 0.2 ML coverage where surface reconstructions are relatively small and using the individually adsorbed O atoms at the respective sites on the surface as reference, we derived a stabilization energy of ~0.3 eV per linear O–Au–O unit. We explain this stabilization by the enhanced orbital overlap between Au 5d and O 2p orbitals in such linear O–Au–O structural units. Analysis of projected density of states (PDOS), Fig. 6, has shown an enhanced hybridization between Au 5d and O 2p states in the structures with O–Au–O bridges in comparison to O-covered surface at 0.1 ML coverage. This enhanced overlap is manifested by sharp peaks in the PDOS, Fig. 6(E), between -7.5 and -6 eV corresponding to Au–O bonding states and near the bottom of the Au d-band corresponding to Au–O antibonding states. Stronger bonding is also reflected in shorter Au–O distances, 1.96-2.10 Å, than the usual range for chemisorbed O, 2.14-2.18 Å.


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Because in all identified stable surface oxide structures the kink atoms of the Au(321) surface are involved in the formation of infinite chains going across terraces, a question arises whether the effect described in this work is something specific for this particular surface and requires the presence of densely spaced kinks. In Fig. S4 we show that a similar stabilization of the subsurface O in linear O–Au–O units is found on the flat Au(111) surface. Therefore, the described effect is general but, of course, the greater flexibility of Au atoms at step edges (a

higher

propensity

to

reconstruction) contributes to the greater stabilization of the chains even at a relatively low O coverage. Similarly to our findings, O– Au–O fragments interconnected in chains of alternating Au and O atoms were

previously

identified

as

Figure 6. (A, B) Representative adsorption geometries of O on Au(321): (A) for coverage 0.1 ML, (B) for coverage 0.3 ML. Positions of relevant Au and O atoms are indicated with numbers. In (A) Au atoms 1,2,3 have only one bond to O. In (B) Au1 and Au2 are bonded to only one O atom, whereas Au3,4,5 are bonded to two O atoms. Atoms 4,8,5,7 form a –(Au–O)– chain. (C, D, E) PDOS: (C) for structure in panel (A), (D, E) for structure in panel (B). The red and black curves indicate the O 2p states and the Au 5d states for Au atoms that are bonded to oxygen, respectively: (C, D) Au is bonded to only one O atom, (E) Au is bonded to two O atoms in a linear O–Au–O unit.

energetically favored structures in the theoretical studies of oxygen chemisorption on Au(111)37 and Au(110)58 surfaces. This type of structures has been termed “surface-oxide-like”37 or “subsurface oxide”38 or “chainlike structures”58 in the literature. Turning to bimetallic surfaces, it is certainly important to find out whether silver also favors linear O–Ag–O fragments and whether the most favorable structures on Ag substituted surfaces are the same as on pure gold. We have addressed these questions by substituting Au with Ag at selected positions in the surface oxide structures, see Fig. S5. Linear chains at coverage 0.2 ML represent the simplest case. By substituting Au with Ag at various locations on the surface, and taking structures with individually adsorbed O as reference we found that O–Ag–O linear units are also stabilized but to a lesser extent than O–Au–O, i.e. by only ~0.05-0.07 eV (versus by 0.3 eV for O–Au–O), which could be



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attributed to more ionic and therefore less directional nature of Ag–O bonds.59 This explanation is supported by significantly more positive Bader charges on Ag ions as compared to Au, see Fig. S5. Because O–Ag bonds on the surface are up to 0.1 eV stronger than O–Au bonds, higher stability is predicted for structures where Ag is bound to one or more O atoms but does not substitute Au within the (–O–Au–) chains. Examples of such structures can be found in Fig. 7. So far, we have drawn conclusions about the stability of O species on our model surfaces solely on the basis of adsorption energies and diffusion barriers without taking into account temperature and partial pressure of O2. To identify, what surface coverage is most thermodynamically favorable for a given temperature and oxygen partial pressure, we plotted in Fig. 7 (A-C) the surface free energies for the most favorable configurations at the O coverage of 0.1-0.4 ML. The surface free energy γ is plotted as a function of the modified chemical potential, ∆µO, which gives the T and p dependence of γ according to Eqs. (3)-(6). At a fixed partial pressure of O2, ∆µO can be directly related to temperature. The most thermodynamically favorable coverage at a given ∆µO is the one with the lowest value of γ. As expected from thermodynamic considerations, higher partial pressures of O2 and lower temperatures favor the dissociative adsorption of O2. We find, Fig. 7(A), that for the pure Au(321) surface, the most thermodynamically stable phase at room temperature is a surface incorporating (–O–Au–) chains at the partial pressure of O2 1 atm, whereas it is the clean surface at UHV conditions. The transition from clean surface to surface oxide occurs above 400 K at ambient conditions (p(O2) = 1 atm) and below 200 K at UHV conditions. At the near-ambient pressure region (p(O2) = 10-3 atm), which may be relevant for making comparisons to studies with in situ X-ray photoelectron spectroscopy60-61 or environmental transmission electron microscopy,40, 62 the surface oxide phase is no longer stable at or above room temperature, consistent with the decomposition of a gold oxide film on gold foil at 373 K and O2 pressure ~10-4 atm as reported in Ref.

