(Nanowerk News) A catalyst using one or only a few palladium atoms removed 90% of unburned methane from natural gas engine exhaust at low temperatures in a recent study. While more research needs to be done, advances in single atom catalysis have the potential to reduce exhaust emissions of methane, one of the worst heat-trapping greenhouse gases at about 25 times the rate of carbon dioxide.
Reporting on Natural Catalysis (“Dynamic and reversible transformation of subnanometer-sized palladium in ceria for efficient methane removal”), a research effort between Washington State University and SLAC’s National Accelerator Laboratory demonstrated that a single-atom catalyst is able to remove methane from engine exhaust at a lower temperature, less than 350 degrees Celsius (662 degrees Fahrenheit), while maintaining reaction stability at higher temperatures.
“It’s almost a self-modulation process that miraculously overcomes the challenges people face — low-temperature inactivity and high-temperature instability,” said Yong Wang, Regents Professor in WSU Gene and Linda Voiland’s School of Chemical Engineering and Bioengineering and corresponding author on the paper.
Natural gas engines are used in an estimated 30 million to 40 million vehicles worldwide and are popular in Europe and Asia. The gas industry also uses it to run compressors that pump natural gas into people’s homes. They are generally considered cleaner than gasoline or diesel engines, emitting less carbon and particulate pollution.
However, when these natural gas powered engines are started, they give off unburned methane and trap heat because their catalytic converters don’t work well at low temperatures. The catalyst for methane removal is inefficient at lower flue gas temperatures or greatly degrades at higher temperatures.
“There is a big push to use natural gas, but when you use it for a combustion engine there will always be unburned natural gas from the exhaust, and you have to find a way to get rid of it. Otherwise, you’re causing more global warming,” said co-author Frank Abild-Pedersen, staff scientist at the SLAC National Accelerator Laboratory. “If you can remove 90% of the methane from the exhaust and keep the reaction steady, that’s great.”
The single-atom catalyst with active metal dispersed singly on the support also uses every atom of the expensive and precious metal, Wang added.
“If you can make them more reactive, that’s the icing,” he said.
In their work, the researchers were able to show that their catalyst made of a single palladium atom on a support of cerium oxide efficiently removes methane from engine exhaust, even when the engine has just been started.
They found that traces of carbon monoxide, which are always present in engine exhaust, play a key role in forming dynamically active sites for reactions at room temperature. Carbon monoxide helps palladium single atoms migrate to form groups of two or three atoms that efficiently break down methane molecules at low temperatures.
Then, as the temperature of the exhaust gas rises, the sub-nanometer clusters disperse back into single atoms again so that the catalyst is thermally stable. This reversible process allows the catalyst to work effectively and use up every atom of palladium during engine running – including during cold start.
“We were actually 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 is,” said Christopher Tassone, staff scientist at SLAC’s National Accelerator Laboratory and one of the paper’s authors.
Researchers are working to further advance catalyst technology. They wanted to better understand why palladium behaves in one way while other precious metals such as platinum act differently.
The research has a ways to go before it’s put in a car, but the researchers are working with industry partners as well as with the Pacific Northwest National Laboratory to one day move the work closer to commercialization.