9032
J. Phys. Chem. 1996, 100, 9032-9037
Metal Deposition on Oxide Surfaces: A Quantum-Chemical Study of the Interaction of Rb, Pd, and Ag Atoms with the Surface Vacancies of MgO Anna Maria Ferrari and Gianfranco Pacchioni* Dipartimento di Chimica Inorganica, Metallorganica e Analitica, UniVersita` di Milano, Via Venezian 21, 20133 Milano, Italy ReceiVed: December 7, 1995; In Final Form: February 23, 1996X
The interaction of Rb, Pd, and Ag atoms with the surface vacancies of MgO, the Fs and the Vs centers, has been studied by ab initio cluster model wave functions. We have considered the interaction of each atom with Fs, Fs+, Fs2+, Vs, Vs-, and Vs2- sites. These sites correspond to the removal of O, O-, O2-, Mg, Mg+, and Mg2+ atoms or ions, respectively, from the surface. The bond with the metal atoms, which is found to be very weak on the regular surface sites, can be very different depending of the formal charge of the vacancy. Neutral Fs centers are in general rather unreactive as their electronic structure resembles that of the regular surface; Fs+ paramagnetic centers have a relatively large electron affinity and tend to ionize metal atoms with low ionization potentials, such as alkali-metal atoms or to form covalent polar bonds with the adsorbed metal atoms; Fs2+ centers have a very high electron affinity so that all metal atoms are ionized when interacting with these sites. Neutral Vs sites are also electron deficient; here the metal atoms tend to form a dication and to replace the missing Mg ion in the lattice with large gain in the electrostatic energy. On Vs- vacancies the metal atoms lose one electron and become singly ionized with formation of strong ionic bonds at the interface. Finally, no charge transfer occurs between the metal atoms and the electronically saturated Vs2- sites; in this case the bond strength is due only to the metal polarizability.
1. Introduction The deposition of metal atoms, clusters, and thin films on oxide surfaces is an important aspect of material science and catalysis.1 Despite its technological relevance, there is little information on the detailed nature of the metal-oxide interface. Just a few theoretical studies have been devoted to the interaction of metals on oxides and the emphasis has been almost exclusively on the adsorption on regular surfaces, such as the MgO(100) one.2-11 On the other hand, there is enough experimental evidence that the seeds of the cluster growth on an oxide surface are the defect sites, in particular the vacancies. This has been recently proved for the case of the adsorption of Cu and Pd atoms and clusters on the MgO (100) surface.12-17 In this work we consider from a quantum-mechanical point of view the interaction of isolated metal atoms with the oxygen and magnesium vacancies of the MgO surface. These vacancies are usually referred to as Fs and Vs centers, respectively, where the subscript s indicates their location on the surface. Depending on the missing atom or ion the following nomenclature is used: Fs2+ (missing O2-), Fs+ (missing O-), Fs (missing neutral O), Vs2- (missing Mg2+), Vs- (missing Mg+), and Vs (missing neutral Mg). In a recent paper18 we have considered the geometric properties and the electronic structure of neutral and charged MgO surface vacancies. Here we analyze the interaction of Rb, Pd, and Ag atoms with these surface sites. The choice to study Pd and Ag is due to the existence of several experimental studies on the deposition of these or of related metals, such as Cu, on MgO.12,13 On the other hand, alkaline metals have been widely used in the doping of MgO to increase the concentration of paramagnetic vacancies19,20 and Rb can be considered as representative of this group of metals. Rb, Pd and Ag atoms have very different ionization potentials (IP) and they differ also for the presence of d electrons in the valence * Corresponding author. X Abstract published in AdVance ACS Abstracts, April 15, 1996.
S0022-3654(95)03633-1 CCC: $12.00
shell. Thus, the study of their interaction with MgO surface vacancies allows one to draw some general conclusions on the behavior of other metal atoms. After a brief description of the computational method and of the cluster models used to simulate the MgO vacancies (section 2) we discuss the interaction of Rb, Pd, and Ag with the regular, nondefective, MgO surface (section 3A); adsorption at both Mg2+ cations and at O2- anions will be considered. Interaction of the metal atoms with the Fs and the Vs centers will be the subject of sections 3B and 3C, respectively. General conclusions are summarized in the last section. 2. Computational Method The adsorption of metal atoms on the surface vacancies of MgO has been simulated by means of cluster models,21 the same approach used for the study of the electronic structure and properties of Fs and Vs sites.18 In an explorative investigation we used nonstoichiometric [O12Mg5]n- (n ) 14, 15, 16) clusters to model the Fsm+ sites (m ) 0, 1, 2) and [Mg12O5]n+ clusters to simulate the Vsm- sites; more extended [O12Mg13]m+ and [Mg12O13]m- clusters (Figure 1) have been used for more refined calculations (see below). The clusters are embedded in a large array of point charges, PC, to represent the Madelung potential of the extended surface. The entire system, cluster plus PCs, is neutral. The basis sets used for the substrate clusters are the same described in ref 18. In particular, different basis sets were employed for the Mg ions depending on their position in the cluster; the five Mg ions at the vacancy site in Fs centers have been treated with a larger basis, [13s8p/6s3p], which includes a good representation of the 3s and 3p Mg orbitals.22 All the other Mg ions have been treated with single-ζ (SZ) basis, [8s4p/ 2s1p].23 The 12 nearest-neighbor O ions have been described with a double-ζ (DZ) [8s4p/4s2p] basis,23 the rest with the same basis contracted to SZ. Similarly, in the Vs centers the 12 Mg and the 5 O ions closest to the vacancy are treated with a DZ basis, the rest with a SZ basis. For the cluster models of the F © 1996 American Chemical Society
Metal Deposition on Oxide Surfaces
J. Phys. Chem., Vol. 100, No. 21, 1996 9033
Figure 1. O12Mg13 cluster model of a surface oxygen vacancy interacting with a metal atom. The cluster is embedded in a large array of point charges (not shown). Similar Mg12O13 clusters have been used for the study of magnesium vacancy sites.
