NiWO 4 was prepared using the polymeric precursor method and studied in terms of physical and chemical properties to verify its stability for industrial applications as pigments. The characterization was accomplished using thermal analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) and UV-Vis spectroscopies, colorimetric coordinates, and Raman spectra. Increasing the temperature, successive exothermic reactions were observed and they are related with thermal decomposition of the organic compound. The stability was reached at ~700 ℃. The material is verified to become completely free of second phase at ~800 ℃. The end NiWO 4 powders showed an intense charge transfer (CT)-related tail centered in the ultraviolet region, resulting in a yellow product. In addition, the powders exhibited broad excitation band and broad deep blue-green emission band, which were enhanced with increasing powders' crystallinity.
Barium molybdate and Barium tungstate are important materials due their photoluminescent properties and they also have catalysis and photocatalysis applications. In this work, powders of these compounds were prepared by microwave-assisted hydrothermal (MAH) method and polymeric precursor method (PPM) and their structural and optical properties were studied. Furthermore, these materials were employed as solid catalysts towards gas phase toluene oxidation reactions. X-ray diffraction confirms the purity of materials at both preparation methods and reveals a preferential growth when the powders are prepared by MAH due polymeric agents and processing using microwave, which was confirmed by Field emission scanning electron microscopy. Photoluminesce emission was attributed to the charge-transfer transitions within the [WO 4 ] 2and [MoO 4 ] 2complexes. The H 2 Temperature-Programmed Reduction (H 2 -TPR), O 2 -chemisorption and extended X-ray absorption fine structure (EXAFS) results indicated that BaWO 4 samples, compared with BaMoO 4 samples, have higher oxygen mobility and oxygen vacancies that appear as key factors for the achievement of better catalytic performances.
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