Synthesis and properties of core-shell metal-ceramic microstructures and their application as heterogeneous catalysts

Article abstract of DOI:10.1002/cctc.201402274

A facile synthesis of core-shell metal-ceramic microcomposites with unique structural and chemical properties and their superior performance as heterogeneous catalysts are reported. These core-shell microstructures were prepared by hydrothermal surface oxidation of Al particles in water with or without the presence of heterometal species followed by calcination. The crystalline porous MgAl₂O₄ spinel, which has complex morphological features, allowed the superior dispersion and stability of Rh clusters, and the core-shell microarchitecture provided facilitated heat and mass transport concomitantly, which enabled a significant enhancement in the glycerol conversion turnover rate to hydrogen compared to the conventional Rh/MgAl₂O₄ catalyst. This simple and scalable heterogeneous catalyst design approach offers practical applicability for catalytic reactions that require extensive heat and mass flux and rapid process response, such as for micro-fuel-processing in fuel cells.

Full text of DOI:10.1002/cctc.201402274

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DOI: 10.1002/cctc.201402274
Synthesis and Properties of Core–Shell Metal–Ceramic
Microstructures and their Application as Heterogeneous
Catalysts
Jieun Kim and Doohwan Lee*[a]
A facile synthesis of core–shell metal–ceramic microcomposites
with unique structural and chemical properties and their supe-
rior performance as heterogeneous catalysts are reported.
These core–shell microstructures were prepared by hydrother-
mal surface oxidation of Al particles in water with or without
the presence of heterometal species followed by calcination.
The crystalline porous MgAl2O4 spinel, which has complex mor-
phological features, allowed the superior dispersion and stabili-
ty of Rh clusters, and the core–shell microarchitecture provided
facilitated heat and mass transport concomitantly, which ena-
bled a significant enhancement in the glycerol conversion
turnover rate to hydrogen compared to the conventional Rh/
MgAl2O4 catalyst. This simple and scalable heterogeneous cata-
lyst design approach offers practical applicability for catalytic
reactions that require extensive heat and mass flux and rapid
process response, such as for micro-fuel-processing in fuel
cells.
Introduction
The heat and mass transfer properties of heterogeneous cata-
lysts and their intrinsic kinetics are important factors that
impact the performance of heterogeneous catalysts in practical
applications that require intensive heat and mass flux.[1] Highly
endothermic or exothermic catalytic reactions provide good
examples, such as fuel reforming and preferential CO oxidation
for H2 production in fuel cell microreactors.[2a,b,c,d] It was report-
ed that the enhancements in the heat transport properties of
a Ru catalyst supported on the Al2O3@Al metal–ceramic core–
shell microcomposite improved the reaction rates of methane
stream reforming considerably. This is a highly endothermic re-
action and often involves heat-transport limitation in practical
applications.[3] The superior heat conductivity of metal–ceramic
composites can also provide a rapid process response, which is
crucial for systems that undergo frequent on–off operations
such as micro-fuel-processors in fuel cells and catalytic convert-
ers in automobiles. A common approach to obtain metal–ce-
ramic composites is the wash-coating of porous metal oxides
over metallic substrates, but the resulting structures are usually
prone to degradation at the interfaces. The anodic surface oxi-
dation of Al substrates is also practiced widely for corrosion
protection of the Al substrate and the production of porous
alumina nanofilters.[4] However, these processes are inefficient
for small substrates such as powders or pellets that are the
typical forms of heterogeneous catalysts.
