Oxygen adsorption on silver (110): Dispersion, bonding and precursor state

Surface Science 175 (1986) 101-122 North-Holland, Amsterdam

101

OXYGEN ADSORPTION ON SILVER (110): DISPERSION, BONDING AND PRECURSOR STATE K.C. PRINCE

*, G. PAOLUCCI

Fritz-Haber-Institut Received

12 February

** and A.M. BRADSHAW

der Max-Planck-Gesellschafr, 1986; accepted

Faradayweg

for publication

4 - 6, D-1000 Berlin 33, Germany

4 April 1986

Angle-resolved photoemission, both in the laboratory and with synchrotron radiation, has been used to investigate three states of adsorbed oxygen on Ag(l10): chemisorbed atomic oxygen, undissociated chemisorbed dioxygen and physisorbed molecular oxygen. For atomic oxygen, dispersion of adsorbate-induced levels was observed indicating strong oxygen-oxygen interaction in the [OOl] direction. Polarised light combined with selection rules was used to determine the symmetry of the adsorbate-derived bands. The adsorption geometry and symmetry of the oxygen-induced levels for the chemisorbed dioxygen species were also investigated with the selection rules. The molecule appears to lie parallel to the surface with the O-O axis oriented in the [liO] direction. The adsorbate-induced feature of lowest ionisation potential at 1.1 eV relative to EF is Ins-derived. The very low frequency reported for the O-O stretch and the analogy with coordination chemistry also suggest that the chemisorbed dioxygen species is lying down on the surface. At sufficiently low temperature, oxygen was found to physisorb on the bare metal also with its O-O axis parallel to the surface. We identify physisorbed oxygen as an intrinsic precursor state for chemisorption.

1. Introduction Three distinct adsorbed species result from the interaction of oxygen with a clean Ag(ll0) surface. On adsorption at 300 K, chains of atoms are formed in the [OOl] direction. The LEED work of Engelhardt and Menzel [l] showed that the spacing of these chains varied with coverage to give diffraction patterns described as (n x l), 7 2 n 2 2. The oxygen-oxygen interaction is clearly attractive in the [OOl] direction but the formation of very widely spaced chains at low coverage indicates a long-range repulsive interaction in the [llO] direction: in the (7 x 1) structure, for example, the chain separation is - 2 nm. On the basis of the covalent radius and the surface geometry Engelhardt and Menzel argued that the long bridge site was probably occupied. This is * Present address: Institut fur Grenzfhachenforschung D-5170 Jtulich, Fed. Rep. of Germany. ** Present address: International School for Advanced Trieste, Italy.

und

Vakuumphysik

Studies,

0039-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

Strada

B.V.

der Costiera

KFA

Jiilich,

11, I-34014

102

K. C. Prmce et al. / Oxygen

adsorption

on Ag(l IO)

10011 T -r1io1 Fig. 1. The (2 X 1) atomic oxygen [l], and subsequently established

y

overlayer on Ag(llO), as suggested by Engelhardt and Menzel as the most likely structure by LEED I-V analysis and SEXAFS.

shown in fig. 1 for the (2 X 1) structure. Supporting arguments included the analogy with bulk Ag,O and the chemical reasonableness of this site: good metal/oxygen orbital overlap can be achieved. The assignment was supported by ion scattering experiments [2] but more recently doubts were expressed as to its validity following a LEED 1-V structure analysis [3]. Although the long bridge site gave the best agreement between experiment and theory, the reliability factor was not considered particularly good. Furthermore, ESDIAD measurements [4] indicate a site of symmetry lower than the C,, of the long bridge. More recently, SEXAFS measurements [5] have supported the original assignment, although a relaxation of up to 0.2 A onto a non-symmetric bonding site would be within the margin of error of SEXAFS and account for the ESDIAD pattern. Recent work by Campbell and Paffett [6] has revealed a further ordered overlayer - c(6 x 2) - obtained after high pressure dosing. Below 170 K, a second, molecular form of adsorbed oxygen has been observed [7]. This is a chemisorbed species in which the oxygen-oxygen bond is intact, although considerably weakened: EELS studies [8,9] have indicated that the oxygen-oxygen stretch frequency is very low, 640 cm-‘. We have previously argued [lo] that the molecular species is formally a peroxo species (O:-), in which the previously half-filled IT, orbital of the gas phase oxygen is filled by charge donation. A recent angle-integrated study [ll] agrees with this

