Catalytic performance of metal nanoparticles supported by ceramic composite produced by partial reduction of solid solution with dopant

Article abstract of DOI:10.1111/j.1551-2916.2005.00503.x

A composite with well-dispersed metal nanoparticles at a ceramic surface was produced by partial reduction of solid solution. It was found that a small amount of dopant, such as A1₂O₃, Cr₂O ₃, or Sc₂O₃, accelerated the precipitation of the metal nanoparticles during the reduction. Catalytic performance of the composite for methanol reforming was evaluated. In the Ni-based catalysts, the dopant decreased the CO production by promoting a methanation reaction, while in the Co-based catalysts, the dopant did it by inducing a water-gas shift reaction. Co/MgO with Se₂O₃ doping showed the most preferable reforming performance, high H₂ production, and CO ₂ selectivity.

Full text of DOI:10.1111/j.1551-2916.2005.00503.x

J. Am. Ceram. Soc., 88 [10] 2938–2941 (2005)
DOI: 10.1111/j.1551-2916.2005.00503.x
r 2005 The American Ceramic Society
ournal
J
Catalytic Performance of Metal Nanoparticles Supported by Ceramic
Composite Produced by Partial Reduction of Solid Solution
with Dopant
T. Fukasawa,w T. Suetsuna, K. Harada, and S. Suenaga
Advanced Functional Materials Laboratory, Corporate Research & Development Center, Toshiba Corporation,
Kawasaki 212-8582, Japan
A composite with well-dispersed metal nanoparticles at a ce-
ramic surface was produced by partial reduction of solid solu-
tion. It was found that a small amount of dopant, such as Al2O3,
Cr2O3, or Sc2O3, accelerated the precipitation of the metal
nanoparticles during the reduction. Catalytic performance of the
composite for methanol reforming was evaluated. In the Ni-
based catalysts, the dopant decreased the CO production by
promoting a methanation reaction, while in the Co-based cata-
lysts, the dopant did it by inducing a water–gas shift reaction.
Co/MgO with Sc2O3 doping showed the most preferable re-
forming performance, high H2 production, and CO2 selectivity.
crease in the diffusion rate of the Ni ions to the ceramic surface
during the reduction.7
Other dopants that have the same effects as Al2O3 were in-
vestigated. As a result, it was found that Cr2O3 and Sc2O3 were
also effective in enhancing the metal nanoparticle precipitation.
It is known that the nanosized Ni- and Co-supported mate-
rials have high potential for the reforming catalysts of methane
and ethanol.8,9 These materials are expected to be stable and
inexpensive catalysts. In this paper, the composites produced by
partial reduction of solid solution with dopant were prepared
and a steam-reforming test for methanol was performed. The
effects of the doping materials on reforming performance are
discussed.
I. Introduction
II. Experimental Procedure
COMPOSITE with nanosized metal particles on the ceramic
surface is useful for the applications to catalysts and func-
MgO powder (1 mm, 4Nup, Kojundo Chemical Lab. Co. Ltd.,
Saitama, Japan), NiO powder (1 mm, 499%, FP-NiO, Sumi-
tomo Metal Mining Co. Ltd., Tokyo, Japan), and CoO powder
(1 mm, 3Nup, Kojundo Chemical Lab. Co. Ltd.) were used as
raw powders. Mixed powders of NiO–MgO and CoO–MgO
were prepared. The molar ratios of NiO:MgO and CoO:MgO
were fixed at 1:2. 0.1 mol% of Al2O3, Cr2O3, or Sc2O3 was
added to the mixed powder. The mixture was cold pressed into a
compact and sintered at 13001C for 5 h. Solid solution (Ni,
Mg)O and (Co, Mg)O with dopant were obtained. The sample
density was calculated from the mass and dimensions of the
sample. The solid solution was reduced at 10001C for 10 min
under a hydrogen flow, and the Ni and the Co particles were
precipitated at the ceramic surface. The weight loss of the sam-
ple was measured during the reduction using a TG machine
(TGD-9600, ULVAC-Riko Inc., Yokohama, Japan). To char-
acterize the composite, the phase composition was analyzed by
an X-ray diffractometer (XRD, Rigaku Corp., Tokyo, Japan).
