Thermal genesis course and characterization of praseodymium oxide from praseodymium nitrate hydrate

Article abstract of DOI:10.1016/S0040-6031(00)00727-9

Pr(NO₃)₃·6H₂O was used as a precursor to produce PrO_1.833 at 600°C in an atmosphere of static air. Thermal processes occurred were monitored by means of thermogravimetry, differential thermal analysis and mass spectrometry. IR-spectroscopy and X-ray characterized the intermediates and final solid products. The results showed that, Pr(NO₃)₃·6H₂O decomposes through 11 endothermic weight loss processes. 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)₂ at 340°C. The latter, decomposes to PrO_1.833 at 465°C, via four different intermediates PrO(NO₃) at 430°C, and nonstoichiometric unstable, PrO_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 gaseous decomposition products identified by mass spectroscopy were water vapor and nitrogen oxides (NO, NO₂ and N₂O₅). The activation energy was determined nonisothermally for the thermal processes monitored throughout the decomposition course. The final product PrO_1.833 has a surface area of 46.3 m²/g.

Full text of DOI:10.1016/S0040-6031(00)00727-9

Thermochimica Acta 369 (2001) 59±66
Thermal genesis course and characterization of praseodymium
oxide from praseodymium nitrate hydrate
a,*
G.A.M. Hussein , B.A.A. Balboul , M.A. A-Warith , A.G.M. Othman
a
a
b
a
Chemistry Department, Faculty of Science, Minia University, EI-Minia 60519, Egypt
b
Ceramic Department, National Research Center, Dokki, Cairo, Egypt
Received 15 June 2000; received in revised form 16 September 2000; accepted 16 September 2000
Abstract
Pr(NO
occurred were monitored by means of thermogravimetry, differential thermal analysis and mass spectrometry. IR-spectroscopy
3
)
3
Á6H
2
O was used as a precursor to produce PrO1.833 at 6008C in an atmosphere of static air. Thermal processes
and X-ray characterized the intermediates and ®nal solid products. The results showed that, Pr(NO ) Á6H O decomposes
3
3
2
through 11 endothermic weight loss processes. Five dehydration steps occurred at 130, 180, 200, 230 and 2508C, leading to
the formation of crystalline nitrate monohydrate, Which decomposes to Pr(NO
3 2
) at 3408C. The latter, decomposes to PrO1.833
at 4658C, via four different intermediates PrO(NO ) at 4308C, and nonstoichiometric unstable, PrO0.25(NO ) at 3628C;
3
3 2.5
Pr(O)0.5(NO
spectroscopy were water vapor and nitrogen oxides (NO, NO
3
)
2
at 3828C and Pr(O)0.75(NO
3
)
1.5 at 4008C. The gaseous decomposition products identi®ed by mass
and N ). The activation energy was determined
2
2 5
O
nonisothermally for the thermal processes monitored throughout the decomposition course. The ®nal product PrO1
2
surface area of 46.3 m /g. # 2001 Elsevier Science B.V. All rights reserved.
has a
.833
Keywords: Pr(NO
3
)
3
2
Á6H O; PrO1.833; Decomposition; Formation; DTA; TG; IR; XRD; Mass spectroscopy
1
. Introduction
Praseodymium oxides comprise, Pr O , PrO plus a
range of intermediate phases: Pr O , PrO1.670,
ductor at values of x < 1:73 and n-type semiconductor
at higher value of x. This behavior was interpreted in
the terms of a hopping or electron transfer model
involving transfer of electrons between trivalent and
tetravalent Pr-ions [1±4].
Several studies [7±14] and review article [6] have
been published concerning with the thermal decom-
position of rare earth metal salts. It was reported [6]
that, rare earth metal oxides obtained from nitrate or
acetate precursors leads to higher surface area com-
pared with those obtained from oxalate precursors.
2
3
2
2
3
PrO1.714, PrO1.780, PrO1.800, PrO1.810, and PrO1.833
1±4]. The PrO phase possesses the ¯uorite structure,
[
2
while PrO1.810, and PrO1.833 are oxygen-de®cient
modi®cations of ¯uorite structure. These observed
phases depend on the precursor used, and the atmo-
sphere and temperature of decomposition [3±6]. It was
also reported [1±4] that, PrO is a p-type semicon-
x
Patil et al. [7] reported that, the formation of M O3
2
*
from rare earth nitrates takes place via the oxynitrate
Corresponding author. Tel.: ‡20-86-345231;
(MONO ). They also reported the possible formation
3
of anhydrous nitrate, while Wendlandt and Bear [8]
fax: ‡20-86-342601.
E-mail address: gamalhus@usa.net (G.A.M. Hussein).
0040-6031/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 0 0 ) 0 0 7 2 7 - 9

