The interaction of potassium submonolayers adsorbed on Pt(111) with oxygen and the adsorption of ethylene on the resulting modified surfaces: a TDS and UPS study

Surface Science 284 (1993) 273-280 North-Holland

The interaction of potassium submonolayers adsorbed on Pt( 111) with oxygen and the adsorption of ethylene on the resulting modified surfaces: a TDS and UPS study A, Cassuto *, S. Schmidt and Mane Mane ~~r~ioire

barge

Retort CNRS, 405 rue de Vandoeuvre,~~-~~~~-~~~-Nu~~,

France

Received 1 September 1992, accepted for publication 16 November 1992

UPS shows that K atoms deposited on Pt(lll) in the submonolayer range strongly interact with oxygen molecules. At 300 K, oxygen molecules dissociate. Oxygen atoms either attach to potassium atoms or lead to K,O. At 9.5 K, depending on the expe~mental conditions (exposure and pressure), potassium peroxide or potassium superoxide, as majority species, form. ‘TDS as well as UPS indicate that on these surfaces ethylene is T-bonded as on Pt(lil) surfaces, partially covered with K atoms. No ethylene adsorption occurs on surfaces fully covered with oxygen atoms or oxides. Ethylene adsorption therefore occurs on the clean part of the sample and is disturbed by the presence of various species of potassium attached to oxygen.

1. Int~duction

Alkali metals or their salts are frequently used as promoters in various catalytic reactions involving hydrocarbon synthesis or reactions (see for example refs. [1,21). In connection, the adsorption modes and the reactions of ethylene, a model moiecule, have been studied with the available surface science techniques on various metals and single crystals (see ref. [3j, and references therein), The adsorption of ethylene on single crystals is not only metal-specific but aIso face-sensitive 141. Recent works have been devoted to the influence of coadsorbed alkali metals on Pt single crystals [5-121 and have led to the major conclusion of the di-a bonding being replaced by v-bonding nearby alkaii adatoms. Simultaneously, on alkalicovered Pt(lll), decomposition reactions are inhibited and ethylene desorbs at lower temperatures than on clean Ptflll). On finely dispersed pure hydrogenation platinum catalysts 1131, w

* To whom ~~es~nden~ ~39-6028/93/$~.~

should be addressed.

bonded ethylene has also been identified as a major adsorbed species. Under reaction conditions, the state of the alkali promoter is usually not well known but probably differs from model studies. For example, it has been recently shown [14] with the help of a combined UHV/high-pressure reactor system and FT-IRAS, LEED, AES and TDS that under CO hydrogenation conditions on Ru(OOl), elemental potassium were found primarily at low coverages (0.10) whereas potassium formate and/ or carbonate were observed at higher coverages, Some alkali metal compounds &OH, KNO,, K&O,) have been studied spectroscopically on model catalysts 1151. The various alkali metal oxides are also possible promoters in hydrocarbon reactions. The preparation of noble metal catalysts with alkali promoters may lead to the formation of ionic oxygen species. This includes the 02-, the O$- and the 0; ions involved in the formation of oxides, peroxides and superoxides. The unambiguous identification of the above oxygen species by means of the multiplet struc-

