Langmuir 1993,9, 3491-3496
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A Vibrational Spectroscopic Investigation of CO and Cs Coadsorption on Pt(l1lj M. Tushaus, P. Gardner? and A. M. Bradshaw' Fritz-Haber-Institut der Max -Planck- Gesellschaft, Faradayweg 4-6,14195 Berlin (Dahlem), Germany Received July 6, 1993. I n Final Form: September 21, 1999 The coadsorption of CO and Cs on Pttlll) at 100 K has been investigated using infrared reflectionabsorption spectroscopy (IRAS) and low energy electron diffraction (LEED).The individual coverages of both adsorbates were varied over a wide range and the order of adsorption was reversed. The adsorption behavior is similar to that of CO coadsorbed with K on Pt(ll1J. The vibrational d a b show four groups of strongly perturbed C-O stretches with frequencies between 1420 and 1810 cm-1 as well as several less perturbed bands between 2000 and 2100 cm-1. Only two ordered coadsorption structures are observed: a (4x2) and a (2X2), the latter existing over a wide Cs coveragerange, indicativeof CO-Cs island formation. As in the case of Pt{111)-CO/K, the IRAS results are interpreted in terms of a local stoichiometrypicture. The differences between the two coadsorption systems appear to be related to the different size of the alkali-metal atoms rather than to electronic effects.
Introduction During the last few years there has been considerable interest in the interaction of alkali metals with small catalytically important molecules, particularly CO, coadsorbed on transition-metal surfaces. These numerous studies were motivated not only by the importance of alkali-metal promoters in heterogeneous catalysis but also by a desire to achieve a more fundamental understanding of simple coadsorption systems. This subject has been reviewed by Bonze11and HesketV further information on this and related topics can be found in the proceedings of a recent discussion meeting.3 It is now well established that the chemisorption behavior of CO dramatically changes upon coadsorption with an alkali-metal atom. The heat of adsorption increases, the frequency of the C-0 stretch vibration decreases, and shifts in the electron binding energies of both core and valence levels are observed. Several models describing the nature of the interaction between the alkalimetal atom and CO molecule has been proposed and generally fall into two categories. Whereas some authors propose the existence of direct chemical bonding between the alkali-metal atom and the CO m~lecule,'~ others explain the experimental results in terms of the electrostatic fields associated with an alkali-metal i0n.B-l" The most sophisticated theoretical study so far is the total t Present address: Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 lEW, Englaad. a Abstract published in Advance ACS Abstracts, November 1,
1993. (1) Bonzel, H. P. Surf. Sci. Rep. 1987,8,43. Bonzel, H. P.; Pirug, G .
In The Chemical Physics of Solid Surfaces; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1993; Vol. 6. (2) Heskett, D. Surf. Sci. 1988, 199, 67. (3) The physics and chemistry of alkali metal adsorption; Bonzel, H. P., Bradshaw, A. M., Ertl, G.,Eds.; Elsevier: Amsterdam, 1989. (4) Eberhardt, W.; Hoffmann, F. M.; de Paola, R.; Heskett, D.; Strathy, I.; Plummer, E. W.; M w r , H. R. Phys. Rev. Lett. 1985,54,1856. (5) Leckey,D. M.; Swman, M.; Jacobs, S.; Griger, D.; King, D. A. Surf. Sci. 1985, 1521153, 513. (6) Jennison, D. R. J. Vac. Sci. Technol. 1987, A5, 684. (7) Schultz,P.A.;Pattereon,C.H.;Messmer,R.P. J. Vac.Sci. Technol. 1987, A5, 1061. (8) Lang, N. D.; Holloway, 5.;Norskov, J. K. Surf. Sci. 1986,150,24. (9) Holloway, S.; Norekov, J. K.; Lang, N. D. J. Chem. SOC., Faraday Tram. 1 1987,83, 1935. (10) Wimmer, E.; Fu, C. L.; Freeman, A. J. Phys. Reo. Lett. 1986,23, 2618.
