Reaction mechanism of dichloromethane oxidation on LaMnO₃ perovskite

The reaction mechanism of dichloromethane (CH₂Cl₂) oxidation on LaMnO₃ catalyst was investigated using density functional theory calculations. The results showed that CH₂Cl₂ dechlorination proceeds via CH₂Cl₂ → CH₂ClO → HCHO. The adsorbed Cl∗ and formaldehyde (HCHO) are identified as the important intermediates of CH₂Cl₂ dechlorination process. The dissociated Cl atoms prefer to adsorb on the surface Mn sites. Surface hydroxyl groups are not directly involved in the CH₂Cl₂ dechlorination process, but react with the adsorbed Cl∗ to form HCl. The energy barrier of HCl formation is lower than that of Cl₂ formation, indicating that hydroxyl groups facilitate the removal of adsorbed Cl∗ species. Three possible pathways of HCHO oxidation with the assist of lattice oxygen, active oxygen atom and hydroxyl groups were investigated. HCHO catalytic oxidation contains four steps: HCHO → CHO → CO → H₂O desorption → CO/CO₂ desorption. Compared with the HCHO oxidation by lattice oxygen and hydroxyl groups, HCHO oxidation assisted with activated oxygen atom is more thermodynamically favorable. A complete catalytic cycle was proposed to understand the preferable reaction pathway for CH₂Cl₂ oxidation on LaMnO₃ catalyst. The catalytic cycle includes CH₂Cl₂ dechlorination, HCl formation and HCHO oxidation. The microkinetic analysis indicates that there are four steps controlling the reaction cycle: CH₂Cl₂∗ + ∗ → CH₂Cl∗ + Cl∗, CH₂OCl∗ + Cl∗ → CH₂O∗ + Cl∗, O₂∗ + ∗ → 2O∗, and CHO₂∗ + OH∗ → CO₂ + H₂O∗.