J. Phys. Chem. 1995,99, 8790-8798
8790
Coadsorption of Cs and CO on Ru(OO1): Formation of Cs Exchange of the CO Located inside the Islands
+ CO Islands and Isotope
H. Kondoh, H. Orita, and H. Nozoye” National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan Received: December 8, 1994; In Final Form: March 16, 1995@
The CO adsorption on Cs-covered Ru(OO1) surfaces has been studied by means of temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). When C O molecules adsorb on the Cs-preadsorbed C O islands are formed for Cs coverages lower than 0.25, which surfaces at 85 K, two-dimensional Cs exhibit p(2 x 2) LEED patterns irrespective of both Cs and C O coverages. The p(2 x 2 ) Cs f CO island has an almost constant [CO]:[Cs] stoichiometry of 2:l at Cs coverages below 0.2. This stoichiometry changes to 4:3 when the islands cover the whole surface (& = 0.25). The local density of Cs in the island is 0.25 independent of Cs coverage. The C-0 stretching frequency of the CO species located outside the island shifts from 2075 to 2000 cm-’ in proportion to Cs coverage. The C-0 stretching mode of the inside C O species appears at much lower frequencies (1800-1580 cm-I), which depend on the [CO]:[Cs] ratio. The former and the latter frequency shifts are associated with a weak long-range and a strong short-range effect of Cs, respectively. The isotope exchange between the isotope-labeled C O species occurs exclusively inside the islands. The temperature onset of the isotope-exchange reaction is estimated to be 450-500 K. This is comparative to the temperature onset of the desorption from the 2: 1 state, while it is lower by 100 K than that from the 4:3 state, which results in a distinct difference in the exchange fraction for these states. The formation mechanism of the p(2 x 2) Cs C O island is discussed based on the recently proposed models of the alkali C O coadsorption process.
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+
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Introduction The coadsorption of alkali metals and CO molecules on transition-metal surfaces has attracted much interest, which is not only due to the technological importance of such processes in heterogeneous catalysis but also due to the scientific importance of fundamental understanding of the strong influence of alkalis on the properties of coadsorbed CO.’.* It was generally observed for the alkali f CO coadsorption systems that the coadsorbed alkali atoms give rise to an increase in the CO adsorption energy and a significant decrease in the C-0 stretch frequency. These observations have been related with a stronger metal-CO bond and a weakened C - 0 bond through an alkali-CO interaction. A great number of studies have been carried out to characterize the nature of the alkali-CO interaction and various models describing the nature of the interaction have been p r o p o ~ e d . ~ -Two * ~ different types of interaction, a short-range and a long-range interaction, have been considered as the alkali-CO interaction. The interpretations of the shortrange interaction are generally further classified into two categories; direct chemical interaction between alkali atoms and C011-17and indirect interaction through substrate^.^^^^-^^ The long-range interaction has been usually explained in terms of the electrostatic fields induced by alkali atom^.',^*'^*^^ Although most of the models adopted a picture that a short-range interaction is dominant and a long-range interaction gives minor effects,’-20 the contribution of electrostatic interactions was emphasized by several theoretical s t ~ d i e s . * ~ - *Recently, ~ a significant long-range electrostatic contribution through a Madelung energy arising from a “charged” alkali-CO lattice has been proposed by Al-Sarraf et aLZ4 The strong short-range interaction has been sometimes evidenced by the formation of domains of fixed local
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, M a y 1, 1995.
[alkali]:[CO] stoichiometries. In the case of coadsorption of alkali atoms (K and Cs) and CO on Pt( 11l), several distinct local [alkali]:[CO] stoichiometries have been found by vibrational spectroscopies; the C-0 stretching vibration exhibits several discrete frequencies depending on the stoichiometry.8-’0 Fixed [alkali]:[CO] stoichiometries were also observed in the other coadsorption systems such as C O / K / R U ( O O ~ )and ~ ~ CO/ ~~~’~ m i (1 These coadsorption systems with fixed stoichiometries have well-ordered structures even at low alkali and CO coverage^,^.^^^-'^ which indicates a strong ordering effect of the short-range interaction. Recently, this ordering effect is associated not only with the strong short-range interaction but also with the long-range Madelung c ~ n t r i b u t i o n . ~ ~ In the case that the alkali-CO interaction is extremely strong, the CO species show significantly low C - 0 stretch frequencies and have been interpreted in terms of side-on bonding of C05326 or formation of polymeric CO anions.I3 Such low-frequency CO species were considered to be a precursor state to dissociation. In fact, isotope exchange of CO has been observed in the presence of coadsorbed alkali atoms on several metal surf a c e ~ . ~ . * ’There - ~ ~ is discrepancy about the isotope-exchange mechanism. High-resolution electron energy loss spectroscopy (HREELS)5 and metastable quenching spectroscopy (MQS)27 provided no evidence for the existence of dissociation products (atomic C and 0) on the surfaces, suggesting a coplanar fourcenter (concerted) mechanism for the isotope exchange. However, Matsushima has proposed a dissociation-recombination mechanism by thermal desorption data on CO/CS/RU(OO~).*~ Thus, the mechanism of the isotope exchange is an open question yet. Recently, we reported an unusual adsorption state of CO on Cs-precovered Ru(001) with one-monolayer coverage.31 In this paper, interactions between CO and coadsorbed Cs with submonolayer coverages on Ru(001) have been studied by using
0022-3654/95/2099-8790$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 21, 1995 8791
Coadsorption of Cs and CO on Ru(001) temperature-programmeddesorption (TPD), low energy electron diffraction (LEED), HREELS, and X-ray photoelectron spectroscopy (XPS). We report facile formation of Cs CO p(2 x 2) islands on Ru(001) irrespective of Cs (below one monolayer) and CO coverages and relation between the p(2 x 2) island formation and the isotope-exchange reaction. These results are discussed comparing with recent models of the alkali CO coadsorption p r o c e ~ ~ e s . 9 ~ ~ ~ ~ ~ ~ ~ ~ ~
Cs/Ru(00 1)
+
t multilayers
+
/2nd
layer
Experimental Section Most of the experiments were carried out in a multilevel ultrahigh-vacuum system (base pressure of 8 x lo-" Torr) which was equipped with facilities for Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), temperature-programmed desorption (TPD), and high-resolution electron energy loss spectroscopy (HREELS). Measurements of X-ray photoelectron spectroscopy (XPS) were performed in a separate UHV chamber (base pressure of 2 x lo-" Torr) equipped with a X-ray source and a 150" sector analyzer (VG. ESCALAB Mark 11). The Ru(001) surface was cleaned by repeating cycles of Ar+ sputtering (3.0 kV, 1.5 PA) and annealing to 1400 K until no impurities could be detected by AES, HREELS, and XPS. The ordering of the surface was confirmed by LEED. The Cs was deposited onto the surface at 85 K from a commercial Cs source (SAES getters). Cs overlayers were prepared by once evaporating multilayers of Cs and then thermally desorbing excess Cs. The Cs coverage was controlled by the desorbing temperature and determined by integrating the corresponding TPD spectra as previously reported by Over et al.32 The CO gas was introduced onto the Cs-precovered surface at 85 K. The CO coverages were determined by TPD peak area and calibrated to that for the saturated adsorption on clean Ru(001) (& = 0.65).33 The coverages of Cs and CO are defined as the ratio of the number of adsorbate atoms or molecules to that of first-layer atoms of the substrate. TPD spectra were taken at a heating rate of 5 Ws. HREEL spectra were measured in the specular mode with an incidence angle of 60" from the surface normal and the typical resolution was 50-70 cm-I with a primary beam energy of 4.1 eV. XP spectra were recorded using A1 K a X-ray radiation.
