Textural, structural, and morphological characterizations and catalytic activity of nanosized CeO_2-MO_x (M=Mg~(2+), Al~(3+), Si~(4+)) mixed oxides for CO oxidation

摘要

The present work focuses on the combination of ceria with another oxide of different ionic valences from period 3 (Mg~(2+), Al~(3+), and Si~(4+)) using coprecipitation method, followed by calcination at 450 and 750°C, respectively. The textural, structural, morphological and redox properties of nanosized ceria-magnesia, ceria-alumina and ceria-silica mixed oxides have been investigated by means of N2 physisorption, XRD, Raman, HRTEM, DRS, FT-IR, and H2-TPR technologies. XRD results of these mixed oxides reveal that only nanocrystalline ceria (ca. 3-6nm for the 450°C calcined samples) could be observed. The grain size of ceria increases with the increasing calcination temperature from 450 to 750°C due to sintering effect. The highest specific surface area is obtained at CeO_2-Al_2O_3 mixed oxides when calcination temperature reaches 750°C. Raman spectra display the cubic fluorite structure of ceria and the existence of oxygen vacancies, and displacement of oxygen ions from their normal lattice positions in the ceria-based mixed oxides. DRS measurements confirm that the smaller the grain size of the ceria, the higher indirect band gap energy. H2-TPR results suggest that the reductions of surface and bulk oxygen of ceria were predominant at low and high calcination temperature, respectively. Finally, CO oxidation were performed over these ceria-based mixed oxides, and the combination of CeO_2-Al_2O_3 exhibited highest activity irrespective of calcination temperature, which may due to excellent textural/structural properties, good homogeneity, and redox abilities.