Visualizing reacting single atoms in chemical reactions: Advancing the frontiers of materials research
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Introduction Reliable in situ observations of single atoms in gas–solid catalyst reactions under controlled reaction conditions of gas environments and elevated temperatures is a key goal in understanding heterogeneous gas–solid catalytic reactions.1 Recent applications exploit the development of atomicresolution environmental (scanning) transmission electron microscopy (E(S)TEM) for understanding the role of gas– surface interactions in nanoscale catalysts in their functioning state. Temperature, time, and pressure-resolved studies on real systems have been realized, with the aim of bridging pressure and materials gaps at operating temperatures.2,3 The use of surface science techniques in an ultra-high vacuum, with extended single crystal surface model systems to understand fundamental aspects of heterogeneous catalysis, has been invaluable. However, direct industrial applications of these studies have been limited by the pressure and materials gaps that exist between them and industrial applications. In addition, practical heterogeneous catalyst surfaces are generally not extended as perfect single crystal faces but the much
more complex shapes of nanoparticles on supports.4,5 Early environmental TEM (ETEM) methods also did not provide atomic resolution.6
Atomic-resolution ETEM In developing the first atomic-resolution ETEM,2,3 we took a new approach to organize and design an instrument dedicated to environmental cell (ECELL or gas reactor system) operations with continuously flowing gas and elevated specimen temperatures. The ECELL facilities are integral to the electron microscope, which has been important in the development of the atomic-resolution ETEM for probing in situ gas–solid reactions directly at the atomic level in real time, under controlled gas atmosphere and temperature conditions, and for in situ nanosynthesis.3 The whole electron microscope column in a modern high-resolution (S)TEM has been modified for the ECELL functionality, rather than only the immediate region around the sample. Highlights of this development have included objective lens pole pieces incorporating radial holes for the critical first
Edward D. Boyes, University of York; [email protected] Pratibha L. Gai, University of York; [email protected] DOI: 10.1557/mrs.2015.141
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MRS BULLETIN • VOLUME 40 • JULY 2015 • www.mrs.org/bulletin
© 2015 Materials Research Society
VISUALIZING REACTING SINGLE ATOMS IN CHEMICAL REACTIONS: ADVANCING THE FRONTIERS OF MATERIALS RESEARCH
stage of differential pumping.3 The basic geometry is a fouraperture system with apertures innovatively mounted inside the bores of the objective lens pole pieces. The regular electron microscope sample chamber is used as the controlled reaction ECELL or reactor and thus, is integral to the core instrument.3 The environmental transmission electron microscope can be operated either with gas environments or as a conventional high-vacuum transmission electron microscope without compromising the atomic-resolution imaging. This system permits high gas pressures
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