11th August 2009

Recent comments in the press have mentioned the possibility of the use of gold in diesel autocatalysts. In December 2007, Nanostellar announced the development of a diesel exhaust catalyst which "uses gold alongside traditional platinum and palladium metals". The patents covering this technology indicate that their pre-commercial catalyst formulation consists of two catalytic components. The first is gold alloyed with palladium on a base metal oxide support in a roughly 1:1 ratio in terms of weight of the two precious metals. This catalyses the oxidation of carbon monoxide particularly effectively but its activity is inhibited by the presence of hydrocarbons.

To counter this effect, the catalyst features a second catalytic component which is physically separated from the first. This second component is a platinum or platinum-rich platinum/palladium catalyst, which carries out some of the carbon monoxide oxidation reaction and, crucially, oxidises most of the unburnt hydrocarbons in the diesel exhaust, allowing the gold/palladium component to function efficiently. The two components are physically separated onto two monoliths or into two layers to prevent the components mixing and losing activity quickly.

The gold/palladium component is also readily poisoned by the presence of sulphur-containing species derived from the diesel fuel. However, when an oxidation catalyst employing this technology is used alongside a diesel particulate filter, the high temperatures used to periodically regenerate the filter will remove sulphur from the gold/palladium catalyst component, restoring its performance. Gold or palladium on their own continue to have low efficiency in diesel exhaust oxidation reactions.

Although simulated catalyst ageing indicates that this platinum/palladium/gold concept shows some promise for diesel oxidation catalysis, durability has to be demonstrated on a vehicle operating in real-world conditions before series production can begin.

Based on the Nanostellar patents, this platinum/palladium/gold concept is likely to require similar levels of overall precious metal content to current catalyst technology. If it is widely adopted, there would be some decrease in the use of platinum on a typical diesel oxidation catalyst (and a possible increase in the use of palladium), offsetting some of the growth in platinum usage from diesel particulate filters on light and heavy duty vehicles expected over the medium term.

Depending on the relative prices of the three precious metal components concerned, this approach could in some cases reduce the total precious metal cost of a diesel catalyst compared to current platinum-only and platinum/palladium formulations.