12 September 2005: Affordable route to greener diesel

US researchers say they might have found a cheap and effective alternative for cleaning up vehicle emissions.

Traditional precious metal oxidation catalysts used to remove nitrogen oxides from diesel emissions are easily poisoned by sulfur. Researchers at Pacific Northwest National Laboratory (PNNL), US, supported by the US department of the energy, have discovered that the mineral cryptomelane, a potassium manganese oxide, is very effective at absorbing SO₂ and SO₃. It could be used in tandem with these catalysts to protect them. 

Cryptomelane can absorb 60 per cent SO₂by weight, which is more than 10 times better than conventional metal oxide-based absorbents. It needs to work under the same conditions as the catalyst to protect it effectively. This includes a temperature range of 200-500?C. Tests showed it is effective at high temperatures but not below 325?C. 

PNNL’s Liyu Li modified the mineral, replacing the potassium with silver. He developed a novel, simple and inexpensive method of synthesising nano-sized silver hollandite. This material absorbs SO₂ well at lower temperatures. 

It is also good at oxidising nitrogen oxides, carbon monoxide and hydrocarbons, giving it potential as an alternative to the catalyst itself. ’We are hoping we could replace precious metals with this material,’ said team leader David King. The researchers are now looking for funding to build prototypes and to explore other uses for cryptomelane. Karen Harries-Rees