A study by a team from The Ohio State University and Ford is providing insight into the deactivation mechanism of rhodium in three-way catalysts (TWC) for automotive emissions control. The study, which could enable more efficient usage of rhodium in TWCs, is published in the ACS journal Chemistry of Materials.

Although there are various types, modern catalytic converters use a combination of three precious metals—palladium, platinum and rhodium—to reduce nitric oxide (NO) and nitrogen dioxide (NO₂) emissions. Rising prices for the three precious metals—especially rhodium—has spurred a sruge in the theft of catalytic converters from parked vehicles.

Rhodium is the rarest of all non-radioactive metals. South Africa is the major source, accounting for almost 60% of the world’s rhodium supply; Russia is the second-largest producer.

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

—Cheng-Han Li, lead and corresponding author

In 2021, the average price of Rhodium was US$20,254.10 per troy ounce, according to the US Geological Survey. Currently, pricing is around $19,000 per ounce (Umicore).

Rhodium-based catalysts are in short supply; it is therefore imperative that they be utilized as effectively as possible. Because the catalysts have been known to deactivate at high temperatures, the researchers investigated how their performance changes over time in the presence of high heat.

To do this, Li’s team performed several tests on the converters, including having them endure temperatures higher than 1600 degrees Fahrenheit. While real catalysts rarely exceed such conditions in a moving car, they may experience those temperatures at least occasionally over their lifetimes, especially as the converters get older.

The researchers used a transmission electron microscope to study the microstructures of the three-way catalysts at the atomic level and how they were affected by the heat.

Li noted that rhodium catalysts are supported by oxides like alumina and ceria-zirconia, which help stabilize them. At high heat with oxygen, rhodium dissolves into the alumina and degrades into the stable solution rhodium aluminate. This solution, however, is chemically inactive, meaning that it can’t scrub away harmful pollutants and gases, making the device effectively useless.

However, when exposed to hydrogen, some of the rhodium becomes active again, although not nearly enough to return the catalyst converter to its former efficiency.

Li et al.

The researchers concluded that in the long run, establishing a new design that prevents the formation of rhodium aluminate could help get the most out of these devices. This in-depth understanding of the device’s structure could also help inform better designs for future catalytic converters.

This study was funded by the OSU-Ford Alliance Project.

Resources

• Cheng-Han Li, Jason Wu, Andrew Bean Getsoian, Giovanni Cavataio, and Joerg R. Jinschek (2022) “Direct Observation of Rhodium Aluminate (RhAlO_x) and Its Role in Deactivation and Regeneration of Rh/Al₂O₃ under Three-Way Catalyst Conditions” Chemistry of Materials 34 (5), 2123-2132 doi: 10.1021/acs.chemmater.1c03513