61

. It is quite remarkable that chain-like

surface oxide phases are significantly stabilized on pure gold with respect to regular onsurface chemisorbed O at low coverage. The latter is thermodynamically unstable at room temperature even at high O2 pressure. On the silver substituted surface, as expected, the transition from clean to oxygen covered surface shifts to higher temperature due to the stronger adsorption of oxygen. At the same time stable phases at coverage 0.1 and 0.2 ML (corresponding to regular chemisorption of O) appear in the plots of Fig.7 (B, C). We predict that for Ag concentration in the surface layer of 30% or above, Fig. 7(B, C), not only the surface oxide but also regular on-surface adsorbed O should be the thermodynamically stable state at ambient conditions. Hence, there


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Figure 7. Surface free energy γ as a function of the modified chemical potential of oxygen, ∆µO, for the most stable structures with O coverage 0.1, 0.2, 0.3, and 0.4 ML. (A) Pure Au(321). (B, C) Au(321) with 30% and 100% Ag in the top layer, respectively. γ = 0 corresponds to the respective surface with no O present. Shaded areas indicate the most favorable coverage for a given range of the chemical potential (or, equivalently, the temperature at a fixed partial pressure of O2).



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should be a thermodynamic driving force for the dissociation of molecular oxygen on bimetallic Au-Ag catalysts at ambient conditions if the critical concentration of Ag has been reached, see also Fig. 9(C) below. Note that because DFT does not yield chemical accuracy of adsorption energies, the experimental values of transition temperatures and Ag concentrations may deviate somewhat from the results of our calculation. Being aware of the problem with the DFT-GGA description of O2, we switched from the O2 reference to H2O and H2 reference. That shifted the temperatures of phase transitions to more negative values of ∆µO by about 0.18 eV and consequently to higher temperatures. Having applied this correction, we expect that our results are at least qualitatively reliable because the relative stability of systems with similar nature is typically well described by DFT. Our thermodynamic picture concurs with the recent results of a surface science study on the oxidation of nanoporous gold.63

Ozone-treated

and

subsequently annealed nanoporous gold samples were characterized by a surface silver content of ~14% and were shown to be richly populated with adsorbed oxygen in form of a thin

surface

oxide

film

and

individually chemisorbed O atoms. Figure 8. Examples of O assisted O2 dissociation: (A) on the pure Au(321) surface, (B) on the Au(321) surface with two Ag atoms at the reaction site. Geometries of the initial state (IS) and of the transition state (TS) and the final state (FS) are depicted. Eads, Ea, and Ediss denote the adsorption energy of O2, the activation energy with respect to the adsorbed O2, and the energy of the dissociative adsorption with respect to the gas-phase O2 and the clean surface.

After three days storage under UHV conditions at room temperature, the oxygen species completely vanished from the surface in agreement with our theoretical predictions on the basis of atomistic thermodynamics.

3.3 Dissociation of O2 on Au(321) with pre-adsorbed atomic O Another interesting aspect revealed by our study is the possible effect of adsorbed atomic O species on the activation barrier of O2 dissociation. Our calculations show that preadsorbed O may significantly reduce the activation energy of O2 dissociation if an oxygen atom is located near the reaction site and binds to the same Au atom as the dissociating O2 molecule. Note that such an arrangement of the reactants is quite probable because both O and O2 preferentially adsorb at the sites near the step edge. With this location of the coadsorbed O, an almost linear O–Au–O unit is formed between one of the dissociating O atoms of O2 and the surface oxygen in the transition state, see Fig. 8. Such an arrangement of 16 Environment ACS Paragon Plus

