Carbon nanotube-supported bimetallic Cu-Fe catalysts for syngas conversion to higher alcohols

Molecular Catalysis 479 (2019) 110610

Contents lists available at ScienceDirect

Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat

Carbon nanotube-supported bimetallic Cu-Fe catalysts for syngas conversion to higher alcohols

T

⁎

Shun Hea,1, Wei Wangb,1, Zheng Shena, Gongzhu Lia, Jincan Kanga, , Zhiming Liua, ⁎ ⁎ Gui-Chang Wangb, , Qinghong Zhanga, Ye Wanga, a

State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China b College of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin, 300071, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Syngas Higher alcohols CO hydrogenation Bimetallic catalysis Carbon nanotube

Catalytic transformation of syngas into higher alcohols is a challenging theme in C1 chemistry. The formation of higher alcohols require the synergy between active sites with abilities for CO dissociative and non-dissociative adsorption. This work reports Cu-Fe bimetallic catalysts loaded on carbon nanotubes (CNTs) for the conversion of syngas to higher alcohols. The structure-performance relationship for the catalysts prepared by different methods including co-impregnation and immobilization of pre-fabricated bimetallic nanoparticles have been investigated to gain insights into the effect of proximity of active sites on the formation of higher alcohols. We found that the Cu-Fe/CNT-co-red-imm catalyst, which was prepared by immobilization of colloidal bimetallic nanoparticles pre-fabricated with a co-reduction method, showed the closest proximity of Cu and Fe domains. This catalyst demonstrates the highest selectivity of higher alcohols. Our DFT calculation suggests that the coupling of surface CHO and CHx intermediates to form CH3CHO is a rate-limiting step. The DFT calculation also shows that the closer proximity between Fe2C (Fe) and Cu species leads to a lower energy barrier for the formation of CH3CH2OH from syngas.

1. Introduction Syngas, the mixture of CO and H2, is one of the most important platforms for the utilization of non-petroleum carbon resources including natural gas or shale gas, coal, biomass and even CO2 to supply energy and chemicals [1,2]. Among various products from syngas, higher (C2+) alcohols, which can be used as fuels, fuel additives, hydrogen carriers, and are also versatile chemicals for the synthesis of plasticizers, detergents, lubricants and cosmetics, are highly attractive [3]. Currently, C2+ alcohols are mainly produced by the fermentation of sugars or the hydration of petroleum-derived alkenes. However, the fermentation of sugars is expensive and energy-inefficient, and the latter process heavily relies on petroleum. The synthesis of C2+ alcohols from syngas that can be produced from various carbon resources would offer a sustainable route for the production of these alcohols. Moreover, as compared to hydrocarbons, the synthesis of C2+ alcohols from syngas is atom-efficient because of the incorporation of oxygen atom of CO into target product. Therefore, catalytic transformation of syngas

into C2+ alcohols has attracted much attention in recent years [3–9]. It is generally accepted that the formation of C2+ alcohols requires the CeC coupling between adsorbed CHx (x = 0–3) and CO species, and thus the synergy between active sites with both CO dissociation and non-dissociation abilities [3,6,7]. This is quite different from the situation in methanol synthesis, where CO is non-dissociatively chemisorbed on catalyst surfaces, and in Fischer-Tropsch (FT) synthesis for the production of hydrocarbons, where only the dissociation of CO is necessary. The requirement of precise CeC coupling between controlled adsorbed species formed on different active sites makes the selective synthesis of C2+ alcohols highly challenging. Several types of catalysts have been reported for the conversion of syngas to C2+ alcohols. Rh-based catalysts are typically selective to C2+ alcohols, especially ethanol, and the ethanol selectivity of 40–50% can be achieved at controlled CO conversions (< 10%) [10–13]. However, the high price and limited availability of Rh restrict its large-scale application. Modified methanol-synthesis catalysts, in particular alkali metal-modified Cu-Zn catalysts, show high selectivity of alcohols, but

⁎

Corresponding authors. E-mail addresses: [email protected] (J. Kang), [email protected] (G.-C. Wang), [email protected] (Y. Wang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.mcat.2019.110610 Received 3 May 2019; Received in revised form 26 July 2019; Accepted 5 September 2019 Available online 14 September 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.

Molecular Catalysis 479 (2019) 110610

S. He, et al.

methanol is the major product in alcohols in most cases [14–17]. Another important type of catalysts for the conversion of syngas to C2+ alcohols is the bifunctional catalyst composed of two active metals, which can work for methanol synthesis and FT synthesis [18–33]. In addition, a recent study has demonstrated that the integration of ZnAl2O4 and zeolite mordenite (H-MOR) separated by quartz wool (denoted as ZnAl2O4│H-MOR) could catalyze the direct conversion of syngas into methyl acetate via dimethyl ether intermediate, and the three-layered ZnAl2O4│H-MOR│ZnAl2O4 combination offered ethanol with a selectivity of 52% at CO conversion of 6% [34]. In spite of these advances, the selectivity of C2+ alcohols still remain limited. Fundamental studies to uncover the key factors that determine the product selectivity would play a key role in developing efficient catalysts for selective conversion of syngas into C2+ alcohols. Among various catalysts reported for syngas to C2+ alcohols, the bimetallic catalysts with both CO non-dissociative adsorption and CO dissociative adsorption abilities have attracted much attention in recent years [18–33], since this type of catalysts offers a clear model for fundamental research to gain insights into structure-performance relationships and to further enhance the selectivity of C2+ alcohols. Actually, many studies have been devoted to elucidating the synergistic effect between Cu and Co, which are responsible for CO non-dissociative and dissociative adsorption, respectively [18–26]. It is demonstrated that the Cu-Co alloy nanocrystals are responsible for the selective formation of C2+ alcohols [20,24,26]. Iron is another active metal for CO dissociative adsorption and carbon-chain growth. As compared to Co, Fe has advantages of lower price and higher compatibility with syngas of broader H2/CO ratios as well as wider range of reaction temperatures [35]. Oxygenates and olefins could also be formed with relatively higher selectivities over catalysts with Fe alone as the active metal [35–37]. Therefore, Cu-Fe bimetallic catalysts are expected to be promising catalysts for the direct conversion of syngas to C2+ alcohols. However, as compared to the Cu-Co catalyst, relatively less studies have been devoted to the Cu-Fe bimetallic catalyst for syngas conversion [27–32]. The fabrication of catalysts with a controllable nanostructure of active phase has been proven to be a useful strategy for FT synthesis [38,39]. However, such studies are still scarce for the conversion of syngas to C2+ alcohols, although the synthesis of C2+ alcohols relies more on the synergy and thus the structure of bimetallic catalysts. In particular, there are few studies on the fabrication of Cu-Fe bimetallic catalysts with controlled nanostructures for syngas conversions and the knowledge of the structure-performance relationship for Cu-Fe-catalyzed syngas to C2+ alcohols is less accumulated. Therefore, the major objective of this work is to understand the structure requirement of CuFe bimetallic catalysts for the formation of C2+ alcohols. In this work, the catalytic performances of CNT-supported Cu-Fe catalysts with different structures synthesized in controllable manners are analyzed to gain insights into structure-property relationships. The evolution of catalyst structure during syngas conversions will also be investigated. The mechanism for the formation of C2+ alcohols and the effect of proximity between Cu and Fe sites will be discussed by spin-polarized density functional theory (DFT) calculations.

(1.0 g) were first added into a mixed aqueous solution (20 mL) containing copper (1.97 mmol) and iron (2.24 mmol) nitrates, followed by stirring for 6 h at room temperature. Then, the slurry was dried at 353 K under vigorous stirring for 4 h and the dried powders were treated at 673 K in a N2 gas flow for 3 h. The obtained sample was denoted as CuFe/CNT-co-imp. The Cu and Fe loadings were both 10 wt% in this catalyst. The Cu/CNT-imp and Fe/CNT-imp catalysts containing Cu and Fe alone, respectively, were also prepared using the same procedure. The co-impregnation method was also employed for the preparation of CuFe catalysts loaded on other supports including SiO2, ZrO2, Al2O3, MgO, NaY, H-ZSM-5, active carbon (AC) and mesoporous carbon (meso-C). The immobilization of preliminarily fabricated colloidal bimetallic nanoparticles onto CNTs was also used to prepare the CNT-supported bimetallic Cu-Fe catalysts. The colloidal bimetallic nanoparticles were fabricated by two different approaches, i.e., stepwise reduction and simultaneous reduction approaches. In the stepwise-reduction approach, the colloidal nanoparticles were obtained by reducing the precursors using NaBH4 with different sequences. For the catalyst denoted as Cu/Fe/CNT-step-red-imm, copper nitrate (1.97 mmol) was first dissolved in 20 mL deionized water and reduced by NaBH4 (19.7 mmol) aqueous solution (50 mL) containing 0.50 g polyvinyl pyrrolidone (PVP), which functioned as a capping agent, at room temperature under N2 atmosphere for 2 h. Then, iron nitrate (2.24 mmol) aqueous solution (30 mL) was added into the colloid and the pH value of the suspension was adjusted to 10 with NH3·H2O, followed by further addition of NaBH4 (22.4 mmol) aqueous solution (60 mL) to reduce iron species at room temperature under N2 atmosphere for 2 h. CNTs (1.0 g) were added into the colloid containing the bimetallic nanoparticles. The suspension was stirred for 4 h and was allowed to rest for another 4 h. The Cu/Fe/CNT-step-red-imm catalyst (Cu, 10 wt%; Fe, 10 wt%) was obtained by filtration, drying and thermal treating in N2 gas flow at 673 K for 3 h to remove the capping agent. The catalyst denoted as Fe/ Cu/CNT-step-red-imm (Cu, 10 wt%; Fe, 10 wt%) was prepared by a similar procedure, but the reduction sequence for the synthesis of colloidal bimetallic nanoparticles was different. Iron nitrate (2.24 mmol) was first reduced for 2 h by NaBH4 (22.4 mmol) aqueous solution (60 mL) containing 0.50 g PVP at room temperature under N2 atmosphere. Then, copper nitrate (1.97 mmol) aqueous solution was added into the colloid containing Fe nanoparticles and was reduced for 2 h by NaBH4 (19.7 mmol) aqueous solution (60 mL) in the second step at room temperature and N2 atmosphere. The bimetallic Cu-Fe colloidal nanoparticles were also fabricated by the co-reduction with NaBH4, followed by immobilization onto CNT, providing the catalyst denoted as Cu-Fe/CNT-co-red-imm (Cu, 10 wt%; Fe, 10 wt%). In brief, the Cu-Fe bimetallic colloidal nanoparticles were first fabricated by reducing the mixed aqueous solution of copper (1.97 mmol) and iron (2.24 mmol) nitrates by NaBH4 (42.1 mmol) aqueous solution (60 mL) in the presence of 1.0 g PVP at room temperature under N2 atmosphere for 3 h. Then, CNTs (1.0 g) were added into the suspension containing the bimetallic nanoparticles, and the resultant was stirred for 4 h and rested for 5 h. The solid product was recovered by filtration, followed by drying and heat treatment under N2 at 673 K for 3 h. The Cu/CNT-redimm and Fe/CNT-red-imm were also prepared by the same procedure.

