Isotherms chemical adsorption

A detailed study of the physical and chemical adsorption of water on three xerogels, ferric oxide, alumina and titania, as well as on silica (cf. p. 272) has been carried out by Morimoto and his co-workers. Each sample was outgassed at 600°C for 4 hours, the water isotherm determined at or near 20°C, and a repeat isotherm measured after an outgassing at 30 C. The procedure was repeated on the same sample after it had been evacuated at a... 

The performance of demulsifiers can be predicted by the relationship between the film pressure of the demulsifier and the normalized area and the solvent properties of the demulsifier [1632]. The surfactant activity of the demulsifier is dependent on the bulk phase behavior of the chemical when dispersed in the crude oil emulsions. This behavior can be monitored by determining the demulsifier pressure-area isotherms for adsorption at the crude oil-water interface. 

In the last two sections the formal theory of surface thermodynamics is used to describe material characteristics. The effect of interfaces on some important heterogeneous phase equilibria is summarized in Section 6.2. Here the focus is on the effect of the curvature of the interface. In Section 6.3 adsorption is covered. Physical and chemical adsorption and the effect of interface or surface energies on the segregation of chemical species in the interfacial region are covered. Of special importance again are solid-gas or liquid-gas interfaces and adsorption isotherms, and the thermodynamics of physically adsorbed species is here the main focus. 

The adsorption of gases on solids can be classified into physical and chemical adsorption. Physical adsorption is accompanied by a low enthalpy of adsorption, and the adsorption is reversible. The adsorption/desorption characteristics are in these cases often described by adsorption isotherms. On the other hand, chemical adsorption or segregation involves significantly larger enthalpies and is generally irreversible at low temperatures. It is also often accompanied by reconstruction of the surface due to the formation of strong ionic or covalent bonds. 

This chapter introduces the topic of adsorption, giving examples of both physical adsorption and chemical adsorption, and discusses the similarities and differences between the two. The standard nomenclature of surface science is given from within this context. The energetics of adsorption are explained in terms of the enthalpies of bond formation A/Tadsr Next, isotherms are discussed. 

Figure 10.2 Schematic isotherm for the simplest cases of chemical adsorption from solution onto a solid substrate. The amount of adsorbate available to adsorb is best gauged by the concentration c...

In order to accurately determine the chemisorbed amount from the overall adsorption isotherm, the sample can be further outgassed at the same temperature to remove the physically adsorbed amount, after which a new adsorption procedure is carried out to obtain isotherm II. The difference between the first and second isotherm gives the extent of irreversible adsorption ( ) at a given temperature (Figure 13.5b), and can be considered as a measurement of the amount of strong sites in the catalyst. However, in the first approximation, the magnitude of the heat of adsorption can be considered as a simple criterion to distinguish between physical and chemical adsorption. 

The differences observed in the adsorption isotherm are also qualitatively and quantitatively significant for the entropy. It has been recently shown that the isotherm of adsorption of an ideal adsorbate on a heterogeneous surface can be appreciably improved by taking into account the exact form of S from Eq. (7) instead of the approximate one arising from F-H theory [22], Results for the coverage dependence of the chemical potential (adsorption isotherm) and entropy per site are shown in figs. 1-2 for various fc-mer s sizes and interaction energies [attractive (w<0) as well as repulsive (w>0)]. 

When reactants or intermediates are adsorbed, the rate of the reaction may no longer be related to the concentration by a simple law. This situation is best understood where a reactant is nonspecifically adsorbed in the outer -> Helmholtz plane. The effect of such adsorption on the electrode kinetics is usually termed the -> Frumkin effect. Physical and chemical adsorption on the electrode surface is usually described by means of an -> adsorption isotherm and kinetic equations compatible with various isotherms such as the - Langmuir, -> Temkin, -> Frumkin isotherms are known. 

In a next step, the part of the isotherm for pressures smaller than p/po = 10 was fitted with various equations. As only a limited number of data points were available (in some cases as few as three points) a reasonable fitting could be achieved with various equations. However, to be accepted, these equations had to obey two supplementary conditions general applicability and physico-chemical consistency. To answer to the first condition the same equation (possibly with different parameters) had to be applicable to all adsorbents, and preferably to both argon and nitrogen (the carbon dioxide data being discarded). The second condition is rather straightforward if the first part of the isotherm really depicts a different adsorption mechanism, the equation that describes this part should represent a physico-chemical adsorption phenomenon. 

Kim, D.J. Non-Isothermal Dynamic Adsorption and Reaction in Hydrocarbon Adsorber Systems. PhD. Thesis, Department of Chemical Engineering, Ajou University, 2002. 

We will provide a succinct introduction to the main textural characterisation techniques for catalysts. As a heterogeneous catalyst comprises a support and an active phase, we will distinguish between techniques intended for studying the support, which will be presented in a first section (Physisorption isotherms and mercury porosimetry) and techniques used to characterise the active phase, in the strict sense of the term, shown in a second section (Chemical adsorption). For each technique, we will show the theoretical principle, the way in which the measurement is carried out and the equipment used. Finally, examples will be used to illustrate the type of response that can be given using these characterisation techniques. 

