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Doctoral Thesis
DOI
10.11606/T.18.2018.tde-11072018-091613
Document
Author
Full name
Lorena Dariane da Silva Alencar
Institute/School/College
Knowledge Area
Date of Defense
Published
São Carlos, 2018
Supervisor
Committee
Bernardi, Maria Inês Basso (President)
Jesus, Lilian Menezes de
Mesquita, Alexandre
Salomão, Rafael
Vicente, Fabio Simões de
Title in English
Characterization of BaMoO4, BaWO4, CaWO4 and CaMoO4 compounds obtained by polymeric precursor method and by microwave-assisted hydrothermal method
Keywords in English
EXAFS
Molydbate
Toluene
Tungstate
XANES
Abstract in English
Molybdates and tungstates belonging to the scheelite family constitute an important class of materials, which have advantages as a relatively low cost and being non-polluting. Barium molybdate (BaMoO4), barium tungstate (BaWO4), calcium molybdate (CaMoO4) and calcium tungstate (CaWO4) have been extensively studied due their photoluminescent properties, besides that they also present catalysis and photocatalysis applications. However, to the best of our knowledge there are no structural characterizations of BaMoO4, BaWO4 and CaMoO4 by x-ray absorption spectroscopy (XAS) in the literature. In this work, powders of these 4 compounds were prepared by microwave-assisted hydrothermal (MAH) method and polymeric precursor method (PPM) and their structural properties were characterized by X-ray diffraction (XRD), X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. The morphology and particle size of these crystalline powders were observed by field emission scanning electron microscopy (FE-SEM). Furthermore, BaMoO4, BaWO4 and CaWO4 were employed as solid catalysts towards gas phase toluene oxidation reactions and their optical properties were investigated by ultraviolet visible (UV-Vis) absorption and photoluminescence (PL) measurements. XRD patterns confirm the phase purity of materials from both preparation methods and reveal a preferential growth when the powders are prepared by MAH due polymeric agents and processing using microwave, which was confirmed by FE-SEM. XANES and EXAFS results show that the preparation method did not introduce high disorders into the structure, however the H2 Temperature-Programmed Reduction (H2-TPR) measurements indicated that the catalyst reducibility is affected by the preparation method of the samples. PL emissions were attributed to the charge-transfer transitions within the [WO4]2- and [MoO4]2- complexes.
Title in Portuguese
Caracterização dos compostos BaMoO4, BaWO4, CaWO4 e CaMoO4 obtidos pelos métodos dos precursores poliméricos e hidrotermal assistido por micro-ondas
Keywords in Portuguese
EXAFS
Molibdato
Molybdate
Toluene
Tolueno
Tungstate
Tungstato
XANES
Abstract in Portuguese
Os óxidos molibdatos e tungstatos, pertencentes a família das scheelitas, constituem urna importante classe de materiais que apresentam a vantagem de possuem relativo baixo custo e não serem poluentes. Molibdato de bário (BaMoO4), tungstato de bário (BaWO4), molibdato de cálcio (CaMoO4) e o tungstato de cálcio (CaWO4) tern sido extensivamente estudados devido as suas propriedades fotoluminescentes, além de apresentarem aplicações em catálise e fotocatálise. No entanto, não foi encontrada na literatura caracterizações estruturais de BaMoO4, BaWO4 e CaMoO4 por espectroscopia de absorção de raios X (XAS). Neste trabalho, partículas destes quatro compostos foram preparados pelo método hidrotermal assistido por micro-ondas (MAH) e método dos precursores poliméricos (PPM). Suas propriedades estruturais foram caracterizadas por difração de raios X (XRD) e espectroscopia de absorção de raios X na região XANES (do inglês X-Ray Absorption Near Edge Structure) e região EXAFS (do inglês Extended X-Ray Absorption Fine Structure). A morfologia e o tamanho de partícula desses pós cristalinos foram observados por microscopia eletrônica de varredura por emissão de campo (FE-SEM). Além disso, BaMoO4, BaWO4 e CaWO4 foram empregados como catalisadores sólidos para as reações de oxidação de tolueno em fase gasosa e as suas propriedades ópticas foram investigadas por medidas de absorção no ultravioleta/visível (UV-Vis) e fotoluminescência (PL). Os padrões XRD confirmam a pureza de fase dos materiais obtidos em ambos os métodos de preparação e revelam um crescimento preferencial dos pós preparados por MAH devido aos agentes poliméricos e ao processamento usando micro-ondas, esse crescimento foi confirmado pelas micrografias obtidas por FE-SEM. Os resultados de XANES e EXAFS mostram que o método de preparação não introduz desordens elevadas na estrutura, no entanto, as medidas de redução à temperatura programada (H2-TPR) indicaram que a redução do catalisador e afetada pelo método de preparação das amostras. As emissões de PL foram atribuídas às transições de transferência de carga dentro dos complexos [WO4]2- e [MoO4]2-.
 
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Publishing Date
2018-12-19
 
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