energy calculations on a Ptl,-Kz-CO cluster by Mtiller." Using the local density approximation (LDA) the bonding sites of the CO molecule and the two K atoms as well as the individual bond lengths were varied. The calculations show that "through-metal" charge transfer occurs from K to CO which strengthens the C-0 bond and leads to a change in the favored adsorption site from on-top to bridge. In the light of these calculations, we have recently carried out a comprehensive study of the CO/K coadsorption system on Pt{lll}using infrared reflection-absorption spectroscopy (IRAS)and LEED.12J3 Although this system has been studied extensively before, by both vibrational1p17 and nonvibrational techniques,laZ1 it was not until our most recent investigation that the full complexity of this system was revealed.l3 Briefly, as the [COl:[Kl ratio is varied, five distinct ordered coadsorption structures are observed. The IR spectra show the sequential appearance of four groups of strongly perturbed C-O stretches between 1390and 1802cm-l, each group consisting of severalpeaks. At low K coverages, OK < 0.2 additional CO peaks are observed with frequencies between 1830 and 2105 cm-l, which differ only slightly from those of the K-free surface. These frequencies are given in Table I. In agreement with the most recent EELS study,Is it was found that there is a short-range attractive interaction between the alkalimetal atoms and CO molecules which affects only those molecules directly adjacent to K atoms and causes the C-0 stretch frequency to change as a function of [COI: [K] stoichiometry. The higher resolution IRAS data, however, revealed that for any given stoichiometry the (11) Mtiller, J. E. In ref 3;In The Chemical Physics of Solid Surfaces; King, D. A., Woodruff, D. P., E&.; Elsevier: Amsterdam, 1993; Vol. 6. (12) Hoge, D.; Tbhaue, M.; Gardner, P.; Bradshaw, A. M. In Structure and Reactivity of Surfaces; Morterra, C., Zechina, A., Costa, G.,Eds.; Elsevier: Amsterdam, 1989. (13) mhaus,M.; Gardner, P.; Bradshaw, A. M. Surf. Sci. 1999,286, 212. (14) Crowell, J. E.; Garfunkel, E. L.; Somorjai, G. A. Surf. Sci. 1982, 121, 303. (15) Garfunkel, E. L.; Crowell, J. E.; Somorjai, G.A. J. Phys. Chem. 1982,86, 310. (16) Pirug, G.;Bonzel, H. P. Surf. Sci. 1988, 199, 371. (17) Schweizer, E. PhD Thesis, Freie UniversiMt Berlin, 1988. (18) Kiekinova, M.; Pirug, G.; Bonzel, H. P. Surf. Sci. 1989,133,321. (19) Greenlief, C. M.:Radloff. P. L.; Akhter, S.; White, J. M. Surf. Sci. 1987,186,563. (20) Dose, V.;Rogozik, J.; Bradshaw, A. M.; Prince, K. C. Surf. Sci. 1987,179, 90. (21) Weaner, D. A. Coenen, F. P.; Bonzel, H. P. Phys. Rev. B 1986,33.
0143-7463/93/2409-3491$04.00/00 1993 American Chemical Society
3492 Langmuir, Vol. 9, No. 12,1993
Tiishaus et al.
Figure 1. LEED patterns from Pt(ll1)as a function of increasing Cs coverage at 100 K. B a values are as follows: a, 0.11 (67 V); b, 0.14 (97 V); c, 0.20 (70 V); d, 0.25 (87 V); e, 0.30 (84 V); f, 0.33 (68 V); g, 0.40 (65 V); h, 0.44 (55 V). (Electron beam energies in parentheses.)
