Elaboração e estudo das propriedades elétricas e catalíticas dos tungstatos (Sr, Ce)WO4: aplicação para a conversão de metano

Abstract

Nanoparticles of cerium and strontium tungstate have been prepared from the method of synthesis of the ceramic powders involved the complexation combining EDTA-citrate in order to determine the catalytic activity and the electrical properties of the obtained materials. The syntheses produced strontium (SrWO4), cerium (Ce2W3O12) tungstates and a ternary tungstate composed of Sr and Ce identified with structure Sr0.5Ce0.35□0.15WO4. In the interest of characterize them, TG-DTG, XRD and MET analyzes were performed. TG-DTG analyzes determined that material with crystalline structure could be obtained from 600°C. The diffractograms indicated tetragonal crystalline structure with spatial group I41/a (N° 88) for SrWO4 and monoclinic structure belonging to space group C2/c (Nº 15) for Ce2(WO4)3. The TEM images allowed us to determine the mean crystallite size that were shown to be compatible with the results calculated from the Williamson-Hall equation. Refined XRD analysis using the Rietveld method, high-resolution transmission electron microscopy (HRTEM) images and electronic diffraction were applied to study the ternary tungstate and to define its composition and crystalline structure as tetragonal belonging to the space group I4̅(N° 82). Analyzes were conducted to determine the catalytic activity in partial oxidation of CH4 where the parameters analyzed were reaction temperature, CH4 concentration and catalyst type at three different levels. It was observed that the most relevant parameters for the CH4 oxidation reaction were the type of catalyst and the reaction temperature, respectively. The concentration was not relevant to the result and the most efficient catalyst to convert CH4 into synthesis gas (H2 + CO) was, in descending order, SrWO4, Sr0.5Ce0.35□0.15WO4 and Ce2(WO4)3. Moreover, the electrical properties analysis determined the activation energies of the samples, the result was 1.42eV, 1.16eV and 0.99eV for SrWO4, Sr0.5Ce0.35□0.15WO4 and Ce2(WO4)3, respectively.