CO adsorption on Rh, Pd and Ag atoms deposited on the MgO surface: a comparative ab initio study

Surface Science 540 (2003) 63–75 www.elsevier.com/locate/susc

CO adsorption on Rh, Pd and Ag atoms deposited on the MgO surface: a comparative ab initio study Livia Giordano a, Annalisa Del Vitto a, Gianfranco Pacchioni Anna Maria Ferrari b a

a,*

,

Dipartimento di Scienza dei Materiali, Istituto Nazionale per la Fisica della Materia, Universit
a di Milano-Bicocca, via R. Cozzi 53, 20125 Milan, Italy b Dipartimento di Chimica IFM, Universit
a di Torino, via P. Giuria 5, I-10125 Torino, Italy Received 11 April 2003; accepted for publication 14 May 2003

Abstract The adsorption properties of CO molecules adsorbed on Rh, Pd, and Ag atoms supported on various sites of the MgO surface have been studied by means of a density functional cluster model approach. The metal atoms are stabilized with different binding energies on the regular and morphological defect sites of the surface. Among others we considered oxide anions, neutral and charged anion vacancies (F centers) located at terraces, steps, edges, and corners. CO is used as a probe molecule to characterize where the metal atoms are located. This is done by analyzing how the metal–CO binding energy and the C–O stretching frequency change as function of the substrate site where the metal atom is bound.
2003 Elsevier B.V. All rights reserved. Keywords: Magnesium oxides; Ab initio quantum chemical methods and calculations; Rhodium; Palladium; Silver; Chemisorption; Carbon monoxide

1. Introduction An emerging field in heterogeneous catalysis is that of supported nanoparticles, often referred to as nanocatalysis or nanoscience in catalysis [1–3]. Catalytically active transition metal particles are dispersed on an inert oxide substrate in order to maximize the surface exposed to the reactants and to increase the catalytic efficiency [4,5]. In recent * Corresponding author. Tel.: +39-026448-5219; fax: +39026448-6400. E-mail address: [email protected] (G. Pacchioni).

years, new techniques have been developed which allow the deposition of mass-selected metal clusters and even single metal atoms on oxide thin films supported on metals [2,6]. The resulting nanostructured materials display very peculiar properties [1,7,8] and represent excellent model systems to investigate reactions at the atomic scale. For example, single Pd atoms, inactive in the gasphase, promote the acetylene trimerization to benzene when deposited on the MgO(1 0 0) surface [9]. It has also been shown that the change in the cluster size by a even single metal atom can modify the chemical properties of the cluster [9–11]. When the particle size becomes of atomic scale, the

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2003 Elsevier B.V. All rights reserved. doi:10.1016/S0039-6028(03)00737-4

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L. Giordano et al. / Surface Science 540 (2003) 63–75

support cannot be considered inert anymore: there is increasing evidence that this can modify the electronic structure of the deposited metal as well as the structure of the particles [9,12]. It is now quite clear that metal atoms and clusters prefer to bind to surface irregularities (extended defects, domain boundaries and point defects). In general these defects play a twofold role. On one side they act as trapping sites and nucleation centers for metal atoms and clusters, as pointed out by several experimental studies [4,13,14]. On the other side, they can modify the oxide–metal interaction and, as a consequence, the catalytic activity of the metal unit [9,11,15–18]. In this context, modern microscopies like scanning tunnelling (STM) and atomic force microscopies (AFM), have confirmed the role of the line and point defects in determining cluster growth [13,19]. Still, our understanding of the nature of these defects is quite low and can be achieved only through indirect information. Only few examples of direct identification of the nature of point defects at the surface of oxide thin films have been reported [20]. A powerful tool to get information on the site where the metal atoms are bound is the use of probe molecules. Among these CO is certainly the most popular because of its low reactivity and high sensitivity of the C–O stretching frequency to changes in the electronic structure of the metal atom to which CO is bound [14,21–23]. The comparison of the calculated and experimental CO binding energy and vibrational frequency for the case of Pd atoms adsorbed on MgO [21] has elucidated the role of surface defects and in particular of the oxygen vacancies. On the other hand, this comparison has left some open questions. The study of different transition metal atoms belonging to the same period but having a completely different valence structure, i.e. Rh(4d8 5s1 ), Pd(4d10 5s0 ), and Ag(4d10 5s1 ) can provide new information on the substrate–metal bonding, thus helping in the identification of the surface defects where the metal atoms are stabilized. Rh, Pd and Ag are representative of various transition metal atoms with unfilled d shell (Rh), with complete d shell (Pd) and with complete d shell and partially filled s shell (Ag). In this paper we present results on the adsorption properties of Rh, Pd and Ag atoms adsorbed

on different sites of the MgO(1 0 0) surface, and the characteristics of the CO adsorption on these supported metal atoms. In a separate paper the temperature desorption spectra (TDS) and Fourier-transform infra-red (FT-IR) experimental results for the same systems, CO/Rh1 /MgO, CO/Pd1 / MgO, and CO/Ag1 /MgO, have been reported and interpreted based on the results of ab initio calculations [24]. In Ref. [24], however, only some general features of the calculations have been discussed, in particular CO binding energies and vibrations for a small number of MgO adsorption sites (mainly located at the MgO terrace). No details about geometry and electronic structure were reported, and no results about vacancies and other defects at low-coordinated sites were discussed. Here we provide a complete account of the geometric, energetic, and vibrational data as well as a deeper analysis of the electronic structure of the CO/M/MgO surface complexes (M ¼ Rh, Pd, Ag).

