Influence of Pd Concentration in Au–Pd Nanoparticles for the Hydrogenation of Alkynes

Supported Au–Pd nanoparticles with low Pd content (between 2% and 25%) are excellent catalysts for the selective hydrogenation of alkynes into alkenes, a crucial step for elimination of alkynes from the reactants for olefin polymerization. They have better selectivity at high conversions for the hydrogenation of 1-hexyne to 1-hexene compared to that of pure Pd. However, the role of Pd concentration in Au–Pd particles in maximizing the activity per gram of Pd remains elusive. This work combines scanning transmission electron microscopy (STEM) and density functional theory (DFT) to determine how Pd concentration in the dilute limit affects the activity and selectivity of Au–Pd particles for the hydrogenation of acetylene. Atomic resolution microscopy shows increased Pd segregation to the surface with increasing Pd concentration (above 9%), and DFT analysis shows that isolated Pd atoms on the Au–Pd surface are highly active for acetylene hydrogenation compared to Pd atoms adjacent to other Pd atoms. A high concentration of isolated and active Pd atoms combined with increased segregation of Pd toward the surface explains the existence of an optimum for catalytic properties. It explains the high performance of Au0.96Pd0.04 compared to Au–Pd particles with lower or higher Pd concentrations.