Density Functional Theory Study of the Adsorption of Nitrogen and

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Density Functional Theory Study of the Adsorption of Nitrogen and Sulfur Atoms on Gold (111), (100), and (211) Surfaces April D. Daigle and Joseph J. BelBruno* Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, United States ABSTRACT: Nitrogen and sulfur atom adsorption on flat and stepped gold surfaces are examined by density functional theory. With detailed investigation of adatom location on the (111), (100), and (211) gold surfaces, nitrogen and sulfur atom adsorption are compared with reference to previous work on the oxygen/ gold system. Sulfur adsorbed most strongly, followed by oxygen and nitrogen. The results demonstrate the preference for 3-fold over 2-fold and single-fold adatom coordination as well as the role of low coordinated gold surface atoms in increasing the adsorption energy for nitrogen, sulfur, and oxygen atoms. Pseudopotential curves, calculated adsorption energy as a function of surface position, and nudged-elastic band calculations explored adatom diffusion along the surface. The results indicated limited diffusion on the (111) and (211) surfaces. On the other hand, while nitrogen and sulfur atoms remained localized on the (100) surface, oxygen atoms showed facile diffusion. These results provide a reference for the interaction of nitrogen, sulfur, and oxygen adatoms with gold nanoparticles that project faces similar to the surfaces studied here.

’ INTRODUCTION Gold has long been known and prized as a noble metal and yet has surprising catalytic ability on the nanoscale. Since the seminal discovery of gold nanoparticle catalysis for the oxidation of carbon monoxide at low temperatures,1 gold nanocatalysis has been applied to both oxidative and reductive chemistry. These studies focused on oxygen-containing systems, especially carbon monoxide oxidation, and included the oxidation of styrene to benzaldehyde,2 the oxidation of ethanol to acetaldehyde,3 the hydrogenation of acrolein,4,5 and the reduction of O2 to H2O2 and H2O.6 During the past several years, experimental investigation of gold catalysis has been extended to nitrogen chemistry. Reports include the oxidation of amines to imines,79 the oxidation of aniline to azo compounds,10 the alkylation of primary amines to secondary amines,11 the hydrogenation of nitro compounds,10,12 and the coupling of amines and aldehydes to yield amides.13 The utility of gold nanoparticle adsorbate interactions beyond oxygen-containing molecules is of interest not only on a fundamental level but also becasue of the important role of nitrogen containing compounds in natural products and pharmaceuticals. A description of the morphology of the catalytic surface, the preferred adsorption sites, and the facility of adsorbate diffusion is fundamental in the analysis of catalysis and other surface phenomena. With nanoparticles, a distribution of sizes is present, and the shape of particles in a given size range varies. However, small gold nanoparticles often exhibit facets identical to cuts from the gold crystal. As a result, surface chemistry provides insight into nanoparticle systems and offers a more defined system of study. Although other flat surfaces may be considered, the most stable gold surfaces are the (111) and (100) crystal planes. Moreover, these two surfaces are combined in the stepped (211) r 2011 American Chemical Society

surface, which is constructed of (111) crystal plane terraces and (100) crystal plane steps. Using these three surfaces, one may provide a model for gold nanoparticles, whose stable bulk crystal plane (111) and (100) facets meet at junctions represented by the (211) step. Compared to the oxygen/gold system, relatively few studies of nitrogen or sulfur atom adsorption on gold have been completed. Due to broad interest in thiol self-assembled monolayers on gold as well as the role of sulfur-containing compounds as a poison in catalytic reactions, a handful of investigations have reported details of sulfur atom adsorption on certain gold surfaces;1419 however, all but one limits examination to the most stable (111) surface. Reports of nitrogen atom adsorption on gold are sparse. Moreover, the available studies do not provide a full examination of binding sites and are limited exclusively to the (111) surface.20,21 A thorough theoretical description of nitrogen and/ or sulfur adsorption on gold is absent. In this study, we facilitate comparison of gold surface interactions with nitrogen to the better known oxygen and sulfur systems. Employing the PBE functional with Gaussian basis sets and core pseudopotentials, we investigate adsorption of nitrogen and sulfur atoms on gold (111), (100) and (211) surfaces. Through optimized calculations at a variety of adsorption sites, pseudopotential curves among these sites, nudged-elastic band calculations of barriers to diffusion along the surface, and postprocessing Lowdin and Mulliken charge analysis, we generate a consistent set of detailed energetics. Here, we present the results of the study of nitrogen and sulfur atom adsorption on the Received: July 26, 2011 Revised: October 10, 2011 Published: October 11, 2011 22987

dx.doi.org/10.1021/jp2071327 | J. Phys. Chem. C 2011, 115, 22987–22997

The Journal of Physical Chemistry C gold (111), (100), and (211) surfaces and compare these results to our previously reported findings for the gold/oxygen system.22

’ COMPUTATIONAL METHODS Density functional theory was implemented in the supercell quantum chemistry code, SeqQuest,23 developed by Schultz and co-workers at Sandia National Laboratories. SeqQuest reduces computing time with algorithms for generation of the Hamiltonian matrix that scales linearly with system size. The PBE functional was applied with functional specific SeqQuest basis sets, defined using contracted Gaussian functions. The PBE functional24 is appropriate for the current problem, since it is known to capture some weak interactions that may be present in surface adsorption. This functional is nonempirical and was developed by applying fundamental DFT principles. It has been shown to be suitable for the determination of total energy dependent properties. Core electrons are described by pseudopotentials. Hard pseudopotentials were generated using the code of Fuchs and Scheffler25 employing the Hamann type of potential26 for gold and sulfur and the Troullier/Martins type27 for nitrogen. Valence electrons were represented by a Gaussian double-ζ basis set with polarization, optimized variationally.23 Adsorption of nitrogen and sulfur on the gold surfaces was optimized without constraints. Gamma-point calculations, using large supercell sizes, provided convergence of relative energetics to within 0.12 eV and are reported here. Absolute binding energies will converge with both increased k-grid sampling and supercell dimensions. However, simultaneous unlimited increases in both parameters for supercells of the size employed in this study are not feasible. We have made tests of the relative effects of increasing the k-grid sampling. Comparison of gammapoint results with our large supercells and results using a 5 5 1 k-point grid for the (111) and (100) surfaces with equivalent surface dimensions, the dimensions of the (111) cell, indicated that relative adsorption energies were identical to within 0.12 eV. The (211) surface could not be included in this test because a surface size equivalent to that employed for the (111) surface was too small for creating all of the adsorption sites reported here. Given our test results, we report the gamma-point data at maximal supercell dimensions. The SeqQuest default convergence criterion, 0.00050 Ry, was used for all reported energies. Test calculations varying this parameter by as much as a factor of 10 resulted in energy differences of the order of 103 Ry. The integration point grid was evenly distributed in all dimensions with spacing of approximately 0.28 bohr. Spin polarization may have a significant effect on the binding energies. Energies were calculated with and without the effects of spin polarization; inclusion of spin polarization lowered the adsorption energies significantly, but the reduction was due solely to the free atom. Our reported energies take into account spin polarization of the free atom. Optimization calculations were completed for nitrogen and sulfur atom adsorption in the symmetric sites on the (111), (100), and (211) surfaces of gold. In addition, pseudopotentials were constructed using the fully optimized bare slab in a series of single point calculations of adatom binding along pathways connecting the sites. These pseudopotentials report the adsorption energy at the optimum distance of the adatom from the gold surface using a grid spacing of 0.1 bohr. Binding along these pseudopotentials was typically 0.3 eV weaker than with full optimization, reflecting the role of surface relaxation in response