6
0
G.A.M. Hussein et al. / Thermochimica Acta 369 (2001) 59±66
stated that the anhydrous nitrates are unstable. Stewart
and Wendlandt [9] studied the decomposition of
La(NO ) Á6H O. They reported that, the dehydration
calcination products were obtained by heating at
various temperatures (200±6008C) for l h in a static
air. The calcination temperatures were chosen on
bases of the thermal analysis results. Prior to analysis,
the calcination products were kept dry over silica-gel.
For simplicity, these products are donated in the text
by PrNit, followed by the calcination temperature.
Thus, PrNit-400 indicates decomposition products of
PrNit at 4008C for 1 h. The abbreviation WL stands
for weight loss.
3
3
2
takes place at 1058C to give La(NO ) Á3H O, which
3
3
2
decomposes at 2008C to form a basic nitrate; and at
008C, La O was the ®nal product. Recently, the
5
2
3
decomposition of Y(NO ) Á5H O was studied [10]. It
3
3
2
was reported that a thermally stable monohydrate was
formed. Also a mixture of Y(NO ) and Y(OH)(NO )
3 3
3
3
were obtained at 2708C; and YONO was detected by
3
XRD as crystalline at 3258C. Y O was the ®nal
2
3
product at 5008C. The texture analysis by nitrogen
2.2. Thermal analysis
adsorption method and SEM [11], revealed that Y O
obtained from Y(NO ) Á5H O at 5008C has a higher
2
3
Thermal analysis were performed using DuPont
9900, Model DTA 951-TG 910 (USA). Thermogra-
vimetry (TG) and differential thermal analysis (DTA)
curves were recorded up to 7008C at different heating
rates (5±508C/min.), in static air atmosphere. Equal
weights of the test sample (ca. 15 mg) were used in TG
measurement. Highly sintered a-Al O was the refer-
3
3
2
2
surface area (S
ˆ 58 m /g) compared with that
BET
2
obtained at 7008C (SBET ˆ 20 m /g). They attribute
the lower surface area as results of sintering. Hussein
et al [12,13] have studied the decomposition course of
Th(NO ) Á5H O [12] and Dy(NO ) Á6H O [13] in air
3
4
2
3 3
2
by TG, DTA, IR, XRD and SEM. They reported that,
the unhydrous nitrates are thermally unstable and the
decompositions gave ThO at 3008C and Dy O at
2
3
ence material for the DTA measurements.
From the thermoanalytical data, the temperature
(Tmax) obtained from the DTA curve, were determined
as a function of the heating rate (y) applied. The
activation energy (DE, kJ/mol) was then calculated
for each process using Kissinger equation [15].
2
2 3
6008C, respectively. Different intermediates of non-
stoichiometric oxynitrates were also detected. The
DyONO is crystalline and thermally stable, com-
3
pared to ThO(NO ) which is unstable and amor-
3
2
phous. Balboul [14] studied the decomposition
course of Ho(NO ) Á5H O in air by TG, DTA, IR,
2.3. Mass spectroscopy
3
4
2
XRD and SEM. It was reported that, the unhydrous
nitrates are thermally unstable and the decompositions
gave Ho O at 5608C. Different intermediates of
All the mass spectra of the volatile decomposition
products were recorded with a Hewlett Packard (HP-
5890 series II) gas chromatograph equipped with a
5971 mass selective detector. An HP-vectra Q5/165F
data station was used for acquisition and processing of
the spectra. Also the data station was equipped with
ATLAS for comparison and identi®cation proposes.
The spectra of the gaseous products of PrNit were
taken using a sample cell Tekmar 5 ml frit sparger (Pn
14-2337-024). Some modi®cations were made by
making a furnace (coil) surrounding the sample area,
with temperature controller. This makes the cell a
variable temperature cell.
2
3
nonstoichiometric oxynitrates were also detected.
The HoONO is crystalline and thermally stable.
3
The present investigation, is intended to character-
ize the thermal decomposition course of
Pr(NO ) Á6H O to the onset of PrO
formation.
1.833
3
3
2
The activation energy (DE) was determined noni-
sothermally for decomposition processes involved.
The oxide formed at 6008C was subjected to surface
area measurements by N adsorption.
2
2
. Experimental
The standard procedure for automated GC±MS
were as follows: powder sample (20 mg) was heated
at different temperatures (150±4508C) for 5 min. At
each temperature, the gaseous products were trapped
using Tekmar LSC2000, dynamic head space con-
centrator. The trapped products were automatically
2
.1. Materials
Pr(NO ) Á6H O abbreviated as PrNit, was 99.9%
3
3
2
pure (Aldrich, USA). It was used as received. Its