Homogeneous hydrothermal synthesis of various oxide
micro- and nanomaterials with specific properties and applica-
bility have been reported extensively.[5a,b] However, studies on
metal–ceramic composites are relatively limited. The mecha-
nism of the heterogeneous formation of an alumina film on
the Al substrate is well established and can be described as
aluminum hydroxide formation through hydrothermal surface
oxidation of the Al substrate followed by phase transformation
into crystalline Al2O3 by dehydration.[6a,b] Recent studies have
reported the mechanisms of Al2O3@Al core–shell structure for-
mation[7] and the morphological variation of the Al2O3 shell
under various physicochemical hydrothermal conditions.[8]
Herein, we first report a direct and facile synthesis of core–
shell microstructures that consists of a highly heat-conductive
Al core with a high-surface-area crystalline MeAl2O4 (Me=Mg,
Co, Zn) spinel oxide shell. Markedly, core–shell microarchitec-
tures of various compositional and morphological features can
be formed by the simple hydrothermal oxidation of Al sub-
strates without using templates or agents to result in unique
structural and surface characteristics. Furthermore, we demon-
strate that these core–shell metal–ceramic microstructures fa-
cilitate the heat and mass transport required for the catalytic
reactions by using MgAl2O4@Al as the support for a Rh catalyst
for glycerol steam reforming to hydrogen [Eq. (1)], which is
a highly endothermic reaction.
C3H8O3þ3 H2O Ð 3 CO2þ7 H2 DHo ¼ 128 kJ molꢀ1
298
ð1Þ
[a] J. Kim, Prof. D. Lee
Department of Chemical Engineering
University of Seoul
Jeonnong-dong, Seoul 130-743 (Republic of Korea)
Fax: (+82)2-6490-2364
The complex and high-surface-area MgAl2O4 shell provided
an enhanced dispersion and stability of Rh clusters against
thermal sintering, and the metal–ceramic microcomposite
structure facilitated the heat and mass transport concomitantly
to give rise to the enhancements in glycerol conversion and
E-mail: dolee@uos.ac.kr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402274.
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reaction turnover rates under various space velocity conditions.
This controlled heterogeneous synthesis of MeAl2O4@Al metal–
ceramic core–shell microcomposites with various chemical and
morphological features is unprecedented and sheds light on
their practical application as heterogeneous catalyst supports
for reactions that require intensive heat and mass flux.
Results and Discussion
Hydrothermal synthesis of MeAl2O4@Al microstructures
The g-Al2O3@Al core–shell microstructures were obtained by
a one-step hydrothermal surface oxidation of Al particles in
water between 423 and 473 K in an autoclave (Scheme 1a).
The hydrothermal reaction in aqueous heterometal solution
gave rise to the core–shell microstructures that consist of an Al
core and a multimetal oxide shell of various compositional and
morphological characteristics (Scheme 1b). The type of addi-
tive heterometal salts was not limited as long as they did not
precipitate homogenously, which depended largely on precur-
sor type, hydrothermal temperature, and solution pH. Hydro-
thermal reactions of Al particles in water (473 K, 3 h) formed
boehmite crystallites on the surface (Supporting Information
Figure A1), which transformed into g-Al2O3 crystallites to estab-
lish a densely packed shell of approximately 2–3 mm thick after
calcination at 823 K (Figure 1a and b). The SEM micrographs
show submicron g-Al2O3 crystallites oriented randomly with
plate and rhombic shapes deposited densely on the surface to
form the Al2O3@Al core–shell metal–ceramic composite
structure.
The composition and surface morphologies of MeAl2O3@Al
(Me=Mg, Zn, Co) microstructures prepared in the heterometal
precursor solutions were markedly different from those of
Al2O3@Al. The properties were dependent on the type of heter-
ometal precursors and their concentration. MgAl2O3@Al pre-
pared in Mg(NO3)2 solution displayed a petal-like surface mor-
phology that consisted of curved MgAl2O4 thin plates packed
densely that are grown perpendicular to the Al core (Fig-
ure 1c) with an average shell thickness of approximately 2–
3 mm (Figure 1d). Similar petal-like morphological features with
Figure 1. SEM micrographs of core–shell microstructures: a,b,e) Al2O3@Al,
c,d) MgAl2O4@Al, f) CoAl2O4@Al, g,h) ZnAl2O4@Al. Cross-sectional features
(b,d) were obtained after sample cutting by FIB. The hydrothermal tempera-
ture was 473 K for samples S1–S2 (3 h) and 423 K for S3–S6 (3 h). Additive
heterometal precursors were nitrates except for that of S3 (MgCl2). The con-
centration of additive metal salts in water for the hydrothermal reaction was
0.93m (S2–S5) and 1.85m (S6). Samples were calcined at 823 K for 4 h.
