Non-isothermal studies of the decomposition course of lanthanum oxalate decahydrate

Article abstract of DOI:10.1016/S0040-6031(01)00834-6

The thermal decomposition of lanthanum oxalate hydrate La₂ (C₂O₄)₃·10H₂O till 900°C, in air, is investigated by non-isothermal gravimetry and differential thermal analyses. Intermediates and final solid products were characterized by X-ray diffraction (XRD) and IR-spectroscopy, the results show that La₂(C₂O₄) ₃·10H₂O dehydrates in stepwise at 86-360°C and decomposes to La₂O₃ at 710°C through different intermediates, La₂(C₂O₄)₃, La₂O(CO₃)₂ and La₂O₂CO₃, that form at 400, 425 and 470°C, respectively. The final product La₂O₃ obtained at 800 °C has a surface area of 13.4 m²/g. The activation energy, ΔE of the observed thermal processes is obtained by the non-isothermal method.

Full text of DOI:10.1016/S0040-6031(01)00834-6

Thermochimica Acta 387 (2002) 109–114
Short communication
Non-isothermal studies of the decomposition course of
lanthanum oxalate decahydrate
Basma A.A. Balboula,*, A.M. El-Roudia, Ebthal Samira, A.G. Othmanb
aChemistry Department, Faculty of Science, Minia University, El-Minia 61519, Egypt
bCeramic Department, Natural Research Center, Dokki, Cairo, Egypt
Received 28 June 2001; received in revised form 23 October 2001; accepted 25 October 2001
Abstract
The thermal decomposition of lanthanum oxalate hydrate La2(C2O4)3ꢀ10H2O till 900 8C, in air, is investigated by
non-isothermal gravimetry and differential thermal analyses. Intermediates and final solid products were characterized by
X-ray diffraction (XRD) and IR-spectroscopy, the results show that La2(C2O4)3ꢀ10H2O dehydrates in stepwise at 86–360 8C
and decomposes to La2O3 at 710 8C through different intermediates, La2(C2O4)3, La2O(CO3)2 and La2O2CO3, that form
at 400, 425 and 470 8C, respectively. The final product La2O3 obtained at 800 8C has a surface area of 13.4 m2/g. The
activation energy, DE of the observed thermal processes is obtained by the non-isothermal method. # 2002 Elsevier Science
B.V. All rights reserved.
Keywords: La-oxalate; La-oxide; Decomposition; DTA; TG; IR; XRD; SBET
1. Introduction
The aim of the present study is to characterize the
thermal decomposition of La2(C2O4)3ꢀ10H2O to the
onset of La2O3 by means of thermogravimetry (TG)
and differential thermal analysis (DTA). The solid
products of the reaction were analyzed by IR-spectro-
scopy and X-ray diffractometry. The surface area
(SBET) was measured by N2-adsorption isotherm.
The activation energy, DE of the observed thermal
processes is obtained by the non-isothermal method of
thermoanalytical data analysis.
The hexagonal [1] structure, lanthanum sesquiox-
ide, La2O3, the final decomposition course studied in
the present investigation, is the only stable oxide of
lanthanum up to 2050 8C. La2O3 has numerous appli-
cations in various industrial and the technological
fields. It is an important component of automobile
exhaust—gas conversion [2], as a catalyst support in
the formation of gas conversion catalyst [3,4], and as a
catalyst of oxidative coupling of methane [5–7]. It is
also used as a refractory oxide for calcium lights,
optical glass [8], and in the formation of ceramics as a
core for carbon arc electrodes [8].
2. Experimental
2.1. Materials
* Corresponding author. Tel.: þ20-86-34-52-31;
fax: þ20-86-34-26-01.
Lanthanum acetate tetrahydrate (LaAc, La(CH3-
COO)3ꢀ4H2O) (99.9% pure from Aldrich, USA),
E-mail address: bsman64@yahoo.com (B.A.A. Balboul).
0040-6031/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 0 1 ) 0 0 8 3 4 - 6

