Effects of Ag on morphology and catalytic performance of iron catalysts for Fischer–Tropsch synthesis

Abstract

Fe₃O₄ nanoparticles with pore size of 12.4 nm were synthesized and employed as catalyst for Fischer–Tropsch (FT) synthesis. The as-prepared Fe₃O₄ catalyst achieved a CO conversion of 98.3% while yielding higher than 50 wt% gasoline range (C₅–C₁₁) hydrocarbons after FT reaction for 48 h. Furthermore, highly activated Ag-doped composites were designed through a one-pot solvothermal method, and then porous core/shell materials were obtained. Interestingly, active metal oxide (Fe₃O₄) nanoparticles were interspersed on the surface of the Ag promoter. Importantly, pores could enhance the dispersion of metal particles and facilitate heat and mass transfer. The addition of Ag promoter decreased the selectivity to CH₄ and enhanced the yield of C₂–C₄ olefins. In particular, 0.8Ag/Fe₃O₄ displayed high CO conversion (96.4%) and optimum selectivity to C₂–C₄ olefins (28.3 wt%) while yielding a low selectivity to CH₄ (12.1 wt%), as well as a good selectivity to C₅–C₁₁. More importantly, 0.8Ag/Fe₃O₄ showed the highest catalytic activity (>1.6 × 10⁻⁴ mol_co g_Fe⁻¹ s⁻¹) and the best total hydrocarbon yield (5.25 × 10⁻³ g_HC g_Fe⁻¹ s⁻¹).

---

1. Introduction

Fischer–Tropsch (FT) synthesis is accepted as an effective method for the production of long-chain hydrocarbons from synthesis gas (a mixture of H₂ and CO) through a surface “alkyl” reaction. In particular, FT synthesis of gasoline range (C₅–C₁₁) hydrocarbons and C₂–C₄ olefins have been considered as a direct and an effective route to meet the high demands for liquid fuels and chemicals industry.^1,2 Due to their low methanation activity and excellent water gas shift reaction activity, Fe-based materials have attracted considerable attention.^2–4 However, bulk Fe catalysts display poor mechanical stability at high temperatures, thus inhibiting the formation of light hydrocarbons. Therefore, structural promoters with different functions are often added to a common catalyst to improve the catalytic performance and promote the selectivity to the desired products.^2,5,6 Nevertheless, the interaction among different modifiers cannot be ignored, and the synergistic effect between different promoters is still in dispute.⁵

Aside from promoters, porous supports are often used to alter the catalytic performance by changing the reducibility and improving the mechanical stability.^2,5,7 Moreover, supports may also influence the CO dissociation ability by changing the electronic state of the active metal.⁸ However, the chemical interaction between the support and the active phase may also significantly constrain the catalytic behavior. Although a very weak interaction may cause a poor dispersion of the active phase, a very strong interaction will lead to difficulty in the reduction of the precursor of the active phase.⁵ Furthermore, it has been demonstrated that activity and selectivity display an inverse relationship for supported iron catalysts.²

Although noble metal (Ru or Re) promoted Co-based catalysts have been widely used and investigated, only a few reports have contributed towards elucidating the role of noble metals on the catalytic behavior of Fe-based catalysts.⁵ Moreover, most of the earlier investigations were carried out on multi-component catalysts including several structural promoters, and all the promoters may work together to affect the catalytic performance. Therefore, the influence of a single promoter on the catalytic performance is rarely reported, in particular the influence of Ag promoter on the catalytic performance of Fe-based catalysts.

To exclude the influence of supports and promoters, promoter- and support-free porous Fe₃O₄ nanoparticles were designed and used as FT catalyst. Furthermore, highly activated Ag-doped composites with low interaction between Ag and iron were directly synthesized using a one-pot solvothermal method, with the aim of achieving high catalytic activity and stability, as well as good selectivity to C₅–C₁₁ in liquid products and C₂–C₄ olefins in tail gas. In particular, the addition of an Ag promoter is expected to lower the selectivity to CH₄. To the best of our knowledge, the morphology and catalytic performance of these kinds of structured composites have not been reported before. In addition, the study on Ag-containing Fe-based catalysts is insufficient. For better comparison, 0.5Ag/Fe₃O₄, 0.8Ag/Fe₃O₄, and Ag/Fe₃O₄ composites were prepared by changing the concentration of AgNO₃ (Table S1†).

