Cryptomelane as High-Capacity Sulfur Dioxide Absorbent for Diesel Emission Control: A Stability Study

A high-capacity sulfur oxide absorbent, cryptomelane, has been described previously. Its SO2 capacity can be as high as 70 wt %, which is more than 10 times as high as the capacities of standard metal oxide-based SO2 absorbents. In this study, the stability of cryptomelane under oxidizing, inert, reducing, and lean-rich cycling conditions was determined. Cryptomelane is stable in oxidizing and inert atmospheres. However, it is unstable under reducing atmospheres, converting to lower valent manganese oxide compounds. These low-valent oxides have very low capacity toward SO2. Upon reexposing the reduced cryptomelane products to an oxidizing atmosphere, cryptomelane may re-form. Cryptomelane exposed to the types of lean-rich cycles that have been proposed for NOx traps for diesel emission control, over the temperature range 250-550 (degree)C, remains stable and maintains its very high SO2 capacity. It appears possible to use cryptomelane to protect the NOx traps from sulfur oxide degradation during cyclic lean-rich operation.