Influence of atmosphere kind on temperature programmed decomposition of noble metal chlorides

Article abstract of DOI:10.1016/j.tca.2005.05.006

The hydrated chlorides of noble metals located in the 8-10 groups of Periodic System were used in this work: Ru, Rh, Pd, Ir and Pt. The common mechanism of chloride compounds decomposition in oxidative, inert and reductive atmospheres was postulated and mutual similarities and differences were discussed. These compounds play a significant role in heterogeneous catalysis, as starting materials for metal/oxide catalysts, and also in analytical chemistry and industry.

Full text of DOI:10.1016/j.tca.2005.05.006

Thermochimica Acta 435 (2005) 151–161
Influence of atmosphere kind on temperature programmed
decomposition of noble metal chlorides
∗
´´
Wojciech K. Jozwiak, Tomasz P. Maniecki
Institute of General and Ecological Chemistry, Technical University of Ło´dz´,
˙
Zeromskiego 116, 90-924 Ło´dz´, Poland
Received 5 February 2005; received in revised form 29 April 2005; accepted 4 May 2005
Available online 29 June 2005
Abstract
The hydrated chlorides of noble metals located in the 8–10 groups of Periodic System were used in this work: Ru, Rh, Pd, Ir and Pt.
The common mechanism of chloride compounds decomposition in oxidative, inert and reductive atmospheres was postulated and mutual
similarities and differences were discussed. These compounds play a significant role in heterogeneous catalysis, as starting materials for
metal/oxide catalysts, and also in analytical chemistry and industry.
© 2005 Elsevier B.V. All rights reserved.
1. Introduction
cheaper than metal nitrates, play an important role in many
stages of catalyst preparation: kind and conditions of proce-
dure, activation, reduction and finally, catalytic activity [4].
Chlorine “free” metal/oxide support catalysts usually behave
differently from those containing chlorine ions. In the case
of alumina, silica and titania supported metal catalysts, even
the high temperature range of treatment (800–1000 ◦C) does
not assure chlorine free catalyst surface. Decomposition of
chlorine-organic compounds, oxy-dechlorination and hydro-
dechlorination (HCFC, dioxins, PCV and others) are carried
out on noble metal/support catalysts [5–10]. Also the effect
of chlorine on catalytic activity in oxidation of methane [11],
reduction of unsaturated aldehydes [12] and WGS reaction
[13].
Relatively little is known about the mechanism of thermal
decomposition of noble metal chlorides. Shaplygin and
Lazarev have investigated the behavior of Ru, Rh and Pd
chlorides in air atmosphere but the intermediate products
of decomposition were not reported [1]. Peczkowski
and Vorobiev [6] published some details referring to the
mechanistic approach to iron II and III chlorides decom-
position in air. Papers by Livingstone [7] and Ginzburg
[8] devoted to analytical chemistry of noble metals do not
contain information about the influence of temperature and
atmosphere on metal chlorides decomposition. This work
Thermal stability of the 8–10 group noble metal chlo-
rides was the subject of many papers in the last 50 years
[1,2]. Most papers refer only to individual compounds, nar-
row range of temperature—usually lower than 500 ◦C and air
atmosphere. However, there is still a shortage of comparative
study describing the thermal behavior of metal chlorides in
the wide temperature range (up to 1500 ◦C) and in differ-
ent atmospheres: neutral—helium or argon, oxidative—air
or oxygen and reductive—hydrogen or carbon monoxide.
Chlorides play a significant role in heterogeneous catal-
ysis, as starting materials for metal/oxide catalysts, also in
analytical chemistry and industry. Knowledge of physic-
ochemical properties of noble metal chlorides is closely
related to many aspects of applied and pure catalysis. Noble
metal chlorides and nitrates are widely used as precursors
for active metal phase highly dispersed on oxide supports.
Also, physico-chemical properties of metal/oxide catalysts
are strongly influenced by the presence of chlorine ions
involved in metal–oxide interactions [3]. Metal chlorides,
∗
Corresponding author. Tel.: +48 426313125; fax: +48 426313127.
E-mail addresses: wjozwiak@mail.p.lodz.pl (W.K. Jo´z´wiak),
tmanieck@mail.p.lodz.pl (T.P. Maniecki).
0040-6031/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2005.05.006

152
W.K. Jo´z´wiak, T.P. Maniecki / Thermochimica Acta 435 (2005) 151–161
Table 1
Hydrated chlorides of noble metals from 8 to 10 groups of Periodic System
No.
