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Atomic and molecular adsorption on Au(111)

Yohaselly Santiago-Rodríguez a ,Jeffrey A.Herron b ,María C.Curet-Arana a ,?,Manos Mavrikakis b

a Department of Chemical Engineering,University of Puerto Rico-Mayagüez Campus,Call Box 9000,Mayagüez,Puerto Rico 00681-9000b

Department of Chemical and Biological Engineering,University of Wisconsin-Madison,1415Engineering Drive,Madison,WI 53706,USA

a b s t r a c t

a r t i c l e i n f o Article history:

Received 11February 2014Accepted 22April 2014

Available online 2May 2014

Keywords:

Density functional theory calculations Chemisorption Gold

Low index single crystal surfaces Thermochemistry

Periodic self-consistent density functional theory (DFT-GGA)calculations were used to study the adsorption of several atomic species,molecular species and molecular fragments on the Au(111)surface with a coverage of 1/4monolayer (ML).Binding geometries,binding energies,and diffusion barriers were calculated for 27species.Furthermore,we calculated the surface deformation energy associated with the binding events.The binding strength for all the analyzed species can be ordered as follows:NH 3b NO b CO b CH 3b HCO b NH 2b COOH b OH b HCOO b CNH 2b H b N b NH b NOH b COH b Cl b HCO 3b CH 2b CN b HNO b O b F b S b C b CH.Although the atomic species preferred to bind at the three-fold fcc site,no tendency was observed in site preference for the molecular species and fragments.The intramolecular and adsorbate-surface vibrational frequencies were calculated for all the adsorbates on their most energetically stable adsorption site.Most of the theoretical binding energies and frequencies agreed with experimental values reported in the literature.In general,the values obtained with the PW91functional are more accurate than RPBE in reproducing these experimental binding energies.The energies of the adsorbed species were used to calculate the thermochemical potential energy surfaces for decom-position of CO,NO,N 2,NH 3and CH 4,oxidation of CO,and hydrogenation of CO,CO 2and NO,giving insight into the thermochemistry of these reactions on gold nanoparticles.These potential energy surfaces demonstrated that:the decomposition of species is not energetically favorable on Au(111);the desorption of NH 3,NO and CO are more favorable than their decomposition;the oxidation of CO and hydrogenation of CO and NO on Au(111)to form HCO and HNO,respectively,are also thermodynamically favorable.

1.Introduction

Interest in the catalytic properties of gold nanoparticles emerged in the late 1980s,when Haruta and coworkers reported that these particles are effective catalysts for the oxidation of CO at low tempera-ture [1,2].In the same decade,another key discovery was made by Hutchings,when he reported that gold nanoparticles are very selective catalysts for the hydrochlorination of acetylene [3].However,the commercialization of gold catalysts did not start until a decade later,c.a.1992,when they were proven to decompose ammonia and trimethylamine at room temperature,and were used mainly to decom-pose odors [4].A particular feature that made these catalysts so effective for this application is that moisture enhances their catalytic activity.Since this commercialization,gold nanoparticles have also showed high activities and selectivities for many other reactions,in particular for selective oxidation and hydrogenation reactions.Some of the reac-tions in which gold catalysts have been proven useful include:propyl-ene epoxidation [5,6],oxidation of glycerol [7–9],oxidation of ethylene glycol [10],direct synthesis of hydrogen peroxide from hydrogen and oxygen [11–13],hydrogenation of acetylene to ethylene [14]and

reduction of NO by H 2[15].Reviews by Hashmi and Hutchings [16],Corma and Garcia [17],Pina et al.[18]and Wittstock and B?umer [19]provide comprehensive summaries of gold catalyzed reactions.

Many adsorbates and reactions have been studied on gold surfaces using a variety of experimental and theoretical methods.Because the main applications of gold catalysts are related to the oxidation of CO and hydrogenation of CO,CO 2[20–25]and NO [20,26],the species involved in those reaction systems have been extensively studied on gold surfaces.Several experimental surface science techniques have been used to study the adsorption of O 2and/or O on Au(111),such as:temperature-programmed desorption (TPD)[20,27–32],high-resolution electron energy loss spectroscopy (HREELS)[28,32],He dif-fraction [33],scanning tunneling microscopy (STM)[34],low-energy electron diffraction (LEED)[31,32],X-ray photoelectron spectroscopy (XPS)[32,34]and Auger electron spectroscopy (AES)[32].The adsorp-tion of atomic and molecular oxygen has been widely analyzed with density functional theory (DFT)calculations [27,28,35–37].Oxygen does not chemisorb,molecularly or dissociatively,on clean gold sur-faces under ultrahigh-vacuum (UHV)conditions,nor at elevated tem-perature and pressure [20,27].To study the interaction between oxygen and gold surfaces,different approaches have been developed such as:thermal dissociation of gaseous O 2on hot ?laments,ozone de-composition,coadsorption of NO 2and H 2O,O +sputtering,electron-

Surface Science 627(2014)57–69

?Corresponding author.Tel.:+17878324040x2569;fax:+17872653818.E-mail address:maria.curetarana@https://www.wendangku.net/doc/fb14462006.html, (M.C.

Curet-Arana).

https://www.wendangku.net/doc/fb14462006.html,/10.1016/j.susc.2014.04.012

0039-6028/?2014Elsevier B.V.All rights

reserved.

Contents lists available at ScienceDirect

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j o u r n a l h o me p a g e :w w w.e l s e v i e r.c o m /l o c a t e /s u sc

induced adsorption of oxygen,and electron bombardment of con-densed NO2[27].Also,the oxidation of CO by gold catalysts has moti-vated a number of reaction studies and fundamental surface science investigations into the relevant intermediates.Stolcic et al.showed ev-idence of CO oxidation on small Au particles proceeding via molecular oxygen rather than atomic oxygen when O2is absorbed on free Au2?and Au4?particles at room temperature[38].Moreover,an18O2isotope study performed by Stiehl and coworkers with nanoparticles of Au sup-ported on TiO2demonstrated a reaction pathway that does not require the dissociation of molecular oxygen,such that O2can react directly with CO to form CO2[39].These results were further substantiated with DFT calculations by Molina and coworkers showing that O2can react with CO adsorbed at the edge sites of Au particles leading to the formation of CO2with a low energy barrier of~0.15eV[40].Among the experimental methods that have been employed for studying CO on Au(111)are:TPD[20,41],infrared re?ection absorption spectrosco-py(IRAS)[41,42],Fourier transform infrared spectroscopy(FTIR)[20, 43],129Xenon nuclear magnetic resonance(129Xe NMR)spectroscopy [20],diffuse-re?ectance infrared Fourier transform spectroscopy (DRIFTS)[20],and STM[42].Fourier transform ion cyclotron resonance (FT-ICR)mass spectroscopy has been applied to study the co-adsorption of oxygen and CO on anionic gold clusters[44].Several DFT studies have been performed for the adsorption of CO and the co-adsorption of CO with O or O2on Au(111)[45–48].

The chemisorption of molecular hydrogen on gold has been achieved on unsintered thin gold?lms at low temperature[49,50]. For supported gold particles,the chemisorption of H2molecule has been reported,but amounts are small[20,51].H2dissociation is favor-able on supported surfaces as proved by Fujitani and coworkers,and the active sites for H2dissociation on gold catalysts are thought to be at the interface between gold and the metal oxide support[52].Despite the limited circumstances under which gold surfaces are found to chemisorb molecular hydrogen,the literature reveals a number of observations in which hydrogen molecules or atoms participate in reactions on gold.Some of these reactions include the hydrogenation of hydrocarbons(e.g.alkenes[53–55],alkadienes[56,57]and alkynes [57]),both saturated and unsaturated ketones[57,58]and carbon oxides[22,57],hydrogenolysis[59,60],dehydrogenation[61,62]and water–gas shift(WGS)reaction[16].Furthermore,Azar and coworkers demonstrated with TPD experiments that hydrogen can regenerate their Au catalyst through the removal of the poisoning carbonate-like species produced during CO oxidation[63].Sugawara et al.studied the reactions of cationic gold clusters Au n+(n=1–12)with H2using FT-ICR mass spectrometry[64].Pessoa et al.analyzed the adsorption of several ionic and non-ionic species,among which is H,on the low-index gold surfaces Au(100),Au(110)and Au(111)using DFT calcula-tions(B3LYP and GGA-PW91)[35].Many other DFT studies of hydrogen adsorption on gold clusters have been performed[65–68].

