Catalytic Activity of Water‐Dispersed Palladium Nanoparticles with Different Sizes and Shapes in the Semihydrogenation of Alkynes

Palladium nanoparticles of different sizes and shapes dispersed in water, stabilized by either polyvinylpyrrolidone or glutathione, were studied in both monophasic and biphasic alkyne hydrogenation reactions at 50 bar hydrogen pressure. Spherical nanoparticles of 2 and 5 nm diameter as well as cubic nanoparticles of 18 and 28 nm edge length were investigated. Structural changes in the particles before and after the catalytic reactions were assessed with respect to agglomeration and particle size. The particle size had the strongest effect on the catalytic efficiency, i.e., spherical nanoparticles of 5 nm were the most active and spherical nanoparticles of 2 nm were the least active. The nanocubes had intermediate catalytic activities. Semihydrogenation occurred within minutes at low palladium loadings. The highest turnover frequencies of about 183,000 h −1 were achieved with 5 nm palladium nanoparticles. Mass transport limitations constrained the reaction rates in aqueous biphasic systems with larger nanoparticles.