MnOx catalysts with different morphologies for low temperature synergistic removal of NOx and toluene: Structure–activity relationship and mutual inhibitory effects

Abstract

The simultaneous catalytic removal of nitrogen oxides (NOx) and toluene, in an existing selective catalytic reduction (SCR) unit, has drawn much attention. Exploring the interaction effects between the catalytic oxidation of toluene and the reduction of NOx is crucial to ensure the successful design and development of catalysts for their simultaneous removal. MnOx nanorod and MnOx nanocuboid were synthesized to compare their catalytic performances for the simultaneous removal of NOx and toluene. The MnOx nanorod catalyst possessed greater catalytic activity than the MnOx nanocuboid catalyst and exhibited a greater than 85% catalytic activity in the simultaneous abatement of NOx and toluene at 275 °C. The MnOx nanorod catalyst had a higher proportion of Mn4+ and a better redox ability compared with the MnOx nanocuboid, resulting in an excellent catalytic activity. Thus, it was selected as a model catalyst to investigate the interaction effects. Compared with the separate NH3-SCR and toluene oxidation reactions, there was a mutual inhibitory effect between the SCR reaction and toluene oxidation in the simultaneous removal process over the model catalyst (MnOx nanorod). This mutual inhibitory effect was investigated systematically using TPD, GC-MS and in situ DRIFTs. Mutual inhibitory effect over the model catalyst mainly manifested as follows: 1) the formation of intermediate products in the oxidation process of toluene covered the active sites; 2) the competitive adsorption of NH3 and toluene occurred; and 3) the SCR reaction and toluene oxidation process competed for reactive oxygen species.