The catalysts that clean up automotive emissions typically consist of particles of platinum and other precious metals anchored on oxides. Because only the metal atoms at the particle surfaces come in contact with reactants and catalyze reactions, catalyst manufacturers strive to make these particles as tiny as possible.
But these supported catalysts come with trade-offs. Dispersing the precious metal as finely as possible can go too far, and the catalysts can become unstable: The metal particles diffuse, coalesce, and lose their catalytic oomph. And the catalysts are often inactive when the temperature of the exhaust is low, which is the case when today’s engines start on a cold morning and will regularly be the case with future energy-efficient engines.
A new study on automobile exhaust cleanup describes a way to bypass those problems in a catalytic two-for-one deal. Researchers have shown that a simple procedure can stabilize a platinum-based automotive catalyst and reduce the temperature at which it can thoroughly strip CO from engine exhaust (Science 2017, DOI: 10.1126/science.aao2109).
Such a treatment could help clean up emissions from future engines designed to recover energy lost in hot exhaust, which results in lower temperatures of the gas that passes through the catalytic converter.
In the run-up to the new study, a team led by University of New Mexico chemical engineer Abhaya K. Datye took dispersing metal particles on an oxide support to the extreme. In 2016, the group reported that isolated platinum atoms on ceria could convert CO to CO2, a key reaction in engine-emissions cleanup. But the catalyst worked weakly.
So the team, which includes Yong Wang of the Pacific Northwest National Laboratory, searched for chemical and physical treatments that would boost the Pt-CeO2catalyst’s activity without causing it to fail quickly, a common occurrence during catalyst development.
Eventually the group found that heating the catalyst to 750 °C in steam drastically improves its ability to mediate CO oxidation. Specifically, in contrast to the untreated catalyst, which needs to be heated to roughly 210 °C to begin oxidizing CO and achieves 100% CO conversion at 320 °C, the treated catalyst begins working at just 60 °C and reaches 100% conversion at 148 °C. Furthermore, the treatment makes the catalyst durable: It showed no signs of deactivation even after 300 hours of testing.
Microscopy and spectroscopy analyses indicate that the steam enhances CO-oxidation performance by creating catalytically active sites featuring ceria-bound Pt-OH groups.
“This discovery could help advance the technology for vehicle exhaust conversion,” remarks Bruce C. Gates, a catalysis specialist at the University of California, Davis. “The authors’ catalyst characterizations provide deep insights and point the way forward.” The characterization work also raises intriguing questions for further study, he adds. For example, Gates proposes that researchers should examine the nature of the sites on ceria at which platinum bonds and determine if they are defects. He also wonders if a metal cheaper than platinum would work similarly.