G.A.M. Hussein et al. / Thermochimica Acta 369 (2001) 59±66
61
desorbed into a HP-5890 GC, interfaced with a 5971A
mass-selective detector.
visual observation revealed that, the material melts at
698C. Therefore, process I is due to melting.
The gaseous decomposition products were injected
at 2008C to the GC equipped with 60 m  0:25 mm
DP5 column, with a 2.5 mm ®lm thickness. It is held at
3.1.1.1. Processes II±VI. From the TG and DTA
curves (Fig. 1), it is clear that processes II±VI are
overlapped endothermic weight variant processes,
maximizes at 130, 180, 200, 230 and 2508C,
respectively. The weight loss associated with each
process (Table 1) was close to that which is equivalent
to the release of 1 mol of water (i.e. for process II, WL
is 4.3% close to that 4.16% theoretically calculated
for release of 1 mol of water). Therefore, processes
II±VI are most likely proceed as follows:
5
2
08C for 10 min after injection, then programmed to
408C. Helium gas was used as a carrier gas at a ¯ow
rate 1.0 ml/min. To prepare for the next experiment
decomposition temperature), the trap (LSC2000) was
baked out at 2508C for 10 min under a ¯ow of N gas.
(
2
2
.4. Infrared spectroscopy
�
1
IR spectra were obtained at a resolution 4 cm
ꢁ
�
1
130 C
between 4000 and 400 cm using a model FT/IR 410
JASCO (Japan). IR spectra of PrNit and its solid
calcination products were obtained from thin, lightly
loaded (<1%) KBr-supported discs.
PrꢀNO † Á 6H O ! PrꢀNO † Á 5H O ꢀStep II†
3
3
2
3 3
2
ꢁ
80 C
1
PrꢀNO3† Á 6H2O ! PrꢀNO3† Á 4H2O ꢀStep III†
220 C
3
3
ꢁ
PrꢀNO † Á 6H O ! PrꢀNO † Á 3H O ꢀStep IV†
3
3
2
3 3
2
ꢁ
30 C
2
2
.5. X-ray diffraction
PrꢀNO3† Á 6H2O ! PrꢀNO3† Á 2H2O ꢀStep V†
3
3
ꢁ
50 C
2
XRD powder patterns were obtained using JSX-60P
JEOL diffractometer equipped with Ni-®lter and gen-
PrꢀNO3† Á 6H2O ! PrꢀNO3† Á H2O ꢀStep VI†
3
3
Ê
The mass spectra up to 3208C (Fig. 2) is consistent
erates a beam of Cu Ka radiation (l ˆ 1:5418 A). The
with the TG/DTA data. Only the mass of H O (ion 18,
operational settings for all the XRD scans are voltage:
4
2
at a retention time of 6.8 min.) and its positive radi-
cals, m=z ˆ 19, 17 and 16 [18] were detected. In
support, the IR spectrum of PrNit-200 (Fig. 3), is very
similar to that obtained for untreated PrNit. It displays
0 kV; current: 30 mA; 2y ˆ range 4±608; scanning
speed: 88/min; slit width: 0.028. For identi®cation
purposes the relative intensities (I/I ) and the d-spa-
0
Ê
cing (A) were compared with standard diffraction
patterns of the ASTM powder diffraction ®les [16].
�
1
absorption bands at 1630 cm (dOH of HOH) and at
�
1
1
480, 1420, 1340, 1040, 815 and 740 cm due to
nitrate groups of a mainly shelving bidentate structure
19,20]. Also, the XRD parttern at 2008C (PrNit200,
Fig. 4), revealed the formation of Pr(NO ) .H O as a
2
.6. Nitrogen adsorption measurements
[
The N sorption isotherms were determined volu-
2
3 3
2
crystalline phase [16].
metrically at � 1968C using a micro-apparatus based
The corresponding activation energy (Table 1) has
values of 54(II), 68 (III), 65(IV), 62(V) and 68(VI) kJ/
mol. They are within the range characteristic of dehy-
dration processes [21,22].
on the design of Lippens et al. [17]. The test sample
(
PrNit-600) was out-gassed at 2008C for 2 h under
evacuation at 10 Torr.
�
5
3
.1.1.2. Process VII. Process VII is an endothermic
3
3
3
. Results and discussions
weight variant process peaking at 3408C. It overlaps
with the succeeding processes (VIII±X). The weight
loss determined (Table 1) for process VII is 24.40%
which is near to that expected 24.8% for the formation
.1. Characterization of the decomposition course
.1.1. Praseodymium nitrate hexahydrate, PrNit,
Pr(NO ) Á6H O
of Pr(NO
3
)
3
, as follows:
3
3
2
The DTA curve (Fig. 1) shows an endothermic
weight invariant process located at 708C. A direct
ꢁ
3
40 C
PrꢀNO † Á H O ! PrꢀNO † ꢀStep VII†
3
2
3 3
3