2. Experimental

2.1 Materials preparation

The catalysts used in this study were synthesized using a one-pot solvothermal method. The detailed preparation procedure is described as follows. In brief, 0.13 M NaBH₄ (2 mL) aqueous solution was added to 30 mL of ethylene glycol (EG) containing 0.3 M PVP and 0.05 M AgNO₃. Then, FeCl₃·6H₂O with a weight ratio of 0.5Ag/Fe₃O₄ was poured into the abovementioned solution, followed by the addition of 0.8 M sodium acetate (NaAc) and 7.0 mL of ethylenediamine. Finally, the mixture was maintained at 50 °C for 30 min, and then transferred to a sealed Teflon-lined stainless-steel autoclave and heated at 200 °C for 15 h. The black products were collected with an external magnet and washed with ethanol three times. In addition, 0.8Ag/Fe₃O₄ and Ag/Fe₃O₄ were also prepared by changing the concentration of AgNO₃.

2.2 Materials characterization

Field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) was used to observe the morphology of the catalysts. X-Ray powder diffraction (XRD) was carried out on an X'Pert Pro MPD X-ray diffractometer (PANalytical). X-Ray photoelectron spectroscopy (XPS) studies were performed on a Thermo ESCALAB 250XI system. Brunauer–Emmett–Teller (BET) surface area data were obtained following N₂ physisorption on an automated surface area and porosity analyzer (SI-MP-10, Quantachrome) after 0.1 g of the sample was dried at 200 °C for 10 h, and the pore size distribution plots were calculated by applying the Barrett–Joyner–Halenda (BJH) model. Fourier transform infrared (FT-IR) spectra were recorded with a Tensor 27 spectrometer (Bruker). Temperature-programmed reduction (TPR) experiments on the freshly prepared catalysts (0.1 g) were carried out using a chemisorption analyzer (CPB-1, Quantachrome) with pure H₂ as the reducing gas and He as the carrier gas.

2.3 Catalytic experiments

The catalytic activity and product selectivity of the as-prepared catalysts were analyzed in a fixed bed. The flow rate of the tail gas was measured using a wet gas flow meter. The catalyst was reduced in syngas (H₂/CO = 1) at 300 °C, 4 bar, and at the rate of 3000 mL⁻¹ h⁻¹ g⁻¹ for 12 h. Subsequently, the FT reaction was processed under the conditions of 280 °C, 20 bar, and H₂/CO = 1 supplied at the rate of 3000 mL⁻¹ h⁻¹ g⁻¹. The tail gas was analyzed using an online gas chromatograph (Agilent, 7890A) equipped with a TCD and a FID. The hydrocarbon products in liquid and water were analyzed using an offline gas chromatograph (Shimadzu, 2010).

