Conversion of CO2, CO, and H2 in CO2 Hydrogenation to Fungible

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Conversion of CO, CO and H in CO hydrogenation to fungible liquid fuels on Fe-based catalysts Miron Victor Landau, Nora Meiri, Natalie Utsis, Roxana Vidruk Nehemya, and Moti Herskowitz Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01817 • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 26, 2017

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Industrial & Engineering Chemistry Research

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Conversion of CO2, CO and H2 in CO2 hydrogenation to fungible liquid fuels on Fe-based catalysts M. V. Landau, N. Meiri, N. Utsis, R. Vidruk Nehemya, M. Herskowitz Chemical Engineering Department, Blechner Center for Industrial Catalysis and Process Development, Ben-Gurion University of the Negev, Beer-Sheva, 84105, Israel Abstract CO2 hydrogenation conducted on Fe-based catalysts consists of a wide range of reactions with CO2 and H2 reacting in the reverse-water-gas-shift (RWGS) to produce CO and CO and H2 react in the Fischer-Tropsch (FT) type reactions leading to hydrocarbons and oxygenates. Methanation and Boudouard side reactions are extremely detrimental to selectivity and stability of the Febased catalysts. The catalytic system is very complex posing challenging issues that require fundamental understanding of the dynamics of changes in the catalytic phases, mechanism of key reactions and effects of catalyst composition including key promoters. A comprehensive analysis of fundamental aspects of catalytic materials, phases and promoters and the catalytic mechanisms are presented in this paper. It was established that the ratio of Fecarbide/Feoxide atoms at the surface of activated catalyst responsible for its selectivity is determined by environment of iron ions in oxide precursors changed by insertion of ions of other metals. Fungible liquid fuels were produced in bench scale reactors and demonstrated to be suitable as blending stock for transportation fuels. The techno-economic analysis of processes using CO2 and either water, biogas or natural gas as feedstock was conducted. As expected, the production of eco-friendly, renewable fuels based on CO2 is not competitive with fuels based on crude oil because of the high cost of production of hydrogen.

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I. Introduction The sustainable production of fungible liquid fuels from renewable and abundant feedstocks is one of the major global challenges of the first half of the 21st century. Another major challenge is the climate change and the related global warming created by the carbon dioxide emissions.1,2 The linear relationship between cumulative CO2 emissions and global temperature rise according to various scenarios3 has been recently re-emphasized, stressing that global net emissions need to reach zero at some point in time1,3. It means that CO2 emissions have to be reduced according to specific quota and that CO2 should be recycled. One of the main options is to react it with hydrogen to produce fungible liquid fuels through catalytic processes. CO2 can be separated from a wide variety of stationary sources: power plants, steel, cement plants and natural gas fields.4 Four main types of technologies4 have been developed to capture CO2: chemical and physical absorption, adsorption and membrane separation. Chemical and physical absorption are the most mature and widely used in various commercial applications. Some of the others have reach commercial use in specific applications. Although the cost of CO2 capture varies with the composition of the gas, the type and magnitude of application and the method of capture, in cases where CO2 content is >10%, the estimated cost5,6 is in the range of 30 – 70 $/ton CO2. Hydrogen production by water splitting has been studied extensively over the past two decades with the purpose to develop a viable commercial process7-10. Although an extraordinary research effort has been made, the only technology commercially applied is alkaline and PEM electrolysis8,9 at a very limited capacity of $6000/ton H2, depending on the electricity cost. Hydrogen production by solar energy (photo-electro-chemical and photovoltaic-electrolytic) is in a very early stage7 and the cost of hydrogen using the currently demonstrated technology is very high. The only scenario that would allow application of those technologies when ready for commercialization would be a high carbon tax, estimated7 to be >$800/ton CO2 to reach price parity with hydrogen produced from natural gas. Another source of renewable hydrogen could be bio-gas11 using steam reforming. CO2 hydrogenation on Fe-based catalysts has been studied extensively12-17. Those catalysts were selected since they contain different active sites needed to perform the two catalytic reactions in this process: CO2 and H2 produce CO by reverse water gas shift reaction (RWGS) which further