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to the adsorption event. Barriers to adatom diffusion along the surface were investigated further with nudged elastic band (NEB) calculations, completed in the climbing mode, which allows an increase in energy for the highest energy image in order to determine the barrier height. Twelve images were utilized, and steps were continued until, for five consecutive steps, the energy of all images changed by no more than 0.00050 Ry, the convergence criterion for geometry optimization. The Mulliken and Lowdin charges were calculated along the pseudopotential trajectories. In order to facilitate comparison, these methodological selections were chosen to be consistent with our previously reported results for oxygen atom adsorption on gold surfaces.22

’ RESULTS AND DISCUSSION Stable surfaces of the bulk metal are often found as facets of the nanoparticle, with junctions among these facets sharing features with stepped surfaces. As a result, close-packed and stepped surfaces serve as models for nanoparticles. Here, we investigate the adsorption of nitrogen and sulfur atoms on gold (111), (100), and (211). Adsorption energy is defined by eq 128 Eads ¼ Eadatom-surf Esurf Eadatom

ð1Þ

where Eads is the adsorption energy (with negative values indicating a favorable process), Eadatom‑surf is the energy of the (optimized) adatom plus slab, and Esurf and Eadatom are the energies of the relaxed slab and the atom in the free state. A detailed examination of nitrogen atom and sulfur atom interaction with gold is made through geometry relaxation calculations, single-point calculations, nudged-elastic band analysis and postprocessing charge analysis. After presenting the results, comparisons between nitrogen and sulfur atom adsorption are drawn with reference to previously reported results of the oxygen atom/ gold system.22 Au (111) Surface. The (111) crystal plane forms the most stable surface for gold. The surface has a close-packed arrangement with four unique sites: fcc hollow (FH), hcp hollow (HH), bridge (B), and ontop (O). The FH and HH sites provide 3-fold coordination with surface gold atoms while the B and O sites have 2-fold and single-fold coordination with the surface, respectively. These sites are depicted in Figure 1a. All four sites were investigated using a 3 2 2 supercell, which yields 0.08 ML coverage. The 72 atom supercell has 12 atoms in each of 6 layers, and was selected for the gold/oxygen system by comparison to larger unit cells as described previously.22 A check with the nitrogen/gold and sulfur/gold systems was made by increasing the width as well as the depth of the cell; the resulting variation in the differences in adsorption energies among the sites was less than 0.1 eV for all but the most weakly bound O site, whose adsorption energy relative to the strongest FH site varied by no more than 0.2 eV. The nitrogen adsorption energies for the FH, HH, B, and O sites are provided in Table 1. The fcc hollow site is the preferred binding site with an adsorption energy of 2.28 eV. Binding at this site is 0.12 eV stronger than at the HH site, 0.41 eV stronger than at the B site, and 1.77 eV stronger than at the O site. Nitrogen distinctly prefers 3-fold coordination to gold on this surface. In spite of the shortest AuN distance for the O site, 1.88 Å, adsorption is considerably weaker. Similarly, the 2.02 Å average AuN distances for the B site do not provide higher adsorption energy than the slightly longer 2.05 and 2.07 Å average AuN distances for the HH and FH sites, respectively. Shorter AuN 22988

dx.doi.org/10.1021/jp2071327 |J. Phys. Chem. C 2011, 115, 22987–22997

The Journal of Physical Chemistry C

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Table 1. Nitrogen Adsorption Energies, Coordination Numbers and Distances for the (111), (100), and (211) Gold Surfaces adsorption

coordination

coordination

nitrogengold

sitea

energy, eV

no. (N)

no. (Au)

distance, Å

FH

2.28

3

9

2.06, 2.06, 2.09

HH

2.16

3

9

2.04, 2.06, 2.06

B O

1.87 0.51

2 1

9 9

2.01, 2.02 1.88

H

1.95

4

8, (12)

2.23, 2.23, 2.23,

B

1.55

2

8

2.00, 2.00

O

0.20

1

8

1.88

3

2.12

3

7,9

2.05, 2.05, 2.10

1

1.92

3

7,9

2.10, 2.07, 2.07

5 4

1.86 1.83

2 2

7 7

2.06, 2.06 1.98, 1.98

7

1.70

3

9,10

2.06, 2.09, 2.09

2

1.52

2

7,9

2.01, 2.02

6

1.16

2

7,10

1.97, 2.05

(111)

(100) 2.23, (3.06)

(211)

a

Definitions of the sites are provided in the text; coordination numbers in order with respect to NAu distances.

Figure 1. Adatom sites on the (a) Au (111) surface, (b) Au (100) surface, and (c) Au (211) surface. The adatom is blue. Au is gold with lighter shades for deeper layers.

distances do not compensate for lower nitrogen coordination to the gold surface. The gold atoms in all sites on this surface are coordinated with nine neighboring gold atoms. The preference for the FH binding site and the ordering of the adsorption sites are in agreement with available studies of the N atom/Au (111) system, though reports are sparse.20,21 Using the PBE functional with a plane-wave basis set in STATE, Wang et al. report the FH site as preferred with an adsorption energy of 2.34 eV on a six layer slab at 0.17 ML coverage.20 Using a ten atom cluster model, the FH, HH, B, and O sites were investigated. Wang et al. note a FH > HH > B > O binding energy ordering, in agreement with our slab results. De Vooys et al. assume binding at the FH site, and model adsorption on a three layer slab with 0.25 ML coverage, but allow no relaxation of the gold surface.21 Using the plane wave VASP software package, they report a 1.68 eV adsorption energy. The less favorable binding may be due to the lack of gold relaxation. These reports are consistent with respect to preferred binding

