Regenerative trapping: How Pd improves the durability of Pt diesel oxidation catalysts

Abstract

Pt is an active component in diesel oxidation catalysts (DOCs) but it ripens to form large particles at elevated temperatures due to significant vapor phase transport of PtO2. Adding Pd improves the durability of the catalyst but there is no clear consensus in the literature for the responsible mechanism. In this study, we examined the role of PtO2 vapor pressure on sintering. The vapor pressure of PtO2 was determined by measuring the emission of Pt using planar model catalysts and electron probe microanalysis (EPMA). Significant emission of Pt to the vapor phase was observed in a Pt/SiO2 model catalyst that was aged in flowing air at 800°C. Adding Pd lowered the rate of emission, and so did decreasing the oxygen concentration. Lowering of the vapor pressure of PtO2 led to a decreased rate of sintering. However, the most significant decrease in sintering rate was observed in the presence of excess PdO. The excess PdO serves to trap mobile Pt species, resulting in significantly smaller Pt-Pd particles after aging in air at 800°C for 10h. When PtO2 is emitted from a metallic Pt-Pd particle it can result in the formation of a separate PdO phase. Hence, in these catalysts the PdO is regenerated in the course of catalyst sintering. We propose that regenerative trapping of mobile Pt species by PdO plays an important role in improving the durability of Pt-Pd bimetallic catalysts.