The preparation and genesis of molybdena-alumina and related catalyst systems
Review articleOpen access
Abstract:

AbstractMolybdena-, tungsta-, chromia-, and vanadia-alumina preparations were made using an equilibrium adsorption technique. The Na+ cation was also tested and other supports (titania, magnesia, and silica gel) were employed. The loading of the support was not very dependent on the solution concentration, but for anions was a strong inverse function of the pH. It is shown that this (and the opposite dependence for cations) reflected the isoelectric point of the support surface. Reflectance spectra of the wet, dried (150 °C), and calcined (500 °C) catalysts are reported for molybdena-alumina preparations made at several pH values. In agreement with our previously published Raman data, the catalyst prepared at pH = 8.6 yielded spectra of a monomeric tetrahedral species while those made at lower pH yielded spectra characteristic of octahedral (polymeric) molybdena. Thus, better defined catalysts could be prepared for further study. The data support a model for the ordinary raw catalyst made by incipient wetness from solutions of ammonium paramolybdate in which the surface of the support becomes populated with randomly distributed small clusters (or islands) of polymolybdate species.

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