Enhanced low-temperature SCR performance of metal Oxide/Cu-SSZ-13 composite catalysts: The role of oxygen species in metal oxide

Abstract

A composite catalyst composed of metal oxide and Cu-SSZ-13 components (OXZEO) presents a promising approach to solve the current challenge of improving the low-temperature efficiency of SCR catalysts. The diverse forms of oxygen present within the metal oxide components impact the formation of nitrate, which plays a critical role in the deNOx pathway, thereby influencing the overall SCR reaction in OXZEO. Despite this, the relationship between the various types of oxygen in metal oxide and their catalytic role in OXZEO catalysts remain not fully understood. To clarify this relationship, the activities of Cu-SSZ-13 combined with MnOx, CeOx, and ZrOx, respectively, were compared and analyzed. The low-temperature pathway (below 200 °C) for the three catalysts is primarily ascribed to the interplay between oxide-derived nitrate precursors and NH3 bound to Brønsted acids within the zeolite. However, MnOx/Cu-SSZ-13 demonstrated a stronger synergistic effect, achieving around 50 % higher NOx conversion than Cu-SSZ-13 alone at 200 °C, which was superior to the other two composite catalysts. Characterizations reveal that MnOx with surface oxygen vacancies can supply a significant quantity of chemisorbed oxygen, thus producing more reactive bridged nitrates and bolstering the reaction between nitrate and NH4+ in MnOx/Cu-SSZ-13. On the contrary, for CeOx/Cu-SSZ-13 and ZrOx/Cu-SSZ-13 catalysts, the lattice oxygen of the metal oxide encourages the formation of less active bidentate and monodentate nitrates, resulting in inferior low-temperature SCR performance.