Selective catalytic reduction (SCR) of NO with NH3 was studied on beta-MnO2 and alpha-Mn2O3 catalysts at 150 degrees C and the formation of N2O from SCR was mainly investigated. The activity evaluation showed that the rates of both NO conversion and N2O formation per unit surface area on beta-MnO2 were much higher than the corresponding values on alpha-Mn2O3, while two catalysts gave same generated rate of N-2. Transient reactions of NO with NH3 showed that N2O predominantly originated from direct reaction of NO and NH3 via an Eley-Rideal mechanism. beta-MnO2 gave higher generated rate of N2O in transient reaction of NH3 and more desorption amount of N2O in temperature-programmed desorption of NH3 than alpha-Mn2O3. The results of temperature-programmed reduction of H-2 exhibited that oxygen species on beta-MnO2 are more active than alpha-Mn2O3. Therefore, beta-MnO2 had higher selectivity to N2O in SCR reaction than alpha-Mn2O3, predominantly resulting from higher activated capability to NH3, and beta-MnO2 can cleave more N-H bonds in NH3 molecules to give more adsorbed nitrogen atom species, which reacted with gaseous NO to form more N2O. (C) 2010 Elsevier B.V. All rights reserved.