Compounds
Formula
Producer
1
2
3
4
5
Ruthenium III chloride
Rhodium III chloride
Palladium II chloride
Iridium III chloride
Platinum IV chloride
RuCl3·nH2O
RhCl3·3H2O
PdCl2
Koch–Light Laboratory
Koch–Light Laboratory
POCH–Gliwice
Merck
IrCl3·3H2O
PtCl4·5H2O
Aldrich
is devoted to the comparative study of thermal behavior of
the noble metal hydrated chlorides: Ru, Rh, Pd, Ir and Pt in
the wide temperature range, up to 1500 ◦C and in different
atmospheres: oxidative, neutral and reductive.
2. Experimental
2.1. Materials
Thecommercialcompounds—noblemetalchlorides, used
in this work, are presented in Table 1 containing name, chem-
ical formula and producer.
2.2. Methods
Thermo-gravimetric TG method, equipped with differen-
tial thermal analysis DTA device SETSYS-16/18 (Setaram)
and combined in line with quadrupole mass spectrometer MS
Thermostar(Balzers)wereusedfortemperatureprogrammed
decomposition of noble metal chlorides in three different
gaseous atmospheres: oxidative 2% O2–98% Ar, neutral Ar
5.0 and reductive 2% H2–98% Ar. The TG–DTA–MS mea-
surements were carried out applying sample weight about
20 mg, linear heating rate of 10 ◦C/min, temperature range
from 25 up to 1500 ◦C. Most samples were used without any
preliminary treatment with the exception of ruthenium III
chloride—RuCl3·nH2O pretreated in vacuum at 50 ◦C. The
order of TG–DTA–MS measurements is the same as the loca-
tion of noble metals in the 8–10 groups of Periodic System
i.e. Ru, Rh, Pd, Ir and Pt, respectively.
Fig. 1. Thermal decomposition of RuCl·nH2O in oxidative atmosphere (2%
O2–98% Ar).
tion is at least a three-step process ending at about 400 ◦C
and comprising about 21% of mass sample decrease. In the
range of temperature 400–700 ◦C the first stage of sample
dechlorination–oxidation takes place manifesting the evolu-
tion of HCl (m/z = 36) and the consumption of O2 (m/z = 32).
The second stage of dechlorination occurs in temperature
range 700–850 ◦C and is accompanied by rather strong DTA
endothermic effect and narrow but comparable evolution
of HCl (m/z = 36) and Cl2 (m/z = 70). Oxygen and water
uptakes can be observed O2 (m/z = 32) and H2O (m/z = 18),
respectively. The weight decrease in this region was about
28%. Above 1000 ◦C sample de-oxidation takes place and
this process is illustrated by TG, DTG changes (ꢀm about
12.9%) and a strong endothermic DTA effect accompanied
by MS oxygen evolution (m/z = 32). These results confirm
RuCl3 → RuO2 transformation with the increase of ruthe-
nium oxidation number Ru(III) → Ru(IV) occurring during
dechlorination-oxidation stage. The evolution of oxygen rep-
resents the last stage of sample decomposition above 1000 ◦C
RuO2 → Ru0 + O2. The total change of sample mass was
about 61.6%. The following sequence of successive events
3. Results
3.1. Ruthenium III chloride—RuCl3·nH2O
The decomposition of ruthenium III chloride RuCl3·nH2O
in oxidative atmosphere 2% O2–98% Ar is presented in
Fig. 1. Curves TG, DTG and DTA are located in upper
part (a), whereas MS profiles (m/z = 18—H2O, m/z = 32—O2,
m/z = 36—HCl, m/z = 70—Cl2) are located in lower part (b)
of this figure. Sample decomposition starts at about 50 ◦C
and the changes of TG and DTG curves are accompanied by
one strong and two much smaller endotermic DTA effects.
The evolution of water represented by: MS profile (m/z = 18),
TG, DTG and DTA curves shows that sample dehydra-

W.K. Jo´z´wiak, T.P. Maniecki / Thermochimica Acta 435 (2005) 151–161
153
Fig. 3. ThermaldecompositionofRuCl3·nH2Oinreductiveatmosphere(2%
H2–98% Ar).
Fig. 2. Thermal decomposition of RuCl3·nH2O in inert atmosphere (Ar).
