The core design for the modern ETEM (1, 2), of which there are now some 20 replicates worldwide, has been modified to support full ESTEM functionality and to be compatible with a different basic instrument, in this case the JEOL 2200, with a series of differentially pumped column sections separated by fixed beamline apertures reconfigured for purpose in size and position.  Single atom resolved HAADF imaging and full analytical functionalities, including wide angle electron diffraction, CBDP, EDX and EELS, are enabled under controlled chemical reaction conditions of high temperatures in a continuously flowing gas atmosphere around the supported nanoparticle catalysts samples while retaining sub-Angstrom resolution and single atom sensitivity for atom-by-atom analysis of critical processes.  The novel AC ESTEM machine at York provides new insights into the processes of metal nanoparticle catalyst activation, operational state and deactivation (3) which have economic and other societal importance, including for scientifically informed environmental management.   At typical 2-20Pa gas pressures the gas supply covers the sample surface with multiple 1000s of monolayers of gas per second and generally fully adequate to continuously flood the surface with gas molecules.  They drive the chemistry under conditions defined in surface science as ‘high pressure’ (4) and minimise e-beam driven artefacts.  Catalysis is a massive industry with highly leveraged investments in processes and plants.  However, in many applications, including automotive emission controls (5), it is still relatively inefficient compared to what might be possible with a better informed knowledge base generating strategies to restrain deactivation processes.   The unique to York AC ESTEM supports quantitative atom-by-atom analysis of the underlying mechanisms of Ostwald Ripening (OR) through atom detachment, migration and re-attachment at lower temperatures around constant particle centres of gravity (6,).  This is compared with (and sometimes mixed with) higher temperature modes of particle migration and coalscence (PMC), with local nanostructure attributes in the round influencing local outcomes.  Neither process is found to be sensitive to gas pressure at 2-20Pa levels.   Both mechanisms have now been studied with new levels of precision using the novel capabilities at York; leading to a better informed understanding of the technical problems and options and helping to specify the enabling developments needed going forward.

References: 
1. E D Boyes and P L Gai, Ultramicroscopy, 67 (1997) 219
2. P L Gai et al, MRS Bulletin, 32 (2007) 1044
3. E D Boyes, M R Ward, L Lari and P L Gai, Ann Phys, 525 (2013) 423
4. G Somorjai et al, Phys Chem Chem Phys, 9 (2007) 3500
5. M R Ward et al Chem Cat Chem, 4 (2012) 1622
6. T E Martin et al Chem Cat Chem. 7 (2015) 3705

Acknowledgement
The EPSRC (UK) is supporting the AC ESTEM development and applications at York under strategic research grant EP/J018058/1 

Figures:

Atomic resolution HAADF STEM image of nascent crystalline nanoparticles, less ordered rafts and single atoms of Pt on a C support under representative reaction conditions of a controlled hydrogen gas environment at 3Pa pressure and specimen temperature of 250C in the novel AC ESTEM system built at York on a JEOL 2200FS base with aberration corrected STEM probe and TEM image.

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