Characterization of oxidized surface phases on VOx/ZrO2 catalysts

VOx/ZrO2 samples were prepared by three methods: (i) adsorption from aqueou s solutions of ammonium metavanadate (AV) at pH 1 or 4; (ii) dry impregnati on with AV aqueous solutions at pH 4; and (iii) adsorption from solutions o f vanadyl acetylacetonate in toluene. The catalysts were characterized as p repared (a.p.), after evacuation at increasing temperature up to 773 K, aft er heating in dry oxygen at 773 K (standard oxidation, s.o.), and after red uction in H-2 (or CO), by means of XPS, ESR, and FT-IR spectroscopies. The morphology of pure ZrO2 and VOx/ZrO2 particles was investigated by HRTEM an d XRD analysis. Both ZrO2 and VOx/ZrO2 samples with V content less than or equal to 3.5 atoms nm(-2) showed only zirconia particles with a monoclinic structure. Samples with higher V content contained segregated vanadium phas es (V2O5 and ZrV2O7). On a.p. and s.o. samples with surface V content 3.5 a toms nm(-2), XPS showed vanadium species, uniformly spread on the zirconia surface. Depending on the V content, on all reduced samples ESR detected mo nonuclear V-IV in a square pyramidal configuration, and magnetically intera cting V-IV. On a.p. samples, FT-IR spectra showed the presence of vanadate or metavanadate-type species for V content up to 1.5 atoms nm(-2) and of de cavanadates for V content in the range 1.5-3.5 atoms nm(-2). Heating in oxy gen at 773 K led to a variety of vanadium species anchored to the zirconia surface: isolated vanadates (prevalent in samples with V content < 0.2 atom s nm(-2)), low-nuclearity polyvanadates (prevalent in samples with V conten t up to 1.5 atoms nm(-2)), and high-nuclearity polyvanadates (prevalent in more concentrated samples, up to 3.5 atoms nm(-2)). The relative amount of surface species on s.o. samples mainly depended not on the preparation meth od, but on the vanadium content.