Solid surfaces, acid-base character Lewis acidity

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 molecules impinging on the surface can have either a Lewis base or Lewis acid character. For instance, CO with a filled orbital at the carbon end displays Lewis basicity, while CO2 shows Lewis acidity [35]. The primary interaction is strictly localized on oxides, and surface adducts are formed with the acidic or basic centers of the solid. An example is the nondissociative adsorption of CO on exposed cationic centers on oxide surfaces ... 

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... 

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. 

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...

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. 

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