Heat of adsorption/desorption

The definition of the various heats of adsorption/desorption and their relationship with thermodynamic quantities directly obtainable by calorimetry are given in Table 9.2 [2]. 

Table 9.2 Definition of the various heats of adsorption/desorption.

Besides steady state measurements, there is probably good reason to use flow micro calorimetry for the study of non-steady-state behavior in systems with immobilized bio catalysts. Here, the mathematical description is more complicated, requiring the solution of partial differential equations. Moreover, the heat response can evolve non-specific heats, like heat of adsorption/desorption or mixing phenomena. In spite of these complications, the possibility of the on-line monitoring of the enzyme reaction rate can provide a powerful tool for studying the dynamics of immobilized biocatalyst systems. 

Measured heats of adsorption/desorption for alkali metals on metal snrfaces. The values given are the zero-coverage limit imless stated otherwise. (FEM) indicates that the measurement was done on an... 

Figure 12. Cumulative heats of adsorption/desorption of oxygen and nitrogen at 25°C on a carbon molecular sieve caused by pressure changes between 0 and 20 bar g.

There are numerous references in the literature to irreversible adsorption from solution. Irreversible adsorption is defined as the lack of desotption from an adsoibed layer equilibrated with pure solvent. Often there is no evidence of strong surface-adsorbate bond formation, either in terms of the chemistry of the system or from direct calorimetric measurements of the heat of adsorption. It is also typical that if a better solvent is used, or a strongly competitive adsorbate, then desorption is rapid and complete. Adsorption irreversibility occurs quite frequently in polymers [4] and proteins [121-123] but has also been observed in small molecules and surfactants [124-128]. Each of these cases has a different explanation and discussion. 

Fig. XVIII-11. Calorimetric differential heat of adsorption of H2 on ZnO. Dashed line differential heat of desorption. (From Ref. 104.)...

Process 2, the adsorption of the reactant(s), is often quite rapid for nonporous adsorbents, but not necessarily so it appears to be the rate-limiting step for the water-gas reaction, CO + HjO = CO2 + H2, on Cu(lll) [200]. On the other hand, process 4, the desorption of products, must always be activated at least by Q, the heat of adsorption, and is much more apt to be slow. In fact, because of this expectation, certain seemingly paradoxical situations have arisen. For example, the catalyzed exchange between hydrogen and deuterium on metal surfaces may be quite rapid at temperatures well below room temperature and under circumstances such that the rate of desorption of the product HD appeared to be so slow that the observed reaction should not have been able to occur To be more specific, the originally proposed mechanism, due to Bonhoeffer and Farkas [201], was that of Eq. XVIII-32. That is. 

In this maimer, it can also be seen that molecules will desorb as the surface temperature is raised. This is the phenomenon employed for TPD spectroscopy (see section Al.7.5.4 and section BT25). Note tliat some adsorbates may adsorb and desorb reversibly, i.e. the heats of adsorption and desorption are equal. Other adsorbates, however, will adsorb and desorb via different pathways. 

This, as is shown by the theory, is due to the evolution of the heat of absorption, during solute adsorption at the front part of the peak. Conversely, the back of the peak is eluted at a lower temperature than the surroundings throughout the length of the column due to the absorption of the heat of solute desorption. As a result, the distribution coefficient of the solute at the front of the peak, and at a higher temperature, will be less than the distribution coefficient at the back of the peak, at the... 

In a recent paper [11] this approach has been generalized to deal with reactions at surfaces, notably dissociation of molecules. A lattice gas model is employed for homonuclear molecules with both atoms and molecules present on the surface, also accounting for lateral interactions between all species. In a series of model calculations equilibrium properties, such as heats of adsorption, are discussed, and the role of dissociation disequilibrium on the time evolution of an adsorbate during temperature-programmed desorption is examined. This approach is adaptable to more complicated systems, provided the individual species remain in local equilibrium, allowing of course for dissociation and reaction disequilibria. 

With the availabihty of computers, the transfer matrix method [14] emerged as an alternative and powerful technique for the study of cooperative phenomena of adsorbates resulting from interactions [15-17]. Quantities are calculated exactly on a semi-infinite lattice. Coupled with finite-size scaling towards the infinite lattice, the technique has proved popular for the determination of phase diagrams and critical-point properties of adsorbates [18-23] and magnetic spin systems [24—26], and further references therein. Application to other aspects of adsorbates, e.g., the calculation of desorption rates and heats of adsorption, has been more recent [27-30]. Sufficient accuracy can usually be obtained for the latter without scaling and essentially exact results are possible. In the following, we summarize the elementary but important aspects of the method to emphasize the ease of application. Further details can be found in the above references. 

Alternatively, peak asymmetry could arise from thermal effects. During the passage of a solute along the column the heats of adsorption and desorption that are evolved and adsorbed as the solute distributes itself between the phases. At the front of the peak, where the solute is being continually adsorbed, the heat of adsorption will be evolved and thus the front of the peak will be at a temperature above its surroundings. Conversely, at the rear of the peak, where there will be a net desorption of solute, heat will be adsorbed and the temperature or the rear of the peak will fall below its surroundings. 

The opposite of adsorption, desorption, represents the end of the catalytic cycle. It is also the basis of temperature-programmed desorption (TPD), an important method of studying the heats of adsorption and reactions on a surface, so that the activation... 

CO oxidation is often quoted as a structure-insensitive reaction, implying that the turnover frequency on a certain metal is the same for every type of site, or for every crystallographic surface plane. Figure 10.7 shows that the rates on Rh(lll) and Rh(llO) are indeed similar on the low-temperature side of the maximum, but that they differ at higher temperatures. This is because on the low-temperature side the surface is mainly covered by CO. Hence the rate at which the reaction produces CO2 becomes determined by the probability that CO desorbs to release sites for the oxygen. As the heats of adsorption of CO on the two surfaces are very similar, the resulting rates for CO oxidation are very similar for the two surfaces. However, at temperatures where the CO adsorption-desorption equilibrium lies more towards the gas phase, the surface reaction between O and CO determines the rate, and here the two rhodium surfaces show a difference (Fig. 10.7). The apparent structure insensitivity of the CO oxidation appears to be a coincidence that is not necessarily caused by equality of sites or ensembles thereof on the different surfaces. 

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