Q 1993 - Elsevier Science Publishers B.V. Ah rights reserved

of the va~en~e-baud spectra has already been made on butk alkali metal oxides (Li, K and Cs) prepared either by de~sition under oxygen pressure or by oxidation of aIkaii metal layers 1161. The two first ions having ciosed shells produce, respectively, one and three final states when photoionized. Due to degeneracy and resolution problems, the superoxidic ions experimentally Iead to four major peaks. A point-charge model has been developed to explain the constant energy separations between the 00s) core levels and the O(2p) valence jevels‘ and between the O(2p) levels for each oxygen species in various impounds (Li, K, Cs oxides) 116,171. The i~t~ra&tion of monolayers or submono~ayers of alkali met& with oxygen has been extensively studied, using various techniques, in recent years. As examples, results have been obtained at nearly full monolayer coverage on K/Pt(lll) [l&19], Cs/W~lOO~ [201, Cs/Mo(lOO) [211, Na/Ni(lll) [221, Cs/Ni(lll) 1231 and Cs/GaAs [24] and in the submonolayer range on &/Au [25], K/ Rut0011 R&27], Cs/Pt(lll) 1281,Li/ W(llO) [293, Naf WC110) [29f, K/ W010) 1303and Cs/ WG 10) [301. This paper is mostly devoted, in the case of K/Ptflll) at about half a monolayer, to the interaction of oxygen with deposited potassium atoms and adso~t~on of ethylene on the resulting surfaces. As in previous works f&101, UPS and TDS have been used to identifv the various species, This overage has been chosen for various reasons: (1) If at full monolayer coverage ~tassium is covalent or nearly metal&, at partial coverage there is covalent bonding with p~ati~urn atoms 131,321 or partial charge transfer f33,34j which may render oxidation difficult. (22)Conversion of di-rr ethylene into T-bonded ethylene on K-covered Pt(lll) surfaces reaches a m~imum at about half a monolayer [5,8,121. The amount of r-bonded ethylene decreases strong@ above this coverage while below it, the simultaneous presence of rr- and di-u ethyiene may obscure the results. Some TDS results at higher potassium coverages will be presented and will confirm the decrease in r-bonded ethylene coverage above this tures

value. A comparison will also be made with ethylene adsorptive of K/Ft~lll~ surfaces.

The experiments have been p~~~~~~ in a home-made ultra-high vacuum apparatus (apparatus A in ref. [lo]>, equipped with LIED, MS, line-of~s~ight mass sp~ctr~met~ and a helium discharge tamp that has been used with He I: unpolarized radiation (21.21 cV) to record ultraviolet photoelectron spectra with a CAM ~A~/~ mode). Work function Y~riatiQ~s could be measured from the shifts of the thr~shoid of the UV spectra, by applying an adequate potari~tion on the sample (about -9 V>. Line-of-sight mass spectrometry was used during TDS (5 K/s) monitored with a XT computer. Simultaneous recording of several ions was made possible.. Cooling the sample to 95 K was available through leads connected to a cold “reservoir”. A platinum single crystal (Material Research) cut within 0.5” paratlel to the (Ill) orientation and mec~~niGaIiy polished was mounted on a X, Y, 2, B manipulator* It was checked by LEED, AES and UPS after the cleaning procedure (oxygen treatment, ion bombardment and annealing to 1000 K using therma radiations. Temperatures were measured with a Ghromei-a~urne~ therm~oup~e spot”w~ided near the sampie on the wire hoIdi~~ the crystal, Potassium deposition rates from carefulfy outgassed SAES getter sources were initially calibrated through AES, work function variations and completion of the monolayer at room temperature. At 300 K, the atomic ratio at saturations is 0.44 135,361. Taking this as a reference for caiibration, we found in consistency with the above authors a work function minimum at 1.2 -i_ 0.2 eV at CJK= 0.17. All K coverages will be given thereafter in monolayers (ML) defined as atomic ratios. Oxygen and ethylene (Messer Griesheim~ purity 99.95% and 99.5%, respectively) were introduced through leak valves. The appearance of potassium-oxygen prsducts was monitored as foiIows, For a given potassium coverage (usually r 0.17, almost haIf a monolayer or above) and a given sample tempers

A. Cassuto et al. / The interaction of K submonolayers adsorbed on Pt(lll)

ature, the surface was exposed to oxygen at a given pressure (between 5 x lo-* and 3 x lo-” mbar) and temperature (300 or 95 K) and a ~ntinuous recording of the W spectra was made. The work function increases during the strong interaction of potassium atoms with oxygen and/or formation of oxides. It almost saturates during adsorption on the remaining bare Pt(ll1) surface. Oxygen adsorption was stopped at this stage to prevent adsorption of oxygen on the Pt(ll1) remaining surface. The development of peculiar structures in the UV spectra allowed us to prepare specific K-O surfaces that will be presented below. Saturation of these surfaces with ethylene (5 Langmuir units) was followed with UPS, and TDS was performed after recording the final spectra.

with oxygen

275

N(E) (arbmdts)

\--, to

16

14

12

to

8.6

6.0

binding energy (eV)

4.0

2.0

0.0

2.0

E*

Fig. 1. Adsorption of oxygen at 300 K, under 5 X lOA8 mbar 0, (exposure 10 L), leading to the formation of K-O as a dominant species - UP spectra (He I) of the valence band of clean Pt(lll), of the surface partially covered with K (@, = 0.17) and of the K-O surface (see text and discussion).

