A continuous and clean one-step synthesis of nano-particulate Ce(1- x)Zr(x)O2 solid solutions in near-critical water

Article abstract of DOI:10.1039/b001424i

Gram-scale quantities of microcrystalline powdered CeO₂-ZrO₂ solid solutions can be produced continuously in a near-critical water flow reactor at ca. 300 °C and 25 MPa; rapid hydrothermal co-precipitation leads to nano- particulate Ce(1-x)Zr(x)O(2) (x = 0 to 1), the composition of which is merely determined by the initial concentrations of Ce⁴⁺ and Zr⁴⁺ ions in the starting solution.

Full text of DOI:10.1039/b001424i

A continuous and clean one-step synthesis of nano-particulate Ce12xZr O
x
2
solid solutions in near-critical water
a
a
b
a
Albertina Cabanas, Jawwad A. Darr, Edward Lester and Martyn Poliakoff*
a
School of Chemistry, The University of Nottingham, University Park, Nottingham, UK NG7 2RD.
E-mail: Martyn.Poliakoff@nottingham.ac.uk
School of Chemical, Environmental and Mining Engineering, The University of Nottingham, University Park,
Nottingham, UK NG7 2RD
b
Received (in Oxford, UK) 16th February 2000, Accepted 10th April 2000
Published on the Web 9th May 2000
2
Gram-scale quantities of microcrystalline powdered CeO –
2
ZrO solid solutions can be produced continuously in a near-
critical water flow reactor at ca. 300 °C and 25 MPa; rapid
hydrothermal co-precipitation leads to nano-particulate
x 2
Ce12xZr O (x = 0 to 1), the composition of which is merely
4+
4+
determined by the initial concentrations of Ce and Zr
ions in the starting solution.
The synthesis of ceria-containing materials has long been of
considerable scientific interest for applications as diverse as UV
absorbers and solid oxide fuel cells.1 Mixed CeO
–ZrO
systems show improved oxygen storage capacity and greater
2
2
–4
itself,2 which is partic-
thermal stability compared to CeO
ularly important for three-way exhaust catalysts. Ce12xZr
2
x 2
O
is
prepared conventionally using various methods including high
5
6
temperature solid-state reactions, use of micro-emulsions,
7,8
9,10
high energy ball milling, the sol–gel method,
and ageing
of solutions.11 However, many of these techniques are quite
Fig. 1 Diagram of the flow reactor for the synthesis of metal oxide powders
O. A solution of Ce4 and Zr salts at room
+
4+
and solid solutions in ncH
temperature is mixed with a stream of H
preheater (PH). The mixture is cooled immediately after the mixing point
X) by a water cooling (C) and is then passed through the filter (F) (to
2
time consuming (involving multi-step syntheses or ageing),
energy intensive and/or environmentally unfriendly. When
extremely small particle sizes are needed, the complexity (and
2
O heated up to 400 °C by the
(
sometimes the cost) of manufacture increases further.
remove any large aggregates) and the back-pressure regulator (BPR).
Particles are then collected as a suspension. P denotes pumps.
Arai and coworkers1
2,13
have recently shown that particulate
CeO
2
and ZrO
2
can be made continuously by decomposition of
