Patent Picks: Catalytic Converters

Catalytic converters, found in all U.S. automobiles made since 1975, help clean engine exhaust by breaking down gaseous molecules that produce smog. These marvels of heterogeneous catalytic chemistry oxidize volatile hydrocarbons and carbon monoxide and reduce nitrous oxides (NO_x). The converters use a three-way catalyst (TWC) consisting of rhodium, palladium, and platinum to turn those smog precursors into water, carbon dioxide, and nitrogen. The three metals are embedded into a porous material such as aluminum oxide (Al₂O₃) to increase the catalyst surface area. Manufacturers also mix in rare-earth metal oxides to serve as another source of oxygen for the catalytic reactions. This is necessary for times when the exhaust has low oxygen levels, such as when the car is accelerating or when the engine is still cold. Here, we highlight three innovative advances in TWCs from the databases of Chemical Abstracts Service.

In TWCs, rhodium is a critical component in the catalytic reduction of NO_x. However, the metal’s activity decreases as the catalyst ages and through chronic exposure to the high temperatures of the exhaust. A research team from N.E. Chemcat found that as rhodium particles age they self-assemble and grow in size, resulting in a significant drop in their activity. The researchers inhibited this mass-transfer process by dispersing neodymium oxide (Nd₂O₃) particles with a diameter of 50–100 nm around the rhodium particles (US 20150038325 ). The rhodium particles maintained their size during simulated aging experiments, which involved the researchers heating the catalyst to 950 °C and running the engine for 50 hours. They also found that the Nd₂O₃ enhanced the catalytic activity of rhodium when compared with similar TWCs without neodymium. N.E. Chemcat researchers wouldn’t comment on the mechanism behind the neodymium effect because of the pending patent. But they say the technology could be used for other catalyst systems that break down NO_x.

Bimetallic oxides of cerium and zirconium (CeO₂-ZrO₂) are widely used as an oxygen storage material in TWCs. As the catalyst ages, or when it is exposed to high temperatures, the surface area of these oxides decreases, which limits their ability to transfer oxygen and impairs catalyst activity. Adding Al₂O₃ to the CeO₂-ZrO₂ oxides, either through physically mixing the oxides or chemically forming the trimetallic oxide, can improve the catalyst’s thermal resistance and enhance its oxygen storage and release properties. Researchers from Johnson Matthey developed a new method to coprecipitate cerium(IV) and zirconium(IV) compounds with Al₂O₃ to form a trimetallic oxide (CeO₂-ZrO₂-Al₂O₃). Compared with a physical mixture of Al₂O₃ and CeO₂-ZrO₂, the trimetallic oxides were thermally more stable and had better oxygen storage and release properties (JP 2014534156). Haiying Chen, director of Johnson Matthey’s research division, tells C&EN that the material was initially developed to meet future stringent government emissions standards.

Advances in engine design to increase fuel efficiency might unintentionally lower exhaust temperatures below the temperature at which TWCs are most efficient. Raoul Klingmann and coworkers at Umicore have developed a double-layered TWC, which is thought to be suitable for cooler working conditions (US 20150125370). The first layer sits directly on the surfaces of the catalytic converter’s honeycomb structures and is composed of CeO₂-ZrO₂ and palladium mixed with Al₂O₃. The second layer, which is coated over the first and is in direct contact with the exhaust, consists of rhodium in Al₂O₃. Although the double-layered TWC is meant to meet the needs of future engines, the researchers say it could help better handle exhaust from existing ones when the engines have just started and are still cold.