The decomposition of ruthenium III chloride RuCl3·nH2O
inreductiveatmosphere2%H2–98%ArispresentedinFig. 3.
The character of reductive decomposition of hydrated ruthe-
nium chloride is completely different. Thermally induced
sample reduction is accomplished up to 400 ◦C and the dehy-
dration above 100 ◦C represented by MS profile of water
(m/z = 18) is heavily overlapped with reduction of RuCl3
taking place in temperature range 200–400 ◦C. The reduc-
tion of RuCl3 seems to be represented by two-step reduction
described by DTG curve and MS profiles for H2 consumption
(m/z = 2) and HCl evolution (m/z = 36). The total decrease in
sample weight about 60% is in a good accordance with that
obtained in oxidative atmosphere being about 62%.
can be anticipated: dehydration, dechlorination plus oxida-
tion and finally thermal dissociation of ruthenium oxide lead-
ing to metal phase.
The decomposition of ruthenium III chloride RuCl3·nH2O
in neutral atmosphere Ar is presented in Fig. 2 and its
comparison with Fig. 1 suggests a close resemblance of
hydrated RuCl3 decomposition in inert and oxidative atmo-
spheres. In Fig. 2 three stages of decomposition are also
visible: first, three-step dehydration up to 400 ◦C, second,
two step dechlorination–oxidation up to 800 ◦C and third
one de-oxidation above 1000 ◦C. The exothermic DTA effect
at 570 ◦C is assigned to crystallization process of amor-
phous fairly dehydrated ruthenium chloride RuCl3. The tem-
perature programmed sequence of events is confirmed by
the MS profiles representing successively the evolution of
H2O, HCl, Cl2 and finally O2. The presence of oxygen in
RuCl3 sample may originate from a successive replacement
reaction Cl− → OH− → O2− taking place during sample
dechlorination–reoxidation process leading to RuOx forma-
tion. The evolution of oxygen represents the last stage of sam-
ple decomposition above 1000 ◦C RuOx → Ru0 + (x/2)O2.
3.2. Rhodium III chloride—RhCl3·3H2O
The decomposition of rhodium III chloride RhCl3·3H2O
inoxidativeatmosphere2%O2–98%ArispresentedinFig. 4.
The process starts at about 100 ◦C and the changes of TG and
DTG curves are accompanied by DTA peaks: three endother-
mic (tmin about 170, 295 and 400) and one exothermic (tmax
about 510), respectively. The corresponding effects of water
evolution (m/z = 18) represents two-step sample dehydration

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W.K. Jo´z´wiak, T.P. Maniecki / Thermochimica Acta 435 (2005) 151–161
Fig. 4. ThermaldecompositionofRhCl3·3H2Oinoxidativeatmosphere(2%
Fig. 5. Thermal decomposition of RhCl3·3H2O in inert atmosphere (Ar).
O2–98% Ar).
up to 500 ◦C followed by intermediate crystallization at
about 500 ◦C (exothermic DTA effect) and completion
of dehydration combined with the first stage of chloride
phase decomposition (530–640 ◦C). Sample dechlorination-
oxidation takes place in temperature range 530–930 ◦C as
a two-step process, comprising ꢀm values 6.5 and 38.1%,
respectively. The examination of MS profiles shows that
the first stage of RhCl3 decomposition involves dechlori-
nation according to reaction H2O + Cl− → HCl + OH− or
OH− + Cl− → HCl + O2−, manifesting the evolution of HCl
(m/z = 36), whereas the second stage is rather dominated
by reaction 0.5O2 + 2Cl− → Cl2 + O2−. The simultaneous
evolution of HCl and Cl2 products (m/z = 36) and Cl2
(m/z = 70) and the consumption of H2O and O2 reagents
(m/z = 18 and m/z = 32) confirm the above pathways of
dechlorination process. Both steps of dechlorination are
accompanied by rather strong DTA endothermic peaks (tmin
about 540, and 850). Above 900 ◦C sample de-oxidation
takes place and this process is illustrated by TG, DTG
changes (ꢀm = 6.2%) and a strong endothermic DTA effect
accompanied by MS oxygen evolution (m/z = 32). The total
change of sample mass was about 56.9%. The following
sequence of temperature programmed successive events
of RhCl3·3H2O decomposition can be anticipated: dehy-
dration, crystallization, dechlorination plus oxidation and
final dissociation of ruthenium oxide resulting in metallic
rhodium phase.
