Acid-pretreated red mud for selective catalytic reduction of NOx with NH3: Insights into inhibition mechanism of binders

Abstract Recently, we proposed an efficient pretreatment method for waste red mud (RM) to be utilized as an SCR catalyst. However, in terms of the shaping process, RM’s hydraulicity and easily pulverization always cause the loss of surface areas and active species, which limits its application. In this study, we prepared shaped RM catalysts using the sesbania powder (sp), guar gum (gg), and carboxymethylcellulose sodium (cs) to investigate the impact of binders on the RM catalysts. The results exhibited that the RM-15sp-550 showed the highest activity with T90 (the reaction temperature with NOx conversions higher than 90%) active window of 325–450 °C. The gg and cs strongly inhibited the surface reducibility and NH3 adsorption, which was caused by the decrease of the Fe(III), absorbed oxygen, and acid sites (especially the Bronsted acid sites). The reactions between adsorbed ammonia species on the Bronsted acid sites and gaseous NO were restricted on the RM-15gg-500 and RM-15cs-500. Compared with the RM-15gg-500 and RM-15cs-500, the RM-15sp-500 exhibited more accumulated pores. Raman experiments proposed that more oxygen vacancies formed on the RM-15sp-500 and NH3 were selectively chemisorbed on the FeTiO3 during the NH3 presorbing process, which benefited SCR reaction. The strong viscosity at low temperatures and low decompose temperature of the sp minimized its impact on the RM catalyst. This made it an excellent binder for industrial RM catalyst. The cost analysis showed that the price of the RM catalyst was 67% lower than that of commercial V-W-Ti catalyst, and the use of RM catalyst can also alleviate the RM accumulation issue.