Combining IR Spectroscopy and Monte Carlo Simulations to Identify

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Combining IR Spectroscopy and Monte Carlo Simulations to Identify CO Adsorption Sites on Bimetallic Alloys Sergio J. Manzi, Mariela A. Brites Helú, Wilfred T. Tysoe, and Florencia C. Calaza J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09682 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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The Journal of Physical Chemistry

Combining IR Spectroscopy and Monte Carlo Simulations to Identify CO Adsorption Sites on Bimetallic Alloys Sergio Manzi1, Mariela A. Brites Helú2, Wilfred T. Tysoe3 and Florencia C. Calaza*2 1Departamento

de Física, Universidad Nacional de San Luis, Instituto de Física Aplicada,

CONICET, Chacabuco 917, 5700 San Luis, Argentina 2

Instituto de Desarrollo Tecnológico para la Industria Química (INTEC), UNL-CONICET,

Güemes 3450, 3000 Santa Fe, Argentina 3

Department of Chemistry and Biochemistry, University of Wisconsin—Milwaukee,

Milwaukee, Wisconsin 53211, United States *Corresponding author email address: [email protected]

-1ACS Paragon Plus Environment

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Abstract The atomic distribution on the surface of alloys dictates the nature of the ensembles available as possible active sites during catalytic reactions. In the present work, an infrared spectroscopic study of carbon monoxide adsorption on the surface of AuPd/Pd(111) alloys, combined with Monte Carlo simulations of the surface and bulk atomic distribution, identifies the correct distribution of available surface adsorption sites. For gold coverages > 0.9 monolayers (ML), CO adsorbs weakly on top of Au atoms and with higher adsorption energy on top of Pd atoms (COtop), distributed mostly as monomers on the surface. For Au = 0.8-0.4 ML, Pd-COtop is the predominant species, even though several other sites with multiple coordination are available. The simulations show no perfect ordering of the surface but a slight tendency to form lines of Pd atoms, thus favoring the appearance of bridge, but not 3-fold hollow sites. Using

13CO:12CO

isotopic mixtures, the frequency shifts due to chemical and intermolecular coupling effects has been determined for the COtop IR signal. These effects mostly cancel each other out, so that only small frequency shifts are seen, implying the presence of significant electronic/ligand effects. At Au< 0.5 ML, hollow sites are experimentally observed in agreement to the simulated model surfaces. Their IR absorption bands are tentatively distinguished as fcc and hcp hollow sites by correlating with the simulated distribution of Au and Pd atoms on subsurface sites, where for Au< 0.5 ML an enrichment by Au atoms is seen in the near-surface region.


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Introduction In order to understand heterogeneous catalytic processes, one needs to correctly identify the reaction ensembles on the catalyst surface, which will in turn control the reaction. To identify such ensembles, a common approach is to use carbon monoxide to observe the CO stretching frequencies measured by infrared spectroscopy.1,2 This strategy can be extremely useful when the material under study is composed of different types of atoms, and is thus suitable for studying bimetallic systems, in particular in cases in which one metal strongly adsorbs CO, while the other only binds CO weakly, such as in gold-palladium alloys. Bimetallic alloys are used in many reactions of industrial interest to increase selectivity and/or activity compared to the pure metals.3-6 Some examples in the case of Au-Pd alloys are CO oxidation, cyclotrimerization of acetylene to benzene, vinyl acetate monomer synthesis, selective oxidation of alcohols to aldehydes or ketones, selective oxidation of hydrogen to hydrogen peroxide, and hydrocarbon hydrogenation.3,7-18 The synergetic effects when using an alloy are explained by electronic or ligand effects that influence the charge on a particular atom in the alloy, to ensemble effects caused by the creation of specific reaction sites, strain effects, seen as a combination of the latter two, or coverage effects.19-24 While the available ensembles can be explored by infrared (IR) spectroscopy of adsorbed CO, this approach can lack the requisite resolution to be able to distinguish between sites that differ only slightly in their CO adsorption energies or are dependent on the local environment. In such cases, Monte Carlo methods are useful for simulating the distribution of adsorption sites on the alloy surface, to provide interaction energies between the elements involved in the formation of the alloy, and help with the analysis of the experimental data, in particular in correlating CO



vibrational frequencies with the presence of a surface ensemble of atoms. An advantage of this approach compared to density functional theory (DFT) studies is that quantum calculations are limited to relatively ordered structures, while Monte Carlo simulations can model more complex surfaces. Previously, this approach of combining experimental results obtained from temperature-programmed desorption (TPD) experiments with Monte Carlo simulations of the sites available on AuPd/Pd(111) alloy model surfaces was used to show that the surface composition presents no clear ordering but is not completely random either24 in accord with the absence of LEED patterns when forming AuPd/Pd(111) alloys. In contrast, in the case of AuPd/Pd(100) alloys, ordering of the surface was found at specific compositional ratios.25 In the present study, the investigation of the AuPd/Pd(111) alloy surfaces has been extended to the adsorption of CO on alloys formed by heating a Pd(111) sample covered by 5 ML of Au to various temperatures. This work also takes into account the subsurface composition (up to several layers below the surface), which can directly affect the binding to surface sites. Because CO adsorbs dynamically on all possible sites, measuring spectral changes during both adsorption and subsequent heating to desorb CO has the advantage of testing all the available sites without having to change the molecular probe. In addition, this work uses isotopic mixtures of carbon monoxide to distinguish spectral shifts due to dipole-dipole interactions and those due to interactions with the different surface sites. In summary, the present work combines experimental and theoretical methods to study the adsorption of carbon monoxide on AuPd/Pd(111) model alloys to elucidate the adsorption sites on the surface as a function of alloy composition. Based on recent findings on the importance of the metal distribution in nanoparticles of different sizes and shapes, we extend our studies to investigate the influence of metal distribution in the subsurface layers, in an effort to -4ACS Paragon Plus Environment

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understand the high reactivity/selectivity of AuPd alloys for several reactions of interest. It is suggested that this approach of combining results from IR spectroscopy with theoretical studies by Monte Carlo simulations of the surface and subsurface structure can be applied to bimetallic interfaces in general.

Experimental Details CO adsorption experiments were performed by means of reflection absorption infrared spectroscopy (RAIRS), either for saturated overlayers (obtained by adsorption at a sample temperature of 90 K), or after heating briefly at different temperatures and then cooling to 90 K to obtain the infrared spectra. The Pd(111) substrate (1 cm diameter, 0.5 mm thick, MATECK) was cleaned using a standard procedure, which consisted of cycles of argon ion bombardment (2 kV, 1 μA/cm2), annealing in 4 × 10-8 Torr of O2 at 1000 K and then annealing at 1200 K in vacuum to remove any remaining oxygen.26 The cleanliness of the sample was judged using Auger spectroscopy and oxygen titrations, where O2 instead of CO desorbs following O2 adsorption at room temperature when the sample is carbon free. Following each RAIR experiment, the surface is briefly annealed once again in O2 to regain the cleanliness. RAIRS data were collected using a Bruker Equinox infrared spectrometer equipped with a liquid-nitrogen-cooled, mercurycadmium telluride detector. The complete light path was enclosed and purged with dry, CO2-free air. Data were typically collected for 1000 scans at 4 cm-1 resolution, except when CO adsorbed on gold sites was investigated, when 2000 scans at 8 cm-1 resolution were acquired due to the small signal observed. The equipment has been described in detail elsewhere.27 -5ACS Paragon Plus Environment


