Characterization and Kinetic Studies of Palladium and Rhodium Catalysts in the Three-Way Catalyst System

The three-way catalyst (TWC) is a critical component in gasoline fueled vehicles’ aftertreat-ment systems, able to simultaneously convert NO, CO and unburnt hydrocarbons into less harm-ful products, when the engine works under stoichiometric conditions. The typical TWC consists of Pt-group metals (Rh, Pt, Pd) dispersed onto alumina, along with other additives aimed at im-proving both catalyst activity and stability. Rh is used because of its ability to reduce NOx, while Pt and Pd are used because of their high activity towards oxidation reactions.
Because of increasingly stringent regulations imposed by the Environmental Protection Agency (EPA) the TWC system continues to evolve to be more efficient and have higher stability. In addition, the price volatility that characterizes the Pt-group metals has challenged automotive companies and catalyst suppliers to seek new solutions to reduce the cost associated with TWC.
To design new catalyst formulations that are efficient, durable and less costly, a detailed under-standing of the reaction mechanism during the TWC process is necessary. Here, we focus on the study of two model catalysts: Rh/Al2O3 and Pd/CeO2.
Rh plays a crucial role in the TWC, even though it is present in low amounts due to its activity, but also high price. Rh catalysts are also characterized by high Rh mobility. The presence of reactants, such as CO, O2, NO and H2O, induces structural changes that influence the catalyst activity and selectivity. Using a combination of kinetics and spectroscopic studies we investi-gated Rh single atoms-nanoparticles interconversion during CO oxidation. We observed that Rh catalysts undergo structural changes even at higher temperature under CO/O2 oxidizing mix-tures, affecting the Rh nanoparticle fraction, which are the active sites for CO oxidation. These structural changes exhibit size dependency, with smaller nanoparticles being more prone to dis-perse into single atoms. In addition, the presence of water, a major component in the exhaust gas, did not have any effect on nanoparticle disintegration. On the contrary, during the NO re-duction by CO reaction, we observed that water led to changes in reaction kinetics and product selectivities when water was present in the feed composition. Preliminary studies suggested that ammonia might induce changes in the fraction of Rh nanoparticle and single atoms present in the catalyst.
The final study of this work focuses on Pd/CeO2 catalysts. Characterizing these catalysts via conventional techniques presents challenges. For example, when using CO chemisorption, the reaction between CO and ceria lattice oxygen results in carbonate formation, leading to an overestimation of the CO uptake. To address this issue, we proposed a modified CO chemisorp-tion method, which involves CO2 exposure prior to CO adsorption, with the goal of hindering further carbonate formation during CO chemisorption. This technique was validated through CO oxidation kinetics and DRIFTS studies. This study aims to facilitate the quantification of exposed metal sites on OSC supports, which is crucial for calculating turnover frequency and comparing different catalyst formulations.