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

Abstract

A high-capacity sulfur oxide absorbent, cryptomelane, has been described previously. Its SO 2 capacity can be as high as 70 wt %, which is more than 10 times as high as the capacities of standard metal oxide-based SO 2 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 SO 2 . 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 NO x traps for diesel emission control, over the temperature range 250-550 °C, remains stable and maintains its very high SO 2 capacity. It appears possible to use cryptomelane to protect the NO x traps from sulfur oxide degradation during cyclic leanrich operation.

Facile One-Step Template-Free Synthesis of Uniform Hollow Microstructures of Cryptomelane-Type Manganese Oxide K-OMS-2

“…The unique physical and chemical properties of cryptomelanetype manganese oxide K-OMS-2 have given this material a wide range of potential applications in heterogeneous catalysis, [30][31][32][33][34][35][36][37] adsorption of metal cations in aqueous systems, 38 and control of gas emissions. 39 K-OMS-2 displays a tunnel structure built up from the arrangement of four double-wide slabs of edge-shared octahedral [MnO 6 ] units. The double-wide slabs, which are rotated 90°each other, are joined through corner oxygens producing the 1D tunnels parallel to the [001] direction.…”

“…In general, the synthesis of this type of manganese oxide is carried out through processes involving oxidation of Mn 2+ cation, , reduction of MnO 4− species, ,, and comproportionation of a mixture of Mn 2+ and MnO 4- ions. ,− Hydrothermal, reflux, ,, and microwave heating , have been the thermal treatments usually performed to drive the reactions of synthesis from solutions of the molecular precursors to the oxide material.…”

Facile Preparation of Nanocryptomelane and Its Application in the Treatment of Aqueous Solutions Containing Basic Fuchsin

“…The synthetic counterpart to cryptomelane is known as octahedral molecular sieve, OMS-2. 14,16,18 Because of its porous structure and mixedvalence manganese species, cryptomelane has been extensively explored as a potential material for ion exchange, 19 batteries, 20 waste immobilization, 21,22 and catalysts. 23−25 Synthetic nanocryptomelane has the environmental advantages of natural cryptomelane 26 but also exhibits other excellent characteristics, including high storage stability, high purity, and lower cost.…”

“…In recent years, several different processes have been proposed to prepare cryptomelane-type materials, as well as a variety of other manganese oxides. The synthetic counterpart to cryptomelane is known as octahedral molecular sieve, OMS-2. ,, Because of its porous structure and mixed-valence manganese species, cryptomelane has been extensively explored as a potential material for ion exchange, batteries, waste immobilization, , and catalysts. − Synthetic nanocryptomelane has the environmental advantages of natural cryptomelane but also exhibits other excellent characteristics, including high storage stability, high purity, and lower cost. Compared with other water-scavenging agents, , synthetic cryptomelane has a higher treatment efficiency and, thus, a greater economic benefit, because of its ability to use a lower dosage in a shorter reaction time.…”