Stability and Reactivity of ϵ−χ−θ Iron Carbide Catalyst Phases in Fischer−Tropsch Synthesis: Controlling μC

Reverse Water-Gas Shift Iron Catalyst Derived from Magnetite

The catalytic properties of unsupported iron oxides, specifically magnetite (Fe3O4), were investigated for the reverse water-gas shift (RWGS) reaction at temperatures between 723 K and 773 K and atmospheric pressure. This catalyst exhibited a fast catalytic CO formation rate (35.1 mmol h−1 gcat.−1), high turnover frequency (0.180 s−1), high CO selectivity (> 99%), and high stability (753 K, 45000 cm3h−1gcat.−1) under a 1:1 H2 to CO2 ratio. Reaction rates over the Fe3O4 catalyst displayed a strong dependence on H2 partial pressure (reaction order of ~0.8) and a weaker dependence on CO2 partial pressure (reaction order of 0.33) under an equimolar flow of both reactants. X-ray powder diffraction patterns and XPS spectra reveal that the bulk composition and structure of the post-reaction catalyst was formed mostly of metallic Fe and Fe3C, while the surface contained Fe2+, Fe3+, metallic Fe and Fe3C. Catalyst tests on pure Fe3C (iron carbide) suggest that Fe3C is not an effective catalyst for this reaction at the conditions investigated. Gas-switching experiments (CO2 or H2) indicated that a redox mechanism is the predominant reaction pathway.

A Critical Look at Direct Catalytic Hydrogenation of Carbon Dioxide to Olefins

One of the main initiatives for fighting climate change is to use carbon dioxide as a resource instead of waste. In this respect, thermocatalytic carbon dioxide hydrogenation to high-added-value chemicals is a promising process. Among the products of this reaction (alcohols, alkanes, olefins, or aromatics), light olefins are interesting because they are building blocks for making polymers, as well as other important chemicals. Olefins are mainly produced from fossil fuel sources, but the increasing demand of plastics boosts the need to develop more sustainable synthetic routes. This review gives a critical overview of the most recent achievements in direct carbon dioxide hydrogenation to light olefins, which can take place through two competitive routes: the modified Fischer-Tropsch synthesis and methanol-mediated synthesis. Both routes are compared in terms of catalyst development, reaction performance, and reaction mechanisms. Furthermore, practical aspects of the commercialization of this reaction, such as renewable hydrogen production and carbon dioxide capture, compression, and transport, are discussed. It is concluded that, to date, the catalysts used in the carbon dioxide hydrogenation reaction give a wide product distribution, which reduces the specific selectivity to lower olefins. More efforts are needed to reach better control of the C/H surface ratio and interactions within the functionalities of the catalyst, as well as understanding the reaction mechanism and avoiding deactivation. Renewable H₂ production and carbon dioxide capture and transport technologies are being developed, although they are currently still too expensive for industrial application.

Enhancement of Light Olefins Selectivity Over N-Doped Fischer-Tropsch Synthesis Catalyst Supported on Activated Carbon Pretreated with KMnO4

Ammonium iron citrate was used as an iron precursor in order to prepare N-doped catalysts supported on KMnO4 pretreated activated carbon (10MnK-AC). Iron nitride was synthesized in company with the formation of α-Fe2O3 on 10MnK-AC. The characterizations of the catalysts show that nitrogen atoms were doped into iron lattice rather than the networks of the carbon support. The performance of Fischer-Tropsch synthesis to light olefins (FTO) suggest an improvement in O/P ratio (olefins to paraffins molar ratio of C2–C4) over the iron catalysts supported on 10MnK-AC. The further promotion of light olefins selectivity (up to 44.7%) was obtained over FeN-10MnK-AC catalyst owing to the collaborative contribution of the electron donor effect of nitrogen and the suppression effect on the second hydrogenation over 10MnK-AC support.

Stabilizing the active phase of iron-based Fischer-Tropsch catalysts for lower olefins: mechanism and strategy

An iron carbonyl-mediated Ostwald-ripening-like growth mechanism of an Fe_xC_y active phase in Fischer–Tropsch synthesis is firstly revealed by in situ mass-spectrometric and theoretical analysis.