3. Results and discussion

3.1 Synthesis and characterization of Ag-doped Fe₃O₄ microspheres

During the solvothermal process, EG was employed as a strong reducing agent because it can react with Fe³⁺ to form Fe₃O₄ at a high temperature, and its strong chelating ability facilitates it to form complexes using its hydroxyl groups as ligands.^9,10 In particular, PVP generally serves as a surface stabilizer to form an organic layer on the surface of the nanocrystals, thus inducing the migration and promoting the growth of Fe₃O₄ nanocrystals. The strong affinity of electron lone pairs on imide groups in the PVP group controls the oriented migration and prevents nanoparticles from agglomeration as a result of the steric effect.¹¹ During the aggregation process, adjacent nanoparticles aggregate by an oriented-attachment mechanism. Thus, the nanoparticles share a planar interface in a common crystallographic orientation. As shown in Fig. 1a, aggregated Fe₃O₄ nanoparticles are observed to be driven by the minimization of total surface energy. In other words, newly formed nanoparticles migrate toward the PVP-stabilized low energy structure. On the basis of this, the fabrication of Ag-doped composites is outlined in Scheme 1. In simple terms, PVP-entangled Ag nanocrystals first nucleate in EG solution because of the reduction of AgNO₃ using NaBH₄ solution. Subsequently, the newly-formed Fe₃O₄ nanocrystals migrate toward PVP-entangled Ag nanoparticles, which are driven by the lone pairs in the organic layer (PVP).^11,12 Therefore, the as-formed composites present a core–shell characteristic. Importantly, the shell material consists of many Fe₃O₄ nanoparticles and an Ag microsphere serves as a core.

Fig. 1 SEM images (a–d) of freshly prepared Fe₃O₄, 0.5Ag/Fe₃O₄, 0.8Ag/Fe₃O₄, and Ag/Fe₃O₄ and TEM images (e and f) of 0.8Ag/Fe₃O₄ and Ag/Fe₃O₄, respectively.

Scheme 1 Schematic illustration of the formation process of Ag-doped composites.

To investigate the morphology of such Ag-doped composite structures, concentration-dependent experiments were carried out. The corresponding morphologies of the as-prepared samples with different mass ratios of Ag to Fe₃O₄ are shown in Fig. 1b–d. Interestingly, all the Ag-doped microspheres consist of many Fe₃O₄ nanoparticles, which randomly intersperse on the surface of an Ag microsphere. For better demonstration the structure of the composites, TEM images of 0.8Ag/Fe₃O₄ (Fig. 1e) and Ag/Fe₃O₄ (Fig. 1f) were obtained. It is clearly observed that many nanoparticles adsorb on a microsphere, suggesting that the synthesized Ag-doped samples had a core/shell structure. Of particular interest is that the shapes of the shell materials in Fig. 1b–d match well with those in Fig. 1a, indicating that Fe₃O₄ nanoparticles are the shell materials.

To further understand the formation mechanism of the Ag-doped structure under the present conditions, the 0.8Ag/Fe₃O₄ microsphere was used as a candidate for the following investigation and time-dependent experiments were carried out. The related SEM images of morphology transformation with reaction time were obtained. As shown in Fig. 2a, Fe₃O₄ nanoparticle-coated Ag microspheres can be found after reaction for 5 h; however, Fe₃O₄ nanoparticles display blurred boundaries and poor crystallinity. This is because directional migration, rather than crystal growth, is considered to be the main factor at the beginning of the formation of Fe₃O₄ nanocrystals. Through the migration process, adjacent nanocrystals will fuse together along the (111) direction and form a large nanoparticle by Ostwald ripening, and thus minimize the surface energy.^12,13 As presented in Fig. 2b, quasi-spherical Fe₃O₄ nanoparticles with an average size of 25 nm are observed after a reaction time of 10 h and the size is increased to 40 nm when the reaction time is prolonged to 15 h (Fig. 1b). The results clearly support the proposed mechanism in Scheme 1. Interestingly, this special core–shell structure is rarely reported.

Fig. 2 SEM images of 0.8Ag/Fe₃O₄ composites obtained at different reaction times: (a) 5 h and (b) 10 h.

The chemical composition of the freshly prepared samples was measured using XRD and the results are shown in Fig. 3a. In the absence of AgNO₃, all the diffraction peaks in the black line are readily indexed to a cubic Fe₃O₄ phase and no impure peaks can be observed. Compared with the Ag-free Fe₃O₄ sample (Fig. 1a), obvious peaks at 37.9°, 44.1°, 64.7°, and 77.5°, corresponding to the (111), (200), (220), and (311) planes of metal Ag are observed. The well-resolved diffraction peaks reveal good crystallinity in the Ag specimens. However, no impurity oxide peaks can be observed except for Fe₃O₄, suggesting that Ag does not occupy the tetrahedral site of Fe³⁺ during the process of nucleation. Furthermore, the ICP results in Table S1† can demonstrate that the weight of Ag in Ag-doped catalysts gradually increases with increasing the concentration of AgNO₃. This reveals that Ag is introduced and successfully participates in the phase composition of the composites.

