Transmission electron microscopy

Nanoparticle

Chemistry

Metallurgy

Materials science

Chromatography

Composite number

Engineering

Chemical physics

Composite material

Physics

Scanning transmission electron microscopy

Nanotechnology

Organic chemistry

Optics

Noble metal

Analytical Chemistry (journal)

Metal

Inert gas

Carbon fibers

Dispersion (optics)

Acetylene

Chemical engineering

Nature Nanotechnology

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. Pd, Pt and Au nanoparticles are encapsulated in ZIF-8. The progressive atomic dispersion of these PGM atoms as single sites in the N-doped carbon derived from ZIF-8 pyrolysis is observed.

Direct observation of noble metal nanoparticles transforming to thermally stable single atoms