Thermodynamic stability of RECl3·xH2O (RE = Ce, Pr) determined by dynamic transpiration technique

Article abstract of DOI:10.1007/s10973-019-08517-w

In the present study, decomposition mechanism of CeCl₃·xH₂O(s) and PrCl₃·xH₂O(s) was investigated using thermogravimetry, differential thermal analysis and evolved gas analysis techniques. Intermediate products formed in the dehydration of RE (Ce, Pr) Cl₃·xH₂O(s) were characterized by X-ray diffraction analyses. Thermodynamic stability of the RECl₃·xH₂O(s) and their intermediate products has been determined from the vapor pressure of water measured over the compounds employing dynamic transpiration technique.

Full text of DOI:10.1007/s10973-019-08517-w

Journal of Thermal Analysis and Calorimetry
https://doi.org/10.1007/s10973-019-08517-w(0123456789(). -, vo Vl
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Thermodynamic stability of RECl ÁxH O (RE = Ce, Pr) determined
3
2
by dynamic transpiration technique
D. K. Sahoo1 • R. Mishra2 • Raj Kumar3 • Vivekanand Kain1
Received: 17 May 2018 / Accepted: 25 June 2019
Ó Akad e´ miai Kiad o´ , Budapest, Hungary 2019
Abstract
In the present study, decomposition mechanism of CeCl ÁxH O(s) and PrCl ÁxH O(s) was investigated using thermo-
3
2
3
2
gravimetry, differential thermal analysis and evolved gas analysis techniques. Intermediate products formed in the
dehydration of RE (Ce, Pr) Cl ÁxH O(s) were characterized by X-ray diffraction analyses. Thermodynamic stability of the
3
2
RECl ÁxH O(s) and their intermediate products has been determined from the vapor pressure of water measured over the
3
2
compounds employing dynamic transpiration technique.
Keywords Vapor pressure Á Transpiration technique Á Free energy Á RECl ÁxH O
3
2
Introduction
SOCl (g), CCl (g), COCl (g) and NH Cl(s) [10, 11]. Most
2 4 2 4
of these chlorinating agents are corrosive at operating
temperature of 600–700 °C and are not cost effective.
Synthesis of anhydrous RECl by dehydration of RECl
The demand of rare earth (RE) elements has been increased
drastically in the last two decades due to their widespread
applications in various emerging and established tech-
nologies [1–3]. There is an urgent need to produce these
metals and their alloys in large quantities in an efficient and
economic way. Metallothermic reduction and molten salt
electrolysis are among the most suitable methods adopted
to produce RE metals and alloys [4–6]. Success of these
methods largely depends on the quality of raw materials
available for the extraction process. Anhydrous rare earth
chloride is an attractive raw material for the production of
RE metals and alloys [7–9]. The production of pure
anhydrous rare earth chloride is therefore an important
aspect in the production of rare earth metals.
3
3-
xH O is a simple and attractive process for the industrial
2
production of rare earth metal [12–14]. Various dehydra-
tion processes, i.e., (1) dehydration in the presence of HCl
vapor (2) in the presence ammonium chloride and (3)
vacuum have been used for the dehydration of RECl Á
3
xH O. The vacuum dehydration is found to be the most
2
suitable method for the preparation of anhydrous rare earth
chloride. However, in this process of dehydration,
REOCl(s) is formed in measurable quantity by the
hydrolysis RECl . The presence of REOCl (s) in molten
3
electrolyte hinders reduction process and thereby affects
yield and purity of the product.
Anhydrous rare earth chlorides can be prepared by
The knowledge of dehydration mechanism and the ther-
modynamic stability of the RE chloride hydrates are there-
fore important for the production of anhydrous rare earth
chlorides. The mechanism of dehydration of hydrated cer-
ium chloride and praseodymium chloride has been studied
by various researchers [15–19]. CeCl Á7H O decomposes to
chlorinating from rare earth oxides using agents like Cl (g),
2
&
D. K. Sahoo
deesahoo@gmail.com
3
2
1
SO/F, Material Processing and Corrosion Engineering
Division (MP&CED), Bhabha Atomic Research Centre,
Trombay, Mumbai 400085, India
CeCl3 in the flowing steps: CeCl Á7H O(s) ? CeCl Á3H
3
2
3
2
O(s) ? CeCl Á2H O ? CeCl ÁH O(s) ? CeCl (s). On the
3
2
3
2
3
other hand, two different dehydration schemes were pro-
2
3
Chemistry Division, Bhabha Atomic Research Centre,
Trombay, Mumbai 400085, India
posed for the thermal decomposition of PrCl Á7H O (1)
3
2
7 ? 6 ? 3 ? 2 ? 1 ? 0 and (2) 7 ? 3 ? 2 ? 1 ? 0.
The present study was undertaken to identify and
Uranium Extraction Division, Bhabha Atomic Research
Centre, Trombay, Mumbai 400085, India
123

D. K. Sahoo et al.
characterize the intermediate hydrates formed during the
12
13
dehydration of CeCl Á7H O and PrCl Á7H O employing
3
2
3
2
thermal and X-ray diffraction technique.
11
In addition, the equilibrium vapor pressures of water
over various hydrates have been measured using dynamic
and isothermal transpiration technique. The standard molar
enthalpy of formation and Gibbs energy of formations of
cerium and praseodymium chloride hydrates have been
derived from the measured vapor pressure data.
