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Thermal decomposition studies. Part XIII. Kinetics of the thermal decomposition of the oxalates of the rare earths, yttrium and titanium
Tllermochimica Acta, 39 (1980) 253-266 0 Elsevier Scientific Publishing Company,
Amsterdam
-
Printed
in Belgium
253
THERMAL DECOMPOSITION STUDIES. PART XIII. KINETICS OF THE THERMAL DECOMPOSITION OF THE OXALATES OF THE RARE EARTHS, YTTRIUM AND TITANIUM
K-G. NAIR,
V.V.
SREERAJAN
Travarzcore Titanium Products C.G.R.
NAIR
Department (Received
and V.S.V. Ltd.,
NAYAR
TriLmndrum-695
021
(India)
* of Chemistry,
University
of Iierala. Z’bivandrum-695
001
(India)
10 January 1980)
-ABSTRACT The thermal decomposition behaviour of the oxalates of La, Ce, Pr, Nd, Sm, Gd, Dy, Y and Ti was studied in detail by TG, DTG and DTA. The kinetic parameters (enerm and entropy of activation) for the important stages of dehydration and decomposition were evaluated from TG traces hy using the Coats-Redfern equation and the modified CoatsRedfern equation_
INTRODUCTION
In the course of a programme to study the thermal decomposition of intimate mixtures of titanium oxalate and oxalates of lanthanides as a feasible route, inter alia, towards the synthesis of mixed oxides of titanium and lanthanides, it became desirable to investigate the decomposition pattern of each individual oxalate. Although some extensive studies had been carried out earlier on the thermal behaviour of oxalates of lanthanides and yttrium, most of these studies were purely phenomenological [l-14]. Only a few kinetic studies have been carried out [9,15,16]. A scrutiny of the results published by earlier workers [ 5-251 revealed several contradictions and conflicting claims. Only preliminary studies have been carried out [26-331 on the thermal decomposition of titanyl oxalates and some peroxytitanyl oxalates. A systematic study of the kinetics of the thermal decomposition of the oxalates of the rare earths, yttrium and titanium was therefore undertaken. In the present paper, we report the results of our studies on the oxalates of La, Ce, Pr, Nd, Sm, Gd, Dy, Y and Ti in an atmosphere of static air. (Static air was employed in order to simulate the conditions programmed for the synthesis of mixed metal oxides via oxalate decomposition.)
* To whom correspondence
should be addressed.
254 EXPERIMENTAL
Preparation
of rare earth oxalates
The oxalates of La, Ce, Pr, Nd, Sm, Gd, Dy and Y were prepared by standard methods [5,34,35]. The rare earth oxides (about 0.5 g) were dissolved in 5 ml cont. HNOJ. (Dissolution of cerium oxide in nitric acid is facilitated by the addition of 2 ml of 1% hydrofluoric acid which acts as a catalyst [35].) The solutions were diluted to 100 ml and 50 ml of a saturated solution of oxalic acid was added at room temperature (30 * 2” C). The solutions were stirred vigorously, heated to boiling and then cooled. The precipitated oxalates were filtered off and washed with a 2% aqueous solution of oxalic acid, followed by distilled water, alcohol being dried over anhydrous calcium chloride.
Preparation
of titanyl
oxalate
and finally ether before
[36]
Titanium tetrachloride in HCl was hydrolysed by adding slowly to an icecooled solution of 5% aqueous ammonia. The orthotitanic acid formed was filtered and washed repeatedly with cold water until it was free from ammonia_ The precipitate was dissolved in a saturated solution of aqueous oxalic acid and evaporated at 50-60” to a small volume. Titanyl oxalate was precipitated when absolute alcohol was added to this solution. The osalate was filtered, washed with alcohol and dried over calcium chloride. Purity The purities of the osalates were determined by the analysis of the metals by ignition of weighed samples to the stable oxides. All the osalates of the lanthanides were decahydrates. Yttrium oxalate was obtained as a nonahyTiO(C20J) - 3.5 H1O. Analysis drate. Titanyl osalate had the composition data are given in Table 1. TABLE Analysis Sample No.
1 data Oxalate
La Ce Pr i\id Sm Gd
DY Y Ti
Metal (%) Calcd.
Found
45.14 47.53 46.99 45.93 46.81 17.78 48.49 37.39 33 39 LA.._
44.55 47.86 46.31 -45.69 46.91 37.52 18.81 3’7.59 22.41
255
Apparatus
[3 7,381
A Stanton simultaneous TG-DTA thermobalance, model TROl, with 0.1 mg sensitivity was used. A heating rate of 4°C min-’ and a chart speed of 3 in. 11-l were used throughout and sample masses of 30 mg were uniformly employed to give consistent and comparable data. Samples were finely powdered in an agate mortar and placed in deep cylindrical dimpled Pt crucibles (8 mm deep and 6.2 mm diameter). The sample temperature was measured directly by a Pt/Pt-Rh thermocouple, the head of which was placed in the dimple of the crucible. As reference substance for DTA, ~-AIzOJ was used.
Pyrolysis
experiments
Independent pyrolyses were carried out by heating weighed samples of the osalates in open porcelain crucibles to a temperature of - 800°C. The percentage loss of weight after completion of the pyrolysis was calculated from the weights of the residues_
TREATNEXT
OF DATA
The instrumental TG curves were redrawn as the mass vs. temperature (TG) curves as well as the rate of loss of mass vs. temperature (DTG) curves. The curves were drawn using standard curve set.s. The plateaus and reaction intervals in TG, the peak temperatures in DTG and DTX, and the peak widths in DTX were noted and compared. Determination
of the order
The order of reaction, witz and Metzger c_ =
of reaction
U, was found
,lli(l--rl)
using t.he equation
suggested
by Horo-
(1)
5
IZ is the order of the reaction, C, is the weight fraction present at the temperature T, corresponding to the DTG peak. .-\sdescribed earlier [ 351, a and IZ obtained from the determined values “master rurve” was constructed of c,. where
Errajuatioll of the kinetic
par-ameters
parameters for the dehydration and main decomposition stages were evaluated using the Coats-Redfern equation [ 391 as well as the equation was njodified Coats-Redfern equation [ 401. The Coats-Redfern used in the form
T]le
kinetic
(2)
TABLE
2
Plateaux,
peaks, reaction
Compound
intervals and peak widths
Plateaux in TG
Peaks in DTG
(“C)
(“C) =
in TG, DTG
Reaction intervals in TG
and DTA
curves
Peaks in DTA (“C) =
Peak widths in DTA
(“C) Lanthanum osalate
up to 120 220-380 -180-660 Above 760
135 412 470 695
I s w(sh) m(br)
(“C)
120-220 380-460 460-480 660-760
165 endo 380 exo 455exo 705 exo
s s w m(br)
100-205 365-415 425-480 685-750
170-260 270-370
187 endo 320exo
s s
140-220 280-380
Cerium osalate
up to 170 250-270 Above 370
211s 341s
Praseodl-mium osalate
Up to 140 240-380
200 408 466 529
5 s w(sh) w
140-240 380-450 450-480 500-560
185 endo 385exo 415exo 475 exo
s s m w
120-225 355-400 400-465 465-515
198 261 -I22 495 620
s m 5 m(br) m(br)
150-230 250-290 390-460 460-530 590-680
197 259 405 535 650
s m s s(br) w(br)
125-225 225-305 395-455 465-605 625-705
480-500 Above 560 Neodymium osalate
up to 150 230-250 290-390 530-590 Xbove 680
endo endo exo exo exo
Samarium osalale
up to 140 220-240 300-380 540-5’70 Above 6-10
200 269 128 591
s m s m
l-lo-220 240-300 380-540 5iO-640
185 endo 265 endo 408exo 560 eso
m m s s
140-235 235-290 385-425 490-580
Gadolinium osalatc
up to 120
163 336 2iO 438 590
m m 5 s m
120-200 200-24 5 215-280 380-540 560-640
168 225 265 427 583
endo endo endo es0 es0
m m s s s
115-195 205-235 245-285 -105-445 505-595
138 19i 363 430 522
5 m m 5 m
100-160 170-210 330-380 380-490 500-650
145 endo 225 endo 355 endo 420exo 525 exo
s w w 5 s
110-165 185-245 325-365 395-440 480-550
132s 187 m 383 m 427 5 601 m
90-l 05 105-220 360-395 395-480 550-650
153 195 392 425 605
endo endo cndo es0 exo
m w m 5 m
110-175 185-230 350-400 400-460 530-620
Up to 60
105 m 296 s
60-l ‘70 1 io-330
330-500 Above 5SO
538 m
500-580
1lOendo 290 endo 320 exo 540 es0
w s m m
60-l 80 180-305 305-340 500-570
2SO-380 540-560 Above 640 Dysprosium osalate
up to 100 160-1’70 310-330 -190-500 Above 650
Yttrium oxalate
up to 90 220-360 480-550 Above 650
Titan?;1 osalnte
n Abbreviations: eso = esothermic; endo = endothermic; strong; vs = very strong; sh = shoulder; br = hroad.
