Synthesis and characterization of a H+ exchanged zirconate

Article abstract of DOI:10.1002/zaac.200570041

A new protonated oxide has been synthesized by ion exchange of lithium by proton in lithium metazirconate, Li₂ZrO₃. Easy and complete ion exchange has been achieved in acidic medium. Characterization of the protonated solid by means of X-ray powder diffraction and thermogravimetric analysis allow to propose the nominal formulation H₂ZrO ₃ . However vibrational spectroscopy suggests that a more appropriate formulation would be as an oxyhydroxide ZrO(OH)₂.

Full text of DOI:10.1002/zaac.200570041

Synthesis and Characterization of a H؉ Exchanged Zirconate
´
A. Orera*, A. Kuhn, and F. Garcıa Alvarado
´
Madrid/Spain, Departamento de Quımica, Universidad San Pablo-CEU
Received December 3rd, 2004; accepted March 15th, 2005.
Abstract. A new protonated oxide has been synthesized by ion ex-
change of lithium by proton in lithium metazirconate, Li2ZrO3.
Easy and complete ion exchange has been achieved in acidic me-
dium. Characterization of the protonated solid by means of XϪray
powder diffraction and thermogravimetric analysis allow to pro-
pose the nominal formulation “H2ZrO3“. However vibrational
spectroscopy suggests that a more appropriate formulation would
be as an oxyhydroxide ZrO(OH)2.
Keywords: Lithium zirconate; Ion exchange; Proton exchange;
Zirconium dioxide
Introduction
In the search of low-temperature proton conductors as solid
electrolytes for fuel cell applications we have recently re-
ported the preparation of a proton exchanged tetragonal
tungsten bronze using nitric acid as exchanging agent to
form HxNa1ϪxNbWO6 [1]. In this compound, high conduc-
tivity has been detected to be dependant on proton ex-
change. However, oxidation state of both transition metals,
NbV and WVI are not likely to be stable under the reductive
conditions present in the anode side of a real fuel cell sys-
tem. In fact the bronze reacts readily with pure hydrogen
inducing electronic conductivity [2]. Regarding our search
for stable metal valencies, we report in this work the pre-
paration and characterization of a protonated zirconium
oxide obtained through a proton exchange reaction starting
from lithium metazirconate, Li2ZrO3 [3].
Fig. 1 Schematic representation of Li2ZrO3 crystal structure.
Small spheres in tunnels represent lithium atoms. Octahedra repre-
sent coordination of Zr in the structure.
Proton conductivity is not new in zirconium oxides. In
fact chemisorption and physisorption of water on Yttrium
stabilised zirconia can be analysed by means of conductivity
measurements. In these cases conduction mechanism takes
place in the absorbed water layer as is probably carried by
protons. In the 150 °CϪ450 °C range, proton conduction
is facilitated by chemisorbed water. At lower temperatures
(T<100 °C), a physisorbed water layer facilitates the proton
conduction [4]. In our work we are trying to obtain a zir-
conium oxide where proton may enter into the framework
structure of the oxide and gives rise to proton conductivity
at low temperature. For this reason a compound as Li2ZrO3
where the Li ions are located in the tunnels of a Zr-O
framework (see Fig.1) has been chosen as a host to proton
exchange reactions.
Experimental
Lithium metazirconate was synthesized by solid state reaction of
Li2CO3 (Aldrich, purity 99ϩ%) and ZrO2 (Alfa Aesar, purity
99.5 %) [3]. Stoichiometric quantities of reagents were ground and
heated at 700 °C for 12 h. The product was pelletised and treated
in air at 1050 °C for 21 h.
Ion exchange of lithium by proton was performed by refluxing the
powdered solid in a 5M aqueous solution of HNO3 at 80 °C for
24 h. The solid product was washed with distilled water until no
trace of LiNO3 was found by IR analysis. The sample was then
dried under vacuum for 7 days.
Powder XϪray diffraction data were collected by means of a
Bruker D8 high-resolution XϪray powder diffractometer, using
monochromatic CuKα1 radiation (λϭ1.5406 A) obtained with a
˚
germanium primary monochromator, and equipped with a position
sensitive detector (PSD) MBraun PSDϪ50M. Structure was ana-
lyzed with the Rietveld method using the Fullprof program [5].
* A. Orera
Universidad San Pablo-CEU
Departamento de Ciencias Quımicas
28668 Madrid/Spain
E-mail: alodia.fcex@ceu.es
Fax: - 00 34 91 351 04 75
´
Lithium analysis was carried out using a VARIAN SpectrAA 220
atomic absorption spectrophotometer. LiNO3 (Merck CertiPUR)
was used a standard solution.
Z. Anorg. Allg. Chem. 2005, 631, 1991Ϫ1993
DOI: 10.1002/zaac.200570041
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
1991

