HREELS investigation of COK coadsorption on Ni(111)

surface s c i e n c e ELSEVIER

Surface Science 371 (1997) 45-52

HREELS investigation of CO-K coadsorption on Ni(111) G. C h i a r e l l o *, A. C u p o l i l l o , A. A m o d d e o , L.S. C a p u t i , O. C o m i t e , S. Scalese, L. P a p a g n o , E. C o l a v i t a Istituto Nazionale di Fisica della Materia - Dipartimento di Fisica, Universit& della Calabria, 87036 Arcavacata di Rende, Cosenza, Italy

Received 5 March 1996; accepted for publication 19 July 1996

Abstract

The coadsorption of CO and K on Ni(111) has been studied by high-resolution electron energy-loss spectroscopy. We used three precoverages, one of them (0.25 ML) corresponding to an ordered surface overlayer (p(2 x 2)) and the other two (0.3 and 0.47 ML), to an incommensurate and a disordered surface layer, respectively. Different local CO adsorption arrangements are suggested for each K coverage. The vibrational spectra show features at 27, 180 and 210 meV. The loss at 27 meV is related to a vibration of the reconstructed N i ( l l l ) surface due to the p(2 × 2)-K layer and is damped by 3 L of CO. For the other two K precoverages, that loss exists regardless of the amount of CO. The losses at 180 and 210 meV are assigned to the C O stretching vibration corresponding, respectively, to the occupation of only one or both sites of the same p(2 x 2)-K cell. The CO saturation of the p(2 x 2)-K/Ni(ll 1) surface gives rise to a short-range interaction among CO molecules as well as between each K atom and CO molecules, which causes an overall shift of the C - O stretching frequency towards higher loss energies. Keywords: Alkali metals; Compound formation; Electron energy loss spectroscopy; Molecule solid reactions; Vibrations of adsorbed molecules

1. Introduction

Recent advances [1-12] in the knowledge of the properties of alkali-metal atoms adsorbed on metal surfaces have stimulated new experimental [13,14] and theoretical [15] works on the coadsorption of alkali metal atoms and CO. Such systems [16-20] have long been studied because of the importance of alkali atoms as promoters in several catalytic reactions. Observations common to all alkali-metal atoms-CO systems are the lowering of the C-O stretching frequency [16,18] and the increased

* Corresponding author. Fax: + 39 984 839389; e-mail: [email protected]

desorption temperature [18] of both CO and alkali atoms. Numerous theoretical and experimental studies have not completely clarified the nature of the alkali-metal atom/CO/metal-surface bonding, and different interpretations for the above effects have been proposed. An explanation for the variation of the C-O stretching frequency is based on a charge transfer model [21] from the alkali atom or from the metal surface into the empty 2re* orbital of CO. In the case of C O + K / P t ( l l l ) , theoretical studies [22] show that the CO vibrational shift may be interpreted as being derived from a change of the CO adsorption site induced by K atoms. Recent experimental results have found that a site change does take place, at least for the CO + Cs/Ru(0001) system [ 14]. Moreover, a den-

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G. Chiarello et al./ Surface Science 371 (1997) 45-52

sity functional study [15] of the coadsorption of K and CO on Pd clusters shows that the vibrational energy shift is due to a polarization of the electron distribution of the metal surface in response to the dipole created by alkali atoms. Another important question is the nature of the interaction between alkali atoms and coadsorbed molecules. Short- and long-range interactions have been discussed, and evidence supporting the formation of K + - C O - complexes have been reported [-23,24]. A1 Sarraf et al. [23] performed calorimetric measurements of the coadsorption of K and CO on the Ni(100) surface and, depending on the initial K coverage, found a different behaviour for the adsorption heat. For a K coverage of 0.13 monolayers the main contribution to the adsorption heat derives from a charge transfer into the CO 2~* orbital, while at higher alkali coverages there is a significant contribution due to the Madelung energy of K + C O - islands. For the same system, the formation of K + - C O - structures was confirmed by low-energy electron diffraction (LEED) measurements [24]. On the clean N i ( l l l ) surface, both the adsorption of CO and K and their coadsorption have been studied extensively. For a coverage of 0.5 ML, CO forms a c(4 x 2) structure with CO molecules that occupy both fcc and hcp three-fold sites [25]. The CO stretching vibrational excitation occurs at 236meV (1910cm-1). K atoms form a p ( 2 x 2 ) structure on the N i ( l l l ) surface, adsorbing at on-top sites, and the surface undergoes a relaxation (or a rumpling) [1,26-28]. When the p(2 x 2)-K phase is coadsorbed [13] with a CO-saturated layer (each p(2 x 2)-K unit cell contains two CO molecules), K and CO occupy the same positions as in the case of the clean surface. The observable [13] effects induced from CO adsorption on the p(2 x 2)-K overlayer are the removal of the vertical reconstruction of the N i ( l l l ) surface and the increase of the K-Ni bond length. Recently, we have investigated the vibrational properties of the K / N i ( l l l ) system [29] by high-resolution electron energy-loss spectroscopy (HREELS) measurements. The HREELS spectra show a feature at 27meV whose intensity is enhanced when a well-ordered p(2 x 2)-K phase

