The role of Ag species on Ag/Al2O3 catalyst for the selective catalytic oxidation (SCO) of NH3 to N-2 was Studied using 10 wt% Ag/Al2O3 Catalysts prepared with impregnation, incipient wetness impregnation and sol-gel methods. The catalyst characterization was preformed using N-2 adsorption-desorption. UV/Vis. TEM and XRD. O-2-chemisorption and H-2-O-2 titration were measured to confirm the metal dispersion on the catalyst. The Ag species state and Ag particle size have significant influence oil the Ag/Al2O3 activity and N-2 selectivity of the SCO of NH3 at low temperature. Ag-0 is proposed to be ail active species oil the H-2 pretreated catalyst at low temperature (<140 degrees C). It is evident that well-dispersed and small particle Ag-0 enhances catalytic activity at low temperature, whereas large particle Ag-0 is relate to a high N-2 selectivity. In contrast, Ag could also be the active species at temperatures above 140 degrees C. (C) 2008 Elsevier Inc. All rights reserved.