AIF will host a seminar this Wednesday Nov. 16th from 11:00 am – 12:00 pm at MRC 136.
STEM Imaging of Catalysts to the Single-Atom Level, via Closed-Cell In Situ Gas Reaction Technology
Dr. Lawrence F. Allard
Materials Science & Technology Division
Oak Ridge National Laboratory, Oak Ridge, TN
In recent years, catalysis by single-atom species dispersed on supports has been shown to be an exciting and viable possibility, for catalytic reactions in many systems. The stability of single metal atoms on a support during elevated temperatures typically seen in “real” reaction processes is a critical issue, and is necessary to characterize appropriately in order to develop robust single-atom catalysts (SACs). Imaging single heavy atoms on lower Z supports via aberration-corrected high-angle annular dark-field (HAADF) imaging techniques in modern electron microscopy has become routine over the past decade, and extending this imaging capability into the realm of in situ gas-reaction technology is a natural goal for the catalytic scientist to better understand dynamical movement and the anchoring of noble metal atoms on specific support sites. This understanding will allow the ability to synthesize SACs with significant loadings of catalytic species while maintaining the full dispersion at the single-atom level.
Novel in situ gas-reaction technologies that utilize MEMS-based heater devices retained in a “closed-cell” reactor specimen holder for use in aberration-corrected (S)TEM instruments have shown the remarkable ability for imaging atomic columns in a crystal structure even at elevated temperatures and at pressures up to a full atmosphere. The possibility to extend this imaging capability to SACs has been a goal of our work, and will be demonstrated in this talk. The effects of the electron beam on the sample, and the effects of electron scattering processes within the gas and window materials that comprise the gas-cell are problematical questions that are being addressed. An “ideal” SAC catalyst comprising Pt atoms on NiO nanocrystals has been used as the model sample for HAADF images to demonstrate cases from material on a standard TEM grid to material in the full geometry of the closed gas-cell reactor.
Dr. Larry Allard obtained all three of his degrees at the University of Michigan in the Materials Science and Engineering Dept. He started his electron microscopy career in 1963 as a sophomore, learning theory and practice under Prof. Wilbur Bigelow. He is currently a Distinguished Research Staff Member in DOE’s High Temperature Materials Laboratory, a National User Facility located at Oak Ridge National Laboratory. His research involves ultra-high-resolution imaging and microanalysis studies of nanophase and nanostructured materials, automotive catalytic materials for exhaust after-treatment, and instrumental developments involving in situ electron microscopy, electron holography, digital imaging and remote instrument operation. He is the chief scientist in charge of the Aberration-Corrected Electron Microscope (ACEM) project at the HTML; his JEOL 2200FS instrument one of the first of the new generation of STEM/TEM instruments with sub-Ångström resolution to be installed in the US (2004). He is also the principal technical designer of ORNL’s new Advanced Microscopy Laboratory, a facility housing the most advanced and sensitive modern electron beam instruments. Dr. Allard is the author or co-author of more than 300 scientific publications, a co-organizer of more than a dozen workshops and symposia on advanced microscopy topics, and has co-edited several conference proceedings and books, including “Introduction to Electron Holography,” the first definitive textbook on electron holography, published by Kluwer/Plenum in 1999. Dr. Allard was elected Fellow of the Microscopy Society of America in 2010, in the second Fellows class of the society. He is still collaborating closely with Prof. Bigelow, who at age 93, continues to contribute to the design and fabrication of a myriad of devices and systems that enable the development of the unique capabilities provided by the microscopy groups at ORNL.