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Au and two O atoms is energetically favorable, as mentioned earlier, and therefore, the barrier for O2 dissociation is lowered. Due to this barrier lowering, the dissociation of O2 may become autocatalytic, once a certain concentration of atomic O on the surface is reached. We find that in the case of a pure Au(321) surface, the activation energy drops by as much as 0.3 eV from 1.1 to 0.77 eV, Fig. 8(A), owing to the assistance of an O atom located near the reaction site. Remarkably,

the

reduced

value

is

comparable to the activation barrier for O2 dissociation on the surface with 80% silver!23 Nevertheless, due to the very low adsorption energy of O2 in this case, -0.03 eV, such a scenario is not very probable. The situation can change if Ag atoms are present at the reaction site. For example, in the case illustrated in Fig 8(B) with two Ag substituents at the reaction site, the adsorption energy of O2 becomes -0.15 eV and the dissociation barrier further drops to 0.63 eV. It makes a difference, however, whether Au in the O–Au–O fragment is replaced by Ag or not. Silver does not seem

to

favor

a

linear

O–Ag–O

arrangement as strongly as gold, which could be attributed to more ionic and therefore less directional nature of Ag–O bonds.59 With the O–Ag–O fragment replacing the O–Au–O fragment, the decrease in the activation barrier is only Figure 9. (A) Adsorption energies of O2, (B) the activation barrier for O2 dissociation, and (C) the energy of dissociative adsorption (with respect to gas-phase O2) as a function of the number of Ag substituents in the unit cell. Red and black lines indicate systems with and without co-adsorbed O, respectively. Transition state structures with coadsorbed O are schematically depicted.

moderate compared to the surface with no pre-adsorbed O and is mainly caused by the destabilization of the initial state compared to the oxygen-free surface. Still for all considered concentrations and



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locations of Ag alloy components, the activation energy of O2 dissociation is lower than the activation energies calculated in the absence of the assisting O species, Fig. 9 (B). Therefore, we conclude that pre-adsorbed O can additionally facilitate the dissociation of O2 on goldbased catalysts, which provides further insight into the mechanism of O2 activation on such catalysts. In this context, it may be interesting to mention the experimental finding of Deng et al.,64 that the dissociation probability of O2 on the Au(111) surface was increased to a measurable value of 10-3, i.e. by at least three orders of magnitude, by the presence of preadsorbed atomic oxygen at a low coverage of 0.02 ML. The authors explained the enhancement of O2 dissociation by roughening of the surface in the process of implanting oxygen via electron-induced dissociation of NO2. However, our results suggest that not only low-coordinated Au atoms but likely also chemisorbed O atoms at low-coordinated sites are responsible for this significant increase of the dissociation rate. 4

Conclusions We have studied the thermodynamic and kinetic stability of various types of O species

on the model Au(321) surface. We find that at low oxygen coverage (0.1–0.2 ML) subsurface O is both thermodynamically and kinetically unstable. Such subsurface oxygen species are expected to easily diffuse to the surface unless located several metal layers below the surface. The presence of Ag in the near-surface region was shown to increase the binding strength of on-surface as well as subsurface O but the activation barrier for releasing subsurface O to the surface remains essentially unaffected by the presence of Ag. When increasing the surface oxygen coverage, mixed arrangements with subsurface and on-surface adsorbed oxygen become energetically more favorable than the structures with on-surface only O adsorption. We find that the most favorable configurations at sufficient O coverage (≥0.2 ML) contain characteristic linear O–Au–O units interconnected in (branched) chains. The binding between Au and O in such a linear arrangement is significantly enhanced by up to 0.3 eV due to a greater overlap of Au 5d and O 2p states. Interestingly, silver does not favor chain structures as strong as gold; therefore, on surfaces with Ag impurities there should be a competition between the formation of (–Au–O–) chains and a regular chemisorption to Ag sites. By using the approach of “ab initio thermodynamics” we have shown that at catalytically relevant pressure and temperature regular chemisorption of oxygen at the pure Au(321) surface is thermodynamically unfavorable. However, surface oxide structures consisting of (–Au–O–) chains are predicted to be thermodynamically stable at ambient conditions. The thermodynamic stability of the oxidized surface can be further increased if silver atoms are introduced in the surface region. The transition from clean to O-covered 18 Environment ACS Paragon Plus

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surfaces on the theoretically predicted phase diagrams shifts to the higher temperature with increasing the silver content, such that for alloys with a surface concentration of Ag of 30% or more, individually chemisorbed O becomes thermodynamically stable at room T and normal pressure. At low pressure in the range typical for UHV studies even this concentration of Ag is not sufficient to render oxygen on Au thermodynamically stable. Our results indirectly support the experimentally observed dissociation of O2 on nanoporous gold in a flow reactor and the lack of such dissociation under UHV conditions36 or in multi-pulse measurements using a temporal analysis of products (TAP) reactor.65 We find that atomic oxygen pre-existent on the pure or alloyed Au surfaces (implanted or formed in the course of the dissociative adsorption of O2) can significantly reduce the barrier for O2 dissociation and can hence facilitate O2 activation. Therefore, in addition to the barrier lowering due to the presence of Ag in Au-Ag bimetallic catalysts, there is an indirect role played by Ag through thermodynamic stabilization of on-surface oxygen species. 5