I I
Temperature [K] Figure 1. TPD spectra of Cs from Cs/Ru(OOl) surfaces with different Cs coverages. At the bottom of the figure, LEED pattems which were observed after a multilayers-covered surface was heated up to certain temperatures are shown as a function of the heating temperature. When the Cs-covered surface was heated to 315 K, clear ( J 3 x J3)R3Oo LEED pattem without a background was observed. The "rotated-p(2 x 2)" LEED pattem is denoted by r-p(2 x 2 ) in the figure.
_I
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A
Results 1. CdRu(OO1). Figure 1 shows TPD spectra of Cs from the Cs/Ru(OOl) surfaces with various Cs coverages and LEED pattern changes as a function of the desorbing temperature. The TPD spectrum from a surface covered with multilayers of Cs exhibits two sharp desorption peaks at 305 and 310 K, which are ascribed to the desorption of Cs from the multilayers and the second layer, respectively. The desorption from the first layer is observed over a wide temperature range from 315 to 1150 K. When the Cs-adsorbed surface was heated to 315 K where the desorption from the second layer was completed, a clear ( 4 3 x 43)R3Oo LEED pattern was observed. Thus, we assumed that the Cs coverage of this structure is 0.33 and used the TPD peak area of this structure as a standard to determine the Cs coverages. This calibration was confirmed by a p(2 x 2) structure which exhibited its highest intensity at a Cs coverage of 0.24, close to the nominal value of 0.25. According to this calibration, the coverage of the two monolayers is estimated to be 0.58, which indicates that the saturation coverage of the second layer (0.25) is lower than that of the first layer. TPD spectra of Cs and LEED patterns as a function of the Cs coverage were well in agreement with those in the work by Over et a1.32
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Temperature [K] Figure 2. TPD spectra of CO for a saturated CO adlayer on the Cspreadsorbed Ru(001) surfaces at 85 K as a function of the Cs coverage.
2. CO Adsorption on Cs/Ru(OOl). Figure 2 shows change of TPD spectra of CO as the Cs coverage is increased to approximately two monolayers, where CO is adsorbed on Cs/ Ru(001) at 85 K to saturation coverages. CO exposure required to saturate the surface depends on the Cs coverage; for example, CO exposures to saturate the Cs/Ru(OOl) surfaces with 8cs = 0.15, 0.24, and 0.33 are 5, 10, and 200 langmuirs (1 langmuir
8792 J. Phys. Chem., Vol. 99, No. 21, 1995
Kondoh et al.
-.-
Cs coverag
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Cs Coverage Figure 3. Cs coverage dependence of the amount of CO adsorbed on the Cs-preadsorbed Ru(001) surfaces at 85 K to the saturation.
Torrs), respectively. The top spectrum for a saturated CO layer on clean Ru(000) exhibits two peaks at 400 and 460 K, which is almost the same as the corresponding spectrum in the work by Weimer et aLI9 When a small amount of Cs is preadsorbed on the surface, a new desorption peak appears as a shoulder at the higher temperature side. This high-temperature peak grows and shifts to higher temperature as increasing the Cs coverage. The high-temperature peak is associated with desorption of Cs-influencedCO. Instead of that, the desorption peaks due to CO adsorbing on the bare Ru surface reduce in intensity with the Cs coverage and finally disappear at around BC, = 0.25. A weak and broad desorption feature observed at 300-500 K in the spectra at high Cs coverages (&, 1 0.24) is due to desorption from the back side of the crystal. At 8cs = 0.24, the high-temperature peak shifts to 680 K and becomes sharp and intense. Beyond this coverage, the desorption peak decreases in intensity with staying the desorption temperature almost constant. The coverage of CO adsorbed on the Cs/Ru(OOl) surfaces to the saturation is plotted as a function of the Cs coverage in Figure 3. The CO saturation coverage initially decreases slowly until the Cs coverage is increased to 0.2. At around a Cs coverage of 0.25, the CO saturation coverage rapidly decreases. Beyond the Cs coverage of one monolayer (&, = 0.33), the saturation coverage further decreases gradually with the Cs coverage and finally reduces to almost zero at a Cs coverage of 0.65 which is close to the two-monolayers coverage (&, = 0.58). Figure 4 shows TPD spectra of Cs from the same CO/Cs/ Ru(001) surfaces as those in Figure 1. At Cs coverages higher than 0.2, a coincident desorption of Cs and CO is observed at around 680 K, which indicates that a strong attractive interaction between Cs and CO exists at the high Cs coverages. Beyond the Cs coverage of 0.24, a new desorption feature appears at 400-600 K. This low-temperature peak is associated with desorption of the second-layer Cs which is displaced from the first layer by the formation of a stable 1:l Cs CO lattice at the first layer.31 HREEL spectra for a saturated CO adlayer on Cs-preadsorbed Ru(001) surfaces at 85 K with increasing the Cs coverage up to one monolayer (OC,= 0.33) are shown in Figure 5. Changes in LEED pattern are also shown at the right side of the figure. The HREEL spectrum for a saturated CO layer on clean Ru(001) exhibits the C-0 and the metal-CO stretching modes at 2075 and 435 cm-I, respectively. The LEED pattern for this surface is a well-known "compression" pattern which was =
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400 600 800 1000 1: 30
Temperature [K] Figure 4. TPD spectra of Cs for the CO(sat.)/Cs/Ru(OOl) surfaces with different coverages of Cs.
