Origin of the redox deactivation phenomena in modified alumina‐supported Ce/Pr mixed oxide

In this study, we have investigated the redox deactivation of a Ce/Pr mixed oxide supported on modified alumina. Two samples have been prepared by using silica‐ or lanthana‐modified alumina as supports: 25% Ce 0.8 Pr 0.2 O 2−x /Al 2 O 3 ‐SiO 2 and 25% Ce 0.8 Pr 0.2 O 2−x /Al 2 O 3 ‐La 2 O 3 . The redox behavior was studied by reduction of the samples at selected temperatures, from 200 to 900 °C. Oxygen Storage Capacity (OSC) of reduced samples was measured by their reoxidation at 200 °C. The reduction degree attained by the supported mixed oxide, after reduction at temperatures up to 800 °C, increases in parallel for both samples. However, at 900 °C, there is a decrease of OSC, which is moderate for silica‐modified sample, while for lanthana‐modified catalyst, the OSC is almost negligible. XPS analysis of the fresh samples shows that the redox state of cerium cations close to the surface is mainly + 4, while praseodymium appears always as Pr 3+ . Samples reduced at different temperatures were also investigated by XPS. In the case of silica‐modified alumina reduced at 900 °C, significant amounts of Ce 4+ can still be detected, which is in good agreement with the OSC value observed at that temperature. However, lanthana‐modified sample reduced at 800 °C shows an important amount of Ce 3+ , being the only oxidation state observed for cerium after reduction at 900 °C. This is in good agreement with the strong drop in OSC observed for this latter sample, which can be thus attributable to the trapping of trivalent cations by the alumina, forming a stable perovskite‐like aluminate, LnAlO 3 , as confirmed by X‐Ray Diffraction (XRD).