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Potential Improvements in SOx and NOx Diesel Emissions Controls

Cryptomelane is an excellent catalyst for oxidizing sulfur oxides.

Researchers at Pacific Northwest National Laboratory (PNNL) are working on two materials that offer the promise of improved diesel emissions control.

The first material, cryptomelane, is an excellent catalyst to trap and oxidize sulfur dioxide, thereby protecting nitrogen dioxide traps from the sulfur dioxide that can degrade them. The second material, silver hollandite, is proving to have catalytic properties that can completely oxidize NOx, CO and hydrocarbons.

Sulfur oxides can poison emissions control systems—that’s just one of the reasons ultra low-sulfur fuel is an important step. (Sulfur can also poison a PEM fuel cell.)

PNNL researchers identified the potential for using cryptomelane on monolith supports—sturdy honeycombed structures composed of small parallel channels—to trap sulfur dioxide (SO2) and sulfur trioxide (SO3) from diesel engine emissions.

Cryptomelane is one of a class of octahedral molecular sieves based on manganese. The material has a total sulfur dioxide capacity of 70 wt.%—very high compared to other adsorber materials.

Its capacity is unaffected by the presence of NO or CO in the feedstream and is stable in conditions where there is cycling between lean and rich fuel mixtures.

The researchers have filed for a patent on this application of cryptomelane. They also believe the catalyst may degrade additional undesirable material.

Silver hollandite is an oxidation catalyst resistant to sulfur poisoning.

The second material, silver hollandite, is not new, but it could become an inexpensive oxidation catalyst that is resistant to poisoning by sulfur oxides. The research team at PNNL developed an inexpensive method of synthesizing nano-sized silver hollandite and found that the material can completely oxidize nitrogens of oxide, carbon monoxide and hydrocarbons.

They also found that silver hollandite is an excellent low-temperature sulfur oxides absorbent. Silver hollandite thus maintains its catalytic activity even while it ages by absorbing sulfur oxides.

Cryptomelane seems to work better at higher temperatures.

Thus the work opens two possibilities. One, a cryptomelane-based sulfur trap that could support a range of existing emissions control materials on the market. Two, a nano silver hollandite oxidation catalyst that is resistant to sulfur poisoning.

PNNL presented the results of both of these projects at the recent meeting of the American Chemical Society in Washington, D.C.



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