Observing gas-catalyst dynamics at atomic resolution and single-atom sensitivity

Abstract

Transmission electron microscopy (TEM) has become an indispensable technique for studying heterogeneous catalysts. In particular, advancements of aberration-corrected electron optics and data acquisition schemes have made TEM capable of delivering images of catalysts with sub-Ångström resolution and single-atom sensitivity. Parallel developments of differentially pumped electron microscopes and of gas cells enable in situ observations of catalysts during the exposure to reactive gas environments at pressures of up to atmospheric levels and temperatures of up to several hundred centigrade. Here, we outline how to take advantage of the emerging state-of-the-art instrumentation and methodologies to study surface structures and dynamics to improve the understanding of structure-sensitive catalytic functionality. The concept of using low electron dose-rates in TEM in conjunction with in-line holography and aberration-correction at low voltage (80 kV) is introduced to allow maintaining atomic resolution and sensitivity during in situ observations of catalysts. Benefits are illustrated by exit wave reconstructions of TEM images of a nanocrystalline Co3O4 catalyst material acquired in situ during their exposure to either a reducing or oxidizing gas environment.

On the role of the gas environment, electron-dose-rate, and sample on the image resolution in transmission electron microscopy

“…0.1 % for a temperature increase from 40 to 400 °C (calculated using a linear expansion coefficient of 2.5 • 10 −6 K −1 [37]) and can therefore be neglected. Sample drift is expected to increase slightly with temperature for the MEMS device and would, if anything, worsen the apparent resolution [38]. In contrast to the thermal expansion and drift, the measured electrical resistivity of the semiconducting SiC film decreased markedly from 12 to 7 kΩ upon heating up from 40 to 400 °C.…”

Observing Atoms at Work by Controlling Beam–Sample Interactions

“…In addition, low-dose-rate in-line holography is exceptionally benefi cial to environmental electron microscopy. [ 16 ] Certainly, its capabilities are needed to better understand artifi cial photosynthetic systems, as described in in ref. [ 17 ] This contribution describes underlying principles in detail and points toward fi rst applications.…”

Visualizing Redox Chemistry in Oxide Surfaces at Atomic-Resolution

“…In reactive environments, the oxide surface can undergo substantial reconstruction and exchange oxygen atoms with the gas phase, and the changes in structural coordination and oxidation state of the metal ions can markedly influence the catalytic properties. These redox processes are, however, scarcely understood, reflecting a lack of atomically resolved observations of oxide surfaces under meaningful reaction conditions.With recent advances, transmission electron microscopy has become capable of delivering atomically resolved images of catalyst in situ during exposure to reactive gas environments and elevated temperatures [1][2][3]. Observations of catalyst surfaces are, however, prone to alterations caused by the electron beam due to their reduced atomic coordination.…”

“…With recent advances, transmission electron microscopy has become capable of delivering atomically resolved images of catalyst in situ during exposure to reactive gas environments and elevated temperatures [1][2][3]. Observations of catalyst surfaces are, however, prone to alterations caused by the electron beam due to their reduced atomic coordination.…”

“…This streaky contrast is therefore attributed to beam-induced atom displacements. From [3]. , vacuum) conditions.…”