Physicochemical investigation of platinum dichloride polymorphism

Article abstract of DOI:10.1016/j.tca.2008.10.020

The physicochemical characteristics of phase transitions of PtCl₂ were investigated for the first time. The irreversible character of the transition from β-modification to α-form of PtCl₂ and the temperature range of process were est

Full text of DOI:10.1016/j.tca.2008.10.020

Thermochimica Acta 482 (2009) 62–65
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Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Physicochemical investigation of platinum dichloride polymorphism
∗
Tamara P. Chusova , Zinaida I. Semenova
A.V. Nikolaev Institute of Inorganic Chemistry Siberian Branch of the Russian Academy of Sciences, Academician Lavrentiev Avenue, 3, Novosibirsk, 630090, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 October 2008
Received in revised form 27 October 2008
Accepted 28 October 2008
Available online 5 November 2008
The physicochemical characteristics of phase transitions of PtCl2 were investigated for the first time.
The irreversible character of the transition from ␤-modification to ␣-form of PtCl2 and the temperature
range of process were established (570–870 K). ␣-PtCl2 has one reversible transition at a temperature of
6
60 ± 5 K; the thermal effect of this process is +167 ± 17 J/mol.
©
2008 Elsevier B.V. All rights reserved.
Keywords:
Platinum dichloride
Polymorphism
X-ray phase analysis
Differential thermal analysis
Differential scanning calorimetry
1
. Introduction
ment with the data of ref. [2]. Differences are observed only for the
first two lines in the region of small angles and for some weak lines.
The indicated line intensities are in good agreement with the values
calculated on the basis of structural data [2]. These facts indicate
that the authors of refs. [2,4] studied the same crystal form of plat-
One of the most important systems in chemistry and technol-
ogy of precious metals is Pt–Cl. Investigation of the polymorphism
of platinum chlorides is especially interesting in view of the possi-
bility to use them in the modern technologies of platinum refining
and secondary raw material processing. As far as the polymor-
phism of platinum tetra- and trichloride is concerned, no data
were reported in literature. The data on platinum dichloride poly-
morphism are scarce and partially contradictory. There are some
indications for the existence of several modifications but only two
polymorphous forms have been characterized by present; accord-
ing to the classification proposed in ref. [1], we will call ␤-PtCl2
inum dichloride, namely ␤-PtCl2 (Pt Cl12). It should be noted that
6
the authors of ref. [4] did not describe synthesis method and the
results of chemical analysis of PtCl2 under investigation.
Platinum dichloride in the ␣-form [1] formed either during the
direct interaction between the elements in a sealed ampoule at
820 K or during annealing ␤-PtCl (Pt Cl12) for 2 days at 770 K (evac-
2
6
uated ampoule). The authors of ref. [5] determined the structure
of this compound and established that it belongs to the mono-
clinic system, with the space group C2/m; unit cell parameters are:
(
Pt Cl12) (low-temperature) and ␣-PtCl2 (high-temperature).
6
The single crystals of ␤-PtCl2 were obtained by the authors of
´˚
´˚
´˚
◦
a = 13.258 A, b = 3.194 A, c = 6.802 A, ˇ = 107.75 , Z = 4.
ref. [2] by transferring ␤-PtCl2 (the latter was synthesized by the
thermal decomposition of H PtCl ·6H O in chlorine flow at 750 K)
The IR spectroscopic data for ␤-PtCl are reported in refs. [6–8];
2
2
6
2
no data on the IR spectra for ␣-form were found in literature.
So, the goal of our work was to establish the regions of exis-
tence of ␣-PtCl2 and ␤-PtCl2 and to determine the quantitative
characteristics of the transition ␤-PtCl → ␣-PtCl .
[
3] at 820 K in a flow system (with Cl2 as a carrier gas) for several
days. Those authors carried out a detailed investigation of the crys-
tal structure of ␤-PtCl and found that the substance belongs to the
2
2
2
rhombohedral system with unit cell parameters a = 13.11 ± 0.02 Å
and c = 8.59 ± 0.03 Å (trigonal arrangement). In addition, they estab-
lished that platinum dichloride is composed as a cluster based on
2
. Experimental
2.1. Synthesis
-PtCl2 was obtained by sublimation of platinum ␤-dichloride
Pt Cl groups. Along with paper [2], publication [4] appeared. The
6
12
interplanar spacings reported therein for PtCl2 are in good agree-
␣
in a sealed ampoule under the temperature gradient along the
ampoule 870 → 820 K (the substance transfer from the hot zone
to the cold one occurred) [1].
∗ Corresponding author. Tel.: +7 383 330 92 59.
E-mail address: chu@che.nsk.su (T.P. Chusova).
0
040-6031/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2008.10.020

