In-situ XPD and XANES studies on Ce0.9Zr0.1-XSmXO2-X/2 (X= 0.1, 0.08, 0.06, 0.04, 0.02, 0) in diluted H2 and CH4

LARRONDO, S.A.; SUAREZ ANZORENA, M.R.; TOSCANI, L. M.

Campinas

Congreso; 29th RAU Anual Users Meeting LNLS; 2019

CNPEM

CeO2-ZrO2 mixed oxides are excellent electro-catalysts for oxidation reactions [1]. It was reported in previous works, that the oxide with composition Ce0.9Zr0.1O2 is active as anode material for IT-SOFCs. Nevertheless, it is important to enhance its mixed conductivity (ionic-electronic) in reductive atmospheres [2]. In this sense, the substitution of Zr4+ by Sm3+ was proposed to incorporate vacancies in the oxide structure because of the different oxidation states of both cations, which in turn could have an impact on ionic conductivity. Ce0.9Zr0.1-xSmxO2-x/2 (x= 0.1, 0.08, 0.06, 0.04, 0.02, 0) mixed oxides were synthesized via the citrate method. The aim of this work is to study the effective incorporation of samarium in the structure of the oxide and its stability by X-ray diffraction with synchrotron radiation in different atmospheres and temperatures. Besides, it is studied the effect of samarium incorporation on redox properties in diluted H2 and CH4 atmospheres (5vol.% in 50 mL.min-1, He balance) from room temperature to 800°C and a heating rate of 10°C.min-1 by ?IN-SITU? X-ray absorption spectroscopy (XAS) measurements performed at the Ce LIII absorption edge with synchrotron radiation. The evolution of Ce3+/Ce4+ ratio with temperature in different atmospheres is studied. All samples showed the presence of a single phase corresponding to fluorite structure, characteristic of CeO2, not only in air at room temperature but also, during thermal treatments with H2 or CH4. For samples calcined at 500°C average crystallite sizes are nanometric (< 10.3 nm) regardless the atmosphere. The increment in crystallite size and lattice parameter with samarium content is observed. Samples calcined at 1100 °C exhibit greater crystallite sizes but they are still nanometric. Experiments performed in diluted H2 (5 vol.%) for samples calcined at 500 °C are in good agreement with traditional TPR. Two reduction steps are observed in the reduction profile of Ce0.9Zr0.1O2, the first one corresponding to the reduction of superficial Ce4+(ca. 550 °C) and the second one, at high temperatures (ca. 750 °C) related to bulk Ce4+ reduction [3]. The incorporation of Sm3+ substituting Zr4+ in the structure has a pronounced effect on the kinetics of reduction process and on the final degree of reduction. The two steps are still visible, but the final degree of reduction achieved is similar for all the samples tested. Samples calcined at 1100 °C show higher onsettemperatures for Ce reduction. It is worth to mention that undoped sample, still presents two reduction steps while the others only show one indicating a surface loss with calcination temperature, compatible with the increment observed in average crystallite size. Experiments carried out with diluted CH4 show similar results but it is worth to mention that lower total reduction values are obtained.