varying in the range 133 210 Å between the temperatures limits 823 K 1073 K. The precursors
were prepared by hydrolysis of CrNO
3 3
.9H
2
O and CuNO
3 2
.3H
2
O and CuCH
3
COO
2
.H
2
O. Working in
different conditions
two polynuclear
coordination precursors [Cr
2
CuNH
3 2
OH
6
]NO
3 2
and [Cr
2
CuOH
8
].4H
2
O were obtained. The following reaction equations 61 and 62 may be
written for precursors’ synthesis: NH
4
Cu
2+
→ [CuNH
3 4
]
2+
→ [Cr
2
CuNH
3 2
OH
6
]NO
3 2
] ~10 11 Precursor A
61 Cr
3+
→ CrOH
3
→ [Cr
2
CuOH
8
].4H
2
O Precursor B
62 Copper chromite was obtained through thermal
decomposition of
the precursors.
Following decomposition mechanism eqns. 63 and 64 has
been predicted for the two compounds: Precursor A :
[Cr
2
CuNH
3 2
OH
6
]NO
3 2
→ [Cr
2
CuOH
6
]NO
3 2
→ Cr
2
CuOOH
6
→ Cr
2
O
2
OH
2
.CuO → Cr
2
O
83
.CuO → Cr
2
O
3
.CuO → CuCr
2
O
4
63 Precursor B:
[Cr
2
CuOH
8
].4H
2
O → Cr
2
CuOH
8
→ Cr
2
O
2
OH
2
CuO → Cr
2
O
3
.CuO → CuCr
2
O
4
64
3.8 Microemulsion method
The use of an inorganic phase in water in oil microemulsions
has received
considerable attention for preparing metal particles. This is a
new technique, which allows preparation of ultrafine metal particles within the size range 5 50
nm particle diameter [342].
Nanoparticles of copper chromium hexacyanide with varying particle size are prepared by Kumar
et al. [343] using the micro emulsion method and Poly vinylpyrrolidone PVP as a protecting
polymer. Two separate microemulsions of CuNO
3 2
and K
3
CrCN
6
with PVP are prepared and subsequently mixed together to get the precipitate
of copper chromium hexacyanide nanoparticles. The nanoparticles are separated out by adding
acetone in the resultant mixture and are washed many times with acetone and demineralized water.
The different mixing ratios of PVP to Cu ion concentration 20 to 200 are used to control the
size of the nanoparticles.
3.9 Combustion synthesis
Combustion synthesis CS [344] has emerged as important technique for the synthesis and
processing of advanced ceramics structural and functional,
catalysts, composites,
alloys, intermetallics and nanomaterials. In CS, the
exothermicity of the redox reduction oxidation or electron transfer chemical reaction is used to
produce useful materials [345]. Depending upon the nature of reactants: elements or compounds
solid, liquid or gas; and the exothermicity adiabatic temperature, T, CS is described as: self
propagating high temperature synthesis SHS; low temperature CS, solution combustion synthesis
SCS,
gel combustion, sol gel
combustion, emulsion combustion, volume combustion thermal
explosion, etc. Combustion synthesis processes are characterised by high temperatures, fast heating
rates and short reaction times. These features make CS an attractive method for the manufacture
of technologically useful materials at lower costs compared to conventional ceramic processes. Some
other advantages [346] of CS are:
i Use of relatively simple equipment ii Formation of high purity products
iii Stabilization of metastable phases and iv Formation of virtually any size and shape
products. Combustion synthesis has been extensively
used to prepare a variety of catalysts. Patil et al. [346] reviewed the recent developments in the field
with special emphasis on the preparation of ‘Catalysts’ and ‘Nanomaterials’ by solid state
combustion and solution combustion.
3.9.1 Self,propagating high,temperature synthesis SHS
The SHS method is being developed for the low cost production of engineering and other functional
materials, such
as advanced
ceramics, intermetallics, catalysts and magnetic materials.
The method exploits self sustaining solid flame combustion reactions which develop very high
internal material temperatures over very short periods. It therefore offers many advantages over
traditional methods, such as much lower energy costs, lower environmental impact, ease of
manufacture
and capability
for producing
materials with
unique properties
and characteristics [347].
Xanthopoulou and Vekinis [348] prepared the SHS catalysts and carriers from initial batch
mixtures consisting of nitrates and sulphates, metals and oxides, compacted under a pressure of 5
10 MPa in the form of rods of diameter 1 5 cm and, in some cases, by extrusion as honeycomb
carrier blocks with diameter 1 5 cm and channel
Copyright © 2011, BCREC, ISSN 1978 2993
size of about 5 mm. The samples were preheated in an electric furnace at temperatures of 700 900
C for a few minutes prior to initiation of SHS. The
specific area was increased by depositing a second oxide layer wash coat on the surface of SHS
carriers of about 0.9 4.9 aluminium oxynitrate denoted as OX, 3.2 for the Cu Cr O catalyst. Pd
was then deposited on the SHS carriers using standard aqueous impregnation followed by
calcination and reduction. Standard 0.05 or 0.5 PdAlNO
3 3
Al
2
O
3
catalyst systems produced by a conventional impregnation calcination reduction
process were used for comparison [348] . The author reported that the Cu Cr O catalyst
prepared by SHS is resistant to fuel impurity poisoning and used as carrier for 0.05 Pd,
achieved
50 conversion
light off at
temperatures about 50 C lower than conventional
0.5 PdAl
2
O
3
catalysts for CO oxidation [348].
3.9.2 Solution combustion synthesis
