W(110) Surface. A Molecular Orbital

The dissociation process for CO adsorbed on Pd/W has been studied using a semiempirical molecular orbital formalism. The behavior of the bimetallic sy...
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Langmuir 1999, 15, 4502-4507

CO Dissociation on the Pd/W(110) Surface. A Molecular Orbital Approach R. M. Ferullo† and N. J. Castellani*,‡ Departamento de Quı´mica e Ingenierı´a Quı´mica, Universidad Nacional del Sur, Av. Alem 1253, 8000 Bahı´a Blanca, Argentina, Planta Piloto de Ingenierı´a Quı´mica (UNS-CONICET), Km 7 Camino de la Carrindanga, 8000 Bahı´a Blanca, Argentina, and Departamento de Fı´sica, Universidad Nacional del Sur, Av. Alem 1253, 8000 Bahı´a Blanca, Argentina Received April 29, 1998. In Final Form: November 30, 1998 The dissociation process for CO adsorbed on Pd/W has been studied using a semiempirical molecular orbital formalism. The behavior of the bimetallic system has been examined comparing it with the pure metal surfaces. The different contributions to the dissociation barrier have been calculated and analyzed according to electronic structure arguments. Over Pd/W the CO dissociation is a very unlikely process, as the experimental evidence demonstrates, due to a lower population of 2π* molecular orbitals in comparison with Pd or W.

Introduction Recently many surface science investigations have been focused on the study of bimetallic systems. The main purpose of these works is to understand the electronic perturbations undergone by metals when they form an alloy or when one of the metals forms an overlayer over the other component. Particularly the systems constituted by a thin film of a LTM (late transition metal) like Pd or Pt, supported on monocrystals of an ETM (early transition metal) like W, Ta, or Nb, have been subject to intensive study. From the electronic point of view it has been observed that the LTM valence band shifts to higher binding energies mainly due to a valence-states rehybridization with a small net electronic transfer between the two components.1 It is well established that at room temperature the CO molecule adsorbs dissociatively over an ETM like W and associatively over a LTM like Pd.2 On the other hand, in comparison with a pure LTM, CO adsorbs weakly over LTM/ETM systems.1 To understand this phenomenon we have recently studied theoretically3 the associative CO adsorption on Pd/W(110) using a semiempirical molecular orbital approach. In that work we concluded that the Pd valence band shift to greater binding energies modifies the local Pd electronic structure in such a way that the hybridization of 2π*(CO) molecular orbital decreases significantly. This lower level of hybridization is accompanied by a lower Pd to 2π* electron back-donation, characteristics of a less bonded CO species. Similar trends were also obtained recently by Pick using a tight-binding self-consistent recursion method for CO adsorption on Pd/ W(110) and other systems.4 In the past the CO dissociation process over pure metals was studied fundamentally using semiempirical molecular orbital methods. We can mention the ASED (atom su* To whom correspondence may be addressed. † Departamento de Quı´mica e Ingenierı´a Quı´mica. ‡ UNS-CONICET and Departamento de Fı´sica. (1) Rodriguez, J. A. Surf. Sci. Rep. 1996, 24, 223. (2) Ishi, S.; Ohno, Y.; Viswanathan, B. J. Sci. Ind. Res. 1987, 46, 541. (3) Ferullo, R. M.; Castellani, N. J. Langmuir 1996, 12, 70. (4) Pick, S. Chem. Phys. Lett. 1995, 239, 84.

perposition and electron delocalization) calculations for Ni(111) and Rh(111)5,6 and the MINDO (modified intermediate neglect of different overlap) results for Fe(100).7 Usually for a given metal the activation energy barriers are calculated considering several possible paths, and then the more likely mechanism can be deduced. The same level of detail has not been reported on ab initio calculations.8 Nevertheless, recently DFT (density functional theory) calculations have been applied to CO on Pt(111) and Ni(111)9 imposing several geometrical constraints. Alternatively the bond order conservation model of Shustorovich has been useful to show a relation between the activation barrier for dissociation and the heat of associative adsorption.10 On the other hand no theoretical works are known to have been performed to study the CO dissociation process on the LTM/ETM bimetallic systems. Here this surface reaction on Pd/W(110) is considered in relation to the dissociative CO adsorption on the pure Pd and W components. Model The cluster approximation and the ASED approach were used to calculate the total energy of systems under study. It is a modified extended Hu¨ckel molecular orbital formalism which includes a interatomic repulsion due to screened cores.11 This repulsion term is calculated in a separated way and allows the determination of equilibrium geometries in adsorbed molecules. For a given geometry of the CO/Mn system (n is the number of atoms of the metal substrate) the adsorption energy Eads was calculated as (5) Van Langeveld, A. D.; de Koster, A.; van Santen R. A. Surf. Sci. 1990, 225, 143. (6) De Koster, A.; van Santen, R. A. J. Vac. Sci. Technol., A 1988, 6, 1128. (7) Blyholder, G.; Lawless, M. Surf. Sci. 1993, 290, 155. (8) Whitten, J. L.; Yang, H. Surf. Sci. Rep. 1996, 218, 55. (9) Morikawa, Y.; Mortensen, J. J.; Hammer, B.; Nørskov, J. K. Surf. Sci. 1997, 386, 67. (10) Bell, A. T.; Shustorovich, E. J. Catal. 1990, 121, 1. (11) Anderson, A. B.; Grimes, R. W.; Hong, S. Y. J. Phys. Chem. 1987, 91, 4245.