npression
(2x2)
Energy Loss [cm-'1 Figure 5. HREEL spectra for saturated CO layers on the Cspreadsorbed Ru(001) surface at 85 K as a function of the Cs coverage. The spectra were measured after annealing the surface at 210 K and recorded in the specular geometry with an incidence angle of 60" from the surface normal. LEED pattern changes are also shown at the right side of the figure (see text).
interpreted in terms of an incommensurate hexagonal CO lattice with a compressed unit cell.33 Coadsorption of a small amount of Cs (&, = 0.06) gives rise to a new C-0 stretching mode at 1800 cm-' and an extra LEED pattern of p(2 x 2) in addition to the "compression" pattern. This low-frequency peak is
J. Phys. Chem., Vol. 99, No. 21, 1995 8793
Coadsorption of Cs and CO on Ru(001) associated with a CO species influenced by Cs. The appearance of the p(2 x 2) pattern at this low Cs precoverage indicates formation of Cs CO islands with the p(2 x 2) lattice. Upon increasing the Cs coverage the low-frequency peak increases in intensity, though it shifts to lower frequency. Furthermore, the p(2 x 2) spots become sharp and intense with the Cs coverage. These observations indicate two-dimensional growth of the Cs CO islands. On the other hand, the C-0 stretching mode and the metal-CO stretching mode due to the CO species adsorbing outside the islands reduce in intensity and finally disappear at around a Cs coverage of 0.25, where the islands cover the whole surface and the sharpest p(2 x 2) LEED pattem is observed. The loss feature of the metal-CO mode of the Cs-influenced CO was not observed in the same way as the CO/K/Ru(OOl) ~ y s t e m . ~This , ~ ~absence . ~ ~ of the metal-CO mode was explained by strong intensity transfer between the C - 0 and the metal-CO stretching modes of tilted or side-on bonded CO5xZ6or screening of the metal-CO mode by the nearby alkali atoms.I9 It is interesting to note that when the Cs coverage was below 0.25 the p(2 x 2) LEED pattern was always observed irrespective of the amount of CO adsorbed on the Cs-preadsorbed surfaces. This indicates that the Cs and the CO adsorbed on Ru(001) have a tendency to form p(2 x 2) islands independent of the Cs and CO coverages. After the Cs CO islands cover the whole surface (&, =- 0.25), the Csinfluenced C - 0 stretching mode rapidly decreases in intensity with the Cs coverage and finally disappears at one-monolayer coverage (&, = 0.33). At this stage, the LEED pattern changes from p(2 x 2) to "rotated-p(2 x 2)".31 A weak loss feature at about 450 cm-' in the 8cs = 0.33 spectrum is due to a metal-0 stretch from a small amount of atomic oxygen which inevitably adsorbs on the surface during the preadsorption of the highcoverage Cs layer. Therefore, at the one-monolayer coverage of Cs, no loss feature is observed other than that from the contamination, though the amount of CO molecules in this surface is as much as approximately two-thirds of coadsorbed Cs ( 8 ~ 0= 0.23) as shown in Figure 3. The' absence of CO vibrational modes has been interpreted in terms of a strong shielding of the CO dipole by the second-layer Cs which is displaced from the first layer.31 Figure 6 shows 0 1 s XP spectra for the CO-saturated Cs/Ru(OOl) surfaces. The spectrum for the CO-saturated Ru(001) exhibits a main peak at 532.2 eV and a weak and broad satellite at around 538.5 eV. The main peak and the satellite peak. have been interpreted as "screened" and "unscreened" states, respectively; in the former state the 2x* orbital has been filled by charge transfer to the screening state, whereas such a screening does not occur in the latter state.34 When a small amount of Cs is preadsorbed on the surface, a new structure due to the Cs-influenced CO species located inside the Cs CO islands appears as a shoulder at 530.8 eV. The low binding energy of the Cs-influenced CO is associated with an increased 2x* back-donation. This structure increases in intensity with the progress of the island growth. Instead of that, the peak at 532.2 eV and the broad satellite from the CO species adsorbing outside the islands reduce in intensity and finally disappear at around 8cs = 0.25, where the island growth is completed. 3. Stoichiometry of the Cs CO Islands. HREELS, X P S , and LEED observations indicated that Cs and CO form p(2 x 2) islands even at very low coverages and the p(2 x 2) islands grow with the Cs coverage. This island growth is completed at around a Cs coverage of 0.25. We estimated the amounts of CO species adsorbing at inside and outside of the islands as a function of the Cs coverage based on the CO TPD and XPS results. The total amount of the adsorbed CO is obtained from
+
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Binding Energy [eV] Figure 6. 01s XP spectra for the saturated CO adsorption on the Cspreadsorbed Ru(001) surfaces at 85 K as a function of the Cs coverage. The spectra were measured after annealing the surface at 210 K.