The decomposition of rhodium III chloride RhCl3·3H2O
in neutral atmosphere Ar is presented in Fig. 5 and this
process is very similar to decomposition described above,
in oxidative atmosphere (Fig. 4). Instead of transfor-
mation RhCl3 into Rh2O3 the lack of gaseous oxygen
favours the direct decomposition according to reaction like
RhCl2OH → Rh0 + HCl + 0.5Cl2 + 0.5O2. This process is
accomplished up to 900 ◦C and the total weight loss is
practically the same (ꢀm = 56.8%). On the basses of the
data of Fig. 5 the sequence of temperature programmed
events can be as follow: dehydration, crystallization and
finally dechlorination leading directly to metallic rhodium
phase Rh0. The proposed mechanism is confirmed by
the MS profiles representing successively the evolution
of H2O, HCl, Cl2 and O2. The presence of oxygen or
hydroxyl group into RuCl3 crystal phase may originate
from replacement reaction Cl− + H2O → OH− + HCl or

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155
Fig. 7. Thermal decomposition of PdCl2 in oxidative atmosphere (2%
O2–98% Ar).
Fig. 6. ThermaldecompositionofRhCl3·3H2Oinreductiveatmosphere(2%
H2–98% Ar).
3.3. Palladium II chloride—PdCl2
dehydration reaction 2OH− → O2− + H2O taking place
during the first step of simultaneously occurring sample
dechlorination–dehydration process [1].
The decomposition of palladium II chloride PdCl2 in
oxidative atmosphere 2% O2–98% Ar is presented in Fig. 7.
The process of water removal taking place in the temperature
range 100–300 ◦C and the changes of TG and DTG curves
are accompanied by two endothermic DTA peaks: (tmin about
160 and 240 ◦C). Although palladium II chloride suppos-
edly was nominally water free but the investigated sample
appeared to contain about 2% of water. Two narrow endother-
mic DTA peaks (tmin about 300 and 410 ◦C) were attributed to
crystal phase transformation of PdCl2. The one stage of palla-
dium chloride phase transformation into metallic palladium
phase takes place in temperature range 600–800 ◦C. Sam-
ple dechlorination results in weight loss ꢀm about 40.6%.
The examination of MS profiles shows that transformation
PdCl2 → Pd0 involves water uptake from gaseous atmo-
sphere and the evolution of O2, HCl and relatively small
amountofCl2. Suchprocesscanbedescribedbygeneralreac-
tion PdCl2 + H2O → Pd0 + HCl + 0.5O2 + 0.5Cl2. The fol-
lowing sequence of temperature programmed decomposition
of PdCl2·nH2O can be anticipated: dehydration, crystal phase
The decomposition of rhodium III chloride RhCl3·3H2O
in reductive atmosphere 2% H2–98% Ar is presented in
Fig. 6. As it was expected the character of reductive decom-
position of hydrated rhodium chloride is completely differ-
ent from that characteristic of decomposition in oxidative
and neutral atmospheres (compare Figs. 4 and 5). Tem-
perature programmed reduction of RhCl3 is represented by
MS profiles of H2 consumption (m/z = 2) and HCl evo-
lution (m/z = 36). The dehydration process starting above
100 ◦C and represented by MS profile of water (m/z = 18)
is heavily overlapped with RhCl3 reduction taking place
in temperature range 200–400 ◦C. The total loss of sam-
ple weight about 56.9% is in a good accordance with
the stoichrometric value 60.8% calculated on the basis of
equation RhCl3·3H2O + 1.5H2 → Rh + 3H2O + 3HCl. Close
resemblance of reduction pathway for hydrated rhodium and
ruthenium chlorides is rather undoubtful (compare Fig. 3).

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W.K. Jo´z´wiak, T.P. Maniecki / Thermochimica Acta 435 (2005) 151–161
Fig. 8. Thermal decomposition of PdCl2 in inert atmosphere (Ar).
Fig. 9. Thermal decomposition of PdCl2 in reductive atmosphere (2%
H2–98% Ar).
transformation and finally dechlorination leading to metallic
palladium phase.
of water (m/z = 18) is heavily overlapped with PdCl2 reduc-
tion. The total loss of sample weight was about 37.8% being
in a good accordance with that stoichrometric value 40.0%
calculated on the basis of equation PdCl2 + H2 → Pd + 2HCl.
Again, a close resemblance of hydrogen reduction pathway
for rhodium, ruthenium and palladium chlorides is un doubt-
ful (compare Figs. 3, 6 and 9).