3. Resutts 3.1. The formation of potassium-oxygen species According to Jupille et al. [16], at low temperatures Oz- and 0; are obtained on a few layers of alkali metal after moderate and high exposures. A similar procedure was attempted with success in the submonolayer range at low temperatures. At 300 K, for all potassium coverages equal or above 0.17 ML, a single characteristic spectrum was obtained with a major peak at 5.1 eV below the Fermi level. Fig. 1 shows such a spectrum where the major peak has saturated (and the work function reached a steady value) after 10 L exposure under 5 x lo-’ mbar. Dividing the number of oxygen atoms on the surface by the exposure leads to a rough estimate of the average reactive sticking coefficient (of the order of 2 x 10T2 or above). A second faint feature at 10.3 eV below the Fermi level was tentatively attributed to CO contamination from the background (2 x 10-l’ mbar) and the strong sensitivity to this species. Indeed, the work function is still low (2.2 eV> and it may correspond to a shift of the (1~ + 50) orbitals due to the electrostatic influence of K6+ ions [37]. However, the 4r+ orbital is not visible in fig. 1. The low sensitivi~ of the

analyzer (AE/E mode) at low kinetic energies may be responsible for this fact. No other features were observed for higher exposures. When adsorption was performed at 95 K, always more transitions were observed, whatever the exposure. For comparable exposures (20 L under 5 x 10m8 mbar), the work function also reaches a steady value and extra features appear in the 5 and 7-10 eV energy range below the Fermi level as indicated in fig. 2. The average reactive sticking #efficient is also of the order of 2 x 10-2. Other interacting conditions were found at much higher exposures and pressures (1200 L under 3 x lop5 mbar). The experimental spectrum is shown in fig. 3 where features in the 5-7, and 9 eV ranges appear. The formation of this third species corresponds to a much lower reactive sticking coefficient of the order of 5 X 10w4. The spectra shown in figs. 1, 2 and 3 (see discussion) were attributed either to oxygen atoms strongly interacting with potassium atoms or K,O (named K-O species), peroxide and superoxide. Difference spectra between the oxygen-covered surfaces and the clean Pt(211) surface are given in fig. 4. The positions of the major features have been reported in figs. 1, 2 and 4.

275

A. Cassuto et al. / The interaction

of ~s~bmQnolaye~

a&orbed on Pt(l 11) with oxygen

I

dN(E) (arb.units)

N(E) (arb.units)

J 18

16

14

12

to

8.0

6.0

4.0

2.0

0.0

12

.*.cl

10

binding binding

energy (eV1

so

a0

4.0

20

M

energy (eV)

E

%

Fig. 2. Adsorption of oxygen at 95 K, under 5 X lO_’ mbar 0, (exposure 20 L), leading to the formation of K202 as a dominant species - UP spectra (He I) of the valence band of clean Pt(lll), of the surface partialiy covered with K (8, = 0.171 and of the oxidized surface.