Acknowledgements We acknowledge the financial support from the German Research Foundation (DFG)

within the Project No. MO 1863/2-2. The calculations were performed at the HPC Cluster HERO, located at the University of Oldenburg and funded by the DFG through its Major Research Instrumentation Program (INST 184/108-1 FUGG) and the Ministry of Science and Culture of the Lower Saxony State. A contribution of the European COST Action MP00903 “Nanoalloy” is also acknowledged. We thank Dr. Andreas Schaefer and Dr. Volkmar Zielasek for critical reading of the manuscript. We gratefully acknowledge the insightful comments and helpful suggestions by three anonymous reviewers. Supporting Information Available: Table S1. Effect of slab thickness on the binding energies. Table S2. Experimental values used to derive the corrected electronic energy of O2. Figure S1. Color plot of the O adsorption energy at various subsurface and surface sites of the Ag-modified Au(321) surface. Figure S2. Top six most stable mixed configurations for O coverage 0.2 ML on pure Au(321). Figure S3. Most stable mixed configurations for O coverage 0.5 and 0.6 ML on pure Au(321). Figure S4. Most stable co-adsorption structured containing subsurface O and individually adsorbed O at various values of coverage on Au(111). Figure S5. Effect of substitution with Ag on the adsorption energies, stabilization energies due to formation of linear O–Au–O or O–Ag–O fragments, and ionic charges. Figures S6 and S7. Surface free energy plots for selected Ag-substituted surfaces. Fig. S8. Examples of O assisted O2 dissociation. This material is available free of charge via the internet at http://pubs.acs.org.



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References 1.

2.

3.

4.

5.

6.

7. 8. 9. 10.

11.

12. 13.

14.

15.

16.

17.

Wichmann, A.; Wittstock, A.; Frank, K.; Biener, M. M.; Neumann, B.; Mädler, L.; Biener, J.; Rosenauer, A.; Bäumer, M., Maximizing Activity and Stability by Turning Gold Catalysis Upside Down: Oxide Particles on Nanoporous Gold. Chem. Cat. Chem 2013, 5, 2037-2043. Wittstock, A.; Zielasek, V.; Biener, J.; Friend, C. M.; Bäumer, M., Nanoporous Gold Catalysts for Selective Gas-Phase Oxidative Coupling of Methanol at Low Temperature. Science 2010, 327, 319-322. Carrettin, S.; McMorn, P.; Johnston, P.; Griffin, K.; Hutchings, G. J., Selective Oxidation of Glycerol to Glyceric Acid Using a Gold Catalyst in Aqueous Sodium Hydroxide. Chemical Communications 2002, 0, 696-697. Mathew, D. H.; Yi-Jun, X.; Patrick, J.; Paul, M.; Philip, L.; Dan, I. E.; Albert, F. C.; Gary, A. A.; Graham, J. H.; Frank, K., et al., Tunable Gold Catalysts for Selective Hydrocarbon Oxidation under Mild Conditions. Nature 2005, 437, 1132-1135. Conte, M.; Davies, C. J.; Morgan, D. J.; Davies, T. E.; Elias, D. J.; Carley, A. F.; Johnston, P.; Hutchings, G. J., Aqua Regia Activated Au/C Catalysts for the Hydrochlorination of Acetylene. J. Catal. 