2. Computational techniques The metal and CO adsorption energies and the CO vibrational frequencies have been calculated within the gradient-corrected density functional theory. The hybrid Becke3 functional for exchange [25] and the Lee–Yang–Parr functional for correlation [26] (B3LYP) were employed. For sake of completeness, we mention here that it has been shown recently that the adhesion of metal layers on oxide surfaces where the bonding is dominated by polarization mechanisms is better described at the local density approximation, LDA, level than with the generalized gradient approximation, GGA [27] (see also Ref. [28] for a discussion of the problem). This conclusion, valid for extended metal overlayers, does not apply to strong covalent bonds as for metal atoms bound at the F centers of MgO. Here, GGA-DFT approaches, and in particular the B3LYP functional, seem to provide sufficiently accurate answers. The MgO surface has been modeled using an embedded cluster approach [29]. A large matrix of point charges (PC ¼ ±2) placed at the lattice positions around the cluster has been used in order to include the long-range Madelung field. The posi-

L. Giordano et al. / Surface Science 540 (2003) 63–75

tive charges at the cluster borders have been replaced by effective core potentials (ECP), which take the finite size of the Mg2þ core into account, in order to prevent the polarization of the oxygen atoms at the cluster border [30]. Different adsorption sites were represented by the following clusters: O13 Mg13 for terrace (O5c ), O10 Mg10 for edge (O4c ), O12 Mg12 for step (O4c –O5c ), and O7 Mg3 for corner (O3c ), Fig. 1. For each case we have considered metal adsorption on the oxide anion and on the corresponding neutral and charged oxygen vacancies, Fnc and Fþ nc , respectively (here nc indicates the coordination number of the oxide anion which has been removed to create the vacancy). For each M/MgO surface complex we have considered the possible adsorption of one, two, and even three CO molecules, (CO)m /M/MgO. The Kohn–Sham orbitals have been expanded on a Gaussian-type atomic orbital basis set. The basis set used for the Mg and O ions of the substrate cluster is the 6-31G [31]; when a O atom is removed to form a F center one has to adopt a sufficiently flexible basis set to describe the electron localization in the vacancy. This can be achieved

65

by adding floating functions at the cavity center or by using a more diffuse basis set on the neighboring Mg ions [32]. Here we adopted this second strategy and we used for the Mg ions around the vacancy a 6-31 + G basis set. The Ag, Pd, and Rh atoms were treated by an 18-electrons ECP, which includes in the valence the 4s2 4p6 4dn 5sm electrons; the basis set is of double-zeta plus polarization type [33]. For CO we used a 6-311G* basis (test calculations show that the use of the larger 6311 + G(2d,2p) basis for CO does not change the properties in a significant way). Within this approach the CO harmonic frequency x0 is 2221 cm
1 , to be compared with the experimental value of 2170 cm
1 [34]. Tests on the adequacy of this computational approach can been found in Ref. [21]. The binding energies, De , of the metal atoms to MgO have been corrected for the basis set superposition error (BSSE) [35]. No correction has been applied to the CO dissociation energies and all the corresponding De values are somewhat overestimated because of this effect; this error has been estimated in a previous paper for the case of Pd and it amounts to 0.2–0.3 eV.

Fig. 1. Structure of the clusters used to simulate the MgO surface: O13 Mg13 for terrace (O5c ), O10 Mg10 for edge (O4c ), O7 Mg3 for corner (O3c ) and O12 Mg12 for step (O4c –O5c ). White spheres: O; dark spheres: Mg; small dark spheres: ECP.

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The positions of the CO molecule, of the metal atoms and of the first, second and third neighbors (metal on oxide anions) or of the first and second neighbors (metals on F centers) have been fully optimized using analytical gradients. In the tables , are reported with two the optimal distances, in A decimal digits except for CO where the equilibrium distance is given with three digits. This is because small changes in the CO re reflect in non-negligible changes in the CO xe . For CO adsorbed normally to the surface the vibrational frequency has been calculated by fitting the potential energy curve with a fifth-degree polynomial, while for more complex adsorption geometries a full vibrational analysis has been performed. All calculations have been performed with the Gaussian98 program package [36].

