Multiple Bonding Configurations of Adsorbed Formate on Ag{111}

The adsorption and reaction of the surface formate species on Ag{111} have ... Ag{111} at 240 K. Bidentate bridging formate with 2-fold symmetry is pr...
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J. Phys. Chem. 1996, 100, 12509-12516

12509

Multiple Bonding Configurations of Adsorbed Formate on Ag{111} W. S. Sim, P. Gardner,† and D. A. King* Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. ReceiVed: January 5, 1996; In Final Form: April 29, 1996X

The adsorption and reaction of the surface formate species on Ag{111} have been studied using a combination of reflection absorption infrared spectroscopy and temperature programmed desorption. Four formate species with distinct bonding configurations have been identified when formic acid is deprotonated by preoxidized Ag{111} at 240 K. Bidentate bridging formate with 2-fold symmetry is present at low coverage, while monodentate formate with three different degrees of tilt are formed sequentially with increasing coverage as a result of dipolar interactions and steric crowding. Coadsorption of methoxy and formate, accomplished by nucleophilic displacement of methyl formate with surface atomic O, yields an intermixed surface phase where interadsorbate interactions are minimized and formate tilting is suppressed. Monodentate formate partially decomposes to CO2 and formic acid at 325 K and partially reverts to bidentate bridging formate which dehydrogenates at 350 K to give CO2 and H2.

1. Introduction The decomposition of formic acid on metal surfaces has long been regarded as a model catalytic reaction due to the simplicity of the intermediates and products involved.1,2 Two major reaction pathways have been established, dehydrogenation to produce H2 and CO2 and dehydration to give H2O and CO, with the selectivity being dependent on the metal. Competition between both processes occurs on Ni,3-5 Pd,6-8 Rh,9 Ru,10,11 and Fe,12 while dehydrogenation is solely observed on Cu 13,14 and Pt.15 The more active surfaces of W 16 and Mo 17,18 completely dissociate formic acid to C, H, and O; a recent study suggests that this step also occurs in the initial formic acid decomposition sequence on Ni (and possibly the other group 8 metals) such that the carbided surface so produced changes the subsequent reaction pathway.19 By contrast, Ag 20 and Au 21 surfaces, in the absence of coadsorbed O, are completely inert and show only reversible molecular adsorption and desorption. Formate and anhydride have been implicated as reaction intermediates on the basis of thermal desorption data of formic acid decomposition on a number of single crystal surfaces.2 In particular, surface formate produced from formic acid has been identified with vibrational spectroscopy on Ag{110} (HREELS22), Cu{110} (RAIRS23), Cu{100} (HREELS14,24), Ni{110} (HREELS,25,26 RAIRS19), Pt{111} (HREELS27,28), Pt{110} (HREELS29), Pd{111} (HREELS8), Pd{100} (HREELS6), Rh{111} (HREELS30), Ru{001} (HREELS,31 RAIRS32), Mo{110} (HREELS18), and Mo{100} (HREELS17). The occurrence of this species as a stable intermediate in surface chemistry is more widespread, however, as exemplified by its formation during CO hydrogenation on K/Ru{001}33 and K/Co{101h0},34 CO2 hydrogenation on Cu{100}35 and Ni{110},36 reaction of CO, H2O, and O on Rh{100},37 oxidation of formaldehyde38 and acetic acid39 on Ag{110}, and oxidation of methanol40 and acetone41 on Ag{111}. Formic acid adsorption and oxidation have been investigated on Ag{110},20,22,42 polycrystalline Ag films,43,44 and silicasupported Ag catalysts.45 In general, preoxidation of these surfaces is a necessary condition for cleavage of the OsH bond to produce surface formate, which then undergoes dehydroge† Current address: Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, U.K. X Abstract published in AdVance ACS Abstracts, July 1, 1996.