site, but give significantly lower adsorption energies than we report. To probe the barriers to diffusion along the (111) surface, pseudopotential energy curves were obtained via single point energy calculations along pathways among the FH, HH, B, and O sites. Binding at the reported sites on the (111) surface was an average of 0.3 eV weaker than in the geometry optimization calculations. This is expected since relaxation of the gold surface should contribute favorably to the adsorption energy. The pseudopotential energy curves are shown in Figure 2a. Starting at the lowest energy FH site and moving to the B site (along the green triangles), there is a smooth and steady increase in energy; a similarly smooth and steady decrease in energy is seen moving from the B site to the HH site (along the purple diamonds). Using the optimized energies, the barrier to diffusion from the FH to HH site over the B site is 0.41 eV; the barrier for the reverse path is 0.29 eV. These significant barriers make diffusion of a nitrogen atom along the gold (111) surface unlikely at reasonable temperatures. Paths from the FH, HH, and B sites to the O site are also smooth, but substantially steeper, with a 1.77 eV barrier to diffusion out of the FH site, over the O site. This prohibits diffusion of nitrogen over the O site. The barrier to diffusion between the HH and FH sites was confirmed with nudged elastic band calculations. Using twelve images spaced approximately 0.1 bohr apart, barriers of 0.51 eV from FH to HH and 0.33 eV from HH to FH were found. These barriers are comparable to those found for the nonrelaxed pseudopotential path. Lastly, Mulliken and Lowdin charges were calculated along the trajectories. The charge on the nitrogen atom varies little along the surface; the range of Lowdin charges is 0.09e. The Lowdin charge is no larger than 0.37e, the value in the FH and HH sites, 22989



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Table 2. Sulfur Adsorption Energies, Coordination Numbers and Distances for the (111), (100), and (211) Gold Surfaces adsorption

coordination

coordination

sulfurgold

sitea

energy, eV

no. (S)

no. (Au)

distance, Å

FH HH

3.94 3.77

(111) 3 3

9 9

2.42, 2.42, 2.46 2.43, 2.43, 2.46

B

3.56

2

9

2.39, 2.40

O

2.55

1

9

2.28

(100) H

3.53

4

8, (12)

2.55, 2.55, 2.55,

B

3.10

2

8

2.36, 2.36

O

1.99

1

8

2.27

2.55, (3.59)

(211) 3

3.60

3

7,9

2.40, 2.40, 2.50

5

3.53

2

7

2.39, 2.39

1

3.43

3

7,9

2.42, 2.44, 2.44

2

3.21

2

7,9

2.37, 2.42 2.36, 2.46

6

2.84

2

7,10

4

(site 3)

4

7

7

(site 5)

3

9,10

a

Definitions of the sites are provided in the text; coordination numbers in order with respect to SAu distances.

Figure 2. (a) Nitrogen atom and (b) sulfur atom adsorption energies for trajectories on the Au (111) surface.

indicating a slight transfer of charge to the nitrogen atom from the gold surface. Sulfur atom adsorption was found to be stronger than nitrogen atom adsorption; the adsorption energies for sulfur are reported in Table 2. Among the sites, FH is preferred with an adsorption energy of 3.94 eV. The FH site adsorption energy is 0.17 eV stronger than at the HH site, 0.38 eV stronger than at the B site, and 1.39 eV stronger than at the O site. In all sites, sulfur binds at a greater distance from the surface than nitrogen, which could be anticipated due to its larger atomic radius. The AuS distance is the shortest for the O site at 2.28 Å in spite of the more than 1 eV weaker binding here than in the FH or HH sites where the average AuS distances are 2.43 and 2.44 Å, respectively. Sulfur, like nitrogen, prefers 3-fold coordination with gold to 2-fold or single-fold coordination. The surface gold atoms in all of the sites are coordinated with nine gold neighbors. As the most stable gold surface, the (111) crystal face has been the focus of most experimental and theoretical studies. Our results for this close-packed surface provide a point of comparison with previous reports. Using a plane-wave basis set and the PBE functional in ESPRESSO, Cometto et al. report most favorable sulfur atom adsorption in the FH site followed by the HH and B sites for a four layer slab, in agreement with our results.16 They find the difference in FH and HH adsorption energies to be approximately 0.3 eV compared to our 0.17 eV difference and the difference between the FH and B sites to be approximately 0.4 eV compared to our 0.38 eV difference,

showing good agreement. Rodriguez et al. studied multiple binding sites with a four layer slab and various coverages using a plane-wave basis set in CASTEP with the PW91, PBE, and RPBE functionals. PBE values at the smallest 0.25 ML coverage are compared here.17 In agreement with our results, they find a binding order of FH > HH > B > O, with an adsorption energy of 3.72 eV for the FH site. Differences in adsorption energy from the FH site of 0.15, 0.41, and 1.55 eV for the HH, B, and O sites, respectively, are in good agreement with our 0.17, 0.38, and 1.39 eV differences. Gottschalck and Hammer investigated multiple adsorption sites using a plane-wave basis set and the RPBE and PW91 functionals, but did not allow relaxation of the geometries.19 They found a 3.51 eV FH site adsorption energy with the RPBE functional. Other studies utilized a plane-wave basis set, and reported adsorption energies for the FH site only: 3.23 eV at 0.25 ML coverage using the RPBE functional in CASTEP18 and 3.38 eV at 0.17 ML coverage using the PBE functional in STATE.15 Our coverage is significantly less than in these studies. Since increased coverage decreases adsorption energy,17 it is expected that we would report greater adsorption energies. It should be noted that our SeqQuest calculations applied a Gaussian basis set and the PBE functional, which also gave higher adsorption energies than plane-wave basis sets methods for the oxygen/gold system, consistent with the current results.22 While our adsorption energies are slightly greater, trends in adsorption and the preferred adsorption site are in good agreement with all previous reports. Single-point calculation results for trajectories among the sites are displayed in Figure 2b. Starting at the preferred FH site and moving to the B site (along the green triangles) and then to the HH site (along the purple diamonds), one can see a steady increase in adsorption energy to the B site and a decrease from the B site to the HH site. No additional barriers beyond the 22990