2. Methods

2.2. Catalyst characterization

2.1. Materials and catalyst preparation

X-ray diffraction (XRD) patterns were recorded with a Rigaku Ultima IV X-ray diffractometer with Cu-Kα radiation (40 kV and 20 mA). N2 physisorption measurements were performed with a Micromeritics Tristar II 3020. Prior to the adsorption, the sample was evacuated at 573 K for 3 h. Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) measurements were performed on a Phillips Analytical FEI Tecnai 20 electron microscope operated at an acceleration voltage of 200 kV. The sample was dispersed ultrasonically in ethanol for 5 min, and a drop of solution was deposited onto a carbon-coated gold grid. X-ray fluorescence (XRF)

Multi-walled CNTs used in this work were synthesized by an established method [40]. The synthesized CNTs were pretreated with concentrated (68 wt%) HNO3 at 413 K under refluxing conditions to remove the remaining Ni catalyst as well as other impurities and to create functional groups for the immobilization of metal precursors in aqueous solutions [41]. CNT-supported bimetallic Cu-Fe catalysts were fabricated by four different methods. For the co-impregnation method, the powdery CNTs 2

Molecular Catalysis 479 (2019) 110610

S. He, et al.

Table 1 Catalytic performances of Cu-Fe bimetallic catalysts loaded on various supports by co-impregnation.a Catalystb

Cu-Fe/SiO2-co-imp Cu-Fe/ZrO2-co-imp Cu-Fe/Al2O3-co-imp Cu-Fe/MgO-co-imp Cu-Fe/NaY-co-imp Cu-Fe/H-ZSM-5-co-imp Cu-Fe/AC-co-imp Cu-Fe/meso-C-co-imp Cu-Fe/CNT-co-imp Fe/CNT-impd Cu/CNT-impe a b c d e

Selectivityc (%)

CO conv. (%)

CO2 select. (%)

CH4

C2-4H

C5+H

MeOH

C2-6OH

C2-6OH yield (%)

30 22 66 4.9 43 15 35 19 57 44 3.4

10 7.2 30 26 21 10 13 9.4 19 12 4.3

16 17 16 23 11 21 17 17 16 15 5.1

31 30 38 46 24 32 37 33 32 36 1.0

38 39 37 7.4 55 28 34 38 35 43 0.1

6.0 4.7 3.1 7.5 3.9 12 4.1 4.2 6.0 2.3 93

8.9 9.6 6.5 16 6.6 7.2 8.0 7.4 11 3.5 0.9

2.4 2.0 3.0 0.6 2.2 1.0 2.4 1.3 5.1 1.4 0.03

Reaction conditions: H2/CO = 2; P = 5 MPa; T =523 K; GHSV =3600 h−1; t =10 h. The loadings of Cu and Fe were 10 wt%. C2-4H, C5+H and C2-6OH denote C2-4 hydrocarbons, C5+ hydrocarbons and C2-6 alcohols, respectively. The loading of Fe was 20 wt%. The loading of Cu was 20 wt%.

functions. All atoms concluding the adsorbates were allowed to relax. The Brillouin zone was sampled using a 2 × 2 × 1 Monkhorst−Pack kpoint mesh [47]. Spin-polarization was considered when we treated iron systems due to the magnetic nature of Fe. Dipole corrections were carried out to prevent the electrostatic field [48]. Transition states (TSs) of the formation of ethanol from syngas were investigated by using the nudged elastic band (NEB) to determine the minimum-energy reaction paths and calculate the energy barriers of each elemental reacts from syngas to ethanol [49]. The activation barrier (Ea) is defined as the total energy difference between the transition state (TS) and initial state (IS). The Ea was calculated on the basis of the following expression: Ea = E (TS) - E(IS). The reaction energy was calculated as ΔE = E(FS) - E(IS), where ΔE refers to the total energy gap between the final state (FS) and initial state (IS). The adsorption energy Eads was calculated using the formula of Eads = Eadsorbate/substrate - (Eadsorbate + Esubstrate). In this definition, Eadsorbate/substrate refers to the total energy of adsorbate-Cu2Fe2/ Fe2C(011) system, and Eadsorbate and Esubstrate refer to the energies of adsorbate species and free substrate, respectively.

spectroscopy was used to measure the elemental compositions of catalysts. The XRF measurements were performed with a Panalytical Axois Petro XRF instrument with rhodium target (50 kV, 50 mA). X-ray photoelectron spectroscopy (XPS) was carried out in an UHV chamber equipped with an Omicron XPS (base pressure 5 × 10−10 Torr). A monochromatized Al Kα X-ray source and a Sphere 2 analyzer were used. Considering that the catalyst was pretreated in H2 gas flow prior to reaction, we also pretreated the sample before XPS measurements in H2 at 573 K within a chamber attached to the spectrometer. The pretreated sample could be transferred directly to the analysis chamber without exposure to open air. The binding energy was calibrated using C1s photoelectron peak at 284.6 eV as a reference. Gas chromatography-mass spectrometry (GC–MS) analysis was performed with a Thermo ISQ7000 instrument with a TS-1 ms capillary column. 2.3. Catalytic reaction The catalytic conversion of syngas was performed in a high-pressure fixed-bed reactor with an inner diameter of 7.0 mm. The catalyst was placed in the middle of the reactor. Prior to reaction, the catalyst was pretreated in a H2 gas flow (40 mL min−1) at 573 K for 4 h. After the reactor was cooled down to 323 K, a syngas with a H2/CO ratio of 2/1 was introduced into the reactor. Argon with a concentration of 4% in syngas was used as an internal standard for the calculation of CO conversion. The pressure of syngas was typically regulated to 5.0 MPa. The reaction was started by raising the temperature to the desired reaction temperature (typically, 523 K) with a ramping rate of 2 K min−1. The products were analyzed by on-line gas chromatography. Ar, CO, CO2, and CH4 were separated by a TDX-01 column and were analyzed by a thermal conductivity detector (TCD). The formed C1 to C9 hydrocarbons and alcohols were separated by a Q-Bond column and analyzed by a flame ionization detector (FID). The selectivity was calculated on a molar carbon basis. The carbon balance was typically better than 95%. The catalytic performance after 10 h of reaction was used for discussion.

3. Results and discussion 3.1. Catalytic performances of Cu-Fe bimetallic catalysts loaded on various supports Catalytic behaviors of Cu-Fe bimetallic catalysts loaded on different supports (including metal oxides, zeolites and carbon materials) by the co-impregnation method for the conversion of syngas are displayed in Table 1. As compared to other supports, the use of Al2O3 and CNT supports provided relatively higher CO conversions. The Al2O3-supported Cu-Fe catalyst showed higher selectivity of C2+ hydrocarbons with a relatively lower selectivity of C2-C6 alcohols (6.5%). On the other hand, the Cu-Fe/CNT-co-imp catalyst offered higher selectivity of C2-C6 alcohols (11%). The MgO-support Cu-Fe catalyst exhibited the highest selectivity of C2-C6 alcohols (16%) among all the catalysts displayed in Table 1, but the CO conversion of this catalyst was quite low (4.9%). The highest yield of C2-C6 alcohols was obtained over the catalyst with CNT as the support. It is noteworthy that the catalyst with CNT as a support has shown promising performances in syngas conversions [10,50,51]. This outstanding catalytic performance of CNT-supported catalysts may arise from the well-defined nanostructure and unique properties of CNTs, such as the high accessibility to the supported active phase, the efficient mass transport due to the absence of microporosity, and the excellent hydrogen spillover property. Hereafter, we focus on CNT-supported Cu-Fe catalysts for the conversion of syngas to

2.4. Computational method Periodic DFT calculations were performed by using the Vienna ab initio simulation package (VASP) [42–44]. The interactions between valence and core electrons was described by the projector augmented wave (PAW) method [45]. The Perdew-Burke-Ernzerhof (PBE) functional was used to treat the exchange-correlation energy [46]. The kinetic cutoff energy is set at 400 eV to represent the electronic wave 3