The Au-Pd(111) alloys were prepared following a recipe first proposed by Baddeley et al.7 Gold is deposited from a home-built evaporation source at a constant rate onto the Pd(111) substrate, which is held at room temperature. After 5 monolayers (ML) of gold were deposited, this surface is annealed to different temperatures for 5 minutes to obtain the desired goldpalladium ratio on the surface.28 For the present work, the gold and palladium coverages were measured by Auger electron spectroscopy (AES) after each annealing cycle and prior to each RAIRS experiment, where the AES results were calibrated using low-energy ion scattering (LEIS) as shown previously.29 It was found that Au starts to diffuse into the palladium bulk at temperatures above 500 K while, after annealing the sample to 1100 K, only small amounts of gold were detected on the surface. Carbon monoxide 12CO and 13CO (Sigma Aldrich, 99% 13C) and oxygen O2 (Praxair, 5.0 Research grade) were dosed from the gas line, and the purity of the gases was checked by mass spectrometry. The 12CO/13CO mixtures with different ratios (75:25, 50:50, 25:75) were prepared in separate glass bottles and dosed directly into the UHV system; their purity and correct ratios of each component of the mixtures was also checked by mass spectrometry.

Monte Carlo Simulations To simulate the adsorption and desorption of carbon monoxide, the structure of the alloy substrate is modeled. The canonical Monte Carlo method allows this to be accomplished in a rather simple way by using a three-dimensional array of 𝐿𝑥 × 𝐿𝑦 × 𝐿𝑧 atoms ordered in an fcc structure, with large enough periodic boundary conditions (with 𝐿𝑥,𝐿𝑦 ≪ 𝐿𝑧, taking 𝐿𝑥,𝐿𝑦 = 40 and 𝐿𝑧 = 5000) such that the results are consistent and do not change for large values of any 𝐿𝑖. -6ACS Paragon Plus Environment

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As in the experimental protocol for preparing the alloys, the first 5 monolayers initially contain Au atoms and the rest of the sample comprises Pd atoms.30-32 The system is governed by the Hamiltonian: 𝑁𝑇

𝐻=

∑𝜀 {𝑖}

𝑁𝑇

𝐴𝑢(𝑖)𝑛𝐴𝑢,𝑖

+

𝑁𝑃

∑𝜀

𝑃𝑑(𝑖)𝑛𝑃𝑑,𝑖 {𝑖} 𝑁𝑃

∑𝑛

+ 𝜀𝑃𝑑 ― 𝑃𝑑

𝑃𝑑,𝑖

+ 𝜀𝐴𝑢 ― 𝐴𝑢

∑𝑛

𝐴𝑢,𝑖

𝑁𝑃

𝑛𝐴𝑢,𝑗 + 𝜀𝑃𝑑 ― 𝐴𝑢

{𝑖,𝑗}

∑(𝑛

𝑃𝑑,𝑖

𝑛𝐴𝑢,𝑗 + 𝑛𝐴𝑢,𝑖 𝑛𝑃𝑑,𝑗)

{𝑖,𝑗}

𝑛𝑃𝑑,𝑗

{𝑖,𝑗}

where 𝑛𝑆,𝑖 is the occupation number for the position 𝑖 in the substrate and is 1 if the position is occupied by an atom type S, and 0 otherwise. 𝑁𝑇 is the total number of atoms in the substrate (equal to 𝐿𝑥 × 𝐿𝑦 × 𝐿𝑧) and NP is the total number of the nearest-neighbor pairs in the substrate. 𝜀𝐴𝑢(𝑖), 𝜀𝑃𝑑(𝑖), are the potential energies of a Au and a Pd atom in the substrate. Furthermore, these energies can depend on their location (𝑖). 𝜀𝐴𝑢 ― 𝐴𝑢, 𝜀𝐴𝑢 ― 𝑃𝑑, 𝜀𝑃𝑑 ― 𝑃𝑑 are the lateral interaction energies between two Au atoms, an Au atom and a Pd atom, and two nearestneighbors Pd atoms. In this context, other types of interaction can be easily added. In this way, by following an algorithm of the Kawasaki type in the canonical ensemble, the system is allowed to relax until it reaches equilibrium, where the transition probabilities depend on the canonical partition function and the Hamiltonian. The simulation allows different kinds of information about the system to be obtained as a function of gold coverage on the surface. For example, the amount of Au in the different substrate layers, the quantity of different types of adsorption sites on the surface (3-fold, 2-fold, atop) and to distinguish if they are formed by Au or Pd atoms, the distribution of Au atoms on the surface, and others. Once the surface structure is established, the adsorption of CO on various sites is simulated to compare with the experimental infrared data. -7ACS Paragon Plus Environment


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For the simulation of adsorption and desorption processes, we work in the grand canonical ensemble, in which the number of adsorbed CO molecules (treated as particles) on the surface are allowed to vary as a function of the chemical potential of the system, in order to obtain adsorption isotherms for CO on the surface. To simulate this process, a triangular lattice, with periodic boundary conditions, composed of two different types of particles (in this case, Au and Pd), is considered. For this simulation, the surface is obtained following the procedure described above. The CO molecules (particles) can be adsorbed on three different sites always comprised of Pd atoms (adsorption on Au sites is not considered in this model): between three surface atoms (3-fold sites), in between two surface atoms (2-fold or bridge sites) and on top of one of the surface atoms (atop or top sites). From the experimental data it is found that adsorption of CO at saturation coverage (with the sample at 80 K), leads to adsorption on 2-fold and atop sites,1,2 formed exclusively by Pd atoms. The system is described by the Hamiltonian. 𝑁𝑡𝑜𝑝

𝐻 = ― 𝜀𝑡𝑜𝑝

∑𝑛

𝑡𝑜𝑝,𝑖

𝑁2𝑓

― 𝜀2𝑓

𝑖=1

∑𝑛

2𝑓,𝑖

𝑁3𝑓

― 𝜀3𝑓

𝑖=1

∑𝑛

3𝑓,𝑖

𝑖=1

where here, 𝑁𝑡𝑜𝑝, 𝑁2𝑓, 𝑁3𝑓are the total number of atop, 2-fold, and 3-fold adsorption sites in the lattice, respectively. 𝜀𝑡𝑜𝑝, 𝜀2𝑓, 𝜀3𝑓are the adsorption energies for a CO atom on atop, 2-fold, and 3-fold sites, respectively. 𝑛𝑡𝑜𝑝,𝑖, 𝑛2𝑓,𝑖, 𝑛3𝑓,𝑖 are the occupation numbers, being 1 if the atop, 2fold or 3-fold sites labeled 𝑖 are occupied by a CO atom, and 0 otherwise. In this work, interactions between adsorbed CO molecules are not considered, but they can easily be added to the Hamiltonian in the same way as adsorption energies on 2-fold and atop sites, which can be composed of a mixture of Au and Pd atoms. -8ACS Paragon Plus Environment

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The advantage of Monte Carlo simulations is that additional restrictions can be added to the system, in response to experimental observations. Some of these possible restrictions include: i) the adsorption of CO in a 2-fold position prohibits the occupation of other 2-fold positions which share Pd atoms; ii) sites that involve atop adsorption on any of the Pd atoms which also form 2-fold sites, iii) the adsorption atop any Pd atom allows the adsorption on either only one (our 2T model, see below) or two (our 3T model, see below) neighboring atop Pd sites. This enables the coverage of CO to be obtained and allows the adsorption sites and distribution on the surface to be calculated. The Monte Carlo simulations of our model allow the values of lateral interactions during substrate (alloy) formation and/or adsorption energies on different adsorption sites to be modified to reproduce the experimental data. For example, Figure 1 shows this dependence when the interactions between any type of substrate atom are zero, and the adsorption on atop and 2fold sites are considered isoenergetic (referred to as the 2T model). It can be seen that, under those conditions, at saturation coverage, carbon monoxide prefers to adsorb mostly on atop positions. To modify this, different values can be used for both the lateral interactions during substrate (alloy) formation and/or for adsorption energies in different adsorption sites.



