Alumina, hydroxylation state

More recently, it was found that the hydroxylation state of the alumina has a determining influence upon the particle size of platinum crystallites. Controlled dehydroxylation of the alumina proved to be the most reliable method for preparing stable catalysts with gradual variations of the dispersion, and a continuous series of Pt/Al2O3 catalysts with dispersions (characterized by the H/Pt ratio) ranging from 0.04 to 1.0 was prepared by... 

Different approaches to the kinetics of alcohol dehydration were attempted by two groups of authors [118,119]. In one case, it has been assumed that the active surface of alumina is formed either by free hydroxyl groups or by surface alkoxyl groups. The rate equation was then derived on the basis of the steady-state assumption a good fit to the experimental data was obtained [1118]. The second model was based on the fact that water influences the adsorption of an alcohol and diminishes the available surface. The surface concentrations of tert-butanol and water were taken from independent adsorption measurements and put into the first-order rate equation a good description of integral conversion data was achieved [119]. 

The steric requirements of the surface during the formation of the adsorption complex or transition state also manifest themselves in the dehydration of rigid alcohols with fixed conformations, e.g. of cyclic alcohols. Cis- and trans-2- and 4-alkylcyclohexanols differ markedly in their rate of dehydration on alumina (see Table 5). Most significant are the data on 4-tert-butylcyclohexanols where the bulky ferf-butyl group is in an equatorial position, and thus the differences in the reactivity of the cis and trans isomers indicate the differences in the reactivity of axial and equatorial hydroxyls. The high reactivity of cis-2-terf-butylcyclohex-anol is caused most probably by steric acceleration of the elimination, which is, however, absent in the case of 2,2-dimethylcyclohexanol. 

Abrasive particles are a key component in CMP slurry. The most commonly used abrasive particles include silica, alumina, ceria, zirconia, titania, and diamond. Table 21.1 listed a set of information on each type of abrasive particles such as density, microhardness, and isoelectric points (lEP). It is important to point out that the specific values for these properties depend highly on the preparation techniques and the specific states of the samples. The values listed in the table represent an average of the most commonly reported data. For example, the isoelectric point for silica is a function of the number of hydroxyl groups, type and level of adsorbed species, metal impurity in the solid matrix, and the treatment history of the materials [1]. There are three major types of silica according to their preparation methods fumed, colloidal, and precipitated. The common sources for obtaining these abrasive particles are listed in Table 21.2. As examples, some of the more specific information on... 

Phosphorus oxo-species adsorbed on alumina are present in a well-dispersed state up to a surface density of 2.9 x 10 P atom/pm (or 2.9 P atom/nm ) 31). IR spectroscopy measurements allowed identification of several types of hydroxyl groups on the y-alumina surface, such as type la (tetrahedral Al —OH, 3780 cm ), type lb (octahedral Al —OH, 3795 cm ), types 11a and Ilb (bridged OH between two Al atoms, 3736... 

Substitution of such ions in the surface or bulk of alumina may be beneficial in several ways. Firstly, as suggested by Johnson [24] the hydroxyl surface concentration may be reduced with a subsequent reduction of inter-particle bridging. Secondly, the foreign ion may occupy a vacancy in the alumina lattice, thereby reducing solid state diffusion, Finally, the production of a new compound - either at the surface or in the bulk - may result in greater support stability. 

Because of the high basicity of MgO, magnesium hydroxyls are not affected by CO (114) (Table 2.9). The Av(OH) measured for the hydroxyls on ZnO is very small (263). Some hydroxyls on chromia are not acidic others demonstrate weak acidity (114). Evidently, the oxidation state is very important, as demonstrated by analysis ofCr/Zr02 (264). Higher acidity is measured for OH groups on covalent oxides (bulk and supported) including vanadia (216,282,283), tungsten oxide (280,281), and niobia (279). Like alumina, other oxides demonstrate enhanced acidity after sulfation... 

For example, the activity of alumina for n-butylene isomerization shows a positive trend with increasing total integrated intensity of the OH stretching band of the surface hydroxyl groups and a negative trend with increasing sodium content (as derived by chemical analysis) (169). As already stated, the sodium content is critical for aluminas appHed for the Claus reaction, because sodium enhances the basicity and thus promotes SO2 adsorption, but it also decreases the reactivity of the adsorbed sulfite species (225). 

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