Síntese e caracterização de nanocatalisadores de ZrO2-CeO2/Ni para aplicação em ânodos de células a combustível de óxido sólido

Abstract

Mesoporous ZrO 2-CeO 2 composites are being developed due to their excellent morphological and structural properties, which are necessary for their use in several applications, including gas sensors, three way catalysts (TWC) and solid oxide fuel cells (SOFCs). In this work a new synthesis method was developed based on a template cooperative sol-gel approach, using the tri-block polymer P-123 and Zr/Ce chlorides as the oxides' precursors. Since high cerium oxide quantities lead to better catalytic performance, the ZrO 2-x(mol)%CeO 2 were synthesized with x = 50, 70 and 90. Two different calcination processes were tested (until 540 and 400 • C). NiO was impregnated in order to obtain enough electronic conductivity for their application as SOFC anodes. X-ray diffraction results showed that these systems are biphasic and crystallized preferentially into cubic fluorite type structure together with smaller quantities of the tetragonal zirconia-ceria phase. A 100% cubic phase was retained for 90% of CeO 2 after 400 • C calcination. Textural and morphological characteristics for 540 • C calcination evaluated from N 2 sorption, electronic microscopy images and small angle X-ray scattering revealed a two-density (pores/particles) random crystalline clusters of mesoporous ZrO 2-CeO 2 , with homogeneous composition, average superficial area (30-40 m 2 /g), high dispersivity of pores/particle sizes. Calcination until 400 • C presented a narrower pore size distribution and smaller pores, with higher superficial area (> 100 m 2 /g). It was observed that NiO particles formed an uniform layer over the ZrO 2-CeO 2 without filling or blocking the zirconia-ceria pores. Temperature programmed reduction experiments showed that for all ceria contents the reduction percentage of Ce 4+ species in the samples was higher and at lower temperatures (beginning of reduction at 300 • C) than standard CeO 2 (750 • C). After NiO impregnation this behavior is similar, with NiO reducing at lower temperatures (320 • C) as well. Catalytic activity for methane total oxidation reaction was similar for both calcination temperatures, for 90% CeO 2 , showing 50% of CH 4 conversion around 540 • C. Absorption X-ray in-situ experiments at Ni K-edge and Ce L III-edge showed that all ceria contents are active for total and partial methane oxidation, CH 4 decomposition and CO oxidation at 600 • C. Electrochemical impedance spectroscopy measurements showed low resistivity for higher ceria content, 0,97 Ωcm 2 at 750 • C in 5% CH 4 /3%H 2 O/N 2 atmosphere. Resuming, the material developed in this work presents the best morphological, structural, electrical and catalytical properties for applications as SOFC anode and catalyst, compared to similar materials reported in the literature.

Zircônia céria mesoporosa para células de combustível e catalisadores

“…Para as espécies inorgânicas foram utilizados precursores clorados: cloreto de zircônio para a remoção completa do polímero [52,53] .…”

“…Adicionalmente, as curvas de SAXS das amostras como sintetizadas (AS) e calcinadas (ver figuras 4.10 e 4.11) indicam que não há ordenamento induzido pelo molde. Esse resultado é coerente com o apresentado na literatura com método de síntese semelhante [52,53] . Tal situação não era esperada nessa composição, conforme o diagrama de fases da figura 1.1 e o resultado de trabalhos anteriores [52,53] .…”

“…Esse resultado é coerente com o apresentado na literatura com método de síntese semelhante [52,53] . Tal situação não era esperada nessa composição, conforme o diagrama de fases da figura 1.1 e o resultado de trabalhos anteriores [52,53] . A presença da fase tetragonal em proporções semelhantes por todas as amostras indica que a formação dos sistemas bifásicos não está relacionada ao TIPB, mas sim ao procedimento de síntese e/ou calcinação utilizado.…”