Formation and characterization of samarium oxide generated from different precursors

Article abstract of DOI:10.1016/s0040-6031(02)00535-x

Sm(NO₃)₃·6H₂O and Sm₂(C₂O₄)₃· 10H₂O were used as precursors for the formation of Sm₂O₃. Thermal processes involved in the decomposition course of both salts up to 800°C in air were monitored by nonisothermal gravimetry and differential thermal analysis. Intermediates and final solid products were characterized by IR-spectroscopy, X-ray diffraction and scanning electron microscopy. The results showed that Sm(NO₃)₃·6H₂O decomposes completely through nine endothermic mass loss processes. The dehydration occurs through the first four steps at 90, 125, 195, and 240°C, culminating in a crystalline nitrate monohydrate, which subsequently decomposes to Sm(OH)(NO₃)₂ at 355°C. The latter decomposes rapidly to form a stable and crystalline SmO(NO₃) at 460°C, through nonstoichoimetric unstable intermediates. Finally Sm₂O₃ forms at 520°C. For the oxalate, the dehydration occurs in five steps: the anhydrous oxalate is thermally unstable and immediately decomposes to Sm₂O₃ at 645°C through two unstable intermediates. The crystalline oxide obtained from the nitrate contains larger pores than the oxide obtained from the oxalate, as indicated from scanning electron microscopy (SEM) results.

Full text of DOI:10.1016/s0040-6031(02)00535-x

Thermochimica Acta 402 (2003) 27–36
Formation and characterization of samarium oxide
generated from different precursors
G.A.M. Husseina,∗, D.J. Buttreya, P. DeSanto Jra, A.A. Abd-Elgaberb,
Heba Roshdyb, Ali Y.Z. Myhoubb
Department of Chemical Engineering, University of Delaware, Center for Catalytic
a
Science and Technology, Newark, DE 19716, USA
Chemistry Department, Faculty of Science, Minia University, El-Minia 61519, Egypt
b
Received 5 June 2002; received in revised form 18 October 2002; accepted 20 October 2002
Abstract
Sm(NO3)3·6H2O and Sm2(C2O4)3·10H2O were used as precursors for the formation of Sm2O3. Thermal processes in-
volved in the decomposition course of both salts up to 800 ◦C in air were monitored by nonisothermal gravimetry and
differential thermal analysis. Intermediates and final solid products were characterized by IR-spectroscopy, X-ray diffraction
and scanning electron microscopy. The results showed that Sm(NO3)3·6H2O decomposes completely through nine endother-
mic mass loss processes. The dehydration occurs through the first four steps at 90, 125, 195, and 240 ◦C, culminating in
a crystalline nitrate monohydrate, which subsequently decomposes to Sm(OH)(NO3)2 at 355 ◦C. The latter decomposes
rapidly to form a stable and crystalline SmO(NO3) at 460 ◦C, through nonstoichoimetric unstable intermediates. Finally
Sm2O3 forms at 520 ◦C. For the oxalate, the dehydration occurs in five steps: the anhydrous oxalate is thermally unstable and
immediately decomposes to Sm2O3 at 645 ◦C through two unstable intermediates. The crystalline oxide obtained from the
nitrate contains larger pores than the oxide obtained from the oxalate, as indicated from scanning electron microscopy (SEM)
results.
© 2002 Elsevier Science B.V. All rights reserved.
Keywords: Sm oxide; Sm nitrate and oxalate; Formation; Characterization DTA, TG, IR, XRD and SEM
1. Introduction
pling of methane [3,5–7]. It is also used as a refractory
oxide in the formation of ceramic cores for electrode
applications [9–11]. Moreover, it is used for produc-
ing samarium–cobalt permanent magnets [12].
