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By using a combination of transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) techniques, we have followed in detail the structural and chemical changes occurring on the very surface layers of powder CexPr1-xO2mixed oxides, used as supports, during the preparation of rhodium-supported catalyst by wet impregnation techniques. Results demonstrate the occurrence of Pr3leaching out of the Ce−Pr solid solution during the impregnation step, which promotes a severe spatial redistribution of lanthanide cations at the surface of the mixed oxide. A highly heterogeneous surface composition is reached after the metal deposition. Mobilization of the support also affects the features of the final surface structure of the metal-supported nanoparticles, which become decorated by patches of support material. All these facts seem crucial to properly understand the redox and catalytic behavior of this kind of materials.

This work reports on the hydrogen interaction with a 3 wt % Au/Ce0.62Zr0.38O2(Au/CZ) catalyst prepared by deposition−precipitation. As deduced from X-ray powder diffraction, electron microscopy (scanning transmission electron microscopy−high-angle angular dark field and high-resolution electron microscopy), and CO volumetric/Fourier transform IR (FTIR) adsorption studies, the investigated catalyst shows a good metal dispersion. By combining FTIR spectroscopy and volumetric chemisorption studies, it is shown that upon treating the Au/CZ catalyst with 40 Torr of H2at room temperature a fast and very intense spillover effect occurs. As determined from the recorded isotherm, very high values of the apparent H/Au ratio (>8.0) and of the atomic hydrogen surface density (>11.0 H/nm2) are reached. In parallel with this observation, the onset of a characteristic IR band at 2133 cm-1shows the occurrence of significant support reduction with inherent appearance of Ce3species. Moreover, the simultaneous growth of the IR band at 1630 cm-1due to molecular water strongly suggests that even at room temperature oxygen vacancies are also formed. Additional FTIR spectroscopy studies have shown that the hydrogen spillover is strongly inhibited by either co-adsorption of CO or a reducing pretreatment with flowing 5% H2/Ar at 673 K. These deactivation effects, however, may be reverted by very mild regeneration treatments at room temperature.

The relationship existing between aging conditions, redox behavior, and surface/bulk structural properties of two thermally aged ceria−zirconia mixed oxides, CZ-MO and CZ-SO, is analyzed. The samples were prepared by applying to a fresh Ce0.62Zr0.38O2 mixed oxide two alternative aging routines consisting of a reduction with H2 at 1223 K (5 h), followed by either a mild, CZ-MO, or severe, CZ-SO, re-oxidation treatment. By combining high-resolution electron microscopy and a number of chemical charcterization techniques, it is shown that the nanostructure of the aged oxides, specifically the total amount and surface presence of the phase exhibiting an ordered cationic sublattice (κ-like phase), is a key factor in determining their redox response. In the low-temperature reduction range (Tredn ≤ 773 K), the enhanced reducibility of the CZ-MO sample is proposed to be kinetically controlled by its surface structure mainly consisting of the κ-like phase. In accordance with the reported results, the surface activation of the H2 molecule, much faster on the CZ-MO sample, is proposed to be the rate controlling step of the overall reduction process. This proposal was further confirmed by the dramatic downward shift observed in the temperature-programmed reduction diagrams recorded for the corresponding oxide-supported rhodium samples. By contrast, in the high-temperature reduction range (Tredn ≥ 973 K), the observed difference of reducibility, higher in the case of the CZ-MO sample, is interpreted as due to thermodynamic factors related to the nature of the predominant cationic sublattice structure, ordered for CZ-MO and disordered in the case of the CZ-SO sample.

The redox behaviour of a Ce0.68Zr0.32O2 mixed oxide is reversibly modified by alternating high temperature (1223 K) reduction with either mild (823 K) or high temperature (1223 K) re-oxidation treatments.

A new method to compare the oxygen buffering capacity (OBC) of catalysts shows that Ce–Tb mixed oxide presents an exceptional capability for attenuating fast oscillations of oxygen partial pressure.