Cu- and Fe-speciation in a composite zeolite catalyst for selective catalytic reduction of NOx: insights from operando XAS

Abstract

Cu-SAPO-34 (Cu-CZC) and Fe-mordenite (Fe-MOR) and their mechanical mixture (50 : 50) have been exhaustively investigated by means of operando X-ray absorption spectroscopy under NH₃-SCR conditions. Fe K-edge XANES and EXAFS analysis revealed similar Fe-speciation in both the pure Fe-MOR catalyst and the mechanical mixture after high-temperature pretreatment and under reaction conditions. In contrast, analysis of the Cu K-edge dataset unveiled different trends in temperature-driven Cu-speciation in the pure Cu-CZC and mechanical mixture under standard NH₃-SCR conditions. This difference is more evident in a low-temperature range. The presence of the Fe-MOR component in the mixed catalyst can result in the increase of NH₃ and NO concentration in the proximity of Cu-sites at a low temperature regime, which facilitates temperature-driven migration of NH₃-solvated mobile di-amino complexes from Cu-CZC to the MOR framework. This decreases the density of mobile Cu^I complexes in the entire system and results in the higher reducibility of Cu in the mixed catalyst. The obtained results help in understanding the precise speciation of the active sites in such composite catalysts and thus provide future guidance on design of such systems.