Generation of WO3-ZrO2 catalysts from solid solutions of tungsten in zirconia

WO3-ZrO2 samples were obtained by precipitating zirconium oxynitrate in presence of WO4= species in solution from ammonium metatungstate at pH=10.0. Samples were characterized by atomic absorption spectroscopy, thermal analysis, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and energy filtered-TEM. The ammonia retained in the dried sample produced a reductive atmosphere to generate W5+ ions coexisting with W6+ ions to produce a solid solution of tungsten in the zirconia lattice to stabilize the zirconia tetragonal phase when the sample was annealed at 560 deg. C. When the sample was annealed at 800 deg. C, the W atoms near crystallite surface were oxidized to W6+, producing patches of WO3 on the zirconia crystallite. The HR-TEM analysis confirmed the existence of the solid solution when the sample was annealed at 560 deg. C, and two types of crystalline regions were identified: One with nearly spherical morphology, an average diameter of 8 nm and the atomic distribution of tetragonal zirconia. The second one had a non-spherical morphology with well-faceted faces and dimensions larger than 30 nm, and the atom distribution of tetragonal zirconia. When samples were annealed at 800 deg. C two different zirconia crystallites were formed: Those where only part of the dissolved tungsten atoms segregated to crystallite surface producing patches of nanocrystalline WO3 on the crystallite surface of tetragonal zirconia stabilized with tungsten. The second type corresponded to monoclinic zirconia crystallites with patches of nanocrystalline WO3 on their surface. The tungsten segregation gave rise to the WO3-ZrO2 catalysts. - Graphical abstract: WO x -ZrO2 catalysts were obtained by precipitating zirconium oxynitrate in presence of WO4=species. Initially, the W atoms remained inside the crystallite after annealing at 560 deg. C in a reduced oxidation state (W5+), whereas, the sample annealed at 800 deg. C, the W atoms migrate from the bulk to the surface, forming a layer of W atoms on a ZrO2 core, with the consequent oxidation to W6+, producing patches of nanocrystalline WO3 with dimension smaller than 3 nm