Among the platinum group metals (PGMs), rhodium (Rh) is known as an exceedingly valuable element for automotive catalysts due to its powerful catalytic function. Because Rh is a costly material, it is paramount to enhance its catalytic function in three-way catalysts (TWCs). This work reports results on the palladium (Pd)-Rh combination which assists the catalytic function of Rh. XPS and XRD are used to observe the Rh characteristics, and engine dynamometer and vehicle testing are conducted to measure catalytic performance and quantify the emission benefits of the Pd-Rh interaction in TWCs.
It is well known that Pd-Rh forms a core-shell structured alloy with Rh in its core. This alloy exerts a large negative impact on NOx performance. However, it is inferred from our analyses that highly-dispersed Pd and Rh particles within a certain Pd/Rh atomic ratio prevent this deterioration phenomenon. In this work XPS analysis shows adding Pd increases the Rh0 concentration on the Rh surface when Pd is allocated in proximity to Rh, and the concentration of Rh0 created through the Pd-Rh interaction reaches a maximum at a certain Pd/Rh atomic ratio. In addition, XRD examination shows the concurrent presence of small particles of Rh metal as well as Pd metal and a small amount of Pd-Rh alloy.
Our findings suggest that a strategically-designed Pd-Rh interaction in a TWC could generate a positive impact on its purification performance. In other words, optimizing Pd-Rh interaction could open a new way of reducing Rh usage in TWCs.