A Mechanism Study On Low-temperature SCR De-NO_x Over Iron-based Catalysts With The Analysis Of DRIFTS

Selective catalysis reduction?SCR?of NO_x by ammonia has become one of the most popular technologies in the de-NO_x of waste gases from stationary sources due to its high efficiency,high selectivity,and low cost.However,the widely used commercial catalyst nowadays is V2O5-WO3/TiO2,which works between 300–400°C.Accordingly,the SCR reactor is installed in a high-dust?HD?arrangement in most thermal power plants at present,where the high-temperature,high-dust and high-sulphur environment leads to the catalyst sintering,blocking and poisoning.The tail end?TE?arrangement in which the SCR reactor is located downstream of the dust and SO2 removal units can avoid the aforementioned problems,but it is needed that catalyst with high activity at low and medium temperatures.In this thesis,therefore,the iron-based catalysts with high selectivity,low cost and low pollution for SCR de-NO_x with NH₃ at low temperatures were developed.The in-situ diffuse reflectance infrared Fourier transform spectroscopy?DRIFTS?experiments were carried out to investigate the mechanism of SCR de-NO_x reaction over iron-based catalysts,which would lay a foundation for the development of high-efficiency SCR de-NO_x catalysts at low temperatures.At first,the nano-?Fe₂O₃,nano-?Fe₂O₃,hematite,limonite and siderite catalysts were prepared in the thesis.Then,the activities of different iron-based catalysts,and sulfur and water resistance performances were experimental studied on the de-NO_x activity test device.Furthermore,the catalysts before and after the reaction were analyzed by means of XRD,TEM,BET,XRF and TG-DTG,and the properties of nano-?Fe2O3,nano-?Fe2O3,hematite,limonite and siderite catalysts were studied by in-situ DRIFTS experiments.Through the DRIFTS research,the adsorption and conversion processes of NH3 and NO on the catalyst surface were studied by in-situ transient and quasi-steady state adsorption experiments.The intermediate products and active centers of SCR de-NO_x reaction over the iron-based catalysts were obtained,and the reaction mechanism over iron-based catalysts such as nano-?Fe2O3 catalyst and siderite catalyst was studied,and the following research conclusions were obtained.?1?The nano-?Fe2O3 catalyst and siderite catalyst prepared by calcination at 450°C have excellent medium and low temperature SCR de-NO_x performance,and the NO conversion efficiency is higher than 95%in the temperature range of 150?-300?.The activation energy of the SCR reaction on the nano-?Fe₂O₃ catalyst is 57.3 kJ/mol.The catalysts characterization results show that the nano-?Fe₂O₃ catalyst has large specific surface area,relatively regular pore structure and exhibits high thermal stability.In addition to?Fe₂O₃,the active component of the siderite catalyst also contains?Fe₂O₃ with higher activity,and a small amount of Mn is also doped in crystal lattice,which improves the catalytic de-NO_x activity of siderite catalyst,but also makes the sulfur and water resistance performance worse than that of nano-?Fe₂O₃ catalyst.?2?There are different Lewis acid sites and Br?nsted acid sites on the surface of nano-?/?Fe2O3catalysts and iron ore catalysts,and coordinated NH₃ and NH₄⁺are formed by adsorption of NH₃,respectively.Lattice oxygen or adsorbed dissociated oxygen can dehydrogenate NH3 adsorbed at Lewis acid acid sites into NH2 species.The descending order of dehydrogenation activity is:adsorption dissociation oxygen>nano-?Fe2O3 catalyst lattice oxygen>nano-?Fe2O3 catalyst lattice oxygen.The nano-?Fe₂O₃ catalyst has higher NH₃ dehydrogenation activation performance.?3?The main component of the siderite catalyst calcined at 450?is?Fe2O3 doped with a small amount of Mn.The nano-scale crystallites of the siderite catalyst fully expose the acid sites of the catalyst,while the doping of Mn and residual CO₃^2-enhances both the acidity of Lewis acid and Br?nsted acid.The doping of CO₃^2-has the effect of fixing lattice oxygen of siderite catalyst,which reduces the oxidative dehydrogenation ability of the lattice oxygen to NH3.However,the excellent electron transfer ability between Mn-Fe increases the oxidative dehydrogenation ability of adsorbed dissociated oxygen to NH₃ because of the Mn doping.?4?The adsorption processes of NO are complicated but have a high similarity between different iron oxide catalysts.O₂ has a great effect on the adsorption of NO on the catalyst surface.O₂ can obviously accelerate the adsorption of NO to the catalysts surface.NO reacts with lattice oxygen or adsorbed oxygen to form NO2,which is the key reaction for NO adsorption to the catalyst surface.NO₂ is further converted to form NO₃^-and NO₂^-on the surface of the catalyst.The higher NO oxidation activity of the lattice oxygen of the nano-?Fe₂O₃ catalyst can promote the oxidation of NO to form NO₂ and further promote the adsorption of NO.The active sites of NO oxidation reaction have been increased since the nano-scale crystallites of the siderite catalyst have more crystal defects,and the doping of Mn strengthens the electron transfer between Mn and Fe.?5?The SCR de-NO_x mechanism of nano-?Fe₂O₃ and siderite catalysts is divided into the E-R reaction mechanism with NH₂NO as the main intermediate in high temperature range,The L-H reaction mechanism withO=N-[O-Fe-NH₃]₂ as the intermediate product in middle temperature range and the L-H reaction mechanism with NH₄NO₃ and NH₄NO₂ as the main intermediates in low temperature range.The SCR reaction-promoting effect of the nano-?Fe₂O₃ catalyst is mainly based on the fact that the nano-?Fe₂O₃ catalyst has a highly reactive lattice oxygen,which can directly oxidize NO to NO₂ to promote the low-temperature L-H mechanism reaction.Besides,it can directly participate in the dehydrogenation activation of NH₃ to form NH₂ species to improve high-temperature E-R mechanism reaction,and can also increase the decomposition of O=N-[O-Fe-NH₃]₂ intermediate species to promote the intermediate-temperature L-H mechanism reaction.Furthermore,the SCR reaction-promoting effect of the siderite catalyst is mainly based on the fact that the catalyst consists of a large number of nano-scale crystallites with many crystal defects and abundant adsorption centers and activation centers.The doping of a small amount of Mn on the catalyst further increases the crystal defects.The excellent electron transfer ability between Fe-Mn promotes the oxidation of NO to NO₂ and also improves the oxidative dehydrogenation of NH₃ by O₂ to NH₂.In summary,based on the experimental study of SCR de-NO_x activity over a series of iron-based catalysts,in-situ DRIFTS experiments were conducted to explore the intermediates of SCR reaction over nano-?Fe₂O₃ catalyst,siderite catalyst and other iron-based catalysts.The reaction pathways of SCR de-NO_x reaction over iron-based catalysts were clarified,and the mechanisms were revealed.The research results have important reference value for the development of high-efficiency,low-cost and environment-friendly low-temperature iron-based catalyst.