Transmission electron microscopy (TEM) was first developed by Knoll and Ruska1. As the electrons interact with any residual gas present in the microscope column, the trend has been towards better vacuum. Lower pressure decreases the amount of random collisions of the electrons with gas molecules and thus decreases noise and improves resolution. However, this studying of static inorganic materials - close to room temperature and away from reacting gases - is a tremendous disadvantage especially for heterogeneous catalysis research. We know the structure of the catalyst changes with adsorbed species and thus also during the catalytic reaction. These changes cannot be studied with the ‘before and after, “frozen” sample’ approach of standard ex situ TEM.
For decades now, scientists have been devising ways to insert gases into TEMs without destroying the resolution of the microscope or the electron source. In order to study catalytic processes in operando, the samples also need to be heated. Heating holders were already available for standard vacuum mode TEM, but the heaters need to be integrated into the holders or devices used in the in operando mode. The first successes with gases present inside the TEM column during the imaging process led to the development of the now commercially available2,3 Environmental TEM (ETEM), where localised gas pressures up to 50 mbar do not destroy the electron source but gas pressures above ~5 mbar do destroy the imaging resolution.
Windowed cell systems resolve both problems - gas cannot reach the electron source and destroy it, and due to the short distance the electrons travel through the gas layer atomic resolution is achievable using a nanoreactor system even at pressures up to several bars. Due to the small heated area and the way the heater is configured, drift due to material expansion and contraction during heating and cooling is minimal.
The presentation will summarise the development of windowed cell systems and will discuss dynamic processes in catalyst preparation and in catalytic processes. The Kirkendall effect has been visualised with atomic resolution in the oxidation of Cu nanoparticles. The oscillations of Pt nanoparticles have been observed during the oxidation of CO with O24,5and during the water-gas-shift reaction.6