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MCM-41 modified SnO2 gas sensors: sensitivity and selectivity properties
Sensors and Actuators B 59 (1999) 1 – 8
MCM-41 modified SnO2 gas sensors: sensitivity and selectivity properties Guangjin Li, Sibudjing Kawi * Department of Chemical Engineering, National Uni6ersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 12 July 1998; accepted 28 September 1998
Abstract MCM-41 modified SnO2 (SnO2/MCM-41) gas sensors were prepared by mechanically mixing SnO2 with MCM-41 (which is a novel high-surface-area mesoporous material) followed with thermal treatment in air at 700°C. H2, CO and CH4 were selected as probing gases to characterize the sensing properties of these sensors. Compared with those of the pure SnO2 sensor, SnO2/MCM-41 sensors showed enhanced sensitivities and selectivity to H2. It was found that the SnO2/MCM-41 sensor having SnO2 content of SnO2/MCM-41=0.7 (weight ratio) possessed the highest H2 sensitivity. The sensitivities to CO and CH4 were also found to be improved on the SnO2/MCM-41 sensors at much higher temperature ranges. The effect of SnO2 content on the sensitivity and selectivity to CO and CH4 were also investigated. The mechanistic aspect due to the enhancement of the sensitivity and selectivity of SnO2/MCM-41 gas sensors to H2 is briefly discussed. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Tin dioxide; MCM-41; Mesoporous materials; Semiconductor oxide; Gas sensor
1. Introduction Among the semiconductor oxide materials used for gas sensors, SnO2 is the most widely used one to detect reducing gases (such as H2, CO, hydrocarbon, and alcohol) as well as oxidizing gases (such as oxygen and NOx) [1,2]. The application of pure SnO2 gas sensors is limited mainly because of the lack of selectivity to a particular gas, i.e. the lack of ability of pure SnO2 to selectively recognize a certain gas from others. To improve its sensing properties, SnO2 was generally modified by doping with transition metals, such as Pt, Pd, Ag, etc. [3,4]. Semiconductor oxide gas sensors are classified into surface-sensitive and bulk-sensitive sensors [5]. For a surface-sensitive sensor, the sensor’s conductivity depends on the interaction between the target gases and the surface of sensors. However, for a bulk-sensitive sensor, its conductivity is based on the lattice defects of the bulk. Comparatively, surface-sensitive sensors are dominant in detecting reducing gases or toxic gases. * Corresponding author. Tel.: +65-8746312; fax: +65-7791936. E-mail address: [email protected] (S. Kawi)
Due to the different sensing performance between surface-sensitive and bulk-sensitive sensors, it is believed that the morphology of semiconductor oxide greatly influences the sensing property of a semiconductor oxide sensor, especially for those surface-sensitive sensors. For an example, Yamazoe et al. [3,4] studied the effect of particle size of SnO2 sensors on the sensor’s sensitivity and found that the gas sensitivity of SnO2 sensors to H2, CO and i-C4H10 increases with the decrease of crystallite size in the range below 10 nm; a model concerning the particle size (D) and thickness of space charge layer (L) was used to illustrate this phenomenon. They proposed that when particle sizes in a SnO2 sensor are smaller enough (DB 2L), the whole resistance and sensitivity of the sensor are controlled by grains themselves, inducing the inverse change of particle size and gas sensitivity. Alternatively, the effect of grain size on the sensitivity of a gas sensor can also be explained by the fact that the surface area of a semiconductor oxide increases when its grain size decreases. It is well known that gas sensing properties of semiconductor oxides are derived from the surface reactions between target gas molecules and surface active sites [6]. Based on this explanation, one may easily find an analogy between the surface
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reactions of semiconductor sensors and those on the surfaces of heterogeneous catalysts. Higher surface area of a semiconductor oxide means that more surface active sites (such as surface oxygen species in the case of SnO2 in air) are available to react with the target gas molecules, resulting in higher activity of a catalyst or in higher sensitivity of a semiconductor sensor. The effect of surface areas on the gas sensing property of mesoporous SnO2 sensors has been recently studied by our group [7,8], where a linear relationship between the surface areas of SnO2 and H2 sensitivity was found. The study revealed that higher surface area of the pure mesoporous SnO2 sensor gives better sensitivity [7,8]. However, at temperatures higher than 450°C, it was found that the surface area of mesoporous SnO2 decreased sharply due to the collapse of the mesoporous structures at high temperatures [9]. In order to synthesize high-surface-area semiconductor oxides, which can be used as gas sensors at temperatures higher than 450°C, a synthetic approach is to support semiconductor oxides onto thermally stable high-surface-area supports, such as Al2O3, SiO2, zeolites, and other inorganic materials. According to our knowledge, no publications have ever reported the preparation of high-surface-area semiconductor oxide gas sensors using a solid state modification method. In this paper, SnO2-based gas sensors were prepared by mechanically mixing SnO2 with MCM-41 materials, which are a series of novel synthetic materials possessing very high surface areas ( 1,200 m2 g − 1) and very uniform mesoporous structures (with pore diameters systematically adjusted from 16 to 100 A, ) [10]. The mechanically mixed samples were thermally treated at temperatures as high as 700°C. Compared with the pure SnO2 sensors, the sensitivity and selectivity of these SnO2/MCM-41 sensors to H2 are shown to be improved.
2. Experimental
2.1. Sample preparation Si-Al-MCM-41 used in this study was synthesized using literature procedure [11] as follows: 1.96 g of NaAlO2 was dissolved in 15.6 g of H2O, and the mixture was then mixed with 37.6 g of TEAOH (tetraethylammonium hydroxide, 20%) and 38.4 g of Ludox (Du Pont, 40% colloidal silica in water). After stirring the mixture for 30 min, 84 g of CTMAOH (20.8 wt% water solution) was finally added in drops into the mixture. The slurry thus obtained was stirred for another 2–3 h before it was transferred to a 500 ml polypropylene bottle. The reaction mixture was then thermally treated at 96°C for 48 h. After hy-
drothermal treatment, the product was centrifuged, washed completely with deionized water, and dried at 50°C overnight. The as-synthesized product was then calcined in air at 600°C for 10 h to remove the organic templating agents completely from the mesopores. For comparison, Si-MCM-41 was also synthesized using literature procedure [11]. SnO2/MCM-41 samples were prepared by mechanically mixing calcined MCM-41 with SnO2 (Merck), followed with grinding the mixtures thoroughly in a mortar. These samples are referred as SnO2/Si-(Al)MCM-41(X), where X refers to the weight ratio of SnO2 to Si-Al-MCM-41 or to Si-MCM-41. Sensors were fabricated by pressing the powder samples into 14 mm diameter and 1 mm thickness pellets using a specially designed pellet die. Two Pt wire electrodes, which were 0.2 mm in diameter and were at a distance of 3–4 mm from each other, were embedded in the sensors during the pressing of the sensor pellets. Before the measurement of the sensing properties of the sensors, all newly-fabricated SnO2/Si-(Al)-MCM41(X) sensors were thermally treated at 700°C in air for 10 h, using a heating rate of 1°C min − 1. A pure SnO2 sensor, which was pretreated under the same procedure as those of SnO2/Si-(Al)-MCM-41(X) sensors, was also prepared and its sensing property was measured for reference purposes.