3. Results 3.1. Metal atoms on MgO The preferred adsorption site for metal adsorption on the regular MgO surface is on top of the O2
anions, as found by several theoretical studies [37,38] and found experimentally for the case of Ag films on MgO [39] (here however one should mention that coverage effects can affect the ad sorption site). Rh adsorbs at an height of 2.26 A from the surface plane, with a binding energy of 0.97 eV, Table 1. The Rh atomic ground state is a 4 F(4d8 5s1 ). When adsorbed on the O5c site the interaction with the surface induces a quenching of the magnetic moment which results in a doublet ground state, separated by 0.36 eV from the lowest quartet. In the ground state the unpaired electron is completely localized on the 4d shell of the Rh atom. The interaction of a Pd atom with the surface anion site is virtually the same as for Rh, 0.96 eV, while Ag has a considerably weaker interaction with the surface (0.48 eV), and exhibits a longer adsorption height. The main contributions to the metal–MgO bonding are the polarization of the metal electrons induced by the ionic substrate and the small mixing between the s and d orbitals of the transition metal with the 2p orbitals of the surface oxygen

[40,41]. For Ag, the reduced strength of the interaction is due to the large Pauli repulsion arising from the electron in the diffuse 5s orbital. For Rh, the possibility to promote the 5s electron into the 4d shell results in a short bond distance and a strong interaction, and explains the change in magnetic moment of the adsorbed atom. For all three metals the charge transfer between the substrate and the adsorbate is very small [41]. In general, the interaction with the substrate increases by reducing the coordination of the Onc anion, by 0.1–0.5 eV passing from 5c to 4c oxide anions, and by about the same amount going from 4c to 3c sites, Table 1. A stronger interaction is observed when the metal atom can interact with more than one oxygen atom, as is the case for line defects like a step, Fig. 2a. In this case, the adsorption energy of Rh, 1.95 eV, is about 1 eV stronger than on the O5c terrace sites; for Pd the gain is of about 0.5 eV (the adsorption energy at a step is 1.41 eV). The same effect has been found for the steps of the a-Al2 O3 surface [42]. This can explain the step decoration observed by STM for Pd and Rh atoms on a-Al2 O3 [22] and for Pd on MgO observed by AFM [13]. For Ag, the interaction at steps is only slightly larger than on the terrace sites (0.67 eV instead of 0.48 eV, Table 1). An interesting and important difference between Ag and Pd on one side and Rh on the other side is the bonding mode with a step site. On a step, a metal atom can in principle interact with two Onc anions, an O4c along the step and an O5c on the terrace underneath, Fig. 2a. While Ag and Pd assume a configuration which clearly indicates that the metal interacts only with the O4c ion, Fig. 2b, Rh acts as a bidentate ligand and interacts simultaneously with both anions. The strong bonding of Rh to a step can explain the different diffusion mechanisms observed for Rh and Pd on the MgO surface [24]. We consider now the interaction of the metal atoms with a neutral oxygen vacancy. It has been shown in several studies that these are the most reactive sites on the MgO surface. The presence of the cavity on the surface allows the atoms to closely approach the surface; the adsorption heights . The hybridization of the are reduced by 0.5–0.7 A metal orbitals with the vacancy level induces a

Table 1 Adsorption properties of Rh, Pd and Ag atoms on various sites of the MgO surface: O anions, F and Fþ centers located at terrace (5c), edge (4c), corner (3c), and step (4c–5c) sites O5c

Edge F5c

Fþ 5c

O4c

Corner F4c

Fþ 4c

O3c

Step F3c

Fþ 3c

O4c –O5c

Cluster

O13 Mg13

O10 Mg10

O7 Mg3

O12 Mg12

Symmetry

C4v

C2v

C3v

C1

Rh Electronic state ) d(Rh–S)a (A De b (eV)

2 B2 2.15 0.97

2 B2 2.74 3.21

3 B1 2.83 2.10

2 A1 2.03 1.48

2 B1 2.61 3.28

3 A2 2.77 2.26

2 A1 1.97 1.58

2 A1 2.75 3.22

3 A2 3.00 2.45

2 A 2.08 1.95

Pd Electronic state ) d(Pd–S),a (A De b (eV)

1 A1 2.15 0.96

1 A1 2.67 3.42

2 A1 2.72 2.10

1 A1 2.05 1.35

1 A1 2.57 3.64

2 A1 2.65 2.41

1 A1 2.00 1.51

1 A1 2.69 3.66

2 E 2.77 2.35

1 A 2.06 1.41

Ag Electronic state ) d(Ag–S),a (A De b (eV)

2 A1 2.39 0.48

2 E 2.96 1.54

1 A1 2.89 2.02

2 A1 2.27 0.60

2 B2 2.67 1.62

1 A1 2.77 2.42

2 A1 2.20 1.04

2 E 2.85 1.82

1 A1 2.92 2.54

2 A 2.29 0.67

a b

L. Giordano et al. / Surface Science 540 (2003) 63–75

Terrace

Shortest distance between the metal atom and the substrate atoms S (O for terrace, edge, corner and step, Mg for F centers). Computed as E(MgO) + E(M) ) E(M/MgO), and corrected for the BSSE.