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B.A.A. Balboul et al. / Thermochimica Acta 387 (2002) 109–114
ammonium oxalate ((NH4)2C2O4), ammonium hydro-
xide (NH4OH) and glacial acetic acid were used for
the preparation of oxalate.
410 JASCO (Japan). IR-spectra of LaOx and its
solid calcination products were obtained from thin
(>20 mg/m2), lightly loaded (<1%) KBr-supported
discs.
2.2. Preparation of oxalate
2.5. X-ray diffraction
Oxalates of La(III) from LaAc was prepared by
dropwise addition of a hot 4% ammonium oxalate
solution to a stirred hot solution of acetates after their
dissolution in glacial acetic acid then neutralized to
pH ¼ 7 with (1:1) NH4OH. The precipitates formed
were left to stand at room temperature for 1 h, filtered
off, washed with a diluted ammonium oxalate solution
and finally dried at 80 8C to a constant weight.
The calcination products were obtained by heating
at various temperatures 200–800 8C, in a static atmo-
sphere of air for 1 h. The calcination temperatures were
chosen on basis of the thermal analysis results. The
calcination products indicate throughout the text by the
oxalate designation and the temperature applied, thus
LaOx-800 indicates the calcination product at 800 8C.
The abbreviation WL stands for weight loss.
X-ray diffraction (XRD) powder patterns were
obtained with JSX-60P JEOL diffractometer (Japan).
The X-ray generator is equipped with Ni-filter and
˚
generates a beam of Cu Ka radiation (l ¼ 1:5418 A).
The operational settings for all the XRD scans are
voltage: 40 kV, current: 30 mA, range: 4–608 (2y),
scanning speed: 88/min, slit width: 0.028. For identi-
fication purpose, the relative intensities (I/I0) and the
˚
d-spacing (A) are compared to standard diffraction
patterns in the ASTM powder diffraction file [10].
2.6. N2-adsorption measurements
N2 sorption isotherms were determined volumetri-
cally at À195 8C using a micro apparatus based on the
design, which was described by Lippens et al. [11].
Test samples were outgassed at 220 8C for 6 h while
evacuation at 10À5 Torr was performed. The reprodu-
cibility of the isotherm measurements was better than
97%.
2.3. Thermal analysis
TG and DTA of lanthanum oxalate were carried out
on heating at various rates (y ¼ 5, 10, 20, 30 8C/min)
upto900 8Cinadynamicatmosphereofair(20 ml/min),
using a 7-series thermal analysis model Perkin-Elmer
Analyzer. Ten to fifteen milligrams portions of the test
sample were used for the TG measurements, and
highly sintered a-Al2O3 was the thermally inert refer-
ence for the DTA. Shifts experienced by the DTA peak
temperature (Tmax) as a function of the heating rate
(y) were implemented to calculate the activation
energy, DE (kJ/mol) corresponding to each of the
thermal events monitored, according to the following
equation [9]:
3. Results and discussion
3.1. Characterization of the decomposition course
TG and DTA curves (Fig. 1) monitor 12 WL events
(designated I–XIII) in the decomposition course of
LaOxꢀ10H2O, only three of these events are exother-
mic (event IX, X and XI), whereas the others are
endothermic. The WL effected via the first eight
events (I–VIII) accounts for a stepwise dehydration
of LaOx.10H2O, in which event I and II (WL ¼ 2:7%,
Tmax ¼ 86 8C) and (WL ¼ 5:2%, Tmax ¼ 110 8C),
respectively, each involves the elimination of 1 mol
of water, and thermal event III (WL ¼ 9:9%,
Tmax ¼ 130 8C) leads to the removal of another
2 mol of H2O. Events (IV–VII) (WL ¼ 12:5, 15,
17.5, 20% ) and (Tmax ¼ 155, 174, 195, 225 8C),
respectively leads to the removal of another 4 mol
of water. The corresponding activation energy values
ꢀ
ꢁ
R
1
DE ¼ À
bd log yd Tmax
where R is the gas constant (¼8.314 kJ/mol) and b is a
unitless constant (¼0.457).
2.4. Infrared spectroscopy
IR-spectra were obtained at a resolution of 4 cmÀ1
over the range 4000–400 cmÀ1, using a model FT-IR-
,

B.A.A. Balboul et al. / Thermochimica Acta 387 (2002) 109–114
111
Fig. 1. TG and DTA curves recorded for LaOx at the heating rates indicated, in a dynamic (20 ml/min) atmosphere of air.
(Table 1) are well within the range (<60 kJ/mol)
encountered for removal of weakly bound water of
crystallization from analogous compounds [12]. The
dihydrate thus formed, LaOxꢀ2H2O, persists upon
further heating to 300 8C at which event VIII
(Tmax ¼ 360 8C) commences to operate. The total
WL effected at 380 8C, (ca. 24%) accounts for the
elimination of the remaining 2H2O and, consequently,
for the formation of the unhydrous lanthanum oxalate.
The IR-spectrum of the solid phase LaOx-200 (Fig. 2)
bears a great deal of similarity to that of unheated
La2(C2O4)3ꢀ10H2O because both show absorption
Table 1
Thermal processes; temperature (8C), composition and activation
energy (DE, kJ/mol) proposed in each process for LaOxꢀ10H2O
bands arising from oxalate anions (at 1750–640 cmÀ1
)
Process Temperature (8C) Composition
DE (kJ/mol)
andwaterofhydration(at3440and1640 cmÀ1)[12,13].
However, the corresponding XRD pattern (Fig. 3)
indicates that the products are amorphous and so
water of hydration is very important for the coherency
of LaOx crystal [14].
I
86
110
130
155
174
195
225
360
400
425
470
710
La2(C2O4)3ꢀ9H2O
La2(C2O4)3ꢀ8H2O
La2(C2O4)3.7H2O
La2(C2O4)3.6H2O
La2(C2O4)3ꢀ5H2O
La2(C2O4)3ꢀ4H2O
La2(C2O4)3ꢀ2H2O
La2(C2O4)3
41
43
II
III
IV
V
94
43
51
VI
VII
VIII
IX
X
54
60
The LaOx-300 solid phase IR-spectrum (Fig. 2)
2À
shows that the C2O4 species are weakened and
108
127
123
145
254
La2(CO3)3
different band structure including absorptions at
2350 cmÀ1 and between 1800 and 400 cmÀ1 which
is similar in shape and position to characteristic
La2O(CO3)2
La2O2CO3
XI
XII
La2O3
2À
absorptions CO3 [12].

