Mesoporous PdO/Pt/Al₂O₃ film produced by reverse-micro-emulsion and its application for methane micro-sensor

Abstract

A simple, versatile and effective reverse micro-emulsion and pyrolysis protocol was presented for in situ growth of a PdO/Pt loaded mesoporous Al₂O₃ film. Noble metal (oxide) nanoparticles with a narrow size distribution were homogeneously dispersed throughout the Al₂O₃ support. Most importantly, the obtained worm-like catalyst network has both a high specific area and highly crystalline, which is favorable for application in methane catalytic combustion. When deposed on a micro-heater and used as a sensor element, the resulting micro-sensor demonstrated a short T₉₀ response time, relatively high signal output, high enough signal/noise ratio and extraordinarily low power consumption for methane detection.

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Introduction

Methane, as a main component of natural gas, is widely used in industry and domestic life. Leakage of methane may cause explosion and fire. The development of gas alarms or systems for monitoring methane concentrations is thus urgent and necessary, especially in some special environments. Among the various detection methods, the most commonly and widely used is the catalytic combustion technique.^1–11 Although traditional methane catalytic combustion sensors are portable, highly accurate and reliable so far, it has been found in practice that the power consumption and the size are still relatively high and not suitable for assembly in miniature sensor devices. Recently, with the rapid development of micro-electro-mechanical systems (MEMS), many micro-machining techniques have been developed and used, which make the miniaturisation, low power consumption and intelligence of sensors possible.^12,13

For catalytic combustion gas sensors, there are two elements strung onto the opposite arms of a balanced Wheatstone-bridge circuit. One is an 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. The sensing mechanism of such gas sensors is based on the changes in the electrical resistance induced by the combustion reaction on the surface of the active-element, which is proportional to the amount of combustible gas present. Obviously, for these catalytic systems, the sensing behavior is associated with the catalytic performance of the catalyst, which is closely related to its specific area and morphology.^14–17 Compared with traditional catalytic combustion sensors, the heating area of a MEMS micro-heater chip is only about one percent of that of traditional sensors. Thus, control of the specific area and morphology of the catalyst is essential in fabrication of micro-sensors.

It is well known that mesoporous materials possess high surface areas and well defined pore structures, which have been proven to be excellent in fabricating highly active catalysts or catalyst supports. When combined with a MEMS micro-heater, the loaded catalyst amount and the contact area between methane and the catalyst for catalytic combustion micro-sensors could be greatly enhanced. However, as a main kind of catalyst for methane catalytic combustion sensors, noble metals (oxides) are difficult to form into meso-structures directly. The loading of catalyst onto another meso-porous matrix, or in situ growth of catalyst from precursor in the presence of reducing agents, is usually accomplished in multiple steps, which are not only time-consuming, but sometimes also not applicable for preparation on the small area of a MEMS micro-heater. In previous studies, we have reported that rhodium oxide can be formed into a uniform mesoporous hybrid with Al₂O₃ in a large mole ratio range when P123 was used as template, and this hybrid can be easily deposed onto a micro-heater for methane detection.^18,19 Unfortunately, this protocol is only effective for the rhodium system.¹⁸ 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.

Palladium/platinum systems are one of the main noble metal catalytic systems for the methane catalytic combustion reaction, in addition to the rhodium system.¹⁴ In this report, a simple, versatile and effective reverse micro-emulsion and pyrolysis protocol was developed for in situ growth of mesoporous PdO/Pt loaded Al₂O₃ films on a MEMS micro-heater chip, which was then used for the detection of methane gas (Scheme 1). Noble metal (oxide) nanoparticles with a narrow size distribution around 4 nm were homogeneously dispersed throughout the Al₂O₃ support. Most importantly, the obtained worm-like catalyst network has both a high specific area and is highly crystalline, which is favorable for catalytic combustion applications. The reverse micro-emulsion technique has formerly been a key technique to synthesize oxide nanoparticles with controlled size and narrow size distribution.^20–22 It has been proven in this paper that this strategy is also feasible for mesoporous metal oxide film applications.

Scheme 1 Reverse micro-emulsion and pyrolysis protocol for fabricating a PdO/Pt/Al₂O₃ porous film on a MEMS micro-heater chip.