Fischer–Tropsch synthesis of lower olefins (FTO) is a classical yet modern topic of great significance in which the supported Fe-based nanoparticles are the most promising catalysts. The performance deterioration of catalysts is a big challenge due to the instability of the nanosized active phase of iron carbides. Herein, by in situ mass spectrometry, theoretical analysis, and atmospheric- and high-pressure experimental examinations, we revealed the Ostwald-ripening-like growth mechanism of the active phase of iron carbides in FTO, which involves the cyclic formation–decomposition of iron carbonyl intermediates to transport iron species from small particles to large ones. Accordingly, by suppressing the formation of iron carbonyl species with a high-N-content carbon support, the size and structure of the active phase were regulated and stabilized, and durable iron-based catalysts were conveniently obtained with the highest selectivity for lower olefins up to 54.1%. This study provides a practical strategy for exploring advanced FTO catalysts.

Preparation of Iron Carbides Formed by Iron Oxalate Carburization for Fischer–Tropsch Synthesis

Different iron carbides were synthesized from the iron oxalate precursor by varying the CO carburization temperature between 320 and 450 °C. These iron carbides were applied to the high-temperature Fischer–Tropsch synthesis (FTS) without in situ activation treatment directly. The iron oxalate as a precursor was prepared using a solid-state reaction treatment at room temperature. Pure Fe5C2 was formed at a carburization temperature of 320 C, whereas pure Fe3C was formed at 450 °C. Interestingly, at intermediate carburization temperatures (350–375 °C), these two phases coexisted at the same time although in different proportions, and 360 °C was the transition temperature at which the iron carbide phase transformed from the Fe5C2 phase to the Fe3C phase. The results showed that CO conversions and products selectivity were affected by both the iron carbide phases and the surface carbon layer. CO conversion was higher (75–96%) when Fe5C2 was the dominant iron carbide. The selectivity to C5+ products was higher when Fe3C was alone, while the light olefins selectivity was higher when the two components (Fe5C2 and Fe3C phases) co-existed, but the quantity of Fe3C was small.

Examining the temporal behavior of the hydrocarbonaceous overlayer on an iron based Fischer-Tropsch catalyst

In order to examine fundamental processes connected with the use of an unpromoted iron based Fischer-Tropsch synthesis (FTS) catalyst, model studies examining the temporal formation of hydrocarbonaceous species that form over the catalyst are undertaken using a combination of temperature-programmed oxidation, powder X-ray diffraction, Raman scattering, transmission electron microscopy and inelastic neutron scattering (INS). Catalyst samples were exposed to ambient pressure CO hydrogenation at 623 K for defined periods of time-on-stream (3, 6, 12 and 24 h) prior to analysis. INS reveals a progressive retention of hydrogenous species that is associated with the evolution of a hydrocarbonaceous overlayer, as evidenced by the presence of sp2 and sp3 hybridized C-H vibrational modes. Correlations between the formation of aliphatic and olefinic/aromatic moieties with post-reaction characterization leads to the proposal of a number of chemical transformations that, collectively, define the conditioning phase of the catalyst under the specified set of reaction conditions. A comparison between the inelastic neutron scattering spectra of the 24 h sample with that of an iron catalyst extracted from a commercial grade Fischer-Tropsch reactor validates the relevance of the experimental approach adopted.

Promoted cobalt metal catalysts suitable for the production of lower olefins from natural gas

Due to the surge of natural gas production, feedstocks for chemicals shift towards lighter hydrocarbons, particularly methane. The success of a Gas-to-Chemicals process via synthesis gas (CO and H2) depends on the ability of catalysts to suppress methane and carbon dioxide formation. We designed a Co/Mn/Na/S catalyst, which gives rise to negligible Water-Gas-Shift activity and a hydrocarbon product spectrum deviating from the Anderson-Schulz-Flory distribution. At 240 °C and 1 bar, it shows a C2-C4 olefins selectivity of 54%. At 10 bar, it displays 30% and 59% selectivities towards lower olefins and fuels, respectively. The spent catalyst consists of 10 nm Co nanoparticles with hcp Co metal phase. We propose a synergistic effect of Na plus S, which act as electronic promoters on the Co surface, thus improving selectivities towards lower olefins and fuels while largely reducing methane and carbon dioxide formation.

Structural evolution of carbon in an Fe@C catalyst during the Fischer–Tropsch synthesis reaction

A pseudo-in situ research method was applied to provide insight into the structural evolution of carbon in an Fe@C catalyst at different stages of the Fischer–Tropsch reaction.

