Synthesis of mixed cerium-zirconium oxides by the sol-gel method

Article abstract of DOI:10.1134/S1070427207090054

Mixed oxides CeO₂-ZrO₂ with the Ce/Zr molar ratios varying in a wide range were synthesized by the sol gel method. The phase composition of the resulting particles was determined. The specific surface area of the mixed oxide calcined at various temperatures was found.

Full text of DOI:10.1134/S1070427207090054

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2007, Vol. 80, No. 9, pp. 1468 1471. Pleiades Publishing, Ltd., 2007.
Original Russian Text N.N. Gavrilova, O.V. Zhilina, V.V. Nazarov, 2007, published in Zhurnal Prikladnoi Khimii, 2007, Vol. 80, No. 9, pp. 1432 1435.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Synthesis of Mixed Cerium Zirconium Oxides
by the Sol Gel Method
N. N. Gavrilova, O. V. Zhilina, and V. V. Nazarov
Mendeleev Russian University of Chemical Engineering, Moscow, Russia
Received January 16, 2007
Abstract Mixed oxides CeO2 ZrO2 with the Ce/Zr molar ratios varying in a wide range were synthesized by
the sol gel method. The phase composition of the resulting particles was determined. The specific surface area
of the mixed oxide calcined at various temperatures was found.
DOI: 10.1134/S1070427207090054
Mixed cerium zirconium oxides are widely used in
various fields of science and engineering [1]. Depend-
ing on composition, they can be employed in manu-
facture of solid-state oxide cells, gas sensors, and
oxygen-conducting materials. Systems with high con-
tent of zirconium dioxide are used in production of
ceramic materials and ionic conductors, and systems
with predominance of cerium dioxide form a basis of
various catalysts.
drosols of CeO ZrO and to determine the basic
2 2
parameters of CeO ZrO powders.
2
2
EXPERIMENTAL
The following chemicals of chemically pure grade
were used in the study: cerium(III) nitrate; zirconium
oxynitrate; sodium, potassium, and nickel nitrates;
and aqueous solutions of ammonium hydroxide and
nitric acid.
Mixed cerium zirconium oxides catalyze numerous
reactions [2, 3]. Owing to their specific structure,
these systems are of particular interest in reactions of
hydrocarbon oxidation, both full and selective [4 9].
The concentration of the dispersed phase (in terms
of the sum of cerium and zirconium oxides) was
found by thermogravimetry, with the dry residue cal-
cined at 650 C. An ultrasonic treatment was carried
out in an ultrasonic bath with a working frequency
of 25 kHz. The optical density was found turbidimet-
rically on a KFK-2 instrument at a wavelength of
440 nm (cell thickness 1 10 mm). The pH value was
measured with a Schott CG 825 pH-meter with a glass
electrode. The electrophoretic mobility was deter-
mined by the method of macroelectrophoresis with
a mobile boundary. A dispersion medium produced by
ultrafiltration of hydrosols served as a contact fluid.
The particle size was found by the method of dynamic
light scattering on an FK-22 installation including
a He Ne laser (wavelength 632.8 nm, power 5 mW),
an F-221 photometer, and a UNICOR SP digital cor-
relometer with 128 high-speed channel processors for
on-line calculation of the correlation function (up to
100 ns). The specific surface area of the powders was
found by the method of thermal desorption of nitrogen
on a GKh-1 installation. The thermogravimetric
measurements were made on an MOM Q-1500D
derivatograph, with temperature elevated at a rate of
The different conditions in which these reactions
are carried out and the use of different types of sup-
ports result in that CeO ZrO are used both as highly
2
2
dispersed powdered catalysts, coatings, materials with
a required pore structure, and high-density materials.
The wide diversity of the properties of the final
materials requires that the synthesis method should
allow effective control over the properties of the prod-
ucts obtained and have high output capacity and low
energy expenditure.
One of the most promising methods for synthesis
of mixed oxides CeO ZrO and materials based on
2
2
these oxides is the sol gel technique. The possibility
of varying the conditions in which separate stages of
synthesis are performed allows control over the phase
composition, size, and shape of the forming particles
in a wide range. Different drying and thermal treat-
ment modes can give products with a required pore
structure.
The aim of this study was to obtain mixed hy-
1468

