VOC-Sensing Properties of Adsorption/Combustion-Type Micro Gas Sensors Using Mesoporous Alumina Co-Loaded with Pt and Metal Oxide
Authors: Nagae Kazunori , Ueda Taro , Sasahara Takahiko , Nakagoe Osamu , Tanabe Shuji , Hyodo Takeo , Shimizu Yasuhiro 
Published: February 2020

Volatile organic compounds (VOCs), which are generated from adhesives, resins and paints of new building materials and furniture, cause sick house syndrome such as headache, dizziness and nausea, and thus portable VOC-detecting devices with high sensitivity and selectivity were required as an alternative to large and expensive VOC-analyzing systems. Among various types of VOC sensors, adsorption/combustion-type micro gas sensors, which were developed by our group, are quite promising to detect a low concentration of VOCs, because the VOCs, which are firstly adsorbed on the sensing materials at lower temperatures and then the adsorbed ones, are catalytically oxidized at pulsed heating to elevated temperatures leading to a large response to the VOCs. For example, the sensors using Pd-loaded mesoporous (mp-) alumina (Al2O3) exhibited excellent performance in detecting VOCs1), and co-loading of Au with Pd onto mp-Al2O3 by impregnation improved the sensing property to ethanol2). In addition, the ethanol-sensing property was further enhanced by highly dispersing Au(core)/Pd(shell) nanoparticles, which were prepared by ultrasonic-assisted reduction of Au and Pd chlorides with polyethylene glycol monostearate, on the mp-Al2O3
3). Recently, we have demonstrated that the loading of Pt nanoparticles on the mp-Al2O3 by utilizing the ultrasonic-assisted reduction is also effective in improving several VOCs such as ethanol, acetone, and ethyl acetate4). Therefore, VOC-sensing properties of adsorption/combustion-type micro gas sensors using mp-Al2O3 co-loaded with 1 wt% Pt and 10 wt% metal oxide (1Pt/10MO/mp-Al2O3 sensor, MO: metal oxide (Bi2O3, CeO2, Fe2O3, NiO, RuO2 or ZrO2)) have been investigated in this study.
The mp-Al2O3 powder (specific surface area: ca. 250 m2 g-1) was prepared by microwave-assisted solvothermal treatment of Al(sec-OC4H9)3 in 1-propanol solution4). The 10 wt% MO-loaded mp-Al2O3 (10MO/mp-Al2O3) powders were prepared by impregnation with the constituent metal salt to the mp-Al2O3 powder, followed by firing at 700ºC for 1 h in air. Thereafter, the mp-Al2O3 and 10MO/mp-Al2O3 powders were loaded with 1.0 wt% Pt nanoparticles, which were synthesized by utilizing the ultrasonic-assisted reduction of H2PtCl6 (10 mM) in deionized water containing an appropriate amount of polyethylene glycol monostearate (MW: 4000). The obtained Pt-loaded powders (1 wt% Pt-loaded mp-Al2O3 (1Pt/mp-Al2O3) or 1 wt% Pt-loaded 10MO/mp-Al2O3 (1Pt/10MO/mp-Al2O3)) and mp-Al2O3 powder were set on the sensing and reference regions of a MEMS platform, respectively, and VOC-sensing properties of the adsorption/combustion-type micro gas sensors were measured with a mode of pulse-driven heating (high temperature (450°C) for 0.4 s after low temperature (150°C) for 9.6 s within a cycle of 10 s). In this operation mode, the target VOCs and/or the partially-decomposed products are adsorbed on the sensing materials at 150°C, and subsequently all the adsorbates abruptly burn upon the pulse heating up to 450°C. Thus, a sensor-signal profile typically consists of one large dynamic response and subsequent static response, which originate from the flash catalytic combustion of these adsorbates and general catalytic combustion, respectively, during the pulse heating at 450°C.
The 1Pt/10CeO2-mp-Al2O3 sensor showed the largest static response to all target VOCs (ethanol, ethyl acetate, acetone, benzene, and toluene) among all the sensors tested, probably because Pt/10CeO2-mp-Al2O3 showed the largest specific surface area (ca. 187 m2 g-1). In addition, the magnitude of the static response was little dependent on the kinds of VOCs. On the other hand, all 1Pt/10MO/mp-Al2O3 sensors showed larger dynamic response to ethanol than other VOCs (ethyl acetate, acetone, benzene, and toluene). These sensors also showed relatively large dynamic responses to acetone and ethyl acetate, and the magnitude of dynamic responses to benzene and toluene was much smaller than that to acetone and ethyl acetate. Among them, the 1Pt/10CeO2-mp-Al2O3 sensor showed the largest dynamic response to most of target VOCs, while 1Pt/10Bi2O3-mp-Al2O3 showed the largest dynamic response to only toluene, even though the specific surface area of 1Pt/10Bi2O3-mp-Al2O3 (ca. 142 m2 g-1) is smaller than that of 1Pt/10CeO2-mp-Al2O3. Moreover, the dynamic response speed of the 1Pt/10Bi2O3-mp-Al2O3 sensor to toluene was faster than that of the 1Pt/10CeO2-mp-Al2O3 sensor. These toluene-sensing properties probably result from higher catalytic combustion behavior of toluene over 1Pt/10Bi2O3-mp-Al2O3 than 1Pt/10CeO2-mp-Al2O3. In addition, the adsorption property of toluene on 1Pt/10Bi2O3-mp-Al2O3 may also influence the dynamic response.
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