2013, 297, 128-136. Barakat, T.; Rooke, J. C.; Genty, E.; Cousin, R.; Siffert, S.; Su, B.-L., Gold Catalysts in Environmental Remediation and Water-Gas Shift Technologies. Energy & Environ. Sci. 2013, 6, 371-391. Ferrando, R.; Jellinek, J.; Johnston, R. L., Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles. Chem. Rev. 2008, 108, 845-910. Wittstock, A.; Wichmann, A.; Biener, J.; Baumer, M., Nanoporous Gold: A New Gold Catalyst with Tunable Properties. Faraday Discuss. 2011, 152, 87-98. Xu, B.; Siler, C. G. F.; Madix, R. J.; Friend, C. M., Ag/Au Mixed Sites Promote Oxidative Coupling of Methanol on the Alloy Surface. Chem. Eur. J. 2014, 20, 4646-4652. Kondarides, D. I.; Verykios, X. E., The Adsorption of Oxygen on Ag and Ag-Au Alloys: Mechanistic Implications in Ethylene Epoxidation Catalysis. Stud. Surf. Sci. Catal. 1994, 82, 471-480. Iizuka, Y.; Kawamoto, A.; Akita, K.; Daté, M.; Tsubota, S.; Okumura, M.; Haruta, M., Effect of Impurity and Pretreatment Conditions on the Catalytic Activity of Au Powder for Co Oxidation. Catal. Lett. 2004, 97, 203-208. Liu, J.-H.; Wang, A.-Q.; Chi, Y.-S.; Lin, H.-P.; Mou, C.-Y., Synergistic Effect in an Au−Ag Alloy Nanocatalyst: Co Oxidation. The Journal of Physical Chemistry B 2005, 109, 40-43. Yen, C.-W.; Lin, M.-L.; Wang, A.; Chen, S.-A.; Chen, J.-M.; Mou, C.-Y., CO Oxidation Catalyzed by Au−Ag Bimetallic Nanoparticles Supported in Mesoporous Silica. J. Phys. Chem. C 2009, 113, 17831-17839. Liu, X.; Wang, A.; Yang, X.; Zhang, T.; Mou, C.-Y.; Su, D.-S.; Li, J., Synthesis of Thermally Stable and Highly Active Bimetallic Au−Ag Nanoparticles on Inert Supports. Chem. Mater. 2009, 21, 410-418. Sandoval, A.; Aguilar, A.; Louis, C.; Traverse, A.; Zanella, R., Bimetallic Au–Ag/TiO2 Catalyst Prepared by Deposition–Precipitation: High Activity and Stability in CO Oxidation. J. Catal. 2011, 281, 40-49. Chaki, N. K.; Tsunoyama, H.; Negishi, Y.; Sakurai, H.; Tsukuda, T., Effect of Ag-Doping on the Catalytic Activity of Polymer-Stabilized Au Clusters in Aerobic Oxidation of Alcohol. J. Phys. Chem. C 2007, 111, 4885-4888. Jiang, H.-L.; Akita, T.; Ishida, T.; Haruta, M.; Xu, Q., Synergistic Catalysis of Au@Ag Core−Shell Nanoparticles Stabilized on Metal−Organic Framework. J. Am. Chem. Soc. 2011, 133, 1304-1306.