2.2. Measurements of gas sensing properties The gas sensing properties of pure and MCM-41 modified SnO2 sensors, which were mounted into a specially designed quartz cell, were measured by observing the changes of the sensor’s resistance in air and in reducing gases. Reducing gases used in this study were H2, CO, and CH4, which concentrations were controlled by adjusting the flow rate ratios of target reducing gases (5% H2 in N2, 4% CO in He, or 58.9% CH4 in N2) to dry air. The sensors were first pretreated in flowing air at 600°C overnight in order to obtain stable resistance readings, and then they were cooled down to the desired temperatures for the measurements of their gas sensing properties. The sensing properties to different gases were measured separately. The changes of the sensor resistance were recorded on a Keithley 6517 electrometer using a constant voltage (2V) applied across the sensor elements. Data were collected and processed by a personal computer connected to the electrometer. The sensitivity of a sensor is expressed as the ratio of the difference of the sensor’s resistance in air and in reducing gases (DR = Rair − Rgas) to the sensor’s resisi.e. tance in reducing gases (Rgas), S= DR/Rgas = (Rair − Rgas)/Rgas.
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2.3. Characterization techniques N2 adsorption, which is a widely used method to characterize the surface areas and pore size distributions of porous materials, was used in this study. N2 adsorption was performed on a Quantachrome NOVA 1000 analyzer using N2 at liquid nitrogen temperature as an adsorbate. Detailed experimental procedure for N2 adsorption measurement can be found elsewhere [7,8]. The surface area was determined by a multipoint BET method [12] using the adsorption data in the partial pressure range (P/P 0) of 0.05 – 0.25. Desorption isotherm was used to determine the pore size distribution using a BJH method [12]. X-ray diffraction (XRD) was also used to characterize the as-synthesized and the calcined Si-Al-MCM-41. XRD patterns were obtained on a Shimadzu XRD6000 diffractometer using Cu Ka radiation (l=1.5418 A, ) with a scan speed of 2° min − 1. Due to the limitation of the machine, the samples were scanned starting from 2u angles of 1.5°.
3. Results and discussion
3.1. Characterization of Si-Al-MCM-41 and Si-MCM-41 Fig. 1 shows the XRD patterns of the as-synthesized and the calcined Si-Al-MCM-41 in the 2u range of 1.5 – 10°. Both samples have typical XRD patterns of those of the MCM-41 materials as described in literature [13,14]. The as-synthesized Si-Al-MCM-41 has a very strong peak at a d100 spacing of 42.1 A, (2u =2.1°), indicating the hexagonal lattice of Si-Al-MCM-41. After calcination, the intense XRD peak of Si-Al-MCM41 was shifted to 39.4 A, , suggesting that the removal of the surfactant from the mesopores reduces the mesopore sizes. Pure siliceous Si-MCM-41 gives similar XRD patterns as those of Si-Al-MCM-41, where d100 spacings of 42.1 A, and 37.6 A, were observed on the
Fig. 1. XRD patterns of (a) as-synthesized and (b) calcined Si-AlMCM-41.
Fig. 2. N2 adsorption and desorption isotherms of calcined Si-AlMCM-41.
as-synthesized and the calcined Si-MCM-41, respectively. Fig. 2 shows the N2 adsorption and desorption isotherms of the calcined Si-Al-MCM-41, which surfactant had been removed by calcination at 600°C. The type IV adsorption and desorption isotherms obtained on calcined Si-Al-MCM-41 materials are typical behavior of those mesoporous materials. The adsorption at low P/P0 is due to the monolayer adsorption of N2 on the wall of mesopores and on the external surfaces, and the steep increase at P/P 0 around 0.3 is due to the Kelvin condensation of liquid N2 in the mesopores. The BET surface area, which is based on the adsorption data in the P/P 0 range of 0.02–0.25, was calculated to be 1292 m2 g − 1 for the calcined Si-Al-MCM-41. The pore diameter distribution of Si-Al-MCM-41 is shown in Fig. 3. The sharp pore diameter distribution at about 28 A, is in good agreement with the XRD data, indicating the uniform mesoporous structure of Si-AlMCM-41. A surface area of 1295 m2 g − 1 and a sharp pore diameter distribution at about 27 A, were also
Fig. 3. BJH pore size distribution of calcined Si-Al-MCM-41.
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G. Li, S. Kawi / Sensors and Actuators B 59 (1999) 1–8
Fig. 4. Response transients to 500 ppm of H2 on (a) the pure SnO2 sensor and (b) the SnO2/Si-Al-MCM-41(0.7) sensor at 400 °C.
obtained on calcined Si-MCM-41 materials, showing similarities between the physical properties of calcined Si-Al-MCM-41 and those of calcined Si-MCM-41. SnO2/Si-Al-MCM-41(0.7), which was thermally treated at 700°C for 10 h, was also studied using N2 adsorption; type IV isotherm was also found on this material, indicating that the mesoporous structures of MCM-41 was also maintained after thermal treatment with SnO2. The surface area and pore size of SnO2/Si-Al-MCM41(0.7) were found to be 592 m2 g − 1 and 24.2 A, , respectively.
In fact, as shown in Fig. 5a, the SnO2/Si-Al-MCM41(0.7) sensor displays great enhancement of sensitivity to H2 in the whole temperature ranges between 250 and 600°C, especially at temperatures above 350°C where its sensitivity reaches above 150. On the contrary, the pure SnO2 sensor has very low sensitivity to H2 (especially at temperatures higher than 450°C); it only has a maximum sensitivity of about 36 at 375°C, consistent with the literature results [3,4]. It is worthy to note that the temperature for the maximum H2 sensitivity of the SnO2/Si-Al-MCM-41(0.7) sensor appears at above 400°C, which is higher than that of the pure SnO2 sensor. Fig. 5b shows the sensitivity of the pure SnO2 and SnO2/Si-Al-MCM-41(0.7) sensors to 500 ppm of CO and to 500 ppm of CH4 as a function of the measurement temperatures. Similar to the results observed in the sensing of H2, the pure SnO2 sensor only showed rather moderate sensitivity to CO (about 15) at temperatures lower than 400°C, while the SnO2/Si-Al-MCM41(0.7) sensor showed moderate sensitivity to CO (about 15–18) in a wide temperature range (up to 600°C). At temperatures higher than 300°C, the im-
3.2. Gas sensing properties of pure SnO2 and SnO2 /Si-Al-MCM-41(0.7) sensors In this study, the gas sensing properties of the pure SnO2 and MCM-41 modified SnO2 gas sensors to reducing gases (such as H2, CO and CH4) were derived from the changes of the sensor’s resistance caused by the contacts of the sensors with the target reducing gas molecules at temperatures between 250 and 600°C. Since the response of the sensor’s resistance to the on/off switching of the target gases is in practice too slow at temperatures lower than 250°C, sensing temperatures above 250°C were chosen in our investigation. Fig. 4 shows the response transients of the pure SnO2 and SnO2/Si-Al-MCM-41(0.7) sensors to 500 ppm of H2 at 400°C. The sharp decrease of the sensor’s resistance, which was observed when H2 was introduced to either sensor, is due to the reaction between H2 gas molecules and sensor’s surface oxygen species; this phenomenon is well known as the source of sensing property for resistive semiconductor gas sensors [3,6,4]. It is obvious to notice that the resistance of the SnO2/Si-AlMCM-41(0.7) sensor in air is much higher than that of the pure SnO2 sensor; this observation is not surprising as Si-Al-MCM-41 is an insulator in nature. More interestingly, it was found that the SnO2/Si-Al-MCM41(0.7) sensor possesses much higher sensitivity to H2 than the pure SnO2 sensor at this temperature.
