(6d) Unraveling the Remarkable Stability of Pt-Pd Nanoparticles during Diesel Oxidation Catalysis

It is well known that adding Pd enhances the thermal stability of Pt-based diesel oxidation catalysts. Pt-only catalysts suffer from severe sintering when heated in air at 800 C forming abnormally large particles (100s of nanometers). Pd-only catalysts are stable since they transform into PdO, which has a very low vapor pressure and hence the catalyst can retain nanosized particles after similar high temperature aging treatments (50 h at 800 C in air). The improved thermal stability of Pt-Pd catalysts is not well understood. Various models have been proposed, including the formation of core shell structures as well as regenerative trapping where PdO traps mobile Pt oxides. In this work, we have investigated the emission of Pt and Pd oxides to the vapor phase as a key indicator of the sintering rate for these catalysts. We found that the rates of emission of Pt oxides to the vapor phase are significantly suppressed when Pd is present. TEM examination shows that both Pt and Pd are present in the form of metallic and oxide phases that co-exist in a single nanoparticle. The internal structures of these nanoparticles provide clues to the stabilization of the Pt-Pd catalysts and their ability to catalyze diesel oxidation more effectively. It is remarkable that metallic Pd can be stabilized under oxidizing ambients, which we feel is a significant contributor to the catalytic performance of these catalysts. We will discuss the unique epitaxial metal-oxide interfaces in these catalysts. These findings are relevant to oxidation of a broad class of bimetallic nanoparticles where Pt is alloyed with a more oxophilic component (for example, Pt-Re, or Pt-Sn) helping to explain the catalytic performance of such bimetallic catalysts.