The decomposition of palladium II chloride PdCl2 in inert
Ar atmosphere is presented in Fig. 8. The process of water
removal taking place in the temperature range 100–300 ◦C
and the changes of TG and DTG curves are accompanied by
endothermic DTA peak: (tmin about 110 ◦C). Although palla-
dium II chloride was to be water free but investigated sample
appeared to contain about 2% of water. Two narrow endother-
mic DTA peaks (tmin about 305 and 410 ◦C) were attributed to
crystal phase transformation of PdCl2. The one stage of pal-
ladium chloride phase transformation into metallic palladium
phase takes place in temperature range 600–800 ◦C. Sample
dechlorination results in weight loss ꢀm about 39.9%. The
examination of MS profiles shows the same kind of transfor-
mation as during PdCl2 → Pd0 decomposition in oxidative
2% O2–98% Ar atmosphere (compare Figs. 7 and 8).
The reduction of palladium II chloride PdCl2 in reductive
atmosphere 2% H2–98% Ar is presented in Fig. 9. Tem-
perature programmed reduction starts at about 50 ◦C and
is accomplished bellow 300 ◦C being represented by both
TG, DTG, DTA curves and MS profiles for H2 consump-
tion (m/z = 2) and HCl evolution (m/z = 36). The dehydration
process starting above 50 ◦C and represented by MS profile
3.4. Iridium III chloride—IrCl3·3H2O
The decomposition of iridium III chloride IrCl3·3H2O
in oxidative atmosphere 2% O2–98% Ar is presented in
Fig. 10. The entire process takes place up to 1200 ◦C and
it can be divided into three not overlapped temperature
regions: dehydration 50–400 ◦C, dechlorination–reoxidation
650–750 ◦C and dissociative deoxidation of iridium
oxide—above 900 ◦C. The corresponding sample weight
losses are presented in Fig. 10. The total mass decrease
of sample was about 50.1%. The above assignment of
IrCl3·3H2O decomposition was done on the changes of TG,
DTG curves and DTA peaks and confirmed by MS profiles
of gaseous products H2O, HCl, O2 and Cl2. The additional

W.K. Jo´z´wiak, T.P. Maniecki / Thermochimica Acta 435 (2005) 151–161
157
Fig. 11. Thermal decomposition of IrCl3·3H2O in inert atmosphere (Ar).
Fig. 10. ThermaldecompositionofIrCl3·3H2Oinoxidativeatmosphere(2%
O2–98% Ar).
IrCl3 + H2O → IrCl2OH + HCl seems to be responsible
for the first replacement stage of subsequent sequence
of events Cl− → OH− → O2− → (1/2)O2 taking place in
the first coordination sphere of iridium ion Ir3+. Thus,
the partial transformation of iridium chloride into none
stoichrometric iridium oxide IrOx results from the equation
Ir(OH)xCl6−x → IrOx + xHCl + (3 − x)Cl2 describing the
stoichrometry of octahedral coordination of iridium ion.
The dissociative decomposition of IrOx phase takes place
at much lower temperature (about 100 ◦C) than in the
oxidative atmosphere (compare Fig. 10). Total loss of
sample weight was about 50% what is not only similar
to the value obtained in oxidative atmosphere but also in
accordance with the stoichrometry of following equation:
IrCl3·3H2O → Ir0 + 3H2O + (3/2)Cl2.
DTA exothermic effect (tmax about 590 ◦C) is assigned to
crystallization process of amorphous IrCl3 phase. This crys-
tallic product undergoes further transformation according to
general reaction IrCl3 + 1.5H2O + 0.25O2 → IrO2 + 3HCl.
In fact dechlorination–oxidation process IrCl3 + 1.5H2O →
1.5Ir2O3 + 3HCl taking place in temperature range
550–700 ◦C (TG decrease) is followed by an addi-
tional oxidation 1.5Ir2O3 + 0.5O2 → IrO2 in temperature
range 700–800 ◦C (TG increase). Thus, oxygen in iridium
oxide phase originates both from gaseous O2 molecules
and water or OH hydroxyl groups. Above 900 ◦C thermal
dissociation takes place IrO2 → Ir0 + O2. Generally, the
sequence of temperature programmed successive events of
IrCl3·3H2O decomposition can be anticipated: dehydration,
recrystallization, dechlorination plus oxidation and final
dissociation of iridium oxide leading finally to metallic
iridium phase.