Fig. 5. Correlation between temperature and sensitivity to (a) 500 ppm of H2; (b) 500 ppm of CO and 500 ppm of CH4 on the pure SnO2 sensor (dashed line with filled mark) and on the SnO2/Si-AlMCM-41(0.7) sensor (solid line with empty mark).
G. Li, S. Kawi / Sensors and Actuators B 59 (1999) 1–8
Fig. 6. Correlation between temperature and sensitivity to 500 ppm of H2, CO and CH4 on the SnO2/Si-Al-MCM-41(2.0) sensor.
provement of CO sensitivity was observed. Slight improvement of CH4 sensitivity at temperatures higher than 450°C was also observed on the SnO2/Si-AlMCM-41(0.7) sensor. However, compared with the changes of the sensitivity to H2, much smaller differences in sensitivities to CO and CH4 were observed between the pure SnO2 and SnO2/Si-Al-MCM-41(0.7) sensors, implying the potential for the SnO2/Si-AlMCM-41(0.7) sensor to be used as a sensitive and selective H2 sensor at high temperatures. (Sensor selectivity to particular gases is discussed in Section 3.4.)
3.3. Effect of the SnO2 content and the presence of Al on sensiti6ity To examine the effect of the amount of SnO2 on the sensor’s sensitivity, SnO2/Si-Al-MCM-41 sensors having various SnO2 contents were prepared. Fig. 6 shows changes of the sensitivity of a SnO2/Si-Al-MCM-41(2.0) sensor to 500 ppm of H2, CO and CH4 with measurement temperatures. Similar to the sensing properties of the SnO2/Si-Al-MCM-41(0.7) sensor as discussed above, the SnO2/Si-Al-MCM-41(2.0) sensor also showed enhanced H2 sensitivity compared with the pure SnO2 sensor. However, there are still some differences between the sensing properties of SnO2/Si-Al-MCM41(2.0) and SnO2/Si-Al-MCM-41(0.7) sensors. Firstly, the SnO2/Si-Al-MCM-41(2.0) sensor has lower H2 sensitivity than that of the SnO2/Si-Al-MCM-41(0.7) sensor (especially at high temperatures). Secondly, the temperature for the maximum H2 sensitivity of the SnO2/Si-Al-MCM-41(2.0) sensor appears at around 350 – 400°C, which is lower than that of the SnO2/Si-AlMCM-41(0.7) sensor. In fact, based on the results listed in Table 1, one may easily find the systematic changes of H2 sensitivity with the changes of the amount of SnO2 in Si-Al-MCM-41. Table 1 shows that there is an optimal SnO2 content for detecting H2 with SnO2/Si-AlMCM-41 sensors, i.e. the SnO2/Si-Al-MCM-41 (0.7)
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sensor possesses the highest H2 sensitivity. On the other hand, CO sensitivity decreases with the decrease of SnO2 content and CH4 sensitivity is insensitive to the content of SnO2.Yamazoe et al. [3,4] reported that the addition of 1 wt% or 5 wt% of trivalent Al3 + into the framework of SnO2 greatly increased the H2 sensitivity of SnO2 sensor. They found that calcination of the sensors at temperatures as high as 1100°C was necessary to force aluminum to enter the frameworks of SnO2 and to enhance the H2 sensitivity [3,4]. Eventually, they concluded that the existence of trivalent cations, such as Al3 + , in the framework of SnO2 decreased the carrier concentration and increased the depth of the space charge layer of n-type semiconductor oxides, resulting in the enhancement of H2 sensitivity of SnO2 sensors. Opposite effects on the sensor sensitivity were observed when pentavalent cations, such as Sb5 + , were doped into the framework of SnO2 [3,4]. Although the Si-Al-MCM-41 material used as a modifying material in this study does contain some amount of aluminum in the framework, the great enhancement of H2 sensitivity observed on the SnO2/SiAl-MCM-41 sensor is believed, however, not to be due to the same phenomena related to the high-temperature leaching of Al3 + into SnO2 as observed by Yamazoe et al. [3,4]. The reasons are given as follows. Firstly, Al in Si-Al-MCM-41 is difficult to be transferred to the framework of SnO2 since Al acts as one of the framework components in Si-Al-MCM-41 and is strongly bonded in the framework. Secondly, the thermal treatment temperature used to prepare SnO2/Si-Al-MCM-41 sensors (i.e. 700°C) is not high enough to leach Al3 + into the framework of SnO2 [3,4].To further exclude the effects of Al3 + , purely siliceous Si-MCM-41 supported SnO2 sensors were also prepared using the same solid state modification method and their sensing properties were studied. Fig. 7 shows the sensitivity of a SnO2/SiMCM-41(1.0) sensor to 500 ppm of H2, CO and CH4 as a function of measurement temperatures. The values and changing trends of sensitivities of the SnO2/SiMCM-41 sensor are very similar to those of SnO2/SiAl-MCM-41 sensors, suggesting the similarity between Si-Al-MCM-41 and Si-MCM-41 as supports for SnO2 sensors. In other words, the result shows that Al does not enhance the H2 sensitivity of SnO2/Si-Al-MCM-41 sensors.
3.4. Sensor Selecti6ity In a catalysis reaction, the selectivity of a particular product is generally defined as the percentage of this product to the total amount of all products, i.e. SELi = (Ai /SAi ) 100%, where SELi is the selectivity of product i, Ai is the amount of product i, and SAi is the total amount of all products. Based on this definition, the selectivity of a gas sensor to a particular gas may be
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Table 1 Sensitivities to 500 ppm of H2, CO and CH4 on different sensors Sensor
SnO2/MCM (weight ratio)
H2
CO
Tmax (°C)
Smax
Tmax (°C)
CH4 Smax
Tmax (°C)
Smax
300–350
36
350
15
400
8
SnO2/Si-Al-MCM-41
2.0 1.0 0.7 0.5
350–400 400 \400 \400
113 131 186 13
350 350–400 350 400
43 20 16 2
500–600 500–600 500–600 –
11 10.5 9 –
SnO2/Si-MCM-41
1.0
400
102
350
18
500–600
10
Pure SnO2
quantitatively described as SELi =(Si /SSi ) ×100%, where Si is the sensitivity of a sensor to gas i and SSi represents the sum of sensitivities of a sensor to different target gases at similar concentrations. It should be noted that the selectivity value calculated based on this definition changes not only with the change of measurement temperature and gas concentration, but also the number of gases used. Based on this definition, the selectivity to H2, CO or CH4 may then be described in this study as SELi =[Si /(SH2 +SCO +SCH4)] ×100%, where i represents one of the gases as H2, CO, or CH4. Fig. 8 shows the changes of selectivity with temperatures of pure SnO2 and SnO2/Si-Al-MCM-41 sensors to H2. The results show that the highest H2 selectivity ( 90%) is obtained on a SnO2/Si-Al-MCM-41(0.7) sensor at 400°C. At temperatures between 350 and 500°C, both SnO2/Si-Al-MCM-41(0.7) and SnO2/Si-AlMCM-41(1.0) sensors still show very high selectivity to H2 ( \ 80%), whereas the pure SnO2 sensor showed very low selectivity to H2 at these high temperature ranges. It was also found that when the amount of SnO2 content in SnO2/Si-Al-MCM-41 sensors decreases, the selectivity of the sensors to H2 increases. Compared with the selectivity to H2, the selectivity to CO is much lower (B50%), suggesting that SnO2/Si-
Al-MCM-41 sensors are not good gas sensors to selectively detect CO in gases containing H2. Contrary to the selectivity of the sensors to H2, when the amount of SnO2 content in SnO2/Si-Al-MCM-41 sensors decreases, the selectivity of the sensors to CO generally decreases. It is observed that good CH4 selectivity was obtained only at very high temperature ranges. This is not surprising as it is difficult to activate CH4 at low temperatures. Since CH4 is a very stable compound, activation of CH4 requires catalysts and very high temperatures to break the CH bonds. Activation of methane with/without the existence of oxygen is, in fact, a big challenge in catalysis research field and has been widely studied; in fact, temperatures higher than 500°C are normally needed to obtain sufficient conversion of methane [15,16]. It was also found that, when the amount of SnO2 content in SnO2/Si-Al-MCM-41 sensors decreases, the selectivity of the sensors to CH4 generally decreases. Based on the characterization data presented in this paper, the data are not enough to clearly elucidate the mechanism showing why the sensitivity and selectivity to H2 could be greatly improved by mechanically mixing SnO2 with MCM-41 materials. However, the coexistence state of SnO2 and MCM-41 is believed to be very
Fig. 7. Correlation between temperature and sensitivity to 500 ppm of H2, CO and CH4 on the SnO2/Si-MCM-41(1.0) sensor.
