- Email: [email protected]
butane mixtures in air by a set of two microcalorimetric sensors
262
Sensors and Actuators
Detection of methane/butane microcalorimetric sensors V. Sommer,
R. Rongen,
P. Tobias
B, 6 (1992) 262-265
mixtures in air by a set of two
and D. Kohl
Institute of Applied Physics, IAP, D-6300 Giessen (FRG)
Abstract Non-diffusion-limited microcalorimetric sensors with a platinum catalyst have been prepared. The methane response is proportional to its concentration, and the butane response can be described by a power law with the exponent 1.3. At concentrations near the stoichiometric ratio between the combustible gas and oxygen, the sensitivity increases markedly. This may be due to blocking of sites by an oxygen excess at low concentrations. In methane/butane mixtures the signals are not additive; the methane sensitivity is increased in the presence of butane. A moderate hexamethyldisiloxane (HMDS) treatment strongly reduces the methane sensitivity but the butane sensitivity remains nearly unchanged. A pair of sensors with and without HMDS treatment allows the composition of a methane/ butane mixture in air to be determined.
Introduction Microcalorimetric sensors (pellistors) measure the heat of combustion on a catalyst to determine the concentration of combustible gases and vapours in air. Usually it is regarded as an advantage of pellistors in comparison to sensor devices based on infrared absorption that the signal is approximately proportional to the sum of the concentrations of all combustible gases present. More precisely, the signal should be proportional to the sum of the concentrations weighted by the respective combustion enthalpy 6H of the gas components. Two types of pellistors are described in the literature [ 11. Type I pellistors feature a catalyst directly exposed to the gas flux. The heat evolved on the catalyst surface depends on the reactivity of the surface. Type II pellistors operate under diffusion-limited conditions. The amount of catalyst is excessive by a factor of about ten and the gas access is limited by a diffusion barrier. Therefore a loss of catalytic activity changes the signal only marginally until about 90% of the original activity is lost. Details are given elsewhere [I]. Since poisoning by sulphur-, silicone- or halogen-containing gases often limits the lifetime, the latter type of device is called ‘poison resistant’. Most of the commercially available pellistors are of this type. Usually they are guaranteed to have a linear response with 10% accuracy for one combustible gas in air. 0925~4005/92/%5.00
Type I pellistors are faster because of the missing diffusion step. Low-power devices with a small amount of catalyst flashed to operating temperature only every few seconds by a current pulse are of type I. They are also commercially available [2]. Prototypes exist as micromechanical devices [3]. In this work prototypes of type I pellistors have been prepared. It will be shown that the signals in methane/butane mixtures in air are not additive. A moderate hexamethyldisiloxane (HMDS) pretreatment decreases the response to methane strongly, whereas the response to butane is influenced only slightly. Generally poisoning of noble-metal pellistors by HMDS reduces the methane response more strongly than the response to more easily oxidizable gases (butane, propene, methanol, hydrogen) [4-61. Therefore, a pair of pellistors, one moderately poisoned by HMDS, should allow the concentrations of methane and butane to be determined separately.
Experimental Sample preparation
A&O3 platelets (Degussa, 4 mm x 6 mm x 1 mm) are used as substrates. An Au meander on the rear side serves for heating up to 900 “C. An Au/Ni thermocouple is attached to the rear surface for temperature measurements. A Pt catalyst @ 1992 -
Elsevier Sequoia.
All rights reserved
263
(Alpha-Ventron No. 89100: 10 wt.% Pt on y-alumina) is mixed into bi-distilled water (1:9 by weight). The surface of the platelet is covered with 4 mg of the catalyst. The water is removed first by a stream of warm air (60 “C) and afterwards by annealing for 30 min at 600 “C under atmospheric conditions. The mean size of the Pt clusters is derived from X-ray diffraction. The broadening of the Pt( 111) Bragg peak at 0 = 19.9” of the freshly prepared sample corresponds to a cluster diameter of 5 nm [7]. After annealing at 800 “C for 60 min in synthetic air (80% Nz, 20% O,), the cluster size increased to 13 nm. This cluster size remains constant during the subsequent measurements. HMDS
h
I
I
-105
-115
-95
BINDING ENERGY
(ev)
Fig. 1. Photoemission spectrum (XPS) near the 2p peak of Si. The difference before and after the HMDS treatment described in the Experimental Section is shown. The experimental result can be reproduced by a superposition of four peaks positioned at 100.8, 101.8, 102.9 and 104.0 eV. The individual peaks are designated by integers corresponding to the number of SiLO bonds [S].
