Ammonia selective catalytic reduction (NH3-SCR) over copper- and iron-exchanged zeolites is a state-of-the-art technology for removal of nitrogen oxides (NOx, NO, and NO2) from exhaust emissions but suffers from poor low-temperature (i.e., 150 °C) activity. Here we show that hydrothermal aging of Fe-beta, Fe-ZSM-5, and Fe-ferrierite at 650 °C or higher leads to a remarkable increase in NOx conversion from ∼30 to ∼80% under fast NH3-SCR conditions at 150 °C. The practical relevance of this finding becomes more evident as an aged Fe-beta/fresh Cu-SSZ-13 composite catalyst exhibits ∼90% conversion. We propose that a neutral heteronuclear bis-μ-oxo ironaluminum dimer might be created within iron zeolites during hydrothermal aging and catalyze ammonium nitrate reduction by NO at 150 °C. Density functional theory calculations reveal that the activation free energy (125 versus 147 kJ mol-1) for the reaction of NO with adsorbed NO3- species, the rate-determining step of ammonium nitrate reduction, is considerably lower on the bis-μ-oxo ironaluminum site than on the well-known mononuclear iron-oxo cation site, thus greatly enhancing the overall SCR activity.