CO Adsorption on Gold Nickel Au-Ni(111) Surface Alloys - The Journal

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

CO Adsorption on Gold Nickel Au-Ni(111) Surface Alloys Christopher C. Leon, Qing Liu, and Sylvia T. Ceyer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00379 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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The Journal of Physical Chemistry

CO Adsorption on Gold Nickel Au-Ni(111) Surface Alloys Christopher C. Leon, Qing Liu, S. T. Ceyer* Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts, USA, 02139-4307

ABSTRACT: The adsorption behavior of a saturated single layer of CO on a Au-Ni(111) random surface alloy at 80 K is probed by vibrational spectroscopy as a function of Au coverage. The effect of Au on the occupancy of the Ni-bridge CO site, characterized by internal stretch frequencies between 1860-1960 cm-1, is observed to extend beyond geometric site blocking while that of the Ni-atop bound CO site, 2060-2120 cm-1, is well-described by an ideal site blocking model up to 0.51 ML Au. Gold coverages beyond 0.51 ML support non-classically bound CO, which has a frequency (2150-2160 cm-1) above that of gas phase CO. Its adsorption site is assigned to the atop site of Ni surrounded by 6 Au atoms, Au6Ni. These observations naturally lead to a contemporary, physical picture of the dominant electronic interactions, combining elements from the Blyholder and d-band models, including 3d/5d, 2π*, and 5σ orbital mixing as well as CO dipole-dipole coupling, that determine the progressively non-adsorptive nature of CO with Au coverage, its Ni binding sites, and stretch frequency behavior. This vibrational study illustrates that quantitative knowledge of coverages combined with a thorough analysis of spectroscopic features is key to accurately describing adsorption in this complex and non-ideal system.

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I. Introduction The study of CO adsorption on Ni by vibrational spectroscopy has an extensive history in surface science, because the frequency of the CO internal vibrational stretch mode is sensitive to its chemical environment, varying over 300 cm-1 with changes in ligancy and long-range interactions and thereby revealing key structural information that is often inaccessible by other means. While vibrational studies of CO adsorption on Ni are plentiful, analogous work on wellcharacterized Ni-based alloys is comparatively lacking.

The need to understand design

principles for catalysis and to correlate CO vibrational bands with surface structure motivates a systematic survey of CO adsorption on Au-Ni(111) alloy surfaces that are unique1 in their ability to mediate low temperature CO oxidation at 70 K. This paper presents vibrational spectra of CO adsorbed on Au-Ni(111) held at 80 K as a function of Au coverage up to 0.71 ML Au. At first glance, these data provide no hint that the spectra encode interpretable fundamental aspects of CO adsorption, but through an atypical analysis, it becomes clear how the presence of alloyed Au imprints strongly and distinctively on each adsorption site. These results substantiate the surface structural assignments in our previous work describing molecular O2 adsorption on Au-Ni(111).2 That report and this one constitute a larger study focused on the mechanism of low temperature CO oxidation on Au-Ni(111). Our analysis is enabled by the sensitivity of CO’s internal stretch frequency to its chemical environment as well as by the random nature of the Au atom locations within the Ni surface that allow the absolute CO coverage at atop and bridge sites to be expressed as simple functions of Au coverage. The analysis begins with the development of a geometric site blocking model of adsorption on Au-Ni alloy surfaces saturated with CO, valid at and below 0.79 ML Au, which predicts the occupation of Ni-bridge and Ni-atop sites as a function of Au coverage. We demonstrate quantitatively how the experimental observations differ substantially from this idealized picture.

This analysis involves a thorough examination of how the absolute Au

coverage is measured and assigned on Au-Ni(111) surfaces, and how the CO loss features’ frequencies, intensities, and various derived quantities such as the configurational entropy 2


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behave as a function of this Au coverage assignment. This effort is used in a central way to validate the modelling that describes the observed absolute CO coverage. Significant differences are found between the atop and bridge sites when compared to ideal adsorption, and reflect how Au interacts uniquely with the CO adsorption sites and how the structural phase transition specific to Au-Ni(111) imprints on CO-related observables. These behaviors are rationalized through qualitative arguments based on Nørskov's d-band model3,4 and the Blyholder model5 extended to surfaces,6 which are used to describe the effect of alloying on the progression from CO adsorptive to non-adsorptive behavior and its impact on CO vibrational frequencies. We also identify factors beyond the d-band model that help explain the origin of trends observed for CO vibrational frequencies and intensities in our experiments. This effort includes an analysis of the absolute frequency shifts for fixed adsorption site as a function of Au coverage, and the origin of adsorbed CO frequencies that exceed the gas phase value – a type of reactive CO species implicated in complementary CO oxidation experiments performed by our research group. The analysis techniques illustrated in this paper are general methods that are applicable to other adsorbate systems on random alloy surfaces.

II. Background Understanding CO adsorption on Au-Ni(111) requires considering CO adsorption on Ni(111), and the same with 0.79 ML Au present, which are two surfaces completely free of, and fully occluded by Au, respectively. CO behavior on these surfaces defines the boundary conditions that a CO adsorption model as a function of Au coverage must satisfy. Idealizations of the experimental knowledge at two limits, that adsorption of an ordered, saturation layer of 0.57 ML of CO occurs on Ni(111), and that non-adsorption of CO occurs on 0.79 ML Au, are used to construct a model for how Au impurities affect the CO spatial arrangement at intermediate Au coverages, with emphasis on individual adsorption sites. This tack is useful because the topmost layer of surface alloy atoms forms a hexagonally closed-packed lattice of Ni and Au atoms at all Au coverages, even above 0.3 ML Au when this top layer coordinates differently to its supporting layer due to a complex phase transition.2 That 3



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is, the surface retains its hexagonally closed-packed motif even as its composition changes with Au coverage. Accordingly, the model as described is based on our prior experience with molecular O2 adsorption on Au-Ni surface alloys,2 but de-emphasizes subsurface reconstruction effects while keeping the salient random distribution of surface atoms. These simplifications enable modeling the observed absolute CO coverage and CO binding site populations as functions of Au coverage via probability-based arguments, which in turn, reveal why the alloy surface exhibits such complex CO adsorption behavior. This key result derives from first considering CO adsorption on Ni(111) in the absence of Au. A. Model of Saturated CO Adsorption on Ni(111)

Figure 1. Model7 of 4/7 ML CO adsorbed on Ni(111). Large and small circles represent Ni atoms and CO molecules, respectively. Black and white pseudo-triangles in between the Ni atoms are respectively face-centered-cubic and hexagonal-closed-packed three-fold hollows of the Ni surface. The c(2×2) Ni unit cell (a) and its Wigner-Seitz cell (b) are drawn with thin lines. The overlayer p(√7×√7)R+19.1º unit cell (a) and its Wigner-Seitz cell (b) are drawn as thick lines. Mirroring this figure yields the equivalent quantities for the -19.1º domain. The CO saturation coverage on Ni(111) is 4/7 ML,8 which is attained when Ni(111), at below 200 K and under UHV conditions, is exposed to CO in excess of 5 L.9 The CO molecules adsorb in p(√7/2×√7/2)R±19.1º domains at bridge and atop sites10 in a 3:1 = (3/7):(1/7) mole ratio11 (although a photoelectron diffraction study suggests that these site assignments are incorrect12). In Figure 1a, the unit cell of one of the two chiral domains that coexists at a saturation coverage of 4/7 ML CO adsorbed on Ni(111) is shown as a thick line. It contains 4 CO molecules and 7 Ni atoms. The CO adsorbate unit cell basis vector lengths are √7/2 larger than those of the supporting Ni lattice and are rotated +19.1º with respect to the underlying lattice.

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The

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corresponding Wigner-Seitz cell is shown in Figure 1b as a thick line. The Wigner-Seitz cell, centered over CO adsorbed in the atop position, has 3-fold (not 6-fold) rotational symmetry. The 6-fold rotational symmetry is broken because face-centered-cubic/hexagonal-closed-packed three fold hollow sites are present. The Wigner-Seitz cell helps highlight the existence of surface atoms differently coordinated to adsorbed CO because of the sharp visual distinction between the one high symmetry atop site at its center from the six lower symmetry bridge sites at its periphery.

Figure 2. Example of an ordered, saturated CO on Ni(111) surface with equivalent Ni surface atoms shaded to help register the CO overlayer with respect to the Ni lattice. Large and small circles represent Ni atoms and CO molecules, respectively. CO adsorbed at atop and bridge sites in the +19.1º domain arrangement are shown. The -19.1º domain arrangement is the mirror image of this figure. As boundaries for both unit and Wigner-Seitz cells cut across atoms, the Ni(111) surface is redrawn in Figure 2 with unobstructed CO molecules at atop and bridge sites. The shaded Ni atoms show that the ratio of atop to bridge site counts is 1:3, and the ratio of CO coordination number at these sites is 1:2. Multiplying these ratios together, the number of Ni atoms involved with atop and bridge bonding is 1:6. Expressed as a probability, a randomly chosen Ni atom coordinates to CO in an atop or bridge fashion with a 1/7 or 6/7 chance, respectively. The total coverage is then (1/1)(1/7) + (1/2)(6/7) = 4/7 ML CO comprising 3/7 ML bridge- and 1/7 ML atop-bound CO. A model predicting the CO saturation coverage should reproduce these values. The next section describes how this model, which treats the surface as 4/7 ML worth of independent CO adsorption sites, is modified to evaluate the CO coverage on Au-Ni(111) alloys. Comparison of this model to the data is shown in the Results section. Because all physical 5



interactions have been removed, deviations from the model inform how CO behaves on the actual Au-Ni alloy surface.

Upon analyzing how to re-introduce physical interactions to

reproduce these real behaviors, excellent agreement of the model with experiment is obtained. B. Model of Saturated CO Adsorption on Au-Ni(111) The key parameter to serve as the independent variable in an adsorption-site-centric model is the surface atomic fraction of Au, denoted by p. This quantity is defined as the number of Au atoms in the top surface layer, divided by the sum of Au and Ni atoms in the same layer. It is not to be confused with the surface area fraction of Au, which is defined as the Au coverage in ML divided by 0.79 ML Au, the saturation coverage of Au in a layer on Ni(111). This distinction between surface atomic fraction and surface area fraction is important because this analysis is based on an understanding of CO behavior at adsorption sites, not CO behavior within a fixed surface area.