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reacts with H2 by Fischer-Tropsch synthesis (FTS) to form liquid hydrocarbons (mainly olefins and paraffins) and oxygenates. In contrast, Co catalysts, extensively used in FTS, perform poorly in CO2 hydrogenation, producing mainly methane46. Fe catalysts handle both mixtures of CO2 and H2 as well as mixtures of CO2, CO and H2.13 The active sites are a complex mixture of iron phases, mainly iron carbide and iron oxide,

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formed when Fe oxide is exposed to gaseous

mixtures containing CO and H2. The nature and catalytic performance of the different iron phases have been studied. Transient kinetic experiments coupled with XRD analysis concluded that RWGS takes place on the surface of oxide iron phases and FTS on the surface of iron carbides14,15. High RWGS activity and lack of FTS was measured on Fe-oxide phases which did not form carbides through carburization at CO2/H2 reaction conditions14. This was further supported14 by the relationship between the ratio of FTS/RWGS rates and the ratio of the relative content of iron ions at low oxidation state (carbide phases) to that of iron ions at high oxidation state (iron oxides). Another possibility18 is the formation of Fe-oxide and Fe-carbide islands on the surface of relatively large Fe-carbide and Fe-oxide nanocrystals due to oxidation/ carburization reactions converting surface atomic layers to dynamic and active surface phases at reaction conditions. This creates bifunctional nanocrystals of both Fe-carbide and Fe-oxide phases. The performance of Fe-based catalysts in the FTS was reported to be highly affected by promoters such as K, Cu, Zn, Mn and Cr. Among them, potassium was found to be the most efficient promoter enhancing the selectivity to olefins, suppressing the formation of methane, shifting the products distribution to higher molecular weight organic products and increasing the FTS and WGS rates of reaction substantially19-23. Simulations and modeling are important tools for developing the basic understanding and potential applications of the CO2 hydrogenation process.

Carbon dioxide utilization by

conversion with hydrogen into liquid fuels was simulated based on the experimental data of a novel process using CHEMCAD.

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A detailed kinetic model of the novel iron-based spinel

catalyst that included reverse water gas shift (RWGS), Fischer-Tropsch synthesis (FTS), C5+ hydrocarbons and oxygenates, oligomerization of olefins, as well as hydrogenation of light olefins (C2-C4) was employed. The RWGS reaction rate was significantly inhibited by steam produced in the process because of the chemical equilibrium limitation and apparent strong adsorption. Therefore, periodical water removal is critical in the process, which required



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operation in several reactors in series or in a reactor with recycle. Those system configurations were examined and compared over a range of temperatures, pressures, weight hourly space velocities and carbon dioxide with hydrogen feed ratios. The other aspect of this process which has a significant impact on performance is the oligomerization of light olefins. Both reactors-inseries and single reactor with recycle improved dramatically the productivity and the selectivity to C5+. A simulation of CO2 hydrogenation to liquid fuels that includes three stages: RWGS to syngas, FTS to wax and its upgrading and separation into the various liquid streams was conducted25 using Aspen Plus in the thermodynamic mode. The price of the synfuels estimated in this theoretical study supported by little experimental data was $460/barrel. A detailed simulation of a modified GTL (gas to liquid) process that combines natural gas with CO2 to produce liquid fuels and convert CO2 yielded interesting results26. The GTL consisted of a reforming and a FT reactor with suitable separators and heat exchangers. The novelty of this simulation is the integration of CO2 either in the feed to the reformer or the FT reactor. In all cases analyzed in this study, the feed to the FT reactor packed with a Fe catalyst contained CO2, CO and H2 at various ratios. Their main findings are the significant improvement of this setup compared with the original GTL process in carbon efficiency (by 20-40%), thermal efficiency (by 15-40%) and reduction in CO2 emission by about 85%. The mass and energy balances in this simulation were further extended to include a techno-economic analysis27,28. Within the assumptions of the simulations28, a 40,000 bbls/day would yield $115/bbl of product fuels. This means that only a high carbon tax will render this operation profitable under the current oil prices. A similar analysis27 was conducted for the associated natural gas (a byproduct of the oil industry normally flared) for which a much smaller plant of 2,500 bbls/day is required. At a natural gas price of $2/MMBTU, it was found that the break-even price was $47/bbl with considerable reduction in CO2 emissions. An economic analysis of CO2 hydrogenation29 indicated, as expected, that the cost of the fuels products depends on the hydrogen cost which makes it difficult to have a profitable operation unless a high carbon tax will be in place. Most studies regarding CO2 hydrogenation have covered the fundamental aspects of Fe-based catalysts with very few studies of the process and the fuels products that relate to potential industrial applications. We studied and report in this paper both the fundamentals of Fe-based


