Identifying Performance Descriptors in CO₂ Hydrogenation over Iron‐Based Catalysts Promoted with Alkali Metals

Abstract

Alkali metal promoters have been widely employed for preparation of heterogeneous catalysts used in many industrially important reactions. However, the fundamentals of their effects are usually difficult to access. Herein, we unravel mechanistic and kinetic aspects of the role of alkali metals in CO2 hydrogenation over Fe‐based catalysts through state‐of‐the‐art characterization techniques, spatially resolved steady‐state and transient kinetic analyses. The promoters affect electronic properties of iron in iron carbides. These carbide characteristics determine catalyst ability to activate H2, CO and CO2. The Allen scale electronegativity of alkali metal promoter was successfully correlated with the rates of CO2 hydrogenation to higher hydrocarbons and CH4 as well as with the rate constants of individual steps of CO or CO2 activation. The derived knowledge can be valuable for designing and preparing catalysts applied in other reactions where such promoters are also used.

Operando DRIFT and In situ Raman Spectroscopic Studies on Aspects of CO2 Fischer‐Tropsch Synthesis Catalyzed by Bulk Iron Oxide‐Based Catalysts

“…The hematite (α-Fe 2 O 3 ) catalyst was prepared by precipitation from an aqueous solution of iron nitrate nonahydrate, Fe(NO 3 ) 3 • 9H 2 O, with ammonia solution at pH 9.5. After filtration of the precipitate, washing with water and drying at 100 °C overnight, the precursor was calcined at 400 °C for 6 h. [18] Raman spectroscopy. In situ Raman experiments were used to monitor the restructuring of the catalysts during the pretreatment step.…”

“…In a preceding research study by some of us it was found that reductions of α‐Fe 2 O 3 in a fixed bed reactor at 300 and 400 °C (50 vol % H 2 , GHSV=1160 ml g cat −1 h −1 ) gave mainly magnetite (97.5 %, 68.9 %) and metallic iron (2.4 %, 31.1 %), respectively [18] . The content of Fe increased even more significantly for treatments at higher temperatures.…”

“…[11,16,17] Methane formation is a typical undesired side-reaction of CO 2 -FT over Fe-based catalysts and depends on the reaction conditions such as CO 2 conversion, partial pressures, ratios of CO 2 /H 2 , space velocities, and the exact catalyst preparation. [5,18,19] To reduce the CH 4 selectivity, catalysts are often modified with promotors such as alkali metals, Cu and Mn.…”

CO2 Conversion by Fischer‐Tropsch Synthesis Using Na‐Modified Fe Catalysts

“…Additionally, the spectra indicate that the Fe 2p peak is slightly shifted to lower binding energy, i.e., from 710.92 to 710.37 eV, when the Na content is increased from 0.9 to 9.8 wt %. This shift is related to the pronounced interaction of the electropositive Na species with the iron oxide surface 18, 26. In contrast to the as‐prepared samples, the XP spectra recorded after the carbidization performed at 300 °C and subsequent FTS investigation ( TOS = 140 h) indicate strong carbon deposition on the catalysts, which makes the unequivocal identification of the Na surface species hardly possible.…”

“…Moreover, the present study also shows that the Na promotion of the Fe sample provides similar C5+ selectivities compared to K-and Na-promoted Fe catalysts reported recently. For instance, at 300 °C the C5+ selectivities of K-and Na-modified Fe catalysts from the literature [26,37] amount to approx. 36 % and 32 %, while that of the Na-promoted Fe catalysts presented in this work range from 24-37 % ( 10).…”