various sizes and packing patterns were also devel-
oped on CoAl2O4@Al (Figure 1 f) and ZnAl2O4@Al (Fig-
ure 1h) if nitrate additive metal salts were used. In
particular, the concentration of Zn(NO3)2 solution af-
fected the shell morphology of ZnAl2O4@Al signifi-
cantly (Figure 1g) to result in dense needle-shaped
crystallites on the Al surface at low precursor concen-
tration. The type of additive heterometal precursor
had significant effects on the resulting structures.
The core–shell microstructure formed in MgCl2 aque-
ous solution (Figure 1e) had an urchin-like shell mor-
phology that consisted of g-Al2O3 oxide rods devel-
oped densely that grew perpendicular to the Al sur-
face, which was completely different from the petal-
like MgAl2O4 shell developed in Mg(NO3)2 solution.
Scheme 1. Synthesis of core–shell microstructures that consist of an Al core and porous
metal oxide shell.
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This was because of the homogenous precipitation of MgCl2
into magnesium hydroxides under these conditions, which pre-
vented interaction with the Al substrate at an atomic level. The
results also reflect that the type of anion in the additive heter-
ometal precursor had significant effects on the composition
and morphological characteristics of metal oxide shells grown
hydrothermally. The anisotropic growth of boehmite crystallites
toward the [100] direction attributes the preferential adsorp-
ed that they were polycrystalline and consisted of small pri-
mary crystallites of 2–17 nm (Supporting Information Table A1).
X-ray photoelectron spectroscopy (XPS) results also indicated
the formation of spinel bimetal crystallites with binding ener-
gies of the constituent atoms in MgAl2O4 (Mg1s=1303.8,
Al2p=74.1 eV), CoAl2O4 (Co2p=780.5, Al2p=74.1 eV), and
ZnAl2O4 (Zn2p3=1022, Al2p=74.2 eV) shells, which agreed
well with the values of the crystalline spinel samples prepared
by a coprecipitation method with the stoichiometric atomic
ratio of the constituents (Supporting Information Figure A3).
The surface composition characterized by XPS (Supporting In-
formation Table A2) and energy-dispersive X-ray spectroscopy
(EDX; Supporting Information Figure A4) also confirmed the
formation of bimetal oxide shells on these samples.
tion of anions (adsorption affinity: SO42ꢀ >Clꢀ >NO3 ) onto
ꢀ
other crystal facets, such as (010) and (001), and has been re-
ported in the homogenous hydrothermal synthesis of various
Al2O3 nanostructures.[9a,b,c] Such analogical effects likely oc-
curred for the unidirectional growth of g-Al2O3 crystallites on
the Al2O3@Al microstructure prepared in aqueous MgCl2 solu-
tion. Notably, the primary core–shell particles of these samples
(S1–S6) agglomerated into a foam-like macrostructure that re-
sulted in macro-to-micro hierarchical pore networks if hydro-
thermal synthesis was conducted under a mechanically com-
pressed environment in a confined space (Supporting Informa-
tion Figure A2).
The TEM micrographs and selected area electron diffraction
results shown in Figure 3 also reveal that the metal oxide mi-
Powder XRD patterns of the samples confirmed the forma-
tion of core–shell metal–ceramic composites (Figure 2). These
Figure 3. TEM micrographs and selected area electron diffraction patterns
(inset) of metal oxide crystallites formed on a,c) Al2O3@Al, b) MgAl2O4@Al,
d) CoAl2O4@Al, and e,f) ZnAl2O4@Al microstructures.