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B.A.A. Balboul et al. / Thermochimica Acta 387 (2002) 109–114
Fig. 2. IR-solid phase spectra for LaOx and its 1 h calcination products at the temperatures indicated.
Upon further heating event VIII is overlapped by two
rapid exothermic WL processes (event IX, Tmax
400 8C and event X, Tmax ¼ 425 8C). The WL for IX
is 36.9% which is close to that theoretically calculated
(36.6%) for the formation of La2(CO3)2, as follows:
similar in shape and position to characteristic absorp-
tions of oxycarbonates [13]. XRD pattern for LaOx-
400 (Fig. 3), reveals that the La2(CO3)3 product
inferred from the TG and IR-data is amorphous.
The WL for event X is 42.5% (expected WL is
42.7%) accounts for the conversion of La2(CO3)3 to
La2O(CO3)2 which converts immediately to La2O2CO3
(thermal event XI, WL ¼ 48:4%, Tmax ¼ 470 8C)
(Fig. 1), as follows:
¼
La2ðC2O4Þ3 ! La2ðCO3Þ3
(1)
The IR-spectrum of LaOx-400 shows relived of the
dHOH absorption of hydration of water [13], which is
strong, broad absorption observed in the spectrum of
both the parent, LaOx-200 and LaOx-300 between
1600 and 1750 cmÀ1. The fundamental modes of
La2ðCO3Þ3 ! La2OðCO3Þ2 ! La2O2CO3
(2)
Events IX, X and XI must have overlapped between
420 and 550 8C. The similarity of the activation
energy values (Table 1) for process IX, X and XI
justifies their overlapping.
2À
vibration of the CO3 species between 1800 and
400 cmÀ1 are weakened and different band structure
begin to appear between 1600 and 1300 cmÀ1 which is

B.A.A. Balboul et al. / Thermochimica Acta 387 (2002) 109–114
113
Fig. 4. N2-adsorption isotherm obtained at À195 8C (using BET
Method ) for LaOx-800.
The IR-spectrum of LaOx-800 (Fig. 2), shows no
detectable absorptions due to oxycarbonate species.
The absorptions below 700 cmÀ1 are related to lattice
vibration modes of La2O3 [15]. The weak bands
around 1600, 1500 and 1380 cmÀ1 are most probably
due to surface contamination by carbonate and moist-
ure since it is known that La2O3 is a basic oxide [16].
In support, the XRD pattern of LaOx-800 detect a
crystalline phase of La2O3 (ASTM no. 5-602).
Fig. 3. X-ray powder diffractograms for LaOx and its 1 h
calcination products at the temperatures indicated.
The IR-spectrum (Fig. 2) and XRD patterns (Fig. 3)
support reaction (2). The IR-spectrum LaOx-550 dis-
plays strong absorptions between 1600 and 1300 cmÀ1
and also at 1060 and 870 cmÀ1 assignable to oxycar-
bonate species [13]. The absorption appearing at
730–500 cmÀ1 are related to La–O vibrational lattice
mode [15].
The corresponding XRD pattern for LaOx-550
shows the pattern for crystalline La2O2CO3 (ASTM
no. 23-320).
At <600 8C, process XI slows down ending up with
a weight invariant behavior at 670 8C. Process XII
takes place endothermally at Tmax ¼ 710 8C. The
maximum WL determined (54.2%) agrees well with
that expected (54.8%) to accompany the overall con-
version of LaOxꢀ10H2O to La2O3, i.e. process XII
include the following pathway:
3.2. Nitrogen sorption and surface area
measurement of La2O3
The N2-adsorption isotherm of LaOx-800 is shown
in Fig. 4. The isotherm generally belongs to type IVof
BET classification [17]. The desorption isotherm
closed at P/P0 > 0:4, which give indication the hys-
teresis loop is nearly of type H3 [17]. The loop
suggested that the surface pores are of slit-shaped
or interplating [18]. The SBET value determined was
13.4 m2/g.
4. Conclusion
La2O2CO3 ! La2O3
(3)
The results of the present studies allow the follow-
ing conclusions to be drawn:
Table 1 indicates that event XII has the highest
activation energy (DE ¼ 254 kJ/mol) in the overall
decomposition course of LaOxꢀ10H2O, a fact that
justifies its lowest kinetics.
1. The thermal decomposition course of hydrated
lanthanum oxalate may include the following path