´
A. Orera, A. Kuhn, F. Garcıa Alvarado
Thermogravimetric experiments were carried out in a SEIKO
TG/DTA 6200 thermobalance under nitrogen flow. Temperature
range of measurements was 25 to 800 °C. Heating rate was set to
10 °C/min.
Infrared spectra were recorded in KBr pellets using a Perkin Elmer
2000 spectrometer, in wavenumber 4500Ϫ370 cmϪ1 range.
Results and Discussion
The X-ray powder diffraction pattern (Figure 2a) was in-
dexed in the monoclinic space group Cc, corroborating the
formation of single phase monoclinic Li2ZrO3. Unit cell
Fig. 3 TG / DTA plot of ZrO(OH)2·0.14H2O
˚
˚
parameters were a ϭ 5.4243(3) A; b ϭ 9.0277(5) A; c ϭ
˚
5.4217(3) A; βϭ 112.700(2)° in good agreement with those
Thermogravimetric analysis of the exchanged product is
shown in Fig. 3. Weight loss covers a rather wide tempera-
ture range, from 150 to 350 °C. The total weight loss
(14.2 %) is slightly higher than expected weight loss for a
quantitative exchange reaction of 2 Liϩ by 2 Hϩ (12.8 %)
as determined by AAS. An explanation for this higher
weight loss is that the exchanged product is obtained as
hydrated form ZrO(OH)2 · y H2O with y ഠ 0.14 (note that
exchange reaction was performed using an aqueous solu-
tion). However, from our thermogravimetric data it is
not possible to distinguish processes corresponding to
hydration water loss and proton loss (also as water and
forming ZrO2). On the other hand only a quite broad mini-
mum can be registered in DTA.Then thermal decompo-
sition reaction can be formulated as
reported in the literature [6].
ZrO(OH)2·0.14H2O Ǟ ZrO2 ϩ 1.14 H2O
Protonated samples treated from ca. 190 °C to 300 °C are
amorphous. The process corresponding to the sharp exo-
thermic peak at 350 °C without further weight loss is due
to the crystallization of amorphous zirconia as it has been
previously reported for related cases [7]. This transform-
ation has been monitored by high temperature in situ
XϪray powder diffraction (HT-PXRD). It can be depicted
from Fig. 4 that heating above the transition temperature
induces the formation of tetragonal ZrO2 which gradually
changes to the more stable monoclinic form. At the maxi-
mum temperature (1000 °C), only monoclinic ZrO2 was
present. It has been reported that low temperature tetra-
gonal zirconia can be obtained by thermal treatment of
various zirconium compounds [7Ϫ14], but the mechanism
of its stabilization is still a matter of controversy. However,
the formation of this metastable tetragonal phase at rela-
tively low temperature can be explained by the presence of
OH groups [13] or by small particle size [14]. Therefore, the
method reported in the present work is a new route to ob-
tain tetragonal zirconia where likely the presence of OH
groups and small particle size is playing a role and confirm
previous reports.
Fig. 2 XϪRay diffraction patterns of nominal (a) Li2ZrO3, (b)
ZrO(OH)2·0.14H2O
Ion exchange of lithium by proton was performed by
using 5 M aqueous solution of HNO3 at 80 °C for 24 h,
according to the general exchange reaction:
Li2ZrO3(s) ϩ x HNO3(aq) Ǟ HxLi2ϪxZrO3(s) ϩ x LiNO3(aq)
Lithium analysis of the filtrate confirmed that complete ion
exchange took place (xϭ2).
Exchanged product, ZrO(OH)2 hereafter, showed a
XϪray diffraction pattern with broad reflections (Fig. 2b)
which are indicative of either low crystallinity or small par-
ticle size in the exchanged material. Nevertheless, the struc-
ture seems to be kept in some way after acidic treatment,
since both diagrams exhibit the same main diffraction max-
ima.
1992
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
zaac.wiley-vch.de
Z. Anorg. Allg. Chem. 2005, 631, 1991Ϫ1993

Synthesis and Characterization of a Hϩ Exchanged Zirconate
indicating that protons are forming OH groups in the struc-
ture.
Conclusions
Li2ZrO3 has been used as a parent compound to perform
proton exchange reactions. An easy and quantitative ion
exchange took place by using aqueous nitric acid at moder-
ate temperature. The as-obtained ZrO(OH)2·0.14H2O de-
composes (with water loss) to give amorphous ZrO2, which
crystallizes at 350 °C via ZrO2 (t) to ZrO2 (m). Electrical
measurements are presently in progress.
´
Acknowledgements. We would like to thank Ministerio de Educacion
y Ciencia for funding the projects MAT2001-3713-C04-01 and
MAT2004-03070-C05-01. Financial support from Universidad San
Pablo is also acknowledged.
Fig. 4 XRD evolution with temperature of ZrO(OH)2·0.14H2O
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Z. Anorg. Allg. Chem. 2005, 631, 1991Ϫ1993
zaac.wiley-vch.de
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
1993