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Abstract: The NH3 reactivity was investigated over H-ZSM-5 and iron oxide deposited H-ZSM-5 by temperature-programmed surface interaction (TPSI) of adsorbed NH3 (NH3(ad)) with NO, O2, and “NO+O2”, and by steady-state dynamic reactions consisting of “NH3 +NO”, “NH3 +O2”, and “NH3 +NO+O2” systems. It was found that O2 primarily interacts with NH3(ad) on Brønsted acid sites. NO mainly interacts with NH3(ad) on Lewis acid sites, whereas its interaction with NH3(ad) on Brønsted acid sites results in N2O formation. TPSI provides direct evidence of the “fast” reaction of NO/NO2 with NH3(ad) on Brønsted acid sites in “NO+O2” environment, where O2 concentration plays a critical role. Iron oxide enhances all interactions, particularly those with NH3(ad) on Brønsted acid sites, as witnessed by both TPSI and steady-state reactions. But, iron oxide does not change the activation energy barrier for NH3 conversion, indicating that both acid sites and iron oxide are essential for NH3 SCR of NO x . As a result, a dual mechanism over Lewis and Brønsted acid sites was proposed. [Copyright &y& Elsevier]