Imaging and chemical probing of catalytical reactions with subnanometer resolution

Catalytic reactions can be imaged with nanoscale resolution using video-field emission techniques. The advantage of this approach over common local-probe techniques is two-fold: first, a single nanosized metal grain (a “tip”) can be imaged with atomic resolution all at once rather than by scanning as in STM and, second, the chemical composition can be probed in selected areas of this metal grain while imaging the ongoing chemical reaction. The unique features of this experimental approach will be demonstrated in several examples.
Catalytic and electrocatalytic reactions are inherently non-linear and frequently involve surface reconstructions, which can be imaged with atomic resolution using field ion microscopy (FIM). Reaction hysteresis may be observed and occasionally runs into very regular self-sustained oscillations along with characteristic spatiotemporal pattern formation. This will be demonstrated for the reduction of nitric oxide with hydrogen on a Pt nanosized crystal [1].
In a second example, we will address the question for the mechanisms and pathways leading to the formation of a surface oxide from chemisorbed oxygen layers. Atom probe microscopy (APM) demonstrates in that case how mainly facets of the ⟨022̅⟩ crystallographic directions of a Rh single crystal grain act as oxygen-permeable gateways [2]. The highly anisotropic spatial distribution of incorporated oxygen atoms is in agreement with FIM analyses according to which the hemispherical crystal is transformed into a polyhedral morphology due to the interaction with oxygen. We will also inspect the catalytic surface reaction when dosing both H2 and O2 to the nanosized Rh grain. In this case, video-FIM shows spatio-temporal patterns which allow identifying the catalytically active sites associated with water formation. Again, oscillatory behavior is being observed. Combining the microscopic evidence of FIM with atom-probe techniques during the ongoing reaction allows determining the local chemical composition and, therefore, the feedback mechanism of the oscillations. Accordingly, a reversible surface oxidation is observed as evidenced by Rh-oxide field evaporation [3].


Publication References

C. Barroo, Y. De Decker, T. Visart de Bocarmé and N. Kruse
Emergence of Chemical Oscillations from Nanosized Target Patterns
Phys. Rev. Lett. 117 (2016) 144501
S. V. Lambeets, T. Visart de Bocarmé, D. E. Perea, and N. Kruse
Directional Gateway to Metal Oxidation: 3D Chemical Mapping unfolds Oxygen Diffusional Pathways in Rhodium Nanoparticles
J. Phys. Chem. Lett. 2020, 11, 3144−3151
J. S. McEwen, P. Gaspard, T. Visart de Bocarmé and N. Kruse
Nanometric Chemical Clocks
PNAS 106 (2009) 3006-3010
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