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Epitaxy of the Metal and Oxide Phases in Pt-Pd ‘Janus’ Particles in 800 °C Air-aged Diesel Oxidation Catalysts

There is a critical need for exhaust gas treatment catalysts that provide high reactivity at low temperatures, which are encountered in modern, efficient internal combustion engines. However, these emission control catalysts must also survive harsh environments encountered in operations at high loads, such as towing a trailer up a mountain pass. Modern diesel oxidation catalysts contain platinum (Pt) and palladium (Pd) to catalyze oxidation reactions of CO, NO and hydrocarbons. Pd is known to improve the performance and durability of the Pt-based catalysts. The nature of Pd-Pt bimetallic particles in catalysts aged at high temperatures is not well understood. When heated in air, Pt forms large metallic particles due to vapor phase transport of volatile PtO 2 [1]. On the other hand, Pd transforms into the tetragonal PdO phase which has very low vapor pressure and hence can stay dispersed [5]. Previous work has suggested that core-shell structures may form where an oxide covers the metallic core [4], however such a structure is not consistent with the catalytic performance, since these catalysts perform better than Pt-only or Pd-only catalysts. Understanding the origins of the synergy between Pt and Pd was the goal of this work. Our approach involves a novel catalyst support where ZrO 2 is deposited as a shell layer on nonporous silica spheres. This catalyst support provides exceptional thermal stability as shown