K. C. Prince et al. / Oxygen adrorpiion on Ag(ll0)

103

overall view of the chemisorption of oxygen, but there are differences in the assignment of UPS peaks to which we will return later. Calculations by Selmani et al. [12], using the SCF-Xa method, have reduced the number of possible geometric structures and provided insight into the nature of the bond to the silver surface. The ESDIAD data for the molecular species [4] were indicative of a tilted configuration in the [OOl] azimuth. ,4t low temperature, another molecular oxygen species with a vibrational frequency of 1053 cm-’ has been identified by SERS [13]. It was assigned to a superoxo species but has to date been observed only on rough films and not on single crystal surfaces. In a preliminary communication [14] we recently showed that it is possible surface. at temperatures below - 40 K to physisorb 0, on the bare Ag(ll0) This state is particularly interesting in view of the prediction of a precursor state for chemisorption in this system [l]. Few previous attempts to observe physisorbed molecules on bare single crystal metal surfaces and their subsequent conversion to a chemisorbed phase have been successful [15-191. Oxygen physisorption on polycrystalline gallium and its subsequent conversion to the chemisorbed state has, however, been reported [16]. More recently, physisorption without chemisorption was observed in the system O/Ni(lll) via work function changes [18]. The subsequent conversion of the physisorbed layer was studied and activation energies estimated. In other cases, the adsorbed gas first formed a chemisorbed layer, followed by growth of a physisorbed layer on top of the chemisorbed layer. Oxygen physisorption on silver films has also been studied by core and valence level photoemission [21]. Finally, a few words may be said about the relevance of these studies to industrial catalysis. Studies in UHV are carried out at low adsorbate partial pressure to allow electron spectroscopic methods to be applied. Thermodynamic considerations indicate that adsorbed species observed at low pressure and temperature can also be present at high temperature and pressure. Thus, a characterisation of the oxygen/silver system in UHV can be helpful for understanding the interaction of oxygen with silver in its most important catalytic application, the epoxidation of ethylene. This subject has been reviewed recently [21]. For many years, molecular and atomic oxygen have competed as proposed intermediates for the epoxidation reaction: C,H, C,H,

+ O(ads)

+ C,H,O(ads),

+ O,(ads)

+ C,H,O(ads)

C2H, + 6 O(ads)

(1) + O(ads),

+ 2 CO, + 2 H,O.

(2a) (2b)

If eq. (1) correctly describes the reaction, then the maximum theoretical efficiency is 100%. If (2a) is valid, then for every ethylene molecule converted to ethylene oxide, an oxygen atom remains on the surface. To prevent a build-up of oxygen on the surface, these must be removed via reaction (2b).

104

K. C. Prince et al. / Oxygen adsorption on Ag(ll0)

Thus, for every 6 molecules of ethylene partially oxidised to C,H,O, one must be fully oxidised to CO, and Hz. The selectivity of the reaction is defined as the percentage conversion of C,H, to C,H,O rather than other products. The maximum possible selectivity is 6/7, or 86%, when molecular oxygen mediates the reaction. The failure of industrial catalysts to reach this selectivity was taken for many years as evidence that eqs. (2a) and (2b) correctly describe the process. The recent report [22] of a silver-based catalyst which is more than 86% selective negates this conclusion. Atomic oxygen appears to be at least as important as molecular oxygen, if indeed it is not the only surface intermediate which reacts to form ethylene oxide. In two previous short communications [10,14] we reported our first photoemission data for oxygen adsorbed on Ag(ll0). In the present paper we extend the data base for the atomic species and correct an error made in the two-dimensional band structure plot. Further, we apply selection rules using polarised He1 to the problems of the chemisorbed dioxygen species, deriving important information on geometry and on the bonding scheme. Finally, we investigate the physisorbed species using synchrotron radiation and determine the orientation of the species.

2. Experimental The work was perfomed with a turbo-pumped beam line version of a VG ADES 400 spectrometer. The analyser had an angular acceptance of 2” and could be moved about two independent axes of rotation to any angle within a quadrant of solid angle 77/2 sterad. Energetic resolution was set typically to 0.3 eV with unpolarised He1 or synchrotron radiation, and 0.6 eV with polarised HeI. The usual surface analytical techniques of TPD, LEED and AES were available; the base pressure was 4 X lOWi mbar. The experiments were performed on two silver (110) crystals cut from the same boule (5N, Metals Research). For the atomic oxygen work, the UV light was incident in the [OOl] azimuth, denoted here as the x direction (fig. 2). The second crystal was mounted with the light incident in the [l%O], or y, azimuth. Data for the atomic species were taken, with the exception of a few measurements, on the first crystal; data for the chemisorbed dioxygen were taken from both crystals, and for the physisorbed oxygen from the second crystal only. The atomic oxygen adlayer was prepared by dosing to 2000 L of oxygen at 300 K, and the chemisorbed dioxygen layer by dosing to 500 L at 110 K. Great care was necessary to avoid contamination by the products of wall and filament reactions (chiefly CO,). Measurements with synchrotron radiation on the physisorbed species were performed at the Berlin storage ring BESSY using the 1 m Seya-Namioka monochromator on beam line 42.10. This is a high flux monochromator with a

K C. Prince et al. / Oxygen aakorption on Ag(l It?)

105

Fig. 2. Experimental geometry. The z direction is the normal to the crystal, x is defined as the [OOl] direction and y is then [liO]. Light was incident in either the xz or yz plane. Photoelectrons were collected at an emission angle 6 from the normal.

high degree of polarisation at lower energies, > 97% for Aw < 23 eV. At higher energies the degree of polarisation falls, but is still reasonable, being - 92% at Ao = 30 eV [23]. The high flux was important for one of the experiments described below. For typical ring current and resolution settings, the count rate was about an order of magnitude higher than that obtained with a helium lamp. To determine the orientation of the physisorbed oxygen molecules, we passed the light through the electron energy analyser and measured photoemission at normal light incidence at normal emission. The light was passed through a small hole normally used for
3. Results 3. I. General remarks The overall picture which emerges from this study of oxygen on silver may be described as follows. At the lowest temperature, - 25 K, oxygen adsorbs into the physisorbed precursor state, with some formation of the chemisorbed

