(693c) Elucidating the Nanoscale Driving Forces for Environment-Driven Rh-Pd Nanoparticle Reconstruction

Bimetallic catalysts are known to frequently exhibit high, synergistic performance as compared to their metal constituents, which is directly related to the composition profiles in the first few atomic layers with the nanoparticles. Notably, when exposed to various reaction environments, such bimetallic nanoparticles can undergo adsorbate-induced surface segregation and reconstruction. Although such phenomena have been extensively studied by both experimental and computational approaches, there is still a lack of nanoscale insights into the adsorbate-surface and adsorbate-adsorbate interactions that drive reconstruction. Without such insights, bimetallic catalyst design that accounts for the in situ surface response to reaction environment is greatly hindered. Here, we address this challenge using a case study of Rh₅₀Pd₅₀ nanoparticle reconstruction under cyclical oxidizing and reducing conditions. Using density functional theory (DFT), we mapped the coverage dependent O* adsorption energies and overall surface formation energies over three facets ((111), (100), (110)) and surface layer compositions (Pd enriched, mixed, Rh enriched). A microkinetic model that predicts the O* coverage and near surface structure over an entire nanoparticle surface was then constructed that incorporated the DFT energies by assuming that the nanoparticle is a paraboloid and using kubic harmonics interpolation. As shown in Figure 1, O* initially populates the (111) facet. Further increasing the gas phase O₂ pressure results in O* population of the (100) and (110) facets. This change in O* coverage directly corresponds to reconstruction of the nanoparticle surface, where the nanoparticle initially has a Pd enriched skin that undergoes inversion to a Rh enriched skin. Overall, our work identifies the environment and nanoscale interactions that lead to Rh₅₀Pd₅₀ nanoparticle reconstruction, as well as establishes an approach for predicting such adsorbate induced reconstructions a priori. This work enables the potential of tunable nanoparticle engineering induced by reactive gases as a novel catalyst design strategy.