The Journal of Physical Chemistry C differences in energy at the labeled sites are found. Cometto et al.16 traced similar pseudopotential curves with the same result. Using optimized energies, there is a 0.38 eV barrier for diffusion from the FH to the HH site and a 0.21 eV barrier for the reverse path. The pseudopotential energy curves to the O site are also smooth with steady increases in energy but are much steeper. The barrier from the FH to the HH sites over the ontop site is 1.39 eV; the reverse path has a 1.22 eV barrier. Diffusion to the FH site is possible at sufficient temperatures via the bridge site, but diffusion to the HH site is unlikely. The greater than 1 eV barrier for diffusion over the O site inhibits this pathway. Nudged elastic band calculations confirm these barriers giving identical results for diffusion between the FH and HH sites. Mulliken and Lowdin charges were calculated. The range of values spanned 0.2e, from 0.1e for the O site to +0.1e for the FH site. Clearly, there is negligible transfer of charge upon binding of sulfur to this surface. Au (100) Surface. The (100) surface is the second most stable gold surface, but much less studied than the (111) surface. Gold atoms on the (100) surface have eight nearest neighbors; they are less coordinated than atoms on the (111). Here, we use a 5 5 3 supercell which was selected as the minimum size supercell for the oxygen/gold system. A check of the nitrogen/gold and sulfur/gold systems was made with supercells of decreased depth and width; the variability in adsorption energy differences was found to be less than 0.15 eV. This supercell is 10 10 atoms wide and 6 layers thick, matching the number of layers in the (111) supercell. The 5 5 3 supercell contains 300 atoms and provides 0.02 ML coverage. There are three unique adsorption sites: hollow (H), bridge (B), and ontop (O). An atom in the H site has 4-fold coordination to surface gold atoms, those in the B site have 2-fold coordination, and an atom in the O site has single-fold coordination. All of these sites are considered in this study, and are depicted on a portion of the supercell in Figure 1b. Adsorption energies and distances to the nearest gold atoms are provided in Tables 1 and 2 for nitrogen and sulfur, respectively. Nitrogen adsorption is most stable at the H site with an adsorption energy of 1.95 eV. Adsorption at the H site is 0.40 eV more favorable than at the B site and 1.75 eV more favorable than at the O site. At the H and B sites, the geometry is symmetric with AuN distances of 2.23 and 2.00 Å, respectively. The 3.06 Å AuN distance to the second layer gold atom directly below the H site indicates there is little interaction with this atom, since a survey of reported AuN distances in large molecules gave typical AuN distances of ∼2 Å.29 Nitrogen prefers greater coordination with gold, with the 4-fold hollow site preferred to the 2-fold and single-fold sites. The greater coordination appears to compensate for longer AuN distances since the shortest distance is found where adsorption is the weakest, the O site with a 1.88 Å AuN distance. We are not aware of previous work for this nitrogen atom/gold (100) surface system. In addition to these optimized adsorption energy considerations, single point energies along trajectories among the sites were completed and are reported in Figure 3a. The curves are smooth and show no barrier to motion along the surface beyond the difference in adsorption energies between the sites. The 0.4 eV optimized barrier between the H and B sites poses a significant enough barrier to deter diffusion along the surface at reasonable temperatures. Diffusion of nitrogen from H site to H site over the O site is precluded by the nearly 2 eV barrier. Nudged elastic band calculations yield a 0.43 eV barrier to diffusion from H site

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to H site over the B site, confirming the pseudopotential barrier value. Charges along these pathways were also calculated. A typical Lowdin charge of 0.4e was found with little variation along the surface: the range of values spanned only 0.1e. Charge transfer to the nitrogen atom is of comparable magnitude as that reported for the oxygen atom, which had an average charge of 0.5e. As with the (111) surface, sulfur adsorption was found to be stronger than nitrogen. The H site is preferred with an adsorption energy of 3.53 eV. Adsorption at the B and O sites is weaker by 0.43 and 1.54 eV, respectively. Binding is symmetric for all three sites with AuS distances of 2.55, 2.36, and 2.27 Å for the H, B, and O sites, respectively. For the H site, the 3.59 Å AuS distance to the gold atom directly below the site is larger than most AuS distances in complex molecules which are in the range of ∼23 Å.30 The interaction with this fifth atom is, therefore, minimal. As with nitrogen, greater coordination of sulfur adatoms to gold surface atoms is preferred and appears to compensate for larger AuS distances. Recently, Slusarski and Kostyrko reported sulfur atom adsorption on this surface implementing the PBE functional in SIESTA.14 For a 3 4 atom repeat unit of nine layers, they found preferred adsorption at the H site with ∼5 eV binding energy. Using pseudopotential energy curves similar to those we report below, they demonstrated weaker adsorption at the B and O sites. Adsorption energies at the B and O sites were found to be approximately 1 and 2.5 eV weaker, but greater than our single point calculation differences of 0.45 and 1.36 eV. 22991


The Journal of Physical Chemistry C The results of our pseudopotential calculations for sulfur on the gold (100) surface are shown in Figure 3b. From the H site, the adsorption energy decreases steadily to the B site (along the black squares) and to the O site (along the blue triangles). It is apparent that no barrier beyond the difference in adsorption energies of the sites is present along the pathways investigated. Thus, diffusion along these paths is governed by the 0.43 eV H site to B site and the 1.54 eV H site to O site optimized energy barriers making diffusion improbable at reasonable temperatures. Nudged elastic band calculations confirm this result, with a 0.45 eV barrier to diffusion between H sites over B sites. Mulliken and Lowdin charges were also calculated along the pseudopotential energy curves. Lowdin charges ranged from 0.15e to +0.1e, revealing little redistribution of charge between the gold slab and sulfur adatom. Au (211) Surface. The (211) surface is a stepped surface; it combines (111) surface terraces with (100) surface steps. The step introduces surface gold atoms with a low coordination number, which are believed to drive the increased catalytic capability of gold nanoparticles compared to bulk gold.31 As a result, extrapolations to adsorbate interaction with gold nanoparticles can be made from the (211) surface model. A 270 atom 5 3 3 supercell was used in this study. It contains three terraces of five atom width and six layers. Selection of the cell has been described previously in the context of the oxygen atom/ gold surface system.22 Variation of nitrogen and sulfur adsorption energy differences among the sites was less than 0.12 eV for changes in supercell width and depth for all but site 1 for nitrogen (with a 0.18 eV difference) and the most weakly bound sulfur site, 7 (which shows variability of 0.67 eV), for a decrease in terrace width. Seven sites on the surface were selected for detailed study; they are depicted on a portion of the supercell in Figure 1c. Sites 1 and 7 are fcc hollow sites on (111) terraces, site 1 is at the top of the step and site 7 is at the bottom of the step. Site 5 is a “hanging fcc hollow” site located at the step where an fcc hollow site would occur were the upper terrace extended over the lower terrace. Site 3 is an hcp hollow site on the terrace at the top of the step while sites 2 and 4 are bridge sites, site 2 on the terrace and site 4 at the edge of the step. Lastly, site 6 is nominally a singly coordinated ontop site at the base of the step where the gold atoms are more highly coordinated. A range of coordination numbers for both surface gold atoms and the adsorbed atom are sampled with these sites. The gold and adatom coordination numbers for these sites are given in Tables 1 and 2 along with the adsorption energies and gold-adatom distances. Results for the (211) surface are reported without comparison to previous reports since this is the first discussion of nitrogen or sulfur atom adsorption on this surface. Nitrogen atom adsorption is stronger for sites with higher nitrogen coordination and lower goldgold coordination. Adsorption is greatest at site 3, 2.12 eV, where there are both 3-fold nitrogen coordination to gold and reduced coordination of the gold atoms. The fcc hollow site, 1, has an adsorption energy lower by 0.2 eV. The fcc > hcp hollow ordering of the (111) surface is reversed on this (111) terrace of the (211) surface. The (111) terrace differs from the (111) surface by exhibiting lower coordination of the gold atoms located at the step, two of which make up the hcp hollow site and only one of which is part of the fcc hollow site. Adsorption at site 3 is slightly asymmetric with 2.05 Å AuN distances to the lower coordinated gold atoms and a 2.10 Å AuN distance to the more highly coordinated gold atoms. On the other hand, site 1 shows a closer approach to the

