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

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