Two sets of environmental regulations are working against each other in the auto industry. Regulators have mandated a 54.5-mpg standard for fuel economy by 2025, and automakers have responded primarily by shrinking engines—replacing six-cylinder models with turbocharged four-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. 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.”
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 thefts in which criminals cut the converters from 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 carbon dioxide (CO2) and water and absorbs more oxygen when converting NOx to nitrogen. Similar to the shrink-and-boost strategy of the engines themselves, Golden 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,” he says.
Start-stop technology, in which car engines shut off at red lights or when stopped in traffic, also can 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, he says the current solution is to make PGMs more effective.
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 appearing on cars in 1975, Lester and his team proposed a copper-iron catalyst to an automaker, assuming major producers wouldn’t want to pay for more expensive PGMs. The original equipment manufacturer (OEM) rejected that approach, he says, because, while costly, PGMs mitigated emissions more effectively and substantially faster.
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,” he says.
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. 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 58 grams per cubic foot of PGMs with one that used 2 grams per cubic foot. On a nonturbocharged car, testing met standards when a 22-grams-per-cubic-foot converter also was replaced with a 2-grams-per-cubic-foot converter.
Golden agrees that getting companies to consider a new way of doing things is going to be a challenge. But he says Spinel can offer cost improvements beyond simply reducing costs for PGMs. Throughout the past 40 years, emissions standards have increased several times as catalytic converter performance has improved, a cycle driven by improvements in coating techniques, not in radical changes to the materials.
Statistics from the Association for Emissions Control by Catalysts, a Europebased organization 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 400 cells per square inch, and modern converters have densities as high as 1,200 cells per cubic inch. 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 companies have used to increase cell density and to improve performance are complex and expensive. Coating companies apply 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.
CDTi Chief Financial Officer David Shea says his company is in the process of converting from a catalyst maker to one that also would make base materials for coating companies—effectively going from a Tier 1 supplier to a Tier 2.
“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,” Shea says.
The key is OEMs requiring the use of Spinel in their order specifications. Automakers already use that ordering system for other components, Shea adds.
Golden says suppliers can challenge conventional OEM supply orders, suggesting different ceramic substrates suppliers than those 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.”
RESPONDING TO PRESSURE
U.S., Chinese, Indian, European and Japanese emissions standards are becoming more stringent. These global shifts likely will pressure PGM prices.
Getting the industry to accept a new way of doing things is 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.