[PtCl6] and [Cd(NH3)4][PtCl6] as precursors for intermetallic compounds PtZn and PtCd

Article abstract of DOI:10.1134/S0036023607040067

Double complex salts (tetraamminezinc and tetraamminecadmium hexachloroplatinates) have been synthesized. Their thermal properties have been studied, as well as the products of their degradation in hydrogen and helium atmospheres. Optimal thermolysis sche

Full text of DOI:10.1134/S0036023607040067

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 4, pp. 500–504. © Pleiades Publishing, Inc., 2007.
Original Russian Text © A.V. Zadesenets, A.B. Venediktov, Yu.V. Shubin, S.V. Korenev, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 4, pp. 556–560.
COORDINATION COMPOUNDS
[
Zn(NH ) ][PtCl ] and [Cd(NH ) ][PtCl ] as Precursors
3
4
6
3 4
6
for Intermetallic Compounds PtZn and PtCd
a
A. V. Zadesenets , A. B. Venediktov, Yu. V. Shubin, and S. V. Korenev
Nikolaev Institute of Inorganic Chemistry, Siberian Division, Russian Academy of Sciences,
pr. Akademika Lavrent’eva 3, Novosibirsk, 630090 Russia
*
e-mail: zadesenez@ngs.ru
Received April 18, 2006
Abstract—Double complex salts (tetraamminezinc and tetraamminecadmium hexachloroplatinates) have been
synthesized. Their thermal properties have been studied, as well as the products of their degradation in hydro-
gen and helium atmospheres. Optimal thermolysis schedules have been determined. Thermolysis under hydro-
gen yields intermetallic compounds PtZn and PtCd.
DOI: 10.1134/S0036023607040067
Catalysis is now the most extensive application field
of the platinum-group metals, consuming more than
one-half of the world-wide production of platinum [1].
Catalysts are often supported nanosized platinum met-
als. A partial substitution of a nonprecious metal for a
platinum metal significantly lowers the catalyst cost,
and it is pertinent to search for new supported catalysts
based on solid solutions of precious and nonprecious
metals. In addition, multicomponent supported metal
systems have several strengths over one-component
systems due to synergism. In our earlier work [2], we
showed for several double complexes of platinum met-
als that their reductive thermolysis under a hydrogen
atmosphere, in many cases, yields metal solid solutions
of various dispersions.
EXPERIMENTAL
The starting K [PtCl ] was prepared by a standard
2
6
procedure [3] from hydrochloroplatinic acid (chemi-
cally pure grade).
The double complex salts (DCSs) were synthesized
at room temperature. Solutions of tetraamminezinc and
tetraamminecadmium were prepared by adding 25%
aqueous ammonia to saturated aqueous solutions of
zinc chloride or cadmium perchlorate until the nascent
hydroxides completely dissolved. To the resulting solu-
tions, a 0.02 M K [PtCl ] solution was added so that the
2
6
amount of zinc or cadmium was 10% over the 1 : 1 sto-
ichiometry. Yellow precipitates appeared instanta-
neously; they were filtered off, washed with a minimal
amount of cold water, and dried in air. The yield was
8
5–90% in both cases.
IR spectra were recorded as KBr pellets on a Scim-
The goal of this work was to explore the feasibility
of manufacturing fine-powdered intermetallic com-
pounds PtZn and PtCd.
itar FTS 2000 spectrometer in the wavenumber range of
–
1
Table 1. IR vibration frequencies for DCSs (cm )
DCS
ν(NH3)
δ (NH )
δ (NH )
ρ (NH )
ν(M–N)
d
3
s
3
r
3
[
Zn(NH ) ][PtCl ]
3334, 3253
3290
1683, 1607
1600
1274, 1245, 1219
735, 714
693
427
410
390
381
3
4
6
[Zn(NH ) ]I *
3
4
2
1242
1194
1207
[Cd(NH ) ][PtCl ]
3 4 6
3357, 3265
3360, 3260
1674, 1599
1608
630
[Cd(NH ) ]Cl *
3 4 2
561
*
Data from work [4].
5
00

