THE SHIFT TO THRIFT01 June 2011
John Kendall reports on the challenge of reducing rare earth elements in exhaust catalysts
Both precious and rare earth metals are required for the manufacture of exhaust catalytic converters. Principally, the precious metals most used – platinum, palladium and rhodium – are needed for both the oxidation and reduction reactions required in a three-way catalytic converter to reduce the amount of toxic pollutants in gasoline exhaust gases. Current systems operate with a conversion rate of around 90%.
In addition to the precious metals, the rare earth metal cerium is the most widely used of its type, because it can assist the catalytic converter in several ways. First, it can store oxygen from the exhaust gases and release it when it's needed. It can also help to stabilise the alumina wash-coat used on the catalyst substrate to produce a rough, instead of a smooth, surface, effectively increasing the surface area of the catalyst material and therefore improving its efficiency.
Cerium can also behave as a promoter, by making the precious metal catalyst even more active and therefore effective. It also acts as a catalyst itself. Although cerium is more common than some other metals, it is not readily found in the kind of concentrations considered to be commercially exploitable, which means that the economically viable deposits are not always to be found in politically expedient or geographically convenient locations. Extracting the metal produces toxic waste that also needs careful disposal.
Manufacturers have a strong interest in finding more cost-effective alternatives. "What we can do is get smarter about how we use materials," says Bob McDowel of Aristo Catalyst Technologies, "We always try to 'thrift' the metals to reduce the cost as much as possible, so I'm sure that other companies are looking at ways to thrift their rare earth content.
"How you can do that in general is to be smarter in how you place it in the catalyst and in the formulation." That means finding ways to retain the durability requirements of the catalytic converter, while using 10
or 20% less cerium. "That thrifting process obviously gets more intense as the price goes up", states McDowel, "There's not a whole lot we can do, in terms of substitution of one rare earth for another."
At the same time, improving the wash-coat that contains the rare earth metals, so that it can store oxygen more effectively, could reduce the amount of precious metals needed. "If you have better storage media, like zeolites or cerium dioxide, and you can improve the wash-coat, it might turn out that you can reduce the precious metals, while still having the same efficiency", suggests Frank Terres, exhaust aftertreatment engineering director for Tenneco's OE Emission Control Europe facility in Germany.
Cerium is already one of the least expensive rare earth metals. Bob McDowel from Aristo continues: "The alternatives are, change your regulations; make them easier, so that you don't need all those rare earth and precious metals, but that's not likely to happen.
"We're fairly far along the trail of thrifting and substituting cheaper materials for the rare earths, if we could. Again, the other alternative is to use more precious metals, but that's not cost effective either.
"People are looking at potentially just using cerium for the precious metal promotion activity, as opposed to the oxygen storage activity. There's a small start-up company looking into replacing the alumina wash-coat material with silicon dioxide. What they have not proven yet is, if they can substitute these silica fibres for the precious metal support – the alumina wash-coat we've been using for 35 years now – you can get a more effective catalyst, with very little use of the precious metal, because of the 'nano' technology."
"Typically, the way the industry responds to increasing prices is to find ways to reduce their usage, not cut their usage to zero, says Dr David Belton, GM Fellow for Emission Controls. "I think that is probably the most likely response we will have for some time. Sometimes, we can make the catalytic converter slightly bigger and, by making it bigger, we can use slightly less precious metals.
"In some cases, there are other technologies or devices we can use, but right now those other devices haven't proved to be cost effective, because the price of, say, palladium and rhodium hasn't gone high enough yet to drive us to use them."
Belton gives the example of an auxiliary pump, designed to pump fresh air into the engine. "Called secondary air injection," he continues, "that enables the catalyst to become hot faster and so it replaces precious metals in the catalyst. The price is about €70 per vehicle and right now the precious metal prices don't really dictate using that device yet."
GM announced last year that it was investigating the use of Perovskite in catalytic converters. Is this material a potential replacement for precious metals? "Perovskite materials are just another in the continuing evolution of catalyst technology improvement," says Belton. Like the rare earth metals, their role is to improve the effectiveness of the catalyst. "I wouldn't say they are particularly a breakthrough, they're more like an incremental improvement. Those materials help the more expensive precious metals remain as very small individual particles on the catalyst, as opposed to coming together in larger sized particles. Since only the precious metals on the outside of the particle are effective for catalysis, then a bunch of small particles are a lot better than a few large particles."
With the emissions focus shifting away from toxic emissions towards carbon dioxide emissions reduction, gasoline engine technology will be drawn more towards lean burn systems. "That actually has a tendency to drive the industry away from platinum and rhodium to some extent and towards other materials," suggests Belton. "So it could turn out to be true, depending on how manufacturers solve that CO2 problem, that CO2 regulations could actually decrease the pressure on the precious metals market. If you go to a true lean-burn gasoline system, then you won't have conventional three-way catalysts. You'll have a system that more closely mimics the diesel after-treatment system."
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