10.1021/la980501m CCC: $18.00 © 1999 American Chemical Society Published on Web 05/27/1999

CO Dissociation on the Pd/W(110) Surface

Langmuir, Vol. 15, No. 13, 1999 4503

Figure 1. 42-atom cluster used to describe the (110) face of W, Pd bcc and Pd/W. Only the first and second layers are shown. The nine-point mesh constructed to analyze the dissociation process is also represented here.

Eads ) ∆Ee +

∑Erep

(1)

where ∆Ee is the total valence electron energy of the CO/ Mn system expressed as a difference with respect to the separate fragments CO + Mn. The second term is calculated summing over all pairwise additive repulsions of the system. The repulsion energy between cores A and B can be calculated in two different ways: with the electronic cloud centered on the atom A or on the atom B. In the formulation given by Anderson11 this author considered the electronic cloud centered on the more electronegative atom. On the other hand, Calzaferri12 suggested to use an arithmetic mean between these two situations. Here, and particularly for monatomic adsorption, it was more convenient to employ a different weight for adsorbate or substrate atoms. The atomic parameters employed by the ASED method (ionization potential and Slater’s exponents) come from empirical and theoretical data. The ionization potentials were modified in order to reduce the exaggerated Pd-W and Pd-CO charge transfers as explained in detail in ref 3. The clusters representing the (110) surface of W and Pd/W have 42 atoms distributed in four layers (12 + 9 + 12 + 9 atoms) (Figure 1). The bimetallic system was represented by a Pd12W30 cluster, where we assumed that the Pd overlayer was in epitaxy with the substrate as empirical evidence indicates. For Pd it was important, as it will be shown in the next section, to consider the facecentered cubic (fcc) (100) surface as well as a fictitious surface with the same symmetry as the (110) face of a body-centered cubic (bcc) structure. This Pd surface is labeled as Pd(110) bcc. The main reason to justify this procedure was that we studied a dissociation pathway with a defined geometry and not the best of the ensemble of possible pathways for CO dissociation. Therefore, to make reasonable comparisons of the chemical reactivity between these metallic surfaces, the same mechanism must be tested for the three systems. The cluster corresponding to Pd(110) bcc is that of Figure 1, while that corresponding to the normal Pd(100) fcc is shown in Figure 2. Notice that it also has 42 atoms distributed in four layers. The Pd-Pd and W-W distances are those of the bulk, and for the Pd-W distance we took a mean between the (12) Calzaferri, G.; Forss, L.; Kamber, I. J. Phys. Chem. 1989, 93, 5366.

Figure 2. As in Figure 1 for the (100) face of Pd.

pure metal values.3 Due to the fact that these distances are very close, the Pd42(bcc), W42, and Pd12W30 clusters are very similar from a structural point of view. The reaction path considered for analyzing the dissociation process was the following: We start with the CO molecule perpendicularly adsorbed to the surface over a long-bridge adsorption site (with the C atom closer to the surface). From this geometry the O atom is directed to a neighboring adsorption site, always remaining within a plane normal to the surface. During this process the C-O bond stretches and cracks concluding with the O atom adsorbed in a long-bridge adsorption site. In this way the final C and O separated atoms are located on high coordination sites, in agreement with the experimental evidence showing this type of occupation for monatomic species at low coverages.13 To study this reaction path a mesh with nine points equidistant in 0.34 Å (labeled as 1-4, 6, and 8-11 in Figure 1) was constructed. For each point the C-surface and O-surface distances were optimized. Between points 4 and 6 and 6 and 8, another two points were considered (5 and 7, respectively) because this is a critical zone in which the C-O bond begins to stretch from its initial length. The examination of the electronic structure was accomplished by means of the concept of the local density of states (LDOS) and projected-LDOS, which can be considered as weighted densities of state like those described in ref 3. We also define the first-order momentum of energy corresponding to the molecular orbital µ of the adsorbed CO molecule as follows

