Ce-Zr mixed oxides prepared in molten nitrates

Article abstract of DOI:10.1016/S0925-8388(02)00011-7

Dispersed Ce_0.75Zr_0.25O₂ and Zr_0.84Ce_0.16O₂ cerium(IV)-zirconium(IV) mixed oxides were prepared by the flux method, from the reaction of hydrated Ce(NO₃)₃ and ZrOCl

Full text of DOI:10.1016/S0925-8388(02)00011-7

Journal of Alloys and Compounds 340 (2002) 74–78
L
www.elsevier.com/locate/jallcom
Ce–Zr mixed oxides prepared in molten nitrates
*
P. Afanasiev
Institut de Recherche sur la Catalyse, 2 Avenue A. Einstein, 69626, Villeurbanne, C e´ dex, France
Received 19 July 2001; received in revised form 13 November 2001; accepted 27 November 2001
Abstract
Dispersed Ce0.75Zr0.25O2 and Zr0.84Ce0.16O2 cerium(IV)–zirconium(IV) mixed oxides were prepared by the flux method, from the
reaction of hydrated Ce(NO ) and ZrOCl in molten NaNO at 450–500 8C in the presence of ammonium fluoride. The temperature and
3
3
2
3
the stoichiometry of reactions were determined by mass spectrometry. Powder X-ray diffraction, specific surface area measurements
BET), and scanning electron microscopy were used to study the morphology and particle size distribution of the solid products.
Preparation conditions were optimized to obtain pure oxides. Reaction at 550 8C in the presence of 1% wt. of ammonium fluoride gave the
best result. 2002 Elsevier Science B .V . All rights reserved.
(

Keywords: Molten salt reactions; Cerium; Zirconium mixed oxides; Fluoride
1
. Introduction
nanometers and high specific surface areas (about 130
2 21
m g ).
Over the past several years, cerium oxide and CeO2-
Preparation of finely divided powders of oxides are of
primary importance in many applications including
ceramics and catalysis. Consequently, numerous chemical
preparation ways have been investigated. Reactions in
molten salts provide an alternative method for the prepara-
tion of solids. Molten salts were used to synthesize a
number of compounds, including binary and ternary oxides
containing materials are intensely studied as catalysts,
structural and electronic promoters. CeO -containing
2
catalysts are involved in various heterogeneous catalytic
reactions. It is recognized as a key material in automotive
exhaust catalysts since it can release and take up oxygen
due to rapid reversible transition between oxidation states
3
1
41
[1], phosphates [2], or sulfides [3], at relatively low
Ce and Ce [8]. Cerium(IV) oxide is also a candidate
material for the gas sensor and fuel cells applications.
However, few works dealt with the molten salt prepara-
temperatures, as compared to solid-state synthesis. Among
the molten salts, nitrates possess low melting points and
considerable levels of oxobasicity, which makes them
convenient solvents for some inorganic syntheses, par-
ticularly the precipitation of oxides. Application of molten
nitrates for the syntheses of inorganic materials has been
recently reviewed [4]. Much attention has been drawn in
tions of CeO . Nitrate fluxes were applied to prepare
2
dispersed CeO from sulfate precursor [9]. Recently, flux
2
preparations of cerium oxide were compared for the KOH/
NaOH, NaNO /KNO , and LiCl/KCl eutectic mixtures
3
3
[10].
the past to preparation of zirconium oxide ZrO in molten
nitrates. In the papers by Durand et al. [5], Inmann et al.
Cerium–zirconium mixed oxides are among the most
intensely studied inorganic materials during the last years,
because of their exceptional oxygen storage and ionic
conductivity properties. Much effort has been devoted to
the development of soft chemistry methods of Ce– Zr
oxide preparation. The recent examples are coprecipitation
from cerium nitrate and different zirconium precursors
such as zirconyl nitrate or zirconium propoxide in the
modified sol–gel method [11]; thermal decomposition of
cerium zirconyl oxalate in an argon flow [12]; or the
surfactant assisted-preparations [13]. To our knowledge no
work on the molten salt preparation of mixed Ce–Zr oxide
2
[6] and our previous work [7], the reaction pathways, the
properties of products, and the influence of reaction
conditions were considered for preparation of pure zirconia
and of this oxide stabilized by small amounts of yttrium or
calcium. Finely divided zirconia has been elaborated in
molten nitrates, having the particles size of several
*
Tel.: 133-04-7244-5339; fax: 133-04-7244-5399.
E-mail address: afanas@catalyse.univ-lyon1.fr (P. Afanasiev).
0
925-8388/02/$ – see front matter
 2002 Elsevier Science B .V . All rights reserved.
PII: S0925-8388(02)00011-7

