Chemical adsorption or chemisorption

The chemisorption characteristic is the strong interaction between molecules and surfaces. The chemisorption enthalpy (—AHads) is of the order of 10-100 kcal/mol, thus, of the same order of the reaction involved in chemical bindings. 

Different from the physisorption, the chemisorption is irreversible and occurs at higher temperatures than the condensation temperature, and since the interaction is specific between molecules and solids, the adsorbed molecules form a monolayer. 

The chemisorption processes need longer times to attain the equilibrium condition, in particular at low temperatures. There are two types of chemisorptions  

Unlike the physisorption, the chemisorption is an irreversible process and takes place at higher temperatures, compared to physisorption temperature, and since it is a specific interaction between the gas and the solid, it forms a single layer. 

The chemisorption requires Imig periods of time to reach the equilibrium, especially at low temperatures. There are two types of chemisorption activated or not. In the activated chemisorption, the rate of adsorption varies with temperature and depends on the activation energy, expressed by Arrhenius equation. In the not activated, the adsorption occurs rapidly, since the activatimi energy is practically nil. 

The adsorption isotherm is represented by the amount of gas adsorbed as a function of the partial equilibrium pressure, for a constant temperature. 

The amount of gas adsorbed by a solid is expressed in terms of weight of sample (g) or volume of gas at STP standard conditions (pressure and temperatiu e), but 

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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. 

Adsorption is the preferential concentration of a species at the interface between two phases. Adsorption on solid surfaces is a very complex process and one that is not well understood. The surfaces of most heterogeneous catalysts are not uniform. Variations in energy, crystal structure, and chemical composition will occur as one moves about on the catalyst surface. In spite of this it is generally possible to divide all adsorption phenomena involving solid surfaces into two main classes physical adsorption and chemical adsorption (or chemisorption). Physical adsorption arises from intermolecular forces... 

The adsorption of gas can be of different types. The gas molecule may adsorb as a kind of condensation process it may under other circumstances react with the solid surface (chemical adsorption or chemisorption). In the case of chemiadsorption, a chemical bond formation can almost be expected. On carbon, while oxygen adsorbs (or chemisorbs), one can desorb CO or C02. Experimental data can provide information on the type of adsorption. On porous solid surfaces, the adsorption may give rise to capillary condensation. This indicates that porous solid surfaces will exhibit some specific properties. Catalytic reactions (e.g., formation of NH3 from N2 and Hj) give the most adsorption process in industry. 

Sorption is a surface phenomenon determined by the surface charges and those of the ions surrounding it. One or more of the following mechanisms can be involved in the removal of species by sorption (i) mechanical entrapment, (ii) absorption, (iii) physical sorption, or (iv) chemical sorption on the surface of the solid particle. Physical adsorption (which is weaker than its chemical counterpart) occurs through Van der Waals forces and it is generally reversible and instantaneous. On the other hand, chemical adsorption or chemisorption occurs through the formation of chemical bonds at specific sites. This is closely related to ion exchange processes and complexation. 

Chemical adsorption or chemisorption—A type of adsorption aided by chemical bonding on the surfaces. 

Two types of adsorption are usually postulated, the one being the physical adsorption which occurs at a lower temperature and is associated with smaller adsorption heats of the magnitude of the latent heat of evaporation of the adsorbate, and the other the chemical adsorption or chemisorption occurring usually at higher temperatures with larger heats of the magnitude of heats of chemical reactions. 

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. ... 

Cations and anions that adsorb by forming short directional bonds with the surface cannot be considered to be indifferent in character. These ions actually alter the surface charge by the very process of adsorption, and their bonding is classified as chemical adsorption (or chemisorption) in this text. Examples of chemisorption include copper and phosphate adsorption on iron oxides ... 

Adsorption is due to attraction between the molecules of the surface, called adsorbent, and those of the fluid, called adsorbate. In some cases the attraction is mild, of the same nature as that between like molecules, and is called physical adsorption. In other cases the force of attraction is more nearly akin to the forces involved in the formation of chemical bonds, so the process is called chemical adsorption or chemisorption. 

