CA1261720A

CA1261720A - Combined sensor device for detecting toxic gases

Info

Publication number: CA1261720A
Application number: CA000474247A
Authority: CA
Inventors: Solomon Zaromb; Joseph R. Stetter; Melvin W. Findlay, Jr.
Original assignee: Arch Development Corp
Current assignee: Arch Development Corp
Priority date: 1984-03-02
Filing date: 1985-02-13
Publication date: 1989-09-26
Also published as: GB8503514D0; IT8519721A0; GB2184244A; GB2184244B; GB2155184A; DE3507385A1; GB8629014D0; GB2155184B; FR2560685B1; JPS60205348A; FR2560685A1; IT1184147B

Abstract

COMBINED SENSOR DEVICE FOR DETECTING TOXIC GASES

ABSTRACT
An instrument capable of detecting low concentrations of a pollutant or other component in air or other gas, including a combination of a heating filament having a catalytic surface of a noble metal for exposure to the gas and producing a derivative product from the component, and an electrochemical sensor responsive to the derivative product or providing a signal indicative of the product.
At concentrations in the order of about 1-100 ppm of tetra-chloroethylene, neither the heating filament nor the electrochemical sensor is individually capable of sensing the pollutant. In the combination, the heating filament converts the benzyl chloride to one or more derivative products which may be detected by the electrochemical sensor.

Description

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CoM~INED ~ R D~NIOE FOR DE~ G T~XIC GAS~S
This invention relates ~o devices for detec~ing pollutants and more particularly to devices capable of detecting pollutants at relatively low concentrations in ambieTlt gases, Elcctrochemical sensing devices, especially amperometric sensors such as those disclos~d in U.S. Patent ~los. 3,776,832, 4,201,634 ar.d 4,32~,9~7 offer the advantages of portabiiity, real-tim~ readout, relativeiy low cost, and fair sensitivity and selectivity to a few specific pollutants such as CO, ~S, I~IO, ~2' SO2, hydrazine, CCC12, HCN, or C12. However, these devices are not applicable, at Ø, present to the detectlon of many pecies that are not electrochemically active. i~
In detec~ing pdl~lutants inc~ding toxic substances, sensing devices have usually been limited with respect to concentration~ of the pollutants partlcularly when Ihe pollutants are essentially not alectrocheT~cally active or have an activity d~fficul~ to detect. Below cert~in ~alues of concentratiGn, e.g., }00 ppm (parts per milllon) o~
benzyl chloride, tetrachloroethylene or the like, presently available portable d~vices are essentially unresponsive to the pollutant. Since some pollutants may be extremely .
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toxic, it i3 important to d velop devicc for detecting varlous polluta~ts at low concentrations.
Accordingly~ one object of this invention is a device for detecting pollutants at low concentrations in gases.
Another object is an electrochemic~l device for detect~ng pollutants in ambient gases where the pollutants are essen~ally electrochemically inactive or have an activity difficult ~o detect. An additional object is a device which ~ay be utilized as a portable instrument to survey an area 10 or as a slte monitor for a wide variety of gases. A further object is a device which is also capable of detecting pollutants at higher concentrations, at which level they nay present an acute toY.icity or flammability hazard.
Briefly, the invention relates to a device for detecting a pollutant in an a~bient gas and comprises (1) electrical conversion means having a catalytic surfacc to che~ically convert the pollutant (e.g., a hydrocarbon) to a derivatlve product (e.g., carbon monoxide) having a characteristic electrochemical activity, and (2) electro-chemicaL sensing means responsive ~o that electrochemicalactivity and providing a signal indicative of the derivativP
product and thereby the original pollutant. In one embodi-m~nc, the conversion means includes a sensor respGnsive to higher concen~rations of an elecero~hemically inactiv2 pollutant, but essentially unresponsive to low~r concen~
trations, and an electrochemical sensor responsive to the derivat~ve product, but essentially unresponsive to the pollutant at lo~ concesltrations. The conversi~n means may , .

