Synthesis of ZrO2 in molten salt mixtures: Control of the evolved gas and the oxide texture

Full text of DOI:10.1006/jcat.1996.0269

JOURNAL OF CATALYSIS 162, 143–146 (1996)
ARTICLE NO. 0269
NOTE
Synthesis of ZrO2 in Molten Salt Mixtures: Control of the Evolved Gas
and the Oxide Texture
Zirconium dioxide presents considerable interest in the ments), equipped with a VG quardrupole analyzer work-
field of heterogeneous catalysis, due to its original acid– ing in Faraday mode. Mixtures containing ca. 1 g of salts
base and redox properties (1). Much work has been done were heated from room temperature to 550ꢀC in a glass
on the preparation of high surface area zirconia, obtained cell at a heating rate of 1.5ꢀ minꢁ1. A silica capillary tube
either from aqueous solutions (2), thermal decomposition heated at 180ꢀC continuously bled off the gaseous reac-
of basic salts (3), or sol–gel methods (4, 5). Recently, the tion products into the spectrometer. Signals were recorded
reactions of Zr salts with molten nitrates have been suc- with m/z = 18, 30, 32, 36, 44, 46, corresponding respectively
cessfully used for the preparation of dispersed ZrO2 for to ionized H2O, NO, O2, HCl, CO2, and NO2 molecules.
catalytic applications (6–10). Yttrium-stabilized zirconia, X-ray diffraction patterns were recorded on a diffractome-
obtained in molten KNO3–NaNO3 mixtures, showed spe- ter (Siemens D500) using Ni-filtered CuKꢀ radiation. Sur-
cific surface area as high as 120–130 m2/g (9), which could be face areas and pore radii distributions were measured by ni-
enhanced to above 200 m2/g if the reaction mixtures were trogen adsorption. Chemical analyses of alkali metals were
doped with oxoanions such as molybdate (11) or vanadate carried out by means of the atomic emission method. SEM
(12). However, a disadvantage of this preparation method images were obtained using a scanning electron microscope
is the production of heavily pollutant NOx gases during the (Hitachi S800).
reaction. Another problem concerns the texture of the ob-
Preliminary mass spectra, discussed in Ref. (15), have
tained oxide. Using molten salt preparations, average pore shown that at least two reactions occur in the mixture of
diameters in the range of 2–3 nm were obtained, whereas a hydrated ZrOCl2 with alkali nitrates, i.e., decomposition of
higher porosity is needed in some catalytic processes such hydrated ZrOCl2 (reaction 1) and Lux–Flood acid–base in-
as the hydrotreatment of petroleum fractions.
teraction of the Zr salt with the molten nitrate (reaction 2):
In order to solve these problems, we investigated a mix-
ture of KNO3–NaNO3–Na2CO3 as a reaction medium. The
formation of ZrO2 in molten nitrate can be considered as
an acid–base reaction in which the zirconium salt plays the
role of an acid and the melt anions that of a base (13). The
higher reactivity of the carbonate ion in comparison to that
of nitrate could modify the reaction mechanism, and differ-
ent physico-chemical properties of the resulting solid could
be expected. Carbonate may also act as a textural modifier
similarly to preparations performed in aqueous media (14).
The preparation procedure was as follows: hydrated
zirconium oxychloride and a mixture of alkali metal salts
containing a tenfold molar excess of nitrate plus carbon-
ate relative to Zr were thoroughly mixed and placed in a
Pyrex reactor. The mixtures were pretreated under a nitro-
gen flow at 150ꢀC for 2 h to remove water from the pre-
cursor salts. Then the reaction was carried out at 500ꢀC for
2 h. After cooling, the solidified melt was washed with dis-
tilled water at room temperature until a test with AgNO3
showed no more residual chloride, then the product was
dried overnight in air at 120ꢀC. The compositions of reac-
tion mixtures are listed in Table 1.
ZrOCl2 ꢂ 8H2O → ZrO2 + 7H2O + 2HCl
[1]
[2]
ZrO2+ + 2NO3ꢁ → ZrO2 + 2NO2 + O2
1
2
Reaction [1] was observed mostly before the nitrate melt-
ing, whereas reaction [2] occurred in the melt. Figure 1a
presents the mass spectra obtained with a mixture corre-
sponding to the ZC0 preparation. A special care should be
taken about the NO/NO2 signals, due to the fragmentation
of NO2 in the electron beam. NO2 is characterized by two
signalsofNOandNO2 withanintensityratio NO/NO2 close
to 10, experimentally observed using the decomposition of
Pb(NO3)2 as a reference compound.
TABLE 1
Composition of the Reaction Mixtures
NaNO3
(g)
KNO3
(g)
Na2CO3
(g)
ZrOCl2 ꢂ 8H2O
Sample
(g)
ZC0
ZC1
ZC2
ZC3
ZC4
9
11
0
3.0
3.0
3.0
3.0
3.0
8.9
8.7
8.1
6.3
10.9
10.7
9.9
0.2
0.6
2.0
6.0
The gases produced during the reaction were analyzed
using a mass spectrometer, Gas Trace A (Fisons Instru-
7.7
143
0021-9517/96 $18.00
c
Copyright ꢃ 1996 by Academic Press, Inc.
All rights of reproduction in any form reserved.

