This document discusses the properties and reactivity of nanoparticles for catalysis applications. It notes that nanoparticles have a high surface area to volume ratio, which allows them to be more reactive. The physical and electronic properties, including reactivity, of particles varies significantly as size is reduced to the nanoscale. Studies have shown that properties like chemisorption and catalytic activity depend on particle size and number of atoms. The reactivity of very small clusters, even when supported, relates to their distinct electronic structure based on atom number. Particle shape, support interactions, and alloy composition also influence catalytic properties at the nanoscale.

1. G.PRASHANTH
1ST M.TECH
NANOSCIENCE AND TECHNOLOGY

2.  Large a surface-to-volume ratio as possible in order to
economise the numberof atoms required.
 This is the solution provided by nanoparticles.

3.  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.

4.  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

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

6.  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

7.  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.

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

9.  Their reactivity with regard to bond breaking
(dissociation of CO) or bond formation (trimerisation
of 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.

10.  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.

11.  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.

12. 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

13.  They can influence particle morphology.
• Electron transfer to or from the
support, so that the particles have
either
an excess or a deficit of electrons and
consequently have less tendency to
accept or donate electrons, respectively.
• An epitaxial stress that can modify the
particle structure the lattice parameter,
and the particle morphology.

14. High-resolution TEM images of Pd particles deposited by atomic
beam
on 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

15.  These different influences can be used to explain the
unexpected properties
 of small gold particles when catalysing the oxidation of
CO at low temperatures

16.  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.

17.  The major factors that influence alloying effects are as
follows
Effect of Surface Segregation
Geometric Effects
Electronic Effects

18.  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).

19.  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) andon
the 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.

20.  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.

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