O.
the corresponding metal nitrates in super-critical water, scH
2
2
the crystallinity of the materials obtained. For pure ZrO , TEM
The reaction is believed to proceed via hydrolysis to the
hydroxides, followed by dehydration to the oxides. Here, we
showed a fluffy ‘cloud-like’ image with nano-particles of ca. 5
nm (5.6 nm by PXD). All materials have medium to high
surface areas as measured by BET, SBET (Fig. 3). It is also clear
from Fig. 3 that there is a correlation between SBET values and
the apparent particle size. SBET values for the samples appear to
reach a maximum for the oxides with higher zirconium content
show for the first time that the mixed oxide Ce12xZr
obtained continuously by hydrolysis of mixtures of
NH [Ce(NO ] (CAN) and [Zr(ac) ]† in near-critical water,
ncH O, in the appropriate ratios. The reactor shown in Fig. 1
gave single phase Ce12xZr
high diffusivity of the reactants and the fast dehydration
reaction in ncH O and scH O result in very high hydrolysis and
x 2
O can be
[
4
]
2
3
)
6
4
2
2
1
x
O
2
at rates close to 10 g h . The
(
181, 184 and 183 m g for the 1+4 and 1+9 Ce–Zr materials
and pure ZrO , respectively).
CeO and Ce12xZr (x = 0–0.5) showed cubic fluorite
2
21
2
2
2
x 2
O
2
dehydration reaction rates. Thus, residence times at the mixing
point of only a few seconds were sufficient. In most cases,
decomposition of the metal precursors appeared to be virtually
PXD patterns. Increasing Zr content in the solid solutions
resulted in a decrease in the lattice parameter, owing to the
4+
incorporation of smaller Zr ions into the fluorite (mainly
complete. The composition in Ce12xZr
be changed from x = 0.2 to 0.9 merely by varying the Ce+Zr
ratios in the metal feed solution. Pure CeO and ZrO oxides
x 2
O mixed oxides could
2
2
could also be synthesised by using just one of the pre-
cursors.‡
The samples of Ce12xZr O were characterised by powder X-
x 2
ray diffraction (PXD), Raman spectroscopy, BET surface area
measurements and, in selected cases, TEM. The PXD patterns
2 2
of the materials indicated the formation of CeO –ZrO solid
solutions (Fig. 2). The individual solid phases were identified
by combining PXD and Raman data. A crude estimate of the
apparent particle size was obtained by application of the
Scherrer equation to the PXD line-widths (Fig. 3). For selected
cases, the particle size was also investigated by TEM. For the
1
+1 mixture (i.e. Ce0.5Zr0.5
particles with well-defined edges, well dispersed, between 3 and
nm in size, values which agree with the estimates from the
2
O ), the TEM image showed
Fig. 2 Powder X-ray diffraction (PXD) patterns for Ce12xZr
x 2
O materials
5
prepared in the near-critical water flow reactor: (a) x = 0 (pure CeO ), (b) x
2
Scherrer equation (4.6 nm). Diffraction rings (TEM) confirmed
= 0.2, (c) x = 0.5, (d) x = 0.8, (e) x = 0.9, (f) x = 1 (pure ZrO
2
).
DOI: 10.1039/b001424i
Chem. Commun., 2000, 901–902
901
This journal is © The Royal Society of Chemistry 2000

