Understanding the direct relationship between various structure-directing agents and low-temperature hydrothermal durability over Cu-SAPO-34 during the NH3-SCR reaction

Abstract

Cu-SAPO-34 catalysts used for the selective catalytic reduction with ammonia (NH₃-SCR) reaction deteriorate when exposed to moisture at low temperature (<100 °C), which seriously hinders their commercial application prospects. In this work, Cu-SAPO-34 catalysts templated using various structure-directing agents (SDAs), i.e., DEA, TEA, PA, and TEAOH, were elegantly fabricated via a “one-pot” protocol. The direct relationship between the various SDAs and low-temperature moisture degradation were determined in this work. Via thorough characterizations, it was evidenced that the Cu-SAPO-34 catalysts with more Si–O(H)–Al bonds, more Cu(OH)⁺ moieties and fewer CuO_x clusters exhibited higher SCR catalytic activities and N₂ selectivity. After low-temperature hydration treatment, the relative crystallinity, porosity, Si–O(H)–Al density, and active Cu(OH)⁺ in the catalysts declined, but in varying degrees, where the Cu-SAPO-34 catalysts possessing more Si–O(H)–Al density and Cu(OH)⁺ moieties being the more susceptible to hydrolysis and structural degradation, and vice versa. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis confirmed that both the fresh and degraded Cu-SAPO-34 catalysts primarily follow the Langmuir–Hinshelwood pathway due to its faster reaction rate and lower reaction barrier than the Eley–Rideal pathway.