Solid surfaces, acid-base character

This chapter begins with a general discussion about acid-base character of solid surfaces. It continues with the information gained from microcalorimetry... 

In the next, more details on solid surface dynamics including acid-base character and energetic distribution will be discussed in the study of gas chromatographic measurement dealing with the gas-solid intermolecular interaction. 

In the next, the values obtained from the intermolecular function state are carefully discussed in terms of the acid-base character (or adsorption enthalpy) and the energetic distribution function of a solid surface. 

Meanwhile, in order to determine the specific component, — AG P, in Eq. (50), which causes the acid-base character of a solid surface, three methods, including the method of intermolecular function state based on the polarizability of molecules, as just described above, are generally considered ... 

In the gas-solid interaction studies at infinite dilution, it is found that the surface free energy and surface enthalpy (or adsorption enthalpy including acid-base character) of a solute are strongly influenced by the most active sites on the solid surface. The surface free energy and surface enthalpy have been applied as a typical value of the surface, which can be directly related to the adhesion level [70]. 

As described above, immersion calorimetry constitutes a powerful technique for the textural and chemical characterization of porous solids. In the absence of specific adsorbate-adsorbent interactions, heats of immersion can be related to the surface area available for the molecules of the liquid. However, the use of polar molecules or molecules with functional groups produces specific adsorbent-adsorbate interactions related to the surface chemical properties of the solid. An adequate selection of the immersion liquid can be used to study hydrophilicity, acid-base character, etc. Table 2 reports the enthalpies of immersion (J/g) into different lineal and branched hydrocarbons (n-hexane, 2-methyl-pentane and 2,2-dimethyl-butane) for Zn exchanged NaX zeolites. 

Surface characterization is needed to answer the question of the parameters that favor such a behavior. As we mentioned just before, we think that that acid/base character is the main responsible of the modifications carried by a solid. 

A more efficient method for adjusting the surface acidity of solids is chemical modification of the outermost atomic layers. Connell and Dumesic [71] showed that acid centers (both Lewis and Bronsted) could be generated by doping small amounts (less than 1%) of a second metal cation on the surface of several oxides with various acid-base character, ranging from the very basic (MgO) to acidic... 

The thermochemical techniques most commonly used to investigate the acid/base character of solid surfaces are DTA, TG, DTG, DSC, and calorimetry. These techniques can be used either by themselves, or combined with other techniques (for instance, TG-DSC, calorimetry-volumetry, DSC-chromatography, etc.) [10]. 

The first part of the book examines the crystal and electronic structure, stoichiometry and composition, redox properties, acid-base character, and cation valence states, as well as new approaches to the preparation of ordered TMO with extended structure of texturally defined systems. The second part compiles practical aspects of TMO applications in materials science, chemical sensing, analytical chemistry, solid-state chemistry, microelectronics, nanotechnology, environmental decontamination, and fuel cells. The book examines many types of reactions — such as dehydration, reduction, selective oxidations, olefin metathesis, VOC removal, photo- and electrocatalysis, and water splitting — to elucidate how chemical composition and optical, magnetic, and structural properties of oxides affect their surface reactivity in catalysis. 

Figure 6.11 illustrates the Lewis acid-base character of the solid surfaces presented in Fig. 6.10. The Lewis acid hA and Lewis base components are related to the...

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. 

CO2 is a poor donor but a good electron acceptor. Owing to its acidic character, it is frequently used to probe the basic properties of solid surfaces. IR evidence concerning the formation of carbonate-like species of different configurations has been reported for metal oxides [31], which accounts for the heterogeneity of the surface revealed by micro-calorimetric measurements. The possibility that CO2 could behave as a base and interact with Lewis acid sites should also be considered. However, these sites would have to be very strong Lewis acid sites and this particular adsorption mode of the CO2 molecule should be very weak and can usually be neglected [32]. 

The acid-base properties of solid oxide surfaces with ionic character are controlled mostly by electrostatic factors. The stronger the ionic character, the stronger is the electric field pointing outward from the surface. The electric field of the surface may enhance (or diminish) the local electron donor (or acceptor) properties... 

Chemisorption [114] on an oxide surface differs significantly from that on metals. One of the main reasons for this difference is the ionic character of the solid, which favors acid-base or donor-acceptor reactions. Lewis sites are localized on the cations and basic sites on the anions. An example of this type of interaction is given by CO2, which reacts with basic to give a surface carbonate COj . Similarly, a donor molecule such as H2O or NH3 can be molecularly adsorbed via its lone-pair electrons, which react with an acidic (cation) site. An alternative to the molecular adsorption is that resulting from the heterolytic dissociation of the molecule. It may occur by abstraction of H atom transferred to a basic site, producing a hydroxyl group. 

Finally, we complete the discussion on polymer adsorption with the Lewis acid-base concept, which is a useful tool for evaluating the relative adsorption of polymers versus solvents on the same solid surface (Figure 7.11). The relative interactions of polymer-solid, polymer-solvent and solvent-solid must be accounted for in the case of adsorption of polymers from solutions on solids. It is important at first to know whether these compounds (sohd surface, solvent, polymer) have an acidic or basic character. Notice (Figure 7.11) that very acid solvents compete (with the acidic surface) for the basic polymer and very basic solvents compete (with the basic polymer) for the acidic surface. It seems that in many cases, solvents of balanced acidity/basicity (i.e. not very acid, not very basic, almost neutral) are the best choices for accomplishing maximum adsorption. 

Commonly the distinction between hydrophobic and hydrophilic substances is based on the analysis of interactions between their molecules and water as a solvent. A more precise classification of liquid and solid substances as hydrophobic and hydrophilic may be constructed basing on the apolar (LW) and polar (AB) components of their surface tensions. This three-parameter approach is of great importance for the understanding of surface behaviour [22,32,39,40]. A non-metallic substance is hydrophobic if it interacts with water by exhibiting only LW character. It has very little (or none at all) Lewis acid or Lewis base character. Typical substances at the hydrophobic end have low y surface parameters and their and components are equal to zero. Hydrophilic substances have non-zero y components of the surface tension and at least one of their y and y parameters is significant. 

One typical TPD experiment is designed in a way to enable the pre-treatment of the sample, in situ the admission of specific adsorbate (probe) up to some specific surface coverage or up to the saturation and subsequently, desorption which is performed in a temperature-controlled regime. Many different chemical species can be used as probes if a chosen probe can titrate acid or basic sites at the surface, TPD can be used for the characterization of acidity/basicity of some adsorbent. In fact, temperature-programmed desorption and adsorption calorimetry are most commonly used for the study of acid/base properties of solid materials [20, 35, 36]. The same probe molecules that are used for adsorption calorimetry experiments are applicable in the case of TPD while the investigation of acidic/basic character of solids is perhaps... 

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