The Au coverage is a function of p and this relationship is given in the

Experimental section. Hence, the quantities p and (1-p) are the respective probabilities that a randomly chosen atom on the hexagonal lattice is Au or Ni. The total, absolute CO coverage as a function of p, θtotal-ideal(p), is given by the sum of the atop and bridge adsorption contributions, θtotal-ideal(p) = θatop-ideal(p) + θbridge-ideal(p). The saturation coverages of CO on the atop and bridge sites (1/7 and 3/7 ML, respectively) on Ni(111), and CO coordination numbers (1 and 2) determine their respective linear and quadratic functional dependencies on p. Accordingly, θtotal-ideal(p) = (1/7)(1-p)1 + (3/7)(1-p)2, with θatop-ideal(p) = (1/7)(1-p)1 and θbridge-ideal(p) = (3/7)(1-p)2 assuming CO adsorbs only at Au-free adsorption sites. The terms (1-p)1 and (1-p)2 denote the CO adsorption site size in atoms, and the likelihood of occurrence on the surface. They are consistent with CO having a strong binding preference for Ni over Au atoms. Calculations13 corroborate our assumptions that CO only binds directly to Ni, indicate that the effect of Au is localized, and furthermore, show that adsorption directly to Au is unstable. At p = 0 and 1, corresponding to the Au-free and Au-saturated Ni(111) surfaces, the limiting behavior of the θ terms is correct. In the latter case, all terms vanish, which is important because 6


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Au atoms substituted into the Ni lattice block CO adsorption sites, as experiments show there is no CO adsorption at 0.79 ML Au, the coverage where there are no Ni atoms to which CO can coordinate. This consistency supports the approximation that CO adsorption at the apex of Au atoms, and bridge sites made of Au-Ni or Au-Au atom pairs cannot take place. The functions θtotal-ideal(p), θatop-ideal(p), and θbridge-ideal(p) represent predictions for the absolute CO coverage based on CO site blocking by Au atoms, which does not extend beyond the size of a Au atom. These functions do not predict the microscopic arrangement of CO with respect to the alloy surface, but when plotted as a function of the Au coverage or p, serve as useful benchmarks to compare with experimental results, as shown and discussed in the Results section. Departures from this ideal model reflect quantitative measures of the effect of Au alloying with Ni and show that the alloy surface is dissimilar to its constituent metals. Carbon monoxide is also adsorbed on sites, termed here as miscellaneous sites θmisc(p), that result in CO stretch frequencies above that of gas phase CO. In Section V.B.3, these sites are assigned to CO adsorbed on a Ni atom surrounded by six Au atoms in a hexagonally closedpacked Au6Ni configuration because of its ability to stabilize high frequency CO adsorption. A complete model for total CO adsorption, θtotal-ideal(p), should include these sites. Their inclusion would result in an additional term proportional to p6(1-p), which is the simplest assumption that best reproduces the CO population adsorbed at these miscellaneous sites. However, their low surface coverage (θmisc(p) < 0.02 ML CO) precludes quantitative experimental verification of this p6(1-p) dependence, so they are excluded from the model. Omission of these sites is not the origin of the deviation of the ideal model from the observations, as demonstrated below. III. Experimental A. Procedure Mirroring earlier studies2,14 of O2 on Au-Ni(111) surface alloys, experiments are performed in a molecular beam ultrahigh vacuum apparatus15 with a base pressure below 5 × 10-11 Torr. A Ni(111) single crystal (oriented to less than 0.2° error) can be cooled to 80 K with liquid nitrogen, heated radiatively to 450 K by a thoriated W filament positioned behind the crystal, or 7



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heated to 1300 K by electron bombardment via biasing the crystal relative to the filament. The crystal is cleaned via repeated cycles of Ar+ ion sputtering followed by annealing to between 1000 and 1300 K. Surface cleanliness is verified by the lack of contaminants as detected by Auger electron spectroscopy (AES) and high-resolution electron energy loss spectra (HREELS). Gold (Alfa Aesar, Premion, 99.999%) is vapor-deposited at an adjustable rate between 0.01 and 0.60 ML of Au/min onto the crystal held at 450 K and then annealed at 773 K for 10 min. Surface cleanliness is checked once more as before. The Au coverage is first estimated during deposition with an AT-cut16 quartz crystal microbalance14 contained within the Au source under conditions that satisfy the Sauerbrey approximation.17 It is more accurately determined after deposition via curve fitting of the intensities of the Au and Ni AES transitions.

Spatial

uniformity of the Au coverage between 0.05-0.58 ML Au is assessed by measuring the Au AES intensity at 19 distinct positions arranged in a cross spanning the Ni crystal face. At 0.71 ML Au, 14 positions are measured. The Au coverage uniformity is deemed acceptable only if the dispersion – defined as the ratio of the standard deviation to the average of all measured Au coverages on a single Au covered surface – is less than 2%. Results presented were measured on surfaces with a 1.6% maximum dispersion, an improvement from 3.8% achieved previously.2 At these dispersion levels, the nonuniformity of the true Au coverage is comparable to the error in a single Au coverage measurement. The larger of these quantities is used to calculate 2σ error bars for the Au coverage. All CO exposures are effected using 99.998% research purity CO (Matheson) supplied as a beam. The exposure time is 8.00 s with a nozzle temperature and pressure of 300 K and 25 psi, respectively. The lower limit of the CO exposure is estimated to be at least 100 L and is in excess of the 5 L needed to saturate a Ni(111) surface held at 80 K with CO. These conditions are identical to CO exposures used to maximize CO2 production in prior CO oxidation experiments.1 Dissociative chemisorption does not occur under the conditions in this work. High resolution electron energy loss spectra are measured with an instrument described previously.18 The 5.5 eV electron beam, incident at 60° to the normal, has a 56 cm-1 full width at 8


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half-maximum average energy spread with a range between 51-65 cm-1. Its average peak elastic intensity is 30 kHz with a spread of 12-51 kHz. Spectral acquisition time is 1 h using a channel width of 8 cm-1. All spectra are measured at the specular angle from a crystal held at 80 K and are normalized to 500 kHz total count rate in the fitted elastically scattered loss feature. Impact energies coincident with CO shape resonances were not pursued.19,20 B. Quantitative Gold Coverage Evaluation Gold coverages are determined by analysis of Auger electron spectra measured in derivative mode using 2 kV electrons incident at the normal angle. All Auger transitions are assumed to have derivative Gaussian profiles with identical widths whose intensities are defined with the peak-to-peak method. This identical width constraint removes a fitting parameter that otherwise increases the Au coverage uncertainty.2 These assumptions leverage the empirical observation that the most intense Au and Ni transitions between 50 to 100 eV have identical lineshapes that are insensitive to Au coverage. Furthermore, the Au and Ni Auger transitions used in this analysis span just 10 eV. The background resulting from backscattered electrons can be assumed constant in this small energy interval. All Au coverages, θ, are defined as the ratio between the Au (74 eV) Auger transition intensity and the sum of the Au (74 eV) plus Ni (64 eV) Auger transition intensities. The Ni signal serves as a proxy for the surface area that is not covered by Au. These intensities are determined from fits of two derivative Gaussians at the Auger transitions.

The boundary

conditions are picked to enclose the two Auger transition features in a symmetrical manner. The Au coverages satisfy the relationship θ = s×c, where s is the surface area fraction of Au, and c is the saturation coverage of Au on Ni(111) in ML of Ni atoms. Hence, for s = 1, 0.79 ML Au denotes 100% coverage by Au. However, the Au coverage determined from the ratio of the Au and Ni Auger intensities needs correction for several biases.21 For one, the measured Ni intensity originates not only from the Ni surface atoms, but also from Ni atoms below the surface. A procedure to separate the surface and bulk Ni atom contributions has been determined elsewhere.14,21 Second, the cross 9



sections for excitation of Au and Ni are not equal. These relative cross sections, previously measured and tabulated,22 are applied to the ratio of the measured Auger intensities. However, the cross sections as tabulated neglect the different bulk densities of Au and Ni. Hence, it is necessary to correct each cross section for the relative Au and Ni bulk densities.21 With these biases removed from the experimental measurements, the Au coverage θ is related to the mole fraction of Au p via the relationship p-1 = θ-1 + 1 – c-1. This expression can be derived from the Au coverage definition θ = m/[n+m/c], where m and n are the number of Au and Ni atoms on the surface and the mole fraction p is equal to m / (m+n). The denominator [n+m/c] is the number of Ni atoms on the surface, plus the number of Ni atoms that could fit on the surface area covered by Au atoms.21 IV. Results The experimental results are presented as follows: frequency analysis of the HREEL spectra, which identifies the characteristic vibrational energy losses of CO adsorbed on Au-Ni(111); intensity analysis of HREEL spectra, which examines the dependence of these quantities and their derived quantities as a function of Au coverage; and additional analysis of HREEL spectra, which examines important nuances hidden in the adsorption system. A. Frequency Analysis of HREEL Spectra Figure 3 presents HREEL spectra measured after exposure of Au-Ni surface alloys at 80 K to an excess of CO. The spectrum at each coverage is normalized to one set value for the integrated area of each elastic feature. The spectra are shown with increasing Au coverage from bottom to top, ranging between 0.15-0.71 ML Au. The highest Au coverage is nearly equal to the 0.79 ML Au needed to uniformly cover the Ni surface with Au atoms. The spectrum at 0.15 ML Au (Figure 3a) is characteristic of all CO adsorption spectra obtained below this Au coverage. Unresolved features below 300 cm-1 comprise frustrated rotational and translational modes of CO, the S2 surface phonon of Ni(111)23 at 250 cm-1, and lower frequency surface phonons. No further attempts to resolve these low frequency features were made. Two very intense loss features centered near 1960 cm-1 and 2110 cm-1 and with an intensity ratio that varies from 2:1 to 10


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3:1 are observable, along with a feature centered near 410 cm-1. These well-resolved vibrational losses are the CO internal stretch at bridge and atop adsorption sites of Ni, and the Ni-CO surface stretch, respectively. These assignments are based on CO group frequencies on low index Ni24,25,26 and other27 metal surfaces.

Figure 3. HREELS of CO on Au-Ni(111) surfaces as a function of Au coverage.