Figure 2. Powder XRD patterns of a,c) Al2O3@Al, b) MgAl2O4@Al,
d) CoAl2O4@Al, and e,f) ZnAl2O4@Al. Samples are designated as S1–S6, which
corresponds to Figure 1. Al metal (c) and the metal oxide (c) XRD
references are shown with JCPDS file numbers.
crostructures formed on Al metal particles consist of primary
crystallites of different shapes. On the surface of Al2O3@Al (Fig-
ure 3a, S1), g-Al2O3 crystallites packed randomly with lamellar
pores, which is known to develop parallel to the [100] direc-
tion of boehmite, were observed. The unique g-Al2O3 rods
formed on Al2O3@Al (S3) exhibited largely anisotropic lamellar
pores in the axial direction perpendicular to the Al surface. The
MgAl2O4 plates that collectively formed a petal-like morpholo-
gy on MgAl2O4@Al (S2) were polycrystalline. They consisted of
samples exhibited characteristic diffraction peaks both for the
Al core and the metal oxide shells. The results revealed that
the shells on Al2O3@Al (S1 and S3) were crystalline g-Al2O3,
whereas those on MgAl2O4@Al (S2), CoAl2O4@Al (S4), and ZnA-
l2O4@Al (S5 and S6) were spinel bimetal crystallites. Monome-
tallic oxides were not observed on these samples (S2, S4–S6).
The sizes of the primary crystallites on oxide shells were deter-
mined from the Debye–Scherrer equation. The results indicat-
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Glycerol steam reforming on Rh/MgAl2O4@Al
Table 1. Specific properties of metal oxide shells on the samples.
Sample
Shell thickness[a] Surface area[b] Average pore size Pore volume[b]
The time on stream of glycerol conversion on the Rh/
MgAl2O4@Al and Rh/MgAl2O4 catalysts is shown in
Figure 5 [723 K, H2O/C3H8O=4.5, weight hourly space
velocity (WHSV)=34000 mLgꢀ1 hꢀ1]. Catalyst deacti-
vation was observed during the glycerol steam re-
forming reaction, and the extent of deactivation was
considerably less on the Rh/MgAl2O4@Al catalyst.
TEM analysis conducted on the fresh and spent cata-
lysts suggested clearly that sintering of the dispersed
Rh clusters occurred substantially less on Rh/MgA-
l2O4@Al (Figure 4b) than Rh/MgAl2O4 (Figure 4d). This
result suggested that the complex morphological
and surface properties of the MgAl2O4 shell devel-
oped hydrothermally hampered the aggregation of
[mm]
[m2 gꢀ1
]
[nm]
[cm3 gꢀ1
]
Al2O3@Al (S1)
2.4
150 (133)
224 (163)
196 (111)
113 (90)
112 (73)
156 (82)
148
3.8
3.6
4.0
3.6
3.6
3.6
4.8
5.1
5.7
0.18 (0.16)
0.44 (0.32)
0.33 (0.19)
0.28 (0.22)
0.29 (0.19)
0.42 (0.22)
0.28
MgAl2O4@Al (S2) 1.5
MgAl2O4@Al (S3) 1.0
CoAl2O4@Al (S4) 1.7
ZnAl2O4@Al (S5) 1.1
ZnAl2O4@Al (S6) 0.8
MgAl2O4 (cop)[c]
ZnAl2O4 (cop)
CoAl2O4 (cop)
–
–
–
76
124
0.13
0.22
[a] Calculated based on an average diameter of 10 mm by using phase contents ob-
tained from XRD results. [b] Values in parenthesis are the overall properties of core–
shell microstructures, which include the Al core. [c] cop: Sample obtained by copreci-
pitation at stoichiometric atomic ratio.
small nanocrystallites in rod and flake shapes that differed
Table 2. Specific properties of metal oxide shells on the samples.