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L. Giordano et al. / Surface Science 540 (2003) 63–75

Fig. 2. Schematic view of M atoms and MCO or M(CO)2 complexes adsorbed on various sites of the MgO surface. (a) and (b) O4c –O5c at step site; (c) O5c at terrace site; (d) Fnþ 5c center at a terrace; (e) O5c at terrace site; (f) and (g) O4c –O5c at step site. White spheres: oxygen; black sphere: Mg; large gray sphere: M ¼ Rh or Pd; small gray and white spheres: C and O of the CO molecule.

stronger orbital mixing between adsorbate and substrate with partial charge transfer to the metal [41,43,44]. The strength of the interaction of the metal atoms with Fnc centers follows the same trend found for bonding on the regular terrace sites, Rh  Pd  Ag. An important effect of the bonding of the metal atoms to surface F centers is that the electrons trapped in the vacancy are partly delocalized over the metal atom (in particular they are transferred to the 5s orbital). While a quantitative measure of this charge transfer depends significantly on the procedure used, there is general consensus that the charge transfer is quite substantial, of the order of 0.5–1 electrons per metal atom [41,43]. It is not surprising that the electronic

properties of the adsorbed metal atom are strongly affected by this charge transfer. The bonding properties of CO to these centers should reflect the change in electron density of the atom. Considering the Fþ S center, the results are less adsorbate-dependent, with an adsorption energy which goes from 2 to 3 eV depending on the metal atom and on the location of the Fþ center (terrace, edge, or corner). The strong bonding, similar for the three metals, has different origins. In the case of Ag there is a direct coupling of the unpaired electron in the cavity and the 5s electron of Ag; for Rh the ground state is a triplet, with one unpaired electron localized in the 4d shell of the metal atom and the second delocalized over the 5s and the

L. Giordano et al. / Surface Science 540 (2003) 63–75

vacancy states; for Pd the ground state is a doublet and the unpaired electron is shared between the adsorbate and the cavity. In all cases the positive charge associated to the defect leads to an additional contribution to the bonding from the metal polarization. The strength of the bond of Rh, Pd, and Ag to Fþ nc centers is intermediate between that found for surface oxide anions and neutral Fnc centers, Table 1. We have also considered the adsorption energy of a metal atom adsorbed in a bridge position between two surface oxygens of the regular (1 0 0) surface. In all cases the energy is higher than for the adsorption on-top of the oxide anion in agreement with previous studies [45]. The bridge sites are therefore saddle points in the diffusion of the metal atoms on the surface and there is no evidence that they can stabilize the adsorbed metal atoms. For this reason they are not considered in the analysis of the CO adsorption. 3.2. CO adsorption on supported metal atoms 3.2.1. CO binding energy We have considered first the adsorption properties of a CO molecule adsorbed with the C-end to the metal atom and the molecular axis normal to the surface plane, Fig. 2c and Table 2. Some results for Pd have been already reported in Ref. [21], and are shown here for comparative purposes. As mentioned above, the calculated C–O harmonic frequency is 2221 cm
1 . This value differs from the experimental harmonic value of 2170 cm
1 by about 50 cm
1 . For this reason, we have chosen to scale the CO frequencies by a factor 2170/2221 ¼ 0.977. With this approach, the computed stretching frequencies for the RhCO, RhCOþ , PdCO and PdCOþ complexes are very close to the experimental values obtained in rare matrices [24]. This shows that the chemical bonding between a metal atom and CO is well described in the present computational scheme. On the other hand, when deposited on the MgO surface, the M– CO complexes exhibit a CO xe which may be slightly red-shifted because of artifacts in the calculation. In particular, the non-uniform electric field generated by the PCÕs induces a red-shift of

