Adsorption Temkin isotherms

Of the two isotherms considered so far (Langmuir and Freundlich), the Freundlich isotherm is likely to be more widely obeyed than the Langmuir isotherm because the heat of adsorption q) normally falls with increasing surface converage. This effect is only allowed in the Freundlich isotherm, because it involves a term n that allows for heterogeneity of the solid surface. However, few q-6 plots are logarithmic in form and, therefore, the Freundlich isotherm shall, with many adsorptions be an approximation to the truth. 

Very often the fall in heat of adsorption is more nearly linear than logarithmic, and it is this type of behavior that led to the derivation of the Temkin adsorption isotherm. The isotherm is, in fact, derived from the Langmuir adsorption isotherm by inserting the condition that the heat of adsorption decreases linearly with surface coverage. Such an effect can arise from repulsive forces on a uniform surface or from surface heterogeneity of the surface. 

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A typical adsorption process in electrocatalysis is chemisorption, characteristic primarily for solid metal electrodes. The chemisorbed substance is often chemically modified during the adsorption process. Then either the substance itself or some fragment of it is bonded chemically to the electrode. As electrodes mostly have physically heterogeneous surfaces (see Sections 4.3.3 and 5.5.5), the Temkin adsorption isotherm (Eq. 4.3.46) is suitable for characterizing the adsorption. 

In the Temkin adsorption isotherm, the amount adsorbed is related to the logarithm of the pressure of the adsorptive... 

This analytical solution revealed well the qualitative critical current behavior seen experimentally (Fig. 32), but did not fully predict the quantitative dependence of the critical current at a Pt anode catalyst on Pco in the anode feed [66]. The latter required assuming a Temkin adsorption isotherm for CO at the anode catalyst, as originally suggested in Ref. 67. By using a Temkin isotherm, Eq. (36) allows the free energy of CO adsorption to decrease linearly with co This assumption is in agreement with literature data for CO adsorption on Pt group metals ... 

This uncertainty makes the kinetic analysis in terms of Frumkin and Temkin adsorption isotherms also uncertain. As long as there is no clear evidence to the contrary, it may be safer to rely on a simple form of a kinetic equation consistent with the Langmuir adsorption isotherm. Therefore, it is usually accepted that in not too concentrated acids the OER occurs on Pt according to... 

The most common way to describe ammonia adsorption and desorption in SCR catalysts, in general, is to use the Temkin type isotherm [10, 13, 24, 27, 29]. The Temkin adsorption isotherm considers the adsorbate-adsorbate interactions, and in the models therefore coverage dependent heat of adsorption is used. This also results in that the activation energy for desorption of NH3 is coverage dependent ... 

This gives rise to the Temkin adsorption isotherm ... 

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