Engineering In-Co 3 O 4 /H-SSZ-39(OA) Catalyst for CH 4 -SCR of NO x : Mild Oxalic Acid (OA) Leaching and Co 3 O 4 Modification.

Zeolite-based catalysts efficiently catalyze the selective catalytic reduction of NO x with methane (CH 4 -SCR) for the environmentally friendly removal of nitrogen oxides, but suffer severe deactivation in high-temperature SO 2 - and H 2 O-containing flue gas. In this work, SSZ-39 zeolite (AEI topology) with high hydrothermal stability is reported for preparing CH 4 -SCR catalysts. Mild acid leaching with oxalic acid (OA) not only modulates the Si/Al ratio of commercial SSZ-39 to a suitable value, but also removes some extra-framework Al atoms, introducing a small number of mesopores into the zeolite that alleviate diffusion limitation. Additional Co 3 O 4 modification during indium exchange further enhances the catalytic activity of the resulting In-Co 3 O 4 /H-SSZ-39(OA). The optimized sample exhibits remarkable performance in CH 4 -SCR under a gas hourly space velocity (GHSV) of 24,000 h -1 and in the presence of 5 vol% H 2 O. Even under harsh SO 2 - and H 2 O-containing high-temperature conditions, it shows satisfactory stability. Catalysts containing Co 3 O 4 components demonstrate much higher CH 4 conversion. The strong mutual interaction between Co 3 O 4 and Brønsted acid sites, confirmed by the temperature-programmed desorption of NO (NO-TPD), enables more stable N x O y species to be retained in In-Co 3 O 4 /H-SSZ-39(OA) to supply further reactions at high temperatures.