Unveiling structural defects by 139La NMR and Raman spectroscopies at the origin of surface stability for the design of cerium-based catalysts

International audienceCerium-based complex oxides are essential materials well-suited for numerous applications related to heterogeneous catalysis and electrocatalysis. However, the impact of structural defects on the thermal stability of the surface has still to be elucidated. Probing lanthanum environments by 139La NMR and analyzing, for the first time in parallel, the associated defects by Raman spectroscopy allow for a better view of the structural defects at the surface and in the bulk for samples annealed at T = 600 and 1200 °C, respectively. Moreover, we propose another indexing of the Raman spectra of cerium-based compounds with a fluorite structure by considering two tetrahedral environments around the cationic defects. We evidence the existence of an unusual isolated La pseudo-cubic site, in a series of cerium-, zirconium-, and lanthanum-based oxides. It is found to be stabilized at the subsurface farther from surface’s Zr atoms and oxygen vacancies. La3+ ions in the bulk are preferentially associated with bulk Zr4+ cations as in the La2Zr2O7 ternary compound. When this peculiar La3+ environment surrounded exclusively by Ce4+ cubic sites is thermally stable, the specific surface area remains interesting for catalytic application at high temperatures. However, enhancing the La and Zr contents tends to increase the association of La3+ and Zr4+ ions in clusters and induce a loss of surface area

Insight into the praseodymium effect on the NH3-SCR reaction pathways over W or Nb supported ceria-zirconia based catalysts

International audienceThe partial substitution of zirconium for praseodymium in a ceria-zirconia support was studied for WO3 or Nb2O5 supported catalysts dedicated to the NOx SCR by NH3. This partial substitution favored the support reducibility, but both niobium and tungsten impregnation strongly inhibited the redox behaviors of the support. Concomitantly, Nb2O5 and WO3 also provided acidic sites but praseodymium noticeably inhibited the ammonia storage, especially for WO3-containing sample. Finally, praseodymium drastically decreased the deNOx performances. Additional NO and NH3 oxidation experiments were performed and the various redox and acidic behaviors of all the studied materials were suitable to provide an overview of the reactional pathways which begins by the oxidative dehydrogenation of NH3 on acid sites. The generated species react either with oxygen species or with gaseous NO, reflecting a strong competition between the NOx SCR and NH3 oxidation together with the formation of a key intermediate in the nitrogen oxides reduction

Catalytic methane combustion at low temperatures over YSZ-supported metal oxides: Evidence for lattice oxygen participation via the use of C18O2

Catalytic methane combustion was carried out over YSZ, Pd/YSZ and Rh/YSZ between 200 and 700 °C. Despite similar light-off curves for Pd/YSZ and Rh/YSZ between 200 and 450 °C, isotopic exchange experiments using CD4 and 18O2 revealed different behaviour of both supported catalysts regarding the activation of reactant molecules. The use of C18O2 isotopic gas was shown to be more appropriate than 18O2 to evaluate the bulk oxygen mobility in YSZ at low temperatures. The exchange of gaseous CO2 with lattice YSZ oxygen atoms, which occurs via surface hydrogen carbonate intermediate species, allowed to demonstrate the contribution of bulk oxygen atoms of YSZ in Rh/YSZ at low temperatures

Pr-rich cerium-zirconium-praseodymium mixed oxides for automotive exhaust emission control

International audienceThis study describes the textural, structural, redox properties, oxygen reactivity of two Pr-rich cerium-zirconium-praseodymium (CZP) oxides (Ce 0.45 Zr 0.10 Pr 0.45 O 2-x and Ce 0.475 Zr 0.05 Pr 0.475 O 2-x) and their catalytic activity for propane and CO oxidation in both stoichiometric and lean conditions, two important reactions for automotive exhaust emission control. The concentration of oxygen vacancies can be tuned by the quantity of Pr 3+ cations which depend both on the preparation method and the composition. The Pr 4+ (4f 1)/Pr 3+ (4f 2) ratio was accurately determined by magnetic measurement. A highly energetic grinding step was found to double this Pr 3+ concentration, resulting in a strong enhancement of the lowtemperature reducibility, bulk oxygen mobility and catalytic activity for propane deep oxidation. Pr-rich CZP oxides seem to be less relevant for CO oxidation, which is mainly driven by the specific surface areas. These Pr-rich CZP mixed oxides present better performances for lean conditions, suggesting their possible role in diesel oxidation catalysts applications