WO1995030145A1

WO1995030145A1 - Gas analysis apparatus

Info

Publication number: WO1995030145A1
Application number: PCT/GB1995/000965
Authority: WO
Inventors: Robert Charles Copcutt; William Charles Maskell
Original assignee: Robert Charles Copcutt; William Charles Maskell
Priority date: 1994-04-28
Filing date: 1995-04-27
Publication date: 1995-11-09
Also published as: GB2288874A; GB9408384D0

Abstract

The apparatus which measures the concentration of reducing gases such as carbon monoxide in a gas mixture containing an oxidising gas such as oxygen has a diffusion barrier that restricts the flow of reducing gas to an electrode (2) which catalyses the oxidation of the reducing gas. Another electrode (6) on the other side of a solid electrolyte membrane (1) is not covered by a diffusion barrier and the reducing gas is strongly absorbed on it so that a mixed potential is created on it. The difference in potential between the two electrodes increases with increased concentration of the reducing gas and therefore provides an indication of the concentration.

Description

Gas analysis apparatus

This invention relates to the measurement of reducing gas concentration.

Carbon monoxide is a toxic gas and there is a need for a sensor for detecting it that can withstand the high temperatures found in the exhaust outlets of the numerous combustion devices which are the main source of CO. Devices constructed from solid electrolytes are ideally suited to high temperature operation.

CO is strongly adsorbed onto platinum. Below about 500 ^°C it is so strongly adsorbed that it effectively excludes oxygen from the surface. The activity of the surface oxygen falls well below that of the gas phase oxygen. When the platinum is in contact with an oxygen ion conductor such as zirconia this drop in surface activity can be measured directly because the potential on the electrode is determined by the oxygen activity. When the activity of the gas on the electrode is different to the activity in the gas phase surrounding the electrode, the potential on that electrode is referred to as a mixed potential.

At low CO concentrations the potential of a platinum electrode on zirconia shows an almost linear dependence upon the CO concentration when the system is at temperatures between about 300 ^°C and 500^° C. If one platinum electrode is exposed directly to the gas being determined and another is covered with a catalyst that promotes the oxidation of the CO a potential difference is created. Figure 1 shows how the voltage from such a sensor increases as the CO concentration increases. In some circumstances it is advantageous to avoid covering the electrodes with a porous catalyst layer.

Accordingly, one aspect of the present invention provides a sensor for measuring a concentration of a gas in a gas mixture comprising: a control chamber enclosing a first volume of gas; a first electrode exposable to the gas in the control chamber; a second electrode exposable to a second volume of gas outside of the control chamber; and a diffusion barrier for restricting a flow of gas into and/or out of the control chamber.

A further aspect of the present invention provides a sensor which measures the concentration of reducing gases such as carbon monoxide in a gas mixture containing an oxidising gas such as oxygen, which sensor has a diffusion barrier that restricts the flow of reducing gas to an electrode which catalyses the oxidation of the reducing gas. Another electrode on the other side of a solid electrolyte membrane is not covered by a diffusion barrier and the reducing gas is strongly absorbed on it so that a mixed potential is created on it. The difference in potential between the two electrodes increases with increased concentration of the reducing gas and therefore provides an indication of the concentration of that gas.

In order that the present invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a graph showing the relationship between the voltage and CO concentration; Figure 2 is a diagrammatic cross-section through an embodiment of the present invention; and

Figure 3 is a diagrammatic cross-section through a further embodiment of the present invention.

Referring to Figure 2, a diffusion barrier 3 is provided. The diffusion barrier limits the rate at which CO and 0₂ can reach the shielded electrode. This platinum electrode is a catalyst for the reaction:

2C0 + 0₂ = 2CO_z

However, the rate at which this reaction can occur on this electrode is limited and if the diffusion pore is small enough all the CO is oxidised and no mixed potential forms on the shielded electrode.

Figure 2 illustrates an embodiment of the invention in a simple form. Two electrodes, 2 and 6, composed of suitable materials such as platinum are deposited onto a disc of a suitable solid electrolyte such as zirconia 1. Electrode 2 is covered by another disc 3 of any suitable material and separated from this electrode by a suitable separating ring 5. A diffusion pore 4 within discs 1 or 3 or ring 5 restricts the flow of gas to electrode 2.

If the diffusion pore is small enough and if electrode 2 has sufficient catalytic ability all the reducing gas such as CO that enters into the enclosed volume or control chamber 7 is oxidised by the excess oxidising gas such as oxygen that must also be present in the gas mixture surrounding the sensor. The gas in the enclosed volume and exposed to electrode 2 is therefore near equilibrium while the gas exposed to electrode 6 contains a larger concentration of unreacted reducing gas. A mixed potential therefore forms on electrode 6 but not to such a great extend on electrode 2. This will crease a potential difference measured by voltmeter 8. Therefore the reading on voltmeter 8 provides a measure of the reducing gas concentration in the gas surrounding the sensor as illustrated in Figure 1.

Optimum performance of the sensor requires careful selection of many parameters such as sensor temperature, electrode surface area, morphology and materials of construction, diffusion pore size, electrolyte material used etc. Between 300 and 500°C platinum electrodes can be used on zirconia solid electrolyte to make a sensor that responds to CO with a small amount of cross-sensitivity to gases such as H₂.