Figure 1. CO coverage versus Au surface coverage obtained from Monte Carlo simulations for the 2T model, without lateral interactions between any pair of atoms in the substrate. Adsorption energies are equal for all adsorption sites.

Results This section presents the results of RAIRS experiments, commencing by briefly reviewing previous results for the adsorption of carbon monoxide on clean Pd(111), followed by those on various surface alloys with gold coverages ranging from 1 to 0.19 ML. In the following, a top site (Pdtop) is defined as adsorption on a single atom of Pd (or Au as Autop). Here, the adsorption site could be surrounded by Au (or Pd) atoms to allow additional CO to adsorb on neighboring Pd sites. A 3-fold hollow site is defined as that formed by a set of three, neighboring palladium atoms (Pd3f) and a bridge site ensemble is one formed by two adjacent palladium atoms (Pd2f). Carbon monoxide does not adsorb neither on bridge (2f) or hollow (3f) sites when one of the atoms comprising the site is gold; therefore, for those sites, only Pd atoms can form a CO adsorption site. The 3f hollow sites can be further classified as hexagonal-close packed (hcp, PdH) depending on whether an atom from the 2nd layer is located directly below the site, or facecentered cubic (fcc, PdF), if the 3rd layer atom is located below.

A. CO adsorption on Pd(111) surface The structures formed by the adsorption of carbon monoxide on clean Pd surfaces have been extensively studied and are briefly summarised in Scheme 1. The Supporting Information section


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summarizes the IR spectra of CO on Pd(111) at 90 K (Figure S1). At low coverages, CO adsorbs preferentially at 3f hollow sites up to a maximum coverage of 0.33 ML (where 1 ML equals the surface atom density of Pd(111)) and shows a characteristic (√3×√3)-R30˚ LEED pattern (Scheme 1A).1,2,33 Scheme 1B depicts the ordered c(4×2)-2CO structure for CO adsorption on bridge sites with a maximum coverage of 0.5 ML. A saturated CO overlayer (with a coverage of 0.57 ML) can be obtained if a compressed overlayer is allowed to relax, where the additional CO occupies atop sites (Scheme 1C).1,2,33 This structure has been also proposed at intermediate-tohigh coverages.34-37 Therefore, the integrated areas under the IR peaks of the saturated layer in Fig. S1c are those that will be used to calibrate the CO coverages on the alloys.

Scheme 1. Ordered structures observed at CO coverages of (A) 0.33 ML, (B) 0.5 ML and (C) 0.57 ML on Pd(111). B. Adsorption of 12CO and 13CO mixtures on Pd(111) By using isotopic mixtures of CO (12CO /13CO) it is possible to decouple frequency shifts due to dipole-dipole coupling from those reflecting the effect of adsorption on specific sites.1 The results of these experiments on Pd(111) will help identify the interactions between adsorbates and, subsequently, when performing the same analysis on the alloys, with sites on AuPd alloys to shed light on the distribution of both metals on the surface. -11ACS Paragon Plus Environment


The spectra due to adsorption of carbon monoxide mixtures (13CO/12CO) at 90 K using different isotopic ratios are displayed in Figures S2 (a) 75:25, (b) 50:50 and (c) 25:75. The spectra of pure 12CO and 13CO and their mixtures are shown at the same CO coverages of 0.33 (Fig. 2a), 0.05 (Fig. 2b) and 0.6 (Fig. 2c) ML, where 3f and 2f sites are occupied for the two highest CO coverages. The band assigned to CO adsorbed on bridge sites (Fig. 2c, 19001950 cm-1) shows a clear effect due to dipole-dipole coupling. The effects due to binding to different surface sites (chemical) effects and those due to dipole-dipole (vibrational coupling) can be disentangled by using isotopic mixtures of CO at different dilution ratios. Bradshaw and Hoffmann used this approach to study the vibrational frequency shifts of CO adsorbed on bridge sites on Pd(100).1,2 In that case, a shift of about 65 cm-1 was found to be due to chemical effects while about 35 cm-1 was due to dipole-dipole coupling, both having the effect of increasing the frequency of the band, accounting for a total shift of about 100 cm-1 when going from low CO coverages to a 0.5 ML coverage.2 The chemical shift is related to the charge that can be backdonated to the anti-bonding molecular 2π* orbital of CO; therefore the more back donation, the greater the shift to lower frequencies.


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Figure 2. RAIR spectra of carbon monoxide isotopic mixtures (13CO (75) :12CO (25), 13CO (50) : 12CO (50), 13CO (25) : 12CO (75)) adsorbed on Pd(111) at 90 K for CO: (a) 0.33 ML, (b) 0.05 ML and (c) 0.6 ML. If we consider the 3f site on Pd(111), the total chemical shift is about 343 cm-1 towards lower frequency, decreasing from 2143 cm-1 for gas-phase CO to 1800 cm-1 for isolated CO adsorbed on a 3f site (Figs. 2a and b). For isolated CO at low coverages (bottom spectra in Fig. 2b), the maximum possible charge is available for donation, but, at higher coverages (Figs. 2a and b), the competition for those electrons is manifest as a shift to higher frequency. Therefore, the positive chemical shift (+20 cm-1) arises because the charge is shared by several molecules comprising the ordered 0.33 ML adlayer (Fig. 2a). If we perform this type of analysis for the data presented in Fig. 2a, we can separate both these effects for the case of the 3-fold hollow sites. In the case of the low-frequency band at 1795 cm-1, due to adsorbed 12CO

13CO

molecules on the 3f sites (mixture

(75) :13CO (25)), a shift of 12 cm-1 is found due to dipole-dipole coupling (band at 1807

cm-1 for

13CO

pure isotope) and ~20 cm-1 due to chemical shifts (band at 1774 cm-1 for the

singleton frequency for that same isotopic mixture), both shifts being towards higher frequency and marked by vertical dashed lines in Fig. 2a,b. The same conclusions can be drawn from the shifts observed by

12CO

(12 cm-1 due to vibrational coupling, 19 cm-1 due to chemical/static

shift). In the case of CO adsorbed on bridge sites (Fig. 2c), a similar conclusion can be obtained from the

13CO

shifts; there is a +38 cm-1 shift due to vibrational coupling (+34 cm-1 if results

from 12CO vibrations are included). This shift is larger than in the case of CO on 3f sites because the saturated coverage is larger and molecules are closer to each other to reach a 0.6 ML coverage. Therefore, coupling between adjacent dipoles is stronger for CO2f than for CO3f. The chemical shift is more difficult to identify on a bridge site because of the lack of clearly distinct -13ACS Paragon Plus Environment


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vibrations for the adsorbed 12CO singleton. By taking the frequency for CO on Pd(100) found at 1895 cm-1,34 which should be close to that on Pd(111), one can calculate a ~21 cm-1 chemical shift for CO2f. For COtop sites, such an analysis is not straightforward because of the low CO coverage on these sites and will be discussed in greater detail for the alloys.

