An illustration depicting individual palladium atoms (white) removing methane (white bubbles) at the surface of the catalyst. Image: Cortland Johnson/Pacific Northwest National Laboratory.
An illustration depicting individual palladium atoms (white) removing methane (white bubbles) at the surface of the catalyst. Image: Cortland Johnson/Pacific Northwest National Laboratory.

Individual palladium atoms attached to the surface of a catalyst can remove 90% of unburned methane from natural-gas engine exhaust at low temperatures, say researchers from the US Department of Energy (DOE)’s SLAC National Accelerator Laboratory and Washington State University (WSU). They report their findings in a paper in Nature Catalysis.

While more research needs to be done, this advance in single-atom catalysis has the potential to lower exhaust emissions of methane, one of the worst greenhouse gases, which traps heat at about 25 times the rate of carbon dioxide.

The researchers showed that the catalyst was able to remove methane from engine exhaust at both the lower temperatures where engines start up and the higher temperatures where they operate most efficiently but where catalysts often break down.

“It’s almost a self-modulating process which miraculously overcomes the challenges that people have been fighting – low-temperature inactivity and high-temperature instability,” said Yong Wang, a professor in WSU’s School of Chemical Engineering and Bioengineering and one of four lead authors of the paper.

Engines that run on natural gas power 30–40 million vehicles worldwide and are popular in Europe and Asia. The natural gas industry also uses such engines to run compressors that pump gas into people’s homes. Natural-gas engines are generally considered cleaner than gasoline or diesel engines, creating less carbon and particulate pollution.

However, when natural-gas engines start up, they emit unburnt, heat-trapping methane because their catalytic converters don’t work well at low temperatures. Today's catalysts for methane removal are either inefficient at lower exhaust temperatures or become severely degraded at higher temperatures.

“There’s a big drive towards using natural gas, but when you use it for combustion engines, there will always be unburnt natural gas from the exhaust, and you have to find a way to remove that. If not, you cause more severe global warming,” said co-author Frank Abild-Pedersen, a SLAC staff scientist and co-director of the lab’s SUNCAT Center for Interface Science and Catalysis, which is run jointly with Stanford University. “If you can remove 90% of the methane from the exhaust and keep the reaction stable, that’s tremendous.”

A catalyst with single atoms of palladium dispersed on a support also uses every atom of the expensive and precious metal. “If you can make them more reactive,” Wang said, “that’s the icing on the cake.”

In their work, the researchers showed that their catalyst, which is made from single palladium atoms on a cerium oxide support, could efficiently remove methane from engine exhaust, even when the engine was just starting.

They also found that the trace amounts of carbon monoxide that are always present in engine exhaust played a key role in dynamically forming active sites for the reaction at room temperature. The carbon monoxide helped the single atoms of palladium to migrate to form two- or three-atom clusters that efficiently break apart the methane molecules at low temperatures.

Then, as the exhaust temperatures rose, these clusters broke up into single atoms that redispersed. This reversible process not only made the catalyst thermally stable but also allowed it to use every palladium atom the entire time the engine was running – including when it started cold.

“We were really able to find a way to keep the supported palladium catalyst stable and highly active and, because of the diverse expertise across the team, to understand why this was occurring,” said SLAC staff scientist Christopher Tassone.

The researchers are now working to further advance the catalyst technology. They would like to better understand why palladium behaves in one way while other precious metals such as platinum act differently.

The research has a way to go before it can be put inside a car, but the researchers are already collaborating with industry partners, as well as with DOE’s Pacific Northwest National Laboratory, to move the work closer to commercialization.

This story is adapted from material from SLAC National Accelerator Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.