69

30–40 cm
1 in the CO computed frequencies. In this respect, the vibrational frequencies reported in Table 2 could be 30–40 cm
1 too low. Stated differently, it is possible that the scaling factor adopted for the MCO molecular complexes is not optimal to describe vibrations of molecules on clusters embedded in PCs. In this respect it is worth mentioning that it has been suggested that the vibrational properties of molecules adsorbed on the MgO surface are better described by bare MgO clusters with no embedding in PCs [46]. This approach, however, is totally unsuited to treat electron trapping sites as the F centers at the MgO surface [47]. Some general trends can be deduced from the analysis of the results of Table 2. The calculations have been performed within C4v , C2v , or C3v symmetry constraint for terrace, edge and corner respectively, whereas no symmetry constraint has been imposed for adsorption on the step site. In some selected cases these constraints have been removed and we observed a tendency of the CO molecule to assume a tilted configuration, as discussed in more detail below. Ag1 /MgO does not exhibit any tendency to bind CO, except for some very specific sites like lowcoordinated F centers. Since the CO binding energies have not been corrected by the BSSE, these small bindings are questionable. It should be mentioned that the gas-phase Ag–CO complex is unbound, Table 2. Thus, the conclusion is that Ag atoms deposited on MgO do not bind CO more strongly than the MgO surface cations (in particular the low-coordinates Mg2þ sites) where CO is bound by 0.2–0.3 eV [48,49]. Indeed, this is consistent with the experimental observation that on Ag1 /MgO all CO has desorbed from the surface for temperatures around 120 K and that the IR spectra are very similar for CO/MgO and CO/Ag1 / MgO [24]. The situation is completely different for Rh and Pd adsorbed on the oxide anions of the surface, Table 2. On both Rh1 /MgO and Pd1 /MgO CO is strongly bound to the metal atoms adsorbed on oxide anions, Onc . In these cases the CO adsorption energies are between 2 and 3 eV, with Rh binding CO slightly more strongly than Pd (by 0.2–0.6 eV, depending on the site). Thus, CO is expected to desorb from metal atoms deposited on

70 Table 2 Properties of a CO molecule adsorbed on Rh, Pd and Ag atoms stabilized at different sites of the MgO surface: O anions, F and Fþ centers located at terrace (5c), edge (4c), corner (3c), and step (4c–5c) sites Free

Terrace O5c

Symmetry

Edge F5c

Fþ 5c

C4v

O4c

Corner F4c

Fþ 4c

C2v

O3c

Step F3c

Fþ 3c

C3v

O4c –O5c C1

– 1.82 1.152 1.94 – 2008

2.16 1.85 1.153 2.74 2.04 1999

2.70 1.84 1.162 1.18 2.87 1930

2.87 1.94 1.140 1.03 1.64 2062

2.07 1.86 1.153 2.51 2.28 2014

2.64 1.86 1.154 1.11 2.76 1957

2.73 1.96 1.134 1.22 1.90 2098

2.04 1.84 1.160 2.96 2.88 1966

2.84 1.87 1.159 1.15 2.71 1931

2.96 1.95 1.137 0.80 1.65 2082

2.08 1.85 1.160 2.61 2.96 1946

Pd ) d(Pd–S)a (A ) d(Pd–C) (A ) d(C–O) (A De (CO)b (eV) De (MCO)c (eV) x0 (cm
1 )

– 1.87 1.143 1.80 – 2072

2.13 1.85 1.149 2.34 1.94 2029

2.80 1.97 1.146 0.63 2.78 2013

2.81 1.97 1.136 0.82 1.69 2091

2.06 1.85 1.147 2.29 2.13 2028

2.69 2.00 1.142 0.45 2.71 2024

2.69 2.01 1.130 0.72 1.81 2122

2.05 1.83 1.156 2.67 2.70 1993

2.91 1.99 1.147 0.39 2.62 1999

2.87 2.00 1.134 0.69 1.69 2104

2.08 1.85 1.149 2.32 2.34 2033

– – –

– – –

2.70 2.27 1.146 0.42

2.80 2.87 1.123 0.14

– – –

d

2.89 3.19 1.125 0.05

– – –

d

2.84 2.23 1.162 0.17

2.90 2.35 1.151 0.32

2.93 3.16 1.125 0.06

2.21 2.18 1.160 0.27

Ag ) d(Ag–S)a (A ) d(Ag–C) (A ) d(C–O) (A De (CO)b (eV)

d

d

The binding energies are not corrected for the BSSE. a Shortest distance between the metal atom and the substrate atoms S (O for terrace, edge, corner and step, Mg for F centers). b Computed as E(M/MgO) + E(CO) ) E(CO/M/MgO). c Computed as E(MgO) + E(MCO) ) E(CO/M/MgO). d System unbound.

L. Giordano et al. / Surface Science 540 (2003) 63–75

Rh ) d(Rh–S)a (A ) d(Rh–C) (A ) d(C–O) (A De (CO)b (eV) De (MCO)c (eV) x0 (cm
1 )