CO chemisorption[a] Rh dispersion Rh cluster size[b]
from the round nanocrystallites developed on CoAl2O4@Al (S4)
and ZnAl2O4@Al (S6). The TEM results of ZnAl2O4@Al (S5; Fig-
ure 3e) suggested that the needle-shaped crystallites observed
in the SEM micrograph (Figure 1g) might be bundles of
ZnAl2O4 nanotubes. The thickness, BET surface area, average
pore size, and pore volume of the metal oxide shells on the
core–shell microstructures are presented in Table 1. For com-
parison, the properties of MgAl2O4, ZnAl2O4, and CoAl2O4 spinel
obtained by the coprecipitation method are also displayed
(complete N2 sorption isotherms and pore size distribution re-
sults are shown in Supporting Information Figure A5). The re-
sults revealed that the surface area of MgAl2O4 and ZnAl2O4
shells on the core–shell microstructures were substantially
larger than their coprecipitated counterparts. The pore size of
core–shell microstructures was much more uniform than that
of the coprecipitated counterparts, with average values
between 3.6 and 4.0 nm.
Catalyst
[m2 gꢀ1
]
[%]
[nm]
Rh/MgAl2O4@Al 109
Rh/MgAl2O4 83
37
29
2.9
3.8
[a] Obtained at 308 K after sample reduction at 823 K in H2. [b] Calculated
with the assumption of Rh hemisphere particles on the surface.
Rh catalyst supported on the MgAl2O4@Al microstructure
The sharp and complex topological features with enhanced
surface area and porosity on these metal oxide shells can pro-
vide superior dispersion of active metals and increase stability
against sintering. Simultaneously, the highly heat-conductive
Al core and hierarchical pore structures on the unique metal
oxide shells can facilitate heat and mass transfer as catalysts.
The constructive effects of the core–shell metal–ceramic com-
posites were observed on the glycerol steam reforming reac-
tion conducted on a Rh catalyst (3.0 wt% Rh) supported on
MgAl2O4@Al (S2) in comparison to that supported on MgAl2O4
(coprecipitated). The specific properties of the catalysts ob-
tained from CO chemisorption measurements are presented in
Table 2. A higher Rh dispersion was observed on the MgA-
l2O4@Al core–shell samples. The average size of the Rh clusters
on fresh Rh/MgAl2O4@Al and Rh/MgAl2O4 was 2.9 and 3.8 nm,
respectively, which agrees well with the observations from
TEM analysis (Figure 4a and c).
Figure 4. TEM micrographs of fresh and spent Rh/MgAl2O4@Al and Rh/
MgAl2O4 catalysts (3 wt% Rh) used for glycerol stream reforming at 823 K
for 24 h (H2O/C3H8O3 =4.5, WHSV=34000 mLgꢀ1 hꢀ1).
dispersed Rh clusters effectively. In the case of Pt clusters, the
markedly high stability of Pt nanoclusters on MgAl2O4 spinel
because of the strong interaction ability of spinel {111} facets
was reported.[10]
The constructive contributions of MgAl2O4@Al, glycerol con-
version, and conversion turnover rate (TOR) on Rh/MgAl2O4@Al
were compared to those on Rh/MgAl2O4 for various space ve-
locity conditions (Figure 6). The dispersion and size of the Rh
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Figure 5. Time on stream of glycerol conversion on Rh/MgAl2O4@Al and Rh/
MgAl2O4(coprecipitated) (3 wt% Rh loading, 723 K, H2O/C3H8O=4.5,
WHSV=34000 mLgꢀ1 hꢀ1).
Figure 7. XRD (top) and SEM (bottom) characterization results for the fresh
and spent Rh/MgAl2O4@Al (823 K for 24h, WHSV=17000 gꢀ1 hꢀ1) samples.
!
*
^
XRD peaks: Al metal ( ), MgAl2O4 ( ), Rh ( ).
precursor. The application of these unique structures and prop-
erties could be versatile and include heterogeneous catalysts
for fuel cells and catalytic converters for automobiles, in which
rapid process response is of great importance under frequent
on–off operations.
Figure 6. Initial glycerol conversion [%] and glycerol conversion TOR [hꢀ1] on
Rh/MgAl2O4@Al and Rh/MgAl2O4 catalysts taken after 4 h on stream (823 K,
reactant H2O/C3H8O3 feed ratio=4.5).
