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
Non alkoxide sol gel
process, involving
hydrolysis and condensation of metal salts, avoids the disadvantage of alkoxide sol gel process high
sensitivity to moist environment, however, has still the disadvantage of different hydrolysis
susceptibilities of the individual components [25]. One of the advantages of this method is the
important reduction of the required calcination temperatures,
minimizing the
undesired aggregation of the particles. This method was
found to be an effective route to synthesize mixed oxide nanoparticles with narrow size distribution
[25]. Ma et al. [369] presented a non alkoxide sol gel route to synthesised highly active and selective
Cu Cr catalysts for glycerol conversion. The synthesis involves dissolving 3.3 g of Cr
NO
3
.3.9H
2
O and 1.0 g of CuNO
3 2
.3H
2
O in 16 mL of ethanol at 60
C to give a clear dark blue solution. After adding 5.0 mL of propylene oxide, a
dark green transparent gel is formed within a few minutes under stirring. After drying overnight in
air at 70 C, the resulting xerogel is transferred to
a quartz reactor inside a tubular resistance furnace. The furnace is then ramped at 1
Cmin to a final temperature and is held for 120 min under
20 O
2
in Ar at a flow rate of 120 mlmin. The yield of copper and chromium in the oxide catalyst
is about to 100, and the ratio of Cu to Cr could be varied by the initial molar ratio of copper nitrate
and chromium nitrate.
The results show that the surface area of the Cu Cr catalyst is adjusted by the hydrolysis
conditions, CuCr molar ratio, and treatment conditions such as gas atmosphere and final
temperature. For the sample with CuCr = 0.5, the surface area of Cu Cr xerogel can reach 94 m
2
g and decreased to only 31 m
2
g after calcination at 500
C. The catalysts show significant catalytic activity and selectivity in glycerol conversion, i.e.
above 52 conversion of glycerol and above 88 selectivity to 1,2 propanediol at 210
C and 4.15 MPa H
2
pressure. CuCr
2
O
4
supported Cu catalysts are much more active than Cr
2
O
3
supported Cu catalysts. This indicates a strong interaction
between Cu and CuCr
2
O
4
that is significantly improving the effectiveness of the catalyst for
glycerol conversion.
Copyright © 2011, BCREC, ISSN 1978 2993
4. Conclusions
