Quantum chemical investigation of reactants in selective reduction of NO_x on ion exchanged ZSM-5

A combination of quantum chemistry, molecular dynamics and computer graphics methods was applied in the investigation of the adsorption and activation mechanism of methane and NO_x molecules on Ga-exchanged ZSM-5 catalyst. For CH₄ we found that the initial step is a weak physical adsorption on the Ga site at the distance between gallium and carbon atoms of 2.9 Å and the adsorption energy was -4.9 kcal/mol. In the next step the dissociative adsorption of methane was studied. The dissociated complex with CH₃ attached to Ga at 2.0 Å and H bonded to the extraframework oxygen appeared to be very favorable and led to energetic stabilization of -63.0 kcal/mol. Another dissociated adsorption state was found when a hydrogen atom was attached to Ga and CH₃ bonded to the extraframework oxygen. In this case stabilization energy was estimated to be -31.4 kcal/mol. Moreover, we described the transition state of methane dissociation. The activation barrier on the Ga site was 31.3 kcal/mol, which means kinetically favorable reaction. For the NO_x molecules there is no energy barrier before chemisorption. The adsorption energy for NO and for NO₂ were -18.4 and -10.3 kcal/mol, respectively. It is found that NO is slightly, while NO₂ is significantly activated. The spin distribution analysis also shows an activated NO₂ part after adsorption. It is also observed that the Ga atom becomes stable, tetrahedrally coordinated after each of the adsorption processes.