Roles of noble metals (M = Ag, Au, Pd, Pt and Rh) on CeO₂ in enhancing activity toward soot oxidation: Active oxygen species and DFT calculations

The effects of noble metal (M = Ag, Au, Pd, Pt, and Rh) on CeO₂ in enhancing the activity toward soot oxidation were studied through experimental methods and density functional theory (DFT) calculations. Each noble metal (3 mol.%) was supported on CeO₂ (M/CeO₂) and the properties of the catalysts were verified by XRD, HRTEM, N₂ physisorption, CO chemisorption, XPS, and H₂-TPR results. The noble metal was highly dispersed over CeO₂, except for Au due to the sintering of Au, and the reducibility of the catalysts was greatly improved according to degree of the interaction between each noble metal and CeO₂. The activities of M/CeO₂ catalysts for soot oxidation were better than that of CeO₂, and followed the order Rh/CeO₂ > Ag/CeO₂ > Pt/CeO₂ > Au/CeO₂ > Pd/CeO₂ > CeO₂. Moreover, our DFT calculations showed that vacancy formation energy was gradually lowered in the following order: CeO₂ > Pd₄/CeO₂ > Pt₄/CeO₂ > Au₄/CeO₂ = Ag₄/CeO₂ > Rh₄/CeO₂, which was similar order with experimental activity. In addition, the electronic states of the p and f orbitals of CeO₂ were studied to compare with the occupied Ce 4f electrons, which affect the redox property. Rh/CeO₂ and Ag/CeO₂ showed the improved soot oxidation activity, with an enhanced ability to generate oxygen vacancy formation and oxygen adsorption and increased electron transfer. Consequently, the experimental and DFT calculation results revealed the roles of noble metals on ceria with respect to catalytic activity.