FORMATION OF THE MO-VI SURFACE PHASE ON MOOX ZRC2 CATALYSTS/

The characterization (chemical analysis, XRD, TEM, HTREM, XPS, ESR, an d IR) of MoOx/ZrO2 is reported. The samples were prepared by (i) equil ibrium 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 in creasing temperature, or after heating in O-2 at 773 K (s.o.). With di lute 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) y ielded samples richer in Mo (high loading). No differences due to the preparation method were found at low loading. XPS showed Mo-VI only, u niformly spread on ZrO2. On evacuation of a.p. samples up to 423 K, su rface molybdates lost water from their outer coordination sphere. At t his stage, interaction with the surface was still weak. After evacuati on at 573 K, Mo species anchored to the surface. On evacuation at 773 K, ESR and IR showed reduction, and the subsequent titration with O-2 suggested the presence of Mo-IV in addition to Mo-V. ESR, titration wi th 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 show ing a strong anchorage of molybdates to the surface. The IR results fo r the low-loading samples showed low nuclearity molybdates (possibly m ononuclear) and polymolybdates. As the Mo content increased, the low-n uclearity species concentration increased slightly, whereas the polymo lybdate concentration increased markedly. Samples prepared by equilibr ium adsorption or impregnation, but having a comparable Mo concentrati on, had the same (molybdates):(polymolybdates) ratio. For high-loading samples, after s.o., IR showed the presence of other bands probably a rising from MoO3 and ZrMo2O8. These two compounds were revealed by the XRD and HRTEM analysis.