Fig. 8. Correlation between selectivity to H2 of the pure SnO2 and SnO2/Si-Al-MCM-41 sensors (having different SnO2 content) with temperature.
G. Li, S. Kawi / Sensors and Actuators B 59 (1999) 1–8
important in affecting the sensing properties of SnO2 sensors. On one hand, part of the SnO2 in SnO2/Si-AlMCM-41 sensors might migrate and be coated on the surface of MCM-41 through high temperature solid state reaction, which has been well used in catalyst preparation to coat active species onto surfaces of catalyst supports [17,18]. The possible surface coating of high-surface-area SnO2 onto MCM-41 may greatly influence the surface reactivity of SnO2 with H2 and produce high sensitivity to H2. On the other hand, SnO2/Si-Al-MCM-41 sensor materials may possibly be only mechanical mixtures of SnO2 powders and MCM41 powders. In both cases, the pore structures and the adsorption properties of MCM-41 may be responsible for the improvement of sensing properties of SnO2 by MCM-41 materials. It is believed that, no matter what the coexistence state of SnO2 on MCM-41 is, the difference in the diffusion of H2, CO and CH4 into the pore structures of sensor particles may play an important role in explaining the change of sensor’s sensitivity. Being very small molecules, H2 molecules diffuse much faster in porous materials than larger molecules, such as CO and CH4. This effect may cause H2 to be much easier than CO and CH4 to access more SnO2 in SnO2/MCM-41 sensors, hence causing the difference in the sensitivity and selectivity among these gases. To confirm this viewpoint, the study of the sensing properties of SnO2 sensors which are modified using porous materials having even much smaller pores — such as zeolites —may be helpful. If the above assumption of the effect of gas diffusion in controlling the sensitivity and selectivity of sensors is correct, the difference between the zeolitebased sensor’s sensitivity to H2 and CO (or CH4) should be even larger. Efforts should be made on the characterization of these SnO2/Si-Al-MCM-41 sensor materials in the future in order to understand better the mechanistic aspects of the preparation and the sensing behavior of these sensor materials.
4. Conclusions By mechanically mixing SnO2 with MCM-41 and treating the solid mixture at high temperatures, SnO2/ Si-Al-MCM-41 and SnO2/Si-MCM-41 sensors were prepared. Based on the sensitivity and selectivity results, several points can be concluded as follows: 1. Compared with the pure SnO2 gas sensor, MCM-41 modified SnO2 sensors posses better H2 sensitivity at temperatures between 250 and 600°C. The moderate enhancement of CO and slight enhancement of CH4 sensitivity are also found at much high temperatures. 2. The enhancement of the gas sensitivity of SnO2/SiAl-MCM-41 sensors to H2 is not caused by the doping effect of Al3 + into SnO2.
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3. The SnO2 content of SnO2/MCM-41=0.7 (weight ratio) is the optimum SnO2 content for SnO2/Si-AlMCM-41 sensors to detect H2. 4. The best H2 selectivity (over 90%) is obtained on the SnO2/Si-Al-MCM-41(0.7) sensor at a temperature of 400°C, implying a great potential for this sensor to be used as a sensitive and selective H2 gas sensor at high temperatures. MCM-41 modified SnO2 sensors are not good sensors to selectively detect CO. CH4 can only be selectively detected by these MCM-41 modified SnO2 sensors at much high temperatures (\ 500°C).
Acknowledgements The research was supported by the National University of Singapore (RP 950669) and by the Process Analysis and Optimization (PAO) Enterprise. The authors express their sincere appreciation to the Shimadzu (Asia Pacific) Pte Ltd for their assistance in XRD experiments.
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[13] X.S. Zhao, G.Q.M. Lu, G.J. Millar, Advances in mesoporous molecular-sieve MCM-41, Ind. Eng. Chem. Res. 35 (1996) 2075. [14] C.-Y. Chen, H.-X. Li, M.E. Davis, Studies on mesoporous materials I. Synthesis and characterization of MCM-41, Microporous Mater. 2 (1993) 17–26. [15] Y. Amenomiya, V.I. Birss, M. Goledzinowski, J. Galuszka, A.R. Sanger, Conversion of methane by oxidative coupling, Catal. Rev. Sci. Eng. 32 (1990) 163. [16] S.T. Wong, Y.D. Xu, L.S. Wang, S.T. Liu, G.J. Li, M.S. Xie, X.X. Guo, Methane and ethane activation without adding oxygen: promotional effect of W in Mo-W/HZSM-5, Catal. Lett. 38 (1996) 39. [17] Y.C. Xie, Y.Q. Tang, Spontaneous monolayer dispersion of oxides and salts onto surfaces of supports: applications to heterogeneous catalysis, Adv. Catal. 37 (1990) 1. [18] M. Ziolek, I. Nowak, H.G. Karge, Solid-state interaction between niobium oxide and Y-type zeolites, Stud. Surf. Sci. Catal. 94 (1995) 270.
Biographies Guangjin Li received his BS degree (1987) in chemistry from the University of Science and Technology of China, Hefei, China. After 5 years postgraduate study in Dalian Institute of Chemical Physics ( July 1987– August 1990) and Catalysis Research Center, Hokkaido University, Japan (September 1990 – May 1992), he was awarded his Ph.D. degree (1992) from Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
.