Molecular Catalysis 479 (2019) 110610

S. He, et al.

imm catalysts. After H2 reduction at 573 K for 4 h, the diffraction peaks belonging to Cu2O and Fe3O4 phases almost disappeared (Fig. 1B). At the same time, the peaks ascribed to metallic Cu (2θ = 43.3, 50.5 and 74.1°) and metallic Fe (2θ = 44.7 and 65.0°) were clearly observed. These results suggest that Cu2O and Fe3O4 were the major crystalline phases in all the fresh catalysts prior to H2 reduction, and these phases were mainly transformed to metallic Cu and Fe after H2 reduction. Fig. 2 shows TEM images and line-scan EDS analyses for the four fresh catalysts prior to H2 reduction. The particle-size distribution for each catalyst was also displayed in the figure. The mean sizes of metal particles evaluated from the particle-size distribution were 15–18 nm in the four catalysts. Only a small fraction of metal particles had sizes < 10 nm. Considering the inner diameters of CNTs, we speculate that most of the metal particles (≥ 95%) are located outside the CNTs. The inter-planar spacings for the crystalline particles were estimated from the high-resolution TEM (HRTEM) image of each catalyst. The values for the Cu-Fe/CNT-co-imp catalyst prepared by the co-impregnation were calculated to be 0.247 and 0.254 nm (Fig. 2b), which could be ascribed to the (111) facet of Cu2O and the (311) facet of Fe3O4, respectively. This is in agreement with the XRD result that this catalyst contained both Cu2O and Fe3O4 particles. The line-scan EDS analyses (Fig. 2c and d) for two particles (marked with red lines across the particles in Fig. 2a) indicate that the copper and iron particles in the CuFe/CNT-co-imp catalyst are mostly separated with each other. For the Cu/Fe/CNT-step-red-imm catalyst prepared by immobilization of bimetallic particles, which were fabricated by stepwise reduction (first Cu and then Fe), a core-shell structure was formed (Fig. 2f and g). The inter-planar spacings for the core and the shell were calculated to be 0.246 and 0.254 nm, which could be ascribed to the (111) facet of Cu2O and the (311) facet of Fe3O4, respectively. Line-scan EDS analyses confirmed that the particles contained mainly copper in the core position and iron in the shell position (Fig. 2h and i), further confirming the formation of the core-shell structure in the fresh catalyst. From HRTEM images, the thicknesses of Cu2O core and Fe3O4 shell were estimated to be 10–12 nm and 3–5 nm, respectively. On the other hand, the formation of core-shell structure could not be observed for the Fe/Cu/CNT-step-red-imm catalyst (Fig. 2k), for which the Fe precursor was first reduced followed by the reduction of Cu(NO3)2 during the fabrication of bimetallic nanoparticles. The interfacial contact between Fe3O4 and Cu2O particles could be discerned from the HRTEM image for this catalyst (Fig. 2l). The line-scan EDS analyses also revealed that the nanoparticles in this catalyst contained overlapped regions with both copper and iron (Fig. 2m and n). For the Cu-Fe/CNT-co-red-imm catalyst with Cu-Fe bimetallic particles fabricated by simultaneous reduction followed by adsorption onto CNTs, HRTEM results indicated that both Cu2O and Fe3O4 domains were contained in a particle of ∼17 nm in size (Fig. 2q). Line-scan EDS analyses for two particles (with red line marked in Fig. 2p) showed that the signals of copper and iron changed simultaneously with the position within each nanoparticle (Fig. 2r and s). These results indicate that copper and iron are distributed uniformly in these nanoparticles. The distribution of copper and iron in the four catalysts was further investigated by using the dark-field TEM with EDS elemental mapping (Fig. S1). For the Cu-Fe/CNT-co-imp catalyst, copper and iron elements distributed separately and irregularly. This further suggests that copper and iron particles are mainly separated with each other in this catalyst (Figs. S1a-1c). For the Cu/Fe/CNT-step-red-imm catalyst, copper was mainly surrounded by iron in a particle, confirming the formation of Cu@Fe core-shell structure (Figs. S1d-1f). There are some overlapped regions for copper and iron elements in the Fe/Cu/CNT-step-red-imm catalyst (Figs. S1g-1i). For the Cu-Fe/CNT-co-red-imm catalyst (Figs. S1j-1l)), the copper and iron elements appeared in the same region, confirming the closest proximity between the two components in this catalyst. Subsequently, we performed TEM studies for the four catalysts after H2 reduction. The typical HRTEM images of these catalysts are

C2+ alcohols. Catalytic behaviors of CNT-supported Fe and Cu alone prepared by the impregnation are also displayed in Table 1. The Fe/CNT-imp exhibited a higher CO conversion, but hydrocarbons were the major products, whereas methanol was the predominant product over the Cu/ CNT-imp catalyst. The CO conversion over the Cu/CNT-imp catalyst was lower under our reaction conditions. These observations confirm that Fe, which is an active component for FT synthesis, is responsible for CO dissociation and chain growth, while the hydrogenation of CO proceeds on Cu surfaces without CO dissociation. 3.2. Structures of CNT-supported bimetallic catalysts prepared by different procedures To gain insights into the effect of structure of Cu-Fe bimetallic nanoparticles on catalytic behaviors, we prepared Cu-Fe bimetallic catalysts loaded on CNTs with four different procedures. Here, we first characterize the fresh CNT-supported bimetallic catalysts and the catalysts after H2 reduction. N2 physisorption measurements showed that the surface areas and pore volumes of the CNT-supported bimetallic catalysts prepared by different procedures were 96-122 m2 g−1 and 0.17-0.19 cm3 g−1, respectively (Table S1), which were somewhat lower than those for CNTs without loading active components. The average pore diameters for these samples were around 7.0 nm. XRD patterns for the CNT-supported Cu-Fe catalysts are displayed in Fig. 1. The diffraction peak observed at 2θ of 26.2° for all the catalysts could be ascribed to the (002) reflection of CNTs. For the fresh catalysts (Fig. 1A), diffractions peaks at 2θ of 36.4, 42.3, 61.4, 70.0 and 73.6° were assignable to Cu2O, while the diffraction peaks at 2θ of 30.1, 35.5, 43.1, 53.7, 57.2, 62.7 and 74.1° could be assigned to Fe3O4. A small diffractions peak at 2θ of 50.3°, which was assignable to Cu0, was also observed for the fresh Fe/Cu/CNT-step-red-imm and Cu-Fe/CNT-co-red-

Fig. 1. XRD patterns for CNT-supported bimetallic catalysts prepared by different procedures before (A) and after (B) H2 reduction. (a) Cu-Fe/CNT-co-imp, (b) Cu/Fe/CNT-step-red-imm, (c) Fe/Cu/CNT-step-red-imp, (d) Cu-Fe/CNT-cored-imm. 4

Molecular Catalysis 479 (2019) 110610

S. He, et al.

Fig. 2. TEM micrographs (a, f, k, p), HRTEM micrographs (b, g, l, q), line-scan EDS analyses (c, d, h, i, m, n, r, s), and particle-size distributions (e, j, o, t) for CNTsupported bimetallic catalysts prepared by different procedures before H2 reduction. (a–e) Cu-Fe/CNT-co-imp, (f–j) Cu/Fe/CNT-step-red-imm, (k–o) Fe/Cu/CNT-stepred-imm, (p–t) Cu-Fe/CNT-co-red-imm.

Fig. 3. HRTEM micrographs for CNT-supported bimetallic catalysts prepared by different procedures after H2 reduction. (a) Cu-Fe/CNT-co-imp, (b) Cu/Fe/CNT-stepred-imm, (c) Fe/Cu/CNT-step-red-imm, (d) Cu-Fe/CNT-co-red-imm.

Inside the iron-containing particle, both metallic Fe with an interplanar spacing of 0.201 nm ascribed to the (110) facet of bcc α-Fe and Fe3O4 with an inter-planar spacing of 0.255 nm belonging to its (311) facet co-existed (Fig. 3a). The core-shell structure could still be clearly

displayed in Fig. 3. For the H2-reduced Cu-Fe/CNT-co-imp catalyst, metallic Cu particles with an inter-planar spacing of 0.209 nm ascribed to the (111) facet of fcc Cu was observed (Fig. 3a). The metallic Cu particle was found to be separated from the iron-containing particles. 5

Molecular Catalysis 479 (2019) 110610

S. He, et al.

the significant lower Cu/Fe molar ratios on surfaces either before or after H2 reduction. This result also suggests that there is no remarkable segregation of copper or iron species on the surfaces of the other three catalysts. We further calculated the ratio of intensities of Fe 2p3/2 peaks ascribed to Fe0 and Fe3O4, denoted as I(Fe0)/I(Fe3O4), for the catalysts after H2 reduction. The I(Fe0)/I(Fe3O4) for the H2-reduced catalysts increased in the following sequence: Cu/Fe/CNT-step-redimm < Cu-Fe/CNT-co-imp < Fe/Cu/CNT-step-red-imm < Cu-Fe/CNTco-red-imm (Table 2). This result indicates that the Cu-Fe/CNT-co-redimm catalyst, whose iron and copper species are in the most intimate contact, possesses the highest fraction of Fe0 on catalyst surfaces after H2 reduction, whereas the Cu/Fe/CNT-step-red-imm catalyst with the core-shell Cu@Fe0-Fe3O4 structure showed the lowest fraction of Fe0.