SYNTHESIS OF MIXED CERIUM ZIRCONIUM OXIDES
1469
1
5 deg min . The phase composition of the powders
It should be noted that xerogels produced by drying
of concentrated hydrosols retain capacity for spontane-
ous dispersion in distilled water with pH adjusted to
1.0 4.5. The xerogels were produced by drying of
hydrosols in air at 25 C.
was studied by X-ray phase analysis on a DRON-3
diffractometer with CuK radiation.
Mixed hydrosols with various Ce/Zr molar ratios
were produced by peptization. In this method, water-
insoluble hydroxides are obtained from a salt solution
and then are peptized in the presence of a strong acid.
The OH/M molar ratio in the precipitation of hy-
droxides was 25. Ammonium hydroxide served as
a precipitating agent. Then the precipitate obtained
was washed with distilled water to pH 9 in order to
remove residual electrolytes and then was peptized
in the presence of nitric acid and treated with ultra-
sound for 2 min. As shown by these experiments,
mixed hydrosols can be synthesized in a rather wide
A thermogravimetric analysis of these xerogels
(Fig. 1) demonstrates that, for all the samples, a sharp
weight loss is observed at 250 C, with the following
two endothermic effects observed. Below 100 C,
capillary and weakly bound water is removed, and
the peak at 220 280 C is associated with the decom-
position of nitrates. The exothermic effect associated
with crystallization of zirconium dioxide is recorded
at 380 C (below this temperature, zirconium dioxide
is X-ray-amorphous) only for the Ce Zr O sam-
0.2 0.8
2
+
range of [H ]/[M] molar ratios.
ple. For other CeO /ZrO molar ratios, a broad exo-
2
2
thermic peak is observed in the temperature range
The procedure developed can yield mixed CeO
ZrO hydrosols with an initial concentration of 1.8 to
6.8 wt %, which can be then concentrated by evapora-
tion or ultrafiltration to 13 43 wt %.
2
4+
300 800 C. This is due to incorporation of Zr into
2
the crystal lattice of cerium dioxide.
The data obtained are in good agreement with
the results of the X-ray phase analysis (Fig. 2). An
analysis of the X-ray diffraction patterns of powders
The resulting hydrosols are stable in acidic media
at pH 1.0 4.5. Particles of the dispersed phase of
the hydrosols are spherical, with an average hydro-
dynamic radius of 18 to 80 nm, depending on the
CeO /ZrO molar ratio.
calcined at 600 C demonstrated that CeZrO solid
2
solutions are present in a wide range of CeO /ZrO
2
2
molar ratios. However, reflections characteristic of
t-ZrO are observed in some cases (Ce Zr O ,
2
2
An important factor in fabrication of supported
catalysts is the adhesion of hydrosol particles to the
support surface. This adhesion is largely determined
by the charge on the surface of hydrosol particles.
Measurements of the electrophoretic mobility demon-
strated that hydrosol particles are positively charged
and the -potential, evaluated using the Helmholtz
Smoluchowski equation, is 11 to 32 mV at pH 1.0
4.5.
2
0.9 0.1 2
Ce Zr O , Ce Zr O ).
0.8 0.2
2
0.2 0.8 2
This may occur because the procedure suggested
here for synthesis of hydrosols cannot provide full
transition of ions into insoluble CeO ZrO com-
2
2
pounds. Therefore, the hydrosols contain a certain
amount of Ce and Zr in the ionic form, which can lead
in further thermal treatment to formation of individual
phases. This can be prevented by washing of xerogels
with distilled water to complete removal of the ions.
When sols are used as a basis of various formula-
tions, it becomes necessary to introduce salts of vari-
ous metals. The basic condition for a high-quality
product to be obtained is the aggregative stability of
the resulting formulation. To determine the maximum
amount of an electrolyte, at which the hydrosols retain
aggregative stability, their coagulation thresholds were
measured in the presence of various electrolytes. It was
found that mixed hydrosols are stable against 1 1 and
2 1 electrolytes. In the presence of NaNO , KNO ,
To obtain information about the thermal stability of
the mixed oxides synthesized, the specific surface
areas of CeO ZrO powders calcined at various tem-
2
2
peratures were found. Figure 3 shows the specific sur-
face areas S and particles sizes D as functions of
sp
calcination temperature and the ZrO content of the
2
system. The average particle size D was evaluated by
the formula D = 6/( S ), where is the density of
sp
3
3
a powder, and S , its specific surface area.
sp
and Ni(NO ) , coagulation is not observed even in
3 2
high-concentration solutions of these electrolytes.
As can be seen from these dependences, the largest
2
1
specific surface area (118 m g ) after calcination at
450 C is observed for the system Ce Zr O . How-
The stability of the hydrosols against electrolytes
of the 1 1 and 2 1 types allows introduction of
required amounts of metal nitrates into the hydrosols,
which enables significant modification of the com-
position of catalytically active formulations.
0.9 0.1
2
ever, it exhibits the strongest decrease in the specific
surface area as the calcination temperature is raised.
An insignificant decrease in the specific surface area
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 9 2007

