Selective CO2 Hydrogenation to Hydrocarbons on Cu-promoted Fe

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Selective CO2 Hydrogenation to Hydrocarbons on Cupromoted Fe-based Catalysts: Dependence on Cu-Fe Interaction Junhui Liu, Anfeng Zhang, Xiao Jiang, Min Liu, Yanwei Sun, Chunshan Song, and Xinwen Guo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01491 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018

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ACS Sustainable Chemistry & Engineering

Selective CO2 Hydrogenation to Hydrocarbons on Cu-promoted Fe-based Catalysts: Dependence on Cu-Fe Interaction Junhui Liua, Anfeng Zhanga, Xiao Jiangb, Min Liua, Yanwei Suna, Chunshan Songac*, Xinwen Guoa* a

State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, P. R. China.

b

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA.

c

EMS Energy Institute, PSU-DUT Joint Center for Energy Research and Department of Energy & Mineral Engineering and Chemical Engineering, The Pennsylvania State University, University Park, PA, 16802, USA.

Corresponding authors, E-mail: [email protected] (X.W. Guo); [email protected] (C.S. Song)

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ABSTRACT Conversion of CO2 to sustainable chemical feedstocks and fuels by reacting with renewable hydrogen is considered to be a promising direction in energy research. The selectivity of desired products, such as C2-C4= and C5+, is low over un-promoted iron-based catalysts for CO2 hydrogenation. Therefore, promoters are often used to tailor and optimize the product distribution. In this work, the effect of doping Cu into Fe-based supported catalysts on the catalytic performance for CO2 hydrogenation to hydrocarbons was studied with a particularly focus on the interaction between Fe and Cu. For this purpose, catalysts with different Fe and Cu distribution were prepared by various impregnation methods. It was found that the selectivity of C2-C4= over Cu-promoted catalysts decreased, but a significant improvement was obtained for C5+. This promoting behavior is different from that of other promoters (e.g., K, Mn, and Zn, etc.). The secondary conversion of produced olefins on Cu-promoted catalysts, which results from the improvement of olefins adsorption, on Cu-promoted catalysts leads to the decrease of C2-C4= (hydrogenation), but the increase of C5+ (oligomerization). Characterization results demonstrate that the catalytic performance is evidently associated with the strength of the interaction between Fe and Cu in the supported catalysts. KEYWORDS: CO2 hydrogenation, Promoters, Cu-Fe interaction, Hydrocarbons distribution, Preparation methods


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INTRODUCTION

Anthropogenic CO2 emissions have increased from approximately 31.9 to 35.5 Gt per year in 2010-2014.1 The continued growth in CO2 emissions has given rise to climate change, which attracted considerable attention of researchers.2-3 Carbon capture and utilization (CCU) is considered as an effective approach to mitigate the CO2 emission and alleviate energy crisis.4-8 Researchers are currently focusing on the reduction of CO2 into fuels and chemicals. The products, such as CO, methanol, methane, higher alcohols, and hydrocarbons can be obtained by hydrogenation of CO2 over varied catalysts.9-16 C2+ hydrocarbons have a wider market (fuels) or a higher added value (chemicals), though the CO2 molecule is stable and the C-C coupling is difficult.17-18 The reports on CO2 hydrogenation to hydrocarbons have concentrated on active phases, supports, and promoters. CO2 hydrogenation is considered as the modified Fischer-Tropsch synthesis (FTS). Cobalt and iron are widely used to produce fuels and chemicals from syngas.19-21 However, cobalt tends to selectively produce methane during the CO2 hydrogenation reaction.22 CO2 hydrogenation over iron-based catalysts proceeds two-step process, with reduction of CO2 to CO via the reverse water gas shift (RWGS) reaction followed by the conversion of CO to hydrocarbons via FTS.23-24 Moreover, magnetite is active for RWGS reaction, while iron carbide is highly catalytically active for FTS.24 Metal oxides, such as Al2O3 and ZrO2, have been widely used as supports because the strong metal-support interaction between iron and metal support can prevent particle agglomeration during the catalyst



