Formation of the MoVI Surfade Phase on MoOX/ZrO2 Catalysts

The characterization (chemical analysis, XRD, TEM, HTREM, XPS, ESR, and IR) of MoOx/ZrO2 is reported. The samples were prepared by (i) equilibrium adsorption (ammonium heptamolybdate solution, AHM, at pH = 1, 2, or 8), (ii) dry impregnation, or (iii) mechanical mixing of ZrO2 and MoO3. Samples were studied as prepared (a.p.), after evacuation at increasing temperature, or after heating in O2 at 773 K (s.o.). With dilute AHM (0.002 M), equilibrium adsorption led to a plateau of about 5 Mo atoms nm-2 (low loading). More concentrated solutions (0.05 M) yielded samples richer in Mo (high loading). No differences due to the preparation method were found at low loading. XPS showed Mo-VI only, uniformly spread on ZrO2. On evacuation of a.p. samples up to 423 K, surface molybdates lost water from their outer coordination sphere. At this stage, interaction with the surface was still weak. After evacuation at 573 K, Mo species anchored to the surface. On evacuation at 773 K, ESR and IR showed reduction, and the subsequent titration with O2 suggested the presence of Mo-IV in addition to Mo-V. ESR, titration with O-2, and LR showed that, in a subsequent treatment in vacuo at 773 K, species anchored to the surface by s.o. were not reduced, thus showing a strong anchorage of molybdates to the surface. The IR results for the low-loading samples showed low nuclearity molybdates (possibly mononuclear) and polymolybdates. As the Mo content increased, the low-nuclearity species concentration increased slightly, whereas the polymolybdate concentration increased markedly. Samples prepared by equilibrium adsorption or impregnation, but having a comparable Mo concentration, had the same (molybdates):(polymolybdates) ratio. For high-loading samples, after s.o., IR showed the presence of other bands probably arising from MoO3 and ZrMo2O8. These two compounds were revealed by the XRD and HRTEM analysis.