Theoretical Investigation of the Mechanism of the Selective Catalytic Reduction of Nitric Oxide with Ammonia on H-Form Zeolites

The selective catalytic reduction of NO with ammonia in the presence of oxygen 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 parts: (1) the oxidation of NO to NO2, (2) the formation of an intermediate (NH2NO), and (3) the decomposition of this species to nitrogen and water. For the second step, three different pathways were studied, differing in the NOx species initially present on the active site: (i) two NO molecules form N2O2, (ii) NO2 and NO form N2O3, and (iii) two NO2 molecules form N2O4. For steps 1 and 2, the crossing of potential energy surfaces was considered for the transition of single molecules to adsorbed clusters. For all three parts of the mechanism, the energy profile of the heterogeneously catalyzed reaction is favorable, as compared to that of the corresponding homogeneous reaction. Due to the strong adsorption of ammonia on the acid site, it is likely that the rate-determining step of the overall reaction is the oxidation of NO to NO2 caused by blocking of the active site by NH3. As far as we have investigated the reaction mechanism of the selective catalytic reduction of NO with NH3 in this work, the results are in agreement with the experimental literature.