Conclusions
The heat and mass transport characteristics of heterogeneous
catalysts are important properties that should be considered
for practical reactor applications as the majority of catalytic re-
actions are coupled with heat transfer, fluid dynamics, and sur-
face reaction kinetics. In this aspect, the core–shell metal–ce-
ramic microstructures prepared by the facile hydrothermal sur-
face oxidation of an Al substrate reported in this work could
provide facilitated heat and mass transfer through the catalysts
because of 1) the enhanced thermal conductivity of the metal–
ceramic composites and 2) the macro-to-micro hierarchical
pore networks on the shell structures. We first reported the
simple and effective hydrothermal synthesis and properties of
MeAl2O4@Al (Me=Mg, Co, Zn) core–shell microstructures that
consist of an Al core encapsulated by MeAl2O4 spinel. As a cata-
lyst support, the high surface area and complex surface mor-
phological features of MgAl2O4@Al provided an improved dis-
persion of Rh clusters and also hampered their thermal sinter-
ing during glycerol stream reforming reactions at elevated
temperatures. Rh/MgAl2O4@Al showed a 20–30% higher glyc-
erol conversion turnover rate than Rh/MgAl2O4 (both have sim-
ilar Rh cluster sizes), which demonstrates that the facilitated
heat and mass transport through the unique MgAl2O4@Al mi-
crostructures combined constructively for the reactions. Some
clusters on these catalysts were similar, and the TOR values
were calculated from the initial rates taken before sizable cata-
lyst deactivation at a time on stream of 4 h. Therefore, TOR
comparison reflected the contribution of heat and mass trans-
port properties on the catalyst structures fairly. The results ex-
hibited approximately 20–30% higher TOR values on Rh/MgA-
l2O4@Al than on Rh/MgAl2O4 at various space velocity condi-
tions. This result demonstrated that facilitated heat transfer
through the MgAl2O4 metal–ceramic composite on Rh/MgA-
l2O4@Al enhanced the extent of the glycerol reforming reaction
significantly with higher conversion TOR values. The core–shell
microstructure and surface morphology of Rh/MgAl2O4@Al re-
mained the same without sizable changes as confirmed from
XRD and SEM results for the fresh and spent (823 K for 24 h)
catalysts (Figure 7). The results suggested that the core–shell
developed hydrothermally and morphological structures were
thermally and chemically stable for the reactions without deg-
radation at the metal–oxide interface and at the shell surface.[3]
The facile and effective synthesis approach for core–shell
metal–ceramic composites presented in this work can be ap-
plied to various architectures, which include plates, tubes, and
monoliths and depended on the initial shape of the Al metal
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cused ion beam (FIB) unit. Powder XRD (ATX-G, Rigaku) patterns of
the samples were obtained using monochromic CuKa radiation at
40 kV and 40 mA with a scan rate of 1 s per step (0.028). The BET
surface area, pore size distribution, and total pore volume of sam-
ples were characterized by N2 adsorption–desorption isotherms ob-
tained by using a volumetric unit (NOVA 2200e, Quantachrome).
The chemical composition and atomic bonding state of metal
oxides were analyzed by XPS (PHI 5000 VersaProbe, Ulvac-PHI) and
EDX. The shell thickness of the core–shell microstructures was cal-
culated from the area ratio of the XRD main peak for Al and metal
oxides with reference values predetermined for known Al and
metal oxide mixtures. The specific surface area and pore volume of
shells were determined by accounting for composite composition.
The primary crystal size of the metal oxide shell was calculated
from the Debye–Scherer equation. The structure and properties of
supported Rh catalysts were characterized by high-resolution trans-
mission electron microscopy (HR-TEM; JEM-3010, JEOL) operated at
300 kV. The dispersion and size of the Rh clusters were calculated
from CO chemisorption uptake on the catalysts obtained at 308 K
after in situ sample reduction in H2 at 823 K for 1 h followed by
purging in a He flow for 2 h (AutoChem 2910, Micromeritics).
further important issues are not discussed in this work, which
include the quantification of the apparent thermal conductivity
of the core–shell microstructures at various oxide layer thick-
nesses and the stability of the structures at the metal–ceramic
interface under harsh thermal cyclic operations. These are con-
sidered as the subject of the future work.
