Adsorption of Pd and Pt atoms on α-Al2O3(): density functional study of cluster models embedded in an elastic polarizable environment
Introduction
Transition and noble metal particles deposited on oxide ceramics are of growing interest due to their numerous technological applications [1], [2], [3]. For instance, supported metals such as Pd and Pt are widely used catalysts [4]. Oxide supports can significantly affect both structural and electronic characteristics of the metal species deposited on them [5]. A detailed understanding of the metal/ceramic interactions at the microscopic level is crucial for studying the chemical reactivity of the supported systems. Unfortunately, it is often complicated to obtain pertinent information from experiments alone because many factors affect the properties of the metal/oxide interface under experimental conditions. On the other hand, computational studies based on first-principles quantum mechanical (QM) methods enable to separate these factors and thus furnish a valuable complementary mean for expanding our understanding.
Single metal atoms and small clusters interacting with ionic metal oxides are key for unraveling the initial stage of interface formation. Moreover, these systems can be considered as elementary building blocks of supported catalysts. The initial steps of small metal species interacting with oxides include: (i) adsorption and diffusion of metal atoms, (ii) homogeneous and heterogeneous (on defects) nucleation, (iii) begin of coalescence. It is important to determine structural and energetic parameters for these steps.
For metal particles on ionic substrates one can differentiate between the interactions with non-polar (stoichiometric) and polar surfaces. Stoichiometric surfaces are represented by essentially unrelaxed (0 0 1) facets of oxides with rock-salt structure, e.g. MgO(0 0 1), widely utilized as metal support [1], [2]. Oxides exhibiting polar surfaces, with features often dependent on the preparation conditions, are also common. Clean polar surfaces are unstable and thus difficult to prepare unreconstructed, dehydroxylated, and free of defects [6]. Corundum, α-Al2O3, is a prototype of such metal oxides. Its most stable Al-terminated (0 0 0 1) surface [2], [7] was extensively studied experimentally as support for metal particles [5]. Strong relaxation is inherent to this surface: it decreases charge separation (polarity) effects [6] and thus reduces the chemical activity. As a surface of a wide gap insulator, α-Al2O3(0 0 0 1) is expected to feature adsorption properties that are not very different from those of MgO(0 0 1) [8].
Periodic slab and embedded cluster models are often used approaches to study metal species supported on oxides [9]. For touchstone complexes of metal atoms on MgO(0 0 1), results of density functional cluster [10], [11] and periodic [12], [13], [14] calculations are in good agreement. In particular, (i) adsorption sites on top of oxygen are preferred, (ii) bonding can be rationalized by the polarization of metal atoms in the electrostatic field of the support with small covalent and only very minor ionic contributions, (iii) adsorption energies are close in these two computational approaches, provided the same exchange-correlation functional is used. Embedded cluster calculations on representative systems of Pd and Pt atoms on MgO(0 0 1) with the gradient-corrected Becke–Perdew (BP86) [15], [16] approximation resulted in binding energies of 1.4 eV [17] and 2.3 eV [18], respectively.
For low coverage of Pd atoms on the Al-terminated surface α-Al2O3(0 0 0 1), one adatom per three atoms of the upper oxygen layer (1/3 ML), a plane-wave periodic approach with ultrasoft pseudopotentials identified adsorption sites on top of O anions as preferred [19], [20]. Despite a non-negligible ionic contribution to the Pd–Al2O3 bond, no ionization of adsorbed Pd atoms was found. Binding energy of about 1.4 eV with a strong contribution of the substrate relaxation was computed using the generalized-gradient functional (GGA) PW91 [21]. Embedded cluster calculations with the same functional resulted in a smaller adsorption energy of 0.86 eV due to neglected substrate relaxation, but revealed a similar binding nature [20]. At variance, pseudopotential Gaussian-type orbital periodic calculations of single Pt atoms on α-Al2O3(0 0 0 1) (for the same 1/3 ML coverage) led to the conclusion that adsorption actually induces ionization of adsorbed Pt [22]. This finding implies a dramatically different interaction mechanism of Pt atoms with non-polar and polar oxide substrates: minor ionic contribution on the one hand, and essentially ionic bonding on the other. Apparently, no such a strong bonding difference is found for adsorbed isolated Pd atoms: the ionic part of the adsorption interaction prevails with neither the stoichiometric MgO(0 0 1) nor the polar α-Al2O3(0 0 0 1) surface; thus, the adsorption mechanism in the complex Pd1/α-Al2O3(0 0 0 1) was calculated similar to that in the complex Pt1/MgO(0 0 1) [18]. Whereas Pd and Pt atoms bond in similar fashion to MgO(0 0 1), they seem to behave quite differently on the non-stoichiometric surface α-Al2O3(0 0 0 1). This issue requires closer inspection.