P. Afanasiev / Journal of Alloys and Compounds 340 (2002) 74–78
75
was done up to date. The present article deals with
pure molten NaNO produce mixtures of CeO and ZrO
3 2
2
preparation of Ce–Zr oxide by means of molten salt
technique.
oxides in the temperature range 400–550 8C. The XRD
patterns show relatively narrow peaks of CeO and broad
2
lines of poorly crystalline ZrO . Mass-spectrometry data
2
show that NO2 and O2 are the main products of both
reactions of Ce and Zr salts, in agreement with simple Lux
Flood interactions (1) and (2).
2
. Experimental
The series of ceria–zirconia samples was prepared in
3
1
2
3
Ce 1 3NO ⇒ CeO 1 3NO 1 1/2O
(1)
(2)
molten NaNO , doped with different amounts of am-
2
2
2
3
monium hydrogen fluoride. The atomic ratios Zr:Ce in the
reaction mixtures were 0.5, 1 and 2. Hydrated zirconium
oxychloride, hydrated cerium nitrate, ammonium hydrogen
2
2
ZrOCl 1 2NO ⇒ ZrO 1 2NO 1 2Cl 1 1/2O
2
3
2
2
2
fluoride, and NaNO in a 10-fold molar excess relative to
The reactions (1) and (2) occur in the weakly overlap-
ping temperature ranges. The reactivity curves are com-
pared in Fig. 1 as the mass spectrometric signals of NO
3
the total amount of Ce and Zr, were thoroughly mixed and
put in a Pyrex reactor. The mixtures were pretreated under
a nitrogen flow at 150 8C for 2 h to remove water from the
precursor salts, then the reaction was carried out at 400–
species, formed from NO in the spectrometer, which are
2
most intense and characteristic for this type of processes.
Zirconium salt began to react even before the nitrate
melting, and had two reaction maxima at ca. 250 and
330 8C. ZrO2 precipitation was virtually completed at
420 8C. By contrast, for the cerium salt the reaction onset
was observed at 360–370 8C, the rate maximum at 450–
460 8C; and the reaction was completed at 500 8C. Due to
such a strong difference of reactivity, ZrO2 is already
completely precipitated when Ce salt only begins to react.
No precipitation of mixed oxide occurs therefore in the
flux.
5
50 8C, for 4 h. Note that the alkali metal nitrates do not
decompose with noticeable velocity up to 600 8C therefore
can be safely used as the solvents at these temperatures.
After cooling, the solidified melt was washed with distilled
water at room temperature until a test with AgNO show
3
no more residual chloride, then the product was dried
overnight in air at 120 8C. X-ray diffraction patterns were
recorded on a diffractometer SIEMENS D500 by using
Ni-filtered Cu Ka radiation. Identification of phases was
made using standard JCPDS files. Surface areas and pore
radii distributions were measured by nitrogen adsorption.
Chemical analyses of alkali metals were carried out by
using the atomic emission method, that of fluoride was
determined electrochemically, using ion selective electrode
To increase the reaction probability, we needed to
improve the conditions for interaction between Zr and Ce
species. This can be done either by trying different
precursors, or by modifying the solubility of the reactants
with LaF membrane.
3
The gaseous products evolved upon heating of the
samples were studied using a mass-spectrometer Gas Trace
A (Fison Instruments) equipped with a quadrupole ana-
lyzer (VG analyser) working in a Faraday mode. The
ionization was done by electron impact with an electron
energy of 65 eV. The samples (ca. 0.1 g) were heated from
room temperature to 550 8C in a glass cell at the heating
2
1
rate of 1.5 8C min . A silica capillary tube heated at
80 8C continuously bled off a proportion of the gaseous
1
reaction products. Several signals were registered, those
with m/e518, 30, 32, 44 and 46 corresponded, respective-
ly, to ionized species H O, NO, O , CO , and NO . The
2
2
2
2
technical questions of mass spectrometric analysis of
molten nitrate reactions is discussed in more detail in Ref.
[14]. Scanning electron microscopy (SEM) images were
obtained on a Hitachi S800 device, at the center of
electronic microscopy of Claude Bernard University
(
Lyon). FT-IR spectra were obtained on a BRUKER
device.
3
. Results and discussion
Fig. 1. Mass-spectroscopic signals of NO evolved during heating of the
mixture of NaNO3 with Ce(NO ) ?6H O (a); ZrOCl ?8H O (b). The
3
3
2
2
2
The reactions of zirconium and cerium precursors in
molar ratio of nitrate to metal precursor is 10:1.