A review of thermal decomposition properties of
rare-earth salts [13] reported that lanthanide oxides
obtained from nitrate or acetate precursors exhibit
higher surface areas than those obtained from oxalate
precursors. Patil et al. [14] reported that the forma-
tion of M2O3 from rare-earth nitrates takes place via
the oxy-nitrate (MONO3). They also reported the
possible formation of an anhydrous nitrate, whereas
Samarium sesquioxides, Sm2O3, with B-type cen-
tered monoclinic and C-type anion-deficient fluorite
structures [1], have diverse applications in environ-
mental and catalytic science [2–10]. It is important
as a catalyst for dehydration of alcohols [3] and as a
catalyst support for metals used in the dehydration of
2-alkanols to 1-alkenes, as well as for oxidative cou-
∗
Corresponding author.
E-mail address: gma156@yahoo.com (G.A.M. Hussein).
0040-6031/03/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0040-6031(02)00535-X

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G.A.M. Hussein et al. / Thermochimica Acta 402 (2003) 27–36
Wendlandt and Bear [15] stated that the anhydrous
nitrates are unstable. Studies on the decomposition
of Y(NO3)3·5H2O and Y2(C2O4)3·8H2O indicated
that a thermally stable monohydrate nitrate and dihy-
drate oxalate were formed [16]. Crystalline YONO3
and Y2O2CO3, respectively, were also detected by
XRD. Y2O3 was the final product at 500 and 640 ◦C,
respectively. The texture analysis by nitrogen adsorp-
tion and scanning electron microscopy (SEM) [17]
revealed that Y2O3 obtained from Y(NO3)3·5H2O at
500 ◦C has a higher surface area (SBET = 58 m2/g)
than that obtained at 700 ◦C from Y2(C2O4)3·8H2O
(SBET = 12 m2/g), which was attributed to sintering.
The decompositon of Pr(NO3)3·6H2O in air to form
Pr6O11 has been studied using TGA, differential ther-
mal analysis (DTA), infrared spectroscopy (IR) and
XRD [18]. The results indicated that Pr(NO3)3·6H2O
decomposes through 11 endothermic mass loss pro-
cesses. Five dehydration steps occurred at 130, 180,
200, 230 and 250 ◦C, leading to the formation of
crystalline nitrate monohydrate, which decomposes
to Pr(NO3)3 at 340 ◦C. The latter, decomposes to
Pr6O11 at 465 ◦C via four different intermediates;
PrO(NO3) at 430 ◦C, a nonstoichiometric unsta-
ble Pr(O)0.25(NO3)2.5 at 362 ◦C; Pr(O)0.5(NO3)2 at
382 ◦C; and Pr(O)0.75(NO3)1.5 at 400 ◦C. The final de-
composition product, Pr6O11, has a porous character
with a large surface area of 46.3 m2/g. Balboul [19]
studied the decomposition course of Ho(NO3)4·5H2O
in air by TGA, DTA, IR, XRD, and SEM. It was re-
ported that the anhydrous nitrates are thermally unsta-
ble and decomposition at 560 ◦C yielded Ho2O3. Dif-
ferent intermediates of nonstoichiometric oxy-nitrates
were also detected.
The decomposition of Sm2(C2O4)3·7H2O has been
studied in air by Moosath et al. [20] and Wendlandt
[21]. Both agree that partial decomposition occurs at
400 ◦C and that the final decomposition to Sm2O3 oc-
curs at 700 ◦C. Above 400 ◦C the decomposition oc-
curs gradually as the temperature increases to 700 ◦C.
Although Moosath et al. [20] reported no lower hy-
drate or carbonate intermediates, Wendlandt reported
the formation of a lower hydrate.
Recently, Hussein et al. [22] studied the decompo-
sition of Gd2(C2O4)3·10H2O to form Gd2O3 up to
800 ◦C in air. The results showed that Gd2(C2O4)3·
10H2O dehydrates in six endothermic steps, then
decomposes to Gd2O3 at 575 ◦C through different in-
termediates. Gd2(C2O4)3·2H2O and Gd2O2CO3 were
found to be stable intermediates, while the anhydrous
oxalate was thermally unstable. The final product
Gd2O3 had a surface area of 23.4 m2/g.