Periodic studies of metal atoms on α-Al2O3(0 0 0 1) [20], [22] emphasized the importance of adsorption-induced relaxation of the substrate. Recently, we implemented an advanced tool for cluster embedding in an elastic polarizable environment (EPE) [17]. This tool is particularly valuable for describing accurately the adsorption on strongly relaxed metal-oxide surfaces. We used our EPE embedding to rationalize the adsorption of Pd atoms on regular and defect sites of MgO(0 0 1) [17] and of CO on α-Al2O3(0 0 0 1) [8]. This work reports on the first application of the EPE embedding scheme to investigate adsorption of single metal atoms, Pd and Pt, on conceivable sites of the Al-terminated α-Al2O3(0 0 0 1) surface. The present work aims at (i) further examining the performance of embedded vs. periodic models, (ii) quantifying adsorption properties of α-Al2O3(0 0 0 1) for noble metal atoms, and (iii) improving our understanding of how metals interact with polar surfaces of main-group oxides at the very first stage of deposition, e.g., whether adsorbed atoms are oxidized.
Section snippets
Computational details
The calculations were performed using the linear combination of Gaussian-type orbitals fitting-function density functional (LCGTO-FF-DF) method [23] implemented in the parallel computer code ParaGauss [24], [25]. The GGA exchange-correlation functional BP86 [15], [16] was used self-consistently. Relativistic effects were taken into account at the scalar relativistic level [26]. We employed the same orbital basis sets as in previous calculations: Al(12s9p2d)→[6s4p2d] [27], O(9s5p2d)→[5s4p2d] [27]
Cluster models
The crystal structure of α-Al2O3 bulk exhibits the symmetry of the space group R3c. A hexagonal unit cell contains 12 Al and 18 O ions, grouped along the c axis in an alternating stacking of two cationic and one anionic layers [2]. The cations are located in octahedral sites of the hexagonal close-packed oxygen sub-lattice and occupy two-third of the octahedral positions. The structure of the most stable Al-terminated (0 0 0 1) surface of this crystal is known from experimental studies [2] and
Results and discussion
Calculated data for the adsorption complexes M/[Al4O6]/(Alpp*)6 (1) and M/[Al10O15]/(Alpp*)n (2; n=13,15) of the atoms M=Pd, Pt are displayed in Table 1, Table 2: distances r between the adsorbate M and its nearest neighboring atoms Al and O, the dynamic dipole moment ∂μ/∂z for the vertical displacement of M (along the z axis), the binding energy Eb, and the contribution to it of the adsorption-induced relaxation of the substrate . The notation f refers to moieties M optimized on
Conclusions
We applied a density functional method with a consistent cluster embedding in an EPE to study adsorption complexes of single Pd and Pt atoms on the relaxed polar Al-terminated surface α-Al2O3(0 0 0 1). The embedding method treats both the active QM region and its classical environment in a variational fashion, without artificial constraints for geometry optimization.
In line with our previous results for metal species on MgO(0 0 1), the most favorable adsorption complexes of atomic Pd and Pt on α-Al2O
Acknowledgments
This work was supported by Volkswagen-Stiftung (Grant I/73653), INTAS-RFBI (Grant IR-97-1071/RBFI 9703-71057), the Krasnoyarsk Regional Scientific Foundation (Grants 1F0161 and 6F0151), Deutsche Forschungsgemeinschaft, and Fonds der Chemischen Industrie (Germany).
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