Adsorption mechanisms physisorption

Another Russian scientist who played a leading role in the advancement of the understanding of adsorption mechanisms was A.V. Kiselev. With the help of a large team of co-workers and by making a systematic investigation of various well-defined adsorbents (notably oxides, carbons and zeolites), Kiselev was able to demonstrate that certain specific interactions were involved in the adsorption of polar molecules on polar or ionic surfaces. At the same time, in the UK the specificity of physisorption was under investigation by Barrer - especially in the context of his pioneering work on the properties of the molecular sieve zeolites. 

A distinction is made between two types of adsorption mechanisms chemisorption and physisorption. One, physisorption, is reversible and involves exclusively physical interaction forces (van der Waals forces) between the component to be adsorbed and the adsorbent. The other, chemisorption, is characterized by greater interaction energies which result in a chemical modification of the component adsorbed along with its reversible or irreversible adsorption. ... 

In the mesoporous range for MCM-41, each steps of the water adsorption mechanism are well identified by neutron diffraction the physisorption of the first molecules on the rough MCM-41 wall, then the whole filling of the MCM-41 mesopore by water molecules. The usual molecular water organisation is measured (confined liquid phase, confined nanociystallites of cubic ice). The confinement effect, the displacement of the whole water phase diagram towards the low temperature side is quantifi decreasing the MCM-41 pore size increases the temperature displacement. 

Fig. 3. Potential curves, describing adiabatic and non-adiabatic adsorption mechanisms, 22, 11 - diabatic terms, responsible for the configurations c -f- S and c-t- 5, respectively for z -> oo (c and S being adsorbed molecule of the sort c and surface) 21, 2 T - adiabatic terms responsible for the scune configurations for z oo Ech, Eph, E t, - energies of chemisorption, physisorption and chemisorption activation A. - nonadiabatic terms interaction parameter z. - term crossing point E, Ez - normal energies of translational motion of molecule in initial and final states. Term 1 corresponds to the chemisorptional state while the term 2 - to the phisisorptional one for z < z,.

It is at once evident that there is a remarkable degree of similarity between the shapes of L-type solute isotherms and Type I physisorption isotherms. However, this similarity is misleading since the adsorption mechanisms involved are likely to be quite different. We have seen already that Type I physisorption isotherms for gas-solid systems are normally associated with micropore filling. In contrast, the plateau of an L-type solute isotherm usually corresponds to monolayer completion. In this respect, solute adsorption appears to correspond more closely to the classical Langmuir mechanism. If this is indeed the case it would seem to be possible to calculate the surface area from nj by the application of a simple equation of the same form as Eq. (2). 

The hydrophobic character exhibited by dehydroxylated silica is not shared by the metal oxides on which detailed adsorption studies have been made, in particular the oxides of Al, Cr, Fe, Mg, Ti and Zn. With these oxides, the progressive removal of chemisorbed water leads to an increase, rather than a decrease, in the affinity for water. In recent years much attention has been devoted, notably by use of spectroscopic and adsorption techniques, to the elucidation of the mechanism of the physisorption and chemisorption of water by those oxides the following brief account brings out some of the salient features. 

As noted before, thin film lubrication (TFL) is a transition lubrication state between the elastohydrodynamic lubrication (EHL) and the boundary lubrication (BL). It is widely accepted that in addition to piezo-viscous effect and solid elastic deformation, EHL is featured with viscous fluid films and it is based upon a continuum mechanism. Boundary lubrication, however, featured with adsorption films, is either due to physisorption or chemisorption, and it is based on surface physical/chemical properties [14]. It will be of great importance to bridge the gap between EHL and BL regarding the work mechanism and study methods, by considering TFL as a specihc lubrication state. In TFL modeling, the microstructure of the fluids and the surface effects are two major factors to be taken into consideration. 

The adsorption action of activated carbon may be explained in terms of the surface tension (or energy per unit surface area) exhibited by the activated particles whose specific surface area is very large. The molecules on the surface of the particles are subjected to unbalanced forces due to unsatisfied bonds and this is responsible for the attachment of other molecules to the surface. The attractive forces are, however, relatively weak and short range, and are called Van der Waals forces, and the adsorption process under these conditions is termed as a physical adsorption (physisorption) process. In this case, the adsorbed molecules are readily desorbed from the surface. Adsorption resulting from chemical interaction with surface molecules is termed as chemisorption. In contrast to the physical process described for the adsorption on carbon, the chemisorption process is characterized by stronger forces and irreversibility. It may, however, be mentioned that many adsorption phenomena involve both physical and chemical processes. They are, therefore, not easily classified, and the general term, sorption, is used to designate the mechanism of the process. 

The majority of physisorption isotherms (Fig. 1.14 Type I-VI) and hysteresis loops (Fig. 1.14 H1-H4) are classified by lUPAC [21]. Reversible Type 1 isotherms are given by microporous (see below) solids having relatively small external surface areas (e.g. activated carbon or zeolites). The sharp and steep initial rise is associated with capillary condensation in micropores which follow a different mechanism compared with mesopores. Reversible Type II isotherms are typical for non-porous or macroporous (see below) materials and represent unrestricted monolayer-multilayer adsorption. Point B indicates the stage at which multilayer adsorption starts and lies at the beginning of the almost linear middle section. Reversible Type III isotherms are not very common. They have an indistinct point B, since the adsorbent-adsorbate interactions are weak. An example for such a system is nitrogen on polyethylene. Type IV isotherms are very common and show characteristic hysteresis loops which arise from different adsorption and desorption mechanisms in mesopores (see below). Type V and Type VI isotherms are uncommon, and their interpretation is difficult. A Type VI isotherm can arise with stepwise multilayer adsorption on a uniform nonporous surface. 