Experimental section

Chemicals and reagents

All chemicals used, including aluminum isopropoxide, isopropanol, poly(propylene glycol)–block–Poly(ethylene glycol)–block–poly(propylene glycol) (P123) etc., were of analytical grade, were purchased from Sigma Chemical Co., Ltd and were used without any further purification.

Synthesis of mesoporous Al₂O₃

Aluminum isopropoxide (6.5 g) was dissolved in isopropanol (125 ml). The solution was introduced into a prepared reverse micro-emulsion [15 g poly(propylene glycol)–block–poly(ethylene glycol)–block–poly(propylene glycol), 214.7 g pentanol, 65.2 g iso-octane]. After ageing and cooling at −5 °C for 24 h, the lower suspension was filtered and calcined at 500 °C for 1 h.

Synthesis of mesoporous PdO and Pt loaded Al₂O₃ hybrid (PdO/Pt/Al₂O₃)

Aluminum isopropoxide (6.5 g) was dissolved in isopropanol (125 ml). The solution was introduced into a prepared reverse micro-emulsion [15 g poly(propylene glycol)–block–poly(ethylene glycol)–block–poly(propylene glycol), 214.7 g pentanol, 65.2 g iso-octane and 12.8 g H₂O with K₂PtCl₆ and H₂PdCl₄ dissolved inside]. After ageing and cooling at −5 °C for 24 h, the lower suspension was filtered and calcined at 500 °C for 1 h.

Characterization of the materials

X-ray diffraction (XRD) data were collected using a Bruker D8 Focus powder diffractometer with graphite monochromatized Cu-Kα radiation (λ = 0.15405 nm). N₂ adsorption and desorption isotherms were measured at 77 K on a Micromeritics ASAP 2020 system. The specific surface area and the pore size distribution 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 spectrometer and energy dispersive X-ray spectrometer.

MEMS sensor fabrication and measurement

A MEMS combustion-type sensor, employing mesoporous PdO/Pt/Al₂O₃ hybrid as the sensor material and mesoporous alumina as the compensating material, was fabricated using a spin-coating method. For the active element (catalytic element), the mesoporous PdO/Pt/Al₂O₃ hybrid suspension was dropped onto the MEMS micro-heater with integrated platinum electrodes. After spin-coating, the micro-heater was slowly and electrically heated up to 500 °C and then maintained at this temperature for 1 min to remove the template. This procedure was repeated to ensure full coverage of the film. For the reference element, the procedure was similar to that of the above active element using mesoporous alumina suspension as the film precursor.

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. The operating mode and data acquisition and processing were controlled through Labview software (National Instrument).

Results and discussion

For the PdO/Pt/Al₂O₃ catalytic system, it has been proven that the optimal mole ratio of Pd/Pt is about 9/1 when used in the methane catalytic combustion reaction, and thus this was applied in our micro-sensor.^23–25 In order to get a stable and detectable output signal with the such small heating area (0.01 mm²) of the MEMS micro-heater, the surface coated catalytic materials were fabricated into mesoporous structures using reverse micro-emulsion as the precursor. Thus, the loaded catalyst amount and contact area between methane and the catalyst could be greatly enhanced.

Fig. 1 shows the transmission electron microscopy (TEM) images of the synthesized PdO/Pt/Al₂O₃ materials with different noble metal/Al mole ratios from 1 : 4 to 2 : 1. All the samples show the characteristics of a worm-like architecture with metal nanoparticles uniformly dispersed throughout the porous support. There are no large noble metal (oxide) particles visible either within or out of the meso-structure. The corresponding selected area electron diffraction patterns of the samples show a clear ring pattern, in which the lattice constant measured agrees with the (101) (110) (112) plane of PdO and the (111) plane of Pt. The lattice spacings of the particles determined through HRTEM analysis (Fig. 2) match well with those of the (200) (111) crystal planes of Pt and the (110) (101) crystal planes of PdO, respectively. Surprisingly, there are no Al₂O₃ nanoparticles distinguishable in the image. This may due to the much lower diffraction contrast or relatively lower crystallinity when compared with the noble metal (oxide). The noble metal (oxide) nano-crystals show a narrow particle size distribution with an average of 4–5 nm.