10
9
Experimental
8
CeCl Á7H O(s) (99.9% purity, LR Grade) and PrCl Á7H O
3
2
3
2
7
(
99.9% purity, LR Grade) were procured and used for
vapor pressure studies without any further purification. The
compounds were characterized by thermogravimetry (TG),
differential thermal analysis(DTA) and evolved gas anal-
ysis (EGA) using SETARAM (Setsys Evolution, France)
instrument. Room-temperature and high-temperature XRD
studies were carried out employing INEL make high-tem-
perature X-ray diffractometer (HT-XRD) (EQUINOX
6
4
3
2
5
1
3
000) using CuK radiation. HT-XRD patterns were
µ
recorded at different temperatures in the 2h range 10°–70°
in static air atmosphere.
Fig. 1 Schematic of the micro thermo-balance used for the vapor
pressure measurement. Where 1—Furnace, 2—Capillary, 3— Sam-
ple, 4—Tired sample holder, 5—Thermocouple, 6—Baffles, 7—
Vapor pressure measurement
2
8mm Id alumina tube, 8—14 mm id silica tube, 9—Silica pan for
Vapor pressure of the H O(g) over the compounds was
2
weight tearing,1 0—Rear port forbalnce housing, 11—Caplillary
flowmeter, 12—Hang down Pt wire, 13— Electronic balance
measured employing a thermo-gravimetric transpiration
instrument described elsewhere [20]. Figure 1 shows the
schematic diagram of the transpiration set-up used in the
present experiments.
measurement. The equilibrium condition for the measure-
ment was established by estimating the apparent pressure
of water over the sample as a function of carrier gas (Ar-
gon) flow rate. The region where the apparent vapor
pressure of water is independent of flow rate of the carrier
gas was chosen as the equilibrium condition. Here, the
measurement was carried out near isothermal condition as
the variation of temperature was expected to be 0.5 °C
which is well within the experimental uncertainty.
In this study, sample was spread on a platinum crucible
-
1
and heated at a rate of 0.5 K min . Argon was used as
carrier gas for sweeping out the vapors evolved from the
samples. Mass loss from the sample was recorded at a
mean temperature for 60 s interval at a constant flow rate
of carrier gas. The apparent pressure of water over the
samples was determined from the mass loss data using the
following equation.
In the present investigation, the coexisting phases in
each dehydration step could not be isolated due to the
instability of the intermediate products on exposure to air,
rather the sample was obtained with desired composition in
the two-phase region for a given vaporization reaction.
This was possible by monitoring the thermogravimetry
Dm
1
RT
 v
PH O ¼
Â
;
2
t
MH2O
where (PH2O) is the vapor pressure of water, Dm is the mass
los (g), t is the known interval of time (60 s in the present
investigation), M
is the molecular weight of water
H2O
-1
18 9 10 kg mol ), R is the universal gas constant
(TG) curve and online measurement of mass loss. The
-
3
(
assumption taken in present measurement is that there is no
mutual solubility among the coexisting phases during the
vaporization reactions.
(
0.082 L atm/moL/K), T is the room temperature (K) and m
is the carrier gas flow rate (lh ).
-
1
The thermocouple (‘S’ type) used for measuring the
sample temperature was placed much closer to the crucible
containing the sample to have better accuracy of
In the present investigation, measurements were carried
out in the temperature range of 325–330 K, 344–347 K,
1
23

Thermodynamic stability of RECl
3
ÁxH
2
O (RE = Ce, Pr) determined by dynamic transpiration…
3
62–376 K and 397–400 K for the two-phase mix-
DTA plot infers to the loss of water molecules in four
consecutive steps.
tures CeCl Á7H O(s) ? CeCl Á3H O(s), CeCl Á3H O(s) ?
3
2
3
2
3
2
CeCl Á2H O(s),CeCl Á2H O(s) ? CedCl ÁH O(s)
and
Figure 4 shows the mass spectrometry analysis of the
3
2
3
2
3
2
CeCl ÁH O(s) ? CeCl (s), respectively. Similar experi-
evolved gas during the decomposition of CeCl Á7H
3
2
3
3
2
-
1
ments were carried out to measure the vapor pressure of
water in the temperature range of 332–337 K, 350–353 K,
O(s) recorded at a heating rate of 5 °C min
under
flowing argon atmosphere. The evolved gas analysis
reveals only water molecule (corresponding to ion frag-
ment m/z = 18) comes out over the entire range of mass
loss steps. Ion fragments m/z 36 and 38 corresponding to
HCl and ion fragments m/z 35 and 37 corresponding to Cl2
molecule were not detected which could be produced by
3
70–374 K and 416–420 K for the two-phase mixtures
PrCl Á7H O(s) ? PrCl Á3H O(s), PrCl Á3H O(s) ? PrCl
3
2
3
2
3
2
3
Á2H O(s), PrCl Á2H O(s) ? PrCl ÁH O(s) and PrCl ÁH
2
3
2
3
2
3
2
O(s) ? PrCl (s), respectively. Two independent sets of
3
experiments were carried out to measure the vapor pressure
of each decomposition steps.
the hydrolysis of the CeCl (s). This substantiates that no
3
The reliability of the present approach in the acquisition
of equilibrium vapor pressure data was further established
by determining the vapor pressure data for the last dehy-
dration step for the two hydrates by adopting dynamic as
well as isothermal mode. It is observed here that isothermal
mode could be adopted only for the fourth mass loss step
where the temperature span for dehydration is sufficiently
large to perform series of isothermal experiments. In
isothermal mode, samples were equilibrated at constant
hydrolysis reaction occurs during the dehydration of
CeCl ÁxH O up to 450 K under inert atmosphere.
