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

TiO2 supported catalysts have been widely studied for the selective catalytic reduction (SCR) of NOx; 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 N-2 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 NOx species. While Ce co-doping with Mn prohibits the surface adsorption and formation of NH3 and NOx derived species, it boosts the N-2 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 N-2 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.