A1258150468aPpTr

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Role of Nanomaterials in Catalysis
S. Navaladian
National Centre for Catalysis Research,
Department of Chemistry, Indian institute of
Technology madras, Chennai-36.
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Introduction
Nanomaterials
Materials of size in the range of nm .i. e., 10-9
m
Nanomaterials are well known in catalysis atom
economy
What is new then? Advancement in the electron
microscopic microscopic techniques
Good example Supported Gold nanoparticles of
size (lt 5 nm) for CO oxidation
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CO oxidation
I, Au/?-Fe2O3 (Au/Fe l/19, coprecipitation,
400C) (2) 0.5 wt Pd-?-Al2O3J (impregnation,
300C) 3. Au fine powder 4, Co3O4 (carbonate,
400C) 5, NiO (hydrate, 200C) 6, Cu-Fe2O3
(hydrate. 400C) 7. 5 wt Au/a-Fe2O3
(impregnation, 200C) 8). 5 wt Au
/?-Al2O3J (impregnation. 200C).
Haruta et al., Chemistry Letters, (1987) 405
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H2 Oxidation
I, Au/?-Fe2O3 (Au/Fe l/19, coprecipitation,
400C) (2) 0.5 wt Pd-?-Al2O3J (impregnation,
300C) 3. Au fine powder 4, Co3O4 (carbonate,
400C) 5, NiO (hydrate, 200C) 6, Cu-Fe2O3
(hydrate. 400C) 7. 5 wt Au/a-Fe2O3
(impregnation, 200C) 8). 5 wt Au
-?-Al2O3J (impregnation. 200C).
Haruta et al., Journal of Catalysis, 115(1989)301
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CO oxidation on various Au/ metal oxide supports
Open symbols reducible Supports Closed symbols
Non-reducible supports
B. Hvolbaeka, and K. Nørskov, Nanotoday, 2 (2007)
14.
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What happens when size decreases?

• Surface area increases
• Co-ordination number of atom decreases (more no
  of exposed atoms)
• Band gap redox potentials HOMO-LUMO gap
  altered-reactivity varies
• Melting point variation
• Enhanced toughness
• Magnetic properties
• Unuasual crystal structures
• Surface plasmon resonance in metals
• Surface enhancement in Raman spectrum
• Morphological effect

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Surface area dependence on particle size
(n- Number of atoms per particle)
G. Bond and D. T. Thompson, Catalysis Reviews, 41
(1999) 319
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Melting point Vs Particle size
Melting point decreases dramatically as the
particle size gets below 5 nm
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Unusual structures
Shape of fcc gold lt 5 nm

• truncated octahedron, (B) icosahedron,
• (C) Marks decahedron and (D) cuboctahedron

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Surfece to bulk ratio with size
Spherical iron nanocrystals
Journal of Physical Chemistry, 100(1996)12142
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Surface to volume ratio
Source Nanoscale Materials in Chemistry, Wiley,
2001
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Pd
M 309
M 561
M 55
PtFe
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Origin of the Properties
Bulk Metal
Nanoscale metal
Decreasing the size
Unoccupied states
occupied states
Separation between the valence and conduction
bands
Close lying bands
Unbound electrons have motion that is not confined
Electron motion becomes confined, and
quantization sets in
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Energy Diagrams of Semiconductor
NANOPARTICLE
Energy
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Magnetic properties

• Bulk Al metal is diamagnetic Al clusters (13
  atoms- 0.8 nm) is magnetic.
• due to the change in the electronic
  configuration.
• Pd and Pt bulk is not magnetic, but
  nanoparticles are ferromagnetic. Due to
• the structural changes associated with size.
• PtFe alloy particles ( 30 -100 nm) are showing
  higher coercively.
• Both Au and Au NPs are non-magnetic poor
  density of states.
• Au NPs capped with thiols are magnetic due to
  eth alteration of d band structure
• Large spin-orbit coupling of noble metal leads
  to anisotropy in the crystals leading
• to the high ordering of spin.