It has been suggested that hydroxyl groups present on the surface of gold in aqueous solution promote reactions such as the oxidation of CO and glycerol[69–72].Sanchez-Castillo and coworkers observed CO oxi-dation by O2at room temperature on gold nanotubes in polycarbonate membranes without the presence of a catalyst support,?nding that the activity can be enhanced by performing the reaction in the aqueous phase,increasing the pH of the solution and using H2O2as the oxidizing agent[73].Ketchie et al.investigated the promotional effect of OH on the oxidation of both CO and glycerol in the aqueous phase,as a function of pH,over Au/C and Au/TiO2using transmission electron microscopy (TEM),near-edge X-ray absorption?ne structure(NEXAFS)and X-ray absorption near edge structure(XANES)spectroscopy,high perfor-mance liquid chromatography(HPLC)and ultraviolet–visible(UV–VIS) spectroscopy[71].They concluded that the hydroxyl groups adsorbed on the Au surface increase CO oxidation rates by facilitating the adsorp-tion and activation of O2on the Au surface.Similarly,OH groups are nec-essary in order to activate glycerol by deprotonation of the primary alcohol group prior to the selective oxidization to glyceric acid over both gold catalysts.The adsorption of the OH ion on Au(100),Au(110) and Au(111)was also analyzed by Pessoa and coworkers using DFT calculations[35].

For many reactions involving CO,CO2,H2and water(e.g.WGS reac-tion),reaction intermediates such as formyl(HCO),COH,formate (HCOO),carboxyl(COOH)and bicarbonate(HCO3)have been proposed as relevant species[69].For that reason,Senanakaye and coworkers have investigated the role of formate,carbonate,and carboxyl as possible intermediates in the WGS reaction through the interaction of CO with OH(OH ads+CO gas→CO2,gas+0.5H2,gas)on Au(111)and O/Au(111)using synchrotron-based core level photoemission, NEXAFS,infrared absorption spectroscopy(IR)and DFT calculations [74].Their results indicate that formate is not a key intermediate at low temperatures because HCOO is stable on Au(111)up to tempera-tures near350K.In fact,they suggest that the formation of this species could provoke surface poisoning.Moreover,the results of IR spectrosco-py and photoemission spectroscopy point to HO–CO interactions,which are consistent with the formation of a COOH intermediate with a short lifetime on the gold surface[74].On the other hand,Bond and Thomp-son,based on a reaction mechanism developed by Costello and co-workers[72],proposed a mechanism for CO oxidation on Au/Al2O3 where a bicarbonate species is involved.In this mechanism,an Au–OH species reacts with a CO molecule adsorbed on an under-coordinated gold atom to form a carboxyl species,which is then oxidized to bicar-bonate.They suggest that the decomposition of bicarbonate would yield CO2,and in the presence of water,COOH is formed.Thus,water and OH groups play an important role on this mechanism to facilitate CO oxidation through the formation of COOH[69].

Gold catalysts are very susceptible to poisoning,particularly with chloride ions.Supported Au catalysts are often prepared using HAuCl4 as a precursor.Yang and coworkers have demonstrated using X-ray ?uorescence(XRF),NEXAFS,XANES and high-resolution scanning transmission electron microscopy(STEM)that residual chloride can cause agglomeration of Au particles during heat treatment and sup-presses its catalytic activity by poisoning active sites[75,76].On the other hand,the adsorption of chloride on gold can be advantageous for various applications in homogeneous and heterogeneous catalysis. Mercury can be removed from coal-?red boiler gas ef?uents by oxidiz-ing it to HgCl2with chlorinating reagents over a gold catalyst[77]and chlorine also increases the selectivity of styrene epoxidation on Au(111)[78].These potential applications of Cl on catalytic systems motivated Baker and coworkers to perform?rst-principles DFT calcula-tions to study the nature of the interaction between Cl and the Au(111) surface,and they have reported evidence that this bonding is primarily of covalent character[79].Lemoine and coworkers also studied the Eley–Rideal type reaction of Cl adsorbed on Au(111)with gas-phase H atoms using DFT[80].The adsorption of chlorine on Au(111)has also been studied by various experimental(e.g.AES,XPS and UV photo-electron spectroscopy(UPS),LEED,TPD[81]and STM[82])and theoret-ical techniques[82].

Although the catalytic properties of nanoscale gold have been exten-sively studied,the intrinsic mechanism of the heterogeneous catalysis at the atomic and molecular level has not been fully clari?ed.DFT studies can provide further insight into the fundamental surface science of gold catalysis.Similar to previous works on Pt(111)[83],Rh(111) [84],Ir(111)[85],Pd(111)[86],Ru(0001)[87]and Re(0001)[88]this work aims at performing a systematic study of the chemisorption of several atomic and molecular species,and molecular fragments in-volved in gold catalyzed reactions.We study the adsorption of these species on the Au(111)facet,which is the thermodynamically most sta-ble and prevalent con?guration of supported gold nanoparticles[27]. While it has been observed that this surface reconstructs to the charac-teristic herringbone structure[89,90]with an additional4%of Au atoms incorporated into the surface layer,our calculations only account for re-actions on the unreconstructed Au(111)surface.For this analysis,we have used periodic self-consistent DFT calculations to determine

58Y.Santiago-Rodríguez et al./Surface Science627(2014)57–69

preferred binding sites,binding energies,vibrational modes and esti-mated diffusion barriers.The thermochemistry of several surface reac-tions on Au(111)is also examined.These DFT-derived results present a database of benchmarks that can be used for comparison with exper-iments,including state-of-the-art single crystal adsorption microcalo-rimetry[91,92],and we compare our results to experimental results when available.Our data can be added to other electronically accessible databases,which have started appearing recently[93].

2.Methods

We have used DACAPO,a periodic self-consistent DFT-based total energy code[94],to analyze the adsorption of atoms,molecules and molecular fragments on the surface of Au(111)and to study the ther-mochemistry of simple reactions,such as hydrogenation of CO,CO2 and NO.For these calculations,a2×2unit cell is used to construct a four-layer Au(111)slab with?ve equivalent layers of vacuum separat-ing successive slabs.The top two layers of the slab are relaxed,and the surface Brillouin zone is sampled at54Chadi–Cohen k-points.The Kohn–Sham one-electron valence states are expanded in a basis of plane waves with kinetic energies up to25Ry[94],and the core elec-trons are described by ultrasoft pseudopotentials[95].The exchange-correlation potential and energy are described self-consistently using the GGA-PW91functional[96].Energies are also calculated non-self-consistently using the RPBE[94]functional,and are provided in the text enclosed by square brackets next to the PW91values (e.g.?2.01[?1.97]eV).We note in passing that we also evaluated the binding energies of NH,HCO3,and HCOO(species that have very large differences between the PW91and RPBE results)self-consistently with the RPBE functional and found that the difference between the self-consistent and non-self-consistent results was b0.1eV.Therefore,we suggest that the differences in the PW91 and RPBE values reported,herein,are due to differences in the func-tional and not due to non-self-consistency.The self-consistent densi-ties are determined by iterative diagonalization of the Kohn–Sham Hamiltonian,Fermi-population of the Kohn–Sham states(k B T= 0.1eV),and Pulay mixing of the resulting electronic density.All total energies are extrapolated to k B T=0eV[94].The results of our calcula-tions are used to determine preferred binding sites,chemisorbed structures,binding energies,estimated diffusion barriers,surface defor-mation energies and the thermochemistry of molecular dissociation/ formation,hydrogenation and oxidation reactions.

The calculated PW91equilibrium lattice constant for bulk Au is 4.18?,which closely agrees with the experimental value of4.0783?[97].The binding energy(E b)of an adsorbate is calculated with respect to a clean relaxed Au(111)slab and the respective adsor-bate in the gas phase as described by the following equation:E b= E substrate+adsorbate?E substrate?E gas-phase adsorbate,where E substrate+adsorbate is the total energy of the relaxed Au(111)surface with an adsorbate,

E substrate is the total energy of the relaxed clean Au(111)surface and

E gas-phase adsorbate is the total energy of the isolated adsorbate.Note,a negative binding energy is associated with an exothermic adsorption process.The deformation energy is de?ned as the change in surface en-ergy upon the adsorption of a species.It is the energy difference be-tween the relaxed clean surface and the deformed surface frozen after the adsorption with the adsorbate removed.Based on this de?nition, deformation energy is a positive quantity.Harmonic vibrational fre-quencies for each adsorbate are calculated by diagonalizing the mass-weighted Hessian matrix.The second derivatives of the energy were evaluated using a?nite difference approximation[98].The diffusion barrier of an adsorbate is estimated by?rst identifying a probable diffu-sion path that connects neighboring minima on the potential energy surface through a metastable state.Then,the energy of the lowest min-imum is subtracted from the energy of the metastable site to give an es-timated diffusion barrier.In general,for the atomic species,the diffusion path is from the fcc site to the adjacent hcp site by passing through a metastable bridge site.

