Improvement of CO Oxidation Performance by Controlling Pd State in DOC

Diesel oxidation catalysts (DOCs) combining the functions of Pd and Pt-Pd alloys have been used in practice to satisfy the strict exhaust emission regulations that have been specified for passenger cars in recent years. Pd is an indispensable component in DOCs because it exhibits superior oxidation activity for CO and HC. To reduce the amount of precious metal used and to improve robustness, it is important to control the electronic state and gas adsorption characteristics of Pd and PdO_x during catalytic reactions.In this study, by investigating the CO adsorption behavior of Pd, it was observed that Pd supported on a CeO₂/ZrO₂ mixed-oxide material (CZ) showed a preferable CO adsorption state and better CO light-off performance. Pd in Pd/CZ became metallic with increasing reaction time, and the CO oxidation performance of Pd/CZ decreased. This change in activity was correlated with CO adsorption on Pd changing from linear-type to bridge-type adsorption. In order to suppress this performance deterioration, a small amount of an additional electron promoter was placed near the Pd particles; this changed the CO adsorption state on Pd to a linear type that is preferable for CO oxidation activity. In addition, it is known that alloying Pd with Pt suppresses Pt sintering. The Pt -Pd alloying mechanism was studied using Extended X-ray Adsorption Fine Structure (EXAFS) spectroscopy. Furthermore, the correlation between the degree of alloying of Pt and Pd and the CO oxidation activity was investigated.