treatment
Two samples, 1Si and 2Si, prepared as described above, are pretreated in a mixture of HMDS and synthetic air. A stream of synthetic air (0.1 l/h) is passed through a bubbler filled with HMDS. The vapour pressure of HMDS at 20 “C amounts to 23 Torr, corresponding to about 3 vol.%. A concentration of 300 ppm is adjusted by mixing the saturated flux with a second flux of 10 l/h synthetic air. The samples kept at a temperature of 600 “C are exposed for 30 min to the 300 ppm HMDS stream. The parameters of this pretreatment are chosen to reduce the sensitivity to butane by about 10% and that to methane by about 80%. Figure 1 shows a photoemission difference spectrum (XPS) around the 2p peak of Si before and after the HMDS treatment of sample 1Si. In the framework of a random bond model [8], a stoichiometry of SiO 1.4can be derived from the four individual peaks forming the experimental peak in Fig. 1. The peak did not change during operation for 300 h at 600 “C. In the following results pairs of pellistors are discussed. The untreated elements of the pairs are denoted by 1 or 2, and the HMDS-treated ones by 1Si or 2Si. Measurements
4.5 0% butane + 3% methane
E
4.0
+
1%butane 3% methane
TIME (s) Fig. 2. Trace of power consumption of a pellistor without HMDS pretreatment exposed sequentially to various mixtures of butane and methane in artificial air. Data points were taken every 5 s. The arrows give the appropriate corrections for variation of thermal conductivities during the three different admixtures. Operating temperature: 600 “C. For comparison, a calculated response level (0) proportional to the sum of the methane and butane concentrations weighted by the respective combustion enthalpies is also shown.
gas is taken as the signal. Corrections for heat conduction are indicated in Fig. 2 and are already considered in Figs. 3 and 4.
of gas response
Each pellistor is installed in a small housing with a volume of 0.5 cm’. A gas flux of 10 l/h through the housing is kept constant by electronic flow regulators. From these data a gas-switching time of 0.2 s is calculated. The temperature of the pellistors is kept at 600 “C, and the loss of power’ consumption during admixture of combustible
Results and discussion
The response of a pellistor without HMDS treatment to three different mixtures of methane and butane in artificial air is shown as a function of time in Fig. 2. The combustion enthalpies for
264
methane and butane amount to 800 and 2660 kJ/ mole at 600 “C [9]. If the signals were additive, weighted by the relative enthalpies, the response to the mixtures with 1 or 2% butane would be given by the levels ‘- O-’ in Fig. 2. Apparently the experimental values deviate markedly from the calculated response. This may be caused by a non-linearity or a non-additivity of the methane and butane signals. Both effects occur, as can be seen from the data in Fig. 3. At low methane concentrations (Fig. 3(a)) the response is approximately linear. However, even in the regime of linear response, the sensitivity to methane depends on the butane concentration. 1.8% butane (LEL value) increases the sensitivity to methane by about 30%. For higher methane concentrations on the right side of the broken line, the signals increase steeply with concentration till saturation is reached. The onset of the steep increase shifts to lower methane concentrations as the butane concentration is increased. The observed behaviour of the sensitivity for methane and butane is qualitatively understandable if three different ranges of concentrations are regarded separately. For low concentrations of combustibles an excess of oxygen chemisorbed on the surface is prob5.0
4.0
3.0
2.0
1.0
0 0
2
4
6
0 10
METHANE (~01%)
0
1.0
2.0
BUTANE (~01%)
Fig. 3. Pellistor without HMDS treatment. The loss of power consumption corrected for heat conductivity effects is given for pulses with different methane and butane concentrations. The lower explosion limit values (LEL) for methane (5 vol.%), butane ( I .8 vol.%) and their admixtures are indicated. (a) The lowest-lying curve belongs to a methane/air mixture. The curves above are taken for pulses with 0.16, 0.40, 0.67, 0.93, 1.18, 1.44, 1.69, 1.95, 2.22 and 2.5 vol.% butane added. To the left of the broken line a linear response to methane is observed. (b) The lowest-lying curve belongs to a butane/air mixture. The curves above are taken for pulses with I, 2, 3, 4, 5.5, 6, 7, 8, 9, IO and II vol.% methane added.
