The catalysts used in the selective catalytic reduction (SCR) of NOx with NH3 were prepared from gamma Fe2O3 nanoparticles. The NH3-SCR activity measurement was carried out in a fixed bed reactor. The adsorption characteristics of gamma Fe2O3 nano-catalysts to NH3 and NO were studied with in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements. Experimental activity measurement indicated that gamma Fe2O3 used as a SCR de-NOx catalyst exhibited excellent low-temperature SCR de-NOx performance. The DRIFTS measurements showed that there existed both NH4+ bonded to Bronsted acid sites and coordinated NH3 bonded Lewisto acid sites on the catalyst surface. Coordinated NH3 was the main adsorption product. O-2 promoted H-abstraction from coordinated NH3 to form NH2 species, as well as O-2 greatly enhancing the adsorption of NO on the catalyst SU rface to form nitrate species and absorbed NO2. Possible reaction paths with the gamma Fe2O3 catalyst were proposed as follows: the SCR process mainly formed NH2 from H-abstraction of coordinated NH3 which reacted with NO to form N-2 and H2O at high and medium tem peratures. o At low temperatures, however, the formation of adsorbed NO2 resulted from NO oxidation by O-2 over Fe3+ sites played an important role. NH4N3 and (NH4+)(2)NO2 as the key intermediate products reacted with NO to form N-2 and H2O at low temperatures.