Adsorption of Pd atoms and Pd4 clusters on the MgO(001) surface: a density functional study

Yudanov, Ilya V.; Vent, S.; Neyman, Konstantin M.; Pacchioni, Gianfranco; Rösch, Notker

doi:10.1016/s0009-2614(97)00772-0

Cited by 88 publications

(48 citation statements)

References 27 publications

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“…[10][11][12][13][14][15] Recent density-functional and other estimates of E a gave 0.9-1.0 eV, 10 with correction downward toward 0.8 eV. 10,11 One-quarter ML Pd was calculated to be bound to the surface by about 1.3 eV/atom, 12 encouraging us to believe that E a is of this order. There are several literature estimates of the binding energy of the diatomic molecule Pd 2 in the gas phase, covering a huge range from 0.73 to 1.69 eV.…”

Section: Fig 1 Arrhenius Representation Of Pd Island Density N X ͑Cmmentioning

confidence: 97%

Nucleation and growth of supported clusters at defect sites: Pd/MgO(001)

et al. 2000

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Nucleation and growth of Pd on cleaved MgO͑001͒ surfaces were studied by variable-temperature atomic force microscopy in the temperature range 200-800 K. Constant island densities (ϳ3ϫ10 12 cm Ϫ2 ) were observed over a wide temperature range, indicating nucleation kinetics governed by point defects with a high trapping energy. These results are compared to a rate equation model that describes the principal atomistic nucleation and growth processes, including nucleation at attractive point defects. Energies for defect trapping, adsorption, surface diffusion, and pair binding are deduced, and compared with recent ab initio calculations.Metal aggregates supported on oxide surfaces have many practical applications due to their catalytic, magnetic, and electrical properties. Consequently, fundamental studies have been carried out on a range of model systems.1 Although the main microscopic steps governing nucleation and growth of the films are now understood, detailed characterization of these processes has proven difficult. In particular, little is known about the energies involved. In recent years, ab initio calculations of the binding of metal atoms and clusters to oxide surfaces have progressed, generating further stimulus for experimental determination of the relevant interactions. On the other hand, a much more complete understanding has been achieved for the case of metallic substrates. In large part, this is due to the application of variable-temperature scanning tunneling microscopy ͑STM͒ for in situ imaging of the nucleation and growth stages, compared to analytic models and numerical simulations. In this work, we adopt a similar approach to metal growth on insulating oxide surfaces. This allows us to determine the principal energies governing nucleation and growth, and in particular to consider the influence of defects. On oxides, defects like vacancies or steps are present even at wellprepared single-crystal surfaces. Frequently the nucleation and growth behavior on these substrates is dominated by the defects. Early transmission electron microscopy studies of metal growth on alkali halide and alkaline earth surfaces 3 indicated defect nucleation in some cases, but relatively little is known about the interaction between defects and adsorbed metal atoms.We have studied the growth of Pd on MgO͑001͒ surfaces, utilizing variable-temperature atomic force microscopy ͑AFM͒ to image the deposits. This is one of the most explored metal/oxide systems, being a model for supported metal catalysts. Palladium grows in three-dimensional clusters, similar to most metal/oxide systems, as the surface energy is usually higher for metals than for oxides.1 We demonstrate that the nucleation kinetics in a large temperature range is determined by attractive point defects. With the help of a simple rate equation model we deduce the relevant interaction energies for the Pd/MgO͑001͒ system. It is expected that the understanding of nucleation at defects can be explored to produce novel nanostructures. The experiments were performed...

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Section: Fig 1 Arrhenius Representation Of Pd Island Density N X ͑Cmmentioning

confidence: 97%

Nucleation and growth of supported clusters at defect sites: Pd/MgO(001)

et al. 2000

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“…Two tetramer configurations were studied on a larger MgO unit cell, with 36 atoms per layer. The tetramer is stable in a flat square planar configuration [53,54], in which each Pd atom is bound to an oxygen site. This structure has an adsorption energy of 7.71 eV (1.93 eV/atom), with Pd-Pd bond distances of 2.55 Å and Pd-O distances of 2.28 Å .…”

