VOx/ZrO2 samples were prepared by three methods: (i) adsorption from aqueous solutions of ammonium metavanadate (AV) at pH 1 or 4; (ii) dry impregnation with AV aqueous solutions at pH 4; and (iii) adsorption from solutions of vanadyl acetylacetonate in toluene. The catalysts were characterized as prepared (a.p.), after evacuation at increasing temperature up to 773 K, after heating in dry oxygen at 773 K (standard oxidation, s.o.), and after reduction in H2 (or CO), by means of XPS, ESR, and FT-IR spectroscopies. The morphology of pure ZrO2 and VOx/ZrO2 particles was investigated by HRTEM and XRD analysis. Both ZrO2 and VOx/ ZrO2 samples with V content e 3.5 atoms nm-2 showed only zirconia particles with a monoclinic structure. Samples with higher V content contained segregated vanadium phases (V2O5 and ZrV2O7). On a.p. and s.o. samples with surface V content e 3.5 atoms nm-2, XPS showed vanadium species, uniformly spread on the zirconia surface. Depending on the V content, on all reduced samples ESR detected mononuclear VIV in a square pyramidal configuration, and magnetically interacting VIV. 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 decavanadates for V content in the range 1.5-3.5 atoms nm-2. Heating in oxygen 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 atoms nm-2), low-nuclearity polyvanadates (prevalent in samples with V content 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 method, but on the vanadium content.
Characterization of oxidized surface phase on VOx/ZrO2 catalysts
PRINETTO, Federica;GHIOTTI, Giovanna;
1998-01-01
Abstract
VOx/ZrO2 samples were prepared by three methods: (i) adsorption from aqueous solutions of ammonium metavanadate (AV) at pH 1 or 4; (ii) dry impregnation with AV aqueous solutions at pH 4; and (iii) adsorption from solutions of vanadyl acetylacetonate in toluene. The catalysts were characterized as prepared (a.p.), after evacuation at increasing temperature up to 773 K, after heating in dry oxygen at 773 K (standard oxidation, s.o.), and after reduction in H2 (or CO), by means of XPS, ESR, and FT-IR spectroscopies. The morphology of pure ZrO2 and VOx/ZrO2 particles was investigated by HRTEM and XRD analysis. Both ZrO2 and VOx/ ZrO2 samples with V content e 3.5 atoms nm-2 showed only zirconia particles with a monoclinic structure. Samples with higher V content contained segregated vanadium phases (V2O5 and ZrV2O7). On a.p. and s.o. samples with surface V content e 3.5 atoms nm-2, XPS showed vanadium species, uniformly spread on the zirconia surface. Depending on the V content, on all reduced samples ESR detected mononuclear VIV in a square pyramidal configuration, and magnetically interacting VIV. 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 decavanadates for V content in the range 1.5-3.5 atoms nm-2. Heating in oxygen 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 atoms nm-2), low-nuclearity polyvanadates (prevalent in samples with V content 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 method, but on the vanadium content.
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