Abstract X‐ray absorption spectroscopy (XAS) is one of the powerful operando tools to track structural variations in heterogeneous catalysts. The nature of active sites in catalyst research is of great relevance, especially given the growing importance of energy storage using CO 2 as feedstock and the need for dynamic availability of electric power. Due to the pressure/temperature prerequisite of catalyst performance, the characterization of catalyst structure during catalysis under such high‐pressure reaction conditions is important to further improve catalyst design at a molecular level. Investigating catalysts in a controlled reaction atmosphere, while probing with X‐rays, offers an excellent opportunity for developing infrastructure at the synchrotron. Herein, a mobile setup with a robust spectroscopic cell for in situ and operando XAS applications, including a high‐pressure gas dosing equipment for such catalytic systems, is presented. The in situ/operando cell is operational for both the transmission and the fluorescence XAS mode at up to 50 bar and 450 °C. The setup comes with a protective box with Kapton windows, which holds the cell and serves as a miniature fume hood, and on‐line product analysis. Furthermore, the gas dosing equipment is compact, light‐weighted and can be easily transported to different synchrotrons and allows an optimum pre‐mix of gas flows and pressure build‐up. Methanol and Fischer‐Tropsch syntheses are used as examples for the highly flexible instrumentation.
Operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) were performed on a Co/TiO2 Fischer–Tropsch synthesis (FTS) catalyst at 16 bar for (at least) 48 h time-on-stream in both a synchrotron facility and a laboratory-based X-ray diffractometer. Cobalt carbide formation was observed earlier during FTS with operando XAS than with XRD. This apparent discrepancy is due to the higher sensitivity of XAS to a short-range order. Interestingly, in both cases, the product formation does not noticeably change when cobalt carbide formation is detected. This suggests that cobalt carbide formation is not a major deactivation mechanism, as is often suggested for FTS. Moreover, no cobalt oxide formation was detected by XAS or XRD. In other words, one of the classical proposals invoked to explain Co/TiO2 catalyst deactivation could not be supported by our operando X-ray characterization data obtained at close to industrially relevant reaction conditions. Furthermore, a bimodal cobalt particle distribution was observed by high-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray analysis, while product formation remained relatively stable. The bimodal distribution is most probably due to the mobility and migration of the cobalt nanoparticles during FTS conditions.
Designing acid-stable oxygen evolution reaction electrocatalysts is key to developing sustainable energy technologies such as polymer electrolyte membrane electrolyzers but has proven challenging due to the high applied anodic potentials and corrosive electrolyte. This work showcases advanced nanoscale microscopy techniques supported by complementary structural and chemical characterization to develop a fundamental understanding of stability in promising SrIrO3 thin film electrocatalyst materials. Cross-sectional high-resolution transmission electron microscopy illustrates atomic-scale bulk and surface structure, while secondary ion mass spectrometry imaging using a helium ion microscope provides the nanoscale lateral elemental distribution at the surface. After accelerated degradation tests under anodic potential, the SrIrO3 film thins and roughens, but the lateral distribution of Sr and Ir remains homogeneous. A layer-wise dissolution mechanism is hypothesized, wherein anodic potential causes the IrOx-rich surface to dissolve and be regenerated by Sr leaching. The characterization approaches utilized herein and mechanistic insights into SrIrO3 are translatable to a wide range of catalyst systems.
Analysis of the oxidation state and coordination geometry using pre-edge analysis is attractive for heterogeneous catalysis and materials science, especially for in situ and time-resolved studies or highly diluted systems. In the present study, focus is laid on iron-based catalysts. First a systematic investigation of the pre-edge region of the Fe K -edge using staurolite, FePO 4 , FeO and α-Fe 2 O 3 as reference compounds for tetrahedral Fe 2+ , tetrahedral Fe 3+ , octahedral Fe 2+ and octahedral Fe 3+ , respectively, is reported. In particular, high-resolution and conventional X-ray absorption spectra are compared, considering that in heterogeneous catalysis and material science a compromise between high-quality spectroscopic data acquisition and simultaneous analysis of functional properties is required. Results, which were obtained from reference spectra acquired with different resolution and quality, demonstrate that this analysis is also applicable to conventionally recorded pre-edge data. For this purpose, subtraction of the edge onset is preferentially carried out using an arctangent and a first-degree polynomial, independent of the resolution and quality of the data. For both standard and high-resolution data, multiplet analysis of pre-edge features has limitations due to weak transitions that cannot be identified. On the other hand, an arbitrary empirical peak fitting assists the analysis in that non-local transitions can be isolated. The analysis of the oxidation state and coordination geometry of the Fe sites using a variogram-based method is shown to be effective for standard-resolution data and leads to the same results as for high-resolution spectra. This method, validated by analysing spectra of reference compounds and their well defined mixtures, is finally applied to track structural changes in a 1% Fe/Al 2 O 3 and a 0.5% Fe/BEA zeolite catalyst during reduction in 5% H 2 /He. The results, hardly accessible by other techniques, show that Fe 3+ is transformed into Fe 2+ , while the local Fe–O coordination number of 4–5 is maintained, suggesting that the reduction involves a rearrangement of the oxygen neighbours rather than their removal. In conclusion, the variogram-based analysis of Fe K -edge spectra proves to be very useful in catalysis research.