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comprise a heated filament made of or coated with a noble metal catalyst, such as Pt, Pd, Ir, Rh, Au, Ag, or an alloy or compound of one of such metals.
Thus broadly, the invention contemplates a device for detecting a gas, vapor, chemical pollutant or other component of interest in a gaseous medium which comprises an electrical heating element means including a noble metal~containing catalytic surface arranged for exposure to a sample gaseous medium including a component of interest for heating the component for catalytically producing a derivative product of the component not present in the sample gaseous medium, with the derivative product having a characteristic electrochemical activity, and an electrochemical sensing means arranged for exposure to the derivative product as it leaves the heating element means, with the electrochemical sensing means being responsive to the electro-chemical activity of the product, and being capable of producing a signal indicative of the product and thereby the component of interest.
In another embodiment the invention provides a device for detecting a component of an ambient gas which comprises a conversion means for chemically changing said component to a derivative product having a characteristic electrochemical activity, with the conversion means having means for generating a signal when the component is present in a concentration above about 0.1%, but essentially unresponsive at concentrations below about 0.1~, and a sensing means responsive to the electrochemical activity of the product for concentrations of the component .~ below about 0.1~ for providing a signal indicative of the product . ' 1 ,;;. .
and thereby the component.

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3a Fig. 1 is an exploded view of one embodiment of the invention.
Fig. 2 is a cross-sectional view of yet another embodi-ment of the invention.
Fig. 3 is a flow diagram of yet another embodiment of the invention.
Figs. 4A, 4B & 4C show representative response curves for benzene from devices constructed according to the invention.
The device of the invention is suitable for use in detecting at least one of a variety of pollutants or otherwise hazardous gases or vapors in ambient gas. These pollutants commonly include various organic compositions such as benzene, benzyl chloride, toluene, methane, tetrachloroethylene, tetrahydrofuran, cyclo-hexane and the like. It is particularly useful for detecting the presence of a pollutant such as benzyl chloride or benzene at low concentrations in the order of about 1-100 ppm where some sensing devices are inoperative. In addition, the device may be constructed of components permitting its use as a portable instrument capable of on-site detection of a pollutant and in some instances fixed site analysis of the general concentration of the pollutant.
The device includes in combination, electrical heating means having a noble metal exposed surface for chemically changing the pollutant to a derivative product having a ,~ .

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characteri6tic electrochemical activity and a sensing ~l~ans responsive to the electrochemical acti~ity of the product and including signal means providing a signal indicative o, the product ana thereby the polluL~n~. The derivative product results from the chemical change ~n the pollutant which may occur rrom the oxidation or other process on the pollutal~t to either form electrochemical activ$ty or chan~e the existing activity of the pollutant. A particularly useful combination for the dev~ce includes a noble metal heating means which also operates as a sensor providing a signal at concentrations of the pollutant above about O.l-l.OZ while providing the derivative product over the overall concentration range r^or detectlon by the electrochemical sensing means. The presence or absence of ~he signal from the noble metal heating means in co~bination with the signal from the electrochemical sensing ~czns may be used in determining the presence or absence of a pollutant and its general concentration range.
The deriva~ive product from the pollutant exhibits a characteristic electrochemical activity which may be detected by ~he sensing means at levels as low as about one ppm. These products may be the oxides of carbo~, sulfur, nit~o~en and the like having electrochemical activity and other compositions indicative of partifil oxidation vr dccomposition of ~he pollu~ant.
The device of Fig. 1 includes the catalytic heating means as illustrated by a hot-wire sensor arranged ~o receive a sample of the ambient gas and senslng ~cans .

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illustrated by an electzochemical sensor arranged to rcceive the ~erivative product. The conversion means may include a signal means fo~ providing a signal apart from the signal fro~ the sensing means. rl'i,e ~ re sensor includes a heating means. and a ca~alyst such as one or more of the oxidation catalysts. In general, hot-wire sensors and catalysts based on Pt, Pd, Rh and ~u are particularly useful. Also useful may be catalysts cor.prising Ir or Ag, or various noble metal allovs, such as Pd-Ag, Pt^Rh, Pt-Ir or Au-A~, or a compound, espec~ially an oxide, of o~e of said metals. The hot wires or filamPnts ~ay be either made of a pure noble metal or, preferably, be coated on a suitable baser metal or alloy. The catalysts may also be dispersed on a support such as C, Si or alù~.ina. As the s2mple is exposed to the hot catalyst, it is chemically changed and pre~erably oxidized or deco~posed to at least one derivative product which may be detec~ed by th~ electrochemical sensor.
~ le electrochemical sensor i8 responsive to low levels of the derivative product and provide~ a signal indicati~e of the product and thereby the pollutant. While the electroche~ical sensor is responsive to some electrv-che~ic~lly active substances at these low l~vels~ its response to compounds such as benzyl chloride and the like which are not primarily characterized by electroche~ical activ~y tends to be so limited that i~ ~ay not be used for detection unless aided by one of the above-mentioned oxidation catalysts.