observed in the Cu/Fe/CNT-step-red-imm catalyst after H2 reduction (Fig. 3b). The HRTEM image indicated that the core-shell particle contained metallic Cu as the core and metallic Fe as the shell, which was further surrounded by un-reduced Fe3O4 shell. For the H2-reduced Fe/Cu/CNT-step-red-imm catalyst (Fig. 3c), we observed metallic Cu particle in contact with the particle composed of both metallic Fe and Fe3O4. The result (Fig. 3d) for the H2-reduced Cu-Fe/CNT-co-red-imm catalyst indicated that this catalyst also contained metallic Cu, metallic Fe and Fe3O4 species, but the Cu and Fe domains were in closer proximity. In short, our HRTEM studies reveal that the four catalysts after H2 reduction all contained metallic Cu, metallic Fe and Fe3O4 species, but the proximity of these species are different in the four catalysts. Metallic Fe and Cu species were in the most intimate contact in the H2-reduced Cu-Fe/CNT-co-red-imm catalyst. We performed XPS measurements for the four catalysts before and after H2 reduction to gain information on the chemical states of Cu and Fe on catalyst surfaces. For each catalyst before H2 reduction, the binding energies of Cu 2p3/2 and Cu 2p1/2 were 933.0 and 953.0 eV, respectively (Fig. S2A), which could be ascribed to either Cu+ or Cu0 [52,53]. The kinetic energy of the Cu L3VV Auger line was centered at 915.9 eV (Fig. S2B), which was assignable to Cu+ [53]. The binding energies of Fe 2p3/2 and 2p1/2 were 711.2 and 724.3 eV (Fig. S2C), respectively, for each catalyst before H2 reduction, suggesting that iron existed as Fe3O4 on the surface [54,55]. After H2 reduction, although the binding energies of Cu 2p3/2 and Cu 2p1/2 were similar to those before H2 reduction (Fig. 4A), the kinetic-energy peak of Cu L3VV Auger line for the four catalysts all shifted to 918.4 eV (Fig. 4B). This indicates the reduction of Cu+ to Cu0 on catalyst surfaces after H2 reduction. The XPS spectra of Fe 2p for the four catalysts after H2 reduction also changed significantly. The peaks for Fe 2p3/2 and Fe 2p1/2 could be deconvolved into two components (Fig. 4C). The component with binding energies of Fe 2p3/2 and 2p1/2 at 711.0 and 724.2 eV were attributable to Fe3O4, whereas that with binding energies of Fe 2p3/2 and 2p1/2 at 707.3 and 720.4 eV could be ascribed to Fe0 [56]. In other words, both Fe3O4 and Fe0 co-existed on catalyst surfaces after H2 reduction. Therefore, the chemical states of surface copper and iron obtained from XPS are almost independent of the catalyst preparation method, and are consistent with those obtained from XRD and HRTEM studies. Table 2 shows the surface compositions for the four catalysts before and after H2 reduction estimated from XPS. The bulk Cu/Fe molar ratio for each catalyst, which was measured by XRF, was close to that used in the preparation stage, indicating that almost all the copper and iron in the precursors used for preparation were incorporated in the catalyst. The surface Cu/Fe molar ratios derived from XPS were similar to those in the bulk except for the Cu/Fe/CNT-step-red-imm catalyst (Table 2), for which the core-shell structure with copper core and iron shell (Cu2O@Fe3O4 before H2 reduction or Cu@Fe0-Fe3O4 after H2 reduction) is formed. The formation of the core-shell structure could interpret

3.3. Catalytic behaviors of CNT-supported bimetallic catalysts Table 3 displays the catalytic behaviors of the CNT-supported bimetallic Cu-Fe catalysts prepared by different methods. Under the same reaction conditions, the Cu-Fe/CNT-co-imp catalyst exhibited the highest CO conversion among the four catalysts. Hydrocarbons, CH3OH and C2-C6 alcohols (C2+ alcohols) were formed over all the four catalysts, but the product selectivity was different. The Cu-Fe/CNT-co-redimm catalyst exhibited the highest selectivity to alcohols and the lowest one to hydrocarbons. In particular, the selectivity of C2-C6 alcohols over the Cu-Fe/CNT-co-red-imm catalyst reached 23%, which is twice of that over the Cu-Fe/CNT-co-imp. The yield of C2-C6 alcohols was also the highest over the Cu-Fe/CNT-co-red-imm catalyst. To compare the selectivity on the same CO conversion level, we performed syngas conversions at a longer contact time for the three catalysts prepared by the immobilization method. At similar CO conversions (53–57%), the selectivity of C2-C6 alcohols decreased in the following sequence: Cu-Fe/ CNT-co-red-imm > Fe/Cu/CNT-step-red-imm > Cu-Fe/CNT-co-imp > Cu/Fe/CNT-step-red-imm (Table 3). GC–MS analyses of liquid products obtained from syngas conversion using the Cu-Fe/CNT-co-red-imm catalyst revealed that n-alcohols dominated the alcohol products and only a very low fraction of branched alcohols (< 0.5%) was detected (Fig. S3). The Cu-Fe/CNT-co-red-imm catalyst demonstrates the highest selectivity (∼21%) and yield (8.7%) of C2-C6 alcohols. The yield of C2+ alcohols over our catalyst is better than those over most of the bimetallic Cu-Fe catalysts reported to date (Table S2). We further investigated the catalytic behaviors of the catalysts prepared by four procedures but with different loadings of Cu and Fe. The single Cu catalyst (either Cu/CNT-imp prepared by impregnation or Cu/CNT-red-imm prepared by immobilization of colloidal Cu nanoparticles preliminarily fabricated by NaBH4 reduction) exhibited higher CH3OH selectivity and lower CO conversion, whereas the single Fe catalyst (Fe/CNT-imp or Fe/CNT-red-imm) displayed higher hydrocarbon selectivity and higher CO conversion (Table 4). For the bimetallic catalysts, an increase in Fe loading or a decrease in Cu loading

Fig. 4. Cu 2p XPS (A), Cu L3VV Auger (B), and Fe 2p XPS (C) spectra for CNT-supported bimetallic catalysts prepared by different procedures after H2 reduction. (a) Cu-Fe/CNT-co-imp, (b) Cu/Fe/CNT-step-red-imm, (c) Fe/Cu/CNT-step-red-imm, (d) Cu-Fe/CNT-co-red-imm. 6

Molecular Catalysis 479 (2019) 110610

S. He, et al.

Table 2 Bulk and surface compositions for CNT-supported bimetallic catalysts measured by XRF and XPS. Catalysta

Cu-Fe/CNT-co-imp Cu/Fe/CNT-step-red-imm Fe/Cu/CNT-step-red-imm Cu-Fe/CNT-co-red-imm a b c d e

Cu/Fe ratiob

Cu/Fe ratioc

1.15 1.17 1.10 1.12

Fresh

Reduced

Used

1.37 0.55 1.20 1.07

0.94 0.44 0.90 0.92

1.43 1.03 1.59 1.31

I(Fe)/I(Fe3O4) after H2 reductiond

I(Fe2C)/I(Fe3O4)after the reactione

0.95 0.27 1.19 1.31

0.08 0.05 0.13 0.19

The molar ratio of Cu/Fe used for preparation was 1.14. Measured by XRF. Measured by XPS. Estimated from the intensities of XPS peaks ascribed to Fe and Fe3O4. Estimated from the intensities of XPS peaks ascribed to Fe2C and Fe3O4.

Table 3 Catalytic behaviors of CNT-supported bimetallic Cu-Fe catalysts prepared by different procedures.a Catalystb

CO conv. (%)

Cu-Fe/CNT-co-imp Cu/Fe/CNT-step-red-imm Fe/Cu/CNT-step-red-imm Cu-Fe/CNT-co-red-imm Cu/Fe/CNT-step-red-immd Fe/Cu/CNT-step-red-immd Cu-Fe/CNT-co-red-immd a b c d

CO2 select. (%)

57 45 41 36 54 54 53

19 25 22 21 25 20 22

Selectivityc (%) CH4

C2-4H

C5+H

MeOH

C2-6OH

C2-6OH yield (%)

16 16 14 15 12 9.6 17

32 33 31 30 36 28 26

35 32 36 23 38 42 31

6.0 6.0 4.3 8.9 4.0 7.5 5.0

11 13 15 23 10 13 21

5.1 4.4 4.8 6.5 4.1 5.6 8.7

Reaction conditions: H2/CO = 2; P = 5 MPa; T =523 K; GHSV =3600 h−1; t =10 h. The loadings of Cu and Fe were 10 wt%. C2-4H, C5+H and C2-6OH denote C2-4 hydrocarbons, C5+ hydrocarbons and C2-6 alcohols, respectively. GHSV =2400 h−1.

and decreases that of hydrocarbons. The selectivity of C2-C6 alcohols over the supported bimetallic catalysts was higher than that over the single supported Cu or Fe catalyst. Thus, there exists a significant synergistic effect on the formation of C2-C6 alcohols. A further comparison among the four series of catalysts prepared by different procedures revealed that the Cu-Fe/CNT-co-red-imm catalyst was the most efficient for the formation of C2-C6 alcohols. The yield of C2-C6 alcohols over the Cu-Fe/CNT-co-red-imm catalyst with any Cu/Fe ratio was higher than those over the other series of catalysts with the same Cu/Fe ratio or with similar CO conversions. This further demonstrates that the CNT-supported bimetallic catalyst with copper and iron

increased CO conversion, irrespective of the preparation method. The CO conversion over the Cu-Fe/CNT-co-imp catalysts (Cu/Fe = 1/1 or 1/ 3 in mass ratio) was even higher than that over the Fe/CNT-imp catalyst. The use of Cu to partially replace Fe in some bimetallic catalysts prepared by the immobilization of pre-fabricated colloidal metallic nanoparticles could also enhance the CO conversion. These results suggest that, although the Cu catalyst alone is less active than the Fe catalyst under our reaction conditions, the bimetallic catalyst composed of Cu and Fe with a proper ratio can enhance or keep the activity. It has been demonstrated that the incorporation of Cu into Fe significantly enhances the selectivities of alcohols, in particular of C2-C6 alcohols,

Table 4 Catalytic behaviors of CNT-supported Cu-Fe bimetallic catalysts with different Cu and Fe loadings.a Catalyst

Cu/CNT-imp Fe/CNT-imp Cu-Fe/CNT-co-imp

Cu/CNT-red-imm Fe/CNT-red-imm Cu/Fe/CNT-step-red-imm

Fe/Cu/CNT-step-red-imm

Cu-Fe/CNT-co-red-imm

a b

Loadings (wt%)

CO conv. (%)

Cu

Fe

20 0 15 10 5 20 0 15 10 5 15 10 5 15 10 5

0 20 5 10 15 0 20 5 10 15 5 10 15 5 10 15

3.4 44 44 57 60 3.0 42 16 45 47 22 41 45 29 36 48

CO2 select. (%)

4.3 12 12 19 20 7.5 21 15 25 26 23 22 14 20 21 22

Selectivityb (%) CH4

C2-4H

C5+H

MeOH

C2-6OH

C2-6OH yield (%)

5.1 15 17 16 15 4.1 20 13 16 17 12 14 17 13 15 16

1.0 36 28 32 29 1.3 34 26 33 34 32 31 30 28 30 32

0.1 43 28 35 43 0.2 40 28 32 36 32 36 41 23 23 29

93 2.3 13 6.0 5.0 94 2.1 19 6.0 5.1 5.1 4.3 3.0 9.8 8.9 6.0

0.9 3.5 14 11 7.8 0.6 3.8 14 13 8.0 19 15 9.0 26 23 17

0.03 1.4 5.4 5.1 3.7 0.02 1.3 1.9 4.4 2.8 3.2 4.8 3.5 6.0 6.5 6.4

Reaction conditions: H2/CO = 2; P = 5 MPa; T =523 K; GHSV =3600 h−1; t =10 h. C2-4H, C5+H and C2-6OH denote C2-4 hydrocarbons, C5+ hydrocarbons and C2-6 alcohols, respectively. 7

Molecular Catalysis 479 (2019) 110610

S. He, et al.

Fig. 5. Catalytic behaviors of Cu-Fe/CNT-cored-imm catalyst (The loadings of Cu and Fe were both 10 wt%) for syngas conversions. (a) Effect of pressure. H2/CO = 2/1; P = 1–7 MPa; T =523 K; GHSV =3600 h−1; t =10 h. (b) Effect of temperature. H2/CO = 2/1; P = 5 MPa; T = 483–563 K; GHSV =3600 h−1; t =10 h. (c) Effect of H2/CO ratio. H2/ CO = 0.5–3.0; P = 5 MPa; T =523 K; GHSV =3600 h−1; t =10 h. (d) Catalyst stability. H2/ CO = 2; P = 5 MPa; T =523 K; GHSV =3600 h-1. C1-4H: C1-4 hydrocarbons. C5+H: C5+ hydrocarbons. Alcohols: Total alcohols.