144
NOTE
FIG. 1. Mass spectra of the gases produced upon heating in vacuum the reaction mixture K–Na–NO3–ZrOCl2 ꢂ 8H2O corresponding to the
preparation ZC0 (a) and the reaction mixture Na2CO3–K–Na–NO3–ZrOCl2 ꢂ 8H2O corresponding to the preparation ZC4 (b).
Multistage reaction of the zirconium salt in the The appearance of both NO2 and HCl confirms that the
K–Na–NO3 melt can be seen in Fig. 1. The signals of m/z 30 total process involves both reactions [1] and [2].
and 46 have similar shapes, the ratio of their intensities be-
The mass spectra of hydrated ZrOCl2 in K–Na–
ing fairly constant. Therefore both signals can be attributed NO3 + Na2CO3 were measured for the reaction mixture
to the formation of NO2 and its subsequent fragmentation. corresponding to the sample ZC4 (Table 1). According to
NO2 appears already at ca. 120ꢀC and its signal increases up the mass-spectral data (Fig. 1b) it appears that the reaction
to 450ꢀC. At somewhat lower temperatures, the side reac- pathway was changed as a result of the addition of Na2CO3.
tion of thermal decomposition occurred and a considerable The amounts of NO2 and HCl evolved were drastically
amount of HCl was observed with a maximum at ca. 270ꢀC. reduced by factors of about 40 and 20, respectively, as

NOTE
145
compared to the non-doped mixture. At the same time,
It was shown earlier that the addition of multidentate
strong peaks of CO2 appear, both below and above the chelating anions such as sulfate, molybdate (15, 17), or
melting temperature (ca. 300ꢀC). Therefore, we can con- phosphate to the nitrate melt leads to the stabilization of
clude that carbonate addition results in a virtual change of the tetragonal variety of zirconia and simultaneously to
the mechanism from reactions [1] and [2] to reaction [3].
a decrease of the ZrO2 crystallite size (18, 19). Accord-
ingly, the stabilization of tetragonal zirconia by the pres-
ence of carbonate in the melt could be expected, as well
as an increase of its surface area. In agreement with this
hypothesis, samples prepared with 10 and 30 wt% carbon-
ate (ZC3, ZC4) have pure tetragonal structure, whereas
the monoclinic variety was observed at low carbonate con-
centration (ZC1, ZC2). At the same time, the presence of
carbonate ions leads to a significant increase of the mean
pore size and pore volume without considerable modifica-
tion of the specific surface area, though the average size
of the zirconia crystallites determined by XRD was in-
creased (Table 2). Such a behavior can be attributed to
the acceleration of zirconia again in the melt due to the
presence of carbonate species. The aging phenomenon, well
known for aqueous precipitates and sol–gel products, con-
sists of the growth of necks between the solid particles due
to the dissolution–precipitation process. Consequently, a
more open pore structure can be obtained, without appre-
ciable decrease of the specific surface area. It is known that
in aqueous media the aging of an amphoteric oxide can be
strongly accelerated by the presence of acids or bases (20).
By analogy with aqueous precipitates, it is proposed that
ZrO2+ + CO32ꢁ → ZrO2 + CO2
[3]
Reaction mixtures, corresponding to the samples ZC1 and
ZC2, produced CO2 together with a considerable amount
of nitrogen oxides, since the amount of carbonate in them
was lower than is required by the stoichiometry of reaction
[3]. The mass spectrum for mixture ZC3 which contained
excess carbonate was similar to that shown in Fig. 1b. As
deduced from Fig. 1b, a high degree of transformation is
already obtained at 300ꢀC. However, complementary ex-
periments showed that a temperature of 500ꢀC is required
to achieve the complete reaction and to obtain zirconia with
good textural properties.
Chemical analysis of the samples prepared at 500ꢀC
shows that all the samples contain residual alkali metals and
carbonate (0.2–0.4 wt% Na; 0.2–0.6 wt% HCOꢁ3 ), which
cannot be eliminated by washing with distilled water at
room temperature. The amount of chloride which comes
fromtheZrOCl2 precursorisnegligible. Theresidualnitrate
level is very low, as it was in the zirconia prepared from pure
nitrates (16).
FIG. 2. SEM images of the samples ZC0 (a) and ZC4 (b).

146
NOTE
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TABLE 2
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144
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carbonate ions increase the basicity of the melt and then
accelerate the mass transfer.
The influence of carbonate ions on the morphology of
zirconia particles can be clearly seen on the SEM pic-
tures. In the ZC0 sample prepared in the K–Na–NO3
mixture (Fig. 2a) the oxide particles have a spherical
morphology, typical of these preparations (16). The sample
consists of spherical agglomerates having a mean diame-
ter of ca. 1–2 ꢁm. By contrast, the sample ZC4 exhibits a
“cheesehole” morphology (Fig. 2b) which can be the result
of the advanced neck growth.
Polydentate oxoanions have been applied earlier to con-
trol the properties of molten salt preparations of oxides
(17). Assuggestedforzirconia(16), theprocessesdetermin-
ing the properties of the resulting oxide are the elimination
of nitrate groups from the amorphous Zr oxonitrate, the
nucleation of zirconia particles, and their extensive growth
and agglomeration. Acting as a strong base, carbonate ions
react chemically with the zirconium salt and accelerate the
subsequent aging of the zirconia precipitate. The new re-
action pathway avoids the production of pollutant gases
such as HCl and NO2. Moreover, the presence of carbon-
ate strongly influences the morphology of the particles. It
becomes possible to prepare solids with the porosity and
the surface area required for some catalytic applications
such as hydrotreatment.
Pavel Afanasiev
Christophe Geantet
Michel Lacroix
ꢄ
Miche`le Breysse
Institut de Recherches sur la Catalyse
2 Avenue A. Einstein
69626 Villeurbanne Ce´dex, France
Received February 13, 1996; revised April 1, 1996; accepted April 25,
1996
ACKNOWLEDGMENTS
The authors thank T. des Courie`res and J. L. Dubois from ELF Antar
for valuable discussions.
ꢄ
E-mail: breysse@catalyse.univ-lyon1.fr.