L. Giordano et al. / Surface Science 540 (2003) 63–75

oxide anions well above room temperature. No large difference is found in the CO adsorption energy for metal atoms adsorbed on terrace, step, edge, or corner sites. In this respect, the analysis of the bonding of CO to the supported metal atom is not diagnostic of the anion site where the metal is bound. For both Rh and Pd the CO bond strength is considerably enhanced when the metal is supported to MgO (the binding energies increase by 0.5–0.8 eV) compared to the gas-phase Rh and Pd atoms. This effect reflects the increased back-donating ability of the metals in contact with the valence band electrons of the oxide support. An important result is that the metal–CO bonding is similar or even stronger than that of the RhCO or PdCO complexes to the surface. On terrace O5c sites the O5c M–CO bond is definitely stronger than the O5c –MCO one. On the low coordinated sites the differences are small. This means that an increase in temperature can lead to diffusion of the MCO complexes before CO desorption occurs. In this respect, we found that the bonding of the RhCO complex with a step is almost an eV higher than with a terrace, while for the corresponding PdCO complex the difference is only 0.4 eV, Table 2 and Fig. 2f. This means that the steps could be trapping sites for the RhCO but not for the PdCO complexes diffusing on the surface. When we consider the neutral F centers, the similarity of Rh and Pd disappears. The bonding of CO to Pd1 /Fnc is always below 0.6 eV, with the terrace F5c defect site showing the strongest bonding. Thus, a dramatic change is found when one compares the CO binding energy to Pd/Onc and to Pd/Fnc : the bond strength is reduced by a factor 4–6. This has allowed in a previous study to clearly rule out the possibility that Pd atoms are bound at oxide anions of the MgO surface [21]; in fact, TDS experiments show that CO desorption occurs already at 250 K, a temperature which correspond to a desorption barrier of 0.6–0.7 eV, totally incompatible with CO being adsorbed on Pd1 /Onc [21]. On Rh1 /Fnc CO is bound by more than 1 eV, showing a stronger affinity of this system for CO compared to Pd/Fnc . No large difference is found for F centers located at various sites, Table 2. The different behavior of Rh and Pd ad-

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sorbed on Fnc centers towards CO can be explained as follows. On Pd the delocalization of the trapped electrons into the 5s level leads to an increased Pauli repulsion with the CO molecule and in a strong weakening of the bond; on Rh this effect is smaller because of the presence of an incomplete d shell and the easier mixing of the 5s with the 4d orbitals to form new hybrid orbitals. On neutral F centers the bonding of the MCO unit to the surface is so strong that no mobility of this species occurs once the complex is trapped at an oxygen vacancy. An increase in temperature will result in the loss of CO (at lower temperature for Pd than for Rh) and a Rh or Pd atom filling the vacancy. þ The bonding of CO to Rh/Fþ nc and Pd/Fnc sites closely follows that described above for the neutral Fnc centers. The bond strength is close to 1 eV for supported Rh, and around about 0.8 eV for supported Pd. The bonding is quite insensitive to the location of the vacancy (terrace, edge or corner) and the M–CO bonding is weaker than that of the complex with the defect. Not surprisingly, the major differences in electronic structure caused by the positive charge of the vacancy reflect on the vibrational frequency of CO, as it will be discussed below. 3.2.2. CO stretching frequency The signature of a change in the back-donation mechanism is provided by the red-shift of the CO stretching frequency. In unsupported MCO complexes the vibrational frequency is shifted with respect to the free CO molecule by about 170 cm
1 for Rh–CO and 100 cm
1 for Pd–CO. Notice that this large difference does not correspond to a similar large change in the CO binding energy (1.94 eV for Rh–CO versus 1.80 eV for Pd–CO, Table 2). Rather, it is the consequence of the shorter metal–CO distance in the Rh complex, , respectively. When supported 1.82 versus 1.87 A on MgO oxide anions, no matter which is the oxide anion coordination, the metal–CO distances are similar in CO/Rh/Onc and CO/Pd/Onc , 1.85 , and the Rh–CO and Pd–CO stretching freA quencies are closer than in the corresponding gasphase M–CO complexes, Table 2. In all cases the

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CO frequency is considerably red-shifted compared to the corresponding unsupported M–CO complex, Table 2. This is not surprising considering that the oxide anion underneath the metal atom reinforces the back donation to the empty CO orbitals. Things are quite distinct for Rh and Pd when one considers adsorption on neutral F centers. For Rh we notice that the Rh–CO distance on Fnc sites remains essentially unchanged compared to the corresponding Onc sites, 1.84–1.87 ; thus, for a nearly constant metal–CO distance A the presence of excess electrons in the cavity results directly in an increased back-donation which lowers the CO frequency by another 30–60 cm
1 . The CO x0 for Rh supported on Fnc centers is therefore well below 2000 cm
1 , Table 2. We noticed already that CO is less strongly bound on Pd/ Fnc than on Rh/Fnc and that this is due to a larger Pauli repulsion in the case of Pd. This has direct, important consequences on the Pd–CO distances. On F centers, the Pd–CO separation increases to  compared to Onc sites where is around nearly 2 A . The enhanced back donation due to the 1.85 A excess of charge in the vacancy is compensated by the longer distance hence by the reduced overlap of the CO 2p levels with the Pd 4d orbitals. The final effect is that virtually no change occurs in the CO vibrational frequency when we compare Pd adsorbed on oxide anions with Pd on Fnc centers. In this respect the behavior of Rh differs from that of Pd. We have seen above that by increasing the temperature migration of MCO species is expected. These species will diffuse on the surface until they became stabilized at F centers where diffusion is stopped. In IR spectra this should result in a shift to smaller frequencies for CO/Rh1 / MgO while no shift should occur for CO/Pd1 / MgO. This is indeed what has been found experimentally [24]. When one considers the charged Fþ nc centers, things are again similar for Pd and Rh: the metal– CO distance is elongated with respect to the adsorption on oxide anions and the CO stretching frequency is close to 2080–2100 cm
1 for most sites. Two mechanisms contribute to this result: (a) the presence of one trapped electron in Fþ nc centers instead of two as in neutral F centers results in a smaller back donation; (b) the electric field asso-