3.Results and discussion

In this section,properties obtained for different adsorbates on the Au(111)surface are described and compared with available experimen-tal results.Results include binding energies(Tables1and6,and Fig.4), adsorption site preferences,adsorption geometries(Table5and Fig.1), surface deformation energies(Table4)and estimated diffusion barriers (Table3and Fig.2).In addition,the vibrational modes and their associated frequencies are given for the most stable con?guration of each adsorbate on Au(111)(Tables2,7and8).Finally,we develop ther-mochemical potential energy surfaces(Figs.5and6)for decomposition of CO,NO,N2,NH3,CH4,for the oxidation of CO,and for the hydrogena-tion of CO,CO2and NO,to assess the favorability of these processes on Au(111).

3.1.Adsorption of atomic species

3.1.1.Hydrogen(H)

Similar to Pt(111),as demonstrated by Ford et al.,hydrogen is the least strongly bound atomic species to the Au(111)surface among those we studied[83].Hydrogen prefers to bind on the fcc site as illus-trated in Fig.1.However,adsorption on the hcp site is only0.04eV higher in energy.As shown in Table1,the calculated binding energy of H on the fcc site is?2.18[?2.00]eV.The calculated Au–H vibrational frequency for the adsorbed H on the fcc site is794cm?1(Table2).The diffusion barrier from an fcc site to its adjacent hcp site passing through a bridge site is estimated to be only0.15[0.16]eV(Table3and Fig.2). The estimated deformation caused by hydrogen adsorption on Au(111) is very small with a value of0.06[0.03]eV(Table4).This deformation is caused by pulling the contacting gold atom up0.07?from its position in a clean relaxed slab and increasing the average distance between two adjacent Au atoms in contact with the adsorbate by0.08?.Additional geometric information,as de?ned in Fig.3,is supplied in Table5.

3.1.2.Oxygen(O)

The preferred adsorption site for atomic oxygen is the fcc site(see Fig.1),with an associated binding energy of?2.77[?2.19]eV (Table1).This value is in good agreement with the Au\O bond dissoci-ation energy of?2.43eV estimated by Saliba and collaborators in their TPD study of the adsorption of oxygen on Au(111)by exposure to ozone at300K[32,35].The calculated vibrational frequency of the Au\O stretch on the fcc site is382cm?1(Table2).Baker et al.observed this mode at380cm?1using HREELS for atomic oxygen deposited by ozone decomposition on Au(111)at a surface temperature of200K and low oxygen coverage,concurring with our result[28].In our analy-sis,the adsorption of the O atoms occurs at a height of1.15?above the Au surface layer,corresponding to an Au\O bond length of2.14?. This result is0.10?longer than the experimental value of Au\O bond length obtained for oxygen coordinated to three Au atoms in bulk gold (III)oxide[99].When oxygen is adsorbed,the contacting Au atoms are displaced0.06?above their original position and the Au\Au bond dis-tance between oxygen-adjacent Au atoms increases by0.18?(Table5).These changes on the Au surface produce a deformation ener-gy of0.22[0.23]eV(Table4).For the O atom,the diffusion path from an fcc site to an hcp site passing through a bridge site has an estimated bar-rier of0.56[0.47]eV(Table3and Fig.2).

3.1.3.Nitrogen(N)

The fcc site is the most stable adsorption site for N with a binding energy of?2.22[?1.77]eV.The calculated Au\N stretching frequency for this site is430cm?1.A diffusion barrier from an fcc to an hcp site passing through a bridge site was estimated and is0.84[0.78]eV, which is the highest barrier among the atomic species that we studied.

Y.Santiago-Rodríguez et al./Surface Science627(2014)57–69

Table 5shows that nitrogen is adsorbed at a distance of 1.05?from the plane of Au(111)while the gold atoms in contact with this adsorbate move up by 0.08?from their position in a clean relaxed surface.These modi ?cations in the structure of Au(111)result in a deformation energy of 0.15[0.14]eV.

3.1.

4.Sulfur (S)

Sulfur is the atomic species with the second strongest binding ener-gy.This energy is ?3.50[?2.98]eV and it corresponds to adsorption at the fcc site.Our ?ndings are in agreement with the experimental results obtained by Kurokawa et al.,who con ?rmed that sulfur adsorbs on the fcc site of Au(111)using STM at 77K [100].Our results are also in agreement with the DFT work done by Rodríguez et al.[101],which showed that the adsorption of sulfur at low coverage on fcc hollow sites of Au(111)is energetically more favorable than adsorption on bridge or top sites.The S atom is adsorbed 1.61?above the surface,cor-responding to a Au \S bond length of 2.40?.McGuirk et al.detected three adsorption phases using LEED:a (1×1)phase at low coverage,

Table 1

Binding energies (PW91[RPBE])and site preferences of atomic species on Au(111);the binding energies for the most stable site for each species are shown in bold.Adsorbate

Preferred site Binding energy (eV)

Calc.

Exp.

top

hcp

fcc

bridge

Exp.

H fcc ?1.87[?1.72]?2.14[?1.96]?2.18[?2.00]C fcc ?2.12[?1.79]?3.86[?3.39]?4.03[?3.54]N fcc ?0.88[?0.64]?1.94[?1.53]?2.22[?1.77]O fcc ?1.41[?1.02]?2.53[?1.97]?2.77[?2.19]?2.43a

F fcc ?2.56[?2.27]?2.75[?2.39]?2.77[?2.41]?2.74[?2.39]S fcc fcc

?1.99[?1.63]?3.30[?2.81]?3.50[?2.98]Cl

fcc

?2.10[?1.79]

?2.29[?1.88]

?2.30[?1.89]

?2.28[?1.89]

?2.34c

a O 2,TPD [35].

b S,STM [100].c

Cl 2,TPD [81]

Fig.1.Top and side views of the most stable con ?gurations for the adsorption of selected adsorbates on Au(111).Table 5provides the geometric details for these structures.

60Y.Santiago-Rodríguez et al./Surface Science 627(2014)57–69

a (5×5)phase above 0.28ML coverage,and a (√3×√3)R30°phase above 0.30ML [102].From the (5×5)-7S phase,they found that the S atoms occupy fcc sites,resting 1.57?above the Au surface (a 2.29?Au \S bond length),in good agreement with our results.Upon adsorp-tion,the contacting plane of Au atoms is moved 0.05?upward from its relaxed position and the distance between the two adjacent Au atoms is increased by 0.14?.This corresponds to a deformation energy of 0.19[0.20]eV.Among the atomic species,sulfur is the adsorbate with the second highest deformation energy.This is consistent with the structur-al changes observed by Min and coworkers on the Au(111)surface when sulfur is adsorbed [89].In particular,they demonstrated that the Au(111)herringbone reconstruction lifts during sulfur adsorption,which releases Au atoms from the surface.These structural changes were attributed to the reduction of tensile surface stress on Au(111)by charge redistribution,which is caused by the electronegativity of sulfur.The adsorption of other electronegative species,such as oxygen,also causes restructuring of Au(111).Min and coworkers indicate that the release of gold atoms caused by the adsorption of sulfur or oxygen on Au(111)enhances the catalytic activity for O 2dissociation and SO 2decomposition because of the formation of low-coordinated Au sites.Our DFT calculations estimated a vibrational frequency for S adsorbed at the fcc site of 289cm ?1.The barrier to diffusion of S from an fcc to an hcp site through a bridge site is estimated to be 0.54[0.47]eV.3.1.5.Carbon (C)

As observed with other metal surfaces,such as Pt(111)[83],carbon binds most strongly to the Au(111)surface among all seven atomic species analyzed in this study,with a binding energy of ?4.03[?3.54]eV at the fcc site.Diffusion of a C atom from fcc to hcp passing through a bridge site has an estimated barrier of 0.72[0.69]eV.