One of the most important thermodynamic properties of a fluid, inhomogeneous or not, is its chemical potential p. This is particularly true in physical adsorption, since measurements of the isotherm of adsorption, which is amount adsorbed as a function of the pressure p of the gas in equilibrium with the adsorbed phase, are widely employed as a method of characterization of the system. However, the chemical potentials jUgas and pads are equal at equilibrium and, if the gas is ideal. 

As a typical example of CEDFT calculations, we present in Fig. 1 the capillary condensation isotherm of N2 in a cylindrical pore mimicking the pore channel in MCM-41 mesoporous molecular sieves. The isotherm is presented in co-ordinates adsorption N versus chemical potential p Calculations were performed at 77 K for the internal diameter of 3.3 nm up to the saturation conditions, point H. We used Tarazona s representation of the Helmholtz free energy [6] with the parameters for fluid-fluid and solid-fluid interaction potentials, which were employed in our previous papers [7]. We distinguish three regions on the isotherm. The adsorption branch OC corresponds to consecutive formation of adsorption layers. Note that the sharp transitions between the consecutive layers are not observed in experiments. They are caused by a well-known shortcoming of the model employed, which ignores intrinsic to real... 

Figure 4 Canonic isotherms of adsorption of H2PdCl4 (1-6), H2PtCl4 (7), HAuCU (8), and H2PtCl6 (9-16) on chemically unmodified carbon supports. Some of the isotherms are reproduced from literature data (9 [152], 10 [147], 11 [46], 14-16 [153]) and the others are obtained by the authors of the present review.

Figure 22.9 Examples of molecular engineering of carbon surface (a) isotherms for adsorption of HjO vapor on various chemically modified carbons, (b) gas drying characteristics of the modified selective surface membrane (SSF) membrane, (c) high-temperature chemisorption of CO2 on MgO-doped activated carhon.

The Langmuir isotherm is commonly nsed in describing chemical adsorption, such as polymer and surfactant adsorption. Therefore, in the following examples, we will use the Langmuir isotherm to discuss the three different types of fronts. 

The signal intensity increased with the stagnation pressure of the molecular beam and finally leveled off. This behavior is well represented by the Langmuir adsorption isotherm which is applicable to chemical adsorption. A monolayer on the substrate is photodissociated to generate Ga atoms. 

Physical and chemical adsorption and desorption isotherms are important in characterizing the overall adsorbent surface. The slightest change in the shape of the plotted isotherm... 

There appears to be a disparity in terminology of isotherms between the hydrogeology and chemistry literature. While chemistry textbooks describe adsorption isotherms (e.g., Drever, 1988, p. 362 Stumm and Morgan, 1996, p. 521), the hydrogeology literature uses the term sorption isotherms (e.g., Domenico and Schwartz, 1998, p.299 Fetter, 1999, p. 122 Freeze and Cherry, 1979, p.403). Manuals for popular transport codes also describe sorption isotherms (Zheng, 1990, pp.2-9). According to Sposito (1984), the term adsorption includes all reactions on the two-dimensional solid-water interface physical and chemical adsorption and ion-exchange the term sorption refers to all processes that transfer an ion from aqueous to solid phase. 

Figure 1. Adsorption isotherms for a pure iron synthetic ammonia catalyst for various gases near their boiling points. Curve lA is for physical plus chemical adsorption of CO. Curve IB is for physical adsorption occurring at -1830C after the evacuation of the samples at -78 C for an hour. The solid symbols are for desorption. (Reproduced from Ref. 22. Copyright 1937, American Chemical Society.)...

Therefore, in the system under study nonaqueous solvents exert an essential effect on proceeding of colloid-chemical processes which result in structurization of a porous material. The outcome of such an effect is, as a rule, a decrease (sometimes rather substantial) in the specific surface value as well as a decrease in the sorptive volume of pores and increase in their size. Analogous inferences were also made by the authors of reference [19] who considered the isotherms of adsorption of n-hexane by samples 13-19. AU the curves (with the exception of the isotherm for sample 19) were S-shaped and were distinguished for a steep rise of hysteresis loop at P/Pq > 0.5. This is known to be characteristic of mesoporous adsorbents [27]. The isotherm for sample 19 prepared in the presence of DMF was more flat and differed markedly from the isotherms of samples 13-18. The observation is in complete agreement with the porous structure parameters of this sample (see Table 33.1). 

Figure 6.2 Five types of isotherms for adsorption. (Adapted with permission from S. Brunauer, L. S. Deming, W. E. Denting, and E. Teller, On a Theory of the Van der Waals Adsorption of Gases, J. Am. Chem. Soc., 62, 1723. Copyright 1940, American Chemical Society.)...

In view of these results, a qualitative model is proposed for the isothermal, static adsorption of sulfonates on mineral solids from microemulsions. We assume that the adsorption behavior is primarily determined by the clay fraction of the adsorbent because of its high specific area. In general, the adsorption process can be regarded as a partitioning of the sulfonate anions between the microemulsion phase and the microemulsion-adsorbent interface. At equilibrium the amount of adsorbed sulfonate is a measure of the relative magnitudes of the sulfonate-microemulsion, sulfonate-adsorbent and microemulsion-adsorbent interactions. Since the sulfonate is charged, one must in principle consider electrostatic as well as chemical forces. 

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