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preparation.25-26 Carbon materials, such as carbon nano-fibers (CNF) and carbon nano-tubes (CNT) are generally used as supports, such that the active iron phase was easy to reduce and carbonize during reaction.27-28 To optimize the product distribution and increase the activity and stability of catalysts, promoters usually need to be added into catalysts to increase the yield of desired products. Alkali metals favor for CO2 hydrogenation. Addition of potassium or sodium can enhance the adsorption of CO2 and accelerate the formation of χ-Fe5C2 during reaction condition. CO2 conversion increased and olefin production as well as that of longer chain hydrocarbons was enhanced over K-modified iron catalysts.26,

29

Manganese is not only a structural promoter but is also an electronic promoter. Mn can promote catalyst reduction, dispersion and carburization, improving the selectivity of olefins.30-31 Cerium is an active low-temperature RWGS catalyst.32 Ce can shorten the initial induction period of catalysts, but it has little effect on products in CO2 conversion.33 Zn, Zr, and Mg can also reduce the methane formation and increase catalytic activity.33-35 Cu is also a widely studied promoter in FTS and CO2 hydrogenation. The synergistic properties of iron and copper have been extensively studied and bimetallic Fe-Cu catalysts were used in many fields.36-38 Cu can improve the catalytic activity of iron-based catalysts in FTS by favoring the reduction and carburization of iron.39-40 Eric van Steen’s group used the delafossite as a model compound in FTS. They deemed that copper facilitated the conversion of magnetite to α-Fe in hydrogen, but not its conversion to predominantly χ-Fe5C2 in CO.41-42 Song et al. found addition


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of transition metals (M = Co, Ni, Cu, Pd) led to the promotion of C2+ hydrocarbons formation for CO2 hydrogenation.43 Most studies focused on the effects of Cu addition on

activity,

stability,

and

long-chain

hydrogenations

selectivity

in

CO2

hydrogenation.43-45 However, little attention was paid to the selectivity of C2-C4= and the reason of the change. In the present work, the effect of Cu on the product distribution over Fe supported catalysts is found to be different from that obtained using the other usual promoters. The selectivity of C2-C4= selectivity decreased while the C5+ selectivity increased obviously. The increase of CO2 conversion and C5+ selectivity was attributed to the strong interaction between Cu and Fe which facilitated the reduction of Fe and enhanced the CO2 adsorption of catalysts. C3H6-TPD demonstrated that the adsorption of primarily formed olefins increased on Cu-promoted catalysts. Hydrogenation of olefins increased the selectivity of paraffins, and the oligomerization of olefins increased the selectivity of C5+. The interaction of Fe and Cu was controlled by changing the distribution of the two metals. The interaction strength of Cu and Fe had an influence on catalytic performance.

EXPERIMENTAL SECTION

Catalyst preparation

10Fe/Al2O3 catalyst was prepared by incipient wetness impregnation method using alkaline Al2O3 (Aladdin Chemicals), which has more surface basic hydroxyl and is in favor of acidic CO2 adsorption, as the support and aqueous solution of



Fe(NO3)3·9H2O as the iron precursor. The impregnated sample was dried at 100 ℃ over night and calcined in air at 540 °C for 4 h and the heating rate was 2 °C/min. The Fe loading of all catalysts is fixed at 10 wt% throughout this work. The co-impregnation method was applied to prepare the promoted catalysts using Fe(NO3)3·9H2O, M(NO3)a·bH2O (M= Cu, Zn, Mn, Zr, Mo, and Cr) and/or KNO3 aqueous solutions by following the same procedure as described above. The prepared catalysts are denoted as 10FexMyK/Al2O3, where x and y represent the mass fraction of the M and K on the basis of support weight, respectively. For comparison, the promoted catalysts were also prepared by the sequential impregnation method. In the case of S-10Fe1K3Cu/Al2O3, Fe/K was impregnated first and calcined, followed by Cu impregnation. On the other hand, S-3Cu10Fe1K/Al2O3 was prepared by a reverse procedure. For clarity, those sequentially prepared catalysts were marked with an “S” in front. All the actual metal loadings of prepared catalysts as detected by ICP, approach the theoretical loading values.

