One-pot synthesis of uniform mesoporous rhodium oxide/alumina hybrid as high sensitivity and low power consumption methane catalytic combustion micro-sensor

Article abstract of DOI:10.1039/c2jm15870a

Uniform mesoporous rhodium oxide/alumina hybrid have been prepared following a facile one-pot self-assembly approach using P123 as template. Such hybrid nanomaterials display a high dispersion of both the noble metal oxide and the alumina within mesoporous structure in a broad Rh/Al mole ratio up to 8:1, which makes them attractive materials for catalytic applications. After coating on MEMS micro-heater, a catalytic combustion type methane gas micro-sensor was fabricated and investigated for its sensing performance. The mesostructure-based sensor demonstrated a short T₉₀ response time, relatively high signal output, high enough signal/noise ratio and extraordinarily low power consumption.

Full text of DOI:10.1039/c2jm15870a

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PAPER
One-pot synthesis of uniform mesoporous rhodium oxide/alumina hybrid as
high sensitivity and low power consumption methane catalytic combustion
micro-sensor†
a
a
a
a
a
a
Liang Li, Shufan Niu, Yan Qu, Qian Zhang, Hua Li, Yongsheng Li, Wenru Zhao and Jianlin Shi
a
ab
*
*
Received 15th November 2011, Accepted 13th March 2012
DOI: 10.1039/c2jm15870a
Uniform mesoporous rhodium oxide/alumina hybrid have been prepared following a facile one-pot
self-assembly approach using P123 as template. Such hybrid nanomaterials display a high dispersion of
both the noble metal oxide and the alumina within mesoporous structure in a broad Rh/Al mole ratio
up to 8 : 1, which makes them attractive materials for catalytic applications. After coating on MEMS
micro-heater, a catalytic combustion type methane gas micro-sensor was fabricated and investigated
for its sensing performance. The mesostructure-based sensor demonstrated a short T90 response time,
relatively high signal output, high enough signal/noise ratio and extraordinarily low power
consumption.
resistance in the circuit that is proportional to the amount of
combustible gas present. Thus, for the same catalytic system, the
performances of catalytic combustion gas sensors are directly
dependent on catalytic performance of the catalysts, which is
closely related to its specific surface area and morphology.12
With the rapid development of micro-electro-mechanical
systems (MEMS), many micro-machining ways were well
developed and used, which made the micromation, low power
consumption and intelligence of sensors possible.13,14 During the
development of MEMS methane catalytic combustion sensor, it
was found that the signal was very low accompanied by high
noise when a traditional matrix and catalyst were used. This can
be attributed to the very limited heating area of the catalytic
element (the heating area of a micro-heater is only about
0.01 mm2), which is only about one percent of that of traditional
catalytic combustion sensor. The straightforward and effective
way to improve the sensitivity of the MEMS sensor is to enlarge
the valid contact area between catalyst and target gases.