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The absolute values of the center frequencies of these three loss features when compared against CO adsorbed on Ni(111) never exposed to Au are blueshifted by 55 cm-1, 52 cm-1, and 35 cm-1 from the Au-free bridge (1905-1920 cm-1), atop (2058-2074 cm-1) and surface (400 cm-1) values, respectively.28,29 The blueshift is ascribed to Au slightly reducing the d-band center energy and is evidence that the Au atoms do not simply passivate the surface by blocking adsorption sites. The vibrational frequencies are also blueshifted compared to measurements on differently prepared Au-Ni thin films,30 which is evidence that the absolute shifts are not only a function of Au coverage, but also sensitive to surface morphology and preparation. A shoulder at 2160 cm-1 is observed at 0.51 ML Au, which develops into a defined loss feature at and above 0.58 ML Au (Figure 3g-h). The atop feature may be obscuring it at lower Au coverages, or its surface concentration may be too low to be detectable. The shoulder stretch frequency is higher than gas phase CO (2143 cm-1) and is assigned below to CO coordinated to a Ni in a Au6Ni hexagonally closed-packed arrangement. Direct coordination of CO to a Au atom is unlikely given the strong experimental evidence for CO bonding to an embedded Au6Ni facet on Au(111),31 calculations showing the same on Au-Ni(111) alloys,13 and the absence of similar CO adsorption stretch features on Au(111)-like surfaces.32 Minority loss features near 750 cm-1 and 1460 cm-1, whose characterizations are ongoing, are observed on Au-Ni alloys even in the absence of adsorbate. The intensity of the bridge feature decreases faster than the atop feature with increasing Au coverage, seen most obviously around 0.30 ML Au, where the integrated bridge and atop intensities are nearly identical (Figure 3b-c). At 0.33 ML Au (Figure 3d), the bridge feature is clearly less intense than the atop feature. Between 0.39 and 0.51 ML Au, the bridge feature vanishes (Figure 3e-f).

Only the atop feature is seen at all Au coverages.

Because the

disappearance of the bridge feature coincides with the appearance of the high frequency 2160 cm-1 feature, one interpretation is that the bridge sites convert into atop sites, which and in turn convert into atop sites of Ni atoms in the Au6Ni configuration. Analysis of this conversion process would require more data points spanning 0.4–0.5 ML Au. 12


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To understand how the vibrational loss features' center frequencies and intensities vary with Au coverage, two unconstrained Gaussian functions with a constant background were fit to each set of HREELS features between 1800-2300 cm-1. Fitted spectra are shown in Figure 4, with panels (a) and (b) showing typical fits above and below 0.39 ML Au. The fits show that the atop feature redshifts 30 cm-1 from 2110 cm-1 to 2080 cm-1. This shift is less than the spectrometer resolution.

Speculatively, if the shift is interpreted as the result of at least two spectral

components whose intensities (but not frequencies) vary with Au coverage, then higher resolution experiments may reveal CO bound to distinguishable atop adsorption sites. Similar arguments can be made for the bridge feature, whose frequency shift in Figure 5 is almost 100 cm-1.

Figure 4. Representative Gaussian fits to CO loss features above 1800 cm-1. Figure 5 is a plot of the fitted CO loss features' center frequency as a function of Au coverage. The center frequency is the maximum value of a Gaussian function fit to a loss feature. The atop feature has an extrapolated frequency of 2110 cm-1 at zero Au coverage. Its frequency exceeds by at least 50 cm-1 the expected CO atop frequency of 2050-2060 cm-1 13



observed for 0.50-0.57 ML CO on Ni(111). A similar 45 cm-1 overestimation is seen for an analogous linear extrapolation performed for the bridge feature, based on an expected CO bridge frequency of 1905-1920 cm-1.

These discrepancies are indicative of the need to perform

additional experiments at low Au coverage to better characterize how the CO stretch frequency varies in the dilute limit.

A spectrum of CO at 0.00 ML Au33 is unattainable in these

experiments due to residual Au dissolved in the Ni crystal after prolonged exposure to Au.

Figure 5. Center frequencies of the most intense CO loss features above 1800 cm-1. The CO adsorption sites are respectively Ni-atop (a), Ni-bridge (b), and other (c). All error bars are two standard deviations. Another interesting observation concerns the onset of a frequency redshift, and the associated redshift rate. The redshifts for the bridge and atop features do not occur in synchrony, indicating that the associated binding sites are differently sensitive to Au. The atop frequency remains constant near 2110 cm-1 up to 0.3 ML Au, while the bridge frequency redshifts. Once a threshold of 0.3 ML Au is reached, the atop frequency redshifts by about 130 cm-1 / ML Au, curiously similar to the redshift rate for the bridge feature observed from 0.00-0.33 ML Au.

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B. Intensity Analysis of HREEL Spectra

Figure 6. Scattering probabilities (see text for definition) calculated from integrated absolute intensities of the high frequency CO loss features and elastic feature. Only 2σ error bars exceeding the plot markers are shown. These probabilities are shown as a function of Au coverage, , on the lower axis and as a function of Au mole fraction, p, on the upper axis. Plotted in Figure 6 are the integrated absolute intensities calculated by integrating the fits to each loss feature at a given Au coverage. These intensities are plotted as experimentally measured absolute integrated scattering probabilities, which are equal to the ratio of integrated intensities of the CO loss feature to that of the zero loss feature. A constant, α = 0.0234 ± 0.00142σ scattering probability per unit ML of CO per unit surface area of the Ni crystal, relates the left and right axes. This constant can be derived from either of two methods that give equal results: by curve fitting (which is discussed in Sections IV.B.1 and IV.C) or by extrapolation to zero CO coverage using the data points at and above 0.15 ML Au, and assuming that 0.57 ML is the saturation coverage of CO on Ni(111), that 0.79 ML Au is the saturation coverage of Au on Ni(111), and that the CO vertical dipole moments are identical at all adsorption sites. To reflect

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these assumptions, the data points at zero ML Au are plotted at the ideal values of 1/7, 3/7 and 4/7 ML CO for the atop sites, bridge sites and total CO coverage. The data points at all other Au coverages (with exceptions noted below) in Figure 6a-c correspond to the integrated intensities of CO bound at respectively Ni-atop, Ni-bridge, and other positions. The total of these quantities is plotted in Figure 6d. Data points above 0.5 ML Au where the bridge feature is not observed (see Figure 3) are not fitted quantities and are set to zero. Data points at 0.05 ML Au are relative (not absolute) scattering probabilities rescaled with a fitting parameter to minimize the error with respect to the solid curves plotted in Figure 6. 1. Modeling Departures from Ideal Adsorption The dotted lines in Figure 6 are the predicted CO coverages based on the ideal model described in Section II.B. As a function of Au coverage, the ideal model qualitatively reproduces both the decreasing monotonicity and positive concavity exhibited by the experimental data, but does not describe the CO coverage of bridge sites or the total CO coverage well. The model noticeably does not track the experimental points at intermediate Au coverages. This observation motivates quantifying these departures as a means towards understanding how interactions induced by alloying with Au manifest in these curves. To do so, two quantities, λatop(p) and λbridge(p), which are the ratios of the experimental (here labeled θatop-fit(p) and θbridge-fit(p), respectively) to ideal CO coverages for atop and bridge adsorption, respectively, are defined. The ratios are intensive quantities because they are independent of the absolute surface area of the Ni crystal. They relate to how the 2π*-derived states of adsorbed CO vary with Au coverage that is discussed in Section

V.A.2.

Using

these

definitions,

θatop-fit(p) = θatop-ideal(p)λatop(p)

and

θbridge-fit(p) = θbridge-ideal(p)λbridge(p), where each term is the product of its ideal CO occupation, weighted by its corresponding λ(p). The ratios λatop(p) and λbridge(p) are interpreted as occupation fractions of CO of a particular type compared to its theoretical upper bound, taken across all CO adsorption sites of the same type. They are bound between 0 and 1 inclusive. Numerical values below unity indicate reduced CO adsorption compared to the non-interacting ideal model. It can 16


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be likened to CO adsorption reflecting the effective repulsive interactions, beyond simple site blocking (termed geometric effects below) by Au atoms, at a particular Au coverage. Multiplying θatop-fit(p) and θbridge-fit(p) by α (defined above as the scattering probability per unit ML of CO per unit surface area of the Ni crystal) yields the atop and bridge adsorption fit curves. That is, the solid curves in Figure 6a-b are determined by fitting αθatop-fit(p) = αθatop-ideal(p)λatop(p) and αθbridge-fit(p) = αθbridge-ideal(p)λbridge(p) to the data points, treating α, λatop(p), and λbridge(p) as fit parameters. The sum of the two curves αθtotal-fit(p) = αθatop-fit(p) + αθbridge-fit(p) results in the solid line in Figure 6d. In Figure 6, the left and right axes are scaled such that any curve with a factor of α plotted on the left axis exactly overlaps with the same curve without the factor α plotted on the right axis. 2. Accurate Parameterization of Adsorption Based on Derived Quantities The parameterization of λatop(p) and λbridge(p) is now discussed. First, the expression θatop-ideal(p) = (1/7)(1-p) is linear in p, as is the qualitative behavior of the atop CO data points in Figure 6a, so setting λatop(p) = 1 yields a good fit to the data, and only one curve is shown in Figure 6a. It is nonlinear when plotted as a function of the Au coverage on the bottom axis, but this nonlinearity is so small that it is immaterial to this analysis. Differences between the measured and predicted ideal CO coverages appear only above 0.51 ML Au. Use of this simple function for λatop(p) is reasonable given the paucity and weak intensity of data points for the CO feature at 2160 cm-1 assigned to Au6Ni-atop in Figure 6c. In contrast, the bridge CO data points in Figure 6b clearly deviate from the ideal model indicated by the dotted line. For this part of the analysis, it is more convenient to switch from using p to using the Au coverage θ as the free variable. Examining the limiting behavior at low and high Au coverages results in two boundary conditions λbridge(0 ML Au) = 1 and λbridge(0.79 ML Au) = 0 that a valid fit to the data must satisfy. The former boundary condition is more important because it corresponds to the case where bridge CO adsorption is a maximum. No additional boundary conditions are needed. To smoothly interpolate between these endpoints, a sigmoidal function based on the complementary error function erfc(.) is used. This choice arises 17



from its small number of adjustable parameters. First, define λbridge(θ) = erfc((θ-θ0)/B)/A, where A, B, θ0 are unknown constants. A is a normalization constant related to the boundary conditions. B is a shape parameter that is inversely proportional to the maximum rate of how quickly the function λbridge(θ) depletes from 1 to 0. This rate is equal to -2/(ABπ1/2), derivable by extremizing dλbridge/dθ. The constant θ0 is the midpoint of this transition, exactly where this maximum occurs. To solve for A, the boundary condition λbridge(0 ML Au) = 1 is strictly held, which implies that A = erfc(-θ0/B), so λbridge(θ) = erfc((θ-θ0)/B)/erfc(-θ0/B). B and θ0 are determined by curve fitting. The final parameter is α, but unlike λatop(θ) and λbridge(θ), α is an extensive quantity that depends on the surface area of the Ni crystal. In order to minimize the fit errors linked to this difference, λbridge(θ) is first determined by fitting to an auxiliary set of data independent of area units, and then α is fit to the data dependent on area units in Figure 6. This auxiliary set of data is the molar configurational entropy that is written in terms of a unitless quantity, the relative fraction of bridge sites, F, defined as the ratio of the measured bridge intensity to the sum of the bridge and atop intensities. Figure 7a shows a plot of the quantity F, calculated from the data points in Figure 6, as a function of Au coverage. In terms of fitting parameters, the relative fraction of bridge sites F is written as θbridge-ideal(θ)λbridge(θ)/[θbridge-ideal(θ)λbridge(θ) + θatop-ideal(θ)λatop(θ)].