Eµ(1) )

∫-∞EfE Fµ(E) dE

(2)

where Fµ(E) is the local density of states. This definition will be useful to take into account the energy distribution of hybrids composing molecular orbital µ. Results and Discussion First we consider the dissociation adsorption on CO/ Pd(110) bcc. The reaction pathway for this system is shown in Figure 3. We observe that during the first steps of CO molecule bending, the C atom remains practically at the same distance from the surface. Afterward, when the C-O stretching process is over the bridge site, this C atom begins to approach closer to the surface. The C-O bond finally collapses when the respective individual atom (13) Mitchell, K. R. Surf. Sci. 1985, 149, 93.

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Figure 3. Schematic representation of the CO dissociation process on Pd(110) bcc. The labels of the abscissa make reference to the points of the mesh in Figure 1.

Figure 4. Energetic changes for the CO dissociation process on Pd(110) bcc. The energies are expressed relative to CO adsorbed as a molecular species. The labels of the abscissa make reference to the points of the mesh in Figure 1. (1) ∆Eads; (2) ∆Eeads; (3) ∆Erads.

resides on neighboring high coordination sites. Similar reaction pathways can be obtained for CO/W(110) and CO/Pd/W(110) systems. The adsorption energy corresponding to the different dissociation steps is shown in Figure 4. Its value has been expressed relative to CO adsorbed as a molecular species and named as ∆Eads. The two components of ∆Eads, i.e., ∆Eeads and ∆Erads, make reference respectively to ∆Ee and ∑Erep terms in eq 1. Furthermore, as a consequence of C-O bond breaking, the oxygen atom carries out an excess of electronic negative charge. The corresponding electron-electron repulsion makes an important contribution to the total electronic energy, nevertheless normally it is not included in the standard EH calculations. Here, to incorporate such effect, a procedure similar to that used in the past for the H-W, H-Ni, and H-Pt interactions14-16 was followed. It consists of the addition of a phenomenological term of the U∆Q form to the total energy of the system. ∆Q is the electron charge excess for the oxygen atom with respect to some (14) Piccitto, G.; Firingo, S.; Baldo, M.; Pucci, R. Surf. Sci. 1986, 167, 437. (15) Ferullo, R. M., Castellani, N. J. J. Alloys Compd. 1993, 191, 173. (16) Castellani N. J.; Le´gare´, P.; Demangeat, C.; Pick, S. Surf. Sci. 1996, 352-354, 148.

Ferullo and Castellani

Figure 5. As in Figure 4 for the CO dissociation process on W(110).

electronic configuration taken as a reference, and U is a semiempirical constant to be adjusted with other source of information. In the present case the ab initio calculations performed by Bagus17 for the diatomic OPd molecule at the equilibrium distance (with Q(O) ) -0.7 electron) were our reference, giving U ) 2.6 eV‚electron-1. Notice that while the repulsive contribution to ∆Eads shows a minimum at step 6 that corresponds to the decrease of C-O interatomic repulsion due to the C-O stretching just over the Pd-Pd bridge, the electronic contribution presents the typical activation barrier showing evidence of two competitive electronic interactions: the C-O bond is cracking (an increase of Eads) and the O-substate bond is forming (a decrease of Eads). The crossover of the two components of ∆Eads at step 8, after the O atom moves through the Pd-Pd bridge, is also remarkable. While the repulsive component remains without significant changes in the 8-11 interval, the electronic one stabilizes significantly giving a 1.5 eV gain of energy. In this way the dissociation barrier is controlled fundamentally by the electronic component, resulting in a transition state placed near the maximum of the ∆Eeads curve. When this situation is reached the C-O distance increases up to 57% of the value for the perpendicular geometry. Several interesting comparisons can be made with the ab initio results of Morikawa et al.9 for CO on Ni(111) and Pt(111), two metals in the same column in the periodic table. These authors obtained, depending on the dissociation pathway, a dissociation barrier of 2.9-3.2 eV for CO/ Pt(111) and of 1.5-2.4 eV for CO/Ni(111). Our result of 2.72 eV for CO/Pd(110) bcc is between these values, in agreement with the experimental CO dissociation trends. On the other hand the CO distances reported in ref 9 for the transition state of CO on Pt or Ni are ∼80% greater than that corresponding to free CO, while in our calculation the transition state species is ∼60% longer. As a consequence, the atomic states of C and O obtained in ref 9 are more similar to the states of dissociated CO, with C and O atoms as separated units, while our 5σ or 2π* projectedLDOS of CO can be more easily related to the molecular states of free CO molecule (see later). Figure 5 shows the dissociation energy curves for CO/ W(110). Notice that the general profiles of repulsive and electronic contributions to ∆Eads are similar to CO/ Pd(17) Bauschlicher, C. W.; Nelin, C. J.; Bagus, P. S. J. Chem. Phys. 1985, 82, 3265.

