Adsorption of carbon monoxide on Pt{100} surfaces: dependence of the CO stretching vibrational frequency on surface coverage

Surface Science 460 (2000) 101–111 www.elsevier.nl/locate/susc

Adsorption of carbon monoxide on Pt{100} surfaces: dependence of the CO stretching vibrational frequency on surface coverage Daniel Curulla, Anna Clotet *, Josep M. Ricart Departament de Quı´mica Fı´sica i Inorga`nica, Universitat Rovira i Virgili, Pl. Imperial Ta`rraco 1, 43005 Tarragona, Spain Received 27 January 2000; accepted for publication 6 April 2000

Abstract We have used the ab initio cluster model approach to study the dependence of the CO stretching frequency on CO surface coverage. We have also investigated the relative importance of the various factors that can affect the position of the CO stretching band as coverage increases. Two effects can change the CO stretching frequency: the adsorbate– adsorbate dipole coupling, which is a purely physical effect, and the changes in the 2p1 CO molecular orbitals, due to the different chemical environment at higher coverages. From our vibrational analysis, we conclude that CO–CO dipole coupling is the main cause of the upward shift of the CO stretching band when the CO coverage is increased. The population of the 2p1 CO molecular orbitals does not change at any coverage within the region considered. We have also estimated the 12CO–13CO dipole coupling, which previous studies have assumed to be weak. Our results demonstrate that the 12CO–13CO dipole coupling is indeed weak compared with the 12CO–12CO dipole coupling. At a CO surface coverage of 0.5 monolayers (ML), we have calculated a band shift of 40 cm−1 to higher frequency. However, we should point out that when one 12CO molecule is surrounded by a 13CO environment, the 12CO stretching band shifts 10 cm−1 upwards. We have also computed the heat of adsorption of CO on Pt{100}-(1×1) as a function of CO coverage. The initial heat of adsorption is calculated to be about 192 kJ mol−1 and then drops to 180 kJ mol−1 at 0.5 ML. These results agree quite well with recent calorimetric measurements. Besides that, we have estimated that the CO–CO interaction energy at 0.5 ML is repulsive and has a value of 5 kJ mol−1. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Carbon monoxide; Chemisorption; Density functional calculations; Low index single crystal surfaces; Platinum

1. Introduction The adsorption of carbon monoxide on singlecrystal platinum surfaces has been widely investigated for the last two decades [1]. The clean Pt{100} surface is reconstructed and the atoms on the surface adopt a quasi-hexagonal structure. This * Corresponding author. Fax: + 34-977-559-563. E-mail address: [email protected] (A. Clotet)

phase was referred to as Pt{100}-hex by Heilmann et al. [2]. However, the adsorption of strongly chemisorbed molecules, such as carbon monoxide, removes the hex reconstruction and the metal surface atoms then exhibit a truncated (1×1) ideal phase. The fact that carbon monoxide has different heats of adsorption on the reconstructed and the ideal (1×1) surfaces is thought to be the reason for the phase transition observed. Recently, Yeo et al. [3,4] found that the initial heat of adsorption

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of CO on a Pt{100}-(1×1) surface is larger than on a Pt{100}-hex surface by about 40 kJ mol−1. Adsorption and phase transition are believed to occur in sequential steps. Behm et al. formulated a mechanism that describes the adsorption of CO and the subsequent surface phase transition [5,6 ]. In the early 1990s, Gardner et al. [7] discussed this mechanism using isotopic dilution measurements. This led to a better understanding of the adsorption of CO on Pt{100}. Recent infrared absorption spectroscopy (IRAS ) experiments [8] show that, when CO is chemisorbed on Pt{100}-hex, the infrared spectrum has a single band at 2083 cm−1 that is assigned to the stretching vibrational mode of linearly bonded CO on the reconstructed surface. This band is 21 cm−1 above the initial frequency observed on the (1×1) surface, which appears at 2062 cm−1. When the CO exposure is increased, the feature at 2083 cm−1 shifts up to 2089 cm−1. Bearing in mind that adsorbate–adsorbate dipole coupling causes the vibrational bands to shift upwards, Gardner et al. [7] concluded that the small shift is proof that CO adsorption induces an immediate local lifting of the reconstruction. There is also a second feature at 1863 cm−1, which corresponds to bridge-bonded species, and a shoulder at 2088 cm−1, which is attributed to the stretching vibrational mode of linearly bonded CO on the truncated (1×1) islands. These two bands overlap because the local coverage on the ideal surface is larger than on the reconstructed surface. These results agree with the mechanism proposed by Behm et al. [5] in which islands of CO adsorbed on the (1×1) surface are thought to have a local coverage of approximately 0.5 monolayers (ML). The dependence of the CO stretching band on surface coverage was studied by Crossley and King [9,10]. They reported a band shift of about 30–40 cm−1 for CO-chemisorbed Pt{111}; the infrared spectrum showed a feature at 2063 cm−1 at low coverage that moved up to 2100 cm−1 at saturation. Results were similar for CO chemisorbed on Pt{100}-(1×1). The band shift was quantitatively reproduced at saturation coverage by varying the isotopic ratio in a 12CO/13CO mixture, and was shown to be due to adsorbate– adsorbate dipole coupling interactions. This con-