Page 20 of 33

Page 21 of 33

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

18.

19.

20.

21. 22. 23.

24.

25. 26.

27.

28. 29.

30. 31. 32. 33. 34. 35.

36.

37.


Menezes, W. G.; Neumann, B.; Zielasek, V.; Thiel, K.; Bäumer, M., Bimetallic Au-Ag Nanoparticles: Enhancing the Catalytic Activity of Au for Reduction Reactions in the Liquid Phase by Addition of Ag. Chem. Phys. Chem 2013, 14, 1577-1581. Liu, X.; Li, Y.; Lee, J. W.; Hong, C.-Y.; Mou, C.-Y.; Jang, B. W. L., Selective Hydrogenation of Acetylene in Excess Ethylene over SiO2 Supported Au–Ag Bimetallic Catalyst. Appl. Catal. A. 2012, 439–440, 8-14. Zheng, J.; Lin, H.; Wang, Y.-n.; Zheng, X.; Duan, X.; Yuan, Y., Efficient Low-Temperature Selective Hydrogenation of Esters on Bimetallic Au–Ag/SBA-15 Catalyst. J. Catal. 2013, 297, 110-118. Wittstock, A.; Wichmann, A.; Bäumer, M., Nanoporous Gold as a Platform for a Building Block Catalyst. ACS Catal. 2012, 2, 2199-2215. Zielasek, V.; Jürgens, B.; Schulz, C.; Biener, J.; Biener, M. M.; Hamza, A. V.; Bäumer, M., Gold Catalysts: Nanoporous Gold Foams. Angew. Chem. Int. Ed. 2006, 45, 8241-8244. Moskaleva, L. V.; Röhe, S.; Wittstock, A.; Zielasek, V.; Klüner, T.; Neyman, K. M.; Bäumer, M., Silver Residues as a Possible Key to a Remarkable Oxidative Catalytic Activity of Nanoporous Gold. Phys. Chem. Chem. Phys. 2011, 13, 4529-4539. Wang, L.-C.; Zhong, Y.; Jin, H.; Widmann, D.; Weissmüller, J.; Behm, R. J., Catalytic Activity of Nanostructured Au: Scale Effects Versus Bimetallic/Bifunctional Effects in LowTemperature CO Oxidation on Nanoporous Au. Beilstein J. Nanotechnol. 2013, 4, 111-128. Verykios, X. E.; Stein, F. P.; Coughlin, R. W., Oxidation of Ethylene over Silver: Adsorption, Kinetics, Catalyst. Catal. Rev. 1980, 22, 197-234. Bao, X.; Muhler, M.; Schedel-Niedrig, T.; Schlögl, R., Interaction of Oxygen with Silver at High Temperature and Atmospheric Pressure: A Spectroscopic and Structural Analysis of a Strongly Bound Surface Species. Phys. Rev. B 1996, 54, 2249-2262. Su, H.-Y.; Zeng, Z.; Bao, X.-H.; Li, W.-X., First-Principles Study of Carbon Monoxide Oxidation on Ag(111) in Presence of Subsurface Oxygen and Stepped Ag(221). J. Phys. Chem. C 2009, 113, 8266-8272. Li, W.-X.; Stampfl, C.; Scheffler, M., Oxygen Adsorption on Ag(111): A DensityFunctional Theory Investigation. Phys. Rev. B 2002, 65, 075407. Rocha, T. C. R.; Oestereich, A.; Demidov, D. V.; Havecker, M.; Zafeiratos, S.; Weinberg, G.; Bukhtiyarov, V. I.; Knop-Gericke, A.; Schlogl, R., The Silver-Oxygen System in Catalysis: New Insights by near Ambient Pressure X-Ray Photoelectron Spectroscopy. Phys. Chem. Chem. Phys. 2012, 14, 4554-4564. Min, B. K.; Friend, C. M., Heterogeneous Gold-Based Catalysis for Green Chemistry: LowTemperature CO Oxidation and Propene Oxidation. Chem. Rev. 2007, 107, 2709-2724. Nagy, A. J.; Mestl, G.; Schlögl, R., The Role of Subsurface Oxygen in the Silver-Catalyzed, Oxidative Coupling of Methane. J. Catal. 1999, 188, 58-68. Qu, Z.; Cheng, M.; Huang, W.; Bao, X., Formation of Subsurface Oxygen Species and Its High Activity toward CO Oxidation over Silver Catalysts. J. Catal. 2005, 229, 446-458. Chevrier, J.; Huang, L.; Zeppenfeld, P.; Comsa, G., Characterization by Scanning Tunneling Microscopy of the Oxygen Induced Restructuring of Au(111). Surf. Sci. 1996, 355, 1-12. Gottfried, J. M.; Elghobashi, N.; Schroeder, S. L. M.; Christmann, K., Oxidation of Gold by Oxygen-Ion Sputtering. Surf. Sci. 2003, 523, 89-102. Zielasek, V.; Xu, B.; Liu, X.; Bäumer, M.; Friend, C. M., Absence of Subsurface Oxygen Effects in the Oxidation of Olefins on Au: Styrene Oxidation over Sputtered Au(111). The J. Phys. Chem. C 2009, 113, 8924-8929. Röhe, S.; Frank, K.; Schaefer, A.; Wittstock, A.; Zielasek, V.; Rosenauer, A.; Bäumer, M., CO Oxidation on Nanoporous Gold: A Combined TPD and XPS Study of Active Catalysts. Surf. Sci. 2013, 609, 106-112. Shi, H.; Stampfl, C., First-Principles Investigations of the Structure and Stability of Oxygen Adsorption and Surface Oxide Formation at Au(111). Phys. Rev. B 2007, 76, 075327. 21 Environment ACS Paragon Plus


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

38.

39.

40.

41. 42. 43. 44. 45. 46. 47. 48. 49. 50.

51.

52. 53.

54. 55. 56.

57. 58. 59.

