How to optimize precious metal usage in catalytic converters

Catalytic converters efficiently strip pollutants and smog-forming compounds from engine exhaust by reacting these species on the surface of precious metals such as rhodium. The expensive metal may last longer and catalyst makers may be able to use it more sparingly thanks to a study that details atomic-level changes that deactivate and reactivate the catalytic metal (Chem. Mater. 2022, DOI: 10.1021/acs.chemmater.1c03513). Gasoline engine emissions are scrubbed by a three-way catalyst (TWC), which takes its name from the system’s ability to scrub three pollutants. TWCs oxidize hydrocarbons and carbon monoxide and reduce nitrogen oxides. Modern TWCs rely on rhodium nanoparticles typically supported on alumina. Earlier studies showed that exposure to oxidizing exhaust gases at high temperature can deactivate the catalyst and that reducing conditions can help restore its activity. But details of these processes have remained elusive. So Cheng-Han Li and Joerg R. Jinschek of the Ohio State University and coworkers at Ford Motor scrutinized TWCs using atomic-resolution microscopy, X-ray spectroscopy, and other methods. They exposed the catalysts to high temperatures and typical exhaust streams, which vary from oxygen rich to oxygen poor during normal engine operation. They showed that oxidizing conditions can cause rhodium nanoparticles to dissolve into the support and form rhodium aluminate, a catalytically inactive material. Reducing conditions help reverse the process. The study suggests that rhodium usage can be optimized by chemically anchoring the nanoparticles more tightly and by using alloys that resist dissolution.