3.2. Ethylene adsorptionon the uariouspotassiumoxygen species

Using similar conditions, the various potassium-oxygen species (named hereafter for conve-

-20 F

Fig. 4. Difference UP spectra (He I) between the surfaces partially covered with the K-O surface, the oxides (0, = 0.171 and the clean Ptflll) surface.

niency K-O, K,O, or peroxide, KO, or superoxide, see discussion) have been obtained at several initial potassium coverages and TDS performed after saturation of the surface with ethylene. Figs. 5 and 6 indicate that ethylene desorption mostly occurs at low temperatures on the first two surfaces, a situation comparable to the K/Pt(lll) surface [5,8,10-123. They also show that di-o bonded ethylene (desorption in the 250-290 K range) has almost disappeared. There

C,H,

pressure

(arb.units)

116 K

-cc-e K?0.45 :8’ “( ,_.’.“‘.“.*. ........_.........._.................. __...” . . ..,............. II..,............ RK: U.36 .,. .,‘.......” .,_,.,.,,,,................. I I.. . .......... . ...” . ..l...........” : 0 R=0.25 ...... ....... .. ........I......._. .,...................,.,..,..... ...:’ ‘, “x ...t.)............

18

14

14

I2

to binding

8.0

6.0

energy (eV)

4.0

2.0

0.0

..

-1.0

Fig. 3. Adsorption of oxygen at 95 K, under 3X 10W5mbar 0, (exposure 1200 L), leading to the formation of KOz as a dominant species - UP spectra (He I) of the valence band of clean Ptflll), of the surface partially covered with K (BK = 0.17) and of the oxidized surface.

aioK ‘i

%

\

‘. . . ..I._.,...._..,_...,,,......,,,.,..,..,,..........

_

.

-...

‘1

““““““~“~.~~~

.. . ..._.”

.

. . .

. . .

aK‘*.‘7 . .

. .

,^,.,,..,

.A-

80

130

180

230

.3tl*

Fig. 5. TDS of ethylene after saturation of K-O surfaces (initial K coverages: @x = 0.17; 0.25; 0.36; 0.451.

A. Cassutoet al. / The interactionof K submonolayersadsorbedon Pt(ll1) withoxygen

C,H,

217

dN(E) (arb.units)

pressure(arb.units)

/ ,125K e K=0.43 .‘.._ : ‘., _.: .., .‘..“._ .,_.,_...,,, _____.. _.,...... __ _..._.._ _.__“.._ .....”._...._......... _._._._......_._.......-...--.” ...._._..

e K=0.28 ‘.., ,.‘_ ._._.__,.” .(.,...._ __.._. _____. _.“” ,,.. _____._ ..............._ ...”...._. I ...” ...” ........I-.. ..: ‘, ‘..._

I8

16

t4

12

to

s.0

6.0

binding energy (eV)

Fig. 6. TDS of ethylene after saturation of K,O, surfaces (initial K coverages: fIK = 0.17; 0.19; 0.28; 0.43).

is almost no adsorption on the fully covered Pt(ll1) surface (0, = 0.44). This proves that ethylene is adsorbed on the clean part of the Pt(ll1) surface and is disturbed by the presence of oxygen atoms or the oxides. The amount of desorbed ethylene decreases above 8, = 0.17 and is at this coverage higher on (K-0) surfaces than on K,O, surfaces. A much smaller amount of ethylene molecules is desorbed from the superoxidic sur-

C,H,

pressure(arb.units)

116K

80

ml

140

170

200

0.0

-1.0

%

Fig. 8. Difference UP spectra (He I) between Pt(ll1) surfaces partially covered with potassium atoms, K-O or potassium peroxide (initial 0 x = 0.17) and saturated with ethylene and the Pt(ll1) surfaces partially covered with potassium atoms, K-O or potassium peroxide (initial OK= 0.17).

faces as indicated in fig. 7 where TDS is shown for the three species formed on a potassium layer corresponding to OK= 0.17. In all cases, minor or no hydrogen desorption was recorded, indicating strong inhibition of the decomposition path.

4. Discussion

C2H4/K02/R(111)

125 K .........._...... _._._.__,_

1.0

3.2.2. UPS results Strong extra features appeared after saturation with ethylene of the first two surfaces (f3, = 0.17) while almost no changes were observed on the third one, in agreement with TDS results. Difference spectra between the first two surfaces after and before saturation with ethylene are indicated in fig. 8 where a comparison is made with the potassium-covered surface at the same coverage.

_‘...

_....+.-.

4.0

__“.__“..“_

ml

.,..,........... _

260

no

“.”

