Solid surfaces, acid-base character basic

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 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]. 

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. 

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... 

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 ... 

---