Electrochemical sensor for determining the oxygen concentration

Details Electrochemical sensor for determining the oxygen concentration Electrochemical sensor for determining the oxygen concentration

1996-01-19

1997-05-20

Bell, Bruce F.

Chemistry: electrical and wave energy

Apparatus

Electrolytic

204425, 204426, 204427, 204428, 204429, 204421, G01N 2726

Patent

active

056309201

DESCRIPTION:

BRIEF SUMMARY
PRIOR ART

The invention is based on an electrochemical sensor for determining the oxygen concentration in gas mixtures.
In the case of known electrochemical sensors for determining oxygen concentration, the emf at the electrodes is measured, a large voltage being produced if the lambda value of the gas mixture is smaller than the stoichiometric ratio. In this case, a material, for example platinum, that catalyzes establishment of equilibrium of the gas mixture is used for the electrode exposed to the gas to be measured. The effect of the catalyzing property of platinum is that the free oxygen occurring in the gas mixture binds to the oxidizable components in the gas mixture, as a result of which the sensor is also sensitive to oxidizable gas components. However, the cross-sensitivity of electrochemical sensors has a disruptive effect in particular application cases.
It is, for example, possible to monitor the operational integrity of catalytic convertors in exhaust emission control systems of internal-combustion engines by measuring the oxygen contained in the exhaust gas downstream of the catalyst in the exhaust gas flow direction. If the catalytic convertor is not operational, then the oxygen in the exhaust gas downstream of the catalyst remains incompletely converted, and oxidizable and reducible fuel components which are still unconverted are found in the exhaust gas. Incomplete conversion means that the residual oxygen content in the exhaust gas is substantially higher than that which corresponds to thermodynamic equilibrium. The sensor according to the invention is, for example, intended to respond to such an increased residual oxygen content. Care must therefore be taken for it to be possible to measure the changes in the oxygen partial pressure at the three-phase boundary of the measuring electrode. To this end, it is known from DE-A 23 04 464 to produce an electrode that does not catalyze establishment of equilibrium of the gas mixture from gold or silver. The effect of these electrode materials is that a competitive reaction between the oxygen and the oxidizable or reducible gas components takes place at the electrode. As a result, even when high lambda values have been tuned, the free oxygen entrained in the exhaust gas is extensively converted by the competitive reaction with, for example, CO, so that only little oxygen reaches the three-phase boundary of the measuring electrode, the result of which is that an approximately equally high positive potential remains obtained from low to high lambda values. In the case of a correctly operating catalytic convertor, the oxygen is converted by the catalyst, so that at least approximately the thermodynamic equilibrium pressure exists downstream of the catalytic converter, the result of which is that the potential difference at the measuring electrode is reduced and a lambda jump occurs like in the case of a conventional lambda probe. With decreasing degree of conversion by the catalytic converter, the lambda jump is shifted to higher lambda values. A disadvantage with this sensor is that no potential difference occurs with lambda values tuned to <1 (rich exhaust gas). The sensor is therefore unsuitable for monitoring a catalytic convertor in this lambda range.


ADVANTAGES OF THE INVENTION

The sensor according to the invention has the advantage that the oxygen in non-equilibrium gas mixtures can be measured, with the greatest possible exclusion of cross-sensitivity to oxidizable gas components. The sensor is particularly suitable for monitoring the degree of conversion by catalytic convertors in exhaust emission control systems of internal-combustion engines with exhaust gases tuned to lambda values of <1. Conclusions regarding the ageing of the catalyst can be drawn from the degree of convergence by the catalytic convertor.
According to other features of the present invention, advantageous further developments and improvements of the sensor according to the invention are possible. The platinum-bismuth measuring electrode uses the properties of

REFERENCES:
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patent: 4547281 (1985-10-01), Wang et al.
patent: 4863583 (1989-09-01), Kurachi et al.
patent: 5314604 (1994-05-01), Friese et al.
patent: 5411644 (1995-05-01), Neukermans
patent: 5427672 (1995-06-01), B ocker et al.
patent: 5486279 (1996-01-01), Friese et al.

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