Facile Synthesis of Highly Efficient Amorphous Mn-MIL-100 Catalysts: Formation Mechanism and Structure Changes during Application in CO Oxidation

A comprehensive study was carried out on amorphous metal-organic frameworks Mn-MIL-100 as efficient catalysts for CO oxidation. This study focused on explaining the crystalline–amorphous–crystalline transformations during thermolysis of Mn-MIL-100 and studying the structure changes during the CO oxidation reaction. A possible formation mechanism of amorphous Mn-MIL-100 was proposed. Amorphous Mn-MIL-100 obtained by calcination at 250 °C (a-Mn-250) showed a smaller specific surface area (4 m² g⁻¹) but high catalytic activity. Furthermore, the structure of amorphous Mn-MIL-100 was labile during the reaction. When a-Mn-250 was treated with reaction atmosphere at high temperature (giving used-a-Mn-250-S), the amorphous catalysts transformed into Mn₂O₃. Meanwhile, the BET surface area (164 m² g⁻¹) and catalytic performance both sharply increased. In addition, used-a-Mn-250-S catalyst transformed from Mn₂O₃ into Mn₃O₄, and this resulted in a slight decrease of catalytic activity in the presence of 1 vol % water vapor in the feed stream. A schematic mechanism of the structure changes during the reaction process was proposed. The success of the synthesis relies on the increase in BET surface area by using CO as retreatment atmosphere, and the enhanced catalytic activity was attributed to the unique structure, a large quantity of surface active oxygen species, oxygen vacancies, and good low-temperature reduction behavior.