w = weak;
m = medium;
s =
-
Ti
DY
oxfllale
oxillnlc
OXillill(!
OXilliltL!
OXZlliIlC
OXillill~~
Gtl
mhtc
oxalulc
Srn
Ntl
Pr
Ce
LIl
Subslancc
4 1~20
Yz(W4 I.1’
Pilrtinl loss
co:!
0r
III
Y?(C?OJh
D~z(G04)3
‘WC204h
Sw 01, CO2
N&O,,, 2CO:
PrzOJ, COz
Lil?OJ, CO*
Al’tcr Shgc
Al’lcr
Slilf.!l! II
1
Ah
Shgc
TiO(C$O,,) s 21120
Pyrolysis thlo
TABLE 3
Al’ler IV
y203, co2
CO,
DYZ%,
CO2
(3120.1,
NdzOJ, CO,
Skigc
TiOz
YZOI
DY2%
TG
Gt1203
TG Calc. Pyrolysis
Pyrolysis 13.8 12.66
15.46 14.91
TG CillC.
14.53 14.05
9.26 9.50
TG Calc. Pyrolysis
Pyrolysis
Calc.
TG Calc. Pyrolysis
15.28 15.72
17.20
Pyrolysis
17.93
CillC.
24.67 24.80
26.67 24,85
23.34 24.94
I
Ah Sl#C
Moss loss
TG
Cdc. Pyrolysis
TG
Pyrolysis
CillC.
TC
Pyrolysis
CillC.
TG
Sni20.,
N&O,
PrbOl I
cc02
Laz0.J
Find
56.05
66.9
19.94 20.88
17.96 18.74
14.35 14.25
23.61 24.19
23.33 24.57
46.0
43.34 42.67
Alter Stage II
(W)
26.84
27.92
23.08 23.42
22.2 23.75
48.61 47.28
41.88 42.05
48.5
49.67
47.34 48.77
Arter Stage III
55.83 66.33
46.15 46.79
48.15 46.42
47.00 47.p5
After Stage IV
62,59
61,9 62.82
62.81 62.61 62041
51.28 51.61 61.19
52.78 52.22 52.48
53.47 63.19 53.09
53.83 54.07 54.31
53.33 53.09 53,69
52.22 52.47 52.14
54.67 64.86 66.45
Final
-
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0 L66’0 I866’0
8966'0
96G6’0
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6’1%
X’9
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L'LFl:
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E”Jsi
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9’869
S’OOX 1’9Cl L’11’1
i;'SOL
L’OE T 9’9 I*L
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2’286 ,,ot x CO'I
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0866’0 LE66’0
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II I
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punotlum~
1966'0 8986’0 1966'0 1666'0
6LL6'0 HEG6'0 OP66'0 2866'0 L966'0
6L66'0 9866'0 L866'0 9P66'0 EL66'0
ZL66'0 PL66'0 PP66'0 6L66'0 L666'0
L986'0 9866'0
1:966’0 s LRG'O 1966'0 966G'O
SGL6.0 ZP66'0 IPGG'O 9966'0 6866'0
6L66'0 9866'0 9666'0 2966'0 SL66.0
ZL66'0 bL66'0 91766'0 8866'0 L666'0
0886'0 0966'0
I'RPP Z’Z61: exisr Z'9L 9'PI
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‘CO1 scol ,;OI g,OT c,ot
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9ciz so1
fl
t [
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RES
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A
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E E 1 I 1
06'; 86 1. OLZ 962 c9r
III II la’099 z.09’; H’LZi, VP6 E’bOI
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9’99 !)H’PZ
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6x!!9 I-‘61*!1 L'ICZ I:'GUZ 6'L61 9'L8
t: t: r I
O'Z96 L’X1.Z [‘l?gZ
G'G96 E'I.(;Z H’89Z L'GG H'I6
a~eluxo
“1I!Il!SO
dlu~l!xo
!lL
s
Aa
i1Jl!ll!XO
PC3
o~u(l2xo
us
260
where g(a) = -ln(l
- (Y)
1 --(1-#--n g(a) = -l-n
when n = 1
when rz # 1
and il is the pre-esponential factor, Q the heating rate, E the energy of activation, R the gas constant and 2” the absolute temperature. The modified Coats-Redfem equation [40] was used in the form log
As E + 2.303R - 2.303RT
go= T3
(3)
where k is the Boltzmann constant, Iz the Planck constant and AS the entropy of activation_ Plotting the left-hand side of eqns. (2) and (3) against l/T gave straight line graphs with slopes of -E/2.303 R. The activation energy, E, was thus calculated from the slope. A was calculated from the intercept of the CoatsRedfern plot, while AS was calculated from the intercept of the modified Coats-Redfem plot. A knowledge of A enables the evaluation of AS and vice versa, as has been shown by earlier workers [ 261, using the equation
(4)
RESULTS
The plateaus and reaction intervals in TG, the peak temperatures in DTG and DTA, and the peak widths in DTA are given in Table 2. Pyrolysis data are presented in Table 3 and the kinetic parameters in Table 4.
THERMAL
Oxalates
BEHAVIOUR
of the rare earths and yttrium
Lanthanum oxalate shows a stability plateau in TG up to 120°C and four DTG peaks at 185, 412, 470 and 695°C; the DTA peaks are at 165, 380, 455 and 705°C. Dehydration takes place in the temperature range 120220°C in a single step. The endothermic DTA peak at 165°C (Stage I) corresponds to this dehydration. The decomposition starts at 380°C and an intermediate compound which appears to be LazOj, 2 CO;! is formed at 460°C (Stage II). The osycarbonate La203, CO, is formed (Stage III) at 480” C and is stable up to 660°C. Further decomposition commences at 660°C and the oside La203 is obtained above 760°C. The exothermic peak in DTA at 380°C denotes the oxidative decomposition and the two peaks of the oxycarbonates. (eso) at 455 and 705°C denote the decomposition
261
Cerous oxalate is stable up to 170°C and the dehydration starts at this temperature. Dehydration is completed in a single stage (Stage I) as in the case of lanthanum and the single endothermic DTA peak at 187OC, which corresponds to the DTG peak at 211”C, denotes the dehydration. The anhydrous oxalate is stable between 250 and 270°C after which decomposition starts. The decomposition is completed in the range 270-370°C and the stable Ce02 is formed above 370°C. The strong exothermic DTA peak at 320°C represents this oxidative decomposition. In this case, the oxalate appears to be directly converted to the Ce(IV) oxide without forming the oxycarbonate. This could mean either the absence of an intervening carbonate stage or a transient existence for this phase which escapes detection under the present conditions. This unique behaviour, shown only by cerous oxalate, distinguishes it from the other rare earth oxalates in the present study. Praseodymium oxalate is stable up to 140°C and dehydration starts at this temperature, continuing up to 240” C. The DTG peak is at 200” C. The endothermic DTA peak at 185°C represents this dehydration (Stage I). The anhydrous praseodymium oxalate is stable in the range 240-380°C and it undergoes decomposition at 380-450,450-480 and 500-560°C. The three DTG peaks at 408,466 and 529°C correspond to the three exothermic DTA peaks at 385, 415 and 475°C and correspond to the decomposition of the anhydrous oxalate in three stages to the oxide. The final product is Pr601 I. Stages II and III are very close and the intermediate product has only a transient existence. The identification of the product after Stage II was therefore not possible. The product after Stage III corresponds to PrlOa, co2 .