Dalian, China. From 1992 to 1996, he worked as a research associate in State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. He worked as a postdoctoral fellow in Department of Chemical Engineering, National University of Singapore from May 1996 to January1998. He is currently working as an Engineer in Hewlett-Packard Singapore Ltd. His main research interests are in heterogeneous catalysis, synthesis of inorganic materials, organometallic chemistry, and semiconductor chemical sensors. Sibudjing Kawi obtained his BA in Chemistry (Hons.) from the University of Texas at Austin (USA), B.S. in Chemical Engineering (Hons.) from The University of Texas at Austin (USA), MS in Chemical Engineering from The University of Illinois at Urbana-Champaign (USA), and Ph.D. in Chemical Engineering from The University of Delaware (USA). He did his two-year postdoctoral study at the University of California at Davis (USA). He is presently an Assistant Professor in the Department of Chemical and Environmental Engineering at the National University of Singapore (Singapore). His field of specialization includes semiconductor oxide gas sensors, heterogeneous catalysts, ceramic adsorbents, and advanced materials. He is a member of the American Institute of Chemical Engineers, the American Chemical Society, and the Materials Research Society.
MCM-41 modified SnO2 gas sensors: sensitivity and selectivity properties Guangjin Li, Sibudjing Kawi * Department of Chemical Engineering, National Uni6ersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 12 July 1998; accepted 28 September 1998
Abstract MCM-41 modified SnO2 (SnO2/MCM-41) gas sensors were prepared by mechanically mixing SnO2 with MCM-41 (which is a novel high-surface-area mesoporous material) followed with thermal treatment in air at 700°C. H2, CO and CH4 were selected as probing gases to characterize the sensing properties of these sensors. Compared with those of the pure SnO2 sensor, SnO2/MCM-41 sensors showed enhanced sensitivities and selectivity to H2. It was found that the SnO2/MCM-41 sensor having SnO2 content of SnO2/MCM-41=0.7 (weight ratio) possessed the highest H2 sensitivity. The sensitivities to CO and CH4 were also found to be improved on the SnO2/MCM-41 sensors at much higher temperature ranges. The effect of SnO2 content on the sensitivity and selectivity to CO and CH4 were also investigated. The mechanistic aspect due to the enhancement of the sensitivity and selectivity of SnO2/MCM-41 gas sensors to H2 is briefly discussed. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Tin dioxide; MCM-41; Mesoporous materials; Semiconductor oxide; Gas sensor
1. Introduction Among the semiconductor oxide materials used for gas sensors, SnO2 is the most widely used one to detect reducing gases (such as H2, CO, hydrocarbon, and alcohol) as well as oxidizing gases (such as oxygen and NOx) [1,2]. The application of pure SnO2 gas sensors is limited mainly because of the lack of selectivity to a particular gas, i.e. the lack of ability of pure SnO2 to selectively recognize a certain gas from others. To improve its sensing properties, SnO2 was generally modified by doping with transition metals, such as Pt, Pd, Ag, etc. [3,4]. Semiconductor oxide gas sensors are classified into surface-sensitive and bulk-sensitive sensors [5]. For a surface-sensitive sensor, the sensor’s conductivity depends on the interaction between the target gases and the surface of sensors. However, for a bulk-sensitive sensor, its conductivity is based on the lattice defects of the bulk. Comparatively, surface-sensitive sensors are dominant in detecting reducing gases or toxic gases. * Corresponding author. Tel.: +65-8746312; fax: +65-7791936. E-mail address: [email protected] (S. Kawi)
Due to the different sensing performance between surface-sensitive and bulk-sensitive sensors, it is believed that the morphology of semiconductor oxide greatly influences the sensing property of a semiconductor oxide sensor, especially for those surface-sensitive sensors. For an example, Yamazoe et al. [3,4] studied the effect of particle size of SnO2 sensors on the sensor’s sensitivity and found that the gas sensitivity of SnO2 sensors to H2, CO and i-C4H10 increases with the decrease of crystallite size in the range below 10 nm; a model concerning the particle size (D) and thickness of space charge layer (L) was used to illustrate this phenomenon. They proposed that when particle sizes in a SnO2 sensor are smaller enough (DB 2L), the whole resistance and sensitivity of the sensor are controlled by grains themselves, inducing the inverse change of particle size and gas sensitivity. Alternatively, the effect of grain size on the sensitivity of a gas sensor can also be explained by the fact that the surface area of a semiconductor oxide increases when its grain size decreases. It is well known that gas sensing properties of semiconductor oxides are derived from the surface reactions between target gas molecules and surface active sites [6]. Based on this explanation, one may easily find an analogy between the surface
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reactions of semiconductor sensors and those on the surfaces of heterogeneous catalysts. Higher surface area of a semiconductor oxide means that more surface active sites (such as surface oxygen species in the case of SnO2 in air) are available to react with the target gas molecules, resulting in higher activity of a catalyst or in higher sensitivity of a semiconductor sensor. The effect of surface areas on the gas sensing property of mesoporous SnO2 sensors has been recently studied by our group [7,8], where a linear relationship between the surface areas of SnO2 and H2 sensitivity was found. The study revealed that higher surface area of the pure mesoporous SnO2 sensor gives better sensitivity [7,8]. However, at temperatures higher than 450°C, it was found that the surface area of mesoporous SnO2 decreased sharply due to the collapse of the mesoporous structures at high temperatures [9]. In order to synthesize high-surface-area semiconductor oxides, which can be used as gas sensors at temperatures higher than 450°C, a synthetic approach is to support semiconductor oxides onto thermally stable high-surface-area supports, such as Al2O3, SiO2, zeolites, and other inorganic materials. According to our knowledge, no publications have ever reported the preparation of high-surface-area semiconductor oxide gas sensors using a solid state modification method. In this paper, SnO2-based gas sensors were prepared by mechanically mixing SnO2 with MCM-41 materials, which are a series of novel synthetic materials possessing very high surface areas ( 1,200 m2 g − 1) and very uniform mesoporous structures (with pore diameters systematically adjusted from 16 to 100 A, ) [10]. The mechanically mixed samples were thermally treated at temperatures as high as 700°C. Compared with the pure SnO2 sensors, the sensitivity and selectivity of these SnO2/MCM-41 sensors to H2 are shown to be improved.
2. Experimental
2.1. Sample preparation Si-Al-MCM-41 used in this study was synthesized using literature procedure [11] as follows: 1.96 g of NaAlO2 was dissolved in 15.6 g of H2O, and the mixture was then mixed with 37.6 g of TEAOH (tetraethylammonium hydroxide, 20%) and 38.4 g of Ludox (Du Pont, 40% colloidal silica in water). After stirring the mixture for 30 min, 84 g of CTMAOH (20.8 wt% water solution) was finally added in drops into the mixture. The slurry thus obtained was stirred for another 2–3 h before it was transferred to a 500 ml polypropylene bottle. The reaction mixture was then thermally treated at 96°C for 48 h. After hy-
drothermal treatment, the product was centrifuged, washed completely with deionized water, and dried at 50°C overnight. The as-synthesized product was then calcined in air at 600°C for 10 h to remove the organic templating agents completely from the mesopores. For comparison, Si-MCM-41 was also synthesized using literature procedure [11]. SnO2/MCM-41 samples were prepared by mechanically mixing calcined MCM-41 with SnO2 (Merck), followed with grinding the mixtures thoroughly in a mortar. These samples are referred as SnO2/Si-(Al)MCM-41(X), where X refers to the weight ratio of SnO2 to Si-Al-MCM-41 or to Si-MCM-41. Sensors were fabricated by pressing the powder samples into 14 mm diameter and 1 mm thickness pellets using a specially designed pellet die. Two Pt wire electrodes, which were 0.2 mm in diameter and were at a distance of 3–4 mm from each other, were embedded in the sensors during the pressing of the sensor pellets. Before the measurement of the sensing properties of the sensors, all newly-fabricated SnO2/Si-(Al)-MCM41(X) sensors were thermally treated at 700°C in air for 10 h, using a heating rate of 1°C min − 1. A pure SnO2 sensor, which was pretreated under the same procedure as those of SnO2/Si-(Al)-MCM-41(X) sensors, was also prepared and its sensing property was measured for reference purposes.
