Core-shell structured microcapsular-like Ru@SiO2 reactor for efficient generation of COx-free hydrogen through ammonia decomposition

Article abstract of DOI:10.1039/c0cc00430h

The core-shell structured microcapsular-like Ru@SiO₂ reactor is proved to be the most efficient material known to date for CO_x-free hydrogen production via ammonia decomposition for fuel cells application. The very active Ru core par

Full text of DOI:10.1039/c0cc00430h

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Core–shell structured microcapsular-like Ru@SiO reactor for efficient
2
generation of CO -free hydrogen through ammonia decompositionw
x
Yanxing Li,ab Lianghong Yao, Yanyan Song, Shunqiang Liu, Jing Zhao, Weijie Ji* and
a
a
a
a
a
Chak-Tong Au*b
Received 17th March 2010, Accepted 19th May 2010
First published as an Advance Article on the web 14th June 2010
DOI: 10.1039/c0cc00430h
9
the reducing agent in the preparation media, the metallic
The core–shell structured microcapsular-like Ru@SiO
2
reactor
is proved to be the most efficient material known to date for
colloids can be directly obtained. Under the refluxing tempera-
0
ture the metallic colloids grow to the Ru SMPs which are
0
served as the Ru cores in the core–shell structures. For the
CO -free hydrogen production via ammonia decomposition for
x
fuel cells application. The very active Ru core particles can
retain good stability even at high temperatures (up to 650 1C)
thanks to the protection of the inert SiO nano-shell.
2
first time, it is found that the space developed between the core
and the internal wall of the shell inside the microcapsular-like
Ru@SiO
The Ru SMPs were synthesized through a solvent-thermal
procedure. Typically, 2.0 g RuCl was added to 100 ml
2
reactor has a critical effect on reactivity.
The processes such as steam reforming, partial oxidation, and
autothermal reforming of methane are the conventional
3
approaches to H production. However, the level of residual
2
ethylene glycol. After stirring the mixture for 1 h at 313 K, a
homogeneous black-brown solution was formed. The solution
was transferred to a three-neck flask and refluxed at 453 K for
3 h. The as-generated grey precipitate was collected by centri-
fugation and washed three times with distilled water and
ethanol. Hereinafter, the solid powder dried at 313 K overnight
in vacuum is denoted as Dried-SMPs, and the Dried-SMPs
calcined in air at 773 K for 4 h as Calcined-SMPs.
CO is unacceptable for current proton-exchange membrane
fuel cells (PEMFCs). In order to make use of the CO-containing
H
2
, one should eliminate residual CO by oxidation of CO in
high hydrogen concentration, which adds substantial cost and
1
complexity to obtain CO -free H . Choudhary et al. also
x 2
reported another way to generate CO-free hydrogen by the
reversible cyclic stepwise steam reforming of methane. The
catalytic decomposition of ammonia has attracted much atten-
The core–shell structures were prepared via a sonication-
1
¨
assisted Stober process. Typically, Dried-SMPs of 0.95 g
0
tion because of the need of producing CO -free hydrogen for
x
2
PEMFCs. It is considered as an economical method, and