Catalytic tests

The catalytic hydrogenation of CO2 reaction was conducted in a pressurized fixed-bed flow reactor with 8 mm inner diameter at 3 MPa. 1 g catalyst was used for each test and was reduced in pure H2 (45 mL min-1) at 400 °C for 8 h prior to reaction. After reaching the reaction temperature, the feed gas was switched to the mixture of CO2 and H2 under the reaction conditions of n(H2)/n(CO2)= 3 (molar ratio) and space velocity= 3600 ml g-1 h-1.


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The products were analyzed on-line using a gas chromatograph (FULI GC 97). CO, CO2, and CH4 were analyzed on a carbon molecular sieve column with TCD, while CH4 and C2-C8 hydrocarbons (C2+) were analyzed by FID with a HayeSep Q column. Chromatograms of FID and TCD were correlated through CH4, and product selectivity was obtained based on carbon balance, C-mol%.

Characterization

X-ray diffraction (XRD) patterns were recorded with a Rigaku SmartLab(9) diffractometer using a Cu Kα radiation (λ= 1.5406 Å) source at the step size of 0.02°over the range between 5° and 80° range. Scanning electron microscopy (SEM) images and energy dispersive spectrometer (EDS) maps were collected using a field emission scanning electron microscopy (NOVA NanoSEM 450). High-resolution transmission electron microscopy (HR-TEM) images were taken using a Tecnai G2 20 S-twin instrument (FEI Company) with the acceleration voltage of 200 kV. The samples were ultrasonicated in ethanol, and a few droplets of ethanol suspension were dropped onto a copper grid, followed by drying at ambient temperature. N2 adsorption-desorption isotherms were obtained at -196 °C on a Quantachromeautosorb analyzer. The power was degassed in vacuum at 300 °C for 10 h prior to the measurement. X-ray photoelectron spectroscopy (XPS) measurement was measured using a Thermo Scientific ESCA Lab250 spectrometer consisting with monochromatic Al Kα



as the X-ray source. All of the binding energies were calibrated using the C 1s peak at 284.6 eV. The reducibility of the catalysts was analyzed by H2 temperature-programmed reduction (TPR) with a ChemBETPulsar TPR/TPD instrument (Quantachrome, USA). Typically, ~0.1 g sample was charged into the quartz tube and pretreated in He at 400 °C for 1 h to remove physically adsorbed water, followed by cooling down to 50 °C. The TPR program was initiated by switching to 5 vol % H2/Ar with a total flow rate of 30 ml min−1 and heating up to 850 °C at 10 °C min−1. CO2 temperature-programmed desorption (CO2-TPD) measurements were conducted using the same equipment as TPR. A sample with the weight of approximately 100 mg was in situ reduced in 5 vol% H2/Ar (ca. 30 ml min-1) at 400 °C for 2 h. The catalyst was subsequently flushed with He (ca. 30 ml min-1) for 30 min at the same temperature. After cooling to 50 °C, the sample was exposed to pure CO2 (30 mL·min-1) for 1 h and then flushed with He flow (30 mL·min-1) for 1 h to remove all physically adsorbed molecules. The TPD program was initiated by heating up to 700 °C at a rate of 10 °C min-1. C3H6 temperature-programmed desorption (C3H6-TPD) was carried out for the spent catalysts. The catalyst was first pretreated at 400 °C in He for 3 h. After cooling to 50°C, the sample was exposed to pure C3H6 (30 mL·min-1) for 1 h and then flushed with He flow (30 mL·min-1) for 1 h. The TPD program was initiated by heating up to 400 °C with a ramp of 10 °C min-1. Elemental analysis was measured by Inductively Coupled Plasma-optical


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emission spectroscopy (ICP-OES) over a Perkin Elmer OPTIMA 2000DV apparatus.