Fig. 3 XRD patterns (a) and FT-IR spectra (b) of freshly prepared samples.

To understand the interaction between PVP and Fe₃O₄ nanoparticles in the products, the FT-IR spectra of the samples were obtained. The pink line in Fig. 3b shows the spectrum of pure PVP. The band that appeared at 3460 cm⁻¹ is assigned to O–H stretching vibration.¹⁴ It is observed that the spectra of the products are considerably similar to that of the pure PVP. The absorption peaks centered around 2860–3000 cm⁻¹ are related to C–H stretching and the peak at 1663.9 cm⁻¹ is attributed to C=O stretching. In particular, a sharpened peak at 584 cm⁻¹ is due to the Fe–O lattice mode of Fe₃O₄ (Fig. 3b).¹⁵ The deformation vibration absorption peak of C–H moves from 1427.6 cm⁻¹ (PVP) to 1442.8 cm⁻¹, and the stretching vibration of C=O shifts from 1663.9 (PVP) to 1632.8 cm⁻¹ in the synthesized products. The migration of C=O and C–H vibrations reveal that PVP coordinates with Fe₃O₄ nanocrystals.¹⁶ The possible interaction between Fe₃O₄ nanocrystals and PVP molecules has been diagrammatically described in Scheme 1. Furthermore, all the FT-IR peaks of the Ag-doped composites match well with that of the Ag-free sample (black line), indicating that no Ag–O bond is formed. The results indicate that Fe₃O₄ is the only oxide in this system and this is in line with the XRD results.

The chemical environment of various surface components in the Ag-doped composites can be further identified by XPS (Fig. 4). As presented in Fig. 4a, the elemental peaks of C, N, Ag, Fe, and O are observed. It is clear that the N–C=O group in PVP is responsible for the peaks of C1s and N1s. In Fig. 4b, Ag3d shows two symmetrical peak shapes and the peak binding energies of Ag3d_3/2 and Ag3d_5/2 for the products are 373.5 and 367.5 eV with a peak separation of 6.0 eV, indicating the presence of Ag.¹⁷ Moreover, the loss feature at 370.9 eV results from the spin–orbit component for metal Ag.¹⁸ As shown in Fig. 4c, the high resolution Fe2p spectra of the samples present two distinct broad peaks at binding energies of 723.9 and 710.3 eV for Fe2p_1/2 and Fe2p_3/2, and the values are consistent with the report for Fe₃O₄.¹⁹ As presented in Fig. 4d, the O1s profile of the samples is asymmetric and it can be fitted to two peaks at 529.4 and 530.5 eV. Oxygen species on the surface of Fe₃O₄ particles match well with that position. Interestingly, the atomic ratio of O to Fe maintains an approximate value of 1.5 with increasing concentration of AgNO₃ (inset in Fig. 4d), indicating that no silver oxide is formed. In particular, the excess percent of atomic O can be attributed to the O–H and C=O groups, as described in Fig. 3b.

Fig. 4 XPS spectra of freshly prepared products (a), high-resolution XPS spectra of Ag3d (b), Fe2p (c) and O1s (d).