(or Calcined-SMPs of 1.17 g) were added to 100 ml absolute
ethanol and the mixture was sonicated for 30 min in an
ultrasound bath (KQ-100DE, 40 kHz, 100W). Then 20 ml
3
NH storage and delivery can be more readily handled. Many
3
metals, alloys, and compounds with noble metal characters
4
have been tested for this reaction. The invention of a carbon-
NH
3
ÁH
2
O and 420 ml tetraethylorthosilicate (TEOS, 98%, the
based Ru catalyst that was commercialized in the mid-1990s
molar ratio of Si/Ru = 0.2) were added under sonication.
After 1 h, the product was collected by centrifugation, washed
twice with distilled water and ethanol, and dried at 313 K for
can be considered as a breakthrough, but the details have not
5
been disclosed.
In the last decade, core–shell structured materials have
attracted wide attention for their unique structural features
6 h. The samples so obtained are denoted as Dried-SMP@SiO
and Calcined-SMP@SiO , respectively. The catalysts subject to
in situ reduction in a 25% H /Ar (v/v) flow at 823 K for 2 h are
denoted as Dried-SMP-R@SiO and Calcined-SMP-R@SiO
adsorption–desorption measurement was performed on
2
2
6
and physicochemical properties. By having a nano-material
2
encapsulated in a stable but porous shell, one can have the
stability and compatibility of the material enhanced; in the
mean time, there could be a change of electron charge,
2
2
.
N
2
an ASAP-2020 instrument at 77 K. XRD analysis was con-
ducted on a Philips X’Pert MPD Pro X-ray diffractometer,
with Cu-Ka radiation (l = 0.1541 nm) in the 2y range of
10–801. Scanning electron microscopy (SEM) measurements
were carried out on a S-4800 scanning microscope whereas
TEM images were taken on JEOL JEM-1010/2010 transmis-
sion electron microscopes operated at 80 or 200 kV.
7
reactivity and functionality of the enwrapped materials.
Nanoparticles (NPs) coated with silica have been studied in
fields such as biology, optics, electronics, magnetism, sensing
8
and catalysis. In this communication, we report the synthesis
of core–shell structured Ru submicroparticles (SMPs) for
ammonia decomposition. By employing ethylene glycol as
Catalytic testing was carried out in a continuous-flow
a
Key Laboratory of Mesoscopic Chemistry, MOE,
quartz reactor (catalyst: 0.1 g, 60–80 mesh) under pure NH
À1
3
School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, China.
E-mail: jiwj@nju.edu.cn; Fax: +86-25-83317761
Department of Chemistry, Hong Kong Baptist University,
Kowloon Tong, Hong Kong. E-mail: pctau@hkbu.edu.hk;
Fax: +852-34117348
(
flow rate: 50 ml min ; GHSVNH3: 30 000 ml/(h gcat)). Before
2
the reaction, the catalyst was reduced in situ in a 25% H /Ar
b
(v/v) flow at 823 K for 2 h before being exposed to a flow of
pure ammonia. The reactivity was evaluated in the 623–923 K
range, and acquisition of activity data was conducted after
establishment of a steady state at a particular tempera-
ture. Product analysis was performed using an on-line gas
w Electronic supplementary information (ESI) available: N
tion–desorption data, H
c0cc00430h
2
adsorp-
-TPD and H -TPR profiles. See DOI: 10.1039/
2 2
5
298 | Chem. Commun., 2010, 46, 5298–5300
This journal is ꢀc The Royal Society of Chemistry 2010