106 ad~or~tja~ at

further adsorption

300K ,s i 3xW3 iseeRet /t/l

*

~~~~.~

3x1

18OK

at 3OOK

*

0

ht.)

2x1

compiete ca~versian, at high caveray~ with desarption

ewpts. s-ra-2

LO K

atmast complete ranvsrsion at tow caverages, at higher Cowages with desarption

Fig. 3. Oxygen species on Ag(l10).

state. On Waring, the ~hysisQrbed state partly desorbs and partly converts. At 170 JS, the c~e~sorb~ state also partly desorbs and partly dissociates into atomic oxygen. Above 580 K, the atomic state r~ornbi~~s and desorbs leaving a clean surface. This scheme is summarised in fig. 3. In the follo~n~, the p~otoe~ssion expe~m~ts on each of the three species are described.

In the apparatus used for our previous study [lo], the light source and electron energy anaIyser were both fixed and the angular dependent was measured by rotating the crystal. This had the disadvantage that the angle of ~nc~de~~ of the light, a, and the angle of emission of the electrons, B, varied simultaneously. In the present study, the angle of incident and the crystal position were set, and only the analyser moved. Spectra from atomic oxygen in the (2 X 1) structure taken with HeI hght for various values of & are shown in fig. 4. The component of photoelectron momentum parallel to the surface is in direction of the surface ~rillo~~n zone (see inset fig. the @Ol] smuts, or

107

K. C. Prince et al. / Oxygen adsorption on Ag(Il0)

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*.. . .. . . .. .. .. . . . . ., .. _ O0 I I I 2 1 EF

(eV)

Fig. 4. Unpolarised He1 spectra of Ag(llO)-0(2x 1). Light was incident along the x or [OOl] azimuth. and emission was also measured along this azimuth, ho = 21.2 eV. 0 = polar angle of emission (see fig. 2). 1.0

a_-_X--

X

x

x

XX

xx

Fig. 5. Photoemission spectra of the 0(2x 1) overlayer on Ag(llO), showing antibonding atomic oxygen levels above the silver d-band. The peak which p-polarised light (see fig. 6) is marked with solid circles and the one that disappears

dispersion of remains with with crosses.

108

K. C. Prince et al. / Oxygen adsorption on Ag(ll0)

5). As reported previously, peaks appear at 8.2, 3.0 and 1.5-3.0 eV binding energy. Here we concentrate on the latter two peaks, since in this energy range the peaks are rather sharp; the peak at 8.2 eV is very weak and only evident in subtraction spectra. Based on the XCZ calculations of Riisch and Menzel [24] we have previously argued that the levels above the d-band are oxygen 2p-derived and anti-bonding with respect to the silver-oxygen bond; this conclusion is supported by recent GVB calculations [25]. The dispersion relation derived from the data in fig. 4 and other spectra is shown in fig. 5. A comparison with thecorresponding plot in our earlier paper [lo] shows that an error of a factor 2 occurred there in the momentum axis. The strong dispersion of the level at lowest binding energy can be traced from the centre of the first surface Brillouin zone F to the centre of the next zone r’ and a little beyond. In a simple tight-binding picture [26] the behaviour of this band with its lowest energy at I? would be symptomatic of an s- or p,-derived band. A search was made for dispersion in the [ITO] azimuth, but within experimental error none was found. A glance at fig. 1 makes the reason for this clear: the separation between oxygen atoms along the chains is 0.41 nm, but between the chains 0.58 nm, and the extent of dispersion is expected to fall exponentially with distance. A further observation regarding the spectra in fig. 4 may be made. At the zone boundary, the strongly dispersing peak is noticeably sharper (FWHM = 0.35 eV) than at the centre (FWHM = 0.55 eV). This may be due to the absence of hybridisation near the zone edge zone, although the calculations of Ho et al. [27] predict no absolute gap for this region in the projection of the bulk band structure. Another possible reason for the variation of peak width is lifetime broadening, the contribution of which to the width of a peak is expected to increase as the square of the energy below the Fermi level [28], and could be four times higher at 3 eV binding energy than at 1.5 eV. Polarised He1 light was used to determine the symmetry of the adsorbate induced levels; fig. 6 shows spectra for p- and s-polarised light incident in the [OOl] azimuth. Photoelectron emission was measured, as above, in the same plane, corresponding to points along TX, where the symmetry is C,. Selection rules tell us that only s-polarised light will give rise to emission from p,-derived states and only p-polarised light to emission from p,- or p,-derived states. The data in fig. 5, which are confirmed at other emission angles, clearly show that the non-dispersing band at - 3 eV binding energy is p,-derived. (The polariser [29] was estimated to be 80-90s efficient.) The strongly dispersing band is p,- or p,-derived: from the direction of dispersion probably the latter. Of course metal states of appropriate symmetry will also be involved. Since these bands are considered to be anti-bonding with respect to the Ag-0 interaction, we expect the third band, probably p,-derived, to be unfilled. The corresponding experiment with p- and s-polarised light in the [liO]