clusion was supported by dipole–dipole coupling calculations using Hammaker’s mathematical model [11]. This model was refined to include the interactions between a dipole and its own image, the images of other dipoles, and the screening of dipole images by the electronic polarizability of the adsorbate [12–14]. In addition, several studies have shown that the adsorption of CO on metal surfaces can be explained in terms of the Blyholder model [1,15,16 ], which suggests that CO bonds to the metal via the 5s molecular orbital, with simultaneous backdonation into the 2p1 CO orbitals. Since the 2p1 orbitals have an antibonding character with respect to the CO molecular bond, the partial occupancy weakens the CO bond, and decreases the CO stretching vibrational frequency. This assumption has been demonstrated theoretically using the projection technique described by Nelin et al. [17] and the constrained space orbital variation (CSOV ) method [1,18–22]. Therefore, if the population of the 2p1 CO molecular orbitals is changed (diminished ) by the increasing coverage, the CO stretching vibrational band may also shift upwards. From now on, this effect will be referred to as the chemical effect, unlike the adsorbate– adsorbate dipole coupling which is purely physical. In this paper we discuss the importance of the adsorbate–adsorbate dipole coupling and also the change in population of the 2p1 antibonding CO molecular orbitals. For our study we used the ab initio cluster model approach in the framework of the density functional theory. The heat of adsorption was also computed as function of the CO coverage, and the CO–CO interaction energy was estimated at a coverage of 0.5 ML. In addition, and due to the lack of quantitative structural information, the equilibrium geometries are discussed briefly.

2. Computational details The cluster model approach was used to study the adsorption of CO on Pt{100}-(1×1). A 41-atom cluster model — with 25 atoms in the first layer and 16 atoms in the second layer — was used to model the Pt{100}-(1×1) surface. We

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Fig. 1. Pt (25,16) cluster model. 41

shall refer to it as Pt (25,16) (Fig. 1). The posi41 tions of the atoms were the same as those in bulk ˚ platinum, with a lattice parameter of 3.9239 A [23]. This cluster model was chosen to define the c(2×2) unit cell that is formed when CO coverage reaches a value of 0.5 ML, at which all of the CO molecules are bound at on-top sites. At 0.5 ML, all of the CO molecules can also occupy bridge sites; in this case a c(2×2) low-energy electron diffraction (LEED) pattern is also observed. However, recent IRAS experiments [8] suggest that the most favourable situation is the adsorption at on-top sites. As in previous studies [1], we divided the cluster model into two regions: the local and the environmental. The former is built up with 13 platinum atoms: nine surface atoms and four atoms in the second layer. We treated the atoms of the local region with the Hay–Wadt relativistic effective core potential [24] (R-ECP) which explicitly includes the 5s25p65d10 electrons and a (8s6p3d/3s3p2d) basis set. The surface atoms, which are part of the