C. CO adsorption on model AuPd/Pd(111) alloys at high gold coverages The RAIR spectra shown in Figure 3 for gold coverages in the alloy between 1 and 0.89 ML were collected after dosing carbon monoxide on alloys with various gold coverages, where the gold coverages are indicated adjacent to the corresponding spectrum. CO was dosed with the sample held at 90 K and was sufficiently high to form a

12CO-saturated

surface. The fully

covered gold surface (θAu = 1 ML) exhibits only one CO stretching feature at 2110 cm-1 assigned to CO adsorbed onto Au top sites.38-43 Decreasing the amount of gold in the alloy leads to the emergence of a feature at 2088 cm-1, first seen as a shoulder at a surface Pd coverage of ~5% (θAu = 0.95 ML), characteristic of CO adsorbed onto Pd atop sites, as shown in the previous section. The simultaneous signals for CO on Au and Pd atop sites is clearly observed for an alloy with θAu = 0.89 ML (second to top spectrum in Fig. 3). Between this range of alloy compositions, the peak corresponding to CO atop Au sites decreases in intensity with increasing Pd coverage, implying that the number of adsorption sites for CO on top of gold also decreases.


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Figure 3. RAIR spectra of a saturated overlayer of CO adsorbed on AuPd alloys at 90 K (Au = 1 - 0.89). In the case of the sample heated to higher temperatures, the annealing temperatures are indicated adjacent to the corresponding spectrum. The effect of heating the saturated overlayer on an alloy with Au coverage of 0.98 ML is also shown in Fig. 3, where the spectra found on heating to 130 K and 175 K are shown in the middle of the figure. CO desorbs over this temperature range (as detected in TPD24), resulting in a decrease of the total CO coverage. The CO stretching peak blue shifts and decreases in intensity, shifting to 2121 cm-1. This effect has been observed previously,41-43 and the red shift seen for CO adsorbed on top of Au at low coverages (about 13 cm-1) in our spectra agrees well with that observed by others.38,39,41-43 This effect is seen for the adsorption of CO on coinage metals (Cu, Ag and Au), where a combination of factors leads to small shifts in the CO stretching mode and result in a strong “wall effect” due to repulsion between the 5 CO molecular orbital and the -15ACS Paragon Plus Environment


filled metal d-band, resulting in weak 5 donation causing weak adsorption on the top site and a stronger contribution of back donation from the metal into the 2* CO orbital.39,44-47

D. RAIRS of CO adsorbed on AuPd alloys for θAu= 0.89 - 0.19 ML. Figure 4 shows the RAIR spectra obtained for a saturated overlayer of 12CO at 90 K for alloy compositions θAu = 0.89 - 0.19 ML. As the gold coverage decreases, the feature corresponding to CO adsorbed on top of Pd (at ~2089 to 2094 cm-1) becomes sharper and the signal of the Au atop sites (at ~2110 cm-1) disappears. For Au = 0.65 and 0.44 ML, the adsorption of CO on top of Pd sites becomes most favorable displaying an intense signal at ~2094 cm-1, implying a large population of CO adsorbed on Pd atop sites. Below a coverage of θAu = 0.5 ML, CO adsorbed on Pd‒Pd bridge sites is detected as a broad feature at 1940 cm-1. This signal becomes sharper and blue-shifts to 1954 cm-1 as the Pd coverage in the alloy increases. Finally, the vibrational frequency for CO on the Pd atop site feature shifts to slightly higher frequencies (+4 cm-1) and decreases in intensity as the gold coverage decreases.


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Figure 4. RAIR spectra of a saturated adlayer of CO adsorbed on AuPd alloys at 90 K (Au = 0.89 - 0.19 ML). The dependences on CO coverage are displayed in Figure 5 (a-f) for alloy compositions from 0.78 - 0.19 ML as a function of CO dose at 90 K (shown as the bottom spectra in each panel), and after annealing to the indicated temperatures (noted adjacent to each spectrum). On most alloys, CO adsorption is favored on Pd atop sites (from the peak at ~2090 cm-1), but heating from 90 to 440 K reveals other vibrational modes. At the highest gold coverage (θAu> 0.78 ML, Fig. 5a), signals associated with CO2f sites (which should appear at 1910 cm-1) were not detected at any temperature. Features due to CO2f sites start to appear ~ 1911 cm-1 when the alloy with θAu = 0.78 ML was warmed to 290 K while, at lower gold coverages, they are detected at 90 K. Peaks corresponding to 3-fold sites (in the region of 1850 cm-1) are observed for alloys with θAu < 0.56 ML (Fig. 5c) at low temperatures (90 K). From the shapes of these peaks, the existence of



different types of 3-fold site is evident. Thus, the broad feature at 1846 cm-1 (Figure 5c, θAu = 0.56 ML) comprises three distinct features at 1848, 1824 and 1809 cm-1, where a shift of ~20 cm1

between them is taken to indicate that they correspond to different 3-fold sites. The population

of each of these sites, as well as the frequencies of adsorbed CO, change with temperature. This trend is better seen in alloys with high gold coverages (Figs.5 d, e, f) and these results are discussed in greater detail below. The occupation of available sites is dictated by the adsorption energies of CO at each site. However, the adsorption energies are coverage dependant, due to repulsive molecular interactions, the effect of dilution of Pd by Au, or electronic effects. In Fig. 5a (θAu = 0.78 ML), the spectra show two features characteristic of CO adsorbing on top of a Au site (shifting from 2116 cm-1 to 2110 cm-1 with increasing coverage) and on top of a Pd site (with a characteristic signal centred at 2088-2089 cm-1). No clearly visible 2f sites are detected, although a weak feature may be visible at ~1950 cm-1. At higher temperatures, some CO adsorbed on Au sites desorbs at ~185 K, causing the peak to shift to its low-coverage value of 2116 cm-1. The signal of CO on Pd atop sites maintains its original intensity. When heating to 295 K, the signal for CO on Au atop sites has disappeared and the intensity of the CO on atop Pd sites decreases by ~50%. The most interesting result is the appearance of a clear signal at 1912 cm-1, which corresponds to CO adsorbed on bridge sites. This signal is quite small because of the low coverage of available bridge sites on this high-gold-coverage alloy but provides a clear indication of CO adsorbed on 2f sites. Finally, heating to 330 K shows that the signal persists, while the signal for CO on atop sites disappears, implying that the adsorption energy for CO on 2f sites is higher than on atop sites. Further heating removes all adsorbed CO.


