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Special Properties of Au Nanocatalysts. Maryam Ebrahimi Chem 750/7530 March 30 th , 2006. • Introduction • Goodman’s Research Laboratory • Gold Nanoparticles • Research Proposal • References. Outline.

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Special properties of au nanocatalysts l.jpg

Special Properties of Au Nanocatalysts

Maryam Ebrahimi

Chem 750/7530

March 30th, 2006


Outline l.jpg

•Introduction

•Goodman’s Research Laboratory

•Gold Nanoparticles

•Research Proposal

•References

Outline


Introduction l.jpg

•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 isheterogeneous 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

Introduction


Gold nanoparticles l.jpg

•Au has long been known as beingcatalytically far less activethan other transition metals.

•Because of its inertness, Au was formerly considered as anineffective catalyst.

•This assumption was based on studies where Au was present as relatively large particles(diameter > 10 nm)or inbulk formsuch as single crystal.

•Haruta et al. have shown exceptionally high CO oxidation activity on supportednano-Aucatalysts even at sub-ambient temperatures(200 K).

Gold Nanoparticles


Gold nanoparticles6 l.jpg

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

Gold Nanoparticles


The structure of catalytically active gold on titania l.jpg

•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

The structure of Catalytically Active Gold on Titania


The most active size 3 3 5 nm science 281 1998 1647 l.jpg

The Most Active Size: 3-3.5 nmScience, 281 (1998) 1647


The most active size 3 3 5 nm catalysis letters 99 2005 1 catalysis today 111 2006 22 33 l.jpg

The Most Active Size: 3-3.5 nmCatalysis Letters,99 (2005) 1 Catalysis Today,111 (2006) 22-33


Slide10 l.jpg

Gold monolayers & bilayers that completely wet the oxide support,

eliminate direct support effects.

Science, 306 (2004) 252


Particle thickness and shape co adsorb strongly on the au bilayer structure l.jpg

• On the basis ofkinetic 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.

Particle thickness and shape(CO Adsorb strongly on the Au bilayer structure)


Science 306 2004 252 catalysis today 111 2006 22 33 l.jpg

Science, 306 (2004) 252 Catalysis Today,111 (2006) 22-33


Strong metal support interaction smsi l.jpg

•A key feature ofAu grown on TiOx/Mo(112)is thestrength of the interactionbetween theoverlayer Auand thesupport comprised of strong bonding between Au andreduced Tiatoms of the TiOx support, yielding electron-rich Au.

• Recenttheoreticalstudies: 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.

Strong metal support interaction (SMSI)


Surface defects l.jpg

•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.

Surface Defects


Essential features of the interaction of au with tio 2 l.jpg

(1)wetting of the support by the cluster

(2)strongbonding between the Au atoms at theinterface 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 bulkdefects, 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.

Essential Features of the Interaction of Au with TiO2


Research proposal l.jpg

•Electronic properties of deposited metal clusters and thin films:

how does theelectronic 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

elementarysteps involved? What is the prevailing nucleation

mechanism? Under whichconditions are ordered/disordered particles formed? Is the growth processinfluenced by an ambient of certain gases?

•Interaction with gases: in which way does the interaction strength/adsorptionenergy change with size? Is the particle shape altered by gas adsorption?

•Catalytic activity: how does the activity/selectivity change with dispersion. Aremetal-support interactions of relevance?

Research Proposal


Research proposal17 l.jpg

•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.

Research Proposal


Slide18 l.jpg

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

Research Proposal


References l.jpg

1. D.W. Goodman et al., Science 281 (1998) 1647-1650

2. D.W. Goodman et al., Science 306 (2004) 252-255

3. D.W. Goodman et al., Catalysis Today 111 (2006) 22-33

4. D.W. Goodman et al., Catalysis Letters 99 (2005) 1-4

5. D.W. Goodman et al., J. Phys. Chem. B 108 (2004) 16339-16343

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

References


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