The present investigation set out to characterize
the thermal decomposition course of Sm(NO3)3·6H2O
and Sm2(C2O4)3·10H2O to form Sm2O3 by means
of thermogravimetric analysis (TG) and differential
thermal analysis (DTA). The reaction products were
characterized by infrared spectroscopy (IR) and X-ray
powder diffraction (XRD). The progressive morpho-
logical development of Sm2O3 was followed using
scanning electron microscopy (SEM).
2. Experimental
2.1. Materials
Samarium nitrate hexahydrate, Sm(NO3)3·6H2O,
abbreviated as SmNit, was used as received and was
99.9% pure (WAKO, Pure Chemical Industry, Japan).
Calcination products were obtained by heating at
various temperatures (200–600 ◦C) for 1 h in air. The
calcination temperatures were chosen on the basis
of the thermal analysis results. Prior to analysis, the
calcination products were kept dry over silica gel.
Samarium oxalate decahydrate, Sm2(C2O4)3·
10H2O, abbreviated as SmOx, was prepared from
samarium acetate by dropwise addition of a hot 4%
ammonium oxalate solution to a stirred, hot solu-
tion of samarium acetate dissolved in glacial acetic
acid. The resulting solution was then neutralized to
pH 7 with 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 ◦C to constant weight [22].
The calcination products were obtained by heating at
various temperatures between 200 and 800 ◦C in air
for 1 h. The calcination temperatures were chosen on
the basis of the thermal analysis results.
For the sake of simplicity, all products are denoted
in the text by SmNit for the nitrate and SmOx for
the oxalate, followed by the calcination temperature.
Thus, SmNit600 indicates decomposition products
of SmNit at 600 ◦C for 1 h. In the case of original
specimens, the suffix Org. will be used in place of
temperature.

G.A.M. Hussein et al. / Thermochimica Acta 402 (2003) 27–36
29
2.2. Thermal analysis
3. Results and discussion
Thermal analysis was performed with a MAC-
Science Model DTA-TG2000 (Tokyo, Japan). Ther-
mogravimetric analysis (TG) and differential thermal
analysis (DTA) curves were recorded over a wide
range of temperatures, up to 800 ◦C at a heat-
ing rate of 10 ◦C/min in flowing air at 50 ml/min.
The sample size used in the TGA measurement
was approximately 15 mg. Highly sintered ␣-Al2O3
was used as a reference material for the DTA
measurements.
3.1. Samarium nitrate hexahydrate, SmNit,
Sm(NO3)3·6H2O
3.1.1. Processes I–V
The DTA curve (Fig. 1A) shows an endothermic,
mass-invariant process located at 70 ◦C (process I). A
direct measurement of the melting point of SmNit was
found to occur between 70 and 73 ◦C. The TG and
DTA curves (Fig. 1A) indicated that processes II and
III are overlapping, endothermic mass loss processes
with maxima at 90 ◦C and 125 ◦C, respectively. Pro-
cess II yielded a mass loss (ML) = 4.1% (Table 1)
corresponding to the release of one moles of water,
while process III yielded a total ML = 12.1% corre-
sponding to the release of two more moles of water.
Processes IV occurred at 195 ◦C with a total ML =
16.1% and process V occurred at 240 ◦C with a total
ML = 20.4%. The ML observed for processes IV and
V each corresponded to the release of one more mole
of water. Hence, the dehydration process most likely
proceeds as follows:
2.3. Infrared spectroscopy
IR spectra were obtained at a resolution of 4 cm−1
over the range 4000–400 cm−1 using a model FT/IR
410 JASCO (Japan). IR spectra of SmNit, SmOx, and
the solid calcination products were obtained from thin
(>20 mg/m2), lightly loaded (<1%) KBr-supported
discs.