RESULTS AND DISCUSSION

Screening tests of various metal-promoted Fe catalysts

Fig. 1 shows the XRD patterns of the catalysts with different promoters. The diffraction peaks at 2θ values of 24.14°, 33.15°, 35.61°, 40.85°, 49.48°, 54.09°, 62.45°, and 64.00° can be assigned to hematite (JCPDS no. 33-0664). The crystalline phase of Fe2O3 in 10Fe1K/Al2O3 is detected according to the XRD patterns, whereas the diffraction peaks of hematite in 10Fe3Zn1K/Al2O3 and 10Fe3Cu1K/Al2O3 become weak, and only Al2O3 phases are presented for the rest of promoted catalysts. Raman spectra showed that the intensity of iron oxide decreased obviously, and CuFe2O4 was formed in Cu-promoted catalyst (Fig. S1).46 The disappearance of the hematite phases may originate from the strong interaction between Fe and the promoters and/or well-dispersed metal nanoparticles.



Fig. 1 XRD patterns of catalysts with different promoters. SEM images and EDS maps of metal-promoted catalysts are depicted in Fig. 2 and Fig. S2. The Al2O3 support is agglomerated bulk composed of small nanoparticles, as well as numerous intracrystalline pores (Figs. 2a-b). SEM/EDS characterization demonstrates that Fe and promoted metals disperse on the supports well, which is in accordance with the weak hematite peaks in the XRD patterns. H2-TPR was used to observe the reduction behavior of the Fe-based catalysts. As shown in Fig. 3, the two H2 consumption peaks of unpromoted 10Fe1K/Al2O3, centered at 479 and 768 °C, are attributed to step reduction, namely, from Fe2O3 to Fe3O4 and finally to α-Fe.47 Interestingly, the addition of Mo shifts the Fe reduction peaks to higher temperatures, while the addition of other promoters results in a shift towards lower temperatures. Among all, the low temperature-shift is more significant for Cu-promoted catalyst, wherein the reduction temperature of iron decreased to 339 °C and 563 °C. Meanwhile, another reduction peak emerges at 200-300 °C, corresponding to the reduction of Cu oxide.40, 48


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Fig. 2 (a,b) SEM images of 10Fe3Mo1K/Al2O3 and 10Fe3Cu1K/Al2O3, and (c-f and g-j) HAADF images and EDS maps of 10Fe3Mo1K/Al2O3 and 10Fe3Cu1K/Al2O3, respectively.

Fig. 3 H2-TPR profiles of catalysts with different promoters. The catalytic performance of these promoted catalysts was investigated, and results are tabulated in Table 1. CO2 conversion over unpromoted 10Fe1K/Al2O3 is 34.6 %, and the selectivity of C2-C4= is 6.7 C-mol%; the C2+ selectivity is 25.7 C-mol% of the products, and the major proportion consists of CO and CH4. On the other hand, the addition of second metal results in a slight increase of CO2 conversion, among



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which that of Mn- and Cu-promoted catalysts exceeds 40 %, in agreement with the fact that Mn is known to be a favorable promoter in CO2 hydrogenation.49 Compared with 10Fe1K/Al2O3, the selectivity of C2-C4= and C5+ remains almost unchanged over 10Fe3Cr1K/Al2O3. For other promoted catalysts, namely Mn-, Mo-, Zn-, and Zr-promoted ones, the selectivity of C2-C4= increases in comparison to the unpromoted one, and a simultaneous increase of C5+ selectivity was observed. However, 10Fe3Cu1K/Al2O3 exhibits a significant enhancement of C5+ selectivity (13.8 C-mol%), but with a drastic drop of C2-C4= selectivity in comparison to the unpromoted catalyst. To interpret such opposite trend, the effect of Cu content on activity and product distribution was examined, along with the evolution of product distribution with temperatures. Table 1 CO2 hydrogenation performance of metal promoted-catalystsa Catalyst 10Fe1K/Al2O3 10Fe3Mn1K/Al2O3 10Fe3Mo1K/Al2O3 10Fe3Zn1K/Al2O3 10Fe3Cu1K/Al2O3 10Fe3Zr1K/Al2O3 10Fe3Cr1K/Al2O3