2.2. Measurements of gas sensing properties The gas sensing properties of pure and MCM-41 modified SnO2 sensors, which were mounted into a specially designed quartz cell, were measured by observing the changes of the sensor’s resistance in air and in reducing gases. Reducing gases used in this study were H2, CO, and CH4, which concentrations were controlled by adjusting the flow rate ratios of target reducing gases (5% H2 in N2, 4% CO in He, or 58.9% CH4 in N2) to dry air. The sensors were first pretreated in flowing air at 600°C overnight in order to obtain stable resistance readings, and then they were cooled down to the desired temperatures for the measurements of their gas sensing properties. The sensing properties to different gases were measured separately. The changes of the sensor resistance were recorded on a Keithley 6517 electrometer using a constant voltage (2V) applied across the sensor elements. Data were collected and processed by a personal computer connected to the electrometer. The sensitivity of a sensor is expressed as the ratio of the difference of the sensor’s resistance in air and in reducing gases (DR = Rair − Rgas) to the sensor’s resisi.e. tance in reducing gases (Rgas), S= DR/Rgas = (Rair − Rgas)/Rgas.
G. Li, S. Kawi / Sensors and Actuators B 59 (1999) 1–8
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2.3. Characterization techniques N2 adsorption, which is a widely used method to characterize the surface areas and pore size distributions of porous materials, was used in this study. N2 adsorption was performed on a Quantachrome NOVA 1000 analyzer using N2 at liquid nitrogen temperature as an adsorbate. Detailed experimental procedure for N2 adsorption measurement can be found elsewhere [7,8]. The surface area was determined by a multipoint BET method [12] using the adsorption data in the partial pressure range (P/P 0) of 0.05 – 0.25. Desorption isotherm was used to determine the pore size distribution using a BJH method [12]. X-ray diffraction (XRD) was also used to characterize the as-synthesized and the calcined Si-Al-MCM-41. XRD patterns were obtained on a Shimadzu XRD6000 diffractometer using Cu Ka radiation (l=1.5418 A, ) with a scan speed of 2° min − 1. Due to the limitation of the machine, the samples were scanned starting from 2u angles of 1.5°.
3. Results and discussion
3.1. Characterization of Si-Al-MCM-41 and Si-MCM-41 Fig. 1 shows the XRD patterns of the as-synthesized and the calcined Si-Al-MCM-41 in the 2u range of 1.5 – 10°. Both samples have typical XRD patterns of those of the MCM-41 materials as described in literature [13,14]. The as-synthesized Si-Al-MCM-41 has a very strong peak at a d100 spacing of 42.1 A, (2u =2.1°), indicating the hexagonal lattice of Si-Al-MCM-41. After calcination, the intense XRD peak of Si-Al-MCM41 was shifted to 39.4 A, , suggesting that the removal of the surfactant from the mesopores reduces the mesopore sizes. Pure siliceous Si-MCM-41 gives similar XRD patterns as those of Si-Al-MCM-41, where d100 spacings of 42.1 A, and 37.6 A, were observed on the
Fig. 1. XRD patterns of (a) as-synthesized and (b) calcined Si-AlMCM-41.
Fig. 2. N2 adsorption and desorption isotherms of calcined Si-AlMCM-41.
as-synthesized and the calcined Si-MCM-41, respectively. Fig. 2 shows the N2 adsorption and desorption isotherms of the calcined Si-Al-MCM-41, which surfactant had been removed by calcination at 600°C. The type IV adsorption and desorption isotherms obtained on calcined Si-Al-MCM-41 materials are typical behavior of those mesoporous materials. The adsorption at low P/P0 is due to the monolayer adsorption of N2 on the wall of mesopores and on the external surfaces, and the steep increase at P/P 0 around 0.3 is due to the Kelvin condensation of liquid N2 in the mesopores. The BET surface area, which is based on the adsorption data in the P/P 0 range of 0.02–0.25, was calculated to be 1292 m2 g − 1 for the calcined Si-Al-MCM-41. The pore diameter distribution of Si-Al-MCM-41 is shown in Fig. 3. The sharp pore diameter distribution at about 28 A, is in good agreement with the XRD data, indicating the uniform mesoporous structure of Si-AlMCM-41. A surface area of 1295 m2 g − 1 and a sharp pore diameter distribution at about 27 A, were also
Fig. 3. BJH pore size distribution of calcined Si-Al-MCM-41.
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G. Li, S. Kawi / Sensors and Actuators B 59 (1999) 1–8
Fig. 4. Response transients to 500 ppm of H2 on (a) the pure SnO2 sensor and (b) the SnO2/Si-Al-MCM-41(0.7) sensor at 400 °C.
obtained on calcined Si-MCM-41 materials, showing similarities between the physical properties of calcined Si-Al-MCM-41 and those of calcined Si-MCM-41. SnO2/Si-Al-MCM-41(0.7), which was thermally treated at 700°C for 10 h, was also studied using N2 adsorption; type IV isotherm was also found on this material, indicating that the mesoporous structures of MCM-41 was also maintained after thermal treatment with SnO2. The surface area and pore size of SnO2/Si-Al-MCM41(0.7) were found to be 592 m2 g − 1 and 24.2 A, , respectively.
In fact, as shown in Fig. 5a, the SnO2/Si-Al-MCM41(0.7) sensor displays great enhancement of sensitivity to H2 in the whole temperature ranges between 250 and 600°C, especially at temperatures above 350°C where its sensitivity reaches above 150. On the contrary, the pure SnO2 sensor has very low sensitivity to H2 (especially at temperatures higher than 450°C); it only has a maximum sensitivity of about 36 at 375°C, consistent with the literature results [3,4]. It is worthy to note that the temperature for the maximum H2 sensitivity of the SnO2/Si-Al-MCM-41(0.7) sensor appears at above 400°C, which is higher than that of the pure SnO2 sensor. Fig. 5b shows the sensitivity of the pure SnO2 and SnO2/Si-Al-MCM-41(0.7) sensors to 500 ppm of CO and to 500 ppm of CH4 as a function of the measurement temperatures. Similar to the results observed in the sensing of H2, the pure SnO2 sensor only showed rather moderate sensitivity to CO (about 15) at temperatures lower than 400°C, while the SnO2/Si-Al-MCM41(0.7) sensor showed moderate sensitivity to CO (about 15–18) in a wide temperature range (up to 600°C). At temperatures higher than 300°C, the im-
3.2. Gas sensing properties of pure SnO2 and SnO2 /Si-Al-MCM-41(0.7) sensors In this study, the gas sensing properties of the pure SnO2 and MCM-41 modified SnO2 gas sensors to reducing gases (such as H2, CO and CH4) were derived from the changes of the sensor’s resistance caused by the contacts of the sensors with the target reducing gas molecules at temperatures between 250 and 600°C. Since the response of the sensor’s resistance to the on/off switching of the target gases is in practice too slow at temperatures lower than 250°C, sensing temperatures above 250°C were chosen in our investigation. Fig. 4 shows the response transients of the pure SnO2 and SnO2/Si-Al-MCM-41(0.7) sensors to 500 ppm of H2 at 400°C. The sharp decrease of the sensor’s resistance, which was observed when H2 was introduced to either sensor, is due to the reaction between H2 gas molecules and sensor’s surface oxygen species; this phenomenon is well known as the source of sensing property for resistive semiconductor gas sensors [3,6,4]. It is obvious to notice that the resistance of the SnO2/Si-AlMCM-41(0.7) sensor in air is much higher than that of the pure SnO2 sensor; this observation is not surprising as Si-Al-MCM-41 is an insulator in nature. More interestingly, it was found that the SnO2/Si-Al-MCM41(0.7) sensor possesses much higher sensitivity to H2 than the pure SnO2 sensor at this temperature.