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6 ~ Z 6 ~7 % 0 Sultably, the electroche~ical sensor m~y be a con tant-potential amperometric sensor. As the derivative product is sensed, a signal is generated indicative of the electrochemlcal ~r~.~vit~ the "working electrod4" of the sensor. In general, the ho~ wire sensor representin~ the conversion means and the ampero~etric chemical sensor representing the sensing means are sufficiently small that they may be bu~lt ineo a single unit where the derivstive product from the hot wire sensor may interact with the electrochemical sensor within the same unit. Preferably, the sa~ple is introduced into a sample channelin~
arrangement by which the sample is first introduced to the hot-wire sensor. The derivative product from the hot-wire sensor is then routed to the electrochemicai sensor either directly, but in a con~rolled manner, so as to enhance the acti~ity of said Qensor by raising its temperature to a preferred value or through a baffle arrangement or other isolation syste~ 80 that the electrochemical sensor need not be directly expos~d to the heater of the hot-w~re sensor.
In Fig. l, an exploded view of the invention is illustrated showing a path including a baffle arrangenent to isolate ~he electrochemical sensor fro~ the heater of the hot-wire sensor and thereby reduce the possibility o~ da~age to the electrochemical sensor.
The inventive device includes electrical heating m~ans with a noble metal surface exposed to the gas for heating the ga3 and catalyzing the combustion o~ the pollutant eo provide a derivative product having a characteristic ., .

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electrochemical activity and electrochemlcAl sensing means responsive to that activity for providing a signal indicative of that activlty. As illustrated in Fig. 1, the heating means is pr~,~ided by a filament 10 composed of a heating element core 12 ana a noble metal s~rface 14 mounted in a reaction chamber 16 of block 15. A sensing ele~ent 18 is mounted in chamber ~0 of block 19 ror detecting the electrochemical activlty of the product derived fro~ the combustion of the pollutant. A reference electrode , ~nd a counter electrode 21 are also provided. Blocks 15 and 19 are joined by block 23 with chamber 24. Inlet 26, outlet ~8 and cha~bers 16 nd 24 provide channeling means for expo~ing the gas to the heating filament 10 and the derivative product to the sensing element 18`. Closure`member 30 provides sealing of chamber 20. A plug~able openir.~ 32 above chamber 20 permits introduction of electrolyte it~to the electrochemical sensor.
In ano~her e~bodiment of the in~ention, shown in Fig. 2, a diffusion-type electrochemical sensor is used to detect concentrations of 0-2.4% me~hane in air by pree~:posing any ambient gas diffusing or convectin~ towards said sensor to an iridium-coat~d filam~nt hea~ed to a te~perature of 300-600C and pre~crably 400-500C. l'he filament i8 ~ade preferably of~ metal or alloy of relatively high resistivity having a coefficien~ of ther~.al expansion close to that of iridium, e.g., commercial grade ti anium. By ad~usting the filametlt length and cross-section t~ yield a reslstance of about ~,000 ohmQ and : . .

~ ~ 26 ~7 2 ~ -temperature of 400-500C when heatcd by a current of about 10 mill~ampcres, it beco~.es possible to achieve a low-power low-curren~-drain methane detector suitable for in~rinsically safe mine-monitoring applications.
In Fig~ 2 the device 36 includes hea~ing filament 3~
mounted in housing 37 wlth electrochemical sensLng elenent 40 hidden from direct exposure to filament 38 by barrier 42 of glass wool or other suitable porous material. In Fig. 2, inLet 52, vent 46, and chanrlels 48, 49 and S0 provide channeling of the gAs and derivaeive product. A sou~ce of electrical current is provided by leads 54 and 56 passing through a ceramic plug 44 to fila~ent 38.
The heating of the iridium-coated filament of Fi~. 2 is prefera~ly governed by a temperature con~rolier, such as that shown in ~he block diagra~ of Fig. 3. To further reduce the heating power requirc~lents, ehe filament may be heated in intermittent pulses, e.g., in pulses of O.S to 3 secondsi duration every 10 seconds. In general, the response time Oc the device is less than about 20 seconds. ~
The heating pulses are ~ufficient to bring the peak filam~ne temperature up to the range of 400-S00C.
It i~ noteworthy ehat electrochemical sensars when~used by themselves such as that used in the de~rice o~ Fig. ~, do not u~ually respont to methane even in concentrstions as high as lOOZ. Yet the signals obtained wieh the heatec iridium filament permit easy measurement o~ methane concentrations a~ lou as 0.05: or less.