local region, define a nine-atom square that constitutes the c(2×2) unit cell. The rest of the atoms, those in the environmental region, were described with a one-electron R-ECP and a double-zeta plus polarization (DZP) basis set, which we had developed in our group [25]. For both the carbon and the oxygen atoms, the all-electron 6-31G1 basis sets were used. The calculations were carried out within the framework of the density functional theory, using the B3LYP hybrid functional. The computational package used was 94 [26 ]. All binding energies were corrected with the basisset superposition error (BSSE), which was calculated with the counterpoise method proposed by Boys and Bernardi [27]. We should point out that, because we did not make periodic calculations, we can observe three different platinum atoms in the unit cell; these three different platinum atoms are labelled as corner, edge and centre (Fig. 1). The different environments of these three atoms may lead to different values for equilibrium geometry of adsorbed CO or for the CO stretching frequency. To determine the extent to which these properties may vary, we calculated the equilibrium geometry and the CO stretching frequency in the three cases: on the corner atom, on the edge atom and on the centre atom ( Table 1). The CO geometry was fully optimized, remaining vertical — in an upright configuration — in all the tests. We did not observe tilting, either when the CO molecule is bound to an edge platinum atom or when it is bound to a corner atom. Only slight differences in the PtMC distance appeared when the CO molecule was bound to the centre atom. In this case, the PtMC ˚ , which is 0.03 A ˚ smaller than distance is 1.83 A that for the corner and the edge atoms. Moreover, differences in the CMO distance are insignificant.

Table 1 Equilibrium geometry, heat of adsorption (BE) and stretching frequency for chemisorbed CO on a corner atom, an edge atom and ˚ ngstro¨ms; d ˚ ngstro¨ms; BE is the binding energy a centre atom; z is the PtMC distance in A is the CMO distance in A C CO in kJ mol−1; and n is the CO stretching frequency in wavenumbers CO Adsorption site

˚) z (A C

˚) d (A CO

n (cm−1) CO

BE ( kJ mol−1)

Corner atom Edge atom Centre atom

1.862 1.862 1.834

1.154 1.155 1.156

2071 2069 2068

199 197 192

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Finally, the CO stretching frequency varies only from 2068 to 2071 cm−1, and the heat of adsorption from 192 to 199 kJ mol−1. Therefore, we believe that our cluster model can be used to study the adsorbate–adsorbate interactions. The CO stretching frequency calculations have been carried out by partial diagonalization of the mass-weighted hessian matrix. We have assumed that there is no coupling between the adsorbate vibrational modes and those of the platinum cluster (surface phonons). This assumption is true for the CO stretching mode, whereas there could be some coupling between less energetic CO vibrational modes and the surface phonons.

Table 2 Structural parameters for different nCO/Pt (25,16) systems; n 41 is the number of CO molecules; z is the PtMC distance in C ˚ ˚ ngstro¨ms. When Angstro¨ms; and d is the CMO distance in A CO n=1, only the centre platinum atom is occupied; for n=2, two opposite corners are occupied; for n=3, two opposite corners and the centre platinum atom are occupied; for n=4, all the corners are occupied; and for n=5, all the corners plus the centre platinum atom are occupied nCO/Pt (25,16) 41

1 2 3 4 5

Centre atom

Corner atom

˚) z (A C

˚) d (A CO

˚) z (A C

˚) d (A CO

1.834 – 1.862 – 1.869

1.156 – 1.155 – 1.156

– 1.856 1.859 1.859 1.862

– 1.154 1.154 1.155 1.154

3. Results and discussion 3.1. Adsorption geometry Reliable quantitative geometrical data for molecular adsorbates on surfaces are difficult to determine experimentally. In contrast, spectroscopic techniques have been widely used to obtain qualitative geometrical information on, for example, the adsorption site. However, some authors have claimed that spectroscopic data, mostly from vibrational spectroscopy, should be used with caution because in some cases the assignment derived from that information fails [28–30]. On the other hand, this lack of quantitative information can be filled up with theoretical studies. We shall briefly discuss the geometrical parameters calculated in this work ( Table 2). Different systems were studied, varying the number of CO molecules ( Fig. 2). The first system contains only one CO molecule, which is adsorbed on the centre atom; we shall refer to it as the low-coverage ˚ system. In this case, we obtained a value of 1.834 A ˚ for the CMO for the PtMC distance and 1.156 A distance. The next system contains two CO molecules, at opposite corners; in this case, the PtMC distance increased slightly compared with the previous system, but the CMO distance remained constant. In the system with three CO molecules, two opposite corners and the centre atom are occupied. However, there is no significant difference between the results for the CO molecules on

