G.PRASHANTH

1ST M.TECH NANOSCIENCE AND TECHNOLOGY

Large a surface-to-volume ratio as possible in order to economise the numberof atoms required.

This is the solution provided by nanoparticles.

It is when the particle dimensions are reduced to the nanoscale that the relative variation of the different sites becomes most marked.

It is also in this size range that the physical and electronic properties of the particles vary the most.

Specific chemisorption and catalytic properties can thus be expected in the ‘nano’ range of particle sizes.

The reactivity depends on the number of atoms contained in the cluster.

It is also tempting to suggest a correlation between the reactivity and the ionisation potential in these small clusters

Dependence of the hydrogen addition rate (k) and ionization energy (IP) on the number of atoms in iron clusters

The binding is strong when the number of electrons is odd.

A possible explanation is that the cluster with an odd number of electrons has a smaller ionisation potential thereby increasing the availability of the HOMO electrons.

It is quite clear that free clusters cannot be used as such in catalysis

Naturally, addition of an electron to a small cluster with discrete and well separated electron energy levels can greatly affect the energy level of the HOMO.

On the other hand, it should be noted that, for clusters comprising a hundred or more atoms, the gap between electron levels close to the Fermi level can be considered as a continuum and an additional electron will not significantly modify their Fermi levels.

In any case, this shows, if such was necessary, that the reactivity and the electronic (and atomic) structure are related.

Some work has been done on the reactivity of mass-selected supported metal particles with very small sizes. It has been shown that the chemisorptionproperties of supported clusters, in the size range from a few atoms to several tens of atoms, varies with the number of constitutive atoms

Their reactivity with regard to bond breaking (dissociation of CO) or bond formation (trimerisationof acetylene [27]) also varies significantly with the number of atoms making up the cluster.

It seems as though the reactivity of very small clusters (at most a few tens of atoms), even supported, is related to their particular and distinct electronic properties for clusters containing a well-defined number of atoms.

Measure of the reactivity of a solid surface.

Destabilising the molecule to be transformed, the catalytic solid succeeds in lowering the energy barrier to be overcome in order to modify it and thereby accelerates the process at a given temperature.

It must depend on the structure of the surface sites and hence on the size of the nanoparticles.

Size reduction can prove useful for catalytic processes, by inducing better activity and better selectivity in the target produce and better stability in the catalyst.

The higher reactivity of atoms with lower coordination.

Relative variation of the turn-over number (TON), the number of molecules transformed per surface atom and per unit time in given reaction conditions as a function of the dispersion for hydrogenation of butene-1 (black squares), butyne-1 (white squares), butadiene-1,3 (plus signs), and isoprene (white triangles). θ is the particle size and Nt is the total number of metal atoms in a particle of size θ

They can influence particle morphology.

• Electron transfer to or from the support, so that the particles have eitheran excess or a deficit of electrons and consequently have less tendency toaccept or donate electrons, respectively.• An epitaxial stress that can modify the particle structure the lattice parameter, and the particle morphology.

High-resolution TEM images of Pd particles deposited by atomic beamon amorphous carbon, illustrating the different shapes: (a) cubo-octahedral (predominant)and (b) icosahedral (with C5 axis perpendicular to the support plane).Reprinted from with the kind permission of Elsevier

These different influences can be used to explain the unexpected properties

of small gold particles when catalysing the oxidation of CO at low temperatures

In alloys, several further parameters must be taken into account: the surface composition, the relative positions of the two components, and the associated changes in electronic properties.

The major factors that influence alloying effects are as follows

Effect of Surface Segregation

Geometric Effects

Electronic Effects

The surface composition is controlled by the effects of segregation of one of the partners on the surface, this happening so as to minimise the energy at thermodynamic equilibrium.

It is usually the element with the lowest surface tension which segregates at the surface.

This phenomenon is enhanced when the segregating element has the largest atomic radius. However, the phenomenon of surface segregation is moderated for A–B alloys with exothermic formation energy, i.e., those with

ΔHA–B >1 /2 (ΔHA–A +ΔHB–B).

Geometric effects are related on the one hand to dilution of an active metal by a less active metal with respect to some given reaction (dilution effect) andonthe other hand to structural changes, such as the appearance of an ordered arrangement of the partners at the surface (surface structures) and/or surface reconstructions.

This heading refers to electronic modifications induced either by interaction between the two metals, e.g., formation of a chemical bond between the two partners, charge transfer, polarisation, etc., or by the stress exerted on the catalytically relevant metal by its partner.

Once again, these influences will modify the valence orbitals of the surface sites of the catalyst, and hence also the interactions between the molecular orbitals of the reactants and the reaction products.

C. Br´echignac P. Houdy M. Lahmani (Eds.) Nanomaterials and Nanochemistry