Experimental Section
Core–shell microstructured metal–ceramic composites were pre-
pared by the hydrothermal surface oxidation of Al particles (Good-
fellow, Øꢁ25 mm) in deionized water or an aqueous solution of
additive metal precursor: Mg(NO3)2·6H2O (Junsei), MgCl2·6H2O
(Junsei), Co(NO3)2·6H2O (Alfa), and Zn(NO3)2·6H2O (Aldrich). In a typi-
cal synthesis, Al powder (2.0 g) was placed in a Teflon-lined auto-
clave (100 mL), and deionized H2O or aqueous metal precursor so-
lution (40 mL) was added. Hydrothermal synthesis was conducted
at temperatures between 423 and 473 K for 3 h under autogenous
pressure conditions. The temperature was maintained constantly
by using a proportional-integral-derivative (PID) controller. The re-
sulting samples were washed with deionized water several times,
filtered, dried at 393 K for 12 h, and calcined at 823 K for 4 h
(ramp=10 Kminꢀ1). For comparison, MgAl2O4, CoAl2O4, and
ZnAl2O4 spinel were prepared by a coprecipitation method. Briefly,
aqueous Al(NO3)3·9H2O (Junsei) solution (200 mL, 0.5m) was mixed
with additive metal precursor solution (200 mL, 1.0m) in a beaker
at RT with vigorous stirring. The pH of this mixture was adjusted
continuously to 9.5 by the dropwise addition of an aqueous am-
monia solution (8.0m). The precipitates were collected and treated
in the same way as the core–shell composites.
Acknowledgements
Financial support was provided by the Korea Research Founda-
tion (KRF 2011-0010095) and the Human Resources Development
Program of the Korea Institute of Energy Technology Evaluation
and Planning (KETEP) under a grant by the Korea Government
Ministry of Trade, Industry and Energy (2012-4010203260).
Rh catalysts supported on MgAl2O4@Al2O3 and MgAl2O4 were pre-
pared by incipient wetness impregnation with an aqueous solution
of RhCl3·xH2O (Alfa). The resulting samples were dried at 393 K for
12 h and calcined at 873 K in air for 4 h (ramp=10 Kminꢀ1). Glycer-
ol stream reforming was conducted with these catalysts at 823 K in
a fixed-bed tubular quartz glass reactor (OD=10, ID=8, L=
450 mm). Catalysts between 250 and 425 mm in size were packed
in the middle of the reactor with a fritted disk and quartz wool.
The reactor was installed vertically in a PID controlled tubular fur-
nace, and the catalyst bed temperature was measured by using
a K-type thermocouple attached onto the outside reactor wall. Cat-
alysts were reduced in a flow of H2 (50 mLminꢀ1) at 823 K for 1 h
and subsequently purged in Ar. Glycerol steam reforming was con-
ducted by the introduction of a glycerol/water reactant mixture
(H2O/C3H8O molar ratio=4.5) by using an HPLC pump (Series II,
LabAlliance). The amount of packed catalyst was fixed, whereas
the reactant flow rate varied with WHSV values of 17000, 34000,
and 68000 mLgꢀ1 hꢀ1. The liquid feed was evaporated by using
a preheater (573 K) under Ar carrier gas flow (1 mol% N2/balance
Ar, Ar/(C3H8O+H2O)=0.74). The condensable liquid effluents were
separated by a condenser maintained at 269 K and analyzed by
using GC (7890A, Agilent) equipped with a flame ionization detec-
tor (FID). Gaseous products were analyzed online by using a GC
(7890A, Agilent) equipped with a thermal conductivity detector
(TCD).
Keywords: aluminum · ceramics · heterogeneous catalysts ·
hydrothermal synthesis · rhodium
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The surface morphology and cross-sectional structure of the sam-
ples were obtained by SEM (XL-30 FEG, FEI) equipped with a fo-
Received: April 25, 2014
Published online on July 24, 2014
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