Silicas amorphous, transition metal ions

XII. Transition Metal Ions Supported on Amorphous Silica ... 

In general, divalent transition-metal ions can be applied using urea DP. For trivalent ions hydrolysis may take place at pH values that are too low, thus preventing the interaction with sihca that typically is favored at pH > 5. Mul and Hirschon [28], however, successfully applied La(III)oxide onto silica by injection of a NH4OH solution starting at pH = 4 and ending at pH = 10. From the pH measurements an interaction between La(OH)3 and Si02 could he inferred, while XRD showed the La phase to he amorphous. 

IR-spectra of the transition metal containing mesoporous silicas are similar to that of amorphous silica. Only an supplementary absorption band at around 940 nm is present in all the spectra of the mono- and bimetallic incorporated samples but its intensity is variable. This band can be attributed to the M-O-Si bond and often used as a prove of the incorporation of metal ions in the tfamework of silicas. 

Alumina is frequently used as a carrier for metals, preferably in the y-modification. Under the prolonged effect of temperature, a transition into the a-modification is possible. Likewise, amorphous silica carrier can evolve into the crystalline form by the effect of temperature or of foreign substances such as gases, or the presence of impurities, such as sodium ions which catalyze nucleation. 

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