K.C. Prince et al. / Oxygen adsorption on Ag(IlO,l

b ]

z

E A

.E

s 0)

2

.

‘I

or P,

P,

A ,.....:. L

109

....*.*--..... I ..

..

*. .* ... *\*, .:....::..: .*.. **.....

*.- . . . . . . .

s light e=20°

p light

e=20”

Binding

energy (eV)

Fig. 6. Polarised He1 spectra of Ag(llO)-O(2 X 1). Light incident at 45” along [OOl] or x aximuth, Aw = 21.2 eV. The emission angle is chosen so that the dispersing peak is at lowest binding energy.

brings off normal no additional information. In normal emission with s-polarised light, where in principle it would be possible to distinguish between pX and p,, the result was not definitive because of the imperfect polarisation. For the related system (2 X l)-O/Cu(lOO) it was possible to map the dispersion of the corresponding bonding levels on copper, at binding energies of 6 to 8 eV [30]. The highest density of states in this case was at about 6 eV. This suggests that the weak feature observed here at 8.2 eV [lo] may actually be the tail of a broader peak extending up to higher energy, but which is obscured by the silver d-band. This view is supported by the angle-integrated results of Barteau and Madix [ll]. Because of the large dispersion of 2 eV observed on copper, DiDio et al. [30] argued that a direct oxygen-oxygen interaction alone is insufficient to explain the data. A substrate-mediated effect has to be operating. The dispersion of the anti-bonding levels on silver is smaller, but at 1.5 eV still quite large. A significant difference between copper and silver is, however, the absence of reconstruction on the silver surface in the presence of oxygen. This makes a direct comparison difficult. azimuth

3.3. Chemisorbed

dioxygen

Since no ordered overlayer was observed in LEED for molecular oxygen, no dispersion of the peaks in UPS was expected. The main aims of the photoemission experiments reported here were to confirm the peak assignment in ref. [lo] and possibly to obtain some information on the orientation of the

110

:.

I

I

I

I

I

I

9876543 Energy below

hu:: 16.8 eV

x

II

2 Fermi

\

1 EF

level

Fig. 7. Nel spectrum of molecular oxygen on Ag(llO). LY = 45”, normal emission, 500 L O2 at 110 K: (a) clean surface; (bf adsorbate covered surface; (c) spectrum (b) - spectrum (a). The d-band is omitted since changes occw in this region associated with scattering of the silver d-electrons, but which do not necessarily indicate the presence of adsorbate-induced states.

dioxygen species. Fig. 7 shows a UPS spectrum taken with Ne I light (16.8 eV) after a dose of 500 L 0,. In agreement with our previous He I data, peaks are seen at energies of I .I, - 4.3 and 8.6 eV. The energy of the second peak is difficult to determine exactly since it appears as a shoulder on the low binding energy side of the d-band. Spectra taken with polarised He I light are shown in fig. 8. The 1.1 eV peak is, allowing for the polariser, absent for s-polarised light incident along the [ITO] azimuth and normal emission but present for p-polarised light. The 4.3 peak is present for both polarisations. In fig. 9,

111

K. C. Prince et al. / Oxygen aa!sorption on Ag(I IO) p-light

hw.

s-light

21 2 ev

_-. ..’

i-- . ...“.....’

(01

;’

s___..:

I-*

I.._-._

‘..

‘._^___

--.--_.,

:.

: _..’

i.,

/.’

. -’

,._.:’

lb1

,*

,,..-

L____.--

‘L.____

8

6

6

~L--c_-__.__

.--

2

EF

8

6

below

EnC!rgy

EF

6

2

EF

leVl

Fig. 8. Polarised HeI spectra of molecular oxygen on Ag(llO), produced by dosing 600 L 0, at 110 K. Light incident along y or [liO] azimuth, a = 45“. (a) Clean surface spectra, normal emission. (b) Adsorbate covered surface, normal emission. (c) Difference spectra, x 8. The d-band region is shaded.

hw:

. .

p laghI

.

.

21.2eV

..* . .

.

. .. . . . . . *

s light

.

. ..*

. . . . . ._. I

I

I

I

I

6

3

2

1

EF

Energy

Fig. 9. As fig. SC, difference

spectra

helow

for light incident emission.

EF

(eV)

at 45O along

x or (Ool] azimuth.

Normal

112

K. C. Prince et al. / Oxygen adsorption on Ag(I 10)

spectra taken with the light incident in the other azimuth are shown. The 1.1 eV peak IS now present in p-polarised light and absent for s-polarised light. The significance of these results for the assignment and orientation problems will be discussed in section 4. 3.4. Physisorbed

oxygen

UPS spectra of oxygen, physisorbed on silver are shown in fig. 10. The energetic spacing of the peaks is the same as in the gas phase within experimental error, but the whole spectrum is shifted by 0.7 eV to lower binding energy taking account of a decrease in work function from 4.3 to 3.9

I

5

I

I

I

I

I

14

12

10

6

6

4

2

Energy

below

Fig. 10. UPS spectra of the physisorbed species (I= 45O, normal emission. The oxygen exposure was approximately 20 K. We have labelled the which the photoelectrons derive, neglecting spin ref.

EF

,

EF

leV1

taken with synchrotron radiation, Aw = 22.2 eV, is indicated for each spectrum. The temperature peaks according to the molecular orbital from multiplicity. Full ion state notations are given in [31].