ably present. Hicks et al. [lo] found that the combustion rate for methane on Pd catalysts under excess oxygen is more than an order of magnitude lower in comparison to the rate under stoichiometric conditions. Under sub-stoichiometric conditions adsorbed oxygen may block sites needed for adsorption and/or reaction of methane and butane. This assumption appears reasonable, since the activation energies for methane combustion, 36 kcal/ mole [lo], and for oxygen desorption, 34 kcal/mole [ 11, 121, nearly coincide. In an analogous manner, on Pt pellistor elements the steep increase of combustion rate shown in Fig. 3(a) could be due to stoichiometric conditions, where oxygen site blocking is less impeding, being reached. At much higher combustible concentrations the signal is reduced because of a lack of oxygen. The reactivity of Pt pellistors to methane is known to be low in comparison to that of higher alkanes [ 131. During methane detection a simultaneously present concentration of butane can consume chemisorbed oxygen and increase the density of adsorption and reaction sites for methane combustion. This explains why the methane sensitivity is increased in the presence of butane. In Fig. 3(b) the results shown in Fig. 3(a) are replotted as a function of butane concentrations. Apparently there is no linear response for small butane concentrations. As long as the methane concentrations remain small (below 5%) the signal is approximately proportional to (cb) 1.3. A simple description of the sensor response below the LEL level in Fig. 3(a) and (b) reproducing the behaviour described above is given by (least-squares fit) 6P = O.l02c, + 0.606(&)‘.3 + o.014c,(c~)‘~3
(1)
where 6P = decrease of power consumption during the gas pulse (W) and c,,,, cb = concentrations for methane and butane in air in vol.%. The last term in eqn. (1) describes the interaction of methane and butane. The general behaviour found for the untreated samples reappears for HMDStreated samples, Fig. 4. The response below the LEL level can be described by 6P = O.Ol&, + 0.521(&3
+ 0.010c,(cb)‘~3
(2)
The sensitivities to methane and butane are reduced to about 18 and 86%, respectively, by the HMDS treatment. The interaction between methane and butane remains strong.
265
is not observed, because the influence of catalytic activity is only weak [ 151. The effects caused by methane/butane interaction should be less pronounced, too.
References
0
0246610
METHANE (~01%)
0
1.0
2.0
BUTANE (~01%)
Fig. 4. Pellistor with the HMDS treatment as described in the Experimental Section. The gas pulses are applied as described in the caption of Fig. 3. Only one different methane concentration was chosen: 5.0 vol.% instead of 5.5 vol.% methane. (b) For clarity, the curves for 2 and 4 vol.% methane are omitted; those for 6 and 8 vol.% methane are only plotted in the region limited by butane concentrations of I.18 vol.% and 2.5 vol.%.
The description by eqns. (1) and (2) allows the methane and butane contents below the LEL limits to be determined with an accuracy of 0.5 vol.% for methane and 0.1 vol.% for butane. It should be emphasized that the proposed evaluation considers only in a simple way the reaction order and an interaction effect. More sophisticated evaluation methods could yield a better approximation to the sensor response [ 141. Such methods are under investigation in our group. Conclusions
The results given here refer to non-diffusionlimited pellistors for which the signal height varies with the catalytic activity. For diffusion-limited (poison-resistant) pellistors the steep signal increase for near-stoichiometric concentrations of combustibles (Figs. 3 and 4)
I S. J. Gentry and P. T. Walsh, The theory of poisoning of catalytic flammable gas-sensing elements, in P. T. Moseley and B. C. Tofield (eds.), Solid Stare Gas Sensors, Adam Hilger, Bristol, 1987, pp. 32-54. 2 GfG, Gesellschaft fib Geratetechnik, Dortmund, Germany. 3 C. Vauchier, D. Charlot and G. Delapierre, Gas catalytic calorimetric microsensor, Proc. Third Int. Meet. Chemical Sensors, Cleveland, OH, USA, 1990, pp. 50-54. 4 Reference I, p. 34. 5 S. J. Gentry and A. Jones, Poisoning and inhibition of catalytic oxidation 1. The effect of silicone vapour on the gas-phase oxidations of methane, propene, carbon monoxide and hydrogen over platinum catalysts, J. Appl. Chem. Biotechnol., 28 (1978) 727-732. 6 C. F. Cullis and B. M. Willat, The inhibition of hydrocarbon oxidation over supported precious metal catalysts, J. Catal., 86 (1984) 187-200. H. Neff, Grundlagen und Anwendung der Riintgen-FeinstrukturAnalyse, R. Oldenbourg, Munich, 2nd edn., 1962, pp. 299-31 I. J. Finster, D. Schulze and A. Meisel, Characterization of amorphous SiO, layers with Esca, Surf. Sci., 162 (1985) 671-679. Landolt-Bornstein, Zahlenwerte und Funktionen aus Physik, Chemie, Astronomie, Geophysik, Technik, Vol. II, Part 4, Springer, Berlin, 6th edn., 1961. 10 R. F. Hicks, H. Qi, M. L. Young and R. G. Lee, Structure sensitivity of methane oxidation over platinum and palladium, J. Catal.,
122 (1990) 280-294.