Product select. (C-mol%)

CO2 conv. (%)

CO

CH4

C5+

C2-C4=

C2-C40

C2+

34.6 42.0 39.2 38.6 41.7 39.6 36.1

41.8 23.0 31.3 33.3 26.5 28.9 36.1

32.5 36.1 31.1 35.8 27.8 37.5 35.2

5.4 11.1 8.9 6.8 13.8 7.6 5.0

6.7 9.4 8.2 7.5 1.6 7.8 6.1

13.6 20.4 20.5 16.6 30.3 18.2 17.6

25.7 40.9 37.6 30.9 45.7 33.6 28.7

a

Reaction conditions:1.0 g catalyst, 400 °C, 3 MPa, H2/CO2 = 3, 3,600 ml h-1 gcat-1, and TOS = 5 h.


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Effect of Cu on activity and product distribution

Fig. 4 Catalytic performance of a) 10FexCu1K/Al2O3 at 400 °C, b) 10Fe3K/Al2O3 and 10Fe3Cu3K/Al2O3 at 400 °C, c) 10Fe1K/Al2O3 and 10Fe3Cu1K/Al2O3 at 320 °C, and d) 10Fe3Cu1K/Al2O3 at different temperatures. To elucidate the role of Cu, catalysts with various Cu doping amounts were prepared and tested. Fig. 4a shows the product distribution of 10FexCuK/Al2O3, along with CO2 conversion in the inset. Moreover, it is found that the C5+ selectivity also increases with the Cu content monotonically until the mass fraction of Cu is 3, and a slight drop is evidenced with a further addition of Cu. When the copper was added into 10Fe3K/Al2O3, the conversion of CO2 shows a distinct increase. With respect to product distribution, the light olefins decrease from 20.3 to 14.5 C-mol%, while the C2-C40 increased (Fig. 4b). A similar trend is also observed for 10Fe/Al2O3



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and 10Fe3Cu/Al2O3 (Fig. S3), as well as for 10Fe1K/Al2O3 and 10Fe3Cu1K/Al2O3 at 320 °C (Fig. 4c). In the following step, the activity test was carried out on 10Fe3Cu1K/Al2O3 at temperatures ranging from 300 to 400 oC. As illustrated in Fig. 4d, the selectivity of C2-C4= is low in the whole range of temperature examined and regardless of the temperature. C5+ exhibits a maximum at T=320 oC, as does C2+ in general. Notably, the desired C2+ is always dominant in the products at all temperatures, the selectivity of which varies between 45.7-57.9 C-mol% range. Clearly, the addition of Cu enables a suppression of C2-C4= formation but an enhancement

of

C5+.

Such

statement

is

further

corroborated

by

the

Anderson-Schulz-Flory (ASF) plots of the 10Fe1K/Al2O3 and 10Fe3Cu1K/Al2O3 catalysts at 400 °C (Fig. S4), wherein the chain-growth probability (α) evidently increases from 0.44 to 0.54 with the addition of Cu. Moreover, the Cu-promoted catalyst 10Fe3Cu1K/Al2O3 exhibits a better stability at the harsh reaction temperature, while the deactivation behavior of 10Fe1K/Al2O3 is obvious (Fig. S5). As well known, Cu is highly active in RWGS reaction9, 50, and Fig. S3 shows that the CO selectivity is 95.1 C-mol% with the 30.8 % CO2 conversion at 400 °C over 10Cu/Al2O3. However, the CO selectivity over Cu-promoted Fe-based catalysts surprisingly decreases (Fig. 4), as well as CH4. Generally, the selectivity of C2-C4= and C5+ increases simultaneously on Mn-, Zn-, Zr- (mentioned in Section 3.1), and K-promoted Fe catalysts,26, 51 making the behavior of the Cu-promoted counterpart unique. The origin of such Cu-dependent behavior will be examined and results will be discussed a later section.