114
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[2] M. Hideaki, S. Hirofami, S. Hideo, F. Yoshiyasu, Ind. Eng.
ways:
La2ðC2O4Þ3 ꢀ 10H2O!La2ðC2O4Þ3 ꢀ 9H2O
Chem. Proc. Res. Dev. 25 (2) (1988) 202.
[3] D. Andriamasinoro, R. Kieffer, A. Kiennemann, P. Poix,
Appl. Catal. 106 (1993) 201.
I
II
III
!La2ðC2O4Þ3 ꢀ 8H2O!La2ðC2O4Þ3 ꢀ 6H2O
[4] M.P. Rosynek, D.T. Magnison, J. Catal. 46 (1977) 402.
[5] K. Okabe, K. Sayama, H. Kusama, H. Arakawa, Bull. Chem.
Soc. Jpn. 67 (1994) 2894.
IV
V
!La2ðC2O4Þ3 ꢀ 5H2O!La2ðC2O4Þ3 ꢀ 4H2O
[6] K. Otsuka, K. Jinno, A. Morikawa, J. Catal. 100 (1986)
353.
VI
VII
!La2ðC2O4Þ3 ꢀ 3H2O!La2ðC2O4Þ3 ꢀ 2H2O
[7] H. Arakawa, Bull. Chem. Soc. Jpn. 21 (11) (1994) 31.
[8] N.I. Sax and R.J. Lewis (Eds.), Hawley’s Condensed
Chemical Dictionary, 11th Edition, Reinhold, New York,
1987, p. 683.
VIII
IX
X
!La2ðC2O4Þ3!La2ðCO3Þ3!La2OðCO3Þ2
XI
XII
!La2O2CO3!La2O3:
[9] J.H. Flynn, J. Thermal. Anal. 27 (1983) 45.
[10] J.V. Smith (Ed.), X-ray Powder Data File, American Society
for Testing and Materials, PA, USA, 1960.
2. The lower hydrate oxalate (dihydrate) is thermally
stable but the unhydrous oxalate is unstable and
the water of hydration is responsible of the crystal
coherency of LaOx.
[11] B.C. Lippens, B.G. Linsen, J.H. De Boer, J. Catal. 3 (1964)
32.
[12] M.E. Brown, D. Dollimore, A.K. Galwey, in: C.H. Bamford,
C.F.H. Tipper (Eds.), Chemical Kinetics, Reaction in the
Solid State, Elsevier, Amsterdam, 1980.
3. Lanthanum carbonates La2(CO3)3 (amorphous),
La2O(CO3)2 (amorphous), La2O2CO3 (crystal-
line), were formed as intermediates during the
decomposition of lanthanum oxalate hydrate but
with no region of stability.
4. A crystalline phase of La2O3 were detectable as
the final decomposition product of La2(C2O4)3.
5. La2O3 obtained at 800 8C has a surface area of
13.4 m2/g.
[13] K. Nakamoto, Infrared Spectra of Inorganic and Coordination
Compounds, Wiley, New York, 1970, p. 253.
[14] G.A. Gadsden, Infrared Spectra of Minerals and Rela-
ted Inorganic Compounds, Butterworths, London, 1975,
p. 65.
[15] J.A. Goldsmith, S.D. Ross, Spectrochem. Acta Part A 23
(1967) 1909.
[16] K. Tanabe, K. Misono, Y. Ono, H. Hatori, New Solid Acids
and Bases, Kodansha, Tokyo; Elseveir, Amsterdam, 1989,
pp. 41–47.
References
[17] S.J. Gregg, K.S.W. Sing, Adsorption Surface Area and
Porosity, 2nd Edition, Academic Press, London, 1982.
[18] K.S.W. Sing, Pure Appl. Chem. 54 (1982) 2201.
[1] P. Kofstad, Nonstoichiometry, Diffusion and Electrical
Conductivity in Binary Metal Oxide, Wiley, New York,
1972, p. 289