1. Role of Nanomaterials in Catalysis
National
Centre for
Catalysis Research
S. Navaladian
National Centre for Catalysis Research,
Department of Chemistry,
Indian institute of Technology madras,
Chennai-36.

2. Introduction Nanomaterials What is new then?
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 (< 5 nm) for CO oxidation

3. CO oxidation I, Au/-Fe2O3 (Au/Fe = l/19, coprecipitation, 400°C);
(2) 0.5 wt% Pd--Al2O3J (impregnation, 300°C);
3. Au fine powder;
4, Co3O4 (carbonate, 400°C);
5, NiO (hydrate, 200°C);
6, Cu-Fe2O3 (hydrate. 400°C);
7. 5 wt% Au/a-Fe2O3 (impregnation, 200°C); 8). 5 wt% Au /-Al2O3J (impregnation. 200°C).
Haruta et al., Chemistry Letters, (1987) 405

4. H2 Oxidation Haruta et al., Journal of Catalysis, 115(1989)301
I, Au/-Fe2O3 (Au/Fe = l/19, coprecipitation, 400°C);
(2) 0.5 wt% Pd--Al2O3J (impregnation, 300°C);
3. Au fine powder;
4, Co3O4 (carbonate, 400°C);
5, NiO (hydrate, 200°C);
6, Cu-Fe2O3 (hydrate. 400°C);
7. 5 wt% Au/a-Fe2O3 (impregnation, 200°C); 8). 5 wt% Au --Al2O3J (impregnation. 200°C).
Haruta et al., Journal of Catalysis, 115(1989)301

5. 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.

6. 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

7. Surface area dependence on particle size
(n- Number of
atoms
per particle)
G. Bond and D. T. Thompson, Catalysis Reviews, 41 (1999) 319

8. Melting point decreases dramatically as the particle
Melting point Vs Particle size
Melting point decreases dramatically as the particle
size gets below 5 nm

9. Unusual structures Shape of fcc gold < 5 nm
truncated octahedron, (B) icosahedron,
(C) Marks decahedron and (D) cuboctahedron

10. Surfece to bulk ratio with size
Spherical iron
nanocrystals
Journal of Physical Chemistry, 100(1996)12142

11. Surface to volume ratio
Source: Nanoscale Materials in Chemistry, Wiley, 2001

12. Pd
M 55
M 309
M 561
PtFe

13. Electron motion becomes confined, and quantization sets in
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

14. CB Energy VB Energy Diagrams of Semiconductor NANOPARTICLE MOLECULEDE
MOLECULE
LUMO
HOMO DE CB VB Eg
BULK SOLID
Energy

15. 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 ( 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

16. Band diagram of CdTe nanocrystals
UV-visible spectra
CdTe nanocrystals
CdSe nanocrystals

17. Colloidal CdSe quantum dots dispersed in hexane
Colour change with particle size
Quantum dots are semiconductors particles that has all three dimensions confined to the nm length scale
Colloidal CdSe quantum dots dispersed in hexane
Size decreasing

18. Nanomaterials with various morphology

19. Density of states of nanoparticles with different morphology
A. P. Alivisatos et al., Journal of Physical Chemistry,100 (1996) 13226

20. Band structure with various morphology
A. P. Alivisatos et al., Journal of Physical Chemistry,100 (1996) 13226

21. 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

22. DFT calculation for two pathways of CO oxidation
favorable
Hvolbaek et al., Nanotoday, 2 (2007)14

23. 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

24. 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

25. 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?.

26. Unsupported Au is also active? yes
CO oxidation – a-gold
Dealloying of Ag/Au alloy
Free corrosion
(128 min)
Anodic corrosion
(15 min)
100 nm
f-gold
a-gold
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

27. 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

28. 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) 5512–5516

29. 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) 5512–5516

30. UPS spectra of Pt and Pt alloys
d band is closer to the Fermi level
Stamenkovic et al., Nature materials, 6 (2007)241

31. UPS spectra of Au, Ag and Cu

32. 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