Engineering In-Co₃O₄/H-SSZ-39(OA) Catalyst for CH₄-SCR of NO_x: Mild Oxalic Acid (OA) Leaching and Co₃O₄ Modification

Zeolite-based catalysts efficiently catalyze the selective catalytic reduction of NO_x with methane (CH₄-SCR) for the environmentally friendly removal of nitrogen oxides, but suffer severe deactivation in high-temperature SO₂- and H₂O-containing flue gas. In this work, SSZ-39 zeolite (AEI topology) with high hydrothermal stability is reported for preparing CH₄-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 Co₃O₄ modification during indium exchange further enhances the catalytic activity of the resulting In-Co₃O₄/H-SSZ-39(OA). The optimized sample exhibits remarkable performance in CH₄-SCR under a gas hourly space velocity (GHSV) of 24,000 h⁻¹ and in the presence of 5 vol% H₂O. Even under harsh SO₂- and H₂O-containing high-temperature conditions, it shows satisfactory stability. Catalysts containing Co₃O₄ components demonstrate much higher CH₄ conversion. The strong mutual interaction between Co₃O₄ and Brønsted acid sites, confirmed by the temperature-programmed desorption of NO (NO-TPD), enables more stable N_xO_y species to be retained in In-Co₃O₄/H-SSZ-39(OA) to supply further reactions at high temperatures.