The specific surface area and pore size of the freshly prepared products were characterized using the N₂ sorption technique. Fig. 5a shows type IV isotherms of Fe₃O₄, 0.5Ag/Fe₃O₄, 0.8Ag/Fe₃O₄, and Ag/Fe₃O₄ microspheres along with obvious hysteresis loops at relative pressures of P/P₀ = 0.7–1.0, indicating the presence of mesopores in the products.^20,21 The BET surface area of the microspheres is calculated to be 53.4, 42.0, 45.7, and 40.4 m² g⁻¹, respectively. It should be noted that the pores formed in this study are attributed to the interspaces of the adjacent Fe₃O₄ particles (Fig. 1). Therefore, the random aggregation of Fe₃O₄ particles is responsible for the wide range of pore sizes (Fig. 5b). As shown in Fig. 5b, the major pore sizes are centered at 12.4, 16.8, 17.3, and 12.4 nm. The results demonstrate that the as-prepared products are mesoporous materials. It is known that mesoporous structure can provide efficient transport pathways to the interior cavities, thus enhancing the catalytic activity.^16,22

Fig. 5 N₂ adsorption–desorption results of freshly prepared products: (a) BET isotherms and (b) BJH isotherms.

3.2 FT performance of as-prepared catalysts

FT synthesis reaction was measured under the conditions of 280 °C, 20 bar, H₂/CO ratio of 1 supplied at the rate of 3000 mL⁻¹ g⁻¹ h⁻¹. Then, catalytic activity and product selectivity of Ag-free and Ag-doped catalysts could be obtained. On the basis of this, the influence of Ag on catalytic performance can be investigated. The corresponding results for all the catalysts are summarized in Table 1. As can be seen, the Fe₃O₄ catalyst achieved a high CO conversion of 98.3% combined with a high selectivity of 50.3 wt% to gasoline range hydrocarbons (C₅–C₁₁) after a 48 h reaction. Moreover, selectivity to CH₄ as a side product was lower than 14.4 wt% while yielding a high C₂–C₄ olefin fraction (22.0 wt%). This catalytic performance is superior to those of previously reported supported iron-based catalysts.^5–7,23 In a previous study, promoter-free Fe catalysts showed a relatively low selectivity of 25.8 wt% to C₅–C₁₁.⁶

Table 1 Catalytic performance of samples. Catalytic tests are performed at 280 °C, 20 bar, a H₂/CO ratio of 1, 3000 mL⁻¹ g⁻¹ h⁻¹ for 48 h

Performance	Fe3O4	0.5Ag/Fe3O4	0.8Ag/Fe3O4	Ag/Fe3O4
CO conversion (%)
	98.3	88.4	96.4	71.8
Product selectivity (wt%)
CH4	14.4	11.5	12.1	11.7
C2–C4 paraffins	9.4	6.5	7.6	6.8
C2–C4 olefins	22.0	24.6	28.3	27.3
C5–C11	50.3	49.1	46.2	46.1
C12+	3.9	8.3	5.8	8.1

---

On the other hand, it can be observed that the introduction of an Ag promoter suppresses the formation of CH₄ but accelerates the selectivity to C₂–C₄ olefins (Table 1). To the best of our knowledge, few reports have referred to the elucidation of the influence of an Ag promoter on the catalytic performance of Fe-based catalysts for FT synthesis. However, it is widely accepted that a noble metal promoter (Ru or Re) is an effective strategy for accelerating the reducibility of Co species into metal Co(0) clusters, and thus suppress the hydrogenation reaction.^5,24,25 On the basis of this, the H₂-TPR profiles of Ag-free and Ag-doped catalysts were carried out in this study to investigate the effect of Ag on the reduction behavior of iron oxide. As shown in Fig. 6a, the reduction process of the Ag-free Fe₃O₄ catalyst displays three distinct stages. The first stage is ascribed to the transformation of Fe₂O₃ to Fe₃O₄, followed by the transformation of Fe₃O₄ to FeO and then transformation of FeO to Fe.⁹ The adsorption peak of the Fe₃O₄ catalyst at 320–400 °C is smaller and only Fe₃O₄ phase is detected in Fig. 3a and b (black line), indicating that the small amount of Fe₂O₃ is the result of partial oxidization of Fe₃O₄ nanoparticles. Significant changes to the peaks took place after the inclusion of Ag. Notably, only two reduction peaks for the conversion of Fe₃O₄ to FeO and FeO to Fe are found, in addition to an obvious blue shift of peak positions in contrast to that of the Fe₃O₄ catalyst (black line). It is a reasonable assumption that the addition of Ag promoter suppresses the oxidization of Fe₃O₄ to Fe₂O₃ and facilitates the reduction of iron oxide to Fe. In addition, H₂ conversion with the Ag-free catalyst is higher than that with the Ag-doped catalyst with time on stream (Table S2†). On the basis of this, it may be speculated that the Ag promoter limits the hydrogenation reaction, thus increasing the FT production of C₂–C₄ olefins without increasing the selectivity to CH₄ (Table 1).

