Synthesis of MnO₂/Mg-Al layered double hydroxide and evaluation of its NO-removal performance

The removal of toxic nitric oxide (NO) from exhaust gas is challenging and expensive. MnO₂/Mg-Al LDH, a composite of manganese dioxide and Mg-Al layered double hydroxide, was synthesized via the rehydration method for potential application in NO removal. MnO₂/Mg-Al LDH was found to be complexed with MnO₂, pillared between the LDH layers and intercalated with CO₃²⁻ and OH⁻. We investigated the performance of a MnO₂/Mg-Al LDH treatment in removing NO from exhaust gas. The exhaust gas was circulated in a reaction tube filled with MnO₂/Mg-Al LDH, CO₃･Mg-Al LDH, or a MnO₂ + CO₃･Mg-Al LDH mixture, and the NO removal efficiency of the MnO₂/Mg-Al LDH composite was compared with that of the other compounds. When NO gas (150 ppm, linear velocity 1.0 m/min) was distributed at 170 °C, treatment with the MnO₂･Mg-Al LDH composite and CO₃･Mg-Al LDH yielded removal rates of 83.9% and 0%, respectively. The significantly higher NO removal rate of MnO₂/Mg-Al LDH was ascribed to the transformation of NO to NO₂ via oxidation over MnO₂. Further, the NO removal rate after 90 min through the application of a simple mixture of MnO₂ and CO₃･Mg-Al LDH (Mn: 4.2 wt%) was only 1.4%, suggesting that a synergistic effect was generated by the MnO₂/Mg-Al LDH composite. Though NO removal by MnO₂/Mg-Al LDH was reduced at temperatures of 140 °C and higher, the results suggest that the synthesized compound can be much more effective than those commonly used for NO removal from exhaust gas.