exists on the N i ( l l l ) surface. We interpreted this feature as a dipole-active vibrational mode which is related to the reconstruction of the N i ( l l l ) surface induced by the p(2 x 2)-K overlayer. The phonon, which was assigned [29] to a gap mode [-30] $2, implies an interaction between the two uppermost layers of the Ni surface. In the present HREELS investigation, we use the behaviour of the above feature to gain information on the properties of the coadsorption of K and CO on the N i ( l l 1) surface. The measurements have been performed for two K precoverage regimes, namely 0.25 ML [31] where K forms a well-ordered p(2 x 2) layer, and two higher coverages, 0.3 ML, where the structure is incommensurate, and 0.47ML, where K atoms form a disordered phase. The choice of definite precoverage regimes allows us to take advantage of the knowledge of the CO adsorption sites within the p(2 x 2)-K cell for the interpretation of the results. HREELS spectra of the coadsorption of CO on a p(2 x 2)-K layer confirm our previous assignment of the feature at 27 meV as being related to surface rumpling. On p(2 x 2)-K such rumpling persists after adsorption of a low CO dose, but disappears at CO saturation. On the basis of these results, we suggest that at low CO doses, only one of the two allowable three-fold sites in the p(2 x 2)-K cell is occupied. For the other two K precoverages, we have HREELS spectra similar to those of low CO doses on the p(2 x 2)-K cell, and we are forced to conclude that the vibrational properties of the surface are not changed, and are again caused by interactions of only one CO molecule per cell with its neighbour K atoms. It is worth stressing that the use of a definite initial K-phase on the N i ( l l l ) surface, as well as the conclusions of PhD measurements [ 13] on the same system, were crucial to interpret the present data.

2. Experimental The experiments were performed in an ultrahighvacuum system equipped with an angular-resolved 5 0 m m mean radius spherical analyzer for HREELS measurements and with facilities for

47

G. Chiarello et al./Surface Science 371 (1997) 45-52

L E E D and AES characterizations. All H R E E L S spectra were recorded in the specular mode with a primary beam energy of 10 eV and an F W H M of 10 meV. No evidence of electron-impact scattering behaviour was observed for these structures from out-of-specular measurements. The Ni(111) surface was prepared by Ar + bombardment and annealing cycles, and surface cleanliness and order were checked by AES and LEED, respectively. Potassium was evaporated from a welloutgassed SAES getter source, and its coverage was obtained from the K (L2.3VV-to-Ni L2,3VV) Auger peak intensity ratio, assuming a coverage of 0.25 ML for the p(2 x 2) phase. Completion [-26] of the first layer occurs at about 0.31 ML.

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3. Results 180

Fig. 1 shows the loss spectra of the p(2 x 2)K/Ni(111) system exposed to CO up to saturation at RT. The loss spectrum obtained before CO exposure is characterized by the feature at 27 meV. The stretching frequency of the CO molecule increases, ranging from 180 up to 210 meV, which is well below the value of 236 meV measured for 0.5 ML of CO on the clean Ni(111) surface. The intensity of the feature at 27 meV first increases for low CO exposures and then rapidly decreases, vanishing almost completely for a CO exposure of 6 L (1 L -10 -6 Torr-s), (Fig. 2, open circles). The same figure (diamonds) also shows the intensity behaviour of the intramolecular CO stretching vibration versus CO exposure. L E E D observations for each stage of the CO coverage show that a low CO dose (1 L of CO) produces sharper spots, while higher CO exposures give rise to diffuse spots. The ordering effect of a small amount of CO from contamination on the p(2 x 2)-K overlayer was also observed in a previous LEED study of this system [32]. The behaviour of the CO uptake on the K-exposed N i ( l l l ) surface depends on the K precoverage. For 0.3 M L of K (Fig. 3), that is for the formation of a full K layer, the structure at 27 meV is still present and the L E E D pattern of the p(2 x 2)-K is still observable, although with

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100 150 200 ENERGY LOSS (meV)