Neodymium oxalate decahydrate is stable up to 150°C and the dehydration starts at this temperature resulting in a horizontal weight level corresponding to NdZ(Cz04)s - 3 H20. The trihydrate loses water in the temperature range 250-290°C. The DTG peaks at 198 and 261°C and the corresponding DTA endothermic peaks at 197 and 259°C represent the two dehydration stages (Stages I and II). There is excellent correspondence between DTG and DTA as far as the first two peaks are concerned. At the first stage, the decahydrate is converted to the trihydrate which, at the second stage, forms the anhydrous salt. The anhydrous osalate is stable in the range 290-390” C and begins to decompose at 390” C. A decomposition peak occurs at 422°C in DTG and at 405°C (exothermic) in DTX. At this stage, the composition corresponds roughly to partial decomposition of the oxalate. A broad DTA exothermic peak occurs in the region 465-605”C and there is a broad DTG peak also in this region. This could represent the formation of Nd,Oa, 2 CO2 (Stage III). At 53O”C, the composition is NdZOJ,, CO, (Stage IV). The final decomposition reaction starts at 590°C and the oside is formed at 680°C. The exothermic DTA peak at 65O”C, which corresponds roughly to the DTG peak at 62O”C, denotes this decomposition. The final residue after heating up to 700” C was analysed and found to be NdzO 3. Samarium oxalate is stable up to 140°C and dehydration starts at this
262
temperature. The two DTG peaks at 200 and 269”C, which have close correspondence with the two endothermic DTA peaks at 185 and 265”C, correspond to the two dehydration stages (Stages I and II). The first of these stages represents the transition from Sm1(&04)3 - 10 H,O to Sml(&OJ)j . 3.5 Hz0 and the second stage refers to the transition to the anhydrous osalate. The anhydrous oxalate is stable between 300 and 380°C and undergoes decomposition in two stages (Stages III and IV). The DTG peak at 428” C and the exothermic DTA peak at 408” C correspond to the first-stage decomposition. The product at this stage (Stage III) has the composition CO?. This osycarbonate then decomposes to the oxide SmzOJ Sm,Gj, (Stage IV). The DTG peak at 591°C and the esothermic DTA peak at 560°C denote this decomposition_ Gadolinium oxalate is stable up to 120” C and dehydration starts at this temperature. The three DTG peaks at 163, 226 and 270°C which are closely paralleled by the three endothermic DTA peaks at 168, 225 and 265”C, correspond to the three dehydration stages I, II and III (10 Hz0 --* 6 Hz0 --, 4 HZ0 + anhydrous salt). The anhydrous oxalate is stable between 280 and 380” C and undergoes decomposition in two stages. The DTG peak at 438” C and the esothermic DTA peak at 427°C correspond to the first stage of decomposition (Stage IV)_ The product after this stage appears to be Gdz03, COz. This then decomposes to the oxide Gd103 (Stage V). The DTG peak at. 590°C and the esothermic DTX peak at 583°C represent the final stage of decomposition. Dysprosium osalate appears to be stable up to 100°C when dehydration begins. The three DTG peaks at 138, 197 and 363”C, which correspond with the three endothermic DTA peaks at 145, 225 and 355”C, refer to the Stages I, II and III of dehydration (10 Hz0 1 4 H,O --, 2 Hz0 + anhydrous salt). The tetrahydrate is stable in the range 210-330”. The anhydrous oxalate undergoes decomposition in two stages (Stages IV and V). The DTG peak at 430°C and exothermic DTA peak at 420°C represent the first of these (Stage IV). The product obtained at this stage has the rough composition DyzOs, CO, which further decomposes to the oside Dy,O, (Stage V). The DTG peak at 522°C and the esothermic DTA peak at 525°C represent this stage of decomposition. Yttrium osalate is stable up to 90°C and dehydration begins at this temperature_ The three DTG peaks at 132, 187 and 383”C, paralleled by the three endothermic DTA peaks at 153, 195 and 392”C, correspond to Stages I, II and III of dehydration (9 H,O --t 4 Hz0 -* 2 Hz0 +- anhydrous salt). The The anhydrous oxalate dihydrate is stable in the range 220-360°C. undergoes decomposition in two stages (Stages IV and V). The DTG peak at 427°C and the esothermic DTA peak at 425°C represent the first of these stages (Stage IV). The product obtained after this stage appears to be Y203, COz. This further decomposes to the oside YzO3 (Stage V). The DTG peak at 601°C and the esothermic DTA peak at 605°C represent this stage of decomposition. TiO(C20d) - 3.5 H,O, is stable up to 60°C and the Titanyl osalate, dehydration starts at about this temperature. The DTG peak is at 105°C. The endothermic DTA peak at 110” C represents this dehydration (Stage I)
263 leading to the formation of the dihydrate, TiO(C104) - 2 H20. The dihydrate begins further dehydration accompanied by decomposition. The DTG peak at 296°C represents this stage (Stage II). In DTA, this stage is rcpresented by two close-lying peaks, an endothermic peak at 290°C and an esothermic peak at. 320°C. The phase formed at 330°C is a basic carbonate of titanium with the empirical formula Ti3COn or TiO(COJ), 2 TiO- and is stable up to 500°C. _.4bove this temperature, it decomposes to give Ti02 . The DTG peak at 538”C, paralleled by the esothemlic peak at- 54O”C, corrcs-ponds to this final stage of decomposition (Stage III). Mass loss clata Mass loss data are given in Table 3. The final residue is the oside &I203 for M = La, Nd, Sm, Gd, Dy and Y; it is MO1 for Ce and Ti and Pr6011 for Pr. The mass loss data from TG, as well as from independent pyrolysis studies. agree closely with calculated vaIues in all cases. It may be seen from Triblc 3 that the agreement is satisfactory between the calculated and obsclved (from TG) values for mass loss dat.a for intermediate stages as well, whcrever intermediate stages are sufficient.& well-defined. h’irlctic
parameter-s
The kinet.ic parameters evaluated from the TG traces arc given in Tahlr -1. The values of E, AS and rl using the Coats-Redfern equation and a similar set using the modified Coats-Redfern equation arc presented. The corrcllation coefficients in each case are also shown.
