Role of Iron on the Structure and Stability of Ni3.2Fe/Al2O3 during Dynamic CO2 Methanation for P2X Applications

An energy scenario, mainly based on renewables, requires efficient and flexible Power-to-X (P2X) storage technologies, including the methanation of CO₂. As active Ni⁰ surface sites of monometallic nickel-based catalysts are prone to surface oxidation under hydrogen-deficient conditions, we investigated iron as “protective” dopant. A combined operando X-ray absorption spectroscopy and X-ray diffraction setup with quantitative on-line product analysis was used to unravel the structure of Ni and Fe in an alloyed Ni-Fe/Al₂O₃ catalyst during dynamically driven methanation of CO₂. We observed that Fe protects Ni from oxidation and is itself more dynamic in the oxidation and reduction process. Hence, such “sacrificial” or“protective” dopants added in order to preserve the catalytic activity under dynamic reaction conditions may not only be of high relevance with respect to fine-tuning of catalysts for future industrial P2X applications but certainly also of general interest

Bridging the gap between industry and synchrotron: Operando study at 30 bar over 300 h during Fischer-Tropsch synthesis

In order to reduce CO$_{2}$ emissions, it is necessary to substitute fossil fuels with renewable energy using CO$_{2}$ as a carbon feedstock. An attractive route for synthetic fuel production is the Fe- or Co-catalysed Fischer–Tropsch process. A profound knowledge of the catalyst deactivation phenomena under industrial conditions is crucial for the process optimisation. In this study, we followed the structural changes of a Co–Ni–Re/γ-Al$_{2}$O$_{3}$ catalyst for >300 hours at 30 bar and 250 °C during the Fischer–Tropsch synthesis operando at a synchrotron radiation facility. The advanced setup built for operando X-ray diffraction and X-ray absorption spectroscopy allows simultaneous and robust monitoring of the catalytic activity even over 300 h time on stream. We found three activity regimes for the Co–Ni–Re/γ-Al$_{2}$O$_{3}$ catalyst during 310 h of operation. Fast decline in activity was observed during the initiation phase in the first hours of operation due to liquid film formation (mass transport limitations). Furthermore, solid state reactions and carbon depositions were found while continuing the exposure of the catalyst to harsh temperature conditions of 250 °C. By using this advanced setup, we bridged the gap between industrially oriented catalysts and fundamental studies at synchrotron radiation facilities, opening up new possibilities for operando characterisation of industrial processes that rely on conditions of up to 450 °C and 50 bar

Structural dynamics in Ni–Fe catalysts during CO₂ methanation - role of iron oxide clusters

Bimetallic Ni–Fe catalysts show great potential for CO$_{2}$ methanation concerning activity, selectivity and long-term stability even under transient reaction conditions as required for Power-to-X applications. Various contrary suggestions on the role of iron in this system on CO$_{2}$ activation have been proposed, hence, its actual task remained still unclear. In this study, we used X-ray absorption spectroscopy (XAS) combined with X-ray diffraction (XRD), XAS in combination with modulation excitation spectroscopy (MES) and density functional theory (DFT) to shed detailed light on the role of iron in a bimetallic Ni–Fe based CO$_{2}$ methanation catalyst. During catalyst activation we observed a synergistic effect between nickel and iron that led to higher fractions of reduced nickel compared to a monometallic Ni-based catalyst. By XAS–XRD combined with DFT, we found formation of FeO$_{x}$ clusters on top of the metal particles. Modulation excitation coupled XAS data complemented with DFT calculations provided evidence of a Fe$^{0}$ ⇌ Fe$^{2+}$+ ⇌ Fe$^{3+}$ redox mechanism at the interface of these FeO$_{x}$ clusters. This may promote CO$_{2}$ dissociation. This is the first time that this highly dynamic role of iron has been experimentally confirmed in bimetallic Ni–Fe based catalysts with respect to CO$_{2}$ activation during the methanation reaction and may also be at the origin of better performance of other CO$_{2}$-hydrogenation catalysts. The insight into the structural surface changes reported in this study show the dynamics of the Fe–Ni system and allow the development of realistic surface models as basis for CO$_{2}$ activation and possible intermediates in these bimetallic systems