3
2
To ascertain the dehydration mechanism, intermediate
products formed in the decomposition steps of CeCl Á
3
7H O(s) were characterized using X-ray diffraction (XRD)
2
technique. Since rare earth chloride hydrates are highly
hygroscopic in nature, it was difficult to isolate the inter-
mediate compounds and take XRD at room temperature.
Therefore, in the present investigation, in situ characteri-
zations of the intermediate products were carried out
employing high-temperature X-ray diffraction (HTXRD)
technique. XRD patterns for CeCl Á7H O recorded at dif-
-
1
temperature and the carrier gas with flow rate 2.5 hr was
passed over the sample. The mass loss was recorded for a
given time interval, and the vapor pressure was calculated
using the relation given in Eq. (1). Experimental conditions
for the isothermal measurement were kept similar to that of
dynamic mode of measurement.
3
2
ferent temperatures were obtained from thermo-gravimet-
ric curve. Figure 5a shows the HTXRD patterns of cerium
chloride hydrates recorded at 303, 343 and 358 K, and
Fig. 5b shows the XRD patterns of CeCl hydrates recor-
3
ded at 387 and 453 K. The XRD patterns recorded at
303 K and 343 K match with the reported pattern of
CeCl Á7H O(s) (JCPDF #00-070-2366) and CeCl Á3H
Results and discussion
3
2
3
2
Establishment of decomposition mechanism
O(s) (JCPDF #00-082-0900), respectively. XRD plot
obtained at 358 K may be of CeCl Á2H O, but there is no
3
2
To start with, thermal decomposition behavior of CeCl Á
reported data available in the literature for this compound.
3
7
H O(s) was investigated employing TG–DTA technique.
2
Figure 2 shows the TG–DTA plot of CeCl Á7H
3
2
-
1
2
2
1
2
0
8
O(s) recorded at a heating rate of 2 °C min up to 450 K
under the flow of argon gas. Four endothermic peaks are
clearly observed in DTA plot (Fig. 2) which indicates that
the dehydration of CeCl .7H O proceeds through four
0
1
TG
Heat flow
362 K
–
Endothermic
382 K 400 K
3
2
successive water loss steps. But the mass loss steps are not
clearly resolved in the TG plot (Fig. 2). Therefore, to have
better resolution between the mass loss steps, TG of
CeCl Á7H O was carried out at a lower heating rate of
– 2
436 K
–
–
3
4
16
14
3
2
-
1
0
.5 °C min . Figure 3 shows the TG–DTA plot of
-1
CeCl Á7H O recorded at a heating rate of 0.5 °C min
– 5
3
2
1
2
0
under the flow of argon gas. The mass loss obtained in the
four successive steps from Fig. 3 was found to be 18.4, 5.4,
–
6
1
5
3
.5 and 4.9% in the temperature ranges 315–338 K,
38–358 K, 358–367 K and 395–418 K, respectively. This
3
25
350
375
400
425
450
475
Temperature/K
may correspond to the loss of 4, 1, 1 and 1 water molecule
in four successive steps. Four endothermic peaks in the
Fig. 2 TG-DTA plot of CeCl
3
Á7H
2
O recorded at the heating rate of
-1
-1
2
°C min under 2.5 l h flowing argon atmosphere
123

D. K. Sahoo et al.
TG-DTA plot of CeCl .7H O@0.5K min
–
1
(a) 1200
3
2
5
2
.0
T = 358 K
TG
Heat flow
1
000
800
600
3
19 K
0
5
Endothermic
1.5
T = 343 K
3
68 K
384 K
415 K
CeCl .3H O
3
2
–
1
0
0
.0
.5
.0
–
–
10
15
4
2
00
00
0
T = 303 K
CeCl .7H O
3
2
325
350
375
400
425
450
475
Temperature/K
20
40
60
2θ/°
Fig. 3 TG-DTA plot for CeCl
min under the flowing argon atmosphere
3
Á7H
2
O at a heating rate of 0.5 °C
-
1
(
b) 600
T = 453 K
Δ: CeCl3
Heating rate = 5 °C min–1
.00E – 010 Start Temp = 315 K
500
m/z = 18
m/z = 36
m/z = 38
m/z = 35
m/z = 37
1
8
6
4
2
400
.00E – 011
3
2
1
00
00
00
0
.00E – 011
T = 387 K
Δ
: CeCl .H O
3
2
.00E – 011
.00E – 011
0.00E + 000
20
40
θ/°
60
0
1000
2000
3000
4000
5000
2
Time/s
Fig. 5 HTXRD patterns of CeCl
recorded at 303, 343 and 358 K. b HTXRD patterns recorded at 387
and 453 K
3
ÁxH
2
O(s) sample. a HTXRD pattern
Fig. 4 Evolved gas analysis (EGA) plot of CeCl
heating rate of 5 °C min under the flowing argon atmosphere
3
Á7H
2
O recorded at a
-
1
The XRD patterns recorded at 387 and at 453 K (Fig. 5b)
Similar investigation was carried out to establish the ther-
are found to match with the reported pattern of CeCl ÁH
3
2
mal decomposition pattern of PrCl Á7H O(s). Figure 6
O(s) (JCPDF #001-0115) and CeCl3 (JCPDF #00-075-
3
2
shows the TG–DTA plot of PrCl Á7H O(s) recorded at a
1
891), respectively.