Enhanced strength and toughness

• Due to the defects present in the nanoparticles
  mainly dislocations

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Band diagram of CdTe nanocrystals
UV-visible spectra
CdTe nanocrystals
CdSe nanocrystals
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Colour change with particle size
Quantum dots are semiconductors particles that
has all three dimensions confined to the 1-100 nm
length scale
Colloidal CdSe quantum dots dispersed in hexane
Size decreasing
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Nanomaterials with various morphology
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Density of states of nanoparticles with different
morphology
A. P. Alivisatos et al., Journal of Physical
Chemistry,100 (1996) 13226
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Band structure with various morphology
A. P. Alivisatos et al., Journal of Physical
Chemistry,100 (1996) 13226
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The dissociative chemisorption energies for
oxygen DFT calculation
bcc (210) surface ( Fe, MO, W)
fcc (211) surface For other metals
Only gold shows endothermic chemisorption- highly
inert for oxygen
Au has d-states so low in energy that the
interaction with oxygen 2p states is net
repulsive.
Hvolbaek et al., Nanotoday, 2 (2007)14
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DFT calculation for two pathways of CO oxidation
favorable
Hvolbaek et al., Nanotoday, 2 (2007)14
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DFT calculation of binding energies
Binding energy is low for low co-ordination of
Au. Hence CO oxidation is facile even at low
temperatures
Hvolbaek et al., Nanotoday, 2 (2007)14
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Particle size and co-ordination number

• Active sites are corner atoms
• Below 5 nm of size CO oxidation activity is
  drastically increases
• Corner and edge atoms increasing. Flat surface
  are remaining same
• and decreasing

Hvolbaek et al., Nanotoday, 2 (2007)14
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• Why Au nanoparticles are active?
• Low co-ordination number. Atoms in the corner
  are
• active sites
• Defect site promoted catalytic effect
• Strong metal-support interaction (SMSI) on
  reducible supports like TiO2, Fe2O3. But,
  unsupported Au NPs is also active?.

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Unsupported Au is also active? yes
CO oxidation a-gold
Dealloying of Ag/Au alloy
After reaction
6 nm pores
25 nm pores
Likewise corner atoms may contribute for the
catalytic activity of Au NPs
Xu et al.,Journal of the American Chemical
Society, 129 (2007) 42-43
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Work function and Fermi level in solids
Work function of Ag Ag 4.26 eV Ag(100)
4.64 eV, Ag(110) 4.52 eV Ag(111) 4.74 eV
Also depends upon packing of The metals bcc
low work function fcc hcp high work function
W - Work function U - potential well EF Fermi
energy
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Important criterion for catalytic activity of
metals

• Position of centre of the d-band
• Shape and width of the d-electron density
• High d-electron density Fermi level is a factor
  indicating
• the catalytic activity
• Example Pt a versatile catalyst

T. Bligaard et al., Electrochimica Acta, 52
(2007) 55125516
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Calculated d-projected densities of states for
different Pt surfaces
Co ordination decreases
A shift of ?d by 1 eV takes place
Catalytically more active- Favorable CO
adsorption
Atomic density decreases
Bandwidth depends on the coordination number of
the metal and this leads to substantial
variations in the d-band centers
T. Bligaard et al., Electrochimica Acta, 52
(2007) 55125516
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UPS spectra of Pt and Pt alloys
d band is closer to the Fermi level
Stamenkovic et al., Nature materials, 6 (2007)241
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UPS spectra of Au, Ag and Cu
Ag
Cu
Au
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Concluding remarks

• The abnormal catalytic activity of Au NPs is
  attributed to
• the various factors such as low
  co-ordination of Au atoms,
• defect sites and metal-support interactions.
• The increase in the atoms with low
  co-ordination number shifts
• the d-band DOS to towards the Fermi level
  and changes the
• shape and width of the d-band in transition
  metals.
• But still clear-cut reasons are not available.
  This may go a long
• way in explaining the catalytic activity of
  nanomaterials.

Thank you