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

Abstract

Abstracts 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 CH 4 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 A and the adsorption energy was −4.9 kcal/mol. In the next step the dissociative adsorption of methane was studied. The dissociated complex with CH 3 attached to Ga at 2.0 A 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 CH 3 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 NO 2 were −18.4 and −10.3 kcal/mol, respectively. It is found that NO is slightly, while NO 2 is significantly activated. The spin distribution analysis also shows an activated NO 2 part after adsorption. It is also observed that the Ga atom becomes stable, tetrahedrally coordinated after each of the adsorption processes.