Mechanism of low temperature selective catalytic reduction of NO with NH3 over carbon-supported Mn3O4 Active phase and role of surface NO species

Abstract

In this work, the interactions at low temperature (125 °C) of gaseous NH₃, NO and O₂ with carbon-supported manganese oxide catalysts have been extensively studied, mainly via step–response experiments, with the objective of explaining the low-temperature mechanism of the SCR reaction on such catalysts. To achieve this aim, the present work analyses the active phase and the interaction of NO species with the active phase both in the absence and in the presence of O₂. The catalyst was found to be Mn₃O₄ composed locally of octahedral Mn₂O₃ and a slightly oxidised (oxygen excess) tetrahedral MnO. Both local species have different though interrelated roles during the SCR process. On the octahedral Mn₂O₃ NO can be adsorbed as nitrosyls (on oxygen vacancies) and as linear and bridged nitrites on oxygen atoms (absent under steady state SCR conditions). On the tetrahedral MnO NO₂ is adsorbed as nitrates on hydroxyl groups previously formed by the reaction of water with the oxygen excess of this phase. NO₂ is produced via the reaction of nitrosyls with O₂. These nitrates are responsible for the partial deactivation of the catalyst under SCR conditions.