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CO gas sensor based on Au–La2O3 added SnO2 ceramics with siliceous zeolite coat
Sensors and Actuators B 45 (1997) 101 – 106
CO gas sensor based on Au–La2O3 added SnO2 ceramics with siliceous zeolite coat Kiyoshi Fukui *, Sachiko Nishida New Cosmos Electric, 2 -5 -4 Mitsuya-Naka, Yodogawa-ku, Osaka 532, Japan Received 17 October 1996; received in revised form 16 September 1997; accepted 18 September 1997
Abstract The CO selective gas sensor based on La2O3 –Au/SnO2 ceramics had a high selectivity to CO among H2, CH4, i-C4H10 and C2H4; while a high sensitivity to C2H5OH. An acidic catalyst layer converting C2H5OH into C2H4 with a low sensitivity by dehydration was coated on the sensing layer of the La2O3 –Au/SnO2 ceramics to reduce the sensitivity to C2H5OH. Ferrierite, one of siliceous zeolites, was found to have a prominent ethanol filtering effect; such a prominent performance was related to a strong acid strength and a large acid amount. The siliceous zeolites have a hydrophobic property, a specific area of more than a few hundreds in m2 g − 1 and a thermal stability. As for the ethanol filtering effects, ka/D] 105 was estimated on the basis of a simple model of chemical dynamics, k: a rate constant of dehydration, a: an effective reaction area of the catalyst layer, D: a diffusion coefficient in the filtering layer of ferrierite. As a result, a CO selective gas sensor with a sensitivity to CO over ten times higher than that to the other gases was obtained at about 300°C. © 1997 Elsevier Science S.A. Keywords: CO selective gas sensor; La2O3 –Au/SnO2 ceramics; Siliceous zeolites; Acidic catalyst; Dehydration; Ethanol filtering effects; A simple model of chemical dynamics
1. Introduction The conventional CO gas sensors have had a poor selectivity to ethanol vapor, which coexists very often in kitchens, causing false alarms; and ethanol absorbents, such as active carbon have always been used. In addition, the CO gas sensor operated at a low temperature has been necessary to be purged periodically to obtain a stable performance. The target in the present study was to develop an alcohol sensitivity depressed, steadily workable and widely available CO gas sensor. The CO selective gas sensor based on La2O3 – Au/SnO2 ceramics, reported in the 5th International Meeting on Chemical Sensor held in Rome [1], had a high selectivity to CO among H2, CH4, i-C4H10 and C2H4: La2O3 depressed the sensitivity to CO, H2, CH4, i-C4H10, and C2H4; and Au sensitized significantly CO alone. As a result, the sensitivity to CO was above ten times as high as that to H2, CH4, i-C4H10 and C2H4; while the sensitivity to C2H5OH alone remained high. * Corresponding author. Tel.: + 81 6 3083154; fax: +81 6 3030622. 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 2 8 0 - 3
Some siliceous zeolites were examined in the present study, as ethanol filtering materials, which were coated on the above sensing layer to depress such a high sensitivity to C2H5OH. The depression of the sensitivity to ethanol is due to the conversion of ethanol into ethylene within the acid catalyst layer covering the sensing layer, that have a low sensitivity to ethylene. Some attempts on the basis of the same principle have
Fig. 1. Sensor structure (a) and bridge circuit for operation (b).
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been made by using WO3 added Al2O3 [2,3], which is well-known as one of the prominent acid catalysts. The siliceous zeolites, which are the zeolites with the SiO2 to Al2O3 molar ratios above five, have generally a hydrophobic property and a thermal stability. The siliceous zeolites used in the present work were ferrierite (18.2) and two Y-types (13.9, 6.2): the values in the parentheses indicate the molar ratios of SiO2 to Al2O3. Ferrierite was found to exhibit the most prominent ethanol filtering effect among the investigated zeolites. The prominent effects were related to a large acid amount and a strong acid strength; as a result, ferrierite had a prominent acid catalyst even at a low temperature (for example, 300°C) desirable for obtaining CO selectivity. The present work proposes an innovative method and a promising material in the development of gas sensors.