T.P. Chusova, Z.I. Semenova / Thermochimica Acta 482 (2009) 62–65
63
Table 1
␤
-PtCl2 was synthesized by means of the thermal decomposi-
Data of X-ray phase analysis for PtCl2 on the basis of literature data.
tion of H PtCl ·6H O in the flow of chlorine (P(Cl ) = 1 atm) at 750 K
2
6
2
2
[
3].
␤-PtCl2
2]
␣-PtCl2
[
[4]
[1,5]
2.2. Identification
´
´
´
d␣ ( A˚ )
hkil
Intensity
d␣ ( A˚ )
Intensity
d␣ ( A˚ )
Middle
Strong
8.16
6.69
100
100
50
10
10
10
30
80
30
30
10
30
50
30
30
10
50
30
10
10
50
30
50
6.482
6.309
5.428
3.322
3.255
3.242
3.156
3.104
2.885
2.713
2.581
2.544
2.236
2.224
2.106
1.909
1.845
1.781
1.700
1.674
1.656
1.622
1.601
The chemical analysis of synthesis products was carried out
6
6
4
.852
.520
.735
1011
1120
0221
0112
3030
2022
2240
1232
1341
0003
4041
1123
3142
3251
using the method of platinum and chlorine determination in plat-
inum chlorides from one weighed portion; the method is based
on the separation of metal platinum from chlorine by means of
reduction:
4.019
3
3
Middle
Middle
Very weak
Middle
4.03
3.78
3.47
3.25
.776
.424
x
2
3.274
PtClx(s) + H (g) = Pt(s) + xHCl(g),
2
3.034
2.953
2.863
2.692
Strong
Middle
2.932
2.675
with the simultaneous absorption of hydrogen chloride in water.
The optimal conditions for the reduction were determined with
the derivatograph “Paulik–Paulik–Erdey” [9]. The results showed
that in the case of platinum dichloride the reduction proceeds with
a noticeable rate even at 340 K; the maximal rate is observed at
a temperature of 400 K. Reduction was carried out as follows. A
weighed portion of the substance (0.08–0.15 g) was loaded into a
quartz cup reduced to a fixed weight; it was placed into a flow
quartz reactor. The system was blown with hydrogen for 10 min and
2.552
2.540
2
Very strong
Very weak
Very weak
Weak
2.536
2.462
2.374
2.286
.475
2
2
.282
.226
3033
2352
3360
1014
5052
6060
4371
5270
3363
4480
2.193
Weak
Weak
Strong
Middle
Very weak
Strong
Very weak
Middle
2.190
2.107
2.008
1.871
1.810
1.772
1.713
1.637
2
2
1
1
.110
.005
.889
.821
the reactor temperature was increased gradually (at a rate of about
◦
5
0 C/h) to 473 K. The reduction was carried out for 40 min. Then
the reactor was cooled in hydrogen flow to the room temperature,
the cup with reduced platinum was taken out of the reactor and
annealed to remove adsorbed hydrogen at 1000 K for 20–30 min.
Platinum was determined in the form of metal platinum using the
gravimetric method. Aqueous solutions from absorption vessels,
as well as rinsing water from the reactor with absorbed hydrogen
chloride, were transferred into a volumetric flask with a volume
of 100–200 ml. Chloride ions were determined by means of the
1.784
1.736
1
.637
ucts were investigated by means of X-ray phase analysis and IR
spectroscopy.
2
.3.2. Differential thermal analysis of the samples in sealed
ampoules
Differential thermal analysis of the samples in sealed ampoules
was carried out using a set-up described in ref. [10]. A sample (∼1 g)
and the reference (annealed aluminum oxide) were placed in ther-
mographic ampoules made of quartz glass; they were placed in an
potentiometric titration with 0.05N AgNO solution under standard
3
conditions.
Relative error of platinum determination was 0.2–0.5% (depend-
ing on the weighed portion), chlorine ± 0.5%.
X-ray investigation was carried out with DRON-2 diffractome-
ter with the filtered copper radiation (Cu K␣, Ni-filter) at a rate of
C/min.
◦
1
3
The IR spectra of samples were recorded with «PerkinElmer
Table 2
−
1
Experimental data of X-ray phase analysis for ␤- and ␣-forms of PtCl2.
25» instrument within the range 200–400 cm using the win-
dows made of CsI, protected with a polyethylene film; the samples
10–15 mg) were ground in a mortar with anhydrous vaseline oil.
comparison of the X-ray data for PtCl2, presented in
Tables 1 and 2, gives us reasons to state that the synthesized
products are identical to ␤- and ␣-forms of platinum dichloride,
respectively. The results of identification of the ␤- and ␣-forms (the
chemical analysis and the IR spectra) are shown in Table 3.
-form*
␣-form**
␤
(
´
´
Intensity
100
d␣ ( A˚ )
Intensity
d␣ ( A˚ )
A
6.87
6.56
100
80
10
5
20
10
5
6.56
6.39
6
0
6
0
4.745
4.040
3.786
3.267
3.038
2.955
2.885
2.690
2.637
2.543
2.475
2.368
2.292
2.186
2.122
2.006
1.877
1.821
1.781
1.636
5.471
3.969
3.262
3.175
3.106
2.719
2.582
2.556
2.238
2.215
2.108
1.985
1.845
1.821
1.781
1.701
1.656
1.623
1.578
1.539
2
1
5
Our results on the IR spectra of ␤-PtCl are in satisfactory agree-
15
5
2
ment with those reported in refs. [6–8]. The IR spectra of ␤- and
2
0
8
8
8
0
8
8
8
8
4
5
␣
-forms of PtCl2 have insignificant differences from each other.
20
20
10
10
5
15
15
10
10
5
Mainly the difference is in the ratios of intensities of the main
maxima of the ␤- and ␣-forms.
6
2.3. Investigation methods
2.3.1. Thermal annealing followed by quenching
The sample (∼0.1 g) was loaded into quartz ampoules with the
1
0
volume about 1 ml, the ampoules were evacuated at room tem-
perature and sealed. The ampoules were placed in an electric
resistance furnace in small metal containers (to achieve temper-
ature leveling) and kept at a fixed temperature for two weeks. After
the experiment, the containers were rapidly cooled by immersing
into cold water, the ampoules were opened, the resulting prod-
8
4
15
8
10
10
5
5
*
−3
−3
drö(␤-PtCl2) = 6.22 g cm
drö(␣-PtCl ) = 6.44 g cm
.
.
*
*
2

