Facet control of manganese oxides with diverse redox abilities and acidities for catalytically removing hazardous 1,2-dichloroethane

Abstract

The preparation of four kinds of MnO₂, namely, α-, β-, γ-, and δ-type MnO₂, with distinct crystal phases and tunnel structures was achieved and they were applied for catalytic combustion of 1,2-dichloroethane (1,2-DCE). The redox ability and acidity of MnO₂ as well as the corresponding reaction mechanism were studied by means of various surface-sensitive techniques, including TPR, TPD, OIE, XPS, and in situ DRIFTS together with DFT calculations. The catalytic activities for 1,2-DCE combustion demonstrated that γ-MnO₂ displayed the most superior activity with the maximum HCl yield of 95% and CO₂ yield of 92% due to its abundant oxygen vacancies on the surface, easy formation of reactive oxygen species, and strong acidity to readily react with the adsorbed reactant to form HCl, which reflects the synergistic effect of its redox properties and acidity. However, the strong Mn–Cl bonding associated with high valence Mn⁴⁺ and the scarcity of acidic sites in β- and δ-MnO₂ hinder the HCl elimination process, even leading to the undesirable chlorine deposition associated with a series of polychlorinated byproducts including 1,1,2-C₂H₃Cl₃ and CCl₄via a Cl substitution process. A mechanism for 1,2-DCE combustion involving dehydrochlorination via C–Cl bond cleavage at acidic sites as well as excess polychlorination by Cl₂ over a relatively oxidative sample was accordingly proposed.