Oxygen sensors with catalytic electrode

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... 

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. 

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. 

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. 

In another approach, a DO sensor was proposed by modifying a glassy carbon (GC) electrode with cobalt tetrasulfonate phthalocyanine (CoTSPc). The direct reduction of oxygen at a solid electrode is a slow process and also requires a high negative potential which can be lowered by electron transfer mediators that can shuttle the electrons between oxygen and electrode. Among these mediators, phthalocyanines acquired a lot of attention because of their catalytic ability and... 

The adsorptive and voltammetric characteristics of Cu(II) complexes with guanine, guanosine and adenosine were exploited [120] in order to detect these bases after separation by capillary zone electrophoresis, and the enzyme-mimic catalytic activity of a DNA-Cu2+ complex [121] was used to develop an amperometric quinacrine sensor using an oxygen electrode covered by the complex entrapped in polyacrylamide gel. 

Porphyrins are often employed in sensors on account of their ability to act as cation hosts and, with a suitable metal ion coordinated, as redox catalysts. Electropolymerised poly(metalloporphyrin)s have been used as potentiometric anion-selective electrodes [131] and as amperometric electrocatalytic sensors for many species including phenols [132], nitrous oxide [133] and oxygen [134]. Panasyuk et al. [135] have electropolymerised [nickel-(protoporphyrin IX)dimethylester] (Fig. 18.10) on glassy carbon in the presence of nitrobenzene in an attempt to prepare a nitrobenzene-selective amperometric sensor. Following extraction of the nitrobenzene the electrode was exposed to different species and cyclic voltammetric measurements made. A response was observed at the reduction potential of nitrobenzene (the polyporphyrin film acts only to accumulate the analyte and not in a catalytic fashion). Selectivity for nitrobenzene compared with w-nitroaniline and o-nitroto-luene was enhanced compared with an untreated electrode, while a glassy carbon-... 

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