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A~ an illustration of the general f low diagram associa~ed with the detection process, F~g. 3 ~llustrates an arrange~ent where the derivative product may be au~omatically routed through a multipath solenoid valve or other diversion means to either the electroche~ical sensor or removed fro~ the device before expo~ure ~o the sensor depending on the concentration levels. In this flow arrangement, ~he signal from the hot-wire sensor may serve to indicate the presence Or the pollutant at a concentration above about 0.5-l.OZ and may be used to direct the flow of derivatLve product away from the device to avoid the possible effects oF an excessive concentration of cert~in pollutants on the electrochemical sensor.
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~ en no signal fro~ the hot-wire sensor'is proYided, the flow pattern will be used to direct the deri~ative product to the electrochemical sensor and $ts s'gn 1 wiIl provide an indication of the presence of the derivative product and thereby the pollutznt. With a combination of signal6, the presence of the pollutant and it~ conc~r.tration may be determined.
~ e general flow pat~ern of the gas and~derivative product are illustrated i~ Fig. 3. As illustrated, a gas~
sample is ad~itted via l~ne 60 to conversion cha~ber 62 having filament 64 whose temperature i9 controlled by co~txoller 6S. S~gnal indicating means is provided by indica~or 66 to provlde any signal a~ailable from filament 64~ .he derivative produc~ i~ channeled via line~
68, vaIve 70 and line 72 to the electrochemlcal sen60r 74 : , ' ~ .
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having a signal ~n~icator 76. Pump 78 provides the suction and detenmines the flow rate within the device. Controller 65 and valve 70 are governed by a microprocessor (computer/
controller) 79 which receives the signals from indicator 66 and 76.
As an illustration of the representative performance o~
the a~ove-disclosed embodiments, Fig. 4 provides re~ponse curves for benzene. In general, b~nzene i5 not readily detected on ele~trochemical sensors at ppm levels and is further essentially undetectable by a hot-wire sensor at ppm levels. However, by combining the ~wo different sensors, it is possible to produce signals indicative of the presence Or benzene at a concentra~ion of less than 100 pp~ level ir, . _ I
air. While ~enzene at such levels~is not detectable by hot-wire or heated semiconductor sensors, the heated catslytic surface of such sensors or other noble metal-co&ted filaments converts the benzene to one or more :.
oxides of carbon or degradation products which are then detected by the electrochemical s~nsor.
In Fig. 4A, the heated filAment was made of a finc platinum wire (0.08 millimeter in diameter) si~lilar eo th~t used in hot-wire fla~able gas sensors, heated to a temperature of about 1000C. The t.wo humps star~ing at about 1.5 D.nutes and 4 minutes correspond to e~posure to samples of 200 ppm and 50 ppm of benzene, respectiveiy.~ ~n Fig, 4B, a commercial ~in:oxide semiconductor sensor, he~ted ;~
to about 300C, was used as a conversion mean~, The three humps st~rting at aboue 1 minute, 6 minutes, and 10 ~inutes~
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~26~ ;72~;1 correspond to samples of 20~ ppm, 50 ppm, and 200 ppm of benzene, respectively. In Fig. 4C, the conversion device consisted of a fine (0.08 milli~eter ~n dia~eter) gold filament heated to 950~50C, and the hu~p star~ing at about 0.6 minute W85 due ~o a sample of 200 pp~ of benzene. The same electrochemical sensor, co~prising a platinu~ black sensing electrode at a potential of about 1.1 volt relative to the standard reversible hydrogen electrode, was used in all three cases. A comparison of the ordinates of ~igs. 4A, B, and C shows that the heated ~in oxide yields an approximately three to four times higher response than the platinum filament, but that the gold filament yields a : ~ three-fold higher response-~than the tin oxide and a ten times higher rèsponse than the platinum.
Other representative compounds which do not appear to be detected by either type of sensor ~ndependently but are :
by the two sensors in combination at concentrations of about 100 pp~ or less are benzyl chloride and tetrachloroethyiene.
The following example is provided or illustrative purposes and is not intended to be restrictive as to the ~cope of the invention~
E~U~PLE I
A ample of alr containing~200 ppm of tetrachloro~ :~
ethylene was tested in an apparatus having a catalytic bead :
sensor available from Rexnord Corporation of Sunnyvale, Cali~ornia-and a Pt-catalyzed CO fil~mcnt available fr~m Energetics Science of ~aw~horne, New York. : The sample was~
tested with each element operatin~ separately, Observed . ~