The entropy per occupied adsorption site, S/k, is given by

S/k = ln{1/[FF(1-F)(1-F)]} as a function of Au coverage θ, where the configurational entropy S is defined as Boltzmann's equation S = k ln W. The entropy connects the local, microscopic details of CO adsorption with global, ensemble effects. Note that the entropy S, plotted in Figure 7b, is a highly nonlinear function of F, and in turn, of p or θ, which makes it the most sensitive physical quantity to fit amongst all quantities considered in this study. Determining λbridge(θ) by fitting it to a nonlinear quantity in θ and then determining α (by fitting to the data points in Figure 6 once λbridge(θ) is determined, and then setting θ = 0) helps constrain the fit parameters to be near solutions that have the most explanatory power.

18


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Figure 7. (a) Mole fraction, F, of CO adsorbed at bridge sites as a function of Au coverage. All error bars are 2 standard deviations. (b) Configurational entropy of bridge and atop sites calculated from adsorbed CO intensities and derived mole fractions. The result of fitting the configurational entropy with the parameterized expression for λbridge(θ) = erfc((θ-θ0)/B)/erfc(-θ0/B), with B and θ0 as fit parameters and λatop(θ) = 1, is shown as the solid curve marked (b) in Figure 7. The fit parameters are determined to have the values B = 0.14 ± 0.012σ ML Au (0.14 ± 0.012σ mole fraction Au) and θ0 = 0.27 ± 0.012σ ML Au (0.29 ± 0.012σ mole fraction Au). It is interesting that B is nearly 1/7, which is the mole fraction of an atom of interest in an environment of its nearest neighbors in a hexagonal lattice, and θ0 is close to the Au coverage of the Au-Ni(111) surface reconstruction. For this set of fit parameters, the boundary values are λbridge(0 ML Au) = 1 by construction, and λbridge(0.79 ML Au) < 1×10-4, which is well below any instrumentation detection threshold in the experiment and is negligibly different from the ideal value of 0. The agreement between the experimentally determined configurational entropy and the fit is excellent. These same parameters are used to predict the solid curve in Figure 7a for the relative fraction of CO bridge sites and the solid curves in Figure

19



6, after determination of the value for α. Again, the agreement between the predictions and experimental observations is excellent. 3. Physical Significance of Parameterization and Configurational Entropy The significance of these excellent fits go beyond quantitative agreement between experimental data and modelling. The results of fitting the intensive quantities of Figure 7 with separable functions suggest that “geometric” and “electronic” effects have convenient and clear definitions. While the definition θbridge-fit(p) = θbridge-ideal(p)λbridge(p) is an approximation, the excellent fit achieved using it implies that θbridge-fit(p) is separable into a product of two independent components θbridge-ideal(p) and λbridge(p).

These two quantities can now be used to define

geometric and electronic effects for adsorption at random surfaces: the former is the contribution to adsorption that arises from counting adsorption sites on an ideal random surface, and the latter is non-ideality that arises from non-geometric effects. The ability to fit experimental data points as a function of coverage then works as a litmus test for the separability of geometrical and electronic effects as defined here. This approach is useful because typically these effects cannot be decoupled nor delineated from one another. The physical significance of the quantitative values of the fitting parameters for λbridge(θ) is now considered. Recall that θbridge-fit(θ) = θbridge-ideal(θ)λbridge(θ) and note that the following results are completely independent of α. Because λbridge(θ) ≤ 1, it means θbridge-fit ≤ θbridge-ideal, and shows that the effect of individual Au atoms does extend beyond its local 1-atom site. To quantify this effect, the focus is directed at comparing the quantities dθbridge-ideal/dθ and dθbridge-fit/dθ, which are the slopes determined by calculating the numerical derivative of the ideal model and of the experimentally fit curves in Figure 6b, respectively. The slopes are negative, and are interpreted as the loss of CO bridge sites per unit increase in Au coverage, at a particular Au coverage. Figure 8 shows these differential losses of bridge-bound CO adsorption sites, defined as the absolute quantities |dθbridge-ideal/dθ| and |dθbridge-fit/dθ|. The two curves do not track each other, indicating that Au atoms affect CO adsorption far beyond just blocking adsorption sites with which they are in direct contact.

This effect is present even in the dilute limit, where 20


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|dθbridge-fit/dθ| is 3% larger than |dθbridge-ideal/dθ|.

Figure 8. Differential loss of bridge-bound CO for the case of Au atoms behaving (a) as in the ideal model and (b) experimentally, as derived from fitted experimental data points. The curves are derived by taking the numerical derivative of the curves in Figure 6b. The impact beyond site blocking is particularly evident for θ < 0.35 ML Au, where |dθbridge-ideal/dθ| < |dθbridge-fit/dθ|. Above this coverage, the surface is so thoroughly affected by Au that additional Au increments no longer remove bridge sites as it once did at lower coverages. The maximum of |dθbridge-fit/dθ| is 1.48 ML CO per ML Au, and occurs at θbridge-fit-max = 0.22 ML Au. It is greater than unity, which is consistent with a quantity of Au added to the surface removing more than its equivalent quantity of bridge-bound CO.

This behavior of

|dθbridge-fit/dθ| > 1 ML CO per ML Au occurs between 0.09 and 0.31 ML Au coverage. Interestingly, examining the total CO coverage shows that |dθatop-fit/dθ + dθbridge-fit/dθ| > 1 whenever θ < 0.33 ML Au. These important inequalities seem to be related to the surface reconstruction coverage of 0.3 ML Au, which introduces geometric effects beyond that of an ideal, hexagonally closed-packed random surface. Their presence is corroborated by the abrupt rearrangement and reduction in CO adsorption per unit area above 0.3 ML Au37 that appears to disproportionately affect bridge 21



sites, as evident in Figures 6 and 8. There are also electronic effects to consider. For atop sites, the deviation of the approximation λatop(θ) ≈ 1 from the experimental data points is largest between 0.5 and 0.6 ML Au in Figure 6a, and hence, |dθatop-fit/dθ| should be maximized near θatop-fit-max ≈ 0.55 ML Au. That θbridge-fit-max is less than θatop-fit-max is discussed in Section V.A.2 and ascribed to site-specific electronic effects that are driven by the dominant CO 2π* interactions. Namely, CO at a bridge site is more strongly affected by perturbations in its local environment because it is more strongly bound than CO at an atop site.34 Therefore, less Au needs to be present for its impact on CO adsorption to be noticeable. This effect is corroborated by the behavior of the bridge and atop frequencies in Figure 5. The former is immediately affected by Au whereas the latter only decreases above 0.3 ML Au. Changes in the configurational entropy as a function of Au coverage shown in Figure 7b also reveal interesting insights into the CO adsorption behavior. First, the entropy satisfies the expression 0 ≤ S/k ≤ ln(2) = 0.693, because atop and bridge are the only 2 types of CO adsorption sites considered here. The maximum is attained when F = ½, and is experimentally attained at 0.29 ML Au, approximately the coverage where the atop and bridge features have equal intensity. The minimum is attained when F = 0, at coverages above 0.51 ML Au when only atop sites are occupied. The case when F = 1 is inadmissible because CO does not adsorb at bridge sites exclusively under the experimental conditions considered. At zero Au coverage, the respective population (mole) fractions of atop and bridge sites are ¼ and ¾ per CO unit cell at 0.57 ML CO coverage, resulting in a configurational entropy of 0.56. Between 0 to 0.29 ML Au, the configurational entropy increases monotonically towards the maximum of 0.693, reflecting the variety of binding site environments increasing with Au coverage, as seen by the growing equivalence of the intensities of the atop and bridge bound CO features in Figure 3. The entropy maximum is attained at 0.29 ML Au, corresponding to a Au atomic fraction of p = 0.31. Attaining this maximum is significant because it corresponds to surface Au coverage where the absolute number of bridge- and atop-bound CO is equal (Figure 6). Moreover, their binding energies to the surface must be degenerate because the differently22


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bound CO molecules are in equilibrium. Corroborating this result are density functional theory calculations showing that such a degeneracy exists when p = 3/9 = 0.33.13 An experimental proof of CO adsorbed in an equilibrium distribution is described in Section IV.C. The entropy maximum attained at 0.29 ML Au is also nearly coincident with the onset of surface reconstruction at 0.3 ML Au. Above this Au coverage, the configurational entropy decreases monotonically towards zero at a rate much faster than the initial increase towards the entropy maximum, reflecting the rapid depletion of bridge-bound CO. This rapid depletion is consistent with both geometric and electronic effects working in synergy, particularly above 0.3 ML Au. Not only are adsorption sites being removed, but of those that remain, their adsorption energies are also being reduced. Focusing on the geometric effects beyond the ideal model, the reconstruction decreases the entropy of the CO overlayer, an observation supported by scanning tunneling microscopy (STM) experiments that reveal CO molecules avoiding the dislocation loop,37 thereby precluding the onset of new adsorption sites. That is, CO appears to prefer bridge and atop sites least perturbed by the reconstruction. The net effect of the reconstruction on CO adsorption sites is removing some fraction of otherwise admissible bridge and atop sites without introducing any new adsorption sites. This effect is further complicated by the increasing fraction of adsorption sites affected by a reconstruction loop with increasing Au coverage, because the surface density of reconstruction loops increases with increasing Au coverage. We emphasize that geometric effects alone cannot account for the observed behavior of the configurational entropy, because the rapid depletion of bridge sites versus atop sites is also determined by the electronic structure of CO interactions with Au-Ni(111). The latter governs the ability of CO to adsorb, and its behavior in response to changes in Au coverage is driven by the CO 2π* interactions with the surface. In particular, the bridge-bound CO exhibits a higher sensitivity to changes in the occupancy of the 2π*-derived states that lead to differing behaviors in bridge and atop adsorption, and how they are reflected in the configurational entropy. These electronic effects are discussed in Section V.A.2. 23