7
6
P. Afanasiev / Journal of Alloys and Compounds 340 (2002) 74–78
CeZrO oxide of continuous solutions was obtained, but
4
always the mixtures of Zr-rich Zr0.84Ce0.16O2 and
Ce0.75Zr0.25O in the proportions depending on the metal
2
precursors loading. For the reaction mixtures with the
metals loading corresponding to the composition of these
two compounds, the latter were obtained in pure state. In
Fig. 2 are depicted the XRD patterns of the solids obtained
with different amounts of fluoride at 550 8C for the
reaction time 8 h. Scanning electron microscopy with
simultaneous EDX analysis, and element selective cartog-
raphy showed good homogeneity of the Zr0.84Ce0.16O and
2
Ce0.75Zr0.25O samples obtained at 550 8C. The morpholo-
2
gy of oxides obtained is shown in Fig. 3. Rodlike
monodisperse particles were observed for Ce0.75Zr0.25O2,
whereas Zr0.84Ce0.16O2 shows agglomerates of slightly
anisotropic particles. The results of phase composition
analysis and BET specific surface areas as a function of
preparation conditions are reported in Table 1 for several
selected samples. We can conclude that finely dispersed
materials are obtained, with specific surface area greater
for Zr-rich oxide than for the Ce-rich counterpart. Chemi-
cal analysis showed low content of alkali metal and
fluoride in the oxides prepared at 500 8C with addition of
Fig. 2. XRD patterns of the products of reactions of equimolar
Ce(NO ) ?6H O and ZrOCl ?8H O mixtures in molten NaNO at 550 8C
3
3
2
2
2
3
as a function of NH HF weight loading. Peak labels: 1—CeF3 (JCPDS
4
2
1
% of ammonium fluoride to the flux (less than 0.2% wt.
3
8-0452), 2—Ce0.75Zr0.25O2 (JCPDS 28-0271), 3—Zr0.84Ce0.16O2
(JCPDS 38-1437).
of Na and less that 0.1% of F).
Though the mixed oxides were obtained at 550 8C, we
characterized the products from lower temperatures to have
an idea about transient processes during the oxide forma-
tion. It follows from the analysis of low temperature
with some chemically active admixtures. Earlier [15] we
have found that addition of fluoride to molten nitrates leads
to the drastic change of the properties of zirconia prepared
in the nitrate fluxes. Fluoride strongly accelerates crys-
tallization of thermodynamically stable monoclinic form.
The reason for such an effect was probably in a partial
solubilisation of oxide due to strong complexation between
preparations that fluoride CeF is an intermediate product
3
which is precipitated prior to formation of mixed oxide.
For the fluoride content of 1%, at reaction temperature
450 8C (Fig. 4a) very sharp and intense lines of CeF are
3
seen, which correspond to the presence of small amounts
of large crystals. Indeed, in the EDX study we observed
rare but large and well shaped hexagonal crystals consist-
ing mostly of Ce and F. Therefore the interaction between
cerium and fluoride species in the melt is probably stronger
than that of zirconium, which at these conditions does not
form bulk fluoride compounds at these conditions. At the
reaction temperature 500 8C the lines become smaller and
disappear completely at 550 8C (Fig. 4b), the product being
transformed to mixed oxides. In the final mixed oxide there
is no hexagonal particles keeping initial morphology of
2
Zr(IV) and F leading to formation of the soluble anion2s
of fluorozirconate (Eq. (3)) and at large amounts of F
even to the pure Na ZrF phase.
3
7
2
32
7
22
ZrO 1 7F ⇔ ZrF 1 2O
(3)
2
We expected therefore that addition of fluoride might
improve the conditions for obtaining Zr–Ce mixed oxide.
Indeed, after the addition of 1 to 15% wt. of NH HF ,
4
2
we observed formation of mixed oxides. No equimolar
Table 1
Preparation conditions and properties of selected Zr–Ce samples
2
21
T, 8C
Ce:Zr, at
NH HF , wt.%
S, m g
Phases, DRX
4
2
5
5
5
5
4
5
50
50
50
50
50
00
1
1
3
8
1
1
0
1
1
1
5
111
55
28
61
42
6
ZrO (T), CeO
2
2
Zr0.84Ce0.16O , Ce0.75Zr0.25O2
2
Ce0.75Zr0.25O2
Zr0.88Ce0.12O2
ZrO (M), CeO , CeF
3
2
2
15
Ce0.75Zr0.25O , Zr0.84Ce0.16O2
2
CeF , Na ZrF
7
3
3
5
50
1
15
7
Ce0.75Zr0.25O , Zr0.84Ce0.16O2
2
Na ZrF , CeF
3
3
7

