Influence of alumina dissolution on the final state of MoO_x/Al_2O_3 catalysts

摘要

This study shows that alumina dissolution controls the speciation of molybdic species deposited on the alumina surface during MoO_x/Al_2O_3 catalysts preparation by the Incipient Wetness method.One hour after the start of the aqueous impregnation step we can easily identify the formation of bulk (NH_4)_3[Al(OH)_6Mo_6O_18] with XRD.This is a clear evidence that there is an extensive reaction of molybdate ions with dissolved aluminum atoms during the preparation procedure.This phenomenon was previously demonstrated in the preparation of MoO_x/Al_2O_3 catalysts by the Equilibrium Adsorption method,and we show here that this occurs also during Incipient Wetness impregnation.In this latter case,the aluminomolybic species deposited on the support is not dispersed at the molecular level but precipitates as a bulk species.The formation of this spedcies can be avoided by a fast impregnation step followed by freezing the solid in liquid nitrogen and drying it under vacuum (freeze-drying).We also show that the nature of the catalyst obtained after calcination at 400 deg C is dependent on the nature of the molybdic speciesobtained after the initial queous impregnation step.For the catalysts obtained by the classical Incipient Wetness impregnation,we can identify bulk MoO_3 by XRD for loadings as low as 7 Mo wt