Selective catalytic reduction of NO with NH3 over HZSM-5/CeO2 hybrid catalysts: Relationship between acid structure and reaction mechanism

Abstract

CeO₂ is active in the NH₃ selective catalytic reduction of NO (NH₃-SCR) reaction due to its excellent redox properties. However, the low surface acidity of CeO₂ limits its NH₃-SCR activity. In this study, highly active HZSM-5-modified HZSM-5/CeO₂ (Z/Ce) mixed oxide catalysts were prepared by a simple physical milling method. The HZSM-5 modification stimulated many thermally stable Brønsted-acid structures and high oxygen mobility on the surface of the Z/Ce catalysts, promoting the acid cycling pathways driven by redox macrocycles, and accelerating NO removal. The NO conversion of 25Z/Ce at 200–400 °C is about 34.8 %-62.3 % higher than that of CeO₂. A series of physicochemical and in situ interfacial reactions were analyzed to explore the medium- and high-temperature reaction mechanism on the CeO₂ surface. The Eley-Rideal reaction mechanism on the Lewis-acid structure with NO_(g) as the reactant is deduced. When HZSM-5 modified CeO₂, more NO was activated to NO_2(g) on the surface of the Z/Ce catalyst at medium and high temperatures (300 °C). This significantly enhanced the Eley-Rideal reaction mechanism with NO_2(g) as the main reactant on the Brønsted-acid structure as well as the occurrence of the “Fast-SCR” reaction. This work provides a simple method to improve the performance of CeO₂ in the NH₃-SCR reaction and elucidates the reaction mechanism.