In situ observation of phase changes of a silica-supported cobalt catalyst for the Fischer-Tropsch process by the development of a synchrotron-compatible in situ/operando powder X-ray diffraction cell

In situ characterization of catalysts gives direct insight into the working state of the material. Here, the design and performance characteristics of a universal in situ synchrotron-compatible X-ray diffraction cell capable of operation at high temperature and high pressure, 1373 K, and 35 bar, respectively, are reported. Its performance is demonstrated by characterizing a cobalt-based catalyst used in a prototypical high-pressure catalytic reaction, the Fischer-Tropsch synthesis, using X-ray diffraction. Cobalt nanoparticles supported on silica were studied in situ during Fischer-Tropsch catalysis using syngas, H₂ and CO, at 723 K and 20 bar. Post reaction, the Co nanoparticles were carburized at elevated pressure, demonstrating an increased rate of carburization compared with atmospheric studies.

Synthesis of stable and low-CO2 selective ε-iron carbide Fischer-Tropsch catalysts

The Fe-catalyzed Fischer-Tropsch (FT) reaction constitutes the core of the coal-to-liquids (CTL) process, which converts coal into liquid fuels. Conventional Fe-based catalysts typically convert 30% of the CO feed to CO₂ in the FT unit. Decreasing the CO₂ release in the FT step will reduce costs and enhance productivity of the overall process. In this context, we synthesize phase-pure ε(')-Fe₂C catalysts exhibiting low CO₂ selectivity by carefully controlling the pretreatment and carburization conditions. Kinetic data reveal that liquid fuels can be obtained free from primary CO₂. These catalysts displayed stable FT performance at 23 bar and 235°C for at least 150 hours. Notably, in situ characterization emphasizes the high durability of pure ε(')-Fe₂C in an industrial pilot test. These findings contribute to the development of new Fe-based FT catalysts for next-generation CTL processes.

Porous Graphene-Confined Fe-K as Highly Efficient Catalyst for CO2 Direct Hydrogenation to Light Olefins

We devised iron-based catalysts with honeycomb-structured graphene (HSG) as the support and potassium as the promoter for CO₂ direct hydrogenation to light olefins (CO₂-FTO). Over the optimal FeK1.5/HSG catalyst, the iron time yield of light olefins amounted to 73 μmol_CO2 g_Fe^-1 s^-1 with high selectivity of 59%. No obvious deactivation occurred within 120 h on stream. The excellent catalytic performance is attributed to the confinement effect of the porous HSG on the sintering of the active sites and the promotion effect of potassium on the activation of inert CO₂ and the formation of iron carbide active for CO₂-FTO.

Probing the Evolution of Palladium Species in Pd@MOF Catalysts during the Heck Coupling Reaction: An Operando X-ray Absorption Spectroscopy Study

The mechanism of the Heck C-C coupling reaction catalyzed by Pd@MOFs has been investigated using operando X-ray absorption spectroscopy (XAS) and powder X-ray diffraction (PXRD) combined with transmission electron microscopy (TEM) analysis and nuclear magnetic resonance (¹H NMR) kinetic studies. A custom-made reaction cell was used, allowing operando PXRD and XAS data collection using high-energy synchrotron radiation. By analyzing the XAS data in combination with ex situ studies, the evolution of the palladium species is followed from the as-synthesized to its deactivated form. An adaptive reaction mechanism is proposed. Mononuclear Pd(II) complexes are found to be the dominant active species at the beginning of the reaction, which then gradually transform into Pd nanoclusters with 13-20 Pd atoms on average in later catalytic turnovers. Consumption of available reagent and substrate leads to coordination of Cl^- ions to their surfaces, which causes the poisoning of the active sites. By understanding the deactivation process, it was possible to tune the reaction conditions and prolong the lifetime of the catalyst.

DFT insight into the effect of potassium on the adsorption, activation and dissociation of CO2 over Fe-based catalysts

