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 mechanisms remain unknown at the molecular scale. We elucidated the interaction of a series of aromatic compounds with octadecylthiolate-modified palladium nanocatalysts in all-atomic detail, explain large increases in product selectivity and yield, and propose a consistent reaction mechanism. Molecular dynamics simulations with cutting-edge potentials reveal surface interactions at the scale of entire nanoparticles, showing adsorption free energies on the order of -5 kcal/mol on the ligand-modified nanoparticles that are much smaller than -10 to -30 kcal/mol on bare metal surfaces for the same molecules. The ligands induce a two-step process of adsorption that involves exothermic condensation into the ligand shell, followed by exothermic binding to the metal surface, and enforces upright molecular orientations. The thermochemical attraction to the ligand shell scales with molecule size while interactions with the metal surface are less molecule-dependent. Total adsorption energies amount to 2/3 compared to bare metal surfaces and are compensated by large entropy losses of the molecules due to condensation in the ligand shell and increased confinement of the alkyl chains. Upright orientations of the aromatic alcohols lower the activation energy of the hydrodeoxygenation (HDO) reaction and suppress competing decarbonylation reactions. Electronic structure calculations uncover acidic and basic properties of hydrogenated Pd surfaces and suggest a mechanism that involves the transfer of an acidic Pd-bound hydrogen atom to the hydroxyl group as a rate-limiting step. The mechanism is consistent with a wide range of experimental data. The methods and results open up new pathways to design metal and alloy catalysts of specific shape, ligand coverage, and reaction conditions for biomass upgrading.

Understanding surface reactivity of ligand-protected metal nanoparticles for biomass upgrading

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.