Structural characterization and catalytic activity of nanosized CexM1-xO2 (M = Zr and Hf) mixed oxides

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 CexZr1-xO2 (CZ) and CexHf1-xO2 (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.