The Study On SCR Catalytic Performance And Mechanism Of The ?-Fe₂O₃-based Catalysts With Regular Morphology

With the increasingly stringent emission control standards of nitrogen oxides being implemented,the control of nitrogen oxides in thermal power plants is facing new challenges.Selective catalytic reduction?SCR?technology has been widely used in the flue gas denitrification of coal-fired power plants due to its technical maturity and high removal efficiency,and catalyst is the heart of the SCR technology.Due to the expense and toxicity of the commercial vanadium-based SCR catalysts,developing non-vanadium-based catalysts is essential.?-Fe₂O₃-based catalysts are promising owing to their low cost,high thermal stability,and low toxicity.In this study,the effects of morphology and facet of ?-Fe₂O₃ catalysts in the NH₃-SCR reaction have been investigated.The synthetic protocols of H₂SO₄ erosion or WO₃ loading are used to prepare well-defined and high-performance ?-Fe₂O₃-based SCR catalysts.Changes in the physical and chemical property of catalysts,the nature of the active sites,and the reaction mechanism of SCR are analyzed by a variety of characterizations.Three important results associated with the Fe₂O₃-based catalysts are as follows:?1?The theoretical calculation shows that the {001} crystal face of the ?-Fe₂O₃ exhibits the highest surface energy and oxygen atom density among the most widely studied crystal faces.Therefore,it is speculated the {001} crystal face obtains high catalytic performance.Comparing the ?-Fe₂O₃ catalysts with well-defined morphologies,including hexagon,rod,and diamond,it is found that the hexagon-shaped ?-Fe₂O₃,exposing mainly {001} crystal facets,exhibits the best SCR activity,which is in accordance with the prediction of theoretical calculation.The {001} surface of ?-Fe₂O₃ possesses strong surface acidity and abundant surface unsaturated oxygen species,which is favorable for the adsorption and the activation of NH₃ species.The ?-Fe₂O₃ surface is dominated by Lewis acid sites.In SCR reaction,the adsorbed NH₃ at Lewis acid sites reacts with the gaseous NO,which is confirmed by diffuse reflectance infrared spectroscopy.However,the {001} surface accumulates nitrate species and consumes the active NH₂^-species,thus resulting in unwanted side reactions and the decrease of SCR activity at high temperatures.?2?Compared with the protocols of HCl or HNO₃ etching,the H₂SO₄ etching of hexagonal ?-Fe₂O₃ catalyst improves the SCR activity much more greatly.The catalyst of ?-Fe₂O₃ eroded by 5 wt% H₂SO₄ obtains good SCR activity,good resistance of H₂O and SO₂ and high N₂ selectivity.The H₂SO₄ etching produces a Fe₂?SO₄?₃ surface layer.The synergistic effect of the Fe₂?SO₄?₃ layer and the uncovered surface ?-Fe₂O₃ components has a particular function on the overall catalytic activity.In SCR reaction,the Fe₂?SO₄?₃ layer participates by providing abundant Br?nsted acid sites for NH₃ adsorption,while the exposed ?-Fe₂O₃ on the surface supplies adsorption and oxidation sites for NO molecules.NH₃ adsorbs on the Br?nsted acid site of the surface,forming NH₄⁺,and the NO₂ formed on the ?-Fe₂O₃ surface sites reacts with the NH₄⁺ to generate H₂O and N₂ molecules.?3?Because hexagonal Fe₂O₃ exposes mainly {001} facts on its surface,the WO₃/Fe₂O₃ catalyst with a single atomic layer structure of WO₃ could be successfully synthesized.The WO₃/Fe₂O₃ catalyst with a nominal WO₃ coverage of 0.05 monolayers?ML?shows excellent catalytic performance,good resistance of H₂O and SO₂ as well as high N₂ selectivity in SCR reaction.The active sites of the WO₃/Fe₂O₃catalyst are located at the contact edge of the WO₃ surface layer and the Fe₂O₃ support.A strong electronics transfer from Fe₂O₃ to WO₃ layer takes place.The content of active surface oxygen species increases greatly for the WO₃-containing sample.A large number of Br?nsted acid sites and Lewis acid sites on the WO₃ surface layer of this 0.05 ML WO₃/Fe₂O₃ render it highly catalytic active.Both NH₃ species that adsorbed on Lewis acid sites and NH₄⁺ that adsorbed on the Br?nsted acid sites are found to participate in the SCR reaction.Moreover,the surface modification protocols,including H₂SO₄ erosion or WO₃ loading,are adopted to improve the SCR catalytic activity of amorphous ?-Fe₂O₃ as well,showing a great promise for practical application.The in-depth understanding of the active sites and SCR reaction mechanism would provide theoretical guidance and data support for the synthesis of other high-performance Fe₂O₃-based SCR catalysts.