2.4. X-ray diffraction
Sm(NO3)3 · 6H2OII (90 ◦C) → Sm(NO3)3 · 5H2O
III (125 ◦C) → Sm(NO3)3 · 3H2O
XRD powder patterns were obtained with a Philips
3520 diffractometer using a fixed copper anode gen-
erating Cu K␣ radiation (λ = 1.5418 Å) as the X-ray
source and equipped with a Ni-filter and graphite
monochromator on the detector arm. The opera-
tional settings for all the XRD scans were: voltage
= 40 kV; current = 30 mA; scan range 4–60◦ 2θ
using a step size of 0.02◦ at 2 s per step. For identi-
fication purposes the relative intensities (I/I0) and the
d-spacings (Å) were compared with standard diffrac-
tion patterns in the ASTM powder diffraction file
[23].
IV (195 ◦C) → Sm(NO3)3 · 2H2O
V (240 ◦C) → Sm(NO3)3 · H2O
In support, the IR spectrum of SmNit200 (Fig. 2A)
is very similar to that obtained for untreated SmNit.
It displays absorption bands at 1640 cm−1 (δOH of
Table 1
Thermal processes in the calcination of Sm(NO3)3·6H2O
Step Tmax Observed Theoretical Composition
(◦C) ML (%)
ML (%)
2.5. Scanning electron microscopy
I
II
III
IV
V
70
90
–
Sm(NO3)3·6H2O (liquid)
Sm(NO3)3·5H2O
Sm(NO3)3·3H2O
Sm(NO3)3·2H2O
Sm(NO3)3·H2O
Sm(NO3)3
SmO0.5(NO3)2
SmO0.75(NO3)1.5
SmONO3
4.1
4.08
12.15
16.2
20.25
24.3
36.5
42.61
48.6
60.7
SEM was performed using a JEOL JXA-840 SEM
operating at 20 keV. Samples of SmNit, SmOx, and
their respective calcination products were mounted
separately on aluminum stubs, evacuated to 10−3 Torr,
and precoated for 5 min in a sputter-coater with thin,
uniform, gold/palladium film to minimize charg-
ing in the electron beam. The applied voltage was
1.2–1.6 kV.
125 12.1
195 16.1
240 20.4
355 24.2
VI
VII 405 36.7
VIII 445 42.5
IX
X
460 48.6
520 60.5
Sm2O3

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G.A.M. Hussein et al. / Thermochimica Acta 402 (2003) 27–36
Fig. 1. TGA and DTA curves recorded for SmNit (A) and SmOx (B) at the heating rates indicated, in flowing air (50 ml/min).

G.A.M. Hussein et al. / Thermochimica Acta 402 (2003) 27–36
31
of water and formation of anhydrous Sm(NO3)3. The
strong overlap between processes VI and VII indicate
that the anhydrous SmNit is thermally unstable. This
is in agreement with previous studies, in which the an-
hydrous PrNit [18] and HoNit [19] are thermally un-
stable as well. The anhydrous SmNit is not detected,
in agreement with Wendlandt and Bear [15], who also
reported that the anhydrous nitrate is thermally unsta-
ble. Its clear that, the water of hydration is responsible
for crystal coherency and thermal stability of SmNit.
Fig. 1A shows that processes VII–IX are over-
lapped, endothermic processes with maxima at 405,
445 and 460 ◦C (Table 1). The total ML associated
with process VII is 36.7% close to the calculated
36.5% for the formation of SmO0.5(NO3)2. For pro-
cess VIII the total ML = 42.5%, close to the calcu-
lated 42.6% for the formation of SmO0.75(NO3)1.5
.
Process IX leads to the formation of SmONO3 as
indicated from the observed weight loss of 48.6%,
which matches the calculated value of 48.6% for this
transition. Non-stoichiometric oxynitrates have pre-
viously been reported during the decomposition of
PrNit [19], ThNit [26] and DyNit [27], while differ-
ent MOx(NO3)y compounds were formed as unstable
intermediates.
The IR spectrum of SmNit400 (Fig. 2A) provides
convincing evidence for the formation of SmONO3.
