XRD, SAXS and XANES studies of mesoporous zirconia-based materials

RABANNI R.; MARTINS T.; D. G. LAMAS; FABREGAS ISMAEL OSCAR; ANDRINI L.; FANTINI, M.C.A

Madrid

Congreso; XXII congress and general assembly IUCR; 2011

The synthesis of ordered mesoporous zirconia based structures for catalytic applications is a research area under development. These systems are also potential candidates as anodes in intermediate temperature solid oxide fuel cells [1-3]. Ordered mesoporous zirconia can be formed  with a polymeric template, but during the calcination process the amorphous walls crystallize, causing the collapse of the ordered network [3]. Nevertheless, many synthesis strategies can beexplored in order to attain high surface areas (50-100 m2/g), even in disordered porous networks.  In this work the zirconia based materials were prepared with ZrCl4, Pluronic P123,  ethanol and water, varying some synthesis parameters. SAXS, XRD, and XANES at Zr L2 and  L3 edges were performed to characterize the samples. For samples heated in water vapor, the  SAXS results showed that the ordered mesoporous structure vanished after calcination. The XRD  data revealed a single tetragonal zirconia crystalline wall, free of chloride species after a 400ºC  calcination process. On the other hand, the SAXS data related to the sample that remained in an  autoclave for 48 hours, showed a partially ordered structure after calcination. The XRD results  showed a mixture of tetragonal and monoclinic phases, besides chloride species that did not  decompose at 540ºC. The use of a less acidic precursor solution overcame this problem. In order  to study probable changes on the Zr first shell in pure ZrO2 samples prepared by different  synthesis processes, XANES data were collect by TEY at Zr L2,L3-edges at the D04A/SXS  beamline of LNLS (Brazil). ZrO2 monoclinic standard (coordination number, CN=7), BaZrO3  and ZrSiO4 standards (CN equal to 6 and 8, respectively) were measured. A calibration curve  was built, fitting each spectrum of the Zr L3 edge and taking the energy split of the molecular  orbitals. The sample synthesized with hydrothermal treatment in water vapor and calcinated at  400ºC should present a CN close to 8, since its XRD data revealed a tetragonal structure, but the  XANES analysis showed a CN close to 7. Previous XAS results of nanocrystalline tetragonal  zirconia at Zr K edge showed a lower coordination number, related to the high disorder of the O  atoms in Zr first shell [4,5], in agreement with the present work. For the samples calcinated at  540ºC the mixture of crystalline phases detected by XRD are in agreement with the obtained CN  close to 7.