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cuxrents from each sensor operating separately were indis-tinguishable from norma} noise levels. In another test, a si~ilarly constructed electrochemical sensor ~n combination with a Pt-catalyzed CO filament wa~ used for an air sample containing only 20 ppm tetrachloroethylene. A signal of about 0.3 microamps was observed for the combina~ion of sensors a~ compared to a value o~ less than 0.2 (noise level) microamps without the filament.
As de~cribed above, the invention provides a useful device for detecting a gas, vapor, chemical pollutant or other component in a gaseous mcdium and is capabie by a combination of a catalytic heating elen~nt and an electrochemical scnsor of de~ecting the component at low concentraticns. -- -The foregoing description of embodiments of theii~vention has been presented for purposes of illustration and description. It is not intended to be exhau~tive or to li~it the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A device for detecting a gas, vapor, chemical pollutant or other component of interest in a gaseous medium comprising:
electrical heating element means including a noble metal-containing catalytic surface arranged for exposure to a sample gaseous medium including a component of interest for heating the component for catalytically producing a derivative product of said component not present in said sample gaseous medium, the derivative product having a characteristic electro-chemical activity, and electrochemical sensing means arranged for exposure to the derivative product as it leaves said heating element means, said electrochemical sensing means being responsive to the electrochemical activity of the product, and being capable of producing a signal indicative of the product and thereby the component of interest.

2. The device of Claim 1 including at least one additional heating element means with a different catalytic surface exposed to said gas.

3. The device of Claim 1 including means for passing an electrical current through said heating means and means for measuring the signal from said sensing means.

4. The device of Claim 3 including means for interrupting the electrical current through said heating means.

5. The device of Claim 4 wherein said interruption means generates heat pulses of a predetermined duration at predetermined time intervals.

6. The device of Claim 1 wherein said catalyst is based on Pt.

7. The device of Claim 1 wherein said catalyst is based on Au.

8. The device of Claim 1 wherein said catalyst is based on Ir.

9. The device or Claim 1 wherein said catalyst is based on Pd.

10. The device of Claim 1 wherein said catalyst is based on Rh.

11. The device o Claim 1 wherein said catalyst is based on a mixture of Pd and Ag.

12. The device of Claim 5 including a plurality of said heating means with different catalytic surfaces.

13. The device of Claim 5 wherein said heating means has a core material with an electrical resistivity above that of the noble metal and with a coefficient of thermal expansion comparable to that of the noble metal.

14. The device of Claim 1 wherein the heating means and sensing means are selected to be capable of providing a signal indicative of said component at concentrations below about 0.1%.

15. The device of Claim 1 including channeling means for directing said gas to said heating means and said product to said sensing means.

16. The device of Claim 15 wherein said channeling means includes means for preventing direct exposure of said sensing element to said heating means.

17. The device of Claim 1 wherein said heating means includes signal means for providing a signal indicative of said component.

18. The device of Claim 16 wherein said heating means includes signal means for providing a signal indicative of said component and said channeling means includes means for diverting said product from exposure to said sensing means.

19. A device for detecting a component of an ambient gas comprising:
conversion means for chemically changing said component to a derivative product having a characteristic electro-chemical activity, said conversion means having means for generating a signal when said component is present in a concentration above about 0.1%, but essentially unresponsive at concentrations below about 0.1%, and sensing means responsive to the electrochemical activity of said product for concentrations of said component below about 0.1% for providing a signal indicative of the product and thereby the component.

20. The device of Claim 19 including means for receiving the signals from said conversion and sensing means and determining the general concentration of said component.

21. The device of Claim 19 where said conversion means is a hot wire sensor having a surface of an oxidation catalyst.

22. The device of Claim 20 wherein said sensing means is an electrochemical sensor operative with oxidized gases of organic compositions.

23. The device of Claim 19 wherein said conversion means is a semiconductor sensor.

24. The device of Claim 23 wherein said semiconductor is a metal oxide.

25. The device of Claim 24 wherein said metal oxide is tin oxide or a mixture thereof.