Vibrational frequency calculations for C at the fcc site give an Au \C stretching frequency of 415cm ?1.The calculated adsorption height for C on Au(111)is 0.96?.In addition,the outward displacement of the Au atoms caused by the adsorbed C is 0.09?and the distance be-tween the two neighboring Au atoms increases by 0.12?.These modi-?cations on the geometry yield a deformation energy of 0.13[0.10]eV.3.1.6.Fluorine (F)

The most stable adsorption site for F is the fcc site with a binding energy of ?2.77[?2.41]eV.Slightly less stable adsorption of F occurs at the hcp and the bridge sites with binding energies of ?2.75[?2.39]and ?2.74[?2.39]eV,respectively.Consequently,the diffu-sion of F on Au(111)from an fcc site to an hcp site through the bridge site has an estimated barrier of only 0.03[0.02]eV.The calculated

f c c -b r -h c p

f c c -b r

b r -f

c c

t o p -b r

t o p -f c c

b r -t o p

f c c -b r -h c p

t o p -b r

f c c -b r -h c p

b r -t o p

f c c -b r -h c p

f c c -t o p

0.0

0.2

0.4

0.6

0.81.0

1.2

1.4C O

C l

C N

O H

H N O

N O

N H 3

N H 2

C N H 2

N O H

H C O 3

N H

C H 2

C H

C O H

D i f f u s i o n B a r r i e r (e V )

PW91RPBE

f c c -b r -h c p

f c c -b r

f c c -b r -h c p Fig.2.Estimated diffusion barriers for various adsorbate species on Au(111).The diffusion pathway for each adsorbate is indicated above each bar.

Table 2

Vibrational frequencies of adsorbed atomic species at fcc sites on Au(111).Adsorbate Calculated (cm ?1)Experimental (cm ?1)

H 794C 415N 430O 382380a

F 292S 289Cl

204

260b

a O,HREELS [28].b

Cl ?,SERS [103].

Table 3

Estimated diffusion barriers for various adsorbates on Au(111).Adsorbate

Diffusion barrier (eV)Diffusion path

PW91

RPBE H 0.150.16fcc-br-hcp b C 0.720.69fcc-br-hcp b O 0.560.47fcc-br-hcp b N 0.840.78fcc-br-hcp b S 0.540.47fcc-br-hcp b Cl 0.020.00fcc-br-hcp F 0.030.02fcc-br-hcp OH 0.100.15

br-fcc CO 0.00a

fcc-br NO 0.140.29top-fcc NH 0.730.63fcc-br-hcp b NH 20.310.17br-top NH 30.210.05top-fcc CH 0.900.90fcc-br-hcp b CH 20.870.81br-top CN 0.070.03fcc-br HNO 0.110.18top-br

NOH 0.510.45fcc-br-hcp b COH 1.32 1.19fcc-top CNH 20.310.29fcc-br-hcp b HCO 3

0.53

0.44

top-br

The diffusion barrier cannot be estimated because the calculated binding energy is positive (i.e.does not adsorb).b

The energy of the adsorbate at the metastable site was calculated by ?xing the x and y coordinate of the atom through which the adsorbate binds over the metastable site with the x,y,and z coordinates of the slab atoms ?xed.All other degrees of freedom were relaxed.

Y.Santiago-Rodríguez et al./Surface Science 627(2014)57–69

Au \F stretching frequency for F adsorbed at the fcc site is 292cm ?1.The adsorption of F takes place at a height of 1.65?from the Au surface.The deformation energy for the interaction of F with the Au(111)sur-face is 0.07[0.10]eV,which is generated by the downward displace-ment by 0.03?of the Au atoms in contact with F and an increase of 0.09?in the distance between two contacting Au atoms.

3.1.7.Chlorine (Cl)

Atomic chlorine binds to the fcc site on Au(111)with a binding en-ergy of ?2.30[?1.89]eV.This calculated energy agrees with a ?2.34eV Cl \Au bond dissociation energy estimated by Kastanas and Koel using TPD [81].Moreover,the preferred site according to our calcu-lations is consistent with the ?ndings of Gao and coworkers in their DFT

study,where they con ?rmed that the adsorption of chlorine on the fcc site is the con ?guration of lowest energy [82].Our calculated difference in energy between the fcc,hcp and bridge sites is within the error of the calculation,with a difference less than 0.02eV.Hence,chlorine is the atomic species with the smallest estimated diffusion barrier,0.02[0.00]eV,to move from fcc to hcp through bridge.The calculated vibra-tional frequency for the Au \Cl stretching is 204cm ?1,which is similar to the experimental value of 260cm ?1obtained by Tadayyoni and Weaver with surface-enhanced Raman spectroscopy (SERS)[103].

Table 4

Deformation energy (ΔE)upon adsorption of each species on Au(111).Adsorbate

Site

ΔE (eV)PW91

RPBE H fcc 0.060.03C fcc 0.130.10N fcc 0.150.14O fcc 0.220.23S fcc 0.190.20Cl fcc 0.070.10F fcc 0.070.10CO fcc 0.090.06CH fcc 0.160.13CH 2bridge 0.200.16CH 3top 0.070.06COH fcc 0.160.12HCO top 0.070.06CN fcc 0.110.10OH bridge 0.090.10NO top 0.040.04NH fcc 0.340.33NH 2bridge 0.200.20NH 3top 0.010.01NOH fcc 0.270.26HNO top

0.020.03COOH top-H down 0.060.05HCOO top 0.030.06CNH 2fcc 0.160.14HCO 3

top

0.03

0.07

Fig.3.Top and side view of geometric parameters for (a)atomic and (b)molecular adsorbates.Z A –Au (?)is the vertical distance between the adsorbate and the plane of Au in contact with it;ΔZ Au (?)is the change in vertical distance of the Au atoms in contact with the adsorbate and the plane of Au atoms on a clean relaxed surface;d Au –Au (?)is the average distance between adjacent Au atoms in contact with the adsorbate,and this distance for two adjacent Au atoms on a relaxed clean surface is 2.96?;d A –B (?)is the bond length within an adsorbed species.

Table 5

Adsorption geometry on Au(111).Adsorbate

Z A –Au (?)ΔZ Au (?)d Au –Au (?)d A –B (?)

H (fcc)0.690.07 3.04C (fcc)0.960.09 3.08N (fcc) 1.050.08 3.09O (fcc) 1.150.06 3.13F (fcc) 1.65?0.03 3.05S (fcc) 1.610.05 3.10Cl (fcc) 2.03?0.01 3.04CO (fcc) 1.360.09 3.05 1.19CH (fcc)

1.040.12 3.09 1.10CH 2(bridge) 1.470.22 3.03 1.10CH 3(top)

2.130.22 2.97 1.09

COH (fcc) 1.110.13 3.09 1.34(CO)0.99(OH)HCO (top) 2.100.24 2.97 1.21(CO)1.11(CH)OH (bridge) 1.760.06 3.040.99NO (top) 2.210.16 2.95 1.18NH (fcc)

1.020.12 3.16 1.03NH 2(bridge) 1.580.15 3.08 1.02NH 3(top)

2.450.03 2.96 1.02

NOH (fcc) 1.120.10 3.14 1.41(NO)0.99(OH)HNO (top) 2.200.09 2.95 1.24(NO)1.05(NH)COOH

(top-H down) 2.130.20 2.96 1.21(CO)0.99(OH)1.33(C \OH)

HCOO (top) 2.280.02 2.93 1.27(CO)1.11(CH)CN (fcc) 1.380.07 3.05 1.20

CNH 2(fcc) 1.260.10 3.08 1.35(CN)1.02(NH)HCO 3(top)

2.26

0.00

2.94

1.27(CO a )1.28(CO b )0.96(OH)1.11(CH)1.36(C \OH)

See Fig.3for geometric de ?nitions of these parameters.a

Refers to CO group of the left side in HCO 3molecule as depicted in Fig.1.b

Refers to CO group of the right side in HCO 3molecule as depicted in Fig.1.

62Y.Santiago-Rodríguez et al./Surface Science 627(2014)57–69

Chlorine is adsorbed at a height of 2.03?from the Au surface.The adja-cent Au atoms are shifted 0.01?downwards from their original https://www.wendangku.net/doc/fb14462006.html,ing XRD,the Au \Cl bond length in bulk Au(III)Cl was found to be 2.23?[104],which is close to our calculated value of 2.69?.The de-formation energy caused by these changes on the surface is 0.07[0.10]eV.