3.4. Characterization of CNT-supported bimetallic catalysts after syngas conversions

species in closer proximity is more efficient for the formation of C2-C6 alcohols. Effects of reaction conditions on catalytic behaviors have been investigated for syngas conversions over Cu-Fe/CNT-co-red-imm catalyst. The increase in pressure from 1.0 to 5.0 MPa increased CO conversion from 14% to 36% and the C2-C6 alcohol selectivity from 11% to 23% (Fig. 5a). At the same time, the selectivity of C1-C4 hydrocarbons decreased, while the selectivities of C5+ hydrocarbons and total alcohols increased. A further increase in pressure rather decreased the selectivities of C2-C6 and total alcohols, although the CO conversion still increased. Upon increasing the reaction temperature from 483 to 563 K, the CO conversion increased significantly from 7.6% to 58% (Fig. 5b). At the same time, the selectivity of C1-C4 hydrocarbons increased and that of C5+ hydrocarbons decreased. The selectivity of total alcohols decreased gradually. The selectivity of C2-C6 alcohols first increased from 16% to 23% as the temperature increased from 483 to 523 K and then decreased slightly with a further increase in temperature. Upon increasing the H2/CO ratio from 0.5:1 to 3:1, the CO conversion increased from 12% to 42%, but the C2-C6 alcohol selectivity decreased from 31% to 15% (Fig. 5c). The selectivities of total alcohols and C5+ hydrocarbons also decreased, whereas the selectivity of C1-C4 hydrocarbons increased. Therefore, a higher pressure, medium reaction temperature and controlled H2/CO ratio are favorable for the conversion of syngas into C2-C6 alcohols. We have further measured the stability of the Cu-Fe/CNT-co-redimm catalyst with Cu and Fe loadings of 10 wt%. The changes in both CO conversion and product selectivity were not significantly after 100 h of reaction (Fig. 5d). The conversion of CO and the selectivity of C2-C6 alcohols were sustained at 33–36% and 21–23%, respectively, in 100 h of reaction. Thus, the present catalyst with the closest proximity of copper and iron species is relatively stable for the synthesis of C2-C6 alcohols from syngas, which would be due to the strong synergistic effect between iron and copper species.

We performed characterizations for the CNT-supported bimetallic catalysts prepared by four procedures with the Cu and Fe loadings of both 10 wt% after syngas conversions for 10 h under the conditions displayed in Table 3. XRD patterns indicated that there were no significant differences for the four catalysts after the reaction (Fig. 6). Metallic Cu remained as the only phase of copper species. This is also the case for the Cu/CNT-red-imm catalyst (Fig. S4A). The crystalline phases related to iron species changed after the reactions. The diffraction lines ascribed to metallic Fe at 2θ of 44.7 and 65.0° in the sample after H2 reduction (Fig. 1B) disappeared. Instead, new diffraction lines at 2θ of 37.0, 41.8, 42.7 and 57.1°, which were assignable to an iron

Fig. 6. XRD patterns for CNT-supported bimetallic catalysts prepared by different procedures after syngas conversions. (a) Cu-Fe/CNT-co-imp, (b) Cu/Fe/ CNT-step-red-imm, (c) Fe/Cu/CNT-step-red-imm, (d) Cu-Fe/CNT-co-red-imm. 8

Molecular Catalysis 479 (2019) 110610

S. He, et al.

Fig. 7. HRTEM micrographs for CNT-supported bimetallic catalysts prepared by different procedures after syngas conversions. (a) Cu-Fe/CNT-co-imp, (b) Cu/Fe/ CNT-step-red-imm, (c) Fe/Cu/CNT-step-red-imm, (d) Cu-Fe/CNT-co-red-imm.

carbide phase, ε-Fe2C [57], was observed. This suggests that Fe0 has been transformed into ε-Fe2C during the syngas conversion. It is noteworthy that χ-Fe5C2 has been reported as the major active phase for Fecatalyzed FT reactions [1]. We speculate that the relatively milder reaction temperature (523 K) and the presence of α-Fe as the major precursor of iron after H2 reduction would both favor the generation of ε-Fe2C during syngas conversions under our reaction conditions [57–59]. Moreover, the use of carbon materials as supports may also be beneficial to the ε-Fe2C phase [32]. The same result was observed for the CNT-supported monometallic Fe catalyst (Fe/CNT-red-imm) (Fig. S4B). TEM measurements showed that the mean sizes of bimetallic nanoparticles were 19.8, 17.2, 19.2 and 20.4 nm for the used Cu-Fe/CNTco-imp, Cu/Fe/CNT-step-red-imm, Fe/Cu/CNT-step-red-imm and Cu-Fe/ CNT-co-red-imm catalysts, respectively (Fig. S5). These mean sizes were only slightly larger than those for the corresponding catalysts before the reaction (Fig. 2). Thus, the bimetallic nanoparticles did not undergo significant aggregation during the reaction. Typical HRTEM micrographs for the four bimetallic catalysts after the reaction are displayed in Fig. 7. The Cu-Fe/CNT-co-imp catalyst was composed of copper particles with an inter-planar spacing of 0.210 nm ascribed to the (111) facet of metallic Cu (fcc) and two types of iron

particles. The bigger iron particle with an inter-planar spacing of 0.485 nm was attributable to Fe3O4 with the (111) facet exposed, while the smaller iron particle with an inter-planar spacing of 0.212 nm corresponded to ε-Fe2C with (111) facet exposed. Similarly, HRTEM measurements for other three catalysts also indicated the co-existence of Cu, Fe3O4 and ε-Fe2C domains. The monometallic Cu/CNT-red-imm and Fe/CNT-red-imm catalysts after the reaction contained metallic Cu and Fe2C as well as Fe3O4 species, respectively (Fig. S6). The core-shell structure observed in the Cu/Fe/CNT-step-red-imm catalyst before reaction could not be clearly observed after the reaction, indicating the reconstruction of catalysts during syngas conversion. We believe that this arises from the transformation of Fe0 to ε-Fe2C. The comparison among the four catalysts indicated that the Cu-Fe/CNT-co-red-imm catalyst contained Cu and Fe2C domains in the closest proximity. This is probably because copper and iron species are uniformly distributed in this catalyst before reaction (Fig. 2r and s). The dark-field TEM micrographs and line-scan EDS analyses further suggest that the proximity between Cu ad Fe species keep close after syngas conversions (Fig. S5). Therefore, although the catalyst underwent reconstruction during the reaction due to the transformation of Fe0 to ε-Fe2C, the proximity between Fe and Cu species could be sustained to some extent. XPS was also used for the characterization of catalysts after the

Fig. 8. Cu 2p XPS (A), Cu L3VV Auger (B), and Fe 2p XPS (C) spectra for CNT-supported bimetallic catalysts prepared by different procedures after syngas conversions. (a) Cu-Fe/CNT-co-imp, (b) Cu/Fe/CNT-step-red-imm, (c) Fe/Cu/CNT-step-red-imm, (d) Cu-Fe/CNT-co-red-imm. 9