Table 1 Specifications of siliceous zeolites of ethanol filtering materials Items
Siliceous zeolites Ferrierite
SiO2/Al2O3 (molar ra18.2 tios) Na2O (wt.%) B0.05 K2O (wt.%) 0.11 Acid amounts (m 2.4 mol/g) Acid strength Strong Specific area (m2/g) 200 Averaged diameter 26 (mm) Cristallite size (mm) 0.5×0.03 (Disk form)
Y-1
Y-2
13.9 0.06 — 2.0
6.2 0.24 — 0.2
Medium Weak 700 700 7 2 0.2
0.2
– 0.3
—0.3
2. Experimental A tin oxide semiconductor was obtained through a calcination (700°C for 8 h) of stannic acid prepared by hydrolysis of tin chloride with an ammonium aqueous solution. The 0.4 atomic% antimony was doped into the lattice of tin oxide to obtain a desirable conductivity. The powder of the tin oxide semiconductor was applied to a platinum wire (the wire diameter was 20 mm) coil in a bead with the diameter of 0.45 mm, as shown in Fig. 1; subsequently, fired by Joule heating of the wire coil at 600°C for 2 h in air. In addition, all the heating processes were always carried out by the Joule heating of the coil in air. The sensor element, called Hot Wire Semiconductor Gas Sensor, thus obtained, the coil of which is a heater and electrode, was operated by a conventional bridge circuit. The sensor temperature was controlled by bridge voltage E and bridge output V was given as follows: V = − E{Rs/(Rs +R0) −1/2}
(1)
where Rs, R0 and R1 are sensor and fixed resistances, respectively: here, R0 =5.6 ohm and R1 =200 ohm. Gas sensitivity DV was defined as follows: DV = V(gas)−V(air)
(2)
where V(gas) and V(air) are the sensor outputs in a sample gas co-existing air and a clean air, respectively. Lanthanum oxide (4 mol%) and gold (0.02 atm%) were added to the tin oxide layer to obtain a CO selectivity: their additions were carried out by impregnation of the aqueous solutions of La(NO3)3 and HAuCl4. Further, an ethanol filtering layer was formed to cover the above sensing layer and their ethanol filtering effects as the ethanol-converting acid catalysts were investigated. All the filtering materials used in the present work were commercially available ones:
siliceous zeolites of ferrierite and two Y-types, which were protonated; g-Al2O3, which was prepared by calcination of commercially available boehmite at 450°C for 1 h; a colloidal silica, which was a 20 wt.% coloidal silica dispersed aqueous solution, was used as a binder of the ethanol filtering materials. The ethanol filtering materials prepared in a paste were applied to cover the above sensing layer; after dried in air, the filtering layer was sintered at 600°C for 10 min in air. The dependence of ferrierite layer-thickness on the ethanol filtering effects was investigated on the basis of chemical dynamics; in addition, ferrierite gave the most prominent ethanol filtering effects. The thickness of the filtering layer was measured by an optical microscope. The sensor temperature was measured by an infrared radiation thermometer. The sensitivity to CO, H2, CH4, C2H4, i-C4H10 and C2H5OH was measured in a 10 l chamber, equipped with a stirring fan; the gas concentration was determined by a volume-mixing method.
3. Results and discussion Table 1 shows the specifications of the siliceous zeolites [4] used in the present work. The large molar ratios of SiO2 to Al2O3 result in a hydrophobic property and a thermal stability desirable for sensing materials. The very small contents of Na2O and K2O indicate that the siliceous zeolites have been modified into Bro¨nsted acids and removal of the alkaline metal ions undesirable for the present gas sensor. As a result, acid amounts and acid strength involving catalytic activity of debydration of ethanol was obtained: the order of the acid strength was estimated from NH3-TPD spectra. An apparent relationship between the specific area or the crytallite size and the ethanol filtering effect was
K. Fukui, S. Nishida / Sensors and Actuators B 45 (1997) 101–106 Table 2 Comparison of ethanol filtering effects on the basis of ethanol concentrations equal to sensitivity to CO (100 ppm) in each materials Filtering materials
Ethanol gas concentration (ppm) equal to sensitivity to CO (100 ppm)
Ferrierite Y-1 Y-2 g-Al2O3
1500 500 40 70
The layer thickness was 75 mm; the bridge voltage was 1.35 V (the sensor temperature ca. 300°C)