6
2
G.A.M. Hussein et al. / Thermochimica Acta 369 (2001) 59±66
Fig. 1. TG and DTA curves recorded for PrNit at the heating rates indicated, in staticair.
Table 1
Thermal processes, experimental and theoretical Weight losses, composition proposed and activation energy for each process
Process
Observed
WL (%)
Theoretical
WL (%)
Chemical
composition
Activation energy
(kJ/mol)
I
±
Melt
4.16
Pr(NO
Pr(NO
Pr(NO
Pr(NO
Pr(NO
Pr(NO
Pr(NO
3
3
3
3
3
3
3
)
)
)
)
)
)
)
3
3
3
3
3
3
3
Á6H
Á5H
Á4H
Á3H
Á2H
2
2
2
2
2
O
O
O
O
O
±
54
II
4.3
III
IV
V
8.2
8.31
68
12.2
16.2
20.5
24.4
30.3
37.5
43.4
50.0
60.5
12.47
16.63
20.78
24.8
65
62
VI
VII
VIII
IX
X
ÁH
2
O
68
210
199
205
212
215
226
30.2
37.25
43.4
PrO0.25(NO
PrO0.5(NO
3 1.5
Pr(O)0.75(NO )
3 2.5
)
3 2
)
XI
XII
49.6
60.8
PrO(NO
3
)
PrO1.833

G.A.M. Hussein et al. / Thermochimica Acta 369 (2001) 59±66
63
Fig. 2. Mass spectra recorded from the gas surrounding a 20 mg portion of PrNit at the temperatures indicated.

