Reversible loss of core–shell structure for Ni–Au bimetallic nanoparticles during CO2 hydrogenation

The high catalytic performance of core–shell nanoparticles is usually attributed to their distinct geometric and electronic structures. Here we reveal a dynamic mechanism that overturns this conventional understanding by a direct environmental transmission electron microscopy visualization coupled with multiple state-of-the-art in situ techniques, which include synchrotron X-ray absorption spectroscopy, infrared spectroscopy and theoretical simulations. A Ni–Au catalytic system, which exhibits a highly selective CO production in CO2 hydrogenation, features an intact ultrathin Au shell over the Ni core before and after the reaction. However, the catalytic performance could not be attributed to the Au shell surface, but rather to the formation of a transient reconstructed alloy surface, promoted by CO adsorption during the reaction. The discovery of such a reversible transformation urges us to reconsider the reaction mechanism beyond the stationary model, and may have important implications not only for core–shell nanoparticles, but also for other well-defined nanocatalysts. The structure of core–shell catalysts is often assumed to be conserved over a reaction. Now, an in situ study reveals that the shell of Ni@Au nanoparticles is reversibly converted into a Ni–Au alloy during CO2 hydrogenation, with important mechanistic implications.