1 Electronic Structure and Chemical Reactivity Chem 7530/750 Maryam Ebrahimi February 21 st,2006.

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Electronic Structure and

Chemical Reactivity

Chem 7530/750

Maryam EbrahimiFebruary 21st,2006

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►The structure & the exceptional physical and chemical properties of small metal particles.

►Deposited metal particles are technically employed in heterogeneous catalysis.

►Since the complex structure of real catalysts often hampers the attempt to connect macroscopic effects with the microscopic process taking place on the surface, MODEL CATALYSTS have been presented.

►Ultra-thin oxide films (a few Angstroms) grown on a metallic substrate are an excellent alternative to circumvent the problems like making single crystal samples, insulating character of the oxides, sample mounting and cleaning which relate to the poor thermal conductivity of oxides.

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Real and Model Catalyst

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• Nanocrystalline materials composed of crystallites in the 1-10 nm size range possess very high surface to volume ratios because of the fine grain size.

• Such materials exhibit chemical and physical properties characteristic of neither the isolated atoms nor of the bulk material.

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Metal Particles: Electronic Structure

influenced by 2 factors

(I) Size dependence of the electronic properties Quantum Size Effect: discrete energy levels in the Valence Band

for very small clusters up to 100 atoms!

Beyond this size, a band of states will be formed.

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Pd/Al2O3/NiAl(110)

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Metal Particles: Electronic Structure

(II) The nature of the metal-support interaction Two cases:1. Localized chemical bonds (ionic/covalent)

→ characteristic chemical shifts of the state involved

2. Charge redistribution between the metal overlayer and the oxide support (or the metal-substrate, if a thin oxide film is used)

→ a simultaneous shift of all core and valence levels of the

substrate should be detectable An example: Pd adsorb CO 1 Mono Layer of Pd on Al can not adsorb CO

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Are the properties of thin oxide films influenced by the metal substrate underneath?

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Metal Particles: Adsorption Behaviour

• Size dependent adsorption• Size dependent catalytic properties• Structural heterogeneity of the composite system can be the origin

of an extraordinary adsorption and reaction behaviour • The factors which influence the bonding of an adsorbate to a

deposited metal particle of varying size :

1. Electronic Effects

2. Geometric Effects: the local electronic structure depends on the local geometric arrangement of the surface atoms at that site. It is possible that a reaction can only occur if an ensemble of atoms in a particular geometry is available on a surface

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CO adsorption(CO as a probe molecule)

• The adsorption energy of CO on Pd increases with decreasing particle size• A shift of the peak towards higher BEs as the particles size decreases

CO adsorption on Pd particles of different size deposited onto Al2O3/NiAl(110): (a) C 1s spectra (taken at 90 K after CO saturation) including the spectrum of (2× 2) CO/Pd(111) (b) TD spectra

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CO adsorption

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CO desorption and dissociation

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Gold particles smaller than about 3-5 nanometers

• CO oxidation at temperatures as low as -70°C • very high selectivity in partial oxidation

reactions, such as 100% selectivity at 50°C for oxidation of propylene to propylene oxide as well as near room temperature reduction of nitric oxide with H2 using alumina-supported gold nanoparticles

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Reference

• M. Baumer, H-J Freund, Progress in Surface Science 61(1999) 127-198

• http://www.wtec.org/loyola/pdf/nano.pdf