3.2.Adsorption of molecules and molecular fragments

3.2.1.Carbon monoxide (CO)

Our DFT calculations of CO adsorption on Au(111)indicate that ad-sorption at the fcc and bridge sites has the same binding energy of ?0.36[0.10]eV,while CO adsorbs at hcp and top sites with binding en-ergies of ?0.32[0.14]eV and ?0.31[0.07]eV,respectively (Table 6).The preferred adsorption site for CO on Au(111)is different for the two DFT functionals used in this study.According to PW91,the pre-ferred adsorption site is either the fcc or the bridge site.However,with the RPBE functional the lowest energy adsorption site for CO is the top site.A previous combined experimental and theoretical study (IRAS and DFT-GGA)concluded that CO adsorbs on top sites at low cov-erage [42],which is not in agreement with the adsorption site prefer-ences obtained with PW91.It has been reported in the literature that DFT with non-hybrid functionals such as PW91and RPBE functionals tend to fail in predicting the correct adsorption site for strong pi-acceptor molecules such as CO [45,46,105].Empirical corrections for the adsorption energy have been developed by Mason et al.[105]to im-prove the prediction of CO adsorption sites on metal surfaces obtaining results in agreement with experimental data.The binding energy for CO has been estimated using TPD and it is ?0.43eV [106].Yim and coworkers also reported adsorption energies close to our results:?0.46eV and ?0.56eV for the low-temperature programmed desorp-tion peak and the high-temperature peak,respectively,which were calculated with experimental data obtained on Au(111)surfaces

roughened by argon ion bombardment using a scanning tunneling microscopy/TPD system [107].Yim et al.also studied the adsorption of CO on this surface using infrared spectroscopy,obtaining a C \O vibrational frequency of 2120cm ?1.Our calculated vibrational fre-quency for the C \O vibrational mode when CO is adsorbed at the bridge site is 1983cm ?1(Table 7),while it is at 1898cm ?1when adsorbed at the fcc site.Our results,however,are in closer agreement to the frequency reported by Sun et al.of 1925–1975cm ?1[108]using surface-enhanced infrared absorption spectroscopy (SEIRAS)on a Au ?lm electrode.In contrast,Piccolo and coworkers found the mode at 2060cm ?1between 1and100Torr pressure using IRAS [42],and they concluded that it is due to linearly adsorbed CO at the top site.This experimental frequency is similar to our calculated C \O stretch fre-quency at the top site,which is 2142cm ?1.We also calculated frequen-cies for the Au \CO stretch at the fcc and bridge site at 233cm ?1and 263cm ?1,respectively.The latter value is close to the 290cm ?1band determined by Tadayyoni and Weaver using SERS,which corresponds to the carbon-surface vibration generated during the CO adsorption on gold electrodes [103].The diffusion path from fcc site to bridge site has an estimated barrier of 0.00eV.This diffusion barrier is the smallest of all the species analyzed in this study (see Fig.2).According to our cal-culations,CO is adsorbed at the fcc site with the C atom bound to the surface at a distance of 1.36?from the surface.The C \O bond is parallel to the surface normal with a C \O bond length of 1.19?.A deformation energy of 0.09[0.06]eV is caused by a vertical outward displacement of the contacting Au atoms from their original positions of 0.09?and a separation of 0.09?between adjacent Au atoms.

3.2.2.Methyl (CH 3),methylene (CH 2)and methylidyne (CH)

According to our calculations,CH 3adsorbs only at the top site of Au(111),with a binding energy of ?1.19[?0.86]eV.CH 2adsorbs most strongly to the bridge site,with a binding energy of ?2.45[?1.97]eV.Similar to other metal surfaces,such as Pt(111)[83],CH

Table 6

Binding Energies (PW91[RPBE])and site preferences of molecules and molecular fragments on Au(111).Entries in bold face signify preferred adsorption con ?gurations,based on the PW91binding energies.Adsorbate

Preferred site Binding Energy (eV)Calc.

Exp.top

fcc

hcp

bridge Exp.

CO fcc/bridge (PW91)top (RPBE)top d

?0.31[0.07]?0.36[0.10]?0.32[0.14]?0.36[0.09]

?0.43a

?0.46b,?0.56c

CH fcc –

?4.34[?3.75]?4.09[?3.54]CH 2bridge ?1.58[?1.16]––?2.45[?1.97]CH 3top ?1.19[?0.86]–

––COH fcc ?0.95[?0.50]?2.27[?1.70]––HCO top ?1.33[?0.97]–

–

CN fcc ?2.52[?2.23]?2.69[?2.27]?2.62[?2.22]?2.62[?2.25]OH bridge ?1.51[?1.08]?1.60[?1.03]?1.55[?1.00]?1.70[?1.18]NO top ?0.30[0.06]?0.16[0.35]?0.10[0.39]–

?0.40e

NH fcc ?0.67[?0.27]?2.22[?1.60]?1.93[?1.34]NH 2bridge ?1.12[?0.73]–

–?1.43[?0.90]NH 3top ?0.29[0.00]?0.08[0.05]–

–

?0.34f

NOH fcc ?1.61[?1.09]?2.26[?1.55]?2.04[?1.35]HNO top

?2.74[?2.26]––?2.63[?2.08]COOH top-H down ?1.47[?1.01]?1.40[?0.98]h –

–

HCOO top i top i,g

?1.86[?1.29]–

–

–CNH 2fcc –

?2.08[?1.53]?1.90[?1.39]–

HCO 3

top i

?2.36[?1.43]

–

?1.72[?0.86]

?1.83[?0.99]

No stable adsorption structure was obtained for entries indicated with “–”.a

CO,TPD [106].b

LTP,CO,STM/TPD [107].c

HTP,CO,STM/TPD [107].d

CO,STM [42].e

NO,TPD [111].f

NH 3,TPD [109].g

HCOOH/HCOO,bidentate (η2-O,O)con ?guration,NEXAFS/IRAS/DFT [74].h

Two stable carboxyl (COOH)binding geometries were found over the top site,but they differ on the orientation of O \H bond.This value corresponds to the binding energy of the top site with the O \H bond oriented away from the surface.i

Two oxygen atoms of the species are respectively adsorbed on two gold atoms by the preferred site speci ?ed.This conformation is also named bidentate (η2-O,O)con ?guration.

Y.Santiago-Rodríguez et al./Surface Science 627(2014)57–69

has the strongest binding among all CHx fragments as illustrated in Fig.4,with a binding energy of ?4.34[?3.75]eV at the fcc site.

The calculated frequency for the C \H stretching mode in CH is 3092cm ?1.For CH 2,the estimated symmetric stretching mode has a frequency of 3041cm ?1.Similarly,the calculated frequency for the C \H asymmetric stretching mode of CH 2is 3137cm ?1,for the scissor-ing mode of CH 2is 1312cm ?1and for the C \H twisting mode of CH 2is 640cm ?1.Our DFT calculations provided frequencies of 3032and 3152–3153cm ?1for the symmetric and asymmetric C-H stretching modes of CH 3,respectively.

As shown in Fig.1,CH 3is bound on a top site in a tetrahedral geom-etry through its carbon atom 2.13?above the surface and the C \H bonds (C \H bond lengths are 1.09?)are oriented 74°from the surface normal.The adsorption of CH 3pulls the bound Au atom 0.22?out of the surface plane at a deformation cost of 0.07[0.06]eV.On bridge sites,CH 2is bound through its carbon atom 1.47?above the surface with the 1.10?C \H bonds oriented 54°from the surface normal.This adsorption deforms the surface by 0.20[0.16]eV.CH is adsorbed on fcc sites through its carbon atom,which is adsorbed 1.04?above the surface,corresponding to a Au \C bond length of 2.07?.The 1.10?C \H bond is oriented parallel to the surface normal.This adsorption slightly displaces the adjacent surface Au atoms by 0.12?vertically,

and expands their Au \Au bond lengths to 3.09?.This deformation costs 0.16[0.13]eV.The diffusion barriers have been estimated as 0.90[0.90]eV for CH and 0.87[0.81]eV for CH 2.We were unable to locate a metastable adsorption site for CH 3,therefore we have not estimated its diffusion barrier.

3.2.3.COH intermediate and formyl (HCO)intermediate

The most stable adsorption site for COH is the fcc site,with a binding energy of ?2.27[?1.70]eV.The formyl intermediate (HCO)has a weaker interaction with Au(111),binding at the top site with a binding energy of ?1.33[?0.97]https://www.wendangku.net/doc/fb14462006.html,paring the total energies of both isomers on the gold surface,HCO adsorbed on Au(111)is more stable than the adsorbed COH isomer by 0.83[1.03]eV.At fcc sites,COH is bound through its carbon atom,with the C \O bond axis parallel to the surface normal and the O \H bond oriented 69°away from the sur-face normal (see Fig.1).The C \O bond length is 1.34?,while the O \H bond length is 0.99?.This adsorption causes a displacement of the gold atom bonded to COH outward by 0.13?and it increases the distance for two adjacent Au atoms by 0.13?.These geometric changes produce a surface deformation of 0.16[0.12]eV on Au(111).HCO bound at the top site,has its C \H bond oriented 69°with respect to the surface nor-mal and the C \O bond oriented 55°from the normal (the H \C \O bond angle is 124°).The C \O bond in adsorbed HCO is 1.21?,signi ?-cantly shorter than in COH,and the C \H bond length is 1.11?.The adsorption of HCO pulls the bound Au atom 0.24?out of the surface plane,which has a deformation cost of 0.07[0.06]eV.Additional geo-metric parameters are reported in Table 5.The diffusion barrier for COH was estimated to be 1.32[1.19]eV,while we were unable to locate a metastable site to estimate the barrier for HCO diffusion.Vibrational modes for COH and HCO are reported in Table 8.For COH,we ?nd the asymmetric O \H stretch at 3717cm ?1,the asymmetric C \O stretch at 1239cm ?1,the Au \COH stretch at 324cm ?1,a scissoring mode at 1120cm ?1,rocking modes at 389and 437cm ?1,and a twisting mode at 153cm ?1.For HCO we found the asymmetric C \H stretch at 2897cm ?1,the Au \HCO stretch at 471cm ?1and 223cm ?1,a scissor-ing mode at 1166cm ?1,a wagging mode at 729cm ?1,and a twisting mode at 85cm ?1.

