Solvothermal preparation and control of phase composition of nanosized rhodium sulfide particles

Article abstract of DOI:10.1246/cl.2009.210

Well-crystallized Rh₂S₃ and Rh₁₇S ₁₅ powders with average particle size of 100nm were solvothermally prepared at 673 K for 10 h with S/Rh ratio of 1.65 and 0.9, respectively. The crystallinity and phase of the r

Full text of DOI:10.1246/cl.2009.210

210
Chemistry Letters Vol.38, No.3 (2009)
Solvothermal Preparation and Control of Phase Composition
of Nanosized Rhodium Sulfide Particles
Wu-xing Zhang,1 Kazumichi Yanagisawa,ꢀ1 Sumio Kamiya,2 and Tatsuo Shou2
1Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, Kochi 780-8520
2Toyota Motor Corp., Toyota 471-8572
(Received September 3, 2008; CL-080838; E-mail: yanagi@kochi-u.ac.jp)
Well-crystallized Rh2S3 and Rh17S15 powders with average
particle size of 100 nm were solvothermally prepared at 673 K
for 10 h with S/Rh ratio of 1.65 and 0.9, respectively. The crys-
tallinity and phase of the rhodium sulfides are closely related to
the reaction temperature and the S/Rh ratio. By thermal treat-
ment in Ar, the Rh2S3 can be transformed into Rh17S15 at
938 K by loss of sulfur.
Rhodium sulfides have attracted more and more attention
because of their particular high catalytic activity in hydrodesul-
furization1,2 and oxygen reduction.3 The rhodium sulfide system
contains three phases, Rh3S2, Rh3S4, and Rh17S15, and their bul-
ky products have been obtained at high temperatures above
1300 K.4–7 The high catalytic activities of rhodium sulfides were
closely related to their surface areas and crystalline structures.
Rh2S3 has a unique layered structure formed by face-sharing
pairs of distorted [RhS6] octahedra, and the layers are loosely
bound to each other only by van der Waals forces,4 while Rh3S4
and Rh17S15 have structure composed of [RhS6] octahedra and
Rh–Rh metal bonds which give them properties of metallic con-
ductor.5–7 However, it is still a tough task to prepare well-crys-
tallized rhodium sulfide nanopowders with controlled crystalline
phases at low reaction temperatures. Until now, nano-sized rho-
dium sulfides were mainly synthesized by refluxing3 and wet
chemistry.8–11 However, these methods only gave amorphous
products, and information on the detailed synthesis and phases
formed was not provided. In this paper, we provide a simple
solvothermal method to prepare nanosized and well-crystallized
rhodium sulfide powders. The phase control and thermal behav-
ior evolution of the rhodium sulfides are discussed for the first
time according to our best knowledge.
Figure 1. XRD patterns of Rh2S3 powders synthesized at dif-
ferent temperatures (S/Rh ¼ 1:65). a) 673 K for 10 h; b) 623 K
for 10 h; c) 493 K for 10 h.
The starting materials were rhodium carbonyl (Rh6(CO)16)
and sulfur powder. For a typical reaction to synthesize Rh2S3,
0.5 g of Rh6(CO)16 and 0.3 g of sulfur powders (S/Rh molar
ratio = 1.65, 10% S in excess) were put into the autoclaves
and solvothermally treated in xylene at temperatures from 493
to 673 K for 10 h. The products were harvested by centrifuge,
washed with acetone, and finally dried at 353 K for character-
ization. The thermal treatments were conducted at temperatures
from 673 to 1023 K in Ar atmosphere. The morphologies and
structures were characterized by X-ray diffraction (XRD, Rigaku
RTP-300 RC), transmission electron microscopy (TEM, JEOL
JEM-2010). The TG-DTA (Seiko TG/DTA 6300) was con-
ducted in N2 atmosphere.
Figure 2. XRD patterns of Rh17S15 powders synthesized at dif-
ferent temperatures (S/Rh ¼ 0:9). a) 673 K for 10 h; b) 623 K for
10 h; c) 493 K for 10 h.
below 493 K resulted in formation of amorphous products
(Figure 1c). Crystalline Rh17S15 was also formed at 673 K with
S/Rh ratio of 0.9 (JCPDS No: 73-1443, Figure 2a), and the
reaction at 623 K gave poorly crystallized products (Figures 2b
and 2c). HRTEM pictures of Rh2S3 and Rh17S15 synthesized
at 673 K show clear lattice fringes. The particle size of the
Rh2S3 powder prepared at 673 K is about 100 nm (Supporting In-
formation, Figure S1).12
The growth mechanism of the rhodium sulfides in solvother-
mal reaction can be explained by a dissolution–precipitation
process, because both rhodium carbonyl and sulfur are separate-
ly dissolved in xylene under solvothermal conditions, and the
nanosized rhodium sulfides have quite different morphology
from that of dozens of micron-sized bulky rhodium carbonyl
Figure 1 shows the XRD patterns of the products obtained at
different temperatures with S/Rh ratio of 1.65. Well-crystallized
rhodium sulfide Rh2S3 was synthesized at 673 K (JCPDS No:
