Imaging at the Single Atom Level in Closed Cell In Situ Gas Reactions
Single atom catalysis is emerging as a new frontier in the field of heterogeneous catalysis [1 4] Singleatom catalysts (SACs) have recently been successfully synthesized and proved to be effective for many important catalytic reactions with high atom efficiency [1 4] A key issue facing the practical applications of SACs is, however, the stability of the metal single atoms during catalytic reactions, especially at elevated temperatures. To develop robust SAC synthesis protocols to stabilize noble metal single atoms on appropriate metal oxide supports requires a fundamental understanding of the dynamics of the supported single atoms under gas environments and at elevated temperatures. Understanding the dynamical movement and the anchoring of noble metal atoms on support materials becomes critical to developing novel strategies to synthesize SACs with high levels of metal loading. Recently developed technologies for in situ reaction studies utilizing a closed cell MEMS based reactor specimen holder [5] in an aberration corrected (S)TEM instrument have shown the capability to image atomic columns on metal particles at pressures up to an atmosphere and temperatures up to 1000degC. However, imaging of single heavy metal atoms on oxide supports and tracking their movements during reaction experiments remains problematical, due to the scattering phenomena relative to the elements of the gas cell geometry, and electron beam effects. In the present study, we demonstrate the capabilities and efficacy for observations of single atom reactions in a closed cell system, using a specimen comprising Pt atoms on NiO nanocrystals. High angle annular dark field (HAADF) mages are shown for 4 cases: (a) the ideal geometry of catalyst support particles suspended over a hole in a holey carbon support film; (b) support particles on the carbon film; (c) support particles on the thin (30nm) SiN membrane of a MEMS heater device with the beam first traversing the SiN (see caption for details) and (d) support particles imaged in the full gas cell geometry at elevated temperature and pressure (see caption)
Microscopy and Microanalysis