| Selective catalytic reduction with NH3(NH3-SCR) is one of the most effective methods in the abatement of NOx from diesel exhaust. Manganese based catalyst has excellent low-temperature SCR performance and is a potential catalyst for the removal of NOx. ZSM-5has been widely used as new catalytic materials due to its high hydrothermal stability and shape-selecting selectivity. Based on this, a series of Mn supported ZSM-5catalysts were prepared. Ce as a promoter and Fe as a second-active component were added respectively to modify the catalysts. Selective catalytic reduction of NOx in the presence of excess oxygen was investigated systemically over all the Mn based catalysts. The purpose of this paper was to optimize the preparing condition and improve the low-temperature SCR activity of Mn based catalysts. Further, the effects of preparation conditions on the performance and structure of catalysts were also investigated in this paper.First of all, Mn/ZSM-5catalysts were prepared through precipitation method and the influence of calcination temperature on its SCR activity, structure, oxidation states and surface atom concentrations were studied. The catalyst calcined at300℃showed the best catalytic activity in the range of150～420℃. The activity of catalysts decreased with the increase of calcination temperature. When calcined at lower temperatures, the catalysts possessed larger specific surface area and higher surface Mn and lattice oxygen concentration, and Mn existed in the form of Mn3O4and amorphous MnO2over catalysts. With calcination temperature increasing, the crystallization degree of Mn3O4increased, but the relative contents of amorphous MnO2and surface Mn and lattice oxygen decreased. When calcined at600℃, the catalyst sintered, its specific surface area dropped dramatically, and Mn2O3species were formed. In SCR process, Mn3O4and amorphous MnO2were the main active substance, while the existence of Mn2O3was unfavorable for the removal of NOX. There were no obvious changes between the fresh and the used MnZ-3catalysts, except the slight decrease of surface Mn concentration.Then, Ce was added as a promoter and Mn-Ce/ZSM-5catalysts were prepared by co-precipitation method. Furthermore, the relationships between calcination conditions and catalytic performance and catalyst structure were also discussed. Mn-Ce/ZSM-5catalyst calcined at300℃exhibited the best SCR performance at low temperature and broader activity window. The catalyst activity decreased with the increase of calcination temperature. Mn3O4was formed over the catalyst calcined at500℃. No Ce species were detected over all the catalysts and this may be due to the low amounts or the good dispersion of Ce. With respect to calcination atmosphere, calcination in air could improve the low-temperature SCR performance of catalyst and broaden the active window range of Mn-Ce/ZSM-5catalysts. XRD and XPS analysis showed that: when catalyst was calcined in air, MnO2existed as the main phase, Mn3O4was the minor phase over catalyst, and the surface Mn and O concentration were comparatively high; when calcined in N2, almost equal amounts of MnO2and Mn3O4existed over catalyst; when calcined in H2, MnO became the main phase, and the surface Mn and O concentrations declined. Over all the catalysts calcined in different atmosphere, Ce all existed in the form of CeO2.At last, Fe was added as a second active component to prepare Mn-Fe/ZSM-5catalysts. The effect of preparation methods, precursors and calcination temperature on the catalytic activity and physicochemical properties of Mn-Fe/ZSM-5catalysts were investigated. Regarding preparation methods, the catalyst prepared by co-precipitation method showed excellent low-temperature SCR activity; when prepared by ion-exchange method, the activity of the catalyst decreased and Fe2O3was formed; the catalyst prepared by impregnation method exhibited an intermediate activity. With regard to precursors, the catalyst with Fe(NO3)3as precursor possessed better SCR activity for NOX removal and smaller particle size than FeCl3; the particle of the catalyst with FeCl3as precursor were agglomerated to some extent; no XRD diffraction peaks of Mn or Fe were detected over both catalysts; Mn existed in the form of Mn+4and Fe existed in the form of Fe+3over both catalysts. On calcination temperature, the catalyst calcined at300℃had better SCR performance and a rougher surface than the catalyst calcined at500℃; when catalyst was calcined at500℃, Fe2O3was formed. |