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chromatograph (GC-122) equipped with a thermal conducti-
vity detector and a Poropak Q column, using He as a carrier
gas. NH3 conversion in a blank reactor was found to be
o1.0% at 823 K.
It was observed that the BET surface areas, pore volumes
and pore size distribution of the two catalysts were similar
(
ESIw). The reaction was thought to be not diffusion limited,
because the silica shells were thin (B50 nm) and rather porous.
The experimental procedures for the silica coating of Ru
cores and the following in situ hydrogen reduction of
core–shell structures are exactly identical for the two catalysts.
Moreover, the catalytic reaction has been conducted over the
Fig. 2 SEM images of Dried SMPs.
Dried-SMP-R@SiO
2
(A-R), still there is no void between SiO
2
and Ru. However, the TEM image of Calcined-SMP-R@SiO
2
(
B-R) shows that there is space developed between the core and
the internal wall of the shell. Of the four samples, the cores are
enclosed in uniform SiO shells of 30–50 nm thickness. The
2
core–shell structured Ru@SiO catalyst at 500–650 1C in a
period of 60–80 h and there is no loss of catalytic activity
observed. The porosity of the silica shell and the particle size
of the Ru cores are essentially maintained even subject to high
temperature process. According to our preparation strategy
of core–shell structures, one can infer that in the case of
2
presence of stable silica shells can also effectively prevent the Ru
0
cores from aggregation; as a consequence, the Ru particle size
can be essentially retained.
Fig. 4 shows the temperature dependence of NH conversion
3
Calcined-SMP@SiO
due to air-calcination of Ru to RuO
tion because of the following reduction of RuO
2
although there is structure expansion
over the Dried-SMP-R@SiO2 and Calcined-SMP-R@SiO2
0
2
and structure contrac-
catalysts. The results show that Calcined-SMP-R@SiO
significantly more active than Dried-SMP-R@SiO . Over the
former, 100% NH conversion is achieved at 823 K while NH
is
2
0
to Ru , the
2
2
0
Ru core particles are similar in size in the two catalysts. Most
importantly, the measured metal surface areas of the two
catalysts are identical on the basis of per unit mass of catalyst
3
3
conversion is only 78.8% at 823 K over the latter. The
observation suggests that the pre-calcination of the unwrapped
cores has a critical effect on catalyst activity.
(ESIw), suggesting that the accessible active metal surfaces are
precisely comparable.
In situ measurement of the metal surface exposure of Ru
From Fig. 1, one can see that the dried SMPs are essentially
in the form of metallic Ru (JCPDS No. 01-1253), while the
cores has been performed by means of in situ H reduction
2
followed by H -TPD. The measured H -TPD peak areas of
2
2
calcined ones are a mixture of Ru and RuO
2
(JCPDS No.
1-1172). After reduction, both Dried-SMP-R@SiO and
show identical patterns of metallic
Calcined-SMP-R@SiO and Dried-SMP-R@SiO are proved
2
2
2
2
to be identical, suggesting that the metal surface area of the
two catalysts is the same. The desorption profiles of the two
catalysts also matched quite well in the range of 50–350 1C,
indicating that the pre-calcination had a slight impact on the
nature of the Ru cores. This experimental evidence suggests
that neither the metal surface area nor the chemical nature of
the Ru cores can be notably modified by the oxidation/
reduction pre-treatments. Based on the characterization results
Calcined-SMP-R@SiO
2
Ru, having crystallinity higher than that of Dried-SMP@SiO
No silica signal was observed over the samples, indicating that
the SiO shells are amorphous.
2
.
2
The SEM images of Dried-SMPs show the Ru particles are
spherical in shape and 100–300 nm in size (Fig. 2). The SEM
image of higher magnification indicates that the SMPs are
made up of smaller Ru NPs (10–20 nm).
of (HR)TEM and H -TPD, one can conclude that the difference
2
Shown in Fig. 3 are the TEM images of the samples of
in catalytic activity is mainly due to the variation in catalyst
2
Fig. 1. It is found that for Dried-SMP@SiO (A) and Calcined-
SMP@SiO (B) the core and shell are in close contact. As for
2
Fig. 1 XRD patterns of (A): Dried-SMP@SiO ; (A-R): Dried-
2
Fig. 3 TEM images of (A): Dried-SMP@SiO
2
; (A-R): Dried-SMP-
SMP-R@SiO
SMP-R@SiO
2
;
.
(B): Calcined-SMP@SiO
2
,
and (B-R): Calcined-
R@SiO
R@SiO
2
;
.
(B): Calcined-SMP@SiO , and (B-R): Calcined-SMP-
2
2
2
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i
m
a
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e
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i
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i
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We acknowledge the financial support of the RGC,
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1
2
Scheme 1 Illustration of enhanced adsorption and reaction in
1
a
microcapsular-like reactor: (A-R) Dried-SMP-R@SiO
2
; (B-R)
Calcined-SMP-R@SiO
2
.
5
300 | Chem. Commun., 2010, 46, 5298–5300
This journal is ꢀc The Royal Society of Chemistry 2010