Development of Novel Mn-based Catalysts for Low Temperature Selective Catalytic Reduction of Nitrogen Oxides with Ammonia

The development of highly efficient and durable low-temperature NH₃-SCR catalysts is crucial for reducing NOx emissions in industrial processes. Over the past few decades, various strategies have been developed and implemented to enhance Mn-based catalysts. However, challenges such as a narrow temperature range, limited resistance to water, and susceptibility to SO₂ poisoning persist, hindering their practical applications. This thesis focuses on the improvement of MnOx catalysts for low-temperature NH₃-SCR, aiming to expand the operational temperature range, improve N₂ selectivity, and increase resistance to water and SO₂. This enhancement is pursued through three distinct strategies: constructing core-shell structures, doping with different metals, and optimizing the synthesis process. In the first part, a novel core-shell-shell catalyst, MnO_x@TiO₂@CeO₂, was fabricated. This catalyst exhibited exceptional low-temperature activity, achieving NOx conversion >80% across a wide temperature range, both under dry (120-260 °C) and wet conditions (180-255 °C), even at high weight hourly space velocity (WHSV) of 240,000 mL/(gꞏh). The inner TiO₂ shell significantly increased the specific surface area, surface Mn4+/Mn ratio and chemisorbed oxygen content, thereby providing more active sites and promoting the oxidation of NO to NO₂. The outer CeO₂ shell not only extensively increased the surface acid sites but also enhanced the acid strength, which was beneficial for ammonia adsorption, resulting in a good water tolerance. This research highlights the importance of a rational core-shell catalyst design to achieve both high low-temperature performance and enhanced durability for Mn-based NH₃-SCR catalysts.
In the second part, MnO_x catalysts doped with Fe or Al were prepared using a preferred solvothermal method. MnFeO_x exhibited superior low-temperature SCR activity compared to MnAlO_x, achieving over 90% NO_x conversion at a broader temperature range of 100-250 °C, even at a high WHSV of 240,000 mL/(gꞏh). Furthermore, MnFeO_x exhibited better tolerance to water and SO₂ as well as thermal regeneration. The improved performance of MnFeO_x was attributed to a higher Mn⁴⁺ ratio, increased reducibility, and more surface chemisorbed oxygen. Moreover, Fe doping facilitated electron transfer between Mn and Fe ions and weakened the interaction between active sites and deposited sulfates during the heating process. This promoted the re-oxidation of Mn²⁺ to catalytically active Mn³⁺/Mn⁴⁺. This study suggests that doping MnO_x catalysts with elements like Fe can improve their SO₂ tolerance by weakening interactions with sulfates, especially when combined with appropriate treatments like thermal regeneration.
In the third part, three Mn-Fe oxide catalysts were synthesized through different methods. Among these catalysts, MnFeO_x-H, prepared via the solvothermal method, exhibited the highest activity at low temperatures (<200 °C). Conversely, the MnO_x@FeOy catalyst, synthesized using a one-pot method combining seed-mediated growth with galvanic replacement, displayed improved tolerance to SO₂ and H₂O in comparison to the other two catalysts. The enhanced SO₂ resistance was attributed to the inhibitory effect of water on the oxidation and deposition of SO₂ on Mn sites, as confirmed by TGA and XPS results. This research sheds light on improving the SO₂ resistance of Mn-based catalysts. With the appropriate catalyst, H₂O, often regarded as an inhibitory component in flue gas, can serve as a protective barrier by impeding the adsorption of SO₂ on active Mn sites.
In summary, this thesis marks significant progress in enhancing the performance of MnO_x-based NH₃-SCR catalysts and uncovers intriguing findings. We anticipate that the insights gained from this work will inspire future catalyst designs with enhanced applicability for removing NO_x from industrial flue gases.