Bridging nitro groups are indicated by new bands at
1190 and 850 cm−1 [24,25], possibly due to a combi-
Fig. 2. IR-spectra for the 1-h calcination products of SmNit (A)
and SmOx (B) at the temperatures indicated.
nation of ␯
outside the bridge and bridging ␯
,
=
O
=
N
O
N
respectively. The strong composite absorption bands
emerging at 600–400 cm−1 are probably related to
metal–oxygen vibrational modes (Sm–O) [28]. Also
the band due to δOH at 1640 cm−1 of SmO(NO3) is
still present. Furthermore, the XRD pattern (Fig. 3A)
of SmNit400 exhibits only diffraction peaks from
SmONO3.
The SEM image of SmNit400 (Fig. 4) shows irreg-
ularly stacked, sheet-like structures with steps, voids,
and large cavities with rough surfaces, clearly differ-
ent morphologically from the original SmNit and Sm-
Nit200.
HOH) and at 1530–1320, 1040, 830 and 730 cm−1
due to the nitrate group in a mainly bidentate struc-
ture [24,25]. The XRD pattern of SmNit200 (Fig. 3A)
indicates the formation of Sm(NO3)3·H2O as a crys-
talline phase. Furthermore, the SEM of the parent Sm-
Nit (Fig. 4) reveals regularly-shaped crystals, while the
SEM micrograph of SmNit200 (Fig. 4) shows large,
irregularly-shaped crystals that exhibit evidence of a
wet surface with holes and bulges as a result of melt-
ing and evolution of water vapour.
3.1.2. Processes VI–IX
3.1.3. Process X
Process VI is an endothermic ML process with a
maximum at 355 ◦C and overlaps the succeeding pro-
cesses (VII–IX). The mass loss determined for process
VI is 24.28%, corresponding to the loss of the last mole
Process X (Fig. 1A) represents endothermic de-
composition of SmONO3 at 520 ◦C. The total ML =
60.5% is close to the calculated 60.7% for the over-
all conversion of Sm(NO3)3·6H2O to Sm2O3. The IR

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G.A.M. Hussein et al. / Thermochimica Acta 402 (2003) 27–36
Fig. 3. X-ray powder diffractograms for the 1-h calcination products of SmNit (A) and SmOx (B) at the temperatures indicated.

G.A.M. Hussein et al. / Thermochimica Acta 402 (2003) 27–36
33
Fig. 4. Scanning electron micrograph obtained for SmNit and SmOx calcined at different temperatures for 1 h.

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G.A.M. Hussein et al. / Thermochimica Acta 402 (2003) 27–36
spectrum of SmNit600 (Fig. 2A) displays only bands
below 800 cm−1 due to Sm2O3 [28]. Support for this
interpretation is provided by the corresponding XRD
pattern of SmNit550 (Fig. 3A) that compares well with
the standard pattern (ASTM 25–749) of the mono-
clinic (B-type) Sm2O3 structure. However, the XRD
pattern of SmNit600 (Fig. 3A) is of the cubic (C-type)
structure (ASTM 15–0813), indicating that the mon-
oclinic to cubic transformation occurs with heating in
the range 550–600 ◦C. The SEM image of SmNit600
(Fig. 4) indicates that the morphology has changed;
the sheet-like particles have grown together, although
some pores, cracks, and fragments remain.
displaying almost all of the characteristic absorption
bands of oxalate anions between 1750 and 640 cm−1
[24,25], as well as those corresponding to the wa-
ter of hydration. The XRD pattern of SmOx200 and
SmOx300 (Fig. 3B) reveals the lack of crystallinity
of the oxalate dihydrate intermediate. The SEM mi-
crograph (Fig. 4) of samarium oxalate decahydrate
reveals uniform small particles with nearly the same
shape and morphology. The SEM micrograph of
SmOx300 reveals some changes in the crystallite mor-
phology as a result of decomposition to the oxalate
dihydrate, specifically the particle size and degree of
agglomeration have decreased relative to SmOxOrg.
It seems likely that in the oxalate decahydrate, some
of the water is present as water of crystallization that
acts to stabilize the morphology, perhaps through an
extensive hydrogen bonding network [26,27].
