Serendipitous discovery could lead to more efficient catalysts

PULLMAN, Wash. – Preparing catalysts by sending hot, steamy car exhaust over them could improve their efficiency and reduce the amount of rare and expensive metals required in vehicle catalytic converters and many other emission control and industrial processes.

Reporting in the journal, Nature, an international team of researchers found that the hot car exhaust containing nitrogen oxides and carbon monoxide caused a previously unknown reaction that, used proactively, can significantly improve catalytic activity. Catalysts are substances that increase the rate of chemical reactions.

The researchers found that hot exhaust encouraged ceria particles, one of the components of the catalyst materials, to form two-dimensional, nano-sized clusters. These clusters, densely covering the surface, create many sites where chemical reactions can happen, increasing the efficiency of the process. The method also creates a large number of loosely bound oxygen ions associated with cerium atoms, which can move easily and further improve many, common catalytic reactions that require either adding or taking away oxygen. 

“They act like an oxygen sponge, and once the oxygen is easily activated, it’s very useful for many reactions requiring oxygen, like oxidation of hydrocarbons and carbon monoxide,” said Yong Wang, one of the study’s corresponding authors and Regents Professor in Washington State University’s Gene and Linda Voiland School of Chemical Engineering and Bioengineering. “This offers better design of a catalyst for multiple reactions.”

Serendipitously discovered, the new treatment method was found to improve catalytic activity by about ten times.

“Luck was a factor in this. Sometimes, we just stumble into a great science discovery. At the same time, this is one which is of practical interest,” said Wang, who also holds a joint appointment with Pacific Northwest National Laboratory.

Researchers are continually trying to improve catalytic converters and other emission control technologies. Over their lifetime, these technologies for removing pollutants from the exhaust streams of cars or power plants become less effective. Manufacturers have to include extra amounts of rare and expensive metals, such as rhodium, platinum, or palladium, in the converter to compensate for the catalyst’s degradation and to meet required emission standards.

One mystery that has befuddled industry for many years is that while nano-sized metal materials in catalytic converters are known to “sinter”, or glob together into larger particles and become ineffective over time when exposed to very hot and harsh conditions of the exhaust steam, the catalytic converters somehow still hold up better than expected.

“If you look at the ceria particle size growth, you would expect at least a hundred times reduction in activity, and that’s not the case,” said Konstantin Khivantsev, a staff scientist and chemical engineer at Pacific Northwest National Laboratory (PNNL) and a corresponding author. “There is a process that was not discovered or recognized, but it contributed to improved dispersion and catalytic activity.”

The researchers, including from WSU, PNNL, University of New Mexico, University of Sofia, Bulgaria, and Purdue University, decided to artificially age a catalyst to study it. However, instead of using just water, which is a typical component of aging tests, they decided to run very hot car exhaust over the catalyst for several hours. They observed that the catalyst’s performance improved rather than degraded.

“So, this is part of the fun of doing research because intuition told us that the catalyst would de-activate, and the results were the complete opposite,” said Abhaya Datye, Distinguished Regents Professor Emeritus at the University of New Mexico and a corresponding author. “We then asked ourselves whether this was real, and repeated the experiment several times. The next step was to figure out the science.”

They discovered that catalytic converters do not degrade as quickly as expected because the hot car exhaust, unbeknownst to the researchers in the field, had actually been aiding the reactions occasionally in short bursts when the car exhaust was at especially high temperatures. In terms of practical impact, the researchers now want to use the treatment process on purpose -- pre-activating the catalyst and purposely forming the reactive state at the beginning of the catalyst’s life.   

The new treatment technique could allow for the reduction of the amount of precious metals, such as rhodium, required in the catalyst, offering significant cost savings. Catalytic converters in a car usually contain about $800 worth of rhodium.

“In the spent catalysts, atomically thin patches of ceria that formed during the dispersion of ceria nanoparticles, are in in contact with precious metals, such as rhodium and platinum,” said János Szanyi, a corresponding author and staff scientist at PNNL.  “This allows the catalysts to survive harsh temperature conditions of vehicle exhaust and still maintain their activity.”

The researchers have been testing their catalyst treatment at the lab scale. They’re also working with industry partners to test them in vehicles under real operating conditions. The work relied on fundamental science that was supported by the U.S. Department of Energy’s Office of Science, Basic Energy Sciences, Catalysis Science program. The research at PNNL was supported by the U.S. Department of Energy’s Energy Efficiency and Renewable Energy Vehicle Technologies Office.