Baker, T. A.; Xu, B.; Liu, X.; Kaxiras, E.; Friend, C. M., Nature of Oxidation of the Au(111) Surface: Experimental and Theoretical Investigation. J. Phys. Chem. C 2009, 113, 1656116564. Baber, A. E.; Torres, D.; Müller, K.; Nazzarro, M.; Liu, P.; Starr, D. E.; Stacchiola, D. J., Reactivity and Morphology of Oxygen-Modified Au Surfaces. J. Phys. Chem. C 2012, 116, 18292-18299. Fujita, T.; Guan, P.; McKenna, K.; Lang, X.; Hirata, A.; Zhang, L.; Tokunaga, T.; Arai, S.; Yamamoto, Y.; Tanaka, N., et al., Atomic Origins of the High Catalytic Activity of Nanoporous Gold. Nature Mater. 2012, 11, 775-780. Fajín, J. L. C.; Cordeiro, M. N. D. S.; Gomes, J. R. B., Adsorption of Atomic and Molecular Oxygen on the Au(321) Surface: Dft Study. J. Phys. Chem. C 2007, 111, 17311-17321. Fajín, J. L. C.; Cordeiro, M. N. D. S.; Gomes, J. R. B., DFT Study of the Au(321) Surface Reconstruction by Consecutive Deposition of Oxygen Atoms. Surf. Sci. 2008, 602, 424-435. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. Kresse, G.; Furthmüller, J., Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15-50. Kresse, G.; Hafner, J., Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal– Amorphous-Semiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 14251-14269. Blöchl, P. E., Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953-17979. Kresse, G.; Joubert, D., From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758-1775. Monkhorst, H. J.; Pack, J. D., Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188-5192. Mills, G.; Jónsson, H.; Schenter, G. K., Reversible Work Transition State Theory: Application to Dissociative Adsorption of Hydrogen. Surf. Sci. 1995, 324, 305-337. Jónsson, H.; Mills, G.; Jacobsen, K. W., Nudged Elastic Band Method for Finding Minimum Energy Paths of Transitions. In Classical and Quantum Dynamics in Condensed Phase Simulations, Berne, B. J.; Ciccotti, G.; Coker, D. F., Eds. World Scientific: Singapore, 1998; pp 385-404. Henkelman, G.; Jónsson, H., A Dimer Method for Finding Saddle Points on High Dimensional Potential Surfaces Using Only First Derivatives. J. Chem. Phys. 1999, 111, 7010-7022. Reuter, K.; Scheffler, M., Composition, Structure, and Stability of Ruo2(110) as a Function of Oxygen Pressure. Phys. Rev. B 2001, 65, 035406. Saidi, W. A.; Lee, M.; Li, L.; Zhou, G.; McGaughey, A. J. H., Ab Initio Atomistic Thermodynamics Study of the Early Stages of Cu(100) Oxidation. Phys. Rev. B 2012, 86, 245429. Martínez, J. I.; Hansen, H. A.; Rossmeisl, J.; Nørskov, J. K., Formation Energies of Rutile Metal Dioxides Using Density Functional Theory. Phys. Rev. B 2009, 79, 045120. Calle-Vallejo, F.; Martínez, J. I.; García-Lastra, J. M.; Mogensen, M.; Rossmeisl, J., Trends in Stability of Perovskite Oxides. Angew. Chem. Int. Ed. 2010, 49, 7699-7701. Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N., JANAF Thermochemical Tables, 3rd ed.; American Institute of Physics: New York, 1985. Gong, J., Structure and Surface Chemistry of Gold-Based Model Catalysts. Chem. Rev. 2011, 112, 2987-3054. Landmann, M.; Rauls, E.; Schmidt, W. G., Chainlike Au−O Structures on Au(110)-(1 × R) Surfaces Calculated from First Principles. J. Phys. Chem. C 2009, 113, 5690-5699. Torres, D.; Neyman, K. M.; Illas, F., Oxygen Atoms on the (111) Surface of Coinage Metals: On the Chemical State of the Adsorbate. Chem. Phys. Lett. 2006, 429, 86-90. 22 Environment ACS Paragon Plus

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60.

61.

62.

63.

64.

65.


Wittstock, A.; Neumann, B.; Schaefer, A.; Dumbuya, K.; Kübel, C.; Biener, M. M.; Zielasek, V.; Steinrück, H.-P.; Gottfried, J. M.; Biener, J., et al., Nanoporous Au: An Unsupported Pure Gold Catalyst? J. Phys. Chem. C 2009, 113, 5593-5600. Klyushin, A. Y.; Rocha, T. C. R.; Havecker, M.; Knop-Gericke, A.; Schlogl, R., A near Ambient Pressure XPS Study of Au Oxidation. Phys. Chem. Chem. Phys. 2014, 16, 78817886. Fujita, T.; Tokunaga, T.; Zhang, L.; Li, D.; Chen, L.; Arai, S.; Yamamoto, Y.; Hirata, A.; Tanaka, N.; Ding, Y., et al., Atomic Observation of Catalysis-Induced Nanopore Coarsening of Nanoporous Gold. Nano Lett. 2014, 14, 1172-1177. Schaefer, A.; Ragazzon, D.; Wittstock, A.; Walle, L. E.; Borg, A.; Bäumer, M.; Sandell, A., Toward Controlled Modification of Nanoporous Gold. A Detailed Surface Science Study on Cleaning and Oxidation. J. Phys. Chem. C 2012, 116, 4564-4571. Deng, X.; Min, B. K.; Guloy, A.; Friend, C. M., Enhancement of O2 Dissociation on Au(111) by Adsorbed Oxygen: Implications for Oxidation Catalysis. J. Am. Chem. Soc. 2005, 127, 9267-9270. Wang, L. C.; Jin, H. J.; Widmann, D.; Weissmüller, J.; Behm, R. J., Dynamic Studies of CO Oxidation on Nanoporous Au Using a Tap Reactor. J. Catal. 2011, 278, 219-227.