C. Additional Analysis of HREEL Spectra The integrated intensity of a CO stretch feature is used as a measure of the amount of CO adsorbed on a site throughout this work. This procedure assumes that the excitation dipoles of CO adsorbed on different sites are equal, which is a valid assumption because the ratio of experimentally derived integrated intensities at zero Au coverage reflects the known 1:3 ratio of CO coverage in the atop versus bridge sites, as shown in Figure 6. Moreover, the experimentally measured excitation dipoles of adsorbed CO on Au-Ni(111) match within 2% tolerance. This value is derived by refitting the data points in Figures 6 and 7 excluding the points at zero Au coverage, with two fit parameters representing the different scattering probabilities for atop and bridge adsorbed CO, rather than one fit parameter representing equal scattering probabilities of the same. Their magnitudes are determined to be 0.0232 ± 0.00242σ and 0.0236 ± 0.00182σ, respectively. The foregoing analyses also presume that the distribution of adsorption sites reflects an equilibrium distribution at 80 K, meaning that the barrier to site conversion from bridge to atop must be small relative to the surface temperature for all Au coverages. Site hopping should be considered,34 given the low barriers to conversion35 on Ni(111) and peculiar CO dynamics on Au(111) even at 5 K.36 If this barrier were large, then the intensities of the EELS loss features may not be a measure of the equilibrium populations of atop and bridge sites. To test whether these measurements reflect an equilibrium distribution of CO, the intensities of the loss features from a 0.47 ML Au surface saturated with CO at 80 K are compared to those measured from an adsorbed CO layer that was cycled between 80 K and 200 K. As all loss features remain unchanged after the temperature cycling, it is concluded that an equilibrium CO distribution is attained. The surface morphology and homogeneity of the surface alloy is now considered. The dissimilarity of the loss features between low and high Au coverage is strong evidence that the surface is not dealloyed because Au aggregation or phase separation would lead to Ni-rich areas with an effective Au coverage lower than that of the whole surface, and hence would be reflected 24


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in the CO loss features. This lack of dealloying is consistent with STM,37 but not LEEM38, 39 observations. Nevertheless, the variance could be ascribed to the alloy preparation method prior to the LEEM measurements, which is different from that employed in our study. To better understand the nature of CO on Au-Ni(111), we now proceed to examine the key factors that contribute to the observed CO adsorption behaviors reported in the Results section. V. Discussion The adsorptive behavior of CO on Au-Ni(111) is characterized by its adsorption energies, vibrational frequencies, adsorption sites, and adsorption site occupancies. All four quantities are interrelated so inferences about one can be made by observing the others. The adsorption energies are the most important because they are related to the local minima of the potential energy surface describing CO chemistry on the surface alloy, and therefore, summarize the fundamental interactions that occur on the surface.

The adsorption frequencies follow in

importance because they are related to the curvature of the potential at those minima. As the adsorption energies are not directly observable via HREELS, a qualitative picture of the electronic structure is first described to establish the foundations that relate to the other three quantities listed above. Next, the origin of the CO frequency shifts for fixed adsorption sites is discussed. Finally, crucial connections are made between these physical quantities and the adsorption model presented in Section II, which originated from analyzing only the CO adsorption frequencies and intensities while making no direct reference to adsorption energies or chemical interactions that occur on the surface. The aim is to show how the behavior of these physical quantities validates the key simplifications of the model that, in turn, results in quantitative agreement between the predicted site occupancies and the observed CO coverage. At 80 K, CO chemisorption occurs readily on Ni(111) but ceases at 0.79 ML Au, implying that the stability of adsorbed CO decreases with increasing Au coverage. On Ni(111), the adsorption energy is 1.26 eV in the dilute CO limit,40 while it is just 0.04 eV on Au(111).41 The progression between the two extremes with increasing Au coverage depends on the relative energetics of both local and delocalized interactions between the alloy d-band and the CO 25



frontier orbitals. Their convolution is reflected in the CO stretch frequency as a function of Au coverage. In addition to these electronic structure considerations, the projected density of states at specific binding sites is also relevant, as well as CO lateral interactions that are dependent on both CO and Au coverage. When these factors are considered as a whole, it enables explaining the origin of the observed CO frequency shifts, and examining in detail the model used to describe CO adsorption. A. Electronic Structure of the Surface Alloy The chemistry of the surface alloy is intimately related to the structure of its d-band. The d-band center, bandwidth, and filling extent are all parameters that affect the surface chemistry. These quantities are coupled and change in a strongly correlated manner,4 so choosing one of these quantities is sufficient to describe chemisorption behavior on metal surfaces and their alloys. An important energy reference in all discussions of electronic structure is the Fermi level, which is the energy level of the highest occupied electronic state in the system. The d-band center is measured with respect to the Fermi level and is affected by both the orbital overlap between the metals in the alloy, and the d-band filling extent. The energy difference between the d-band center and the adsorbate molecular orbitals determines the CO chemisorptive strength and stability, and the effectiveness of the overlap between the surface and CO molecular orbitals. This stability decreases both from left-to-right and top-to-bottom across the periodic table dblock. In the first case, the increased d-band filling helps populate the 2π* antibonding states of CO. In the second case, the diffusivity of the d orbitals increases so the surface-adsorbate interaction becomes more repulsive for fixed adsorbate geometry, causing the adsorbate to adjust its geometry to minimize these repulsions. Gold is below and to the right of Ni on the periodic table, so alloying Au into Ni results in a surface that interacts more weakly with adsorbates when compared to a pure Ni surface. Upon alloying Au with Ni, the entire d-band center decreases and its bandwidth increases, with little change to its filling. The d-band center decreases because the Au 5d electrons form a band with a large fraction of density of states below the Ni 3d band.42 These lower energy Au 5d electrons 26


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replace the higher energy Ni 3d electrons, lowering the valence d-band of the alloy. This substitution reduces the Ni 3d electron density at the surface. The d-band bandwidth increases because the combination of Ni and Au d-bands span a larger energy range. The bandwidth is a qualitative indicator of the extent of the d-d orbital mixing. Introducing diffuse Au 5d orbitals into an environment that normally accommodates compact Ni 3d orbitals results in more repulsive interactions that broaden the alloy d-band. The same effect can be achieved if the surface is compressively strained (as it occurs in the Au-Ni(111) alloy) because the Au atom is larger than the Ni atom. A phase transition driven by the interfacial strain between the top two layers results in a slight expansion of the surface lattice above 0.3 ML Au, which counteracts this strain, but does not fully counteract the broadening d-band. The d-band filling does not change appreciably because alloying with Au results in some simultaneous depletion and compensatory gain of its 5d and 6s electron density, respectively.43,44 Since the d-band filling is about constant after alloying, the d-band widening and d-band center lowering occurs in tandem. These factors combined together explain qualitatively why increased Au coverage on Ni(111) decreases the d-band center energy. Adjusting the d-band level with alloying can modify surface-CO interactions and their adsorptive behavior. In the following discussion, the d-band is considered to change linearly with Au coverage. While it is an oversimplification,13 this presumption is used here because it excludes detailed interactions between Au and Ni to which our measurements are insensitive. 1. CO Interactions with Ni(111) The binding energy of CO to Ni(111) and changes in thereof are strongly dependent on surfaceCO interactions occurring mainly between the Ni and C atom of this surface-CO complex. Of interest is the qualitative picture6,45 of the relative overlap and energetics between the Ni valence 4s and 3d bands, and the frontier orbitals of a single CO molecule, which are its 5σ and 2π* orbitals. Understanding them explicates why the stretch frequency of CO adsorbed on AuNi(111) exceeds that on Ni(111) and why the populations of the different CO binding sites are affected by Au coverage in an a priori non-expected fashion. The effect of lateral interactions at 27



higher CO coverages is addressed in a separate section concerned with absolute CO frequencies. In the entire discussion, the total number of electrons below the Fermi level is always conserved, and is particularly relevant when any electronic states are shifted above or below the Fermi level due to orbital mixing. While these states will be represented pictorially below with rectangles and give the illusion that there is a net loss or gain of electrons in the system, the true density of states adjusts so that the total number of electrons stays fixed. These changes are not shown because they do not affect the qualitative electronic structure arguments in this paper. The rectangles merely represent an energy span where the density of states’ distribution is highest in a particular band. The CO frontier orbitals have the electronic configuration (5σ)2(2π*)0, meaning that the highest occupied molecular orbital is 5σ, and the lowest unoccupied molecular orbital is 2π*. Now consider the interaction of an isolated CO molecule with the Ni(111) surface as a hypothetical two-step process46 that involves orbital mixing of the 5σ and 2π* CO molecular orbitals with the Ni 4s orbitals that form the Ni 4s band, followed by further orbital mixing with the Ni 3d orbitals that form the Ni 3d band. In the first step, the 5σ and 2π* orbitals interact both in-phase and out-of-phase with the Ni 4s band and broaden into new 4s-5σ and 4s-2π* bands. For simplicity, only bonding interactions of the new 4s-5σ and 4s-2π* bands are considered here. These bands are drawn on the right and labeled 5σ and 2π* in Figure 9 for brevity. Because the original 5σ and 2π* molecular orbitals are completely occupied and unoccupied, the resulting new 4s-5σ and 4s-2π* band centers are located below and above the Fermi level, respectively. Their band centers are more energetically stable than their respective original orbitals in the gas phase, and are equal to the stability gained from gas phase CO adsorbing on a simpler,41 hypothetical Ni metal surface that has no d electrons (which is approximated here by considering CO adsorption on metal surfaces such as Al(111)47, 48). The degree of this stability is different for the 4s-5σ and 4s-2π* interactions. In the first case, the in-phase interaction between the 4s and 5σ orbitals that are of the same symmetry is a bonding interaction, and their favorable overlap leads to a very stabilizing surface28


