Performance Of Iron-based Catalyst Supported On SAPO-34 For Low-temperature Selective Catalytic Reduction Process

The selective catalytic reduction?SCR?of NOx with NH₃ is one of the most effective and common methods to reduce NOx emissions from stationary sources.The typically commercial catalysts for this process are mainly V₂O₅-WO₃?MoO₃?/TiO₂.Although these catalysts performance a high SCR activity and a well resistance to H₂O/SO₂,they remain a series of unavoidable drawbacks.For instance,the biotoxicity of vanadium in V₂O₅-WO₃?MoO₃?/TiO₂ could cause secondary pollution.On the other hand,the operating temperatures?300⁴00 ??of the catalysts are high.The SCR device was placed in front of the particulate removal and desulfurizer device in order to avoid heating the flue gas again,but the high concentration of SO₂ and dust in this situation accelerated the inactivation of the catalysts.The temperature of flue gas was too low while the SCR device was placed behind the particulate removal and desulfurizer device,and the catalytic activity of traditional V-W-Ti catalysts was poor at low-temperature.Therefore,it makes much more sense to develop an efficient and environmental-friendly catalyst with a low operating temperature?100²50 ??.Due to their low-prices and environmentally friendly,iron-based catalysts become an appropriate potential replacements.In this paper,we improved preparation method of catalysts and doped Ce to enhance the SCR activity of iron-based catalysts on low-temperatures.As a catalyst support,SAPO-34 possessed the large specific surface area and adjustable surface acidity,which were beneficial for well dispersion of the active species.In this paper,an improved urea-precipitation method was applied to prepare a series of Fe₂O₃/SAPO-34 catalysts.Its low-temperature SCR activity was studied and its structure and properties were analyzed.We discussed the influence and revealed the optimization mechanisms of the different calcination temperature and the addition of Ce on the SCR activity of iron-based catalysts on low-temperature,in term of surface structure,oxidation-reduction and difference of surface acidity.Firstly,orthogonal experiment was used to investigated the effects of solution concentration?precipitation temperature and other conditions in SCR reaction.The results showed that the effects of precipitation conditions on catalytic activity were arrayed in order as follow: precipitation temperature > solution concentration > mass fraction > reaction time > stirring speed.The optimal precipitation conditions were as follow: 0.05 mol/L solution concentration? 20 r/s stirring speed?80 ?reaction temperature?18 h reaction time?30 %mass fraction.Then the low-temperature SCR activity of catalyst was the highest under this conditons.Secondary,the influence of calcination temperature on low-temperature SCR activity of Fe₂O₃/SAPO-34 was investigated in particular.The results demonstrated that the iron species were highly dispersed on the surface of SAPO-34,and then the catalysts exposed more strong acid sites and active sites,which were favorable for the oxidation-reduction ability and NH₃ adsorption capability of catalysts;meanwhile,catalysts possessed the larger specific surface area and higher proportion of chemisorbed oxygen,which were helpful for NO oxidation to NO₂ and then accelerated the low-temperature SCR reaction.Particularly,catalysts calcined at 300 ?performanced the best SCR activity on low-temperature.At last,the characteristics and SCR catalytic activity of Fe-Ce/SAPO-34 catalysts were compared with that of the Fe/SAPO-34 catalysts.The results showed that the addition of cerium oxides could improve the SCR activity and the resistance to SO₂ and H2 O of Fe/SAPO-34 catalysts,and the optimal doped proportion of Ce/Fe was 0.2?molar ratio?.On the one hand Fe/SAPO-34 catalysts?Ce/Fe=0.2?achieved 90 %NO conversion at 170 ?,on the orther hand SCR activity of this catalysts were only reduced about 8 %in the presence of SO₂ and H2 O.The addition of Ce could reduced the loss of specific surface area of Fe₂O₃/SAPO-34 catalysts in the calcined process,which was beneficial for well dispersion of active species on the support.Meanwhile,the co-existence of Fe²⁺ and Fe³⁺ enhanced the oxidation-reduction ability of catalysts.The addition of cerium also could improve the resistance of catalysts to H2 O and SO₂ by protecting iron from being sulfated.