Better Use of Rhodium in Future Catalytic Converters

A new study proposes that advanced catalytic converters could have longer lifespans and need a lesser amount of rare materials to work.

Catalytic converters convert unsafe gases from the exhaust of a car, including carbon monoxide and other impurities, into steam and other safer derivatives, such as nitrogen and carbon dioxide.

An efficient catalytic converter can endure for over a decade, but according to Cheng-Han Li, the study’s lead author, there is room for development. He stated future catalytic technologies could be engineered to excellently scrub pollutants for a longer timeframe.

We want to have a better lifetime for catalytic converters. Otherwise, they will have to be replaced or won't pass the government’s emission tests.

Cheng-Han Li, Study Lead Author and Doctoral Student in Materials Science and Engineering, The Ohio State University

Details of the study were reported in the journal Chemistry of Materials recently. 

Based on where a person lives, federal emission standards can differ. In 1975, to fight the increasing smog issue in cities throughout the United States, Congress sanctioned legislation stated that it was mandatory for all vehicles to have catalytic converters.

Although there are several types, contemporary catalytic converters use a blend of three precious metals: platinum, palladium and rhodium. These three-way catalysts can decrease nitrogen dioxide (NO₂) and nitric oxide (NO) emissions – two substances, if placed together, can form NOx, a chemical compound that possesses both direct and indirect damaging effects on human health.

Increasing prices for the three rare metals – particularly rhodium – is why felons all over have chosen to steal catalytic converters. The element is found mostly in the river sands of North and South America. Rhodium is said to be the world’s rarest element and is more valuable than platinum and gold.

The cost of rhodium has risen dramatically over the past years due to increasing demand coupled with a fundamental supply deficit.

Cheng-Han Li, Study Lead Author and Doctoral Student in Materials Science and Engineering, The Ohio State University

That means catalytic converters can be costly to manufacture, and doubly costly to replace.

Furthermore, since rhodium-based catalysts are in short supply, it is vital that they be used as efficiently as possible. As the catalysts have been proven to neutralize at high temperatures, scientists examined how their performance alters over time when exposed to high heat.

To achieve this, Li’s team carried out many tests on the converters, including having them undergo temperatures over 1600 °F. While real catalysts hardly surpass such conditions in a moving automobile, they may undergo those temperatures nonetheless sometime over their lifespans, particularly as the converters begin to age.

Scientists employed a transmission electron microscope to examine the microstructures of the three-way catalysts at the atomic scale and how they were influenced by the heat.

“By observing the microstructure, we can make the connection between high heat, the converter’s real performance, and its microstructure.”

Cheng-Han Li, Study Lead Author and Doctoral Student in Materials Science and Engineering, The Ohio State University

Li observed that rhodium catalysts are backed by oxides such as ceria-zirconia and alumina, which assist in stabilizing them.

When exposed to high heat along with oxygen, rhodium dissolves into the alumina and deforms into the stable solution of rhodium aluminate. This solution, nevertheless, is chemically inactive, meaning that it cannot scrub away unsafe gases and pollutants, rendering the device effectively impractical.

However, this is reversible.

When in the presence of hydrogen, some of the rhodium turns active again, but it is not practical enough to change the catalyst converter to its initial efficiency. 

The study’s discoveries determined that in the long run, creating a new design that stops the development of rhodium aluminate could help obtain the maximum out of these devices. This exhaustive understanding of the device’s structure could also help provide insight for improved designs for future catalytic converters.

Our results give car manufacturers a specific direction to follow to optimize the use of rhodium-based catalysts.

Cheng-Han Li, Study Lead Author and Doctoral Student in Materials Science and Engineering, The Ohio State University

The study’s co-authors include Jason Wu, Giovanni Cavataio, Andrew Bean Getsoian of the Ford Motor Company, and Joerg Jinschek, an associate professor of materials science and engineering at the Ohio State University.

This study received funding from the OSU-Ford Alliance Project.

Journal Reference:

Li, C-H., et al. (2022) Direct Observation of Rhodium Aluminate (RhAlOx) and Its Role in Deactivation and Regeneration of Rh/Al₂O₃ under Three-Way Catalyst Conditions. Chemistry of Materials. doi.org/10.1021/acs.chemmater.1c03513.