K. C. Prince et al. / Oxygen adorption on Ag(llQ)

Oxygen

exposure

113

(L)

Fig. 11. Coverage of physisorbed and chemisorbed dioxygen as a function of exposure. The chemisorbed oxygen coverage was measured from the height of the peak at 1.1 eV binding energy, while the integrated area of the 1~~ + 30s peaks was used to measure the coverage of physisorbed oxygen.

eV on adsorption [31]. This difference in binding energy is due to screening of the final hole state. After a dose of - 1.5 L, the spectrum starts to shift to higher binding energy. A new set of oxygen peaks grows in displaced 0.5 eV to higher energy. This corresponds to completion of the first layer and growth of the second layer. In the second layer, the photoholes created in the photoemission process are screened largely by neighbouring oxygen molecules, whereas photoholes in the first layer are screened by the underlying metal. Oxygen molecules are much less polarisable than the metal and so provide poorer screening. The peaks from the second layer thus shift to higher binding energy. This effect is well known, e.g. ref. [15], and for our purposes provides a convenient monitor of coverage. Even at the lowest coverage, some chemisorbed dioxygen was also present. The respective coverages of physisorbed and chemisorbed dioxygen could be monitored from the integrated area of the 3~s + 1~” peak and the height of the 1.1 eV feature. The coverage of both species increased simultaneously from zero coverage (fig. 11). This is in contrast to the case of oxygen/Al(lll) [15] where the surface first saturates with a chemisorbed species, and only then begins to adsorb the physisorbed state. We conclude that, at least at low coverages, the physisorbed molecules are in contact with the bare metal surface. As previously reported [14], anomalous intensity ratios of the peaks in the first layer suggest that the physisorbed molecules are oriented on the surface.

114

K. C. Prince et

Oxygen adsorption an Ag(l IO)

Al. /

s-light

*.-* -.. .-* -*._.*+.*_

: ‘....

I

13

I

I

I

12 11 10

,

,

,

I,

g

8

7

6

5

,

I

L

3

Energy below EF (eW Fig. 12. Polarised He1 light photoemission spectra of molecular oxygen on Ag(ll0). a= 45O, normal emission 25 K, Ao = 21.2 eV. Note the disappearance of the peaks of (I symmetry for s-polarised light.

This was confined by the use of polarised NeI light (fig. 12). It can be seen that in s-polarised light, the 3u,-derived peak disappears, indicating that the molecules are not randomly oriented. A considerable reduction in intensity of the In,-derived feature also occurs with s-polarised light, but this is partly associated with changes in the Ag d-band. To supplement these data synchrotron radiation was used to excite the 2~s” level. Fig. 13 shows spectra for normal phot~~ssion, with the light incident at normal and at near-to-gr~ng incidence (SO*). The Zcr,-derived peak is suppressed by a factor of - 5 in going from normal to grazing incidence. As will be discussed below, this is strong evidence for an orientation parallel to the surface. Some limited experiments were carried out to investigate the conversion of the physisorbed state to the chemisorbed, undissociated state. Heating a sub-monolayer coverage of physisorbed oxygen invariably resulted in an increase in photoemission intensity at 1.1 eV binding energy. This is interpreted as conversion of the physisorbed state into the chemisorbed state. The

115

K. C. Prince et al. / Oxygen odrorpiion on Ag(I IO]

hw =29 8eV

L

I

22

20

18

18 16 Energy

Fig. 13. Synchrotron

I,

1,

14

12

below

10 EF

8

,

I

6

L

I

(

2 EF

(eVJ

radiation photoemission spectra of one monolayer of physisorbed (1.4 L dose), trw = 29.8 eV, normal emission.

oxygen

design of the holder did not allow accurate thermal desorption, so that it was not possible to compare quantitatively how much oxygen converted and how much desorbed.

4. Discussion Having already dealt with the main features of the spectra from the atomic oxygen species in section 3.2, the discussion here concentrates on the chemisorbed dioxygen molecule and the physisorbed species. To address the question of assignment of orbitals and orientation of chemisorbed dioxygen we consult the correlation table shown in table 1. Here the reduction in symmetry for various (symmetric) adsorption sites of an 0, species and the corresponding behaviour of the orbitals is represented. Note that for the C,, lying-down geometry, the molecular axis can either be in the x direction or in the y direction. A possible C, configuration is illustrated in fig. 14. C, symmetry would pertain when the molecules were tilted, or lying down in one of the azimuths other than at a bridge site. The spectra of figs. 8 and 9 show first of all that the molecule is oriented and adsorbed in some kind of symmetric site, otherwise the virtual disappearance of the 1.1 eV feature, in one case with s-polarised light, in the other case with p-polarised light, would be highly

K. C. Prince et al. / Oxygen adsorption on Ag(l IO)

116

Table 1 Correlation table for the molecular orbitals species) in four lower symmetry configurations