TABLE 1: First and Second Ionization Potentials (IP) and Polarizabilities (r) of Rb, Pd, and Ag Atoms and Ions from HF Calculations IP1, eV
IP2, eV
R(M), Å3 R(M+), Å3
all electron ECP smalla ECP largea exp all electron ECP smalla ECP largea exp ECP largea expb ECP largea
Rb
Pd
Ag
3.7
5.8 10.2 6.4 8.3 20.7 23.9 20.5 19.4 0.8 4.8 0.4
5.8 6.3 6.3 7.6 26.1 24.4 24.4 21.5 8.7 7.2-8.6 0.3
3.2 4.2 29.8 26.4 27.3 47.3 47.3 0.2
a
ECP small: Pd 10-electron ECP, Ag 11-electron ECP. ECP large: Rb 9-electron ECP, Pd 18-electron ECP, Ag 19-electron ECP. b From ref 34; when experimental polarizabilities are not available, accurate computed values are given.
and V centers, Gaussian functions have been placed at the vacancy site. In particular, we used the same basis of the missing atom, the [8s4p/4s2p] O basis for Fs and the [13s8p/ 6s3p] Mg basis for Vs centers.18 The metal atoms have been treated with relativistic effective core potentials (ECP).24-26 The ECP for Rb includes in the core the 1s2 to 3d10 electrons and treats explicitly the 4s2, 4p6, and 5s1 as valence electrons; we denote this ECP as 9-electron ECP. In the first set of calculations, the 1s2 to 4p6 electrons of Pd and Ag have been included in the core and only the 4d10 (Pd) and the 4d105s1 (Ag) electrons have been treated explicitly in the valence (10-electron and 11-electron ECPs, respectively). However, these ECPs are not flexible enough to accurately reproduce the first and second IPs of the isolated atoms (see Table 1); in particular for Pd there are substantial differences, of the order of 3 eV, with respect to an all-electron calculation of comparable quality. Clearly, a more extended definition of the valence is necessary. This has been considered in the second series of calculations, where larger clusters have also been used. In the second set of calculations the surface Fs and Vs sites have been represented by stoichiometric [O13Mg13] and [Mg13O13] clusters where the central surface atom has been removed (Figure 1). This eliminates the problem of nonstoichiometric clusters of adding or removing electrons to maintain the (2 formal ionic charges. Second, we have used another set of ECPs for Pd and Ag where the 4s2, 4p6, 4d10, and 5s electrons have been treated explicitly in the valence shell; we denote these ECPs as 18-electron ECP (Pd) and 19-electron ECP (Ag). The ECP basis sets are Rb [5s5p/4s3p], Pd [5s5p4d/4s3p3d], and Ag [5s5p4d/4s3p3d].24-26 With these ECPs and basis sets the
first and second IPs of Rb, Pd, and Ag are repoduced with acceptable errors; also the atomic polarizabilities are in good agreement with the experimental data, with the exception of Pd where the computed value is too small (Table 1). Unless differently specified, in the following we report and discuss only the results from these more extended calculations. The results obtained with the smaller clusters and ECPs are qualitatively similar, except for a few cases where the use of small ECPs is not recommended. The geometry of the adsorbed metal atoms and of the four surface magnesium or oxygen atoms defining the Fs or Vs vacancy site, respectively, has been fully optimized by means of analytical gradients. All the other atoms of the cluster have been fixed at the positions of the ideal MgO lattice. The adsorption energies have been corrected by the basis set superposition error (BSSE)27 in all cases where this is significant with respect to the interaction energy. When electrostatic effects dominate the interaction, the BSSE is just a small fraction of the total stabilization; in these cases the BSSE correction has not been applied. The interaction energies have been computed at the HF level and correlation effects have not been taken into account. It will be clear from the following discussion, however, that in most cases these are not essential since the interaction is largely ionic. When this is not the case, as for metal atoms adsorbed on oxide anions of the MgO terraces, one should keep in mind the qualitative character of the predicted interaction energies. The calculations have been performed with the program package Hondo 8.528 on IBM Risc 6000 workstations. 3. Results and Discussion 3.A. Adsorption at the Nondefective MgO Surface. The adsorption of metal atoms and clusters on the regular (100) surface of MgO has been studied experimentally12-17,29,30 and theoretically.3-5,9 Medium-energy ion-scattering experiments have shown that only 50% of the initially incident Cu atoms stick to the MgO (100) surface,29 indicating a weak adsorption. More recent experiments on the same system give a somewhat higher value of the sticking probability, but in any case this remains