Introduction
Over the last decade, the development of sensitive and reliable
sensors for combustible gases and organic vapours below the
lower explosion limit (LEL) has gained considerable attention
for social safety, energy saving and environmental applica-
tions.1–11 There are many gas detection methods (such as thermal
conductivity, optical interferometry, IR absorption, ultrasonic
measurement, catalytic combustion and so on.), however, the
most common and widely used in industry is the catalytic
combustion technique due to its portability, high accuracy and
reliability, which is based on the exothermic reaction between the
detected gases and oxygen. In catalytic combustion gas sensors,
there are two elements strung onto the opposite arms of
a balanced Wheatstone-bridge circuit. One is called ‘‘active’’
element coated with catalyst that allows combustion to occur on
the surface. The other, ‘‘reference’’ element, lacks the catalyst
outer coating but in other respects exactly resembles the active
element. If combustible vapours are present, they will be cata-
lytically combusted on the surface of the active element, heating
it to a higher temperature. The difference between the tempera-
tures of the two elements produces a change in the electrical
Mesoporous material has high surface area and well defined
pore structure which has been proven to be excellent in fabri-
cating highly active catalysts or catalyst supports, and also shows
very high gas responsivity and rapid gas responding kinetics in
various gas sensor devices.1,2,13–17 If such mesostructure can be
intruduced into MEMS catalytic combustion sensor, the loaded
catalyst amount and contacted area between methane and the
catalyst will be greatly enhanced compared with that using
traditional catalyst structure, and much higher sensitivity can be
obtained. However, as a main kind of catalyst for methane
catalytic combustion sensor, noble metals or noble metal oxides
are difficult to form mesoporous structure directly. When loaded
on other mesoporous matrix, the inculsions of either preformed
metal nanoparticles, or in situ grown ones from metal precusors
aKey Laboratory for Ultrafine Materials of Ministry of Education, School
of Materials Science and Engineering, East China University of Science
and Technology, Shanghai 200237, China. E-mail: liliang@ecust.edu.cn;
jlshi@sunm.shcnc.ac.cn
bState Key Laboratory of High Performance Ceramic and Superfine
Microstructure, Shanghai Institute of Ceramics, Chinese Academy of
Sciences, Shanghai 200050, China
† Electronic supplementary information (ESI) available: [HRTEM
images and pore volumes of the hybrids; TEM image, N2
adsorption-desorption isotherms and the corresponding pore size
distribution and XRD patternt of compensating materials (mesoporous
alumina)]. See DOI: 10.1039/c2jm15870a
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 9263–9267 | 9263

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in the presence of reducing agents, are usually accomplished in
multi-steps, which is not only time-consuming, but sometimes
also not applicable for the preparation of the composite meso-
porous films on the small area of a MEMS micro-heater.
Moreover, the loaded metal or metal oxide nanoparticles will
inevitably block the mesopore channels, especially at high
loading content, leading to greatly reduced specific area of the
film. Therefore, the fabrication of such a mesoporous film with
high content of catalyst and controlled porosity by a simple and
flexible method, is still a challenge.
MEMS sensor preparation and measurement
A MEMS combustion-type sensor, employing mesoporous
rhodium oxide/alumina hybrid as a sensor materials and meso-
porous alumina as a compensating materials was fabricated by
spin-coating method. For active element (catalytic element),
mesoporous rhodium oxide/alumina hybrid colloidal were
dropped onto the MEMS micro-heater with integrated platinum
electrodes. After spin-coating, the micro-heater was slowly and
electrically heated up to 550 ꢀC and then maintained at this
temperature for 1 min to remove the template. This procedure
was repeated twice to ensure a fully coverage of the film. As to
reference element, the produce was similar to that of the above
active element using mesoporous alumina colloidal as film
precursor.
Rhodium catalyst is one of the main noble metal catalytic
systems for methane catalytic combustion reaction, in addition
to the palladium/platinum system.12 In this report, we present
that rhodium can directly fabricated into uniform mesoporous
structure with poly(ethylene glycol)–poly(propylene glycol)–
poly(ethylene glycol) template only after adding some aluminium
as additives. Such a novel co-assembly route endows the
composite Rh2O3/Al2O3 catalyst with high noble metal content,
high specific surface area and thermal stability, which is neces-
sary for its application in methane gas sensors of extra-high
sensitivity and fast sensor response.
A computer-controlled gas test bench was used to characterize
the sensor materials. It consisted of a gas delivery system, Teflon
sensor chambers, and a Wheatstone-bridge measurement for
voltage determinations. Operating mode and data acquisition
and processing were controlled through Labview software
(National Instrument).
