Adsorption acid-base surface properties

Pulse chemisorption in flow Active sites surface area Degree of dispersion Determination of strong gas-solid interaction Acid/base surface properties Isostheric heat of adsorption... 

Hydroxylated groups, which arc the cause of the surface charge (see Chapter 6), may al.so act as coordination sites for di.s.solved cations, or may be substituted by anions (surface coordination). They may also act as nucleation Sites for a solid phase in the case of surface precipitation. Therefore, several possible mechanisms should be considered in the overall phenomena of adsorption, and most of them may be described in terms of the classical concepts of coordination chemistry. However, adsorption exhibits some specific characteristics because one of the partners is a solid with structural and physical properties that may play an important role. Although adsorption often involves only the surface of the particles, if the solid exhibits appreciable ionic mobility (in the case of an ion-exchange material), or electronic mobility (for a mixed-valence material), acid-base surface reactions may trigger various phenomena in the core of the particles, reactions which in turn may cause ionic and/or electron transfers through the. solid-.solution interface. 

The observed complexity of the Se(IV) electrochemistry due to adsorption layers, formation of surface compounds, coupled chemical reactions, lack of electroactivity of reduction products, and other interrelated factors has been discussed extensively. Zuman and Somer [31] have provided a thorough literature-based review with almost 170 references on the complex polarographic and voltammetric behavior of Se(-i-IV) (selenous acid), including the acid-base properties, salt and complex formation, chemical reduction and reaction with organic and inorganic... 

Acid-base reactivity is an important property of oxide catalysts, and its control is of interest in surface chemistry as well as being of importance in industrial applications. The exposed cations and anions on oxide surfaces have long been described as acid-base pairs. The polar planes of ZnO showed dissociative adsorption and subsequent decomposition of methanol and formic acid related with their surface acid-base properties[3]. Further examples related to the topic of acid-base properties have been accumulated to date[ 1,4-6]. 

In all above mentioned applications, the surface properties of group IIIA elements based solids are of primary importance in governing the thermodynamics of the adsorption, reaction, and desorption steps, which represent the core of a catalytic process. The method often used to clarify the mechanism of catalytic action is to search for correlations between the catalyst activity and selectivity and some other properties of its surface as, for instance, surface composition and surface acidity and basicity [58-60]. Also, since contact catalysis involves the adsorption of at least one of the reactants as a step of the reaction mechanism, the correlation of quantities related to the reactant chemisorption with the catalytic activity is necessary. The magnitude of the bonds between reactants and catalysts is obviously a relevant parameter. It has been quantitatively confirmed that only a fraction of the surface sites is active during catalysis, the more reactive sites being inhibited by strongly adsorbed species and the less reactive sites not allowing the formation of active species [61]. 

The snrface acid-base properties of supported oxides can be conveniently investigated by studying the adsorption of suitably chosen basic-acidic probe molecules on the solid. As shown, acidic and basic sites are often present simultaneously on solid surfaces. The knowledge of the detailed amphoteric character of supported metal oxides is of extreme interest due to the possibility of using them as catalysts in different reactions in which acidity governs the reaction mechanism. 

The first two terms represent van der Waals interactions between the adsorbed SOC and the surface, which would apply to all SOC. The second two terms represent Lewis acid-base interactions, which can be important for compounds containing O, N, or aromatic rings, for example, the adsorption of alkyl ethers on the polar surface of quartz. The y coefficients (in mJ m 2) describe the surface properties, where yvdw is associated with its van der Waals interactions with adsorbing gases, y describes its electron-acceptor interactions, and y describes the electron-donor interactions of the surface. On the other hand, the properties of the adsorbing species are described by In pL for the van der Waals interactions and by the dimensionless parameters ft and which relate to the electron-donor and electron-acceptor properties (if any), respectively, of the adsorbing molecule. 

In starting a residue analysis in foods, the choice of proper vials for sample preparation is very important. Available vials are made of either glass or polymeric materials such as polyethylene, polypropylene, or polytetrafluoroethylene. The choice of the proper material depends strongly on the physicochemical properties of the analyte. For a number of compounds that have the tendency to irreversible adsorption onto glass surfaces, the polymer-based vials are obviously the best choice. However, the surface of the polymer-based vials may contain phthalates or plasticizers that can dissolve in certain solvents and may interfere with the identification of analytes. When using dichloromethane, for example, phthalates may be the reason for the appearance of a series of unexpected peaks in the mass spectra of the samples. Plasticizers, on the other hand, fluoresce and may interfere with the detection of fluorescence analytes. Thus, for handling of troublesome analytes, use of vials made of polytetrafluoroethylene is recommended. This material does not contain any plasticizers or organic acids, can withstand temperatures up to 500 K, and lacks active sites that could adsorb polar compounds on its surface. 

The alumina or silica-alumina supports used in bifunctional catalysts have been shown to be acidic in nature. The acidic properties are readily demonstrated by the affinity of these solids for adsorption of basic compounds such as ammonia, trimethylamine, re-butylamine, pyridine, and quinoline (01, R5). Furthermore, adsorption of certain acid-base indicators such as butter yellow gives a coloration similar to that observed in acid media (B3, B4). With regard to the origin of the acidity, Tamele (Tl) has suggested in the case of silica-alumina that aluminum atoms replace silicon atoms in the surface of the silica structure, giving rise to surface sites of the form... 

In order to obtain quantitative measurements of hydrogenation activity and acidity, various schemes are employed. For example, metal surface area has been related to hydrogenation activity and the adsorption of bases such as pyridine and ammonia have been correlated with acidity ((3). Some authors have used certain key reactions involving pure compounds as an indication of catalytic properties (16). Each of these methods is useful however, because of the complex interdependence of the catalytic functions of the hydrocracking catalysts and changes in these functions with catalyst aging, results from each method must be interpreted with caution. 

Figure 13.6 shows a schematic for IGC operation. Inverse, in this instance, refers to the observation that the powder is the unknown material, and the vapor that is injected into the column is known, which is inverse to the conditions that exist in traditional gas chromatography. After the initial injection of the known gas probe, the retention time and volume of the probe are measured as it passes through the packed powder bed. The gas probes range from a series of alkanes, which are nonpolar in nature, to polar probes such as chloroform and water. Using these different probes, the acid-base nature of the compound, specific surface energies of adsorption, and other thermodynamic properties are calculated. The governing equations for these calculations are based upon fundamental thermodynamic principles, and reveal a great deal of information about the surface of powder with a relatively simple experimental setup (Fig. 13.6). This technique has been applied to a number of different applications. IGC has been used to detect the following scenarios ...

This paper focuses on the influence of the support on the H/D exchange of CP over supported Pt catalysts. It will be shown that kinetics and selectivities are largely affected by the support material. Particle size effects are separated from support effects. The activity shows a compensation effect, and the apparent activation energy and pre-exponential factor show an isokinetic relationship . This can be explained by different adsorption modes of the CP on the metallic Pt surface. The change in adsorption modes is attributed to a change in the electronic structure of the Pt particles, which in turn is induced by changes in the acid/base properties of the support. 

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