S0022-3654(96)00072-X CCC: $12.00

nation upon heating. A striking feature among these studies is the multitude of adsorption geometries that can be adopted by the surface formate species. This behavior manifests itself in the absorption frequencies and dipole activity of the vibrational modes, which can be easily probed using infrared spectroscopy. In this paper, we present a detailed characterization of the surface formate species generated on Ag{111} by reaction of formic acid and methyl formate with preadsorbed atomic O using a combination of reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). We show that even on a well-defined, close-packed basal crystal plane, it is possible for formate to assume multiple bonding configurations. We also investigate the effect of coadsorbed methoxy on the adsorption behavior of formate. 2. Experimental Section The ultrahigh vacuum (UHV) system used for these experiments, based around a Mattson Cygnus 100 Fourier-transform spectrometer, has been described in detail elsewhere.46 RAIR spectra were obtained by the coaddition of 500 scans at 4 cm-1 resolution over the spectral range 750-4000 cm-1 and presented as a ratio against a clean surface spectrum. All RAIR spectra were acquired under isothermal conditions at the temperatures indicated. TPD spectra were recorded with a linear heating rate of 2 K s-1. The Ag{111} crystal was cleaned by repeated cycles of Ar sputtering at 600 K followed by annealing to 750 K. The atomically adsorbed O-covered surface, characterized by a p(4 × 4) LEED pattern, was formed by exposing the clean surface to ∼1 mbar of O2 at 425 K for 5 min, followed by a momentary anneal in vacuum to ∼470 K.40 Formic acid (HCOOH, Aldrich, 96%, dried over anhydrous MgSO4), formic-d acid (DCOOH, Merck, 99% D), and methyl formate (HCOOCH3, Aldrich, 99%) were degassed by several freeze-pump-thaw cycles prior to dosing via a leak valve. Exposures are reported in langmuirs (1 langmuir ) 1 × 10-6 Torr‚s) and are based on uncorrected ion gauge readings. 3. Results 3.1. RAIR Spectra of Formic Acid Adsorbed on Ag{111}. Figure 1 shows the RAIR spectra of HCOOH and DCOOH adsorbed on Ag{111} at 130 K, obtained by exposing the clean © 1996 American Chemical Society

12510 J. Phys. Chem., Vol. 100, No. 30, 1996

Sim et al.

TABLE 1: Vibrational Frequencies and Mode Assignments for HCOOH and DCOOH HCOOH (cm-1)

DCOOH (cm-1)

assgnmt

R-crystallinea,b

Ag filma,c

Ag{110}d,e

V(CH)/V(CD) V(OH) V(CdO) δ(CH)/δ(CD) V(C-O) π(CH)/π(CD) π(OH) δ(OCO)

2958 2892, 2712, 2532 1703, 1609 1391, 1381, 1370 1255, 1224 1083 974 721

obscured 2881 1641 1368, 1338 1196, 1164 1057 902 684

2950 2640 1720

a

Ag{111}a

1770-1660

1240 960 730

1078 978

R-crystallinea,b 2273, 2246 2896, 2708, 2550 1680, 1593 1006 1258 899 981 710

Ag{111}a

1740-1580 897 978

From infrared spectra. b Reference 49. c Reference 44. d From HREEL spectra. e Reference 22.

Figure 1. RAIR spectra of clean Ag{111} in the presence of 10-8 mbar of HCOOH and DCOOH at 130 K.

crystal surface to an ambient pressure of 1 × 10-8 mbar of the respective formic acid isotopomer. The absorption bands are characteristic of molecular formic acid, as assigned in Table 1, and the invariance of their intensities with increasing gas pressure suggests that saturated monolayers have been formed. The spectra are dominated by the out-of-plane OsH deformation π(OH) at 978 cm-1 and the out-of-plane CsH and CsD deformations π(CH) at 1078 cm-1 and π(CD) at 897 cm-1, respectively. The CdO stretch, V(CdO), which normally shows a very strong absorption band for formic acid, is highly attenuated in this case and appears as a broad envelope from 1660-1770 cm-1 for HCOOH, and from 1580-1740 cm-1 for DCOOH. No other features are apparent in the spectra. 3.2. RAIR Spectra of Formate Adsorbed on Ag{111}. The RAIR spectra of preoxidized Ag{111} dosed with increasing exposures of HCOOH and DCOOH at 240 K are shown in Figures 2 and 3, respectively. An entirely new set of coveragedependent absorption features is observed that can be attributed to the formation of several different species of surface formate, as indicated by the splitting of the bands in various regions of the spectra. A synopsis of the spectral interpretations of several related systems is provided in Table 2. Characteristic bands are observed for the symmetric OsCsO stretch Vs(OCO) at 1260-1360 cm-1, and the asymmetric OsCsO stretch Vas(OCO) at 1530-1610 cm-1. Each of these bands in the HCOO spectra can be correlated with a corresponding one in the DCOO spectra, and they all exhibit downward frequency shifts of 5-25 cm-1 as a result of deuteration. This means that the vibrational modes involve some contribution from H/D and rules out surface species of composition CxOy, for example carbonate (CO32-) and oxalate (C2O42-). Formic acid anhydride (assigned from the RAIR

Figure 2. RAIR spectra of preoxidized Ag{111} exposed to increasing doses of HCOOH at 240 K.

