Modified red mud catalyst for the selective catalytic reduction of nitrogen oxides: Impact mechanism of cerium precursors on surface physicochemical properties

Red mud, as industrial solid waste, causes severe environmental problems such as soil alkalization and groundwater pollution. In this work, we researched and developed the red mud as a selective catalytic reduction catalyst for NOx removal with NH3 (NH3-SCR). After selective dissolution and specific heat treatment, different Ce precursors were used to modifying its physical and chemical properties. The results showed that Ce(NO3)3 and Ce(NH4)2(NO3)6 modified red mud (RMcn and RMcan) had excellent SCR performance below 300 °C. Ce(SO4)2 modified red mud (RMcs) showed relatively low NOx conversions at 200–300 °C. The redox property was improved with the Ce(NO3)3 and Ce(NH4)2(NO3)6, while depressed with the Ce(SO4)2. Agglomerates generated on the RMcs and blocked the accumulated pores due to the formation of Ce2(SO4)3. The surface acidity of RMcs enhanced with increased adsorption for ammonia. However, these new adsorbed ammonia species, highly related to the sulfate from the Ce2(SO4)3, were inert and did not react with the adsorbed or gaseous NO species at 200–300 °C. The abundant surface lattice oxygen from CeO2 microcrystals improved the catalytic oxidation capacity of the RMcn and RMcan.