not seen in this study. The average diameter of secondary particles closely relates to diffusion of ethanol. Some of the above factors are supposed to govern the ethanol filtering effects or the chemical dynamics occurring within the filtering layer. It is noted that ferrierite have a larger acid amount and a stronger acid strength than those of the other siliceous zeolites. On the other hand, g-Al2O3 is well-known as one of the prominent acid catalysts in dehydration of alcohol; however, a corresponding high ethanol filtering effect was not obtained, as shown in Table 2. Table 2 shows a comparison of performance among ethanol filtering materials: ferrierite, two Y-types (1 and 2) and g-Al2O3; the performance was evaluated by ethanol gas concentrations (ppm) equal to the sensitivity to CO (100 ppm): the sensors were operated at about 300°C. As seen in Table 2, ferrierite gave the highest value and the most prominent ethanol filtering
103
effects, in other words, the highest activity as an acid catalyst for converting ethanol into ethylene. Such a prominent performance is supposed to be due to the large acid amounts and the strong acid strength. Siliceous zeolite of Y-1 gave the lowest value, which was nearly equal to that of the original sensor without the filtering layer, such that no filtering effects should be related to the small acid amounts and the weak acid strength. In other words, the ethanol filtering effect is supposed to be mainly due to the chemical activity of the layer. Fig. 2 shows the dependence of the bridge voltage on the sensitivity to CO (100 ppm), H2 (1000 ppm), C2H5OH (200 ppm in the Y-2 coat; 1000 ppm in the ferrierite one), C2H4 (1000 ppm), i-C4H10 (1000 ppm) for the gas sensors with the Y-2 and ferrierite coats under an operation temperature of about 300°C. The sensor with the Y-2 coat, as mentioned above, had nearly the same sensitivity characteristics as the original sensor without the coat. The thickness of the filtering layer was 75 mm. In the case of the ferrierite coat, the sensitivity to C2H5OH alone was significantly reduced in comparison with the Y-2 coat. Further, Fig. 3 shows the dependence of the gas concentration on the sensitivity to the same sample gases as shown in Fig. 2, which was obtained at a bridge voltage of 1.35 V, corresponding to a nearly optimum operating temperature of 300°C. The sensitivity to C2H5OH in the ferrierite coat, as suggested from Fig. 2, was significantly reduced down to less than 1/8, resulting in below 1/10 in comparison with CO; here, the lower limit of the sensi-
Fig. 2. Bridge voltage dependence of sensitivity to CO, H2, C2H4, i-C4H10 and C2H5OH. The ethanol filtering layers were of Y-2 (left) and ferrierite (right); the layer thickness was 75 mm.
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Fig. 3. Gas concentration dependence of the sensitivity to CO, H2, C2H4, CH4, i-C4H10 and C2H5OH. The ethanol filtering layers were Y-2 (left) and ferrierite (right); the bridge voltage 1.35 V (ca. 300°C); the layer thickness was 75 mm.
tivity to C2H5OH is in principle expected to be that to C2H4. Fig. 4 shows the response profiles to CO, H2 and C2H5OH in various gas concentrations for the gas sensor with the ferrierite coat under the same operation conditions as shown in Fig. 3: quick responses were obtained in all gases and the sensitivity to C2H5OH of 1000 ppm was constantly depressed at a low level for 10 min, that showed a stable activity for dehydration of ethanol. Fig. 5 shows the dependence of the ferrierite layer thickness on the sensitivity to ethanol, when the sensor temperature was nearly the same at each point between thickness values from zero (the original sensor without the coat) to 125 mm. The ethanol sensitivity versus gas concentration curves shifted rapidly towards a region of high gas concentration with increase in the thickness and then stayed at a limiting value: the ethanol gas concentrations equal to the sensitivity to CO (100 ppm) were 40, 900, 950, 1500, 1700 and 1600 ppm with increase in the layer thicknesses of 0 (an original sensor without the coat), 20, 50, 75, 100 and 125 mm, respectively. Fig. 6 shows the dependence of the sensitivity ratios of ethanol to ethylene at each concentration of 1000 or 500 ppm on the layer thickness: the ratios decreased monotonically with increase in the layer thickness and became nearly one at 125 mm; where an almost 100% conversion of ethanol into ethylene was established: in other words, the above results are due to an upper limit of ethanol filtering effects. In addition, the response rate to CO is expected to be affected by diffusion through the filtering layer: the 90% response times to CO (100 ppm) increased monotonously from 10 to 24 s with increase in the layer thickness described above. From the above results, the evaluation of the ethanol filtering effects were carried out on the basis of a simple