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Figure 1. Top view of the Au(321) surface. The (2 × 1) surface unit cell is indicated. 75x75mm (300 x 300 DPI)

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Figure 2. (A) Three types of subsurface sites considered. (B) Various on-surface (lower terrace and step) and subsurface (upper terrace) sites on the Au(321) surface. Dashed lines help to visualize three-fold onsurface sites f and g. Yellow, green, and light blue indicate octahedral, tetrahedral “vertex down”, and tetrahedral “vertex up” sites, respectively. 46x26mm (300 x 300 DPI)


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Figure 3. O adsorption energy (relative to O2 and the clean surface) as a function of the number of Ag atoms at the binding site. Panels A and B compare on-surface site e with the corresponding subsurface site O3. Panels C and D compare on-surface site c with the corresponding subsurface site dT2. Positive values of Eads indicate endothermic adsorption. Positions of Ag substituents relative to O are schematically shown at the bottom of each panel. 135x121mm (300 x 300 DPI)



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Figure 4. Color plot of the O adsorption energy (eV) at various (A) subsurface and (B) on-surface sites of the pure Au(321) surface. Colored triangles mark O adsorption sites. In each adsorption structure only one O atom is considered per (2×1) unit cell. Adsorption energy is calculated with respect to gas-phase O2 and the clean Au(321) surface. Negative values indicate exothermic adsorption. An analogous diagram for Agmodified Au(321) is shown in the SI, Fig. S1 67x55mm (300 x 300 DPI)


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Figure 5. Most stable mixed configurations containing subsurface O for a given O coverage. The coverage as well as the average adsorption energy (per O atom) are indicated. Large and small spheres represent Au and O atoms, respectively. Arrows point to the location of subsurface O. 68x56mm (300 x 300 DPI)



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Figure 6. (A, B) Representative adsorption geometries of O on Au(321): (A) for coverage 0.1 ML, (B) for coverage 0.3 ML. Positions of relevant Au and O atoms are indicated with numbers. In (A) Au atoms 1,2,3 have only one bond to O. In (B) Au1 and Au2 are bonded to only one O atom, whereas Au3,4,5 are bonded to two O atoms. Atoms 4,8,5,7 form a –(Au–O)– chain. (C, D, E) PDOS: (C) for structure in panel (A), (D, E) for structure in panel (B). The red and black curves indicate the O 2p states and the Au 5d states for Au atoms that are bonded to oxygen, respectively: (C, D) Au is bonded to only one O atom, (E) Au is bonded to two O atoms in a linear O–Au–O unit. 103x130mm (300 x 300 DPI)


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Figure 7. Surface free energy γ as a function of the modified chemical potential of oxygen, ∆µO, for the most stable structures with O coverage 0.1, 0.2, 0.3, and 0.4 ML. (A) Pure Au(321). (B, C) Au(321) with 30% and 100% Ag in the top layer, respectively. γ = 0 corresponds to the respective surface with no O present. Shaded areas indicate the most favorable coverage for a given range of the chemical potential (or, equivalently, the temperature at a fixed partial pressure of O2). 202x273mm (300 x 300 DPI)



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Figure 8. Examples of O assisted O2 dissociation: (A) on the pure Au(321) surface, (B) on the Au(321) surface with two Ag atoms at the reaction site. Geometries of the initial state (IS) and of the transition state (TS) and the final state (FS) are depicted. Eads, Ea, and Ediss denote the adsorption energy of O2, the activation energy with respect to the adsorbed O2, and the energy of the dissociative adsorption with respect to the gas-phase O2 and the clean surface. 54x36mm (600 x 600 DPI)


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Figure 9. (A) Adsorption energies of O2, (B) the activation barrier for O2 dissociation, and (C) the energy of dissociative adsorption (with respect to gas-phase O2) as a function of the number of Ag substituents in the unit cell. Red and black lines indicate systems with and without co-adsorbed O, respectively. Transition state structures with co-adsorbed O are schematically depicted. 191x443mm (300 x 300 DPI)


Chemisorbed Oxygen on the Au(321) Surface ... - ACS Publications

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