Unravelling the New Roles of Na and Mn Promoter in CO₂ Hydrogenation over Fe₃O₄‐Based Catalysts for Enhanced Selectivity to Light α‐Olefins

Abstract

The direct production of light α‐olefins (C2=‐C4=) from CO2 is of great importance as this process can convert the greenhouse gas into the desired chemicals. In this study, the crucial roles of Na and Mn promoter in CO2 hydrogenation to produce light α‐olefins via the Fischer‐Tropsch synthesis (FTS) over Fe3O4‐based catalysts are investigated. The results indicate that both Na and Mn promoter can enhance the reducibility of Fe3O4. In situ XPS and DFT calculations show that Na facilitates the reduction by electron donation from Na to Fe as the oxygen vacancy formation energy is reduced by Na. In contrast, Mn promotes the reduction by the presence of oxygen vacancy in MnO as the oxygen in Fe oxide can spillover to the vacancy in MnO spontaneously. For un‐promoted Fe3O4 catalysts, CO2 hydrogenation dominantly produces light n‐paraffins. The addition of Na remarkably shifts the selectivity to light α‐olefins with a sharp decline in the selectivity to light n‐paraffins, which is attributed to the electron donation from Na to Fe resulting in the promoted CO dissociation and the favorable β‐H abstraction of surface short alkyl‐metal intermediates. The addition of Mn into Na‐containing Fe3O4 catalysts can obviously further enhance the selectivity to light α‐olefins as the spatial hindrance of Mn suppresses the chain growth to increase the amount of surface short alkyl‐metal intermediates.

Transforming carbon dioxide into jet fuel using an organic combustion-synthesized Fe-Mn-K catalyst

“…4f). This has been proposed as the source of the catalytically active sites for CO and/or CO 2 hydrogenation to hydrocarbons, as previously reported [54][55][56][57] .…”

“…The reflections in the powder diffractogram from the used catalyst phase were considerably more complex, consisting of mixtures of Fe 3 O 4 , Fe 2 O 3 , and χ-Fe 5 C 2 . Importantly, χ-Fe 5 C 2 is widely acknowledged to be the active catalytic species in the in situ hydrogenation of CO and/or CO 2 and this iron carbide phase plays a crucial role in the subsequent C-C chain growth reactions [54][55][56][57] .…”

“…The reaction scheme. Hägg carbide (χ-Fe 5 C 2 ) is widely accepted to be the active catalytic species in the FTS and this iron carbide phase plays a crucial role in the C-C chain growth reactions [54][55][56][57] . The χ-Fe 5 C 2 usually prepared by activation of hematite under syngas atmosphere at temperature of 200-450°C, the whole activation process contains a three-step reduction process of Fe 2 O 3 to iron (Fe 2 O 3 → Fe 3 O 4 → FeO → Fe) and then followed by carburisation of Fe to χ-Fe 5 C 2 77 .…”

Highly Selective Olefin Production from CO2 Hydrogenation on Iron Catalysts: A Subtle Synergy between Manganese and Sodium Additives

“…Inspired by the advanced studies of FTS,the introduction of alkaline species (e.g.Naand K) [6] and transition metals (e.g. Mn, Co,a nd Cu) [6c-e, 7] has been attempted to modify the Fe catalysts for CO 2 hydrogenation.…”

Effects of Sm on Fe–Mn catalysts for Fischer–Tropsch synthesis

“…Both factors hindered CO adsorption and inhibited iron carbide formation, nally leading to low FTS activity. 48 The FTS product selectivity of the catalysts with different Sm loadings and their detailed data are also listed in Table 3. The catalysts presented selectivity of C [2][3][4] in the HC distribution, which increased from 26.6% to 32.6% aer the addition of samarium from 0 to 1%, but it decreased to 27.7% as the Sm amount continued to increase to 2%.…”