II M. Alnot, A. Cassuto, J. Fusy and A. Pentenero, Comparative adsorption of 0, and N,O on platinum recrystallized ribbons, Jpn. J. Appl. Phys., Suppl. 2, (2) (1974) 79-84.
I2 J. J. Ehrhardt, L. Colin, A. Accorsi, M. Kazmierczak and I. Zdanevitch, Catalytic oxidation of methane on platinum thin films, Sensors and Acruators B, 7 (1992) 656-660. I3 R. B. Anderson, K. C. Stein, J. J. Feenan and L. J. E. Hofer, Catalytic oxidation of methane, Ind. Eng. Chem., 53 (1961) 809812. 14 H. Sundgren, F. Winquist and I. Lundstrom, Artificial neural networks and statistical pattern recognition improve MOSFET gas sensor array calibration, Proc. 6th Int. Conf. Solid-State Sensors and Actuators 1991, pp. 574-577.
(Transducers
‘91), San Francisco, CA, USA,
15 H.-J. Bahs, personal communication.
Sensors and Actuators
Detection of methane/butane microcalorimetric sensors V. Sommer,
R. Rongen,
P. Tobias
B, 6 (1992) 262-265
mixtures in air by a set of two
and D. Kohl
Institute of Applied Physics, IAP, D-6300 Giessen (FRG)
Abstract Non-diffusion-limited microcalorimetric sensors with a platinum catalyst have been prepared. The methane response is proportional to its concentration, and the butane response can be described by a power law with the exponent 1.3. At concentrations near the stoichiometric ratio between the combustible gas and oxygen, the sensitivity increases markedly. This may be due to blocking of sites by an oxygen excess at low concentrations. In methane/butane mixtures the signals are not additive; the methane sensitivity is increased in the presence of butane. A moderate hexamethyldisiloxane (HMDS) treatment strongly reduces the methane sensitivity but the butane sensitivity remains nearly unchanged. A pair of sensors with and without HMDS treatment allows the composition of a methane/ butane mixture in air to be determined.
Introduction Microcalorimetric sensors (pellistors) measure the heat of combustion on a catalyst to determine the concentration of combustible gases and vapours in air. Usually it is regarded as an advantage of pellistors in comparison to sensor devices based on infrared absorption that the signal is approximately proportional to the sum of the concentrations of all combustible gases present. More precisely, the signal should be proportional to the sum of the concentrations weighted by the respective combustion enthalpy 6H of the gas components. Two types of pellistors are described in the literature [ 11. Type I pellistors feature a catalyst directly exposed to the gas flux. The heat evolved on the catalyst surface depends on the reactivity of the surface. Type II pellistors operate under diffusion-limited conditions. The amount of catalyst is excessive by a factor of about ten and the gas access is limited by a diffusion barrier. Therefore a loss of catalytic activity changes the signal only marginally until about 90% of the original activity is lost. Details are given elsewhere [I]. Since poisoning by sulphur-, silicone- or halogen-containing gases often limits the lifetime, the latter type of device is called ‘poison resistant’. Most of the commercially available pellistors are of this type. Usually they are guaranteed to have a linear response with 10% accuracy for one combustible gas in air. 0925~4005/92/%5.00
Type I pellistors are faster because of the missing diffusion step. Low-power devices with a small amount of catalyst flashed to operating temperature only every few seconds by a current pulse are of type I. They are also commercially available [2]. Prototypes exist as micromechanical devices [3]. In this work prototypes of type I pellistors have been prepared. It will be shown that the signals in methane/butane mixtures in air are not additive. A moderate hexamethyldisiloxane (HMDS) pretreatment decreases the response to methane strongly, whereas the response to butane is influenced only slightly. Generally poisoning of noble-metal pellistors by HMDS reduces the methane response more strongly than the response to more easily oxidizable gases (butane, propene, methanol, hydrogen) [4-61. Therefore, a pair of pellistors, one moderately poisoned by HMDS, should allow the concentrations of methane and butane to be determined separately.
