High catalytic activity of PdO_x/MnO₂ for CO oxidation and importance of oxidation state of Mn

Catalytic oxidation is used as the most effective to remove CO in the exhaust gases from automobiles and power plants. PdO_x/MnO_x was examined as a catalyst for CO oxidation using a conventional flow reactor at 50°-150°C. In the reaction conditions of GHSV of 1.22 x 10⁵/hr and CO concentration of 2000 ppm, PdO_x/MnO₂ showed higher catalytic activity compared with PdO_x/Mn₂O₃, which was previously reported as an effective catalyst due to the cooperative action of palladium and Mn₂O₃ for this reaction. The reason for higher activity of PdO_x/MnO₂ than PdO_x/Mn₂O₃ was studied using TPR and XPS studies. TPR showed that PdO_x/MnO₂ could be reduced by CO at much lower temperature than PdO_x/Mn₂O₃. XPS showed that the valence of manganese in the PdO_x/MnO₂ was between 4+ and 3+, which was higher than that of manganese in the PdO_x/Mn₂O₃ catalyst of which the valence was between 3+ and 2+. The readiness of MnO₂ to be reduced with maintaining a higher oxidation state showed higher CO oxidation activity than Mn₂O₃ as support for PdO_x.