3.2. Samarium oxalate decahydrate, SmOx,
Sm2(C2O4)3·10H2O
The dihydrate thus formed persists to 360 ◦C, at
which temperature process V occurs endothermi-
cally with a maximum at 385 ◦C and total ML =
24.4% (Table 2), which is consistent with the ex-
pected 24.03% for the removal of 10 moles of H2O
from the oxalate decahydrate. This indicates that
Sm2(C2O4)3·2H2O is the only lower hydrate which
is thermally stable. Fig. 1B also shows that after
complete dehydration the decomposition takes place
immediately through three overlapping exothermic
processes. These processes show maxima at 405 ◦C
for VI, 430 ◦C for VII and 455 ◦C for VIII (Table 2).
Process VI brings the total ML to 35.1%, close to
that calculated for the formation of samarium carbon-
ate, Sm2(CO3)3. Process VII is weakly exothermic
and results in a total ML of 41.5%, correspond-
ing to the formation of samarium monoxycarbonate,
Sm2O(CO3)2 (Table 2). Process VIII brings the total
mass loss to 47.5%, which is close to the 47.2% total
ML expected for the formation of Sm2O2CO3 from
the oxalate decahydrate.
The TG and DTA curves in Fig. 1B reveal the
presence of nine ML processes, designated I–IX, that
occurred during the decomposition of Sm2(C2O4)3·
10H2O (SmOx) in an atmosphere of air. Three of
these processes (VI, VII and VIII) are exothermic and
overlap each other, while all the other processes are
endothermic and resolved. The ML resulting from the
first four processes (I–IV) suggests a step-wise dehy-
dration of Sm2(C2O4)3·10H2O leading to the forma-
tion of samarium oxalate dihydrate (Table 2). The first
four processes result in the loss of eight moles of water
with processes I and II removing two moles of water
each, process III removing three moles of water, and
process IV removing one mole of water.
In support of the above results, the IR spectra
obtained for SmOx200 and SmOx300 (Fig. 2B) are
both similar to that of untreated Sm2(C2O4)3·10H2O,
Table 2
Thermal processes in the calcination of Sm2(C2O4)3·10H2O
Step Tmax
Observed
ML (%)
Theoretical
ML (%)
Composition
The formation of Sm2O2CO3 is supported by the IR
spectrum (Fig. 2B) of SmOx400, which does not ex-
hibit the ␦HOH absorption band of water that is present
as a strong, broad absorption band in the IR spectra
of SmOx200 and SmOx300. The fundamental modes
(◦C)
I
II
78
100
120
222
385
405
430
4.7
9.6
4.8
9.6
Sm2(C2O4)3·8H2O
Sm2(C2O4)3·6H2O
Sm2(C2O4)3·3H2O
Sm2(C2O4)3·2H2O
Sm2(C2O4)3
Sm2(CO3)3
Sm2O(CO3)2
Sm2O2CO3
Sm2O3
III
IV
V
VI
VII
16.6
19.3
24.4
35.1
41.5
47.5
52.8
16.9
19.3
24.1
35.4
41.3
47.2
53.1
2−
of the CO3 species between 1600 and 1300 cm−1
were present in SmOx400, as well as the absorption
bands appearing at 800–400 cm−1, which are related
to the Sm–O vibrational lattice modes [28]. The char-
acteristic bands of the oxy-carbonates are weaker as
VIII 455
IX 645

G.A.M. Hussein et al. / Thermochimica Acta 402 (2003) 27–36
35
the temperature goes up to 500 ◦C, as seen in the IR
spectrum (Fig. 2B) of SmOx500, while all other Sm–O
bands remain the same.
The XRD pattern (Fig. 3B) for the solid decompo-
sition product of SmOx450 supports the TG (Fig. 1B)
and IR (Fig. 2B) data. It reveals the formation of crys-
talline Sm2O2CO3 (ASTM 23-0624). The correspond-
ing SEM micrograph (Fig. 4) of SmOx450 reveals
irregularly-shaped, large particles with rough surfaces
that have a tendency to form relatively large agglom-
erates with large cavities and holes.