the corner atoms and on the centre atom. The fourth system has four CO molecules, each one occupying one corner atom. The last system we studied contains five CO molecules, four of which are at the corners and one of which is on the centre atom; this system reproduces the c(2×2) unit cell observed by LEED and has a coverage of 0.5 ML. From now on, we shall refer to this system as the high-coverage system. These results show that there is no significant change in the adsorption geometry between the low-coverage limit and a value of 0.5 ML. This conclusion would be reasonable if the interaction energy between CO molecules were small in the coverage range considered, in agreement with the microcalorimetric experiments carried out by Yeo et al. [4]. 3.2. Heat of adsorption The heat of adsorption of carbon monoxide was calculated for the low- and high-coverage systems. In the system with only one CO molecule, it was 192 kJ mol−1; this value is in good agreement with that of 215 kJ mol−1 reported by Yeo et al. [4] from microcalorimetric measurements. Yeo et al. observed that the heat of adsorption decreases slightly as CO exposure is raised, and it was about 195 kJ mol−1 when the CO coverage was between 0.25 and 0.5 ML. In the

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Fig. 2. nCO/Pt (25,16) systems. 41

high-coverage system, which has five CO molecules, we obtained a value of 180 kJ mol−1. We also calculated the heat of adsorption when three molecules are considered, and we obtained again a value of 180 kJ mol−1. Hence, the cluster models reproduce the experimental observations, and this shows that our systems are quite good models, despite the limitations of the cluster model approach. In addition, we estimated the CO–CO interaction energy at a coverage of 0.5 ML. We should note that the CO–CO interaction energy does not depend on the inital phase of the clean surface — reconstructed or ideal-truncated, it only depends on the distance to the neighbouring adsorbates and the number of them. In order to calculate the interaction energy between two adjoining CO molecules, we used the following set of equations: E [(CO) Pt (25,16)]=5×E [COPt (25,16)] INT 5 41 INT 41 +4×E (CO–CO), (1) rep E [(CO) Pt (25,16)]=E[(CO) Pt (25,16)] INT 5 41 5 41 −E[Pt (25,16)]−5×E[CO] (2) 41

and [COPt (25,16)]=E[COPt (25,16)] INT 41 41 −E[Pt (25,16)]−E[CO], 41

E

(3)

where E [(CO) Pt (25,16)] is the interaction INT 5 41 energy of five CO molecules with the Pt (25,16) 41 cluster; E [COPt (25,16)] is the interaction INT 41 energy of one CO molecule with the cluster; and E[(CO) Pt (25,16)], E[COPt (25,16)], E[Pt 5 41 41 41 (25,16)] and E[CO] are total energies. Using this set of equations, a repulsion energy of approximately 5 kJ mol−1 was obtained for the CO–CO interaction energy. This result is the same as that reported by Yeo et al. [4] at saturation coverage (0.75 ML). Since our cluster model is intended to simulate the c(2×2) unit cell, which corresponds to a coverage of 0.5 ML, this coincidence means that our calculations slightly overestimate the interaction energy between CO molecules. In the n=3 system, the CO–CO interaction energy is about half that computed for the n=5 system (2.3 kJ mol−1). These results suggest that the

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nature of the CO–CO repulsive energy seems to be purely electrostatic. 3.3. Vibrational analysis In this section we shall focus on the linearbonded CO stretching frequency. Experimental studies have shown that the CO stretching band shifts to higher frequencies as CO exposure increases [8,10]. The band shift has been treated theoretically by using a mathematical model based on adsorbate–adsorbate dipole interactions [11– 14]. However, a second explanation, based on the metal–adsorbate bonding model proposed by Blyholder [15], may also be able to predict the shift upwards of the CO stretching band. As CO exposure increases, the amount of charge backdonated per CO molecule is likely to diminish and then, as the population of the 2p1 molecular orbitals decreases, the CMO bond strengthens and the CO stretching band shifts to higher frequencies. As we have mentioned in the Introduction, we refer to the last model as the chemical effect, contrary to the adsorbate–adsorbate dipole coupling, which is a physical effect. The CO stretching frequency shift for CO chemisorbed on Pt(100) and Pt(111) was studied by Crossley and King [9,10]. In both cases, a shift of approximately 40 cm−1 was observed as the coverage went from zero to saturation. In order to quantify the importance of the adsorbate–adsorbate dipole coupling in the shift upwards, Crossley and King recorded a series of spectra at saturation coverage but changed the 12CO/13CO ratio. They started with a solution of 100% 12CO, and ended with a solution of 8% 12CO and 92% 13CO. During this process, they froze the chemical contribution –

