To understand the effect of H 2 on the selective catalytic reduction of NO x with C 2 H 5 OH over Ag/Al 2 O 3 , surface intermediates were examined using in situ DRIFTS spectra, and by-products were identified using GC–MS. Results showed that H 2 addition promoted the partial oxidation of C 2 H 5 OH to form enolic species, and enhanced the reaction of NCO with NO+O 2 at low temperature. We propose that the enhancement of the enolic species was the main contributor in accelerating NO x reduction under the presence of H 2 over Ag/Al 2 O 3 at low temperatures.