Efficient CO activation on Rh particles promoted by Mn cocatalysts is important to the activity for the conversion of the syngas (CO and H(2)) to hydrocarbon and oxygenates. To study the effect of the step edge and promotion of Mn cocatalysts on CO activation, we studied the CO dissociation on Mn-decorated Rh(1 1 1) and stepped Rh(5 5 3) surfaces using density functional theory calculations. We found that the presence of the step edge and Mn stabilizes the transition state and reaction products: compared to clean Rh(1 1 1), calculated barrier for CO dissociation on Mn-decorated Rh(5 5 3) is lowered by about 1.60 eV, and corresponding reaction energies with respect to CO in gas phase changes from endothermic (0.21 eV) to strong exothermic (-1.73 eV). The present work indicates that the addition of Mn cocatalysts and decrease of Rh particle sizes improves greatly the activity of CO dissociation. (C) 2010 Elsevier B.V. All rights reserved.