CO Dissociation on the Pd/W(110) Surface

Langmuir, Vol. 15, No. 13, 1999 4505

Table 1. Maximum Values of ∆Eads, ∆Eeads, and ∆Erads (eV) during the CO Dissociation Process ∆Eads ∆Eeads ∆Erads

Pd(110) bcc

Pd(100)

W(110)

Pd/W(110)

2.72 2.00 2.93

1.97 1.14 2.51

1.92 0.49 2.57

5.73 5.40 1.79

(110) bcc. The main difference is the greater stabilization of the electronic component. The crossover of ∆Eeads and ∆Erads is not present, and both the electronic and total dissociation barriers are ∼1 eV lower in comparison with Pd. Hence, this dissociation process is noticeably easier on W than on Pd. This observation can be appreciated in Table 1, where the ∆Eeads, ∆Erads, and ∆Eads maxima are summarized for the transition state region. Moreover the minimum of the repulsive component at step 6 is less deep than for CO/Pd(110) bcc due to the smaller O-metal repulsion. At the maximum of ∆Eads the C-O attains a value ∼55% longer than that for the initial perpendicular geometry. The easier CO bond cleavage on a W surface in comparison with a Pd surface can be adscribed to the d-band occupation of W with respect to Pd. Indeed, Anderson claimed that when CO adsorbs in a side-on orientation over the late transition metals the antibonding states formed between the atomic d-orbitals and the π molecular levels are more occupied than the same hybrids on early transition metals.18 Therefore if a CO molecule bends down over a LTM the bonding between the molecule and metal weakens and an important activation barrier is produced. At the same time, Hoffmann showed that when CO adsorbs with its molecular axis normally to the surface, the 2π* occupation decreases along the 3d transition metal series.19 This fact is due to a fall of the Fermi level and the difusseness of d-orbitals, making the CO molecule more able to be dissociated. The results corresponding to CO on Pd(100) are shown in Figure 6. Notice in comparison with Figure 4 that for this geometry the minimum of the repulsive component (now at step 5) is much less smooth and the subsequent repulsive barrier much wider than those on the bcc symmetry. Furthermore the barrier due to the electronic contribution is centered more to the left, with its falling down at step 6, resulting a relatively lower and narrower energy barrier. The reason for this behavior is that the C atom approaches its final site earlier in comparison with the other mechanism and the C-O bond stretches more suddenly between steps 5 and 6. The net dissociation barrier is smaller than that on the bcc symmetry and close to the case of W(110). However looking at Table 1, we observe that the maximum of the electronic component for CO on Pd(100) remains 0.65 eV higher than that corresponding to CO on W(110). It must be underlined, regarding the initial molecular (i.e., not dissociative) state of CO, that the Eads values and equilibrium properties obtained for the fictitious Pd(110) bcc surface are almost the same as those corresponding to the (100) fcc surface. The dissociation over the Pd/W(110) surface can be studied in Figure 7 and Table 1. We note there the very important dissociation barrier in comparison with pure Pd in Figure 4 (nearly 100% greater, see first row in Table 1). Here at the maximum of ∆Eads the C-O distance is ∼90% greater than the value for the perpendicular geometry. Moreover the final dissociated state is notably unstable (∆Eads ∼ 4 eV greater). This result is in agreement with the experimentation: the UV photoelectron spectra (18) Anderson, A. B.; Dowd, D. Q. J. Phys. Chem. 1987, 91, 869. (19) Sung, S.; Hoffmann, R. J. Am. Chem. Soc. 1985, 107, 578.

Figure 6. As in Figure 4 for the CO dissociation process on Pd(100). The labels of the abscissa make reference to the points of the mesh in Figure 2.