Table 3 Adsorption properties of CO molecule on Rh, Pd and Ag atoms supported on terrace F centers, optimized without symmetry constraints ) d(M–Mg) (A ) d(M–C) (A ) d(C–O) (A \M–C–O De (CO)a (eV) De (MCO)b (eV)

Rh

Pd

Ag

2.83 1.84 1.168 178.5 1.48 3.17

2.78 1.99 1.153 143.9 0.50 2.64

2.87 2.49 1.158 145.4 0.22 c

Computed as E(M/MgO) + E(CO) ) E(CO/M/MgO). Computed as E(MgO) + E(MCO) ) E(CO/M/MgO). c The gas-phase AgCO complex is unbound. a

b

ciated to the positively charged defect center interacts electrostatically with the CO dipole leading to a net increase in the CO stretching frequency (first-order Stark effect) [50]. For selected adsorption sites, namely O5c and F5c , the geometries have been reoptimized by removing all symmetry constraints, Table 3. For both Rh–CO and Pd–CO at O5c the geometry with CO normal to the surface is the most stable, while for Rh–CO and Ag–CO at F5c centers a tilted geometry turns out to be more stable, see Table 3 and Fig. 2d. The energy difference is quite pronounced for Rh, 0.3 eV, and very small for Ag, 0.05 eV (we mentioned above that the bonding of CO to Ag1 /MgO is so weak that the existence of the complex is not proved). For Pd on F5c centers the tilted and the normal geometries are nearly degenerate. However, the tilted geometry allows a better overlap of the metal d orbitals and the CO empty states, leading to a larger back-donation which induces a further shift of the CO frequency to smaller values. 3.3. Dicarbonyl formation Experimentally, the formation of surface complexes where more than one CO molecule is bound on the supported metal atom has been proven by isotopic exchange for Pd and Rh on MgO and Al2 O3 thin films [14,21,22]. The theoretical study of the ability of a supported atom to bind two or even three CO molecules is important not only for the interpretation of the TDS and IR spectra, but

L. Giordano et al. / Surface Science 540 (2003) 63–75

also for the rationalization of the reactivity of the system. In fact, both CO oxidation [11,51] and acetylene trimerization [9], two reactions which have been studied on single atoms deposited on MgO, imply the ability of the metal atom to bind simultaneously two or even three reactant molecules. We have considered the formation of Rh(CO)2 and Pd(CO)2 complexes on the O5c terrace site, on the O4c –O5c step site, and on the neutral and charged oxygen vacancy, F5c and Fþ 5c respectively. On terrace sites we observe the formation of a symmetric gem-dicarbonyl, see Table 4 and Fig. 2e. For Rh bonded to oxide anions, the binding energy of the second CO molecule is of 0.6–0.7 eV (0.6 eV for a terrace complex, 0.7 eV for a step complex, Table 4). Thus, the energy required to detach a CO molecule from Rh(CO)2 is reduced compared to the gas-phase complex by more than a factor of two. When a second CO molecule is added to the CO/Rh/F5c complex the energy gain, close to 2 eV, is larger than for the unsupported molecule. The ground state is a doublet for both oxide anions and neutral F centers. On a CO/Rh/ Fþ 5c complex a second CO molecule is bound even more strongly, 2.5 eV. Notice that a change in electronic configuration accompanies the addition of the second CO molecule in this case. The ground state, which is a triplet in CO/Rh/Fþ 5c (with the singlet state 0.26 eV higher in energy) becomes

73

a closed-shell singlet on (CO)2 /Rh/Fþ 5c . The Rh–C distance decreases going from the unsupported to the supported Rh(CO)2 complex but the C–Rh–C angle becomes smaller than 180. On a step, the C– Rh–C angle is close to 90 and Rh assumes an effective square planar coordination, with two CO ligands on one side and two oxide ligands on the other, Fig. 2g. These results clearly show a strong tendency of Rh to bind more than one CO molecule. In fact, we have checked for Rh deposited on O5c and F5c centers that even a Rh(CO)3 complex is stable, Table 4; the reaction (CO)2 /Rh/ MgO + CO fi (CO)3 /Rh/MgO is exothermic by about 1 eV. The situation is completely different for Pd. On a terrace O5c site Pd binds a second CO molecule by 0.28 eV only, and this binding is largely an artifact due to the BSSE. On a step, the Pd(CO)2 complex is unbound. Also on F5c and Fþ 5c centers the second CO molecule is weakly bound, 0.45 and 0.69 eV, respectively, Table 4. It is not surprising therefore that the changes in Pd–C distance by depositing the complex are small; the C–Pd–C angle becomes close to 120 for all sites, Table 4. Thus, there is a different behavior of Rh and Pd atoms supported on MgO in binding CO. Notice that this difference is a direct effect of the interaction with the substrate. In fact, Rh(CO)2 and Pd(CO)2 gas-phase complexes exhibit similar stabilities, Table 4.

