Japanese chemists have discovered a more effective way of removing nitrogen oxides (NOx) from diesel engine exhausts.

Atsushi Satsuma and Ken-ichi Shimizu at Nagoya University, working with Shigeo Satokawa of Tokyo Gas Co. Ltd., found that when they added hydrogen to the exhaust they could improve the conversion of NOx to nitrogen gas using a catalyst of silver on an alumina surface. NOx need to be reduced because they can cause smog and acid rain.

Satsuma described that when hydrogen was added the amount of NOx converted to nitrogen increased from nearly nothing to 50%. He also found that hydrogen decreased the temperature at which this reaction could occur, into the usual temperature range of diesel engines, 200 to 400°C. 

The oxygen content in diesel engine exhaust favours the catalytic converter oxidising the poisonous carbon monoxide to dioxide, but this can inhibit the reduction of NOx. Using unburned hydrocarbons in the exhaust as reducing agents, along with suitable catalysts, could help overcome the problem, said Satsuma. 

According to Satsuma metal oxide catalysts have shown high stability and moderate tolerance to sulfur oxides and water vapour (both present in diesel exhaust), of which the silver on alumina is thought to be the most promising. One point needing further improvement was the low selective catalytic reduction at low temperature.

Robbie Burch at Queen's University, Belfast, UK, said that the most significant discovery is the fact that small amounts of hydrogen can dramatically increase the ability of silver catalysts to remove toxic NOx at low temperatures. 'Normally you would expect the hydrogen to burn to give water, but this does not happen,' he said.

Although the effect of hydrogen is not fully understood, Satsuma thinks that the hydrogen supports the reduction of the NOx by the hydrocarbon, rather than being a reducing agent itself.

Michael J Smith