[
Zn(NH ) ][PtCl ] AND [Cd(NH ) ][PtCl ] AS PRECURSORS
501
3
4
6
3 4
6
–
1
4
00–4000 cm . Assignment was done with reference
T, °C
800
to work [4]. The assignment results are displayed in
Table 1.
2
4
2.5/48.12
7
00
00
Chlorine was determined routinely according to
Schoniger. Separate analyses of complexes and metal
solid solutions for the metals were carried out by
atomic absorption spectrometry (AAS) Zeeman back-
ground correction on a Hitachi Z 8000 spectrometer
with after solubilizing the solid phases with aqua regia.
Element interference was eliminated by buffering the
solutions with LaCl3.
6
~
70/48.12
500
00
300
~
62/48.12
3
4
6
4
4.0/48.12
4
1
47.8/48.12
2
1
00
00
0
For [Zn(NH ) ][PtCl ] anal. calcd., %: Cl, 39.30;
Zn, 12.08; Pt, 36.04.
5
3
4
6
4
8.9/48.12
Found, %: Cl, 39.1; Zn, 12.2; Pt, 36.1.
0
0.5
1.0
1.5
2.0
2.5
3.0
τ, h
For [Cd(NH ) ][PtCl ] anal. calcd., %: Cl, 36.16;
3
4
6
Cd, 19.11; Pt, 33.16.
Fig. 1. Thermolysis of [Zn(NH ) ][PtCl ] in hydrogen.
The weight of the residue (in percent of the starting weight)
as a function of heating schedule (obs./calcd.).
3
4
6
Found, %: Cl, 36.3; Cd, 19.3; Pt, 33.0.
Metal powders were prepared by reducing samples
of the complexes (~100 mg) in flowing hydrogen at var-
ious temperatures and hearing rates. Figure 1 displays
the weight of the residue (in percent of the starting
weight) as a function of heating schedule for
[
4]) in the spectra of the DCSs in question provides cir-
cumstantial evidence in favor of the individuality of the
compounds.
[
Zn(NH ) ][PtCl ].
3 4 6
Powder X-ray diffraction. We failed to grow DCS
crystals suitable for a single-crystal X-ray diffraction
experiment. We also failed to find compounds isostruc-
tural to our synthesized complexes in the JCPDS data-
base [6]. The X-ray diffraction pattern for a polycrys-
talline sample of the complex was indexed assuming a
monoclinic unit cell with the parameters a = 12.134(3)
Å, b = 7.813(2) Å, c = 7.787(2) Å, β= 119.4(2)°. The
positions and intensities of X-ray diffraction reflections
for this complex repeat from synthesis to synthesis.
Reflections other than pertaining to the chosen unit cell
were not found.
Thermoanalytical curves (Figs. 2, 3) were obtained
on a Q-1000 derivatograph modified to operate in dif-
ferent gas atmospheres.
Powder X-ray diffraction analysis of polycrystal-
line samples of the complex salts and their thermolysis
products was carried out on a DRON-Seifert RM4 dif-
fractometer (CuK radiation, graphite monochromator
α
on a diffracted beam, scintillation detector with ampli-
tude discrimination). Test samples were prepared by
spilling an ethanol suspension onto the polished side
of a fused silica cell. A polycrystalline silicon sample
a = 5.4309 Å) prepared in the same manner was used
as an external reference. The X-ray diffraction patterns
(
The X-ray diffraction pattern of the complex
were recorded in a discrete mode in the 2θ range from [Cd(NH ) ][PtCl ] differs strongly from that of the
3
4
6
5
° to 60° for the complex salts and from 5° to 135° for
thermolysis products.
complex [Zn(NH ) ][PtCl ] in both the positions and
3
4
6
intensities of the reflections. Broad, poorly resolved
reflections did not allow us to index the unit cell.
RESULTS AND DISCUSSION
Thermolysis in helium. The thermolysis of the
DCSs under a helium atmosphere involves three
stages. The weight loss at the first stage corresponds
to ~17 amu in both cases. Powder X-ray diffraction
shows that the intermediate produced at this stage is
amorphous. There are low-intensity reflections, which
are assigned to the (NH ) [PtCl ] phase. This stage is
accompanied by a noticeable exotherm in the DTA curve,
which is characteristic of ligand exchange between the
inner spheres of the complex ions in the DCS.
IR spectra. In the IR spectra of the DCSs, the
bands of the bending vibrations of coordinated ammo-
nia molecules shift toward higher wavenumbers com-
pared to simple salts of zinc and cadmium tetraam-
mines. The δ (NH ) bands are split in both compounds.
d
3
4
2
6
The δ (NH ) and δ (NH ) bands are split for the zinc
s
3
r
3
compound. We observed split and shifted modes of
coordinated ammonia molecules in DCSs in our previ-
ous work [5]; this was due to the distortion of the sym-
metry of complex ions in the crystal structure of DCSs.
The nonappearance of the vibrations intrinsic to the
ammonium ion and other complex cations
The second stage also produces a hygroscopic inter-
mediate, which contains metallic platinum and zinc or
cadmium chloride hydrates as shown by powder X-ray
2+
–1
(
[Cd(NH ) ] with the following wavenumbers (cm ): diffraction. Weight losses at this stage well match the sto-
3 6
δ (NH ), 1091; δ (NH ) 1585; ρ (NH ) 613; νMN, 298 ichiometry of the PtMCl residue, where M = Zn or Cd.
s
3
d
3
r
3
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 4 2007