Results and discussion
Experimenal
Fig. 1 depicts the TEM images of the hybrids Rh2O3/Al2O3 with
different Rh/Al mole ratios. The uniform sponge-like structure is
somewhat similar to that of mesoporous alumina.† Different
from other noble metal (oxide)/alumina hybrid, there is not any
large noble metal (oxide) nano-particle visible either within or
out of the mesoprous structure. The mesostructues are uniform
and the pore network distribute homogeneously throughout the
particles, as can be found in the TEM images, in the Rh/Al ratio
form 1 : 1 to 8 : 1. The corresponding selected-area electron
diffraction (SAED) patterns of the samples show a faint ring
pattern, lattice constant measured agree with (110) plane in the
standard data (JCPDS, 36-1451), revealing the existence of
Synthesis of mesoporous alumina
In a typical synthesis, 1.0 g of poly(ethylene glycol)–poly-
(propylene glycol)–poly(ethylene glycol) (P123) and 0.01 M
AlCl3 were successively added to 10 ml anhydrous ethanol. The
resultant mixture was vigorously stirred at room temperature for
60 min. The final mesoporous alumina colloidal was transferred
ꢀ
to an oven and underwent heat treatment at 60 C for 48 h in
static air for complete solvent evaporation. The resulting xerogel
ꢀ
was calcined at 550 C for 1 h to remove the template.
Synthesis of mesoporous rhodium oxide/alumina hybrid
A series of mesoporous rhodium oxide/alumina hybrid were
synthesized in a co-self-assembly route of rhodium and alumina
precursors. RhCl3$xH2O was first dissolved in water, and then
solution was added into above mesoporous alumina colloidal.
The mixture was sonicated by ultrasonic for 30 min. The final
mesoporous rhodium oxide/alumina hybrid colloidal was trans-
ꢀ
ferred to an oven and heated at 60 C for 48 h to evaporate the
solvent, and finally the product was calcined at 550 ꢀC for 1 h to
remove the template.
Characterization of the materials
X-ray diffraction (XRD) data were collected using a Bruk D8
Focus powder diffractometer with graphite mono chromatized
Cu-Ka radiation (l ¼ 0.15405 nm). N2 adsorption and desorp-
tion isotherms were measured at 77 K on a micromeritics ASAP
2020 system. The specific surface area and the pore size distri-
bution were calculated using the BET and Barrett-Joyner-
Halenda (BJH) methods, respectively. Transmission electron
microscopy (TEM) observations were performed on a field
emission JEM-3000F (JEOL) electron microscope operated at
300 kV equipped with a Gatan-666 electron energy loss spec-
trometer and energy dispersive X-ray spectrometer.
Fig. 1 TEM images of the hybrids with different Rh/Al mole ratios: (a)
Rh/Al ¼ 1/1, (b) Rh:Al ¼ 2/1, (c) Rh/Al ¼ 4 : 1, (d) Rh/Al ¼ 8 : 1.
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multi-crystalline Rh2O3. The size, distribution and morphologies
of the rhodium oxide crystalline were further analyzed by
HRTEM analysis.† A homogeneous distribution of small Rh2O3
crystalline, with different density but similar size (about 2 nm),
can be determined. It seems the content of crystalline varies
directly with the mole ratio of Rh/Al. The effect of the alumina
content on N2 adsorption isotherms and the corresponding pore
size distributions of the hybrid materials are shown in Fig. 2. In
all cases, the isotherms are type IV suggesting mesoporous
structure and the appearance of H4 hysteresis loops indicate the
formation of very narrow slit-like mesopores. The specific areas
of the hybrids are to be expected to be substantially lower than
that of the pure mesoporous alumina at increased rhodium
content.† Meanwhile, the average pore sizes are all at about
4 nm. It seems that the Rh/Al mole ratio hardly affects the pore
size of the final materials.
Fig. 3 Wide angle XRD pattern of mesoporous Rh2O3/Al2O3 hybrid.
a: Rh/Al ¼ 1/1, b: Rh:Al ¼ 2/1, c: Rh/Al ¼ 4 : 1, d: Rh/Al ¼ 8 : 1.
The XRD patterns of the hybrid materials are shown in Fig. 3.
