Mechanism of Selective Catalytic Reduction of NO with NH3 over MnOX/Al2O3, I. Adsorption and Desorption of the Single Reaction Components

The adsorption of NO and NH3on MnOx/Al2O3catalysts, used for the low temperature selective catalytic reduction of NO, was studiedseparately by use of TPD (with labelled compounds) and FTIR. Besides,the influence of O2on the adsorption of the reactants wasinvestigated. At 323 K, NH3can adsorb as coordinated NH3and ammonium ions, which both have comparable thermal stability. Hence, both of them, as well as amide species, can be present at reaction temperature (423 K). In the presence of O2the relative distribution of these three surface species does not change. NO adsorbs in small quantities on the surface of these catalysts after an inert treatmentas Mn3+–NO nitrosyls and some nitrites/nitrates. However, it adsorbs in significant amounts after an oxidative pretreatment and in high amounts (NO/Mn ≈ 1) in the presence of gas phase O2. At 423 K, the following compounds can be present, in increasing order of thermodynamic stability: linear nitrites, bridged nitrites, monodentate nitrites < bridged nitrates < bidentate nitrates. The formation of these five species is strongly enhanced in the presence of O2, and probably proceeds by NO oxidation. In contrast, nitrosylic compounds are unstable in O2containing atmospheres. The uptake of NO in the presence of O2is lower than the NO2uptake, and relativelymore stable nitrates are formed in the latter case. The role of O2is to oxidise NO at the surface rather than in the gas phase.

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