Just as we suspected, Mitsui's silver based catalyst has little hope of displacing PGMs in auto catalysts any time soon, though it may, if it works out some time in the next 20 years, end up in a very limited role in diesel catalysts. Whoopee.

29th April 2008

Mitsui Mining of Japan announced on 23rd April 2008 that it has developed silver-based technology which could be used as a replacement for platinum in a diesel particulate filter (DPF). Mitsui claims that as the market price of silver is much lower than that of platinum, this technology could reduce the pgm cost by more than 90%. Mitsui also said that it intended to use the technology for industrial diesel applications – agricultural machinery and generators – after 2012.

In diesel engine exhaust, a DPF is used to trap particles of soot emitted as a result of combusting diesel fuel. Accumulated soot is periodically burned off the filter (i.e. the filter is regenerated). A platinum catalyst is used to generate this combustion at relatively low temperature. The DPF is preceded by or combined with a diesel oxidation catalyst (DOC) using platinum and palladium to oxidise hydrocarbons (HC) and carbon monoxide (CO) produced by the engine.

Is this a high potential technology?

Mitsui hopes to extend the application of this catalyst to trucks, buses and passenger cars. This will require extensive testing and development in order to prove that silver can be a more cost-effective material than already existing platinum DPF systems and to overcome its known drawbacks. Silver can easily sinter and may even vaporise at elevated temperature. It is also susceptible to poisoning by sulphur, reacting with sulphur to form silver sulphate (AgSO4), a very stable compound which cannot be decomposed or reduced back to silver except at high temperature. Stabilisation of silver is essential in order to provide the thermal durability required for commercial use.

Will platinum be superseded in diesel applications because of this technology?

Assuming the above challenges are solved, there is a possibility that this technology could be used for combusting soot in a DPF for some diesel on-road or non-road applications. However, unlike platinum, silver is a poor catalyst for oxidising nitric oxide (NO) to nitrogen dioxide (NO2), which is essential for regenerating the filter in circumstances where the vehicle operating temperature is low, as in city driving.

Silver is also not a good catalyst for oxidation of HC and CO, thus cannot be used instead of platinum and palladium in the oxidation catalyst (DOC). Moreover, a second DOC containing platinum would be needed to oxidise any residual HC and CO which would pass through a silver-based DPF unconverted. Any use of silver in a DPF is thus likely to be in conjunction with a pgm catalyst, while any pgm replaced by silver in a DPF would to a greater or lesser extent need to be added back into the system elsewhere.

Mitsui is targeting tractors and generators for the first application of the silver catalyst. Currently these engines are not required to have catalysts so if Mitsui's catalyst is used it will not replace any existing demand for platinum.

In summary, the technology may, if successfully developed, have limited application in diesel engine emission control for off-road vehicles in several years' time but it is too soon to know whether it could challenge platinum in filters on wider diesel engine applications in the long term. It is very unlikely that silver will replace any of the platinum and palladium used in diesel oxidation catalysts or gasoline three-way catalysts.