Abstract
The raise of regenerative but unsteadily produced energy demands a highly flexible way to store the energy for time periods when less energy is produced than consumed. In the current study, it is investigated if catalysts based on environmentally more attractive and less hazardous to health Fe might be able to be considered as an alternative to Ni catalysts in the CO2 methanation at elevated pressure. For this a set of catalysts with 1–10 wt % Fe supported on the zeolite 13X is analysed in CO2 methanation at 1–15 bar. The trends of activity as well as selectivity with varying Fe loading and pressure are presented. Correlation with thorough characterization of the materials shows that a very high dispersion of Fe in octahedral sites within the zeolite is necessary to generate CH4 as the main reaction product and suppress the Fischer–Tropsch activity towards CC coupling reactions at elevated pressure. Especially with low Fe loading such as 1 wt % high reaction rates of 42 mmol(CO2)/(mol(Fe)∙s) with a CH4 selectivity of 76 % at 300 °C and 10 bar are obtained. In contrast to that, highly Fe loaded catalysts tend to form increasing amounts of Fischer–Tropsch products at increasing pressure. In addition, highly Fe-loaded catalysts are much more susceptible to destruction of the zeolite under reaction conditions. At the same time, highly loaded catalysts form a Fe3C shell around the remaining support. Hence, avoiding the formation of a Fe3C phase is crucial for high CH4 selectivity. The results presented here therefore show that catalysts with a very high Fe-dispersion in particular can gain considerably in importance as alternatives to Ni-methanation catalysts at elevated pressure.
Highlights
Utilization of CO2 is currently a hot topic in catalysis due to the chance to decrease anthropogenic CO2 emissions on the one hand and to recycle it as a C1 source in exchange to fossil fuels on the other hand
Since the zeolite framework is prone to destruction by iron, the integrity of the structure was validated via XRD analysis
Comparable product selectivites of S(CO) = 11 % and S(CC) = 14 % are observed at 10 and 15 bar. This trend of decreasing selectivity towards C–C coupled products and increasing CH4 selectivity with increasing pressure is opposed to the general trend of Fe-based Fisher–Tropsch catalysts reported in literature [26], in which Fe3C is regarded as active species
Summary
Utilization of CO2 is currently a hot topic in catalysis due to the chance to decrease anthropogenic CO2 emissions on the one hand and to recycle it as a C1 source in exchange to fossil fuels on the other hand. With the aim of tailoring Fe-based materials as CO2 methanation catalysts, studies on increasing the C2–C4 fraction in CO-FTR, with CH4 as an undesirable product, provide information on the direction of necessary properties for high CH4 yields: In general, iron carbides are considered as the active phase in FTR and active carbon sites contribute to the chain growth mechanism [26]. As main focus the trends in activity and selectivity with increasing pressure as well as iron loading are carefully analysed and correlated with the properties of the catalysts This leads to a justification if CO2 methanation on Fe-based catalyst will become feasible as an attractive alternative
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