DISCUSSION
A survey of the results in Tables 2 and 3 reveals an interesting fnc~. Thr hydrated osalates of the lighter lanthanides (La, Ce and Pr) undergo sin&hstage dehydration from the decahydrate to t.he anhydrous salt. In Nd and Sm, we have two-stage dehydration, whereas in the heavier lanthanidcs Gd and Dy, we have three-stage dehydration . Xs espected, Y falls in line wiLh the heavier lanthanides [41] and shows a three-stage dehydration. It. is interesting to note that these definitive trends observed in the thermal behaviour are compatible with the well-known increase in coordinal ion ability (e.g. hydration energies) of the lauthanide M3’ ion from La t.o Lu. The final residues are normal osides, M1OJ, in the case of La, Nd, Sm, Grl. Dy and Y. In the case of Pr, it is PrciO 1I and for Ce it is CeOL. This is 110~ surprising because, under open static air conditions, one espects the Inost stable oxide to be formed, which is Pr,O 11 for Pr ancl Ce02 for Cc. Thc~ residue for titanyl osalate is TiOz as expected. A similar, although less clear-cut, tendency could be noted as regards the final decomposition to the oside stage. The oxide stage is attained at 760°C
264
for La, 680°C for Nd and at about 640°C for Sm, Gd, Dy and Y. This indicates that the thermal stability of the carbonates decreases marginally from La through Nd to Sm, Gd, Dy and Y. Such an order would reflect the increasing Lewis acidities of the M3’ ions from La3’ to Lu3’. The exceptional behaviour in the case of Ce and Pr could be attributed to the formation of oxides of formulae other than M,Oa. The DTA peaks for dehydration are endothermic as expected. In the case of further decompositions, whereas one may expect endothermicity as a general tendency, one finds exothermicity in all the cases. This may be explained by osidation processes (such as combustion of the liberated CO to COZ) accompanying the decomposition of the oxalates in the oxidizing atmosphere (static air) employed for the studies. (It is well known that CaC,Oq gives exothermic DTA peaks in air, whereas in nitrogen, the DTA peaks reveal their true endothermicity.) In the present studies, static air was deliberately employed as mentioned earlier. Kinetic parameters Larztharzides and y ttrirmz The first dehydration stages are found to have E values of the order of -100 kJ mole -I_ The second dehydration stages also generally have values of although the third dehydration stages (e.g. Gd and Y) 100-200 kJ mole-‘, have higher values (300-450 kJ mole-‘). This implies that the dehydration of the lower hydrate is kinetically less favoured than that of the higher hydrates_ The decomposition stage, especially the final ones leading to oxide formation, show higher values of E (700-800 kJ mole-‘). This is to be espected because these decomposition stages involve rupture of stronger bonds than in the case of dehydration_ It is difficult to draw definitive conclusions from AS and -4 data which must await further theoretical developments in this field. One may note, however, that the dehydrations generally tend to have lower values of AS and A. A negative value for AS further shows a more ordered activated comples and a less than normally fast reaction. On t.he other hand, positive values of ilS indicate less ordered activated compleses and faster than normal reactions [421. Titanyl oxala te The first dehydration stage of titanyl osalate has a low value for the j, whereas the second stage has a slightly energy of activation ( -30 kJ mole-' higher value (-70 kJ mole-‘). It may be noted that the order of magnitude of both values is the same as, or one less than, that for similar dehydration stages of lanthanide oxalates. The values for the entropy of activation for the dehydration stages of hydrated titanyl osalate are found to be negative, indicating that the activated comples is more ordered than the reactants and that the reaction is less than normally fast [421.
265 ACKNOWLEDGEMENTS
We thank Dr. P.M. Madhusudanan and Dr. P. Indrasenan for helpful discussions and the Space Science and Technolow Centre, Trivandrum-22 for instrumental facilities made available to us.
REFERENCES 1 H.A. Flaschka and A.J. BarnardJr. (Eds.), Chelates in Analytical Chemistry, Vol. I, Marcel Dekker, New York, 1967. Thermal Methods of Analysis, Interscience, New York, 1964. 1 W.W. Wendlandt, 3 R-F. Schwenker, Jr. and P.D. Gam (Eds.), Thermal ..4naIysis, Vols. 1 and 2, Academic Press, New York, 1969. Analysis, Elsevier, Amsterdam, 1963. 4 C. Duval, Inorganic Thermogravimetric W.W. Wendlandt, Anal. Chem., 30 (1958) 58. Anal. Chem., 31 (1959) 408. ; W.W. Wendlandt, Wendlandt, T.D. George and G-R. Horton, J. Inorg. Nucl. Chem., 1'7 (1961) 7 W.W. 2i3. g _A.Glasner and M. Steinberg, J. Inorg. Nucl. Chem., 32 (1961) 39. 9 A. Glasner and M. Steinberg, J. Inorg. Nucl. Chem.. 16 (1960) 2'79. 10 A. Glasner, E. Levy and M. Steinberg, J. Inorg. Nucl. Chem., 25 ( 1963) 1119. 11 =\. Glasner and M. Steinberg. J. Inorg. Nucl. Chem.. 32 (1961) 136. 1” _A_ Glasner, E. Levy and MI. Steinberg. J. Inorg. Nucl. Chem., 25 (19G3) l-4 15. 13 A. Glasner. TalanLa, 11 (1964) 405. 1-I O.K. Srivastava and A.R. Vasudeva Murthy, Curr. Sci.. 39 (1960) 450. 15 V.V. Suhba Rao, R.V.G. Rao and X.B. Biswas, J. Inorg. Nucl. Chem.. 27 (1965) 23%. S.C. Saraiya and A.K. Sundaram, J. Inorg. Xucl. Chem., 11! 16 \‘.M. Padmanabhan, f 19tiO) 336. 17 R. Aloebius. W. Dietzold and F. RIatthes, J. Iuorg. Kucl. Chem., 28 (19GG.l I S-Ii. iS R. XItibius and F. blatthes. J. Inorg. Nucl. Chem., 28 (19GG) 1601. 19 J-K. Marsh. J. Chem. Sot., (1943) 40. 20 R. Miihius. Wiss. Z. Tech. Hochsch. Chem. Carl Schorlemmer Leena-Merseburg, 6 (1964) 359. 21 S.S. Xloosath, J. _-\Ixaham and T.V. Swaminathan. Z. :\norg_ Xllg. Chem.. 3% (19G3) 90. ‘1’) __ S.S. ~Ioosath. .J. i\lx-aham and T.V. Swal~lini~tll;~~l. Z. i\norg. .-\llg. Chem., 32-l (1963) 96. S.V. Kalal~ukhovn and L.S. Brykina. Zh. Neorg. ‘) 3 \-a.S. Savitskaya. 3.X. Tvorogov. Khim., i (1962) 2029. ‘24 P. Cara and J. Loriers, J. Rech. C.N.R.S.. 39 (195i) IO;. a5 _-1.Glasner and M. Steinberg, Bull. Res. Count. Isr., Sect. _-I, s (1959) 17-1. ‘76 H.S. Gopalalirishnamurtll~-, >I. Subha Rae :lnd T.R. Xaray-allnnkutLy. J. Inorp. Nucl. Chenl., 3i (19';s) S91_ %I. SuI~b;l Rae :lnd T.R. Nar;ly;nnankutly, Thermochim. r?i H.S. Gopnlakrishnamurt1~~, ~\CLiI. 13 (19i5) 183. S‘S H.S. Gopnlakrishnamurth~-, %I. Subha Rae and T.R. Narayannnkutty. J. InorE-. Nucl. Chctn.. 38 (19iG) -117. “9 C.Gh. Macrovici and Gh. idorm-, Bolyai Ser. Chem.. 1’7 (2) (1972) 5. A.V. Lapitskii, L.G. Vlasov and A-1. Tsvetrov, Dokl. :\kad. Xauk 30 B.V. Strizhakov, SSSR. 133 (1960) 1347. 31 P-K. Gallagher and F. Schrcy, J. Am. &ram. Sot.. 46 t 1963) 367. 3” P-K. Gallagher and J. Thomson. Jr., J. Am. Ceram. Sot.. -1s f 1965) G-l-l. 33 G-L:‘. derc and C.C. Pate], J. Inorg. Nucl. Chem.. 20 (1961) 343.