5
02
ZADESENETS et al.
m, %
2
42°C, 96.7%
TG
1
00
2
00°C, 100.0%
8
0
0
0
0
0
4
12°C, 61.0%
6
4
2
DTA
DTG
5
85°C, 37.0%
1
50
200
250
300
350
400
450
500
550
600
T, °C
Fig. 2. Thermoanalytical curves for [Zn(NH ) ][PtCl ] thermolysis in helium at 10 K/min.
3
4
6
m, %
TG
1
00
1
18°C, 99.8%
2
05°C, 97.1%
3
34°C, 84.6%
3
44°C, 77.7%
90°C, 64.0%
8
0
0
0
0
0
3
DTA
DTG
6
4
2
5
0
100
150
200
250
300
350
400
450
T, °C
Fig. 3. Thermoanalytical curves for [Cd(NH ) ][PtCl ] thermolysis in helium at 10 K/min.
3
4
6
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 4 2007

[
Zn(NH ) ][PtCl ] AND [Cd(NH ) ][PtCl ] AS PRECURSORS
503
3
4
6
3 4
6
The final stage involves the sublimation (vaporiza- T, °C
tion) of zinc or cadmium chloride. The boiling temper-
atures are 733 and 975°ë for ZnCl and CdCl , respec-
2
2
4
00
tively. The final product of [Zn(NH ) ][PtCl ] thermol-
3
4
6
ysis in helium is platinum metal as identified by powder
X-ray diffraction; the weight of the residue (37%)
θ(PtZn)
matches the calculated platinum proportion in this salt 300
36.04%).
Thermolysis in hydrogen. When the DCSs degrade
PtZn1.7
(
(Pt)
(Zn)
γ1
γ
Pt Zn
3
in hydrogen at temperatures above 500°C, the resulting
metal phases are depleted in the nonprecious compo-
nent (we called this phenomenon “oveheating” of the
samples). Therefore, we had to choose optimal reduc-
tion parameters to ensure the 1 : 1 stoichiometry of the
product.
2
00
0
Pt
10 20 30 40 50 60 70 80 90 100
2 4
5 6
Zn
at. %
Fig. 4. Fragment of the Pt–Zn phase diagram [7]. Dashed
lines indicate the compositions of the metal phase as deter-
mined from TG data for the relevant sample (Fig. 1).
Figure 1 displays several examples of the reduction
of [Zn(NH ) ][PtCl ]. Salt samples were heated at 10
3
4
6
or 20 K/min. Faster heating resulted in either incom-
plete reduction (in sample 1) or loss of zinc (in sample (Fig. 4), the homogeneous range of the PtZn phase at
). Slower heating (5 K/min) to 400°ë also did not room temperature is 31–47.5 at. % Zn. As temperature
2
ensure full reduction (in sample 3); however, subse- increases, the homogeneous range widens to reach
quent exposure at this temperature (in sample 4) or 31−51 at. % Zn at 900°C. Samples 4–6 have only reflec-
slow cooling (in sample 5) diminished the deviation of tions from the PtZn phase. The unit cell parameters of
the product weight from the value calculated for the PtZn in all samples are practically the same; they do not
sum of the metals. We believe that the schedule we differ from the parameters of the reference in the
employed for sample 6 is optimal: first, a DCS was JCPDS database (Table 2). Reflections of the PtZn1.7
heated at a rate of 10 K/min to 300°ë; then, the heating
rate was lowered; after reaching 400°ë, the sample was
exposed for at least 1 h.
phase (JCPDS 6-619) are not found in equiatomic sam-
ples 5 and 6. The absence of this phase is likely due to
nonequilibrium synthesis conditions. Sample 5 dis-
The Zn : Pt ratio in these products determined by plays very weak (<5%) reflections of platinum, which
AAS is displayed in Table 2. One can see that “over- is also because of its metastable state.
heated” samples are depleted in zinc. Powder X-ray dif-
For [Cd(NH ) ][PtCl ], 400°ë is also the optimal
fraction shows that sample 2 (with the lowest zinc con-
3 4
6
centration) consists of pure platinum and intermetallic reduction temperature. However, despite several
compound PtZn (JCPDS 6-604), which matches the attempts, we failed to obtain the single-phase interme-
phase diagram [7]. According to the phase diagram tallic compound. Powder X-ray diffraction shows
Table 2. Analytical data for the thermolysis products of DCSs in hydrogen
Percentage con-
Sample no.
M : Pt (AAS)
0.73
Phase
a, Å
c, Å
MCS, Å
tent of the phase
2
PtZn
Pt
~45
~55
100
>95
<5
4.025(3)
3.919(2)
4.030(3)
4.025(3)
3.500(5)
90–120
90–120
4
5
0.82
1.00
PtZn
PtZn
Pt
3.500(3)
3.491(3)
120–170
120–200
Not determined
3.485(3)
6
1.00
–
PtZn
–
100
–
4.030(3)
4.0247
150–220
–
Reference PtZn*
Pt–Cd**
3.491
0.995
PtCd
Pt
>95
<5
4.185(3)
3.825(3)
Not determined
3.817
Reference
PtCd***
–
–
–
4.174
–
*
Reference data for Pt52.8Zn47.2 (JCPDS 6-604) [6].
*
* Thermolysis product of [Cd(NH ) ][PtCl ] in hydrogen.
3
4
6
*
** Reference data fo Pt51.5Cd48.5 (JCPDS 14-7) [6].
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 4 2007

