UNIDEF   23986
UNIDAD DE INVESTIGACION Y DESARROLLO ESTRATEGICO PARA LA DEFENSA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Temperature-resolved XANES of mesoporous zirconia-ceria
Autor/es:
R.BACANI; M.F.C.A. FANTINI; T.S. MARTINS; D.G. LAMAS; S.A. LARRONDO.
Lugar:
Santa Fe
Reunión:
Encuentro; VIII Reunión Anual de la Asociación Argentina de Cristalografía; 2012
Institución organizadora:
Asociación Argentina de Cristalografía
Resumen:
Since the discovery of mesoporous silica structures in the 90’s there is much interest in mesoporous mixed oxides for catalytical applications. Ordered mesoporous ZrO2-CeO2 mixed oxides are potential candidates for anodes in solid oxide fuel cells (SOFC), aiming to achieve a lower working temperature [1]. The structural features of zirconia-ceria materials in combination with oxygen storage/release capacity (OSC) are crucial for various catalytic reactions [3-5]. The main goal of this investigation is evaluate the reduction capacity of Ce4+ to Ce3+ and Ni2+ to Ni0 in ZrO2-90%CeO2 sample impregnated with 60%NiO. The sample was prepared with 90% CeO2 (from chloride precursors), the triblock copolymer (Pluronic P-123) as a structure directing agent in acidic media (pH~3). A Teflon autoclave was used to perform a hydrothermal treatment (80ºC/48 hours). The calcination to remove the polymer was made until 540ºC in N2 atmosphere (1ºC/min), followed by the change to air, remaining at this temperature for 4 hours. The NiO impregnation was made with an ethanol dispersion of Ni(NO3).6H2O. The resultinge powder sample was calcinated in air until 350ºC for 2 hours. Temperature-resolved XANES data at the Ce LIII-edge and Ni K-edge were collected at the DXAS dispersive beam line of the Brazilian National Synchrotron Facility (LNLS) in transmission mode, using a Si(111) monochromator and a CCD camera detector. Sample preparation consisted of mixing ~6mg of the powder samples with boron nitride (BN) and pressing into pellets (one for each edge). The data were acquired during an experiment of temperature programmed reduction (TPR) under a 5% H2/He until 600ºC and mixtures of 20%CH4:5%O2/He with 2:1, 1:1 and 1:2 ratios, with total flow of 200mL/min, at temperatures from 400 to 600ºC. After each process with CH4 and O2, a TPR procedure was performed in order to evaluate the reduction capacity of the sample after reactions with methane. All the reactions were monitored with a mass spectrometer. The data was analyzed with a linear combination fit of 2 standards for each valence number using Athena software [2]. The Ni K-edge experiments demonstrated that NiO embedded in the porous zirconia-ceria matrix reduces at lower temperatures than pure NiO, revealing that the ZrO2-90%CeO2 support improves the reduction of impregnated NiO. NiO was reduced to Ni after all reactions with methane and oxygen. Hydrogenated carbonaceous species were detected suggesting that a reactivation of the Ni catalyst happened after hydrocarbon reactions under reducing atmosphere. Previous Ce LIII-edge experiments demonstrated that the Ce4+ to Ce3+ complete reduction occurs at higher temperatures [6]. The Ce3+ species reach a maximum of 39% at 600ºC, a practically constant reduction factor after all the reducing reactions with methane and oxygen. In this work, the total oxidation of methane CH4:O2 (1:2 ratio) was observed at lower temperatures (below 440ºC) for both edges