The catalytic activity of Ce-Hf, Ce-Hf-Mg mixed oxides and RuO₂/HfO₂ deposited on CeO₂: Role of superoxide/peroxide in soot oxidation reaction

The synthesis of binary mixed oxide Ce-Hf (CH), ternary mixed oxide Ce-Hf-Ru (CHR), and Ce-Hf-Mg (CHM) has been attempted using the PVP-assisted sol-gel method. The structural and surface morphology of the prepared catalysts were investigated and studied for the generation of superoxides (O₂²⁻)/peroxides (O₂²⁻) species and availability of lattice oxygen (O⁻) during soot oxidation reactions. The successfully incorporated Hf⁴⁺/Mg²⁺ into ceria (CH, CHM) enhances the redox potential sites on nano-flake morphology, thus produces more oxygen vacancies (□). However, Ru⁴⁺/Hf⁴⁺ was not doped into the ceria structure in CHR catalyst; it showed lesser structural distortions generating fewer oxygen vacancies. In addition, it was observed that better performing catalysts should possess lower oxidation temperature and be catalytically stable. Indeed, the ternary oxide CHR featured excellent catalytic properties when compared with the others. However, CHM was found structurally and catalytically stable with self-regenerative capability even after the repeated soot oxidation experiments. Thus, the possible soot oxidation mechanism has been proposed on the prepared catalysts.