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more highly coordinated gold atoms, 2.07 Å, while maintaining a greater, 2.10 Å, AuN distance to the lower coordinated gold atom. This unexpected approach to more highly coordinated gold may contribute to the lower binding energy at fcc hollow site 1 than at the hcp hollow site 3. Adatoms at sites 5 and 4 are next most strongly bound and are degenerate with binding energy 0.28 eV lower than at the preferred adsorption site. Though these sites have only 2-fold coordination of nitrogen to gold, the gold atoms have coordination numbers reduced by two from the flat (111) surface and the nitrogen atom has a closer approach to the surface with symmetric AuN distances of 1.98 Å and 2.06 Å for sites 4 and 5, respectively. These sites are closely related; site 5 is simply pushed back from the step. The lowest energy adsorption sites, sites 7, 2, and 6, adsorb with energies lower than the most favored site by 0.42, 0.60, and 0.96 eV, respectively. Among these low energy sites, site 7 involves the most highly coordinated gold atoms, but is 3-fold coordinated to the gold surface and has the strongest adsorption energy. The binding has a slight asymmetry with a closer approach to the gold atom with 9-fold coordination, 2.06 Å, than to the gold atoms with 10-fold coordination, 2.09 Å, in keeping with the preference for adsorption to low coordinated gold atoms. Sites 2 and 6 have 2-fold coordination to gold. They differ by 0.36 eV, with weaker binding for site 6 which involves a more highly coordinated gold atom. Adsorption at site 6 is asymmetric with a 1.97 Å AuN distance to the 7-fold coordinated gold atoms and a 2.05 Å AuN distance to the 10-fold coordinated gold atom. Site 2, on the other hand, is symmetric with a 2.02 Å AuN distance to the 7-fold and 9-fold coordinated gold atoms. The slightly shorter 1.97 Å AuN distance to the low coordinated gold atom of site 6 apparently does not sufficiently compensate for the effect of binding to the 10-fold coordinated gold atom. The effect of low coordinated gold can be further demonstrated by comparing sites of the same adatom coordination type for varying gold atom coordination on this stepped surface. First, comparing the fcc hollow sites, we see stronger adsorption, by 0.22 eV, at site 1 versus site 7. The coordination numbers of the gold atoms are seven, nine and nine for site 1, but nine, ten and ten for site 7. The relative binding energies are in agreement with the trend of greater binding for decreased coordination of gold. Second, comparing hcp hollow sites, there is stronger adsorption at site 3 where gold coordination numbers are seven, seven and nine than at the hcp hollow site located at the base of the step (not one of the numbered sites) where gold coordination numbers are nine, nine, and ten. The difference in adsorption energy is 0.46 eV. Third, we compare bridge sites: site 4, site 2, and the bridge site at the base of the step (not a numbered site). These sites have adsorption energies of 1.83, 1.52, and 1.25 eV, respectively. While site 4 involves 7-fold coordinated gold atoms, site 2 involves a 7-fold and a 9-fold gold atom and the bridge site at the base of the step is made up of 9-fold and 10-fold coordinated gold atoms. Thus, we again observe that adsorption energy decreases as gold coordination number increases. Lastly, comparing the ontop site in the middle of the terrace (not a numbered site), at the edge of the step (site 4) and at the base of the step (site 6), we find that adsorption energy increases first by 0.37 eV and then by 1.04 eV. This correlates with changes in gold coordination number of nine to seven to ten. At first glance, the 1.04 eV increase in adsorption energy with increasing coordination from seven to ten appears to violate the preference for low coordinated gold. However, site 6, though nominally an ontop site, is sufficiently close to the step to allow for significant 22992


The Journal of Physical Chemistry C interaction with both the 10-fold coordinated gold atom below and the 7-fold coordinated gold atom in the step. Thus, the stronger adsorption energy reflects both 2-fold coordination of the nitrogen atom and coordination to a low coordinated gold atom. Comparison of the adsorption sites on the (211) surface demonstrates that binding to low coordinated gold atoms leads to stronger nitrogen atom adsorption. Though significant, it is not the sole factor determining to adsorption energy. One must also consider the coordination number of the atom. Extrapolating to nanoparticles, one might expect strongest binding where nitrogen atoms bind with high coordination number to low coordination number gold atoms. The interaction of nitrogen atoms with the step of the (211) surface was investigated in more detail through single point calculations. Pseudopotential energy curves from site 1 to sites 6 and 7 are reported in Figure 4a. Starting at fcc hollow site 1 and moving to bridge site 2 (along the black squares), there is a smooth increase in energy which is steadily lowered with motion toward the hcp hollow site 3 (along the red circles). Moving toward the step and site 4 (along the green triangles), there is a smooth, small increase in energy. This energy is maintained along the energy pathway to the degenerate site 5 (along the blue x). Continuing over the step toward site 6 (along the orange +) there is a steep increase in energy as nitrogen becomes bound to more highly coordinated gold atoms. A significant barrier is encountered along this path, 0.50 eV greater in height than the difference in nitrogen adsorption energy at sites 5 and 6. Motion over the step along an alternate pathway from site 5 to site 7 (along the purple diamonds) begins as an isoenergetic path, but then encounters a steep increase in energy as the step is crossed with nitrogen reaching the fcc hollow site 7 after passing over a barrier 0.16 eV greater than the difference in nitrogen adsorption energy at sites 5 and 7. Adding these additional barriers to the difference in optimized adsorption energies of the sites, we find 1.2 and 0.32 eV barriers to diffusion from site 5 to sites 6 and 7, respectively. The pseudopotential trajectories represent straight-line paths of the nitrogen atom between sites with only optimization of the height above the terrace. Deviations from these pathways parallel to the surface were not permitted in the calculation of pseudopotential curves and may occur to partially avoid these barriers. Notably, the portion of the trajectory from site 5 to site 6, where adsorption is least favorable, correlates with a maximum in the distance of the nitrogen atom from the plane of the (100) step. As the atom reaches a point 0.8 of the distance to site 6, a steep drop in energy is apparent. The onset of this decline is coincident with a rapid drop in the distance to the plane of the (100) step. Considering the depiction of these sites in Figure 1c, we postulate that the nitrogen atom is kept at a greater distance from the slab by the presence of the neighboring gold atom in the step. Once the pathway passes beyond this atom, closer approach to the (100) surface is possible and the adsorption energy increases. If this is the case, the nitrogen atom should prefer a path that arcs away from the gold atom in the slab to avoid a repulsive approach to the atom. An NEB calculation with twelve images was completed between these sites to provide confirmation, and resulted in the expected arched path with no greater barrier than the difference in adsorption energies of sites 5 and 6. A barrier of 0.70 eV is, therefore, expected for diffusion to site 6 which would occur in an arched pathway around the gold atom in the step. Note that NEB calculations revealed similar behavior in