266 3-l W.R. Schoeller and -4-R. Powell, The Analysis of Minerals and of lhe Rarer El-zments. Griffin, London. 195%. 35 I.M. Iiolthoff. P-J_ Elvinp and E.B. Sandell (Eds.). Treatise on Analytical Chemistry. Vol. S. Part II. Interscience. Kerv York, 1963. 3fi D.P. Kharkar and CC. Patel. J. Indian Inst. Sci.. Sect. A. 39-A (1957) -41. in R.A. MacKenzie (Ed.). Differential Thermal Analysis. Vol. I, 3i J.P. Redfern. _kademic Press, New Torli;, 19iO. 3s P.M. Nadhusud:uxtn. P.S.K. Xamhisan and C.G.R. Nnir. Thermochim. .-\cta. 9 I 197-I ) l-19. :39 ;\.\\-. Coats and J.P. Redfern. Sature (London). 201 (196-I) 6s. -10 C.G.R. Xair and P.M. ~Iatlusuda~xm. Curr. Sri., 4.1 ( 19’75) 31 2. _-\dvaucr~cl Inorganic Chemistq-. Wiley. New Tork. .I 1 F-h. Cotton and G. Wilkinson. 196(i. Kinetics ;incl 1Iechanism. Wiley, New \-ark. 19f-i 1. p_ 42 _-\.;\. Frost and R.G. Pearson. 101.
Amsterdam
-
Printed
in Belgium
253
THERMAL DECOMPOSITION STUDIES. PART XIII. KINETICS OF THE THERMAL DECOMPOSITION OF THE OXALATES OF THE RARE EARTHS, YTTRIUM AND TITANIUM
K-G. NAIR,
V.V.
SREERAJAN
Travarzcore Titanium Products C.G.R.
NAIR
Department (Received
and V.S.V. Ltd.,
NAYAR
TriLmndrum-695
021
(India)
* of Chemistry,
University
of Iierala. Z’bivandrum-695
001
(India)
10 January 1980)
-ABSTRACT The thermal decomposition behaviour of the oxalates of La, Ce, Pr, Nd, Sm, Gd, Dy, Y and Ti was studied in detail by TG, DTG and DTA. The kinetic parameters (enerm and entropy of activation) for the important stages of dehydration and decomposition were evaluated from TG traces hy using the Coats-Redfern equation and the modified CoatsRedfern equation_
INTRODUCTION
In the course of a programme to study the thermal decomposition of intimate mixtures of titanium oxalate and oxalates of lanthanides as a feasible route, inter alia, towards the synthesis of mixed oxides of titanium and lanthanides, it became desirable to investigate the decomposition pattern of each individual oxalate. Although some extensive studies had been carried out earlier on the thermal behaviour of oxalates of lanthanides and yttrium, most of these studies were purely phenomenological [l-14]. Only a few kinetic studies have been carried out [9,15,16]. A scrutiny of the results published by earlier workers [ 5-251 revealed several contradictions and conflicting claims. Only preliminary studies have been carried out [26-331 on the thermal decomposition of titanyl oxalates and some peroxytitanyl oxalates. A systematic study of the kinetics of the thermal decomposition of the oxalates of the rare earths, yttrium and titanium was therefore undertaken. In the present paper, we report the results of our studies on the oxalates of La, Ce, Pr, Nd, Sm, Gd, Dy, Y and Ti in an atmosphere of static air. (Static air was employed in order to simulate the conditions programmed for the synthesis of mixed metal oxides via oxalate decomposition.)
* To whom correspondence
should be addressed.
254 EXPERIMENTAL
Preparation
of rare earth oxalates
The oxalates of La, Ce, Pr, Nd, Sm, Gd, Dy and Y were prepared by standard methods [5,34,35]. The rare earth oxides (about 0.5 g) were dissolved in 5 ml cont. HNOJ. (Dissolution of cerium oxide in nitric acid is facilitated by the addition of 2 ml of 1% hydrofluoric acid which acts as a catalyst [35].) The solutions were diluted to 100 ml and 50 ml of a saturated solution of oxalic acid was added at room temperature (30 * 2” C). The solutions were stirred vigorously, heated to boiling and then cooled. The precipitated oxalates were filtered off and washed with a 2% aqueous solution of oxalic acid, followed by distilled water, alcohol being dried over anhydrous calcium chloride.
Preparation
of titanyl
oxalate
and finally ether before
[36]
Titanium tetrachloride in HCl was hydrolysed by adding slowly to an icecooled solution of 5% aqueous ammonia. The orthotitanic acid formed was filtered and washed repeatedly with cold water until it was free from ammonia_ The precipitate was dissolved in a saturated solution of aqueous oxalic acid and evaporated at 50-60” to a small volume. Titanyl oxalate was precipitated when absolute alcohol was added to this solution. The osalate was filtered, washed with alcohol and dried over calcium chloride. Purity The purities of the osalates were determined by the analysis of the metals by ignition of weighed samples to the stable oxides. All the osalates of the lanthanides were decahydrates. Yttrium oxalate was obtained as a nonahyTiO(C20J) - 3.5 H1O. Analysis drate. Titanyl osalate had the composition data are given in Table 1. TABLE Analysis Sample No.
1 data Oxalate
La Ce Pr i\id Sm Gd
DY Y Ti
Metal (%) Calcd.
Found
45.14 47.53 46.99 45.93 46.81 17.78 48.49 37.39 33 39 LA.._
44.55 47.86 46.31 -45.69 46.91 37.52 18.81 3’7.59 22.41
255
Apparatus
[3 7,381
A Stanton simultaneous TG-DTA thermobalance, model TROl, with 0.1 mg sensitivity was used. A heating rate of 4°C min-’ and a chart speed of 3 in. 11-l were used throughout and sample masses of 30 mg were uniformly employed to give consistent and comparable data. Samples were finely powdered in an agate mortar and placed in deep cylindrical dimpled Pt crucibles (8 mm deep and 6.2 mm diameter). The sample temperature was measured directly by a Pt/Pt-Rh thermocouple, the head of which was placed in the dimple of the crucible. As reference substance for DTA, ~-AIzOJ was used.
