Structural changes of Rh/Al2O3 during CO oxidation

Silvia  Marino, University of Virginia

Rhodium (Rh) is one of the active components used in commercial three-way catalysts (TWCs) to remove NOx, CO and unburnt hydrocarbons, when the engine operates under stoichiometric conditions. Due to the high price of Rh, a detailed understanding of the reaction mechanisms is necessary to optimize rhodium use in these catalyst designs.

An aged Rh/γ-Al2O3 catalyst was studied, with the intent to calculate kinetic parameters. CO oxidation in the presence of water was investigated under lean, stoichiometric and rich conditions. The apparent activation energies calculated for CO oxidation corresponds to the heat of adsorption of CO on the metal. However, at higher temperatures different activation energies were observed under lean conditions, suggesting structural changes.  As shown by spectroscopic studies, Rh particles can be re-dispersed into single atoms in the presence of CO at low temperature. However, at high temperature CO is able to aggregate Rh single atoms into particles, which have been proposed to be the active sites for CO oxidation. Rh single atoms aggregation might be the process causing different activation energy values calculated at higher temperature during lean CO oxidation and leading to a higher active sites density.