Figure 3. RAIR spectra of preoxidized Ag{111} exposed to increasing doses of DCOOH at 240 K.

spectra of formic acid decomposition on Ni{111}47) is also unlikely as the absorption frequencies fall well outside the corresponding range of its matrix-isolated infrared spectra48 (CdO stretches at 1762 and 1812 cm-1, CsO stretches at 776 and 998 cm-1). On the other hand, they are in good agreement

Multiple Bonding of Adsorbed Formate on Ag{111}

J. Phys. Chem., Vol. 100, No. 30, 1996 12511

TABLE 2: Vibrational Frequencies and Mode Assignments for HCOO and DCOO HCOO (cm-1) assgnmt

DCOO (cm-1)

Ag{110}a,b

Cu{110}c,d

Ni{110}c,e

Ru{001}c,f

Ag{111}c

2900

2950, 2890

2942, 2840

2930, 2857

2936-2886, 2840-2808 1608 (γ) 1580 (β) 1548 (δ) 1351 (δ) 1340 (β) 1331 (R) 1280 (γ) obscuredg 752h

V(CH)/V(CD) and Vas(OCO) + δ(CH) or 2δ(CH) Vas(OCO)

1640

-

-

-

Vs(OCO)

1340

1350

1352

1361

δ(CH)/δ(CD) π(CH)/π(CD) δ(OCO)

obscuredg 1050 770

-

770

784

Ag{110}a,b

Ni{110}c,e

Ru{001}c,f

Ag{111}c

2150

2180

2186

2164-2136

1640

-

-

1310

1330

1329

750

774

1010 obscured 770

1595 (γ) 1570 (β) 1543 (δ) 1325 (δ) 1315 (β) 1301 (R) 1268 (γ) 1000 750h

a From HREEL spectra. b Reference 22. c From infrared spectra. d Reference 23. e Reference 19. f Reference 32. g Expected at 1360 cm-1. h Tentative assignment.

with the Vs(OCO) and Vas(OCO) frequencies of formate on other metal surfaces.6,8,14,17-19,22-32 At higher formic acid exposures, the in-plane CsD deformation δ(CD) is visible at 1000 cm-1, while the corresponding CsH deformation δ(CH), expected at 1360 cm-1, is probably masked by the Vs(OCO) absorption. Neither π(CH) nor π(CD) is observed, in contrast to the RAIR spectra of formic acid. The in-plane OsCsO deformation δ(OCO) occurs between 750-790 cm-1 for formate on most metal surfaces. Due to poor signal-to-noise below 800 cm-1 in our spectra, we can only make a tentative assignment of a band at ∼750 cm-1 (not shown) to this mode. The doublet at 2800-2900 cm-1 in the low-coverage HCOO spectra (which splits into a pair of doublets at high coverage) can be assigned to the CsH stretch V(CH) in Fermi resonance with either the 2δ(CH) overtone or the combination band Vas(OCO) + δ(CH). On clean metal surfaces, the doublet occurs above 2800 cm-1, and the majority of the studies have preferred an assignment involving Vas(OCO) + δ(CH).19,23 On K-covered surfaces, however, they appear below 2800 cm-1, and coupled with isotopic substitution studies, 2δ(CH) has been implicated instead.32,34 Both of these conclusions are in accord with the vibrational frequencies of the respective modes. In the present case, the unperturbed vibrational frequency of V(CH) is estimated to be at ∼2850 cm-1. Given that Vas(OCO) ranges from 1530 to 1610 cm-1 and taking δ(CH) as 1360 cm-1, it would thus appear that Vas(OCO) + δ(CH) ({1570 ( 40} + 1360 ) {2930 ( 40}) is more likely to be the mode involved in the Fermi resonance rather than 2δ(CH) (2 × 1360 ) 2720). The assignment of each doublet to a Fermi resonance pair as opposed to two different V(CH) modes is substantiated by the spectra of DCOO, where each set of bands collapses into a single CsD stretch V(CD) at 2140-2170 cm-1. The near degeneracy in energy levels is lifted here as the V(CD) frequency no longer matches any of the possible overtones or combination bands. 3.3. Identification of Multiple Formate Species on Ag{111}. On the basis of the evolution of the absorption bands in the HCOO spectra, we can identify four different surface formate species, denoted R, β, γ, and δ, which exhibit different Vs(OCO) and Vas(OCO) frequencies. The major species present in the low-exposure regime (5 L, δ-formate (Vs(OCO) at 1351 cm-1, Vas(OCO) at 1548 cm-1) is formed as well. All of these features are reflected by