Molecular Catalysis 479 (2019) 110610

S. He, et al.

or H2-reduced sample also exhibited relatively lower selectivity of C2-C6 alcohols because of the enrichment of iron species in the shell. Although the structural reconstruction during the reaction changed the core-shell structure, the contact between Cu0 and Fe2C species was limited. The Fe/Cu/CNT-step-red-imm catalyst demonstrated relatively higher selectivity of C2-C6 alcohols probably owing to more interfacial contact between Cu0 and Fe2C species after the reaction. The distribution of copper and irons species was uniform in the fresh and the H2-reduced Cu-Fe/CNT-co-red-imm catalyst. The Cu/Fe ratio on surfaces of this catalyst was quite stable even after the reaction. This implies that, although the iron species undergoes reconstruction to Fe2C species during the reaction, the intimate contact between Cu0 and iron species could be sustained to some extent, leading to the highest selectivity of C2-C6 alcohols for this catalyst. The fast formation of Fe2C species [62], and the keeping of close proximity between Cu0 and Fe2C species during the reaction would contribute to the long-term stability of the Cu-Fe/CNTco-red-imm catalyst (Fig. 5d). As compared to the other catalysts, the Cu-Fe/CNT-co-red-imm catalyst also possesses higher fraction of Fe0 after H2 reduction and higher fraction of Fe2C species after the reaction, which may also contribute to the higher C2-C6 alcohols. In brief, the experimental results enable us to conclude that the closer proximity between Cu and Fe2C domains results in higher C2-C6 alcohols from syngas over the supported Cu-Fe bimetallic catalysts. The synergistic effect between the closely contacted Cu and Fe2C domains would favor the coupling between CHxO and CHx intermediates, which are formed on Cu0 and Fe2C sites, respectively. 3.5. DFT calculations for syngas to higher alcohols over Fe2Cu2/Fe2C(011) surfaces Model: The Fe2C(011) was chosen due to its relatively stable character compared to other faces based on the previous theoretical study [63]. For modeling the Fe2C(011), the periodic slab models were chosen in this work. The Fe2C(011) surface was modeled using a 2 × 2 unit cell, the slabs consisting of five-layer iron and three-layer carbon with 15 Å of vacuum in the z direction. For Cu-Fe2C(011) surface, a model of Cu2Fe2/Fe2C(011) was constructed by substituting two Fe atoms with two Cu atoms on the top most layer of Fe2C(011) surface based on the surface composition of Cu2Fe2 (Fig. 9). In all calculations, the top three-layer iron and one-layer carbon were allowed to relax, while the corresponding bottom layers were fixed. Adsorption geometry and energy on Cu2Fe2/Fe2C(011) surface: The calculated adsorption energy of possible species is given Table S3, and the related adsorption configurations are displayed in Fig. 10. CO species prefers to adsorb at the top site of the Fe atom with the adsorption energy of -1.41 eV. H prefers to adsorb at the bridge site between Fe and Cu atoms with the adsorption energy of -2.36 eV. CH species tends to bind at the 3-fold hollow sites, CH2 prefers to adsorb at the 3-fold hollow sites. CH3 prefers to adsorb at the top of the Fe atom. The adsorption energies of CHx (x = 1, 2, 3) are -6.06, -3.29 and -1.54 eV, respectively. HCO prefers to bind via both C (bridge) and O (bridge), with both O and C interacting with the substrate, yielding adsorption energy of -1.68 eV. For other C2 oxygenates like CH3CHO, CH3CH2O and CH3CH2OH, CH3CHO prefers to bind via O (top) at the top site of the Fe atom, CH3CH2O adsorbs via O atom at the top site of the Cu atom, CH3CH2OH adsorbs at the top site of the Fe atom with O atom, the adsorption energies are -0.37, -2.26 and -0.29 eV respectively. The reaction pathways from syngas to ethanol: after getting the stable adsorption configuration of possible species, we calculated reaction paths from CO and H, and the calculated results are given in Table 5, and Fig. 10 as well as Fig. 11. The first step is the activation of CO to form active C species and then undergo a hydrogenation to form CHx (x = 1–3) species for the higher alcohols synthesis from syngas [43,44]. It was reported that there were two possibilities for formation of CHx (x = 1–3): one is CO hydrogenation to form HCO, then HCO dissociation to produce CHx

Fig. 9. Top view and side view of Cu2Fe2/Fe2C(011) surface model of Cu-Fe/ CNT-co-red-imm catalyst.

reaction. Although it is hard to discriminate Cu0 and Cu+ from the Cu 2p spectra (Fig. 8A), the Cu L3VV Auger spectra for all the catalysts confirm that only the Cu0 species with the signal at 918.5 eV exists on catalyst surfaces after the reaction (Fig. 8B). The Fe 2p XPS spectra showed that iron carbide with binding energy of 706.0–707.0 eV and Fe3O4 were the major species on catalyst surfaces after the reaction [60,61]. This agrees well with the XRD result (Fig. 6). The quantitative result uncovered that the Cu/Fe ratios on the surfaces of Cu-Fe/CNT-coimp, Cu/Fe/CNT-step-red-imm and Fe/Cu/CNT-step-red-imm after the reaction increased to some extent as compared to those prior to the reaction, whereas that on surfaces of the Cu-Fe/CNT-co-red-imm catalyst only changed slightly (Table 2). This increase in the surface Cu/Fe ratio suggested a significant reconstruction of nanostructures of the former three catalysts, whereas the structure of the Cu-Fe/CNT-co-redimm catalyst changed insignificantly after the reaction. The ratio of intensities of Fe 2p3/2 peaks ascribed to Fe2C and Fe3O4, denoted as I (Fe2C)/I(Fe3O4), was estimated. The result showed that the used Cu-Fe/ CNT-co-red-imm catalyst had the highest I(Fe2C)/I(Fe3O4) value, whereas the Cu/Fe/CNT-step-red-imm catalyst had the lowest one (Table 2). In short, our XPS results demonstrate that the Cu-Fe/CNT-cored-imm catalyst possesses the closest proximity even after the reaction and the highest fraction of Fe2C species. All the experimental results described above suggest that the proximity between copper and iron species is the most important parameter that determines the selectivity of C2-C6 alcohols. Most of the copper and iron species in the Cu-Fe/CNT-co-imp catalyst were separated with each other before and after the reaction, and this catalyst showed quite lower selectivity of C2-C6 alcohols (Tables 3 and 4). The Cu/Fe/CNT-step-red-imm catalyst with a core-shell structure in the fresh 10

Molecular Catalysis 479 (2019) 110610

S. He, et al.

Fig. 10. Optimized configurations for the main TSs involved in reactions on Cu2Fe2/Fe2C(011) surface of Cu-Fe/CNT-co-red-imm catalyst. Table 5 Energetic results of each elemental step for C2H5OH formation over Cu-Fe/CNTco-red-imm and Cu-Fe/CNT-co-imp catalysts. Elemental steps

CO+H→HCO HCO→CH + O CH+H→CH2 CH2+H→CH3 CH3+CHO→CH3CHO CH3CHO+H→CH3CH2O CH3CH2O+H→CH3CH2OH

Cu-Fe/CNT-co-red-imm

Cu-Fe/CNT-co-imp

ΔH (eV)

Ea (eV)

d(CH/ C-O) (Å)

ΔH (eV)

Ea (eV)

d(CH/CO) (Å)

0.67 −0.03 −0.19 −1.03 −1.47 −0.69 −0.51

0.97 0.99 0.24 0.12 0.87 0.41 0.43

1.15 1.95 1.78 1.75 2.15 1.63 1.46

0.17 0.16 −0.03 −0.67 −0.87 −0.62 −0.57

0.53 1.21 0.99 0.26 1.85 1.19 0.31

1.15 1.95 1.98 1.85 2.35 1.98 1.37

(x = 1–3); the other is direct CO dissociation to form C and O atoms, followed by the hydrogenation of C to form CHx (x = 1–3). Because of the high activation barrier of direct CO desorption, C–O bond cleavage is very difficult to achieve, we study the case of H-assisted CO dissociation via the reaction of HCO → CH + O [64]. For HCO formation, CO adsorbs at the top site of Fe atom and H adsorbs at the bridge site of

Fig. 11. Potential energy profiles for ethanol synthesis mechanism over Cu-Fe/ CNT-co-red-imm and Cu-Fe/CNT-co-imp catalysts.

Fe and Cu atoms. It was found that the reaction energy for the HCO formation is 0.67 eV and its corresponding activation barrier is 0.97 eV. At TS1, the C–H length is 1.15 Å with HCO adsorbs at the top site of Fe 11

Molecular Catalysis 479 (2019) 110610

S. He, et al.

was quite different for the four type of catalysts. The Cu-Fe/CNT-co-redimm catalyst prepared by immobilization of colloidal Cu-Fe nanoparticles fabricated by co-reduction with NaBH4 possessed the closest proximity. The structure-performance correlation reveals that the proximity plays a pivotal role in determining the C2+ alcohol selectivity. The yield of C2-C6 alcohols reached 8.7% with selectivity of 21% over the Cu-Fe/CNT-co-red-imm catalyst. The present work demonstrates that the synergistic effect between Cu and Fe2C species is the key to the conversion of syngas into C2+ alcohols. DFT calculation results elucidated that starting from CO and H, CH3 was generated via the route of CO + H →HCO → CH + O → CH2 → CH3, and the formation of CH3CH2OH proceeded via the reaction pathway of CH3 + HCO → CH3CHO → CH3CH2O → CH3CH2OH. The CeC coupling reaction to form CH3CHO was the rate-limiting step with the highest activation barrier. The energy barriers of transforming of HCO to CH3CHO species and subsequently hydrogenated to CH3CH2O species on the Cu-Fe/CNT-co-imp catalyst with less intimate of copper and iron species are all higher than those on the Cu-Fe/CNT-co-red-imm catalyst with closer proximity of Fe2C (Fe) and Cu species.