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the case of oxygen where diffusion from site 5 to site 6 occurs through an arced trajectory to give a 0.58 eV barrier. The path from site 5 to site 7 was also investigated with a twelve image nudged elastic band calculation. In this case, the nudged elastic band calculation did not find a path to avoid the barrier in the pseudopotential energy curve, but gave a total barrier of 0.49 eV. A third nudged elastic band pathway directed from site 5 to the point midway between sites 6 and 7 was also investigated. The calculation yielded an energy curve with no barrier beyond the difference in the adsorption energy at the end points of the path. However, this difference in energy is large: 0.66 eV. Diffusion is therefore less hindered when it takes place on a pathway from site 5 to site 7; however, the 0.49 eV total barrier obtained with the nudged elastic band calculation makes this improbable. We see that, although adsorption at site 7 is preferred to adsorption at site 6 and the difference in adsorption energy between sites 5 and 7 is 0.16 eV, the additional barrier makes diffusion between these sites improbable and illustrates the need for considerations beyond adsorption at symmetrical sites on the surface to obtain an accurate description of adatom diffusion. To continue, we consider diffusion of nitrogen on the surface. Diffusion from the upper terrace to the step requires motion from site 1 to site 3 over the bridge site 2; the barrier to this process is 0.40 eV, and will, therefore, remain improbable at reasonable temperatures. Diffusion from site 3 to site 4 requires 0.29 eV and is also unlikely. On the other hand, diffusion between 22993


The Journal of Physical Chemistry C sites 4 and 5 has no barrier and is facile. Diffusion between sites 4 and 5, however, provides only local motion of the nitrogen atom on the surface. As discussed above, diffusion over the step is most likely toward site 7; the barrier of 0.49 eV, however, will prevent diffusion to these more highly coordinated gold atoms. In summary, nitrogen diffusion on the (211) surface is significantly hindered. Nitrogen atoms have highly constrained motion and will likely be found in the stronger binding sites which occur along the step. Mulliken and Lowdin charges were calculated for this surface. A small negative charge was found on nitrogen with no variation among the sites. The range of Lowdin charges was 0.05e with a typical charge of 0.4e. Adsorption of sulfur is preferred for sites that include low coordination gold atoms. Site 3, an hcp hollow site, has the most favorable adsorption energy, 3.60 eV. Binding is asymmetric with AuS distances of 2.40 Å, 2.40 Å and 2.50 Å. The shorter distances are to the less coordinated gold atoms where 9-fold coordination is reduced to seven by the step. Adsorption at site 5, the “hanging” fcc hollow site lies nearby in energy, 3.53 eV. Here, there is symmetric binding with AuS distances of 2.39 Å. The adsorption energy is only 0.07 eV less than that of site 3 in spite of the 2-fold coordination of sulfur to gold. The lower coordinated gold appears to compensate for the lower coordination of sulfur. Adsorption at site 1 is also strong, with energy only 0.17 eV less than the preferred site for this surface. The AuS distance to the 7-fold coordinate gold atom is 2.42 Å compared to the slightly longer 2.44 Å AuS distance for the 9-fold coordinated gold atoms. Asymmetry is also seen for site 2 where AuS distances are 2.37 Å and 2.42 Å with closer approach to the lower coordinated gold atom. The adsorption energy for this site is 0.39 eV less than the most stable site on this surface. Site 6 has the lowest adsorption energy, 0.76 eV below the most stable site, and exhibits strong geometrical asymmetry: AuS distances are 2.36 and 2.46 Å for the 7-fold and 10-fold coordinated gold atoms, respectively. Sites 4 and 7 are not reported since relaxation yielded geometries corresponding to adsorption in sites 3 and 5, respectively. The role of low coordinated gold in increasing sulfur adsorption energy is seen in the ordering of these sites and the shorter AuS distances for lower coordinated gold. As for nitrogen, direct comparison of sites with the same sulfur coordination, but varying gold atom coordination further demonstrates the effect. We do not make comparisons of the fcc hollow sites, site 1 and 7, since the fcc hollow site at the base of the step, site 7, is unstable. The hcp hollow sites do, however, demonstrate greater binding for lower coordinated gold: 3.60 eV for gold coordination numbers of seven and nine versus 3.01 eV for gold coordination numbers of nine and ten, a 0.59 eV difference. Comparing the bridge sites, we observe that adsorption on the terrace (site 2) is 0.39 eV lower in energy than at the step edge (site 4) where gold coordination is reduced from nine to seven and 0.34 eV lower than adsorption at the base of the step (not one of the labeled sites) where gold coordination is increased from nine to ten. Similarly, ontop sites show greater adsorption to gold with lower coordination number. Adsorption energies are 1.91, 2.22, and 2.83 eV, for ontop sites on the terrace, on the step edge and at the base of the step. The adsorption energy is 0.69 eV greater at the step than the terrace, where the gold is fully coordinated. The further increase in adsorption energy, 0.61 eV, at the ontop site at the base of the step results from 2-fold coordination to not only to the gold atom at the base of the step