The process of iridium III chloride IrCl3·3H2O in reduc-
tive atmosphere 2% H2–98% Ar is presented in Fig. 12. Once
again it is very similar to the reduction profiles of described
above ruthenium, rhodium and palladium chlorides (compare
Figs. 3, 6 and 9). Temperature programmed reduction of IrCl3
begins above 200 ◦C and is accomplished bellow 350 ◦C and
is represented by MS profiles for H2 consumption (m/z = 2)
and HCl evolution (m/z = 36). The dehydration process
The decomposition of iridium III chloride IrCl3·3H2O
in inert Ar atmosphere is presented in Fig. 11. The
process takes place up to 1000 ◦C and the only con-
siderable differences result from the obvious lack of
oxygen in gaseous atmosphere. The intermediate reaction

158
W.K. Jo´z´wiak, T.P. Maniecki / Thermochimica Acta 435 (2005) 151–161
Fig. 12. ThermaldecompositionofIrCl3·3H2Oinreductiveatmosphere(2%
Fig. 13. Thermal decomposition of PtCl4·5H2O in oxidative atmosphere
H2–98% Ar).
(2% O2–98% Ar).
starting above 100 ◦C and represented by MS profile of water
(m/z = 18) is heavily overlapped with IrCl3 reduction. The
total loss of sample weight about 44.5% is in a good accor-
dance with that stoichrometric value 46.1% calculated on the
basis of equation IrCl3·3H2O + 1.5H2 → Ir0 + 3H2O + 3HCl.
MS profiles of gaseous products H2O, HCl, O2 and Cl2. The
successive stages can be described by following equations:
(1) four-step dehydration: PtCl4·5H2O → PtCl4·H2O +
4H2O, ꢀm = 18.6%;
(2) dechlorination: PtCl4·H2O + (1/2)O2 → PtCl2O1/2(OH)
3.5. Platinum IV chloride—PtCl4·5H2O
+ HCl + (1/2)Cl2, ꢀm = 15.3%;
(3) dechlorination–reduction:
PtCl2O1/2(OH) → Pt0 + HCl + (1/2)Cl2 + (3/4)O2,
The decomposition of platinum IV chloride PtCl4·5H2O
in oxidative atmosphere 2% O2–98% Ar is presented
in Fig. 13. The entire process takes place in tempera-
ture range 100–550 ◦C, much narrower than those found
for described above ranges of decompositions of other
noble metal chlorides. The process of complete decom-
position PtCl4·5H2O → Pt + 2Cl2 + 5H2O can be divided
into three isolated temperature regions: dehydration pro-
cess 100–300 ◦C and completely separated two stages
of dechlorination-oxygenation and dechlorination-reduction
processes 280–400 ◦C and 420–550 ◦C, respectively. The
total decrease of sample mass was about 54.1%. The above
assignment of PtCl4·5H2O decomposition was based on the
changes of TG, DTG curves and DTA peaks and confirmed by
ꢀm = 17.5%.
In fact, stage 2 is a partial replacement of chlorine ions
by oxygen atom taken from the first coordination sphere of
Pt4+ whereas final stage 3 leads to decomposition of Pt(IV)
complex into metallic platinum and gaseous products: HCl,
Cl2 and O2. The total decrease of sample mass is about
51.4% being the sum of the above ꢀm values and is in a
good accordance with stoichrometry of PtCl4·5H2O com-
plete decomposition ꢀmth = 54.2%.
The decomposition of platinum IV chloride PtCl4·5H2O
in inert Ar is presented in Fig. 14 and it is very similar
to that experiment carried out in oxidative atmosphere 2%

W.K. Jo´z´wiak, T.P. Maniecki / Thermochimica Acta 435 (2005) 151–161
159
Fig. 14. Thermal decomposition of PtCl4·5H2O in inert atmosphere (Ar).
Fig. 15. Thermal decomposition of PtCl4·5H2O in reductive atmosphere
(2% H2–98% Ar).