Pyrolysis
experiments
Independent pyrolyses were carried out by heating weighed samples of the osalates in open porcelain crucibles to a temperature of - 800°C. The percentage loss of weight after completion of the pyrolysis was calculated from the weights of the residues_
TREATNEXT
OF DATA
The instrumental TG curves were redrawn as the mass vs. temperature (TG) curves as well as the rate of loss of mass vs. temperature (DTG) curves. The curves were drawn using standard curve set.s. The plateaus and reaction intervals in TG, the peak temperatures in DTG and DTX, and the peak widths in DTX were noted and compared. Determination
of the order
The order of reaction, witz and Metzger c_ =
of reaction
U, was found
,lli(l--rl)
using t.he equation
suggested
by Horo-
(1)
5
IZ is the order of the reaction, C, is the weight fraction present at the temperature T, corresponding to the DTG peak. .-\sdescribed earlier [ 351, a and IZ obtained from the determined values “master rurve” was constructed of c,. where
Errajuatioll of the kinetic
par-ameters
parameters for the dehydration and main decomposition stages were evaluated using the Coats-Redfern equation [ 391 as well as the equation was njodified Coats-Redfern equation [ 401. The Coats-Redfern used in the form
T]le
kinetic
(2)
TABLE
2
Plateaux,
peaks, reaction
Compound
intervals and peak widths
Plateaux in TG
Peaks in DTG
(“C)
(“C) =
in TG, DTG
Reaction intervals in TG
and DTA
curves
Peaks in DTA (“C) =
Peak widths in DTA
(“C) Lanthanum osalate
up to 120 220-380 -180-660 Above 760
135 412 470 695
I s w(sh) m(br)
(“C)
120-220 380-460 460-480 660-760
165 endo 380 exo 455exo 705 exo
s s w m(br)
100-205 365-415 425-480 685-750
170-260 270-370
187 endo 320exo
s s
140-220 280-380
Cerium osalate
up to 170 250-270 Above 370
211s 341s
Praseodl-mium osalate
Up to 140 240-380
200 408 466 529
5 s w(sh) w
140-240 380-450 450-480 500-560
185 endo 385exo 415exo 475 exo
s s m w
120-225 355-400 400-465 465-515
198 261 -I22 495 620
s m 5 m(br) m(br)
150-230 250-290 390-460 460-530 590-680
197 259 405 535 650
s m s s(br) w(br)
125-225 225-305 395-455 465-605 625-705
480-500 Above 560 Neodymium osalate
up to 150 230-250 290-390 530-590 Xbove 680
endo endo exo exo exo
Samarium osalale
up to 140 220-240 300-380 540-5’70 Above 6-10
200 269 128 591
s m s m
l-lo-220 240-300 380-540 5iO-640
185 endo 265 endo 408exo 560 eso
m m s s
140-235 235-290 385-425 490-580
Gadolinium osalatc
up to 120
163 336 2iO 438 590
m m 5 s m
120-200 200-24 5 215-280 380-540 560-640
168 225 265 427 583
endo endo endo es0 es0
m m s s s
115-195 205-235 245-285 -105-445 505-595
138 19i 363 430 522
5 m m 5 m
100-160 170-210 330-380 380-490 500-650
145 endo 225 endo 355 endo 420exo 525 exo
s w w 5 s
110-165 185-245 325-365 395-440 480-550
132s 187 m 383 m 427 5 601 m
90-l 05 105-220 360-395 395-480 550-650
153 195 392 425 605
endo endo cndo es0 exo
m w m 5 m
110-175 185-230 350-400 400-460 530-620
Up to 60
105 m 296 s
60-l ‘70 1 io-330
330-500 Above 5SO
538 m
500-580
1lOendo 290 endo 320 exo 540 es0
w s m m
60-l 80 180-305 305-340 500-570
2SO-380 540-560 Above 640 Dysprosium osalate
up to 100 160-1’70 310-330 -190-500 Above 650
Yttrium oxalate
up to 90 220-360 480-550 Above 650
Titan?;1 osalnte
n Abbreviations: eso = esothermic; endo = endothermic; strong; vs = very strong; sh = shoulder; br = hroad.
w = weak;
m = medium;
s =
-
Ti
DY
oxfllale
oxillnlc
OXillill(!
OXilliltL!
OXZlliIlC
OXillill~~
Gtl
mhtc
oxalulc
Srn
Ntl
Pr
Ce
LIl
Subslancc
4 1~20
Yz(W4 I.1’
Pilrtinl loss
co:!
0r
III
Y?(C?OJh
D~z(G04)3
‘WC204h
Sw 01, CO2
N&O,,, 2CO:
PrzOJ, COz
Lil?OJ, CO*
Al’tcr Shgc
Al’lcr
Slilf.!l! II
1
Ah
Shgc
TiO(C$O,,) s 21120
Pyrolysis thlo
TABLE 3
Al’ler IV
y203, co2
CO,
DYZ%,
CO2
(3120.1,
NdzOJ, CO,
Skigc
TiOz
YZOI
DY2%
TG
Gt1203
TG Calc. Pyrolysis
Pyrolysis 13.8 12.66
15.46 14.91
TG CillC.
14.53 14.05
9.26 9.50
TG Calc. Pyrolysis
Pyrolysis
Calc.
TG Calc. Pyrolysis
15.28 15.72
17.20
Pyrolysis
17.93
CillC.
24.67 24.80
26.67 24,85
23.34 24.94
I
Ah Sl#C
Moss loss
TG
Cdc. Pyrolysis
TG
Pyrolysis
CillC.
TC
Pyrolysis
CillC.
TG
Sni20.,
N&O,
PrbOl I
cc02
Laz0.J
Find
56.05
66.9
19.94 20.88
17.96 18.74
14.35 14.25
23.61 24.19
23.33 24.57
46.0
43.34 42.67
Alter Stage II
(W)
26.84
27.92
23.08 23.42
22.2 23.75
48.61 47.28
41.88 42.05
48.5
49.67
47.34 48.77
Arter Stage III
55.83 66.33
46.15 46.79
48.15 46.42
47.00 47.p5
After Stage IV
62,59
61,9 62.82
62.81 62.61 62041
51.28 51.61 61.19
52.78 52.22 52.48
53.47 63.19 53.09
53.83 54.07 54.31
53.33 53.09 53,69
52.22 52.47 52.14
54.67 64.86 66.45
Final
-
!
,
0 L66’0 I866’0
8966'0
96G6’0
H’XRl~
6’1%
X’9
P’99H
L'LFl:
L'H-
L”L6l
T’O6’j
H’ I,LH l.‘Z
E”Jsi
;,OI x 91,‘[
,,o I x GL’I.
9’869
S’OOX 1’9Cl L’11’1
i;'SOL
L’OE T 9’9 I*L
G'LG
2’286 ,,ot x CO'I
u,,OT ): C6’T
,,,o I x 9 I'9 ,,OL j: tiB'Z
([_,,),JW
,I ?I--3
"rOI % 49'X
( ,_s (II-, )v’lu
II U-D I4
&It x 9O'L
” u-0 r)
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-!131! JO &klU~
;a/ ‘LlO!ll!A
( , _optu I’q)
I, %I--,3
E’!)OH
L’OL9
0866’0 LE66’0
6’5 I-
9666'0
9666’0 8%66’0 L866’0 la’006 6’61 0’161
Z’lO-
9266'0 9866'0
ZE96’0 ZS66’0 9’ZHl
L966'0 8966'0
0686’0 8i766’0 LL66’0
G
(l,aot ;c i;E’I c,ot )’ l-l,‘I:
E966’0 l~‘l-Ls
1
PI 1-N ‘uo!y!h
111
G’6 I:-
” a-3
sp JO kh.llU~
(I-vu
-!lN!