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To identify the morphology and phases of Fe species, HR-TEM measurements were performed. HR-TEM images of 10Fe1K/Al2O3 (Figs. 5a-1 and a-2) show the lattice fringe with spacings of 0.27 and 0.17 nm, corresponding to (104) and (116) planes in α-Fe2O3. The lattice fringes with the spacing of 0.26 nm in 10Fe3Cu1K/Al2O3 (Figs. 5b-1 and b-2) are consistent with the (103) plane of CuFe2O4, indicating the intimate contact between Cu and Fe and possible strong interaction.

Fig. 5 HR-TEM images of a) 10Fe1K/Al2O3 and b) 10Fe3Cu1K/Al2O3. Fig. 6 presents the XP spectra of the catalysts in the region of Cu 2p and Fe 2p. For all Cu-containing catalysts (Fig. 6a), the binding energy of Cu 2p3/2 and Cu 2p1/2 at 934.3 eV and 954.3 eV, along with two distinct shakeup satellites at 942.1 eV and 962.1 eV, indicates the presence of Cu2+ species.52 The characteristic shakeup satellite peaks can be attributed to the charge transfer between the transition metal 3d and surrounding ligand oxygen 2p orbitals.53 Compared with Cu 2p in 10Cu/Al2O3, the doublet of Cu 2p in 10FexCu1K/Al2O3 shifts slightly to a higher binding energy.



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Clearly, there should exist an interaction between Fe and Cu on the surface. Meanwhile，as shown in Fig. 6b, 10Fe1K/Al2O3 exhibits the binding energy of Fe 2p3/2 at 712.1 eV, corresponding to Fe2O3, along with a satellite peak at 720.0 eV.54 Notably, with the gradual increase of Cu content, the peaks of Fe species shifts to the lower binding energy, as well as a drastic decrease of the peak intensity. The observed results indicate the existence of a strong interaction between these metals, and a modification of the electron density on the metal sites exists, such as the electron transfer from Cu to Fe.

Fig. 6 XPS results of catalysts, a) Cu 2p and b) Fe 2p. To further examine the interaction of Cu and Fe, H2-TPR was carried out. The resultant profiles are shown in Fig. 7. The reduction peaks of Fe species shift to higher temperature with the addition of K, the retardance of which due to the incorporation of alkali metals as has been reported in the literature.40,


55-56

In

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comparison of 10Cu/Al2O3, the reduction peaks of Cu species in 10Fe3Cu1K/Al2O3 shift to higher temperatures, while reduction peaks of Fe species move towards lower temperatures considerably with respect to those in 10Fe1K/Al2O3. The TPR data clearly indicates that addition of Cu significantly promotes Fe reduction, while with itself being retarded in return. Such characteristic reduction behavior provides additional corroboration for t the strong interaction between Fe and Cu.

Fig. 7 H2-TPR profiles of catalysts with different promoters. CO2-TPD profiles of Cu-promoted and unpromoted catalysts are shown in Fig. 8 and S6. Generally, two broad peaks are evidenced in the whole range of temperature, wherein the one desorbed at lower temperatures corresponds to weakly-bonded CO2 species, while the other at higher temperatures can be attributed to the chemisorbed CO2 or carbonate species.26,

57

It is found that the peak intensities of these

two-adsorbed species are increased with the addition of Cu, which can be associated with the enhancement of quantity as well. Evidently, the introduction of Cu enables an improvement of CO2 adsorption on the surface.



Fig. 8 CO2-TPD profiles of 10Fe1K/Al2O3 and 10Fe3Cu1K/Al2O3. C3H6-TPD was also measured to identify the lower olefins adsorption on catalysts,58 and the results are depicted in Fig. 9. The desorption temperatures of the major peak of three catalysts are approximately identical, except that the peak intensity increases with increasing Cu content. Evidently, the Cu addition greatly improves the C3H6 adsorption on the surface. Such Cu-induced strong adsorption toward produced olefins on the surface likely results in the reduction of olefin content in the effluent, while, on the other hand, it may also be conducive to the synthesis of higher hydrocarbons via secondary conversion of adsorbed olefins.42

Fig. 9 C3H6-TPD profiles of different catalysts.