CO molecules experiencevarying degrees of perturbation dependent upon the local CO/K structure. This is most pronounced at low coverages where the IRAS spectra exhibit distinct fine structure which was interpreted in terms of short, linear CO-K chains,with the C - 0 stretching frequency being sensitive to the position of the molecule within the chain. In addition, the IRAS study showed that, irrespective of stoichiometry, the C-0 frequency is dependent, to some extent, upon the total amount of K present on the surface. This established the existence of a weaker long range effect on the C-0 stretch, explained by the vibrational Stark e f f e ~ t . The ~ ~ ,IRAS ~ ~ and LEED results were also shown to be consistent with the LDA clustercalculations,llwhich predict a switchof the favored CO adsorption site from on-top on the clean surface to bridge in the coadsorbed layer. As a continuationof this study we now turn our attention to CO coadsorption with other alkali-metal atoms and present in this paper the corresponding results for Cs.
Experimental Section The main features of the experimental apparatus are a UHV chamber (base pressure < 10-l') mbar) equipped with rear-view LEED optics,a quadrupole mass spectrometer, and an evacuable FT-IR spectrometer. A detailed description of the apparatus and the crystal cleaning procedure has been given elsewhere." All the spectra were recorded at a spectral resolution of 4 cm-1 and a crystal temperature of 100 K. The Cs was dosed from a carefully outgassed SAES-getter source located 2 cm in front of the crystal. The Cs deposition was carried out a t 100 K and consisted of a number of exposure cycles (doses). Higher exposures were obtained by increasing the number of cycles. Between each cycle the getter source was allowed to cool so that the amount of Cs evaporated per cycle remained constant. For Cs adsorption on the clean surface the series of LEED patterns observed as a function of exposure were in good agreement with the work of Cousty and Riwan= (seebelow). From the diffraction patterns, the Cs coverage was determined assuming each unit (22) Uram, K. J.; Ng, L.; Folman, M.; Yates, J. T. J. Chem.Phys. 1986, 84,2891. (23) Uram, K. J.; Ng, L.; Yates, J. T. Surf. Sci. 1986, 177, 253. (24) Schweizer,E.; Perason, B. N. J.; Tiishaus, M.; Hoge, D.; Bradshaw, A. M.Surf. Sci. 1989, 213,49. (25) Cousty, J.; Riwan, R. Surf. Sci. 1988,204,45.
cell contains a single Cs atom. A linear relationship between the number of doses and Cs coveragewas derived enabling overlayers of intermediate coverage to be prepared. All coverages quoted are relative to the number of surface substrate atoms. = 0.41 represents one monolayer of Cs. All the data shown are for alkalimetal adsorption followed by CO exposure. Reversing the order of adsorption led to qualitatively similar results but the LEED patterns were generally more diffuse indicativeof a poorer quality overlayer.
Results Figure 1shows the series of LEED patterns observed for Cs adsorption on Pt(ll1) at 100 K as a function of exposure. The structures observed are (Figure la) a very weak (3X3),8 a = 0.11, followed by (Figure lb) a (4'7x4'7)R19",8Ce = 0.14, and (Figure Id) a (2X2), 8cs = 0.25. At higher exposures, 8Ce > 0.25, the ( 2 x 2 ) structure changes into a ( 4 3 x 4 3 ) structure exhibiting a high degree of rotational disorder (Figure 1 e-h). Although the most intense part of the pattern corresponds to the R O O orientation, the appearance of a weak ring indicates a range of orientations. This adsorption sequence is similar to that observed by Cousty and R i ~ a except n ~ ~that the weak ( 3 x 3 ) structure was not seen and the (47X1/7)R19° was not specifically identified as such. The rotational disorder observed at the higher coverages was explained by the fact that in the ( 4 3 x 4 3 ) structure, the Cs atom must be compressed by =lo% compared with bulk Cs, thereby restricting diffusion. Figures 2 and 3 show the IRAS spectra of CO adsorbed on Cs-covered Pt(ll1) for Oca = 0.03 and OC, = 0.06, respectively. At these low Cs coverages no ordered overlayer structures are observed. The IRAS spectra as a function of CO exposure are extremely complex revealing a richness of fine structure. These spectra are similar to those for low coverage K coadsorption showing groups of peaks equivalent to the strongly perturbed species 2-4 and the essentially unaffected species7, as shown in Table I. The frequencies of the groups of bands for Cs coadsorption are also given in Table I. For the strongly perturbed species the C-0 stretching frequencies are 2030 cm-l higher for coadsorption with Cs than for coadsorption with K.
