XANES studies on zirconia-based materials for catalysis

REBECA BACANI; T.S. MARTINS; M.C.A. FANTINI; R.O. FUENTES; R.J. PRADO; I.O. FÁBREGAS; L.M. ACUÑA; L. ANDRINI; F.F. MUÑOZ; D.G. LAMAS

Campinas

Workshop; Reunião Anual de Usuários (21º RAU); 2011

ABTLus

Ordered mesoporous silica, with 2-D hexagonal (SBA-15, MCM-41) structures areextensively studied since the 90´s, but only few works dealt with ZrO2 orderedmesoporous structures for catalytic applications and their use as anode in solid ox-ide fuel cells (SOFC). These systems may lead to better performance in the masstransport of fuel/oxidation agent, in the electron conductivity and charge transfer,due to an enhancement on surface area, aiming to achieve a lower operation tem-perature limit. These zirconia-based materials synthesized with polymer templates,the mesoporous network usually collapses due to the crystallization of the walls atlow temperature (around 300oC). In our previous work, a silica palisade was addedto the zirconia-based matrix to improve the mechanical stability. Although the ¯-nal material has high super¯cial area (100m2/g), this method was not enough tomaintain the ordered mesoporous structure after the calcination at 540oC, a goalthat has been pursued in our present research.In order to study changes on the Zr ¯rst shell coordination in pure ZrO2 sam-ples due to the di®erences on the synthesis processes (hydrothermal treatment inTe°on autoclave at 80oC/48h and water vapor at 40oC/48h, calcination at 400oCin air and at 540oC in N2 and air), XANES data were collected at Zr L2,L3-edges in the D04A/SXS-9197 beamline, using total electron yield detection. Theresults showed that a larger calcination temperature lead to a more ordered ¯rstZr-O shell, but without any di®erence in the spectra that could be related to aspeci¯c zirconia nanocrystalline phase (cubic or tetragonal). In order to analyzethe Ce3+/Ce4+ ratio in ZrO2-90%CeO2 samples, prepared with a silica palisade,temperature-resolved XANES data were collected at the Ce L3-edge spectra in theD06A/DXAS-10900 beamline. The experiments were performed under He/(5%)H2(°ux of 20mL/min), detected through transmission mode in the temperature rangeof 25oC up to 500oC (ramp of 12.5oC/min). The results demonstrated that the in-crease of silica content (from 10 to 30 mol%) and the improvement on the super¯cialarea promoted a higher reduction of Ce4+ under lower temperature.