Getting platinum out of catalytic converters

Start-stop engine technology and turbocharging are lowering exhaust temperatures, reducing the effectiveness of catalytic converters as emissions standards rise. California’s Clean Diesel Technologies Inc. may have a solution – a new catalyst chemistry that lowers emissions while reducing the use of costly platinum.

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September 22, 2015

Two sets of environmental regulations are working against each other in the auto industry. Regulators have mandated a 54.5mpg standard for fuel economy by 2025, and automakers have responded primarily by shrinking engines – replacing 6-cylinder models with turbocharged 4-cylinder options. Smaller engines tend to burn less gas, so fuel efficiency goes up.

However, exhaust from turbocharged engines is a separate problem. In 2017, the U.S. Environmental Protection Agency’s Tier 3 emissions standards go into effect, requiring manufacturers to mitigate more of the harmful material that leaves tailpipes. Turbocharging – and other fuel-saving technologies that automakers are implementing – lowers exhaust temperatures, reducing the effectiveness of catalytic converters. So paradoxically, standards to make cars greener in terms of carbon dioxide emissions could create problems for nitrogen oxide (NOx) and carbon monoxide (CO) emissions.

“During an acceleration with a turbocharged engine, NOx goes from low to extremely high. At the same time, the gas flow rate over the catalyst goes extremely high, and that has a multiplier effect on the catalyst. So the amount of NOx flowing over the platinum particles gets insanely high. And yet, going from Tier 2 to Tier 3, the NOx emissions are going to have to be much lower,” says Stephen Golden, chief technology officer for Oxnard, California-based Clean Diesel Technologies Inc. (CDTi), a company that develops and produces catalysts for diesel and gasoline engines.

“Turbo downsizing is a lot more fuel efficient, but there’s a lot less heat going into the exhaust,” Golden says. “You can’t have many seconds when the catalyst is not really awake, because you’ll blow by the tailpipe standards before you really get started.”
 

Costly catalysts

So far, the industry’s reaction to rising emissions standards has been to use the same materials that have dramatically improved emissions levels since the 1970s – platinum group metals (PGMs) such as rhodium, palladium, and platinum. PGMs are great at pushing oxygen into unburned fuel in emissions, effectively converting hydrocarbons to water vapor, and at pulling oxygen out of NOx and converting that into nitrogen. But PGMs are very expensive, leading to higher material costs and design challenges – such as automakers making catalytic converters less accessible following a rash of component thefts in which criminals would cut the converters off SUVs.

Golden says CDTi’s response is a new catalyst chemistry that requires significantly less PGM materials, while improving the performance of the converter to meet the rigorous upcoming standards. Called Spinel, CDTi’s material uses base metals in a crystalline structure that stores more oxygen than other catalysts. Spinel provides more oxygen to the PGMs when converting hydrocarbons to CO2 and water and absorbs more oxygen when converting NOx to nitrogen. Similar to the shrink-and-boost strategy of the engines themselves, he says Spinel reduces the amount of PGM material but turbocharges the performance.

“As you look at the cooling effect of turbo downsizing on the cold start, the knee-jerk reaction is to increase the PGM loading by a factor of three or four,” Golden says. “It could happen, but it would be very expensive, and it would pressure the PGM supply.”

Start-stop technology, in which car engines shut off at red lights or when stopped in traffic, can also strain catalytic converters, Golden says, because the engines stay cooler, preventing traditional catalysts from hitting their most efficient states.

“With start-stop and some of the hybrids out there, you never get warm enough to fully activate the catalyst,” Golden says. While he dreams of producing a converter with no PGMs, Golden says the solution for now is to make PGMs more effective.
 

Challenging proposition

George Lester, president of consulting company George Lester Inc. and an adjunct professor at the Center for Catalysis and Surface Science at Northwestern University in Evanston, Illinois, was on the team at Universal Oil Products (now part of Honeywell) that developed some of the first catalytic converters for cars in the 1960s and 1970s. Lester says the auto industry has studied and rejected low-PGM and PGM-free catalysts many times throughout the past 40 years.

About six years before catalytic converters started showing up on cars in 1975, Lester and his team proposed a copper-iron catalyst to an automaker, assuming that major producers wouldn’t be willing to pay for more expensive PGMs. The original equipment manufacturer (OEM) rejected that approach, he said, because while costly, the PGMs mitigated emissions more effectively and substantially faster than other systems. Since then, the industry has never turned back.

“If you have something that’s marginally different, it’s a terrible matter to try to get it produced,” Lester says. “The car company is manufacturing hundreds of thousands of these units per year, and it’s got something on its vehicles that’s generated 600 million customer miles with a satisfactory experience.”

While he agrees that lower-temperature emissions from turbocharging are creating a challenge for catalysts, he says many OEMs will simply increase the amount of PGM material to compensate. It’s a costly solution, but companies know it will work, Lester says.