Experimental Sample preparation
A&O3 platelets (Degussa, 4 mm x 6 mm x 1 mm) are used as substrates. An Au meander on the rear side serves for heating up to 900 “C. An Au/Ni thermocouple is attached to the rear surface for temperature measurements. A Pt catalyst @ 1992 -
Elsevier Sequoia.
All rights reserved
263
(Alpha-Ventron No. 89100: 10 wt.% Pt on y-alumina) is mixed into bi-distilled water (1:9 by weight). The surface of the platelet is covered with 4 mg of the catalyst. The water is removed first by a stream of warm air (60 “C) and afterwards by annealing for 30 min at 600 “C under atmospheric conditions. The mean size of the Pt clusters is derived from X-ray diffraction. The broadening of the Pt( 111) Bragg peak at 0 = 19.9” of the freshly prepared sample corresponds to a cluster diameter of 5 nm [7]. After annealing at 800 “C for 60 min in synthetic air (80% Nz, 20% O,), the cluster size increased to 13 nm. This cluster size remains constant during the subsequent measurements. HMDS
h
I
I
-105
-115
-95
BINDING ENERGY
(ev)
Fig. 1. Photoemission spectrum (XPS) near the 2p peak of Si. The difference before and after the HMDS treatment described in the Experimental Section is shown. The experimental result can be reproduced by a superposition of four peaks positioned at 100.8, 101.8, 102.9 and 104.0 eV. The individual peaks are designated by integers corresponding to the number of SiLO bonds [S].
treatment
Two samples, 1Si and 2Si, prepared as described above, are pretreated in a mixture of HMDS and synthetic air. A stream of synthetic air (0.1 l/h) is passed through a bubbler filled with HMDS. The vapour pressure of HMDS at 20 “C amounts to 23 Torr, corresponding to about 3 vol.%. A concentration of 300 ppm is adjusted by mixing the saturated flux with a second flux of 10 l/h synthetic air. The samples kept at a temperature of 600 “C are exposed for 30 min to the 300 ppm HMDS stream. The parameters of this pretreatment are chosen to reduce the sensitivity to butane by about 10% and that to methane by about 80%. Figure 1 shows a photoemission difference spectrum (XPS) around the 2p peak of Si before and after the HMDS treatment of sample 1Si. In the framework of a random bond model [8], a stoichiometry of SiO 1.4can be derived from the four individual peaks forming the experimental peak in Fig. 1. The peak did not change during operation for 300 h at 600 “C. In the following results pairs of pellistors are discussed. The untreated elements of the pairs are denoted by 1 or 2, and the HMDS-treated ones by 1Si or 2Si. Measurements
4.5 0% butane + 3% methane
E
4.0
+
1%butane 3% methane
TIME (s) Fig. 2. Trace of power consumption of a pellistor without HMDS pretreatment exposed sequentially to various mixtures of butane and methane in artificial air. Data points were taken every 5 s. The arrows give the appropriate corrections for variation of thermal conductivities during the three different admixtures. Operating temperature: 600 “C. For comparison, a calculated response level (0) proportional to the sum of the methane and butane concentrations weighted by the respective combustion enthalpies is also shown.
gas is taken as the signal. Corrections for heat conduction are indicated in Fig. 2 and are already considered in Figs. 3 and 4.
