Mechanistic study on the role of oxygen vacancy for methylene chloride oxidation over La-Mn perovskite

Abstract

Oxygen vacancy (OV) has a close relationship with chlorinated volatile organic compounds catalytic oxidation. The role of OV for methylene chloride (DCM) decomposition over defective La-Mn perovskite catalyst was investigated using Density Functional Theoretical calculations. The adsorption characteristics and dissociation processes of reactants (DCM, O2 and H2O) on perfect and defective surfaces were comparatively studied. In DCM dechlorination process, the unoccupied orbitals of OV interact with binding orbitals of Cl atoms, and thus facilitate Cl abstraction. The dissociated Cl atoms are trapped by the OV on the defective surface. The activation energy of HCl formation (144.31 kJ/mol) with the assistant of hydroxyl groups is lower than that of Cl2 formation (250.86 kJ/mol) over defective LaMnO3 surface. OV (Lewis acid) and its proximal surface hydroxyls (Lewis base) tend to form the frustrated Lewis acid-base pairs, which can capture the dissociated Cl atoms with the assistance of protons into HCl. The surface hydroxyls can be regenerated readily from H2O dissociation at OV sites, thus achieving a sustainable Cl remove. Molecular O2 is easily activated and dissociated into O atoms by OV on defective surface. The atomic O adsorbed on surface Mn sites are the primary oxygen active species for DCM deep oxidation.