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Cs Coverage Figure 7. Change of the coverage of the CO species adsorbing outside (solid circles) and inside (open circles) the islands as a function of the Cs coverage. These CO coverages were determined by the TPD and XPS results.
the TPD peak area (see Figure 3) and the ratio between the amounts of CO adsorbed inside and outside the islands is deduced from intensity ratio between the 01s XPS peaks at 530.8 and 532.2 eV by assuming the same cross section for the two CO species.35 Hereafter, we denote the CO located inside and outside the islands by inside CO and outside CO, respectively. The changes of the amounts of inside and outside CO thus determined are shown as a function of the Cs coverage in Figure 7. The amount of outside CO decreases almost linearly with the Cs coverage and reduces to zero at 8cS= 0.25. On the other hand, the amount of inside CO increases with the Cs coverage until Bcs = 0.21, and then it decreases after passing through a maximum. Since the "compression" LEED pattern, which is characteristic of the saturated CO layer on clean
8794 J. Phys. Chem., Vol. 99, No. 21, 1995
1
Kondoh et al.
0 0
0
0
0.1
0.2
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Cs Coverage Figure 8. Changes of the [CO]/[Cs] ratio for the CO and Cs located inside the islands determined from the data in Figure 7 as a function of the Cs coverage.
Cs Coverage Figure 9. Change of the C - 0 stretch frequencies for the outside CO species (solid circles) and the inside CO species (open circles) as a function of the Cs coverage.
TABLE 1: Observed C-0 Stretch Frequencies and Ru(OOl), is observed during the island-growth process, the [CO]:[Cs] Stoichiometriesin the CO/Cs/Ru(OOl) and density of the CO layer at the outside of the islands is the same CO/Cs/Pt(111) Systems as that on the clean surface. Therefore, the linear decrease in co/cs/Ru(oo1) CO/Cs/Pt(111)" the outside CO indicates that the total area of the islands stoichiometry freq [cm-I] stoichiometry freq [cm-'1 increases linearly with the Cs coverage. This implies that all 2:3 1245h 1:2 1420- 1480 the adsorbed Cs atoms form the islands and the Cs density in 4:3 1580 1:l 1550-1630 the islands keeps constant over the full range of the island1680-1800 2:1 1680-1730 2: 1 growth process; here, the "density" is defined as the ratio of 3: 1 1780-1810 the Cs atoms to the surface Ru atoms in the island. Since the uninfluenced 2050-2 100 uninfluenced 2000-2075 Cs coverage at the completion of the island growth is 0.25, the a Quoted from ref 10. Obtained by the off-specular measurement local density of Cs in the islands is a constant value of 0.25 of HREELS in ref 31. irrespective of the Cs coverage, which corresponds well to the constant observation of the p(2 x 2) LEED pattern during the frequencies of outside (solid circles) and inside CO species (open island-growth process. Thus, we can conclude that CO and Cs circles) as a function of the Cs coverage. The C-0 stretch form the islands which contain the p(2 x 2)-Cs lattice with frequency of outside CO decreases from 2075 to 2000 cm-' as the Cs density of 0.25, for all the Cs coverages below 0.25. the Cs coverage is increased to 0.21. This small shift, which We can also determine the [CO]/[Cs] ratio of the islands as is almost proportional to the Cs coverage, is due to an electric a function of the Cs coverage from the amount of inside CO as field induced by coadsorbed Cs atoms, Le., a long-range Stark shown in Figure 8. In the Cs-coverage range below 0.2 the effect on the C-0 stretching v i b r a t i ~ n . ~ , The ~ . ' ~C-0 . ~ ~ stretch [CO]/[Cs] ratio keeps almost constant at 2.0-2.2, though the frequency of inside CO slightly shifts to lower frequency with ratio slightly changes depending on the Cs coverage. Beyond the Cs coverage until BcS= 0.21 (1800 1680 cm-I) and above the Cs coverage of 0.2, the ratio decreases rapidly and reaches this coverage it steeply decreases until 8cs = 0.33 (1680 to a value of */3 at one-monolayer coverage (&, = 0.33). At 1245 cm-I). This is the same behavior as that for the [CO]/ low Cs coverages, the [CO]/[Cs] ratio might be influenced by [Cs] ratio shown in Figure 8. Therefore, the C-0 stretch a peripheral effect for the Cs CO islands because of small frequency of inside CO species is apparently related with the sizes of the islands; at the rim of the islands the local [CO]/ [CO]/[Cs] ratio. Such a close relation between the CO [Cs] ratio is increased by the peripheral CO. This might be a frequency and the [CO]/[Cs] stoichiometry has been reported possible reason for the small change of the ratio at low for the CO/K/Pt( 111) systems,9and the CO/Cs/Pt(l 11) system.I0 coverages. Not withstanding this possible minor effect, the It is interesting to compare the relation for the present system [CO]:[Cs] stoichiometry of the islands below Ocs = 0.2 is with that for the CO/Cs/Pt( 111) system.l0 The comparison is considered to be basically 2:l. Above Bcs = 0.2, the stoichishown in Table I. The frequency-stoichiometry relation for ometry decreases and has a ratio of 4:3 at Ocs = 0.24, where the present system almost corresponds to that for the CO/Cs/ the islands growth is almost completed, and finally reaches to Pt( 111) system except for the extremely low-frequency (1245 the Cs-richest stoichiometry, 2:3, at Ocs = 0.33. The formation cm-') phase of the present system. of the islands with a fixed local [CO]:[Cs] stoichiometry even 4. Isotope experiments of CO on Cs/Ru(OOl). Several at low Cs coverages far below the one-monolayer coverage experiments using CO isotopes were performed to investigate indicates a strong short-range interaction between Cs and CO. (1) the diffusion of CO admolecules between the inside and The change of the [CO]/[Cs] ratio at high Cs coverages is related the outside of the islands and (2) the isotope-exchange reaction with an additional long-range Madelung c o r ~ t r i b u t i o nas~ ~ ~ ~ ~with bond breaking of CO isotopes adsorbed on Cs-precovered discussed later. Ru(001) surfaces. The C - 0 stretch frequency of inside CO should reflect the We prepared a Cs-preadsorbed Ru(001) surface with a Cs change of the [CO]/[Cs] ratio. Figure 9 shows the C-0 stretch coverage of 0.15 and then exposed the surface to 1 langmuir of
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Coadsorption of Cs and CO on Ru(001)
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J. Phys. Chem., Vol. 99, No. 21, 1995 8795
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Temperature [IS] Figure 10. TPD spectra of 13C180for (a) after exposing a Cs(& = 0.15)/Ru(OOl) surface to 1 langmuir of l3CI8O at 85 K, (b) after exposing the Cs/Ru(OOl) surface to 5 langmuirs of 13C180,(c) after exposing the Cs/Ru(OOl) surface to 1 langmuir of 13C'80and then to 5 langmuirs of I2CI6O.
