The methanation of CO2, as a part of the power-to-gas concept, was studied under various industrially relevant feed compositions with a focus on the formation and influence of carbonaceous species. For this purpose, 5 wt. % Ni/AL(2)O(3), 5 wt.% Ni3Fe/Al2O3 and 3.4 wt.% NiRh0.1/Al2O3 catalysts were prepared and characterized by Xray diffraction (XRD), temperature-programmed reduction (TPR), scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). During the methanation of CO2, the Ni3Fe catalyst emerged as the most active and selective catalyst in the mid-temperature regime (300-350 degrees C). At 400 degrees C, all three tested catalysts showed high conversion of CO2 (67-75%; Ni > Ni3Fe > NiRh0.1) and selectivity towards CH4 (95-98%). Operando Raman spectroscopy was applied to elucidate the possible influence of carbonaceous species on the performance of the catalysts. Notably, no carbon deposition was observed under various feed compositions, even in CO2 or CO2/CH4 mixtures, e.g. as provided by biogas plants. Only in pure CH4 atmosphere an intensive carbon deposition with graphitic structure occurred as uncovered by operando Raman spectroscopy. Experiments in the lab-scale reactor and a spectroscopic microreactor could be correlated and revealed a strong catalytic deactivation of the carbon covered catalysts including a pronounced shift of the selectivity towards CO. The initial activity could be recovered after reactivation in H-2 at elevated temperatures, which led to a removal of the deposits especially from the metal particles. Raman spectroscopy, supported by the results from high -resolution transmission electron microscopy (HRTEM) and EELS, revealed that carbon remained on the support material. The latter did not have any significant influence on the catalytic activity and could be removed in an oxidizing atmosphere.

The methanation of CO2 within the power-to-gas concept was investigated under fluctuating reaction conditions to gather detailed insight into the structural dynamics of the catalyst. A 10 wt % Ni/Al2O3 catalyst with uniform 3.7 nm metal particles and a dispersion of 21% suitable to investigate structural changes also in a surface-sensitive way was prepared and characterized in detail. Operando quick-scanning X-ray absorption spectroscopy (XAS/QEXAFS) studies were performed to analyze the influence of 30 s and 300 s H-2 interruptions during the methanation of CO2 in the presence of O-2 impurities (technical CO2). These conditions represent the fluctuating supply of H-2 from renewable energies for the decentralized methanation. Short-term H-2 interruptions led to oxidation of the most reactive low-coordinated metallic Ni sites, which could not be re-reduced fully during the subsequent methanation cycle and accordingly caused deactivation. Detailed evaluation of the extended X-ray absorption fine structure (EXAFS) spectra showed surface oxidation/reduction processes, whereas the core of the Ni particles remained reduced. The 300-s H-2 interruptions resulted in bulk oxidation already after the first cycle and a more pronounced deactivation. These results clearly show the importance and opportunities of investigating the structural dynamics of catalysts to identify their mechanism, especially in power-to-chemicals processes using renewable H-2.

A promising bimetallic 17 wt % Ni3Fe catalyst supported on gamma-Al2O3 was prepared via homogeneous deposition-precipitation for the application in the methanation of CO2 to gather more detailed insight into the structure and performance of the catalyst compared to state-of-the-art methanation systems. X-ray diffraction (XRD) analysis, detailed investigations using scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy analysis (EDX) of single particles as well as larger areas, high-resolution transmission electron microscopy (HRTEM) imaging, temperature-programmed reduction (H-2-TPR), and in-depth interpretation of Raman bands led to the conclusion that a high fraction of the Ni and Fe formed the desired Ni3Fe alloy resulting in small and well-defined nanoparticles with 4 nm in size and a dispersion of 24%. For comparison, a monometallic catalyst with similar dispersion using the same preparation method and analysis was prepared. Using a fixed-bed reactor, the Ni3Fe catalyst showed better low-temperature performance compared to a monometallic Ni reference catalyst, especially at elevated pressures. Longterm experiments in a microchannel packed bed reactor under industrially relevant reaction conditions in competition with a commercial Ni-based methanation catalyst revealed an improved performance of the Ni3Fe system at 358 degrees C and 6 bar involving enhanced conversion of CO2 to 71%, selectivity to CH4 > 98%, and most notably a high stability. Deactivation occurred only at lower temperatures, which was related to carbon deposition due to an increased CO production. Kinetic measurements were compared with literature models derived for Ni/Al2O3 catalysts, which fit well but underestimate the performance of the Ni3Fe system, emphasizing the synergetic effect of Ni and Fe.

