Physisorption phenomena

Based on the classic models for the physisorption phenomena on solid surfaces, the coverage of the physisorbed molecules is a function of the sticking probability and/or adsorption potential and depends on the molecular weight and molecular volume in the liquid phase. It is obvious that these characteristics are related to the equivalent constant in the adsorption isotherm of the contaminants. 

In this chapter we will discuss some of the basic concepts which are used to describe adsorption phenomena of pure and mixed gases on the surface of solids. We here prefer a physical point of view, restricted to physisorption phenomena where adsorb molecules (admolecules) always are preserved and are not subject to chemical reactions or catalysis. Also, we always have industrial applications of physisorption processes in mind, i. e. we prefer simple and phenomenological concepts based on macroscopic experiments often being embedded within the framework of thermodynamics. That is, we prefer to take only those aspects of the molecular situation of an adsorption system into account which have been or at least can be proved experimentally and are not subject to mere speculation. 

We here restrict in what follows to physisorption phenomena. However, some of the examples presented in the subsequent Chapters refer to physicochemical adsorption systems, cp. Fig. 6.29, and chemisorption systems, cp. Fig. 3.24. 

Physisorbates, i. e. adsorbed phases caused by physisorption phenomena can exhibit many different structures reflecting the underlying molecular mechanism. The most often types of these can be described as follows ... 

The Steele theory is, therefore, the simplest treatment possible for a soundly based statistical mechanics description of the physisorption phenomenon and may be taken as a referenee point for the perturbation development given by Eq. (21). The influenee of both the periodieity of the solid structure and the deviations with respeet to the 2D approximation have been studied theoretically [182,224], although their practieal application and comparison with... 

To determine AHads of a heavy molecule on a complex surface is still a formidable task for quantum chemical calculations. Fully relativistic methods for calculating systems of the heaviest elements interacting with a (complex) surface are not yet available. Especially difficult is the prediction of the physisorption phenomenon caused by weak interactions, where the DPT generally fails. 

When gaseous or liquid molecules adhere to thesurface of the adsorbent by means of a chemical reaction and the formation of chemical bonds, the phenomenon is called chemical adsorption or chemisorption. Heat releases of 10 to 100 kcal/g-mol are typical for chemisorption, which are much higher than the heat release for physisorption. With chemical adsorption, regeneration is often either difficult or impossible. Chemisorption usually occurs only at temperatures greater than 200 C when the activation energy is available to make or break chemical bonds. 

Hydrogen uptake two adsorption phenomenons may be due to physisorption or chemisorption. The hydrogen uptake for the uncatalysed samples (samples without Pt or Pd) was assumed to be due to physisorption because the experiment was undertaken at 77 K. The additional uptake for the samples in which the catalyst was present is therefore due to some specific interaction with the catalyst particles... 

The term adsorption is the complex phenomenon of the interaction of an organic or inorganic molecule with a surface. Physisorption and chemisorption are generally distinguished according to the nature of forces involved. 

Adsorption is defined as the concentration of gas molecules near the surface of a solid material. The adsorbed gas is called adsorbate, and the solid where adsorption takes place is known as the adsorbent. Adsorption is a physical phenomenon (usually called physisorption) that occurs at any environmental condition (pressure and temperature), but it becomes measurable only at very low temperatures. Thus physisorption experiments are performed at very low temperatures, usually at the boiling temperature of liquid nitrogen at atmospheric pressure. Adsorption takes place because of the presence of an intrinsic surface energy. When a material is exposed to a gas, an attractive force acts between the exposed surface of the solid and the gas molecules. The result of these forces is characterized as physical (or van der Waals) adsorption, in contrast to the stronger chemical attractions associated with chemisorption. The surface area of a solid includes both the external surface and the internal surface of the pores. 

Because of the weak bonds involved between the gas molecules and the surface (<15 KJ/mol), adsorption is a reversible phenomenon. Gas physisorption is considered nonselective, thus filling the surface step-by-step (or layer by layer) depending on the available solid surface and the relative pressure. Filling the first layer enables the measurement of the surface area of the material, because the amount of gas adsorbed when the monolayer is saturated is proportional to the entire surface area of the sample. The complete adsorption/ desorption analysis is called an adsorption isotherm. The six lUPAC (International Union for Physical and Applied Chemistry) standard adsorption isotherms are shown in Figure 5 they differ because the systems demonstrate different gas/solid interactions (4). ... 

