5 wt.% of Mn(NO3)2.4H2O was used as a precursor for the wet impregnation synthesis of MnOx/SiO2. After dehydration at 70 °C, a white paste was obtained and thereafter dried in an oven overnight at 90 °C. After drying, a black powder is obtained.
Why is the powder not retaining its white colour? A colour change of the supported metal nitrate precursor is often associated with metal oxide formation after calcination.
Waters of hydration and physisorbed water would be removed during calcination, under air, at around 100-150 °C, and the decomposition of the Mn(NO3)2 is reported only to occur around 300 °C.
Try drying it under inert atmosphere. If oxidation takes place, it should help.
Air Drying or Heat drying provides enough oxidation to complete the process for the formation of ur composite.. if the drying rxn does not favor ur formation or color change of composite then it means ur preparation of the composite is not the right pathway u used.
My opinion
The silica surface may catalyze the decomposition of the manganous nitrate with oxidation of Mn(2+) to some Mn(3+), forming for example black Mn3O4. If you want to prevent this "early" oxidation, working under nitrogen may help; however, oxygen is also available from the decomposition of nitrate, so I would expect some darkening to still occur.
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