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catalysts including optimization of the Fe-catalytic materials as well as engineering aspects of production of blending stock for transportation fuels and the techno-economic analysis of CO2 conversion with hydrogen from various sources. The potential application of catalytic processes that convert CO2 to liquid fuels is specifically addressed with recommendations for further work in this area. II. Fundamentals of CO2, CO and H2 reactions on Fe-based catalysts The conversion of CO2 to liquid fuels is accomplished either by its reaction with hydrogen or by reactions in mixtures of CO2, CO and H2, depending on the source of hydrogen. A number of catalytic reactions take place on Fe-based catalysts: reverse-water-gas shift (RWGS) on Fe-oxide phases to convert CO2 and H2 to CO and H2O, Fischer-Tropsch reactions of CO and H2 to olefins, paraffins and oxygenates and methanation (M) of CO and H2 on Fe-carbide phases (Scheme 1).

14,15,22,30,31

Moreover, oligomerization of lower olefins to higher olefins and

hydrogenation of olefins to paraffins were also found to be important reactions13.

Scheme 1. Main reactions involved in CO2 hydrogenation to hydrocarbons. The interrelation of iron oxide and carbide species in catalysts obtained by partial carburization of oxide precursor is based on the dynamic nature of iron carburization – oxidation governed by thermodynamic equilibrium32. X-ray adsorption spectroscopy measured the alteration of iron oxide precursor during the CO/H2 reaction18. The results indicated that FTS is catalyzed by the dynamic and active surface phase containing nonstoichiometric FeCx species formed by conversion of surface layers in Fe3O4 phase or oxidation of iron carbide phase18. It was concluded that the catalytic behavior of these dynamic surfaces is largely independent of the carbide or oxide nature of the particle core. The unresolved issue is the capability of such



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dynamic oxycarbide phase, being the only one stable structure at the working catalyst surface, to catalyze the RWGS reaction. The FTS activity of catalytic phases in partially carburized iron oxide was studied by monitoring the CO/H2 conversion and iron state by XPS and Mössbauer spectroscopy as a function of reaction time33. XPS did not detect carbide iron on catalysts surface layers while Mössbauer spectroscopy found iron carbide as a bulk phase concluding that magnetite Fe3O4 is active in FTS. Identification of catalytic phases was based on measurements conducted with multiphase iron materials that reacted with CO, CO2 and H2O during self-organization of their phase composition at reaction conditions34. The functions of catalytic phases constituting the iron-based catalytic material were further identified employing: i) measurements of transient kinetics of CO2 and CO hydrogenation during the initial hour of catalytic run with pure iron oxide and carbide phases minimizing the effects of their carburization/oxidation on catalytic performance14; ii) testing the same phases in steady state experiments at conditions that diminished carburization/oxidation22. The results of transient kinetic measurements presented in Figure 1 were done with potassium promoted catalysts. Testing K/Ba-Fe-hexaaluminate that does not react with CO/H2 at selected conditions produced only CO, as expected since only RWGS took place (Figure 1a). CO did not appear at the beginning of run with K/Fe3O4 since it adsorbed. Subsequently, the main product was CO (Figure1b) with small amounts of hydrocarbons attributed to CO/H2 conversion according to FTS catalyzed by 4 wt% Fe5C2 formed as a result of carburization. Conducting CO2 hydrogenation at similar conditions22 with water in the feed (H2O/CO2 = 0.9) that completely depressed carburization of Fe-Al-O spinel material yielded only CO. Those results provide a strong basis to conclude that Fe-oxide phase in the Fe catalyst provides the catalytic function of RWGS.