if there is one – to the observed shift, since the coverage remained fixed in all of the spectra recorded. In contrast, adsorbate–adsorbate dipole interactions vary as the 12CO/13CO ratio changes. In these experiments Crossley and King reproduced the shift upwards observed when coverage increases, concluding that the adsorbate–adsorbate dipole coupling was responsible for the band shift upwards and that the 12CO–13CO dipole coupling was weak. In addition, the spectra showed two features above 2000 cm−1. One feature appeared at 2100 cm−1 when a solution of 100% 12CO was used. This band moved to lower frequencies as the percentage of 12CO diminished; they referred to this band as the high-frequency band. The second feature was observed when the composition of the mixture was 50% 12CO, and it appeared in between 2020 and 2040 cm−1. They referred to this second band as the low-frequency band. The low-frequency band gained in intensity at the expense of the high-frequency band as the percentage of 12 CO diminished. Therefore it is clear that the 12CO stretching vibration contributes to the high-frequency band, while the 13CO stretching vibration contributes to the low-frequency band. In order to separate the chemical and physical contributions to the upward band shift, we have copied the procedure carried out by Crossley and King, from a computational point of view. We first calculated the CO stretching frequency in the five systems described above, under Computational details, to determine whether we could reproduce the band shift observed when CO coverage increases ( Table 3). In the low-coverage system, when only one CO molecule is chemisorbed, we obtained a value of 2068 cm−1 for the CO stretching frequency; this result agrees with the one

Table 3 CO stretching frequency as a function of the number of CO molecules chemisorbed on the cluster; n is the CO stretching frequency; Dn is the shift with respect to the previous system. Results are given in wavenumbers n12CO/Pt (25,16) 41

n (cm−1)

Dn (cm−1)

n13CO/Pt (25,16) 41

n (cm−1)

Dn (cm−1)

1 2 3 4 5

2068 2093 2098 2102 2108

– 25 4 4 6

1 2 3 4 5

2020 2044 2049 2052 2058

– 24 5 3 6

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reported in the experimental studies of 2062 cm−1 [8,10]. On the other hand, in the highcoverage system, which contains five CO molecules, we obtained a value of 2108 cm−1 for the symmetric stretching mode; this result is also in good agreement with that reported by Crossley and King [10] at saturation. Hence, we obtain a shift of 40 cm−1, which is in good agreement with experimental studies, and so we can conclude that we have reproduced the observed band shift. At this point, we should point out some important features: (1) our results correspond to harmonic frequencies; (2) the high-coverage system models the c(2×2) unit cell, which has a coverage of 0.5 ML, whereas the saturation coverage reported by Crossley and King is 0.75 ML; and (3) in systems containing more than one CO molecule, there will be more than one CO stretching mode (i.e., if there five CO molecules then there will be five CO stretching modes). However, since we are interested in the band shift, we only discuss the results for the most intense feature, the symmetric stretching frequency ( Fig. 3).

107

Next, we calculated the CO stretching frequency for the other three systems, which have two, three and four CO molecules. Perhaps the most interesting feature is that the most important shift between two successive systems is the one between the low-coverage system and the two-molecules system. In this case the shift was calculated to be 25 cm−1 upwards. The rest of the shifts are smaller, about 4 cm−1. We also analysed the dependence of the 13CO stretching frequency on the number of 13CO molecules ( Table 3), because it will be helpful for later discussion. We obtained a value of 2020 cm−1 in the low-coverage system, and 2058 cm−1 in the high-coverage system. Again, our results are in good agreement with those reported by Crossley and King [10] and, one more, the greatest frequency shift between two successive systems is between the low-coverage and the two-molecules systems. So, the adsorbate–adsorbate dipole interaction seems to be important when the exposure increases initially, but its influence diminishes as the exposure increases further.

Fig. 3. Picture of the CO stretching modes obtained in the high-coverage system. The symmetry point group is C ; both A and E 4v 1 modes are IR-active.

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Fig. 4. Vibrational frequencies for the low- and high-frequency bands as a function of the number of 12CO molecules; for the 13CO stretching frequency and for the 12CO stretching frequency.