The physical surface area and the active metal surface area were
measured by the gas absorption method (AUTOSORB-1C,
Quantachrome Instruments Corp., Boynton Beach, FL), using
nitrogen and hydrogen, respectively. The reduction samples
were directly observed using a scanning electron microscopy
(SEM; Model JSM-840F, JEOL Co. Ltd., Tokyo, Japan). Al-
though some samples were coated with a gold by a sputtering
device to impart more electroconduction, it is not considered to
influence the observation. The acceleration voltage was 5 kV.
The steam reforming test of methanol was performed using
the reduced sample. Before reduction, the solid solution was
crushed into pieces and sieved at over 200 mm through a mesh.
About 2 g of the sample (including ceramic support) was fixed
by glass wool in a stainless-steel tube of 10 mm i.d. as shown in
Fig. 1. A liquid mixture of CH3OH and H2O was fed at a speed
of 0.15 cc/min using an automated syringe and vaporized at
1501C. A molar ratio of H2O:CH3OH5 4:1 was used (gaseous
flow rates of 120 cc/min H2O and 30 cc/min CH3OH). The
temperature of the reactor was controlled by a thermocouple
A
tional devices.1,2 As the properties of the composite are influ-
enced by dispersion morphology of the metal particles, such as
the particle size, the number density, and homogeneity, it is im-
portant to control it. In general, an impregnation method is used
to prepare fine catalysts on the ceramic substrate. However, us-
ing the conventional method, it is difficult to control the dis-
persion and aggregation of the metal particles. The partial
reduction method makes it possible to provide composite ho-
mogeneously dispersed metal nanoparticles at the ceramic sur-
face.3,4 In this method, an oxide solid solution, composed of
reducible oxide and irreducible oxide, is used as a precursor. By
heating the precursor in a reductive atmosphere, only unstable
oxide is reduced, producing metal nanoparticles in situ. This
method is effective in making a homogeneous texture without
aggregation for the samples in any form. The composite has
several features, notably fine and well-dispersed metal nanopar-
ticles and strong bonding with ceramic substrate. Dispersion
morphology of the metal particles is controllable by changing
the composition and reduction conditions. Furthermore, it is
thermally stable because it is produced at a temperature higher
than that of use. In our previous paper, we studied Ni/MgO
catalysts fabricated by partial reduction of NiO–MgO solid so-
lution.5,6 We found that a small amount of Al2O3 contained in
the MgO raw powder drastically changed the dispersion mor-
phology of the Ni nanoparticles.6 It is considered that the Al31
doped into MgO created lattice defects and promoted an in-
J. H. Adair—contributing editor
Manuscript No. 20161. Received October 18, 2004; approved February 15, 2005.
This work has been supported by the New Energy and Industrial Technology Devel-
opment Organization (NEDO), as part of the Synergy Ceramics Project promoted by the
Ministry of Economy, Technology and Industry (METI), Japan.
wAuthor to whom correspondence should be addressed. e-mail: takayuki.fukasawa@
toshiba.co.jp
2938

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Communications of the American Ceramic Society
2939
N
2
Thermocouple
(a)
(c)
Sample
Syringe pump
Vaporizer Reformer
Out gas
GC
CH OH+H O
150°C 250-400°C
3
2
mixture
Cooling condenser
1µm
1µm
Fig. 1. Schematic of the methanol reforming test.
(b)
(d)
located in the catalyst bed. The reactor was heated by an electric
furnace. Vapors (unreacted CH3OH and H2O) were trapped
using a cooling condenser. The gas lines were heated at about
1501C by heating tapes. 50 cc/min N2 was used as both the car-
rier gas and the reference gas for the gas analyzer. Gas analysis
was performed using a gas chromatography (micro-GC; CP400,
Varian Inc., Palo Alto, CA), equipped with a thermal conduc-
tivity detector. CP-Cox column (Ar carrier) was used for the
1µm
1µm
Fig. 2. Scanning electron microscopy micrographs of Ni/MgO com-
posites after the reduction. (a) No doping, (b) Al2O3 doping, (c) Cr2O3
doping, (d) Sc2O3 doping.
˚
separation of H2 and CO2, while Molsieve5A column (He
carrier) was used for the separation of CO, CH4, and N2.