35-0736, Figure 1a). The reaction at 623 K gave poorly crystal-
lized Rh2S3 (Figure 1b), while the reaction at low temperatures
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.3 (2009)
211
Figure 4. TG-DTA results of the amorphous rhodium sulfide
powders prepared at 493 K for 10 h (S/Rh ratio = 0.9).
Figure 3. TG-DTA results of the amorphous rhodium sulfide
powders prepared at 493 K for 10 h (S/Rh ratio = 1.65).
crystals. Good crystallinity of the rhodium sulfides needs high
reaction temperature up to 673 K for more than 2 h under solvo-
thermal conditions (we use 10 h in our experiments to make sure
the complete reaction). The amorphous rhodium sulfides ob-
tained at 493 K can also crystallize by calcination at 673 K.
The S/Rh ratio affects the crystallization after calcination.
Especially for Rh2S3, too excessive sulfur content in the starting
materials can produce sulfur abundant rhodium sulfide that has
poor crystallinity even after calcination at 673 K (Supporting
Information, Figure S2).
In conclusion, both Rh2S3 and Rh17S15 crystalline powders
are prepared for the first time by solvothermal reaction at
673 K. They consist of fine particles with average particle size
of 100 nm. The crystallinity and phase composition of the rhodi-
um sulfides are closely related to the reaction temperature and
the S/Rh ratio. During heat treatment in Ar, the Rh2S3 can be
transformed into Rh17S15 at 938 K by loss of sulfur. These rho-
dium sulfides are expected to present high catalytic activities in
hydrodesulfurization and oxygen reduction reaction.
The TG-DTA measurements were conducted for the amor-
phous Rh2S3 and Rh17S15 powders prepared at 493 K. For
Rh2S3 powders, the weight loss up to 623 K accompanied by
two exothermic peaks at 542 and 608 K is ascribed to the decom-
position of the organic compounds (Figure 3). For the exother-
mic peaks at 697 K, we ascribed it to the crystallization of Rh2S3
since the XRD results show that amorphous rhodium sulfide syn-
thesized at 493 K can be crystallized to Rh2S3 by calcinations at
673 K in Ar (Supporting Information, Figure S3a). It is also ob-
served that a large weight loss occurs at around 873 K owing to
the release of sulfur. Accompanied by this weight loss, an exo-
thermic peak appears at 938 K in the DTA curve, which corre-
sponds to the phase formation of Rh17S15. This phase formation
is supported by the XRD pattern of product calcined at 1023 K
for 5 h in Ar (Supporting Information, Figure S3b). The calcined
rhodium sulfide is pure Rh17S15. The total weight loss up to
1023 K is about 28.2%.
The authors are very grateful for the support of the Youth
Chengguang Project of Science and Technology of Wuhan City
of China under Grant No. 200850731354, and for the help from
Analytical and Testing Center in Huazhong University of Sci-
ence & Technology.
References and Notes
1
R. Oviedo-Roa, J.-M. Martinez-Magadan, F. Illas, J. Phys.
Chem. B 2006, 110, 7951.
2
3
A. Tan, S. Harris, Inorg. Chem. 1998, 37, 2215.
R. W. Reeve, P. A. Christensen, A. J. Dickinson, A. Hamnett,
K. Scott, Electrochim. Acta 2000, 45, 4237.
´
4
5
6
7
E. Parthe, E. Hohnke, F. Hulliger, Acta Cryst. 1967, 23, 832.
J. Beck, T. Hilbert, Z. Anorg. Allg. Chem. 2000, 626, 72.
S. Geller, Acta Cryst. 1962, 15, 1198.
H. R. Naren, A. Thamizhavel, A. K. Nigam, S. Ramakrishnan,
Phys. Rev. Lett. 2008, 100, 026404.
For the amorphous Rh17S15 powder (Figure 4), two exother-
mic peaks were observed at 588 and 721 K in the DTA curve, in
which the exothermic peak at 588 K corresponded to the decom-
position of the organic compounds, while the exothermic peaks
at 721 K was ascribed to the formation of Rh17S15 phase because
the XRD results show that amorphous rhodium sulfide synthe-
sized at 693 K can be crystallized to Rh17S15 by calcinations at
713 K in Ar (Supporting Information, Figure 3c). The large
weight loss of the amorphous Rh2S3 at around 873 K is not ob-
served in amorphous Rh17S15. After calcination at 1023 K for 5 h
in Ar, the product is still pure Rh17S15 (Supporting Information,
Figure 3d).
8
9
I. B. Rufus, V. Ramakrishnan, B. Viswanathan, J. C.
Kuriacose, Langmuir 1990, 6, 565.
M. Cattenot, J.-L. Portefaix, J. Afonso, M. Breysse, M.
Lacroix, G. Perot, J. Catal. 1998, 173, 366.
10 E. J. M. Hensen, H. J. A. Brans, G. M. H. J. Lardinois, V. H. J.
de Beer, J. A. R. van Veen, R. A. van Santen, J. Catal. 2000,
192, 98.
11 Y.-M. Tsou, H. Deng, G. N. Martelli, R. J. Allen, E. S. De
Castro, U.S. Patent 141108, 2002.
12 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
Published on the web (Advance View) January 31, 2009; doi:10.1246/cl.2009.210