Fig. 6 (a) H₂-TPR profiles of freshly prepared catalysts, (b) ASF plots of catalysts after a 48 h reaction.

Fig. 6b displays the ASF plots of catalysts after 48 h of FT reaction. The Ag-free Fe₃O₄ catalyst shows product selectivity, which can be modelled with a chain growth probability (α) of 0.40. Although 0.5Ag/Fe₃O₄ and Ag/Fe₃O₄ achieve lower CO conversions of 88.4% and 71.8%, they show high selectivity to C₂–C₄ olefins combined with a lower CH₄ product fraction at calculated α values of 0.34 and 0.35, respectively, (Fig. 6b). Furthermore, 0.8Ag/Fe₃O₄ provides an α value of 0.38, which is close to the value for Ag-free catalyst. Moreover, it achieved a CO conversion of 96.4%, which is comparable to that of Fe₃O₄ catalyst. More importantly, 0.8Ag/Fe₃O₄ exhibited the maximum selectivity to C₂–C₄ olefins (28.3 wt%). This result can be rationalized using the “alkyl” mechanism.^2,4 During FT synthesis, a CH₃ group adsorbs on the surface of the catalyst following CO dissociation and hydrogenation on carbon, which acts as a chain initiator. The chain growth is promoted via the insertion of CH₂ monomer units into the adsorbed alkyl species. The chain growth can be terminated by the abstraction of β-hydride to produce α-olefins or by hydrogenation to form paraffins. Then, the hydrogenation reaction is restrained (Table S2†) but the olefin selectivity is improved. The suppression of the CH₄ and paraffin reactions may be understood to occur due to the introduction of the Ag promoter (Table S2†).

The catalytic activity is expressed as the number of CO moles converted to hydrocarbons per gram of iron per second and thus is noted as the iron-time-yield (FTY) over time on stream. The weight of Fe per gram of catalyst is calculated according to the ICP analysis presented in Table S1.† It has been reported that the transformation of Fe₃O₄ to FeCx in syngas is of great importance, probably because FeCx is believed to be an active phase.⁵ In the case of the XRD patterns of the Ag-free catalyst after a 72 h reaction (Fig. 7a), obvious C peaks can be found. Carbon deposition may inhibit the interaction between the FeCx phase and syngas and thus result in the coexistence of the FeCx and Fe₃O₄ phases. As presented in Fig. 7a, the C peaks appear to become weaker as the weight of Ag increases. The result suggests that the addition of Ag promoter may inhibit the deposition of carbon. More importantly, weak FeCx phases in the blue and green pattern (Fig. 7a) demonstrate that the Ag promoter accelerates the reoxidation of FeCx to Fe₃O₄. Then, the new FeCx phase is re-formed on the surface of the catalysts with time on stream, thus improving the catalytic activity. Fig. 7b shows the FTY values of the catalysts with time on stream. As expected from the calculation results, the catalytic activity in all the Ag-doped catalysts is superior to that of the Ag-free Fe₃O₄ catalyst, thus demonstrating that the catalytic activity can be quickly activated with Ag. In particular, 0.8Ag/Fe₃O₄ showed the highest FTY value of 1.6 × 10⁻⁴ mol_co g_Fe⁻¹ s⁻¹ for up to 72 h. This FTY value is much higher than the previously reported highest value (8.48 × 10⁻⁵ mol_co g_Fe⁻¹ s⁻¹) among the various supported iron nanostructures.²³ Therefore, the enhanced activity mainly originates from improved activation in the presence of the Ag promoter.