of gas response
Each pellistor is installed in a small housing with a volume of 0.5 cm’. A gas flux of 10 l/h through the housing is kept constant by electronic flow regulators. From these data a gas-switching time of 0.2 s is calculated. The temperature of the pellistors is kept at 600 “C, and the loss of power’ consumption during admixture of combustible
Results and discussion
The response of a pellistor without HMDS treatment to three different mixtures of methane and butane in artificial air is shown as a function of time in Fig. 2. The combustion enthalpies for
264
methane and butane amount to 800 and 2660 kJ/ mole at 600 “C [9]. If the signals were additive, weighted by the relative enthalpies, the response to the mixtures with 1 or 2% butane would be given by the levels ‘- O-’ in Fig. 2. Apparently the experimental values deviate markedly from the calculated response. This may be caused by a non-linearity or a non-additivity of the methane and butane signals. Both effects occur, as can be seen from the data in Fig. 3. At low methane concentrations (Fig. 3(a)) the response is approximately linear. However, even in the regime of linear response, the sensitivity to methane depends on the butane concentration. 1.8% butane (LEL value) increases the sensitivity to methane by about 30%. For higher methane concentrations on the right side of the broken line, the signals increase steeply with concentration till saturation is reached. The onset of the steep increase shifts to lower methane concentrations as the butane concentration is increased. The observed behaviour of the sensitivity for methane and butane is qualitatively understandable if three different ranges of concentrations are regarded separately. For low concentrations of combustibles an excess of oxygen chemisorbed on the surface is prob5.0
4.0
3.0
2.0
1.0
0 0
2
4
6
0 10
METHANE (~01%)
0
1.0
2.0
BUTANE (~01%)
Fig. 3. Pellistor without HMDS treatment. The loss of power consumption corrected for heat conductivity effects is given for pulses with different methane and butane concentrations. The lower explosion limit values (LEL) for methane (5 vol.%), butane ( I .8 vol.%) and their admixtures are indicated. (a) The lowest-lying curve belongs to a methane/air mixture. The curves above are taken for pulses with 0.16, 0.40, 0.67, 0.93, 1.18, 1.44, 1.69, 1.95, 2.22 and 2.5 vol.% butane added. To the left of the broken line a linear response to methane is observed. (b) The lowest-lying curve belongs to a butane/air mixture. The curves above are taken for pulses with I, 2, 3, 4, 5.5, 6, 7, 8, 9, IO and II vol.% methane added.
ably present. Hicks et al. [lo] found that the combustion rate for methane on Pd catalysts under excess oxygen is more than an order of magnitude lower in comparison to the rate under stoichiometric conditions. Under sub-stoichiometric conditions adsorbed oxygen may block sites needed for adsorption and/or reaction of methane and butane. This assumption appears reasonable, since the activation energies for methane combustion, 36 kcal/ mole [lo], and for oxygen desorption, 34 kcal/mole [ 11, 121, nearly coincide. In an analogous manner, on Pt pellistor elements the steep increase of combustion rate shown in Fig. 3(a) could be due to stoichiometric conditions, where oxygen site blocking is less impeding, being reached. At much higher combustible concentrations the signal is reduced because of a lack of oxygen. The reactivity of Pt pellistors to methane is known to be low in comparison to that of higher alkanes [ 131. During methane detection a simultaneously present concentration of butane can consume chemisorbed oxygen and increase the density of adsorption and reaction sites for methane combustion. This explains why the methane sensitivity is increased in the presence of butane. In Fig. 3(b) the results shown in Fig. 3(a) are replotted as a function of butane concentrations. Apparently there is no linear response for small butane concentrations. As long as the methane concentrations remain small (below 5%) the signal is approximately proportional to (cb) 1.3. A simple description of the sensor response below the LEL level in Fig. 3(a) and (b) reproducing the behaviour described above is given by (least-squares fit) 6P = O.l02c, + 0.606(&)‘.3 + o.014c,(c~)‘~3
(1)
where 6P = decrease of power consumption during the gas pulse (W) and c,,,, cb = concentrations for methane and butane in air in vol.%. The last term in eqn. (1) describes the interaction of methane and butane. The general behaviour found for the untreated samples reappears for HMDStreated samples, Fig. 4. The response below the LEL level can be described by 6P = O.Ol&, + 0.521(&3
+ 0.010c,(cb)‘~3
(2)
The sensitivities to methane and butane are reduced to about 18 and 86%, respectively, by the HMDS treatment. The interaction between methane and butane remains strong.