the corresponding Vs(OCO) and Vas(OCO) bands in the DCOO spectra, where similar assignments can be made for the various deuterated analogues. Due to the close proximity of the bands within each OsCsO stretching region, we expect significant dipole coupling and intensity transfer between the absorption bands. An accurate quantitative analysis of each formate species is thus not possible. Further corroboration for the production of multiple formate species is evident from the splitting of the bands in the CsH and CsD stretching regions at high formic acid exposures. Both R- and β-formate give rise to the lower frequency set of bands (2902, 2827, and 2146 cm-1), while γ- and δ-formate are responsible for the higher frequency bands (2936, 2840, and 2164 cm-1). In addition, we can associate the appearance of δ(CD) (and the masked δ(CH) by inference) with one or more of β-, γ-, or δ-formate. The sequential formation of each formate species also explains the behavior of the Fermi resonance doublet which goes through a series of intensity fluctuations that implies switching of the relative positions of the unperturbed frequencies of the fundamental and combination bands involved. This can be attributed to the different Vas(OCO) and hence variable Vas(OCO) + δ(CH) frequencies due to the various formate coordination states, coupled with coveragedependent shifts of Vas(OCO) and V(CH). 3.4. Thermal Decomposition of Formate on Ag{111}. Figures 4 and 5 show a series of RAIR spectra collected after initially dosing the preoxidized Ag{111} surface with saturation doses (>5 L) of HCOOH and DCOOH, respectively, and then annealing to the temperatures indicated. Features of the different formate species are apparent at 180 K, with an additional band at 1670-1690 cm-1 that can be attributed to V(CdO) of coadsorbed formic acid. On heating, the formic acid desorbs or reacts by 200 K, leaving the formate bands which become resolvable into their individual components at 240 K, probably as a result of heat-induced ordering of the surface species. Further heating causes the sequential disappearance of bands due to δ-, γ-, β-, and eventually R-formate, in reverse fashion to their formation. There is also a distinct downward shift in Vs(OCO) of R-formate by ∼10 cm-1 upon desorption of the other formate species, compared to the initial dosing case. TPD spectra obtained by heating the formate-covered surface from 240 K at a linear heating rate of 2 K s-1 are shown in Figure 6. The pair of peaks at 350 K signify the concurrent evolution of CO2 and H2 (D2) and can be ascribed to the decomposition of R- and β-formate (via R-formate). A smaller CO2 peak (with no H2 (D2) counterpart) precedes the major desorption peak by ∼25 K and correlates with the removal of γ- and δ-formate. This peak is absent if the surface is first

12512 J. Phys. Chem., Vol. 100, No. 30, 1996

Figure 4. RAIR spectra of a saturated HCOO overlayer on Ag{111} annealed to and maintained at the temperatures indicated.

Figure 5. RAIR spectra of a saturated DCOO overlayer on Ag{111} annealed to and maintained at the temperatures indicated.

annealed to 300 K, cooled, and then reflashed. The isothermal conditions under which the RAIR spectra were acquired is responsible for the temperature lag between the disappearance of the RAIR features and the appearance of the TPD peaks. 3.5. RAIR Spectra of the Reaction between Methyl Formate and Preoxidized Ag{111}. Surface formate can also be generated by dosing the preoxidized Ag{111} surface with methyl formate, as shown in Figure 7. For an exposure of 0.2 L at 180 K, the resulting RAIR spectrum can be assigned to a mixture of R-formate (Vs(OCO) at 1331 cm-1, V(CH) and Vas(OCO) + δ(CH) at 2888 and 2808 cm-1) and methoxy (the CsO stretch V(CO) at 1028 cm-1, the symmetric methyl stretch Vs(CH3) at 2792 cm-1, and the overtones of the symmetric and asymmetric methyl deformations 2δs(CH3) at 2875 cm-1 and 2δas(CH3) at 2912 cm-1, respectively). The absorption bands grow in size with methyl formate dose and saturate at an exposure of ∼5 L, whereupon the inherently weak methoxy δs(CH3) absorption becomes visible at 1431 cm-1. It is interesting

Sim et al.

Figure 6. TPD spectra of CO2 and D2 (from DCOO) obtained without and with preannealing of a saturated formate overlayer on Ag{111} to 300 K following adsorption at 240 K.