Figure 7. As in Figure 4 for the CO dissociation process on Pd/W(110).

of CO/Pd/W(110) show only the presence of molecular CO with any traces of C or O atoms.20,21 Related to this we mention that the thermal desorption spectra obtained by Goodman22 for CO/Pd/W(110) show CO desorption maxima in a temperature range approximately 400 K below that corresponding to the β-CO states for dissociated CO on pure W(110) and W(100).23 Moreover the photoelectron valence band measured by Gomer24 for CO adsorbed on submonolayer Pd deposits on W reveals the presence of both R (associated) and β (dissociated) states. The 5σ-4π states of R-CO become progressively more important as the Pd coverage on W increases. Looking in detail at the two components of ∆Eads, we notice that their profiles are different in comparison with the corresponding components over pure Pd. Indeed, the repulsive contribution shows a deeper minimum at step 6 and a much lower final value at step 11. On the other hand the electronic component is always positive with a large repulsion barrier centered around step 7 (∆Eeads ∼ 3.4 eV greater, see second row in Table 1). While the behavior of ∆Erads can be (20) Ruckman, M. W.; Johnson P. D.; Strongin, M. Phys. Rev. B 1985, 31, 3405. (21) Ruckman, M. W.; Strongin, M. Acc. Chem. Res. 1994, 27, 250. (22) Berlowitz, P. J.; Goodman, D. W. Langmuir 1988, 4, 1091. (23) Wang, C.; Gomer, R. Surf. Sci. 1979, 90, 10. (24) Zhao, Y. B.; Gomer, R. Surf. Sci. 1990, 239, 189.

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Ferullo and Castellani Table 2. Integrated Values for the Projected LDOS ()pop) of CO Molecular Orbitals for the Perpendicular Adsorption (Step 1) and at the Transition State Region (Step 7)a pop (CO-5σ) step 1 pop (CO-5σ) step 7 ∆pop (CO-5σ) pop (CO-2π*) step 1 pop (CO-2π*) step 7 ∆pop (CO-2π*) OP (C-O) step 1 OP (C-O) step 7 ∆OP

Pd(110) bcc

W(110)

Pd/W(110)

1.803 1.856 +0.053 1.338 2.360 +1.022 1.399 0.570 -0.829

1.759 1.753 -0.006 1.169 2.356 +1.187 1.486 0.546 -0.940

1.738 1.854 +0.116 0.626 1.436 +0.810 1.606 0.702 -0.904

a The C-O overlap population values for both geometries are also reported. ∆ refers to the changes between these two geometries.

Figure 8. Projected local density of states (projected-LDOS) corresponding to the 2π* (long-dashed line) and 5σ hybrids (short-dashed line) of CO/Pd(110) bcc at step 7 and LDOS at the metal atom corresponding to the shortest metal-C bond (continuous line). Energy values are referred to the Fermi level.

Figure 9. As in Figure 8 for CO/W(110) at step 7.

Figure 10. As in Figure 8 for CO/Pd/W(110) at step 7.

explained simply considering the greater C-Pd or O-Pd distances (∼0.2-0.3 Å greater in steps 4-8) the profile of ∆Eeads is due to the presence of a very weak bonding of CO with the surface during the overall C-O bond stretching. This last observation can be understood using electronic structure ideas, as will be shown later. The different behaviors observed in the dissociation process of CO on Pd, W, and Pd/W can be analyzed more deeply in terms of the electronic structure. For that purpose in Figures 8-10 the projected-LDOS for 5σ and 2π* molecular orbitals of CO at step 7 (near the maximum of ∆Eeads, in the transition state region) are shown. The LDOS of the nearest metallic atom is also exhibited in the same figures. Notice the much higher value of LDOS (EF) for the Pd surface atoms in comparison with surface