The process of ion exchange will affect the available pore volume inside the zeolite. The micropore volume has been measured by Na physisorption. The Na physisorption isotherms of the CuHis loaded zeolite samples are of Langmuir type I. The evolution of the micropore volume as a function of the initial copper concentration in the ion exchange solution is presented in Figure 4. The micropore volume decreases from 0.34 ml/g for a pure Y zeolite to 0.29 ml/g for the highest copper loading. The curve displays a bend in going from an initial copper concentration of 0.25 Cu/UC to an initial copper concentration of 0.50 Cu/UC. This phenomenon may be attributed to an increase in the relative amount of the more bulky complex B in the pore system of the zeolite. 

Gas adsorption is a phenomenon in which a gas molecule is removed from the gaseous phase by a solid surface. (Fig. 3.4.3) The gas itself is called the adsorptive and the concentrated gas on the surface in high density is called the adsorbate. The substance adsorbing the gas is called the adsorbent and the phenomenon in which the gas adheres to the surface (pores) is called adsorption. The phenomena in which the adsorbents are taken into the solid are called absorption or occlusion. Occlusion is a special case where gas is reversibly adsorbed into a crystal lattice with intrinsic saturated composition. These phenomena are further divided into physisorption and chemisorption based on the strength of the interaction energy between the gas and the solid. When the heat of adsorption is greater than 20 kj mol-1 it is classified as chemical adsorption, but the classification is not strict. 

Because physisorption is a surface phenomenon, research interest has focused on high surface area materials. A means of maximising the available surface is to increase the porosity. Porous materials which have received considerable attention are high surface area carbons (Yang et al, 2007), carbon nanotubes (CNTs) (Poirier et al, 2004), zeolites (Langmi et al,... 

In contrast to current interest in the use of high surface area solids such as zeolites for the physisorption of hydrogen under cryogenic conditions, the earliest work in the storage of hydrogen in zeolites was concerned with a completely different phenomenon. At room temperature and below hydrogen molecules do not enter certain zeolite cages, but at elevated temperatures... 

In the absence of chemical reaction between adsorbed species, it is instructive to analyze adsorption/desorption equilibria via steps 3 and 6. The overall objective here is to develop expressions between the partial pressnie / a of gas A above a solid surface and the fraction of active sites a on the catalyst that are occnpied by this gas when it adsorbs. The phenomenon of chemisorption and the relation between pa and a apply to a unimolecular layer of adsorbed molecnles on the catalytic surface. This is typically referred to as a monolayer, where the intermolecular forces of attraction between adsorbed molecules and active snrface sites are characteristic of chemical bonds. When complete monolayer coverage of the surface exists, subsequent adsorption on this saturated surface corresponds to physisorption, which is analogous to condensation of a gas on a cold snbstrate. The enthalpy change for chemisorption is exothermic with valnes between 10 and 100 kcal/mol. The Langmuir adsorption isotherm, first proposed in 1918 (see Langmuir, 1918), is based on the following reversible elementary step that simulates single site adsorption on a catalytic surface when there is only one adsorbate (i.e., gas A) present ... 

It seems obvious that the phenomenon involved in the reversible adsorption of hydrogen at low temperature is physisorption. 

Physical adsorption is very effective particularly at a temperature close to the critical temperature of a given gas. Chemisorption usually occurs at temperatures much higher than the critical temperature and - by contrast to physisorption is a specific process which can occur only on some solid surfaces for a given gas. In other words, chemisorption only occurs if the fluid is capable of forming a chemical bond with the adsorbent. Under favourable conditions both processes can occur simultaneously or alternately. Adsorption of oxygen on the active carbon can serve as an example. At a temperature of liquid air it has a character of the physical process, at temperatures 100 200°C it has a mixed character and over 200 C the phenomenon of oxygen uptake by active carbon is complicated due to the dominant oxygen chemisorption [55]. 

According to the nature of the gas-solid interaction one speaks of chemisorption, for specific forces involving chemical bonds, or physisorption, for non-specific forces of electrostatic or Van der Waals nature [1]. Since these secondary forces are manifested among all atoms, physisorption is an ubiquitous phenomenon. However, since they are usually weak, physisorption is an important phenomenon only at low temperature. 

Adsorption designates the fixation of fluid molecules ( or particles) on a solid surface, without there being any penetration of these particles into the solid bulk. Desorption is the reverse phenomenon. Two types of adsorption are to be distinguished, according as the interaction between the adsorbent and the adsorbate is of physical or chemical nature. In the former case, the adsorption is called physisorption ( or physical adsorption), in the latter, it is called chemisorption ( or chemical adsorption). Table I below summarizes the main features by which these two types can be distinguished, although no sharp distinction can be made and intermediate cases exist. ... 

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