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Figure 1. Results of the transient experiments of CO2 hydrogenation conducted with Kpromoted Fe-oxide and Fe-carbide materials (T=320oC, P=20 bar, H2/CO2 = 3, WHSVCO2=3 h-1: (a) K/Fe–Ba–hexaaluminate; (b) K/Fe3O4; (c) K/Fe5C2. (Adapted from14)

Testing K/Fe5C2 synthesized by preliminary complete carburization of magnetite precursor yielded hydrocarbons (Figure 1c). Production of CO went through maximum, attributed to oxidation of iron35 (FeCx + CO2 C + CO + FexOy) in the Fe-carbide phase by CO2 and further enhanced by water produced by FTS (FeCx + H2O C + H2 + FexOy). Testing the same catalyst in CO hydrogenation at similar conditions and inlet H2/CO ratio of 10 yielded hydrocarbons by FTS with negligible WGS activity. Therefore, the contribution of the iron carbide phase in multiphase Fe-oxide / Fe-carbide catalyst to the CO production by RWGS is at least an order of magnitude less than of Fe-oxide phases. Mechanistic aspects of RWGS/FTS/Methanation reactions: nature of active sites. The rational design of bi(multi)phase iron catalysts for CO2 and CO hydrogenation should consider the catalytic functionality of surfaces of corresponding catalytic phases as well as promoters. It



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should be based on the understanding of their chemical reactivity and the nature of catalytic active sites, determining selection of promoters. The RWGS reaction on metal oxides is based on a redox cycle36. It consists of H2 reducing Fe(+3) to Fe(+2) and CO2 oxidizing Fe(+2) with formation of CO closing the reaction cycle that may proceed via formation of surface carbonate intermediate (Figure 2). The direct evidence for this mechanism is Fe(2+) and Fe(3+) located in octahedral sites of the magnetite spinel structure that can function as a redox couple at RWGS conditions37. Isotope shift experiments reacting C18O2/H2 on Fe-oxide catalyst yielded

16

O- containing products (H216O, C16O2 and C16O18O)

using 16O atoms of only one external atomic layer of the oxide38.

Figure 2. Surface vacancies regenerative mechanism of RWGS reaction at the surface of Fe-oxide22

Oxygen from catalysts surface layers was exchanged with oxygen of CO2 during the reaction. According to CO2 thermo-desorption data, two types of adsorption sites with low and high adsorption energy were found on the surface of iron oxide. CO2 desorbed unchanged at relatively low temperature while at higher temperature CO was observed22. This was considered as evidence for strong adsorption of CO2 at oxygen vacancies with subsequent oxidation of nearby iron ions generating CO as shown in Figure 2. A correlation between the Fe(2+)/Fe(3+) ions ratio in the Fe-oxide surface layer (XPS) (indicate the content of oxygen vacancies) and the amount of strongly adsorbed CO2 was consistent with this model22. These data demonstrate that the iron -oxygen pairs at the surface of iron oxide phases are active sites for RWGS with electron hopping between ferrous and ferric cations.