Fig. 5. Correlation between temperature and sensitivity to (a) 500 ppm of H2; (b) 500 ppm of CO and 500 ppm of CH4 on the pure SnO2 sensor (dashed line with filled mark) and on the SnO2/Si-AlMCM-41(0.7) sensor (solid line with empty mark).
G. Li, S. Kawi / Sensors and Actuators B 59 (1999) 1–8
Fig. 6. Correlation between temperature and sensitivity to 500 ppm of H2, CO and CH4 on the SnO2/Si-Al-MCM-41(2.0) sensor.
provement of CO sensitivity was observed. Slight improvement of CH4 sensitivity at temperatures higher than 450°C was also observed on the SnO2/Si-AlMCM-41(0.7) sensor. However, compared with the changes of the sensitivity to H2, much smaller differences in sensitivities to CO and CH4 were observed between the pure SnO2 and SnO2/Si-Al-MCM-41(0.7) sensors, implying the potential for the SnO2/Si-AlMCM-41(0.7) sensor to be used as a sensitive and selective H2 sensor at high temperatures. (Sensor selectivity to particular gases is discussed in Section 3.4.)
3.3. Effect of the SnO2 content and the presence of Al on sensiti6ity To examine the effect of the amount of SnO2 on the sensor’s sensitivity, SnO2/Si-Al-MCM-41 sensors having various SnO2 contents were prepared. Fig. 6 shows changes of the sensitivity of a SnO2/Si-Al-MCM-41(2.0) sensor to 500 ppm of H2, CO and CH4 with measurement temperatures. Similar to the sensing properties of the SnO2/Si-Al-MCM-41(0.7) sensor as discussed above, the SnO2/Si-Al-MCM-41(2.0) sensor also showed enhanced H2 sensitivity compared with the pure SnO2 sensor. However, there are still some differences between the sensing properties of SnO2/Si-Al-MCM41(2.0) and SnO2/Si-Al-MCM-41(0.7) sensors. Firstly, the SnO2/Si-Al-MCM-41(2.0) sensor has lower H2 sensitivity than that of the SnO2/Si-Al-MCM-41(0.7) sensor (especially at high temperatures). Secondly, the temperature for the maximum H2 sensitivity of the SnO2/Si-Al-MCM-41(2.0) sensor appears at around 350 – 400°C, which is lower than that of the SnO2/Si-AlMCM-41(0.7) sensor. In fact, based on the results listed in Table 1, one may easily find the systematic changes of H2 sensitivity with the changes of the amount of SnO2 in Si-Al-MCM-41. Table 1 shows that there is an optimal SnO2 content for detecting H2 with SnO2/Si-AlMCM-41 sensors, i.e. the SnO2/Si-Al-MCM-41 (0.7)
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sensor possesses the highest H2 sensitivity. On the other hand, CO sensitivity decreases with the decrease of SnO2 content and CH4 sensitivity is insensitive to the content of SnO2.Yamazoe et al. [3,4] reported that the addition of 1 wt% or 5 wt% of trivalent Al3 + into the framework of SnO2 greatly increased the H2 sensitivity of SnO2 sensor. They found that calcination of the sensors at temperatures as high as 1100°C was necessary to force aluminum to enter the frameworks of SnO2 and to enhance the H2 sensitivity [3,4]. Eventually, they concluded that the existence of trivalent cations, such as Al3 + , in the framework of SnO2 decreased the carrier concentration and increased the depth of the space charge layer of n-type semiconductor oxides, resulting in the enhancement of H2 sensitivity of SnO2 sensors. Opposite effects on the sensor sensitivity were observed when pentavalent cations, such as Sb5 + , were doped into the framework of SnO2 [3,4]. Although the Si-Al-MCM-41 material used as a modifying material in this study does contain some amount of aluminum in the framework, the great enhancement of H2 sensitivity observed on the SnO2/SiAl-MCM-41 sensor is believed, however, not to be due to the same phenomena related to the high-temperature leaching of Al3 + into SnO2 as observed by Yamazoe et al. [3,4]. The reasons are given as follows. Firstly, Al in Si-Al-MCM-41 is difficult to be transferred to the framework of SnO2 since Al acts as one of the framework components in Si-Al-MCM-41 and is strongly bonded in the framework. Secondly, the thermal treatment temperature used to prepare SnO2/Si-Al-MCM-41 sensors (i.e. 700°C) is not high enough to leach Al3 + into the framework of SnO2 [3,4].To further exclude the effects of Al3 + , purely siliceous Si-MCM-41 supported SnO2 sensors were also prepared using the same solid state modification method and their sensing properties were studied. Fig. 7 shows the sensitivity of a SnO2/SiMCM-41(1.0) sensor to 500 ppm of H2, CO and CH4 as a function of measurement temperatures. The values and changing trends of sensitivities of the SnO2/SiMCM-41 sensor are very similar to those of SnO2/SiAl-MCM-41 sensors, suggesting the similarity between Si-Al-MCM-41 and Si-MCM-41 as supports for SnO2 sensors. In other words, the result shows that Al does not enhance the H2 sensitivity of SnO2/Si-Al-MCM-41 sensors.