of oxygen

D mh

C 2v

C 2v

Gas phase

Molecular axis normal to the surface

Molecular axis parallel to the surface

(physisorbed

a2 +bl aI +b2 al b,

or chemisorbed

c2

CS

a+b a+b a b

a” + a’ a’ + a” a’ a’

dioxygen

unlikely. Further, the 4.3 eV feature appears to be present for both polarisations in both cases. This behaviour is only possible if the 1.1 eV feature is assigned to the lT,-derived level in the lying-down C,, geometry. Only in this configuration is a single b, (or b2) state possible: the selection rules tell us that emission from an a2 state is forbidden normal to the surface. This evidence for the lying-down geometry is supported by the vibrational frequency and the comparison with IR and Raman data for dioxygen-containing compounds. At 640 cm-’ the observed O-O stretch is 130 cm-’ lower than that for the dioxygen species measured on Pt(ll1) [32-341 and in transition metal dioxygen complexes [35]. Table 2 summarises this data. The O-O stretch in oxyhemocyanin, the only known dioxygen complex with a Group IB metal (copper), is the lowest known frequency for a dioxygen moiety yet recorded [36]. Oxyhemocyanin, by way of interest, is a protein responsible for oxygen transport in the blood of a large number of invertebrates. Two copper atoms from the centre at which the dioxygen is bonded and the protein acts as a large ligand. The O,-Cu, configuration has C, symmetry, similar to that shown in

Fig. 14. Possible adsorption sites of C, symmetry for chemisorbed have been chosen to give approximately the metal-oxygen bond compound oxyhaemocyanin.

dioxygen on Ag(ll0). The sites lengths observed in the cluster

117

K. C. Prince ei al. / Oxygen adsorption on Ag(ll0) Table 2 O-O stretching

frequency

in various

dioxygen

complexes

Species

C (cm-t)

Species

0, gas phase Superoxo, end on Superoxo, bridged Peroxo, monodentate

1580 1103-1195 1075-1122 807- 932

Peroxo, bidentate Oxyhemocyanin o,/Pt(lll) WAg(ll0)

ir (cm-‘) (no 1B metals)

790-844 746 870 640

fig. 14; the copper atoms have a separation of 0.35 to 0.5 nm [34]. The assignment of the dioxygen species on silver (110) to a bidentate peroxo-like species, in keeping with the proposed Czv configuration, is thus quite reasonable. In a previous publication [lo] we have pointed out that if both the b, and a, orbitals are below the Fermi level the dioxygen molecule is best described as a peroxo species (formally Ol-). The fact that the 1.1 eV peak is considerably broader for off-normal emission suggests that the a2 orbital is also present in this region. Table 3 shows the peak assignment resulting from the present work as well as that of Barteau and Madix [ll]. We do not observe the shoulder at 3.0 eV which the latter authors ascribe to one split component of the lT,-derived level. There is also disagreement to the extent of 1.6 eV on the binding energy of the 3us-derived level. Our assignments compare better, however, with theory [12]. Energies agree to within - 0.6 eV and the orbital symmetry is correct, when the molecule is placed lying down with its axis parallel to the [OOl] direction. Such a site would perhaps explain the ESDIAD data of Bange et al. [4] in which desorption of O+ ions is observed in the [OOl] azimuth. Assuming a lying-down C,, configuration it is actually possible using the data of figs. 8 and 9 to determine the orientation of the O-O axis. Considering only the b, state at 1.1 eV, the various possibilities can be explored using the selection rules (table 4). From the comparison with the experimental data one is unmistakably led to the conclusion that the molecular axis is parallel to the y axis or [llO] azimuth, i.e. the molecule lies in the grooves of the surface. This Table 3 UPS data and assignments: Present

chemisorbed

work

Peak energy

molecular

dioxygen

Barteau Assignment

(ev)

and Madix [11]

Peak energy

Assignment

(ev)

1.1 4.3

b, at +b2

ns-derived n”-derived

8.2

at

as-derived

0.8 3.0 4.0 6.6

ns-derived ns-derived a,-derived ok-denved

(doublet)

K. C. Prince et al. / Oxygen adsorption on Ag(ll0)

118

Table 4 Application assumption

of the selection rules to the problem of a C,, “lying-down” configuration.

Light incident [lOOI azimuth

[Ii01 azimuth

in

of the orientation of the dioxygen b, state: allowed ( +), forbidden s

P

II X

_

” Y

+

+ _

II X II y

+ _

O-O

axis

species on the (-)

_ +

