Precise regulation of acid pretreatment for red mud SCR catalyst: Targeting on optimizing the acidity and reducibility

• The optimum SCR activity was realized by tuning the acid pretreatment. • Optimized catalysts showed NO_x conversion above 90%. • The NH₃ and NO adsorption capacity of Al-O₃-Fe is stronger than Fe-O₃-Fe. • The formation of almandine consumes Fe³⁺ and Al³⁺ and weakens their interaction. Red mud (RM), as an alkaline waste, was recently proved to be a promising substitute for the SCR catalyst. Dealkalization could improve the acidity and reducibility of red mud, which were critical for SCR reaction. However, the dealkalization effect depended on the reaction between acid solution and red mud. In this study, we realized the directional control of the chemical state of active sites through tuning the acid pretreatment (dealkalization) process. The pretreatment endpoint was controlled at pH values of 3–5 with diluted nitric acid. When the pH values of red mud were 3 and 5 (CRM-3 and CRM-5), activated catalysts showed NO_x conversion above 90% at 275°C–475°C. The high initial reaction rate, Ce³⁺/(Ce³⁺ + Ce⁴⁺) ratio, and surface acidity accounted for the excellent SCR performance of CRM-5 catalyst. Meanwhile, more Fe³⁺ on the CRM-3 surface improved the NH₃ adsorption. There was a strong interaction between Al and Fe in both CRM-5 and CRM-3 catalysts. DFT results showed that the adsorption capacity of the Al-O₃-Fe for NH₃ and NO is stronger than that of Fe-O₃-Fe, which enhanced the NO_x conversion of the catalyst. However, the almandine was formed in CRM-4, consumed part of Fe³⁺ and Al³⁺, and the interaction between Al and Fe was weakened. Also, deposited almandine on the catalyst surface covered the active sites, thus leading to lower NH₃-SCR activity.