Unveiling Structural Defects by 139La NMR and Raman Spectroscopies at the Origin of Surface Stability for the Design of Cerium-Based Catalysts

Cerium-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.