Langmuir, Vol. 9, No. 12,1993 3493
Coadsorption of CO and Cs on Pt(ll1) Pt{l 1 I} - co/cs T = lOOK
e,
Pt{1 1 1) - co/cs T = 100K
CO - exposure (nbar s)
= 0.03
e,
I
2098
CO - exposure (nbar s)
= 0.06
2091
T
10
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1844
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/L.
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I
I
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I
6 0 0 1400 1600 1 i O O 20-00 2ioo Wavenumber (cm-I) Figure 2. IRAS spectra of CO on a Pt 111) surface precovered with Cs at 100 K shown as a function of 0 exposure. k. = 0.03.
I
At higher Cs coverages,ordered coadsorption overlayers
are observed in LEED but there are only two compared
to five for coadsorption with K. These are the (2x2) and (4x2) structures, the latter always giving a rather diffuse diffractionpattern (Figure4). The (2x2) pattern, however,
is stable over a much wider coverage range than any of the structures observed for K coadsorption,being seen clearly a = 0.11 + (0.2 nbar-s CO), Oca= 0.14 + (0.2-0.4 nbaros at B CO), Bcs = 0.2 + (0.1-0.7 nbms CO), and Bcs = 0.25 + (04.6nbar-s CO). By use of the classification of Table I, the vibrational spectra in these ranges indicate that a (2x2) structure canexist for [CO]:[Kl stoichiometriesfrom 1:2 to 2:1, although the predominant band present is that of species 2, indicative of alocal1:l stoichiometry. Figure 5 shows a typical series of spectra for an intermediate a = 0.14, and corresponds to the sequence coverage of B of LEED patterns of Figure 4 in which both the (4x2) and (2x2) structures are observed. The spectra again show the sequential appearance of groups of bands as in Figures 2 and 3, but the fine structure within the groups, especially at low CO coverage, is not so pronounced. At even higher Cs coverage, the fine structure is lost, as all the bands broaden considerably. At Bcs= 0.25 (Figure6) broad bands associated with all the highly perturbed species, 1-4, are still observed, but at Bcs = 0.30 (Figure 7) the spectra at all CO coverages are dominated by a single broad band at 1450-1480 cm-l, indicative of species 1. At higher Cs coverages still,correspondingto one monolayer (ea= 0.41),
I
I
I
I
1
li00 1400 1600 1800 2600 2ioo Waven umber (cm-l)
Figure 3. IRAS spectra of CO on a Pt(ll1) surface precovered with Cs at 100 K shown ae a function of CO exposure. = 0.06. Table I. Observed C-O Stetching Bands (cm-1) in the Pt(lllJ-CO/K and Pt(lll)-CO/Cs Systems species pro+ CO/K COICS (C-0 band) stoichometry ref 20 ref 13 this work 1 1:2 1390-1420 1390-1420 1420-1480 2 1:l 1510-1610 1505-1605 1550-1630 3 21 1640-1730 1645-1715 1680-1730 4 31 1800-1820 1765-1805 1780-1810 5 1830-1850 1840-1865 6 1990-2030 1990-2030 7 a -2100 2050-2100 2050-2100 a Not
affected by K.
no CO is observed. (Note that the small negative feature observed in Figures 6 and 7, at -2100 cm-l, is due to a small amount of CO on the “clean surface” background spectrum which from the integrated intensity represents less than 0.05% of a monolayer.) Discussion From the vibrational spectra presented in Figures 2,3, and 5 it is clear that the behavior of CO on a Cs-covered Pt(ll1j surface is quite similar to that on a K-covered surface at 100 K. Essentially, the same groups of C-O stretch bands are observed for coadsorption with each of the alkali metals. The similarities are most prominent at very low alkali-metalcoverageswhere essentiallythe same fine structure is observed and the frequency difference
Tiishaus et at.
