According to a new paper in Science, a team from GM’s Global R&D operations may have found a significantly less expensive way to treat vehicle exhaust via a catalytic converter that uses a strontium-doped perovskite instead of costly platinum-based catalyst.
For some context, it’s important to know that while newer diesel engines have attained or even surpassed the miles-per-gallon levels in available hybrid vehicles, these “lean-burn” engines come with a price: hard-to-treat NOx emissions. (There also are some fuel-efficient gasoline-powered lean-burn engines, i.e., direct injection, available.)
As a result of this NOx problem, vehicle makers have had to add costly NOx aftertreatment systems that use either an expensive platinum-based catalyst or a complicated urea-based system (gee, honey, did you remember to top off and preheat our car’s urea supply today?).
Thus, it’s major news that the GM research team, led by Chang Hwan Kim, demonstrated that perovskite oxides, La1–xSrxCoO3 and La1–xSrxMnO3 (using simulated diesel exhaust) were able to achieve conversions of NOx comparable to that of commercial platinum-based catalysts.
Unfortunately, this story is a little more complicated. One bit of bad news is that Kim and the rest of the team discovered that these perovskite materials are less effective at oxidizing the other major problematic exhaust components: hydrocarbons and carbon monoxide. Sulfur contaminants in fuel can also deactivate these catalysts.
The good news is that they found a way around that problem. They learned that the NOx oxidation activity of the catalyst could be improved – even in the presence of sulfur – by adding palladium.
While palladium is part of the same group of metals as platinum and isn’t cheap, it is a far less expensive replacement.
James E. Parks II, a scientist at Oak Ridge National Lab, in a companion article explained the significance of all this, noting, “With volatile and escalating prices, catalyst product costs vary dramatically during the course of a product development cycle. As a result, engineers are constantly chasing a moving cost target during development. The catalyst developed by Kim et al. greatly reduces the amount of [platinum group metals] while still maintaining their effectiveness for NOx reduction from lean engines. This alternative technology will allow engineers greater flexibility as they work to develop better catalysts in a market where volatile PGM prices have made commercial introduction of fuel-efficient lean vehicles challenging. It is possible that these catalysts may allow lean-burn technology to be used with minimal added cost compared to conventional engines,” he writes.
An article in Nature about these discoveries pointed out that perovskite oxides have been studied for their use as catalysts for decades, however the magazine reports that other researchers in this field think Kim’s group has broken new ground. For example, Louise Olsson, who investigates catalysts at the Chalmers University of Technology in Gothenburg, Sweden, tells Nature, “I have not seen anything else with such good oxidation using a non-noble metal [platinum] catalyst.”
Likewise, Owen Bailey, who studies similar catalysts for Umicore, comments that, “I worked on these types of catalysts 13 years ago and I couldn’t get these results . . . How far it can be pushed, I don’t know.”