atom via C atom. Then HCO dissociation into CH and O, the reaction energy is -0.03 eV and its corresponding activation barrier is 0.99 eV. At TS2, the distance between C and O is 1.95 Å, CH adsorbs at the bridge site between Fe and C atoms, and the O adsorbs at the top site of C atom. After formation of CH, CH prefers to be hydrogenated to CH2, the reaction energy is -0.19 eV, and its corresponding activation barrier is 0.24 eV. In the TS3, the C–H bond distance is 1.78 Å, CH binds at the 3hollow site, and H adsorbs at the top site of Fe atom. CH2 tends to be hydrogenated to CH3, the reaction is exothermic with the reaction energy of -1.03 eV, and the activation barrier is 0.12 eV. At TS4, the C–H bond length is 1.75 Å, CH2 binds at the bridge site between Fe and Cu atoms, and H adsorbs at the top site of Fe. The CH3 species is propagated to form the C2 oxygenate precursors, and then the chain growth will be achieved by hydrogenation to C2H5OH. After the formation of CH3, CHO is inserted into CH3 to form CH3CHO which mainly contributes to C2 oxygenates formation [65]. For this reaction process, HCO prefers to bind via C atom at the top site of Fe atom and CH3 binds at the top site of Fe atom at the initial configuration. The reaction energy is -1.47 eV, and its corresponding activation barrier is 0.87 eV. At TS5, the CeC length is 2.15 Å. CH3CHO then be hydrogenated to CH3CH2O, which can further hydrogenation to form CH3CH2OH. The reaction energy is -0.69 eV, and its activation barrier is 0.41 eV. At TS6, the C–H length is 1.63 Å. Then CH3CH2O species is hydrogenated to CH3CH2OH, the reaction energy is -0.51 eV, and it needs to overcome an activation barrier of 0.43 eV. At TS7, the OeH length is 1.46 Å. Totally, starting from CO and H, CH3CH2OH is formed via the reaction pathway of CO + H → HCO → CH + O → CH2 → CH3, and CH3 + HCO → CH3CHO → CH3CH2O → CH3CH2OH. The CeC coupling reaction to form CH3CHO is the rate-limiting step with the highest activation barrier (0.87 eV) over Cu2Fe2/Fe2C(011) surface for the Cu-Fe/CNT-co-red-imm catalyst. As mentioned above, the Cu-Fe/CNT-co-red-imm catalyst with the closer proximity of copper and iron species exhibited higher selectivity to C2+ oxygenates than that to Cu-Fe/CNT-co-imp catalyst with copper and iron species separated. To reveal the difference for the formation of ethanol and C2+ oxygenates between these two catalysts, we further calculated the potential energy diagrams for the formation of CH3CH2OH over Cu-Fe/CNT-co-red-imm and Cu-Fe/CNT-co-imp catalysts. As displayed in Fig. 11 and Table 5, compared to Cu-Fe/CNT-cored-imm system, it was found that the energy barriers of transforming of HCO and CH3 to CH3CHO species and subsequently hydrogenating to CH3CH2O species on Cu-Fe/CNT-co-imp catalyst were 1.85 and 1.19 eV, respectively, which were all significantly higher than those on Cu-Fe/ CNT-co-red-imm catalyst. These results suggest that the formation of CH3CH2OH is relatively difficult for Cu-Fe/CNT-co-imp catalyst, which may interpret the lower selectivity of C2+ oxygenates over this catalyst compared to Cu-Fe/CNT-co-red-imm catalyst. It was reported that the iron carbide favored CO dissociation and carbon-chain propagation for Fe based FT catalysts [15,66]. We consider that the Cu-Fe/CNT-co-imp catalyst would undergo CeC coupling in the presence of the separated Fe2C species, and then form much heavier hydrocarbons during CO hydrogenation (Table 3).

Declaration of Competing Interest These authors declare no competing interests. Acknowledgments This work was supported by the National Key Research and Development Program of the Ministry of Science and Technology of China (No. 2017YFB0602201), the National Natural Science Foundation of China (Nos.21433008, 21872112, 91545203, 21673188and21690082), and the Fundamental Research Funds for the Central Universities (No. 20720170025). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mcat.2019.110610. References [1] K. Cheng, J. Kang, D.L. King, V. Subramanian, C. Zhou, Q. Zhang, Y. Wang, Advances in catalysis for syngas conversion to hydrocarbons, Adv. Catal. 60 (2017) 125–208. [2] W. Zhou, K. Cheng, J. Kang, C. Zhou, V. Subramanian, Q. Zhang, Y. Wang, New horizon in C1 chemistry: breaking the selectivity limitation in transformation of syngas and hydrogenation of CO2 into hydrocarbon chemicals and fuels, Chem. Soc. Rev. 48 (2019) 3193–3228. [3] J.J. Spivey, A. Egbebi, Heterogeneous catalytic synthesis of ethanol from biomassderived syngas, Chem. Soc. Rev. 36 (2007) 1514–1528. [4] V. Subramani, S.K. Gangwal, A review of recent literature to search for an efficient catalytic process for the conversion of syngas to ethanol, Energy Fuel 22 (2008) 814–839. [5] K. Fang, D. Li, M. Lin, M. Xiang, W. Wei, Y. Sun, A short review of heterogeneous catalytic process for mixed alcohols synthesis via syngas, Catal. Today 147 (2009) 133–138. [6] M. Gupta, M.L. Smith, J.J. Spivey, Heterogeneous catalytic conversion of dry syngas to ethanol and higher alcohols on Cu-based catalysts, ACS Catal. 1 (2011) 641–656. [7] H.T. Luk, C. Mondelli, D.C. Ferré, J.A. Stewart, J. Pérez-Ramírez, Status and prospects in higher alcohols synthesis from syngas, Chem. Soc. Rev. 46 (2017) 1358–1426. [8] Y. An, T. Lin, F. Yu, Y. Yang, L. Zhong, M. Wu, Y. Sun, Advances in direct production of value-added chemicals via syngas conversion, Sci. China Chem. 60 (2017) 887–903. [9] M. Ao, G.H. Pham, J. Sunarso, M.O. Tade, S. Liu, Active centers of catalysts for higher alcohol synthesis from syngas: a review, ACS Catal. 8 (2018) 7025–7050. [10] X. Pan, Z. Fan, W. Chen, Y. Ding, H. Luo, X. Bao, Enhanced ethanol production inside carbon-nanotube reactors containing catalytic particles, Nat. Mater. 6 (2007) 507–511. [11] N.D. Subramanian, J. Gao, X. Mo, J.G. Goodwin Jr., W. Torres, J.J. Spivey, La and/ or V oxide promoted Rh/SiO2 catalysts: effect of temperature, H2/CO ratio, space velocity, and pressure on ethanol selectivity from syngas, J. Catal. 272 (2010) 204–209. [12] J. Wang, Q. Zhang, Y. Wang, Rh-catalyzed syngas conversion to ethanol: studies on

4. Conclusions Bimetallic Cu-Fe catalysts loaded on CNTs could catalyze the conversion of syngas into C2+ alcohols with better selectivity than those on other supports. The Cu-Fe/CNT catalysts, which were prepared by four different procedures including co-impregnation, immobilization of colloidal Cu-Fe nanoparticle preliminarily fabricated by stepwise NaBH4 reduction (first Cu then Fe; first Fe then Cu) and simultaneous NaBH4 reduction, were studied to gain key structural factors for the selective formation of C2+ alcohols. All these catalysts contained metallic Cu, metallic Fe and Fe3O4 species prior to reaction, and the catalyst underwent reconstruction because of the evolution of metallic Fe to ε-Fe2C during the reaction. The proximity between Cu and Fe species 12

Molecular Catalysis 479 (2019) 110610

S. He, et al.

[39] H. Wang, W. Zhou, J.X. Liu, R. Si, G. Sun, M.Q. Zhong, H.Y. Su, H.B. Zhao, J.A. Rodriguez, S.J. Pennycook, J.C. Idrobo, W.X. Li, Y. Kou, D. Ma, Platinummodulated cobalt nanocatalysts for low-temperature aqueous-phase FischerTropsch synthesis, J. Am. Chem. Soc. 135 (2013) 4149–4158. [40] P. Chen, H.B. Zhang, G.D. Lin, Q. Hong, K.R. Tsai, Growth of carbon nanotubes by catalytic decomposition of CH4 or CO on a Ni-MgO catalyst, Carbon 35 (1997) 1495–1501. [41] P. Serp, M. Corrias, P. Kalck, Carbon nanotubes and nanofibers in catalysis, Appl. Catal. A Gen. 253 (2003) 337–358. [42] G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (1996) 11169–11186. [43] W. Wang, Y. Wang, G.C. Wang, CO dissociation mechanism on Pd-doped Fe(100): comparison with Cu/Fe(100), J. Phys. Chem. C 121 (2017) 6820–6834. [44] W. Wang, Y. Wang, G.C. Wang, Ethanol synthesis from syngas over Cu(Pd)-doped Fe(100): a systematic theoretical investigation, Phys. Chem. Chem. Phys. 20 (2018) 2492–2507. [45] G. Kresse, D. Joubert, From ultrasoft pseudopotentials to the projector augmentedwave method, Phys. Rev. B 59 (1999) 1758–1775. [46] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865–3868. [47] H.J. Monkhorst, J.D. Pack, Special points for Brillouin-zone integrations, Phys. Rev. B 13 (1976) 5188–5192. [48] L. Bengtsson, Dipole correction for surface supercell calculations, Phys. Rev. B 59 (1999) 12301–12304. [49] G. Henkelman, H. Jónsson, Improved tangent estimate in the nudged elastic band method for finding minimum energy paths and saddle points, J. Chem. Phys. 113 (2000) 9978–9985. [50] J. Kang, S. Zhang, Q. Zhang, Y. Wang, Ruthenium nanoparticles supported on carbon nanotubes as efficient catalysts for selective conversion of synthesis gas to diesel fuel, Angew. Chem. Int. Ed. 48 (2009) 2565–2568. [51] X. Dong, X.L. Liang, H.Y. Li, G.D. Lin, P. Zhang, H.B. Zhang, Preparation and characterization of carbon nanotube-promoted Co-Cu catalyst for higher alcohol synthesis from syngas, Catal. Today 147 (2009) 158–165. [52] J. Morales, J.P. Espinos, A. Caballero, A.R. Gonzalez-Elipe, J.A. Mejias, XPS study of interface and ligand effects in supported Cu2O and CuO nanometric particles, J. Phys. Chem. B 109 (2005) 7758–7765. [53] L. Yang, J. He, Q. Zhang, Y. Wang, Copper-catalyzed propylene epoxidation by oxygen: significant promoting effect of vanadium on unsupported copper catalyst, J. Catal. 276 (2010) 76–84. [54] Z.S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng, K. Müllen, 3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles as efficient electrocatalysts for the oxygen reduction reaction, J. Am. Chem. Soc. 134 (2012) 9082–9085. [55] J. Lu, X. Jiao, D. Chen, W. Li, Solvothermal synthesis and characterization of Fe3O4 and γ-Fe2O3 nanoplates, J. Phys. Chem. C 113 (2009) 4012–4017. [56] G. Sun, B. Dong, M. Cao, B. Wei, C. Hu, Hierarchical dendrite-like magnetic materials of Fe3O4, γ-Fe2O3, and Fe with high performance of microwave absorption, Chem. Mater. 23 (2011) 1587–1593. [57] K. Xu, B. Sun, J. Lin, W. Wen, Y. Pei, S. Yan, M. Qiao, X. Zhang, B. Zong, ε-iron carbide as a low-temperature Fischer-Tropsch synthesis catalyst, Nat. Commun. 5 (2014) 5783. [58] Q. Chang, C. Zhang, C. Liu, Y. Wei, A.V. Cheruvathur, A.I. Dugulan, J.W. Niemantsverdriet, X. Liu, Y. He, M. Qing, L. Zheng, Y. Yun, Y. Wang, Y. Li, Relationship between iron carbide phases (ε-Fe2C, Fe7C3, and χ‑Fe5C2) and catalytic performances of Fe/SiO2 Fischer−Tropsch catalysts, ACS Catal. 8 (2018) 3304–3316. [59] D.B. Bukur, K. Okabe, M.P. Rosynek, C. Li, D. Wang, K.R.P.M. Rao, G.P. Huffman, Activation studies with a precipitated iron catalysts for Fischer-Tropsch synthesis, J. Catal. 155 (1995) 353–365. [60] C. Yang, H. Zhao, Y. Hou, D. Ma, Fe5C2nanoparticles: a facile bromide-induced synthesis and as an active phase for Fischer-Tropsch synthesis, J. Am. Chem. Soc. 134 (2012) 15814–15821. [61] D. Fu, W. Dai, X. Xu, W. Mao, J. Su, Z. Zhang, B. Shi, J. Smith, P. Li, J. Xu, Y. Han, Probing the structure evolution of iron-based Fischer-Tropsch to produce olefins by operando Raman spectroscopy, ChemCatChem 7 (2015) 752–756. [62] J.W. Niemantsverdriet, A.M. van der Kraan, W.L. van Dijk, H.S. van der Baan, Behavior of metallic iron catalysts during Fischer-Tropsch synthesis studied with Mössbauer spectroscopy, X-ray diffraction, carbon content determination, and reaction kinetic measurements, J. Phys. Chem. 84 (1980) 3363–3370. [63] C.F. Huo, Y.W. Li, J. Wang, H. Jiao, Insight into CH4 formation in iron-catalyzed Fischer-Tropsch synthesis, J. Am. Chem. Soc. 131 (2009) 14713–14721. [64] Y. Zhao, S. Li, Y. Sun, CO dissociation mechanism on Cu-doped Fe(100) surfaces, J. Phys. Chem. C 117 (2013) 24920–24931. [65] H. Zheng, R. Zhang, Z. Li, B. Wang, Insight into the mechanism and possibility of ethanol formation from syngas on Cu(100) surface, J. Mol. Catal. A Chem. 404–405 (2015) 115–130. [66] T.H. Pham, X. Duan, G. Qian, X. Zhou, D. Chen, CO activation pathways of FischerTropsch synthesis on χ-Fe5C2 (510): direct versus hydrogen-assisted CO dissociation, J. Phys. Chem. C 118 (2014) 10170–10176.