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5σ interaction. In the second case, the different symmetries of the 4s and 2π* orbitals make their overlap less efficient.49 Consequently, the 4s-5σ band is energetically farther away from the Fermi level compared to the 4s-2π* band. This description forms the basis for the second step of orbital mixing that involves these bands interacting with the 3d orbitals. Shown on the left in Figure 9 is the metal d-band, represented by a blue rectangle labeled with its constituent orbitals. About 90% of it is below the Fermi level because 9 out of 10 Ni 3d orbitals in the bulk are occupied.50 Shown on the right are the CO-derived bands, arising from the bonding 4s-5σ and 4s-2π* interactions just discussed. Ensuing references to 5σ and 2π* bands without the 4s prefix refer to the bands that result from 4s-5σ orbital mixing and 4s-2π* orbital mixing. They are both narrower50 than the Ni 3d, but this detail is immaterial here. What matters is that the Ni 3d band center, which is 1.29 eV below the Fermi level,51 is between the hypothetical 5σ–2π* bandgap, and is energetically closer to the 2π* than the 5σ bands. Twophoton photoemission52 from adsorbed CO on Ni(111) shows that the onset of antibonding 2π* derived states are at most 2 eV above the Fermi level and the onset of the 5σ state is approximately 8 eV below.53 Knowing the relative energetics of these bands helps one to correctly order the energies of the resulting orbitally mixed surface-CO bands shown in the middle of Figure 9. Because the 5σ and 2π* orbitals have different symmetries and because their relative energy difference is large, there is no mixing between 5σ and 2π* states, even after orbital mixing with the 3d band. Therefore, the 3d-5σ and 3d-2π* interactions are considered independently, as is emphasized by showing the two interactions separately in Figures 9a and 9b. Orbital mixing between states of two different bands can be attractive or repulsive, and the sum of these interactions results in two new bands, shown in the middle of Figure 9a, that reflect the antibonding (3d-5σ)* and bonding (3d-5σ) states of the surface-CO bond, respectively. Orbital mixing between the 3d and 2π* bands is drawn in Figure 9b analogously. They are colored to represent the degree of similarity with their parent 3d and 5σ bands or 3d and 2π* bands, and arranged from bottom to top with increasing energy. The dashed, thin, and thick lines 29



connecting two bands together represent increasing degrees of orbital similarity. The relative placement of the rectangles qualitatively follows the band structure previously described54 and calculated55,56 when the local density of states after CO adsorption on Ni(111) are projected onto the 5σ and 2π* orbitals. The (3d-5σ)—(3d-5σ)* band splitting exceeds that of the (3d-2π*)—(3d-2π*)* band, as shown in Figure 9, because both the exchange interaction and orbital overlap between the 3d and 5σ orbitals are larger than those between the 3d and 2π* orbitals.41 The splitting is a measure of the overall energetic gain attained through orbital mixing and orthogonalization of the orbitals involved. Within an effective medium approximation,57 these quantities are proportional41 to the overlap matrix elements between the CO adsorbate and Ni. If r is the Ni-CO distance, this matrix element scales51 as 1/r1+M+L, where M and L are defined respectively as the orbital angular momentum of the Ni 3d (M = 2) and CO adsorbate (L = 0 for 5σ, 1 for 2π*). The lower sum of the orbital angular momenta of the metal 3d and ligand 5σ states leads to a larger splitting of the (3d-5σ)—(3d-5σ)* bands despite the 3d and 2π* states being energetically closer together. Figure 9 also depicts interactions of a weak or non-interacting character that are termed pseudo-non-bonding interactions to emphasize that they are not true non-bonding interactions. They involve pairwise interactions that lack the correct symmetry or spatial proximity for good orbital overlap and are indicated by faded rectangles in the center of Figure 9a (3d and 5σ) and Figure 9b (3d and 2π*). They retain the character of the original 3d, orbitally mixed 4s-5σ, and orbitally mixed 4s-2π* bands. Overlap of the pseudo-non-bonding 2π* band with the antibonding (3d-2π*)* band has been interpreted as a factor that contributes to broad resonance features observed for the 2π* state.58,59,60

30


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Figure 9. Schematic showing the orbital mixing between the metal 3d band and the CO 2π* and 5σ bands (recall these bands are orbitally mixed with 4s, see text). (a) 3d and 5σ on Ni. (b) 3d and 2π* on Ni. Orange lines denote the Fermi level, labeled EF. Dashed, thin, and thick black lines connecting two bands indicate increasing degrees of orbital similarity. Within each panel, orbital energies are ordered from lowest to highest, reading from bottom to top. The spacing between the bands is qualitative and there is no relationship between the energy scales of the two panels. Overall, Figure 9 shows the eight distinct interactions that result from the orbital mixing of the Ni 3d band with the 5σ and 2π* bands. The four strongest interactions are the bonding and antibonding 3d-5σ (Figure 9a), and the bonding and antibonding 3d-2π* (Figure 9b), respectively denoted (3d-5σ), (3d-5σ)*, (3d-2π*), and (3d-2π*)*. These interactions occur mainly between two orbitals with favorable symmetry and motivate shading their corresponding bands with a darker color in Figure 9. For example, when CO adsorbs at atop sites, the Ni 3dz2 – CO 5σ interaction

31



contributes strongly to the (3d-5σ) and (3d-5σ)* bands, and likewise, when CO adsorbs at bridge sites, both Ni 3dxz – CO 2π* and Ni 3dyz – CO 2π* interactions contribute strongly to the (3d-2π*) and (3d-2π*)* bands. With the description of the band structure complete, the key interactions that determine the surface-CO bond stability can be examined. It is strongly correlated with the fractional filling of all bonding bands, less the fractional filling of all antibonding bands. A part of this difference is straightforward to calculate because the bands that are sufficiently above or below the Fermi level are guaranteed to be either completely empty or full regardless of any alloying or orbital mixing. Therefore, only the bands closest to the Fermi level require special consideration. As Figure 9 shows, orbital mixing results in a bonding (3d-2π*) and an antibonding (3d-5σ)* band near the Fermi level. A fraction of both bands cross the Fermi level, which leads to populating some bonding (3d-2π*) and depopulating some antibonding (3d-5σ)* states. Both changes strengthen the surface-CO bond. The dominant4 contribution to this strengthening is mediated by the (3d-2π*) band because the 2π* band is closer to the Fermi level (and the Ni 3d states) than the 5σ, which increases its degree of orbital mixing. Hence, the fraction of the (3d-2π*) band below the Fermi level is larger than the fraction of the (3d-5σ)* band above. Calculations and experiments concur that the (3d-5σ)* states penetrate above the Fermi level and are key61 to forming the surface-CO bond, but their overall contribution towards Ni-CO bonding is small to negligible.62 This picture shows that even on a surface, the CO 5σ orbital retains some of its gas phase nonbonding to slightly antibonding character. It remains nonbonding in the sense that formation of the surface-CO bond does not significantly change the total occupancy of the 5σ-derived states. It is antibonding in that any increased population of the 5σderived states must occur in the antibonding (3d-5σ)* band. This description of the interactions in terms of the d-band model essentially extends the simpler picture of the Blyholder model.5 2. CO Interactions with Au-Ni(111) Alloying Au into Ni(111) affects the surface-CO bond directly. As Au atoms have a filled 5d shell, alloying decreases the metal d-band position with respect to the Fermi level, which 32


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changes the relative energetics, and consequently, the orbital mixing between the metal d-band and CO 5σ and 2π* molecular bands. Alloying also replaces the compact Ni 3d orbitals with more diffuse Au 5d orbitals, decreasing Pauli repulsion63 and overlap between the CO orbitals and surface. Of the resulting interactions in Figure 9, the antibonding (3d-5σ)* and bonding (3d-2π*) bands that are nearest the Fermi level experience the most consequential changes. These two bands on Au-Ni(111) are shifted with respect to both the Fermi level and their original positions on Ni(111), leading to fractional filling changes that ultimately weaken the surface-CO bond. These changes manifest in the transition from CO adsorption towards non-adsorption with increased Au coverage.

Figure 10. Schematic showing the relative energetics of the orbitally mixed bands with respect to the Fermi level on Ni(111) and Au-Ni(111). (a) Bonding (3d-5σ) and antibonding (3d-5σ)*. (b) Bonding (3d-2π*) and antibonding (3d-2π*)*. The thick grey line represents the Fermi level. Within each panel, orbital energies are ordered from highest to lowest, reading from top to bottom. The spacing between the bands is qualitative and there is no relationship between the energy scales of the two panels. Only interactions between the CO 5σ and 2π* bands with the Ni(111) and Au-Ni(111) dbands are shown in Figure 10. The band positions are qualitatively the same as those in Figure 9 but drawn differently to highlight the band behaviors near the Fermi level. Ensuing references to the Au-Ni(111) alloy d-band should be understood as having both 3d and 5d character even if references to 5d states are omitted. In Figure 10a, the qualitative positions of the bonding (3d-5σ) and antibonding (3d-5σ)* states with respect to the Fermi level are shown for CO

33



adsorption on Ni(111) and Au-Ni(111). Their positions are particularly sensitive to Au coverage changes because both Ni and Au are metals whose d-bands are more than half full.64 Note that the energy splitting between these two bands increases with adding Au. In contrast, in Figure 10b, the splitting between the bonding (3d-2π*) and antibonding (3d-2π*)* bands decreases. The opposite impact of the splitting upon adding Au originates from the interaction strength between the d-band center and 5σ and 2π* orbitals being inversely proportional to the energy difference of their centers.65 Recall that the Ni 3d band center is between but energetically closer to the 2π* than the 5σ band center. With increasing Au coverage, the d-band center shifts away from the Fermi level, moving energetically closer to the 5σ and away from the 2π* orbital, so the energy difference between 3d and 5σ decreases, while the same between 3d and 2π* increases. Hence, 3d orbital mixing with the 5σ and 2π* orbitals results in a greater energy difference between the (3d-5σ)—(3d-5σ)* bands and a smaller energy difference between the (3d-2π*)—(3d-2π*)* bands with increasing Au coverage. Crucially, the Fermi level crosses the bands nearest to it differently. As shown in Figure 10, in response to adding Au, the antibonding (3d-5σ)* and bonding (3d-2π*) bands both shift to energetically higher positions, so the fractional occupation of both bands decreases. While these two effects are separate components that respectively strengthen and weaken the surface-CO bond, only the bonding (3d-2π*) interaction is dominant in making the surface-CO bond because the fraction of orbitally mixed 5σ states above the Fermi level is quite small compared to the fraction of orbitally mixed 2π* states below it.61

While this relative relationship remains

unperturbed with increasing Au coverage because of the particular energetics and orbital overlap matrix elements for this system,41,51 the higher Au coverage does lead to stabilizing CO bound to an atop site with a stretch frequency above the gas phase value, as discussed below. However, the overall surface-CO interaction still weakens with increasing Au coverage. In summary, all these mechanisms explain how the potential energy well depth describing the surface-CO bond becomes shallower with increasing Au coverage, and hence, why CO readily adsorbs on Ni(111) but not on Au-Ni(111) at 0.79 ML Au. 34


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While the bonding (3d-2π*) interaction dominates over the (3d-5σ)* interaction, the smaller contribution of the latter to the overall surface-CO bond is not zero which leads to nuanced differences between atop- versus bridge-bound CO. Calculations on stepped Au surfaces exhibit an analogue of the (3d-5σ) state only for atop-bound CO, and it resides energetically below any bridge-related feature.66 Thus, for atop-bound CO, its complementary (3d-5σ)* state is more likely to be above the Fermi level due to orbital mixing. Both would contribute to strengthening the surface-CO bond, and this effect can dampen the overall weakening of the surface-CO bond from the loss of (3d-2π*) interactions with increasing Au coverage. Even in the absence of Au, when CO is moved from an atop to bridge site, the electron occupancy of the 5σ-derived states remain essentially constant, while that of the 2π*-derived states increases by a factor of 1.6.49 This change shows that variations in surface bound CO behavior are primarily driven by changes in the 2π*-derived states, even if 5σ-derived states are helping to anchor the molecule to the surface. Recall that changes in the 2π*-derived states are dominated by changes in the bonding (3d-2π*) interaction. Because this interaction is the most sensitive to Au coverage changes, it explains the electronic structure origins for the enhanced Au sensitivity of bridge-bound as compared to atop-bound CO. This sensitivity has implications for how the configurational entropy behaves as a function of Au coverage as shown in Figure 7, where non-ideal geometric effects manifest as a steep decrease in configurational entropy. However, electronic structure effects also work in synergy with it to rapidly convert the binding preference of CO from bridge to atop sites. This transition is largely mediated by the 2π*-derived states. Only when both electronic and geometric effects are considered is agreement between modelling and the data attained. B. Absolute CO Frequencies Having described how CO adsorbs on Ni(111) and Au-Ni(111) with qualitative electronic structure arguments, and having shown how CO adsorption goes from adsorption to nonadsorption through alloying with Au, the roots of the observed CO frequencies and frequency shifts are now discussed.

