The Research Of Preparation And Performance Of Ceria-based Catalyst In The NH₃-selective Catalytic Reduction

Coal based energy structure in China will not change in a long time in the future,however,the burning of coal will produce large amounts of nitrogen oxides(NOx),which results in serious environmental problems.The total amount of NOx in our country is high and emission control is imperative.Ammonia selective catalytic reduction(NH3-SCR)has become a mainstream denitration technology in coal-fired power plants due to its relatively mature technology and high NOx removal efficiency.Ceria based catalysts are widely used in NH3-SCR catalysts because of their good redox properties,appropriate surface acidity,high storage/release oxygen capacity,and abundant resource storage.In this paper,the performance of ceria based SCR denitrification was optimized from three aspects of preparation method,additives and morphology control.The mechanism of SCR removal by cerium based catalysts was also studied.The specific contents are as follows:1.The MoO3-CeO2 catalysts were prepared respectively by impregnation and coprecipitation mehods and the effects of different preparation methods on texture properties,redox perporty,the surface acidity,adsorption ability of nitrate species and the NH3-SCR performance was investigated.The obtained conclusions were as follows:1)the catalyst prepared by precipitation method(MoCe-cp)had larger surface area,more surface oxygen species and better reducility than that of the catalyst prepared by impregnation method(Mo-CeO2),but these factors were not the decisive factors that influenced the performance of catalyst.2)The Mo-CeO2 catalyst had higher concentration of surface molybdenum species than MoCe-cp,which resulted in weaker adsorption ability of nitrate species and more Bronsted acid sites.While the acid sites on MoCe-cp catalysts were dominated by Lewis acid sites.3)The competition adsorption existed between bridged nitrates and NH3 gas bound to Lewis acid sites,causing the surface acid sites to be hindered by nitrate in the NH3-SCR reaction.4)The NH3-SCR reaction on MoCe-cp and Mo-CeO2 catalysts was followed by Eley-Rideal mechanism,and the weaker adsorption ability of nitrate species and more Bronsted acid sites on Mo-CeO2 catalyst facilitated the NH3-SCR reaction proceed smoothly via the Eley-Rideal pathway,which resulted in better catalytic activity.2.The TiO2/CeO2 catalyst was modified with ultra-low content of CuO and the influence of CuO content on NH3-SCR catalytic performance of TiO2-CuO/CeO2 catalysts was investigated.The study found that even the minor change of CuO content had significant influence on the catalytic activity of TiO2-CuO/CeO2 catalyst and the optimized Ti1Cu0.005/CeO2 catalyst had good NO removal efficiency in the temperature range of 200 ?-400 ? and showed excellent sulfur resistance at 300 ?.3.Exploring the reasons for the influence of CuO content on the catalytic activity of TiO2-CuO/CeO2 catalyst.we got the following conclusions:1)The addition of CuO significantly improved the reducibility of TiO2/CeO2 catalyst,but strong redox properties could lead to excessive NH3 oxidation which caused the decline of NO conversion and N2 selectivity.2)The addition of CuO increased the content and mobility of adsorbed oxygen species on the catalyst surface,which was beneficial to oxidation of NO to NO2 to promote the occurrence of "fast SCR" reaction,and then improve the low temperature activity of the catalyst.3)The Br(?)nsted acid site on the catalyst is more favorable for the improvement of low temperature activity.4)The addition of CuO weakened the adsorption ability of nitrates species which is conducive to prevent the surface acid sites from being hindered by nitrates species in the NH3-SCR reaction.This could facilitate the NH3-SCR reaction proceed smoothly via the Eley-Rideal pathway.4.A novel MnO,-CeO2 nanosphere catalyst(Mn/CeO2-NS)with assembled structure from tiny particles was prepared and showed superior DeNOx performance compared with the catalyst composed of MnOx supported on traditional CeO2 nanoparticles(Mn/CeO2).Meanwhile,the Mn/CeO2-NS exhibited 80%NO conversion at 100 ?and good resistance to high space velocity and SO2.The catalyst also displayed excellent stability,as no deactivation was observed during the 100 h testing at 150 ?.The characterization results showed that:compared to the traditional CeO2 powder,the larger surface area and abundant pore structure of spherical CeO2 was conducive to the dispersion of Mn species,which resulted in enhancing the interaction between the carrier and the active species of manganese oxide,thus improving the redox performance of Mn/CeO2-NS catalyst.Moreover,the better reducibility and more surface active oxygen species on Mn/CeO2-NS catalyst was beneficial to oxidation of NO to NO2,which in turn could promote the occurrence of "fast SCR" reaction,and then improved the low temperature activity of the catalyst.