Adsorption

As adsorption (from Latin adsorptio of adsorbere "(to ) suck " ) refers to the accumulation of substances from gases or liquids on the surface of a solid, generally at the interface between two phases.

Of the absorption, in which the substances to penetrate into the interior of a solid body or liquid is different. The generic term for adsorption and absorption is sorption.

Adsorb two or more species to a surface, so this is called coadsorption. This plays an important role, in which the different types of atoms are adsorbed on the same surface, where they react primarily with catalysts.

• 4.1 physisorption
• 4.2 chemisorption

Clarification

In the general sense is a physical adsorption process, adhere to the surface of another material and accumulate on the surface thereof wherein the substance (molecules generally ). The forces cause the adhesion, no chemical bonds but physical forces. Therefore, this form of physical adsorption or physisorption adsorption exact called. The physical adsorption generally includes the reverse process, the desorption as a system always seeks an equilibrium between the adsorption and desorption of a substance. The surface on which the adsorption takes place, the surface of a solid or a liquid surface can be. Generally called the surface interfaces. The phase from which the material arrives at the interface may be a gas phase or a liquid ( solution). The particles pass through its omni-directional, thermally driven molecular movement to the interface. In the following paragraphs, however, the adsorption of gases onto solid surfaces is described mainly. The physisorption is a common process in addition to the absorption and takes place on almost any surface, the gases ( air) or liquid (water ) temperatures.

In the particular case of the chemical adsorption, also called chemisorption materials are bonded by chemical bonds to the surface of a solid. The chemisorption may have significant consequences for the adsorbed material and therefore differs from a physisorption. Breaking of chemical bonds and knotting other substances can form and substance desorbed ( desorbate ), a product of a chemical reaction be. Usually occur in the chemisorption on high heats of reaction, which can be used as a criterion to distinguish between physisorption and chemisorption. Chemisorption is often not an equilibrium reaction, that is irreversible, may take place only at high temperatures, and often does not lead to an accumulation of substances on the surface. Chemisorptions are important steps that occur in processes of heterogeneous catalysis.

Generally, the term adsorption should be used only for the routine processes the physisorption. Specific adsorption of chemical processes should be explicitly referred to as chemisorption.

Physical adsorption

The physical adsorption ( physisorption ) of a substance on a surface similar to a chemical equilibrium reaction. However, the adsorbed substance ( adsorbate ) with the surface is no chemical bond, but rather adheres by weaker forces similar to the adhesion. So only van der Waals forces occur in the rule. The adsorption is physisorption in the range of 4 to 40 kJ / mol. Persist Chemical bonds within an adsorbed particle, but are polarized. For this equilibrium reaction can set up an equation between the substance which is adsorbed ( adsorbate ), the surface of the solid ( adsorbent) and the adsorbate:

Where:

Make particles to the surface (Fig. 1), they are either reflected or absorbed. Adsorption occurs when the particles can submit their kinetic energy and the adsorption energy (Eb), which is released in form of heat ( Fig. 2). Thus, the reaction is exothermic. The adsorption is reversible, since the particles with a similar amount of energy can leave the surface again, since these reactions are inhibited usually not transition states. The activation energy (Ea ) of the reaction is, therefore, equal to the adsorption energy (Eb). Adsorbed particles have no fixed binding site on the surface, do not linger at the spot where they were adsorbed, but move freely along the surface.

The position of equilibrium depends on the characteristics and size of the surface, the properties and pressure (or concentration ) of the Adsorbtivs, as well as the temperature. At 300 ° C and normal pressures balance is quite often on the left side. The equilibrium lies completely on the right side, the particles have a monomolecular layer on the surface form of the gas phase from the ideal case.

Practically can after the formation of the monolayer more layers form ( Fig. 3). Strictly speaking, one counts only the formation of the first layer for adsorption, since only here interactions between surface and adsorbate act. In the other layers consist only interactions between the adsorbates. Strictly speaking, then is a condensation of the adsorptive. The heat of condensation ( binding energy of the adsorbates with each other ) is significantly lower in the rule than the adsorption energy of an adsorbate to the adsorbent.

In general, the surfaces of the adsorbent are not ideal planar structure, but have many defects ( lattice defects ). These defects form points at which the adsorbate is particularly tightly bound (Fig. 5). The mobility of the adsorbate allows these positions, regardless of the place where the adsorbate is adsorbed. The performance of an adsorbent, therefore, increases with the number of his defects. For this reason, the activated carbon is substantially amorphous constructed a particularly powerful adsorbent in comparison to graphite. Other, high-performance materials such as zeolites are not amorphous but have a very narrow channel system ( Fig. 6), a so-called inner surface in which only a few molecules ( adsorbates ) are next to each other. For such substances, to adsorption and condensation can hardly be distinguished. Sometimes called such processes also capillary condensation (see capillarity ).

Surfaces are always occupied under normal conditions due to substances. On the surface of solids in contact with solvents sit solvent molecules. Surfaces in contact with air are occupied eg with water molecules. Practically suitable adsorbents are characterized by selectivity (Fig. 4). From solutions of solutes displace the solvent and from the air displace certain substances from the surface of the water, as they are more strongly bound ( Adsorptionsverdrängung ). This accumulation of certain substances on the surface allows the production of gas filters (eg filter respirators ) and method of adsorption chromatography.

Adsorption isotherm

For the adsorption of monomolecular layers, a degree of coverage Θ of the surface can be defined:

The degree of occupancy Θ in a gas system - interface is dependent on the partial pressure p of the gas and the temperature T.

