Study On The Surface Acid Modification And Sulfur Resistance Of Iron-based Catalysts

Combustion of coal,oil and other substances releases pollutants and gases that damage the environment..For example,the sulfur oxides and nitrogen oxides emitted from coal combustion will form environmental problems such as acid rain.The removal technology of sulfur oxides has been relatively perfect at present,and the elimination of nitrogen oxides is still being explored by human beings.NH₃ selective catalytic reduction of NO_x（NH₃-SCR）is recognized as a way to effectively control NO_x emissions.The catalyst is the core of this process,and the development of new non-toxic and high-performance denitrification catalysts is an important task at present.In actual application,the tail gas contains a amount of H₂O and SO₂,which requires a denitration catalyst with high NO_x conversion rate and good property of H₂O and SO₂ resistance.In this paper,the effects of surface acid modification（sulfation treatment and impregnation with MoO_x on catalyst surface）on NH₃-SCR performance,water and sulfur resistance of iron-based catalyst were investigated,and an in-depth study of the catalytic mechanism was performed.The following is the specific research content:（1）Theα-Fe₂O₃ catalyst prepared by the reverse precipitation method was treated by one-step sulfation to obtain a sulfation catalyst with excellent N₂ selectivity and medium-high temperature activity.For all sulfatedα-Fe₂O₃,the catalytic activity shows a trend of first increasing and then decreasing with the sulfation temperature increasing.Theα-Fe₂O₃-350 catalyst shows a NO conversion rate of almost 100%in the range of 275℃-350℃,and exhibits excellent water and sulfur resistance at 300℃.With the increase of sulfation temperature,the increase of sulfation degree leads to the increase of sulfate species and even the formation of massive bulk-like sulfate.Hydrogen temperature program reduction（H₂-TPR）and ammonia temperature program desorption（NH₃-TPD）suggest that sulfation reduces the redox properties ofα-Fe₂O₃,however,it significantly changes the surface acid properties and increases its surface acid sites.In situ diffuse reflectance Fourier transform infrared spectroscopy（In situ DRIFTS）shows that the sulfation treatment produces more Lewis acid and Br（？）nsted acid sites on theα-Fe₂O₃-350 catalyst surface,which was the main reason for the increased activity.In the NH₃-SCR reaction,the Eley-Rideal（E-R）mechanism and the Langmuir-Hinshelwood（L-H）mechanism may act simultaneously.It is worth noting that sulfidation hinders the adsorption of NO_x on theα-Fe₂O₃-350 surface,which reduces the thermal stability of nitrate,thereby hindering its L-H mechanism.（2）The Fe₁₅Mn₁O_x sample prepared by the reverse co-precipitation method was loaded with different mass fractions of MoO_x by the impregnation method,and the effects of different MoO_xloadings on the catalyst’s SCR activity,N₂ selectivity,and sulfur resistance were explored.The0.62 wt%Mo catalyst(the mass fraction is calculated by the mass ratio of Mo atom to Fe₁₅Mn₁O_x)showed the best low temperature activity.XPS shows that the addition of Mo reduces the surface oxygen species content.In situ DRIFTS shows that Mo can effectively increase the Br（？）nsted acid sites on the catalyst surface,thereby promoting the low temperature activity of SCR.