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but also to the low coordinated gold atom in the adjacent step, the latter compensates for the increased gold coordination at the base of the step. Low coordination of surface gold atoms plays a clear role in sulfur atom adsorption; however, sulfur coordination number is also significant. Higher coordination of the sulfur atom leads to stronger adsorption as is evidenced by the most favorable adsorption energy at the 3-fold coordinated site 3 and the preference for adsorption at 3-fold coordinated site 1 to 2-fold coordinated site 2. In addition to optimized adsorption energy calculations, pseudopotential energy curves were generated and are reported in Figure 4b. Starting at fcc hollow site 1 and moving to bridge site 2 (along the black squares), there is a smooth increase in energy which is steadily lost with motion toward the hcp hollow site 3 (along the red circles) where adsorption is most favorable. Moving away from site 3, toward the step and site 4 (along the green triangles), there is a small increase in energy to this nearly isoenergetic site. The path to site 5 (along the blue x) is flat; adsorption energy is invariant to motion between degenerate sites 4 and 5. Continuing over the step toward site 6 (along the orange +) yields a steep increase in energy as sulfur becomes bound to more highly coordinated gold atoms. No barrier beyond the difference in adsorption energies of the sites is found, yielding a 0.69 eV barrier based on the optimized values. On the other hand, continuing over the step in the direction of site 7 yields a decrease in energy sufficiently strong to reach the greatest adsorption energies along these trajectories. As site 7 is attained, a significant barrier, 0.11 eV greater than the difference in single point energies of sites 5 and 7, is encountered. Optimized adsorption values are not utilized to estimate this barrier since the site 7 geometry was unstable to relaxation. In agreement with these pseudopotential results, nudged elastic band calculations show no additional barriers beyond the difference in adsorption energies among the sites yielding a 0.70 eV barrier from site 5 to site 6 and a 0.74 eV barrier from site 5 to site 7. Diffusion of sulfur over the step toward site 6 or site 7 is improbable due to substantial energetic barriers. Mulliken and Lowdin charges along these trajectories showed little to no redistribution of charge upon adsorption of sulfur. Typical Lowdin charges of 0.0e were found with a range of approximately 0.1e about this value. Comparison of Nitrogen, Sulfur, and Oxygen Adsorption. On all three gold surfaces—(111), (100), and (211)—the adsorption energy of atomic nitrogen, sulfur and oxygen follows the ordering S > O > N. Adsorption of a sulfur atom is preferred to an oxygen atom by 0.56, 0.65, and 0.36 eV on the (111), (100), and (211) surfaces, for the preferred binding site. We find a more substantial preference for sulfur to nitrogen, 1.66, 1.58, and 1.48 eV, on these surfaces, respectively. While prior reports are incomplete in consideration of the range of gold surfaces and restricted to a single site and small unit cells, they are consistent with our detailed study in the ordering of atom adsorption energies. On the (111) surface, Xue and co-workers found sulfur adsorption preferred to oxygen by 0.59 eV for 0.17 ML coverage utilizing STATE, the PBE functional and a plane wave basis set, in agreement with our results.15 Similarly, Liu et al. reported stronger sulfur than oxygen atom adsorption on the (111) surface, but to a greater extent, 1.21 eV, at 0.25 ML coverage using CASTEP with the RPBE functional and a plane wave basis set.18 Nitrogen atom adsorption was also considered by AbildPedersen et al. on minimally sized unit cells with the RPBE functional using 0.25 ML coverage for the (111) surface and 0.5 22994


The Journal of Physical Chemistry C ML coverage for the (211) surface.32 Atom adsorption energies on various surfaces of gold can be estimated from the reported data and yields an ordering of S > O > N, in agreement with our results. It is clear that sulfur consistently binds more strongly to the gold surfaces than oxygen or nitrogen, but shows this trend is less prominent on the stepped surface where low coordinated gold atoms are introduced. Comparing adsorption energy as a function of the surface for nitrogen, sulfur and oxygen, we find the same trend for all atoms: (111) > (211) > (100). This result is surprising since the gold atoms are most highly coordinated on the (111) surface, each surface atom has a coordination number of nine. Clearly, adsorption strength is not automatically increased with reduced gold coordination. Comparing sites of varying gold coordination number within the (211) surface sites, however, demonstrates the trend of stronger adsorption with lower coordination of gold, as has been discussed in detail for each atom. Adsorption trends on the gold (111) surface are similar for nitrogen, sulfur and oxygen. In all cases, the ordering of FH > HH > B > O persists. The three atoms show a preference for higher coordination to gold on this surface, since 3-fold hollow sites are unanimously preferred to 2-fold bridge and single-fold ontop sites. Adatom-gold distances in each of these sites have the trend S > O > N with values of 2.4, 2.2, and 2.1 Å for the FH site. The distances show a slight reversal of nitrogen and oxygen from the trend in atomic radii: S > N > O with sizes of 104, 75, and 66 pm, respectively.33 Nitrogen binds with a slightly closer approach to the surface than would be anticipated from its atomic radius, but displays the weakest binding among these three atoms. While the FH and HH sites differ most in energy for oxygen atom adsorption with a 0.24 eV difference, the range of adsorption energies is greatest for nitrogen where the FH and O site energies differ by 1.77 eV. There is a more distinct preference for 3-fold coordination in the case of nitrogen. Comparison of the pseudopotentials shows qualitative conformity among the three atoms. None of the pseudopotential curves show a barrier to diffusion greater than the difference in adsorption energies at the labeled sites. Diffusion is, therefore, controlled by the difference in adsorption energies of the hollow and bridge sites in the case of all three adatoms. The barrier for diffusion from the favored FH site is greatest for nitrogen and sulfur at 0.41 and 0.38 eV. The barrier for oxygen is 0.34 eV. These barriers are all significant enough to prevent diffusion of the atoms away from the FH site at reasonable temperatures. Diffusion from the HH to the FH site, on the other hand, has barriers of 0.3, 0.2, and 0.1 eV, for nitrogen, sulfur and oxygen, respectively. As a result, diffusion to the FH site from the HH site may be facile for oxygen atoms, but improbable for nitrogen. For the (100) surface, differences among nitrogen, sulfur and oxygen adsorption energies are large enough to show qualitative distinctions. While oxygen binds with equivalent adsorption energy at the B and H sites, nitrogen and sulfur have the most favored adsorption at the H site with a 0.4 eV preference over the B site. For all three atoms, the O site is least preferred, indicating decreased preference for single-fold adsorption. Adatom-gold distances are ordered S > O > N on this surface. As with the (111) surface, nitrogen binds with a slightly closer approach to the surface than would be anticipated from its atomic radius. Consideration of the pseudopotential curves reveals barriers to diffusion for nitrogen, sulfur and oxygen atoms of 0.40 eV, 0.38 and 0.03 eV, respectively. The difference between the H and B site barriers for nitrogen and sulfur is significant. It is sufficient to

