Structure-Dependent CO and NO Oxidation over Bimetallic PtPd/Al$_{2}$O$_{3}$ Catalysts: Impact of Pt–Pd Mixing and Alloying on Activity and Stability

The influence of the noble metal nanoparticle structure on the activity and stability of bimetallic PtPd/Al$_2$O$_3$ catalysts was systematically investigated. Various conventional and advanced synthesis methods were used to tune the initial Pt–Pd mixing/alloying extent. By using complementary characterization techniques, pronounced differences in the CO and NO oxidation activity could be correlated with variations in the noble metal oxidation state, particle size, and composition. The beneficial effect of Pt–Pd mixing/alloying in catalysts predominantly containing noble metal nanoparticles with a surface-averaged size smaller than ∼5 nm was revealed for the CO, NO, and combined CO/NO oxidation reactions. In this regard, colloidal methods enabled the formation of homogeneously alloyed PtPd particles with a narrow size distribution and superior catalytic activity at low temperatures. However, the presence of uniform PtPd bimetallic particles did not prevent catalyst deactivation, as pronounced noble metal sintering and segregation phenomena still occurred during hydrothermal aging. In contrast, for applications operating at temperatures exceeding 600 °C, catalysts with a less uniform initial structure exhibited improved stability.