The CO vibrational frequencies are functions of its surface 35



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coordination number and surface concentration. Both quantities are decreasing functions of Au coverage but the former causes the CO frequencies to shift up, while the latter causes the CO frequencies to shift down. Thus, the measured frequencies reflect the competition between the binding strength of CO to the surface versus the lateral interactions between CO molecules. For brevity, these interactions are referred to as surface-CO and CO-CO interactions.

As this

competition is not apparent in the band structure diagrams and electronic structure arguments summarized in Figures 9 and 10, it is discussed below by examining how the CO stretch frequencies relate to the surface-CO bond order (largely governed by dominant 2π* interactions with a small 5σ component) and their sensitivity towards the underlying phase transition of the Au-Ni(111) alloy as a function of Au coverage dependence. Ultimately, CO-CO interactions are shown to have a dominant effect over the CO frequencies for a fixed adsorption site. Finally, the origin of the CO stretch frequencies exceeding the gas phase value at high Au coverage is discussed. This phenomenon on Au-Ni(111) does not involve lateral interactions, but rather electrostatic effects that would otherwise be hidden at low Au coverage. 1. Origin of Adsorbed CO Frequency Trends The electronic structure arguments of the previous section can be modified to rationalize adsorbed CO frequency trends by relating the potential energy well-depth (or equivalently, the CO adsorption energy) of the surface-CO bond with the potential energy curvature of the internal CO stretch. Relating the two parameters is necessary because the frequency is directly related to the local curvature, rather than the depth, of the potential energy well that describes the CO internal stretch. That the frontier orbitals of CO, particularly the 2π* orbital, participate in both the surface-CO and internal CO bond is insufficient to establish a correlation between the two, so an appeal to empirical results is helpful. The internal CO stretch frequency of gas phase transition metal carbonyls increases linearly with decreasing 2π* orbital occupancy.67 Similar trends hold for adsorbed CO species.68 With increasing Au coverage, the metal d and CO 2π* orbital overlap decreases, which in turn reduces the charge transfer from the metal to CO as fewer (3d-2π*) orbitals are now orbitally mixed 36


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below the Fermi level. The charge transfer reduction weakens the surface-CO bond, but also strengthens the internal C-O bond. The charge transfer involves electron density concentrated where the CO 2π* and the metal d states overlap, evident in calculations of CO adsorption on Group 10 metal clusters.55,69,70 The C-O bond strengthening is reflected in the curvature of the CO stretch potential energy surface narrowing, which increases the CO stretch frequency. Adsorbed CO with a higher stretch frequency is bound more weakly compared to adsorbed CO with a lower frequency. Because the d-band in the alloy is lower compared to Ni, the stretch frequencies of CO bound to the Ni-bridge and Ni-atop sites are higher on Au-Ni than on Ni(111). The Au6Ni-atop stretch frequency is highest because it involves a Ni adsorption site that is maximally surrounded by Au. Specifically, the frequencies of CO bound to the Ni-bridge (19201960 cm-1), Ni-atop (2060-2110 cm-1), and Au6Ni-atop (2150-2160 cm-1) site moieties increase in the order listed, tracking with the lower degree of orbital mixing of the CO 2π* orbital with the Ni 3d electrons at these binding sites where the coordination number decreases from 2 to 1.49 The nearly 100 cm-1 difference between the group frequencies of the differently bound CO shows that the degree of charge transfer permitted by the CO coordination geometry also plays a large role in determining the absolute frequency of bound CO. Occluded by these dominant effects of the CO 2π* state is a small contribution from the CO 5σ state that regulates the 3d-2π* interactions. Because the CO 5σ orbital is completely full, the 3d-5σ interactions are largely repulsive. Reducing these interactions allows CO to approach closer to the surface and improve the spatial overlap between the attractive 3d-2π* interactions.47,71 As shown in Figure 10 and discussed previously, this reduction occurs with increasing Au coverage because the upper part of the antibonding (3d-5σ)* band shifts above the Fermi level. Without electrostatic effects, this phenomenon contributes a small redshifting component to the CO stretch frequency for fixed adsorption site, but it can become blueshifting when it activates electrostatic effects at high Au coverage, which is discussed below. It suffices to say that the true nature of the 5σ is enigmatic. Continuing with a deeper analysis of the frequencies, Figure 11 is a plot of the average CO 37



frequency as a function of Au coverage. The ordinate is calculated by averaging over the fitted center frequencies (Figure 5) weighted by their fitted intensities (Figure 6). It is equivalent to an intensity-weighted average of all CO adsorption vibrational stretch frequencies spanning 18002200 cm-1 while neglecting the different CO adsorption sites. The weighted average CO stretch frequency increases monotonically in a highly nonlinear way, indicating a nontrivial modification of the surface electronic structure by the Au atoms. The low slope between 0.05 and 0.29 ML Au shows that the effective bond order of the CO molecule, as reported by the weighted average CO stretch frequency, has not changed much. At 0.29 ML Au, the behavior of the curve abruptly changes, coincident with a geometric phase transition occurring on the surface alloy. Plotting the average frequency highlights this dramatic change that is not readily apparent in the HREEL spectra. The rapid increase in average frequency indicates a significant increase in CO bond order, consistent with the surface becoming progressively less adsorptive towards CO due to Au. At the saturation value of 0.79 ML Au, the extrapolated value is 2120 cm-1, which is comparable to the CO vibrational frequency measured on roughened Au(111)-derived surfaces.32,72

This coincidental result is interesting given the very different experimental

conditions used. From 0.29–0.71 ML Au, the approach towards 2120 cm-1 seems asymptotic.

Figure 11. Average adsorbed CO stretch frequency across all adsorbed CO species. Error bars are 2σ. 38


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The demonstration of a connection between CO and the geometric phase transition of the AuNi(111) surface at 0.3 ML involving its top two layers of atoms is significant. Up to this point – despite CO being in intimate contact with the surface – it has been ambiguous whether CO is sensitive to any morphological details of Au-Ni(111) beyond the approximation that its surface layer is a hexagonally closed-packed array of random Au and Ni atoms. This effort now evidences and clarifies that CO is indeed sensitive to even morphological changes beneath the surface layer, and one manifestation of it is in the average adsorbed CO stretch frequency across all adsorbed CO species. 2. Absolute Frequency Shifts for Fixed Adsorption Site as a Function of Au Coverage Unlike the theoretical behavior ascribed to surface-CO considerations alone, the experimental CO stretch frequency with increasing Au coverage for fixed adsorption site actually decreases (Figure 5) due to CO coverage dependent, CO-CO lateral interactions that originate from the nonzero vibrational polarizability of CO.73 With increasing Au coverage, the decrease in lateral interactions causes the CO frequency to drop faster than the frequency gain that can be had as the surface-CO bond weakens from reducing the degree of 3d-2π* orbital mixing. The drop can be rationalized by considering Ni(111) whose CO saturation coverage and dipole coupling between CO molecules is highest and hence, whose blueshift is maximized.

With increasing Au

coverage, less CO is adsorbed at saturation coverage, so the blueshift is reduced. This reduction also occurs if the CO coverage is lowered relative to saturation for a fixed Au coverage.

Figure 12. Schematic showing the energy span of the CO 2π* derived bands on: (a) Ni(111) at high CO coverage. (b) Ni(111) at low CO coverage. (c) Au-Ni(111) at low CO coverage. Dotted lines indicate the derived band centers. As both the surface-CO and CO-CO interactions change with Au coverage and affect the absolute CO frequency, determining the absolute blueshift is not trivial. However, it is still useful to estimate the blueshift tendency qualitatively74 and understand how the two interactions 39



contribute to it. In Figure 12, the CO 2π* derived band is shown with the correct relative energetic placement of the 2π* band centers at different CO75,76 and Au coverages. The 2π* states below the Fermi level can be tracked with surface Penning ionization electron spectroscopy.77 The many lateral interactions on Ni(111) at saturation coverage result in a broadened band that is shifted above the Fermi level (Figure 12a). The broadening originates from the orbital mixing between CO molecules, which is distinct from that incurred by a single CO orbital interacting with the metal surface. The shift is away from the Fermi level because the lateral interactions are repulsive and destabilizing. A comparison of Figures 12a and 12b shows how the degree of blueshifting is reduced from its maximum possible extent.