3494 Langmuir, Vol. 9, No. 12,1993
Pt(ll1) - co/cs T = lOOK Ocs= 0.14
I
CO - exposure (nbar s)
0.003
e
10
10
d
0.6
0.5 c
-.-.-/
0.3
1480 lW1 146111
A
b
d
L
0.15
0.1
0.05 12bo 1600 6 0 0 d o 0 2doo 2ioo Wavenumber (cm-l) Figure 5. IRAS spectra of CO on a Pt{l11)surface precovered with Cs at 100 K shown as a function of CO exposure. 8a = 0.14.
Figure 4. LEED patterns from a Pt(ll1)surface precovered with Cs (ea= 0.14)at 100 K shown as a function of CO exposure: a, Cs layer, 97 V; b, 0.15 nbar-s, 79 V; c, 0.30 nbms, 85 V; d, 0.60 nbaros, 81 V; e, 10 nbar-s, 84 V.
between the highly perturbed CO species 1-4 for the two systems is of the order of 20-30 cm-I. At higher Cs coverages, however, where the ordered overlayers are observed, important differences become apparent. Although the sequence of bands remains the same, the band indicative of a [CO]:[Cs] stoichiometry of 1:2 appears at a lower Cs coverage compared to coadsorption with K (Oc, = 0.14, OK = 0.2). Similarly, the relatively unperturbed
on-top band, species 7,disappears at OC, = 0.2 compared with OK = 0.25. In addition, at higher Cs coverages the difference in frequency of the highly perturbed CO species in the two systems increases, especially that of species 1. The similarities and differences, as a function of alkalimetal coverage, can be rationalized by considering the LEED patterns observed and taking into account size of the Cs adatom, Throughout the whole range of Cs and CO coverages, only two ordered coadsorption structures are observed, with the (2x2)structure being stable over a wide range of [CO]:[Cs] stoichiometries. At no point were LEED patterns indicative of a ( 4 3 x 4 3 ) structure observed, whereas such patterns were exhibited at all K coverages above OK = 0.2. This difference is not too surprising when one considersthe LEED patterns of thePt(lll)-Cs system along (Figure 1). As discussed above, according to Cousty and Riwan, the high degree of rotationaldisorder observed at OC, > 0.33 is due to lack of diffusion within the "compressed" Cs ~ v e r l a y e r .It~is ~ very difficult therefore, considering the tightly packed nature of such layers, to see how CO molecules could be incorporated within such a structure. Thus, by taking into account purely the size of the Cs atom, we can essentially rule out coadsorption structures in which Cs-Cs distances as short as 4 3 occur. This restriction explains, in part, the small number of ordered coadsorption structures observed, since real space structures analogous to the (43xd3)rect and (3x3)
Coadsorption of CO and Cs on Pt(ll1)
Langmuir, Vol. 9,No. 12,1993 3495
T
0.0006 95
I
Pt{1 1 1} - co/cs T = 100K O,, = 0.3
't{ 1 1 1} - co/cs r = IOOK 3,, = 0.25
I
0.0004
CO - exposure (nbar s)
'
1480
1643
CO - exposure (nbar s)
10 10
2 1
a 0
1
c
(5
e0 0.8
v)
I )
a
0.8 0.6
0.7 0.6
0.5
I I I 1 I 1 1200 1400 1600 1800 2000 2200 Wavenumber (cm-l) Figure 6. IRAS spectra of CO on a Pt(ll1) surface precovered with Cs at 100 K shown as a function of CO exposure. 8 a = 0.25.