A novel nanoparticulate catalyst of copper (Cu) and ruthenium (Ru) was designed for low-temperature ammonia oxidation at near-stoichiometric mixtures using a bottom-up approach. A synergistic effect of the two metals was found. An optimum CuRu catalyst presents a reaction rate threefold higher than that for Ru and forty-fold higher than that for Cu. X-ray absorption spectroscopy suggests that in the most active catalyst Cu forms one or two monolayer thick patches on Ru and the catalysts are less active once 3D Cu islands form. The good performance of the tuned Cu/Ru catalyst is attributed to changes in the electronic structure, and thus the altered adsorption properties of the surface Cu sites.

The methanation of CO2 as a relevant strategy for energy storage has been studied by operando X-ray absorption spectroscopy under dynamic H-2/CO2 and CO2 reaction atmospheres. A typical CO2 conversion of 81% was reached at 400 degrees C with a 23 wt.-% Ni/CaO-Al2O3 catalyst, yielding 80% of CH4. The operando XAS experiment under working conditions revealed pronounced structural changes, e.g., a fast bulk oxidation of the Ni particles after removal of H-2 from the H-2/CO2 (4:1) gas stream. A lower performance of the catalyst was observed in the subsequent methanation cycle due to the presence of a residual oxidized fraction of Ni. (C) 2015 Published by Elsevier Inc.

Conversion of electric power into chemical energy carriers plays a key role in many concepts for a future energy supply based on renewable sources. ́Power-to-X ́ technologies like the methanation of CO2 allow long-term energy storage and compensation of fluctuations in renewable energy. Structural characterization of methanation catalysts under realistic and dynamic reaction conditions [1] using methods like X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) provides valuable information for a knowledge-based optimization. Ni-based catalysts for the methanation of CO2 were characterized in quartz capillaries heated by a gas blower with operando XAS at the KIT synchrotron (XAS and CAT-ACT beamlines, [2]), and with Quick Scanning XAS at the Swiss Light Source (SuperXAS, [3]). A fluctuating H2 supply was simulated by temporary removal of H2 from the feed gas [4]. Within the German BMBF project “Kopernikus Power-to-X” these studies were continued at elevated pressure to approach industrial relevant conditions and by including a commercial Ni-based catalyst used in three-phase methanation of CO2 [5]. At CAT-ACT, a set-up for combined XAS and XRD was implemented. Operando XAS studies on Ni-based catalysts for twoand three-phase methanation of CO2 at atmospheric pressure provided insights into deactivation mechanisms during methanation under dynamic feed conditions. The results revealed that all investigated catalysts were stable under methanation conditions at atmospheric pressure. H2 dropouts caused partial oxidation of the catalytically active Nito Ni-species resulting in a lower activity during subsequent methanation cycles. XRD data indicated formation of a NiO phase. No NiCO3 reflections were observed. A regeneration of the catalysts without a loss in activity was achieved by reduction in H2/N2. The combined XAS-XRD experiments provided detailed complementary structural information about the changes in Ni-based catalysts during methanation of CO2 and under more dynamic feed conditions. All catalysts were stable during CO2 methanation but immediately oxidized in CO2/N2 (H2 dropout, technical CO2). A regeneration to restore the initial activity was achieved by reduction in H2/N2. In order to further approach industrially relevant conditions (elevated pressure, low space velocity) a new high pressure operando cell for combined XAS-XRD is now tested, and the operando studies on Ni-based catalysts for twoand three-phase methanation of CO2 will be continued to further investigate the structural changes. More complex processes like three-phase methanation will be studied in liquid phase using an in situ batch cell based on ref. [6].