completely homogenous. The particles are highly crystalline
TEM) despite the relatively low temperatures of formation, and
the primary particle sizes are extremely small imparting
(
2
21
relatively high surface areas of up to ca. 180 m g for the
materials. Currently we are exploring the potential of our
technique for synthesising other solid solutions.
We gratefully acknowledge the financial support of the
EPSRC: Clean Technology Fellowship (M. P.), Grant No. GR/
K76023 (J. A. D.) and ICI (A. C.). We thank Dr M. G. Barker
and his group, Dr A.A. Galkin, Dr D. A. Graham, Dr. P. A.
Hamley, S. J. Barlow and M. Guyler for their assistance and
discussions.
Notes and references
Fig. 3 SBET area measurements (-) and estimated particle sizes by PXD (:)
of the materials vs. zirconium content (%Zr).
†
Zirconium acetate/dilute acetic acid solution with a 15% Zr content as
supplied by Aldrich.
CAUTION: this experiment involves high pressure and temperature and
‡
4
+
requires appropriate apparatus.
Ce ) lattice. For the samples of Ce+Zr ratios 1+4 and 1+9, a
tetragonal phase was assigned.§ For pure ZrO , PXD and
The total concentration of the metal solution (Ce + Zr) was kept constant
and equal to 0.21 M. The flow rates of the metal salt and water streams were
2
Raman data revealed a mixture of monoclinic and tetragonal
phases. Considering the relative intensities of the most intense
reflection in the PXD of both phases, the ratio of monoclinic to
tetragonal phases was estimated to be 40+60.
The precise phase assignment is slightly different from that
reported in the literature.3 One possible explanation is that the
Zr content in the solid solutions is lower than the expected.
21
5
.0 and 10.0 mL min , respectively. Pressure was kept constant at 25 ± 1
MPa and the H O inlet temperature was ca. 360 °C. At the mixing point, the
2
temperature was close to 300 °C. The resulting coloured suspensions of
metal oxides were separated from the clear aqueous phase by decantation,
and the solids were dried at 100 °C after washing with water.
,14
§ Although the characteristic tetragonal splittings of the reflections (200),
(220), (311) and (400) in the fluorite structure (Fig. 2) were not visible
owing to the broadness of the PXD peaks, the appearance of bands at 625,
However, this would not explain why our samples of pure ZrO
2
21
4
60 and 315 cm
in the Raman spectra of the fresh materials (not
contained a mixture of monoclinic and tetragonal phases rather
than the monoclinic (thermodynamically stable) phase reported
in the literature.14 Therefore is likely that the differences are due
to the small particle sizes of the materials produced. Fornasiero
8
illustrated) suggests a tetragonal phase.
1
T. Masui, K. Fujiwara, K. Machida and G. Adachi, Chem. Mater., 1997,
, 2197.
9
3
et al. have reported that below a critical crystalline size, the
2
3
A. Trovarelli, Catal. Rev., 1996, 38, 439.
P. Fornasiero, G. Balducci, R. DiMonte, J. Kaspar, V. Sergo, G.
Gubitosa, A. Ferrero and M. Graziani, J. Catal., 1996, 164, 173.
tetragonal phase is favoured over the monoclinic and, with
extremely small particles, the cubic phase is favoured over the
tetragonal, which is consistent with our phase assignment. As
discussed before, the extremely small particle sizes may be as a
result of the rapid precipitation and relatively low synthesis
temperature. All samples were studied by TGA. In each case,
there was a weight loss (2–12%) at @100 °C, presumably due to
adsorbed water. Some samples showed smaller weight losses
4 G. Vlaic, R. DiMonte, P. Fornasiero, E. Fonda, J. Kaspar and M.
Graziani, Stud. Surf. Sci. Catal., 1998, 116, 185.
5
6
7
8
M. Yashima, K. Morimoto, N. Ishizawa and M. Yoshimura, J. Am.
Ceram. Soc., 1993, 76, 1745.
T. Masui, K. Fujiwara, Y. Peng, T. Sakata, K. Machida, H. Mori and G.
Adachi, J. Alloys Compd., 1998, 269, 116.
C. de Leitenberg, A. Trovarelli, F. Zamar, G. Maschio, G. Dolcetti and
J. Llorca, J. Chem. Soc., Chem. Commun., 1995, 2181.
A. Trovarelli, F. Zamar, J. Llorca, C. de Leitenburg, G. Dolcetti and J. T.
Kiss, J. Catal., 1997, 169, 490.
(
4–7%) around 250 and 310 °C; IR spectra suggest that theses
losses may be associated with residual nitrate and acetate.
In conclusion, we have reported a new and extremely rapid
one-step synthesis of ceria–zirconia solid solutions (at the
concentrations and flow rates employed in the experiments,
9 T. Settu and R. Gobinathan, Bull. Chem. Soc. Jpn., 1994, 67, 1999.
10 T. Masui, Y. Peng, K. Machida and G. Adachi, Chem. Mater., 1998, 10,
4005 and references therein.
2
1
between ca. 8 and 11 g h , depending on the composition of
, can be produced continuously). The chemistry is
1
1
1
1 B. Djurcic, D. McGarry and S. Pickering, J. Mater. Sci. Lett., 1993, 12,
320.
2 T. Adschiri, K. Kanazawa and K. Arai, J. Am. Ceram. Soc., 1992, 75,
019.
3 Y. Hakuta, S. Onai, S. Terayama, T. Adschiri and K. Arai, J. Mater. Sci.
Lett., 1998, 17, 1211.
x 2
Ce12xZr O
1
simple and clean. The reaction does not require the addition of
organic solvents, modifiers (to control pH) or prolonged
reaction times. The temperatures are low (300 °C) compared to
those used in conventional techniques. Most importantly, the
composition of Ce12xZr
amounts of precursors in the starting solution. The products are
1
x
O
2
can be tuned simply by varying the
14 M. Yashima, K. Morimoto, N. Ishizawa and M. Yoshimura, J. Am.
Ceram. Soc., 1993, 76, 2865.
902
Chem. Commun., 2000, 901–902