If the CO surface concentration is decreased, the CO-CO

interactions and its corresponding 2π* bandwidth decreases. The 2π* band center also moves closer to the Fermi level, so the fraction of the 2π* band above the Fermi level decreases, resulting in less blueshift of the CO frequency. Under influence of Au, a comparison of Figures 12b and 12c (which both have the same CO surface concentration) shows that the band center shifts further away from the Fermi level due to less favorable Au-CO interactions compared to Ni-CO, which leads to the CO frequency to blueshift, opposite that of decreasing the CO concentration at fixed Au coverage. Their relative magnitudes determine the observed frequency dependence of CO adsorbed on Au-Ni(111) at saturation as a function of Au coverage. As emphasized, the lateral interactions are empirically dominant over the surface-CO effects. There is an interesting consequence of this competition between the surface-CO and CO-CO interactions. The two effects causing the CO frequency to shift in opposite directions have a tendency to keep the center of the 2π* states around the same energy above the Fermi level for a surface of any Au coverage with its corresponding saturation CO coverage, which is a feature reflected in Figure 9. That the 2π* level stays approximately the same despite the changes occurring with Au coverage can be placed on a more quantitative footing by noting that the energetics of CO adsorption on Ni(111) at saturation is similar to CO adsorption on Au-Ni(111) at high Au coverage where the CO concentration is low, which is a consequence of the 40


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adsorption process being strongly linked to the involvement of the 2π* orbitals. To wit, the adsorption energy of CO in the atop site of Au-Ni(111) at 0.57 ML Au is 0.82 eV,13 which is nearly identical to the CO adsorption energy of 0.87 eV78 at 0.53 ML CO on Ni(111) (which is nearly the saturation coverage of 0.57 ML CO). At intermediate Au coverages, the adsorption energy is assumed to contain a weighting of the two contributions. In summary, because the CO binding energy is not changing very much with Au coverage, the absolute frequency shift is dominated by lateral interactions. 3. Origin of Adsorbed CO Frequencies Exceeding the Gas Phase Value The phenomenon of CO behaving as a ligand with frequencies exceeding the gas phase value79,80 is sometimes described as “non-classical”81 because simple molecular orbital arguments alone are insufficient to account for it. However, non-classical bonding, such as observed for CO adsorbed with frequencies between 2150-2160 cm-1 at high Au coverages on Au-Ni(111), can be rationalized as the competing effects of a combination of three factors: strong surface-5σ interactions, weak surface-2π* interactions, and strong electrostatic effects. On Au-Ni(111), the non-classically bound CO adsorption site must contain both Au and Ni atoms because such CO moieties are unknown on both Ni(111) and Au(111). The Au6Ni adsorption site is the simplest site assignment consistent with the stated factors for non-classical CO adsorption. Consider a gas phase CO molecule whose stretch frequency is 2143 cm-1. Removing an electron from its 5σ orbital yields [CO]+, whose stretch frequency is higher at 2214 cm-1, despite its bond order having decreased from 3 to 2½. Because the electron density of the 5σ orbital is biased towards the C atom, its removal upon ionization results in a C atom that is transiently more positively charged than the O atom. This instantaneous polarization then reduces the C-O equilibrium distance82,83 and modifies the curvature of the C-O bond interaction potential in a specific manner that involves the second derivatives of the electric dipole moment and electric polarizability along the vibrational coordinate.84 The magnitudes of these contributions are such that the frequency increases for [CO]+ compared to CO. This example shows that electrostatic effects can override the correlation between decreasing CO stretch frequency and increasing 2π* 41



orbital occupancy.67 In analogy to this example, if through adsorption, some fractional charge δ- is removed from the 5σ orbital that contributes to the original triple bond of CO, the resulting [CO]δ+ species could conceivably have a stretch frequency between 2143 and 2214 cm-1. Increasing the strength of the metal-5σ interaction is positively correlated with increasing the CO stretch frequency,85 which is achieved for CO on Au-Ni(111) when the fraction of antibonding (3d-5σ)* states orbitally mixed above the Fermi level is increased. This situation reduces the occupation of electrons in these antibonding states. Because the 5σ is so deeply bound compared to the Fermi level, this reduction can only occur at high Au coverages where the d-band is broad enough to reach and overlap with the 5σ state. Recall that only above 0.51 ML Au is CO observed with a stretch frequency above the gas phase value (Figure 5). Figure 10a shows a schematic of some (3d-5σ)* states that are repelled above the Fermi level,50,61 which lowers the 5σ orbital filling. Since the system remains charge neutral, the apparent loss of electrons in these states is exactly compensated by gains that occur in the lower lying orbitals, but these changes are not material to the discussion here. That empty antibonding (3d-5σ)* states contribute to CO bonding is typically masked by the bonding (3d-2π*) interactions, which are usually dominant. At high Au coverage, this dominance no longer holds because of the poor energetic overlap between a 2π* orbital with a 3d orbital that has acquired 5d character through orbital mixing in the Au-Ni(111) alloy. The 5d character of the Ni 3d orbitals is maximized for a Ni atom whose neighbors are all Au atoms. These arguments show that strong surface-5σ interactions do occur in conjunction with weak surface-2π* interactions at the Au6Ni adsorption site. This weakness is particularly important because it allows the electrostatic effect86 not to be obscured. Recall that the emptying of antibonding (3d-5σ)* states reduces the CO 5σ orbital filling. Because the (3d-5σ)* states are themselves orbitally mixed, the projected density of states of the Ni 3d electrons is also reduced.50 When the surface-CO interaction is dominated by the σ bond that is formed between the CO 5σ orbital and the Ni atom, the adsorption itself causes the CO 42


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molecule to be anchored to a slightly positively charged Ni atom, here denoted Niδ+. The positive charge on this anchor point is key. This arrangement of atoms can be thought of as a Niδ+–CO complex that favors the polarized (rather than neutral) resonance structures of CO, which then leads to CO adsorption frequencies above the gas phase value. Thus, it is important that forming this Niδ+ is energetically favorable; it is foremost aligned with the direction of charge transfer occurring from Ni to Au according to a Bader87 charge analysis of model AuNi(111) alloys.13 The arrangement of atoms in Au-Ni(111) with the highest propensity to stabilize this Niδ+ center is a Ni atom surrounded by 6 Au atoms. The formation of Niδ+, a positive metal center, in analogy to many metal-CO complexes, is ultimately the mechanism that creates an electric field that blueshifts the CO stretch mode,88 giving rise to surface-bound CO with measured frequencies of 2150-2160 cm-1 at high Au coverage. VI. Conclusions Carbon monoxide in a 0.57 ML saturated and ordered layer adsorbed on Ni(111) occupies bridge and atop sites in a 3:1 ratio at 80 K. As Au atoms are randomly substituted for Ni surface atoms to form a Au-Ni(111) surface alloy, they block adsorption sites and result in weakening of the Ni-CO surface bond. While CO continues to occupy atop and bridge Ni sites of the alloy, bridge site occupation drops more rapidly than predicted by an ideal geometric site blocking model, while the atop site occupation follows the predictions up to 0.51 ML Au coverage.

A

quantitative comparison (employing the experimentally measured configurational entropy) of the observed bridge site occupation to the ideal model demonstrates that the geometric site blocking effects are reliably separable from electronic effects despite the many changes in the electronic surface structure, in the CO orbital mixing with the substrate, and in the CO-CO lateral interactions as the Au coverage is increased. The electronic effects are responsible for removing up to 1.48 bridge CO sites per Au atom. The major contributor to these multi-site electronic effects is ascribed to the energetic decrease in the metal d-band position due to interactions of the Au 5d with the Ni 3d electrons. This modification of the Ni electronic structure with increasing Au coverage results in a less energetic increase in the CO antibonding (3d-5σ)* band above the 43



Fermi level than that of the bonding (3d-2π*) band, thereby weakening the surface-CO bond. Given the dominance of the 2π* orbital in governing the bonding of CO to the bridge site, an increase in the frequency of the internal CO stretch with Au coverage might be expected. However, diminishing CO dipole-dipole lateral interactions as a result of lower CO coverage override the emptying of the (3d-2π*) band causing the frequency to fall from 1960 to 1840 cm-1 as the Au coverage climbs to 0.71 ML. Moreover, in-depth analyses into the rate of change of the frequency dependence and the rate of change of the bridge site occupation with Au coverage also strongly suggest major perturbations on both by the reconstruction of the first subsurface layer of the Au-Ni surface alloy at 0.3 ML Au. In contrast to bridge bound CO, the atop site CO occupancy falls as predicted by the geometric site blocking model up to 0.51 ML Au. The atop site differs from the bridge site in substantially lower occupancy of the bonding (3d-2π*) band, while the occupancy of the antibonding (3d-5σ)* band is essentially unchanging. This fact, coupled with the larger effect of Au on the energetic position of the (3d-2π*) band relative to the antibonding (3d-5σ)* band, results in the relative insensitivity of the atop bound CO occupancy to the addition of Au up to 0.51 ML. The vibrational frequency of the atop CO decreases from 2110 to 2060 cm-1 as the Au coverage increases to 0.50 ML as a result of decreasing lateral interactions associated with lower CO coverage. Above 0.51 ML Au, the occupancy of atop CO at 2110 to 2060 cm-1 deviates from the geometric model, and a new CO feature, characterized by an internal stretch frequency of 21502160 cm-1 appears. It is assigned to CO bound to the atop site of Ni surrounded by 6 Au atoms in a Au6Ni configuration. Its higher frequency than that of gas phase CO is a result of an electrostatic effect that polarizes the CO charge distribution towards an effectively positively charged Ni atom. The charge depletion on the Ni atom arises both from the much higher electron affinity of Au compared to Ni as well as the continued increase in energy of the (3d-5σ)* band above the Fermi level as the Au coverage is increased. While the electrostatic effect is present even in the case of CO adsorbed on Ni(111), it is unmasked in the case of CO adsorbed 44


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on Ni in a Au6Ni configuration because of the substantially reduced (3d-2π*) bonding interactions. This study serves a multitude of purposes. It is foremost a demonstration of the depth and breadth of understanding that is accessible through a vibrational study of adsorbates on quantitatively-characterized surfaces. It defines and applies a precise notion of separability of geometric and electronic effects, derived from ideal adsorption. It advances a self-consistent picture of adsorption on a catalytically relevant surface. But more importantly, its overall success in enabling the description of the highly non-ideal behavior of CO adsorption on AuNi(111) represents a tangible step towards benchmarking and speculating about the behavior of other more complex systems yet to be considered and illustrates the intrinsic value brought forward in considering the detailed origins of the many surface-adsorbate, adsorbate-adsorbate, and electrostatic interactions in this system. This pursuit remains fundamental and acutely important given the ubiquity and importance of surface-related phenomena. AUTHOR INFORMATION *[email protected],

Phone: (617) 253-4537

ACKNOWLEDGEMENTS This research was supported by DoE, DE-FG02-05ER15665 and by the Shell-MITEI Seed Fund Program. C. C. Leon thanks the Natural Sciences and Engineering Research Council of Canada for a Postgraduate Scholarship PGS D3-374322-2009.

References (1) Lahr, D. L.; Ceyer, S. T. Catalyzed CO Oxidation at 70 K on an Extended Au/Ni Surface Alloy. J. Am. Chem. Soc. 2006, 128, 1800–1801. (2) Leon, C. C.; Lee, J. G.; Ceyer, S. T. Oxygen Adsorption on Au-Ni(111) Surface Alloys. J. Phys. Chem. C 2014, 118, 29043–29057. (3) Nørskov, J. K. Electronic Factors in Catalysis. Prog. Surf. Sci. 1991, 38, 103–144.

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A Topological Theory of Molecular

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