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Fig. 10 Plausible reaction pathways of CO2 conversion to hydrocarbons on Cu-promoted FeK/Al2O3 catalysts. HR-TEM and H2-TPR results indicate presence of a the strong interaction between Fe and Cu in Cu-promoted catalysts. The addition of Cu made it easier for iron reduction, and the electron-rich iron is more favorable for CO2 adsorption. Therefore, the CO2 conversion improved, the selectivity of CH4 decreased and the selectivity of C5+ increased. The introduction of Cu enhanced the adsorption of primarily formed olefins and the secondary conversion of produced olefins was enhanced. The hydrogenation of olefins increased the selectivity of paraffins, and the oligomerization of olefins increased the selectivity of C5+ (Fig. 10). This is the origin of the simultaneous increase of C5+ the selectivity and decrease of C2-C4= selectivity.

Dependence of product distribution on Fe and Cu interaction

Eric van Steen’s group reported that a physical mixture of Cu and Fe oxide barely showed advanced catalytic performance in comparison to delafossite CuFeO2 in FTS.42 In the present work, the Cu-induced improvement in terms of both activity and product distribution is evidenced, the behavior of which is associated with the strong interaction between Cu and Fe. Clearly, to optimize the function of Cu, it



should be in close proximity of the Fe species in the catalysts. To further confirm this statement, we adopted the co-impregnation and sequential impregnation methods to adjust the distribution of Cu and Fe, as well as the interaction between Cu and Fe.

Fig. 11 N2 adsorption-desorption isotherms and corresponding pore size distribution (inset) of catalysts. Fig. 11 shows the nitrogen adsorption-desorption isotherms and the corresponding BJH pore size distribution plots of catalysts prepared by different methods, along with the pristine support material for comparison. All isotherms are type IV with a H3-type hysteresis loop. The hysteretic loops between the adsorption and desorption curves concentrate at medium relative pressures, indicating the high quality of mesoporous materials. Combined with Figs. 2a and 2b, this indicates that the pores are contributed by the accumulation of small nanoparticles. The pore size distribution of the samples is displayed in the inset, and textural property is tabulated in Table S1. The average pore sizes of 10Fe1K/Al2O3 and 10Fe3Cu1K/Al2O3 are 4.96 nm, which approach to that of Al2O3 (4.95 nm). While the average pore sizes of S-10Fe1K3Cu/Al2O3 and S-3Cu10Fe1K/Al2O3 are 5.69 nm, which are larger than others. The BET specific surface areas of metals supported Al2O3 decrease compared


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with Al2O3 (Table S1). The BET specific surface areas of the catalysts prepared by sequential impregnation are smaller than that prepared by co-impregnation. The nitrogen adsorption-desorption indicates that the two preparation methods affected the structure of the catalysts.

Fig. 12 XRD patterns of catalysts prepared by different methods. The diffraction peaks of hematite phases could hardly be observed in 10Fe3Cu 1K/Al2O3, while S-10Fe1K3Cu/Al2O3 and S-3Cu10Fe1K/Al2O3 show intense diffraction peaks of hematite (Fig 12). It is very likely that the separate employment of Fe and Cu results in the relatively intact growth of hematite phases. Fig. 13 exhibits HR-TEM images of S-10Fe1K3Cu/Al2O3 and S-3Cu10Fe1K/Al2O3. Evidently, Fe exists mainly in the form of α-Fe2O3, while, interestingly, CuO particles are in intimate contact with Fe oxide particles. A schematic illustration of this phenomenon is presented in Fig. 14. H2-TPR was also measured to further clarify the interaction of Cu and Fe in terms of reduction behavior. As presented in Fig. 15, the reduction of Fe oxide on sequentially prepared catalysts is clearly retarded in comparison to that on co-impregnated catalyst,

as the

reduction

temperatures


of

Fe

oxide

on


S-10Fe1K3Cu/Al2O3

and

S-3Cu10Fe1K/Al2O3

are

higher

Page 22 of 34

than

those

on

10Fe3Cu1K/Al2O3. These results demonstrate that the interaction between Fe and Cu was successfully tuned among the catalysts, and the co-impregnated catalyst exhibits much stronger metallic interaction than the sequentially prepared counterparts.