13C180, resulting in a 13C180 coverage of 0.11. LEED pattern of the surface was "ring" before the CO adsorption. After the exposure to the 13C180,the p(2 x 2) LEED pattern appeared in addition to the "ring", which indicates that the 13C180forms p(2 x 2) islands with Cs. A TPD spectrum of 13C180from this surface is shown in Figure loa. The TPD spectrum exhibits a single desorption peak at 600 K. The HREEL spectrum for this surface (not shown) also gives a single CO stretching mode at 1460 cm-I. These observations indicate that the 13C180 adsorbs only in the islands. Figure 10b shows a TPD spectrum from a saturated 13C'80layer on the Cs(6cs = 0.15)/Ru(001) surface formed by exposing to 5 langmuirs of I3Cl8O. Lowtemperature desorption peaks due to the I3Cl8Ospecies adsorbing outside the islands are also observed. To sequentially fill the inside and the outside sites of the islands with two different CO isotopes, we exposed the Cspreadsorbed surface (&, = 0.15) to 1 langmuir of I3CIsO and then 5 langmuirs of 12C160 at 85 K. If the diffusion of CO between the islands and the outside does not occur at 85 K, the outside of the islands is occupied only by the l2CI6Ospecies. By measuring TPD of I3ClSOfrom this surface, we can obtain information about the mutual diffusion of the CO molecules. The TPD spectrum thus obtained is shown in Figure 1Oc. Desorption of 13C1s0is not limited to the temperature range of the desorption from the islands at 550-700 K (Figure loa) but also observed at a low-temperature range. The shape of desorption spectrum at the low-temperature range is the same as that for the saturated 13Ci80layer (Figure lob). The drop of desorption rate observed at 550-700 K is associated with the decrease in the amount of 13C1x0 due to isotope exchange between 13C180 and l2CI6Olocated inside the islands. These results clearly indicate that at the saturation coverage the isotopelabeled CO molecules almost completely mix by the onset of desorption (300 K) due to fast mutual diffusion. A similar result was observed for the CO/K/Ru(OOl) system by dePaola et aL5 To investigate isotope-exchange reaction of the adsorbed CO molecules, a 1:l mixture of I3Cl6Oand 12C1s0was adsorbed on the Cs-covered surfaces (&, = 0.06, 0.15, 0.24, and 0.33) at 85 K to the saturation and TPD spectra of initially adsorbed
1
I
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I
300
500
700
900
Temperature [K] Figure 11. TPD spectra of CO isotopes for the adsorption of a 1:l mixture of I3Cl6Oand '2C'80on the Cs/Ru(OOl) surfaces with different Cs precoverages, (a) 0.06, (b) 0.15, (c) 0.24, and (d) 0.33. The initially adsorbed CO species (I3Ci6Oand 12C'80)and the exchanged CO species (I2CI6O and 13C'80)are indicated by solid lines and broken lines, respectively. Since 7% of the isotope mixture was exchanged on the ionizing filament of the mass spectrometer, the TPD spectra were corrected to remove such an extrinsic contribution.
species ('3C160and 12C180)and of isotope-exchanged species (l2CI6Oand I3Cl8O)were measured. The results are shown in Figure 11. At low Cs coverages (&, = 0.06 and 0.15), below the completion of the island formation, the desorption of the isotopeexchanged species appears only at the high-temperature range corresponding to the desorption from the islands as shown in Figure lla,b. This observation indicates that only the inside CO species can be exchanged. If the isotope exchange began to occur in the islands well below the high-temperature range, exchanged CO species also should desorb at the low-temperature range corresponding to the desorption from outside of the islands because the mutual diffusion of CO between inside and outside of the islands is fast even below 330 K. Therefore, the temperature onset of the exchange of the CC) located inside the islands should be the same as that of the desorption of the CO species; that is, 480-500 K. At high Cs coverages (&, = 0.24 and 0.33), around and above the completion of the island formation, most of the CO species adsorbed on the surfaces exhibit isotope exchange as shown in Figure 1lc,d. However, in the case of the high Cs coverage, the temperature onset cannot be inferred from the TPD spectra because of the absence of desorption of the CO species adsorbing outside the islands. We estimated the fraction of the exchanged CO as a function of the Cs coverage. The fraction was obtained by comparing the actual 13C'80yield with the 13C180 yield expected when the complete isotope scrambling occurs in the islands and the outside CO molecules do not participate in the isotope scrambling reaction; only the inside CO species can be exchanged as described above. The fractions thus obtained are shown in Figure 12. The most notable point is a steep increase in the fraction at a critical coverage; the fraction is about 0.5 below 6cs = 0.2, while it is steeply increased to about 0.85 above this
Kondoh et al.
8796 J. Phys. Chem., Vol. 99, No. 21, 1995
I
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A *..............
-
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0.2
0.3
0.4
Cs Coverage Figure 12. Fraction of the exchanged CO with respect to inside CO as a function of the Cs coverage. A 1:l mixture of I3Ci6Oand 1 2 C i 8 0 was initially adsorbed on the Cs/Ru(OOl) surfaces to the saturation at 85 K. The fraction was estimated as a ratio between the amount of and the amount of 1 3 C 1 8expected 0 when the l3Cl6O desorbed 13C180 and i 2 C 1 8 0molecules adsorbing inside the islands are completely exchanged.
