Understanding the Surface Reactivity of Ligand-Protected Metal Nanoparticles for Biomass Upgrading

Ligand-protected metal nanoparticles are widely used in heterogeneous catalysis and biomass upgrading. Thiolate surfactants can greatly improve the overall yield; however, the dynamics of the reacting species and the reaction mechanism have remained unknown at the molecular scale. We elucidated the interaction of a series of aromatic compounds with octadecylthiolate-modified palladium nanocatalysts in atomic detail and explain large increases in product selectivity and yield through a detailed reaction mechanism. Molecular dynamics simulations reveal adsorption free energies on the order of −5 kcal/mol on the ligand-modified nanoparticles, which are significantly smaller than those on bare metal surfaces, where −10 to −30 kcal/mol are found. The ligands induce a two-step process of condensation in the ligand shell and adsorption, leading to upright molecular orientations, in contrast to single-step adsorption on bare metal surfaces. Exothermic condensation into the ligand shell and binding to the metal surface are accompanied by large entropy losses due to the reduced mobility in the ligand shell and increased confinement of the alkyl chains. Results from molecular dynamics simulations using the interface force field (IFF) show impressive agreement with available thermochemical reference data from experiments. Upright orientations of aromatic alcohol reactants lower the activation energy for the hydrodeoxygenation (HDO) reaction and suppress competing decarbonylation reactions. The analysis of the HDO reaction mechanism by QM/MM calculations in the presence of the ligands as well as by DFT calculations under vacuum uncovers the acidic and basic properties of hydrogenated Pd surfaces. The rate-limiting step involves the transfer of Pd-bound hydrogen atoms to hydroxyl groups in the alcohol reactants. The mechanism explains prior experimental data and supports the rational design of metal and alloy catalysts of specific shape, ligand coverage, and reaction conditions for biomass upgrading.