O2–98% Ar (compare Fig. 13). Once again, dehydration
is four-step process and the accompanying mass loss ꢀm
is about 18.6% The remarkable differences are limited to
the source of origin for oxygen evolution in last stage of
PtCl4·5H2O decomposition, taking into account the absence
of oxygen in argon atmosphere and very small contribution
of chlorine in gaseous products observed during both stages
of platinum chloride dechlorination. The proposed explana-
tion is supported on the larger involvement of water in the first
stage of dechlorination. Thus, both water is the source of oxy-
gen ligands in first coordination sphere of platinum ion Pt4+
and the same water molecule is responsible for HCl forma-
tion. The first stage of dechlorination should be modified in
following way: PtCl4 + H2O → PtCl2O + 2HCl, ꢀm = 15.3%
whereas the final stage of decomposition according to equa-
tion: PtCl2O + H2O → Pt0 + 2HCl + O2, ꢀm = 17.5%. Such
approach also explains the presence of small amount of
chlorine in gaseous products of platinum chloride decompo-
sition in inert atmosphere. The total decrease of sample mass
being the sum of the above ꢀm values and it is in a good
accordance not only with the stoichrometry of PtCl4·5H2O
complete decomposition ꢀmth = 54.2% but also with value
51.4% obtained for oxidative atmosphere.
The process of platinum IV chloride PtCl4·5H2O in reduc-
tive atmosphere 2% H2–98% Ar is presented in Fig. 15.
Reduction behavior is very similar to the reduction pro-
files of described above ruthenium, rhodium, palladium and
iridium hydrated chlorides (compare Figs. 3, 6, 9 and 12).
Temperature programmed reduction of platinum IV chloride
PtCl4·5H2O occurs between 50 and 350 ◦C and is repre-
sented by MS profiles for H2 consumption (m/z = 2) and
HCl evolution (m/z = 36). The dehydration process begins
above 100 ◦C and takes place up to 350 ◦C being heavily
overlapped with PtCl4 reduction. The total loss of sample
weight about 54.2% is in a good accordance with that sto-
ichrometric value 52.4% calculated on the basis of equation
PtCl4·5H2O + 2H2 → Pt + 2H2O + 3HCl.
4. Discussion
The comparative study of thermal behavior of the noble
metal hydrated chlorides Ru, Rh, Pd, Ir and Pt in the wide
temperature range, up to 1500 ◦C and in different atmo-
spheres: oxidative, neutral and reductive has been presented.

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The process of decomposition in oxygen appeared very sim-
ilar to decomposition in inert atmosphere. In both cases
thermal decomposition of noble metal hydrated chloride
Mex+Clx·nH2O is characterized by several following inter-
mediate processes:
canbedifferentdependingonbothoxygencontentanddegree
of compound dehydration in the system.
Assuming the octahedral coordination of hydrated noble
metal chlorides the general pathway of Mex+Clx·nH2O
decomposition in oxygen and argon atmospheres can be
described by following reaction equations:
1. Several-step successive dehydration Mex+Clx·nH2O →
Mex+Clx·pH2O + (n − p)H2O combined with the forma-
tion of amorphous metal chloride phase in temperature
range from 100 up to about 500 ◦C.
(1) dehydration:
Mex+Clx·nH2O → Mex+Clx(H2O)6−x
+
(n + x − 6)H2O;
(2) partial dechlorination in argon: Mex+Clx(H2O)6−x
→
2. Crystallization of amorphous Mex+Clx·pH2O taking place
in temperature range 500–600 ◦C for Ru, Rh and Ir
(exothermic peaks on DTA curve).
Mex+Cly(OH)6−y + (x − y)HCl;
(3) partial dehlorination in oxygen: Mex+Clx(H2O)6−x
+ (z/4)O2 → Mex+Clx−z
Oz/2(OH)6−y + (x − y)HCl +
3. One-stepforPdandIrortwo-stepforRuandRhdecompo-
sition of Mex+Clx·pH2O in temperature range 550–900 ◦C
depending on the kind of noble metal. In oxygen atmo-
sphere only metallic Pd and metal oxides: RuO2, Rh2O3
and IrO2 are formed.
4. Thermally induced dissociation of MeOx/2 into Me metal
and oxygen occurs above 950 ◦C for Rh2O3 and IrO2 and
above 1100 for RuO2.
(z/2)Cl2;
(4) dissociation in argon: Mex+Cly(OH)6−y → Me0 +
(6 − y)HCl + (y/3)Cl2 + (3 − y/2)O2;
(5) dechlorination–oxidation:
Mex+Clx−z
Oz/2(OH)6−y
+
((x − z)/2)O2 → Mex+Ox/2 + (6 − y)HCl +
((x + y − z − 6)/2)Cl2;
(6) thermal dissociation: Mex+Ox/2 → Me0 + (x/4)O2.
