Origin and prevention of broad particle size distributions in carbon-supported palladium catalysts prepared by liquid-phase reduction

Carbon-supported palladium (Pd/C) catalysts often display broad or multimodal Pd particle size distributions detrimental for their performance. Therefore, the formation of large particles during preparation should be better understood and avoided. We prepared catalysts with up to 10 wt% Pd supported on oxygen-functionalized carbon nanotubes and activated carbon via liquid-phase reduction. It was inferred that small Pd particles (∼1 nm) were formed from Pd ions adsorbed on the carbon. At higher loadings, up to 65% of the Pd ended up in larger particles (>10 nm) probably formed from Pd ions in solution, lowering the Pd-normalized activity in the hydrogenation of cinnamaldehyde 4-fold compared to samples with small particles only. Three key factors were identified for preparing Pd/C catalysts with exclusively ∼1 nm Pd particles: a support with a high density of acid sites and high specific surface area, and a metal loading at molar ratio Pd/acid sites <0.5.