Fig. 7 Catalytic activity with time on stream (a) and XRD patterns of catalysts after a 72 h FT reaction (b).

The total hydrocarbon (HC) product yields (gram of generated hydrocarbons per gram of iron per second) for all the catalysts were measured after 48 h of FT reaction, which were calculated by the sum of specific product yields (Fig. 8 and Table 2). The specific product yield for a catalyst was obtained by analyzing the feed gas (H₂, CO) and the tail gas (H₂, CO, CO₂, and C_1–4 hydrocarbon) using an online GC equipped with a TCD and a FID. Moreover, the liquid products were detected using an offline GC. It can be observed from Fig. 8 that the total HC product yield for Ag-doped catalysts is higher than that for the Ag-free Fe₃O₄ catalyst. In particular, the 0.8Ag/Fe₃O₄ catalyst displays the optimum total HC productivity, which is calculated to be 5.25 × 10⁻³ g_HC g_Fe⁻¹ s⁻¹. The effect of the Ag promoter on the specific product yield is dramatic. The Ag-doped catalysts show high catalytic activity (Fig. 7b) combined with a high specific product yield of the desired products, including C₂–C₄ olefins and C₅–C₁₁ HC (Table 2). In particular, 0.8Ag/Fe₃O₄ exhibits a high productivity to gasoline range (C₅–C₁₁) products (2.43 × 10⁻³ g_HC g_Fe⁻¹ s⁻¹), while showing a product yield of 1.48 × 10⁻³ g_HC g_Fe⁻¹ s⁻¹ for C₂–C₄ olefins. Thus, the enhancement of total HC product yields, particularly for C₂–C₄ olefins and C₅–C₁₁ hydrocarbons, may be understood to occur due to the introduction of Ag promoter, indicating that Ag may be promising for use in the FT synthesis of desired HC products.

Fig. 8 Total hydrocarbon product yield for the catalysts after a 48 h reaction.

Table 2 Specific product yields of catalysts after 48 h on stream

Catalyst	Specific product yield (10^−4 gHC gFe^−1 s^−1)
	CH4	C2–C4 paraffins	C2–C4 olefins	C5–C11	C12+
Fe3O4	4.90	3.16	7.45	17.0	1.35
0.5Ag/Fe3O4	4.80	2.72	10.3	20.6	3.53
0.8Ag/Fe3O4	6.35	4.03	14.8	24.3	3.04
Ag/Fe3O4	5.41	3.15	12.7	21.3	3.75

---

4. Conclusions

Unprompted Fe₃O₄ nanoparticles were synthesized and used as FT catalysts for the production of C₅–C₁₁ hydrocarbons (50.3 wt%). Furthermore, Ag-doped porous catalysts consisting of active iron oxide and Ag promoter were successfully fabricated after the introduction of AgNO₃. In particular, Fe₃O₄ nanoparticles were interspersed on the surface of Ag core and pores were formed in the interspaces of the neighboring Fe₃O₄ nanoparticles. More importantly, the addition of the Ag promoter not only modified the morphology of the catalyst but also influenced the catalytic performance. The Ag-doped catalysts exhibited high selectivity toward C₂–C₄ olefins and lower selectivity to CH₄. In particular, 0.8Ag/Fe₃O₄ showed a high CO conversion of 96.4% with a good selectivity to C₂–C₄ olefins (28.3 wt%) and the CH₄ product fraction decreased to 12.1 wt%. Moreover, it displayed the highest FTY values with the best total HC product yield and the highest C₂–C₄ olefin and C₅–C₁₁ productivity. It is anticipated that the Ag promoter will be promising for FT synthesis.

Acknowledgements

This work was supported financially by the National Key Basic Research Program of China (973 program, 2013CB228105) and the National Natural Science Foundation of China (51302263), (U1362109) and (51206172).

