Rhodium (Rh) is one of the key components used in commercial three-way catalysts (TWCs) to remove NOx, CO and unburnt hydrocarbons from engine exhaust, when the engine operates under stoichiometric fuel/air combustion conditions. With the recent increase in Rh price, a detailed understanding of the reaction mechanisms is necessary to optimize Rh use in the TWC catalyst design.  We first studied the kinetics of CO oxidation under rich, stoichiometric and lean conditions on an aged Rh supported on γ-alumina catalyst. The calculated apparent activation energy corresponds to the heat of CO desorption from the metal particle, as previously reported. However, higher temperatures showed a different activation energy under lean conditions. High temperature kinetics combined with spectroscopic studies suggests that structural changes might occur during CO oxidation under lean conditions. At low temperature, CO is able to disperse Rh particles into single atoms through an oxidative process, while at high temperature, CO is able to reaggregate Rh single atoms into nanoparticles. The latter process leads to an increase in the number of active sites for CO oxidation, which is reflected in a higher pre-exponential factor at high temperature. Finally, it was found that the temperature of the change in the kinetic behavior has a particle size dependence. Lean CO oxidation kinetics studied on catalysts characterized by different dispersions showed that the temperature at which the change occurs decreases with the increase in particle size.

Rh catalyst structural changes during CO oxidation

ACS Spring 2022

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