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FTS is considered to proceed at the surface of iron carbide phases as a result of dissociation/ hydrogenation of surface carbonyls formed after adsorption of CO on iron atoms at near-metallic state. DFT calculations indicated that CO adsorption followed by formation of CHx species is energetically preferable on regions of surface iron atoms capable to adsorb CO forming surface carbonyls39. The accepted model assumes generation of CH or CH2 species – monomers for oligomerization to higher hydrocarbons, from CO and H2 dissociative adsorption (Figure 3)35,40,41. This mechanism includes insertion of CO to the growing chains thus explaining production of oxygenated hydrocarbons39,40,42,43

Figure 3. Representation of surface carbide mechanism of FTS reaction (adapted 35). The products formation in the FTS proceeds according to condensation polymerization mechanism following the Anderson–Schulz–Flory theory44,45. However, methane is formed mainly via a specific methanation mechanism instead of FTS46. Alkali promotion of iron catalysts increases the average chain length and the olefin content of the products, whereas the activity and methane selectivity decrease13,47,48. These points to the different nature of active sites and mechanisms for FTS and methanation. According to recent findings, there are three sources of carbon in iron carbide or carburized Febased catalysts as intermediates in hydrocarbons production from CO/H2 with different reactivity and selectivity. Application of SSTIKA and TPH methods indicated atomic C1 carbons adsorbed on surface C-vacancies or on iron atoms (Cα), surface polymerization products CsCH (Cβ) and carbon in the bulk carbide phase (Cγ), together with nonreactive graphitic carbon.49-53



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Dissociative CO adsorption assisted by hydrogen as well as deposition via the Boudouart reaction are the source of adsorbed Cs carbons.

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Carbons formed according to this route

participate in coupling reaction with other adsorbed carbons producing CsCH (Cβ) intermediates. Changing

12

CO/H2 feedstock to

13

CO/H2 with carburized Fe-catalyst yielded enhancement of

13

CH4 outlet concentration in fast and slow steps corresponding to presence of two types of

surface carbons different by as much as 25-50 times in reactivity to methanation.54 Isotopic experiments on CO hydrogenation conducted on Fe-catalyst carburized with

14

C showed

contribution of surface carbon of Fe-carbide to production of C2+ hydrocarbons of C=O species formed after CO adsorption on Cvacancies should be observed at temperatures higher than desorption of CO by decomposition of surface Fe-carbonyl species. Ways of catalysts rational design. Conversion of CO2, CO and H2 requires high active Fe-based catalysts in RWGS, FTS to C5+ and negligible activity in methanation. Enhancing the RWGS catalytic function can be accomplished by increasing the content and surface area of Fe-oxide phase and the surface concentration of Fe-O pairs with low energetic requirements to the formation of oxygen vacancies. The methanation reaction on carbide catalysts is structuresensitive. Decreasing crystal size of Fe5C2 carbide nanoparticles to 1500 h, the catalyst activated with CO demonstrated higher stability.65 At similar activation conditions, treatment of Fe/SiO2 with CO enhanced carburization by a factor of 3.68 Reduction with hydrogen converts hematite to metallic iron in the sequence: α-Fe2O3 Fe3O4 FeO α-Fe increasing carburization as α-Fe > FeO > Fe3O4 .69 The performance and surface intermediates of Fe-Cu-K-Si-O after activation at 280ºC and atmospheric pressure with three different gases: CO, H2 and syngas was compared.70 The most active catalyst was obtained after activation with H2, containing α-Fe and Fe3O4. Increasing temperature to 350oC and switching to syngas increased FTS activity.

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This effect of pretreatment temperature is related to formation

of more active non-stoichiometric Fe-carbide phase at FTS conditions after H2 reduction at the lower temperature compared with stoichiometric Fe5C2 after H2 reduction at higher temperature.72 Fe-oxide catalysts cannot be efficiently carburized with CO2 and H2 mixtures because of lack of CO. If those catalysts are first reduced with H2 at high temperature converting them to α-Fe, carburization at low CO concentrations is feasible. We performed experiments to determine effects of the activation conditions of a K-promoted Fe-oxide catalyst on its performance in CO2 hydrogenation. Experimental details are given in supplement S1.