Figure 7. RAIR spectra of preoxidized Ag{111} exposed to methyl formate at 180 and 280 K.

to note that R-formate is the only type of formate species formed at all exposures and that the methoxy bands, although in excellent agreement with the RAIR spectra of methoxy produced from methanol deprotonation on preoxidized Ag{111},40 exhibit a V(CO) frequency that by contrast shifts downward with increasing surface coverage. Performing the experiment with the surface held at 280 K (the decomposition temperature of methoxy) prior to a saturation dose (>5 L) of methyl formate yields the distinctive spectrum of R-formate on clean Ag{111}, which is identical to that obtained if HCOOH were used instead. The Vs(OCO) frequency of 1320 cm-1 is indicative of minimal perturbation by, or the absence of, other coadsorbed species. 4. Discussion 4.1. Nature of Adsorbed Formic Acid on Ag{111}. Molecular formic acid is known to exist in several forms, as shown in Figure 8, with each possessing characteristic OsH stretching and bending frequencies that are dictated by the degree of intermolecular H bonding.44,49,50 In particular, the

Multiple Bonding of Adsorbed Formate on Ag{111}

J. Phys. Chem., Vol. 100, No. 30, 1996 12513 TABLE 3: Symmetry Species of the Fundamental Internal Modes of Formate under the Possible Symmetry Groups of the Surface-Adsorbate Complex for given symmetry group

Figure 8. Structures of the various forms of molecular formic acid.

π(OH) frequency increases from 636, 917, and 947, to 974 cm-1 for monomeric, dimeric, β-polymeric, and R-polymeric formic acids, respectively, and serves as a useful indicator to the state of molecular aggregation.28 The weak bonding of formic acid to Ag{111} necessitates the use of dynamic gas pressures of ∼10-8 mbar in order to sustain a saturated monolayer on the surface at 130 K. On the basis of the RAIR spectra, we deduce that formic acid adopts the R-polymeric chain structure on Ag{111}, as the π(OH) frequency at 978 cm-1 is in excellent agreement with that of solid R-crystalline formic acid. Moreover, the broad V(CdO) absorption envelope is consistent with the splitting in V(CdO) of the R-polymorph (but absent for the β-polymorph) due to the coupled in- and out-of-phase vibrations of adjacent polymeric chains. A similar situation has been observed on Pt{111},28 where the R-polymorph was also formed at high submonolayer coverages. However, while the π(OH) frequency on Pt{111} shifts downward with decreasing coverage, indicating the transition from the R- to the β-polymorph and then to formic acid dimers, it remains constant at all coverages on Ag{111}, suggesting that no phase changes occur in the latter case. Unlike that for Pt{111}, the Ag{111}-formic acid interaction is probably so weak that the surface is unable to perturb to any significant extent the intermolecular H bonding, thus allowing the formic acid molecules to continue to aggregate into densely packed islands of the R-polymorph even at the lowest surface coverages. Paradoxically, formic acid assumes the less compact β-polymeric structure on clean Ag films,44 which could be due to the less homogeneous, polycrystalline nature of this surface. Since the out-of-plane vibrations π(OH) and π(CH)/π(CD) are infrared-active, while the in-plane vibrations are unobservable, we conclude from the metal-surface selection rule that the formic acid molecules lie with their molecular planes essentially parallel to the surface. At high coverages, the appearance of weak absorption bands due to V(CdO) implies a slight degree of puckering within the polymeric chains, presumably a result of steric crowding or interchain repulsion. 4.2. Bonding Configurations of Formate on Ag{111}. Carboxylate species are able to bind to metal centers in a variety of ways, the most commonly encountered forms being the monodentate, bidentate chelating, and bidentate bridging configurations. Depending on the mode of coordination, the carboxylate species can possess CsO bond orders that range from 1 to 2. This would in turn be reflected by the bond strengths and, hence, vibrational frequencies of Vs(OCO) and Vas(OCO). Analysis of the infrared spectra of a large number

mode

C2V

Cs(1)

Cs(2)

