Transition metal catalysts typically take the form of metal nanoparticles supported on inexpensive oxide or carbon materials but are often modeled as flat, periodic surfaces in density functional theory (DFT) studies. The curvature of nanoparticle surfaces introduces a diverse set of sites and interfaces to nanoparticle catalysts that are not present with flat models. DFT calculations for adsorption of H* on Pt, Ir, Pd, and Rh; CO* on Rh and Ru; NO* on Rh; and S* on Re, Pt, and Ru indicate that curved nanoparticles (38–800 atoms in size) can accommodate coverages &gt; 1 ML (A*/M<sub>surf</sub>) while surface models saturate at 1.0 ML for H* and near 2/3 ML for CO*, NO*, and S*. Higher (supramonolayer) coverages on particles arise because adsorbates cause metal-metal bonds to expand (weakening repulsion between coadsorbates) and particle curvature allows adsorbate adlayers to laterally relax; these effects allow undercoordinated sites on corners and edges to bind multiple adsorbates. For example, H* coverage on small Ir<sub>38</sub> nanoparticles can remain thermodynamically favorable at coverages above 3 ML. These effects are unique to nanoparticle models which are less constrained than periodic surface models. Dense, supramonolayer coverages can also alter catalytic behavior; for example, DFT-predicted rates more closely match experimental observations when using models that include the dense CO* adlayers that form during Fischer-Tropsch synthesis and H* adlayers that form during ethane hydrogenolysis. In addition to expanding M–M bonds, adsorbates can also cause surfaces to restructure, the effects of which can be more pronounced on nanoparticles by exposing more undercoordinated sites or higher-index facets. Finally, the support for these catalysts is not inert, even for C supports; for example, functionalizing or defunctionalizing the C support (via oxidation and thermal reduction) can alter charge transfer to Pd nanoparticles and, in turn, the selectivity of partial hydrogenation of cinnamaldehyde to hydrocinnamaldehyde. The complexities these nanoparticle models can be captured in state-of-the-art DFT studies and open new vistas for the rigorous study of heterogeneous catalysts and their properties.

Adsorption, restructuring, and support interactions on transition metal nanoparticle catalysts

ACS Spring 2022

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