Reaction-Driven Formation of Ag-Cu Alloy Nanostructures from Cu@Ag Core-Shell Nanoparticles Analyzed by Moirè Patterns Using Environmental TEM Images

Well defined Cu@Ag core@shell (or core-shell) nanoparticles have been synthesized through a carbon monoxide (CO)-assisted solution phase method. These Cu@Ag nanoparticles transformed into Ag-Cu alloy nanostructures under propylene epoxidation conditions. Environmental transmission electron microscopy (ETEM) revealed the structural evolution of Cu@Ag nanoparticle under different propylene-to-oxygen (PP/O₂) ratios based on the Moirè pattern analysis of the micrographs obtained in situ. The counter diffusion of Ag and Cu atoms, from the shell and core respectively, was observed at the PP/O₂ ratio of 2:1. When the PP/O₂ ratio was 1:1, Cu@Ag core-shell nanoparticles turned into alloy nanostructures. The transformation from Cu@Ag to Ag-Cu alloy nanoparticles changed the catalytic performance in the production of propylene oxide from partial oxidation of propylene, resulting in a yield of 1.14 min⁻¹ and selectivity of 79% when the PP/O₂ ratio was 2:1. This study shows besides temperature and pressure, other less-studied factors of reaction conditions such as composition of reactants may play critical roles in the dynamics of bimetallic nanostructures, thus their corresponding catalytic performances. This study develops the techniques on the applications of Moirè patterns for analyzing the dynamics of bimetallic core-shell nanostructures at atomic scale using (S)TEM micrographs.