A number of possible methods for modifying the sensitivity of the sensor to the gas of primary interest and reducing its cross-sensitivity to other gases can be envisaged. For instance a second diffusion barrier could be placed over electrode 6 so that both electrodes are covered by diffusion barriers with different diffusion rates. Another option is to use different materials for the

construction of electrodes 2 and 6 to enhance the effect of the diffusion barrier. For instance if electrode 2 has a strong catalytic action it will ensure that all the reducing gas diffusing through pore 4 is oxidized by the oxidizing gas that also diffuses through pore 4. If sufficient catalytic action cannot be achieved using electrode 2 alone a catalytic surface could be added to any other part of the enclosed volume. If cross-sensitivity with any other gas is a problem, selective catalysts that promote a reaction that removes the unwanted gas could be used either as a covering over the relevant electrode or anywhere within the enclosed volume. For instance a catalyst that oxidizes hydrogen but not CO could be used to cover electrode 6 with the result that the mixed potential on electrode 6 would be due to CO only.

Figure 3 illustrates a more advanced device with increased functionality over that of figure 2. The covering disc 3 is now also made from a solid electrolyte and two extra ring shaped electrodes 9 and 10 have been added. A current is passed through electrodes 9 and 10 to modify :he gas concentration within the enclosed voiume 7. A reference electrode 13 has also been added to the pump cell consisting of electrodes 9 and 10 and electrolyte disc 3. The operation amplifier 12 maintains the potential set by voltage source 14 between electrodes 9 and 13 by passing whatever current is required through ammeter 1 1 and electrodes 9 and 10. The passage of current through electrode 9 increases or decreases the catalytic activity of this electrode depending upon die direction of the current. This can be used to bring the gas within the enclosed voiume closer or further from equilibrium as desired. This can be used to enhance the gas-sensing ability of electrodes 2 and 6 as measured by voltmeter 7.

Definitions:

Mixed potential. A mixed potential is caused by two or more species competing for adsorption sites on an electrode to the extent that the activity of the potential determining species (oxygen in the case of an oxygen ion conducting solid electrolyte) on the electrode is changed by a measurable amount compared to ihai in the fluid phase burrounding the electrode.

Gauge cell. An electrochemical cell consisting of two electrodes deposited onto a single piece of a solid electrolyte. The open circuit voltage between the two electrodes is the gauge cell voltage.

Pump cell. A gauge cell with the difference that a current is passed between the two electrode to pump mobile ions from the one electrode to the other.

Claims

CLAIMS ;

1. A sensor for measuring a concentration of a gas in a gas mixture comprising: a control chamber enclosing a first volume of gas; a first electrode exposable to the gas in the control chamber; a second electrode exposable to a second volume of gas outside of the control chamber; and a diffusion barrier for restricting a flow of gas into and/or out of the control chamber.

2. A sensor according to Claim 1, wherein the diffusion barrier comprises a single diffusion pore.

3. A sensor according to Claim 1 or 2, wherein the diffusion barrier is spaced from the first electrode.

4. A sensor according to Claim 3, wherein the diffusion barrier is spaced from the first electrode by an annular spacer.

5. A sensor according to any preceding claim, wherein the potential of the first electrode with respect to the second electrode provides an indication of the concentration of a particular gas in the second volume of gas.

6. A sensor according to Claim 5, wherein the potential difference between the first and second electrodes provides an indication of the concentration of a particular gas in the second volume of gas.

7. A sensor according to any preceding claim, wherein the concentration of a particular gas in the control chamber is substantially in equilibrium, the concentration of that gas being measured by the sensor.

8. A sensor according to any preceding claim, wherein one or both of the electrodes are manufactured from a material which catalyses a reaction between a particular gas, the concentration of which is to be measured by the sensor, and another gas.

9. A sensor according to Claim 8, wherein one or both electrodes are manufactured from platinum and catalyse the reaction:

2C0+0₂=2C0₂

10. A sensor according to any preceding claim, wherein the first electrode is mounted on a solid electrolyte.

11. A sensor according to Claim 10, wherein the second electrode is mounted on the solid electrolyte.

12. A sensor according to Claim 10 or 11, wherein the electrodes are mounted on a Zirconia electrolyte.

13. A sensor according to any preceding claim, wherein the sensor is for measuring reducing gas concentrations in a gas mixture containing free oxidising gas, the diffusion barrier for reducing the difference between an equilibrium and a mixed potential on the first electrode, so that the difference in potential between the first electrode and the second electrode can be used to measure the concentration of any gas capable of creating a mixed potential.

14. A sensor according to Claim 13, wherein two or more diffusion barriers with different diffusion rates are provided to create two or more different mixed potentials.

15. A sensor according to Claim 13 or 14, wherein the flow of free oxidising gas into and/or out of the control chamber is controlled by electrochemical pumping, using AC or DC on a pump cell, to enhance the effect of the diffusion barrier and increase the sensitivity or accuracy of gauge cell voltage as a measure of the concentration of the reducing gas being measured.

16. A sensor according to Claim 15, wherein a reference electrode is added to the pump cell in order to determine the overvoltage on either or both of the electrodes and thus enhance the gas sensing ability of the sensor.

17. A sensor according to any one of Claims 13 to 16, wherein different materials are used for constructing each of the electrodes in order to enhance the effect of the diffusion barrier, modify the overvoltages on the pump electrodes or to enable several different gases to be detected by a single multi-electrode sensor.

18. A sensor according to any preceding claim, wherein a catalytic layer is added to any part of the control chamber or coated over any external electrode in order to increase the selectivity or sensitivity of the sensor, or to enable more than one different gas to be detected by a single sensor.

19. A sensor according to any preceding claim, wherein a heating device is incorporated in the sensor to raise the sensor to a suitable operating temperature.

20. A sensor substantially as hereinbefore described, with reference to and as shown in Figures 2 and 3.

21. Any novel feature or combination of features disclosed herein.