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.

核壳纳米粒子的高催化性能通常归因于其独特的几何和电子结构。在这里，我们揭示了一种动态机制，通过直接环境透射电子显微镜可视化以及多种同步原位技术（包括同步加速器X射线吸收光谱法，红外光谱法和理论模拟），推翻了这种传统理解。Ni-Au催化系统，在CO _2中表现出高度选择性的CO生成氢化反应前后，在镍核上均具有完整的超薄金壳。然而，催化性能不能归因于Au壳表面，而是归因于在反应期间通过CO吸附促进的瞬时重构的合金表面的形成。这种可逆转化的发现促使我们重新考虑稳态模型以外的反应机理，不仅对核壳纳米粒子，而且对其他定义明确的纳米催化剂也可能具有重要意义。