6
4
T.P. Chusova, Z.I. Semenova / Thermochimica Acta 482 (2009) 62–65
Table 3
Identification of platinum dichloride.
Chemical analysis (mass%)
Color
IR spectrum, ꢀ (cm−1)
␤
-PtCl2
Found
Olive green
Experimental data: 280 (shoulder), 302 (middle), 323 (middle), 327 (strong), 352
(
strong), 368 (shoulder).
Literature data: 329 (strong), 345 (strong), 357 (weak) [6], 200 (weak), 318 (very
strong) [7], 317 (strong) [8].
Pt 73.42
Calculated
Pt 73.34
Cl 26.47
Cl 26.66
␣
-PtCl2
Dark violet
Experimental data: 267 (weak), 304 (middle), 330 (strong), 350 (strong), 367
(
shoulder).
Found
Literature data: –
Pt 73.26
Calculated
Pt 73.34
Cl 26.67
Cl 26.66
aluminum unit. The unit was situated in the resistance furnace.
Linear heating was carried out with the help of the linear volt-
age device and a precision [11] isodromic thermoregulator PIT-3.
Temperature of the effects was recorded with the help of Pt–PtRh
thermocouples. A direct thermocouple was calibrated over melting
The IR spectra of the samples from three groups exhibit almost
no differences from each other. With an increase in annealing tem-
perature, only some changes are observed in the bands and in the
−
1
ratio of the intensities of the main maxima at 327 and 352 cm
(see Fig. 2 and Table 3).
and solidification temperatures of Sn, Pb and K Cr O7. Total error
So, it may be stated on the basis of the results of thermal anneal-
ing that the ␤ → ␣ transition occurs within temperature range
570–870 K and is accompanied by amorphization of the initial
structure. As temperature increases, the nucleation and accumu-
2
2
◦
of temperature measurement was estimated to be ± 1 C. Heating
◦
rates were varied within the range 5–10 C/min. Experiments were
carried out in sealed ampoules with a capillary to seal off chlorine.
Thermographic ampoules were filled with the substance in a dry
box in nitrogen; then the ampoules were evacuated and sealed.
2.3.3. Differential scanning calorimetry
Using instrument of “Setaram” company was investigated
curves of heating and cooling of ␣-PtCl2 in evacuated ampoule.
Instrument of «Setaram» company [12].
3. Discussion of results
Thermal annealing of ␤-PtCl2 followed by quenching demon-
strated changes in the character of diffraction patterns depending
on annealing temperature (Fig. 1). The samples may be convention-
ally divided into three groups.
The first group includes the samples of the initial and annealed
at 510 K ␤-PtCl2 (curves 1 and 2, respectively). These products are
similar in the appearance, color and have similar diffraction pat-
terns. The latter is an evidence of the fact that annealing at a
temperature up to 510 K causes no structural changes in ␤-form.
The second group includes the samples annealed at 590, 660,
760 K. The observed shifts of some most intensive maxima, disap-
pearance of a number of reflections and the appearance of two new
◦
◦
lines at 2ꢁ = 12.4 and 24.95 with the intensity increasing with an
increase in annealing temperature provide evidence of a rearrange-
ment occurring in the structure of ␤-PtCl . In addition, a number of
2
reflections belonging to the samples of the third group appear on
curve 5 (Fig. 1, annealing at 760 K). Their intensity increases as the
annealing temperature increases. The color of samples annealed
at 590 and 660 K is almost the same as that of the ␤-form, while
the product obtained after annealing at 760 K becomes dark brown
(
Table 3).
The third group includes the samples kept at 830 and 890 K.
The diffraction patterns of the samples (curves 6 and 7, Fig. 1)
kept at these temperatures almost coincide with those for the ␣-
form (curve 8). Some difference is observed for the sample annealed
◦
at 830 K: its diffraction patterns contain a maximum at 2ꢁ = 24.95 ,
belonging to the substances of the second group, while the max-
imum at 2ꢁ = 22.45 characteristic of the ␣-form. The samples of
the third group are dark violet.
Fig. 1. Diffraction patterns of the products of thermal annealing at a temperature
of: 510 K (2), 590 K (3), 660 K (4), 760 K (5), 830 K (6), 890 K (7). (1 and 8) Diffraction
patterns of ␤-PtCl2 and ␣-PtCl2 at room temperature, respectively.
◦