model of chemical dynamics involving conversion of ethanol into ethylene via dehydration, which was assumed to proceed according to a first order chemical reaction accompanied with diffusion into a spherical porous catalyst layer of the diameter of R (cm). The ratio of the ethanol concentration of C (mol cm − 3) in the catalyst layer over Cs (mol cm − 3) on the outside surface of the layer is given by the following equation [5]: C/Cs = (r/R) − 1sinh{m(r/R)}/sinh m,
(3)
m=R ka/D
(4)
−2
−1
where D (cm s ) is a diffusion coefficient in the porous layer, k (cm − 3 s − 1) is a rate constant of the first order chemical reaction in the porous layer; a (cm2 cm − 3) is the effective reaction area in the layer; m is a dimensionless parameter, which governs the depth distribution of the ethanol gas concentration within the porous catalyst layer, as seen in Eq. (3). The following experimental results were applied to evaluate the ethanol filtering effects: the sensitivity ratio of ethanol to ethylene was one at the thickness of 125 mm, accordingly, R= 325 mm; and further, the concentration of ethanol should decrease from 1000 ppm to, at least, below a few ppm from the dependence curve of the gas concentration on the sensitivity to ethanol, as a necessary condition: thus, each value of C0, C0/Cs, m and ka/D were 1, 10 − 3, 21, 3.5× 105; and 0.1, 10 − 4, 27, 6.0× 105, respectively. That is to say, a value of ka/D in the order of 105 indicates that the ferrierite has a very high activity as an acid calalyst, converting ethanol into ethylene via dehydration; the relation of ka/D] 105 is supposed to be a standard value for design of the filtering layer: ka is a chemical activity of filtering material and diffusion coefficient of D is governed by the porosity of the filtering layer.
K. Fukui, S. Nishida / Sensors and Actuators B 45 (1997) 101–106
105
Fig. 4. Response profiles to CO, H2 and C2H5OH. The ethanol filtering layer was ferrierite; the layer thickness was 75 mm; the bridge voltage was 1.35 V (ca. 300°C).
4. Conclusion The Au–La2O3/SnO2 system with the ferrierite coat have given a prominent CO selective gas sensor, which will be steadily workable and easily available for various applications. The system is composed of the composite effects of the following several functions: a gas sensing material of n-type semiconductor SnO2, giving a high gas-sensitivity; a basic metal oxide of La2O3 for basic characterization of the surface of SnO2, resulting in reduction of the sensitivity to H2, CH4, i-C4H10, C2H4, which are very often interference gases; and a prominent acid catalyst of ferrierite, which is one of the siliceous zeolites for reduction of the sensitivity to ethanol via conversion of ethanol into ethylene. The siliceous zeolites are hydrophobic Fig. 6. Layer-thickness dependence of the sensitivity ratios of C2H5OH to C2H4 at each gas concentration of 500 ( ) or 1000 ppm() under nearly the same sensor temperatures of 300°C in each thickness.
and thermally stable, which are desirable properties for sensor materials. The sensor operating temperature of about 300°C will be a lower limit for maintaining stability in steady operation; which corresponds to a sensor power consumption of about 80 mW. In future, the long term stability and other performance data will be investigated from the viewpoint of practical use. Fig. 5. Layer thickness dependence of sensitivity vs. gas concentration curves for C2H5OH. The curves and values of sensitivity to CO were almost the same regardless for all the layer thicknesses: for instance, the sensitivity to CO (100 ppm) fell into 5393 mV; the axis of ordinate is calibrated by the sensitivity to CO in 400 ppm; the bridge voltage was 1.35 V (ca. 300°C).
References [1] K. Fukui, M. Nakane, CO gas sensor based on Au–La2O3 loaded SnO2 ceramic, Sensor and Actuators B 24 – 25 (1995) 486 – 490.
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[2] K. Suzuki, S. Sakai, T. Takada, M. Nakane, Detection of offensive odor using ZnO thick film type gas sensor, in: Proceedings of the Korea-Japan Joint Symposium on Chemical Sensors 1991, Korea, May, 1991, pp. 152–158. [3] H. Mitsuhashi, K. Fukui, M. Nakane, Multi-layer structure CO selective gas sensor, in: Digest of the 18th Chemical Sensor Symposium, Japan, April, 1994, pp. 29–32. [4] T. Fukushima, H Miyazaki, S. Asano, Properties of HSZ Series (Synthetic High Silica Zeolites), Journal of TOSOH Res. 33 (2) (1989) 155 – 166.
.
[5] R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena, Wiley, 1960, pp. 542 – 547.
