Reaction mechanism of dichloromethane oxidation on LaMnO perovskite.

The reaction mechanism of dichloromethane (CHCl) oxidation on LaMnO catalyst was investigated using density functional theory calculations. The results showed that CHCl dechlorination proceeds via CHCl → CHClO → HCHO. The adsorbed Cl∗ and formaldehyde (HCHO) are identified as the important intermediates of CHCl dechlorination process. The dissociated Cl atoms prefer to adsorb on the surface Mn sites. Surface hydroxyl groups are not directly involved in the CHCl dechlorination process, but react with the adsorbed Cl∗ to form HCl. The energy barrier of HCl formation is lower than that of Cl 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 → HO desorption → CO/CO 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 CHCl oxidation on LaMnO catalyst. The catalytic cycle includes CHCl dechlorination, HCl formation and HCHO oxidation. The microkinetic analysis indicates that there are four steps controlling the reaction cycle: CHCl∗ + ∗ → CHCl∗ + Cl∗, CHOCl∗ + Cl∗ → CHO∗ + Cl∗, O∗ + ∗ → 2O∗, and CHO∗ + OH∗ → CO + HO∗.