ACTIVITY AND SELECTIVITY OF PURE MANGANESE OXIDES IN THE SELECTIVE CATALYTIC REDUCTION OF NITRIC-OXIDE WITH AMMONIA

Manganese oxides of different crystallinity, oxidation state and speci fic surface area have been used in the selective catalytic reduction ( SCR) of nitric oxide with ammonia between 385 and 575 K. MnO2 appears to exhibit the highest activity per unit surface area, followed by Mn5 O8, Mn2O3, Mn3O4 and MnO, in that order. This SCR activity correlates with the onset of reduction in temperature-programmed reduction (TPR) experiments, indicating a relation between the SCR process and active surface oxygen. Mn2O3 is preferred in SCR since its selectivity toward s nitrogen formation during this process is the highest. In all cases the selectivity decreases with increasing temperature. The oxidation s tate of the manganese, the crystallinity and the specific surface area are decisive for the performance of the oxides. The specific surface area correlates well with the nitric oxide reduction activity. The nit rous oxide originates from a reaction between nitric oxide and ammonia below 475 K and from oxidation of ammonia at higher temperatures, pro ven by using (NH3)-N-15. Participation of the bulk oxygen of the manga nese oxides can be excluded, since TPR reveals that the bulk oxidation state remains unchanged during SCR, except for MnO, which is transfor med into Mn3O4 under the applied conditions. In the oxidation of ammon ia the degree of oxidation of the nitrogen containing products (N2, N2 O, NO) increases with increasing temperature and with increasing oxida tion state of the manganese. A reaction model is proposed to account f or the observed phenomena.