ABSTRACT
Basic Sites on the Ideal Surface of a Solid Oxide . . . . . . . . . . . . . . 253 9.3 Techniques for the Characterization of Surface
Acidity and Basicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 9.3.1 Molecular Probes for Surface Acidity and Basicity
Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 9.3.2 Quantitative Adsorption of Probe Molecules from Gas and
Liquid Phases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 9.3.3 Calorimetric Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 9.3.4 Temperature Programmed Desorption Methods . . . . . . . . . . . . . . . . 258 9.3.5 IR Spectroscopic Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 9.3.6 Nuclear Magnetic Resonance (NMR)
Spectroscopic Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 9.3.7 Theoretical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 9.3.8 Catalytic Probe Reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265
9.4 Composition Related Surface Properties of Oxide Materials . . . . . . . . . . . 266 9.4.1 Lewis Acid Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266
“DK3029_C009” — #2
9.4.2 Density of Lewis Sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268 9.4.3 Lewis Acidity and Basicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 9.4.4 Brønsted Acidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269
9.5 Structure-Related Surface Properties of Metal Oxides. Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 9.5.1 The Aluminas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272
9.5.1.1 Structure-related and morphology-related surface properties of aluminas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272
9.5.1.2 The “active sites” of aluminas . . . . . . . . . . . . . . . . . . . . . . . . 276 9.5.1.3 Impurity related surface properties of aluminas . . . . . . 279
9.5.2 The “Mixed Oxides” of Silicon and Aluminum: Composition and Structure Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . 280 9.5.2.1 The Brønsted acidic sites of protonic zeolites . . . . . . . . 282 9.5.2.2 The acidic sites of silica-aluminas . . . . . . . . . . . . . . . . . . . . 287 9.5.2.3 The external surface of protonic zeolites . . . . . . . . . . . . . 290 9.5.2.4 Extraframework material in protonic zeolites . . . . . . . . 291
9.5.3 Cationic Zeolites and Other Metal-Containing Zeolite-Like Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293
9.5.4 Sulfated and Tungstated Zirconia: True Superacids? . . . . . . . . . . . 296 9.5.5 Solid Basic Catalysts: Oxides Containing Alkali,
Alkali-Earth, and Rare Earth Cations . . . . . . . . . . . . . . . . . . . . . . . . . . . 298 9.6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300
Powders constituted by metal oxides are relevant products of the inorganic chemical industry and find a number of applications, as such or as precursors of dense sintered ceramics. Surface acid-base interactions are involved in many aspects. They are not only fundamental phenomena involved in heterogeneous catalysis [1] and adsorption [2], but are also of interest in pigment technologies [3] as well as in particle sintering [4], to produce bulky ceramics for electronic, magnetic, and optical applications. For these reasons, the acid-base properties of oxide powders have been the object of many investigations. Some comprehensive papers have been published in the last 10 years [5-9]. “Metal oxides” are a wide family of materials that can differ significantly from the point of view of their structure. In Table 9.1, some industrially applied metal oxide catalysts are summarized to provide evidence for the significant difference of their structure.