3
2
-
1
heating rate of 2 °C min up to 450 K under argon flow.
Based on the above observations, the decomposition
Figure 7 shows the evolved gas analysis of PrCl Á7H
mechanism of the CeCl Á7H O may be described as
3
2
3
2
-
1
O(s) recorded at a heating rate of 5 °C min under argon
flow. Figure 8a shows the HTXRD patterns of praseody-
mium chloride hydrates recorded at 303, 348 and 368 K,
CeCl Á 7H OðsÞ ! CeCl Á 3H OðsÞ þ 4H O ðgÞ
3
2
3
2
2
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð315\T=K\338Þ
CeCl Á 3H OðsÞ ! CeCl Á 2H OðsÞ þ H O ðgÞ
whereas Fig. 8b shows XRD patterns of PrCl
recorded at 408 and 453 K. The XRD patterns recorded at
03 K (Fig. 8a) and at 453 K (Fig. 8b) are found to match
well with the reported pattern of PrCl O(s) (JCPDF
Á7H
084-2188) and PrCl (JCPDF #084-1900), respectively.
hydrates
3
2
3
2
2
3
ð338\T=K\358Þ
3
CeCl3 Á 2H2O ðsÞ ! CeCl3 Á H2OðsÞ þ H2O ðgÞ
3
2
ð358\T=K\367Þ
#
3
XRD data recorded at 348, 368 and 408 K may correspond
to the intermediate compounds PrCl O, PrCl Á2H
Á3H
and PrCl ÁH O, respectively. However, no XRD patterns
CeCl3 Á H2OðsÞ ! CeCl3ðsÞ þ H2O ðgÞ
ð395\T=K\418Þ
O
2
3
2
3
3
2
are available in the literature for comparison. These
1
23

Thermodynamic stability of RECl
3
ÁxH
2
O (RE = Ce, Pr) determined by dynamic transpiration…
patterns are quite different from each other and also dif-
Establishment of equilibrium condition
for measurement
ferent from the patterns of PrCl Á7H O and PrCl .
3
2
3
Based on above observations, the decomposition
mechanism of the PrCl ÁxH O may be described as
Effect of flow rate is one of the most important parameters
in the transpiration technique. The gas flow rates are con-
trolled within a range called ‘‘plateau region’’ to achieve
equilibrium condition. The plateau region was obtained
here by studying the mass loss per unit volume of the
carrier gas as a function of carrier gas flow rate. Figure 9
shows the variation of the apparent pressure of water vapor
over the compounds versus flow rate of the carrier gas
3
2
PrCl Á 7H OðsÞ ! PrCl Á 3H OðsÞ þ 4H O ðgÞ
3
2
3
2
2
ð5Þ
ð6Þ
ð7Þ
ð8Þ
ð323\T=K\348Þ
PrCl Á 3H OðsÞ ! PrCl Á 2H OðsÞ þ H O ðgÞ
3
2
3
2
2
ð348\T=K\368Þ
PrCl Á 2H OðsÞ ! PrCl Á H OðsÞ þ H O ðgÞ
3
2
3
2
2
ð368\T=K\383Þ
PrCl Á H OðsÞ ! PrCl ðsÞ þ H O ðgÞ
(argon). The black curve (Fig. 9) represents the vapor
3
2
3
2
pressure of water measured for CeCl Á7H O(s) ? CeCl Á
3
2
3
ð413\T=K\443Þ
3H O(s) ? 4H O (g) reaction at 330 K, and the red curve
2 2
(
Fig. 9) shows vapor pressure measured for PrCl Á7H
3
2
The above mechanism of decomposition of CeCl Á7H O
3
2
O(s) ? PrCl Á3H O(s) ? 4H O (g) reaction at 335 K. It is
3
2
2
and PrCl Á7H O matches well with the reported data [17].
