Abstract：
Carbon-supported metal nanoparticles (NPs) comprise an important class of heterogeneous catalysts. The interaction between the metal and carbon support influences the overall material properties, viz., the catalytic performance. Herein we use in situ and ex situ transmission electron microscopy (TEM) in combination with in situ X-ray spectroscopy (XPS) to investigate the encapsulation of metallic iridium NPs by carbon in an Ir/C catalyst. Real-time atomic-scale imaging visualizes particle reshaping and increased graphitization of the carbon support upon heating of Ir/C in vacuum. According to in situ TEM results, carbon overcoating grows over Ir NPs during the heating process, starting from ca. 550 °C. With the carbon overlayers formed, no sintering and migration of Ir NPs is observed at 800 °C, yet the initial Ir NPs sinter at or below 550 °C, i.e., at a temperature associated with an incomplete particle encapsulation. The carbon overlayer corrugates when the temperature is decreased from 800 to 200 °C and this process is associated with the particle surface reconstruction and is reversible, such that the corrugated carbon overlayer can be smoothed out by increasing the temperature back to 800 °C. The catalytic performance (activity and stability) of the encapsulated Ir NPs in the hydrogen evolution reaction (HER) is higher than that of the initial (nonencapsulated) state of Ir/C. Overall, this work highlights microscopic details of the currently understudied phenomenon of the carbon encapsulation of supported noble metal NPs and demonstrates additionally that the encapsulation by carbon is an effective measure for tuning the catalytic performance.
摘要：
碳负载的金属纳米颗粒(NP)包含一类重要的非均相催化剂。金属和碳载体之间的相互作用会影响整体材料性能，viz.,催化性能。在这里，我们使用原位和非原位透射电子显微镜（TEM）结合原位X射线光谱（XPS）来研究碳在Ir/C催化剂中对金属铱NP的封装。实时原子级成像可在真空中加热Ir/C后可视化颗粒重塑和碳载体的石墨化增加。根据原位TEM结果，在加热过程中，碳覆盖层在IrNP上生长，从CA开始。550°C。随着碳覆盖层的形成，在800°C下没有观察到IrNP的烧结和迁移，然而，初始IrNPs在550°C或以下烧结，即，在与不完全的颗粒封装相关的温度下。当温度从800°C降低到200°C时，碳覆盖层会起皱，并且该过程与颗粒表面重建有关并且是可逆的，这样可以通过将温度升高回到800°C来平滑波纹碳覆盖层。包封的IrNP在析氢反应（HER）中的催化性能（活性和稳定性）高于Ir/C的初始（非包封）状态。总的来说,这项工作强调了目前研究不足的负载贵金属NP的碳封装现象的微观细节，并证明了碳封装是调节催化性能的有效措施。