The opposite situation of slow adsorption more frequently prevails in heterogeneous catalysis, where chemically adsorbed molecules undergo reaction on the surface. The rate of the process is determined by the chemical adsorption or chemisorption, where the adsorbed molecules react chemically with the surface. Since chemical bonds are broken, an activation energy is required and the adsorption is relatively slow. Because of the activation energy requirement, chemisorption is also often called activated adsorption. For our unimolecular example the reaction rate is equal to the chemical adsorption rate or... 

The other case is that of slow adsorption of the substance, with fast chemical reaction at the surface. Such process is known as chemical adsorption or chemisorption. Here adsorbed molecules are glued to the surface by chemical forces of the same type as the forces of valence bonds. In order for these forces to reveal themselves, a molecule should come into a deformed state and overcome the activation barrier. Therefore the process of chemical adsorption requires a certain activation energy. Sometimes chemical adsorption is called activated adsorption. Chemical adsorption is closely related to the process of heterogeneous catalysis. The reaction rate of chemical adsorption is given by the expression ... 

It is also possible in adsorption phenomena to distinguish between physical and chemical adsorption. Chemical adsorption or chemisorption is characterized by a simple electron transfer between the gas in physisoibed state and the solid. This transfer results in the forming of a reversible chemical bond between the two compounds (see Figure 1.1b). Once again, the appearance of the chemisorption process is directly related to the environment s thermodynamic conditions. 

Adsorption is the process whereby a chemical species moves from one phase and is held on the surface of another where it accumulates. Chemical adsorption, or chemisorption, involves molecular interaction, while physical adsorption relies on bonding due to electrostatic forces (e.g. van der Waals forces). Adsorption may be important in the removal of ammonia from the liquid phase under anaerobic conditions and may therefore also be important in the immobilisation or remobilisation of metals. 

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... 

Chemical adsorption or chemisorption where the adsorbed gases will have developed strong chemical bonds with the surface... 

Adsorption can result either from the van der Waals universal interactions (physical adsorption, physisorption) or it can have a character of th chemical process (chemical adsorption or chemisorption). Contrary to physisorption, the chemisorption only occurs as a monolayer [14]. Physical adsorption can be compared to the condensation process of the adsorptive. As a rule, it is a reversible process occurring at a temperature lower or close to the critical temperature of an adsorbed substance. 

The forces operating between the molecules of an adsorbate and the adsorbent are mostfy short-range forces as suggested by I. Langmuir. These forces may be non-specific like the dispersion forces, the orientationforces and the inductionforces. Sometimes chemical bonds like ionic and covalent bonds which are specific in nature may also be involved in the adsorption process. In such a case, when a strong chemical bond appears between a molecule of the adsorbate and the surface of the adsorbent, the phenomenon is called chemical adsorption or chemisorption. 

Physical adsorption can occur from three different effects an orientation effect, a dispersion effect, or an induction effect. For polar molecules, attraction to each other occurs because of an orientation effect. The orientation effect describes the attraction that occurs between the dipoles of two polar molecules. The negative area of one is attracted to the positive area of the other. Chemical adsorption, or chemisorption, results from the chemical interaction between a molecule and a solid. The molecule is held to the surface of the adsorbate by the formation of a chemical bond. Adsorbents used in chemisorption can be either pure substances or chemicals deposited on an inert carrier material. ... 

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). 

In practice, there are two kinds of adsorptioa Interaction of the adsorbate with the surface may take place simply through forces of physical attractiorr, i.e. intermolecular forces, and this type of adsorption is known as physical adsorption. It occurs on siufaces where the valency reqrrirements of the atoms in the surface have already been satisfied by bonding with neighbotrring atoms. If, on the other hand, a surface is unsaturated, that is, the atorrrs in the strrface are not fully satisfied by bonding with neighbouring atoms, chemical adsorption or chemisorption occurs. Here, chemical bonds form between the adsorbate arrd the strrface. 

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