Fig. 13 HR-TEM images of a) S-10Fe1K3Cu/Al2O3, b) S-3Cu10Fe1K/Al2O3.

Fig. 14 Schematic illustration of metal distribution on support.

Fig. 15 H2-TPR profiles of catalysts with different ways of Cu addition.


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Table 2 CO2 hydrogenation performance of Cu-promoted catalysts prepared by different methodsa Product select. (C-mol%)

Catalyst

CO2 conv. (%)

CO

CH4

C5+

C2-C4=

C2-C40

C2+

10Fe3Cu1K/Al2O3 S-3Cu10Fe1K/Al2O3 S-10Fe3Cu1K/Al2O3

41.7 38.6 40.1

26.5 36.1 36.7

27.8 28.3 26.1

13.8 9.5 10.1

1.6 2.6 2.4

30.3 23.6 24.7

45.7 35.6 37.7

a

Reaction conditions: 1.0 g catalyst, 400 °C, 3 MPa, H2/CO2 = 3, 3,600 ml h–1 gcat-1, and TOS = 5 h. Table 2 shows the catalytic performance of the Cu-promoted catalysts prepared by different methods. Generally, CO2 conversion of all catalysts changes little. On the contrary, in spite of slightly lower selectivity of C2-C4=, the selectivity of C2-C40 and C5+ of 10Fe3Cu1K/Al2O3, prepared by co-impregnation, is more prominent than those of sequentially prepared counterparts. Moreover, the sequentially prepared catalysts exhibit very similar product distribution. Such distinct variation may originate from the alienated contact between two metals. Therefore, the intimate Fe and Cu interaction is indispensable for the obtained superior performance in CO2 hydrogenation to hydrocarbons.

CONCLUSIONS

Copper was added into iron-based supported catalysts to investigate the function of Cu for CO2 hydrogenation. The addition of copper increased the CO2 conversion. Compared with other usual promoters, selectivity of C2-C4= decreased while the C5+ selectivity increased obviously. The strong interaction between Cu and Fe facilitated the reduction of Fe and the CO2 adsorption was enhanced on Cu-supported catalysts, which were contributed to the improvement of CO2 conversion, decrease of CH4, and



Page 24 of 34

increase of C5+. The enhanced adsorption of primarily formed olefins on Cu-promoted catalysts gave rise to secondary conversion of the produced olefins. Hydrogenation of olefins increased the selectivity of paraffins, and the oligomerization of olefins increased the selectivity of C5+. Co-impregnation and sequential impregnation methods were used to prepare the catalysts with different Fe and Cu distribution, respectively. The interaction strength of Cu and Fe, which is correlated with the contact of Fe and Cu, also has an effect on the catalytic performance.

SUPPORTING INFORMATION

Raman spectra of 10Fe3Cu1K/Al2O3 and 10Fe3Cu1K/Al2O3. EDS maps of a) 10Fe3Mn1K/Al2O3,

b)

10Fe3Zr1K/Al2O3,

c)

10Fe3Zn1K/Al2O3,

and

d)

10Fe3Cr1K/Al2O3. Catalytic performance of 10Fe/Al2O3, 10Fe3Cu/Al2O3, and 10Cu/Al2O3 at 400 °C. ASF plots of 10Fe1K/Al2O3 and 10Fe3Cu1K/Al2O3. Catalytic performance over 10Fe1K/Al2O3 and 10Fe3Cu1K/Al2O3 with TOS. CO2-TPD profiles of 10Fe/Al2O3 and 10Fe3Cu/Al2O3. Textural properties of catalysts.

ACKNOWLEDGMENTS

This work was financially supported in part by the National Key Research and Development Program of China (2016YFB0600902-5).

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Synopsis: Catalysts with different Fe and Cu distribution were prepared by various impregnation methods for sustainable production of value-added hydrocarbons.


Page 34 of 34

Selective CO2 Hydrogenation to Hydrocarbons on Cu-promoted Fe

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