When More Is Less: Nonmonotonic Trends in Adsorption on Clusters in Alloy Surfaces

Single-atom alloys can be effective catalysts and have been compared to supported single-atom catalysts. To rationally design single-atom alloys and other surfaces with localized ensembles, it is crucial to understand variations in reactivity when varying the dopant and the ensemble size. Here, we examined hydrogen adsorption on surfaces embedded with localized clusters and discovered general trends. Counterintuitively, increasing the amount of a more reactive metal sometimes makes a surface site less reactive. This behavior is due to the hybridization and splitting of sharp peaks in the electronic density of states of many of these surfaces, making them analogous to free-standing nanoclusters. Further, single-atom alloys have qualitatively different behavior than larger ensembles. Specifically, the adsorption energy is U-shaped when plotted against the dopant’s group for single atom alloys. Additionally, adsorption energies on single atom alloys correlate more strongly with the dopant’s p-band center than the d-band center.