A dynamic structural behavior of Pt nanoparticles on the ceria surface under reducing/oxidizing conditions was found at moderate temperatures (<500 °C) and exploited to enhance the catalytic activity of Pt/CeO -based exhaust gas catalysts. Redispersion of platinum in an oxidizing atmosphere already occurred at 400 °C. A protocol with reducing pulses at 250-400 °C was applied in a subsequent step for controlled Pt-particle formation. Operando X-ray absorption spectroscopy unraveled the different extent of reduction and sintering of Pt particles: The choice of the reductant allowed the tuning of the reduction degree/particle size and thus the catalytic activity (CO>H >C H ). This dynamic nature of Pt on ceria at such low temperatures (250-500 °C) was additionally confirmed by in situ environmental transmission electron microscopy. A general concept is proposed to adjust the noble metal dispersion (size, structure), for example, during operation of an exhaust gas catalyst.

Structural characteristics of ceria−titania and vanadia/ceria−titania mixed oxides have been investigated using X-ray powder diffraction (XRD), Raman spectroscopy (RS), and X-ray photoelectron spectroscopy (XPS) techniques. The (1:1 mole ratio) mixed oxide was obtained by a coprecipitation method, and a nominal 5 wt % V2O5 was deposited over its surface by a wet impregnation technique. Both of the materials were then subjected to thermal treatments from 773 to 1073 K and were characterized by the above-mentioned techniques. The XRD results suggest that the CeO2−TiO2 mixed oxide calcined at 773 K primarily consists of poorly crystalline CeO2 and TiO2-anatase phases and that a better crystallization of these oxides occurs with increasing calcination temperature. The “a” cell-parameter values suggest some incorporation of titanium into the ceria lattice. Impregnation of vanadia on ceria−titania enhances the crystallization of CeO2 and TiO2 oxides. However, no crystalline V2O5 could be observed from XRD and RS measurements. Furthermore, the dispersed molecular vanadium oxide (polyvanadate), evidenced by Raman measurements, interacts preferentially with the CeO2 portion of the mixed oxides and forms the CeVO4 compound at higher calcination temperatures. The XRD and RS results provide direct evidence about the formation of CeVO4. The XPS electron-binding energies indicate that ceria, titania, and vanadia are mainly in their highest oxidation states, Ce(IV), Ti(IV), and V(V). The formation of Ce(III) has also been noticed in both CeO2−TiO2 and V2O5/CeO2−TiO2 samples at all temperatures.

Microstructure evolution of ceria-based mixed oxides CeO2−MO2 (M = Si4+, Ti4+, and Zr4+) after thermal treatments in the temperature range of 773−1073 K were investigated by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and other techniques. The CeO2−SiO2 was synthesized by a deposition precipitation method, and a coprecipitation procedure was adopted to make CeO2−TiO2 and CeO2−ZrO2 binary oxides. The XRD measurements revealed the presence of crystalline cubic CeO2 on the surface of SiO2 in CeO2−SiO2, CeO2 and TiO2 (anatase) in CeO2−TiO2, and Ce0.75Zr0.25O2 and Ce0.6Zr0.4O2 phases in CeO2−ZrO2 samples. The crystallinity of these phases increased as the calcination temperature increased. Estimations of the cell parameter a indicated an expansion of the CeO2 lattice in the case of CeO2−TiO2 samples, whereas a contraction was noted in the case of CeO2−ZrO2. Some incorporation of Si4+ ions into the CeO2 lattice was noted at higher calcination temperatures for the CeO2−SiO2 samples. Raman measurements revealed the presence of oxygen vacancies, lattice defects, and the displacement of oxide ions from their normal lattice positions in the case of the CeO2−TiO2 and CeO2−ZrO2 samples. The XPS studies revealed the presence of silica, titania, and zirconia in their highest oxidation statesSi4+, Ti4+, and Zr4+at the surface of the materials. Cerium is present in both Ce4+ and Ce3+ oxidation states, but in different proportions, depending on the mixed-oxide system and the calcination temperature used.