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Figure 5. RAIR spectra of CO adsorption at increasing CO coverage and after annealing the sample to the indicated temperatures (noted adjacent to each spectrum) for alloys with Au: (a) 0.78, (b) 0.65, (c) 0.56, (d) 0.44, (e) 0.32 and (f) 0.19 ML.



Analogous behaviour is found for an alloy with ~65% Au (Fig. 5b). In this case, some 2f sites are available for adsorbed CO at 90 K because a peak at ~1910 cm-1 is observed together with signals corresponding to CO atop Au and Pd sites. It appears that, although 2f sites are available, CO preferentially adsorbs on atop Pd sites. The signal due to CO adsorbed on Au atop sites follow the same trends seen above; a red shift with increasing coverage and a disappearance of CO when heating the overlayer to ~215 K. At this temperature, the CO on Pd atop sites retains its intensity and a small feature at ~1910 cm-1 is detected, which evolves into a symmetric feature on heating to 297 K and where the CO atop signal decreased by ~50%. The feature at 1912 cm-1 is due to CO adsorbed on bridge sites and is clearly stable upon heating to at least 337 K. It is interesting to note here that the available bridge sites are distant from each other because the IR signal shows no asymmetry or frequency shift due to dipole-dipole coupling. When the gold coverage is decreased further (θAu = 0.56 ML, Fig. 5c), similar trends are observed, with CO showing the presence of bridge sites and some 3f sites (evidenced by a small feature at ~1950 cm-1), and a very small contribution of COtop on Au. At saturation coverage, no bridge sites are occupied, and all CO occupies atop sites. This result implies that the system is stabilized by accommodating all CO molecules on top of Pd atoms instead of occupying the available bridge sites. It is likely that, in order to attain the high coverage, some of the bridge sites now need to accommodate dimers of COtop. In the case of spectra at lower gold coverages (Figs. 5d, e and f), the trend is the same in that range of CO adsorption sites are detected and now the COtop signal decreases in intensity and there are more occupied bridge sites. These bridge sites are closely spaced and start to interact by dipole-dipole CO coupling. For example, in Fig. 5d (θAu = 0.44 ML), CO initially adsorbs preferentially in all three types of possible sites (at 1820-1850 cm-1 for CO3f, at 1910 cm-1 for -20ACS Paragon Plus Environment

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CO2f, and at 2091 cm-1 for COtop), implying that larger areas of pure Pd are present, although they are not sufficiently large to completely resemble the behaviour seen for pure Pd(111), and are still influenced by the Au. The signal due to CO adsorbed on top of Pd sites (COtop at 2091 cm-1) is slightly shifted from that seen at lower Pd coverages (2088 cm-1 for alloys with θAu = 0.89 ML, Fig. 4) and could be due to a larger COtop coverage that increases the dipole-dipole coupling, or repulsive interactions. The larger the Pd coverages, the higher the number of 3f CO adsorption sites, more closely resembling the behaviour seen for clean Pd(111) for the alloys with low gold coverages, θAu = 0.32 and 0.19 ML (Fig. 5 e and f, respectively), even though there is still some influence of Au in the alloy.

Discussion To be able to quantitatively compare the available adsorption sites with the observed infrared spectra, an initial reference is required for a sample in which the distribution of adsorption sites is known. This is accomplished by assigning the RAIR spectra of CO on clean Pd(111). Considering the integrated areas of the signals observed for the saturated overlayer at 90 K (Fig. S1c) as representative of a maximum coverage of 0.6 ML of adsorbed CO (0.5 ML arising from the signal at 1953 cm-1 (CO2f), and 0.1 ML from the signal at 2090 cm-1 (COtop)), the integrated areas for those same infrared signals observed on the alloys can be compared.1,33-36 A. Identifying adsorption sites on AuPd(111); θAu = 1 - 0.89 ML. The alloys with the highest Au coverage, going from a completely gold covered surface to θAu = 0.89 ML (Fig.4), are analysed first. The observation of only signals corresponding to CO adsorbed on top of gold atoms at the highest gold coverages, which follows the behaviour seen -21ACS Paragon Plus Environment


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previously by others,39,40 is a good test for our model system. There is no clear indication in the literature for ordered CO structures on Au(111) surfaces and CO has been observed on Au(111) facets only at very low temperatures (much lower than those used in this study), or above 80 K on high-coordination number sites (steps, island borders, kinks and nanoparticle edges), or on terraces of Au thin films on another metal substrate which stabilizes adsorbed CO.38,

40-43,48,49

Thus, it is not possible to make direct assignments of Au sites, and it can only be qualitatively shown that, as the Au coverage in the surface alloy decreases, the relative coverage of CO adsorbed on Au sites decreases. In addition, the Au infrared signals are broad (with a FWHM of ~17-20 cm-1), suggesting no ordering, perhaps implying that CO agglomerates on the surface, possibly growing from step edges which have low-coordinated Au atoms. At Au coverages of about 0.95 ML, a small feature (~2090 cm-1, Fig. 3) is seen as a low-frequency shoulder on COAu signal at CO saturation, demonstrating the presence of Pd sites (Fig. 3). At θAu = 0.89 ML (Fig, 4), a distinct CO-Pd signal is clearly observed, indicating that CO adsorbs on top of Pd predominantly distributed as monomers. B. Identifying adsorption sites on AuPd(111); θAu = 0.89 - 0.19 ML We now address the alloys with θAu from 0.89 to 0.19 ML. Figure 4 summarizes the data for a saturated CO monolayer on the alloys where there is a clear enhancement signal for CO on top of Pd atoms. Thus, the total atop CO on Pd (displaying a ~2090 cm-1peak) is larger than would be expected if there were only monomers present on the alloy. For instance, for a gold coverage of 0.65 ML (Fig. 5b), the amount of CO adsorbed on top sites is slightly above 0.33 ML (if all Pd sites were distributed as monomers and isolated from each other by Au atoms). However, it is known that the system shows no ordered structures for any Au/Pd ratios, but is not strictly random either.24 Therefore, if there were other available adsorption sites, either bridge or -22ACS Paragon Plus Environment

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hollow (3f), then CO should adsorb on them and not on atop sites if the CO adsorption energies on these sites did not change from those on the Pd(111). Thus, from the results in Figure 5, it is noted that, for θAu = 0.65 ML (Fig. 5b), some bridge sites are occupied by CO at low coverages from the characteristic band at ~1910 cm-1. Furthermore, the results for θAu = 0.78 ML (Fig. 5a) show a small number of 2f sites (at ~1912 cm-1) when heated to 295 and 330 K. For θAu = 0.56 ML (Fig. 5c), the maximum CO atop signal is observed at a saturation coverage with no evidence of CO adsorbed on any other site, even though there are available CO sites. For instance, some 3f sites are clearly evident (at ~1910 cm-1) and are stable up to ~ 400 K (the characteristic CO desorption temperature from Pd hollow sites is 450-490 K33). These observations indicate that the CO adsorption energies on the alloy differ from those found on Pd(111), where CO adsorbs preferentially on atop sites instead of bridge or hollow sites at high coverages. This could be due to a strong electronic effect because of the presence of surrounding Au. When the gold coverage decreases to 0.44 – 0.19 ML, the CO adsorbed Pd atop sites at saturation also decreases, showing not only that more 2f and 3f sites are available, but also that they seem not be strongly affected by surrounding Au sites. C. Site distribution as a function of alloy composition To summarize the results shown in Figs. 5 a-f, the data for the infrared features for CO adsorbed on atop and bridge at 90 K and after heating are depicted versus CO dose (in Langmuirs, where 1L = 1x10-6Torr.s) in Figures 6a (COtop) and b (CO2f), where the last point (10 L dose) represents CO saturation. Figure 6a is subdivided into a plot showing the frequencies (top panel) and the relative coverages (bottom panel) of atop CO versus exposure. Plotting the data in this way shows that, for Au coverages of 0.78, 0.65 and 0.56 ML, the CO atop coverage increases linearly to a maximum value (Fig. 6a, bottom panel), while only a few bridge sites are -23ACS Paragon Plus Environment