Fe-oxide precursor was

prepared by combustion synthesis using glycolic acid as organic complexant promoted with 3 wt% potassium as described elsewhere73. The material consisted of two phases – magnetite and hematite with crystal size 15 and 30 nm, respectively, at weight ratio of 35:65 (XRD) and surface area of 110 m2/g. Characteristics of activated materials measured after 150 h on run of CO2 hydrogenation in a fixed-bed reactor (T = 300oC; P = 10 bar, H2/CO2 = 6; WHSVCO2 = 1 h-1) are shown in Table 1. Four activated K-Fe-O precursors were employed, depending on the pretreatment, as shown in Table 1. The phase composition and surface area changed significantly



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during the initial period due to self-organization of catalytic material at reaction conditions producing CO and water by RWGS. The self-organization without pre-activation yields low carburization extent while application of three activation protocols listed in Table 1 yields similar phase compositions and surface areas for the catalysts. The performance of K-Fe-O catalyst strongly depended on the activation protocol (Table 2). The catalysts activity and selectivity to C5+ hydrocarbons increased in the activation order H2 < H2/CO < (H2 H2/CO). The reason is the different morphology of those materials, their carbon content and assembling modes of Fe5C2-Fe3O4 nanoparticles. EFTEM micrographs display spatial elements distributions in corresponding catalysts (Figure 5). The untreated catalysts consisted of large crystallites of magnetite aggregated with small iron carbide particles and little carbon (Figure5a). After H2 reduction, the material consisted of Fe-carbide and magnetite nanoparticles of similar size encapsulated in carbon shells which decreased activity (Figure 5b). Carburization with syngas yielded iron carbide and magnetite nanoparticles partially embedded in carbon patches leaving their surface available for CO2/H2 (Figure 5c). Application of a hybrid activation protocol produced iron carbide nanocrystals decorated with a layer comprising small (2-3 nm, XRD) nanoparticles of magnetite (Figure 5d). Implementation of hybrid activation protocol minimizes the spatial separation of the two active phases responsible for RWGS and FTS catalytic functions thus creating close contact.


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Table 1 Surface area and phase composition of catalysts derived from K-Fe-O precursor after activation in CO2 hydrogenation (T = 300oC; P = 10 bar, H2/CO2 = 6, WHSVCO2 = 1 h-1). Catalyst pretreatment mode

No pretreatment H2 reduction at 450oC, 6 h, 1 bar H2/CO(1:1) carburization at 300oC, 3 h, 1 bar H2 reduction followed by H2/CO carburization

Surface area, m2/g 110

Catalysts characteristics After pretreatment After CO2 hydrogenation (150 h) Phase composition (XRD) Surface Phase composition area, (XRD) m2/g α-Fe α-Fe2O3 Fe3O4 FeO Fe5C2 Fe3O4 Fe5C2 -65 35 --7 75 25

--

70

--

19

11

--

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15

85

92

--

--

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--

86

14

22

78

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--

--

10

--

90

9

14

86

Table 2. Steady state performance in CO2 hydrogenation of catalysts derived from K-Fe-O precursor after activation (T = 300oC; P = 10 bar, H2/CO2 = 6; WHSVCO2 = 1 h-1).

Results of CO2 hydrogenation Catalyst pretreatment mode

Not treated H2 reduction at 450oC, 6 h, 1 bar H2/CO(1:1) carburization At 300oC, 3 h, 1 bar H2 reduction followed by H2/CO carburization

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CO

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Fe3O4

Carbon Fe3O4

Fe5C2

Fe5C2

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b

Fe3O4 carbon Fe5C2

Fe5C2

Carbon

Fe3O4

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Figure 5. EFTEM images of steady state catalyst derived from K-Fe-O precursor after activation: (a) no pretreatment; (b) H2 –reduction; (c) H2/CO carburization; (d) H2 reduction H2 reduction followed by H2/CO carburization.