Biographies Kiyoshi Fukui, Doctor of Science (Physical and inorganic chemistry), Chief researcher Sachiko Nishida, University graduate (Chemical engineering), Researcher
CO gas sensor based on Au–La2O3 added SnO2 ceramics with siliceous zeolite coat Kiyoshi Fukui *, Sachiko Nishida New Cosmos Electric, 2 -5 -4 Mitsuya-Naka, Yodogawa-ku, Osaka 532, Japan Received 17 October 1996; received in revised form 16 September 1997; accepted 18 September 1997
Abstract The CO selective gas sensor based on La2O3 –Au/SnO2 ceramics had a high selectivity to CO among H2, CH4, i-C4H10 and C2H4; while a high sensitivity to C2H5OH. An acidic catalyst layer converting C2H5OH into C2H4 with a low sensitivity by dehydration was coated on the sensing layer of the La2O3 –Au/SnO2 ceramics to reduce the sensitivity to C2H5OH. Ferrierite, one of siliceous zeolites, was found to have a prominent ethanol filtering effect; such a prominent performance was related to a strong acid strength and a large acid amount. The siliceous zeolites have a hydrophobic property, a specific area of more than a few hundreds in m2 g − 1 and a thermal stability. As for the ethanol filtering effects, ka/D] 105 was estimated on the basis of a simple model of chemical dynamics, k: a rate constant of dehydration, a: an effective reaction area of the catalyst layer, D: a diffusion coefficient in the filtering layer of ferrierite. As a result, a CO selective gas sensor with a sensitivity to CO over ten times higher than that to the other gases was obtained at about 300°C. © 1997 Elsevier Science S.A. Keywords: CO selective gas sensor; La2O3 –Au/SnO2 ceramics; Siliceous zeolites; Acidic catalyst; Dehydration; Ethanol filtering effects; A simple model of chemical dynamics
1. Introduction The conventional CO gas sensors have had a poor selectivity to ethanol vapor, which coexists very often in kitchens, causing false alarms; and ethanol absorbents, such as active carbon have always been used. In addition, the CO gas sensor operated at a low temperature has been necessary to be purged periodically to obtain a stable performance. The target in the present study was to develop an alcohol sensitivity depressed, steadily workable and widely available CO gas sensor. The CO selective gas sensor based on La2O3 – Au/SnO2 ceramics, reported in the 5th International Meeting on Chemical Sensor held in Rome [1], had a high selectivity to CO among H2, CH4, i-C4H10 and C2H4: La2O3 depressed the sensitivity to CO, H2, CH4, i-C4H10, and C2H4; and Au sensitized significantly CO alone. As a result, the sensitivity to CO was above ten times as high as that to H2, CH4, i-C4H10 and C2H4; while the sensitivity to C2H5OH alone remained high. * Corresponding author. Tel.: + 81 6 3083154; fax: +81 6 3030622. 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 2 8 0 - 3
Some siliceous zeolites were examined in the present study, as ethanol filtering materials, which were coated on the above sensing layer to depress such a high sensitivity to C2H5OH. The depression of the sensitivity to ethanol is due to the conversion of ethanol into ethylene within the acid catalyst layer covering the sensing layer, that have a low sensitivity to ethylene. Some attempts on the basis of the same principle have
Fig. 1. Sensor structure (a) and bridge circuit for operation (b).
102
K. Fukui, S. Nishida / Sensors and Actuators B 45 (1997) 101–106
been made by using WO3 added Al2O3 [2,3], which is well-known as one of the prominent acid catalysts. The siliceous zeolites, which are the zeolites with the SiO2 to Al2O3 molar ratios above five, have generally a hydrophobic property and a thermal stability. The siliceous zeolites used in the present work were ferrierite (18.2) and two Y-types (13.9, 6.2): the values in the parentheses indicate the molar ratios of SiO2 to Al2O3. Ferrierite was found to exhibit the most prominent ethanol filtering effect among the investigated zeolites. The prominent effects were related to a large acid amount and a strong acid strength; as a result, ferrierite had a prominent acid catalyst even at a low temperature (for example, 300°C) desirable for obtaining CO selectivity. The present work proposes an innovative method and a promising material in the development of gas sensors.
