NOx adsorption on MnO2/NaY composite: an in situ FTIR and EPR study

Abstract

The NO, NO/O2, and NO/O2/H2O adsorption on MnO2/NaY (5 and 15wt.% MnO2) composite catalyst and NaY has been studied by means of in situ FTIR and EPR spectroscopy at elevated temperatures and during heating under reaction-like conditions. NO adsorption and co-adsorption of NO and O2 on NaY and MnO2/NaY proceeds via oxidation of NO forming NO2− and NO3− species. Whereas the manganese dioxide preferably acts as oxidising agent, the zeolite stores the NOx species as nitrite and nitrate ions in the solid. In the presence of oxygen, the nitrate formation is enhanced due to additional oxidation of NO through gaseous oxygen leading to NO2. Dimerisation of NO2 to N2O4 and following disproportionation of the latter causes the formation of NO+ and NO3− species which are associated with nucleophilic zeolitic oxygen and especially alkali cations of the zeolite, respectively. The presence of oxygen facilitates reoxidation of Mn2+ which keeps more Mn ions in the active state. Pre-adsorbed water and higher amounts of water vapour in the feed hinder the NO adsorption by blocking the adsorption sites and shift the nitrate formation to higher temperatures. The quantities and thermal stability of the nitrates formed during NO and NO/O2 adsorption differs which points to a different mechanism of nitrate formation. In the absence of gaseous oxygen, nitrates are formed by participation of only lattice oxygen. In the presence of oxygen, nitrate formation by dimerisation and disproportionation reactions of NO2 dominates. The manganese component of the composite catalyst supports the oxidation of NO to nitrite and subsequently to nitrate. During this process Mn4+ is reduced to Mn2+ as evidenced by in situ EPR measurements.