Theoretical Investigation of the Mechanism of the Selective Catalytic Oxidation of Ammonia on H-Form Zeolites

The selective catalytic oxidation (SCO) of ammonia has been investigated on a portion of the H-ZSM5 framework which contains 5T atoms by using density functional theory, representing H-form zeolites. The mechanism was subdivided into three main blocks: (I) the direct reaction of NH3 with oxygen, leading either to HNO or to NH2OH, (II) the decay of nitroxyl (HNO), and (III) the decay of intermediately produced hydroxylamine (NH2OH). For the decay of HNO and NH2OH the catalyst is active for reactions with both NO and HNO, which leads to a reaction network with several energetically similar and accessible pathways. The initial reaction of oxygen with adsorbed ammonia exhibits the highest energy barrier in the mechanism and thus is likely to be the rate-limiting step. For all three parts, the crossing of potential energy surfaces was considered if necessary. The investigation of potential reaction pathways of N2O with NH3 and NO reveals a low activity of H-ZSM5 for them, and thus, they only have a minor relevance within the SCO. The results of the investigated reaction network of the selective catalytic oxidation of ammonia are in agreement with the experimental literature.