Acid-base properties, of surfaces

EQUILIBRIA OF SILICA SURFACE SITES COMPLEXING WITH THE ONE-VALENCE CATIONS IN AQUEOUS SOLUTIONS (ACID-BASE PROPERTIES OF SURFACE ADSORPTION SITES)... 

Acid-base properties of oxide surfaces are employed in many fields and their relationship with PZC has been often invoked. Adsorption and displacement of different organic molecules from gas phase was proposed as a tool to characterize acid-base properties of dry ZnO and MgO [341]. Hammet acidity functions were used as a measure of acid-base strength of oxides and some salts [342]. Acidity and basicity were determined by titration with 1-butylamine and trichloroacetic acid in benzene using indicators of different pAg. There is no simple correlation between these results and the PZC. Acid-base properties of surfaces have been derived from IR spectra of vapors of probe acids or bases, e.g. pyridine [343] adsorbed on these surfaces. The correlation between Gibbs energy of adsorption of organic solvents on oxides calculated from results obtained by means of inverse gas chromatography and the acceptor and donor ability of these solvents was too poor to use this method to characterize the donor-acceptor properties of the solids [344],... 

Methanol is a key compound in Cl chemistry because it allows the conversion of raw materials, from which it is produced, into more valuable organic chemicals. However, the main application of heterogeneous catalysts for the activation of CH3OH is related to their transformation into hydrocarbons. For these technologies, the catalytic reactions are based on the acid-base properties of surfaces, and the catalytic materials consist of zeolites (such... 

This review focuses on the acid-base properties of surfaces of porous solids. In the context of the above discussion, it is inevitable that established practices will require some modifications. It is obvious that solids possess acidity and basicity. The challenge in characterizing their acid-base behavior results from the presence of two phases and from location of the acid-base sites at the interface between the solid and either a gaseous or liquid phase. Moreover, when acid-base chemistry occurs in spaces confined to the micropores or interlayers of nanostructured materials, the rules are broken a second time because all references to acid-base properties of macroscopically honlogeneous phases based on the classical approach become inconsistent. 

The chemical specificity of XPS again provides in this area a powerful route to the determination of the acid-base properties of surfaces. Indeed there appears to be a direct linear correlation between certain XPS data and the isoelectric... 

The (I)-(III)-samples sorption ability investigation for cationic dyes microamounts has shown that for DG the maximum rate of extraction is within 70-90 % at pH 3. The isotherm of S-type proves the physical character of solution process and a seeming ionic exchange. Maximal rate of F extraction for all samples was 40-60 % at pH 8 due to electrostatic forces. The anionic dyes have more significant affinity to surface researching Al Oj-samples comparatively with cationic. The forms of obtained soi ption isotherms atpH have mixed character of H,F-type chemosorption mechanism of fonuation of a primary monolayer with the further bilayers formation due to H-bonds and hydrophobic interactions. The different values of pH p for sorbents and dyes confirm their multifunctional character and distinctions in the acid-base properties of adsoi ption centers. 

Referring to the ionic effects, measuring of swelling in solutions which closely model real ones can provide reliable estimates. The papers [58, 132] can serve as examples of such an approach. In choosing a type of SAH suitable for some particular soil it is necessary to take into account the acid-base properties of the gel and the soil because otherwise collapse phenomena are likely to result from common counterions and the sorption on solid surfaces. 

The field of surface-mediated synthesis of metal carbonyl clusters has developed briskly in recent years [4-6], although many organometallic chemists still seem to be unfamiliar with the methods or consider themselves ill-equipped to carry them out. In a typical synthesis, a metal salt or an organometallic precursor is brought from solution or the gas phase onto a high-area porous metal oxide, and then gas-phase reactants are brought in contact with the sample to cause conversion of the surface species into the desired products. In these syntheses, characteristics such as the acid-base properties of the support influence fhe chemisfry, much as a solvenf or coreactant influences fhe chemisfry in a convenfional synfhesis. An advanfage of... 

The acid-base oxides such as aluminas were used as catalysts, adsorbents or catalyst supports and it was interesting to know the surface acid-base properties of these catalysts. 

The pH optical fiber sensor without any pH-sensitive dye was also described70. Porous silica layer made by the sol-gel method was cladded onto optical fibre core and was exploited as the optical transducer. Acid-base properties of silica surface caused that the surface charge of silica changed with pH of the solution. For example saturation of the sol-gel layer with cations leads to an increase of the electron density of the film, hence, the refractive index of the film. Since the surface charge of silica depends on pH, the refractive index of silica film varies also with pH. Thus, changes of... 

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. 

When immersed in aqueous solutions, the surfaces of metal oxides are expected to be hydroxylated (see Section 3.1). However, metal oxides may also react with aqueous solutions leading to dissolution, which can effectively be thought of as mass transfer from the solid to aqueous phase. The rate and extent of dissolution reactions depend on a number of factors, including solution pH, acid-base properties of oxo groups on the metal oxide surface, types of ligands present in solution, metal... 

Fowkes, F. M., Quantitative characterization of the acid-base properties of solvents, polymers, and inorganic surfaces . In Acid-Base Interactions - Relevance to Adhesion Science and Technology, K. L. Mittal and H. R. Anderson, Eds., VSP, Utrecht, 1991, pp. 93-115. 

It took some time to adopt a similar view of other heterogeneous elimination and substitution reactions. Most efficient experimental tools have been found in stereochemical studies, correlation of structure effects on rates and measurement of deuterium kinetic isotope effects. The usual kinetic studies were not of much help due to the complex nature of catalytic reactions and relatively large experimental error. The progress has been made possible also by the studies of surface acid—base properties of the solids and their meaning for catalysis (for a detailed treatment see ref. 5). 

Liquid-phase hydrogenation of 1,4 butynediol to cis-1,4-butenediol and 1,4-butanediol has been carried out on nickel catalysts supported on thirteen different supports. Some commercial nickel catalysts were used as references. Furthermore, metal loading and Ni-Cu alloying have also been studied. The results obtained indicates that catalytic activity, selectivity and metal surface area of catalysts are closely correlated to some textural and/or acid-base properties of the corresponding support. Similarly, the influence of Cu as a second metal in catalyst behaviour is also related to the nature of the support. 

Similar correlations between the acid-base properties of catalysts and activ-ity/selectivity were earlier observed in the rearrangement of simple oxiranes (refs. 5-8). In our case it seems reasonable to suppose that the observed changes are due to the different competing mechanisms discussed above. WO, with strong acidic sites in high concentration, is able to form the carbenium ion. Since the density and the strength of the basic sites on WO are low, formation of the double-bonded surface species depicted in Fig. 3 has only a low probability. The single-bonded open carbenium ion is then mainly transformed to ketone 3. In harmony with this, the isomers exhibit identical selectivity, a... 

The surface area is a function of the temperature of preparation of the oxides [169, 486]. Thus, the apparent electrocatalytic activity decreases with increasing temperature of calcination (which is usually in the range 350°-500 °C) [227, 475]. However, if the calcination is carried out at too high temperature, the electrode surface is deactivated probably as a consequence of some dehydration, and the observed Tafel slope can be very high [487], The important relationship between the acid-base properties of an oxide surface in solution and its electrocatalytic properties has been pointed out by the present author [488]. 

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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