Versatile spectroscopic cell for tender X-ray operando studies in heterogeneous catalysis at ID26

The tender X-ray range between 1.5 to 4.5 keV covers the absorption L-edges of 4d transition metals that often serve as highly efficient catalysts. Beamline ID26 features an emission spectrometer that works in the tender X-ray range and that renders low-noise and high-energy-resolution absorption and emission spectra even on samples with low absorber concentration in complex matrices. However, until recently, it had not been possible to carry out operando studies inside the vacuum chamber of the tender X-ray spectrometer. In the context of the InnovaXN programme, the ID26 beamline team and collaborators have developed a cell that fulfils all the requirements of a catalytic reactor and is fully compatible with the X-ray spectroscopy environment. First experiments were carried out on a catalyst containing 1 wt.% of Rh on an Al₂O₃ support.

During the commissioning of the cell, it was vital to ensure that the windows could withstand the pressure difference with sufficient X-ray transmission for the tender X-rays (Figure 1). The count rates were found to be high enough to obtain good data quality after a few minutes of acquisition time. A thermographic camera was used to confirm that the heating was uniform across the catalyst bed with precise temperature readings from the thermocouples. Catalysis experiments are often carried out under chemically harsh conditions inside the reactor, and it was crucial that the windows remain intact throughout an experimental session under the extremely brilliant X-ray beam.

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Fig. 1: Spectroscopy in-situ /operando cell for tender X-ray investigations. a) 3D catalytic reactor design with its main parts, and (b) longitudinal section of the reactor: 1: Window; 2: Metallic sample cover; 3: Graphite gasket; 4: Gas inlet; 5: Gas outlet; 6: Thermocouples; 7: Screw holes (six units); 8: Air cooling inlet; 9: Air cooling outlet; 10: Resistor; 11: Electric connections. c) Positioning of the in-situ /operando cell (red) inside the tender X-ray emission spectrometer chamber. 

The potential of this setup for catalysis research was then demonstrated by recording high-energy-resolution fluorescence-detected (HERFD) X-ray absorption spectroscopy (XAS) data at the Rh L₃-edge during catalytic CO oxidation. A high-energy shoulder (peak C in Figure 2) was resolved, which is indicative of rhodium carbonyls. By recording spectra at different positions along the catalytic bed and comparing the contribution of the carbonyl species, the progress of the reaction could be followed.

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Fig. 2: Rh L₃-edge HERFD-XANES spectra for the CO oxidation catalytic light-off cycle between 100°C and 350°C for 1wt. % Rh/γ-Al₂O₃. a) Inlet position, (b) middle position, and (c) outlet position of the catalyst bed. The inset shows peak C, which indicates bonding of CO to Rh. d) Normalised intensity of peak C at different positions across the catalysis bed with CO conversion as function of temperature.

The combination of a highly efficient tender X-ray emission spectrometer optimised for HERFD-XAS studies with an operando cell is currently unique in the world. Besides 4d transition metals, the spectrometer can also reach the energies of the K-edges of sulfur, chlorine, potassium and calcium, as well as the M-edges of actinides. A future upgrade will reduce the emission energy to cover the Al Kα lines. The setup creates opportunities for a very large number of studies in catalysis research. If necessary, absorption edges in the hard X-ray range can also be measured inside the chamber such that multi-edge studies are possible in one experimental session. The setup can be modified to host other in-situ cells, e.g., for electrochemical studies. Work is also underway to implement multi-reactor cells inside the chamber, with the goal of greatly reducing the downtime.

Principal publication and authors
Versatile spectroscopic cell for operando studies in heterogeneous catalysis using tender X-ray spectroscopy in fluorescence mode, H.A. Suarez Orduz (a,b), S.-L. Heck (b), P. Dolcet (b), Y. Watier (a), M. Casapu (b), J.-D. Grunwaldt (b,c) and P. Glatzel (a), Chem. – Methods, e202300044 (2024); https://doi.org/10.1002/cmtd.202300044
(a) ESRF
(b) Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT), Karlsruhe (Germany)
(c) Institute of Catalysis Research and Technology (IKFT), Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen (Germany)