Table 1 Specifications of siliceous zeolites of ethanol filtering materials Items
Siliceous zeolites Ferrierite
SiO2/Al2O3 (molar ra18.2 tios) Na2O (wt.%) B0.05 K2O (wt.%) 0.11 Acid amounts (m 2.4 mol/g) Acid strength Strong Specific area (m2/g) 200 Averaged diameter 26 (mm) Cristallite size (mm) 0.5×0.03 (Disk form)
Y-1
Y-2
13.9 0.06 — 2.0
6.2 0.24 — 0.2
Medium Weak 700 700 7 2 0.2
0.2
– 0.3
—0.3
2. Experimental A tin oxide semiconductor was obtained through a calcination (700°C for 8 h) of stannic acid prepared by hydrolysis of tin chloride with an ammonium aqueous solution. The 0.4 atomic% antimony was doped into the lattice of tin oxide to obtain a desirable conductivity. The powder of the tin oxide semiconductor was applied to a platinum wire (the wire diameter was 20 mm) coil in a bead with the diameter of 0.45 mm, as shown in Fig. 1; subsequently, fired by Joule heating of the wire coil at 600°C for 2 h in air. In addition, all the heating processes were always carried out by the Joule heating of the coil in air. The sensor element, called Hot Wire Semiconductor Gas Sensor, thus obtained, the coil of which is a heater and electrode, was operated by a conventional bridge circuit. The sensor temperature was controlled by bridge voltage E and bridge output V was given as follows: V = − E{Rs/(Rs +R0) −1/2}
(1)
where Rs, R0 and R1 are sensor and fixed resistances, respectively: here, R0 =5.6 ohm and R1 =200 ohm. Gas sensitivity DV was defined as follows: DV = V(gas)−V(air)
(2)
where V(gas) and V(air) are the sensor outputs in a sample gas co-existing air and a clean air, respectively. Lanthanum oxide (4 mol%) and gold (0.02 atm%) were added to the tin oxide layer to obtain a CO selectivity: their additions were carried out by impregnation of the aqueous solutions of La(NO3)3 and HAuCl4. Further, an ethanol filtering layer was formed to cover the above sensing layer and their ethanol filtering effects as the ethanol-converting acid catalysts were investigated. All the filtering materials used in the present work were commercially available ones:
siliceous zeolites of ferrierite and two Y-types, which were protonated; g-Al2O3, which was prepared by calcination of commercially available boehmite at 450°C for 1 h; a colloidal silica, which was a 20 wt.% coloidal silica dispersed aqueous solution, was used as a binder of the ethanol filtering materials. The ethanol filtering materials prepared in a paste were applied to cover the above sensing layer; after dried in air, the filtering layer was sintered at 600°C for 10 min in air. The dependence of ferrierite layer-thickness on the ethanol filtering effects was investigated on the basis of chemical dynamics; in addition, ferrierite gave the most prominent ethanol filtering effects. The thickness of the filtering layer was measured by an optical microscope. The sensor temperature was measured by an infrared radiation thermometer. The sensitivity to CO, H2, CH4, C2H4, i-C4H10 and C2H5OH was measured in a 10 l chamber, equipped with a stirring fan; the gas concentration was determined by a volume-mixing method.
3. Results and discussion Table 1 shows the specifications of the siliceous zeolites [4] used in the present work. The large molar ratios of SiO2 to Al2O3 result in a hydrophobic property and a thermal stability desirable for sensing materials. The very small contents of Na2O and K2O indicate that the siliceous zeolites have been modified into Bro¨nsted acids and removal of the alkaline metal ions undesirable for the present gas sensor. As a result, acid amounts and acid strength involving catalytic activity of debydration of ethanol was obtained: the order of the acid strength was estimated from NH3-TPD spectra. An apparent relationship between the specific area or the crytallite size and the ethanol filtering effect was
K. Fukui, S. Nishida / Sensors and Actuators B 45 (1997) 101–106 Table 2 Comparison of ethanol filtering effects on the basis of ethanol concentrations equal to sensitivity to CO (100 ppm) in each materials Filtering materials
Ethanol gas concentration (ppm) equal to sensitivity to CO (100 ppm)
Ferrierite Y-1 Y-2 g-Al2O3
1500 500 40 70
The layer thickness was 75 mm; the bridge voltage was 1.35 V (the sensor temperature ca. 300°C)
