Mechanism Study Of NO Catalytic Oxidation And SO₂ Tolerance Over Nanometer Metal Oxide Catalysts

In this paper, α-, β- and γ- MnO₂ nanorods were prepared by hydrothermal method and used for NO catalytic oxidation. Catalytic of NO over MnO₂ nanorods could be regarded as a temperature-dependent process. Among the three MnO₂ nanorods, γ- MnO₂ was proven to be of the highest intrinsic activity for NO catalytic oxidation, under the same conditions. According to the characterization results, the NO_x adsorbed onγ- MnO₂ nanorod was very unstable, which could easily decompose to NO₂, and the low crystallinity, high redox ability, high concentrations of Mn4+ and chemisorbed oxygen, and the strong NO adsorption ability should be the major factors leading to the excellent catalytic performance of γ- MnO₂ nanorod. The resistance to SO₂ of the three nanorods was poor. After the SO₂ removed, there is some promotion of the activity for the γ- MnO₂ sample, but β- and γ- MnO₂ samples were almost unchanged.Ce O₂ nanorods, nanospheres and nanoparticles were prepared by hydrothermal method and used for catalytic oxidation of NO. The experimental results showed that Ce O₂ nanorods were of the best catalytic performance. From the characterization results of XRD, H2-TPR, NO-TPD and XPS, the excellent catalytic performance of Ce O₂ nanorods could be ascribed to low crystallinity, high reducibility, strong NO adsorption ability and the presence of rich surface chemisorbed oxygen.Ce O_x@MnO_x catalyst with core-shell structure was prepared and used for catalytic oxidation of NO. The experimental results showed that the intrinsic catalytic activity of Ce O_x@MnO_x was higher than that of Ce MnO_x prepared by citric acid method. From the characterization results, we can see that the high intrinsic activity of Ce O_x@MnO_x could be attributed to its low crystallinity, good reducibility, and high concentrations of Mn4+ and active oxygen species. The SO₂-resistance experimental results showed that Ce O_x@MnO_x had better SO₂-resistance than Ce MnO_x. After the SO₂ removed, all the two samples could not restore to their original levels. For Ce MnO_x there was about 10% lower than its original level, but the activity of Ce MnO_x was almost unchanged.Finally, the reaction mechanism of NO catalytic oxidation over Ce O_x@MnO_x catalyst was studied thoroughly. NO was first absorbed on the suface of the catalysts, and then nitrosyls were readily oxidized to nitrates by active lattice oxygen. Subsenqently, nitrates were decomposed to the final product NO₂, and the consumed lattice oxygen was supplemented by gaseous O₂.