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Fig. 1. HREELS spectra at room temperature of p(2x 2)-K and CO + p (2 x 2)-K phases on Ni( 111). split and broadened spots. Again, the LEED pattern becomes much better defined after a small amount of CO. Contrary to the previous precoverage, the peak at 27 meV still exists for a CO dose of 10 L. The intensity of the vibrational feature at 180meV follows the same trend of the 27 meV loss, being a distinct peak for 3 L CO which remains after further CO exposures. The stretching frequency of CO starts at 176meV (3 L of CO), moves to 182 meV (10 L of CO) and does not change with further coverage. The results obtained by exposing at CO one and half layers of K are shown in Fig. 4. These H R E E L S measurements were carried out at 200 K in order to easily condense more than one K layer. For this K precoverage the vibration at 27 meV disappeared, but returned again after CO exposure.

G. Chiarello et al./Surface Science 371 (1997) 45 52

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CO Dose (L) Fig. 2. Intensity of the feature at 27 meV (open circle), and of the C O v i b r a t i o n (diamond), as function of the C O dose. The intensities are n o r m a l i z e d to the elastic peak. For low C O doses we used a n e x p a n d e d scale in the 0-6.5 M L range.

The CO stretching frequency shifts from 176 to 181meV throughout the investigated adsorption range. Moreover, the initial LEED pattern, composed of large Ni spots and diffuse background, evolved with CO exposures in a rather diffuse p(2 x 2) pattern and brighter Ni spots.

4. Discussion For convenience, we briefly recall some peculiar points characterizing the adsorption of K on N i ( l l l ) . They will be useful in interpreting the behaviour of the C O - K interaction on the Ni(111) surface. A p ( 2 × 2 ) LEED pattern is formed [1,32] on adsorbing 0.25 M L of K atoms at room temperature, as well as at lower temperatures. All investigations [1,26-28] agree that K atoms adsorb at on-top sites, inducing a rumpling [ 1] or a relaxation [-27,28] of the N i ( l l l ) surface. The surface rumpling provides a screening-out of the direct adsorbate-adsorbate repulsion [6]. The on-top positions were found to be still occupied [26] for K coverages well above and

well below the p(2 × 2) phase. However, due to the high lateral mobility of K atoms inside the flat potential well associated with on-top sites, a significant fraction of adatoms can be thermally excited into sites which are different from the on-top ones [26]. An important feature of the present CO-K/Ni(111) investigation is that the coadsorption of small quantities of CO (less than 1 L of CO) on a p(2 x 2)-K layer gives rise to a better L E E D pattern. Together with the improvement of the LEED image, we observe that the same CO dose gives rise to an increase of the feature at 27 meV; then for further CO adsorption, the vibration is highly damped. The C - O stretching frequency moves to higher energies (up to 210 meV) with CO exposure of the p ( 2 x 2 ) - K / N i ( l l l ) surface. At low CO coverages, very probably only one of the two three-fold sites for CO in the p(2 x 2)-K cell is mostly occupied, and each CO molecule interacts with K atoms. In this state, the surface appears to be still relaxed. We do not know what the adsorption site is of CO on the p ( 2 × 2 ) - K / N i ( l l l ) cell before CO saturation. In an LEED study performed by Over

49

G. Chiarello et al./Surface Science 371 (1997) 45-52 ENERGY LOSS (cm 1)

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Fig. 3. HREELS spectra at room temperature of 0.3 ML of K and CO + 0.3 ML of K on Ni(111). This coverage corresponds to the completetion of the first K layer.

Fig. 4. HREELS spectra of 0.47 ML of K and CO+0.47 ML of K on Ni(lll). These measurements were performed at T= 200 K in order to condense the second K layer.

et al. [14] for C O + C s / R u ( 0 0 0 1 ) , the CO was sitting on an hcp site. For the reader's convenience we suppose that initially, CO may adsorb in the hcp site of the p(2 x 2 ) - K / N i ( I l l ) cell for all the three precoverages studied, although our conclusions will also apply for the initial fcc site. We suggest that the C - O stretching frequency at 180 meV is peculiar to CO molecules scarcely interacting each other. The continuous shift of the CO stretching frequency towards higher values should be indicative of a change in the local CO environment, because a filling of fcc sites is occurring. In other words, at CO saturation both hcp as well as fcc sites are occupied, and each p ( 2 x 2)-K cell contains two C O molecules [13] which interact not only with each other but also with neighbour K atoms. We assume that the shift

of the vibrational frequency from 180 to 210 meV is due to a C O - C O interaction as well as to a K - C O interaction. On the other hand, a similar lateral interaction has recently been suggested for CO adsorbed on an Ru(0001) surface by He et al. [33] in order to interpret the C O stretching vibrational energy shift versus coverage. For CO doses higher than 6 L, C O saturation is not yet reached, but the majority of p(2 x 2)-K cells are now occupied by two C O molecules. A short-range interaction [ 15 ] among C O molecules adsorbed on the K-precovered surface and between each K atom and CO molecules, inhibits a filling up of the 2~* antibonding orbitals of CO, strengthening the C O interatomic bond, and also provides a screening-out of the repulsion between K atoms. All this leads to a release of the vertical reconstruc-