not seen in this study. The average diameter of secondary particles closely relates to diffusion of ethanol. Some of the above factors are supposed to govern the ethanol filtering effects or the chemical dynamics occurring within the filtering layer. It is noted that ferrierite have a larger acid amount and a stronger acid strength than those of the other siliceous zeolites. On the other hand, g-Al2O3 is well-known as one of the prominent acid catalysts in dehydration of alcohol; however, a corresponding high ethanol filtering effect was not obtained, as shown in Table 2. Table 2 shows a comparison of performance among ethanol filtering materials: ferrierite, two Y-types (1 and 2) and g-Al2O3; the performance was evaluated by ethanol gas concentrations (ppm) equal to the sensitivity to CO (100 ppm): the sensors were operated at about 300°C. As seen in Table 2, ferrierite gave the highest value and the most prominent ethanol filtering
103
effects, in other words, the highest activity as an acid catalyst for converting ethanol into ethylene. Such a prominent performance is supposed to be due to the large acid amounts and the strong acid strength. Siliceous zeolite of Y-1 gave the lowest value, which was nearly equal to that of the original sensor without the filtering layer, such that no filtering effects should be related to the small acid amounts and the weak acid strength. In other words, the ethanol filtering effect is supposed to be mainly due to the chemical activity of the layer. Fig. 2 shows the dependence of the bridge voltage on the sensitivity to CO (100 ppm), H2 (1000 ppm), C2H5OH (200 ppm in the Y-2 coat; 1000 ppm in the ferrierite one), C2H4 (1000 ppm), i-C4H10 (1000 ppm) for the gas sensors with the Y-2 and ferrierite coats under an operation temperature of about 300°C. The sensor with the Y-2 coat, as mentioned above, had nearly the same sensitivity characteristics as the original sensor without the coat. The thickness of the filtering layer was 75 mm. In the case of the ferrierite coat, the sensitivity to C2H5OH alone was significantly reduced in comparison with the Y-2 coat. Further, Fig. 3 shows the dependence of the gas concentration on the sensitivity to the same sample gases as shown in Fig. 2, which was obtained at a bridge voltage of 1.35 V, corresponding to a nearly optimum operating temperature of 300°C. The sensitivity to C2H5OH in the ferrierite coat, as suggested from Fig. 2, was significantly reduced down to less than 1/8, resulting in below 1/10 in comparison with CO; here, the lower limit of the sensi-
Fig. 2. Bridge voltage dependence of sensitivity to CO, H2, C2H4, i-C4H10 and C2H5OH. The ethanol filtering layers were of Y-2 (left) and ferrierite (right); the layer thickness was 75 mm.
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Fig. 3. Gas concentration dependence of the sensitivity to CO, H2, C2H4, CH4, i-C4H10 and C2H5OH. The ethanol filtering layers were Y-2 (left) and ferrierite (right); the bridge voltage 1.35 V (ca. 300°C); the layer thickness was 75 mm.
tivity to C2H5OH is in principle expected to be that to C2H4. Fig. 4 shows the response profiles to CO, H2 and C2H5OH in various gas concentrations for the gas sensor with the ferrierite coat under the same operation conditions as shown in Fig. 3: quick responses were obtained in all gases and the sensitivity to C2H5OH of 1000 ppm was constantly depressed at a low level for 10 min, that showed a stable activity for dehydration of ethanol. Fig. 5 shows the dependence of the ferrierite layer thickness on the sensitivity to ethanol, when the sensor temperature was nearly the same at each point between thickness values from zero (the original sensor without the coat) to 125 mm. The ethanol sensitivity versus gas concentration curves shifted rapidly towards a region of high gas concentration with increase in the thickness and then stayed at a limiting value: the ethanol gas concentrations equal to the sensitivity to CO (100 ppm) were 40, 900, 950, 1500, 1700 and 1600 ppm with increase in the layer thicknesses of 0 (an original sensor without the coat), 20, 50, 75, 100 and 125 mm, respectively. Fig. 6 shows the dependence of the sensitivity ratios of ethanol to ethylene at each concentration of 1000 or 500 ppm on the layer thickness: the ratios decreased monotonically with increase in the layer thickness and became nearly one at 125 mm; where an almost 100% conversion of ethanol into ethylene was established: in other words, the above results are due to an upper limit of ethanol filtering effects. In addition, the response rate to CO is expected to be affected by diffusion through the filtering layer: the 90% response times to CO (100 ppm) increased monotonously from 10 to 24 s with increase in the layer thickness described above. From the above results, the evaluation of the ethanol filtering effects were carried out on the basis of a simple
