Transcript Catalysis on the Nanoscale

Special Properties of Au
Nanocatalysts
Maryam Ebrahimi
Chem 750/7530
March 30th, 2006
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Outline
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Introduction
Goodman’s Research Laboratory
Gold Nanoparticles
Research Proposal
References
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Introduction
• Metal oxide interface, metal coatings or dispersed metals on oxide supports
play an important role in many technological areas.
• One of the areas where deposited metal particles are technically employed
to a large extent is heterogeneous catalysis.
• There is still a lack of fundamental knowledge about the essential properties
of thin metal films and small metal particles on oxide supports. So, an
increasing number of model studies like “model catalysis” have been
introduced. One approach comes from ultrahigh vacuum (UHV) surface
science aiming at an understanding of the elementary steps involved on a
microscopic level.
• Particle-size effects and the role of metal-support interactions
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Gold Nanoparticles
• Au has long been known as being catalytically far less active
than other transition metals.
• Because of its inertness, Au was formerly considered as an
ineffective catalyst.
• This assumption was based on studies where Au was present as
relatively large particles (diameter > 10 nm) or in bulk form
such as single crystal.
• Haruta et al. have shown exceptionally high CO oxidation
activity on supported nano-Au catalysts even at sub-ambient
temperatures (200 K).
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Gold Nanoparticles
Supported Nano-Au catalysts exhibit:
• an extraordinary high activity for low-temperature catalytic
combustion
• Partial oxidation of hydrocarbon
• Hydrogenation of unsaturated hydrocarbons
• Reduction of nitrogen oxides
• Propylene epoxidation
• Methanol synthesis
• Environmental catalysis
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The structure of Catalytically
Active Gold on Titania
• Cluster size and morphology, particle thickness and
shape
• Support effects:
Nature of the support material, Surface defects, Metal-Support charge
transfer, Au- support interface.
• Metal oxidation state
• Au-oxide contact area
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The Most Active Size: 3-3.5 nm
Science, 281 (1998) 1647
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The Most Active Size: 3-3.5 nm
Catalysis Letters,99 (2005) 1
Catalysis Today,111 (2006) 22-33
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Gold monolayers & bilayers that completely wet the oxide support,
eliminate direct support effects.
Science, 306 (2004) 252
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Particle thickness and shape
(CO Adsorb strongly on the Au bilayer structure)
• On the basis of kinetic studies
and scanning tunneling
microscopy (STM): Au consists
of bilayer islands that have
distinctive electronic and
chemical properties compared
to bulk Au.
• Two well-ordered Au films
(monolayer and bilayer)
completely wet an ultrathin
titania surface.
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Science, 306 (2004) 252
Catalysis Today,111 (2006) 22-33
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Strong metal support interaction (SMSI)
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A key feature of Au grown on
TiOx/Mo(112) is the strength of the
interaction between the overlayer Au
and the support comprised of strong
bonding between Au and reduced Ti
atoms of the TiOx support, yielding
electron-rich Au.
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Recent theoretical studies: importance
of reduced Ti defect sites at the
boundary between Au clusters and a
TiOx interface in determining the Au
cluster shape and electronic properties
via transfer of charge from the support
to Au.
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Surface Defects
• The introduction of defects into a crystal can dramatically
change its electronic properties
• Defects can affect the chemistry of bare metal-oxide surfaces
• Au particles bind more strongly to a defective surface than to a
defect deficient surface. There is significant charge transfer
from the support to the Au particles. Au particles don’t bind to
a perfect TiO2 surface.
• Defect sites on the oxide support play an important role in the
wetting of Au particles yielding electron-rich Au. But the
support itself need not be directly involved in the CO
oxidation reaction sequence.
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Essential Features of the Interaction of Au with
TiO2
(1) wetting of the support by the cluster
(2) strong bonding between the Au atoms at the interface with
surface defects (reduced Ti sites)
(3) electron-rich Au
(4) annealing at temperatures in excess of 750 K, sufficient to
create and mobilize surface and bulk defects, is crucial in
preparing an active catalyst
(5) oxidation leads to deactivation via sintering of Au
Goodman: Au particle size is related to activity, bilayer
Au structure and the strong interaction between Au
and defect sites on the TiO2 surface and critical for
CO oxidation activity.
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Research Proposal
• Electronic properties of deposited metal clusters and thin films:
how does the electronic structure develop with increasing
size/thickness?
• Metal-oxide interface: what is the nature and strength of the bonding?
• Adsorption and adhesion energies.
• Diffusion of metal atoms on oxide supports.
• Nucleation and growth: what are the activation energies for the
elementary steps involved? What is the prevailing nucleation
mechanism? Under which conditions are ordered/disordered particles
formed? Is the growth process influenced by an ambient of certain
gases?
• Interaction with gases: in which way does the interaction
strength/adsorption energy change with size? Is the particle shape
altered by gas adsorption?
• Catalytic activity: how does the activity/selectivity change with
dispersion. Are metal-support interactions of relevance?
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Research Proposal
• The purpose of this program is to explore and manipulate the size,
morphology and chemical environment of gold-containing
nanoparticles with the goal of optimizing their reactivity with
respect to elementary reactions that are of widespread interest in
heterogeneous catalysis.
• The materials focus is on nanoscale molecular catalysts
incorporating the early transition metals (like: Ti, V, Cr, Mn, Fe) or
late transition metals (like: Rh, Pd, Pt) which may have promising
catalytic properties and may offer significant advantages over
more commonly used noble metals.
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Research Proposal
The main steps of the research program involve:
(1) the development of new methodologies for the preparation of well-defined
nanoparticles
(2) reactivity studies as a function of size, morphology and chemical environment
chemical environment : modification of the surface
(a) adding electropositive or electronegative elements
(b) Deposing transition metals, rare earth elements (Ce in the metalic or oxidized
form)
(c) depositing Au nanoparticles on the functionalized substrate
(3) the development and application of new theoretical methods for understanding and
predicting the structure and reactivity of metal-containing nanoparticles. Current
methods being explored for nanoparticle preparation include templating on
strained metal surfaces, deposition of size-selected clusters and impregnation into
nanoporous materials (collaboration with Prof. Uzi Landman at Georgia Tech., Prof.
Jense Norskov in Denmark, and Dr. Pacchioni in Italy)
(4) Methods of characterization: STM, STS, UPS, XPS,FT-IR, HREELS, STM-IETS, TPD
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References
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D.W. Goodman et al., Science 281 (1998) 1647-1650
D.W. Goodman et al., Science 306 (2004) 252-255
D.W. Goodman et al., Catalysis Today 111 (2006) 22-33
D.W. Goodman et al., Catalysis Letters 99 (2005) 1-4
D.W. Goodman et al., J. Phys. Chem. B 108 (2004) 1633916343
6. D.W. Goodman et al., Surface Science 600 (2006) L7-L11
7. D.W. Goodman et al., Applied Catalysis A 291 (2005) 32-36
8. D.W. Goodman et al., Science 310 (2005) 291-293
9. M. Baumer & H-J Freund, Progress in Surface Science 61
(1999) 127-198
10. G.A. Somorjai et al., Topics in Catalysis 24 (2003) 61-72
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