QM/MM Study of the Effect of Local Environment on Dissociative Adsorption in BaY Zeolites

Quantum chemical calculations were carried out to study the characteristics of dissociative adsorption on BaY zeolites, that is, adsorption of neutral HX versus H+X-. The calculations were performed using the QM/MM method ONIOM with the B3LYP functional, SDD basis set, and UFF force field. Key intermediates from the NOx reduction reaction were studied, including nitromethane, aci-nitromethane, acetic acid, CH2COOH, HNO3, HNO2, H2O, and O2NCH2COOH., Two factors were found to affect dissociative adsorption: the local environment in the zeolite and the acidity of the adsorbate molecule, The local environment can vary due to different aluminum arrangements around the active site. It was found that adsorbates in both neutral and dissociated forms are further stabilized as aluminum atoms are further apart because of the electrostatics. Generally, neutral intermediates adsorb with one oxygen atom coordinated to barium, whereas the dissociated intermediates adsorb with two oxygen atoms coordinated to barium in a bidentate configuration. For all of the intermediates studied except HNO3, the neutral form is favored over the dissociated form, and adsorbates with higher acidity have stronger tendencies to dissociate.