II N-0
6’905 l.‘Lcj-
11-3
9866’0
u
08667-l
Yy
(4 1lp3 UO!~l?~ii.L!O~
,I’, .‘ill’,l(-j
uo!l3eit.l
(I’)
‘3366’0 f4366’0 la-,a
-!JJ JO3
(3,) AlIll
r/
A
AI III II I
AI i/ III II I
II I
AI q III I1 I
-iuoaaa
Uoysotl
&ll!lS
yl!“tl
-.ladluJ1
3lLcl
dll:II!xo
PN
punotlum~
1966'0 8986’0 1966'0 1666'0
6LL6'0 HEG6'0 OP66'0 2866'0 L966'0
6L66'0 9866'0 L866'0 9P66'0 EL66'0
ZL66'0 PL66'0 PP66'0 6L66'0 L666'0
L986'0 9866'0
1:966’0 s LRG'O 1966'0 966G'O
SGL6.0 ZP66'0 IPGG'O 9966'0 6866'0
6L66'0 9866'0 9666'0 2966'0 SL66.0
ZL66'0 bL66'0 91766'0 8866'0 L666'0
0886'0 0966'0
I'RPP Z’Z61: exisr Z'9L 9'PI
E’RSL 6’915 8’89E 8'086'S&
NC0 I ,,OL grOL ;,or ,OL
‘CO1 scol ,;OI g,OT c,ot
x x x x
x ZO’ 1 x’ OZ’I x ZZ’S ): ~7 :{ ET’1
x x x x x
,]cotx t'S'!i firO[x OZ.9 ,+I1 x EL'8 IJOIx LO'Z ,,ot x l-t:‘1
I:'LL [9'GZ9’Gl:P 9’110(; 9'69E 8'68P’kC-
6'CE)G L'I:9 IJ'OLZ 6'6In’9H-
vsO[ ,),OT ,,Ol ‘01
05.1 50’1. !)I,‘1 99’1:
[l;‘Z %‘I 86’9 W’Z OH’I:
8'89k 9'BZb 6’991 E’99 z-9
I’IER L’Z9 6'IOZ t"tiG-
H'TZH 6'59 9'66I O'ZOI-
9ciz so1
fl
t [
TO9 LZI &BE L81 ZBI
RES
H'OL O'RZ
I’ E 1 1 1
A AI III II I
A AI III II I
AI III II I
A
I 9'L(;9 1'99s Z'EEF 9'8G 9'LOL
E E 1 I 1
06'; 86 1. OLZ 962 c9r
III II la’099 z.09’; H’LZi, VP6 E’bOI
9'069 6'80s' 9'8GZ Z'ZLI I'(;Z[
E F: 1 1 1
AI III II I
9’99 !)H’PZ
8’EHC! O’EO(; Z’t’6Z Z’89 1 S’[ZI
6'Lc;Z H'COL' O’ZI-L I't6
Itir; HZ I695 OOZ
6x!!9 I-‘61*!1 L'ICZ I:'GUZ 6'L61 9'L8
t: t: r I
O'Z96 L’X1.Z [‘l?gZ
G'G96 E'I.(;Z H’89Z L'GG H'I6
a~eluxo
“1I!Il!SO
dlu~l!xo
!lL
s
Aa
i1Jl!ll!XO
PC3
o~u(l2xo
us
260
where g(a) = -ln(l
- (Y)
1 --(1-#--n g(a) = -l-n
when n = 1
when rz # 1
and il is the pre-esponential factor, Q the heating rate, E the energy of activation, R the gas constant and 2” the absolute temperature. The modified Coats-Redfem equation [40] was used in the form log
As E + 2.303R - 2.303RT
go= T3
(3)
where k is the Boltzmann constant, Iz the Planck constant and AS the entropy of activation_ Plotting the left-hand side of eqns. (2) and (3) against l/T gave straight line graphs with slopes of -E/2.303 R. The activation energy, E, was thus calculated from the slope. A was calculated from the intercept of the CoatsRedfern plot, while AS was calculated from the intercept of the modified Coats-Redfem plot. A knowledge of A enables the evaluation of AS and vice versa, as has been shown by earlier workers [ 261, using the equation
(4)
RESULTS
The plateaus and reaction intervals in TG, the peak temperatures in DTG and DTA, and the peak widths in DTA are given in Table 2. Pyrolysis data are presented in Table 3 and the kinetic parameters in Table 4.
THERMAL
Oxalates
BEHAVIOUR
of the rare earths and yttrium
Lanthanum oxalate shows a stability plateau in TG up to 120°C and four DTG peaks at 185, 412, 470 and 695°C; the DTA peaks are at 165, 380, 455 and 705°C. Dehydration takes place in the temperature range 120220°C in a single step. The endothermic DTA peak at 165°C (Stage I) corresponds to this dehydration. The decomposition starts at 380°C and an intermediate compound which appears to be LazOj, 2 CO;! is formed at 460°C (Stage II). The osycarbonate La203, CO, is formed (Stage III) at 480” C and is stable up to 660°C. Further decomposition commences at 660°C and the oside La203 is obtained above 760°C. The exothermic peak in DTA at 380°C denotes the oxidative decomposition and the two peaks of the oxycarbonates. (eso) at 455 and 705°C denote the decomposition
261
Cerous oxalate is stable up to 170°C and the dehydration starts at this temperature. Dehydration is completed in a single stage (Stage I) as in the case of lanthanum and the single endothermic DTA peak at 187OC, which corresponds to the DTG peak at 211”C, denotes the dehydration. The anhydrous oxalate is stable between 250 and 270°C after which decomposition starts. The decomposition is completed in the range 270-370°C and the stable Ce02 is formed above 370°C. The strong exothermic DTA peak at 320°C represents this oxidative decomposition. In this case, the oxalate appears to be directly converted to the Ce(IV) oxide without forming the oxycarbonate. This could mean either the absence of an intervening carbonate stage or a transient existence for this phase which escapes detection under the present conditions. This unique behaviour, shown only by cerous oxalate, distinguishes it from the other rare earth oxalates in the present study. Praseodymium oxalate is stable up to 140°C and dehydration starts at this temperature, continuing up to 240” C. The DTG peak is at 200” C. The endothermic DTA peak at 185°C represents this dehydration (Stage I). The anhydrous praseodymium oxalate is stable in the range 240-380°C and it undergoes decomposition at 380-450,450-480 and 500-560°C. The three DTG peaks at 408,466 and 529°C correspond to the three exothermic DTA peaks at 385, 415 and 475°C and correspond to the decomposition of the anhydrous oxalate in three stages to the oxide. The final product is Pr601 I. Stages II and III are very close and the intermediate product has only a transient existence. The identification of the product after Stage II was therefore not possible. The product after Stage III corresponds to PrlOa, co2 .
