In Situ images reveal that nanoparticles can be transformed into more active atomic species at high temperatures

Original article by Huang Zhou, Yafei Zhao, Jie Xu, Haoran Sun, Zhijun Li, Wei Liu, Tongwei Yuan, Wei Liu, Xiaoqian Wang, Weng-Chon Cheong, Zhiyuan Wang, Xin Wang, Chao Zhao, Yancai Yao, Wenyu Wang, Fangyao Zhou, Min Chen, Benjin Jin, Rongbo Sun, Jing Liu, Xun Hong, Tao Yao, Shiqiang Wei, Jun Luo & Yuen Wu. Published in Nature Communications volume 11.

Supported metal catalysts have important applications in many industrial processes like the production of chemicals, pharmaceuticals and clean fuels, and the purification of vehicle emissions. At elevated temperatures the small catalyst particles tend to form bigger particles due to a process called sintering, which decreases their active surface areas and diminishes the catalytic activity. Replacing the deactivated metal nanoparticles is a costly process. Therefore researchers are looking for ways to improve the sustainability of these catalysts. In this study, scientists from multiple Chinese institutes, including DENSsolutions customer Tianjin University of Technology, further researched a method to recover or regenerate the activity of sintered and deactivated catalysts.

Figure 1a. Clusters of Pd nanoparticles, as seen in the upper left picture, are thermally diffused in N-doped carbon layers at 900 °C under Ar atmosphere – images taken from a video. Scale bar, 5 nm.

Figure 1b. Detailed view of Pd single atoms in the N-doped carbon layers after the in situ observation in Figure 1a.

Findings

In this in situ experiment the researchers discovered that supported palladium/gold/platinum nanoparticles distributed at the interface of oxide supports and nitrogen-doped carbon shells would undergo an unexpected nitrogen-doped carbon atomization process against the sintering at high temperatures, during which the nanoparticles can be transformed into more active atomic species.

In Situ TEM study

In order to study the thermal diffusion of the Pd nanoparticles within N-doped carbon layers, a sample environment with an inert gas like Argon needs to be created. The big advantage of the Climate system is that it can create this sample environment inside a normal TEM without the need of an ETEM. Furthermore, the high stability of the Climate Nano-Reactor allowed the researchers to record the N-doped carbon atomization process with sufficient detail in order to get valuable insights.

Figure 2. Representative in-situ TEM images of Pd NPs/TiO2@PDA-Pd NPs/TiO2@C. (a-h) Different temperatures, (i-l) different times at 900 °C. Scale bar, 20 nm.

“Thanks to the wonderful gas cell system from DENSsolutions, we can directly observe and record the sintering process of metal catalysts from 100 to 900 °C under 1 bar Ar by transmission electron microscopy (TEM).

We also succeeded in tracking the N-doped carbon atomization process and the evolution of metal nanoparticles into metal single atoms. During these processes, the response of this gas cell system was very fast and perfectly stable.”

Prof. Jun Luo
Professor at the Center for Electron Microscopy, Tianjin University of Technology.

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