Oxygen-Driven Surface Evolution of Nanoporous Gold: Insights from Ab Initio Molecular Dynamics and Auger Electron Spectroscopy

摘要

Nanoporous gold (np-Au) has recently emerged as a highly selective environmentally friendly catalyst for low-temperature applications. Despite the seeming simplicity of this material, which consists of almost pure gold, its surface chemistry turns out to be more complex than anticipated. Interactions among gold, chemisorbed O atoms generated and consumed during catalysis, and trace amounts of Ag impurities present in np-Au lead to complex surface dynamics. In this work, theoretical modeling by means of ab initio molecular dynamics (AIMD) is combined with an Auger electron spectroscopic study to investigate oxygen-driven Ag surface diffusion on Au model surfaces exhibiting structural characteristics of np-Au. AIMD simulations reveal that surface O atoms dynamically form -(Au-O)- chain structures on the stepped Au(321) surface and lead to surface restructuring, but no chain formation is found on the flat Au(111). Ag impurities at low concentration lower the activation barrier for -(Au-O)- chain formation, whereas the formation of -O-Ag-O- links is energetically slightly unfavorable, especially at high Ag concentration. Furthermore, our study reveals the migration of subsurface Ag atoms onto the surface toward O-rich areas. Using the stepped Au(332) surface with Ag impurities under UHV conditions as a model system, we show that atomic oxygen is able to induce surface segregation of Ag at 200 K. Our results suggest that atomic surface oxygen should be one of the driving forces leading to the ligament coarsening in np-Au.