C1

V(CH) Vas(OCO) Vs(OCO) δ(CH) π(CH) δ(OCO) no. of infrared active modes

A1 B1 A1 B1 B2 A1 3

A′ A′′ A′ A′′ A′ A′ 4

A′ A′ A′ A′ A′′ A′ 5

A A A A A A 6

of carboxylate complexes51,52 has shown that typically the singly O-bound species, with one CsO and one CdO bond, has a larger separation of Vs(OCO) and Vas(OCO) (∼300 cm-1) than the doubly O-bonded species with equivalent CsO bonds (∼200 cm-1). The diagnostic value of the CsO stretching frequencies is further complemented by the infrared activity of the corresponding vibrational modes as governed by the metal-surface selection rule for carboxylate species adsorbed on metal surfaces. The free formate ion has C2V symmetry that can be reduced by bonding via one or both of its O atoms to a solid surface. Retention of both mirror planes preserves the C2V symmetry and implies that the CsH bond must be perpendicular to the surface. Tilting about the molecular plane while maintaining equivalence of the O atoms results in the loss of one mirror plane and a reduction in symmetry to Cs(1). Tilting along the molecular plane while keeping it perpendicular to the surface removes the other mirror plane and changes the symmetry to Cs(2). If both mirror planes are lost as a result of random tilting, then the symmetry is lowered to C1. Thus, C2V and Cs(1) symmetry are consistent with a symmetrical bidentate configuration with both O atoms in identical environments, while Cs(2) and C1 symmetry indicate an asymmetrical monodentate bonding configuration with different O environments. These arguments are strictly valid only when the lateral surface potential does not perturb the formate orbitals to any significant extent. If, however, this is not the case, and the symmetry of the metal atoms becomes important, then the symmetry of the surface-adsorbate system for each formate orientation will be the same or lower than in the respective case discussed. Surface-bound formate possesses six internal vibrations that have been classified under various irreducible representations of the possible symmetry groups in Table 3. According to the metal-surface selection rule, only vibrations that are totally symmetric (A, A1, or A′ ) can have a dynamic dipole moment normal to the surface and exhibit infrared activity. Therefore, the expected number of observable fundamental modes in the RAIR spectra are three, four, five, and six for formate with C2V, Cs(1), Cs(2), and C1 symmetries, respectively. With R-formate there are three infrared-active fundamental bands, V(CH), Vs(OCO), and δ(OCO). The normally strongly absorbing Vas(OCO) is absent, indicating that it, and hence δ(CH) implicitly since they both share the same symmetry under all point groups, are dipole-forbidden. The failure to observe π(CH) may be due to either symmetry restrictions (C2V symmetry) or its inherently small dynamic dipole moment (Cs(1) symmetry). Photoelectron diffraction studies of formate on Cu{110} and Cu{100} have shown that it possesses C2V symmetry and bridges two nearest neighbor metal atoms.53,54 NEXAFS and SEXAFS studies of formate on these same two surfaces agree with the C2V symmetry but differ in the adsorption site assignment;55,56 however, this discrepancy has been resolved by reinterpreting the results in light of the photoelectron diffraction work.53 Analysis of the dipole-active molecular and

12514 J. Phys. Chem., Vol. 100, No. 30, 1996 surface phonon modes in the HREEL spectrum of formate on Ni{110} has been shown to be consistent with a C2V bridging species.26 In a NEXAFS study of formate on Ag{110}57 a species was found that appears to be tilted about the molecular plane due to dynamical motion, although the time-averaged inclination is still consistent with C2V symmetry. RAIR spectra of formate on Ni{110}19 and Ru{001},32 which have been ascribed C2V symmetry, show the same features as that of R-formate on Ag{111}. In view of the evidence from the literature, and taking into consideration the fact that the infraredactive π(CH) band is detectable for adsorbed formic acid (which has a tilted molecular plane) on Ag{111}, we propose that R-formate has C2V symmetry and adopts a vertical orientation and bidentate bonding configuration that bridges two adjacent Ag atoms. This implies that R-formate must see the surface with C6V (and not C3V) or C∞V symmetry, which is consistent with the O atoms bonding to top sites where the influence of the underlying layers of metal atoms is expected to be minimal. Molecular orbital calculations for C2V formate on Cu{100},54,58 Ru{001},59 and Rh4 clusters60 also conclude that this (the short bridge configuration) is the most energetically favorable structure. The emergence of Vas(OCO) for β-, γ-, and δ-formate implies a symmetry of Cs(2) or C1 for these species. Its appreciable intensity strongly suggests that the symmetry lowering is the result of some significant perturbation of the electronic structure such as would be caused by tilting of the formate, as opposed to the relatively weaker effects of the lateral surface potential corrugations. If Vas(OCO) - Vs(OCO) is taken as a measure of the degree of equivalence of the two CsO bonds, then frequency separations of 197, 240, and 328 cm-1 for δ-, β-, and γ-formate, respectively would indicate an increasing propensity for one of the O atoms to tilt away from the surface. Thus, δ-, β-, and γ-formate can be classified as essentially monodentate species where the interaction of the “free” O atom with the surface varies from moderate to weak to insignificant. Monodentate formate species have been found on Ag{110},22 Cu{100},14,24 Pt{111},27 Pd{100},6 Ru{001},31 Mo{110},18 and Mo{100},17 where they are all characterized by strong dipole activity of Vas(OCO) in their vibrational spectra. 4.3. Coverage-Dependent Behavior of Formate on Ag{111}. A schematic diagram depicting the coverage-dependent behavior of formate on Ag{111} is shown in Figure 9. Formic acid reacts with surface atomic O in a H-abstraction process that produces formate, OH, and H2O. For Ag surface temperatures above 200 K, the H2O thus formed is known to desorb during the dose:22,42

