A systematic study was conducted to understand the influence of two different dopant cations (Zr4+ and Hf4+) incorporated into the ceria lattice. A modified coprecipitation technique was employed to make the investigated Ce x Zr1-x O2 (CZ) and Ce x Hf1-x O2 (CH) mixed oxides. The study was comprised of extensive characterization of the prepared catalysts using different techniques, namely, X-ray powder diffraction (XRD), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), transmission electron microscopy (TEM), UV−vis diffuse reflectance spectroscopy (UV−vis DRS), and BET surface area method. To assess the usefulness of these catalysts, oxygen storage−release capacity (OSC) and CO oxidation activity measurements were performed. The XRD analyses reveal that the CZ sample bears Ce0.75Zr0.25O2 and Ce0.6Zr0.4O2 phases and the CH sample possesses only the Ce0.8Hf0.2O2 phase after calcination at different temperatures (773−1073 K). RS measurements suggest a defective structure of the mixed oxides resulting in the formation of oxygen vacancies. The TEM results indicate nanometer-sized crystallites and there is no appreciable increase in the particle size even after high temperature treatments. The XPS studies reveal the presence of cerium in both Ce3+ and Ce4+ oxidation states. The ISS results indicate surface enrichment of cerium in the case of the CH sample, while such surface enrichment of cerium is not observed for the CZ sample. The UV−vis DRS measurements provide information about Ce4+ ← O2− and Ce3+ ← O2− charge transfer transitions. The absence of free ZrO2 and HfO2 in the mixed oxides tenders the clue about the formation of respective solid solutions. The CH catalyst exhibited better OSC and CO oxidation activity compared to that of the CZ sample. The OSC and CO oxidation activity results correlate well with the structural characterization data. The influence of ionic radii of dopant cations on the overall performance of the ceria-based mixed oxides is contemplated.

Catalytic oxidation of CO over Ce x Hf1   -   x O2 (CH; 8:2 mole ratio) and Ce x Zr1   -   x O2 (CZ; 1:1 mole ratio) nanocomposite oxides reveals that the former combination exhibits more conversion at much lower temperature than the latter. Thermal and textural stability of both the combinations, obtained by a coprecipitation method and calcined at 773 and 1073 K, were examined by X-ray diffraction, Raman spectroscopy, and CO-temperature programmed reduction techniques. The XRD results disclose formation of a stable Ce0.8Hf0.2O2 cubic phase in the case of the CH sample, whereas CZ suffers phase segregation from Ce0.75Zr0.25O2 to Ce0.6Zr0.4O2 at 1073 K. Oxygen storage capacity (OSC) and CO-TPR measurements reveal a high OSC and low-temperature reducibility for the CH sample due to the defective fluorite structure generated upon incorporation of smaller Hf4+ cations (0.78 Å ionic radius), in comparison to Zr4+ (0.84 Å), into the ceria cubic lattice. The observed high activity of Ce0.8Hf0.2O2 was proven to be due to the generation of lattice defects, formation of more oxygen vacancies, and easy reducibility.

Structural characteristics and catalytic activity of nanosized ceria-terbia mixed oxides have been investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy, temperature-programmed reduction/oxidation, and Brunauer-Emmett-Teller surface area techniques. The catalytic usefulness has been evaluated for oxygen storage-release capacity (OSC) and CO oxidation activity. The XRD and TEM results suggest that the crystallite sizes of these nano-oxides are in the range 5-12 nm within the investigated temperature range of 773-1073 K. The mixed oxide solid solutions adopted a fluorite-type structure and exhibited cell parameters with respect to Vegard's rule. The XPS measurements revealed that both cerium and terbium are in 3+ and 4+ oxidation states, 4+ being dominant in both cases. The reduction temperature of the Ce-Tb oxide is observed to be lower than that of the pure ceria and exhibited better redox properties due to the formation of solid solution. The OSC is substantially higher for the mixed oxide, and the presence of oxygen vacancies favored better CO oxidation activity. The nanosized ceria-terbia mixed oxide is also found to be thermally quite stable and capable of manifesting redox behavior after severe heat treatment.