3.2.

4.Ammonia (NH 3),amide (NH 2)and imide (NH)

NH 3prefers to bind at the top site of Au(111)with a binding energy of ?0.29[0.00]eV.This low binding energy is in excellent agreement with the experimental value of ?0.34eV estimated by Kay and co-workers using TPD [109].NH 2and NH bind more strongly to Au(111),with binding energies of ?1.43[?0.90]eV at the bridge site and ?2.22[?1.60]eV at the fcc site,their preferred adsorption

-5-4.5-4

-3.5-3-2.5-2-1.5-1-0.50

0.5

N H 3

N O

C O (b r i d g e )

C O (f c c )

C H 3

H C O

N H 2

C O O H

O H

H C O O

C N H 2

N H

N O H

C O H

C l

H C O 3

C H 2

C N

H N O

C H

B i n d i n g E n e r g y (e V )

PW91RPBE

Fig.4.Binding energies on Au(111)for the most stable site of each species.

Table 7

Vibrational frequencies (in cm ?1)of adsorbed diatomic species in their lowest energy con ?gurations on Au(111).Adsorbate

Calculated (cm ?1)Experimental (cm ?1)(IM)

(AS)(IM)

(AS)CO (fcc)189********a

CO (br)198********–1975b ,2060a ,2120c 290d

CH (fcc)3092500CN (fcc)20122392100370e,h ,300e,i

OH (br)37763323750f NH (fcc)3500423NO (top)

1800

342

1810g

IM stands for intramolecular,and AS stands for adsorbate-surface.a

CO (top site),IRAS [42].b

CO (bridge site),SEIRAS [108].c

CO,FTIR [107].d

CO,SERS [103].e

CN ?,SERS [114].f

O 2,FTIR [115].g

NO,DRIFTS [112].h

ν(Au \CN).i

δ(Au \CN).

64Y.Santiago-Rodríguez et al./Surface Science 627(2014)57–69

sites respectively.The calculated vibrational frequencies for NH are3500cm?1for the N\H stretching and423cm?1for Au\NH stretching.The symmetric and asymmetric mode of NH2,and the symmetric stretching for NH3have frequencies of3473,3598and 3442cm?1,respectively.NH3shows the N\H asymmetric stretching mode at3609and3611cm?1.NH3is adsorbed at a height of2.45?from the surface and the N\H bond lengths are1.02?,which are in good agreement with the experimental gas phase value of1.012?[110].The adsorption of NH3slightly distorts the surface,with a deforma-tion energy of0.01[0.01]eV.NH2is adsorbed through its N atom1.58?above the surface with a Au\N bond length of2.20?and the1.02?N\H bonds oriented55°from the surface normal.The Au atoms bound to NH2 are pulled0.15?out of the relaxed surface and their Au\Au bond dis-tance expands to3.08?,which causes a surface deformation of0.20 [0.20]eV.NH binds1.02?above the surface,corresponding to a Au\N bond length of2.09?.The N\H bond is1.03?and is oriented parallel to the surface normal.Interestingly,NH adsorption induces the most-positive deformation energy of all adsorbates in this study,with a value of0.34[0.33]eV.This adsorption stretches the Au\Au bond lengths be-tween N-adjacent Au atoms by0.21?.Diffusion barriers are estimated for NH3,NH2,and NH as0.21[0.05]eV,0.31[0.17]eV,and0.73[0.63]eV.

3.2.5.Nitric oxide(NO)

Nitric oxide adsorbs at the top,fcc,and hcp sites with binding ener-gies of?0.30[0.06],?0.16[0.35],and?0.10[0.39]eV,respectively.An experimentally estimated binding energy of?0.4eV was reported by McClure and coworkers using TPD[111],in agreement with our calcu-lated top site adsorption mode.We calculated the N\O stretching frequency at1800cm?1and the Au\NO stretching at342cm?1. Debeila and coworkers reported an experimental N\O stretching fre-quency of1810cm?1on Au/TiO2using DRIFTS[112],in reasonable agreement with our results.NO adsorbs on Au(111)through its N atom at a height of2.21?from the surface.The N\O bond axis is tilted 63°away from the surface normal with an N\O bond length of1.18?(see Fig.1).The deformation on the surface caused by the NO adsorp-tion has an energy of0.04[0.04]eV.NO adsorbed on Au(111)is estimat-ed to diffuse from the top site to the fcc site with an energy barrier of 0.14[0.29]eV.

3.2.6.Nitrosyl hydride(NOH,HNO)

The minimum energy site for the adsorption of NOH on Au(111)is the fcc site,while HNO adsorbs most strongly at the top site.The binding energies of these species were calculated with respect to isolated NO and isolated H,both in the gas phase.They are?2.26[?1.55]eV and ?2.74[?2.26]eV,respectively.Since these binding energies are refer-enced to the same state,we can conclude that HNO is more stable on Au(111)than NOH.The vibrational modes calculated for NOH are at 3704cm?1,1271cm?1,896cm?1,399cm?1,393cm?1,247cm?1, and166cm?1,while HNO has the following vibrational frequencies: 3078cm?1,1504cm?1,1381cm?1,353cm?1,194cm?1,112cm?1, and55cm?1(see Table8).NOH binds through its N atom1.12?above the surface with the N\O bond oriented nearly parallel with the surface normal.The N\O bond length is1.41?,which is signi?-cantly longer than that of NO.The N\O\H bond angle is106°and the O\H bond is0.99?.HNO is adsorbed through its N atom at a height of2.20?above the surface.The N\H bond length is1.05?and it is oriented65°away from the surface normal.The N\O bond length is 1.24?and it is oriented48°away from the surface normal.The Au surface is deformed more signi?cantly upon the adsorption of NOH compared to the adsorption of HNO,with calculated deformation ener-gies of0.27[0.26]eV and0.02[0.03]eV,respectively.The diffusion bar-rier for NOH is estimated to be0.51[0.45]eV and for HNO0.11[0.18]eV.

3.2.7.Carboxyl(COOH),formate(HCOO)and bicarbonate(HCO3)

The minimum energy adsorption structure for carboxyl is obtained when the species adsorbs at the top site through its carbon atom at a

Table8

Vibrational frequencies(in cm?1)of adsorbed polyatomic species in their lowest energy con?gurations on Au(111).

Modes NH3NH2CNH2NOH HNO CH3CH2HCO COH HCOO COOH HCO3 top bridge fcc fcc top top bridge top fcc top a Exp.top-H down top a Exp.

Symmetric IM stretch344234733486d

1298e 303230411308h

2935f

1332h,j

2824f,j

1821g

3618b

1023i

1394h

3782b

1440h,k

3610b,k

Asymmetric IM stretch3609

361135983602d896c

3704b

1504c

3078d

3152

3153

31372897f

1821g

3717b

1239g

1574h1115

1183

1558h1650h,k

AS stretch152373321247353

194432478471

223

32472

248

230

222

Scissoring1570

157114501545127113811388

1391

131211661120732633598

650

Rocking389

4055511072

398

399

393

112709

714

636389

437

259

1285

219190

Wagging929617291166110577872996544391

537

1199

Twisting4367139155640851536386

592

54 750

IM stands for intramolecular,and AS stands for adsorbate-surface.

a Means that two atoms of the species are adsorbed on the preferred site speci?ed. bν(O\H).

cν(N\O).

dν(N\H).

eν(C\N).

fν(C\H).

gν(C\O).

hν(O\C\O).

iν(CO3).

j HCOOH/HCOO,IRAS[74].

k CO,FTIR[113].65

Y.Santiago-Rodríguez et al./Surface Science627(2014)57–69

distance of2.13?from the bound Au atom,with the hydrogen atom of the hydroxyl group oriented towards the surface(see Fig.1).The bond lengths of C\O,O\H and C\OH for COOH are:1.21?,0.99?and 1.33?,respectively.The binding energy for this con?guration is ?1.47[?1.01]eV.When the hydrogen atom of the hydroxyl group is oriented away from the surface,the binding energy is0.07[0.03]eV more positive(see Table6).Formate adsorbs in a bidentate(η2-O,O) con?guration as illustrated in Fig.1,with its two oxygen atoms bonded to the surface by a Au\O length of2.28?.This result is in agreement with the experimental observations reported by Senanayake and co-workers using NEXAFS and IR spectroscopy[74].This preferred con?gu-ration has both C\O bond lengths corresponding to1.27?and a C\H bond length of1.11?.The binding energy for HCOO adsorbed on Au(111)is?1.86[?1.29]eV.Adsorbed formate and carboxyl are isomers of each other,and in that form,adsorbed HCOO is more stable than adsorbed COOH by0.12[0.10]eV.The binding energy for HCO3 was calculated with respect to isolated CO2and OH gas phase species, and it is?2.36[?1.43]eV.The preferred site for the adsorption of bicarbonate is the top site,also with two O atoms absorbed on Au(111)at a height of2.26?,see Fig.1.Details of calculated bond lengths of adsorbed HCO3are provided in Table5.

