Structural characterization and catalytic performanceThe synthesis steps of hydrophilic and hydrophobic catalysts were shown in Supplementary Fig. 1. Fe2O3 nanoparticle was synthesized by a hydrothermal method, and Mn promoter was deposited on Fe2O3 surface via deposition-precipitation. The hydrophilic SiO2-coated catalyst (Fe@Mn@xSi) was obtained by the controllable hydrolysis of x mL of tetraethyl orthosilicate (TEOS) on Fe@Mn. Transmission electron microscope (TEM) images and corresponding energy dispersive X-ray spectroscopy (EDS) elemental mapping confirmed that Fe@Mn nanoparticle was coated homogeneously by amorphous SiO2 (Supplementary Fig. 2). Besides, the content and thickness of SiO2 shell were effectively adjusted by altering the dosage of TEOS (Supplementary Table 1). The hydrophobic SiO2-coated catalyst (Fe@Mn@xSi-c) was further prepared via the surface silanization treatment (Supplementary Figs. 3–6).After the FTS reaction, the FeMn species were still well encapsulated by the SiO2 shell (Fig. 1a–f). Notably, the core of the Fe@Mn@0.2Si-c catalyst shrank obviously compared with the Fe@Mn@0.2Si catalyst. Considering the spatial dimensions of different iron phases vary greatly (Supplementary Fig. 7 and Supplementary Table 2), the change in core size of catalyst reflected the phase evolution of iron species during reaction. The detailed phase composition of catalyst was further characterized by X-ray diffraction (XRD) and Mössbauer spectrum (Supplementary Figs. 8,9 and Supplementary Table 3). During reaction, the Fe2O3 phase in the hydrophilic Fe@Mn@0.2Si catalyst was transformed into a mixture of 59.7% χ-Fe5C2, 34.3% Fe3O4 and 6.0% Fe3+(spm), while that in the hydrophobic Fe@Mn@0.2Si-c catalyst was converted into 74.1% χ-Fe5C2 and 25.9% θ-Fe3C (Fig. 1g). These results suggested that the surface hydrophobization had obvious effects on the phase evolution behavior of iron-based FTS catalyst during syngas conversion.Fig. 1: Structural characterization and catalytic performance.a–c TEM image (a), EDS elemental mapping (b), and structural model (c) of the spent Fe@Mn@0.2Si catalyst. d–f TEM image (d), EDS elemental mapping (e), and structural model (f) of the spent Fe@Mn@0.2Si-c catalyst. g Phase composition of the spent catalysts determined by Mössbauer spectroscopy. h, i CO conversion (h) and hydrocarbons distribution (i) of the Fe@Mn@0.2Si and Fe@Mn@0.2Si-c catalysts.Since hydrocarbon products are produced on iron carbide (FexC) rather than Fe3O418,19, the phase composition of catalyst obviously influenced the catalytic performances (Fig. 1h, i). The selectivity of CH4 in hydrocarbons decreased from 21.5% on the Fe@Mn@0.2Si catalyst to 14.1% on the Fe@Mn@0.2Si-c catalyst, accompanied with the increase of olefins selectivity from 49.8% to 64.3%, implying the enhancement of C − C coupling ability and inhibition of olefins hydrogenation activity during syngas conversion. In addition, the CO conversion to CO2 on the Fe@Mn@0.2Si catalyst was 28.9%, while that on the Fe@Mn@0.2Si-c catalyst was only 7.9%, which was attributed to that the hydrophobic shell inhibited water adsorption and hindered the water-gas shift (WGS) side reaction related to CO2 formation19,24. Moreover, the Fe@Mn@0.2Si-c catalyst presented a good catalytic stability during 110 hours of continuous reaction (Supplementary Fig. 10). The above results displayed the importance of surface hydrophobization in stabilizing the FexC active phase for enhancing olefins production.Insights into the phase evolution behavior of catalystTo illustrate the regulation effect of hydrophobic surface on the phase composition of catalyst, we investigated the phase evolution behaviors of the hydrophilic Fe@Mn@0.2Si and hydrophobic Fe@Mn@0.2Si-c catalysts in the CO + H2O model experiment under the reaction temperature and pressure of syngas conversion (Fig. 2a). The Fe2O3 phase in the two catalysts was reduced into metallic iron (Fe0) phase by reducing in pure H2 (Fig. 2b, c). After switching to the CO atmosphere, a low CO conversion of about 5% was observed, which decreased continuously with time on stream (Fig. 2d, e). Moreover, only Fe5C2 phase was detected in the two catalysts after the CO treatment, suggesting that CO molecules could diffuse through the hydrophilic and hydrophobic SiO2 shell and thus carbonize the internal Fe0 phase.Fig. 2: Phase evolution behaviors of the hydrophilic and hydrophobic