detected at low coverage (Fig. 6b). At intermediate coverages, CObr switches adsorption sites and starts to occupy atop Pd sites. Thus, for Pd‒Pd dimers, the preference is to occupy adjacent Pd sites with 2 CO molecules, rather than one CO occupying a bridge position (2f). This model has been suggested by Behm et al. to explain the HREELS and TPD results for CO on PdAg alloys,50,51 where a high occupancy of atop Pd sites was also observed. This conclusion was reached previously by this group for model PdAu alloys,52 although only two compositions (2% and 12 % Pd) were investigated and concluded the main influence on CO adsorption on the alloys is an ensemble effect.52 The COtop frequencies (Fig. 6a, top panel) are plotted together with the integrated areas for atop CO on AuPd alloys (Fig. 6a, bottom panel). For CO doses below 4 L, and for Au> 0.5 ML, the band is at 2088 cm-1 and increases slightly in frequency when more CO is adsorbed. This suggests dipole-dipole coupling between CO molecules adsorbed on neighboring Pd sites. However, this shift (2-3 cm-1) is less than that seen in Fig. 2 for bridge and 3f sites (with shifts of 12 cm-1 and 38 cm-1). For these low CO coverages, the CO2f signal is initially detected but then disappears as the COtop band reaches its maximum frequency (Fig. 6b, ▲, ●). The same trend is seen for Au< 0.5 ML at low CO coverages, where the CO frequency increases linearly to a maximum of 2090-2094 cm-1. For alloys with higher Pd coverages, CO adsorption is more energetically favorable than for the Au-rich alloys, and the occupation of 2f sites is observed at saturation after passing through a maximum value. It is clear the COtop sites are filled at the expense of the CO2f sites, as in the case of other Pd-based alloys50-52. A maximum number of COtop sites is found at Au = 0.44 ML, but its value decreases steadily towards its reference value of 0.1 ML seen on Pd(111) surfaces for alloys with surface Au< 0.44 ML.


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Figure 6. Summary of the infrared data for CO as a function of CO dose (in Langmuirs) for adsorption at 90 K and subsequent annealing to 290 K and 350 K for adsorption on a) atop sites and b) 2-fold sites; the top panel (open symbols) in (a) plot the vibrational frequencies of COtop sites. The symbols are summarized in the bottom panel of Figure 6a.



Figures 6 a-b compare the integrated areas of COtop and CO2f sites when heating to 290 K and ~350 K. On heating to 290 K, a decrease of COtop signal to half its original value is seen, while the CO2f signal increases to twice its original value, except for Au = 0.32-0.19 ML, where the increase is less. This implies that the CO2f site is populated by COtop, while some CO desorption is detected at 260 K in TPD.33 This suggest that for dimer (Pd‒Pd) sites, half the CO desorbs, and the remainder occupies bridge sites: OC-Pd-Pd-CO  CO(g) + Pd-(CO)-Pd It is also the possible that some 3-fold hollow sites are present even though they are not filled by CO, and that some trimers of CO are actually present: three CO molecules adsorbed on the 3f sites, one on top of each Pd atom. In this way, a portion of the COtop desorbs when heating to 290 K, another CO switches to a bridge site, while the rest CO is adsorbed on a top Pd atom. D. Comparison with simulated surfaces Using the above assignments as a background, a Monte Carlo model was developed for CO adsorption on the alloys, where CO adsorption on each site is dictated by its adsorption energy. To compare with the experiment, the integrated areas of the feature characteristic for atop CO (at 2090 cm-1) are plotted in Figure 7 (COtop: red filled circle) versus Au coverages. There is an increase in COtop intensity up to Au ≈ ½ ML. The results of the MC simulations are shown (COtopM: red empty circles) with only nearest-neighbor lateral interactions between Pd atoms (𝜀𝑃𝑑 ― 𝑃𝑑/𝑘𝐵𝑇= 6, where 𝑇 is the temperature and 𝑘𝐵 is the Boltzmann constant). Several sets of lateral interactions were used to obtain different surface structures (data not shown), including that used in previous work24 which yielded very similar results to those shown in Fig. 7. -26ACS Paragon Plus Environment

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a)

Figure 7. (a) Experimental CO coverages as a function of the gold coverage in the alloy (filled symbols) compared with the simulated (empty symbols) for different CO adsorption sites (COtop and CO2f). COTotal represents the sum of the CO coverage on all sites. (b) Plot of the energy difference ∆𝐸 between CO on 2-fold and atop sites as a function of Au. Two adsorption models were tested for COtop on neighboring sites; one in which only one next-nearest neighbor Pd site is occupied to form a CO dimer (2CO/2Pd), and another one in which two neighboring Pd sites are occupied to form COtop trimers (3CO/3Pd, model 3T, Fig. 7a). The 3T model resulted in a good fit to the experimental results and reproduced the preferential occupation of COtop rather than bridge sites, even though they are present on the surface. It should be pointed out that the integrated areas of COtop signal may overestimate the amount of adsorbed CO.53 To obtain these results, the CO adsorption energy on 2-fold sites must depend on the gold coverage. By comparing the curves with (Fig. 7b, ●) and without interactions (Fig. 7b, ■), the energy difference for CO on 2-fold and atop sites (∆𝐸) is almost independent of the substrate



arrangement, and the energies are in accord with those obtained by Ruff et al. using DFT (≈0.05 eV).52 For Au> 0.6 ML, the occupancy of 2-fold sites is negligible so that ∆𝐸 is not meaningful. E. AuPd(111) Alloy Structures The surface distribution of Au and Pd was investigated without (𝜀𝑃𝑑 ― 𝑃𝑑 = 0, Fig. 8 a, b 𝜀𝑃𝑑 ― 𝑃𝑑

and c) and with (

𝑘𝐵𝑇

= 6, Fig. 8 d, e and f) interactions. The alloy with a Au coverage of 0.56

ML (Fig. 8 b) without interactions shows more Pd agglomeration to form a significant number of 3f sites, different from the case with interactions (Fig. 8 e), where Pd tends to form linear structures which extend over the whole surface to avoid forming 3f sites. This allows higher CO coverages to be obtained than when the nearest-neighbor PdPd lateral interactions are repulsive. For higher gold coverages, (Au = 0.78 ML, Figs. 8 c and f), the trend of forming lines is seen for the surfaces simulated with interaction (Fig. 8f) compared with no interaction (NI, Fig. 8c). At higher Pd coverages (Au = 0.20 ML, Fig. 8 a and d), the differences are less significant, and the surfaces structures are similar. Figure 8 also shows the distribution of adsorbed CO (red dots). When the Pd atoms are separated, more CO is adsorbed because the number of 2-fold sites is lower and CO adsorbs on atop sites. The results of the MC simulations are compared with the experimental CO coverage and the available 2f and 3f sites. Figure 9 compares the relative coverages of 2f and 3f adsorption versus gold coverage with (solid symbols) and without (open symbols) interactions. The observed Pd agglomeration found with no interactions leads to more bridge (open squares) and 3-fold hollow sites (open triangles). With lateral interactions, the coverage of 2f sites decreases and, because of the formation of Pd rows, fewer 3f sites are observed. The results are compared with experiment, in Figure 9, by plotting the coverages of CO on 2f (at 1910-1950 cm-1) and 3f -28ACS Paragon Plus Environment