There is not any diffraction peaks in the small angle range indi-
cating the disordered pore arrangement. A very broadened peaks
at 2q value of 35ꢀ can be found in the wide angle area, which can be
attributed to the characteristic diffractions of the (110) plane of
Rh2O3 crystal, suggesting the nano-crystallized state of rhodium
oxide. From the full width at half-maximum of the (110) diffrac-
tion peak, the calculated crystallite size of rhodium oxide are only
about 2 nm. All these corresponding well with that of SAED and
HRTEM analysis. Fig. 4 shows the typical X-ray photo-electron
spectrum of Rh2O3/Al2O3 hybrid in the Rh 3d5/2, Rh 3d3/2 binding
energy region with a spin separation of 5 eV. The Rh 3d3/2 peak is
centred at 314 eV and the Rh 3d5/2 peak at 309 eV, these matchwell
with Rh3+ 3d3/2 and Rh3+ 3d5/2 and no obvious peak of Rh0 was
observed. This confirms that all the rhodium elements in the
hybrid exist in the form of sesquioxide. This result consists well
with the above XRD analysis.
micro-heater and then assembled as MEMS methane catalytic
combustion sensors (Fig. 5), employing mesoporous alumina as
compensating material. Fig. 6 shows the response signals of
MEMS sensors with different Rh/Al mole ratio for 2 vol% (50%
LEL) methane concentration at working temperature of 400 ꢀC.
It clearly shows that the output single increases with the increase
of Rh content up to Rh/Al mole ratio of 4/1 due to the higher
content of catalyst. However, when Rh/Al mole ratio reach to
8/1, slight decreased in response signal was observed. this can be
ascribed to the highly decreased of the hybrid pore volume. In
general, for methane catalytic combustion sensor, the larger
content of catalyst, the stronger output signals. But for meso-
porous Rh2O3/Al2O3 hybrid, when mole ratio of Rh/Al increased
from 4/1 to 8/1, although the Rh content doubled, the pore
volume decreased from 0.16 cm3 gꢁ1 to 0.06cm3 gꢁ1 at the same
time, it’s only about one third of that of Rh/Al ¼ 4/1. In a word,
the effective contact area between catalyst and methane
decreased at that time resulting the decreased output signal.
Thanks to the porous catalyst structure and high content of
rhodium oxide, the MEMS sensor with hybrid Rh/Al ¼ 4/1 has
the highest response signal of 6.0 mV. Only about 1% heating
The catalytic efficiency and gas sensor properties of the hybrid
Rh2O3/Al2O3 materials were evaluated after coated as catalyst on
Fig. 2 N2 adsorption-desorption isotherms and corresponding pore size
distributions of mesoporous Rh2O3/Al2O3 hybrids of varied Rh/Al
mole ratios: (a) Rh/Al ¼ 1/1, (b) Rh:Al ¼ 2/1, (c) Rh/Al ¼ 4 : 1, (d)
Rh/Al ¼ 8 : 1.
Fig. 4 Typical XPS spectrum of Rh2O3/Al2O3 hybrid.
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exposure and insulation at all methane concentrations. The
signal output increased linearly with increasing the methane
concentration. Generally, for methane gas, the lower explosion
limit threshold is about 4%. According to the industry standard
for catalytic combustion sensor, a gas warning system is
demanded to trigger a pre-alarm at 0.4% (corresponding to 10%
of the lower explosion limit, 10% LEL) and the T90 response time
must be less than 15 s.19 For the mesoporous Rh2O3/Al2O3
hybrid Rh/Al ¼ 4/1 film-based MEMS sensor, the T90 response
time lies between 3–9 s at all methane concentrations. The signal
output is about 3.1 mV for 10% LEL methane concentration, the
signal noise ratio is high enough for detecting when assembled in
an instrument.
Fig. 5 Schematic diagrams and the photograph of the MEMS methane
catalytic combustion sensor.
The influences of ambient temperature and humidity on the
sensing performance were also investigated to check the appli-
cability for practical use. The operating temperature range is the
span of ambient temperatures given by their upper and lower
extremes. Fig. 8a shows the influence of ambient temperature on
the peak responses to 50% LEL methane in the range of ꢁ20–
40 ꢀC at the humidity of 25% RH. The measurement error is
about ꢂ1.7% mV and it equals to 0.05% methane concentration.
