Product selectivity, constant potential conditions

Figure 8.47 shows the effect of the dimensionless potential THFUwr/RT, on product selectivity, S, under constant feed conditions. The selectivity to h2co can be varied deliberately between 0.35 and 0.60 by varying the catalyst potential. 

If the current is held constant at I0reducible compound is kept at such a value that the limiting current is higher than the applied current, the reaction will remain selective. Such a situation requires a continuous addition of substrate and removal of products, and this approach is the most practical one for an industrial application of controlled potential reactions. 

Steady state measurements of NO decomposition in the absence of CO under potentiostatic conditions gave the expected result, namely rapid self-poisoning of the system by chemisorbed oxygen addition of CO resulted immediately in a finite reaction rate which varied reversibly and reproducibly with changes in catalyst potential (Vwr) and reactant partial pressures. Figure 1 shows steady state (potentiostatic) rate data for CO2, N2 and N2O production as a function of Vwr at 621 K for a constant inlet pressures (P no, P co) of NO and CO of 0.75 k Pa. Also shown is the Vwr dependence of N2 selectivity where the latter quantity is defined as... 

But if reactants or products of the desired reaction are lost and/or if undesrred compounds are formed by side reactions (i.e. yield and selectivity will be reduced), it is necessary to avoid any overstepping of the limiting current density. Especially in case of changing conditions (batch operation) with a rising degree of conversion - here, the most significant parameter will be the decreasing reactant concentration - a continuous adjustment of the current density is indispensable. Then it will be better to work at a constant electrode potential than at constant cell current (see Sect. 23.23). 

By conditioning of the poIyHEMA layer in a buffer of fixed pH and primary ion concentration prior to use, the activity of the ions (u,) in the polyHEMA layer is constant. This fixes the inner boimdary potential of the ion sensitive membrane Fot an accurate measurement of an unknown activity of the primary ion in the sample solution based on Nemst law, it is essential that the primary ion activity in the membrane phase (a,) remains constant. This can be achieved by the introduction of proper amounts of primary ion-selective receptor molecules (ligands, L) in the membrane. The extraction of a particular ion in the membrane phase is then determined by the product of the partition of the ion over membrane and aqueous phase (/fp) and the association constant of the ion ionophore complex in the membrane (, ). The association constant /I, determines the free cation activity in the membrane ... 

Note that the calculated value of n might be a fraction if the current efficiency is less than 100 % or the mechanism followed is complex. Preparative electrolysis can be performed under potentiostatic or galvanostatic conditions. In the former case, the use of a potentiostat ensues that the potential of the working electrode relative to a reference electrode is held at a constant value during the whole electrolysis process independently of the current supplied. Preferentially, the potential should be selected so that the substrate can be electrolyzed without the products or the medium being reduced or oxidized at the same time. It is possible by proper potential adjustment to control the fraction of electrolysis X which is defined according to Eq. 98 as the fraction of the substrate transformed during electrolysis. 

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