A Study On Developing Multi-composition SCR Catalyst With Wide Operation Temperature Window Via Ball-milling Method

As the backbone of energy structure,combustion and utilization process of coal has caused serious environmental pollution problems.As the main pollutant of coal-fired emission,nitrogen oxides（NO_x）would cause many environmental problems such as acid rain and photochemical smog,which directly lead great harm to the environment and human health.The selective catalytic reduction（SCR）is the most effective mean for removing nitrogen oxides with ammonia,which the core is regarded as the catalyst used in this technology.However,due to the frequent fluctuations of boiler,the commercial vanadium-tungsten-titanium catalyst,which has a narrow operation temperature window（300-400^oC）,has a sharp attenuation of SCR activation and cannot meet the requirement of current emission regulations at low temperatures.To develop efficient NH₃-SCR catalysts with wide temperature range,a series of MnCe（x）-VW（y）/Ti catalysts based on manganese-cerium-titanium（MnCe/Ti）and vanadium-tungsten-titanium（VW/Ti）with various mixing-ratios were prepared via ball-milling-mixed method.The MnCe（x）-VW（y）/Ti catalyst,which prepared by ball-milling with 6 hours,achieved a high level of more than 90%of NO conversion with a well N₂ selectivity（above80%）in a wide temperature range of 200-400^oC.By regulating the mixing-ratio,the best SCR efficiency of the milling-mixed catalyst could be achieved at particular aim-temperature range to meet the needs of the specific application.Moreover,N₂-sorption,X-ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）et.al were used to characterize the MnCe（x）-VW（y）/Ti catalysts.It was found that the ball-milling process for a certain period of time（no more than 6h）could induce the interaction between the metal oxides supported on the surface of the catalyst.The ratios of(V³⁺+V⁴⁺)/Vⁿ⁺,Mn⁴⁺/Mn³⁺and Ce³⁺/Ce⁴⁺increased significantly after milling-mixed,which effectively improved the activation ability of the catalyst for Fast SCR reaction.Moreover,it was observed that the surface acidity and acidity intensity of the MnCe（5）-VW（5）/Ti samples were improved compared to the original MnCe/Ti and VW/Ti catalysts.In-situ diffuse reflectance infrared transform spectroscopy（in-situ DRIFTS）was used to find out the key reactive intermediates on the surface of the ball-milling-mixed catalysts,which were[（NH₄）₂NO₂]and[NH₄NO₃]bounded to the Br?nsted acid sites,and[NH₂NO]occurring on the Lewis acid sites.Due to the former route is based on the Langmuir-Hinshelwood mechanism and meets the requirements of the“fast SCR”,it would dominate the total reaction below 350^oC.The latter is controlled by the Eley-Rideal mechanism and follows the“standard SCR”,ensuring the stable efficiency of SCR reaction at high temperature.At the same time,it may suggest that the parallel working mechanism between the two reaction routes at a wide temperature range might result in effective utilization of the active sites on the MnCe（x）-VW（y）/Ti surface and efficient SCR reaction.