coverage. Similar characteristic change is also seen in the exchange-fraction curve for the CO/Cs/Ru(OOl) system reported by M a t s ~ s h i m a ,though ~~ the experimental conditions were different and the fraction was calculated with respect to the total adsorbed CO. To estimate the temperature onset of the isotope-exchange reaction for the high Cs coverage, we measured the change of full width at half-maximum (fwhm) of the C-0 stretching mode of a 1:l isotope mixture of 12C160and 13C1s0after flashing the surface up to a predetermined temperature. The HREEL spectra were measured at 85 K. If the isotope exchange occurs by the flashing, the fwhm of the C-0 stretch will decrease due to the formation of the exchanged CO species (13C160and 12C180).We used a saturated CO layer on Cs-preadsorbed Ru(001) with a coverage of 0.24, where the C-0 stretch peak was intense and the sharpest. As a reference, the fwhm changes for pure l2CI6Oadsorbed on the CsRu(OO1) surface were also measured. The results are shown in Figure 13 together with the ratio between the two fwhms, (fwhm(mixture)/fwhm(pure)). A decrease in the fwhm at 250 K observed for both the mixture and the pure species is due to an increased homogeneity of the adlayer caused by the heating. Except for the initial decrease, the fwhm for the pure l2CI6Ochanges little until the temperature is increased up to 580 K. On the other hand, the fwhm for the isotope mixture keeps almost constant until 450 K and begins to decrease from this temperature. This change is more clearly seen in the ratio (top of Figure 13). The decrease in the fwhm for the isotope mixture corresponds to the progress of the isotope exchange. Therefore, the temperature onset of the isotope exchange of the CO/Cs/Ru(OOl) (&, = 0.24) is estimated to be 450 K. This temperature onset is slightly lower than that for the lower Cs coverages (480-500 K). When CO was adsorbed on the Cs-preadsorbed surfaces at 85 K and the coadsorbed surfaces were annealed up to 650 K, the HREELS and XPS results indicated that the CO adsorbs molecularly and provided no evidence for dissociation into atomic carbon and oxygen. Furthermore, a recombinative desorption peak due to atomic C and 0, which usually appears
desorption
90’ 100
’
I
200
’
’
300
’ 400
’
I
500
’
’
600
’
.-0 U
’
0.8 700
Temparature [K] Figure 13. Change of the full width of the half-maximum (fwhm) for the C - 0 stretching mode of pure l2CI6O (solid circles) and of a 1:1 mixture of 1 2 C 1 6and 0 I3Cl8O(open circles) adsorbed on Cs/Ru(OOl)(&, = 0.24) as a function of the flashing temperature. The “flashing” procedure was done by once heating the surface with a heating rate of 1 IUSup to a predetermined temperature and immediately cooling the surface down to 85 K. At the top of the figure, the ratio between the two fwhm’s (fwhm(mixture)/fwhm(pure)) is also plotted (solid triangles).
above 750 K,36was not observed in any conditions studied here. Thus, large amounts of C and 0 do not exist in the present system.
Discussion
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1. Structure of the Cs CO Islands. It is considered that during the island-growth process, at least two different Cs CO coadsorption states with different [CO]:[Cs] stoichiometries are formed; the 2:l state (Oc, = 0-0.2) and the 4:3 state (00 0.25). At present, it is not clear whether or not there is another coadsorption state with a different stoichiometry. Hereafter, we will discuss on these two coadsorption states in detail. Both these states give the p(2 x 2) LEED pattern and have the p(2 x 2)-Cs lattice as stated above. In the case of the 2:1 state, a p(2 x 2) unit cell contains two CO molecules. The HREEL spectra for this state exhibits a single symmetric C - 0 stretching peak as shown in Figure 5, from which we can consider equivalent adsorption sites for the two CO molecules within the resolution limit of the HREELS. According to the recent detailed LEED analyses,32the Cs adatoms of the p(2 x 2) structure on clean Ru(001) occupy on-top sites. Assuming that the Cs atoms in the p(2 x 2)-Cs lattice adsorb at the on-top sites and considering the sizes of Cs atoms and CO molecules, 3-fold hollow sites are most plausible for the two equivalent CO sites. From the comparison of stretching frequencies of CO molecules adsorbed on metal surfaces with those of organometallic complexes, it has been considered that CO molecules adsorbed at multiply (23) coordinated sites have C-0 stretching frequencies lower than 1880 cm-’.37 The stretching frequencies of 1680- 1800 cm-I for the 2: 1 state are consistent with this interpretation. Recently, however, based on a combination of kinematic LEED simulations and infrared reflection-adsorption spectroscopy results for CO/K(Cs)/Pt( 11l), Tiishaus et al. proposed that the CO molecules adsorb at 2-fold bridge sites in the presence of alkali ad atom^.^^'^ Moreover, recent quantitative structural studies have revealed that it is not always valid to use vibrational spectroscopy for adsorption site
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Coadsorption of Cs and CO on Ru(001) determinati~n.~’-~~ Therefore, further experiments are needed to determine the adsorption site. In the case of the 4:3 state, a p(2 x 2) unit cell contains 4/3 CO molecules. This fractional number suggests that two types of CO, i t . , CO molecules adsorbing with one molecule per unit cell and with one molecule per three unit cells, coexist in this state. In fact, the HREEL spectrum for the 4:3 state taken at 85 K exhibits a shoulder structure at 1430 cm-’ in addition to the main peak at 1580 cm-’ and the intensity of the shoulder structure is about one-third of that of the main peak. Furthermore, after this state is heated up to 200 K, a weak ( 2 4 3 x 243)R30° LEED pattern starts to appear in addition to the p(2 x 2) pattern. If the ( 2 4 3 x 21/3)R3Oo unit cell contains one CO molecule, this CO coverage is ’/12, which is the same as the coverage for one CO molecule er three p(2 x 2) unit cells. The appearance of the ( 2 4 3 x 2 3)R3Oo pattern is associated with a long-range ordering of the I / I Z coverage CO by the heating of the surface. Furthermore, in the TPD spectrum for this state shown in Figures 2 and 11 (e,-, = 0.24), a shoulder structure is observed at lower temperatures than that for the main peak and the area of this shoulder structure is about onethird of that of the main peak. While the main-peak CO desorbs simultaneously with Cs, the shoulder structure is due to desorption of pure CO. If the CO species associated with the shoulder structure is removed from the surface by heating up to 650 K, the ( 2 4 3 x 2J3)R3Oo LEED pattern disappears. Therefore, the 4:3 state has a Cs CO 1:l lattice with the p(2 x 2) periodicity and additional CO molecules which form an ordered structure with the ( 2 4 3 x 21/3)R3Oo periodicity above 200 K. 2. Adsorption Process of CO on CdRu(OO1). Although both of the 2:l state and the 4:3 state are formed by saturated adsorption of CO, the CO densities in these two states are different. This