HCOOH (g) + O (a) f HCOO (a) + OH (a) HCOOH (g) + OH (a) f HCOO (a) + H2O (g) The sequential appearance of each formate species can be accounted for by considering both steric and electronic factors that manifest themselves as the surface coverage is increased. At low coverages, there are many available sites for formate to symmetrically bridge two adjacent Ag atoms, which makes R-formate the most abundant species. As the surface becomes more crowded, neighboring R-formate species within domains of high local coverage experience mutual repulsive dipolar interactions that force them to tilt in an effort to minimize this effect. Presumably tilting along the molecular plane is more favorable as this also frees up top sites for subsequent formate bonding and opens up the possibility of H bonding between the “free” formate O atoms and H atoms of neighboring monodentate formate species. There is thus a gradual conver-

Sim et al.

Figure 9. Coverage-dependent behavior of formate on Ag{111}.

sion of R- to β-formate above a certain critical local surface coverage. Only after the surface is saturated with β-formate is the truly monodentate γ-formate formed, and this can be associated with a species that is bonded to isolated Ag atoms where steric hindrance from surrounding surface species prevents any interaction of its free O atom with the surface. When all suitable top sites have been filled, any subsequent adsorption, such as that of δ-formate, will have to involve bonding of the O atoms to less favorable 2- and 3-fold sites. Monodentate formate species on other surfaces are typically formed at high coverages and low temperatures, conditions that tend to limit adsorbate mobility and promote site blocking. On heating the formate-covered surface, the following sequence of events is proposed to occur. The most thermodynamically unstable states, δ- and γ-formate, dehydrogenate first to give CO2 and surface H, which recombines preferentially with the high concentration of β-formate still present to give formic acid. The strong tendency for formic acid to adhere to the chamber walls (and possibly decompose) as compared to CO2 and H2, coupled with the long and nonlinear path from the crystal surface to the mass spectrometer in the UHV system,46 is believed to be responsible for our inability to track its desorption:

δ-,γ-HCOO (a) f CO2 (g) + H (a) β-HCOO (a) + H (a) f HCOOH (g) This would explain why no H2 accompanies the first CO2 desorption peak in the TPD spectra. The recombination reaction has been reported as a minor reaction pathway which follows formate dehydrogenation on Ag{110},22,42 where all products desorb simultaneously at 410 K. As the temperature increases, the freeing up of sites and enhanced mobility of the surface species enable the remaining β-formate to convert back to R-formate, which then decomposes to produce CO2 and H2 at 350 K:

β-HCOO (a) f R-HCOO (a) R-HCOO (a) f CO2 (g) + H (a) 2H (a) f H2 (g)

Multiple Bonding of Adsorbed Formate on Ag{111}

Figure 10. Frequency of the V(CO) band of methoxy as a function methoxy coverage in the presence and absence of an equivalent amount of formate.

This desorption profile is highly characteristic of the dehydrogenation of R-formate on Ag{111} (as verified by RAIRS) produced by the partial oxidation of a variety of other organic molecules, including methyl formate, methanol, and acetone. 4.4. Coadsorption of Formate and Methoxy on Ag{111}. Methyl formate undergoes nucleophilic attack by surface atomic O at the CdO group such that methoxy is displaced and O is incorporated into the resulting formate:

HCOOCH3 (g) + O (a) f HCOO (a) + CH3O (a) This reaction is well-established on both single crystal and polycrystalline Ag 20,61 and Cu 62,63 surfaces. It can be inferred from the stoichiometries of the two sets of reactions above that, for a fixed initial O coverage, a saturation exposure of either formic acid or methyl formate should produce the same total adsorbate coverage in each case. Methoxy is believed to bond with C3V symmetry to fcc 3-fold hollow sites on Ag{111} via its O atom in an upright configuration with the CsO bond axis normal to the surface.40 The current RAIR spectra indicate that this is still the case in the presence of coadsorbed formate. By considering the structural parameters and bonding sites of methoxy and formate, together with the lattice dimensions of the Ag{111} surface, it becomes clear that the methoxy is less efficient than R-formate at blocking symmetrical bridging sites as far as subsequent formate adsorption is concerned. This could partially account for the absence of monodentate formate when methoxy is present, if RAIR spectra with the same total adsorbate coverage are compared. However, inspection of RAIR spectra with the same total formate coverage reveals that methoxy actually promotes the amount of R-formate produced at the expense of β-formate. We propose that coadsorbed methoxy and formate are randomly intermixed in the adsorbed layer, resulting in smaller formate domains where dipolar interactions are not so severe. Moreover, tilting of the formate species is further inhibited by steric interaction of adjacent methoxy CH3 groups. Evidence for the intermixing of the two adsorbate phases can be seen from the effect that coadsorbed formate has on the frequency of the methoxy V(CO) band and its coverage-dependent shift, as shown in Figure 10. For a pure methoxy overlayer generated from methanol deprotonation,40 there is a proportionate increase in the V(CO) frequency with increasing coverage from 1026 to 1040 cm-1 at about half a monolayer, caused by a combination of dipole coupling and an upward chemical shift. On the other hand, the presence of an equivalent quantity of formate depresses the V(CO) frequency from 1028 to 1024 cm-1 in the same methoxy coverage range. The frequency decrease with increasing coverage suggests that coadsorbed formate enhances the electron population of the methoxy 2e orbitals, which are CsO antibonding in nature.40 Although work function measurements