3
2
observed from Fig. 9 that the plateau region falls within the
(
a) 700
2
2
1
2
0
8
TG
3
42 K
Endothermic
0
1
2
3
4
5
6
Heat flow
6
5
4
3
2
1
00
00
00
00
00
00
0
T = 368 K
–
–
–
–
–
–
434 K
400 K
384 K
16
T = 348 K
1
1
1
4
2
0
PrCl .7H O@2 K/min
T = 303 K
PrCl ·7H O
3
2
3
2
325
350
375
400
425
450
475
20
40
60
Temperature/K
2θ/°
Fig. 6 TG-DTA plot of PrCl
3
Á7H
2
O recorded at the heating rate of
-1
-1
(b)
2
°C min under 2.5 lh flowing argon atmosphere
500
PrCl3 T = 453 K
Heating rate = 5 °C min–1
m/z = 18
m/z = 36
m/z = 38
m/z = 35
m/z = 37
1
.40E – 010
400
Start Temp = 315 K
1
1
8
6
4
2
.20E – 010
.00E – 010
.00E – 011
.00E – 011
.00E – 011
.00E – 011
300
200
T = 408 K
100
0
20
40
60
2θ/°
80
100
0
.00E + 000
0
1000
2000 3000
Time/s
4000
5000 6000
Fig. 8 HTXRD patterns of PrCl
recorded at 303, 348 and 368 K. b HTXRD patterns recorded at 408
and 453 K
3
ÁxH
2
O(s) sample. a HTXRD pattern
Fig. 7 Evolved gas analysis (EGA) plot of PrCl
heating rate of 5 °C min under the flowing argon atmosphere
3
Á7H
2
O recorded at a
-
1
123

D. K. Sahoo et al.
0
0
.0085
.0080
Table 1 Mass loss data for the step-I, step-II, step-III and step-IV of
CeCl3 at 330 K
PrCl3 at 335 K
the dehydration of CeCl
1
2.5 and 2.7 L h
3
ÁxH
2
O measured at carrier gas flow rate of
-
Mass
loss
step
Flow
rate/
Lh
Avg.
Mass
Time/ Apparent
0
0
0
0
0
.0075
.0070
.0065
.0060
.0055
temp./T/ loss/Dm/ Dt/s
K
pressure/PH2O
atm
/
-
1
lg
Step-I
325
160.8
169.2
180.6
187.2
211.2
222.6
173.4
182.4
195.6
199.2
228
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
0.00525
0.00552
0.00588
0.0061
3
26
2
.5
327
3
3
3
3
3
28
29
30
25
26
0.00687
0.00725
0.00525
0.00550
0.00591
0.00601
0.00688
0.00724
0.00423
0.00461
0.00489
0.00516
0.00421
0.00462
0.00487
0.00518
0.00371
0.00397
0.00421
0.00452
0.00483
0.00371
0.00396
0.00423
0.00449
0.00483
0.00269
0.00285
0.00301
0.00318
0.00269
0.00280
0.00304
0.00315
1
.5
2.0
2.5
3.0
3.5
Flow rate/L h–1
2
.7
327
28
329
30
3
Fig. 9 The apparent pressure versus flow rate of the carrier gas for
CeCl O(s) ? CeCl Á3H O(s) ? 4H O (g) reaction measured at
Á7H
30 K and PrCl O(s) ? PrCl Á3H O(s) ? 4H O (g) reaction
Á7H
measured at 335 K
3
2
3
2
2
3
3
2
3
2
2
3
240
Step-II
344
345
130.2
141.6
150.6
158.4
139.2
153.0
160.8
171.6
114.0
121.8
129.6
139.2
148.2
123.0
129.6
140.4
149.2
160.0
82.8
-
flow rate range of 2.1–2.7 lh in both the cases. This
1
2.5
3
46
indicates that maintaining the argon gas flow rate in the
-
1
347
range 2.1–2.7 lh in the present set-up will establish the
equilibrium conditions for the first decomposition step in
the dehydration of CeCl Á7H O(s) and PrCl Á7H O(s).
3
3
44
45
3
2
3
2
2
.7
346
47
362
63
364
Experiments were also carried out to achieve the equilib-
rium conditions for other vaporization reactions mentioned
in Eqs. (2)–(4) and Eqs. (5)–(8). It was observed that in all
these steps, apparent pressure is independent of flow rate in
3
Step-III
3
-
1
2.5
the region of 2.0–2.7 lh . Therefore, two sets of experi-
ments were carried out at carrier gas flow rate of 2.5 and
-
.7 lh for the vapor pressure measurement in the dehy-
3
3
3
3
65
66
62
63
1
2
dration of CeCl Á7H O. Similarly, another two set of
3
2
experiments were carried out at flow rates of 2.4 and
-
1
2
.7
364
65
366
97
2
.6 lh in the dehydration of PrCl Á7H O.
