Tuning the micromorphology and exposed facets of MnOx promotes methyl ethyl ketone low-temperature abatement: boosting oxygen activation and electron transmission

Abstract

MnO_x oxides with different morphologies (nanowire (MnO_x-W), nanocube (MnO_x-C), nanorod (MnO_x-R), and nanosphere (MnO_x-S)) and exposed facets were synthesized via a solvothermal method. The catalytic performance of the synthesized MnOx materials for methyl ethyl ketone (MEK) destruction was investigated. Results show that the activity of MnO_x-W with highly exposed {101} facets of Mn₃O₄ is superior to that of MnO_x-C, MnO_x-R, and MnO_x-S exposing {321} facets of Mn₂O₃, {110} facets of MnO₂, and {101} and {112} facets of Mn₃O₄, respectively. MEK can be completely mineralized into CO₂ at 195 °C over MnO_x-W under a relatively high gas hourly space velocity of 37 200 h⁻¹, which is even better than some typical noble metal loaded catalysts. The lowest apparent activation energy of MnO_x-W (27.7 kJ mol⁻¹) for MEK destruction also confirms its excellent catalytic activity. Density functional theory (DFT) results reveal that the {101} facets of Mn₃O₄ have the highest MEK adsorption energy (0.79 eV), which indicates that MEK molecules have a high affinity to adsorb onto the MnO_x-W surface, promoting the oxidation process of MEK. In situ DRIFTS and TPSR results indicate that the mineralization of MEK into CO₂ over MnO_x-W goes through an oxidation route with diacetyl as the primary intermediate. We found that the highly exposed {101} active facets, abundant oxygen vacancies, and excellent low-temperature reducibility are responsible for the superior oxidation performance of MnO_x-W. This finding may bring new insights into the designing of highly effective catalysts and has implications for a wide range of reactions not limited to MEK oxidation.