It is obvious that the variation in sensor response is negligible
under the temperature range. The voltage output errors by varied
temperature has been well compensated by compensating
element (mesoporous alumina film coated MEMS micro-heater),
which had been pre-incorporated directly into the MEMS sensor.
Fig. 8b shows the influence of humidity on the sensor peak
Fig. 6 Responses of the MEMS sensor based on Rh2O3/Al2O3 hybrids
with different mole ratio. (50%LEL methane, 25 ꢀC, 25% RH).
area was needed to reach 25% signal output as compared with the
traditional methane catalytic combustion sensor. In the mean
time, the power consumption is 25 mW, which is only one fifth of
that of traditional.18
The response magnitudes of MEMS sensor with hybrid
Rh/Al ¼ 4/1 at different concentrations of methane were further
recorded at the working temperature of 400 ꢀC (Fig. 7). The
sensor shows a fast response and decay toward the methane
Fig.
7
Sensor responses to the methane inputs of different
Fig. 8 Effect of the temperature (a) and relative humidity (b) on the
concentrations.
signal output of the sensor (methane concentration: 50% LEL).
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micro-heater and assembled as methane catalytic combustion
sensor, it demonstrated a very short T90 response time of less
than 9 s for all the methane concentrations, a high signal output
of about 3.1 mV for pre-alarm 10% LEL methane concentration,
high enough signal noise ratio in practical detecting, and, even
more importantly, a low power consumption of 25 mW, which
was about one fifth of that of a traditional LEL sensor.
Acknowledgements
This study was supported by the National Nature Science
Foundation of China, Grant No. 21073059, Program for
Changjiang Scholars and Innovative Research Team in Univer-
sity (IRT0825), the Fundamental Research Funds for the Central
University No. WK1013001 and the Nature Science of Shanghai
Grant No. 10ZR1407500.
Fig. 9 Response of the sensor (methane concentration: 50%LEL) before
(A) and under (B) the exposure to 100 ppm H2S, and the response under
the exposure to 100ppm H2S mixed with 50%LEL methane (C).
Notes and references
response to 50% LEL methane in the humidity range of 0.5%–
98% RH. The responses are also plotted against the relative
humidity with respect to the saturated vapour pressure at 25 ꢀC.
The voltage output linearly and slightly decreased with the
increase in humidity, but the voltage output variation is not as
strong as that of traditional catalytic combustion sensor.12,18
Only 9.8% sensitivity lost from 0% to 98% RH. All these indicate
that the mesoporous hybrid Rh2O3/Al2O3 film based MEMS
sensor has a strong ability against the change of the environment.
As methane detecting equipment, the catalytic combustion
sensor is often used in hostile environment. Catalyst poisoning or
performance degradation can occur when combustible sensor are
exposed to certain substances. Among them, the most commonly
encountered are sulfur containing compounds. Fig. 9 shows the
effect of typical poison H2S on the performance of MEMS
sensors. It clearly indicates that a 40 min exposure to 100 ppm
H2S mixed in 50% LEL methane does not cause any change in
sensitivity and response time. In our MEMS sensor, the catalyst
consists of a low-density mesoporous structure and has a large
surface area. This highly porous structure ensures the quick
recovery of the sensor even if some of the active sites of the
catalysts were poisoned when exposured under poisoning
environment.
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18 In general, the power consumption of single element of traditional
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Conclusions
A one-pot approach has been developed for the synthesis of an
uniform mesoporous rhodium oxide/alumina hybrid following
a facile method using P123 as template. Such hybrid nano-
materials display high dispersions of both the noble metal oxide
and alumina in a broad scope of Rh/Al mole ratio up to 8 : 1.
Their interesting porosity properties make them attractive
materials for catalytic applications. After coated on a MEMS
19 T90: Time needed to reach 90% of the highest signal.
This journal is ª The Royal Society of Chemistry 2012
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