Chemistry Reference
In-Depth Information
Scheme 16.16
Sonochemical
formation of cobalt nanoclusters.
equally by sonochemical decomposition of Ni(CO)
4
as neat liquid or in decalin solution [74] and
nanophase amorphous nickel supported on silica
microspheres also has been obtained [75].
Nanosized amorphous oxides such as NiFe
2
O
4
powder can be prepared equally by sonochemical
decomposition of Fe(CO)
5
and Ni(CO)
4
in decalin
solutions under an oxygen pressure of 100-150 kPa
[76]. Microscopic characterisation gives no evidence
of crystalline formation and the material is an
agglomerate of nanoparticles with diameters of
>10 nm. Magnetic measurements also indicate
that NiFe
2
O
4
particles are superparamagnetic.
Ultrafine powders of Cr
2
O
3
and Mn
2
O
3
have been
prepared by sonochemical reduction of aqueous
solutions of ammonium dichromate and potassium
permanganate, respectively [77]. The powders are
nanosized (50-200 nm) and crystallisation may be
induced by heating. A continuous process based on
hydrodynamic cavitation
can be employed to prepare a
wide variety of metal oxides in grain sizes of 1-
10 nm, such as iron oxide, bismuth molybdate, per-
ovskites, platinum-loaded zeolite and other ceramics
and superconductors [78]. The method uses a
microfluidiser for mechanically generating hydrody-
namic cavitation and the internal pressure of the
liquid media is elevated from ambient pressure to
between 1000 to 25 000 psi. Nanocrystalline oxides
(Y
2
O
3
,TiO
2
, ZrO
2
, Cr
2
O
3
, Fe
2
O
3
, Co
3
O
4
, NiO, CeO
2
)
also have been prepared by hydrodynamic cavitation
[79].
A recent investigation has demonstrated the use-
fulness of ultrasonic irradiation in the preparation
of delaminated zeolites, which are a particular type
of modified oxides—microporous crystalline alumi-
nosilicates with three-dimensional structures—
having a greater catalytic activity than the layered
structures (clays) and mesoporous catalysts. In an
attempt to increase the pore size of zeolites, a layered
zeolite precursor was delaminated in an ultrasonic
bath operating at 40 kHz for 1 h [80]. This layered
aluminosilicate and a typical zeolite have similar
activities for
n
-decane cracking. In addition, delami-
nation provides greater site accessibility for larger
molecules, catalysing the formation of more liquid
(gasoline) and less gaseous products and coke.
The preparation of cobalt nanoclusters has been
performed by sonicating basic solutions of Co(II)
and hydrazine [81] (see Scheme 16.16). The col-
loidal cobalt nanoclusters consist primarily of cobalt,
although a small amount of oxygen, possibly as a
very thin oxide coating, also is present. The cobalt
nanoclusters are ferromagnetic and may be useful in
the construction of high-density recording media or
permanent magnets.
Recently, the amorphous magnetic metals (Fe, Ni,
and Co), and some amorphous magnetic alloys have
been used as catalysts in the aerobic oxidation of
hydrocarbons such as cyclohexane, methyl cyclo-
hexane, and adamantane [82]. Thus, the oxidation
of cyclohexane is carried out up to 40% conversion
with 80% selectivity for cyclohexanone and cyclo-
hexanol using nanostructured amorphous metals
like Fe and Co, and amorphous alloy like Fe
20
Ni
80
with oxygen (40 atm) at 25-28°C in the absence of
any solvent. In the aerobic oxidation isobutyralde-
hyde as co-reductant and a catalytic amount of acetic
acid are used. Such oxidations can be accomplished
at room temperature under 1 atm of oxygen, albeit
oxygen at 40 atm gives better conversion (see
Scheme 16.17).
Noble metal particles and bimetallic nanoparticles
in aqueous solutions can also be obtained by sono-
chemical procedures. Palladium metallic clusters
have been prepared at room temperature by sono-
chemical reduction of Pd(OAc)
2
and a surfactant,
myristyltrimethylammonium bromide, in THF or
methanol [83]. It is noteworthy that nanosized
amorphous Pd is obtained in THF but it is obtained
in crystalline form in methanol. In this solvent, and
in higher homologous alcohols, sonolyis of tetra-
chloropalladate(II) leads to Pd nanoclusters in which
carbon atoms, formed by complete decomposition of
the solvent, can diffuse. This results in an interstitial
solid having the formula PdC
x
(0 <
x
< 0.15) [84].
Noble metal nanoparticles of Au, Pd and Ag are
