Reversible adsorbed

Alpert and Regnier [37] found that polyethyleneimine (PEI, M = 600) was readily adsorbed by various porous silicas from a methanol solution. After the reversibly adsorbed portion of the polymer was washed out, the content of... 

These sorbents may be used either for selective fixation of biological molecules, which must be isolated and purified, or for selective retention of contaminants. Selective fixation of biopolymers may be easily attained by regulation of eluent polarity on the basis of reversed-phase chromatography methods. Effective isolation of different nucleic acids (RNA, DNA-plasmid) was carried out [115, 116]. Adsorption of nucleosides, nucleotides, tRN A and DNA was investigated. It was shown that nucleosides and nucleotides were reversibly adsorbed on... 

The authors [35] emphasize that their result regarding the first HgS monolayer, which involves reversible underpotential adsorption, suggests that nucleation cannot be considered as a universal mechanism for the formation of anodic films. Analogous conclusions have been inferred for cathodic HgSe films electrodeposited on mercury electrode by the reduction of selenous acid [37] the first monolayer appeared to be reversibly adsorbed, while formation of the following two layers was preceded by nucleation. 

Static Chemisorption. Measurements were made by two procedures. In the first, the catalyst was evacuated at ca. 250°C for at least 8 hrs and cooled to the measurement temperature under vacuum. Hydrogen was then admitted at progressively higher pressures and the amount of gas adsorbed after 15-30 min at each pressure recorded. The sample was then evacuated for 30 min and the dosing procedure repeated so as to obtain a measure of the reversibly adsorbed gas. In the second (saturation) procedure, after reduction and evacuation, the catalyst was cooled to the... 

To illustrate the influence exerted by the energy of adsorption of an intermediate on the rate of an electrocatalytic reaction, consider a very simple two-step reaction of the type A —> X —> B where X, the intermediate, is reversibly adsorbed on the electrode (with a degree of surface coverage 9x). For the sake of simplicity, the electrode surface will be assumed to be homogeneous (i.e., conditions of Langmuir adsorption hold), while the system lacks adsorbed species other than X. The rate, of the adsorption step (the first step) is then proportional to the bulk concentration of the starting material, c, and to the free surface part (1 - 9x) (the part not taken up by species X), while the rate of further transformation of intermediate X, which is tied to its desorption, will be proportional to the surface fraction, 9x, taken up by it ... 

It is useful to consider the relative ease of skeletal reactions compared with adsorption and desorption indicated by deuterium exchange. Here one must ensure that exchange and skeletal reactions involve the same adsorbed intermediate, and this requires working at comparable temperatures, and assessing the adsorbed intermediate from the initial exchange products (cf. 116a). Skeletally important intermediates appear, for instance, to be reversibly adsorbed on platinum and palladium, but irreversibly adsorbed on iron and cobalt. 

The calorimetric method which has been outlined in this section is not applicable to the study of surface interactions or of reaction mechanisms which occur between reversibly adsorbed species. But, even in these unfavorable cases, heat-flow microcalorimetry may still yield useful information concerning either the nature of the adsorbed species, the distribution of sites, or the irreversible modifications which occur frequently on the catalyst surface during the course of the reaction. 

When a monolayer at equilibrium, with surface pressure ir, is subjected to compression at a speed of v cm /sec, the surface pressure, it, is increased because of the reduction of surface area, A. The increase in surface pressure will change the free energy of the adsorbed molecules and lead to a desorption of whole molecules or segments of molecules if these are reversibly adsorbed. The relationship between surface pressure and surface area for a monolayer may be given by an equation of state ... 

In a hysteresis experiment, the movable barrier would be reversed at a time, designated as t, so that the monolayer comes under an expansion process at the same speed, v. The increase of surface area causes a reduction in the surface pressure. For a reversibly adsorbed monolayer, the desorption of segments may continue during the first period of expansion until the surface pressure is reduced to its equilibrium value. On further expansion, readsorption occurs because the surface pressure is below its equilibrium value. 

Poisoning is caused by chemisorption of compounds in the process stream these compounds block or modify active sites on the catalyst. The poison may cause changes in the surface morphology of the catalyst, either by surface reconstruction or surface relaxation, or may modify the bond between the metal catalyst and the support. The toxicity of a poison (P) depends upon the enthalpy of adsorption for the poison, and the free energy for the adsorption process, which controls the equilibrium constant for chemisorption of the poison (KP). The fraction of sites blocked by a reversibly adsorbed poison (0P) can be calculated using a Langmuir isotherm (equation 8.4-23a) ... 

Case Studies Adsorption of Acetic Acid on Pt(111) Single Crystal Electrodes. Acetic acid is one of a very few organic compounds which is reversibly adsorbed on platinum at room temperature (20,25). We report below our radiochemical results on adsorption of this compound on Pt(111) and on polycrystalline Pt. 

Thus, transition metal cations in the lower valence state may also act as Lewis bases. Factors that affect the reactions promoted by Lewis acidity are listed in Table I. Lewis acid sites reversibly adsorb water (6s 9, 42), which may thus strongly compete with organic compounds that have weaker Lewis base properties, such as aromatic hydrocarbons. Lewis acidity depends on the degree of hydration and is strongest under desiccating conditions. Examples of reactions that are promoted by Lewis acidity are summarized in Table II. Other examples have been reviewed by Solomon and Howthorne (37). 