Notes and references

1. J. Zhang, K. Fang, K. Zhang, W. Li and Y. Sun, Korean J. Chem. Eng., 2009, 26, 890.
2. H. M. T. Galvis, J. H. Bitter, C. B. Khare, M. Ruitenbeek, A. I. Dugulan and K. P. de Jong, Science, 2012, 335, 835.
3. C. Wang, L. Xu and Q. Wang, J. Nat. Gas Chem., 2003, 12, 10.
4. Y. Jin and A. K. Datye, J. Catal., 2000, 196, 8.
5. Q. Zhang, J. Kang and Y. Wang, ChemCatChem, 2010, 2, 1030.
6. H. Wan, B. Wu, C. Zhang, H. Xiang and Y. Li, J. Mol. Catal. A: Chem., 2008, 283, 33.
7. H. Xiong, M. Moyo, M. A. Motchelaho, Z. N. Tetana, S. M. A. Dube, L. L. Jewell and N. J. Coville, J. Catal., 2014, 311, 80.
8. A. Y. Khodakov, W. Chu and P. Fongarland, Chem. Rev., 2007, 107, 1692.
9. H. Deng, X. L. Li, Q. Peng, X. Wang, J. P. Chen and Y. D. Li, Angew. Chem., Int. Ed., 2005, 44, 2782.
10. J. Chen, T. Herricks, M. Geissler and Y. Xia, J. Am. Chem. Soc., 2004, 126, 10854.
11. V. G. Pol, H. Grisaru and A. Gedanken, Langmuir, 2005, 21, 3635.
12. B. Jia and L. Gao, J. Phys. Chem. C, 2008, 112, 666.
13. Q. Q. Xiong, J. P. Tu, Y. Lu, J. Chen, Y. X. Yu, Y. Q. Qiao, X. L. Wang and C. D. Gu, J. Phys. Chem. C, 2012, 116, 6495.
14. S. Xuan, Y.-X. J. Wang, J. C. Yu and K. C.-F. Leung, Langmuir, 2009, 25, 11835.
15. T. Fan, D. Pan and H. Zhang, Ind. Eng. Chem. Res., 2011, 50, 9009.
16. M. Zhu and G. Diao, J. Phys. Chem. C, 2011, 115, 18923.
17. B. Chai, X. Wang, S. Cheng, H. Zhou and F. Zhang, Ceram. Int., 2014, 40, 429.
18. K. Lalitha, J. K. Reddy, M. V. P. Sharma, V. D. Kumari and M. Subrahmanyam, Int. J. Hydrogen Energy, 2010, 35, 3991.
19. S. Liu, R. Xing, F. Lu, R. K. Rana and J.-J. Zhu, J. Phys. Chem. C, 2009, 113, 21042.
20. D. H. Chen, L. Cao, F. Z. Huang, P. Imperia, Y. B. Cheng and R. A. Caruso, J. Am. Chem. Soc., 2010, 132, 4438.
21. S. Xuan, F. Wang, J. M. Y. Lai, K. W. Y. Sham, Y.-X. J. Wang, S.-F. Lee, J. C. Yu, C. H. K. Cheng and K. C.-F. Leung, ACS Appl. Mater. Interfaces, 2011, 3, 237.
22. H. Li, Z. Bian, J. Zhu, D. Zhang, G. Li, Y. Huo, H. Li and Y. Lu, J. Am. Chem. Soc., 2007, 129, 8406.
23. J. C. Park, S. C. Yeo, D. H. Chun, J. T. Lim, J.-I. Yang, H.-T. Lee, S. Hong, H. M. Lee, C. S. Kim and H. Jung, J. Mater. Chem. A, 2014, 2, 14371.
24. Q. Cai and J. Li, Catal. Commun., 2008, 9, 2003.
25. G. Jacobs, M. C. Ribeiro, W. Ma, Y. Ji, S. Khalid, P. T. A. Sumodjo and B. H. Davis, Appl. Catal., A, 2009, 361, 137.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra10105k

---