Nanocrystalline Ce0.45Zr0.45La0.10O2-ä for partial methane oxidation: preparation characterization and the origin of improved catalytic performance
Frolova, Elena1; Ivanovskaya, Marya1; Yaremchenko, Alexey2; Kharton, Vladislav2; Sadykov, Vladislav3
1Belarus;
2Portugal;
3Russian Federation

The present work was focused on the characterization of nanocrystalline Ce0.45Zr0.45La0.1O2-ä, prepared via inorganic sol-gel method, and evaluation of its electrocatalytic activity in a solid oxide fuel cell (SOFC) type reactor.
As were detected by EPR lanthanum as well as Pt (1,5 %) doping of the Zr0.5Ce0.5O2 lattice increases also the concentration of Ce3+ defects with different g-factor anisotropy that depends on the location of it. These structural features can be considered as coordinately unsaturated Ce3+ ions in contact with oxygen vacancies [Ce3+–Vo] located at surface and in near-surface layers. The signal with g^ = 1.968 and gII = 1.947 is assigned to Ce3+ centres in a bulk of oxide matrix. The incorporation of La(III) into Zr0.5Ce0.5O2 lattice was found to increase concentration of these centers with respect to the parent phase up to ten times and platinum solution impregnation leads to the extra 5 times increasing in Ce3+ concentration. It is caused by the incorporation of La(III) into Ce-Zr complex hydroxide structure that alters the dehydration process and suppresses crystallization. Also, the presence of La(III) enables to retain cubic fluorite structure of the nano-scale particles up to 1200°C.
High defect concentration seems responsible for the attractive catalytic activity of Ce0.45Zr0.45La0.10O2-ä-based anodes towards partial oxidation of methane (POM). As an example, at the O2:CH4 ratio equal to 0.5, stoichiometric for the POM reaction, CH4 conversion and CO selectivity in the SOFC-type reactor achieved 54 and 73%, respectively. Such a behavior is rather unusual for ceria-based catalysts active to total CH4 combustion. Most likely, substantially high performance of Ce0.45Zr0.45La0.10O2-d results from the formation of Ce3+ states on the surface of nanocrystallites, stabilized by ZrO2 and La2O3 additions, and/or surface instability of the lattice characteristic of morphotropic phase transformations. Both these factors may also promote oxygen atoms activation by neighboring Pt, incorporated into near-surface layers of the nanostructured catalyst.
The work was supported by ISTC (Project Nr.3234).
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