nano size metal particles for catalysis [357]. The ultrafine copper chromite catalyst is obtained
using this method [45]. Liaw and Chen [45] prepared the catalyst by reducing copper nitrate
0.1 M in an aqueous solution with sodium borohydride
1 M.
The aqueous
sodium borohydride solution was added into the copper
nitrate solution by a micropump under a flow of nitrogen. Cr introduced to copper by reducingco
precipitating with sodium borohydride in an aqueous solution of copper nitrate and an
appropriate amount of Cr salt. The resulting black precipitate was thoroughly washed with distilled
water three times to remove the residual ions and then washed with ethanol to remove water. The
precipitate are then dried and calcined. They studied catalytic properties of catalyst prepared by
this method for hydrogenation on monofunctional olefinic and carbonyl and bifunctional conjugated
and nonconjugated compounds and compared with those of a commercial catalyst of copper chromite.
The ultrafine catalysts of Cr CuB containing a much lower content of chromium Cr5mol were
more active than the commercial copper chromite Cr 50mol. The authors proposed that these Cr
CuB catalysts are highly promising for replacing copper chromite for liquid phase hydrogenation
reactions.
3.15 Sol,gel method
The sol gel methods show promising potential for the synthesis of mixed oxides catalysts. The
versatility of the sol gel techniques allows control of the texture, composition, homogeneity, low
calcination temperatures
minimizing the
undesired aggregation of the particles, and structural properties of solids, and makes possible
production of tailored materials such as dispersed metals, oxidic catalysts and chemically modified
supports [358].
Such methods are used primarily for the fabrication of materials typically a metal oxide
starting from a chemical solution sol which acts as the precursor for an integrated network or gel
of either discrete particles or network polymers [359]. Typical precursors are metal alkoxides and
metal nitrates, which undergo hydrolysis and polycondensation reactions to form either a
network ‘elastic solid’ or a colloidal suspension or dispersion a system composed of discrete often
amorphous submicrometer particles dispersed to various degrees in a host fluid.
The hydrolysis of precursor molecules and the condensation between the resulting reactive
species, are the essential reactions of the sol gel process [360]. The resulting processes involved and
the properties of the precursor molecules have a decisive influence on the resulting material
properties [360]. On addition of water, the metal alkoxides
[MORn] readily
hydrolyze as
represented by Eqn. 65. MORn + H
2
O → MOR
n 1
OH + ROH 65
Hydrolysis is followed by condensation to form M O M
bonds via
either dehydration
or dealcoholation as described in Eqs. 66 and 67,
respectively: RO
m
M OH + HO MOR
m
→ RO
m
M O MOR
m
+ H
2
O 66
M OH + RO M → M O M + ROH 67 In this manner, inorganic polymeric oxide
networks are
built up
progressively. The
hydrolysis, condensation
and polymerization
reactions are governed by several factors, including the molar ratio of water to alkoxides, choice of
solvents, temperature and pH or concentration of acid or base catalysts. There are essentially three
different kinds of sol gel or gel technology for preparation of catalysts.
3.15.1 Citric acid complexing method
Citric acid CA assisted sol gel method namely Pechini approach is a facile synthesis for
producing homogeneous nanocomposites [10], in which the use of citric acid as chelating agent
ensures the formation of homogeneous transparent metal citrate gels, and the intimate mixing of
components ensures homogeneity of the final product. Li et al.[10] have prepared Cu Cr O
nanocomposites by citric acid CA complexing approach in which 0.01 mol CuNO
3 2
and 0.02 mol CrNO
3 3
are dissolved in 100 mL deionized water to obtain a mixed metal nitrate solution. Then
citric acid is added to this solution and the molar ratio of citric acid to the total metal ions is fixed to
be 2:1. After stirring for 30 min, the solution is heated at 95
C for several hours to evaporate the water solvent to produce dark brown transparent
viscous gels. The gels are then dried at 160 C for 2
h to obtain the foamy dark powders, which are denoted as precursors of Cu Cr O nanocomposites
CA Cu Cr. After grinding, the precursors are successively heated at 600
C for 3 h to obtain the final black Cu Cr O nanocomposites.
Li and cheng [229] have prepared Bi
2
O
3
CuCr
2
O
4
coreshell nanomaterials following the
Copyright © 2011, BCREC, ISSN 1978 2993
facile synthesis and show that the nanomaterials demonstrate high catalytic activities towards the
oxidation of CO. Yan et al. [12] also synthesized CuCr
2
O
4
TiO
2
heterojunction via a facile CA assisted sol gel method for photocatalytic H
2
evolution. The optimized composition of the nanocomposites
has been
found to
be CuCr
2
O
4
.0.7TiO
2
. And the optimized calcination temperature and photocatalyst mass concentration
are 500 C and 0.8 gl, respectively.
3.15.2 Alkoxide sol,gel method Pechini Method
The Pechini method [361,362] based on polymeric precursors, is used to prepare spinels
and it does not require high temperature calcinations and permits good stoichiometric
control as well as reproducibility. This method consists of the formation of a polymeric resin
between a metallic acid chelate and polyhydroxide alcohol by polyesterification. The metal nitrate
solution is mixed with a stoichiometric amount of citric acid. The resulting solution is stirred for
about 1 hour on a hot plate and the temperature is stabilized at 70
C. The mixture is heated to 900 C, at which point ethylene glycol is added at a
mass ratio of 40:60 with respect to citric acid. The temperature is maintained constant up to resin
formation, which polymerizes at 300 C. The
precursor powders are then calcined for 4 hours at various temperatures, ranging from 500 to 900
C, or at 900
C for 8 hours [363]. The crystallization of the spinel structure starts upon calcining at 700
C. Cu
0.8
Ni
0.2
Cr
2
O
4
is the only phase present upon calcination at 900
C. The process of the Pechini method is almost the same as that of the citrate gel
method, except that metal nitrates are dissolved in alcohols, instead of water [364].
The major disadvantages of using the metal alkoxide based sol gel process are its moisture
sensitivity and the unavailability of suitable commercial precursors especially for mixed metal
oxides. The sol gel synthesis of mixed oxides from alkoxide mixture usually suffers from the different
hydrolysis
susceptibilities of
the individual
components and
the benefits
of improved
homogeneity can be lost during the hydrolysis of the alkoxides, which may ultimately lead to
component segregation and mixed phases in the final materials. To achieve homogeneous mixed
oxides with predetermined compositions, the difference in reactivity has been minimized by
controlled prehydrolysis of the less reactive precursor [365], by chemical modification of the
precursors
[366], by
using single source
heterobimetallic alkoxide precursors [367], or by non hydrolytic sol gel processes [368].
3.15.3 Non,alkoxide sol,gel method