310

_

IO

_

380

Temperature(K)

Fig. 7. TDS of ethylene after saturation the Pt(ll1) surface partially covered with the various potassium-oxygen surfaces (initial 0 K = 0.17).

A point-charge model [38-401 which considers all charges located at an ion center has been applied with success to the 02-, O$- and 0; ions in bulk oxides (Li, K, Cs> [16,17]. Within this model the binding energies of the electronic levels of an ion, either valence or core, are expected

278

A. Cassutoet al. / The ~~tera~t~on of K s~~on~~~ers adsorbedon Pt fill)

to be affected in the same way in any environment and to show constant energy spacings 1421 either in gas or condensed phase. The 02- ion or 0 atoms must lead to a unique feature in the valence band. In figs. 1 and 3, such a feature appears at 5.1 + 0.05 eV below the Fermi level). McBreen et al. [25] have shown using SIMS that oxygen interacts readily with potassium monolayers and potassium submonolayers on Au and that various K,O: ions are formed, including K,O+. Garfunkel and Somoraji [18] deduced from TDS and LEED results that interaction of potassium atoms (at 8, = 0.33) adsorbed on Pt(ll1) with oxygen was strong and that a planar K,O layer might be formed by annealing coadsorbed potassium and oxygen to 750 K. However, Pirug et al. El91 disagreed with their conclusions. Two UPS features named A and B, respectively, in fig. 4 in ref. 1191 appear sequentially when a potassium monolayer (6, = 0.33) deposited on Pt(ll1) reacts with oxygen at 300 K. The binding energies are 4.9 and 6.5 eV below E,, respectively. These energies (as well as our value, 5.1 eV) differ strongly from the energy of the prominent peak at 2,9 [26] or 3.2 eV [161 below E,, characteristic for bulk potassium oxide, K,O. Pirug et al. [19] therefore attributed these features to oxygen atoms adsorbed near potassium atoms or on the bare Pt surface, uncovered by the contraction of the potassium Iayer. A simiiar point of view has been developed recently by Rocker et al. [27] (oxygen + K/Ru(OOl)~ and by Maus-Friedrichs et al. 1301 (oxygen -t- Cs or K/W(llO)) who both used metastabIe impact electron spectroscopy to investigate the interaction of submonolayers of alkali metals with oxygen. Riwan et al. found also (after adsorption of oxygen on Pt(lllI covered by an incomplete Cs(2 x 2) overlayer) a feature shifting between 5.8 and 4.2 eV with oxygen exposure (P,) but it grew along with two other peaks (P3 and P4). Under our experimental conditions, oxygen dosing has been stopped when the work function had almost reached a steady value, to avoid adsorption on the bare Pt(ll1) surface. Our single peak at 5.1 eV below E, compares well with peak A in ref. [191(4.9 eV1, peak ~3in ref. [27](5.0 eV> and peak 0, in ref. [30] (5.0 eV>. Efowever, the

withoxygen

position of the high-lying orbital in bulk oxides is variable, depending on the structure and the purity of the layers (see tables 1, 2 and 3 in ref. [Xi]). Moreover, in submonolayer peroxide and superoxide obtained at 95 K (see below) its position is shifted towards higher binding energies by an amount of the order of 2-3 eV, when compared to bulk oxides. A similar shift wouId lead for submonolayer I&O to a peak located in the 5 eV range. Under these conditions, it is not possible to decide wether oxygen interacting with potassium atoms at 300 K leads to the formation of strongly interacting oxygen atoms [19,27,301 or to K,O, the monoxide [l&25]. This is why this species has been called (K-O). However, r-bonded ethylene molecules adsorbed on the various oxygen-potassium interacting species (see below) exhibit position of their molecular orbitals comparabie to those observed on K/Pt(lll) surfaces (see fig, 8). This indicates a strong effect of the electrostatic potential [l&12] induced by the various potassium-oxygen species on ethylene, including the one obtained at 300 K (K-0) and the potassium fo~ation of KS”. Let us now examine the results obtained at 95 K‘ O$- has a closed shell configuration 3a$?r,:’ 1~2. Its UP spectrum is thus expected to invalve onIy three features, namely “Iii,‘, ‘II, and ‘IIg corresponding to the ionization of the 3a,, lrr, and 1~~ orbitals, respectively. Figs. 2 and 4 indicate their presence at 4.7, 7.8 and 9.1 + 0.05 eV. The presence of some (K-O) is revealed through a shoulder at 5.1 eV. The spacings between the main features, referred to the closest orbital to the Fermi level, 3.1 and 4.4 eV are in good agreement with theoretical calculations with an undisturbed O-O distance equal to 0.149 nm during the ionization process 1421and experimental data on bulk oxides (see table 2 in ref. [It;]) while some discrepancy exists with more recent caiculations 1431 leading to a 211g-211u energy separation equal to 4.0 eV. It does not question our assignment to the peroxide since the valence spectra shown in figs. 2 and 4 produce three well-shaped features and energy separation in concordance with experimental data.