3
2
3
Estimation of vapor pressure
Step-IV
3
Mass loss data for vaporization reactions expressed in
2.5
2.7
398
399
400
397
398
399
87.6
-
1
Eqs. 1–4 for flow rate of 2.5 and 2.7 lh are given in
Table 1. The plot of lnpH2O versus 1/T for the above
vaporization reactions is shown in Fig. 10. The least-
square-fitted linear equations obtained from Fig. 10 can be
expressed as
92.4
97.8
89.4
93.6
100.8
104.4
4
00
À7073ðÆ487Þ
ln pH O=atmðÆ0:02Þ ¼
ð325\T=K\330Þ
þ 16:5ðÆ1:7Þ
þ 17:3ðÆ1:8Þ
2
ð9Þ
T
À8676ðÆ130Þ
À7828ðÆ604Þ
ln pH2O=atmðÆ0:02Þ ¼
ð362\T=K\366Þ
and
þ 18:0ðÆ0:35Þ
ln pH2O=atmðÆ0:02Þ ¼
ð344\T=K\347Þ
T
ð10Þ
T
ð11Þ
1
23

Thermodynamic stability of RECl
3
ÁxH
2
O (RE = Ce, Pr) determined by dynamic transpiration…
–
–
–
–
–
4.9
5.0
5.1
5.2
5.3
Step I
0
.00303
0.00304
0.00305
0.00306
0.00307
0.00308
–
–
–
5.3
5.4
5.5
Step II
0
.002885
0.002890
0.002895
0.002900
0.002905
Step III
–
–
–
–
5.3
5.4
5.5
5.6
0
.002735
0.002740
0.002745
0.002750
0.002755 0.002760
Step IV
–
–
–
–
5.76
5.82
5.88
5.94
0
.002500
0.002505
0.002510
0.002515
0.002520
–1
1/T (K
)
Fig. 10 Plots of lnp versus 1/T for the vaporization reaction CeCl
Á2H O(s) ? H O(g) (Step-II), CeCl Á2H O(s) ? CeCl ÁÁH
3
Á7H
2
O(s) ? CeCl
O(g) (Step-III) and CeCl
3
Á3H
2
O(s) ? 4H
ÁH O(s) ? CeCl
2
O(g) (Step-I), CeCl
3
Á3H
2
O(s) ? CeCl
3
2
2
3
2
3
2
O(s) ? H
2
3
2
(s) ? H O (g) (Step-IV)
3 2
Eqs. (5)–(8) calculated from the slope and intercept of the
plots are shown in Table 3 and are compared with the
available data in the literature [21, 22]. It can be observed
from the table that the standard molar entropy values for
the vaporization reaction mentioned in Eqs. (5)–(8) deter-
mined in the present investigation are in excellent agree-
ment with the data available in the literature.
À9281ðÆ828Þ
ln pH2O=atmðÆ0:02Þ ¼
ð397\T=K\400Þ
þ 17:4ðÆ2:08Þ
T
ð12Þ
Similarly, the vapor pressure of water as a function of
temperature for vaporization reactions expressed in Eqs. 5–
-
for flow rate of 2.4 and 2.6 lh is given in Table 2. The
1
8
plot of lnpH2O versus 1/T for the above processes is shown
in Fig. 11 for the two-phase mixtures PrCl Á7H
The standard molar enthalpy of the vaporization reac-
tions expressed in Eqs. (1)–(8) could not be compared as
there were no reported data available in this regard.
3
2-
O(s) ? PrCl Á3H O(s), PrCl Á3H O(s) ? PrCl Á2H O(s)
3
2
3
2
3
2
PrCl Á2H O(s) ? PrCl ÁH O(s)
and
PrCl ÁH
3
2
3
2
3
2-
The equation for vapor pressure of H O(g) over
2
O(s) ? PrCl (s), respectively. The least-square-fitted linear
3
CeCl ÁH O(s) ? CeCl (s) measured by isothermal method
3
2
3
equations obtained from Fig. 11 can be expressed as
could be expressed as:
À7168ðÆ398Þ
À9381ðÆ15Þ
ln pH O=atmðÆ0:02Þ ¼
þ 16:3ðÆ1:2Þ
þ 17:6ðÆ1:7Þ
þ 17:3ðÆ0:9Þ
ln pH2O=atmðÆ0:02Þ ¼
ð395\T=K\405Þ
þ 17:1ðÆ0:03Þ
2
ð13Þ
ð14Þ
ð15Þ
ð17Þ
T
T
ð332\T=K\337Þ
À8066ðÆ588Þ
Similarly, the equation for vapor pressure of H O(g) over
2
ln pH2O=atmðÆ0:02Þ ¼
ð350\T=K\353Þ
T
PrCl ÁH O(s) ? PrCl (s) measured by isothermal method
3
2
3
could be expressed as:
À8301ðÆ332Þ
À9231ðÆ32Þ
ln pH O=atmðÆ0:02Þ ¼
2
ln pH2O=atmðÆ0:02Þ ¼
ð415\T=K\423Þ
The plot of lnp(H O) versus 1/T measured by the isother-
þ 16:8ðÆ0:007Þ
T
T
ð370\T=K\374Þ
ð18Þ
À8848ðÆ862Þ
and ln pH2O=atmðÆ0:02Þ ¼
ð416\T=K\420Þ
þ 16:1ðÆ1:1Þ
ð16Þ
2
T
mal method along with the dynamic method for the fourth
mass loss step is compared in Figs. 12 and 13. The close-
ness of isothermal and non-isothermal vapor pressure data
The average standard molar enthalpy and entropy of
dehydration reactions described in Eqs. (1)–(4) and in
123

