Understanding of the Fate of α-Fe₂O₃ in CO₂ Hydrogenation through Combined Time-Resolved In Situ Characterization and Microkinetic Analysis

Abstract

CO2 hydrogenation over Fe-based catalysts is a promising pathway to mitigate emissions of this greenhouse gas and provides a possibility for crude-oil-free production of chemicals and fuels. Understanding of (i) the role of crystalline phases or/and surface species in the working catalysts, (ii) the factors affecting their formation under reaction conditions, and (iii) the kind and reactivity of surface precursors of gas-phase products is vital for controlling the efficiency of CO2 hydrogenation. In this study, we applied time-resolved in situ characterization techniques for monitoring phase transformations during Fe2O3 reduction, starting of CO2 hydrogenation, steady-state operation, and finally catalyst deactivation. The obtained structural information after different times on stream was related to kinetic data obtained from temporal analysis of H2 and CO2 activation as well as from steady-state isotopic transient kinetic analysis (SSITKA). Fe2O3 is easily reduced to Fe3O4 and Fe in H2 above 300 °C. Fe5C2 and Fe3C, which are quickly formed from metallic Fe/Fe3O4 under CO2 hydrogenation conditions, do not undergo oxidation with rising time on the reaction stream under ambient-pressure conditions. Nevertheless, the catalyst loses its initial activity and, particularly, the selectivity to hydrocarbons in favor of CO. Thus, we do not confirm the well-recognized deactivation mechanism of CO/CO2 hydrogenation through oxidation of iron carbides. Instead, surface carbonaceous species identified by in situ Raman and pseudo in situ XPS measurements were concluded to cause catalyst deactivation and deselectivation due to hindering the catalyst ability to generate surface species from H2 and CO2. Specifically, the strength of CO2 adsorption and the catalyst activity to dissociate adsorbed CO2 to adsorbed CO decrease in the presence of carbon deposits. Kinetic evaluation of SSITKA tests revealed the presence of (i) only one kind of surface intermediate yielding gas-phase CO after 1.5 h on reaction stream but (ii) at least two kinds (short-lived and long-lived) of surface intermediates participating in CH4 formation in parallel. Carbon deposits seem to block the sites responsible for the formation of short-lived intermediates.

CO2 Hydrogenation to Hydrocarbons over Fe‐Based Catalysts: Status and Recent Developments

“…By employing time-resolved in situ techniques (XRD, XPS, and Raman), very recently, our group contributed to revealing deactivation mechanisms of Febased catalysts in CO 2 -FTS. [22] The observed decrease in CO 2 conversion and changes in product selectivity with time-onstream cannot be explained by the re-oxidation of Fe x C y . Instead, in situ XPS and Raman confirmed the formation of carbon depositions that negatively affect the activation of H 2 / CO 2 and the formation of active intermediates as rationalized by transient kinetic studies.…”

An Overview on Dynamic Phase Transformation and Surface Reconstruction of Iron Catalysts for Catalytic Hydrogenation of COx for Hydrocarbons

“…Further contributions from Diego et al identified strongly adsorbed carboxylate species on spent catalyst surfaces, indicating potential obstacles to further catalyst carburization during FTS due to their strong interaction with iron oxide sites. Additionally, findings from Yang et al’s in situ Raman and pseudo in situ XPS analyses highlighted that carbon deposits impede the catalyst’s ability to generate surface adsorbed species from H 2 and CO 2 . A proposed solution from Han et al involved introducing H 2 O into the syngas, expediting the removal of surface carbon deposits and thereby shortening the reaction induction period by exposing more active sites .…”

“…Most recently, Prof. Kondratenko’s group investigated the phase transformation of α-Fe 2 O 3 , synthesized via chemical precipitation from iron nitrate nonahydrate, employing a series of time-resolved in situ Raman, in situ XRD, and in situ NAP-XPS methodologies . Their results echoed our findings, observing the evolution pattern of Fe 2 O 3 → Fe 3 O 4 → Fe(0) → Fe 5 C 2 /Fe 3 C. However, the catalyst exhibited selectivity loss to hydrocarbons over time on stream.…”

Synergistic effect of K and Zn on Fe-based catalysts for efficient CO2 hydrogenation

“…6–9 Thermal catalytic hydrogenation of CO 2 to produce high value-added chemicals (such as hydrocarbons) is one of the most profitable pathways. 10–14 However, it is still extremely difficult to achieve the transformation of CO 2 into hydrocarbons by a one-step reaction due to the chemical inertness of CO 2 .…”

“…Sodium, Na, was widely used as a promotor to improve the ability of iron carbide to resist the oxidation of H 2 O and CO 2 , as well as the reduction of H 2 , leading to the improvement of selectivity to the desired products. 11,26 For example, a multifunctional Na-Fe 3 O 4 /HZSM-5 catalyst was reported to convert CO 2 into a gasoline fuel with a selectivity up to 78% for all hydrocarbons while only 4% for methane. 12 Another kind of alkali, potassium, K, was reported to be able to enrich the surface basicity of the catalyst and accelerate the carburization of Fe-based catalysts to form active iron carbides, thus assisting in CO 2 conversion.…”