Sensing electrode desorption

A typical electrochemical NOx sensor design involves the use of two electrodes on an oxygen-ion conducting ceramic, such as yttria-stabilized zirconia (YSZ), as shown in Fig. la. Both chemical and electrochemical reactivity at each electrode is critical to sensor performance [8, 18-20]. We have obtained optimal results with a Pt electrode covered with Pt-containing zeolite Y (PtY) as the reference electrode and WO3 as the sensing electrode [17, 21, 22]. These electrodes were identified by temperature programmed desorption of NO from NOx/02-exposed PtY and WO3, and the ability of PtY and WO3 to equilibrate a mixture of NO and O2. Significant reactivity differences were found between the PtY and WO3, with the latter... 

Figure 3 illustrates the effect of the adsorption/desorption kinetics on the transient profile, in the absence of surface diffusion, where KJKd = 1, i.e., half the surface sites are occupied at equilibrium. The results are presented as i/i() versus r 1/2 in order to emphasize the short-time behavior. At very short times, i.e., the largest r 1/2, the UME response is identical for all values of Ka, since under these conditions, the diffusion field adjacent to the electrode is much smaller than the tip/sample separation and so does not sense the presence of the substrate (30,33). At times sufficient for the dif-... 

Since the electrocatalytic reaction implies the existence of an adsorbed species as an intermediate, reactant, or product, the direct interaction with the electrode surface has to be considered first. In this sense, the kinetics of the formation and the stability of the adsorbate are of great importance and may be the determining step for the final value of The slow adsorption kinetics in the case of a reactive adsorbate will make the reaction at the electrocatalyst not fast enough to become operative. However, the same situation can occur in the case of an adsorbed product with a slow desorption kinetics. The most problematic situation can arise due to the stability of an adsorbed intermediate on the surface, which is the rate-determining step of the whole process. In the case of an anodic process, the species desorption can be aided by the presence of a metal oxide on the surface. An interesting example of stable and efficient anodes is the dimensionally stable electrodes (DSE) used in brine... 

The membrane mimetic fructose dehydrogenase system for fructose sensing described in this paper is stable for at least several days. Activity loss appears to occur primarily due to desorption of Qa from the electrode surface since much electrode activity could be restored by exposure of the electrode to a solution containing an alternate lipophilic mediator, decylubiquinone. Efforts are underway to improve mediator retention with the hope that electrode stability can be increased further to a practical level. 

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