Role of Acidic Proton in the Decomposition of NO over Dimeric Cu(I) Active Sites in Cu-ZSM-5 Catalyst: A QM/MM Study

The influence of protons on the mechanism of the direct decomposition of NO over adjacent dimeric Cu(I) active sites in zeolite is theoretically investigated by using ONIOM (QM/MM) calculations with two dicopper model systems IT and 2T, where the Cu(I) atoms are separated by one and two SiO4 tetrahedra, respectively. The reaction proceeds through the formation of N2O as a reaction intermediate and further its decomposition into oxygen and nitrogen. The present study shows that the presence of proton plays an important role in the production of N2O from two NO molecules. In the proton-free mechanism, this process requires a large activation barrier of 56.3 and 55.3 kcal/mol on IT and 2T, respectively, while the inclusion of protons reduces it to 31.4 and 17.3 kcal/mol. The significant decrease in the activation barrier is due to the strengthening of the N-N bond of the formed NO dimer upon protonation, which facilitates the formation of N2O. On the other hand, the presence of protons disfavors the decomposition of N2O and needs an activation barrier of 6-9 kcal/mol higher than that of the corresponding reaction in the absence of protons. The stable intermediate Cu-OH+-Cu formed in the proton-assisted mechanism is responsible for the larger activation energy for N2O decomposition. The proton-assisted NO decomposition mechanism is in agreement with the experimental observation that the decomposition of N2O as well as O-2 desorption are the governing reaction steps in the decomposition of NO. The present study explains the role of the Cu-O-Cu species in the NO decomposition reaction. The results disclosed herein will also pave a way to understanding the mechanism of the reductive N-N coupling of NO molecules catalyzed by metalloenzymes and transition-metal catalysts.