Transcript Reactivity Properties

Electronic Structure
and
Chemical Reactivity
Chem 7530/750
Maryam Ebrahimi
February 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
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thermal conductivity of oxides.
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
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