Silver Catalyst Reduces Cost of Purifying Diesel Exhaust

Apr 25, 2008 19:06 Motohiko Hamada, Nikkei Automotive Technology

Mitsui Mining & Smelting Co Ltd developed a new catalyst using cheaper silver, instead of the more expensive platinum, as the purification catalyst for diesel engine exhaust gas.

The company succeeded in reducing the noble metal cost by more than 90% without sacrificing the performance to purify particulate matters (PM).

Pt is widely used in catalyst for diesel exhaust gas, but it is currently extremely expensive. While the reduction in cost of noble metals remains as a major challenge, it has been believed that the adoption of materials other than Pt is difficult because their purification performance and durability is insufficient.

Mitsui Mining & Smelting developed a technology to stabilize Ag in high temperatures by combining Ag with a metal composite oxide. The company has already confirmed the heat resistance at up to about 800°C in the test environment. It repeatedly conducted the evaluation using an actual engine and proved that the new catalyst has a PM combustion performance that is on par with that of the existing platinum catalyst.

The new silver catalyst provides a higher performance in absorbing and releasing oxygen than the existing platinum catalyst. Thus, the new catalyst can effectively burn soot using the active oxygen present on the catalyst surface.

As the platinum catalyst burns soot using nitrogen dioxide (NO2), the NOx content in the exhaust gas should be larger than the theoretical reaction amount. It is difficult to burn PM at a low NOx concentration. In contrast, the silver catalyst is capable of burning PM using oxygen alone.

The existing platinum catalyst should be at a temperature higher than 600°C to burn PM when there is no NOx present, whereas the silver catalyst can burn PM at about 400°C. When incorporated in the actual engine, the Ag catalyst can reduce the amount of PM even at a low NOx concentration.

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