A facile one-step synthesis of polymer supported rhodium nanoparticles in organic medium and their catalytic performance in the dehydrogenation of ammonia-borane

Article abstract of DOI:10.1039/c1cc15864c

A new type of supported rhodium nanoparticles were reproducibly prepared from N₂H₄BH₃ reduction of [Rh(μ-Cl)(1,5-cod) ]₂ without using any solid support and pre-treatment technique. Their characterization shows the formation of well dispersed rhodium(0) nanoparticles within the framework of a polyaminoborane based polymeric support. These new rhodium(0) nanoparticles were found to be the most active supported catalyst in the catalytic dehydrogenation of ammonia-borane in water at room temperature. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c1cc15864c

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COMMUNICATION
A facile one-step synthesis of polymer supported rhodium nanoparticles
in organic medium and their catalytic performance in the
dehydrogenation of ammonia-boranew
¨
Senem Karahan,z Mehmet Zahmakıran*y and Saim Ozkar
Received 21st September 2011, Accepted 17th November 2011
DOI: 10.1039/c1cc15864c
A new type of supported rhodium nanoparticles were reproducibly
prepared from N2H4BH3 reduction of [Rh(l-Cl)(1,5-cod)]2 without
using any solid support and pre-treatment technique. Their
characterization shows the formation of well dispersed
rhodium(0) nanoparticles within the framework of a polyaminoborane
based polymeric support. These new rhodium(0) nanoparticles were
found to be the most active supported catalyst in the catalytic
dehydrogenation of ammonia-borane in water at room temperature.
dehydrogenation of ammonia-borane (H3NBH3), which has
recently been considered as a suitable hydrogen storage material
because of its low molecular weight (30.9 g molꢀ1) and high
hydrogen content (19.6 wt%), that is greater than the 2015 target
of the U.S. Department of Energy (9 wt% hydrogen for a
material to be practically applicable).9
When a 0.15 mol% solution of [Rh(m-Cl)(1,5-cod)]2 in THF
was treated with a concentrated solution of hydrazine borane
([H4N2BH3] = 5.0 M) at 25 1C, a vigorous bubble formation
was observed, presumably due to H2 evolution from the
dehydrocoupling/dehydrogenation of hydrazine borane.10 During
the course of reaction, a brown solid precipitate was also formed,
which was isolated at the end of the reaction by the evaporation
of solvent and found to be insoluble not only in water but also in
the most common organic solvents.11 It is noteworthy that recent
studies10 have also reported the formation of insoluble, poorly
defined polymeric materials, along with H2 from the thermolysis
of hydrazine borane. The 1H NMR spectrum of the aliquot taken
at the end of the reaction shows only the formation of cyclooctane
(1.3 ppm) from the hydrogenation of a cyclooctadiene ligand,
whereas the 11B{1H} spectrum of the same sample shows no
signal indicating that boron containing a THF-soluble product
is not formed. The compositional investigation of the insoluble
product in terms of inductively coupled plasma optical emission
spectroscopy (ICP-OES) and elemental analyses (EA) shows the
existence of an equimolar amount of boron and nitrogen plus a
very small amount of rhodium metal (1.5 wt%). The solid state
11B{1H}-NMR spectrum (Fig. S1, ESIw) of the same material
shows two signals at 0.7 and ꢀ11.7 ppm assignable to singly
dehydrogenated boranes12,13 and high molecular weight amorphous
polyaminoboranes [BNRHx]y formed from the dehydrogenation of
analogous amine-borane derivatives.14 The analyses done by matrix
assisted laser desorption ionization-time of flight (MALDI-TOF)
mass spectroscopy (Fig. S2, ESIw) and powder X-ray diffraction
(P-XRD) for our insoluble product provide compelling evince for
the formation of amorphous polymeric material with a molecular
weight of >5000. In addition, the attenuated total reflectance
infrared (ATR-IR) spectrum (Fig. S3, ESIw), collected from
the same sample, shows the characteristic bands between
3325–3083 cmꢀ1 for N–H, at 2494 cmꢀ1 for B–H13 and
between 1588–821 cmꢀ1 for B–N stretching and bending
modes.15 The oxidation state of rhodium in Rh@PAB was
investigated by using X-ray photoelectron spectroscopy (XPS)
Today, the preparation of metal nanoparticles with high stability
against bulk formation is of great importance for their potential
applications in many fields,1 including catalysis.2 The majority
of the protocols followed in the synthesis of metal nanoparticles
include the use of surfactants or ligands that can stabilize them
in the embryonic stage against agglomeration.3 Although they
attain colloidal stability with the help of ligands/surfactants,
they still have a tendency to form clumps and eventually to
agglomerate into bulk metal during the catalytic processes.4 The
prevention of nanoparticles agglomeration is one of the most
important challenges in the field of nanocatalysis.5 In this
context, the use of nanoparticles on solid matrices of support
materials represents especially promising directions for preventing
aggregation.6 Moreover, it can also provide the kinetic control of
the catalytic reaction, plus simple isolation from the reaction
solution and efficient reusability.7
Herein, we report one step synthesis of a new type of supported
rhodium(0) nanoparticles, hereafter referred to as Rh@PAB,
without requiring any pre-treatment such as impregnation,
ion-exchange or solid grinding for grafting of metal nanoparticles
to the support. Rh@PAB were prepared reproducibly in a single
step hydrazine borane (N2H4BH3) reduction of the chloro-
(1,5-cyclooctadiene)rhodium(I) precursor ([Rh(m-Cl)(1,5-cod)]2)
and characterized by multipronged analyses.8 The catalytic
performance of Rh@PAB was investigated in the hydrolytic
Department of Chemistry, Middle East Technical University,
06531, Ankara, Turkey
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization/catalytic test results. See DOI:
10.1039/c1cc15864c
z On the leave absence from Department of Chemistry, Dokuz Eylul
¨
˙
University, 35160, Izmir, Turkey.
y Current address: Department of Chemistry, Faculty of Science, Yuzuncu
¨
¨
¨
Yıl University, 65080, Van, Turkey. E-mail: zmehmet@yyu.edu.tr
c
1180 Chem. Commun., 2012, 48, 1180–1182
This journal is The Royal Society of Chemistry 2012

