Abstract
Analytical methods that provide direct real-space information about the dynamics of catalysed reactions often require simplified model systems and operate under high-vacuum conditions. There is thus a strong need for the development of methods that enable observation of active catalysts under relevant working conditions. Here, in situ scanning electron microscopy is employed to study reaction dynamics and structure–activity correlations on surfaces. High sensitivity to changes in the work function and surface composition enables the detection of monolayers of adsorbed molecular species on metal surfaces, which is used here to visualize catalytic NO2 hydrogenation on platinum. The initiation of reactive behaviours and propagation of reaction fronts, as well as the spillover of activated species revealed in real-time and across a large pressure range, demonstrate the power of in situ scanning electron microscopy as a surface science tool in the study of gas-phase- and temperature-induced processes.
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All recorded images and data that support the findings of this study are available from the corresponding author on reasonable request.
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Acknowledgements
The authors want to acknowledge Dr Karsten Kunze for recording the EBSD map and ScopeM for the use of the equipment. C.B. thanks the Fonds de la Recherche Scientifique (F.R.S.-FNRS) and Wallonie–Bruxelles International (Excellence grant WBI.WORLD) for financial support.
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C.B., Z.-J.W. and M.-G.W. designed and performed the experiments and analysed the data. Z.-J.W. and M.-G.W. modified the ESEM set-up for in situ observations of gas–solid interactions. All authors contributed to the writing of the manuscript, and all authors have given approval to the final version.
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Supplementary Figs. 1–3
Supplementary Video 1
An in situ SEM movie that shows the initiation of nonlinear behaviours. Hydrogen was slowly added to the NO2 flow during the experiment. The recording of the movie started at the moment where surface reactivity could be detected. The flow of H2 was fixed at this point. The movie shows propagating reaction fronts and grain-dependent dynamics, it also illustrates the presence of spillover effects that stimulate reactivity on initially non-reactive grains. The movie plays at 7 fps and shows the dynamics observed during 13 min. The images were recorded at a scanning speed of 2.86 s per frame. Experimental conditions: T = 175 °C, \(p_{{\mathrm{NO}}_2}\):\(p_{{\mathrm{H}}_{\mathrm{2}}}\) ≈ 1:10, Pto ≈ 10−2 Pa.
Supplementary Video 2
A mobile phone movie that was recorded while a small subframe of around 75 × 75 pixels was scanned at a pixel dwell time of 0.3 µs. A mobile phone was used because the ESEM does not currently allow recording of such subframes. Scanning at a reduced pixel resolution allows imaging at several hundred frames per second and thus allows us to capture relatively fast propagating wavefronts without aliasing artefacts.
Supplementary Video 3
An in situ SEM movie that shows propagating reaction fronts and meandering spiral waves. The movie plays at 7 fps and individual images were recorded at a scanning speed one frame per 0.97 s. Experimental conditions: \(p{\mathrm{NH}_2}:p{\mathrm{H}_2}\)≈ 1:8, ptot = 8.7 × 10−3 Pa, T = 192 °C.
Supplementary Video 4
Surface reactivity and propagation of reaction fronts during NO2 hydrogenation on a polycrystalline platinum foil recorded at a total pressure of 13 Pa. The movie plays at 7 fps, individual images were recorded at a scanning speed of one frame per 2.86 s.
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Barroo, C., Wang, ZJ., Schlögl, R. et al. Imaging the dynamics of catalysed surface reactions by in situ scanning electron microscopy. Nat Catal 3, 30–39 (2020). https://doi.org/10.1038/s41929-019-0395-3
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DOI: https://doi.org/10.1038/s41929-019-0395-3