Table 4 Properties of M(CO)2 and M(CO)3 complexes stabilized at various sites of the MgO surface M

M/O5c (terrace)

M/O4c –O5c (step)

M/F5c (terrace)

M/Fþ 5c (terrace)

) d(Rh–S) (A ) d(Rh–C) (A \Rh–C–O \C–Rh–C De [Rh(CO)2 fi Rh(CO) + CO] (eV) De [Rh(CO)3 fi Rh(CO)2 + CO] (eV)

– 1.96 180.0 180.0 1.47 1.11

2.26 1.93 167.2 147.2 0.61 1.00

2.13 1.88/1.91 171.0/156.2 92.9 0.71 –

2.87 1.89 170.8 126.7 1.97 1.07

2.99 1.91 160.2 137.5 2.24 –

) d(Pd–O) (A ) d(Pd–C) (A \Pd–C–O \C–Pd–C De [Pd(CO)2 fi Pd(CO) + CO] (eV)

– 1.97 180.0 180.0 1.32

2.33 1.95 171.4 125.5 0.28

– – – –

2.94 1.97 178.0 125.4 0.45

3.02 1.98 175.8 128.4 0.69

a

Unbound.

a

74

L. Giordano et al. / Surface Science 540 (2003) 63–75

4. Conclusions The bonding properties of CO adsorbed to Rh, Pd, and Ag atoms deposited on the MgO surface have been studied with the aim of identifying the sites on the oxide substrate where the metals are bound. The analysis includes a number of regular and defect sites on the MgO surface, in particular oxide anions and anion vacancies at terraces, steps, edges, corners. Rh and Pd atoms interact with the oxide anions of the MgO surface with binding energies close to 1 eV, whereas the Ag atoms exhibits only a weak interaction. The lowcoordinated sites enhance the interaction in all cases. On a step site Rh is bound by almost 2 eV, suggesting a tendency of these extended defects to act as trapping sites. This tendency is less pronounced for Pd. As discussed in other studies [41,43], the neutral oxygen vacancies act as pinning points for the metal atoms, increasing considerably the adsorption energy (up to 3 eV and more for Rh and Pd). On the charged vacancy, Fþ nc the interaction is adsorbate-independent, with adsorption energies of the order of 2 eV. The main question is whether CO adsorption can provide an useful, although indirect, tool to determine the actual site where the metal is bound. Ag does not bind CO and in this respect is a completely inert species. Rh and Pd form strong bonds with one CO molecule when sitting on oxide anions at regular or low-coordinated sites. Rh binds CO slightly more strongly than Pd. The resulting Rh–CO and Pd–CO complexes are so stable that it is possible that a temperature increase induces diffusion of these species on the surface before CO desorption. Things are completely different when the metal is stabilized at oxygen vacancies. In this case Rh and Pd behave quite differently. In fact, on Rh/Fnc the bonding of CO is still substantial, close to 1 eV, while on Pd weaker interactions are observed. This suggests that in TPD experiments, assuming that the metal atoms are sitting on F centers, CO desorption should occur at lower temperatures for Pd1 /MgO than for Rh1 /MgO complexes, as observed experimentally [21,24]. The second difference between Rh and Pd is in the ability to bind more CO molecules. While Rh forms easily Rh(CO)2 and even Rh(CO)3

complexes, Pd shows a very small tendency to bind more than one CO molecule. Some indication about the metal adsorption site can also be obtained from the analysis of the CO stretching frequency. The calculations indicate that, compared to the free MCO complexes, the deposition on the oxide sites results in a lowering of the frequency because of the enhanced back donation of charge. The effect is more pronounced on F centers where the electrons trapped in the cavity are more easily redistributed over the CO empty levels. On charged Fþ centers the presence of an electric field counteracts the effect of the back donation and leads to blue shifts in the CO frequency compared to the unsupported MCO case. In general, the comparison of computed and measured frequencies for adsorbed CO presents more difficulties than for binding energies. One reason is that the frequency of adsorbed CO is a quantity very sensitive to several effects, like the model used, the details of the calculations, but also the CO–CO dipole coupling present in the geminal complexes, etc. Being the shifts relatively small, their interpretation is often non-unambiguous. In this respect, the relative stabilities seem to provide a firm basis for discriminating the MgO sites where the metal atoms are bound.

Acknowledgements We would like to thank Prof. U. Heiz for useful discussions. The work has been supported by the Project of Parallel Computing of the Italian INFM.

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