The effects of H2O on catalytic performance of hollandite manganese oxide (HMO) and beta-MnO2 are comparatively studied in selective catalytic reduction of NO by NH3 (NH3-SCR). The results showed that the HMO possessed higher catalytic activity than beta-MnO2 in the presence of H2O, and the NH3-SCR reaction occurring on the HMO predominantly followed an Eley-Rideal mechanism via reacting adsorbed NH4+ with gaseous NO. According to the NH3-TPD analyses, XPS results and the HRTEM observations coupled with the theoretical calculations, the results elucidated that (i) the initial formation of the NH4+(H2O)(3)OH-double ionic clusters were energetically favorable in the presence of H2O, (ii) the OH- ions from the double ionic clusters reacted with Lewis acid sites of the manganese oxides to transform into Bronsted acid ones, concomitant with release of the NH4+ ions, and (iii) the NH4+ ions both from the double ionic clusters and from reaction of NH3 with Bronsted acid sites can be efficiently adsorbed on the {1 1 0} facets of the HMO to complete the NH3-SCR reaction cycles, whereas the NH4+ ions are not stable on the {1 1 0} facets of beta-MnO2. Therefore, the excellent catalytic performance of the HMO predominantly originates from the special structural {1 1 0} facet due to it being suitable for adsorption of NH4+ active species. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.