Gases released during the conversion of NH4Zr2(PO4)3 to HZr2(PO4)3

Article abstract of DOI:10.1023/A:1011517811132

Identification of gases released during the conversion of NH₄Zr₂(PO₄)₃ to HZr₂(PO₄)₃ was performed by thermogravimetric analysis and infrared spectroscopy. Acidic gases such as n

Full text of DOI:10.1023/A:1011517811132

Journal of Thermal Analysis and Calorimetry, Vol. 65 (2001) 305–308
GASES RELEASED DURING THE CONVERSION OF
NH Zr (PO ) TO HZr (PO )
4
2
4 3
2
4 3
T. Oi, K. Horio, R. Kikuchi and H. Takahashi
Department of Chemistry, Sophia University, 7-1 Kioicho, Chiyoda, Tokyo 102-8554, Japan
(
Received December 11, 2000; in revised form March 5, 2001)
Abstract
4 2 4 3 2 4 3
Gases released during the conversion of NH Zr (PO ) to HZr (PO ) were identified using an appa-
ratus in which gases released from a sample placed in a thermogravimetric analyzer were directly in-
troduced to a gas cell of an IR spectrometer. Such acidic gases as N O and NO were detected besides
the basic NH gas, and their formation mechanism was discussed.
2
3
2 4 3 4 2 4 3
Keywords: HZr (PO ) , NH Zr (PO ) , TG analyser connected to IR
Introduction
The rhombohedral HZr (PO ) , which has originally been studied as an ion conductor
2
4 3
[
toward alkali metal ions at pH 3.4 is in the decreasing order of Na >Li >Cs >Rb >K
[
1], shows some unique properties as a cation exchanger. The affinity of HZr (PO )
2 4 3
+
+
+
+
+
+
+
2], which should be compared with the sequence of Cs >Rb >K >Na >Li of com-
+
+
+
mercially available organic strongly acidic ion exchangers. Its lithium isotope selec-
tivity is more than ten times larger than those of organic ion exchangers [3].
Experimental
The rhombohedral HZr (PO ) is prepared by a heat treatment of NH Zr (PO ) ;
2
4
3
4
2
4 3
NH Zr (PO ) →HZr (PO ) +NH ,
(1)
4
2
4 3
2
4 3
3
3−
and NH Zr (PO ) is obtained as precipitate from aqueous solution containing PO ,
4
2
4
3
4
2
+
ZrO and oxalic acid with the mole ratio of ZrO and (COOH) being about 2:3
2+
2
[
4, 5].
Concerning conversion (1), Clearfield et al. [1] reported a very interesting ob-
servation. When NH Zr (PO ) was heated at 450°C, the off-gases were acidic while
4
2
4 3
the gases were basic when heated at 700°C. They speculated that acidic gases (nitro-
gen oxides) were formed by the oxidation of NH , with strongly acidic sites created
by the removal of NH (i.e. HZr (PO ) phase) acting as catalyst.
3
3
2
4 3
1
418–2874/2001/ $ 5.00
Akadémiai Kiadó, Budapest
©
2001 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

3
06
4 2 4 3 2 4 3
OI et al.: CONVERSION OF NH Zr (PO ) TO HZr (PO )
In order to identify acidic and basic gases released accompanying the conversion
of NH Zr (PO ) to HZr (PO ) and discuss their formation mechanism, we analyzed
4
2
4
3
2
4 3
the released gases directly by infra-red (IR) measurements during thermogravimetric
TG) measurements of NH Zr (PO ) . The apparatus used consisted of a Perkin
(
4
2
4 3
Elmer 2000 IR spectrometer and a Perkin Elmer TGA7 TG analyzer, in which gases
released from a sample placed in a pan of the TG analyzer were directly introduced to
a gas cell of the IR spectrometer by means of a N flow without contacting with the
2
air.
Results and discussion
Figure 1 shows the IR absorbance profiles, as functions of temperature and released
gases in TG measurements of NH Zr (PO ) using a ceramics pan with the heating
4
2
4 3
–
1
rate of 50°C min (Fig. 1a) or 10°C min (Fig. 1b). Contrary to the results of Clear-
–1
–
field et al. [1], only NH (monitored at 966 cm ) is detected regardless of the heating
1
3
rate. It is, thus, reasonable to conclude that HZr (PO ) is formed by reaction (1) un-
der those conditions.
2
4 3
Similar TG measurements were made using a platinum pan instead of the ceram-
ics one, and the results are depicted in Figs 2a and 2b. When the heating rate is fast at
–
1
–1
0°C min (Fig. 2a), releases of N O (monitored at 2244 cm [6]) and NO (moni-
5
tored at 1917 cm [7]) in addition to that of NH are observed between about 300 and
2
–
1
3
–1
Fig. 1 The IR absorbance profiles of off-gases; a – with the heating rate of 50°C min
–1
and with a ceramics pan, and b – with the heating rate of 10°C min and with a
ceramics pan. Note that the scales of the vertical axis are different from figure to
figure and from gas to gas
J. Therm. Anal. Cal., 65, 2001

OI et al.: CONVERSION OF NH
4
Zr
2
(PO
4
)
3
TO HZr
2
(PO
4
)
3
307
7
lease of H O (monitored at 1597 cm ). A close examination of Fig. 2a reveals that the
00°C and between about 350 and 800°C, respectively, both accompanied by the re-
1
–
2
NH profile has two peaks corresponding to those of N O and NO. The most plausible
reactions of the formations of the two acidic gases, N O and NO, are:
3
2
2
2
NH +2O →N O+3H O
(2)
3
2
2
2
and
4
NH +5O →4NO+6H O
(3)
3
2
2
–
With the decrease in heating rate to 10°C min and with a platinum pan, the
1
peaks of NO and NH disappear and the release of N O is diminished (Fig. 2b). It is
inferred that, when the heating rate is low, the major reaction is;
NH →N +3H
3
2
2
(4)
3
2
2
which is undetectable by IR monitoring. Some catalysts must be required for reac-
tions (2)–(4) to occur.
Our above observation shows that platinum used as the material of the pan of the
TG analyzer acted as a catalyst for the oxidation of NH and contrary to the specula-
tions of Clearfield et al. [1], the HZr (PO ) phase did not catalyse the oxidation. It is
3
2
4 3
unknown what kind of materials participated in the heating experiments by Clearfield
et al. [1]. We claim that HZr (PO ) did not act as catalyst in their experiments, either.
2
4 3
–1
Fig. 2 The IR absorbance profiles of off-gases; a – with the heating rate of 50°C min
–1
and with a platinum pan, and b – with the heating rate of 10°C min and with a
platinum pan. Note that the scales of the vertical axis are different from figure to
figure and from gas to gas
J. Therm. Anal. Cal., 65, 2001

3
08
4 2 4 3 2 4 3
OI et al.: CONVERSION OF NH Zr (PO ) TO HZr (PO )
To summarize, we would like to state that N O and NO were identified as acidic
off-gases accompanying the conversion of NH Zr (PO ) to HZr (PO ) and that the
2
4
2
4
3
2
4 3
HZr (PO ) phase of NH Zr (PO ) had little catalytic capability for the oxidation of
NH3.
2
4
3
4
2
4 3
References
1
2
3
4
5
6
7
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J. Therm. Anal. Cal., 65, 2001