Performance Study For Low Temperature NH₃-SCR Reaction With NiMnTi Mixed Oxides Catalysts Derived From LDHs

Mn-based oxides catalysts have aroused special attention owing to their excellent denitration?DeNO_x?performance in low temperature for selective catalytic reduction of NO_x by NH₃?NH₃-SCR?.Despite that,their limited SO₂resistance at low temperature and poor N₂ selectivity at high temperature caused by the strong oxidation ability remain the main restricting factors for its practical application.As reported,it is an effective way to optimize the denitration performance of MnO_x catalyst by introducing supporter and promoter,at the present stage,many researchers have tried to improve the DeNO_x performance with the help of sulfur-resistant carrier TiO₂ and excellent redox promoter NiO.The results showed that different preparation methods have significant effects on the DeNO_x performance of catalysts.Thence,how to superiorly assemble active centers,promoters and supporters of catalysts is still worthy exploring.This paper took these as the research background,base on the unique structural characteristics and excellent physic-chemical properties of Layered Double Hydroxides?LDHs?materials,synergeticly took the full play of Ni-based?Mn-based oxides and TiO₂ as the starting point,effectively assembled the catalytic active center,catalytic promoter and carrier in-situ by virtue of preparing NiMnTi-LDH precursor,investigated the influence on the composition and structure of NiMnTi mixed oxides catalysts and its DeNO_x performance,which were caused by the calcination temperature of precursor,the ratio of active component with promoter and the assembly strategy of LDHs precursors.The main contents and as-acquired conclusions are displayed as the following:?1?The Ni₃Mn₁Ti₁-LDH precursor was synthesized by the homogeneous precipitation method,they were calcined at different temperatures of 400?,500?and 600?,which could be marked as Ni₃Mn₁Ti₁-400,Ni₃Mn₁Ti₁-500 and Ni₃Mn₁Ti₁-600,respectively.Combined with the DeNO_x performance of catalysts at different calcination temperatures,the physic-chemical properties of the catalysts were characterized and analyzed specifically,meanwhile,the regulating effect of calcination temperature on the redox performance of the catalysts were investigated.As the research results presented,the calcination temperature,which has great effect on the catalytic activity of catalyst at low temperature,but was not sensitive to the selectivity of N₂;it primarily influenced the crystallinity,phase component and dispersity,and further indirectly affected the redox property of catalysts.Especially,it is showed that the Ni₃Mn₁Ti₁-400 catalyst displayed high dispersion,favorable phase composition,abundant acid amount,excellent redox performance,simultaneously,the conversion of NO_x and the selectivity of N₂were higher than 90%in the temperature range of 180-360?.?2?With the aid of the homogeneous precipitation method,the Ni_4-xMn_xTi₁-LDH precursor with different Mn/Ni molar ratios were prepared,the composition,phase and thermal decomposition behavior of the LDHs precursors were characterized through various techniques,and then the Ni_4-xMn_xTi₁-LDO catalysts were obtained by calcining the precursor at 400?.Combined with the DeNO_x performance of catalysts at different Mn/Ni molar ratios,the physic-chemical properties of the catalysts were characterized and analyzed specifically,at the same time,the regulating effect of Mn/Ni molar ratios on the redox performance of the catalysts were investigated.The catalytic performances results exhibited that the NO_x conversion and N₂ selectivity were very sensitive to the Mn/Ni molar ratios for Ni_4-xMn_xTi₁-LDO catalysts;by the detailed characterization,it could be found that there were some differences in compositon phase and thermal decomposition behaviour among various LDHs prepared with different Mn/Ti molar ratio;the different Mn/Ni molar ratios primarily influenced the distribution of metal ions valence states on the surface of catalysts,and further affected the redox property of catalysts.Notably,the molar ratio of Mn/Ni/Ti?2:2:1?would induce more favorable concentration of Ni³⁺,Mn⁴⁺and O_?in catalysts surface due to the stronger interaction among nickel,manganese and titanium oxides,and further promoted the excellent DeNO_x properties.?3?The optimum molar ratio of Ni:Mn:Ti was selected as 2:2:1.Firstly,the NiTi-LDH,NiMn-LDH,NiMnTi-LDH precursors were prepared by coprecipitation;then,NiTi-LDH was impregnated with Mn and NiMn-LDH was impregnated with TiO₂,respectively;finally,three kinds of NiMnTi mixed oxides catalysts were synthesized by calcining the LDHs precursors at 400??named as Mn/NiTi-LDO,TiO₂/NiMn-LDO and NiMnTi-LDO,respectively?.Combined with the DeNO_x performance of NiMnTi mixed oxides catalysts,which were prepared by different assembly strategy of LDHs precursors,the physic-chemical properties of the catalysts were characterized and analyzed specifically,in the meantime,the impact of assembly strategy on the interaction among Ni,Mn and Ti oxides were investigated.The results indicated that the different NiMnTi mixed oxides catalysts derived from LDH precursors by different assembly strategy revealed diverse DeNO_x performance,which could be related to the exposure degree of the active components on catalyst surface and the strength of interaction among metal component.As the characterization results presented,for Mn/NiTi-LDO catalyst,most of MnO_x were dispersed on the surface of catalyst,which could led to the weak interaction between Mn and?Ni,Ti?,however,it possessed the strong redox capacity and thence showed excellent low temperature activity.For TiO₂/NiMn-LDO catalyst,the NiMn-LDH precursor were covered by TiO₂ at the process of immersion,which might brought about the weak interaction between?Ni,Mn?and Ti,however,there are abundant acid sites on the surface of catalyst,so TiO₂/NiMn-LDO catalyst showed good activity at high temperature.In contrast,the NiMnTi-LDO catalysts,which derived from the LDHs via one-step in-situ method,which exhibited the highly dispersed and strong interaction among Ni,Mn and Ti,thus showing the best DeNO_x performance.?4?The Ni₂Mn₂Ti₁-LDO,which was obtained by calcining Ni₂Mn₂Ti₁-LDH at 400?,was identified as the most optimized catalyst for NH₃-SCR.It afforded the best NO_x conversion beyond 90%and over 95%N₂ selectivity within the broad temperature window of 150-360?.The DeNO_x performance was almost impervious in the range of GHSV experiment,besides,the catalyst exhibited the excellent H₂O/SO₂ tolerance.