Oxygen sensors with catalytic

Crucible-type oxygen sensor with catalytic electrode. In this case, the solid electrolyte is non-porous and the sensor current 1=0. 

Thick film oxygen sensor with catalytic electrode (when 1-0). Substituting 1=0 into Equation 20 yields... 

Crucible-type oxygen sensor with non-catalytic electrode. A non-catalytic electrode (e. g. Au) is thought to delay the reaction rate in the following reaction... 

Fig. 9. Principle of single catalytic bed for simultaneous reduction and oxidation with oxygen sensor and feedback control on air-to-fuel ratio.

The lambda sensor, which is found in cars with catalytic converters, is an example of an oxygen probe based on the principle of selective electrodes. This sensor, which looks like a spark plug, has a zirconium sleeve (Zr02) that behaves as a solid electrolyte. The external wall is in contact with emitted gas while the internal wall (the reference) is in contact with air. Two electrodes measure the potential difference between the two walls, which is indicative of the difference in concentration of oxygen. 

Fig. 8.6 Calibration plot of oxygenate sensor-1 for total oxygenate compounds produced by catalytic propane oxidation the yield of each oxygenate was determined with a conventional FID gas chromatograph (reproduced by permission of Elsevier from [19]).

A common type of oxygen sensor takes the form of an yttria stabilized zirconia (YSZ, see earlier) tube electroded on the inner and outer surfaces with a porous catalytic platinum electrode. The electrode allows rapid equilibrium to be established between the ambient, the electrode and the tube. Such a system is shown schematically in Fig. 4.36. 

Finally, it should be noted that numerous perovskite-related materials with relatively low oxygen ionic conductivity at 700-1200 K have been excluded from consideration in this brief survey, but may have potential electrochemical applications in fuel cell anodes, current collectors, sensors, and catalytic reactors. Further information on these applications is available elsewhere 1-4, 11, 159, 217-219]. 

The behaviour of TWC s under continuous operation has been extensively studied. Due to the step-like response of the oxygen sensors the gas composition oscillates with a frequency of about 1 Hz around the stoichiometric set point. Therefore, most studies focus on the behaviour of catalytic converters under oscillating exhaust gas composition. In particular, the contribution of ceria to the dynamic behaviour of automotive catalysts under transient air/fuel conditions [3, 4, 5, 6, 7, 8] has been investigated. Binary gas mixtures have been applied to clarify the mechanisms of the periodic operation effects over different noble metal catalysts [9,10,11,12]. Muraki et al. used simulated exhaust gas to examine the performance of noble metals on a-Al Oj [9]. 

If two such electrodes are separated by a thin layer of only zirconia, the application of a potential will lead to the pumping of oxygen from the cathode to the anode. This device can be used as an amperometric sensor for oxygen if a diffusion barrier restricts the flux of oxygen to the cathode. Note that similar devices are also often used as potentiometric sensors according to the Nernst equation (i.e., the lambda-probe in cars with catalytic converters). In this case one side of the cell has to act as a reference, e.g., by using ambient air. 

Since TWC catalytic conversion is most efficient at A=l, it is necessary to use an oxygen sensor in the exhaust stream to maintain the air/fiiel ratio as close to the stoichiometry as feasible. A TWC combined with a closed-loop control system featuring an oxygen sensor has been successfully employed in achieving adequate reduction in all 3 pollutants with high conversion rates for CO, NOx and HCs as shown in fig. 33 (Wiedenmann et al. 1994). 

In parallel with the development of the membrane reformer system, a new concept membrane module, which has a palladium alloy membrane coated on the porous support tube with catalytic activity has been developed (Nishii, 2009). This membrane module is expected to provide a more compact reactor because the reactor does not require a separate catalyst. It is also expected that this module can be manufactured at low cost by applying the industrially-established mass production process used to make oxygen sensors for combustion control in vehicles with internal combustion engines. 

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