model of chemical dynamics involving conversion of ethanol into ethylene via dehydration, which was assumed to proceed according to a first order chemical reaction accompanied with diffusion into a spherical porous catalyst layer of the diameter of R (cm). The ratio of the ethanol concentration of C (mol cm − 3) in the catalyst layer over Cs (mol cm − 3) on the outside surface of the layer is given by the following equation [5]: C/Cs = (r/R) − 1sinh{m(r/R)}/sinh m,
(3)
m=R ka/D
(4)
−2
−1
where D (cm s ) is a diffusion coefficient in the porous layer, k (cm − 3 s − 1) is a rate constant of the first order chemical reaction in the porous layer; a (cm2 cm − 3) is the effective reaction area in the layer; m is a dimensionless parameter, which governs the depth distribution of the ethanol gas concentration within the porous catalyst layer, as seen in Eq. (3). The following experimental results were applied to evaluate the ethanol filtering effects: the sensitivity ratio of ethanol to ethylene was one at the thickness of 125 mm, accordingly, R= 325 mm; and further, the concentration of ethanol should decrease from 1000 ppm to, at least, below a few ppm from the dependence curve of the gas concentration on the sensitivity to ethanol, as a necessary condition: thus, each value of C0, C0/Cs, m and ka/D were 1, 10 − 3, 21, 3.5× 105; and 0.1, 10 − 4, 27, 6.0× 105, respectively. That is to say, a value of ka/D in the order of 105 indicates that the ferrierite has a very high activity as an acid calalyst, converting ethanol into ethylene via dehydration; the relation of ka/D] 105 is supposed to be a standard value for design of the filtering layer: ka is a chemical activity of filtering material and diffusion coefficient of D is governed by the porosity of the filtering layer.
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Fig. 4. Response profiles to CO, H2 and C2H5OH. The ethanol filtering layer was ferrierite; the layer thickness was 75 mm; the bridge voltage was 1.35 V (ca. 300°C).
4. Conclusion The Au–La2O3/SnO2 system with the ferrierite coat have given a prominent CO selective gas sensor, which will be steadily workable and easily available for various applications. The system is composed of the composite effects of the following several functions: a gas sensing material of n-type semiconductor SnO2, giving a high gas-sensitivity; a basic metal oxide of La2O3 for basic characterization of the surface of SnO2, resulting in reduction of the sensitivity to H2, CH4, i-C4H10, C2H4, which are very often interference gases; and a prominent acid catalyst of ferrierite, which is one of the siliceous zeolites for reduction of the sensitivity to ethanol via conversion of ethanol into ethylene. The siliceous zeolites are hydrophobic Fig. 6. Layer-thickness dependence of the sensitivity ratios of C2H5OH to C2H4 at each gas concentration of 500 ( ) or 1000 ppm() under nearly the same sensor temperatures of 300°C in each thickness.
and thermally stable, which are desirable properties for sensor materials. The sensor operating temperature of about 300°C will be a lower limit for maintaining stability in steady operation; which corresponds to a sensor power consumption of about 80 mW. In future, the long term stability and other performance data will be investigated from the viewpoint of practical use. Fig. 5. Layer thickness dependence of sensitivity vs. gas concentration curves for C2H5OH. The curves and values of sensitivity to CO were almost the same regardless for all the layer thicknesses: for instance, the sensitivity to CO (100 ppm) fell into 5393 mV; the axis of ordinate is calibrated by the sensitivity to CO in 400 ppm; the bridge voltage was 1.35 V (ca. 300°C).
References [1] K. Fukui, M. Nakane, CO gas sensor based on Au–La2O3 loaded SnO2 ceramic, Sensor and Actuators B 24 – 25 (1995) 486 – 490.
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[2] K. Suzuki, S. Sakai, T. Takada, M. Nakane, Detection of offensive odor using ZnO thick film type gas sensor, in: Proceedings of the Korea-Japan Joint Symposium on Chemical Sensors 1991, Korea, May, 1991, pp. 152–158. [3] H. Mitsuhashi, K. Fukui, M. Nakane, Multi-layer structure CO selective gas sensor, in: Digest of the 18th Chemical Sensor Symposium, Japan, April, 1994, pp. 29–32. [4] T. Fukushima, H Miyazaki, S. Asano, Properties of HSZ Series (Synthetic High Silica Zeolites), Journal of TOSOH Res. 33 (2) (1989) 155 – 166.
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[5] R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena, Wiley, 1960, pp. 542 – 547.
Biographies Kiyoshi Fukui, Doctor of Science (Physical and inorganic chemistry), Chief researcher Sachiko Nishida, University graduate (Chemical engineering), Researcher