Catalytic conversion of CO2 including hydrogenation has attracted great attention as a method for chemical fixation of CO2 in combination with other techniques such as CO2 capture and storage. Potassium is a well-known promotor for many industrial catalytic processes such as in Fischer-Tropsch synthesis. In this work, we performed density functional theory (DFT) calculations to investigate the effect of potassium on the adsorption, activation, and dissociation of CO2 over Fe(100), Fe5C2(510) and Fe3O4(111) surfaces. The function of K was analyzed in terms of electronic interactions between co-adsorbed CO2 and K-surfaces which showed conspicuous promotion in the presence of K of the adsorption and activation of CO2. The adsorption strength of CO2 on these surfaces ranks as oct2-Fe3O4(111) > Fe(100) > Fe5C2(510). Generally, we observed a direct proportional correlation between the adsorption strength and the charges on the adsorbates. Adding K on the catalyst surface also reduces the kinetic barrier for CO2 dissociation. CO2 dissociation is more facile to occur on Fe(100) and Fe5C2(510) in the presence of K whereas the Fe3O4(111) surfaces impede CO2 dissociation regardless of the existence of K. Instead, a stable CO3- species is formed upon CO2 adsorption on Fe3O4(111) which will be directly hydrogenated when sufficient H* are available on the surface. Our results highlight the origin of the promotion effect of potassium and provide insight for the future design of K-promoted Fe-based catalysts for CO2 hydrogenation.

Quantum-Chemical DFT Study of Direct and H- and C-Assisted CO Dissociation on the χ-Fe5C2 Hägg Carbide

The first step in the Fischer-Tropsch reaction is the production of C1 monomers by the dissociation of the C-O bond. Although Fe is the active metal, it is well known that under typical reaction conditions, it changes into various carbide phases. The Hägg carbide (χ-Fe5C2) phase is usually considered as the catalytically active phase. We carried out a comprehensive DFT study of CO dissociation on various surface terminations of the Hägg carbide, selected on their specific site topology and the presence of stepped sites. Based on the reaction energetics, we identified several feasible CO dissociation pathways over the Hägg carbide. In this study, we have compared the direct CO dissociation with H- and C-assisted CO dissociation mechanisms. We demonstrated that the reaction rate for CO dissociation critically depends on the presence and topology of interstitial C atoms close to the active site. Typically, the CO dissociation proceeds via a direct C-O bond scission mechanism on the stepped sites on the Fe carbide surface. We have shown a preference for the direct CO dissociation on the surfaces with a stepped character. The H-assisted CO dissociation, via a CHO intermediate, was preferred when the surface did not have a clear stepped character. We have also shown that activation barriers for dissociation are highly dependent on the surface termination. With a consistent data set and including migration corrections, we then compared the CO dissociation rates based on a simplified kinetic model. With this model, we showed that besides the activation energy, the adsorption energy of the CO, the C and the O species are important for the reaction rate as well. We found that the most active surface termination is a (111̅) surface cut in such a way that the surface exposes B5 sites that are not occupied by the C atoms. On these B5 sites, the direct CO dissociation presents the highest rate.

A Linear Scaling Relation for CO Oxidation on CeO2-Supported Pd

Resolving the structure and composition of supported nanoparticles under reaction conditions remains a challenge in heterogeneous catalysis. Advanced configurational sampling methods at the density functional theory level are used to identify stable structures of a Pd8 cluster on ceria (CeO2) in the absence and presence of O2. A Monte Carlo method in the Gibbs ensemble predicts Pd-oxide particles to be stable on CeO2 during CO oxidation. Computed potential energy diagrams for CO oxidation reaction cycles are used as input for microkinetics simulations. Pd-oxide exhibits a much higher CO oxidation activity than metallic Pd on CeO2. This work presents for the first time a scaling relation for a CeO2-supported metal nanoparticle catalyst in CO oxidation: a higher oxidation degree of the Pd cluster weakens CO binding and facilitates the rate-determining CO oxidation step with a ceria O atom. Our approach provides a new strategy to model supported nanoparticle catalysts.

A short review of recent advances in CO2 hydrogenation to hydrocarbons over heterogeneous catalysts

CO2 hydrogenation to hydrocarbons is a promising way of making waste to wealth and energy storage, which also solves the environmental and energy issues caused by CO2 emissions. Much efforts and research are aimed at the conversion of CO2 via hydrogenation to various value-added hydrocarbons, such as CH4, lower olefins, gasoline, or long-chain hydrocarbons catalyzed by different catalysts with various mechanisms. This review provides an overview of advances in CO2 hydrogenation to hydrocarbons that have been achieved recently in terms of catalyst design, catalytic performance and reaction mechanism from both experiments and density functional theory calculations. In addition, the factors influencing the performance of catalysts and the first C-C coupling mechanism through different routes are also revealed. The fundamental factor for product selectivity is the surface H/C ratio adjusted by active metals, supports and promoters. Furthermore, the technical and application challenges of CO2 conversion into useful fuels/chemicals are also summarized. To meet these challenges, future research directions are proposed in this review.