Anti-alkali Metal Poisoning Mechanism Of Iron-based Oxide Catalyst In NH₃-SCR

Nitrogen oxides（NO_x）are one of the main atmospheric pollutants,causing many environmental issues including acid rain,photochemical smog and haze.Selective catalytic reduction of NO_xwith NH₃has been widely applied as an efficient technology for controlling NO_xemissions from stationary or mobile sources.As the key to this technology,the research and development of NH₃-SCR catalysts with excellent performance has attracted continuous attention from researchers all over the world.Iron-based catalysts have become a prospective alternative of commercial vanadium-based catalysts owing to their good activity at medium and high temperatures,high N₂selectivity,strong resistance to SO₂poisoning,environmental friendliness and low price.However,they are still confronted with problems of undesired low-temperature activity,narrow temperature window and poor resistance to alkali metals poisoning.Therefore,it is of great significance to develop a novel iron-based catalyst with good low temperature activity and anti-poisoning performance as well as to explore its reaction and resistance mechanism.In this paper,novel highly alkali-resistant iron-based catalysts were originally developed,and a series of characterization and tests were employed to study the structure and surface properties,reaction mechanism and resistance mechanism of the catalysts.The content of this paper is as follows:（1）Two supported iron oxide catalysts（α-Fe₂O₃/TiO₂andγ-Fe₂O₃/TiO₂）with different crystal structures were prepared.The structure,surface acidity and redox properties of the active components of the two catalysts all showed significant differences.The reaction paths at low temperatures and high temperatures were also significantly different so that they exhibited different activity trends at different temperature sections,respectively.（2）A novel phosphate-modified iron-based catalyst with excellent resistance to alkali metal poisoning was originally developed.The structure,surface physical and chemical properties as well as anti-poisoning mechanism of the catalyst were studied in depth.The study found that the enhanced surface acidity and reducibility of the catalyst significantly improved the SCR activity and broadened the temperature window.The combination of the in-situ generated phosphate(PO₄^3-)and K⁺protected the active species from poisons,thereby ensuring the redox capability and avoiding the accumulation of inert nitrate species.The remained active nitrate species adsorbed on the poisoned catalyst surface and the retained NH₃adsorption capacity allowed the Langmuir-Hinshelwood reaction pathway to proceed smoothly so that it could maintain a highly de-NO_xactivity.（3）A TiO₂ supported iron-vanadium catalyst（FeV/TiO₂）with excellent low-temperature activity and a wide temperature window was prepared and its anti-alkali metal poisoning performance and alkali-resistance mechanism were explored.After introducing V species,a large number of Br（?）nsted acid sites newly generated in the catalyst enhanced the surface acidity,and the strong interaction between Fe and V species improved the redox property,thereby significantly promoting the adsorption and activation of NH₃in SCR reaction.After the catalyst was poisoned by K,the surface acidity was still maintained to ensure the adsorption capacity for NH₃and there were still active nitrate species,so that the progress of the L-H reaction path was hardly affected by alkali metal poisoning.In summary,crystal structure adjustment,phosphate modification and doping were employed to effectively improved the low-temperature activity and alkali-resistance of the iron-based catalysts,and the corresponding reaction path and anti-poisoning mechanism were discussed in this paper.We believe that the research in this paper could provide guidance for the design,preparation and application of novel iron-based de-NO_xcatalysts with high efficiency and alkali resistance.