A deformation energy of only0.06[0.05]eV was estimated for the adsorption of COOH on Au(111).This deformation results by the out-ward displacement of0.20?for the bound Au atom from its relaxed position.The Au\Au bond lengths between neighboring Au atoms remained unchanged.The adsorptions of HCOO and HCO3cause sim-ilar surface deformations,with energies of0.03[0.06]eV and0.03 [0.07]eV,respectively.When HCOO is adsorbed on Au(111),the ver-tical displacement of the Au atoms is only0.02?and the distance be-tween two adjacent Au atoms decreases by0.03?.Similarly,when HCO3is adsorbed,no vertical displacement of the upper gold plane in the surface is observed and the Au\Au bond length is contracted by0.02?.

The vibrational modes for COOH were calculated obtaining the C\O symmetric stretching at1821cm?1and the O\H symmetric stretching at3618cm?1.Senanayake et al.found the O\C\O symmetric stretching mode and the C\H stretching mode for HCOO at1332and 2824cm?1using IRAS[74].These experimental values are in good agreement with our vibrational frequencies of1308and2935cm?1 that correspond to the symmetric stretching of the bonds O\C\O and C\H in HCOO,respectively.HCO3has three symmetric stretching modes at1023cm?1for CO3,1394cm?1for O\C\O and3782cm?1 for O\H,and an asymmetric stretching mode at1558cm?1for O\C\O.Similar vibrational modes were obtained by Roze and co-workers for the O\C\O and O\H symmetric stretching,and the O\C\O asymmetric stretching in HCO3using FTIR:1440,3610and 1650cm?1,respectively[113].Other vibrational modes for COOH, HCOO and HCO3are summarized on Table8.The diffusion of HCO3on the surface through the path from top site to bridge site has an esti-mated energy barrier of0.53[0.44]eV.Diffusion barriers for COOH and HCOO were not calculated because we were unable to locate a metastable site for the adsorption of these species.

3.2.8.Hydrogen cyanide(HCN),cyanide(CN)and aminomethylidyne (CNH2)

According to our calculations,HCN does not adsorb on Au(111), while CN adsorbs with a binding energy of?2.69[?2.27]eV and CNH2adsorbs with a binding energy of?2.08[?1.53]eV.Both of these species prefer adsorption at fcc https://www.wendangku.net/doc/fb14462006.html, is adsorbed on Au(111) through its C atom1.38?above the surface.The C\N bond length is 1.20?and it is oriented parallel to the surface normal.The plane of Au atoms in contact with CN is moved0.07?upward and the distance between the contacting Au atoms increases by0.09?.The overall cost of this deformation is0.11[0.10]https://www.wendangku.net/doc/fb14462006.html,H2adsorbs through its carbon atom at1.26?above the surface with the C\N bond axis(bond length of1.35?)oriented parallel to the surface normal.The N\H bonds lengths are1.02?and the H\N\H bond angle is117°.Adsorption causes the Au atoms in contact with CNH2to rise by0.10?from their relaxed positions with an associated deformation energy of0.16 [0.14]eV.The calculated vibrational frequencies for CN include the C\N stretching mode at2012cm?1and the Au\CN stretch at 239cm?1.Beltramo et https://www.wendangku.net/doc/fb14462006.html,ed SERS to evaluate the vibrational modes of CN?adsorbed on a gold electrode?nding the C\N stretch at around 2100cm?1and the Au\CN stretching and bending modes at~370and ~300cm?1,respectively[114],which are in reasonable agreement with our calculated results.We?nd a number of CNH2vibrational modes, including a symmetric N\H stretch at3486cm?1,a symmetric C\N stretch at1298cm?1,an asymmetric N\H stretch at3602cm?1,the Au\CNH2stretch at321cm?1,a scissoring mode at1545cm?1, rocking modes at1072and398cm?1,a wagging mode at291cm?1, and a twisting mode at391cm?https://www.wendangku.net/doc/fb14462006.html, diffuses easily from the fcc to the bridge site with an estimated energy barrier of0.07[0.03]eV. CNH2has an estimated diffusion barrier from the fcc to the hcp site of 0.31[0.29]eV.

3.2.9.Hydroxyl(OH)

OH adsorbs preferentially at the bridge site with a binding energy of ?1.70[?1.18]eV.The OH adsorption occurs at a height of1.76?from the Au(111)surface and the bond length between O and H is0.99?. Adsorption pulls the O-adjacent Au atoms out from the surface by 0.06?and expands their Au\Au bond distance by0.08?.The energy associated with this deformation is0.09[0.10]eV.The vibrational frequency corresponding to the OH stretching is3776cm?1,which approaches a sharpυ(OH)band at3750cm?1observed by Brooker and coworkers analyzing the reduction of O2on a Au electrode in alkaline solution using FTIR[115].The stretching mode for Au\OH is 332cm?1.The diffusion barrier of OH from the bridge site to the fcc site is estimated to0.10[0.15]eV.

3.3.Thermochemistry of possible surface reactions

The PW91binding energies of all the species in their preferred adsorption sites,along with their gas phase energies,were used to calculate reaction energies for the decomposition,hydrogenation,and oxidation of various species.Energy diagrams which describe possible pathways for these surface reactions are shown in Figs.5and6.Some of these reactions are discussed below.

?CO2hydrogenation to carboxyl or formate;and OC\O bond dissociation:

CO2

geT

→CO2

geT

tH aeT→COOH aeT

CO2

geT

→CO2

geT

tH aeT→HCOO aeT

ΔE rxn=0.46eV

ΔE rxn=0.34eV COOH(a)→CO(a)+OH(a)ΔE rxn=1.33eV HCOO(a)→HCO(a)+O(a)ΔE rxn=2.70eV CO2(g)→CO(a)+O(a)ΔE rxn=3.04eV

This thermochemistry suggests that CO2hydrogenation is unfavor-able on Au(111).For hydrogenation,?rst hydrogen must dissociatively adsorb on Au(111)(H2(g)→2H(a)),which is endothermic by0.20eV. Subsequently,CO2is hydrogenated to COOH or HCOO.These reactions are endothermic by0.36eV and0.24eV,respectively(see Fig.5).The direct scission of the OC\O bond in CO2is endothermic by3.04eV. The OC\O bond in COOH can be broken to produce CO and OH,which is endothermic by1.33eV,while the decomposition of HCOO to HCO and O is endothermic by2.70eV.Overall,these results suggest that Au(111)would be a poor catalyst for CO2hydrogenation.It is thermo-dynamically favorable for any adsorbed hydrogen to desorb rather than to hydrogenate CO2.Furthermore,our results suggest that the most thermochemically ef?cient method of breaking the OC\O bond is through the COOH intermediate,producing CO(a)+OH(a).

66Y.Santiago-Rodríguez et al./Surface Science627(2014)57–69

?Formation of COH and HCO via hydrogenation of CO;and C \O bond scission:

CO g eTt12H 2g eT→CO a eTtH a eTΔE rxn =?0.26eV CO g eTt12H 2g eT→COH a eTΔE rxn =0.47eV CO g eTt12H 2g eT→HCO a eTΔE rxn =?0.37eV COH (a)→C (a)+OH (a)ΔE rxn =2.44eV HCO (a)→CH (a)+O (a)ΔE rxn =2.86eV CO (a)→C (a)+O (a)

ΔE rxn

4.52eV

The hydrogenation of CO requires the adsorption of CO (ΔE rxn =?0.36eV)and the adsorption of H 2(ΔE rxn =0.20eV),as illustrated in Fig.5.CO hydrogenation can occur at the carbon atom to produce HCO (ΔE rxn =?0.11eV)or at the oxygen atom to produce COH (ΔE rxn =0.73eV).The net reaction energies (starting from CO (g)and 1/2H 2(g))are ?0.37eV and 0.47eV,respectively.The direct dissociation of adsorbed CO is endothermic by 4.52eV (see Fig.6).Alternatively,the C \O bond can be broken by the decomposition of COH or HCO to C +OH or CH +O,respectively.The reaction energies for these respective pathways are 2.44eV and 2.86eV.Overall,these results show that CO hydrogenation to HCO is more thermodynamically favor-able than to COH on Au (111).In fact,it is more favorable for CO to be hydrogenated to HCO than for CO and H to desorb from Au(111).Also,the C \O bond is most favorably broken by proceeding through a HCO intermediate on Au(111).

