Catalytic efficiency of ceria-zirconia and ceria-hafnia nanocomposite oxides for soot oxidation

The catalytic oxidation of soot particulates has been investigated over CeO2, CeO2-ZrO2 and CeO2-HfO2 nanocomposite oxides. These oxides were synthesized by a modified precipitation method employing dilute aqueous ammonia solution. The prepared catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and BET surface area methods. The soot oxidation has been evaluated by a thermogravimetric method under 'tight contact' conditions. The XRD results revealed formation of cubic CeO2, Ce0.75Zr0.25O2 and Ce0.8Hf0.2O2 phases in case of CeO2, CeO2-ZrO2 and CeO2-HfO2 samples, respectively. TEM studies confirm the nanosized nature of the catalysts. Raman measurements suggest the presence of oxygen vacancies, lattice defects and oxide ion displacement from normal ceria lattice positions. UV-Vis DRS studies show presence of charge transfer transitions Ce3+<- O2- and Ce4+<- O2- respectively. The catalytic activity studies suggest that the oxidation of soot could be enhanced by incorporation of Zr4+ and Hf4+ into the CeO2 lattice. The CeO2-HfO2 combination catalyst exhibited better activity than the CeO2-ZrO2. The observed high activity has been related to the nanosized nature of the composite oxides and the oxygen vacancy created in the crystal lattice.