difference in the CO density is associated with different formation processes for these two states. At low Cs precoverages below 0.2, the LEED pattern changes from “ring” to p(2 x 2) upon exposing the Cs/Ru(OOl) surface to CO. In a recent study on the CO/Cs/F’t(lll) system,’O it was suggested that Cs adatoms and CO molecules form Cs-COCs chains even at low coverages. One-dimensional (1D) structures formed by low-coverage K and CO have been also proposed for the C O K systems on Ru(OO~)~ and Pt(lll).9 Considering the [CO]/[Cs] ratio of 2, a 1D structure such as Cs-(C0)z-Cs might be formed on the Ru(001) surface at low Cs coverages. However, no LEED pattern associated with such a 1D structure was observed during the CO adsorption process. Therefore, even though such a 1D structure is once formed, those structures immediately condense together and form a 2D stable island. This facile 2D-island formation in the CO/Cs/ Ru(001) system is in contrast to the formation of chain structures observed in the CO/K/Ru(OOl) ~ y s t e m .This ~ may be related with a strong attractive interaction between two adjacent Cs( c 0 ) z - C ~ chains and a relatively high mobility of Cs adatoms and a larger Madelung energy gain expected at the formation of the 2D Csd+-C06- lattice due to a smaller electronegativity of Cs. In any case, a condensation process is needed to form the 2:l state. On the other hand, in the case of high Cs precoverages, since the p(2 x 2)-Cs lattice is already formed before the CO adsorption, CO molecules presumably adsorb at vacant sites within the Cs lattice. The TPD spectra for the 4:3 state indicated that, after the desorption of one-fourth of the total CO, a stable 1:l Cs CO phase remains, from which Cs and CO desorb simultaneously at high temperatures. This stabilization of the 1:1 phase should be considerably contributed from the Madelung
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J. Phys. Chem., Vol. 99, No. 21, 1995 8797 energy arising from the Css+-COd- lattice because of the largest size of the i ~ l a n d s . ~Thus, ~ . ~during ~ the CO adsorption process on the p(2 x 2)-Cs lattice, this stable 1:l phase is probably formed at first. This 1:1 phase is capable of accommodating a little more CO molecules within its lattice. CO molecules impinged on the 1:l phase randomly occupy energetically preferred sites, but the further adsorption of CO should significantly destabilize the stable phase resulting in the saturation of CO after accommodating the l/12 coverage CO. This destabilization may be due to a loss of the Madelung energy which arises from the perturbation to the 1:1 Cs CO “charged” lattice. The large sizes of the islands at high Cs coverages make the Madelung contribution significant, while it is less for the small islands formed at low Cs coverages. Therefore, the formation of the different states for the low and high Cs coverages even after saturated adsorption of CO can be explained by the difference in the Madelung contribution due to the different sizes of the islands. 3. Isotope Exchange of CO Located inside the Islands. The isotope exchange of the CO species in the 2: 1 islands starts at about 500 K, while that of the CO species in the 4:3 state starts at 450 K. The isotope-exchange reaction occurs exclusively in the islands and is probably facilitated by the significant C - 0 bond weakening of the inside CO. The small difference in the temperature onset may be related to the difference in the degree of the C - 0 bond weakening between the 2: 1 state and the 4:3 state. In fact, the C - 0 stretch frequency for the 2:l state is 1700-1800 cm-’ whereas that for the 4:3 state is 1580 cm-I. The fraction of the exchanged CO increases steeply around the Cs coverage of 0.2. Below this critical coverage the 2:l state is formed, while above this coverage the 4:3 state is formed. The critical coverage corresponds well to the coverage where the coadsorption state changes from the 2:l to the 4:3 state. Therefore, the sharp increase in the fraction of the exchanged product is unequivocally related with the change of the Cs CO islands from the 2:l state to the 4:3 state. There is a large difference in the fraction between the two states even though both of the CO species are strongly influenced by Cs. This difference is not only due to the difference in the degree of the C-0 bond weakening but also due to the difference in the desorption temperature of the CO species. Since the CO desorption from the 2:l state starts simultaneously with the isotope exchange at about 500 K, the desorption process and the exchange process occur competitively, which results in less exchanged products. On the other hand, the desorption from the 4:3 state starts at about 550 K, which is higher by 100 K than the temperature onset of the exchange reaction. The lower temperature commencement of the exchange process prior to the desorption process efficiently increases the fraction of the exchanged products in the desorbed CO. Matsushima found an angular distribution of the CO desorption flux varying as COS^)^,^ for the CO/Cs/Ru(OOl) system with a high Cs coverage of 0.26 and explained this sharp distribution in terms of desorption of CO immediately after re~ombination.~~ Our present results indicate that the CO molecules do not desorb just after isotope-exchange reaction at a Cs coverage near 0.26. The sharp angular distribution of CO is accompanied by simultaneous desorption of Cs. Thus, the sharp angular distribution might be resulted from the coincident desorption of Cs and CO and not necessarily from the recombinative desorption. As for the mechanism of the isotopeexchange reaction, we cannot determine at present whether the concerted mechanism or the dissociation-recombination mechanism is correct.
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8798 J. Phys. Chem., Vol. 99, No. 21, 1995
Kondoh et al.
Conclusions The coadsorption of CO and Cs on Ru(001) induces the formation of p(2 x 2) Cs CO islands at Cs coverages below 0.25. In the case of the saturated adsorption of CO, at least two coadsorption states with different [CO]:[Cs] stoichiometries, 2:l and 4:3, are found for the p(2 x 2 ) islands. For both the two states, the local density of Cs is 0.25. The desorption energy and the C - 0 stretch frequency depend on the stoichiometry. The isotope exchange of the adsorbed CO molecules occurs exclusively in the islands and the temperature onset of the isotope exchange is estimated to be 450-500 K, which also changes slightly depending on the stoichiometry. This temperature onset is comparative to the desorption temperature for the 2:l state, while it is lower by 100 K than the desorption temperature for the 4:3 state, which results in a distinct difference in the exchange probability. The two Cs CO states are formed by different coadsorption processes. The difference in the attainable stoichiometry between the two Cs CO states is explained in terms of a difference in the Madelung contribution arising from the different sizes of "charged" Cs -t CO lattices.
