Redox Chemistry of CoZSM-5 Zeolite

Abstract

The redox chemistry and the changes that occurred within the solids were studied for an excessively exchanged and a nearly stoichiometric CoZSM-5 zeolite as a function of pretreatment, reduction, and reoxidation conditions. Both gravimetric and volumetric techniques were used to determine this chemistry and the stoichiometry of the processes, which became reversible after the first couple of oxidation/reduction cycles. The catalyst was examined in its several states by diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), X-ray photoelectron spectroscoy (XPS), and both the Faraday and Gouy magnetic susceptibility methods. With the excessively exchanged sample, it was found that Co 3 O 4 formed after pretreatment with O 2 and that Co metal was produced on reduction with either CO or H 2 at 773 K in amounts corresponding to only a portion of the Co 2+ originally present. The stoichiometric sample showed evidence for the presence of Co 3+, but crystalline Co 3 O 4 was not detectable. However, two different products were formed from the original freshly exchanged (and hydrated) zeolite, depending upon whether it was dried in an inert gas or in O 2 as the temperature was raised to 773 K.