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material. However the SEM-EDX spectrum in Fig. 1d confirmed
the existence of rhodium in the analyzed region as judged by
La1, Lb1, Lb1, Lb2 lines of rhodium in the range of 2.3–3.0 keV.18
Indeed, low resolution TEM images of Rh@PAB given in
Fig. 2a and b show the presence of well dispersed rhodium
nanoparticles in the range of 1.2–4.9 nm with a mean diameter of
2.7 ꢁ 0.6 nm (Fig. 2c). In addition, the high resolution-TEM
images shown in Fig. 2d–f reveal the highly crystalline feature of
these rhodium nanoparticles with a crystalline spacing of 0.23 nm,
which can be assigned to Rh(111) face.19 In addition, the
dispersion of rhodium nanoparticles was also investigated by
STEM modulated EDX (Fig. S5, ESIw), which shows the good
dispersion of rhodium nanoparticles across the framework of
the support. These results are solid evidence for the fact that
polymeric material acts only as a support for guest rhodium
nanoparticles and not as a surfactant that can stabilize supported
free rhodium nanoparticles by capping them as known in the
case of PVP stabilized metal nanoparticles.20 The porosity and
the surface area of Rh@PAB were also investigated under
mild pretreatment conditions by nitrogen adsorption–desorption
analyses. Fig. 3 shows a representative nitrogen adsorption–
desorption isotherm and corresponding pore size distribution
of Rh@PAB. The shape of the isotherm follows type III,
which is indicative of multilayer formation at the interface with
high coverage and characteristics for certain kinds of polymeric
materials.21 The BET (Brunauer–Emmett–Teller) surface area and
Fig. 1 (a)–(c) SEM images of Rh@PAB at different magnifications
and (d) SEM/EDS spectrum of Rh@PAB.
(Fig. S4, ESIw). The XPS survey scan spectrum shows the signals
of N 1s at 398.2 and B 1s at 190.1 eV for boron nitride (BQN).16
The high resolution Rh 3d XPS spectrum of Rh@BNp shows two
prominent signals at 307 and 311 eV that can readily be assigned
to Rh 3d5/2 and Rh 3d3/2, respectively.17 The size, morphology and
regional composition of Rh@PAB were investigated by scanning
electron microscopy (SEM), SEM modulated by energy dispersive
X-ray spectroscopy (SEM/EDS), transmission electron micro-
scopy (TEM), high resolution transmission electron microscopy
(HRTEM), scanning transmission electron microscopy (STEM),
and STEM modulated by local energy-dispersive X-ray micro-
analysis (STEM/EDX). Fig. 1a–c show the SEM images of
Rh@PAB with a rhodium loading of 1.5 wt% in different
magnifications, clearly indicating that there is no bulk rhodium
formed in observable size on the surface of the resulting polymeric
single point total pore volume are 44 m2 gꢀ1 and 0.21 cm3 gꢀ1
,
which are considerably large values, as the rhodium nanoparticles
were included in the calculations. The average BJH (Barret–
Joyner–Halenda) pore diameter calculated from the
desorption branch of the isotherm is determined to be 2.5 nm.
The catalytic performance of Rh@PAB was tested in the
hydrolytic dehydrogenation of ammonia-borane (Scheme 1).22
We found that Rh@PAB provide high activity even at high
substrate/catalyst ratios and low temperatures (see Fig. S6, ESIw).
Fig. 2 (a) and (b) Low resolution TEM images of Rh@PAB, scale bars correspond to 100 and 20 nm, respectively; (c) size histogram of rhodium
nanoparticles constructed from counting >100 particles given in (b); (d)–(f) High resolution TEM images of Rh@PAB in which scale bars are
equal to 10, 5, and 2 nm, respectively.
c
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Chem. Commun., 2012, 48, 1180–1182 1181

