Generation of WO{sub 3}-ZrO{sub 2} catalysts from solid solutions of tungsten in zirconia
- Instituto Mexicano del Petroleo. Prog. Ingenieria Molecular, Eje Central L. Cardenas 152, 07730 Mexico D.F. (Mexico)
- Institute of Physics, National University of Mexico (UNAM), A. P. 20-364, 01000 Mexico D.F. (Mexico)
WO{sub 3}-ZrO{sub 2} samples were obtained by precipitating zirconium oxynitrate in presence of WO{sub 4} {sup =} 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 W{sup 5+} ions coexisting with W{sup 6+} 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 W{sup 6+}, producing patches of WO{sub 3} 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 WO{sub 3} on the crystallite surface of tetragonal zirconia stabilized with tungsten. The second type corresponded to monoclinic zirconia crystallites with patches of nanocrystalline WO{sub 3} on their surface. The tungsten segregation gave rise to the WO{sub 3}-ZrO{sub 2} catalysts. - Graphical abstract: WO {sub x} -ZrO{sub 2} catalysts were obtained by precipitating zirconium oxynitrate in presence of WO{sub 4} {sup =}species. Initially, the W atoms remained inside the crystallite after annealing at 560 deg. C in a reduced oxidation state (W{sup 5+}), 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 ZrO{sub 2} core, with the consequent oxidation to W{sup 6+}, producing patches of nanocrystalline WO{sub 3} with dimension smaller than 3 nm.
- OSTI ID:
- 20905422
- Journal Information:
- Journal of Solid State Chemistry, Vol. 179, Issue 8; Other Information: DOI: 10.1016/j.jssc.2006.03.050; PII: S0022-4596(06)00216-7; Copyright (c) 2006 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved; Country of input: International Atomic Energy Agency (IAEA); ISSN 0022-4596
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ABSORPTION SPECTROSCOPY
ANNEALING
CRYSTALS
MONOCLINIC LATTICES
NANOSTRUCTURES
OXYNITRATES
RAMAN SPECTROSCOPY
SOLID SOLUTIONS
SURFACES
TEMPERATURE RANGE 0400-1000 K
TEMPERATURE RANGE 1000-4000 K
THERMAL ANALYSIS
TRANSMISSION ELECTRON MICROSCOPY
TUNGSTATES
TUNGSTEN
TUNGSTEN IONS
TUNGSTEN OXIDES
X-RAY DIFFRACTION
X-RAY PHOTOELECTRON SPECTROSCOPY
ZIRCONIUM OXIDES