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sites (at 1850-1810 cm-1) after heating to 290 K for CO2f sites (Fig. 6 c) and 350 K for CO3f sites (taken from spectra in Figure 5). The experimental results are close to those predicted from simulations, corroborating the formation of Pd rows.

a)

d)

b)

e)

c)

f)

Figure 8. Simulated surfaces with adsorbed CO for different Au as displayed in each panel, using no lateral interactions (a, b and c) and with interactions (d, e and f) showing the distribution of gold (yellow circles) and palladium (black circles) sites. Sites occupied by CO are shown as red dots. -29ACS Paragon Plus Environment


Figure 9. Coverage of 2-fold and 3-fold hollow sites (plotted as the ratio between 2f (3f) possible adsorption sites and total 2f (3f) sites on surface) against gold surface coverage for the cases with lateral interactions (filled symbols) and no interaction (empty symbols); crossed empty symbols are experimental data for the available sites.

F. Vibrational and chemical shifts observed on COtop band. In order to understand the origin of the small frequency shifts seen for COtop on the alloys, isotopic experiments using 12CO/13CO mixtures were carried out. Figure 10 illustrates the Pd-COtop frequency shifts as a function of 12CO/13CO ratio for alloys in the range Au = 0.65 to 0.19 ML, for two cases: a) for a saturated layer at 90 K and b) after warming to 290 K. The same type of analysis is used as in Section B of the Results section. The singleton frequency for each CO isotopomer is not known, so the analysis uses the values for the small peak (at 2087 cm-1) detected on heating to 330-350 K. These values are in good agreement with those at low CO coverages on the Au = 0.89 and 0.78 ML alloys (Fig. 3 and 4).


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Figure 10. Frequencies of the band corresponding to 12COtop and 13COtop against percentage of 13CO

in the isotopic mixtures for an adlayer (a) at 90 K and (b) after annealing to 290 K; (c)

vibrational and chemical shifts and (d) total shifts observed as a function of Au coverage.



The results plotted in Figure 10a reveal that there is a strongly negative chemical shift for all alloys as well as significant dipole-dipole coupling. The negative shift could be due to more back donation into the 2* orbitals, and the strong dipole-dipole coupling due to COtop adsorbing on neighboring sites (as Pd dimers, trimers and rows of palladium). In Figure 10b, for an overlayer that has been annealed to 290 K to desorb ~50% of the COtop molecules, the chemical shifts are mostly absent for this CO coverage, but the vibrational shifts are still significant because the COtop molecules are surrounded or being affected by other CO molecules. To further analyse these shifts, they are plotted as the total chemical and vibrational shifts versus alloy composition in Figure 10c, where both types of shift are most pronounced at Au = 0.44 ML, which also corresponds to the maximum COtop coverage (see Figure 7a). In this case, the chemical and vibrational shifts are -20 cm-1 and +20 cm-1, respectively. This means that the shifts are opposite over the entire range of alloy composition, so that only a 2-3 cm-1total shift is detected at most in some cases. As the gold coverages increase (to Au = 0.32-0.19 ML), both shifts decrease at 90 K (Fig. 10a) and after annealing to 290 K (Fig. 10b). Now there are larger Pd agglomerates where the CO is not influenced by the surrounding Au. This could be an indication of a significant electronic effect for the AuPd alloys. The data are also shown as the total shift as a function of gold coverage in Figure 10d. Because the chemical shifts are negative, they tend to cancel, leading only to small overall shifts in frequency, as observed experimentally (with shifts of 2-6 cm-1). Similar cancelling shifts were also observed for CO/Cu(111).43-47 In addition, the 37 cm-1 shift for a saturated COtop surface was investigated with

12CO/13CO

mixtures on Pt(111),54 and was found to be exclusively due to

dipole-dipole coupling (vibrational shift).55-57 This could indicate that the chemical shift on


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Pd(111) is strongly affected by the presence of the surrounding Au, implying the presence of electronic/ligand effects for a certain range of alloys composition. G. Correlating CO vibrational frequencies to ensembles of atoms in the alloy The ideas presented above on the available adsorption sites and their relationship to the experimental CO vibrational frequencies on AuPd model alloy surfaces are summarized in Scheme 2 along with the CO desorption temperatures (from TPD).28 At high gold coverages (Au = 0.95-0.78 ML), isolated Pd predominates (Scheme 2a), with some bridge sites (Scheme 2b) being present at lower gold coverages (for Au ~ 0.78 ML) Here, CO desorbs as a single peak at 295 K, shifting to 300-310 K as the Pd coverage increases,28 and is assigned to CO desorption from atop sites (Scheme 2a). Compared to COtop desorption from Pd(111) at ~260 K, CO is slightly more stable on monomers surrounded by Au. At higher Pd coverages (Au = 0.65-0.44 ML), desorption occurs in two states at 255 and 335 K.28 However, both are due to COtop sites and is confirmed by the presence of an atop CO vibrational signal at 2090 cm-1 after annealing to 290 K (Fig. 5 b-d), which then disappears when heating to 350 K. This is assigned to CO on adjacent Pd trimers (3CO/3Pd), either in linear or bent arrangements, where both ensembles are identified on the simulated surfaces (see, for example, Fig. 8d). The concomitant appearance of a vibrational mode for CO on bridge sites results in CO desorption at 383 K, Scheme 2(d-f). At a gold coverage of Au = 0.44 ML, the CO desorbs at ~255 K and correlates with the strongest COtop IR band at 2094 cm-1. At higher Pd coverages, the COtop infrared features decrease in intensity, while the intensities of features due to CO2f and CO3f sites increase and CO3f desorbs at 450 K, implying that the surface contains Pd islands which resemble clean Pd(111). Here, CO adsorbed on 3-fold sites becomes evident and is -33ACS Paragon Plus Environment


represented by Scheme 2(g), allowing CO to adsorb in a hollow site. However, since no CO3f infrared features are detected for saturated CO overlayers over the entire compositional range, it is proposed that the correct structures for CO adsorbed at 90 K on the alloy are those depicted in Schemes 2(g) or (h), with either 2 or 3 COtop adsorbed CO molecules. As the CO coverage decreases, the system evolves as shown in Schemes 2g-2h until only a single CO molecule remains adsorbed at the three-fold hollow site to desorb at 450 K.28 The proportion of Pd increases over this range of alloy compositions and two Pd dimers or parallel Pd rows are observed in the simulations. Thus, the possible existence of Pd tetramers is considered as illustrated in Schemes 2 (k-l).

