| Selective catalytic reduction of nitrogen oxide(NOx) with ammonia(NH3-SCR) is the most widely used, maturest technology for NOx removal from lean exhaust of stationary plants, where NOx is reduced to N2 by NH3 on catalysts in excess of oxygen. The catalyst, as the most important part of SCR technology, directly influences the NOx removal efficiency of SCR system. The commercial favored catalysts(V2O5/Ti O2) show their optimum performance in the temperature range 300~400°C. So the SCR system has to be placed in the upstream of the desulfurizer and electrostatic precipitator device, which makes the catalyst susceptible to deactivation owing to the high concentrations of dust and SO2. To solve this problems, the SCR system must be placed downstream of desulfurizer and electrostatic precipitator device, but temperature of the flue gas would drop to below 200°C. Hence, the synthesis of active catalsyts for low temperature SCR is crucial.Manganese oxides have drawn broad attention as they contain various types of labile oxygen and oxidation states of manganese, which are necessary and significant to complete the catalytic cycle in SCR reactions. Cerium oxides with advantages of unique oxygen storage capacity and excellent redox properties have also been attracting attention for SCR. Carbon materials have wide applications as catalyst supports for low-temperature SCR because of their large surface area, high adsorption capacity and special porous structure. Graphene, as a new member of carbon material family, possessing more special large surface area, high electron mobility and conductivity, excellent mechanical and thermal stability, is good catalyst support which may promote catalytic reaction by synergetic effect with active components.In this paper, graphene oxide(GO) was synthesised by an improved Hummers method. Subsequently, catalysts of manganese oxides(at varying loadings) supported on graphene(Mn Ox/GR) were prepared by hydrothermal reaction for low temperature NH3-SCR.Firstly, SCR activity tests were carried out in a fixed-bed reactor. The catalytic activity, structure and surface properties of Mn Ox/GR catalysts with different manganese oxides loadings were investigated. The results showed that manganese oxide entities, with different crystallinities(Mn O, Mn3O4, or Mn O2), were dispersed on the surface of graphene. The catalyst with 20 wt.% Mn Ox displayed the highest activity, which was attributed to the high content of Mn4+ and oxygen adsorbed onto the catalyst surface, as well as the enhancement in redox abilities and the addition of active sites at low temperatures.Secondly, in order to improve SCR activity of Mn Ox/GR catalyst at lower temperature, the catalyst was modified by doping cerium oxides. The structure and SCR performance of Ce Ox-Mn Ox/GR catalysts were investigated. The results showed the addition of cerium oxides could not only improve the SCR activity but also broaden temperature window of Mn Ox/GR catalysts. The Ce Ox-Mn Ox/GR catalyst with molar ratio of Ce/Mn 0.8 had the highest activity. The doping of cerium oxides could enhance the dispersion of metal oxides on the surface of graphene, as well as increase the specific surface area and decrease the average pore size. The concentrations of surface absorded oxygen and Mn4+ were increased and the crystallinities of Mn Ox were lowered after doping cerium oxides.Finally, the effect of H2 O and SO2 on the catalysts possessing hightest SCR performance was investigated. Experimental results showed that the H2 O and SO2 resistance ability of Ce Ox-Mn Ox/GR catalyst significantly enhanced compared to Mn Ox/GR catalyst, because cerium oxides could decrease the sulfating of active center atoms of Mn and the production of ammonium sulfate. |
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