Size-Dependent Hydrogenation Activity of Cobalt Nanoparticles

Cobalt nanoparticles are widely used as hydrogenation catalysts but the chemical nature of the particle size effect in hydrogenation reactions remains a matter of debate. Here, the hydrogenation activity of 3.5–40 nm large Co particles supported on C3N4 was probed employing acetone hydrogenation because two distinctive acetone conversion pathways exist: its direct hydrogenation to isopropanol and alternatively, its initial dehydration to mesityl oxide and subsequent hydrogenation to methyl isobutyl ketone. The acetone hydrogenation turnover frequency per Co surface atom increased with Co particle size up to 40 nm. In contrast, the acetone conversion rates were an order of magnitude smaller and declined with increasing particle size in the absence of H2. Moreover, only particles larger than ∼10 nm converted a 1:1 H2/acetone feed efficiently to isopropanol (>90%) between 150 and 250 °C. Acetone dehydration to mesityl oxide was increasingly favored on the smaller Co particles. Interestingly, the subsequent hydrogenation of mesityl oxide to methyl isobutyl ketone exhibited a Co particle size dependence analogous to isopropanol formation, while the secondary dehydration of isopropanol to propene was rather insensitive to the particle size. H–D exchange experiments demonstrated that H2 activation via dissociative adsorption was increasingly retarded with decreasing Co particle size. Conversely, acetone activation became more difficult with increasing particle size according to its in parallel diminishing dehydration rate in the absence of H2. Hence, the activation of H2 on the metal particles has to play a key role in the Co particle size dependence of acetone hydrogenation.