Solid surfaces, acid-base character oxides

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 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. 

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, calorimetric measurements can also be used to monitor adsorption phenomena on the surface of solid catalysts in contact with a liquid phase (in a solvent). For example, the so-called cal-ad method [30-33] has been used to measure the adsorption heats evolved upon addition of dilute solutions of pyridine in -hexane to a solid acid catalyst (TS, H-ZSM-5) in a slurry wifli -hexane. The amount of free base in solution is measured separately using a UV-Vis spectrophotometer [30,31]. A similar technique has been used to determine the acidic character of niobium oxide and niobium phosphate catalysts in different solvents [34,35], using aniline and 2-phenyl-ethylamine as probe molecules. 

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 ... 

---