6
4
G.A.M. Hussein et al. / Thermochimica Acta 369 (2001) 59±66
The mass spectra up to 4508C, Fig. 2, detects the
masses due to N O (m=z ˆ 108, 47, 46, 45, 32, 31,
2
5
3
0, 28, 16 and 14) [18]. Also NO (m=z ˆ 30, 16 and
1
as a result of the decomposition of N O , as follows:
5) and NO (m=z ˆ 46, 30, 16 and 15) were detected
2
2
5
N2O5 ! NO ‡ NO2 ‡ O2
Also the IR spectrum of the solid phase at 4008C
Fig. 3) displays bands mainly due to nitrate [21,22].
(
However, the evidence for the formation of oxynitrato
anion PrONO , is quite convincing. The composite
3
�
1
absorption emerging at 700±400 cm is most prob-
ably related to Pr±O type vibration [23].
The formation of nonstoichiometric oxynitrate has
been reported before [13]. During the decomposition
of Dy±nitrate, a different forms of DyO (NO ) was
x
3 y
formed as an unstable intermediate which decom-
posed either to oxide or to the higher oxynitrate
(
x @ y). The similarity between the activation energy
Fig. 3. IR Solid phase spectra for PrNit and its l h, calcination
products at the temperatures indicated.
values (Table 1) (210 Æ 10 kJ/mol) may give the
reason for the overlapping with the thermal unstability
of these nonstochoimetric forms.
Above 4508C (Fig. 1), the latter PrONO decom-
3
The strong overlapping between processes VII and
VIII give indication that, anhydrous Pr±nitrate is
thermally unstable.
ꢁ
poses through process X (Tmax ˆ 465 C) to give the
®
6
nal decomposition product PrO1.833 (WL determined
0.5% is close to that expected 60.8% form the
It must be noted that, the anhydrous PrNit was not
detected, in agreement with previous studies [8,12,13].
The higher activation energy value (210 kJ/mol) give
indication that the removal of the ®rst 5 mol of water is
energetically easier than that the removal of the last
mol of water. This may give the reason for the thermal
stability of the Pr±nitrate monohydrate.
decomposition of PrNit to PrO1.833. The strong sup-
port for the formation of PrO1.833 was provided by the
XRD, Fig. 4, of PrNit-600, which displays only the
patterns PrO1.833 (ASTM 6-329) [16]. Moreover, the
IR spectrum of the solid product at 6008C (Fig. 3),
reveals the disappearance of oxynitrate absorptions
� 1
bands. It displays absorption bands below 800 cm
due to metal±oxygen vibrations (lattice modes) of
3
.1.1.3. Processes VIII±XI. Fig. 1 shows that,
(
Pr±O) [23]. The activation energy value for pro-
processes VIII±XI are strongly overlapped
endothermic, maximizes at 362, 382, 400 and
cess XII is 226 kJ/mol. The higher value of the
activation energy may also support the formation
of oxygen de®cient modi®cation of ¯uorite structure
4
308C. The weight loss associated with each
process (Table 1) was close to that equivalent to the
formation of nonstiochoimetric form of oxynitrate,
as follows:
(
PrO1.833).
3.1.2. Surface characterization of PrO1.833 nitrogen
adsorption and surface area measurements
ꢁ
3
62 C
PrꢀNO † ! PrO ꢀNO † ‡ N O5 ꢀStep VIII†
3
3
0:25
3 2:5
2
The N adsorption isotherm of PrNit-600 is shown
2
ꢁ
3
82 C
in Fig. 5. The isotherm generally belongs to type IVof
the BET classi®cation [24]. The desorption isotherm
PrO0:25ꢀNO3†2 ! PrO0:5ꢀNO3† ‡N2O5 ꢀStep IX†
:5
2
ꢁ
00 C
4
0
PrO0:5ꢀNO3† ! PrO0:75ꢀNO3† ‡N2O5 ꢀStep X†
closed at P=P > 0:4, which gives indication that the
hysteresis loop is nearly of type H3 [24]. The sorption
isotherm and hysteresis loop suggested that the
2
1:5
ꢁ
30 C
4
PrO0:75ꢀNO3†1:5 ! PrOꢀNO3† ‡ N2O5 ꢀStep XI†

Fig. 4. X-ray powder diffractograms for PrNit and its 1 h calcination products at the temperatures indicated.
Fig. 5. Nitrogen adsorption isotherm obtained at � 1958C (using BET method) for PrNit-600.

