Desorption/adsorption processes principles

The last years have witnessed tremendous progress in the theoretical description of surfaces and processes on surfaces. A variety of surface properties can now be described from first principles, i.e. without invoking any empirical parameters [1], In particular, whole potential energy surfaces (PES) can nowadays be mapped out by total energy calculations based on ab initio electronic structure theory. This development has also motivated new efforts in the dynamical treatment of adsorption/desorption processes in the last decade such as the development of efficient schemes for high-dimensional quantum dynamical simulations [2, 3]. 

Many authors have proposed reactors with similar basic principles. The best known are those of Garanin et al. [44], Livbjerg and Villadsen [45] and new versions of Berty reactor [34]. Variants of internal recycling reactors have also been proposed by Bennett et al. [43] who tried to decrease the ratio of reactor volume to catalyst volume. In this arrangement the amount of reactant adsorbed increases compared to that in the gas space as a result the dynamics of the adsorption - desorption processes can be detected through the gas phase measurements. 

FIG. 2 Principles of SECMID using H+ as a model adsorbate. Schematic of the transport processes in the tip/substrate domain for a reversible adsorption/desorption process at the substrate following the application of a potential step to the tip UME where the reduction of H+ is diffusion-controlled. The coordinate system and notation for the axisymmetric cylindrical geometry is also shown. Note that the diagram is not to scale as the tip/substrate separation is typically <0.01 rs. 

For the investigation of adsorption/desorption kinetics, SECM is employed to locally perturb adsorption/desorption equilibria and measure the resulting flux of adsorbate from a surface. In this application, the technique is termed SECM-induced desorption (SECMID) [5], but this represents the first use of SECM in an equilibrium perturbation mode of operation. The principles of SECMID are illustrated schematically in Figure 13.1, with specific reference to proton adsorption/desorption at a metal oxide/aqueous interface. For this type of investigation, the tip UME is placed close to the surface of the substrate, such that the tip/substrate separation, d, is of the order of, or less than, the electrode radius, a. The substrate is immersed in a solution of the adsorbate of interest and the adsorption/desorption process is initially at equilibrium. 

The principle we have applied here is called microscopic reversibility or principle of detailed balancing. It shows that there is a link between kinetic rate constants and thermodynamic equilibrium constants. Obviously, equilibrium is not characterized by the cessation of processes at equilibrium the rates of forward and reverse microscopic processes are equal for every elementary reaction step. The microscopic reversibility (which is routinely used in homogeneous solution kinetics) applies also to heterogeneous reactions (adsorption, desorption dissolution, precipitation). 

This chapter describes basic physico-chemical relations between the gas phase transport of atoms and molecules and their thermochemical properties, which are related to the adsorption-desorption equilibrium. These methods can either be used to predict the behavior of the adsorbates in the chromatographic processes, in order to design experiments, or to characterize the absorbate from its experimentally observed behavior in a process. While Part I of this chapter is devoted to basic principles of the process, the derivation of thermochemical data is discussed in Part n. Symbols used in the following sections of Part I are described in Section 5. For results, which were obtained applying the described evaluation methods in gas-adsorption chromatography, see Chapters 4 and 7 of this book. 

Adsorption (desorption) energies or enthalpies of molecules and atoms on various surfaces are of primary and major interest in the experimental gas-phase radiochemical studies of the heaviest elements. In practice, pertinent data can be obtained almost exclusively in the experiments based on chromatographic principles. In the pioneering works [1-3] the required values were derived using the simplest description of the processes in columns in terms of molecular kinetics (see Sect. 4.2). Later [4] the task of finding the adsorption enthalpies was examined using a thermodynamic approach. It revealed that the molecular-kinetic treatment... 

Cerofolini and Rudzihski [43] have reviewed the theoretical principles of single gas and mixture adsorption on heterogeneous surfaces. Their review is chronologically arranged from the earliest to the latest approaches. In the same book, Tovbin [44] reported the application of lattice-gas models to explain mixed-gas adsorption equilibria on heterogeneous surfaces he also discussed [45] the kinetic aspects of adsorption-desorption on flat heterogeneous surfaces. The book [46] also contains other papers on different aspects of adsorption for the reader interested in surface diffusion processes. 

Although this chapter has focused on phase transfer reactions at solid/ liquid interfaces, many of the techniques and principles are generally applicable to such processes at liquid/liquid and air/liquid interfaces. Studies of adsorption/desorption, absorption, dissolution, and lateral interfacial diffusion at these types of interface are of considerable fundamental and practical importance, and SECM studies in these areas are already appearing. 

Since this book is dedicated to the dynamic properties of surfactant adsorption layers it would be useful to give a overview of their typical properties. Subsequent chapters will give a more detailed description of the structure of a surfactant adsorption layer and its formation, models and experiments of adsorption kinetics, the composition of the electrical double layer, and the effect of dynamic adsorption layers on different flow processes. We will show that the kinetics of adsorption/desorption is not only determined by the diffusion law, but in selected cases also by other mechanisms, electrostatic repulsion for example. This mechanism has been studied intensively by Dukhin (1980). Moreover, electrostatic retardation can effect hydrodynamic retardation of systems with moving bubbles and droplets carrying adsorption layers (Dukhin 1993). Before starting with the theoretical foundation of the complicated relationships of nonequilibrium adsorption layers, this introduction presents only the basic principles of the chemistry of surfactants and their actions on the properties of adsorption layers. 

In earlier sections we have discussed the speciation, adsorption, and desorption processes of dissolved Mo in soil solutions. In this section we review the principles of precipitation and dissolution processes and discuss the potential Mo solid phases that may control dissolved Mo in alkaline soil solutions. 

By applying an appropriate perturbation to a relevant parameter of a system under equilibrium, various frequency modulation methods have been used to obtain kinetic parameters of chemical reactions, adsorption-desorption constants on surfaces, effective diffusivities and heat transfer within porous solid materials, etc., in continuous flow or batch systems [1-24]. In principle, it is possible to use the FR technique to discriminate between all of the kinetic mechanisms and to estimate the kinetic parameters of the dynamic processes occurring concurrently in heterogeneous catalytic systems as long as a wide enough frequency range of the perturbation can be accessed experimentally and the theoretical descriptions which properly account for the coupling of all of the dynamic processes can be derived. 

Two principle methods of surface modification are well known they can be summarized as the physical adsorption method and the chemical immobilization technique of the organic modifier.Each method is experienced with certain advantages and disadvantages over the other. For example, the physical adsorption approach is commonly accomplished in a single-step reaction this requires less time for obtaining the final modified solid extractor, but the active donor centers or atoms in the physically modified phases may be consumed in the adsorption process. In addition, these modified phases were found to suffer from leaching or desorption processes under the influence of... 

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