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European Microscopy Congress 2020 Abstract Database

Rapid In-situ XANES Imaging of Chemical Gradients during Catalytic Partial Oxidation of Methane

Rapid In-situ XANES Imaging of Chemical Gradients during Catalytic Partial Oxidation of Methane

Session

PST.6 - In-situ and in-operando microscopy

Authors

Ms Saba Alizadehfanaloo (1), Dr Andreas Schropp (1), Mr Martin Seyrich (1), Dr Jan Garrevoet (1), Scientist Vadim Murzin (1), Mr Johannes Becher (2), Dr Dmitry Doronkin (2), Dr Thomas L. Sheppard (2), Prof Jan-Dierk Grunwaldt (2), Prof Christian G. Schroer (1, 3)

Affiliations

1. DESY
2. KIT
3. University of Hamburg

Keywords

In-situ X-ray absorption spectroscopy; Synchrotron radiation; Full-field imaging; Spatially and temporally resolved XANES; Catalytic partial oxidation of methane; Heterogeneous Catalysis

Abstract text

Supported Rh- and Pt-alumina catalysts are gaining attention for their application in the catalytic partial oxidation (CPO) of methane to synthesis gas (CO + H₂), which is a potential alternative to large-scale and energy-intensive steam reforming plants. These catalysts display gradients in oxidation state and temperature along the catalyst bed, during the switch between methane combustion and ignition/extinction of the CPO reaction [1,2]. To obtain meaningful data for this catalytic reaction, it is important to study it in a spatially-resolved manner (in at least two dimensions), and with time and energy resolution under reaction conditions. However, combining all these concepts is a significant challenge. During previous chemical imaging experiments [1-3], the CPO reaction was studied under static operando conditions with data acquisition in the order of a few hours [1,2] or was limited to a single energy point [3], not allowing for spectroscopic imaging. Now by technical advances in imaging (fast high-resolution detectors) and beamlines at PETRA III, it is possible to study chemical reactions under different (non-static) conditions by spatiotemporal chemical imaging, in order to probe the oxidation state and coordination environment of catalysts in a spatially-resolved way. 

Here we demonstrate new opportunities for rapid spatiotemporal chemical imaging, with CPO of methane over a Pt-alumina catalyst as a model study. The method is also readily applicable for multidimensional (spatially, time, energy resolved) imaging of the changes occurring in other complex catalytic materials under operating conditions. We report on X-ray absorption imaging experiments carried out at the advanced spectroscopy beamline P64 at PETRA III. The beamline is equipped with a fast scanning QEXAFS monochromator, which was synchronized with a high-resolution X-ray detector running continuously at 50 Hz full-frame rate. By measuring a rapid sequence of 2D transmission images of the reactor bed around the Pt L₃ absorption edge at E = 11.564 keV, the chemical state of the platinum catalyst was obtained for every pixel in the image series. In fact, a whole XANES spectrum could be measured in about 4 s for all pixels simultaneously. This allowed us to follow in situ the transition between standard combustion of methane (products CO₂ + H₂O), and the CPO reaction (products CO + H₂). These results indicate that during the partial oxidation of methane over Pt particles, the ignition front moves from the outlet towards the inlet of the catalyst bed with a speed of about 5 µm/s for the given setting.

We can claim that this method allowed following the ignition of the CPO reaction within the catalytic reactor in a spatially and temporally resolved manner and unlocks new possibilities for chemical imaging of catalysts. In fact, the operando imaging method presented here can readily be adapted in the future to examine other rapid chemical reactions with strong structural changes.

References

[1] Jan-Dierk Grunwaldt, et al., “ 2D-Mapping of the Catalyst Structure Inside a Catalytic Microreactor at Work:  Partial Oxidation of Methane over Rh/Al2O3”. J.Phys. Chem. B, 110 (2006), p. 8674. 

[2] Stephan Hannemann, et al.,“ Distinct spatial changes of the catalyst structure inside a fixed-bed microreactor during the partial oxidation of methane over Rh/Al₂O₃”. Catalyst Today, 126 (2007), p. 54.

[3] Bertram Kimmerle, et al., “ Visualizing a Catalyst at Work during the Ignition of the Catalytic Partial Oxidation of Methane”. J.Phys. Chem. C, 113 (2009), p. 3037.