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.

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

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

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

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.

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

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

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

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

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

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

Pd M 55 M 309 M 561 PtFe

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

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

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

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

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 1-100 nm length scale Colloidal CdSe quantum dots dispersed in hexane Size decreasing

Nanomaterials with various morphology

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

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

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

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

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

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

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?.

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

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

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

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

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

UPS spectra of Au, Ag and Cu

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