result appears to be at variance with the ESDIAD data. Unfortunately, the corresponding NEXAFS experiments which could resolve this problem, have so far been thwarted by contamination problems [37]. We turn our attention now to the physisorbed species. Here we are fortunately confronted with relatively sharp levels which lend themselves more readily to the application of the selection rules. As explained above, earlier work had already indicated that the physisorbed 0, molecules are oriented. The disappearance of the emission from the 3us-derived orbital with s-polarised light indicates that this level belongs to the totally symmetric representation. As table 1 shows, however, this applies to all three possible point groups. (Because of the very weak interaction with the surface we assume that only three are relevant here: C,,, C,, and C,, whereby the latter, involving a tilted species, is unlikely. For C,,, the b, + b, is replaced by 71 and a, by IJ in the C,, column of table 1.) The 2a,-derived orbital provides the real test and allows an unequivocal distinction between the orientations. For the standing (C,,) or tilted (C,) molecule 2a, correlates with u or a’, for the lying-down C,, configuration with b,. Using synchrotron radiation it is possible to probe the 2a,-derived orbital relatively easily (if this level were sharp and intense enough it would have been useful to have done this experiment on the dioxygen species). Unfortunately, the configuration of our spectrometer does not allow us to change between s- and p-polarised light on the storage ring as with a rotating polariser on a laboratory source. We have thus taken a special case of the “forbidden geometry” experiment (see section 2) in which the emission plane is orthogonal to the E vector with s-polarised light. Totally symmetric states are then forbidden. This normal incidence/normal emission geometry is shown as the (Y= 0’ curve in fig. 13. The 2u,-derived feature is clearly visible and is in fact the most prominent feature in the spectrum. This indicates the b, assignment and thus a lying-down geometry. A totally symmetric state could only be observed in this geometry if there were some misalignment of the sample. A further test is provided by the spectrum at a = 80”. Using the Scheffler formulae [38] and the optical constants of silver [39] it is possible to calculate the intensity ratio of the peak for the two angles assuming a b,

K. C. Prince et al. / Oxygen ahorption

on Ag(l IO)

119

assignment. In fact, the ratio turns out to be about twice as large as observed. This may seem a large error, but the ratio is a sensitive function of angle at high (Yand small misalignments play an important role. We should, however, remember that the intensity ratio would be the other way round if the 2u,-derived feature had u or a’ symmetry. Another possibility is that a lT,-derived satellite is also present as a shoulder on the 2u, feature [16]. The physisorption potential energy well can be regarded as the sum of the attractive (asymptotic) Van der Waals term (- l/t3) and the steeply rising repulsive term. The former arises from dispersion forces and the latter from the overlap of filled orbitals of the molecule and of the substrate. Harris and Feibelman [40] show that the Van der Waals interaction favours the upright orientation of a diatomic molecule. Simple overlap considerations indicate that a dominant repulsive term would favour the lying-down configuration. In the present case the energy difference between the configurations is clearly dictated by the repulsive term. The herring-bone structures observed with LEED for oxygen and nitrogen on graphite (e.g. ref. [41]) also indicate a preferred parallel orientation. Since the physisorbed species are in contact with the bare metal surface and, on the time scale of the experiment, also stable with regard to conversion into the chemisorbed dioxygen state, we conclude that they are trapped in a potential well. We identify the physisorbed species as an intrinsic precursor state for chemisorption [14]. It was noted earlier that on adsorption of the physisorbed phase, there is always some formation of the chemisorbed species. In addition, the sticking coefficient is - 1, as indicated by monolayer formation at 1.5 L. This data can be explained with the aid of a potential energy diagram (fig. 15). When an oxygen molecule at 300 K impinges on a surface at 25 K, it must dissipate kinetic energy and momentum before it can adsorb. The high sticking probability indicates that both processes are efficient, and in particular, that sufficient phonons can be generated on impact for the molecule to lose its momentum normal to the surface. At higher temperature, the sticking coefficient is low but this is not due to a failure of the surface to thermally accommodate the impinging molecule. Rather, -it is due to a residence time effect whereby the molecule spends only a very short time in the precursor state, which is in general insufficient for conversion to the chemisorbed dioxygen or atomically adsorbed states. The formation of some chemisorbed oxygen during adsorption indicates that thermal accommodation is not perfect, and that some molecules survive the collision with the surface in a thermally excited state. This is not surprising since a molecule requires an energy of E, to overcome the potential barrier. E, is an energy corresponding to a temperature of - 25-40 K, whereas the molecules impinge on the surface at 300 K. We speculate that if cold oxygen were adsorbed on the surface, there would be a lower percentage of conversion to the chemisorbed state. From the literature and the present work, we can assign limits to the

120

K. C. Prince et al. / Oxygen adwrption

on Ag(ll0)

PRECURSOR OXVGEN ASSOCIATIVELY OXYGEN DISSOCIATIVELV OXYGEN

(PHVSISORBED) ADSORBED ADSORBED

