Strength directed metal oxidation

FIGURE 23. Four point flexural strength as a function of temperature for various CMCs fabricated via directed metal oxidation. 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

The data determined directly by Knudsen cell measurements, plus a strong correlation between the bond strengths of metal hydroxide bonds and metal halide (in particular, chloride and fluoride bonds) in the gaseous metal hydroxides and halides were developed and allow us to more reliably estimate the enthalpy of formation of many hydroxide and oxyhydroxide metal compounds whose values of thermochemical heat and formation were previously unknown. These thermochemical properties were then used to estimate volatility of various supporting oxide substrates and catal)dically-active solids that were relevant for the fabrication of catalytic combustors. 

Our knowledge of these factors influencing reaction rates is quite elementary in many respects. Direct study is difficult in many instances, because of the complex nature of the hydrous metal oxide surface. Much of our current understanding comes from using indirect methods. One useful approach is to make small changes in reactant structure, then examine the effect on reaction rate. Another approach is to systematically examine how reactant concentration, medium composition (pH and ionic strength), and the presence of other adsorbing solutes influence reaction rate. 

Preparation of metal catalysts is an old field with few new preparative directions. Most supported metal catalysts are prepared by reducing the corresponding supported metal oxides. The metal oxides in turn are classically prepared by incipient wetness impregnation of solutions containing metal salts, followed by drying and calcination to form the oxides. Changes in the preparation method affect the interaction strength between the supported oxide... 

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