structures observed for the CO/K coadsorption system cannot occur. It seemsclear therefore,from this discussion, and borne out by experiment, that the highest local coverage of Cs that can support ordered coadsorption is 8Ce = 0.25, arranged in a (2x2) unit mesh. Such a unit mesh, incorporating coadsorbed CO molecules, is shown in Figure 8. Note, that in order to incorporate the CO molecules into the overlayer, the Cs is shown with an ionic radius of 1.7 A rather than the larger metallic radius of 2.6 A.26 The implication that the Cs-metal bond is strongly ionic in nature is not unreasonable consideringthe results of two LEED structural studies of pure (2x2) Cs layers on Cu(ll1)and Ru(OOO1)in which the Cs radius was found to be 1.73 and 1.90 A,r e ~ p e c t i v e l y . ~Furthermore, ~*~ the addition of CO to the ordered Cs layer would tend to increase rather than decrease the degree of Cs ionicity.29 The fact that this (2x2) structure, having a local coverage of 8Ce = 0.25, is observed at much lower total coverages, Le., 8cs = 0.11, indicates that there is a net attractive interaction in the adlayer which leads to island formation, i.e., the addition of CO results in the stabilization of the (2x2) structure compared to the pure Cs adlayer of the
1200 1400 1600 1800 2000 2200 Wavenumber (cm-l) Figure 7. IRAS spectra of CO on a Pt(ll1) surface precovered with Cs at 100 K shown as a function of CO exposure. 8a = 0.30.
nnnnnn
Figure& Proposedmodel for the Pt(lll)(2X2)-CO/Csstructure. (26) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Prese: Ithaca, NY, 1960. (27) Lindgren, S. A.; Walldh, L.; Rundgren, J.; Westrin, P.; Neve, J. Phys. Rev. B 1983,28,6707. (28) Over, H.; Biudau, H.; Skottlte-Klein, M.; Ertl, G.; Moritz,W.; Campbell, C. T. Phys. Rev. B 1992,45,8638. (29) Al-Sarraf, N.; Stuckless, J. T.; King, D. A. Nature 1992,360,243.
same local coverage. According to the interpretation of a recent microcalorimetry study of the coadsorption of CO and K on Ni{100),this stabilization energy results from a Madelung energy associated with a "charged" alkali metal-CO lattice.29 A t each coverage where the (2x2)
3496 Langmuir, Vol. 9, No. 12, 1993 structure is clearly observed, the most prominent band in the spectra is at 1565-1595 cm-l, indicative of species 2 associated with a [COl:[Csl stoichiometry of 1:l. This local stoichiometry presumably leads to the optimum Madelung energy contribution. The IRAS data can be rationalized as follows. At low Cs coverages the overlayer consists of essentially isolated Cs atoms ionically bonded to the surface. As CO is adsorbed, there is a net attractive interaction, causing the CO molecules to occupy sites directly adjacent to these Cs atoms. Hence the first C-0 stretching band to appear is indicative of species 2, corresponding to a [COl:[Csl stoichiometryoa 1:l. The existence of f i e structure within the band suggests that even at these low coverages some clustering or the formation of CO-Cs-CO chains occurs. With further CO exposure, additional CO molecules are also attracted to sites directly adjacent to Cs atoms leading to the appearance of species 3 and then species 4. At this point, all the sites adjacent to Cs atoms are blocked, leaving sites which are only weakly perturbed. This behavior is very similar to coadeorption with K indicating that at low alkali-metal coveragea, size effects do not play an important role. At first sight, it would be reasonable to expect lower, not higher, frequencies for the C-0 stretch in this low coverage regime, since the lowering of the work function of transition-metal surfaces is greater for Cs than K. However, because the Cs+ ion is larger than the K+ ion, it is likely that the two centers of charge are further apart, thereby counteracting any increased electron donation from Cs to the metal. At intermediate Cs coverages, low coverages of CO give rise to species 1, indicating that