the promoting effect of FeOx, Catal. Today 171 (2011) 257–265. [13] P. Carrillo, R. Shi, K. Teeluck, S.D. Senanayake, M.G. White, In situ formation of FeRh nanoalloys for oxygenate synthesis, ACS Catal. 8 (2018) 7279–7286. [14] A.M. Hilmen, M. Xu, M.J.L. Gines, E. Iglesia, Synthesis of higher alcohols on copper catalysts supported on alkali-promoted basic oxides, Appl. Catal. A Gen. 169 (1998) 355–372. [15] J.M. Beiramar, A. Griboval-Constant, A.Y. Khodakov, Effects of metal promotion on the performance of CuZnAl catalysts for alcohol synthesis, ChemCatChem 6 (2014) 1788–1793. [16] J. Sun, S. Wan, F. Wang, J. Lin, Y. Wang, Selective synthesis of methanol and higher alcohols over Cs/Cu/ZnO/Al2O3 catalysts, Ind. Eng. Chem. Res. 54 (2015) 7841–7851. [17] J. Sun, Q. Cai, S. Wan, L. Wang, J. Lin, D. Mei, Y. Wang, The promotional effects of cesium promoter on higher alcohol synthesis from syngas over cesium-promoted Cu/ZnO/Al2O3 catalysts, ACS Catal. 6 (2016) 5771–5785. [18] J. Wang, P.A. Chernavskii, A.Y. Khodakov, Y. Wang, Structure and catalytic performance of alumina-supported copper-cobalt catalysts for carbon monoxide hydrogenation, J. Catal. 286 (2012) 51–61. [19] J. Wang, P.A. Chernavskii, Y. Wang, A.Y. Khodakov, Influence of the support and promotion on the structure and catalytic performance of copper-cobalt catalysts for carbon monoxide hydrogenation, Fuel 103 (2013) 1111–1122. [20] G. Prieto, S. Beijer, M.L. Smith, M. He, Y. Au, Z. Wang, D.A. Bruce, K.P. de Jong, J.J. Spivey, P.E. de Jongh, Design and synthesis of copper-cobalt catalysts for the selective conversion of synthesis gas to ethanol and higher alcohols, Angew. Chem. Int. Ed. 53 (2014) 6397–6401. [21] Y. Xiang, V. Chitry, P. Liddicoat, P. Felfer, J. Cairney, S. Ringer, N. Kruse, Longchain terminal alcohols through catalytic CO hydrogenation, J. Am Chem. Soc. 135 (2013) 7114–7117. [22] Y. Xiang, R. Barbosa, X. Li, N. Kruse, Ternary cobalt-copper-niobium catalysts for the selective CO hydrogenation to higher alcohols, ACS Catal. 5 (2015) 2929–2934. [23] Z. Wang, N. Kumar, J.J. Spivey, Preparation and characterization of lanthanumpromoted cobalt-copper catalysts for the conversion of syngas to higher oxygenates: formation of cobalt carbide, J. Catal. 339 (2016) 1–8. [24] A. Cao, G. Liu, L. Wang, J. Liu, Y. Yue, L. Zhang, Y. Liu, Growing layered double hydroxides on CNTs and their catalytic performance for higher alcohol synthesis from syngas, J. Mater. Sci. 51 (2016) 5216–5231. [25] M. Zhang, H. Gong, Y. Yu, DFT study of key elementary steps for C2+ alcohol synthesis on bimetallic sites of Cu-Co shell-core structure from syngas, Mol. Catal. 443 (2017) 165–174. [26] A. Cao, J. Schumann, T. Wang, L. Zhang, J. Xiao, P. Bothra, Y. Liu, F. AbildPedersen, J.K. Nørskov, Mechanistic insights into the synthesis of higher alcohols from syngas on CuCo alloys, ACS Catal. 8 (2018) 10148–10155. [27] K. Xiao, X. Qi, Z. Bao, X. Wang, L. Zhong, K. Fang, M. Lin, Y. Sun, CuFe, CuCo and CuNi nanoparticles as catalysts for higher alcohol synthesis from syngas: a comparative study, Catal. Sci. Technol. 3 (2013) 1591–1602. [28] K. Xiao, Z. Bao, X. Qi, X. Wang, L. Zhong, K. Fang, M. Lin, Y. Sun, Structural evolution of CuFe bimetallic nanoparticles for higher alcohol synthesis, J. Mol. Catal. A Chem. 378 (2013) 319–325. [29] W. Gao, Y. Zhao, J. Liu, Q. Huang, S. He, C. Li, J. Zhao, M. Wei, Catalytic conversion of syngas to mixed alcohols over CuFe-based catalysts derived from layered double hydroxides, Catal. Sci. Technol. 3 (2013) 1324–1332. [30] Y. Lu, B. Cao, F. Yu, J. Liu, Z. Bao, J. Gao, High selectivity higher alcohols synthesis from syngas over three-dimensionally ordered macroporous Cu-Fe catalysts, ChemCatChem 6 (2014) 473–478. [31] Y. Lu, R. Zhang, B. Cao, B. Ge, F.F. Tao, J. Shan, L. Nguyen, Z. Bao, T. Wu, J.W. Pote, B. Wang, F. Yu, Elucidating the copper-Hägg iron carbide synergistic interactions for selective CO hydrogenation to higher alcohols, ACS Catal. 7 (2017) 5500–5512. [32] H.T. Luk, C. Mondelli, S. Mitchell, S. Siol, J.A. Stewart, D. Curulla Ferré, J. PérezRamírez, Role of carbonaceous supports and potassium promoter on higher alcohols synthesis over copper-iron catalysts, ACS Catal. 8 (2018) 9604–9618. [33] T. Lin, X. Qi, X. Wang, L. Xia, C. Wang, F. Yu, H. Wang, S. Li, L. Zhong, Y. Sun, Direct production of higher oxygenates by syngas conversion over a multifunctional catalyst, Angew. Chem. Int. Ed. 58 (2019) 4627–4631. [34] W. Zhou, J. Kang, K. Cheng, S. He, J. Shi, C. Zhou, Q. Zhang, J. Chen, L. Peng, M. Chen, Y. Wang, Direct conversion of syngas into methyl acetate, ethanol, and ethylene by relay catalysis via the intermediate dimethyl ether, Angew. Chem. Int. Ed. 57 (2018) 12012–12016. [35] Q. Zhang, J. Kang, Y. Wang, Development of novel catalysts for Fischer-Tropsch synthesis: tuning the product selectivity, ChemCatChem 2 (2010) 1030–1058. [36] H.M.T. Galvis, J.H. Bitter, C.B. Khare, M. Ruitenbeek, A.I. Dugulan, K.P. de Jong, Supported iron nanoparticles as catalysts for sustainable production of lower olefins, Science 335 (2012) 835–838. [37] J.M. Cho, S.R. Lee, J. Sun, N. Tsubaki, E.J. Jang, J.W. Bae, Highly ordered mesoporous Fe2O3-ZrO2 bimetal oxides for an enhanced CO hydrogenation activity to hydrocarbons with their structural stability, ACS Catal. 7 (2017) 5955–5964. [38] V.R. Calderone, N.R. Shiju, D. Curulla-Ferré, S. Chambrey, A. Khodakov, A. Rose, J. Thiessen, A. Jess, G. Rothenberg, De novo design of nanostructured iron-cobalt Fischer-Tropsch catalysts, Angew. Chem. Int. Ed. 52 (2013) 4397–4401.

13