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make diffusion of nitrogen and sulfur along the (100) surface improbable, while oxygen diffusion is quite facile. Consideration of the adsorption trends among the adatoms studied on the (211) surface reveals similar factors governing adsorption. While nitrogen and sulfur bind most strongly at the hcp hollow site 3, the “hanging” fcc hollow site 5 is slightly preferred for oxygen atoms with site 3 lower in adsorption energy by only 0.1 eV. Site 3, then, which combines high adatom coordination number with low gold coordination number yields consistently strong adsorption among the adatoms studied. Nitrogen, sulfur and oxygen all follow the trend of stronger adsorption to low-coordinated gold atoms, and show a preference for 3-fold to 2-fold coordination to the surface. This is evidenced by the fact that sites 1, 3, 4, and 5 are the preferred adsorption sites for all three adatoms. These sites either have 2-fold coordination to strictly 7-fold coordinated gold or 3-fold coordination to a mixture of 7-fold and 9-fold coordinated gold. On the other hand, the least preferred adsorption sites are consistently sites 6 and 7. These sites are located at the bottom of the step where the gold atoms are more highly coordinated. We also observe that site 3 is consistently more stable than site 1 for nitrogen, sulfur and oxygen atoms in spite of the fact that the fcc hollow site is preferred on the (111) surface and site 3 is an hcp hollow site. As with the close-packed surfaces, adatom-gold distances follow the same trend of S > O > N for all sites except site 5. The relatively closer approach of oxygen to gold in site 5 may partially account for the slightly greater oxygen adsorption energy relative to nitrogen and sulfur in this site. The pseudopotentials coupled with NEB calculations reveal the variations among nitrogen, sulfur and oxygen diffusion over the (211) surface steps. Diffusion from site 1 on the terrace to site 3 over site 2 has barriers of 0.40, 0.22, and 0.24 eV for nitrogen, sulfur, and oxygen, respectively. The barriers to traverse from site 3 to site 1 in the opposite direction are, respectively, 0.60, 0.39, and 0.40 eV for nitrogen, sulfur and oxygen. These barriers are sufficient to hinder diffusion along the terrace in either direction. Moving from site 3, the lowest energy site for nitrogen and sulfur, toward the step and sites 4 and 5 poses a barrier of 0.29 eV for nitrogen, while sulfur and oxygen show nearly flat energy curves and facile diffusion. Nitrogen will be more localized in its preferred binding site, site 3, than sulfur and oxygen, which will be able to diffuse small distances perpendicular to the step edge. All three adatoms have significant barriers to further diffusion over the step to sites with higher coordinated gold at the bottom of the step. The smallest barriers are 0.49, 0.70, and 0.58 eV for nitrogen, sulfur and oxygen, respectively, for diffusion from site 5 to site 7 for nitrogen and oxygen atoms, and from site 5 to sites 6 for sulfur atoms. This suggests that these adatoms will remain at the top of the step in an adsorption experiment, nitrogen localized at site 3, sulfur and oxygen in the region between site 3 and 5. The typical Lowdin charges for nitrogen, sulfur and oxygen were 0.4e, 0.0e, and 0.5e, respectively. While nitrogen and oxygen showed a small negative charge, sulfur remained neutral upon adsorption to gold. These results suggest a small transfer of charge upon adsorption of atoms on gold surfaces with the relative magnitude dependent on electronegativity of the atoms. This ordering of magnitude of charge transfer is consistent with the electronegativity of these atoms since nitrogen and oxygen have more substantial electronegativities than sulfur. All three adatoms showed minimal variation of charge both among the sites on a given surface or among the three surfaces studied. 22995


The Journal of Physical Chemistry C Regardless of the adatom type, the degree of ionicity/covalency of bonding is independent of adsorption site along these gold surfaces. The comparison of binding sites and surfaces resulted in two observations that require some additional discussion. It is notable that the shortest adsorbatesurface bond, to the ontop site on all three surfaces, does not correspond to the greatest adsorption energy. Although theoretically dependent upon several simplifying assumptions, effective medium theory34 provides a qualitative framework for consideration of this result. In that model, adsorption energies for adatoms depend on two terms; one term is the (attractive) energy of interaction with the electron cloud of the surface. This attractive term improves as the electron density increases. The second term is a repulsive interaction between the adatom and the surface atoms. Mathematically, this term is due to the kinetic energy of the electrons and will become more repulsive with decreasing bond length. Highly coordinated binding increases the attractive energetics of the first term by providing greater binding density at a longer distance, therefore minimizing the repulsive term. The ontop binding, with only a single adsorbatesurface interaction, must be adsorbed at a shorter distance to compensate for the decrease in available electron density. This shorter distance results in greater repulsion and, hence, a lower adsorption energy. We speculate that similar competing effects are responsible for the higher binding energy on the (111) surface, even though the gold surface atoms have a greater coordination number in that case. If one compares the energies for adsorption at the preferred binding site on the (111) and (100) surfaces, it may be shown that the adatom-surface distance is significantly shorter on the (111) surface. Moreover, the intergold atom distance is greater for the (100). These two factors, in terms of the model, provide greater electron density for adsorption on the (111) surface and negate the general observation that binding is stronger for less coordinated surface atoms. A similar observation was reported for adsorption of hydrogen on the Ni (111) and (100) surfaces, where the binding energies were found to be identical.35

’ CONCLUSION Investigation of nitrogen, sulfur and oxygen atom adsorption on the (111), (100), and (211) surfaces of gold revealed a number of trends. First, the increased capacity of low coordinated gold to bind adsorbates was confirmed, but was nonuniversal. Adsorption to the highly coordinated gold atoms of the (111) surface consistently yielded stronger adsorption than on the (211) surface where the step introduced reduced gold coordination number. On the other hand, comparison of adsorption energies at equivalent types of sites on the (211) surface demonstrates stronger binding for lower coordination numbers gold atoms. In addition, nitrogen, sulfur and oxygen show closer adatom-gold distances to lower coordinated gold in nearly all instances. Second, we find a preference for higher coordination of the adatom to the surface for nitrogen, sulfur and oxygen. 3-fold coordination is consistently favored over 2-fold and singlecoordination. Likewise, 4-fold coordination is preferred to 2-fold coordination on the (100) surface for nitrogen and sulfur. The equivalent adsorption energy of oxygen in the 4-fold and 2-fold coordination sites on the (100) surface of gold does not follow the trend, but may be understood in light of the significantly shorter, 2.06 Å, AuO distance for the 2-fold site versus the calculated AuO distance, 2.33 Å, for the 4-fold site.

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Along with these coordination number trends, we find a nearly universal increase in adsorption energy for closer approach of the adatom to the gold surface and shorter adatom-gold distances. We also obtain a consistent adsorption preference among the surfaces, (111) > (211) > (100). For all three gold surfaces, adsorption energy follows the ordering of S > O > N. Adatom diffusion along the gold surfaces shows both points of similarity and distinction. On the (111) surface, diffusion from the FH to the HH site is improbable at reasonable temperatures for all three atoms, while diffusion from the HH to the FH site is hindered with a 0.3 eV barrier for nitrogen, but possible with barriers of 0.2 and 0.1 eV for sulfur and oxygen. Sulfur and oxygen atoms may, therefore, more easily locate in the most favored FH site. On the (211) surface, none of the atoms diffuse easily between sites 1 and 3; however, sulfur and oxygen find facile diffusion among sites 3, 4 and 5, while nitrogen diffusion is deterred. As a result, nitrogen will be located more consistently in site 3, while oxygen and sulfur are less limited to site 3. Diffusion over the step to the lower terrace where gold atoms are more highly coordinated is substantially obstructed for all three atoms by energetic barriers of 0.50.7 eV. While the (111) and (211) surfaces show little significant adatom mobility, the (100) surface shows facile long-range diffusion for the oxygen atom. Diffusion between H sites on the surface has no barrier (0.03 eV) for oxygen. On the other hand, nitrogen and sulfur will be localized in the H sites with a 0.4 eV barrier to diffusion out of the site. In general for these gold surfaces, we find greater mobility of oxygen atoms than nitrogen and sulfur atoms. Low coverage of nitrogen or sulfur atoms on gold provides localized nitrogen or sulfur that may not diffuse to active catalytic sites. A low coverage of oxygen atoms on gold, on the other hand, may provide a more effective source of oxygen at the site of catalysis.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: 1-603-646-2270. Fax: 1-603-646-3946.

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Density Functional Theory Study of the Adsorption of Nitrogen and

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