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Physical Adsorption or Physisorption

Physisoiption is a process having low degree of interaction between the adsorbed molecules and the soUd surface. The forces involved are of the same order of magnitude as the van der Waals forces and the enthalpy of adsorption is in the range of the condensatimi or evaporation enthalpy of the gases (from 0.5 to 5 kcal/mol). In this type, the molecular adsorption can form overlapping layers, and the adsorption force decreases with increasing number of layers. [Pg.99]

According to London-van der Waals, the strengths between molecules and atoms can be represented in function ( ),as equation [1] [Pg.100]

The total energy between the molecules is the contribution of dipole-dipole interaction and dipole-induced interaction, which is a function of radius, that is, E = —C X r, where E is the energy of interaction and r the radius of the molecule. Care should be taken when the interactiOTi is of atoms or ions with the surface. [Pg.100]

In conclusion, the majority of molecules interact with the surface atoms and between the molecules. [Pg.100]

The physisorption takes place at low temperatures and is high when the temperature is close to the condensation temperature of the gas. Due to the low energy of interaction with the surface and the absence of activation energy, the physisorption quickly reaches equilibrium and is a reversible process. However, in the materials with very small pores (zeolites, carbons), the physisorption is slow and thus, the process is limited by the rate of gas diffusion into pores. [Pg.100]


The presence of unbalanced attractions at the surface of a solid—say, a metal such as nickel—means that small molecules will tend to become rather loosely attached to the surface in one or (more likely) several molecular layers with an exothermic adsorption energy ranging to about —20 kJ mol-1 for nonpolar molecules. (The term adsorption is used to denote surface sorption without penetration of the bulk solid, which would be called absorption.) No chemical bonds are formed or broken. This state is usually called physical adsorption or physisorption. If, however, the adsorbate forms chemical bonds with the surface atoms, the adsorption process is called chemisorption. Chemisorption can be quite strongly exothermic (—40 to —800 kJ mol-1) but involves only the first monomolecular layer of adsorbate. [Pg.116]

For practical purposes all adsorptions can be classified as one of two types. It can involve merely the van der Waals interaction between the substrate and the catalyst, a process that is termed physical adsorption or physisorption. Alternately, it can involve the formation of catalyst-substrate bonds as discussed above. This is termed chemical adsorption or chemisorption. While the latter is the basis for the chemistry of catalysts, physisorption is the basis for the BET procedure which is commonly used to measure the surface area of solids. ... [Pg.13]

Chemisorption (Chemical adsorption) The adsorption forces are of the same kind as those involved in the formation of chemical bonds. The term chemisorption is used to distinguish chemical adsorption from physical adsorption, or physisorption, in which the forces involved are of the London-van der Waals type. Some guidelines for distinguishing between chemisorption and physisorption are given by IUPAC in reference 1. [Pg.724]

Some adsorbates do not form strong chemical bonds with substrate atoms. This situation is called physical adsorption or physisorption. For these adsorbates the adsorbate-adsorbate interactions can dominate the adsorbate-substrate interactions, and the optimum adsorbate-substrate bonding geometry can be overridden by the lateral adsorbate-adsorbate interactions, yielding, for example, incommensurate structures in which the overlayer and the substrate have independent lattices. When... [Pg.54]

In general terms, physical adsorption, or physisorption, refers to weak bonding of molecules to surfaces through the interactions of induced or permanent dipoles and/or quadrupoles, whereas chemisorption describes adsorption where transfer of chemical charge between adsorbate and surface takes place. Physisorption is characteristically observed at low temperatures, is not an activated process and is completely reversible. Chemisorption, by contrast, involves the formation of bonds, persists to elevated temperatures and can lead to chemical changes. For the adsorption of molecules on microporous solids, important physisorption interactions include the uptake of simple non-polar molecules such as dinitrogen and dioxygen on cationic forms of zeolites whereas the adsorption of molecules onto acid sites is the most important type of chemisorption, because of its importance in catalysis. [Pg.258]

Atoms and molecules frequently adsorb on surfaces, where they may decompose and/or react with other adsorbed species. Modern technology is increasingly dependent on surface chemistry which underlies many industrially important processes as well as destructive processes such as corrosion. It is useful to distinguish two types of adsorption physical adsorption, or physisorption, and chemical adsorption, or chemisorption. Physisorption is similar in nature to condensation and involves little chemical interaction with the surface, being associated with van der Waals forces. Chemisorption involves a tme chemical interaction with the surface, with the formation of a chemical bond. Thus, the enthalpy of physisorption is usually of the order of 20 kJ mol while for chemisorption values are in the region of200 kJ mol . A chemisorbed molecule may either remain intact in molecular chemisorption, or fall apart in dissociative chemisorption. In an important recent development, it is now possible to identify individual molecular bonds of adsorbed molecules using STM... [Pg.259]

There are two generally recognized modes for adsorption of the reactant(s) at the active sites. Physical adsorption or physisorption involves moderately weak interactions such as hydrogen bonding, electrostatic dipole-dipole forces and/or van der Waals forces. Physisorption causes... [Pg.400]

The term physical adsorption or physisorption refers to the phenomenon of gas molecules adhering to a surface at a pressure less than the vapor pressure. The attractions between the molecules being adsorbed and the surface are relatively weak and definitely not covalent or ionic. In Table 1 definitions used in this book and in most of the hterature on physisorption are given [1]. [Pg.1]

So far we have considered the structure and properties of solid surfaces which are clean, i.e., they do not contain any foreign atoms or molecules. Such surfaces can be prepared under UHV conditions. However, in many practical cases, surfaces contain atoms and molecules from the surrounding gas phase which form more or less strong bonds with surface atoms. This phenomenon is called adsorption. Depending on the nature of the forces between the gas atoms or molecules (adsorbate) and the surface (adsorbent) one recognizes physical adsorption (or physisorption) and chemical adsorption (or chemisorption). [Pg.27]


See other pages where Physical Adsorption or Physisorption is mentioned: [Pg.276]    [Pg.250]    [Pg.4]    [Pg.146]    [Pg.276]    [Pg.1318]    [Pg.1799]    [Pg.407]    [Pg.289]    [Pg.769]    [Pg.1791]    [Pg.1500]    [Pg.165]    [Pg.41]    [Pg.67]    [Pg.6306]    [Pg.177]    [Pg.28]    [Pg.433]    [Pg.99]    [Pg.251]   


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