Fig. 1 TEM images of mesoporous PdO/Pt/Al₂O₃ hybrids with different mole ratios of noble metal/Al: (a) 1/4, (b) 1/2, (c) 1/1 and (d) 2/1.

Fig. 2 HRTEM images of mesoporous PdO/Pt/Al₂O₃ hybrids with different mole ratios of noble metal/Al: (a) 1/4, (b) 1/2, (c) 1/1 and (d) 2/1.

Fig. 3 presents the X-ray diffraction (XRD) data for these mesoporous structured catalytic materials. There are no diffraction peaks in the small angle range indicating the disordered pore arrangement. The peaks at 2θ values of 33°, 42°, 54°, 61° and 71° can be assigned as (101), (110), (112), (200) and (202) diffractions of the PdO crystals (PDF no. 43-1024), and an additional two reflection peaks at 2θ values of 40° and 46° can be readily assigned as (111) and (200) diffraction of platinum (PDF no. 04-0802), suggesting the nano-crystallized state of Pt and PdO. From the full width at half-maximum of the (110) diffraction peak of PdO and (111) diffraction peak of Pt, the calculated crystallite size of the noble metal (oxide) is only about 4 nm, corresponding well with that from the TEM analysis.

Fig. 3 Wide angle XRD patterns of mesoporous PdO/Pt/Al₂O₃ hybrids with different mole ratios of noble metal/Al: (a) 0/1, (b) 1/4, (c) 1/2, (d) 1/1 and (e) 2/1.

The N₂ adsorption isotherms and corresponding pore size distributions of the synthesized hybrids are shown in Fig. 4. In all cases, the isotherms are type IV suggesting a mesoporous structure and the appearance of H4 hysteresis loops indicates the formation of very narrow slit-like mesopores. The specific areas of the hybrids are expected to be substantially lower than that of the pure mesoporous alumina at increased noble metal contents. The corresponding specific surface area and the pore size of the hybrids can also be found in Fig. 4. Although the hybrids have a much higher noble metal content, the materials still have much higher specific area, narrow pore distribution and much larger pore volume compared with materials synthesized by other methods.¹⁸ Meanwhile, the average pore sizes are all at about 4 nm. It seems that the noble metal/Al mole ratio hardly affects the pore size of the final materials. It can be expected that the synthesized mesoporous structured PdO/Pt/Al₂O₃ materials are more suitable for the methane catalytic combustion reaction.

Fig. 4 N₂ adsorption–desorption isotherms and corresponding pore size distributions of mesoporous PdO/Pt/Al₂O₃ hybrids with different mole ratios of noble metal/Al: (a) 1/4, (b) 1/2, (c) 1/1 and (d) 2/1.

The catalytic efficiency and gas sensor properties of the meso-structured PdO/Pt/Al₂O₃ materials were evaluated after being coated as catalyst onto micro-heaters and then assembled as MEMS methane catalytic combustion sensors, employing mesoporous alumina as the compensating material. Fig. 5 shows the response signals of the MEMS sensors with different noble metal/Al mole ratios for 2 vol% (corresponding to 50% of the lower explosion limit, 50% LEL) methane concentration at a working temperature of 400 °C. This clearly shows that the output single increases with the increase of noble metal content, due to the higher content of catalyst. However, when the mole ratio of noble metal/Al reached 1/2, a slight increase in the response signal was observed, which also means that there is a slight increase in the oxidized methane per unit time within the catalytic element. In general, for methane catalytic combustion sensors, the larger the content of catalyst, the stronger the output signals. However, the MEMS sensor with noble metal/Al = 1/2 has a high enough response signal of 10.1 mV for methane detection when compared with that using Rh₂O₃ as catalyst (6 mV).¹⁹ Thus, this mole ratio was selected for further investigation after considering the synthesis from the perspective of cost and intensity of signal output. Due to the porous catalyst structure and high content of noble metal, only about 1% of the heating area reaches 25% signal output as compared with traditional methane catalytic combustion sensors.^14,26

Fig. 5 Responses of the MEMS sensors based on PdO/Pt/Al₂O₃ hybrids with different mole ratios (25 °C, 25% RH, 2 vol% methane or 50% of the lower explosion limit, 50% LEL).

