A promising bimetallic 17 wt % Ni3Fe catalyst supported on γ-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 (H2-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. Long-term 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 °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.

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The selective oxidation of propylene to acrolein is an important reaction in the chemical industry which has been extensively studied over the last few decades. Today, spectroscopic, computational, and synthetic approaches allow a renewed view of this established and well-understood catalytic process at a fundamental level. Consequently, a revised mechanistic pathway for the selective propylene oxidation over bismuth molybdates has been suggested recently. Furthermore, studies concerning the local interaction of specific surface entities as well as concepts from semiconductor science have provided valuable information to describe the operation mode of oxidation catalysts. New synthetic methods can be used not only to tune the specific surface area and surface species of a catalyst but also to give direct access to distinct metal oxide phases or specific crystalline phases with a synergetic interplay on the nanoscale. Since complex multicomponent systems, which exhibit both higher selectivity and activity in comparison to pure bismuth molybdates, are used for industrial applications, it is important to transfer the research concepts from such model systems to those more complex systems. This also involves operando characterization techniques on multiple length scales. Recent research activities shine a renewed light on this well-studied reaction, which therefore may become one of the drivers in selective oxidation catalysis to apply and further establish new tools that have been developed in theory, modeling, synthesis, and operando spectroscopy.

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MoO 6 as target bismuth molybdate phases were prepared by hydrothermal synthesis and flame spray pyrolysis and tested for their catalytic performance in the selective oxidation of propylene. Their structure and reactivity during temperature-programmed reaction (TPR) and under reaction conditions were investigated by in situ and operando X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and Raman spectroscopy. To gain insight into amorphous and crystalline structures at the same time, XAS and XRD as well as XAS and Raman spectroscopy were combined in one experiment. TPR in propylene revealed that the reduction of Mo 6+ to Mo 4+ occurred at lower temperatures than from Bi 3+ to Bi 0 in scheelite-structured systems. In a reaction cycle, mainly reduction of molybdenum was observed and EXAFS fitting confirmed the removal of oxygen from MoO 4 2− entities. Minor structural transformations were detected by XRD and Raman spectroscopy. The catalytic performance of aurivillius-structured systems was more diverse than for scheelite-based ones and ranged from highest to lowest observed acrolein yield, probably due to a synergy effect of two or more bismuth molybdate phases. For phase pure systems, bismuth was more easily reduced than molybdenum. In contrast, aurivillius structures with additional phases showed reduction and oxygen removal from both metal centers under steady-state conditions, but molybdenum was in most cases more easily reduced. A high catalytic activity mostly coincided with low reduction temperatures, except for the unselective pure γ-Bi 2 MoO 6 that showed a facilitated reduction of bismuth compared to molybdenum. Hence, the combination of operando methods led to an understanding of the redox behavior of bismuth and molybdenum and their influence on the catalytic performance.