Synergistic promotion of transition metal ion-exchange in TiO2 nanoarray-based monolithic catalysts for the selective catalytic reduction of NOx with NH3

Abstract

TiO₂ supported catalysts have been widely studied for the selective catalytic reduction (SCR) of NO_x; however, comprehensive understanding of synergistic interactions in multi-component SCR catalysts is still lacking. Herein, transition metal elements (V, Cr, Mn, Fe, Co, Ni, Cu, La, and Ce) were loaded onto TiO₂ nanoarrays via ion-exchange using protonated titanate precursors. Amongst these catalysts, Mn-doped catalysts outperform the others with satisfactory NO conversion and N₂ selectivity. Cu co-doping into the Mn-based catalysts promotes their low-temperature activity by improving reducibility, enhancing surface Mn⁴⁺ species and chemisorbed labile oxygen, and elevating the adsorption capacity of NH₃ and NO_x species. While Ce co-doping with Mn prohibits the surface adsorption and formation of NH₃ and NO_x derived species, it boosts the N₂ selectivity at high temperatures. By combining Cu and Ce as doping elements in the Mn-based catalysts, both the low-temperature activity and the high-temperature N₂ selectivity are enhanced, and the Langmuir–Hinshelwood reaction mechanism was proved to dominate in the trimetallic Cu–Ce–5Mn/TiO₂ catalysts due to the low energy barrier.