metallic atoms of W and Pd/W systems. Furthermore it is also remarkable that for the three metallic systems the 2π* hybrids are more spread in energy than the 5σ ones. This observation is due to the fact that 5σ molecular orbitals are more localized and that for this geometry 2π* states undergo a very important hybridization. Moreover, in general, bonding hybrids between 2π* and metallic orbitals for this quasi-end-on geometry over Pd, W, or Pd/W surfaces are nearly 1 eV (with respect to EF) more stable than those for the perpendicular orientation (see ref 3). The existence of an activation barrier for the C-O bond is related to the destabilization of the molecule when 2π* orbitals are populated. This phenomenon can be observed in Table 2 where the integrated values for the projectedLDOS of CO molecular orbitals at step 7 (near the maximum of ∆Eeads, in the transition state region) are summarized together with their changes from the perpendicular geometry. For all the situations considered a noticeable increase of 2π* (a CO antibonding molecular orbital) population is observed. Moreover, in the same table, the values of the C-O overlap population are reported. Notice that a weakness of the C-O bond can be appreciated at step 7. If, now, the variations corresponding to the CO/Pd and CO/W systems are compared in a relative way, we could infer that the CO molecule undergoes a much greater destabilization (i.e., an increase of 2π* population) for CO on W, in agreement with a lower electronic barrier. On the other hand, for the CO/Pd/W system, this destabilization is much smaller than for CO on Pd or on W. Furthermore, the 5σ population is slightly greater. This whole picture is consistent with an activation electronic barrier much higher for the bimetallic system in comparison with the pure metals. The different behavior of CO/Pd, CO/W, and CO/Pd/W systems could also be analyzed by comparing the orbital populations corresponding at step 7. In this way, if the pure metals Pd and W are compared, we will infer that while the 2π* population is nearly the same, the 5σ is ∼ 5% lower on W. Notwithstanding the fact that the second of these results is in agreement with the concept of a weaker C-O bond, the important aspect to underline here is the nonmodification of the 2π* population. As that Pd and W do not belong to the same transition series and W is not in the very beginning of its proper series, it is not surprising that a direct comparison of 2π* populations does not give a correlation with the height of the dissociation barrier. It is better to check the change of this population, as mentioned in a preceding paragraph. On the other hand, notice that the 2π* population on Pd/W is 40% lower than that on Pd or W, while the 5σ one is very close to that obtained on Pd. The first of these results is

CO Dissociation on the Pd/W(110) Surface

Langmuir, Vol. 15, No. 13, 1999 4507

Table 3. First-Order Momenta Corresponding to 5σ and 2π* Orbitals of CO (in eV per Unit of Electron Population and Taking the Fermi Level as the Binding Energy Reference) E(1)5σ E(1)2π*

Pd(110) bcc

W(110)

Pd/W(110)

-3.238 -0.503

-3.313 -1.893

-2.625 -1.789

an indication of a C-O bond remarkably stronger and a less perturbed molecule. Looking at Figures 8 and 9, the projected-LDOS curves for 5σ and 2π* hybrids, we can observe that the former do not undergo any important modification when the CO/ Pd system is compared with CO/W. However, the 2π* orbitals for CO/W seem to be somewhat more stabilized. Moreover, taking into account the shape of projected-LDOS profiles, it is difficult to compare Pd and W only in terms of the width of the 2π* distribution. To quantify these effects the first-order momenta of the energy distribution corresponding to these orbitals are computed (see eq 2). The corresponding values are shown in Table 3, expressed in electronvolts per unit of electronic population and taking the Fermi level as the binding energy reference. We can appreciate that both E(1)5σ and E(1)2π* values for CO on W are stabilized with respect to Pd, this effect being nevertheless much more important for the second than for the first molecular orbital (near 1.39 and 0.07 eV, respectively). Then, from these considerations, we infer that it is not sufficient to perform an analysis of electronic structure changes residing only on the comparison of molecular orbital populations when they are close and, at the same time, the distribution in energy of fragment

hybrids shows a noticeable modification. Regarding the CO/Pd/W system in Figure 10, the 5σ distribution is destabilized and the 2π* one is stabilized. The last observation is in agreement with the above-reported results for the non dissociative CO adsorption where we showed that the 2π* distribution moved to higher binding energies due to the shift of the Pd 4d-band (see Ref 3). We can state that what actually moves is the tail of this distribution and that in this case the decrease of 2π* hybrid population is essential in order to explain the weak COmetal surface interaction. Conclusions In this work several aspects of CO dissociation on transition metal surfaces have been theoretically studied. We could establish that for a similar geometrical symmetry a late transition metal like Pd exhibits a greater dissociation energy barrier than an early transition metal like W. On the other hand the dissociation process on the Pd/W system has an important barrier, making this process very improbable. Of the two components in which these barriers have been divided, electronic and repulsive, the first one defines the general profile of the calculated total energy differences. The behavior of the electronic component, in its turn, can be accounted for by considering the change in population of 2π* hybrids. The formation of the barrier is accompanied by an increase of this population according to the following sequence: CO/W > CO/Pd > CO/Pd/W. It is also essential to perform a careful analysis of the distribution energy of these hybrids if the transition states are directly compared. LA980501M