1470
GAVRILOVA et al.
(a)
(b)
DTA
TG
DTA
TG
DTA
DTA
(c)
(d)
TG
TG
T, C
T, C
Fig. 1. Thermograms of CeO ZrO xerogels at various CeO /ZrO molar ratios. (T) Temperature. (a) Ce Zr O ,
2
2
2
2
0.9 0.1 2
(b) Ce Zr O , (c) Ce Zr O , and (d) Ce Zr O .
0.8 0.2
2
0.5 0.5
2
0.2 0.8 2
is observed for the systems containing more than
CeO ZrO /Al O catalysts. The activity of these cat-
2 2 2 3
50 mol % ZrO .
alysts was studied in the oxidation of CO. The maxi-
mum conversion of carbon monoxide was 98%. It was
observed in the temperature range 450 650 C,
depending on the catalyst composition [10].
2
The hydrosols obtained using this procedure were
used to produce various catalysts.
The method of impregnation of a commercial
Al O -based support was used to obtain CeO
1
Ssp, m2 g
2
3
2
3
D, nm
ZrO /Al O , CuO CeO ZrO /Al O , and Co O
2
2
3
2
2
2
3
2
2 , deg
[ZrO2], mol %
Fig. 2. X-ray diffraction patterns of mixed oxides CeO
2
ZrO at various CeO /ZrO molar ratios. (I) Intensity and
Fig. 3. (1 3) Specific surface area S and (4 6) particle
2
2
2
sp
(2 ) Bragg angle. Calcination temperature 600 C. (1) CeO ,
size D of mixed oxides CeO ZrO calcined at different
2
2
2
(2) Ce Zr O , (3) Ce Zr O , (4) Ce Zr O ,
temperatures vs. the ZrO content of the system. Tempera-
0.9 0.1
0.2 0.8
2
2
0.8 0.2
2
0.5 0.5 2
2
(5) Ce Zr O , and (6) ZrO .
ture, C: (1, 6) 450, (2, 5) 600, and (3, 4) 750.
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 9 2007

SYNTHESIS OF MIXED CERIUM ZIRCONIUM OXIDES
1471
2. Glinski, M. and Kijenski, J., React. Kinet. Catal. Lett.,
The selective oxidation of propane was carried out
both on powdered CeO ZrO NiO catalysts and on
2000, vol. 69, no. 1, p. 123.
2
2
ceramic membrane units with a catalytically active
bed of the same composition. The reaction was
studied in the temperature range 300 500 C. It was
shown that higher selectivity and output capacity for
propylene are achieved with membranes with a cata-
lytically active bed [11].
3. Fiziko-khimicheskie svoistva okislov (Physicochemical
Properties of Oxides), Samsonov, G.V., Ed., Moscow:
Khimiya, 1978.
4. Jibril, B.Y., Al-Zahrani, S.M., and Abassed, A.E.,
Catal. Lett., 2001, vol. 74, nos. 3 4, p. 145.
5. Deshmukh, S.S., Zhang, S.M., Kovalchuk, ??, et al.,
Appl. Catal., 2003, vol. 45, pp. 135 145.
CONCLUSIONS
6. Sadykova, V.A., Kuznetsova, T.G., Veniaminov, S.A.,
et al., React. Kinet. Catal. Lett., 2002, vol. 76, pp. 83
92.
(1) The peptization method can be used to obtain
aggregatively stable mixed hydrosols of cerium and
zirconium dioxides in a wide range of CeO /ZrO
molar ratios. The resulting hydrosols are stable against
introduction of metal nitrates, which allows significant
modification of the composition of catalytically active
formulations. The hydrosols synthesized can be used
for fabrication of both powdered and supported cata-
lysts.
7. Passos, F.B., Oliveira, E.R. de, Mattos, L.V., et al.,
2
2
Catal. Today, 2005, vol. 101, pp. 23 30.
8. Breen, J.P., Burch, R., and Coleman, H.M., Appl.
Catal. B: Environ., 2002, vol. 39, pp. 65 75.
9. Srinivas, D., Satyanarayana, C.V.V., Potdar, H.S.,
et al., Appl. Catal. A: Gen., 2003, vol. 246, pp. 323
334.
10. Kudinov, R.F., Liberman, E.Yu., Mikhailichen-
ko, A.I., et al., Abstracts of Papers, 19-aya Mezhdu-
narodnaya konferentsiya molodykh uchenykh po khi-
mii i khimicheskoi tekhnologii (19th Int. Conf. of
Young Scientists on Chemistry and Chemical Tech-
nology), Moscow, 2005, vol. 19, no. 10, p. 22.
(2) The powders produced by calcination of xero-
gels have large specific surface area. Their phase
composition is mainly represented by CeO ZrO
solid solutions. The highest thermal stability is ob-
served for systems with high content of zirconium
dioxide.
2
2
11. Gavrilova, N.N., Zhilina, O.V., Nazarov, V.V., et al.,
Nauchnye issledovaniya i ikh primenenie: Sovrenmen-
noe sostoyanie i puti razvitiya (Scientific Studies and
Application of Their Results: Modern State and Ways
of Development), Odessa: Chernomor’e, 2005, p. 89.
REFERENCES
1. Trovarelli, A., Catal. Rev., 1996, vol. 38, no. 4,
pp. 439 520.
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