Section: Tetramermentioning

confidence: 99%

Pd diffusion on MgO(100): The role of defects and small cluster mobility

Henkelman

Campbell

et al. 2006

Surface Science

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Density functional theory is used to explore the energy landscape of Pd atoms adsorbed on the terrace of MgO(1 0 0) and at oxygen vacancy sites. Saddle point finding methods reveal that small Pd clusters diffuse on the terrace in interesting ways. The monomer and dimer diffuse via single atom hops between oxygen sites with barriers of 0.34 eV and 0.43 eV respectively. The trimer and tetramer, however, form 3D clusters by overcoming a 2D-3D transition barrier of less than 60 meV. The trimer diffuses along the surface either by a walking or flipping motion, with comparable barriers of ca. 0.5 eV. The tetramer rolls along the terrace with a lower barrier of 0.42 eV. Soft rotational modes at the saddle point lead to an anomalously high prefactor of 1.3 · 10 14 s À1 for tetramer diffusion. This prefactor is two order of magnitude higher than for monomer diffusion, making the tetramer the fastest diffusing species on the terrace at all temperatures for which diffusion is active (above 200 K). Neutral oxygen vacancy sites are found to bind Pd monomers with a 2.63 eV stronger binding energy than the terrace. A second Pd atom, however, binds to this trapped monomer with a smaller energy of 0.56 eV, so that dimers at defects dissociate on a time scale of milliseconds at room temperature. Larger clusters bind more strongly at defects. Trimers and tetramers dissociate from monomer-bound-defects at elevated temperatures of ca. 600 K. These species are also mobile on the terrace, suggesting they are important for the ripening observed at P600 K during Pd vapor deposition on MgO(1 0 0) by Haas et al.

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“…23,[25][26][27][28][29] The ideal, undefective MgO͑100͒ surface is a prototype of such a simple system. 12 In order to construct a single cluster model representation of the relaxed ␣-Al 2 O 3 (0001) surface the following strategy was used.…”

Section: A Embedded-cluster Model Approachmentioning

confidence: 99%

“…In general the BSSE is important in weak interactions only but in the special case of oxide surfaces the diffuse nature of the oxide anion results in BSSE's of several tenths of an eV. 12,23,[25][26][27][28][29] Therefore, only the BSSE corrected adsorption energy computed following the standard BoysBernardi counterpoise method 44 is reported in the different tables. This BSSE corrected adsorption energy is computed as E ads ϭE͑PdϪcluster͒ϪE͑clusterϩPd basis at R e ͒ ϪE͑Pdϩcluster basis at R e ͒.…”

Section: The Embedded-cluster Model Approach Description Of the mentioning

confidence: 99%

“…On the MgO surface, the computed adsorption energy for Pd adsorbed on the basic sites is almost three times the one calculated for Pd adsorbed on the acidic sites and the same behavior is also followed by different adsorbed metal atoms. 23,[27][28][29] However, the data discussed so far concerning the adsorption of metals on the ␣-Al 2 O 3 (0001) surface indicates that even for a clean and nearly perfect surface different surface sites may compete. The order of stability of metal adatoms on the clean ␣-Al 2 O 3 (0001) surface may influence the growth mechanism of the metal cluster and has implications for the diffusion of metal atoms on bulk ␣-Al 2 O 3 .…”

Section: Introductionmentioning

confidence: 99%

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Interaction of Pd withα−Al2O3(0001):

et al. 2002

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The Pd/␣-Al 2 O 3 (0001) interface at low Pd coverage has been studied by a variety of theoretical methods and models at this metal-oxide interface. All results are consistent and predict a noticeable interaction dominated by the metal polarization in response to the presence of the substrate. A significant contribution of the charge transfer from the transition metal to the surface is also observed. The periodic fully relaxed calculations show that the most favorable adsorption site for the interaction of Pd with corundum involves the anionic surface sites, in particular the on-top oxygen site. It is also shown that adsorption of Pd atoms on the surface induces a significant relaxation of the aluminum oxide substrate, especially for the outermost aluminum layer. The small differences observed in the adsorption energies near the oxygen atoms suggest a high mobility of Pd atoms on the surface.

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Adsorption of Pd atoms and Pd4 clusters on the MgO(001) surface: a density functional study

Cited by 88 publications

References 27 publications

Nucleation and growth of supported clusters at defect sites: Pd/MgO(001)

Nucleation and growth of supported clusters at defect sites: Pd/MgO(001)

Pd diffusion on MgO(100): The role of defects and small cluster mobility

Interaction of Pd withα−Al2O3(0001):

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