Morrall2 used a HPLC system with two columns. The first column was loaded with the controlled pore glass (CPG) to be modified. The second column was used for separation of the reaction effluents. This column was coupled to a refractive index detector, allowing for quantitative detection of the effluents. The reaction was initiated by injecting an APTS/toluene mixture and stopped by injection of pure toluene. With this so-called stop-flow mechanism reaction times down to 18 seconds could be used. From these analyses it became evident that upon mixing of the aminosilane with the silica, a very rapid physisorption occurs. The initial adsorption of the APTS (from toluene solution on dried CPG) occurred before the 18 second minimum time delay of the stop-flow apparatus. For non-aminated silanes the adsorption proved to be much slower. This study also revealed the pivotal role of surface water in the modification of siliceous surfaces with alkoxysilanes, as discussed in the previous chapter. 

The rapid decrease in So(E ) observed below 0.15 eV on Pt(5 3 3) (Fig. 18) has also been observed on the Pt(l 1 1) surface [134] and is consistent with a trapping mechanism where the need to dissipate energy limits the probability of adsorption, and subsequent dissociation, via the physisorbed precursor. In order to assess the contribution of the physisorption mediated channel, the contribution to sticking directly via the chemisorbed channel must be subtracted from the measured So. The proportion of So derived from the direct chemisorption channel on Pt(5 3 3) at Ex = 0.05 eV is significantly higher than on Pt(l 1 1) (ca. 10%) [137]. Once this direct contribution is subtracted, the dependence S0(Ts)can be used to obtain kinetic parameters relating to the partition of the physisorbed precursor. This is achieved... 

These limits are to some extent arbitrary since the pore filling mechanisms are dependent on the pore shape and are influenced by the properties of the adsorptive and by the adsorbent-adsorbate interactions. The whole of the accessible volume present in micropores may be regarded as adsorption space and the process which then occurs is micropore filling, as distinct from surface coverage which takes place on the walls of open macropores or mesopores. Micropore filling may be regarded as a primary physisorption process (see Section 8) on the other hand, physisorption in mesopores takes place in two more or less distinct stages (monolayer-multilayer adsorption and capillary condensation). 

It is generally recognized that the mechanism of physisorption is modified in very fine pores (i.e. pores of molecular dimensions) since the close proximity of the pore walls gives rise to an increase in the strength of the adsorbent-adsorbate interactions. As a result of the enhanced adsorption energy, the pores are filled with physisorbed molecules at low p/p°. Adsorbents with such fine pores are usually referred to as microporous. 

Provided that the experimental measurements are made under carefully controlled conditions and that the adsorption systems are well characterized, energy of adsorption data can provide valuable information concerning the mechanisms of physisorption. 

The integral molar quantities are of importance for modelling adsorption systems or in the statistical mechanical treatment of physisorption. For example, they are required for comparing the properties of the adsorbed phase with those of the bulk... 

As explained in Chapter 1, the shape of an adsorption isotherm provides useful preliminary information concerning the mechanisms of physisorption, and hence the nature of the adsorbent. For example, a reversible Type II adsorption-desorption isotherm is generally associated with the formation of an adsorbed layer which progressively thickens as the equilibrium pressure is increased up to the saturation pressure this form of monolayer-multilayer physisorption is observed on an open and stable surface of a non-porous adsorbent. 

Although the simple Langmuir equation is more applicable to some forms of chemisorption, the underlying theory is of great historical importance and has provided a starting point for the development of the BET treatment and of other more refined physisorption isotherm equations. It is therefore appropriate to consider briefly die mechanism of gas adsorption originally proposed by Langmuir (1916, 1918). 

In principle, the as-method is not restricted to nitrogen adsorption and can be applied to any gas-solid physisorption system irrespective of the shape of its isotherm it can be used to check the validity of the BET area and also to identify the individual mechanisms (monolayer-multilayer adsorption, micropore filling or capillary condensation). Numerous examples of different as-plots are to be found in subsequent chapters. Here, we are concerned with the general principles of the as-method of isotherm analysis with particular reference to the evaluation of surface area. The distinctive features of various hypothetical as-plots are revealed in Figure... 

In a critical appraisal of the different methods for determining surface fractal dimensions, Neimark (1990) has stressed the importance of taking account of the different mechanisms of physisorption (e.g. at high p/p° the combination of multilayer adsorption and capillary condensation). Conner and Bennett (1993) have also warned of the risk of an oversimplistic interpretation of a linear log-log fractal plot. 

Physisorption or physical adsorption is the mechanism by which hydrogen is stored in the molecular form, that is, without dissociating, on the surface of a solid material. Responsible for the molecular adsorption of H2 are weak dispersive forces, called van der Waals forces, between the gas molecules and the atoms on the surface of the solid. These intermolecular forces derive from the interaction between temporary dipoles which are formed due to the fluctuations in the charge distribution in molecules and atoms. The combination of attractive van der Waals forces and short range repulsive interactions between a gas molecule and an atom on the surface of the adsorbent results in a potential energy curve which can be well described by the Lennard-Jones Eq. (2.1). 

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