In order to model the isotopic experiments, we used a constant number of CO molecules — five, but changed the n12CO/n13CO ratio. This enabled us to study, as Crossley and King did, the effect of dipole coupling on the band shift, since the chemical environment is constant. We started with five 12CO molecules, which we then replaced one by one with a 13CO molecule until we ended up with five 13CO molecules (Fig. 4). In all systems studied, when possible, we calculated both the low-

frequency band and the high-frequency band. In the high-coverage system, when we had five 12CO molecules, we obtained a value of 2108 cm−1 for the high-frequency band, as we have mentioned above; since there were no 13CO molecules, there was only one vibrational frequency ( Table 4). Then we replaced one 12CO molecule by one 13 CO molecule; in this case there were two vibrational features, one at 2105 cm−1 and the other at 2016 cm−1. The former corresponds to the 12CO

Table 4 CO stretching vibrational frequency, for the high- and low-frequency bands, as a function of the 12CO/13CO ratio, at a fixed number of five CO molecules; n is the 12CO stretching frequency; n is the 13CO stretching frequency. Results are given in wavenumbers high low n12CO

n13CO

n (cm−1) high

Dn (cm−1)

n (cm−1) low

Dn (cm−1)

0 1 2 3 4 5

5 4 3 2 1 0

– 2078 2097 2101 2105 2108

– – 19 4 4 3

2058 2054 2050 2046 2016 –

– −4 −4 −4 −30 –

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stretching frequency and the latter to the 13CO stretching frequency. For the system containing three 12CO molecules and two 13CO, again, the 12CO stretching frequency diminished and the 13 CO frequency increased; the calculated frequencies were now 2101 and 2046 cm−1, respectively. At this point, it is noticeble that although the 12CO stretching frequency diminished slowly, there was a large shift in the 13CO stretching frequency. This is exactly the same behaviour as we observed before. In the system with two 12CO and three 13CO molecules, the 12CO stretching frequency was 2097 cm−1 and a frequency of 2050 cm−1 was calculated for the 13CO stretching mode. The system with only one 12CO molecule but four 13 CO molecules had values of 2078 and 2054 cm−1 for the high- and the low-frequency bands. We should point out that the value for the highfrequency band is 10 cm−1 above the value for the one-molecule system. So, it seems that there is some 12CO–13CO dipole coupling. Nevertheless, the 12CO–13CO dipole coupling is relatively weak compared with the 12CO–12CO coupling, which causes a shift of 40 cm−1. Another important feature is that, again, the largest shift is between the low-coverage and the two-molecules systems of the same isotopic species. And finally, when we studied the system with five 13CO molecules, we obtained only one feature at 2058 cm−1. The most interesting results are as follows. (1) We have shown that there is a frequency shift of 40 cm−1 for the CO stretching band and that this shift depends on the number of CO molecules. This shift has been reproduced with a constant number of CO molecules, but varying the n 12CO/n13CO ratio. So it seems that the adsorbate– adsorbate dipole coupling has the most important effect on the band shift within the range considered. (2) The CO stretching frequency was slightly different for the system with only one 12CO molecule and the system with one 12CO molecule surrounded by four 13CO molecules. This difference may be due to two things: there may be some 12CO–13CO dipole coupling, which is weaker than 12CO–12CO coupling, or it may be caused by the different chemical environment. So it is important to know to the extent to which the chemical effect influences the band shift. Finally, (3) we repro-

duced the shift not only for the high-frequency band, but also for the low-frequency band. 3.4. Chemical effects The discussion above shows that adsorbate– adsorbate dipole interaction is the main contribution to the band shift observed as exposure increases. However, we observed a small difference in the CO stretching frequencies for only one 12 CO molecule and for one 12CO surrounded by four 13CO molecules. This difference, of 10 cm−1, may be due to either the 12CO–13CO dipole coupling or changes in the chemical environment; it is also reasonable to assume that part of this difference arises from artefacts in the model. In order to find out if changes in the chemical environment alter the position of the CO stretching band, we analysed the amount of population in the 5s and 2p1 CO molecular orbitals as a function of the number of CO molecules chemisorbed on the cluster. More precisely, we calculated the population of the 5s and the 2p1 orbitals, using the projection technique [17], for the CO molecule directly adsorbed on the centre platinum atom in the low-coverage and high-coverage systems, and also for the intermediate case with three CO molecules ( Table 5). Calculations show that there is no change in the amount of charge donated to the d orbitals of the metal, or backdonated into 2p1 orbitals, for any of the three systems considered; the populations of the 5s and the 2p1 CO molecular orbitals are about 1.5 and 0.7 electrons, respecTable 5 Population analysis of the CO molecule directly adsorbed on the centre platinum atom nCO/Pt (25,16) 41

