Study On The Denitration And Sulfur Resistance Of Spinel Type Iron Based Catalyst NH₃-SCR

Nitrogen oxide（NO_x）is one of the main air pollutants,which can cause acid rain and photochemical smog,and is also an important cause of regional fine particle harm and haze.With the increasingly strict control index of nitrogen oxide emission in China,it is urgent to deeply denitrate coke oven flue gas.The working temperature range of traditional commercial vanadium-based denitration catalyst is 300～420℃,which is difficult to meet the denitration conditions of coke oven low-temperature flue gas,and vanadium-based catalyst has strong biological toxicity and is easy to cause secondary pollution.The low temperature NH₃-SCR denitration performance of Fe-based catalyst is outstanding,but it has not been industrialized.The key reason is that the flue gas contains SO₂,which is easy for Fe sites on the catalyst surface to combine to generate sulfates,resulting in sulfur poisoning and deactivation of the catalyst.In order to solve the problems of low denitration activity and poor sulfur resistance of iron-based catalysts,spinel CoFe₂O₄ catalyst was prepared by hydrothermal method.Its rich acidic sites and proper redox ability are beneficial to the adsorption and activation of NH₃ and NO_x.After modification with Ti O₂,not only the low-temperature denitration activity of the catalyst was improved,but also the poisoning of SO₂ to the surface active components of the catalyst and the formation of surface sulfate were inhibited,and the sulfur tolerance of the catalyst was enhanced.The specific research results are as follows:（1）A series of spinel-type iron-based catalysts doped with different transition metals were prepared,and the effects of different transition metal elements,preparation methods,calcination temperature and reaction conditions on the denitration performance of the catalysts were studied.The results show that the Co/Fe molar ratio is 1/2 prepared by hydrothermal method,and the CoFe₂O₄ catalyst calcined at 500℃has the best denitration activity,and the NO_x conversion rate and N₂ selectivity are close to 100%at 200℃.Through characterization and analysis,it is found that the catalyst surface contains a large number of L-acid and B-acid sites,and the introduction of Co improves the oxidation ability of Fe.The results of in situ DRIFTS show that the NH₃-SCR reaction path on the catalyst conforms to the L-H mechanism,-NH₂ and monodentate nitrite are the main intermediate species,and the formation of various sulfate substances on the catalyst surface is the main cause of catalyst sulfur poisoning.（2）Based on the poor sulfur resistance of CoFe₂O₄,CoFe₂O₄/Ti O₂ catalyst was optimized,and the low-temperature denitration activity and sulfur resistance were further studied.The results show that the denitration efficiency of the catalyst loaded with 20 wt.%CoFe₂O₄ is 93%at 160℃,and the N₂ selectivity is close to 100%.Through the evaluation experiment of denitration performance,it is concluded that under the conditions of NH₃/NO=1～1.2 and O₂ content of 3～5%,CoFe₂O₄/Ti O₂catalyst has excellent denitration performance at low temperature and high space velocity resistance.The results of sulfur resistance test showed that the sulfur resistance of the catalyst was improved by 20%after the introduction of Ti O₂,and the denitration activity of the catalyst remained at about 90%after the introduction of SO₂for 12 h.Through analysis and characterization,it is found that the reduction temperature of Fe in CoFe₂O₄/Ti O₂ catalyst is lower than that of CoFe₂O₄,and there are more oxygen species adsorbed on the surface,which indicates that CoFe₂O₄/Ti O₂has stronger redox ability.In addition,the content of L-acid on the surface of CoFe₂O₄/Ti O₂ catalyst increased by 8.6%,which further promoted the efficient NH₃-SCR reaction.Combined with the characterization analysis of NO_x-TPD and in situ DRIFTS,it is shown that the NH₃-SCR reaction on the catalyst follows the L-H mechanism,and the catalyst has a strong ability to activate NO_x.Bidentate nitrate is an important intermediate species,and the monodentate nitrate generated by its decomposition can react with NH₃ activated by adsorption.In the experiment of sulfur resistance of catalyst,it is found that the catalyst has strong sulfur resistance because of its large specific surface area and pore volume.In addition,the chelated bidentate sulfate species formed on the surface of the catalyst will occupy some Fe sites,but will not affect the NH₃ and NO_x species adsorbed on the surface,and HSO₄^?species can increase the acidic sites on the surface of the catalyst,promote the activation and dehydrogenation of NH₃,and further promote the NH₃-SCR reaction.