A. Cassuto et al. / The interaction of K submonolayers adsorbed on Pt(lll)

The ground state of the 0; ion is 3a$$l,rr,4 172. The emission of a 1~~ electron leads to the high-lying 3X;, lA, and ‘2: multiplets with energy separations 1.05 (‘2; to ‘An) and 0.550.70 eV (iAg to ‘X2) as derived from calculations when taking the O-O distance equal to 0.129 nm [42,44,45]. The next states towards higher binding energies are nearly degenerate (‘Xi, 3A, and ‘2:). A combination of 3X; and 311, orbitals is expected at a higher binding energy. The spectra in figs. 3 and 4 partially fulfill the expectation of five major orbitals. The valence band of the species called superoxide (KO,) exhibits four major features with spacings referred to the high-lying one equal to 1.1, 4 and 6.6 f 0.05 eV, respectively. However, comparison with existing experimental data (see table 3 in ref. [16]), where currently only four orbitals are also indicated, unambiguously shows that the species obtained at high exposures is a superoxide. It must be attributed to potassium superoxide and not to the r-bonded molecular species that is obtained on clean Pt(ll1) at 95 K [46,471. Peroxide features are no longer visible in figs. 3 and 4 and conversion into the molecular species adsorbed on clean Pt(ll1) (desorbing and dissociating at about 150 K) at the expense of the peroxide is not expected. Moreover, on clean Pt(lll), the molecular species is obtained at low exposures and pressures and only exhibits a broad feature at about 7 eV below the Fermi level [45] while 1200 L and 3 X 10e5 mbar were necessary to record the more complex spectra shown in figs. 3 and 4. It must be noticed that, compared to bulk peroxide and superoxide [16], the positions of the high-lying orbitals in submonolayer oxides are shifted towards higher binding energies. This calls for theoretical work on “planar” K-O compounds. After adsorption of ethylene on the three surfaces, TDS only indicates desorption of ethylene in the 110-130 K temperature range, as on K/Pt(lll) surfaces [5,8,10-121. It is a strong indication of the replacement of the di-a species formed on clean Pt(ll1) surface by a r-bonded species. UPS confirms this result. Despite small shifts in positions, the three spectra shown in fig. 8 exhibit almost the same spacings. NEXAFS [12]

with oxygen

219

has shown that ethylene adsorbed on the Kcovered Pt(ll1) surface was keeping its C-C double bond. The similar UV spectra recorded with the surfaces partially covered with K-O and peroxide lead to the conclusion that this is also the case here. It is confirmed by the spacings between the various molecular orbitals (rcc, abu,, according to ref. [48]) which %r*, UC, and &, agree closely to those of the multilayer [3] and the gas phase values [491. Ethylene does not adsorb on platinum surfaces fully covered with the oxygen-potassium species or potassium atoms [12], as shown by TDS results. As a result, w bonded ethylene must be adsorbed on the clean Pt(ll1) surface, near the potassium-oxygen species, as in the case of K/Pt(lll) surfaces and the effect of the electrostatic potential [11,12] is similar. As a consequence, potassium atoms (or other alkali metals) strongly interacting with oxygen atoms under catalytic conditions using promoted catalysts may lead to easy desorption of unsaturated hydrocarbons. 5. Conclusions Potassium adsorbed on Pt(ll1) at a coverage leading to the work function minimum (0, = 0.17) is converted into various species identified as either strongly interacting K, 0 atoms or K,O (K-O), K,O and KO,. It shows that potassium atoms, bonded to platinum atoms through partial charge transfer or covalent bonding, are able to fully ionize in the presence of oxygen at least at low temperatures. As on K-covered surfaces, ethylene, di-o bonded on the clean Pt(ll1) surface, is converted on such surfaces into a more loosely bonded, r-bonded species. Possible consequences under catalytic conditions with promoted catalysts have been underlined. Acknowledgements

