Catalytic Finite-Size-Effects

Charge density difference plots when oxygen is adsorbed seen from above (upper panel) and the side (lower panel). The white and magenta color scale indicates areas where electrons are accumulated and depleted, respectively. The plotted contours are chosen to be (0.001e/A3).

Today, metal nanoparticles have become popular catalysts for a wide range of industrial processes. Since only the surface is exposed to reactants, it is clear why their high surface-to-volume ratio makes nanoparticles attractive catalysts. In addition, as size decreases, the catalytic and material properties of metal particles were observed to deviate from the bulk behavior. This difference is due to a combination of factors such as distribution of surface sites, changes in local coordination, and quantum size effects. Through large-scale density functional theory (DFT) calculations, scientists at SLAC and Stanford University, in collaboration with researchers from the Technical University of Denmark and Argonne National Lab, were able to deconvolute the effects mentioned and isolate the intrinsic electronic finite-size effects on gold and platinum clusters.

Surface catalytic properties can be measured as binding energies of key adsorbates that serve as descriptors for a wide range of reactions. In this recent study, CO molecule and atomic oxygen were used to probe the close-packed (111) and edge catalytic sites on metal clusters containing up to 1415 atoms. Calculation at this size was made possible through using a highly parallelizable DFT code, GPAW, as well as, access to the leadership computing resources, containing up to 40,960 nodes, at Argonne National Laboratory. In the study, electronic finite size effects on platinum clusters appeared to vanish beyond 147 atoms, where the adsorption energies of CO and O match that on the extended surface. However, in the case of gold clusters, this agreement with surface slabs was reached only after the clusters were above 561 atoms1. In both cases, the convergence of the adsorption energy was observed to coincide with the convergence of the charge density response to the adsorption.

The difference between the convergence behavior between gold and platinum is due to differences in their electronic structures. When looking at their electronic structures, small metal clusters have discrete, molecular-like, density of states, instead of a continuous band. Additionally, for metals with fully filled d-electrons, in which d-states do not contribute to the density of states near the Fermi-level, s-shell effect also plays a role in the catalytic properties of small clusters. Due to fractional d-band filling, there are high densities of states around the Fermi level in platinum systems. In contrast, due to s-state filling, gold clusters can develop to be more alkali-metal-like or halogen-like characteristics, depending on whether the filling is above or below a shell closing2. For this reason, the electronic structure of platinum converges to that of the bulk more quickly, and oscillation in the adsorption energies on gold was not observed on platinum.