Emissions control -- as good as gold

  • 25-Apr-2008 04:24 EDT
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Nanostellar uses “rational design” to develop new catalyst materials for diesel emissions control. Shown is the final snapshot of a simulation that predicts the surface composition of a nanoparticle alloy under CO environment.

As the President and CEO of four-year-old Nanostellar, Inc. points out, nanotechnology is nothing new in emissions control; nano-sized precious-metal particles have been present in catalytic converters since their introduction in the mid-1970s. “So what we work on is, ‘How do we design new materials so that they can overcome some of the limitations of the materials that are being used today?’” Pankaj Dhingra said.

The answer to that question is what Nanostellar refers to as Rational Catalyst Design, which combines computational nanoscience and advanced synthetic chemistry.

“We get asked quite frequently, ‘How come Nanostellar claims to have come up with these new materials that [companies] with a lot bigger R&D funds have not been able to come up with?’” said Dhingra. “Working computationally, rational design allows you to really understand how the reactions take place on the surface of the catalyst and how the material properties change in the operating environment. This knowledge gives a very strong guide for our chemists in terms of the different directions of research that they should be pursuing.”

Substituting computational cycles for lab work can help shorten the catalyst development process, according to Nanostellar, much like computer simulation of automotive components or systems can save time and money by reducing the number of physical prototypes.

Through this emerging area of science, adopted from Stanford University, the Redwood City, CA-based company determined that the addition of gold to the platinum/palladium alloy used in more recent diesel catalytic converters is beneficial in terms of cost and performance.

“For NS Gold, a tri-metal formulation consisting of platinum, palladium, and gold, our focus has been to take some of the platinum out of the 2:1 platinum/palladium alloy and replace that with gold,” explained Dhingra. “It has a positive impact on cost, because gold is about half the cost of platinum, and you get improved performance because of the synergistic behavior between the three metals.”

The ratio between platinum, palladium, and gold is roughly 1:1:1.

This synergy results in a 20-25% reduction in emissions compared to equivalent-cost platinum/palladium alloys, according to Dhingra. Other benefits include improved thermal stability, greater resistance to poisoning, and better light-off temperature.

Rational design helped overcome a technical challenge that previously prevented gold’s use in catalysts, Dhingra said. “Gold is a very volatile metal, so it does not last by itself in the diesel oxidation catalyst under oxidizing conditions and where temperatures can reach 600 to 700°C during regeneration. So we spent quite a bit of time to determine, ‘How do we include gold in this mix so that it is stable?’” The answer is by alloying gold with another metal.

This NS Gold formulation is suitable for both light- and heavy-duty diesel engines, including with HCCI (homogeneous-charge compression-ignition). Now in the technology-evaluation stage of development, the catalyst material is currently being evaluated by about seven OEMs and four catalyst companies, said Dhingra. It could end up in production vehicles as early as 2009 or 2010.
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