The authors thank J. Jupille for fruitful discussions and the referees for their helpful comments.

280

A. Cassuto et al. / The interaction

ofK

submonolayers

References [l] W.M. Ross, Catal. Rev. Sci. Eng. 25 (1983) 591. [2] R. Schiogi, in: Physics and Chemistry of Alkali Metal Adsorption, Eds. H.P. Bonzei, A.M. Bradshaw and G. Erti (Eisevier, New York, 1989) p. 347. [3] M.B. Hugenschmidt, P. Doiie, J. Jupiiie and A. Cassuto, J. Vat. Sci. Technoi. A 7 (1989) 3312. [4] E. Yagasaki, A.L. Backman and RI. Masei, Vacuum 57 (1990) 57. [5] R.G. Windham, M.E. Bartram and R.E. Koei, J. Vat. Sci. Technoi. A 5 (1987) 457. [6] R.G. Windham, M.E. Bartram and R.E. Koei, J. Phys. Chem. 92 (1988) 2862. [7] R.G. Windham and B.E. Koei, J. Phys. Chem. 94 (1990) 1489. [S] X.L. Zhou, X.-Y. Zhu and J.M. White, Surf. Sci. 193 (1988) 387. [9] T. Sekitani, Y. Yoshinobu, M. Ouchi and N. Nishijima, J. Phys. Chem. 94 (1990) 6847. [IO] A. Cassuto, Mane Mane, M. Hugenschmidt, P. Dolie and J. Jupiiie, Surf. Sci. 237 (1990) 63. [II] A. Cassuto, Mane Mane, V. Kronneberg and J. Jupiiie, Surf. Sci. 251/252 (1991) 1133. (121 A. Cassuto, Mane Mane, J. Jupiile, G. Tourilion and Ph. Parent, J. Phys. Chem. 96 (1992) 5987. [13] C. Brunet, A. Jadi and J.C. Lavailey, J. Chim. Phys. 86 (1989) 471. [14] F.M. Hoffmann and M.D. Weisei, Surf. Sci. 269/270 (1992) 495. [15J H.P. Bonzei and H.J. Krebs, in: Physics and Chemistry of Alkali Metal Adsorption, Eds. H.P. Bonzel, A.M. Bradshaw and G. Erti (Elsevier, New York, 1989) p. 331. [16] J. Jupiiie, P. Doiie and M. Besaqon, Surf. Sci. 260 (1992) 271. [17] P. Doiie, S. Drissi, M. Besangon and J. Jupilie, Surf. Sci. 270 0992) 687. [18] E.L. Garfunkel and G.A. Somorjai, Surf. Sci. 11.5(1982) 441. [19] G. Pirug, H.P. Bonzei and G. Brodin, Surf. Sci. 122 (1982) 1. [ZOJP. Soukassian. R. Riwan, Y. Borenstein and J. Lecante, J. Phys. C 17 (1984) 1761. [Zl] R. Riwan, P. Soukassian, S. Zuber and J. Cousty, Surf. Sci. 146 (1984) 382. 1221 P. Doiie, P. Louis and J. Jupiile, Vacuum 41 (1990) 174. 1231 P. Doiie, M. Tommasini and J. Jupiiie, Surf. Sci. 211/212 (1989) 904.

adsorbed on Pt(ll1)

with oxygen

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