Surface Ligands on Pd Nanocrystals Dominate Reaction Kinetics of Catalytic Cinnamaldehyde Hydrogenation Reaction

Capping ligands are widely used in the solution-phase colloidal synthesis of catalytic nanocrystals with uniform sizes and shapes, and their influences on the catalytic performance of as-synthesized catalytic nanocrystals (NCs) are an interesting topic. Herein, via a systematic reaction kinetic study of the cinnamaldehyde (CAL) hydrogenation reaction catalyzed by Pd NCs with well-defined cubic and octahedral shapes and sizes ranging from 9 to 34 nm prepared with polyvinylpyrrolidone (PVP) as the capping ligand, we unveil the dominant role of PVP-Pd interaction in determining the reaction kinetics. The shape-dependent relative reaction rates of competing reaction pathways lead to a higher hydrocinnamaldehyde selectivity over octahedral Pd NCs than cubic Pd NCs. The size-dependent coverage and adsorption configurations of PVP on Pd NCs dominate the size-dependent catalytic activity and reaction kinetics. The densely adsorbed PVP molecules on fine Pd NCs result in strong competitive adsorption of CAL and H₂, leading to low catalytic activity and negative reaction orders of CAL, while the loosely adsorbed PVP molecules on large Pd NCs allow effective coadsorption of CAL and H₂, leading to high catalytic activity and positive reaction orders of CAL.