D. K. Sahoo et al.
Table 2 Mass loss data for the step-I, step-II, step-III and step-IV of the dehydration of PrCl
3
ÁxH
2
O measured at carrier gas flow rate of 2.4 and
-1
2
.6 L h
-1
Mass loss step
Step-I
Flow rate/L h
Avg. Temp./T/K
332
Mass loss/Dm/lg
Time/Dt/s
Apparent pressure/PH2O/atm
148.5
159.3
174.4
183.3
190.6
206.6
159.9
173.7
189.0
197.2
206.5
223.7
125.7
135.8
145.6
152.6
136.2
146.8
157.8
165.2
173.9
187.0
196.2
210.8
221.1
185.3
202.6
212.2
228.4
239.6
152.0
162.0
174.2
179.6
186.2
164.7
176.2
188.0
194.7
201.6
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
0.00504
0.00541
0.00592
0.00622
0.00647
0.00701
0.00501
0.00544
0.00592
0.00618
0.00647
0.00701
0.00427
0.00461
0.00494
0.00518
0.00427
0.00459
0.00495
0.00517
0.0059
3
33
2
.4
334
3
3
3
3
3
35
36
37
32
33
2
.6
.4
334
3
3
3
35
36
37
Step-II
350
351
2
3
3
3
3
52
53
50
51
2
2
.6
.4
352
53
370
71
372
3
Step-III
3
0.00635
0.00666
0.00715
0.0075
3
3
3
3
73
74
70
71
0.0058
0.00635
0.00665
0.00716
0.00751
0.00517
0.00552
0.00592
0.0061
2
2
2
.6
.4
.6
372
3
3
73
74
Step-IV
416
17
418
4
4
4
4
4
19
20
16
17
0.00633
0.00517
0.00553
0.00590
0.00611
0.00632
418
4
4
19
20
suggests that the dynamic method employed in this study is
a equilibrium measurement.
Thermodynamic stability of CeCl ÁxH O
3
2
and PrCl ÁxH O
3
2
The Gibbs energy of formation values (D G°) of CeCl Á
f
3
7
H O and its decomposition products such as CeCl Á3H O,
2
3
2
1
23

Thermodynamic stability of RECl
3
ÁxH
2
O (RE = Ce, Pr) determined by dynamic transpiration…
–
–
–
4.8
Step I
5.0
5.2
0
.00295
0.00296
0.00297
0.00298
0.00299
0.00300
Step II
0.00301
–
–
–
–
5.25
5.32
5.39
5.46
0
.002835
0.002840
0.002845
0.002850
0.002855
–
4.9
5.0
5.1
C
Step III
–
–
0
.002674
0.002681
0.002688
0.002695
Step IV
–
–
–
5.1
5.2
5.3
E
0.002380
0.002385
0.002390
0.002395
0.002400
–
1
1/T (K
)
Fig. 11 Plots of lnp versus 1/T for the vaporization reaction PrCl
O(s) ? H O(g) (Step-II), PrCl O(s) ? PrCl ÁH O(s) ? H
Á2H
3
Á7H
2
O(s) ? PrCl
O(g) (Step-III) and PrCl
3
Á3H
2
O(s) ? 4H
2
O(g) (Step-I), PrCl
3
Á3H
2
O(s) ? PrCl
3
Á
2
H
2
2
3
2
3
2
2
3
2
Á H O(s) ? PrCl
(s) ? H O(g) (Step-IV)
3 2
Table 3 Comparison of standard molar enthalpies and entropies for vaporization reaction steps 1, 2, 3 and 4 in the dehydration process of
CeCl O and PrCl
3
Á7H
Vaporization reactions
2
3
Á7H
2
O
0
-1
0
-1 -1
K
0
-1 -1
DH r/kJ mol
this work)
DS r/J mol
(this work)
DS r/J mol K
(literature [22–24])
(
CeCl
CeCl
CeCl
CeCl
3
3
3
3
Á7H
Á3H
Á2H
2
2
2
O(s) = CeCl
O(s) = CeCl
O(s) = CeCl
3
3
3
Á3H
Á2H
2
O (s) ? 4H
O (s) ? H O (g)
O (g)
2
O (g)
235.2 (± 16)
65.0 (± 05)
72.1 (± 01)
77.1 (± 07)
238.4 (± 12)
67.0 (± 05)
69.0 (± 03)
73.5 (± 07)
548.8 ± 56
143.8 ± 15
149.6 ± 03
144.6 ± 17
542.0 ± 40
146.3 ± 14
143.8 ± 08
133.8 ± 09
2
2
ÁH
2
O (s) ? H
O (g)
O (s) ? 4H
O (s) ? H O (g)
O (g)
2
ÁH
2 3 2
O(s) = CeCl (s) ? H
PrCl
PrCl
PrCl
PrCl
3
Á7H
Á3H
Á2H
2
2
2
O(s) = PrCl
O(s) = PrCl
O(s) = PrCl
3
3
3
Á3H
Á2H
2
2
2
O (g)
571.4
142.9
142.8
142.9
3
3
3
2
ÁH
2
O (s) ? H
2
ÁH
2
O(s) = PrCl
3
(s) ? H O (g)
2
0
À1
CeCl Á2H O and CeCl ÁH O was estimated using the
3
2
3
2
D G
ðÆ0:03ÞðkJ mol Þ
f
CeCl Á2H O
3
2
standard Gibbs energy of formation values of CeCl (s) and
3
¼
À1682ðÆ10Þ þ 0:62ðÆ0:03Þ Â T
ð20Þ
ð21Þ
ð22Þ
H O(g) from the literature [23] and the measured equilib-
2
ð362\T=K\366Þ
rium vapor pressures of water.