Generally, for methane gas, the lower explosion limit threshold is about 4%. According to the industry standard for catalytic combustion sensors, a gas warning system is required to trigger a pre-alarm at 0.4% (corresponding to 10% of the lower explosion limit, 10% LEL) and the T₉₀ (time needed to reach 90% of the highest signal) response time must be less than 15 s.¹⁴ For the fabricated MEMS sensor with a mole ratio of noble metal/Al = 1/2, the response magnitudes at different concentrations of methane were further recorded at the working temperature of 400 °C (Fig. 6). The sensor shows a fast response and decay toward methane exposure and insulation at all methane concentrations. The signal output increased linearly with increasing methane concentration. The T₉₀ response time lies between 3–10 s at all methane concentrations. The signal output is about 4.3 mV for 10% LEL methane concentration, so the signal to noise ratio is high enough for detection when assembled in an instrument.

Fig. 6 Sensor responses to methane inputs of different concentrations (environmental condition: 25 °C, 25% RH).

The power consumption of such micro-sensors can be calculated from the resistance of the micro-heater under a given potential. The resistance of the Pt micro-heater in the sensor device is temperature dependent. Thus, the working temperature, T (°C), can be determined by the resistance, R, of the micro-heater, or

T = T₀ + (R − R₀)/αR₀

where α is the temperature coefficient of Pt resistance, and R₀ is the original resistance at room temperature, T₀ (25 °C). Therefore, by measuring the resistance of the micro-heater under a given potential, the temperature (T) in the device can be calculated. Fig. 7 depicts the heating power consumption as a function of the temperature of the sensor, showing a nearly linear relationship. At the working temperature T = 400 °C, the power consumption of the micro-heater is only about 25 mW, which is about 5 times lower than that of commercial methane catalytic combustion sensors.¹⁴

Fig. 7 Power consumption of the micro-heater as a function of the working temperature of the gas sensor.

To check the applicability for practical use, the influences of ambient temperature and humidity on the sensing performance were also investigated. The operating temperature and humidity range are the span of the ambient temperatures and humidity given by their upper and lower extremes. Fig. 8A depicts the influence of ambient temperature on the responses to 50% LEL methane in the range of −20–40 °C at the humidity of 25% RH. The measurement error is about 1.2% mV and it equals 0.02% methane concentration. The variation in the sensor response is negligible under the given temperature range. The voltage output errors caused by the varying temperature have been well compensated for by the compensating element (meso-structured alumina film coated MEMS micro-heater), which was pre-incorporated directly into the MEMS sensor. Fig. 8B shows the influence of humidity on the sensor peak 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 vapor pressure at 25 °C. The signal 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 sensors. Only 5% sensitivity is lost from 0% to 98% RH. All these results indicate that the mesoporous PdO/Pt/Al₂O₃ film based MEMS sensor has a strong ability to resist changes in the environment.

Fig. 8 Effect of the temperature (A) and relative humidity (B) on the signal output of the sensor (methane concentration: 50% LEL).

Generally, catalyst poisoning or performance degradation can occur when combustible sensors are exposed to certain substances. As methane detecting equipment, the catalytic combustion sensor is often used in hostile environments. Among the poisons that the sensor could be exposed to, the most commonly encountered are sulfur containing compounds. Fig. 9 shows the effect of the typical poison H₂S on the performance of the 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 or 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 exposed to a poisoning environment.

Fig. 9 Response of the sensor (methane concentration: 50% LEL) before (A and B) and after (E and F) exposure to 100 ppm H₂S, and the response under exposure to 100 ppm H₂S mixed with 50% LEL methane (C and D).

Conclusions

A simple and versatile reverse micro-emulsion and pyrolysis protocol has been developed for growth of PdO/Pt loaded mesoporous Al₂O₃ films. The interesting porosity properties and narrow noble metal (oxide) particle distribution make it an attractive material for catalytic applications. When coated on a MEMS micro-heater and assembled as a methane catalytic combustion sensor, it demonstrated a very short T₉₀ response time of less than 9 s for all the methane concentrations tested, a high signal output of about 4.3 mV for pre-alarm 10% LEL methane concentration, a high enough signal to noise ratio in practical detecting, and even more importantly, a low power consumption of 25 mW, which was about one fifth of that for traditional sensors.

Acknowledgements

This study was supported by the National Basic Research Program of China 2013CB933201.

Notes and references

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