Scheme 2. Adsorption ensembles proposed as a function of CO and Au coverage.


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As the Pd coverage in the alloy increases, the proportion of 3-fold hollow sites increases but will differ from that expected for a random distribution of gold and palladium in the alloy that causes the formation of linear Pd features (see Fig. 9). Furthermore, the CO binding energies on 3f sites can be influenced by the subsurface composition, as fcc sites (with no metal atoms directly below in the 2nd layer) or hcp sites (with a metal atom directly below in the hollow site). These sites have been distinguished for CO on Ni(111), where a difference in CO frequency of 15 cm-1 was found 58 where a CO3f fcc (COF) site has a lower frequency than the CO2f hcp (COH) site. The IR spectra display several bands when CO occupies 3f sites on the AuPd alloy. One is at 1850 cm-1 (Fig. 5 c-f) and grows independently of the lower-frequency features (1830-1810 cm-1), especially for gold coverages of 0.56 and 0.44 ML (Fig. 5 c and d). This may indicate that CO adsorbs on 3-fold sites, but the frequency may be too high to be due to an isolated CO3f adsorption site. An alternative possibility is that these frequencies can be assigned to CO2f features, which are highly compressed as illustrated in Scheme 2 (i) or (m), thus lowering the frequency of the isolated CO2f site to 1900 cm-1. However, dipole-dipole coupling may not account for such a large frequency and it would have the effect of increasing the frequency rather than lowering it. An additional possibility is that portrayed in Scheme 2 (o) where two CO molecules are slightly displaced from their bridge sites towards 3f sites, thereby moving towards a gold atom in the alloy. This structure has been proposed previously for CO domain walls observed by STM when compressing a CO adlayer at a coverage of 0.57 ML on Pd(111).59 This will result in a CO vibrational frequency intermediate between CO adsorbed on bridge and 3f sites. However, we exclude this possibility, first because the mode should appear at lower Pd coverages on the alloy,



and second, the interaction of CO with Au is very weak, as seen for the case of CO adsorbed on top of Au (Fig.3). It is possible that the feature is due to the adsorption of two CO molecules in 3f sites, one on a COH site and the other on a COF site. Note that a band at 1860-1870 cm-1 has been assigned to the simultaneous adsorption of CO in hcp + fcc sites on Pd(111) at saturation CO coverage (CO = 0.75 ML), together with the participation of COtop sites. This postulate requires the presence of both types of proximal hollow sites, thus requiring the addition of another Pd dimer to the ensemble to form a Pd hexamer (Pd6), (Scheme 2 q). However, such ensembles are not found in the simulations for Au coverages of ~0.56 ML and can therefore be excluded. It might also be possible to have hcp + fcc 3f sites occupied as in the Pd4 ensemble shown in Scheme 2 (p), similar to the structure proposed for the adsorption of hydrogen atoms on adjacent 3-fold hollow sites.60,61 However, the repulsion between two adjacent adsorbed CO molecules suggests that this structure would be energetically unfavorable. Unfortunately, TPD results provide no additional clues to the origin of the 1810 to 1850 cm-1 vibrational modes because there are no well-resolved CO desorption features between 390 K and 440 K, the temperature range over which these IR bands disappears.28 It may be that this state is obscured by CO2f and CO3f desorption at 383 K and 450 K, or that the CO converts to more stable CO3f states when a portion of the CO in CO2f states has desorbed. H. Effect of Subsurface Composition on CO Adsorption From the discussion above, the adsorption states giving rise to the pair of vibrations at 1830-1810 cm-1 are due to ensembles with almost identical heats of CO adsorption. It is suggested that the subsurface composition of the alloy might influence the heat of adsorption of -36ACS Paragon Plus Environment

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CO on the surface. For example, if the subsurface of the alloy consisted entirely of gold, this would weaken the surface binding of CO and shift the COF vibration to higher frequencies. To explore this issue, Fig. 11 shows the results of Monte Carlo simulations of the distribution of subsurface Au and Pd.

Figure 11. Distribution of Au atoms in deeper layers, from 2nd to 5th layer, plotted as the relative amount of Au with respect to the Au surface coverage (Layer 1: surface). The results reveal that the subsurface layers are enriched with gold when Au ≤ 0.5 ML, the range of gold coverages for which 3f sites predominate. To more clearly show the sites, the results of simulations for Au = 0.15 and 0.2 ML are depicted in Figs. S3 b, d and c, e for the cases without (NI) and with repulsive lateral interactions (WI). Including repulsive Pd‒Pd interactions causes an enrichment of the subsurface layers by gold compared to simulations without lateral interactions. This leads to the appearance of more hollow sites when Au is present in deeper layers and could influence the adsorption of CO on that site. A similar enrichment of subsurface layers by Au has been found for AuPd nanoparticles where an enrichment of the core by Au for core (Au)/shell(Pd) nanostructures has been proposed -37ACS Paragon Plus Environment


to explain their high activity and/or selectivity. The influence of such core-shell structures on the catalytic activity is still not well understood but there is a clear correlation between these types of structures and their activity. More recently, this work has been extended to synthesizing coreshell nanoparticles where the Au is located in the near-surface region of the bulk.62,63 We believe the combination of experimental IR results showing the available adsorption sites with simulations of the surface and subsurface distribution of the components of bimetallic alloys is an excellent approach to obtain a complete structural picture to guide the synthesis of new catalysts.64,65

Conclusions By combining infrared spectroscopy and Monte Carlo simulations results we have completely identified all the adsorption sites available on the surface of AuPd alloys. We observe CO adsorbs weakly on top of Au atoms at Au> 0.9 ML and on atop Pd atoms, distributed mostly as monomers on the surface. For Au= 0.8-0.4 ML, CO adsorbs predominantly on atop Pd sites (COtop) even though 2-fold sites are available in high coverage. The simulations show no perfect ordering of the surface but a slight tendency to form lines of Pd atoms, thus favoring the appearance of bridge, but not 3-fold hollow sites. The preferential adsorption on atop sites is favored for this range of alloy compositions, implying the presence of strong electronic/ligand effects, which were further corroborated by experiments of CO isotopic mixtures adsorption on the alloys where their corresponding vibrational and chemical effects where identified and measured.


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Acknowledgements F.C.C. gratefully acknowledges support of this work by Agencia FONCyT under project PICT 2014-0497. S.M. gratefully acknowledges insightful discussion with Dr. Victor Pereyra. We gratefully acknowledge support of this work by the U.S. Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences, under Grant No. DE-FG02-92ER1428. M. B.H. acknowledges a postdoctoral fellowship by CONICET.

Supporting Information Available: RAIR spectra of carbon monoxide adsorption (12CO, 13CO and mixtures) on Pd(111) at 90 K as a function of dose. Simulated substrates where subsurface layers atomic distribution can be seen through the holes of the first layer, with and without inclusion of lateral interactions. This material is available free of charge via the Internet at http://pubs.acs.org.

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Combining IR Spectroscopy and Monte Carlo Simulations to Identify

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