Promoters strongly affect the ability of Fe-oxide phases to transformations at reduction/ carburization conditions as discussed subsequently, for K-Fe-Al-O spinel direct carburization with syngas found to be an optimal activation procedure.12 III.2. Modification of iron environment in Fe-oxide catalysts precursor as matrix for iron ions The content of iron carbide and oxide phases and their morphology determines performance and may be controlled to a certain extent by insertion of cations of other elements to the structure of Fe-oxide precursor. This could be done by isomorphous substitution of Fe(2+) or / and Fe(3+) ions in magnetite with Co(2+)74, Zn(2+)

21

, Al(3+)12,75 or other ions in the spinel structure or

synthesis of iron containing mixed oxide materials like Fe-Ba- or Fe-La-hexaaluminates14, FeCu-delafossite14,17, Fe-La-perovskite14 or K-ferrite14 with different crystalline structures. Insertion of elements with different electronegativity to the near environment of Fe-ions in a crystalline solid changes the redox properties of iron that may have a two-fold effect in catalysis:


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•

change iron carburization extent that affects FTS and methanation

•

change content of surface oxygen vacancies of iron oxide phase that affects the RWGS.

Co-Fe-O spinel phase mixed with metallic Co-Fe alloy particles prepared by disproportionation of Fe(2+) ions in basic solution of Fe-and Co-salts played a critical role in CO2 hydrogenation.74 Its main function was converting of CO2 to CO. The performance of this catalyst was not compared with pure magnetite phase. Transition metals are efficient promoters of Fe-oxide catalysts in RWGS38. Insertion of zinc or aluminum ions in the magnetite spinel structure depresses reducibility of iron shifting the peaks at TPR to higher temperatures (Figure 6). In the case of Zn, this strongly reduces the carburization of iron with syngas.21 This reduces the FTS activity of the K/Fe-Zn-O catalyst compared with K/Fe3O4 decreasing the selectivity to C5+ hydrocarbons. Substitution of 50% Fe(3+) ions in magnetite with Al(3+) optimizes the carburization activation step.12 The iron in K/Fe-Al-O spinel is not completely carburized at activation step (Figure 7a) yielding a highly porous material (Figure 7b). K/Fe2O3 is completely carburized at activation step yielding nanoparticles of Fe-oxide and carbide phases embedded in low-porous carbon produced by partial oxidation of excessive carbide phase at self-organization step at reaction conditions (Figure 5c).

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b

Figure 6. TPR profiles of magnetite recorded before and after partial substitution of iron ions for Zn(2+)21 (a) and Al(3+)12 (b).



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b

a Fe3O4 Fe5C2

Fe3O4 Carbon

Fe5C

Figure 7. TEM-EELS images of Fe-Al-O spinel catalyst after H2/CO carburization (a)12 and after reaching steady state in CO2 hydrogenation at 320oC (b)14.

The texture of low porosity catalyst derived from K/Fe3O4 that limits diffusion of hydrocarbons formed in the bulk of catalysts pellets was studied (experimental details are given in supplement S1). Reaction products create large channels shown in the SEM micrograph (Figure 8a). In contrast, the highly porous pellets of K/Fe-Al-O spinel facilitate diffusion of reaction products (Figure 8b). This is in agreement with measured texture parameters of these catalysts. Activation-carburization of K/Fe3O4 strongly reduced the surface area by a factor of 15, decreasing pore volume and diameter compared with oxide precursor (Table 3). The surface area and pore volume of K/Fe-Al-O catalyst were lower by a factor of 2 and its pore size higher in agreement with recorded N2-adsorption-desorption isotherms and pore size distributions shown in Figure 9.

- 20µm -

- 20µm -

Figure 8. SEM micrographs of steady state catalysts derived from K/Fe3O4- and K/Fe-Al-O-oxide precursors after activation / self-organization at 300oC.


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Table 3. Texture parameters of fresh oxide K/Fe3O4 and K/Fe-Al-O oxide precursors and catalysts.