→ Sm (NO3)3 · 3H2O (195 ◦C)
→ Sm (NO3)3 · 2H2O (240 ◦C)
→ Sm(NO3)3 · H2O(355 ◦C)
→ Sm2(NO3)3 (405 ◦C)
→ SmO0.5(NO3)2 (445 ◦C)
→ SmO0.75(NO3)1.5 (460 ◦C)
→ SmONO3 (520 ◦C) → Sm2O3
Sm2(C2O4)3·10H2O:
As the temperature is increased to 600 ◦C, process
IX occurs endothermically with a total ML = 52.8%,
which is close to the calculated 53.1% for the overall
conversion of Sm2(C2O4)3·10H2O to Sm2O3. This is
confirmed by the IR-spectrum at 800 ◦C (Fig. 2B),
in which absorption bands due to the oxy-carbonate
species are no longer present. The absorption bands
observed below 700 cm−1 are related to the lattice
vibrational modes of Sm2O3. The weak bands around
1660, 1490 and 1410 cm−1 are most probably due to
the surface contamination with carbonate and water.
The XRD pattern of the SmOx calcination prod-
uct at 600 ◦C (FFig. 3B) provides further evidence for
the complete formation of the oxide, as all diffrac-
tion peaks appearing in the XRD pattern correspond
to the characteristic diffraction peaks of the two dif-
ferent crystalline Sm2O3 phases (monoclinic, ASTM
no. 19-1114 and cubic, ASTM 15-0813). The XRD
patterns of SmOx700 and SmOx800 indicate that only
cubic Sm2O3 is present as expected (Tmax = 645 ◦C).
The SEM micrograph of SmOx800 also reveals that
the Sm2O3 is well crystallized and shows large aggre-
gates of particles with highly porous surfaces. It has
previously been established that the release of volatile
components creates a porous pathway through the ma-
terial bulk [29,30].
Sm2(C2O4)3 · 10H2O (78 ◦C)
→ Sm2(C2O4)3 · 8H2O (100 ◦C)
→ Sm2(C2O4)3 · 6H2O (120 ◦C)
→ Sm2(C2O4)3 · 3H2O (222 ◦C)
→ Sm2(C2O4)3 · 2H2O (385 ◦C)
→ Sm2(C2O4)3 (405 ◦C)
→ Sm2(CO3)3 (430 ◦C)
→ Sm2O(CO3)2 (455 ◦C)
→ Sm2O2CO3 (645 ◦C) → Sm2O3
The oxalate dihydrate and nitrate monohydrate
were found to be thermally stable. Sm2(NO3)3 and
Sm2(C2O4)3 were formed only as unstable interme-
diates in the decomposition of Sm(NO3)3·6H2O and
Sm2(C2O4)3·10H2O. The stable anhydrous interme-
diate in the decomposition of Sm(NO3)3·6H2O was
found to be SmONO3, while Sm2O2CO3 was found
to be the stable anhydrous intermediate in the decom-
position of Sm2(C2O4)3·10H2O. Finally, the Sm2O3
formed from Sm(NO3)3·6H2O was observed to pos-
sess larger, more porous, sheet-like particles com-
pared to Sm2O3 obtained from Sm2(C2O4)3·10H2O,
which contained smaller particles in large aggregates
with less porosity. Consequently, for catalytic pur-
poses, samarium nitrate is a better precursor for the
formation of Sm2O3 than samarium oxalate.
4. Conclusions
The thermal decomposition of SmNit and SmOx in
air to form Sm2O3 involves the following pathways.
Sm(NO3)3·6H2O:
Sm(NO3)3 · 6H2O(s)(70 ◦C)
→ Sm(NO3)3 · 6H2O(l) (90 ◦C)
→ Sm(NO3)3 · 5H2O (125 ◦C)
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