Temkin adsorption

Diagnostic criteria (3r /01og/ and dr)/d ogJ slopes, and J against/(/) relationships) for the various mechanisms of the h.e.r. for Langmuir and Temkin adsorption have been derived and tabulated by McBreen and Genshaw . 

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. 

M. Temkin, Adsorption Equilibrium and the Kinetics of Processes on Non-Homogeneous Surfaces and in the Interaction Between Adsorbed Molecules, Zkumal Fizichesko i Khimii 15 296 (1941). 

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

The mechanism of hydrogen evolution has been investigated by impedance measurements [371] and hydrogen-tritium kinetic isotope effects [375]. The effect of halides dissolved in solution has been studied [372, 376] these ions increase the overpotential in the sequence Cl- Adsorption isotherms for halides have been derived. They conform to the Temkin adsorption model with partial charge transfer. The lateral interaction between adsorbed particles has been calculated. It is higher for Br- than for I- and increases with overpotential on account of the weakening in the metal-halide bond. Thus, halides are substantial poisons for hydrogen evolution on iron. Poisons also include metal ions such as Cd2+, Zn2+, and Mn2+ [26]. 

This is the Frumkin-Temkin adsorption equation considering adsorption heat,7). Substituting Eq.(l I) and Eq,(l6) into Eq.(19), we get a normalized reaction rate at a poisoned steady state as follows. 

Several other types of adsorption isotherms have been described one applicable to fractional monolayers in which there is linear variation between the amount adsorbed and log C is called Temkin adsorption. 

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 ... 

Observed E (kcal/mol) Temkin adsorption Langmuir adsorption... 

Since Steps (32)—(34) are in quasi-equilibrium and follow the rds. they are not proven mechanistic paths. However, evidence of H2O2 in solution (164-166) and the presence of surface [OH] discussed earlier support this mechanism. Kinetic evidence is consistent with the above mechanism, assuming Temkin adsorption on oxygen-free platinum surfaces at low potentials, < 1 V (77). Similar steps can be written for alkaline electrolytes, with reactions (29), (31), (32), or (34) involving HjO instead of H. ... 

Some agreement exists on the effects of Sn, although the results are not reproducible. The UPD of tin differs from the majority of the UPD systems [106]. Its oxidation state is extremely potential-dependent. The effects of Sn on the oxidation of CH3OH on Pt are attractive because they occur at very low potentials. The system Pt-Sn has been investigated in three forms Sn irreversibly adsorbed, Sn adsorbed by UPD, and ordered singlecrystal PtsSn alloy [107]. Vassiliev and coworkers [100] found a Temkin adsorption behavior of neutral Sn at Pt in the potential region from 0 V to 0.2. Above 0.2 V, tin undergoes a partial oxidation to Sn" ", or SnOH at 0.5 V. A tenfold increase of the rate in the presence of Sn has been found and has been ascribed to the bifunctional mechanism [97]. Frehnk and coworkers [108,109] demonstrated by ellipsometry that a submonolayer of Sn oxide disappears from Pt surfaces in the presence of methanol. The reports of considerable catalytic effects of Sn [53, 101, 102] have not been reproduced in recent studies [24, 105, 110]. Inhibition was observed for the methanol oxidation on... 

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... 

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. 

Mechanism Rate deteimining step Langmuir adsorption condition 2h 2h 0 1 Temkin adsorption conditions... 

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 ... 

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