(301c) Theoretical Investigation of the Mechanism of the Selective Catalytic Reduction of Nitric Oxide with Ammonia in the Presence of Oxygen On H-Form Zeolites

Nitrogen oxides (NO, NO₂ and N₂O) are a major source of air pollution as they cause photochemical smog, acid rain and contribute to the greenhouse effect. They are mainly a product of the combustion of fossil fuels in power plants and automobiles, and emission standards have been legislated to regulate exhaust gases. An efficient method to reduce NO_x is the selective catalytic reduction (SCR) with ammonia in the presence of oxygen. Especially iron exchanged zeolites like the Fe/H-ZSM5 were successfully applied as catalysts since the last couple of years. The necessity to avoid ammonia slip is a drawback of this method, usually causing a non optimal dosing of NH₃. Furthermore, most catalysts active for the SCR, also exhibit significant activity for the selective catalytic oxidation (SCO) of ammonia, especially at elevated temperatures.

Usually, Fe-catalysts are prepared by ion exchange, starting with an H-form. Since not all Brønsted acid sites are removed, the final catalysts exhibit two different kinds of active sites, Fe and H. The ambition of research at present is the investigation of the influence of the two different active sites on the SCR of NO_x and N₂O with ammonia as well as on the SCO of ammonia and the clarification of the corresponding reaction mechanisms.

In this work the complex reaction network of the SCR of NO with ammonia in the presence of oxygen on H-ZSM5 was investigated by using the density functional theory (DFT). All reaction steps were calculated on a portion of the catalyst, containing 5T atoms and the crossing of potential energy surfaces was considered if necessary. The network can be subdivided into three main parts: (1) The direct reaction between the species initially present, forming essential intermediates (HNO_x with x=1-3, NH₂NO and NH₂NO₂), (2) the decomposition of the molecules NH₂NO and NH₂NO₂ into water and N₂ and N₂O respectively and (3) the bimolecular reactions of HNO_x (nitroxyl, nitrous acid and nitric acid) in either a self-reaction, with each other or with ammonia. For the first part it was found that, though the H-ZSM5 catalyses the oxidation of NO, the active site is blocked by adsorbed ammonium. Thus it can be concluded that the adsorption of oxygen and the subsequent oxidation of NO is the rate determining step of the SCR. Furthermore, the reaction of NO_x with ammonia was studied for three cases, differing in the species, initially present on the active site: two NO, NO and NO₂ and two NO₂ co-adsorbed on ammonium [1]. The reaction without O₂ or NO₂ turned out to be of no relevance for the SCR, whereas the other two cases are significantly enhanced by the Brønsted acid compared to the corresponding gas phase reactions. The direct oxidation of ammonia as well as the reaction of N₂O with NH₃ can be concluded to be of only minor relevance in the SCR network. For the second part, the investigation of the decomposition of NH₂NO and NH₂NO₂ revealed that a sequence of proton transfers with the catalyst leads to water and N₂ and N₂O respectively. For the third part, H-ZSM5 was shown to be active for the several bimolecular reactions of HNO_x, leading to NO_x by hydrolysis. In case of the self-reaction of HNO, two competitive pathways leading to either N₂O and H₂O or to NO and the intermediate HNOH were found. Furthermore, both nitrous and nitric acid can react with adsorbed ammonia, forming NH₂NO and NH₂NO₂ respectively.

The results of the investigation of the complex reaction network are in agreement with the experimental literature and account for the observed phenomena.