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Newly Developed Catalyst is Effective at Cleaning Emissions from Natural Gas Engines

A newly developed catalyst with unique, atomic-sized "rafts" does a better job than current technology for cleaning up emissions from natural gas engines.

The work, reported in Nature Catalysis, could make natural gas-powered technology cleaner and more viable for trucks, off-road vehicles and equipment powertrains. Researchers developed catalyst "rafts" of palladium (Pd) oxide that are held together with single atoms of platinum. Their catalyst is effective at cleaning up the natural gas and allows the catalytic reaction to be more tolerant of water vapor, reducing the amount of unburnt methane that would be emitted.

While natural gas engines are cleaner than gasoline or diesel engines, creating about 25% less in carbon dioxide emissions and less particulate pollution, they emit unburnt methane because their exhaust emission catalytic converters are not efficient at low temperatures. The new development was shown to perform at higher reaction rates than current technology.

"The improvements in energy efficiency have to go hand in hand with the after-treatment technologies," said Yong Wang, Voiland Distinguished Professor in Washington State University's Gene and Linda Voiland School of Chemical Engineering and Bioengineering and one of the corresponding authors on the paper. "Currently, combustion from methane to generate power is not able to use the most efficient combustion technology. So it works, but there is room for further improvement in that efficiency."

The team was led by researchers from WSU and the University of New Mexico with a number of collaborators in the United States, European Union and China.

While not as widely used in the U.S., natural gas engines are commonly used in vehicles worldwide, especially in China, Iran and India. Because they're less polluting than diesel engines, they are often used in trucks and buses in urban areas. Natural gas-powered engines are also used in the gas industry to run thousands of compressors that pump natural gas to people's homes.

However, these natural gas-powered vehicles emit unburnt methane because their exhaust emission catalytic converters are not efficient at low temperatures. The more efficiently the engines work and the cleaner they burn, the lower the exhaust temperature becomes and the poorer the catalysts perform at cleaning up pollutants. Unburnt methane from the engine, in particular, is a potent greenhouse gas which is about 25 times worse than carbon dioxide, contributing to climate change.

Furthermore, one of the byproducts of methane combustion is water, and conventional catalysts are "notoriously bad" when it comes to working in the presence of water, said Wang. The cleaner burning fuel ends up working against itself in removing pollutants.

Compared to typically used catalysts made of Pd oxide nanoparticles, the rafts the researchers developed provide better tolerance to water vapor with improved reactivity.

"The strongly bound platinum (Pt) can serve as a nucleation site for added metal atoms," said Abhaya K. Datye, professor in UNM's Department of Chemical and Biological Engineering and one of the corresponding authors of this study. "Using trapped Pt atoms, we were able to demonstrate the formation of Pt as well as Pd oxide two-dimensional rafts which modify the oxidation state and reactivity of the active phase."

"Our theory calculations suggested that the raft does not readily dissociate water, thus inhibiting the adverse effect of water poisoning in the catalysis of methane oxidation," said Hua Guo, professor in UNM Department of Chemistry and Chemical Biology.

The researchers are now working to further advance the catalyst technology and are hoping to eventually work with industry to commercialize it.

The work was partially funded by the U.S. Department of Energy Office of Science, Catalysis Science Program, the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy/Vehicle Technologies Office, the Advanced Manufacturing Office, the U.S. Air Force Office of Scientific Research, Chinese National Natural Science Foundation, and the NSF Engineering Research Center CISTAR.

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