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References and Notes (1) Bonzel, H. P. Surf: Sci. Rep. 1987, 8, 43. (2) Physics and Chemistry of Alkali Metal Adsorption; Bonzel, H. P., Bradshaw, A. M., Ertl, G., Eds.; Elsevier: Amsterdam, 1989. (3) Crowell, J. E.; Garfunkel, E. L.; Somorjai, G. A. Surf: Sci. 1982, 121, 303. (4) Kiskinova, M.; Pirug, G.; Bonzel, H. P. Surf: Sci. 1983, 133, 321. ( 5 ) dePaola, R. A,; Herbek, J.; Hoffmann, F. M. J. Chem. Phys. 1985, 82, 2484. (6) Whiteman, L. J.; Ho, W. J. Chem. Phys. 1985, 83, 4808. (7) Uram, K. J.; Ng, L.; Yates, J. T., Jr. Surf: Sci. 1986, 177, 253. (8) Pirug, G.; Bonzel, H. P. Surf: Sci. 1988, 199, 371. (9) Tushaus, M.; Gardner, P.; Bradshaw, A. M. Surf: Sci. 1993, 286, 212. (10) Tushaus, M.; Gardner, P.; Bradshaw, A. M. Langmuir 1993, 9, 349 1. (1 1) Somerton, C.; Mcconville, C. F.; Woodruff, D. P.; Grider, D. E.; Richardson, N. V. Surf: Sci. 1984, 138, 31. (12) Hoffmann. F. M.: Herbek. J.: dePaola. R. A. Chem. Phvs. Lett. 1984, 106, 83. (13) Lackey, D.; Surman, M.; Jacobs, S.;Grider, D.; King, D. A. Surf. Sci. 1984, 152/153, 513.
(14) Eberhardt, W.; Hoffmann, F. M.; dePaola, R.; Heskett, D.; Strathy, I.; Plummer, E. W.; Moser, H. R. Phys. Rev. Lett. 1985, 54, 1856. (15) Seip, U.; Bassignana, I. C.; Kiippers, J.; Ertl, G. Surf: Sci. 1985, 160, 400. (16) Madey, T. E.; Benndorf, C. Surf: Sci. 1985, 164, 602. (17) Schultz, P. A.; Patterson, C. H. J. Vac. Sci. Technol. 1987, AS, 1061. (18) Weimer, J. J.; Umbach, E. Phys. Rev. B 1984, 30, 4863. (19) Weimer, J. J.; Umbach, E.; Menzel, D. Surf: Sci. 1985, 155, 132; Ibid. 1985, 159, 83. (20) Muller, J. E. In ref 2, p 271. (21) Nwskov, J. K.; Holloway, S.; Lang, N. D. Surf: Sci. 1984, 137, 65; J. Vac. Sci. Technol. 1985, A3, 1668. (22) Lang, N. D.; Holloway, S . ; Nwskov, J. K. Surf: Sci. 1985, 150, 24. (23) Wimmer, E.; Fu, C. L.; Freemann, A. J. Phys. Rev. Lett. 1985.55, 2618. (24) Al-Sarraf, N.; Stuckless, J. T.; King, D. A. Nature 1992,360, 243. (25) Murray, S. J.; McGrath, R. Surf: Sci. 1994, 307-309, 668. (26) Hoffmann, F. M.; dePaola, R. A. Phys. Rev. Lett. 1984, 52,1697. (27) Lee, J.; Arias, J.; Hanrahan, C. P.; Martin, R. M.; Metiu, H. P hys. Rev. Lett. 1983, 51, 1991; J. Chem. Phys. 1985, 82, 485. (28) Crowell, J. E.; Tysoe, W. T.; Somorjai, G. A. J. Phys. Chem. 1985, 89, 1598. (29) Matsushima, T. Z. Phys. Chem. (NF) 1988, 158, 175. (30) Matsushima, T. Z. Phys. Chem. (NF) 1989, 162, 1. (31) Kondoh, H.; Nozoye, H. J. Phys. Chem. 1994, 98,390, Surf. Sci., in press. (32) Over, H.; Bludau, H.; Skotte-Klein, M.; Ertl, G.; Moritz, W.; Campbell, C. T. Phys. Rev. B 1992, 45, 8638. (33) Williams, E. D.; Weinberg, W. H. Surf: Sci. 1979, 82, 93. (34) Nilsson, A,; Mirtensson, N. Phys. Rev. B 1989, 40, 10249. (35) The "unscreened" satellite at 538.5 eV is coupled with the main "screened" peak at 532.2 eV. This splitting is due to a final state effect. On the other hand, the peak at 530.8 eV does not exhibit such a splitting probably because of a strong screening effect by the increased 2x* donation. Therefore, the intensity of the satellite is added to that of the main peak at 532.2 eV. (36) For example: Benziger, J.; Madix, R. J. Surf: Sci. 1980, 94, 119. Zaera, F.; Kollin, E.; Gland, J. L. Chem. Phys. Lett. 1985, 121, 464. Colaianni, M. L.; Chen, J. G.; Weinberg, W. H.; Yates, J. T., Jr. J. Am. Chem. SOC. 1992, 114, 3735. (37) Mapledoram, L. D.; Bessent, M. P.; Wander, A,; King. D. A. Chem. Phys. Lett. 1994, 228, 527. (38) Becker, L.; Aminpirooz, S.; Hillert, B.; Pedio, J.; Haase, J.; Adams, D. L. Phys. Rev. B 1993, 47, 9710. (39) Davila, M. E.; Asensio, M. C.; Woodruff. D. P.; Schindler, K.-M.; Hofmann, Ph.; Weiss, K.-U.; Dippel, R.; Gardner, P.; Fritzsche, V.; Bradshaw, A. M.; Conesa, J. C.; Gonzblez-Elipe, A. R. Surf: Sci. 1994, 311, 337.
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