“If you try to downsize and turbocharge at the same time you’re trying to reduce precious metals, you’re handcuffing yourself,” Lester notes.

To win support for a new technology, he says CDTi will have to show either a significant cost reduction or a dramatic improvement in catalyst performance – preferably both. And even then, it will have to convince automakers to rigorously test the material for a long time in various environmental conditions. Given those testing expenses and the fact increasing PGM use is effective, he says convincing companies to change is going to be tough.

“If it’s sizable enough, if it makes enough of a difference, they’ll listen. They’ll give you an honest look, but it has to be pretty significant,” Lester says.

In testing, CDTi was able to meet emissions standards on one turbocharged car – replacing a converter that used 58g/ft3 of PGMs with one that used 2g/ft3. On a non-turbocharged car, testing met standards when a 22g/ft3 was also replaced with a 2g/ft3 converter.
 

Lower-cost manufacturing

Golden agrees that getting companies to consider a new way of doing things is going to be a challenge. But they say Spinel can offer cost improvements beyond simply reducing material costs for PGMs. Throughout the past 40 years, emissions standards have increased several times as catalytic converter performance improved, a cycle driven by improvements in coating techniques, not in radical changes to the materials themselves.

Statistics from the Association for Emissions Control by Catalysts, a European association set up in 1978 to represent catalyst companies, show that in the mid-1970s, catalytic converters had 200 cells per square inch of PGM materials cleaning emissions. Within a decade, that figure was up to 400cpsi, and modern converters have densities as high as 1,200cpsi. More cells in less space has allowed manufacturers to increase performance of converters while reducing their size and weight, but they still use a lot of PGMs.

The techniques that companies have used to increase cell density and improve performance are complex and expensive. Coating companies apply different catalytic materials to converters in multiple layers, leading to repeated coat-and-bake cycles.

“If you look at cross-sections of substrates, you’ll see many layers of materials, lots of architecture and zoning to get more performance from PGM,” Golden says. “It’s become less about the materials and more about the process complexity that’s involved to keep the performance going forward without just increasing the PGM levels.”

He adds that because Spinel’s molecular structure contains the oxygen storage needed to boost PGM performance, many of those complex stages can disappear. Coaters could apply a layer of Spinel followed by a layer of PGMs – a vastly simplified two-stage process.
 

Supplier to materials company

CDTi Chief Financial Officer David Shea says his company is in the process of converting from a company that makes catalysts for automakers to one that would in addition make base materials for coating companies – effectively going from a Tier 1 position in the supply chain to a Tier 2.

“We’ve been doing this for 15 years, and we believe that we’ve been making the highest-performing, lowest PGM catalyst on the market, but we’re still a niche player,” Shea says. “OEMs want to see scale, global presence, strong balance sheet – then you get into key concerns like quality and technology.”

To roll out the technology, he says the company needs to supply the manufacturers that are already working with the OEMs.

“We would supply these highly enabling powders to other catalyst coaters, taking advantage of their infrastructure which is global and right next to every automotive plant in the world. There is enough PGM savings coming out of this that we’ll motivate the OEMs to embrace this strategy,” he adds.

The key is having the OEM require the use of Spinel material in their order specifications. Automakers already use that ordering system for other components, Shea adds.

“The substrates, the ceramic filters that we all coat, it works the same way,” Shea explains. “We buy those and coat those, but the OEMs direct us to buy this substrate from this supplier at this price. The OEM negotiates that and tells us what we’re going to buy for a vehicle. That model, in the emissions part of the automotive industry, already exists. So we’re working within an existing framework, but we’re moving our spot in the supply chain.”

Golden adds that in the case of substrates, new suppliers sometimes challenge conventional OEM supply orders, suggesting different suppliers of ceramic substrates than the ones on the spec sheets.

“That conversation usually lasts a very brief period,” Golden says. “The mechanical engineers at the OEM make those decisions after a lot of study and testing. They’re pretty confident that they have the right material when they spec it.”
 

Conclusion

U.S., Chinese, Indian, European, and Japanese emissions standards are becoming more stringent. These global shifts – such as moving from Tier 2 to Tier 3 implementation, with some manufacturers adopting an accelerated phase-in – will likely pressure PGM prices.

Getting the industry to accept a new way of doing things is always a challenge, but Shea says CDTi’s engineers believe they can prove that Spinel will be a more cost-effective solution – in terms of material costs and manufacturing – for automakers.

“Even though Steve’s Holy Grail is zero PGMs, if you can get it down to 2g/ft3, that’s massive improvement,” Shea says.

 

Clean Diesel Technologies Inc.
www.cdti.com

George Lester Inc.
www.georgelesterinc.com

 

About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or rschoenberger@gie.net.