Neodymium oxalate decahydrate is stable up to 150°C and the dehydration starts at this temperature resulting in a horizontal weight level corresponding to NdZ(Cz04)s - 3 H20. The trihydrate loses water in the temperature range 250-290°C. The DTG peaks at 198 and 261°C and the corresponding DTA endothermic peaks at 197 and 259°C represent the two dehydration stages (Stages I and II). There is excellent correspondence between DTG and DTA as far as the first two peaks are concerned. At the first stage, the decahydrate is converted to the trihydrate which, at the second stage, forms the anhydrous salt. The anhydrous osalate is stable in the range 290-390” C and begins to decompose at 390” C. A decomposition peak occurs at 422°C in DTG and at 405°C (exothermic) in DTX. At this stage, the composition corresponds roughly to partial decomposition of the oxalate. A broad DTA exothermic peak occurs in the region 465-605”C and there is a broad DTG peak also in this region. This could represent the formation of Nd,Oa, 2 CO2 (Stage III). At 53O”C, the composition is NdZOJ,, CO, (Stage IV). The final decomposition reaction starts at 590°C and the oside is formed at 680°C. The exothermic DTA peak at 65O”C, which corresponds roughly to the DTG peak at 62O”C, denotes this decomposition. The final residue after heating up to 700” C was analysed and found to be NdzO 3. Samarium oxalate is stable up to 140°C and dehydration starts at this
262
temperature. The two DTG peaks at 200 and 269”C, which have close correspondence with the two endothermic DTA peaks at 185 and 265”C, correspond to the two dehydration stages (Stages I and II). The first of these stages represents the transition from Sm1(&04)3 - 10 H,O to Sml(&OJ)j . 3.5 Hz0 and the second stage refers to the transition to the anhydrous osalate. The anhydrous oxalate is stable between 300 and 380°C and undergoes decomposition in two stages (Stages III and IV). The DTG peak at 428” C and the exothermic DTA peak at 408” C correspond to the first-stage decomposition. The product at this stage (Stage III) has the composition CO?. This osycarbonate then decomposes to the oxide SmzOJ Sm,Gj, (Stage IV). The DTG peak at 591°C and the esothermic DTA peak at 560°C denote this decomposition_ Gadolinium oxalate is stable up to 120” C and dehydration starts at this temperature. The three DTG peaks at 163, 226 and 270°C which are closely paralleled by the three endothermic DTA peaks at 168, 225 and 265”C, correspond to the three dehydration stages I, II and III (10 Hz0 --* 6 Hz0 --, 4 HZ0 + anhydrous salt). The anhydrous oxalate is stable between 280 and 380” C and undergoes decomposition in two stages. The DTG peak at 438” C and the esothermic DTA peak at 427°C correspond to the first stage of decomposition (Stage IV)_ The product after this stage appears to be Gdz03, COz. This then decomposes to the oxide Gd103 (Stage V). The DTG peak at. 590°C and the esothermic DTX peak at 583°C represent the final stage of decomposition. Dysprosium osalate appears to be stable up to 100°C when dehydration begins. The three DTG peaks at 138, 197 and 363”C, which correspond with the three endothermic DTA peaks at 145, 225 and 355”C, refer to the Stages I, II and III of dehydration (10 Hz0 1 4 H,O --, 2 Hz0 + anhydrous salt). The tetrahydrate is stable in the range 210-330”. The anhydrous oxalate undergoes decomposition in two stages (Stages IV and V). The DTG peak at 430°C and exothermic DTA peak at 420°C represent the first of these (Stage IV). The product obtained at this stage has the rough composition DyzOs, CO, which further decomposes to the oside Dy,O, (Stage V). The DTG peak at 522°C and the esothermic DTA peak at 525°C represent this stage of decomposition. Yttrium osalate is stable up to 90°C and dehydration begins at this temperature_ The three DTG peaks at 132, 187 and 383”C, paralleled by the three endothermic DTA peaks at 153, 195 and 392”C, correspond to Stages I, II and III of dehydration (9 H,O --t 4 Hz0 -* 2 Hz0 +- anhydrous salt). The The anhydrous oxalate dihydrate is stable in the range 220-360°C. undergoes decomposition in two stages (Stages IV and V). The DTG peak at 427°C and the esothermic DTA peak at 425°C represent the first of these stages (Stage IV). The product obtained after this stage appears to be Y203, COz. This further decomposes to the oside YzO3 (Stage V). The DTG peak at 601°C and the esothermic DTA peak at 605°C represent this stage of decomposition. TiO(C20d) - 3.5 H,O, is stable up to 60°C and the Titanyl osalate, dehydration starts at about this temperature. The DTG peak is at 105°C. The endothermic DTA peak at 110” C represents this dehydration (Stage I)
263 leading to the formation of the dihydrate, TiO(C104) - 2 H20. The dihydrate begins further dehydration accompanied by decomposition. The DTG peak at 296°C represents this stage (Stage II). In DTA, this stage is rcpresented by two close-lying peaks, an endothermic peak at 290°C and an esothermic peak at. 320°C. The phase formed at 330°C is a basic carbonate of titanium with the empirical formula Ti3COn or TiO(COJ), 2 TiO- and is stable up to 500°C. _.4bove this temperature, it decomposes to give Ti02 . The DTG peak at 538”C, paralleled by the esothemlic peak at- 54O”C, corrcs-ponds to this final stage of decomposition (Stage III). Mass loss clata Mass loss data are given in Table 3. The final residue is the oside &I203 for M = La, Nd, Sm, Gd, Dy and Y; it is MO1 for Ce and Ti and Pr6011 for Pr. The mass loss data from TG, as well as from independent pyrolysis studies. agree closely with calculated vaIues in all cases. It may be seen from Triblc 3 that the agreement is satisfactory between the calculated and obsclved (from TG) values for mass loss dat.a for intermediate stages as well, whcrever intermediate stages are sufficient.& well-defined. h’irlctic
parameter-s
The kinet.ic parameters evaluated from the TG traces arc given in Tahlr -1. The values of E, AS and rl using the Coats-Redfern equation and a similar set using the modified Coats-Redfern equation arc presented. The corrcllation coefficients in each case are also shown.
DISCUSSION
A survey of the results in Tables 2 and 3 reveals an interesting fnc~. Thr hydrated osalates of the lighter lanthanides (La, Ce and Pr) undergo sin&hstage dehydration from the decahydrate to t.he anhydrous salt. In Nd and Sm, we have two-stage dehydration, whereas in the heavier lanthanidcs Gd and Dy, we have three-stage dehydration . Xs espected, Y falls in line wiLh the heavier lanthanides [41] and shows a three-stage dehydration. It. is interesting to note that these definitive trends observed in the thermal behaviour are compatible with the well-known increase in coordinal ion ability (e.g. hydration energies) of the lauthanide M3’ ion from La t.o Lu. The final residues are normal osides, M1OJ, in the case of La, Nd, Sm, Grl. Dy and Y. In the case of Pr, it is PrciO 1I and for Ce it is CeOL. This is 110~ surprising because, under open static air conditions, one espects the Inost stable oxide to be formed, which is Pr,O 11 for Pr ancl Ce02 for Cc. Thc~ residue for titanyl osalate is TiOz as expected. A similar, although less clear-cut, tendency could be noted as regards the final decomposition to the oside stage. The oxide stage is attained at 760°C
264
for La, 680°C for Nd and at about 640°C for Sm, Gd, Dy and Y. This indicates that the thermal stability of the carbonates decreases marginally from La through Nd to Sm, Gd, Dy and Y. Such an order would reflect the increasing Lewis acidities of the M3’ ions from La3’ to Lu3’. The exceptional behaviour in the case of Ce and Pr could be attributed to the formation of oxides of formulae other than M,Oa. The DTA peaks for dehydration are endothermic as expected. In the case of further decompositions, whereas one may expect endothermicity as a general tendency, one finds exothermicity in all the cases. This may be explained by osidation processes (such as combustion of the liberated CO to COZ) accompanying the decomposition of the oxalates in the oxidizing atmosphere (static air) employed for the studies. (It is well known that CaC,Oq gives exothermic DTA peaks in air, whereas in nitrogen, the DTA peaks reveal their true endothermicity.) In the present studies, static air was deliberately employed as mentioned earlier. Kinetic parameters Larztharzides and y ttrirmz The first dehydration stages are found to have E values of the order of -100 kJ mole -I_ The second dehydration stages also generally have values of although the third dehydration stages (e.g. Gd and Y) 100-200 kJ mole-‘, have higher values (300-450 kJ mole-‘). This implies that the dehydration of the lower hydrate is kinetically less favoured than that of the higher hydrates_ The decomposition stage, especially the final ones leading to oxide formation, show higher values of E (700-800 kJ mole-‘). This is to be espected because these decomposition stages involve rupture of stronger bonds than in the case of dehydration_ It is difficult to draw definitive conclusions from AS and -4 data which must await further theoretical developments in this field. One may note, however, that the dehydrations generally tend to have lower values of AS and A. A negative value for AS further shows a more ordered activated comples and a less than normally fast reaction. On t.he other hand, positive values of ilS indicate less ordered activated compleses and faster than normal reactions [421. Titanyl oxala te The first dehydration stage of titanyl osalate has a low value for the j, whereas the second stage has a slightly energy of activation ( -30 kJ mole-' higher value (-70 kJ mole-‘). It may be noted that the order of magnitude of both values is the same as, or one less than, that for similar dehydration stages of lanthanide oxalates. The values for the entropy of activation for the dehydration stages of hydrated titanyl osalate are found to be negative, indicating that the activated comples is more ordered than the reactants and that the reaction is less than normally fast [421.
265 ACKNOWLEDGEMENTS
We thank Dr. P.M. Madhusudanan and Dr. P. Indrasenan for helpful discussions and the Space Science and Technolow Centre, Trivandrum-22 for instrumental facilities made available to us.
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