(190f) A First-Principles Study on O2 Activation on Pd Adclusters and Embedded Clusters in Pd-Au Bimetallic Catalysts

Recent advances in surface science have demonstrated the existence of undercoordinated clusters of adatoms (adclusters) on Ni and Cu nanoparticles with marked differences in activity compared to conventionally modeled extended facets. These findings may have particular ramifications for catalytic materials containing morphological heterogeneities, inducing Pd-Au bimetallics, which are among the most well-studied bimetallic systems and have demonstrated improved catalytic performance for a number of oxidative reactions.

Inspired by these results, we examine differences in O₂ activation on Pd-Au bimetallics between surface-segregated ad- and embedded clusters of Pd supported on Au(111) using density functional theory (DFT) calculations. In assessing activity differences between the two surface models, we studied the barriers for O₂ dissociation on and desorption from small Pd adclusters and embedded clusters ranging from 1-9 Pd atoms in size in a surface unit cell containing 16 metal atoms. While the barriers for dissociation are similar in both models, undercoordinated adclusters possess noticeably larger desorption barriers (exceeding 0.3 eV) than the embedded clusters. These results suggest that Pd-Au surfaces dominated by Pd adclusters are significantly more selective for O₂ dissociation as compared to desorption and may yield faster rates for reactions limited by the activation of O₂. Furthermore, we examined the surface stability of Pd-Au bimetallics by calculating the required energy to induce Pd adatom formation through the ejection of Pd atoms embedded in the Au substrate. Overall, our results show that surface roughness can be used to tune the reactivity of Pd-Au bimetallic catalysts towards O₂ activation, which may prove relevant for industrial oxidative applications.