Investigation On NH₃-SCR Performance For Ni?Mn,Cu?Fe Hydrotalcite Derived Catalysts

The NH₃-SCR technique has been widely used for the controlment of NO_x emissions from stationary sources.Suitable catalyst is the key to achieve high performance of this technology.With the deepening of NO_x treatment for coal-fired flue gas,developing a denitrification?DeNO_x?catalyst with excellent low-temperature activity,high N₂ selectivity,wide temperature windows and superior H₂O+SO₂ resistance performance possesses important practical significance.It has been accepted that the Mn?Cu?-based oxide catalysts possess excellent redox ability are favorable to the low-temperature SCR activity.Fe-based oxide catalysts have high DeNO_x activity and good SO₂ resistance poisoning activity at medium to high temperature.In addition,Ni-based oxide catalysts are inexpensive,Ni-doped can effectively tune the redox performance and acidity&basicity of the catalyst,inhibit the overoxidation of NH₃,and improve the N₂ selectivity.Based on the advantages of transition metal oxides,the assembly and preparation of potential high performance Ni,Mn?Cu?,Fe composite oxide DeNO_x catalysts are worthy of further study.Based on the merits of hydrotalcite-like derivatives,such as large specific surface area,uniform crystal size distribution,good thermal stability and anti-sintering properties,Ni?Mn,Cu?Fe-LDHs were gained by in-situ preparation and after calcination process acquired Ni?Mn,Cu?Fe-LDO catalyst.Additionally,the structure and performance of the catalysts were tested by a series of characterization techniques,and the superior catalyst was obtained.The main research contents and conclusions are as follows:?1?Ni₄Fe₁-LDH was prepared by urea hydrolysis method,and the effect of calcination temperature of Ni₄Fe₁-LDH on the NH₃-SCR DeNO_x performance of NiFe-LDO catalyst was investigated.The results show that the phase composition of the catalyst was determined by the calcination temperature of the precursor.The single NiO phase was obtained at lower temperature,and the mixed phase of NiO and NiFe₂O₄ spinel was gained at higher temperature.NiFe-500 catalyst can better coordinate the merits of NiO and NiFe₂O₄ spinel,makes the redox reactionNi^2?10??10?Fe^3?10??Ni^3?10??10?Fe^2?10?occur quickly,then the best NH₃-SCR performance was achieved,the NO conversion was over 85%and the N₂ selectivity is up to 96%in a wide temperature range of 210?-360?.?2?The transition metal Mn,Cu was in situ doped into the Ni₄Fe₁-LDH layers and after isomorphous substitution process the Ni₃Mn₁Fe₁-LDH and Ni₃Cu₁Fe₁-LDH was prepared.Ni₃Mn₁Fe₁-LDO and Ni₃Cu₁Fe₁-LDO catalysts were prepared by calcination at 500?,further the effect of the introduction of active component Mn,Cu on low temperature denitration performance of the catalyst was investigated.The results show that compared with the Ni₄Fe₁-LDO catalyst,the Mn,Cu-doped Ni₃?M?₁Fe₁-LDO?M=Mn,Cu?catalyst has better redox capacity and excellent DeNO_x activity at low temperature.Specifically,the Ni₃Cu₁Fe₁-LDO catalyst displayed beyond 90%NO conversion and 95%N₂selectivity in the region of 210-360?.Comparatively,Ni₃Mn₁Fe₁-LDO catalyst has a wider temperature window?180-360??and higher NO conversion?nearly 99%at 180??,N₂ selectivity exceed 96%.The good DeNO_x activity of Ni₃Mn₁Fe₁-LDO catalyst is closely related to the electron transfer between Ni³⁺and Mn³⁺and then accelerate the redox cycle.?3?The pure NiMnFe-LDHs was prepared by adjusting the molar ratio of Ni:Mn:Fe.The effects of calcination temperature?500?,600?and 700??on the DeNO_x performance of Ni₃Mn₁Fe₁-LDO catalyst were investigated.The results show that the Ni₃Mn₁Fe₁-600 catalyst possesses the best DeNO_x activity.The NO conversion is more than 90%and the N₂ selectivity as high as 98%in the range of 150-360?.The excellent DeNO_x activity is related to the composition of the mixed phase of Ni₆MnO₈ and NiFe₂O₄ spinel.The mixed phase provides more surface active centers,which makes the electron transfer between Ni,Mn ion and Ni,Fe ion have synergistic effect and accelerates the redox cycle.Furthermore,the DeNO_x performance of the catalyst was improved.At the same time,the H₂O+SO₂ resistance and stability of Ni₃Mn₁Fe₁-600catalyst were tested at 210?.Experimental results conveyed that the initial SCR activity of Ni₃Mn₁Fe₁-600 was very high?99.5%NO conversion?and remained at around 99.0%after 50 h continuous lifetime.After the introduction of SO₂?200 ppm?and H₂O?g??10%?,the NO conversion decreased slightly,and once the introduction of SO₂ and H₂O?g?was cut off,the NO conversion was restored to the initial value,indicating that the catalyst has better H₂O+SO₂resistance and stability.