?Formation of NOH and HNO through the hydrogenation of NO;and N \O bond scission:

NO g eTt12H 2g eT→NO a eTtH a eTΔE rxn =?0.20eV NO g eTt12H 2g eT→NOH a eTΔE rxn =0.02eV NO g eTt12H 2g eT→HNO a eTΔE rxn =?0.45eV NOH (a)→N (a)+OH (a)ΔE rxn =0.48eV HNO (a)→NH (a)+O (a)ΔE rxn =0.74eV NO (a)→N (a)+O (a)

ΔE rxn

2.05eV

NO adsorbs on Au(111)with a binding energy of ?0.30eV (see Fig.5),while the dissociative adsorption of H 2requires 0.20eV.As

shown in Fig.5,NO (a)would more favorably be hydrogenated at the N atom to produce HNO (ΔE rxn =?0.25eV)than on the O atom to produce NOH,which has a reaction energy of 0.22eV.The net reaction (beginning from NO (g)and 1/2H 2(g))for the formation of HNO is thermochemically favorable,while the corresponding net reaction for NOH is practically thermoneutral.The N \O bond in adsorbed NO can be directly dissociated with a reaction energy of 2.05eV.The N \O bond can be dissociated in hydrogenated NO via NOH to N +OH (ΔE rxn =0.48eV)or HNO to NH +O (ΔE rxn =0.74eV).Similar to CO,all of this thermochemistry suggests that NO,in the presence of hydro-gen,is more likely to be hydrogenated to HNO than to desorb from Au(111).Furthermore,these results indicate that the most ef ?cient method of breaking the N \O bond is by ?rst hydrogenating to HNO and then breaking the bond to form NH +O.?CO oxidation to CO 2:

CO (g)→CO (a)ΔE rxn =?0.36eV O 2(g)→2O (a)

ΔE rxn =0.10eV CO (a)+O (a)→CO 2(g)

ΔE rxn =?3.04eV

The oxidation of CO proceeds by adsorption of CO and O 2on the sur-face.CO adsorbs mildly with an adsorption energy of ?0.36eV,while the activated dissociative adsorption of O 2is endothermic by 0.10eV.The reaction between adsorbed CO and adsorbed atomic oxygen to lib-erate gas phase CO 2is very exothermic,with ΔE rxn =?3.04eV (see Fig.6).Compared to other reactions studied on Au(111),this is the most thermodynamically favorable reaction.This result is in agreement with the experiments that show that this catalyst has high catalytic activity for the oxidation of CO [16,116,117].Clearly,kinetic limitations to activating the O \O bond control the overall reaction rate.?Decomposition of CH 4,NH 3,and N 2:

We ?nd that N 2does not adsorb on Au(111)and that its activated dissociative adsorption is highly unfavorable (ΔE rxn =5.20eV),as illus-trated in Fig.6.Also,methane does not adsorb on Au(111),and its acti-vated dissociative adsorption,CH 4(g)→CH 3(a)+H (a)is endothermic by 1.31eV.All subsequent dehydrogenation steps,leading to the formation

-1

E n e r g y (e V )

NO(g)+1/2H 2(g)N(a)+OH(a)NH(a)+O(a)NOH(a)NO(a)+H(a)

HNO(a)

NO(g)NO(a)N(a)+O(a)

CO 2(g)+1/2H 2(g)HCO(a)+O(a)

COOH(a)

CO 2(g)+H(a)CO(a)+OH(a)HCOO(a)0.74

0.48

0.360.24

1.33

2.05

0.10

CO(g)+1/2H 2(g)COH(a)CO(a)+1/2H 2(g)CH(a)+O(a)

C(a)+OH(a)

HCO(a)

CO(a)+H(a)0.22 2.86

0.102.44

0.73 2.70

-0.25-0.20-0.36-0.11-0.30Fig.5.Thermochemistry for the hydrogenation of NO,CO and CO 2,and for the molecular decomposition of NO.The reference zero corresponds to the energy of the isolated species in gas-phase and the energy of the clean relaxed Au(111)slab at in ?nite separation.Energies are calculated using the PW91functional and the change in energy is labeled next to each arrow on the diagram.Gas-phase species are indicated with (g)and adsorbed species are indicated with (a).The energy of states with multiple adsorbed species are calculated by assuming the adsorbates are at in ?nite separation on the surface.

Y.Santiago-Rodríguez et al./Surface Science 627(2014)57–69

of adsorbed atomic carbon,are also endothermic with the reaction ener-gies of 1.47eV,0.77eV and 1.77eV for the second,third and fourth dehydrogenation steps,respectively.Ammonia binds to Au(111)with a binding energy of ?0.29eV.The dehydrogenation of ammonia is endothermic with reaction energies of 1.47eV,1.24eV,and 1.57eV for the ?rst,second,and third dehydrogenation steps,respectively.As a result,it is more favorable for ammonia to desorb from Au(111)than to be dehydrogenated.4.Conclusions

The adsorption of several atomic (H,O,N,S,C,F and Cl)and molec-ular species (CO,NO,N 2,HCN and NH 3),and molecular fragments (CH 3,CH 2,CH,COH,HCO,NH 2,NH,OH,NOH,HNO,COOH,HCOO,HCO 3,CN and CNH 2)on Au(111)has been studied using periodic self-consistent DFT calculations.The preferred binding sites,binding energies and binding geometries of the species along with their estimated diffusion energy barriers were determined.For all the adsorbates on their most energetically stable adsorption site,the intramolecular and adsorbate-surface vibrational frequencies were calculated.The theoretical results were compared with experimental values found in the literature,when available,and most of these values are in good agreement,indi-cating that our calculations can correctly predict the experimental observations.In general,the PW91functional is more accurate than RPBE for estimating the binding energies of the species adsorbed on Au(111).The RPBE functional tends to underestimate these energies.Our results demonstrate that atomic species tend to bind on Au(111)more strongly than molecular fragments.All the atomic species ana-lyzed adsorb preferentially at fcc sites.Molecular species and fragments do not show any clear preference for the adsorption site.

The thermochemistry for the adsorption and decomposition reac-tions of CH 4,CO,NO,NH 3and N 2,hydrogenation of CO,CO 2and NO,and oxidation of CO were also studied on Au(111).Reaction energies were calculated and used to determine the most thermodynamically favorable reactions.These reaction energies demonstrate that the decomposition of species is not favorable on Au(111),showing that the desorption of NH 3,NO and CO is preferable to the decomposition

of these species.Thus,these theoretical results suggest that in general,bond-making events are facile on Au(111)while bond-breaking events are more dif ?cult.Moreover,the oxidation of CO and hydrogenation of CO and NO to HCO and HNO,respectively,are thermodynamically favored on Au(111).The intermediates COOH and HCOO can be formed by the hydrogenation of CO 2requiring moderately endothermic reac-tion energies,HCOO being more stable than COOH.Of all the reactions analyzed on Au(111),the oxidation of CO is the most favored on this surface,in accord with numerous experimental reports on the subject.Acknowledgments

This work has been supported by the National Science Foundation (DMR-0934115).JAH was partially supported by an Air Products gradu-ate fellowship.Work at UW-Madison was also supported by Depart-ment of Energy-Basic Energy Sciences,Division of Chemical Sciences (grant DE-FG02-05ER15731).The computational work was performed in part using supercomputing resources at the following institutions:EMSL,a National scienti ?c user facility at Paci ?c Northwest National Laboratory (PNNL);the Center for Nanoscale Materials at Argonne Na-tional Laboratory (ANL);and the National Energy Research Scienti ?c Computing Center (NERSC).EMSL is sponsored by the Department of Energy's Of ?ce of Biological and Environmental Research located at https://www.wendangku.net/doc/fb14462006.html,M and NERSC are supported by the U.S.Department of Energy,Of ?ce of Science,under contracts DE-AC02-06CH11357,and DE-AC02-05CH11231,respectively.References

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期刊影响因子的“含金量”是多少

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