In-situ studies of the redox behavior of O60wt.%)/Ce0.9Zr0.1O2 nanopowders

M.G. ZIMICZ; R.J. PRADO; D.G. LAMAS; LARRONDO S.A.

Santa Fe

Encuentro; VIII Reunión Anual de la Asociación Argentina de Cristalografía; 2012

Asociación Argentina de Cristalografía

The development of catalysts with high catalytic activity in methane total and partial oxidation has great interest in the development of anode materials for IT-SOFCs. The synthesis and the high catalytic activity, selectivity and stability of Ce0.9Zr0.1O2 (ZDC) oxides obtained by glycine-nitrate combustion route were reported in previous studies [1, 2]. Solids with a 60wt% NiO content (ZDC6) were prepared by incipient impregnation of ZDC powders with ethanol solutions of Ni(NO3)2.6H2O of the appropriate concentration, followed by drying and 2hs calcination at 1000°C. This temperature is used to fix the anode material during IT-SOFCs anode manufacturing process. In this work we present conventional lab-scale catalytic studies, performed in a fixed bed reactor, and in-situ XANES experiments to study the percentage of reduction of Ce4+ and Ni2+ cations during methane oxidation reactions. The experiments were carried out at the D06A-DXAS dispersive beam-line of the Brazilian Synchrotron Light Laboratory (LNLS), Campinas, Brazil, equipped with a Si(111) monochromator and a CCD detector to collect the transmission spectra. The data were collected at Ce-LIII and Ni-K edges, and analyzed with a linear combination fit of standards for each cation-valence. In situ experiments at the DXAS facility were performed with a mass spectrometer connected to the reactor exit to identify the products obtained with different feed compositions. The ZDC and ZDC6 catalysts showed high levels of methane conversion. In methane total oxidation (TOM) experiments (CH4:O2 = 1:2), with ZDC as catalyst, a low and stable level of reduction of 7% of Ce4+ was observed by XANES. When the solid was exposed to highly reducing atmospheres (10%CH4 without oxygen), the Ce4+ cations exhibited a gradual reduction, while in the reaction products an increase in the production of CO and a decrease in the formation of CO2 were observed, in full agreement with the constant increase in Ce4+ reduction percentage. The catalyst was active until it deactivated when reaching the maximum level of reduction (50%). These tests undoubtedly indicated that the lattice oxygen participates in the oxidation reaction according to the Mars-Van Krevelen model. For sample ZDC6, XANES experiments confirmed that Ni° is the active center to produce the partial oxidation of methane (POM). This data is in agreement with the results obtained in the tests performed in a fixed-bed reactor. The occurrence of TOM or POM reactions is dependent on the CH4:O2 feed ratio. Besides, their excellent catalytic properties under real conditions of IT-SOFCs operation, these materials exhibited a very good performance as anode materials in a non-optimized dual chamber IT-SOFCs operated with H2 at 650°C (70mW.cm-2, 150mA.cm-2)