A series of nickel-manganese bi-metal oxide catalysts of different Ni/Mn ratios were prepared by a co-precipitation method for the low temperature selective catalytic reduction (SCR) of NO with NH3 in the presence of excess O-2. The NO conversion over different catalysts decreased in the following sequence of Ni(0.4)-MnOx > MnOx >> Ni(1)-MnOx > Ni(2.5)-MnOx > NiOx. The Ni(0.4)-MnOx catalyst showed the highest catalytic activity, 85% of NO conversion at 95 degrees C and 100% from 120 to 240 degrees C, among the catalysts investigated. The TPR, XPS, NH3-TPD and in situ FTIR results revealed that Mn4+ was the main active species for SCR reaction and the addition of nickel species enhanced the surface concentration and acidity of Lewis acid sites. A possible synergetic catalytic effect was proposed for the low temperature SCR reaction through the electron transfer between Mn and Ni ions. (C) 2013 Elsevier B.V. All rights reserved.