0
À1
D G
ðÆ0:05ÞðkJ mol Þ
The standard molar Gibbs free energy of formations of
cerium chloride hydrates derived using measured equilib-
rium vapor pressure data following the procedure similar to
our previous work [24, 25] is given below.
f
CeCl Á3H O
3
2
¼ À1990ðÆ10Þ þ 0:80ðÆ0:03Þ Â T
ð344\T=K\348Þ
0
À1
D G
ðÆ0:08ÞðkJ mol Þ
f
CeCl Á7H O
3
2
0
À1
DfG
ðÆ0:03ÞðkJ mol Þ
CeCl3ÁH2O
¼ À3199ðÆ17Þ þ 1:6ðÆ0:09Þ Â T
¼
À1367ðÆ09Þ þ 0:42ðÆ0:02Þ Â T
ð19Þ
ð325\T=K\330Þ
ð396\T=K\400Þ
Similarly, the standard Gibbs energy of formations of PrCl3
hydrates estimated is given below.
123

D. K. Sahoo et al.
–
–
–
–
–
–
–
–
2
3
4
5
6
7
8
9
0
À1
DfG
ðÆ0:07ÞðkJ mol Þ
PrCl3Á7H2O
CeCl .H O →CeCl +H O
ð26Þ
3
2
3
2
¼ À3198ðÆ13Þ þ 1:4ðÆ0:08Þ Â T
ð332\T=K\337Þ
In PH2O(Non iso) = –9281/T + 17.4
± 828) (± 2.08)
(
The standard molar enthalpy of formation of CeCl Á7H
3
2
O(s), CeCl Á3H O(s), CeCl Á2H O(s) and CeCl ÁH O is
3
2
3
2
3
2
found to be - 3199 ± 17 (328 K), - 1990 ± 10 (345 K),
1682 ± 10 (364 K) and - 1367 ± 09
-
-
1
In PH2O(Iso) = –9381/T + 17.3
± 15) (± 0.03)
(398 K) kJ mol , respectively. In the absence of heat
capacity information of hydrated cerium chloride system,
the values of standard molar enthalpy of formation at mean
temperature of measurement were compared with the
(
–
10
.00246 0.00247 0.00248 0.00249 0.00250 0.00251 0.00252 0.00253 0.00254
available standard molar enthalpy of formation data of
0
298.15
0
CeCl Á7H O(s) (D H
) reported in the literature. The
3
2
f
1
/T (K–1
)
standard molar enthalpy of formation of CeCl Á7H O(s) is
3
2
found to be comparable with the reported value
Fig. 12 Comparison of isothermal vs non-isothermal plots of lnp
versus
CeCl
-
- 3178.6 ± 2.5 kJ mol [26].
1
1/T
O(s) ? CeCl
for
the
O(g)
vaporization
reaction
3
ÁH
2
3
(s) ? H
2
Similarly, the standard molar enthalpy of formation of
PrCl Á7H O(s), PrCl Á3H O(s), PrCl Á2H O(s) and PrCl Á
3
2
3
2
3
2
3
–
–
–
–
–
–
–
–
2
3
4
5
6
7
8
9
H O is found to be - 3198 ± 13 (335 K), - 1988 ± 10
2
PrCl .H O →PrCl +H O
3
2
3
2
(
352 K), - 1678 ± 09 (372 K) and - 1366 ± 08
In pH2O(Non iso) = –8848/T + 16.1
± 862) (± 1.1)
-
(418 K) kJ mol , respectively. The standard molar
1
(
enthalpy of formation of PrCl Á7H O(s) at mean tempera-
3
2
ture of measurement (335 K) is found to be comparable
0
with
the
reported
D H
data,
i.e.,
f
298.15
-
- 3182 ± 2 kJ mol [27].
1
In pH2O(Iso) = –9231/T + 16.8
± 32) (± 0.007)
(
Conclusions
Thermal decomposition behavior of CeCl Á7H O and
3
2
PrCl Á7H O was studied under the flow of argon gas
–
10
3
2
0
.00236
0.00238
0.00240
/T (K–1
0.00242
employing thermogravimetry and differential thermal
analysis techniques. The evolved gas analysis reveals that
no hydrolysis reaction occurs during the decomposition of
CeCl Á7H O and PrCl Á7H O up to 450 K under inert
1
)
Fig. 13 Comparison of isothermal vs non-isothermal plots of lnp
3
2
3
2
versus
PrCl
1/T
O(s) ? PrCl
for
(s) ? H
the
O(g)
vaporization
reaction
ÁH
2
3
2
atmosphere. Subsequently, HTXRD analysis was carried
out to ascertain the dehydration mechanism. The thermo-
dynamic stability of CeCl Á7H O and PrCl Á7H O along
3
3
2
3
2
with their intermediate compounds was determined
employing dynamic transpiration technique. Standard
molar enthalpy of formation of CeCl Á7H O and PrCl Á
0
À1
DfG
ðÆ0:03ÞðkJ mol
Þ
PrCl3ÁH2O
¼
À1366ðÆ08Þ þ 0:43ðÆ0:02Þ Â T
ð23Þ
ð24Þ
ð25Þ
3
2
3
ð416\T=K\420Þ
7
H O determined in the present investigation was found to
2
0
À1
agree well with the available data in the literature.
D G
ðÆ0:03ÞðkJ mol Þ
f
PrCl Á2H O
3
2
¼
À1678ðÆ09Þ þ 0:43ðÆ0:01Þ Â T
ð370\T=K\374Þ
0
À1
DfG
ðÆ0:06ÞðkJ mol
Þ
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3
-6H
2
O, TbQ
3
Á6H
2
O and DyCl
3
Á6H
2
O using a fluidized bed.
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