For characterization ( measurement) of the surfaces of materials so-called adsorption isotherms are preferably added, in which the temperature is kept constant. As adsorbate often gases such as nitrogen or noble gases can be used. The position of the equilibrium of a system is then only dependent on the pressure.

A practically important isotherm is the Langmuir isotherm, which can be represented in this way:

The size B can be regarded as the ratio of the rate constant of desorption and adsorption of the system. Figure 7 shows the course of the assignment. For small p (p << b ) the adsorption isotherm is proportional to p. For large p (p >> b ) the curve of occupancy monomolecular Nm approaches. At higher pressures there is condensation, which is not described by the Langmuir isotherm.

In addition to the langmuir adsorption isotherm between can also be described with the friendly 's equation, the chemical adsorption.

Materials with a rough or inner surface often show a different adsorption behavior, which can not be described by the Langmuir isotherm. Often in the isotherm by the Brunauer, Emmett and Teller ( BET isotherm ) use is made:

Adsorption isotherms of materials such as silica gel show a steep rise at low pressure (Fig. 8), but only have a tendency to approach Nm and exceed this value early. Here, the capillary condensation comes into play, which is energetically close to the heat of adsorption of a monomolecular occupancy. For other forms of adsorption isotherms and practical presentation of the above isotherms see here.

Chemical adsorption

The chemical adsorption ( chemisorption) is an accumulation of substances on the surface of solids, which is similar chemical bonding forces. While the binding forces in the physical adsorption of up to 40 kJ / mol, are chemical adsorption reaction with heating in the range of 40-420 kJ / mol. In the chemisorption bonds are formed to the surface and changes the electronic structure within the adsorbate. Through the direct bonds to the surface to a maximum of monomolecular layers can form. Despite a chemical bond to the surface to the adsorbate can - as in a physisorption - mostly along the surface move.

The chemisorption may have significant consequences for the adsorbed material, which depend on the properties of the adsorbate, the adsorbent and the temperature of the system. Chemisorption is a fundamental process in the catalysis with solid catalysts, heterogeneous catalysis so-called. Bonds within the adsorbed substance can be weakened. Chemical reactions with other reactants can now easily expiration; one speaks of a catalytic activation of the reaction.

Under certain circumstances, bonds can be broken within the adsorbate, which can lead to disintegration of the adsorbate in two or more particles ( dissociative chemisorption). So, for example, dissociated hydrogen (H2) to individually on metals such as iron - platinum - and palladium -bonded hydrogen atoms (Fig. 9 (2) ). Dissociative chemisorptions can cause other particles are desorbed, as were adsorbed. Chemisorption has triggered chemical reaction ( catalyzed).

Carbon monoxide ( CO) is dissociated upon adsorption to the metals mentioned above only at high temperatures. Usually a so-called directed chemisorption occurs: CO binds via the C - atom to the metal surface (Fig. 9 (3) ), which is present similar to the binding position in metal carbonyls.

Adsorption is an exothermic reaction, which can, however, extend over energetically unfavorable high transition states (activated adsorption). When activated adsorption adsorption takes place - in contrast to physisorption - only at higher temperatures, and the rate increases with increasing temperature.

In contrast to the activation energy of desorption physisorption is often not equal to the activation energy of the adsorption. The activation energy of desorption resulting from the changes in the adsorbate. While some adsorbed particles (eg, H2 or O2) dissociated, have changed (such as CO2) their spatial configuration of other particles. The magnitude of the activation energies and the heats of adsorption depend not only on the nature of the adsorbed species or their reaction products strongly on the chemical composition and the (electronic) structure of the adsorbate from (Table 1). For chemisorption, adsorption may be irreversible, where a (too) high activation energy required for the desorption of certain particles. If such chemisorption at catalysts, this leads to deactivation and one speaks of a catalyst poison.

Applications

Physisorption

As adsorbents are, for example, activated carbon, silica gel or molecular sieves (zeolites) in the form of beds or in a structured form for practical use. In physisorption based gas filters, such as filter respirator and method for drying moist air ( Adsorption ).

Many methods for the separation or purification, and analysis of mixtures of substances based on chromatographic methods, the adsorption chromatography. All of these methods make use of the equilibrium reaction of the material-specific physical adsorption between an interface and a mobile phase.

For technical and separation of gas mixtures physisorption is used because the gases accumulate varying degrees of pressure and temperature depends on the surface of an adsorbent. We distinguish the modes changes in temperature and pressure swing adsorption. Thus, for example, be obtained from synthesis gas high-purity hydrogen.

In water treatment, wastewater treatment and industrial water management adsorption for the removal of harmful substances in water or material recovery will be used.

Since the adsorption heat is released and in the desorption heat is absorbed, a rating based on the adsorption heat pump ( adsorption chiller ) can be constructed.

Chemisorption

Chemisorption is the essential process for heterogeneous catalysis. Catalysts are suitable for this purpose solids. For the hydrogenation of C = C double bonds and hydrogen is used as the catalyst is Raney nickel or nickel -aluminum alloys. The chemisorption of hydrogen on catalyst plays in the Haber -Bosch process for the recovery of NH3 an important role. The chemisorption of carbon monoxide and hydrogen allows for the Fischer-Tropsch synthesis, the synthesis of hydrocarbons. Also automotive catalysts based on a heterogeneous catalysis.