Mixed Nanooxides

Nuclear Magnetic Resonance Studies of Interfacial Phenomena 

Structural Characteristics and the Heat of Immersion of Nanooxides in Water 

FIGURE 2.1 Relationship between the specific surface area of nanooxides and the enthalpy of immersion of them in water. 

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The structural features of mixed oxides with a mosaic surface of nanoparticles, including patches of different oxide phases or a solid solution of a lower concentrated oxide in a more concentrated oxide, can strongly affect the interfacial phenomena in any media. For instance, the enthalpy of immersion in water (Table 2.1 is greater for mixed nanooxides than nanosilica. However, surface nonuniformity and the differences in the properties of a variety of surface sites result in a significant scatter in the relationship between the and 5bex values (Figure 2.1), despite a tendency of a decrease in the AH value (calculated per surface area unit) with increasing value. 

Thus, investigations of a variety of mixed nanooxides using several methods reveal that the surface properties of SA, ST, and AST materials (such as content of SiOH, Si(OH)2, (-0)4Si, A1(V1), A1(V), and Al(IV), intensity of the FTIR bands of surface hydroxyls, heat of immersion in water, desorption temperature, and content of desorbed water) are complex functions of the specific surface area, total and surface content of all oxide components, composition of the surface (both oxide phase distributions and distributions of the Ti and A1 atoms in the silica phase and vice versa), and treatment temperature (differently affecting SA, ST, and AST samples because ternary oxides can lose hydroxyl coverage at lower temperatures than binary oxides). Many of the mentioned properties very clearly correlate with the surface content of alumina and/or titania in mixed oxides. Despite these differences, nanooxides are morphologically similar even at different specific surface areas because of features of HT synthesis of these oxides. 

Many physicochemical properties of mixed nanooxides depend not only on the surface composition but also on the morphological and structural characteristics of whole nanoparticles and then-aggregates (Figure 2.11c and d), their crystallinity, size distribution, shape, etc. The XRD patterns (Figures 2.11 and 2.12) show that the crystallinity degree of titania and alumina depends on the... 

Results in Figure 2.20 show that the FTIR determined silanol content nearly quantitatively correlates to the samples surface area. For pure silicas, a correlation of surface silanol content and surface area has been described earlier. For mixed nanooxides, at least two factors (5 bet and Cx) affect the silanol content (Figure 2.20). The value (similar to that for pure nanosilica) is the... 

A multiple-factor influence of the surface composition of mixed nanooxides is observed for such interfacial phenomenon as release of heat of immersion in water (Figures 2.44 and 2.45). For SA samples, two maxima are observed on the AH Cx) graphs, at low alumina content and for pure alumina. Both maxima correspond to maximal amounts of hydroxyls (see relative desorption of... 

FIGURE 2.46 Surface content of (a) A1 in fumed silica/alumina and (b) Ti in titania/silica and the plateau adsorption (A) of (a) Pb(II) and (b) Ni(II) as a function of the total (a) alumina and (b) titania content in mixed nanooxides. (Adapted bom Appl. Surf. Sci., 253, Gun ko, V.M., Nychiporuk, Yu.M., Zarko, V.I. et ah, Relationships between surface compositions and properties of surfaces of mixed fumed oxides, 3215-3230, 2007b, Copyright 2007, with permission from Elsevier.)... 

Mixed porous oxides can be divided into two main classes (similar to nanooxides) with mixed oxides synthesized simultaneously or matrix oxides (host oxides) modified by grafting of another oxide (guest oxide) using CVD, PVD, precipitation, or other methods. Clearly, the properties of the materials with the same gross composition but from different classes can be strongly different. Many of regularities found for mixed nanooxides can be true for porous oxides. However, the difference in the textural organization of these materials can play an important role, especially for... 

Different silicas, such as porous silica gels (Tables 4.1 through 4.4), fumed silicas with nonporous primary particles and mixed nanooxides (Table 4.5), natural minerals, and metals deposited on... 

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