Thermal spreading of MoO3 in H–ZY

This work provides a structural, optical and kinetics approach to the molybdenum oxo-species formed during thermally driven migration on H–ZY starting from mechanical mixtures with MoO3. The samples were characterized as a function of time of treatment by UV–vis diffuse reflectance, X-ray diffraction, N2 adsorption and scanning transmission electron microscopy (STEM). Local analysis of elemental compositions obtained from linear scan of characteristic X-ray signal show a direct evidence of molybdenum presence into the zeolite crystals. Ultraviolet absorption spectra were used to determine both the kinetics of the spreading and the speciation of MoOx in the H–ZY. Besides MoO3, three surface molybdenum oxo-species were identified according to the edge energy (Eg) values of bulk molybdenum oxide reference compounds. This study shows that the tetrahedral species prevailed on H–ZY. This is consistent with limitations in the migration and growth of MoOx in the channel structure of the zeolite. Kinetic study suggest that migration of MoOx in the H–ZY at low temperature (ca. 723 K) occurs across the formation and diffusion of hydrated species such as MoO2(OH)2, which interact with the zeolite and form monomeric and dimeric structures (like (MoO4)2− and (Mo2O7)2−). Migration of MoOx species in the H–ZY studied is significant even at 723 K and after very short periods of treatment (<5 min).