Fig. 15. Schematic potential energy diagram for oxygen adsorption on silver.

energies in the potential energy diagram. The depth of the potential well E, is of the order 4 kJ/mol, for a temperature of 25 K. Since heating a mixed layer causes conversion of the physisorbed to the chemisorbed species then E, = E, or E, < E,. Barteau and Madix [7] have indicated that the activation energy for desorption of the molecular species is 20-30 kJ/mol; complications in the desorption mechanism prevent a more definitive value being given. We identify this energy with E,. The energy E, is unknown, but is probably a large fraction of E,. At higher coverages, the molecular species may dissociate or desorb during heating, indicating comparable activation energies for the two processes. Finally, Bowker et al. [42] have estimated an activation energy for desorption of 173 + 5 kJ/mol for atomic oxygen, which we identify with E,.

5. Conclusions Using polarised light we have determined the initial state symmetry of the higher lying photoemission peaks of both atomic and molecular oxygen. These are consistent with previously proposed adsorption and valence electronic structures and measured values agree quantitatively with calculations. On the basis of photoemission selection rules and the analogy with model dioxygen complexes, the oxygen is proposed to be multiply coordinated to the metal surface. Physisorbed molecular oxygen in contact with the bare metal surface

K. C. Prince et al. / Oxygen aakorption on Ag(lIQ,

121

has been identified as an intrinsic precursor state for adsorption. The physisorbed molecule is oriented with its molecular axis parallel to the surface.

Acknowledgements This work was supported in part by the Deutsche Forschungsgemeinschaft through Sfb 6. Thanks are due to the BESSY staff for invaluable technical support.

References [l] H.A. EngeIhardt and D. Menzel, Surface Sci. 57 (1976) 591. [2] W. Heiland, F. Iberl, E. Taglatter and D. Menzel, Surface Sci. 53 (1975) 383. [3] E. Zanazzi, M. Maghetta, U. Bardi, F. Jona and P.M. Marcus, J. Vacuum Sci. Technol. Al (1983) 7. [4] K. Bange, T.E. Madey and J.K. Sass, Chem. Phys. Letters 113 (1989) 56. [5] A. Puschmann and J. Haase, Surface Sci. 144 (1984) 559. [6] CT. Campbell and M.T. Paffett, Surface Sci. 143 (1984) 517. [7] M.A. Barteau and R.J. Madix, Surface Sci. 97 (1980) 101. [S] C. Backx, C.P.M. de Groot and P. Biloen, Surface Sci. 104 (1981) 300. [9] B.A. Sexton and R.J. Madix, Chem. Phys. Letters 76 (1980) 294. [lo] K.C. Prince and A.M. Bradshaw, Surface Sci. 126 (1983) 49. [ll] M.A. Barteau and R.J. Madix, Chem. Phys. Letters 97 (1983) 85. [12] A. Selmani, J.M. Sichel and D.R. S&hub, Surface Sci., submitted. [13] C. Pettenkofer, I. Pockrand and A. Otto, Surface Sci. 135 (1983) 52. [14] K.C. Prince, G. Paolucci, A.M. Bradshaw, K. Horn and C. Mariani, Vacuum 33 (1983) 867. [15] P. Hofmann, K. Horn, A.M. Bradshaw and K. Jacobi, Surface Sci. 82 (1979) 610. [16] D. Schmeisser and K. Jacobi, Surface Sci. 108 (1981) 421. [17] M.J. Gnmze, G. FuhIer, M. Neumann, C.R. Brundle, D.J. Auerbach and J. Behm, Surface Sci. 139 (1984) 109. [18] M. Shayegan, J.M. CavaIlo, R.E. Glover III and R.L. Park, Phys. Rev. Letters 53 (1984) 1578. [19] B.E. Hayden, K. Kretzschmar and A.M. Bradshaw, Surface Sci. 155 (1985) 553. [20] J. E&mans, A. Otto and A. Goldman, Surface Sci. 149 (1984) 293. [21] M.A. Barteau and R.J. Madix, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1982). [22] D. Bryce-Smith, E.J. Blues and B.G. de Martinez, Chem. Ind. (London) (1983) 717. [23] H. Petersen, E. Dietz and U. Sowada, BESSY Jahresbericht 1983 (BESSY, Berlin, 1984) p. 192. [24] N. Riisch and D. Menzel, Chem. Phys. 13 (1976) 243. [25] R.L. Martin and P.J. Hay, Surface Sci. 130 (1983) L283. [26] A.M. Bradshaw and M. Scheffler, J. Vacuum Sci. Technol. 16 (1979) 447. [27] K.M. Ho, B.N. Harmon and S.H. Liu, Phys. Rev. Letters 44 (1980) 1531. [28] J.B. Pendry, in: Photoemission and the Electronic Properties of Surfaces, Eds. B. Feuerbacher et al. (Wiley-Interscience, New York, 1978) p. 97. 1291 K. Jacobi, P. Geng and W. Ranke, J. Phys. El1 (1978) 982. [30] R.A. DiDio, D.M. Zehner and E.W. Plummer, J. Vacuum Sci. Technol. A2 (1984) 852. [31] D.W. Turner, C. Baker, A.D. Baker and C.R. Brundle, Molecular Photoelectron Spectroscopy (Wiley-Interscience, New York, 1970).

122 [32] [33] [34] [35] [36] [37] [3S] [39]

K. C. Prince et al. / Oxygen adsorption on Ag(ll0)

J.L. Gland, B.A. Sexton and G.B. Fisher, Surface Sci. 95 (1980) 587. H. Steininger, J. Lehwald and H. Ibach, Surface Sci. 123 (1982) 1. N.R. Avery, Chem. Phys. Letters 96 (1983) 371. L. Vaska, Act. Chem. Res. 9 (1976) 175. T.B. Freedman, J.S. Loehr and T.M. Loehr, J. Am. Chem. Sot. 98 (1976) 2809. J. Haase, A. Puschmamr and A.M. Bradshaw, unpublished results. M. Scheffler, K. Kambe and F. Forstmann, Solid State Commun. 23 (1977) 789. Physik Daten/Physics Data, Vol. 18-2 (compiled by J.H. Weaver et al.) (ZAED, Karlsruhe, 1981). [40] J. Harris and P.J. Feibelman, Surface Sci. 115 (1982) L133. [41] R.D. Diehl, M.F. Toney and S.C. Fain, Jr., Phys. Rev. Letters 48 (1982) 177. [42] M. Bowker, M.A. Barteau and R.J. Madix, Surface Sci. 92 (1980) 528.