P. Afanasiev / Journal of Alloys and Compounds 340 (2002) 74–78
77
Fig. 3. SEM images of the products of reaction of Ce(NO ) ?6H O and ZrOCl ?8H O mixtures (Ce:Zr53 or 0.125) in molten NaNO with 1% wt. of
3
3
2
2
2
3
NH HF at 550 8C: (a) Ce0.75Zr0.25O , (b) Zr0.84Ce0.16O2.
4
2
2
CeF . The last is obviously redissolved in the melt at
3
higher temperatures (Eq. (5)).
3
1
2
Ce 1 3F ⇒ CeF
(4)
(5)
3
2
3
2
CeF 1 3NO ⇒ CeO 1 3F 1 3NO 1 1/2O
3
2
2
2
If fluoride content in the reaction is very high, bulk
fluoride is not redissolved even at 500 8C and both Zr and
Ce fluoride compounds are seen in the XRD pattern (Fig.
4
(c)). Only a small amount of fluoride (0.5–1% wt.) is
necessary to obtain mixed oxides at 550 8C. The excess of
fluoride (5–15% wt.) remains as large particles of CeF3
and Na ZrF . This excessive fluoride phases do not affect
3
7
the breadth of peaks and intensities of reflections for the
mixed oxides, i.e. it seems to be present in the reaction
mixtures as an inert bulk phase. Therefore we suppose that
rather small equilibrium concentrations of fluoride com-
plexes in the flux induce formation of mixed oxides.
4
. Conclusions
Fig. 4. XRD patterns of the products of reactions of equimolar
Ce(NO ) ?6H O and ZrOCl ?8H O mixtures in molten NaNO at 450 8C
3
3
2
2
2
3
Individual mixed oxides or the intimate mixtures of
Zr–Ce oxide dispersions were prepared by the flux method
(a) and 500 8C (b). Insert (c)—product of reaction at 550 8C and 15% wt.
of NH HF . Peaks labels: 1—CeF ; 2—ZrO (JCPDS 37-1484). 3—
4
2
3
2
Ce0.75Zr0.25O ; 4—Na ZrF (JCPDS 74-0808).
at 550 8C, utilizing cerium(IV) and zirconium(IV) pre-
2
3
7

7
8
P. Afanasiev / Journal of Alloys and Compounds 340 (2002) 74–78
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3
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relatively low temperature method for Ce–Zr mixed
oxides.
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