T.P. Chusova, Z.I. Semenova / Thermochimica Acta 482 (2009) 62–65
65
Fig. 3. Heating and cooling curves for ␣-PtCl2 (differential scanning calorimetry):
a) heating; (b) cooling.
(
Thermal annealing followed by quenching of ␣-PtCl2 showed that
this phase does not exhibit any changes of the crystal structure
during annealing at a temperature up to 900 K: the IR spectra and
diffraction patterns of the samples before and after annealing were
identical. This circumstance is likely to point to the monotropic
character of the transition ␤-PtCl → ␣-PtCl .
2
2
Fig. 2. IR spectra of the products of thermal annealing at a temperature of 510 K (2),
90 K (3), 660 K (4), 760 K (5), 830 K (6), 860 K (7), 890 K (8). (1 and 9) IR spectra of
-PtCl2 and ␣-PtCl2 at room temperature, respectively.
5
␤
References
[
[
[
1] U. Wiese, H. Schafer, Angew. Chem. 82 (1970) 135.
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3] G. Brauer, Handbuch der preparativen anorganischen Chemie, Ferdinand Enke
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may be conserved.
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The data on the temperature and time of ␤ → ␣ transition was
obtained also by means of differential thermal analysis of the sam-
ples in sealed ampoules within temperature range 500–900 K. The
appearance of the samples after experiment pointed to the pres-
ence of the ␣-form, which was then confirmed by the results of
X-ray phase analysis and IR spectroscopy. However, within the
instrument sensitivity limits, no thermal effects were observed on
S.S. Batsanov, E.D. Ruchkin, J. Inorg. Chem. U.S.S.R. 10 (1965) (Engl. Transl.).
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Khim. Nauk 4 (1974) 107–111 (Engl. Transl.).
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[
[
[
[
Khim. Nauk 2 (1974) 106–108 (Engl. Transl.).
␤
-PtCl2 thermograms.
With the help of the differential scanning calorimeter of
[11] Ya.V. Vasilyev, A.F. Neermolov, V.A. Gerasimov, Pribory i tekhnika eksperimenta
(1969) 187–190 (in Russian).
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999;
L.N. Zelenina, T.P. Chusova, Yu.G. Stenin, V.V. Bakovets, J. Phys. Chem. 80–83
2006) (Engl. Transl.).
5
[
«Setaram» [12] company within temperature range 570–900 K, a
1
reversible phase transition was discovered for ␣-PtCl2 at a tem-
perature of 660 ± 5 K and thermal effect +167 ± 17 J/mol (Fig. 3).
(