Effect of H2O on catalytic performance of manganese oxides in NO reduction by NH3

The effects of H2O on catalytic performance of hollandite manganese oxide (HMO) and β-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 β-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)3OH− 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 Brönsted 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 Brönsted 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 β-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.

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► The {1 1 0} facets of hollandite manganese oxide (HMO) are catalytically active sites in the NH3-SCR reaction.
► NH4+ ions as active species are more favorably adsorbed on the {1 1 0} facets of HMO than β-MnO2.
► Brönsted acid sites on the {1 1 0} facets of HMO increase in the presence of H2O.