The above mechanism ignores the possibility of the
increase of oxidation number of metal atom in oxygen
treatment of metal chlorides as it was observed during
dechlorination-oxidation stage of Ru3+Cl3 → Ru4+O2 and
Ir3+Cl3 → Ir4+O2 displacement transformations.
Generally, the mechanism of platinum IV chloride
PtCl4·5H2O decomposition in oxidative O2 or inert atmo-
sphere is analogical to above findings. The only differ-
ence refers to much lower temperature range of PtCl4·5H2O
decomposition 100–500 ◦C. In the case of palladium II chlo-
ride two additional endothermic effects were observed and
those peaks were assigned to crystallic PdCl2 phase trans-
formations [10]. Finally, temperature dependent formation
of metallic Pt and Pd and metal oxides: RuO2, Rh2O3 and
IrO2 reflects increasing order of noble metal oxide thermal
stability: Pt < Pd < Ru < Rh < Ir.
The differences between decomposition of hydrated noble
metal chlorides Mex+Clx·nH2O in oxidative O2 and inert Ar
atmospheres result from different level of gaseous oxygen
content. In oxygen-rich atmosphere the sequence of suc-
cessive events in coordination sphere of noble metal ion
is following: Cl− → H2O → OH− → O2− → O2. Final oxy-
gen ions may originate both from gaseous oxygen and water
molecules or hydroxyl groups. The decomposition pathway
The approach based on the above stages of metal hydrated
chlorides comprises the characteristic sequence of simple
successive steps: Cl− (H2O) → OH− → O2− → O2 occur-
ringinthefirstcoordinationsphereofMex+ ionleadingfinally
to metallic phase formation. The quantitative comparison of
the experimental and theoretical data is presented in Table 2.
The evaluation is based on simple three stage decomposition
sequence consisting of following partial processes: dehy-
dration, dechlorination–oxidation and final de-oxidation. An
acceptable mutual accordance between experimental and the-
oreticaldateconfirmsthevalidityofpostulatedgeneralmech-
anism of noble metal hydrated chlorides.
The hydrogen reduction of noble metals Ru, Rh, Pd, Ir and
Pt hydrated chlorides can be described by two simultaneously
occurring processes according to reaction equations:
Table 2
Theoretical and experimental weight loses for hydrated chlorides from 8 to 10 groups of Periodic System
Compound
ꢀm
Dehydration (%)
Dechlorination–oxidation (%)
Deoxydation (%)
Total (%)
RuCl3·3H2O
Theoretical
Experimental
20.6
20.7
28.5
28.0
12.2
12.9
61.3
61.6
RhCl3·3H2O
PdCl2
Theoretical
Experimental
20.4
12.6
31.3
38.1
9.1
6.2
60.8
56.9
Theoretical
Experimental
–
1.2
40.0
40.6
–
–
40.0
41.8
PtCl4·5H2O
IrCl3·3H2O
Theoretical
Experimental
21.0
18.6
16.6, 16.6
15.3, 17.5
–
–
54.2
51.4
Theoretical
Experimental
15.3
15.4
23.3
26.5
9.0
8.3
47.6
50.2

W.K. Jo´z´wiak, T.P. Maniecki / Thermochimica Acta 435 (2005) 151–161
161
• dehydration: Mex+Clx·nH2O → Mex+Clx + nH2O;
• reduction: Mex+Clx + (x/2)H2 → Me0 + xHCl.
ofThermalAnalysisIChOEcol. TechnicalUniversityofLodz
for carrying out some of TG–DTA–MS measurements.
The hydrogen reduction of hydrated noble metal chlorides
Mex+Clx·nH2O depends only slightly on the kind of metal.
All hydrated chlorides are easily reducible in temperature
range from 50–200 up to 300–400 ◦C depending on the kind
of noble metal. Dehydration process is heavily overlapped
with hydrogen reduction. Despite the obvious similarities the
order of decreasing affinity of reduction in hydrogen is fol-
lowing: Pd > Pt > Rh > Ru > Ir.
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Deeper insight into thermal decomposition of the noble
metal hydrated chlorides Ru, Rh, Pd, Ir and Pt was achieved
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Acknowledgements
This work was partially supported by Grant No. 4 T09B
146 25 of the State Committee for Scientific Research. The
´
authors are indebted to Mr. Witold Sowinski from Laboratory
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