3.4. Sensor Selecti6ity In a catalysis reaction, the selectivity of a particular product is generally defined as the percentage of this product to the total amount of all products, i.e. SELi = (Ai /SAi ) 100%, where SELi is the selectivity of product i, Ai is the amount of product i, and SAi is the total amount of all products. Based on this definition, the selectivity of a gas sensor to a particular gas may be
G. Li, S. Kawi / Sensors and Actuators B 59 (1999) 1–8
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Table 1 Sensitivities to 500 ppm of H2, CO and CH4 on different sensors Sensor
SnO2/MCM (weight ratio)
H2
CO
Tmax (°C)
Smax
Tmax (°C)
CH4 Smax
Tmax (°C)
Smax
300–350
36
350
15
400
8
SnO2/Si-Al-MCM-41
2.0 1.0 0.7 0.5
350–400 400 \400 \400
113 131 186 13
350 350–400 350 400
43 20 16 2
500–600 500–600 500–600 –
11 10.5 9 –
SnO2/Si-MCM-41
1.0
400
102
350
18
500–600
10
Pure SnO2
quantitatively described as SELi =(Si /SSi ) ×100%, where Si is the sensitivity of a sensor to gas i and SSi represents the sum of sensitivities of a sensor to different target gases at similar concentrations. It should be noted that the selectivity value calculated based on this definition changes not only with the change of measurement temperature and gas concentration, but also the number of gases used. Based on this definition, the selectivity to H2, CO or CH4 may then be described in this study as SELi =[Si /(SH2 +SCO +SCH4)] ×100%, where i represents one of the gases as H2, CO, or CH4. Fig. 8 shows the changes of selectivity with temperatures of pure SnO2 and SnO2/Si-Al-MCM-41 sensors to H2. The results show that the highest H2 selectivity ( 90%) is obtained on a SnO2/Si-Al-MCM-41(0.7) sensor at 400°C. At temperatures between 350 and 500°C, both SnO2/Si-Al-MCM-41(0.7) and SnO2/Si-AlMCM-41(1.0) sensors still show very high selectivity to H2 ( \ 80%), whereas the pure SnO2 sensor showed very low selectivity to H2 at these high temperature ranges. It was also found that when the amount of SnO2 content in SnO2/Si-Al-MCM-41 sensors decreases, the selectivity of the sensors to H2 increases. Compared with the selectivity to H2, the selectivity to CO is much lower (B50%), suggesting that SnO2/Si-
Al-MCM-41 sensors are not good gas sensors to selectively detect CO in gases containing H2. Contrary to the selectivity of the sensors to H2, when the amount of SnO2 content in SnO2/Si-Al-MCM-41 sensors decreases, the selectivity of the sensors to CO generally decreases. It is observed that good CH4 selectivity was obtained only at very high temperature ranges. This is not surprising as it is difficult to activate CH4 at low temperatures. Since CH4 is a very stable compound, activation of CH4 requires catalysts and very high temperatures to break the CH bonds. Activation of methane with/without the existence of oxygen is, in fact, a big challenge in catalysis research field and has been widely studied; in fact, temperatures higher than 500°C are normally needed to obtain sufficient conversion of methane [15,16]. It was also found that, when the amount of SnO2 content in SnO2/Si-Al-MCM-41 sensors decreases, the selectivity of the sensors to CH4 generally decreases. Based on the characterization data presented in this paper, the data are not enough to clearly elucidate the mechanism showing why the sensitivity and selectivity to H2 could be greatly improved by mechanically mixing SnO2 with MCM-41 materials. However, the coexistence state of SnO2 and MCM-41 is believed to be very
Fig. 7. Correlation between temperature and sensitivity to 500 ppm of H2, CO and CH4 on the SnO2/Si-MCM-41(1.0) sensor.
Fig. 8. Correlation between selectivity to H2 of the pure SnO2 and SnO2/Si-Al-MCM-41 sensors (having different SnO2 content) with temperature.
G. Li, S. Kawi / Sensors and Actuators B 59 (1999) 1–8
important in affecting the sensing properties of SnO2 sensors. On one hand, part of the SnO2 in SnO2/Si-AlMCM-41 sensors might migrate and be coated on the surface of MCM-41 through high temperature solid state reaction, which has been well used in catalyst preparation to coat active species onto surfaces of catalyst supports [17,18]. The possible surface coating of high-surface-area SnO2 onto MCM-41 may greatly influence the surface reactivity of SnO2 with H2 and produce high sensitivity to H2. On the other hand, SnO2/Si-Al-MCM-41 sensor materials may possibly be only mechanical mixtures of SnO2 powders and MCM41 powders. In both cases, the pore structures and the adsorption properties of MCM-41 may be responsible for the improvement of sensing properties of SnO2 by MCM-41 materials. It is believed that, no matter what the coexistence state of SnO2 on MCM-41 is, the difference in the diffusion of H2, CO and CH4 into the pore structures of sensor particles may play an important role in explaining the change of sensor’s sensitivity. Being very small molecules, H2 molecules diffuse much faster in porous materials than larger molecules, such as CO and CH4. This effect may cause H2 to be much easier than CO and CH4 to access more SnO2 in SnO2/MCM-41 sensors, hence causing the difference in the sensitivity and selectivity among these gases. To confirm this viewpoint, the study of the sensing properties of SnO2 sensors which are modified using porous materials having even much smaller pores — such as zeolites —may be helpful. If the above assumption of the effect of gas diffusion in controlling the sensitivity and selectivity of sensors is correct, the difference between the zeolitebased sensor’s sensitivity to H2 and CO (or CH4) should be even larger. Efforts should be made on the characterization of these SnO2/Si-Al-MCM-41 sensor materials in the future in order to understand better the mechanistic aspects of the preparation and the sensing behavior of these sensor materials.
4. Conclusions By mechanically mixing SnO2 with MCM-41 and treating the solid mixture at high temperatures, SnO2/ Si-Al-MCM-41 and SnO2/Si-MCM-41 sensors were prepared. Based on the sensitivity and selectivity results, several points can be concluded as follows: 1. Compared with the pure SnO2 gas sensor, MCM-41 modified SnO2 sensors posses better H2 sensitivity at temperatures between 250 and 600°C. The moderate enhancement of CO and slight enhancement of CH4 sensitivity are also found at much high temperatures. 2. The enhancement of the gas sensitivity of SnO2/SiAl-MCM-41 sensors to H2 is not caused by the doping effect of Al3 + into SnO2.
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3. The SnO2 content of SnO2/MCM-41=0.7 (weight ratio) is the optimum SnO2 content for SnO2/Si-AlMCM-41 sensors to detect H2. 4. The best H2 selectivity (over 90%) is obtained on the SnO2/Si-Al-MCM-41(0.7) sensor at a temperature of 400°C, implying a great potential for this sensor to be used as a sensitive and selective H2 gas sensor at high temperatures. MCM-41 modified SnO2 sensors are not good sensors to selectively detect CO. CH4 can only be selectively detected by these MCM-41 modified SnO2 sensors at much high temperatures (\ 500°C).
Acknowledgements The research was supported by the National University of Singapore (RP 950669) and by the Process Analysis and Optimization (PAO) Enterprise. The authors express their sincere appreciation to the Shimadzu (Asia Pacific) Pte Ltd for their assistance in XRD experiments.
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Biographies Guangjin Li received his BS degree (1987) in chemistry from the University of Science and Technology of China, Hefei, China. After 5 years postgraduate study in Dalian Institute of Chemical Physics ( July 1987– August 1990) and Catalysis Research Center, Hokkaido University, Japan (September 1990 – May 1992), he was awarded his Ph.D. degree (1992) from Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
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Dalian, China. From 1992 to 1996, he worked as a research associate in State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. He worked as a postdoctoral fellow in Department of Chemical Engineering, National University of Singapore from May 1996 to January1998. He is currently working as an Engineer in Hewlett-Packard Singapore Ltd. His main research interests are in heterogeneous catalysis, synthesis of inorganic materials, organometallic chemistry, and semiconductor chemical sensors. Sibudjing Kawi obtained his BA in Chemistry (Hons.) from the University of Texas at Austin (USA), B.S. in Chemical Engineering (Hons.) from The University of Texas at Austin (USA), MS in Chemical Engineering from The University of Illinois at Urbana-Champaign (USA), and Ph.D. in Chemical Engineering from The University of Delaware (USA). He did his two-year postdoctoral study at the University of California at Davis (USA). He is presently an Assistant Professor in the Department of Chemical and Environmental Engineering at the National University of Singapore (Singapore). His field of specialization includes semiconductor oxide gas sensors, heterogeneous catalysts, ceramic adsorbents, and advanced materials. He is a member of the American Institute of Chemical Engineers, the American Chemical Society, and the Materials Research Society.