Mechanism of the Selective Catalytic Reduction of NO with NH3over MnOx/Al2O3

The mechanism of the selective catalytic reduction of NO withNH3over low loadedMnOx/Al2O3catalysts has beeninvestigated using TPRD, FTIR, and step-response studies withlabelled components. Attention has been focused on the reactivityof the adsorbed NO and NH3complexes, identified in thepreceding paper, at reaction temperatures below 473 K. FTIR showsthat the reaction starts with adsorption of NH3on Lewisacid Mn3+sites and its subsequent transformationto NH2species; NH4+ions,formed at Brønsted acid sites, hardly participate in the reaction.The NH2species can react with both gas phase NO(Eley–Rideal mechanism) and reactive nitrite intermediates(Langmuir–Hinshelwood mechanism). Bridged nitrites andmonodentate nitrites are able to react at these low temperatures.The role of O2cannot simply be explained by reoxidation ofthe surface to close the catalytic cycle; O2appears tobe important both for H-abstraction from adsorbed NH3andin assisting in the formation of reactive nitrites. Bidentate nitratesare formed at a slow rate in a NH3/NO/O2mixture and partly deactivate the catalysts at temperatures below500 K. In contrast, they serve as reactive intermediates for the SCRreaction at higher temperatures. On the basis of these results, areaction scheme in elementary steps is proposed for this type ofcatalyst.