Removal of Dilute Nitrogen Oxide by the Absorption in Mn–Zr Oxide

Abstract

The removal of NO by absorption in manganese zirconium oxide (Mn–Zr oxide) was investigated in both the presence and absence of gaseous O₂. The Mn–Zr oxide at Mn/Zr = 1 exhibited a high NO absorption rate and capacity at 200 °C. The absorption was promoted in the presence of O₂. The absorption capacity was not strongly affected by the NO concentration (75—900 ppm) and W/F (0.125—1 g s cm⁻³). An infrared analysis of the absorbed species indicated that the absorption proceeds as the formation of nitrate in the bulk solid. The absorption and desorption were reversible, and the oxide solid was recovered upon heating a preabsorbed sample at 400 °C. Although the amorphous phase of the Mn–Zr oxide which is formed after heating at 450 °C is effective for NO absorption, due to good mixing of manganese oxide and zirconium oxide, heating at higher temperatures (≥550 °C), leading to a decrease in the surface area and crystallization of single oxides, resulted in a decrease in the absorption capacity. NO removal appears to proceed by the oxidation of NO on the Mn sites, and a subsequent absorption at the Zr sites as (NO₃)⁻ species.