50

G. Chiarello et al./Surface Science 371 (1997) 45-52

tion and to the disappearance of the vibration at 27 meV. This finding is in agreement with PhD results of Davis et al. [13] which show that a p ( 2 x 2 ) - K layer on N i ( l l l ) , saturated with CO, does not exhibit the surface relaxation of the p ( 2 x 2 ) - K layer alone. Thus the behaviour of the loss peak at 27 meV as a function of CO exposure supports the assignment [29] of this feature to a vibration of the reconstructed N i ( l l l ) surface induced by the p(2 x 2)-K layer. The chemisorption behaviour of CO on the N i ( l l l ) surface precovered with 0.3 ML of K (Fig. 3) is in agreement with the previous interpretation. We start from a K-covered surface showing a broad feature at 27meV and a p ( 2 x 2 ) diffuse LEED pattern. The CO uptake gives rise to an intensity increase of the vibrational loss at 27 meV and, moreover, to the CO-related feature at 180 meV. The loss at 27 meV does not disappear upon CO saturation. The adsorption of CO on this metal-like K layer induces a're-ionization of K adatoms, restoring the K atom polarization status that is typical of lower K coverages, hence increasing the K surface bond [-23]. In this condition the feature at 27 meV is more intense due to the strong K-Ni interaction. By using the previous finding, we are led to the conclusion that at this K coverage, CO molecules may occupy only one site in the unit surface cell, and the Ni(111) surface is reconstructed. We may wonder why one of the two sites stay free even at high CO coverages provided the surface is precovered with 0.3 M L of K. One explanation may come from LEED studies of the C O + K / P t ( l l l ) system [34], where the authors found that high precoverages of alkali-metal atoms imply a shrinkage of the unit cell to accomodate K alkali atoms in excess with respect to the ordered phase. If this is the case, evidently one of the two three-fold sites, very probably the fcc site, becomes inaccessible to CO molecules and, consequently, the K - K repulsion is not completely screened out. The interaction of CO molecules in hcp sites with K-alkali atoms is thus confined in each unit cell and the coordination of each K atom turns out to be similar to the case of low CO doses on the 0.25 ML p(2 x 2 ) - K / N i ( l l l ) surface. For this

arrangement of atoms on the surface, the uppermost Ni layer is not only relaxed, as evidenced by the 27 meV peak, but its vibrational motion is also more intense, as the normal component of the dipole to the surface is higher. The results of Fig. 4 confirm the above interpretation, because for 0.47 ML of K, the second layer starts to be built up. The unit cell may be now randomly occupied by one more K alkali atom, and neither the 27 meV loss nor the p(2 x 2) LEED pattern are observed. By exposing the precovered surface to 50 L of CO, saturation status of the surface is reached and the LEED pattern is recovered. Evidently, the overall effect of CO molecules on the surface unit cell is still effective, stabilizing not only the K atoms of the first layer, but also those alkali atoms of the second layer which were mobile and not bound to a specific site. Each CO molecule occupies the hcp site and interacts very probably with two K atoms at different heights, but its stretching frequency does not change appreciably, within our experimental uncertainty. Thus the CO molecule also interacts strongly with the Ni surface for this phase, in agreement with the known result that CO does not adsorb on K atoms. The 27 meV loss is always observed, indicating the presence of a reconstructed surface. We exclude the possibility that the K - O stretching frequency might give a contribution to the 27 meV structure because CO coadsorbed with K does not dissociate at 200 K. The above results for 0 . 4 7 M L of K are in agreement with literature results on similar systems. Christensen et al. [35], for example, suggested that K atoms diffuse on the Ni(100) surface to form K - C O structures. Moreover, Murray et al. [24] performed an LEED investigation of K - C O on Ni(100) and found that CO has an ordering effect on preadsorbed K atoms. Finally, Pirug et al. [34] found an increase in the local K density after the coadsorption of CO and K on P t ( l l l ) . They proposed the formation of K - C O islands, as well as of clean Pt(111) patches. In the present case, the K density may increase locally due to the ordering effect of CO, but the final arrangement of species in the surface unit cell requires one CO molecule in an hcp site interacting

51

G. Chiarello et al./Surface Science 371 (1997) 45-52

with one K atom of the first layer and with one K atom of the second layer. However, from an energetic point of view, the CO stretching frequency is not appreciably affected by this interaction, while the 27 meV vibrational frequency of the uppermost layer against the underlying one is not.