6
6
G.A.M. Hussein et al. / Thermochimica Acta 369 (2001) 59±66
surface pores are slit-shaped or interplating [25]. The
2
SBET value determined for PrNit-600 was 46.3 m /g.
References
[
[
1] P. Kofstad, Nonstoichiometry, Diffusion and Electrical
Conductivity in Binary Metal Oxide, Wiley, New York,
1
972, p. 289.
2] Y. Wilbert, A. duquesnoy, F. Marion, C.R. Acad. Sci. (Paris)
63 (1966) 1539.
4
. Conclusion
2
The thermal decomposition of PrNit in atmosphere
of air involves the following pathways:
[3] J.M. Honig, A.F. Clifforf, P.A. Faeth, Inorg. Chem. 2 (1963)791.
[4] B.G. Hyde, D.J.M. Bevan, L. Eyring, Philos. Trans. R. Soc.
London 259 (1966) 583.
ꢁ
1
30 C
[
5] G.A.M. Hussein, J. Chem. Soc., Faraday Trans. 90 (1994)
693.
PrꢀNO3† Á 6H2O ! PrꢀNO3† Á 5H2O
3
3
3
ꢁ
80 C
1
[
[
6] G.A.M. Hussein, J. Anal. Appl. Pyrol. 37 (1996) 111.
7] K.C. Patil, R.K. Gosavi, C.N.R. Rao, Inorg. Chem. Acta 1
(1967) 155.
PrꢀNO † Á 6H O ! PrꢀNO † Á 4H O
3
3
2
3 3
2
ꢁ
00 C
2
PrꢀNO3† Á 6H2O ! PrꢀNO3† Á 3H2O
3
3
[8] W.W. Wendlandt, J.L.Bear, J.Inorg.Nucl. Chem. 12(1960)276.
[9] D.F. Stewart, W.W. Wendlandt, J. Phys. Chem. 63 (1959)
1330.
ꢁ
30 C
2
PrꢀNO3† Á 6H2O ! PrꢀNO3† Á 2H2O
3
3
ꢁ
50 C
[10] G.A.M. Hussein, Thermochim. Acta 244 (1994) 139.
[11] H.M. Ismail, G.A.M. Hussein, Powder Technol. 84 (1995) 185.
2
PrꢀNO3† Á 6H2O ! PrꢀNO3† Á H2O
3
3
[
12] G.A.M. Hussein, H.M. Ismail, Colloid Surf. 99 (1995) 129.
ꢁ
40 C
3
PrꢀNO † Á 6H O ! PrꢀNO †
[13] G.A.M. Hussein, H. Korban, B. Goda, K. Miyaji, Colloid
Surf. 125 (1997) 63.
3
3
2
3 3
ꢁ
62 C
3
[
[
[
14] B.A.A. Balboul, Powder Technol. 107 (2000) 168.
15] H.E. Kissinger, J. Anal. Chem. 29 (11) (1957) 1526.
16] J.V. Smith (Ed.), X-ray Powder Data File, Amercain Society
for Testing and Materials, Philadelphia, USA, 1960.
PrꢀNO3† Á 6H2O ! PrO0:25ꢀNO3†
3
2:5
1:5
ꢁ
3
82 C
PrꢀNO † Á 6H O ! PrO ꢀNO †
3
3
2
0:5
3 2
ꢁ
4
00 C
[17] B.C. Lippens, B.G. Linsen, J.H. de-Boer, J. Catal. 3 (1964)
2.
PrꢀNO3† Á 6H2O ! PrO0:75ꢀNO3†
3
3
ꢁ
4
30 C
[18] F.W. Mclafferty, D.B. Stauffer (Eds.), The Wiley/NBS
Registry of Mass Spectral Data, Vol. 1, Masses from 2 to
198, New York, 1989.
PrꢀNO † Á 6H O ! PrOꢀNO †
3
3
2
3
ꢁ
65 C
4
PrꢀNO3† Á 6H2O ! PrO1:833
3
[19] K. Nakamoto, Infrared Spectra of Inorganic and Coordination
Compounds, Wiley, New York, 1970, p. 253.
[
20] G.A. Gadsden, Infrared Spectra of Minerals and Related
Inorganic Compounds, Butterworths, London, 1975, p. 6511.
21] M.E. Brown, D. Dollimore, A.K. Galwey, in: C.H. Bamford,
C.F.H. Tipper (Eds.), Chemical Kinetics Reactions in the
Solid State, Vol. 22, Elsevier, Amsterdam, 1980, p. 130.
ꢂ The monohydrate of Pr±nitrate is thermally stable,
while the unhydrous is unstable.
ꢂ No hydroxynitrate was detected as an intermediate.
ꢂ A nonstoichiometric form of oxynitrate was
detected as an unstable intermediate.
[
[22] G.A.M. Hussein, J. Phys. Chem. 98 (1994) 9657.
[
[
[
23] J.A. Goldsmith, S.D. Ross, Spectrochem. Acta A 23 (1967)
909.
ꢂ The gases detected by mass spectroscopy were:
1
H O vapor, NO, NO , and N O .
2
2
2 5
24] S.J. Gregg, K.S.W. Sing, Adsorption Surface Area and
Porosity, 2nd Edition, Academic press, London, 1982.
25] K.S.W. Sing, Pure Appl. Chem. 54 (1982) 2201.
ꢂ PrO
obtained at 6008C has a surface area of
1
.833
2
6.3 m /g.
4