The methanol conversion XCH OH is assumed by Eq. (1), con-
3
sidering that CH3OH was converted to carbon-containing prod-
ucts (CO, CO2, and CH4). And the selectivity of the carbon-con-
taining products, SCO, SCO , and SCH , is given by Eqs. (2)–(4).
amount of Al2O3, Cr2O3, or Sc2O3 drastically changed the dis-
persion morphology of the Ni nanoparticles. These dopants can
be dissolved into MgO substrate and they create the lattice de-
fects (Shottky defects), replacing Mg21 to Me31 (Me: Al, Cr,
Sc). Therefore, it is thought that the defects made the diffusion
rate of the Ni ions to the ceramic surface (or grain boundary)
increase during the reduction process. Henriksen reported that
the solid solubility of Cr2O3 and Sc2O3 into MgO was much
larger than that of Al2O3.10 This suggests that more defects were
formed for the materials with Cr2O3 and Sc2O3 than those with
Al2O3, and it promoted the production of more Ni particles.
Furthermore, the matrix grain size of the composite was
smaller than that of the material without doping. This is con-
sidered to be because the dopant that exceeded the solution limit
remained at the grain boundary of the matrix and prevented the
grain growth of the MgO.
2
4
XCH OH ¼ ðFCO þ FCO þ FCH Þ=FCH OH ꢀ 100
(1)
(2)
(3)
(4)
3
2
4
3
SCO ¼ FCO=ðFCO þ FCO þ FCH Þ ꢀ 100
2
4
SCO ¼ FCO =ðFCO þ FCO þ FCH Þ ꢀ 100
2
2
2
4
SCH ¼ FCH =ðFCO þ FCO þ FCH Þ ꢀ 100
4
4
2
4
where F represents the molar flow rates of the gas products.
Every reforming test was performed under the condition of
W/F 5 4 gcat ꢁ h ꢁ molꢂ1 (W is the catalyst weight and F the total
flow rate of feed gas), and the gas hourly space velocity was
about 7000 hꢂ1
.
The amount of precipitation and the size of metal particle are
controllable by changing the material composition and reduc-
tion conditions. In this study, the materials reduced at 10001C
were used for methanol reforming test to clarify the effect of
doping materials.
Table II shows the reforming performance at 3501 and 4001C
for the Ni-based catalysts. Methanol was completely converted
at these temperatures. The CO production rapidly decreased at a
certain temperature. The temperature was shifted to a lower
III. Results and Discussion
The weight loss of the NiO–MgO solid solution with no doping
was 0.4% at 10001C during the reduction. Various oxide do-
pants were examined before this study. Among them, it was
found that three dopants containing trivalent cation, Al2O3,
Cr2O3, and Sc2O3, showed incomparable weight loss during the
reduction, promoting the precipitation of the Ni particles. Ex-
istence of the nickel was confirmed by XRD. The material prop-
erties used in this study are shown in Table I. The weight loss is
the total amount after the reduction at 10001C for 10 min. The
precipitation of the metal particles started from about 7001C for
all the samples. It was confirmed by weight loss in TG meas-
urement. It suggests that the materials should be steadily used at
a temperature lower than that. The active metal surface area for
the materials with dopant, which is related to catalytic activity,
increased remarkably. Figure 2 shows the microstructures of the
Ni/MgO composites with each dopant, comparing them with
doping-free material. It was difficult to catch the Ni particles in
the dopant-free material because they were very fine in spite of
reducing at 10001C. On the other hand, the addition of a small
Table II. Methanol Conversion and Product Composition
on the Ni-Based Catalysts Produced by Partial Reduction
Reaction
Ni
Product composition
No doping Al2O3 doping Cr2O3 doping Sc2O3 doping
3501C
Conversion (%) 100
H2 (%)
CO (%)
CO2 (%)
CH4 (%)
CO2 selectivity
100
63.0
2.2
23.2
11.6
62.7
100
65.0
5.4
21.2
8.4
100
63.6
0.5
24.3
11.7
66.5
64.9
28.7
4.6
1.9
13.1
60.6
Table I. Material Properties Used in This Study
4001C
Conversion (%) 100
Doping amount Density
(g/cm3)
Weight loss during Active metal surface
area (m2/g)
100
100
100
Dopant
(mol%)
reduction (%)
H2 (%)
