The selective catalytic reduction of NOx with CH4 on Mn-ZSM5: a comparison with Co-ZSM5 and Cu-ZSM5

The abatement of NO with CH4 in the presence of O2 was studied on Mn-ZSM5 (Mn percent exchange 2 to 75%), prepared from Na-ZSM5 by the ion-exchange method. Samples were characterised by FTIR, ESR and magnetic measurements. Both ESR and magnetic measurements showed that all manganese was present as Mn(II). The intensity of FTIR bands from carbonyls was almost proportional to the manganese content. The selectivity for NO abatement, reaction orders in O2, NO or CH4, and apparent activation energy were independent of the manganese content, suggesting that the same surface complexes were formed on all Mn-ZSM5 catalysts. On Mn-ZSM5, turnover frequencies (molecules sÿ1 Mn-atomÿ1) for both NO abatement and CH4 reaction were nearly independent of the manganese content and close to the relevant values on Co-ZSM5, but substantially higher than those on Cu-ZSM5. On Mn-ZSM5, Co-ZSM5 and Cu-ZSM5, the adsorption at RT of NO‡O2 or NO2 caused the formation of nitrates. The normalised intensity (per atom) of monodentate nitrate bands (1520 and 1320 cmÿ1 ) was (i) independent of the metal content, (ii) the same in Mn-ZSM5 and Co-ZSM5, and (iii) much higher in Mn-ZSM5 and Co-ZSM5 than in Cu-ZSM5. On all these catalysts, NO abatement rates were proportional to the concentration of these monodentate nitrates. On FTIR evidence, these species reacted with methane at the same temperature at which the various catalysts were active. These findings suggest a role of monodentate nitrates in the SCR reaction.