In situ and operando laboratory X-ray absorption spectroscopy at high temperature and controlled gas atmosphere with a plug-flow fixed-bed cell

TL;DR

The study demonstrates a laboratory-based X-ray absorption spectroscopy workflow using a von Hámos spectrometer coupled to a plug-flow fixed-bed cell, enabling operando measurements up to 1000 °C and 10 bar with online GC activity readouts. By examining Mn K-edge and Ni K-edge transitions, the authors monitor MnO oxidation to Mn2O3 in situ and NiO reduction to metallic Ni during catalytic activation and CO2 methanation in Ni-containing systems. The work highlights both the capabilities and limitations of lab-based operando XAS, including temperature-related damping and capillary-induced spectral artifacts, while showing that meaningful redox and activity information can be obtained on minute timescales. These results position laboratory XAS as a practical, accessible complement to synchrotron studies for time-resolved catalysis investigations.

Abstract

The capabilities of a plug-flow fixed-bed cell for operando studies of heterogeneous catalysts are demonstrated using laboratory-based X-ray absorption spectroscopy (XAS) with a von Hamos spectrometer. The cell operates at temperatures up to 1000 deg C and pressures up to 10 bar, equipped with three mass flow controllers and two infrared lamps for rapid heating under inert/reactive gas atmospheres. Proof-of-principle studies include in situ MnO oxidation in 5% Ni/MnO and operando Ni nanoparticle evolution in 20-NiO/COK-12 (20.2% NiO on SiO2) during CO2 methanation before/after activation. Within 5-15 min per spectrum, oxidation state changes are resolved while catalytic activity is simultaneously quantified by online GC. Extended datasets and methods are available in the ancillary file SI.pdf (Supplementary Information file). A shortened version is submitted to Journal of Analytical Atomic Spectrometry as a Technical Note.

Figures (7)

• Figure 1: a: Schematic view of sample, quartz wool and TC position inside the capillary surrounded by the SiC tube. b: Top view of the capillary in the SiC tube showing the X-ray beam path. c: CAD model to the orientation of the IR tube furnace in the von Hámos set up.
• Figure 2: P&ID of the In situ/operando reactor setup. With (i) gas flow control unit, (ii) reaction cell and (iii) gas analysis unit. MFC: mass flow controller; CV: check valve; TIC: temperature indicator and controller; GC: Gas chromatograph. Bubbler B-01 is empty to trap water during reaction. Bubbler-B02 contains water to indicate the gas flow. Optional: a pressure control unit (PC) can be installed between the reaction cell and Bubbler B-01.
• Figure 3: K-edge XAS spectra (Mn, Ni, Se and Zr) of 5 % Ni/MnO @ 6539 eV, 20-NiO/COK-12 @ 8333 eV, SnSe @ 12 658 eV and ZrO2 @ 17 998 eV. The Mn, Ni and Se K-edge measurements were performed with the capillary with 0.8 mm inner diameter (IN) and 1.0 mm outer diameter. The Zr K-edge measurement was performed using the capillary with 1.5 outer diameter and 1.0 mm inner diameter. Vertical offsets to the overall absorption ($\mu Q$) of the different spectra have been introduced for better comparison with each other. The unmodified spectra are shown in the SI in Fig. S6--S9. The dark lines on top of the capillary spectrum represent the same materials prepared as a pellet, without distortion introduced by the capillary.
• Figure 4: In situ XAS measurement of 5 % Ni/MnO at the Mn K-edge during heating, with a measurement time of 15 min per spectrum. a: XANES region at RT and elevated temperatures, shown alongside reference spectra of Ni/MnO and NiO/Mn2O3. An offset has been applied for clarity. b: Enlarged view of the absorption edge, highlighting the Mn K-edge shift toward higher oxidation states upon heating.
• Figure 5: Operando XAS measurements of 20-NiO/COK-12 at the Ni K-edge. a: Before catalyst activation, measured at 350 °C for 1 h under H2/CO2 flow at a 4:1 ratio. b: Catalyst reduction/activation at 600 °C for 1 h under 5 % H2/Ar flow. c: After activation, measured again at 350 °C for 2 h under the same H2/CO2 (4:1) conditions as the initial state. Each spectrum was acquired with a measurement time of 300 s (5 min). The results are shown alongside the reference spectra of 20-Ni0/COK-12 prepared as pellet and a 10 µm Ni(0) metal foil with a total acquisition time of 3.5 h and 1.5 h, respectively.