Catalyst

Surface area,

Pore volume,

Average pore

m2/g

cm3/g

diameter, nm

K/FeOx fresh

110

0.49

5.5

K/FeOx steady-state

14

0.07

2.1

K/Fe-Al-O fresh

128

0.53

3.5

K/Fe-Al-O steady-state

66

0.20

6.5

120

5.0E-03

K/Fe-Al-O spinel K/Fe3O4

K/Fe-Al-O spinel

4.5E-03

K/Fe3O4 100

4.0E-03 3.5E-03

Desorption Dv(d) (cc/Å/g)

80

Volume (cc/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

3.0E-03 2.5E-03 2.0E-03 1.5E-03 1.0E-03 5.0E-04

0

0.0E+00 0

0.2

0.4

0.6

Relative pressure (P/P0)

0.8

1

0

5

10

15

20

Pore diameter (nm)

a

b

Figure 9. N2-adsorption-desorption isotherms recorded with K/Fe3O4 and K/Fe-Al-O (a) and their pore size distributioins (b).

Changing the environment of iron ions in oxide precursors using Fe-containing mixed oxide materials with different composition and crystal structure regulates the relative content of iron carbide and oxide14. This changes the ratio of the oxide /carbide iron ions at the catalysts surface that controls RWGS and FTS. Carburizing and stabilization of phase composition in K-promoted catalysts based on magnetite, Fe-Al-O spinel, Fe-Ba- or Fe-La-hexaaluminates, Fe-Cudelafossite, Fe-La-perovskite and potassium ferrite yielded different selectivity to CO and productivity of C5+ hydrocarbons (Figure 10). In this catalysts sequence the ratio (RFTS+M /



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RRWGS) increased from 0.15 to 0.8014. These results correlated with the increased ratio of the iron ions of oxide Fe(2,3+) and carbide Fe(δ+) phases in the catalysts surface layers Fecarb./Feoxide (XPS) (Figure 11).

Figure 10. RWGS(RRWGS), FTS(RFTS), methanation (RM) rates and hydrocarbons productivity (PC5+) for catalysts derived from K-promoted Fe-oxide precursors (320oC, 20 bar, WHSWCO2 = 3 h-1, H2/CO2 =3).14

Changing the environment of iron ions in oxide precursor controls the iron carburization extent enhancing FTS relative to RWGS. This yields higher conversion of CO intermediate increasing the hydrocarbons productivity. Moreover, it also affects the selectivity of iron carbide phase in FTS. It was demonstrated that pre-reduction of CuFe-delafossite in hydrogen at 400oC followed by its partial carburization at CO2 hydrogenation conditions yielded so-called “CO-free” C5+ selectivity of 60-66%17. This type of selectivity does not take into account the CO selectivity in the product that exceeded 30% of the total CO2 conversion in agreement with the results obtained for CuFe-delafossite material in14. But in spite of the misbalancing of RWGS / FTS, this result demonstrates a significant shift of hydrocarbon products distribution from C2-C4 to C5+.


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This is opposite to the shift of hydrocarbon products distribution to C2-C4 observed after insertion of Zn cations to the magnetite spinel structure depressing iron carburization.21 The authors explained it to be a result of increasing iron carburization promoted by copper in delafossite phase.17 Promoters are normally added to Fe-oxide in the range of 1-20 wt%. Most important promoters for Fe-based catalysts are alkaline metals, silica, alumina and transition metal oxides.

1.0 0.9 0.8 0.7

(RFTS+RM)/RRWGS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

0.1

0.2

0.3

0.4

0.5

Surface ratio Fecarb / Feoxide (XPS)

Figure 11. Correlation of R(FTS+M) / RRWGS and relative abundance of Fe-oxide and Fe-carbide atoms in the catalysts surface layer.14

III.3. Control of catalysts functions by addition of promoters III.3.1. Alkaline promoters: effect of potassium. Alkaline additives affect the performance of Fe-based catalysts reducing methane selectivity, increasing hydrocarbons chain growth probability and alkenes/alkanes ratio of products.47,76,77 The surface basicity of alkaline- and alkaline-earth-promoted

Fe

catalysts

depresses

methane

selectivity

in

the

order

Ba

Conversion of CO2, CO, and H2 in CO2 Hydrogenation to Fungible

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