J. Phys. Chem., Vol. 100, No. 30, 1996 12515 on Ag{110} indicate that both surface species are anionic and withdraw electron density from the metal surface,64 it would seem that there is a small degree of charge transfer from formate to methoxy when they are coadsorbed on Ag{111}. However, we do not expect this charge transfer, and its associated downward chemical shift, to be sufficiently large to negate the significant upward shift due any inter-methoxy interactions that may be operative if large methoxy domains exist. Since the influence of coadsorbed formate is allowed to manifest itself in the RAIR spectra, we conclude that the inter-methoxy interactions must be minimal. Our observation would then be most consistent with either a completely mixed adlayer or small methoxy domains screened by small formate domains, such that both intra- and interdomain dipole coupling between the methoxy species are suppressed. The charge transfer mechanism probably stabilizes the intermixed adlayer to some extent by reducing electrostatic repulsion between the adsorbate domains. 5. Conclusions We have studied the adsorption characteristics of formic acid and formate on Ag{111}. Formic acid has been found to be weakly bound and lies with its molecular plane roughly parallel to the surface, forming H bonded chains with the R-polymeric structure. We have identified four surface formate species, denoted R, β, γ, and δ, with different bonding configurations when formic acid reacts with preoxidized Ag{111}. By considering the symmetry of the various formate adsorption geometries and the separation of the CsO stretching frequencies, we have shown that R-formate, which is the only species present at low coverage, has effective 2-fold symmetry and bridges two adjacent Ag atoms, while β-, γ-, and δ-formate are monodentate with different degrees of inclination from the surface. On the basis of the evolution of the RAIR bands, we propose that as the formate coverage increases, dipolar interactions within large domains of R-formate cause tilting and conversion to β-formate, while surface crowding in the highest coverage regime results in the formation of γ- and δ-formate. The tilting of R-formate can be suppressed by the coadsorption of methoxy, which dissipates the formate domains and reduces the interadsorbate interactions. We have also investigated the reactivity of preoxidized Ag{111} toward formic acid and methyl formate. Surface atomic O acts as either a Bro¨nsted base or nucleophile, attacking the OsH or CdO groups, respectively, to produce surface formate. On heating, γ- and δ-formates and some β-formate decompose at 325 K to give CO2 and formic acid, while the remaining β-formate is converted to R-formate, which dehydrogenates at 350 K. Acknowledgment. We thank the EPSRC for an equipment grant and a postdoctoral assistantship (P.G.). W.S.S. acknowledges a scholarship from the Cambridge Commonwealth Trust and St John’s College, Cambridge. References and Notes (1) Mars, P.; Scholten, J. J. F.; Zwietering, P. AdV. Catal. 1963, 14, 35. (2) Madix, R. J. AdV. Catal. 1980, 29, 1. (3) Madix, R. J.; Falconer, J. L. Surf. Sci. 1975, 51, 546. (4) Benziger, J. B.; Madix, R. J. Surf. Sci. 1979, 79, 394. (5) Benziger, J. B.; Schoofs, G. R. J. Phys. Chem. 1984, 88, 4439. (6) Jorgensen, S. W.; Madix, R. J. J. Am. Chem. Soc. 1988, 110, 397. (7) Aas, N.; Li, Y. X.; Bowker, M. J. Phys.: Condens. Matter 1991, 3, S281. (8) Davis, J. L.; Barteau, M. A. Surf. Sci. 1991, 256, 50. (9) Solymosi, F.; Kiss, J.; Kovacs, I. Surf. Sci. 1987, 192, 47. (10) Larson, L. A.; Dickinson, J. T. Surf. Sci. 1979, 84, 17. (11) Sun, Y. K.; Weinberg, W. H. J. Chem. Phys. 1991, 94, 4587.

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