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5. Conclusions We have found that, depending on the K precoverage, the coadsorption of CO affects the behaviour of the substrate phonon at 27 meV in a different manner. The feature at 27 meV is related to the substrate reconstruction which depends on the polarization of the K adsorbed atoms and hence, on the K-Ni bond. Fig. 5 is an attempt to summarize the evolution of the chemisorption system at low CO exposures (Fig. 5a) and at saturation (Fig. 5c) for the p(2 x 2)-K phase. The model of the saturated surface was taken from Ref. [13]. Also shown in Fig. 5 are the relative positions of adsorbed species along the [121] direction. When the K unit cell houses one CO molecule (Fig. 5a), K atoms are not completely screened out each other by CO molecules, and the Ni(111) surface is reconstructed (Fig. 5b). At CO saturation the p(2 x 2)-K unit cell contains two CO molecules which completely screen out the K K repulsion (Fig. 5c) and the bulk interplane distance of the Ni(111) uppermost layers is restored (Fig. 5d), as evidenced by the disappearance of the loss at 27 meV. At higher K coverages, whatever the CO dose, the feature at 27 meV is not damped. In that case, one CO molecule per cell polarizes K atoms, strengthening the K-Ni bond, but does not screen out the repulsion between them. The substrate answers to this dielectric status, contributing to the screening by means of a relaxation.

Acknowledgements We would like to thank E. Li Preti and V. Fabio for their technical assistance.

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000 Fig. 5. A model of the CO adsorption on the p(2x2)K/Ni(111) surface: (a) at low CO exposures each surface unit cell contains one CO molecule, very probably in hcp sites; (b) the same picture, drawn along the [121] direction, shows the surface rumpling. (c) At CO saturation, both hcp and fcc sites are occupied by CO molecules as shown in Ref. [-13]; (d) schematic view of (c) along the 1-121] direction. In this case the Ni(111) surface is unrelaxed.

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G. Chiarello et al./Surface Science 371 (1997) 45-52

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[20] H. Bonzel, Surf. Sci. Rep. 8 (1987) 43. [21] E. Wimmer, C.L. Fu and A.J. Freeman, Phys. Rev. Lett. 55 (1985) 2618. [22] J.E. Muller, in: The Chemical Physics of Solid Surface, Vol. 6, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1993). [23] N. A1 Sarraf, J.T. Stuckless and D.A. King, Nature 360 (1992) 243. [24] S.J. Murray and R. McGrath, Surf. Sci. 307 (1994) 668. [25] L. Becket, S. Aminpirooz, B. Hillert, M. Pedio, J. Haase and D.L. Adams, Phys. Rev. B 47 (1993) 9710. [26] D.L. Adler, I.R. Collins, X. Liang, S.J. Murray, G.S. Leatherman, K.D. Tsuei, E.E. Chaban, S. Chandavarkar, R. McGrath, R.D. Diehl and P.H. Citrin, Phys. Rev. B 48 (1993) 17445. [27] R. Davis, X.M. Hu, D.P. Woodruff, K.U. Weiss, R. Dippel, K.M. Schindler, Ph. Hoffmann, V. Fritzsche and A.M. Bradshaw, Surf. Sci. 307-309 (1994) 632. [28] Z. Huang, L.Q. Wang, A.E. van Wittenau, Z. Hussain and D.H. Shirley, Phys. Rev. B 47 (1993) 13626. [29] G. Chiarello, A. Cupolillo, A. Amoddeo, L.S. Caputi, L. Papagno and E. Colavita, Phys. Rev. B 52 (1995) 4752. [30] W. Menezes, P. Knipp, G. Tisdale and S.J. Sibener, Phys. Rev. B 41 (1990) 5648. [311] ML is defined as the number of atoms that equals that of the N i ( l l l ) surface. [32] S. Chandavarkar and R. Diehl, Phys. Rev. B 38 (1988) 12112. [33] P. He, H. Dietrich and K. Jacobi, Surf. Sci. 345 (1996) 241. [34] G. Pirug and H.P. Bonzel, Surf. Sci. 199 (1988) 371. [35] O.B. Christensen and J.K. Norskov, Chem. Phys. Lett. 214 (1993) 443.