Application of SR Methods for the Study of Nanocomposite Materials for Hydrogen Energy
In the emerging field of Hydrogen Energy, tailor-made design of the active components of monolithic catalysts for selective oxidation/autothermal reforming of hydrocarbons into syngas at short contact times, water gas shift reaction and preferential oxidation of CO in the hydrogen ex-cess is also based upon the concept of the bifunctional reaction mechanism, in which oxygen mobil-ity in complex oxide fluorite-like support (doped ceria or ceria-zirconia) plays an important role. Complex oxide nanocomposites with perovskite-like and fluorite-like structures are known as good ionic or mixed ionic-electronic conductors. This ensures their broad application in the rapidly de-veloping technologies based upon solid state ionic devices including fuel cells (as electrolytes, components of cathodes and anodes), catalytic membrane reactors for ultra-pure oxygen and syn-gas generation. Defect structure of these complex oxide systems is considered as an important factor in ensuring their high performance. However, this restricts ability of traditional structural techniques to elucidate atomic-scale details of their real structure responsible for the transport properties of these systems. This is the reason why Synchrotron Radiation studies (XANES, EXAFS, XRD) of the real/defect structure of complex oxide nanocomposites become tremendously important. This presentation summarizes results of these SR studies for such nanocomposite systems as ceria doped by Me; ceria-zirconia doped by Ln and PrNi0.5Co0.5O3 ? (PNC), Ce0.9Y0.1O2 ? (YDC), Ce0.65Pr0.25Y0.1O2 ? (YPDC), CeO2 and Pr6O11 powders were
synthesized by modified Pechini route. Complimentary methods such as HRTEM, Raman, UV-Vis, XPS, magnetic measurements etc are applied as well to verify different hypothesis on the type of the real structure. As follows from analysis data, Ce cations charge was generally 4+ for PNC YDC, while Pr charge was commonly 3+ for PNC and both 3+ and 4+ for its nanocomposite.
Since Pr charge var-ies differing from Ce, it can be Pr which is generally responsible for oxygen vacancies formation observed. This is in agreement with data on oxygen mobility and surface reactivity studies being carried out previously. Pr and Ce coordination numbers obtained from radial distribution functions are close to the ones in slightly distorted P and F structures. Ce LIII edge spectra for nanocomposite is similar to YPDC with difference explained by local Pr:Ce
ratio variation and structure distortion. Thus, generally Pr3+ migration from P to F phase and its charge variation may cause additional va-cancies formation. This agrees with data obtained in the current work and previous studies. For nanocrystalline doped ceria-zirconia system, the most important factor controlling the lattice oxygen mobility appears to be length of Ce-O (Zr-O) bond and distortion of respective coordination spheres, while free anion vacancies are less important if present at all. Instead, the fast oxygen diffu-sion pathways could be associated with disordered domain boundaries including those between do-mains of different chemical compositions. In this case, the trend in variation of the real structure pa-rameters with the content and size of a doping cation is much more complex due to coexisting in the host fluorite-like lattice of small (Zr4+) and big (Ce4+) cations with different modes of the first coor-dination sphere distortion. Moreover, this affects even the mode of the cations spatial distribution in the lattice or in the surface layer. Thus, doping of ceria-zirconia (1:1) solid solution by Ca or Gd re-sults in depletion of the surface layer by Zr. For these systems, more symmetric coordination envi-ronment around Zr cations in doped samples suggests some ordering due to incorporation of a big doping cation, which results in decreasing the lattice oxygen mobility. The work was done using the infrastructure of the Shared-Use Center Siberian Synchrotron and Terahertz Radiation Center (SSTRC) based on VEPP-3 of BINP SB RAS. This work was sup-ported by BIOGO FP7 Project, Russian Academy of Sciences, and Federal Agency of Scientific Or-ganizations (project V.44.1.17).