Structural evolution of WO 3 nanoclusters on ZrO 2
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Structural evolution of WO3 nanoclusters on ZrO2 C. Angeles-Chavez, M.A. Cortes-Jácome, E. Torres-Garcia, and J.A. Toledo-Antonioa) Instituto Mexicano del Petróleo. Prog. Ingeniería Molecular, 07730 México D. F, México (Received 12 July 2005; accepted 8 December 2005)
Direct evidence of the transformation of WOx species in WO3 nanoclusters on WOx–ZrO2 system was achieved by high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy on samples obtained by a conventional precipitation method and annealed from 560 to 800 °C. WO3 Nanoclusters with 2-nm crystal size orthorhombic structure were identified on the ZrO2 surface after annealing at 800 °C.
Tungsten oxide dispersed on zirconia in WO3–ZrO2 system is one of the more stable mixed oxides with strongly acidic properties, which are of interest in catalytic processes.1,2 The high acid site density has been explained by assuming that zirconia crystallites are covered with a WOx monolayer.3–5 However, direct evidence of the structure and nature of these species on the ZrO2 surface is scarce. In this work, experimental evidence of the growth of WOx species incorporated into the tetragonal zirconia phase into well-dispersed WO3 nanoclusters on ZrO2 surface was obtained by high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. WOx–ZrO2 samples were prepared by precipitation method as was reported previously.6 An aqueous 0.007 M solution of zirconia oxynitrate, ZrO(NO3)2⭈6H2O (99%, Aldrich, Milwaukee, WI) and ammonium metatungstate hydrate, (NH4)6W12O39⭈xH2O (99%, Aldrich) solution 0.007 M was precipitated at pH ⳱ 9.5 to 10 with a solution of ammonium hydroxide 14 vol% NH4OH. The obtained white slurry was aged for 24 h at room temperature. The obtained gel was dried at 110 °C for 18 h. The dried powders were annealed at 560, 700, and 800 °C for 4 h in air flow. Bulk tungsten content was 11.6 wt% determined by atomic absorption spectroscopy in the sample after annealing at 800 °C. The x-ray diffraction (XRD) patterns were recorded at room temperature with Cu K␣ radiation in a Bruker Advance D-8 diffractometer (Karlsruhe, Germany). High-resolution (HR)
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0100 J. Mater. Res., Vol. 21, No. 4, Apr 2006
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transmission electron microscopy (TEM) images were performed in a field emission gun (FEG) JEOL 2010F (Tokyo, Japan) transmission electron microscope operating at 200 kV and chemical analysis by x-ray energy dispersive spectroscopy (EDX) was performed in a TEM Tecnai F30 Super Twin (Eindhoven, Netherlands). The Raman spectra were recorded at room temperature using a Jobin Yvon Horiba spectrometer (T64000, Lille, France), equipped with a confocal microscope (Olympus, BX41, Lille, France) with a laser 514.5 nm at a power level of 30 mW. The spectrometer is equipped with a charge-coupled detector (CCD). XRD patterns showed tetragonal phase for the samples calcined at 560 and 700 °C, while
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