The results of this TIR/FRAP study are that 80% of the nonspecific binding of fluorescein-labeled insulin to the external face of red cell membranes is reversible within <100s, and the mean residency time of the reversibly adsorbed insulin ranges from 0.4 s to 20 s. Surface diffusion of nonspecifically adsorbed insulin (as investigated by an TIR intersecting beam interference fringe pattern see Figure 7.5) was immeasurably small it is insufficient to carry a typical insulin more than 0.3 /xm before desorption. 

Overall the period since the 1980s can be described as a period of explosion in the discovery of new compositions and structures of molecular sieves. This can perhaps be seen most vividly by comparing the numbers of structure types contained in the various editions of the Atlas of Zeolite Structure Types [4]. The first edition (1978) contained 38 structure types, the second edition (1987) 64, the third edition (1992) 85 and the most recent edition (2007) 176. Thus 112 new structure types have been discovered since 1978. However, the reader should be cautioned that a significant number of the structure types included in the Atlas are not truly microporous or molecular sieve materials (i.e., they are not stable for the removal of as-synthesized guest species, typically water or organic templates) and therefore carmot reversibly adsorb molecules or carry out catalytic reactions. Unfortunately, the Atlas gives only limited information on the stability of the structures described. 

I Total amount adsorbed n Amount reversibly adsorbed n Amount irreversibly adsorbed... 

Coordination Poiymers as CO2 Absorbents. This is a long-term (2002-2006) basic research project conducted at the University of Oslo. In this project new and novel adsorbent materials (coordination polymers) will be studied that can selectively and reversibly adsorb/desorb carbon dioxide from ambient to 300°C. 

Adsorption and reaction of C2H4. Ethylene reversibly adsorbed on clean Au(lll) at 95 K. Desorption of ethylene in TPD from the clean Au(lll) surface was observed from approximately 100 - 250 K, with a large peak at 104 K. The reason for the broad desorption peak is uncertain. Outka and Madix (8) have shown that a broad range of desorption is typical of C2 hydrocarbons on Au(llO). They propose that these molecules are weakly bound and therefore occupy a variety of binding sites with different activation energies, consequently they desorb at different temperatures. We observed no H2 evolution from the surface up to temperatures of 700 K and AES showed no residual carbon on the surface after heating. Thus, C2H4 is reversibly, and most likely molecularly, adsorbed on clean Au(lll). 

A very important and confusing observation is that the charge involved in the unusual states in various solutions is nearly independent of the nature of the anion, including organic anions. This aspect of the unusual states carmot be easily reconciled with the charge transfer to, or from the anion as a mechanistic origin of the pseudocapacitive current generated by the reversibly adsorbed anions on Pt(l 11). 

The importance of catalyst stability is often underestimated not only in academia but also in many sectors of industry, notably in the fine chemicals industry, where high selectivities are the main objective (1). Catalyst deactivation is inevitable, but it can be retarded and some of its consequences avoided (2). Deactivation itself is a complex phenomenon. For instance, active sites might be poisoned by feed impurities, reactants, intermediates and products (3). Other causes of catalyst deactivation are particle sintering, metal and support leaching, attrition and deposition of inactive materials on the catalyst surface (4). Catalyst poisons are usually substances, whose interaction with the active surface sites is very strong and irreversible, whereas inhibitors generally weakly and reversibly adsorb on the catalyst surface. Selective poisons are sometimes used intentionally to adjust the selectivity of a particular reaction (2). 

Removal of reversibly adsorbed (physisorbed) cyanine-dye molecules, D, led to skeletonized, SA silane monolayers having pinholes in the shape of D (Fig. 18) [185]. Such pinholes can, in turn, serve as templates for similarly shaped guest molecules. This approach opens the door for molecular recognition and device construction based on molecular recognition [186-190]. The idea relies upon the construction of a well-packed, SA monolayer from a mixture of OTS molecules and a solvent removable guest species. Removal of the guest molecules leaves pinholes with precise dimensions which only accept molecules with... 

Wijmans, C.M., Dickinson, E. (1999a). Brownian dynamics simulation of a bonded network of reversibly adsorbed particles towards a model of protein adsorbed layers. Physical Chemistry Chemical Physics, 1,2141-2147. 

Some components in a gas or liquid interact with sites, termed adsorption sites, on a solid surface by virtue of van der Waals forces, electrostatic interactions, or chemical binding forces. The interaction may be selective to specific components in the fluids, depending on the characteristics of both the solid and the components, and thus the specific components are concentrated on the solid surface. It is assumed that adsorbates are reversibly adsorbed at adsorption sites with homogeneous adsorption energy, and that adsorption is under equilibrium at the fluid- adsorbent interface. Let (m" ) be the number of adsorption sites and (m 2) the number of molecules of A adsorbed at equilibrium, both per unit surface area of the adsorbent. Then, the rate of adsorption r (kmol m s ) should be proportional to the concentration of adsorbate A in the fluid phase and the number of unoccupied adsorption sites. Moreover, the rate of desorption should be proportional to the number of occupied sites per unit surface area. Here, we need not consider the effects of mass transfer, as we are discussing equilibrium conditions at the interface. At equilibrium, these two rates should balance. Thus,... 

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