at least some of the Cs atoms are sufficiently close together to strongly influence two CO molecules. Over a narrow coverage range a (4x2) coadsorption structure exists, but as the CO exposure is increased the adsorbate layer rearranges to form stable (2x2) islands as shown in Figure 8. These islands having a local [COl:[Cs] stoichiometry of 1:1,are always present for Qc8 = 0.11 up to QcB= 0.25. At higher CO coverages CO molecules are presumably forced into the vacant adsorption sites within the lattice, but this leads to a destabilization of the structure indicated by the transformation to a diffuse LEED pattern. Above Qc8 = 0.25 it becomes increasingly difficult for CO to adsorb within the Cs adlayer. This is due not only to the increased number of Cs atoms on the surface but also to the fact that the Cs radius expands as it starts to become less ionic and more metallic. At a Cs coverage close to Qcs= 0.41, no CO adsorbs (at least up to CO partial pressure of 1 X 10-6 mbar) since all free Pt sites are completely blocked. In our work on the CO/K coadsorption system the vibrational spectra were interpreted in terms of a change
Tiishaus et al. of CO bonding site from on-top to bridged in the presence of K.l3 This assignment is consistent with the LDA cluster calculationslland with kinematic LEED simulationswhich always favored real space structures in which K and CO occupy the same type of adsorption site. Therefore, although recent structural studies have questioned the validity of using vibrational frequencies for adsorption site determination,m2 we believe that in this case the assignment was correct. In the case of the CO/Cs system, however, no additional supporting information, e.g. cluster calculations of LEED simulations, is available, thereby making an adsorption site assignment more difficult. From the reduction in frequencyand behavior of the C-0 stretch as a function of [COl:[Csl stoichiometry, we can presume that a similar switch of the favored CO adsorption site, from on-top to bridged, takes place in the presence of Cs. We have therefore placed CO in bridge sites, Figure 8. The linear chain structure, proposed for the CO/K system, however, cannot form since the larger Cs atoms cannot fit in between the adjacent CO molecules. We therefore propose that the Cs atoms are displaced toward the 3-fold hollow site, thereby forming a buckled chain structure. The formation of short buckled chains at lower coverages would also explain the observation of fine structure similar to that observed for the CO/K system. We implicitly assume in this discussion that the Cs atoms will occupy multiple coordination sites. Recently, however, examples of atop adsorption of alkali metals have been found, e.g. refs 27, 28, and, 33. In the case of K, MWer finds a preference for the hollow site (by 0.1 eV) but this may not be true of Cs. We feel, however, that in the absence of any structural information or detailed calculations for the Pt(111)-Cs system, the model suggested is not unreasonable.
Conclusion This work shows that the behavior of CO coadsorption with Cs on Pt(ll1j at 100 K is similar to that that of coadsorption with K and that the differences between the two systems can largely be explained by differences in the size of the alkali-metal atom. Acknowledgment. Financial support from the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 6-81 as well as from the Fonds der Chemischen Industrie is gratefully acknowledged. (30) Aminopirooz, S.; Schmalz, A.; Becker, L.; Haaee, J. Phys. Rev. B 1992,45,6337. (31) Aaensio, M.C.; Woodruff, D. P.;Robineon, A. W.;Schindler, K.M.; Weies, K.-U.; Gardner,P.; Bradshaw, A. M.; Coneaa,J. C.; GonzalesElipe, A. Chem. Phys. Lett. 1992,192,259. (32) Wander, A.; Hu, P.;King, D. A. Chem.Phys. Lett. 1993,201,393. (33) Fieher, D.;Chandavarkar, S.;Collins, 1. R.;Diehl, R.D.; Kaukasoina, P.;Lmdrooe, M. Phys. Rev. Lett. 1992,68, 2786.
