Abstract
Single noble metal atoms and ultrafine metal clusters catalysts tend to sinter into aggregated particles at elevated temperatures, driven by the decrease of metal surface free energy. Herein, we report an unexpected phenomenon that noble metal nanoparticles (Pd, Pt, Au-NPs) can be transformed to thermally stable single atoms (Pd, Pt, Au-SAs) above 900 °C in an inert atmosphere. The atomic dispersion of metal single atoms was confirmed by aberration-corrected scanning transmission electron microscopy and X-ray absorption fine structures. The dynamic process was recorded by in situ environmental transmission electron microscopy, which showed competing sintering and atomization processes during NP-to-SA conversion. Further, density functional theory calculations revealed that high-temperature NP-to-SA conversion was driven by the formation of the more thermodynamically stable Pd-N4 structure when mobile Pd atoms were captured on the defects of nitrogen-doped carbon. The thermally stable single atoms (Pd-SAs) exhibited even better activity and selectivity than nanoparticles (Pd-NPs) for semi-hydrogenation of acetylene.
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Acknowledgements
This work was supported by China Ministry of Science and Technology under contract 2016YFA (0202801), the National Natural Science Foundation of China (21521091, 21390393, U1463202, 21590792, 91645203), 111 Project (B16028) and the China Postdoctoral Science Foundation (2017M620736). The calculations were performed by using supercomputers at Tsinghua National Laboratory for Information Science and Technology. We thank H. B. Pan, X. S. Zheng and J. F. Zhu from NSRL in Hefei, China for their cooperation on XPS measurements.
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Y.L. and Z.L. conceived the idea and co-wrote the paper. S.W. and Z.L. performed most of the reactions, collected and analysed the data. A.L. and X.H. performed the in situ ETEM characterizations. Y.G., Q.Z. and L.G. performed the aberration-corrected scanning transmission electron microscopy characterizations. J.-C.L., H.X. and J.L. proposed the structural model for the active sites and finished the DFT calculations. W.C., Y.W. and L.Z. carried out the XAFS characterization. The other authors performed some of the experiments, discussed the results and commented on the manuscript.
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Supplementary Figures 1–21, Supplementary Tables 1–2 and Supplementary References
Supplementary Video 1
A movie of TEM images showing Pd-NPs transforming to Pd-SAs taken in the temperature window 100 °C to 1,000 °C under an Ar atmosphere.
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Wei, S., Li, A., Liu, JC. et al. Direct observation of noble metal nanoparticles transforming to thermally stable single atoms. Nature Nanotech 13, 856–861 (2018). https://doi.org/10.1038/s41565-018-0197-9
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DOI: https://doi.org/10.1038/s41565-018-0197-9
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