5
04
ZADESENETS et al.
1
00
100
90
80
70
60
50
40
30
20
10
0
First, zinc and cadmium chlorides were heated to
4
00°ë in flowing hydrogen. As expected, reduction did
90
80
70
60
50
40
30
20
10
0
not occur. We repeated the experiments adding a sto-
ichiometric amount of K [PtCl ] to zinc and cadmium
2
6
chlorides and carefully stirring the blend. In this case,
zinc and cadmium did not reduce either; disperse plati-
num was the only metallic product. The reason for this,
as well as for the conservation of the PtZn structure at
different zinc concentrations, is associated with the
high thermodynamic stability of this intermetallic com-
pound. The reduction of [Zn(NH ) ][PtCl ] supported
3
4
6
on γ-Al O with hydrogen yielded a catalyst containing
2
3
2
wt % PtZn. Tests for butadiene hydrogenation (Fig. 5)
9
0
110 130
150
T, °C
170 190 210
showed that the activity of this catalyst is comparable
with that of a pure platinum catalyst.
Fig. 5. Catalytic activity of the intermetallic compound
PtZn in hydrogenation of butadiene. Selectivity is measured
relative to butenes.
REFERENCES
1
2
. Platinum 2005 (Johnson Matthey, London, 2005), p. 2.
. S. V. Korenev and A. B. Venediktov, et al., Zh. Strukt.
Khim. 44 (1), 58 (2003).
. The Synthesis of Complexes of the Platinum-Group Met-
als, Ed. by I. I. Chernyaeva (Nauka, Moscow, 1964),
p. 96 [in Russian].
. K. Nakamoto, Infrared and Raman Spectra of Inorganic
and Coordination Compounds (Wiley, NewYork, 1997),
p. 4.
a PtCd phase in all runs and a minor amount of plati-
num (in all runs, less than 5 wt %). However, AAS
3
4
(
Table 2) shows insignificant sublimation of cadmium
during the experiment, and the excess of platinum is
0.5 at. %. In the Pt–Cd phase diagram, this composi-
~
tion falls in the field of PtCd-base solid solution. Thus,
the presence of a platinum phase in the reduction prod-
ucts of [Cd(NH ) ][PtCl ] is due to incomplete equili-
3
4
6
5. A. B. Venediktov, S. V. Korenev, Yu. V. Shubin, et al.,
Zh. Neorg. Khim. 48 (3), 448 (2003) [Russ. J. Inorg.
Chem. 48 (3), 379 (2003)].
bration in the mixture.
It is noteworthy that under the chosen conditions,
hydrogen reduces active metals, such as zinc and cad-
mium, from similar compounds. To confirm the unique-
ness of this preparation process of intermetallic com-
pounds, we carried out the following experiments.
6
. PDF Database (PCPDFWIN v. 2.02, JCPDS-ICDD,
999).
. Phase Diagrams for Binary Metal Systems: A Hand-
book, Ed. by N. P. Lyakishev (Mashinostroenie, Mos-
cow, 2001), Vol. 3, Part 2, p. 80 [in Russian].
1
7
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 4 2007