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Notes and references
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Scheme 1 The catalytic dehydrogenation of ammonia-borane
(NH3BH3) in water.22
11 Rhodium nanoparticles were formed from the hydrazine-borane
reduction of the Rh(I) precatalyst [Rh(m-Cl)(COD)]2. The resulting
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value of 130 mol H2/mol Rh min in air at 25 ꢁ 0.1 1C, which is
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retains >50% of the initial activity even at the fifth run with the
complete hydrogen generation. The decrease in the activity in
the fifth run can be attributed to the decrease in the number of
active surface atoms due to their clumping as evidenced by
TEM (Fig. S7, ESIw) plus a slight Rh leaching (only 3% of total
Rh) from Rh@PAB to reaction solution, which was detected by
ICP-OES analysis of the filtrate harvested from the fifth run.
In summary, we have demonstrated one-step synthesis of
polymer supported rhodium(0) nanoparticles in organic medium
by the reduction of a rhodium(I) precursor with hydrazine borane.
To the best of our knowledge, this is the only example for the
preparation of supported metal nanoparticles in a one-step
reaction in solution without requiring any additional process
(impregnation, ion-exchange, grafting, solid grinding etc.). The
preliminary results for the characterization of Rh@PAB show the
formation of well dispersed rhodium(0) nanoparticles within the
framework of a polymeric support comprised of [BNRHx]y units.
The catalytic application of these novel materials was investigated
in the hydrolytic dehydrogenation of ammonia-borane, where
they provide record activity among all the supported metal
catalysts reported up to date for the same reaction. Moreover,
they were found to be highly durable which makes them reusable.
Unsolved problems remain in the complete identification of
insoluble solid material formed from the dehydrocoupling of
hydrazine borane and the use of other metal precatalysts in
this method. The work is ongoing to explore the formation
mechanism of Rh@PAB during the dehydrogenation of
hydrazine borane in detail and expand the same methodology
for the synthesis of other metal nanoparticle catalysts.
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c
1182 Chem. Commun., 2012, 48, 1180–1182
This journal is The Royal Society of Chemistry 2012