265
is not observed, because the influence of catalytic activity is only weak [ 151. The effects caused by methane/butane interaction should be less pronounced, too.
References
0
0246610
METHANE (~01%)
0
1.0
2.0
BUTANE (~01%)
Fig. 4. Pellistor with the HMDS treatment as described in the Experimental Section. The gas pulses are applied as described in the caption of Fig. 3. Only one different methane concentration was chosen: 5.0 vol.% instead of 5.5 vol.% methane. (b) For clarity, the curves for 2 and 4 vol.% methane are omitted; those for 6 and 8 vol.% methane are only plotted in the region limited by butane concentrations of I.18 vol.% and 2.5 vol.%.
The description by eqns. (1) and (2) allows the methane and butane contents below the LEL limits to be determined with an accuracy of 0.5 vol.% for methane and 0.1 vol.% for butane. It should be emphasized that the proposed evaluation considers only in a simple way the reaction order and an interaction effect. More sophisticated evaluation methods could yield a better approximation to the sensor response [ 141. Such methods are under investigation in our group. Conclusions
The results given here refer to non-diffusionlimited pellistors for which the signal height varies with the catalytic activity. For diffusion-limited (poison-resistant) pellistors the steep signal increase for near-stoichiometric concentrations of combustibles (Figs. 3 and 4)
I S. J. Gentry and P. T. Walsh, The theory of poisoning of catalytic flammable gas-sensing elements, in P. T. Moseley and B. C. Tofield (eds.), Solid Stare Gas Sensors, Adam Hilger, Bristol, 1987, pp. 32-54. 2 GfG, Gesellschaft fib Geratetechnik, Dortmund, Germany. 3 C. Vauchier, D. Charlot and G. Delapierre, Gas catalytic calorimetric microsensor, Proc. Third Int. Meet. Chemical Sensors, Cleveland, OH, USA, 1990, pp. 50-54. 4 Reference I, p. 34. 5 S. J. Gentry and A. Jones, Poisoning and inhibition of catalytic oxidation 1. The effect of silicone vapour on the gas-phase oxidations of methane, propene, carbon monoxide and hydrogen over platinum catalysts, J. Appl. Chem. Biotechnol., 28 (1978) 727-732. 6 C. F. Cullis and B. M. Willat, The inhibition of hydrocarbon oxidation over supported precious metal catalysts, J. Catal., 86 (1984) 187-200. H. Neff, Grundlagen und Anwendung der Riintgen-FeinstrukturAnalyse, R. Oldenbourg, Munich, 2nd edn., 1962, pp. 299-31 I. J. Finster, D. Schulze and A. Meisel, Characterization of amorphous SiO, layers with Esca, Surf. Sci., 162 (1985) 671-679. Landolt-Bornstein, Zahlenwerte und Funktionen aus Physik, Chemie, Astronomie, Geophysik, Technik, Vol. II, Part 4, Springer, Berlin, 6th edn., 1961. 10 R. F. Hicks, H. Qi, M. L. Young and R. G. Lee, Structure sensitivity of methane oxidation over platinum and palladium, J. Catal.,
122 (1990) 280-294.
II M. Alnot, A. Cassuto, J. Fusy and A. Pentenero, Comparative adsorption of 0, and N,O on platinum recrystallized ribbons, Jpn. J. Appl. Phys., Suppl. 2, (2) (1974) 79-84.
I2 J. J. Ehrhardt, L. Colin, A. Accorsi, M. Kazmierczak and I. Zdanevitch, Catalytic oxidation of methane on platinum thin films, Sensors and Acruators B, 7 (1992) 656-660. I3 R. B. Anderson, K. C. Stein, J. J. Feenan and L. J. E. Hofer, Catalytic oxidation of methane, Ind. Eng. Chem., 53 (1961) 809812. 14 H. Sundgren, F. Winquist and I. Lundstrom, Artificial neural networks and statistical pattern recognition improve MOSFET gas sensor array calibration, Proc. 6th Int. Conf. Solid-State Sensors and Actuators 1991, pp. 574-577.
(Transducers
‘91), San Francisco, CA, USA,
15 H.-J. Bahs, personal communication.