CO (%)
CO2 (%)
CH4 (%)
CO2 selectivity
68.7
60.0
0.6
24.3
15.1
60.8
59.9
0.9
24.2
15.0
60.3
59.3
0.7
24.0
16.0
59.0
1.4
23.8
6.1
Non
0
3.75
3.34
2.94
3.73
ꢂ0.4
ꢂ6.3
0.20
—
1.45
1.47
Al2O3
Cr2O3
Sc2O3
0.1
0.1
0.1
ꢂ10.3
ꢂ10.1
76.0

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Communications of the American Ceramic Society
Vol. 88, No. 10
CH4_s
CO2_s
CO_s
Table III. Methanol Conversion and Product Composition
on the Co-Based Catalysts Produced by Partial Reduction
Reaction
100%
90%
80%
32.0
66.5
39.3
Co
70%
60%
No doping (1C)
350 400
Sc2O3 doping (1C)
350 400
Product composition
50%
40%
30%
20%
10%
0%
Conversion (%)
H2 (%)
CO (%)
CO2 (%)
CH4 (%)
CO2 selectivity
100
69.7
100
72.6
100
100
73.3
1.2
71.9
1.6
59.0
16.2
13.6
0.5
6.8
20.4
0.3
23.9
1.7
23.6
2.8
44.9
74.2
89.4
84.1
1.7
400
1.5
350
Temperature/°C
CH4_s
CO2_s
CO_s
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Fig. 3. Selectivity of the carbon-containing products for the reforming
on the Ni-based catalysts with the Sc2O3 dopant.
6.2
10.1
temperature by the dopant. In particular, the Sc2O3 doping ma-
terial showed the lowest CO production at 3501C. However, it
seemed that the enhanced Ni particles also caused the increase of
the CH4 production. This is not preferable because it leads to the
decrease of the H2 production as follows (Eq. (5)):
89.4
84.1
CO þ 3H2 , CH4 þ H2O
(5)
The selectivity of the carbon-containing products for the ma-
terial with Sc2O3 dopant is shown in Fig. 3. The CH4 production
increased with the reforming temperature.
5.8
4.3
350
400
On the other hand, these dopants were applied to the Co-
based catalysts. Figure 4 shows the microstructure of the Co/
MgO with Sc2O3 dopant. Enhanced Co precipitation was ob-
served at the CoO–MgO solid solution surface as well as in the
Ni-based materials. The product composition of methanol re-
forming is shown in Table III, compared with that of the ma-
terial without doping. It was observed that the Sc2O3 doping
material was active at 3501C, indicating 100% methanol con-
version. And the CO production decreased, maintaining low
CH4 production. This is thought to be because the water–gas
shift (WGS) reaction occurred simultaneously with the reform-
ing reaction.
Temperature/°C
Fig. 5. Selectivity of the carbon-containing products for reforming on
the Co-based catalysts with the Sc2O3 dopant.
The selectivity of the carbon-containing products for the Co-
based catalysts with Sc2O3 dopant is shown in Fig. 5. As for this
material, no change in performance and no significant deacti-
vation were observed in experiments repeated many times. At
present, it is well known that the Cu-based catalyst is the most
effective for methanol reforming. However, it has low thermal
stability, and significant deactivation occurs after the oxidiza-
tion. So, the developed material deserves attention as a non-
precious-metal catalyst with outstanding reforming perform-
ance. Although the specific surface area of the material is as
small as about 1 m2/g, the dispersion morphology of the metal
nanoparticles makes high catalytic activity possible. Optimiza-
tion of the dispersion morphology of the metal particles is nec-
essary to further improve performance. This will be clarified in
the next paper.
CO þ H2O , H2 þ CO2
(6)
The WGS reaction takes advantage of the extra H2 produc-
tion. Consequently, high CO2 and H2 selectivity were achieved.
The Ni/MgO and Co/MgO systems are also among the prom-
ising candidates for methane and ethanol-reforming catalyst.
Regarding the material we developed, applications to these var-
ious fuels are expected.
We developed a new composite material having a high cat-
alytic performance for methanol reforming. This technology is
not limited to methanol reforming. We expected it to lead to
development of other new functional materials.
Acknowledgments
The authors are Members of the Joint Research Consortium of Synergy
Ceramics.
1µm
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