Preparation of Mn/TiO₂ catalysts using recovered manganese from spent alkaline batteries for low-temperature NH₃-SCR

Black mass (BM) from spent alkaline Zn-MnO₂ batteries was used for the first time as a Mn source in the preparation of Mn/TiO₂ catalysts for low-temperature NH₃-selective catalytic reduction (SCR) of NO_x. To recover Mn species and eliminate alkali and Zn species, BM powder underwent DI-water washing, followed by carbothermal reduction. The resulting slags were further dissolved in HNO₃, loaded onto TiO₂ particles with ball milling, and then subjected to calcination. Nearly 100% of Zn species were removed from the BM via carbothermal reduction at 950 °C for 4 h with 5.0 wt% activated carbon. The resulting catalyst, derived from the treated BM, achieved similar NO_x conversion (97%) as the catalyst prepared using a reagent-grade Mn chemical at 160 °C but a higher NO_x-to-N₂ conversion rate at 78%. The promoted N₂ selectivity was attributed to a high Mn⁴⁺/Ti ratio and the presence of impurities from BM, such as Fe³⁺ ions, which enhanced oxidation ability of the catalyst. Conversely, insufficient removal of Zn or carbon additives in the slags led to a decreased Mn concentration, an increased proportion of Mn²⁺/Mn³⁺ species, increased surface OH groups, and reduced oxidation ability on the surface, thus reducing NO_x conversion and N₂ selectivity.