1s 2s 3s 4s 1p 5s 2p n (cm−1)

n=1

n=3

n=5

2.00 2.00 2.00 2.00 3.99 1.41 0.70 2068

2.00 2.00 2.00 2.00 3.99 1.49 0.67 2102

2.00 2.00 2.00 2.00 3.99 1.51 0.68 2108

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tively. The most important difference is between the populations of the 5s molecular orbital in the low-coverage system and the other two systems. However, this small difference arises from the fact that the adsorption geometry, particularly the PtMC distance, is just a bit shorter for the lowcoverage system than for the others; this shorter distance means that the overlap is a bit larger between the 5s orbital of the CO and the d orbitals of the metal, and this allows a larger charge transfer to the metal. None the less, the difference is small. Therefore, the projection analysis reveals that the chemical effects make no important contribution to the vibrational band shift to higher frequencies as the number of CO molecules is increased. This is clearly shown by the fact that, in terms of charge transfer, there is no change between the different systems studied. Hence, if there is a difference between the 12CO stretching frequency in the low-coverage system and the frequency with one 12CO molecule surrounded by four 13CO molecules, it may come from the 12CO–13CO dipole interaction. Nevertheless, we presume that the 12CO–13CO dipole coupling is weak. However, these results cannot be extrapolated to coverages greater than 0.5 ML. At saturation coverage, the population of the 2p1 molecular orbitals can vary due to geometrical changes. We can make this assumption because: (1) at coverages greater than 0.5 ML, Yeo et al. [4] observed a sharp decrease in the heat of adsorption; and (2) some experimental studies suggest that at high coverage the CO molecules are pushed away from their symmetric adsorption sites [31–33].

CO dipole coupling is the phenomenon responsible for the shift upwards of the CO stretching band when CO coverage increases. The study of the partial occupancy of 2p1 CO molecular orbitals shows that there is no change in the population of these orbitals for any coverage within the coverage range considered. This shows that the frequency shift cannot be due to the chemical effect. Of course, these results do not mean that at any coverage, chemical effects are negligible, or that they should not taken into account with other substrates. We have also quantified the 12CO–13 CO dipole coupling that previous studies have assumed to be weak. Our results demonstrate that 12CO–13CO dipole coupling is weaker than the 12CO–12CO dipole coupling. Nevertheless, we should point out that when one 12CO molecule is surrounded by a 13CO environment, there is a shift upwards of 10 cm−1, in contrast to the calculated value of 40 cm−1 between the low- and highcoverage systems. None the less, we think that part of this difference is due to the limitations of the model, and so we presume that the 12CO–13 CO dipole interaction is weak. We have also computed the heat of adsorption of CO on Pt{100}-(1×1) as a function of CO coverage. The initial heat of adsorption calculated is about 192 kJ mol−1, dropping to 180 kJ mol−1 at a coverage of 0.5 ML. These results agree quite well with the calorimetric measurements carried out by Yeo et al. [4]. Finally, we have also estimated that the CO–CO interaction energy is repulsive and small, about 5 kJ mol−1. It should be pointed out that one of the most important conclusions of the present study is that we can study high-coverage arrangements with the cluster model approach.

4. Conclusions In this study we have analysed the different contributions that can affect the position of the CO stretching band as coverage increases. Two effects may change the CO stretching frequency: the first one is adsorbate–adsorbate dipole coupling, which is a purely physical effect, and the second is population variation of the 2p1 orbitals, which is a chemical effect. Our vibrational analysis clearly shows that CO–

Acknowledgements We are grateful for financial support from the Spanish ‘Ministerio de Educacio´n y Ciencia’, project CICyT PB95-0847-C02-02, and for partial support from ‘Generalitat de Catalunya’, project 97SGR000. D.C. is also grateful to the ‘Generalitat de Catalunya’ for a predoctoral fellowship. We

D. Curulla et al. / Surface Science 460 (2000) 101–111

would also like to thank Professor F. Illas for helpful discussions.

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