catalysts.a The schematic diagram of the CO + H2O model experiment. Firstly, the catalyst was reduced in pure H2 at 350 °C, 0.1 MPa for 20 h. Subsequently, the catalyst was exposed to the CO or CO + H2O atmosphere at 320 °C, 2.0 MPa for 20 h. b, c XRD patterns of the Fe@Mn@0.2Si (b) and Fe@Mn@0.2Si-c (c) catalysts at different stages. d, e CO conversion of the Fe@Mn@0.2Si (d) and Fe@Mn@0.2Si-c (e) catalysts during the model experiment.When co-feeding CO and H2O in the reactor, the CO conversion of the Fe@Mn@0.2Si catalyst reached about 30% (Fig. 2d) and a mixture of Fe3O4 and Fe5C2 phases was detected (Fig. 2b), implying that H2O molecules could easily diffuse through the hydrophilic SiO2 shell and thus oxidize internal iron species. By contrast, the stable CO conversion of the Fe@Mn@0.2Si-c catalyst was less than 2% (Fig. 2e) and no Fe3O4 phase was detected after the CO + H2O treatment (Fig. 2c). As illustrated in our previous work24, the diffusion of water molecules through hydrophilic SiO2 is bidirectional, while the diffusion through hydrophobic SiO2 is unidirectional. Thus, the H2O molecules outside catalyst (H2Ooutside) could hardly diffuse in and influence the phase composition of the hydrophobic Fe@Mn@0.2Si-c catalyst. During syngas conversion, even though H2O molecules were produced on the internal FexC active sites, the hydrophobic surface hindered the entry of H2Ooutside, which reduced the water concentration in the core vicinity of catalyst. Thus, surface hydrophobization of catalyst could inhibit the oxidation of iron species and stabilize the FexC active phase during reaction.To further illustrate the protective effect of hydrophobic surface on iron carbide, in situ XRD characterizations on the Fe@Mn@0.2Si and Fe@Mn@0.2Si-c catalysts in the CO + H2O atmosphere were conducted. Before test, the iron species in the two catalysts were transformed into iron carbide by the H2 reduction and CO carbonization procedures in Fig. 2. Then, the two catalysts were exposed to the CO + H2O atmosphere at 320 °C to observe the influence of H2O on the iron carbide in the two catalysts. As shown in Fig. 3a, the iron carbide in the hydrophilic Fe@Mn@0.2Si catalyst gradually evolved once introducing H2O into the reactor chamber. With time on stream, the intensity of diffraction peaks related to iron carbide decreased, while new diffraction peaks at 35.4°, 57.0°, and 62.5° attributed to Fe3O4 phase appeared (Fig. 3b). In addition, the phase evolution proceeded rapidly and reached an equilibrium state within one hour (Fig. 3c). These results suggested that H2O molecules adsorbed easily on the hydrophilic Fe@Mn@0.2Si catalyst and thereby oxidized the internal iron carbide into Fe3O4 phase. As for the hydrophobic Fe@Mn@0.2Si-c catalyst, this phase evolution process was not observed (Fig. 3d). With time on stream, the intensity of diffraction peaks related to iron carbide in this hydrophobic catalyst remained stable and no diffraction peak related to Fe3O4 phase was detected (Fig. 3e, f). The above results of in situ XRD characterization clearly demonstrated the different phase evolution process of iron carbide in the hydrophilic Fe@Mn@0.2Si and hydrophobic Fe@Mn@0.2Si-c catalysts when exposing to the CO + H2O atmosphere, confirming that the oxidation of iron carbide by H2O molecules could be effectively inhibited via surface hydrophobization.Fig. 3: Inhibiting the oxidation of iron carbide by water via surface hydrophobization.a–c Heatmap (a), in situ XRD patterns (b), and the relative intensity between the diffraction peak of Fe5C2 at 44.2° and the diffraction peak of Fe3O4 at 35.4° (c) of the Fe@Mn@0.2Si catalyst when exposing to the CO + H2O atmosphere at 320 °C. d–f Heatmap (d), in situ XRD patterns (e), and the relative intensity between the diffraction peak of Fe5C2 at 44.2° and the diffraction peak of Fe3O4 at 35.4° (f) of the Fe@Mn@0.2Si-c catalyst when exposing to the CO + H2O atmosphere at 320 °C.Effects of shell thickness on the phase structure and catalytic performanceThe effects of the thickness of SiO2 shell on the reduction-carburization behaviors of internal iron species in catalyst were characterized by carbon monoxide temperature-programmed reduction (CO-TPR). As the increase of TEOS dosage, the reduction peaks shifted towards higher temperature and the peaks area gradually decreased (Supplementary Fig. 11), suggesting that the increase of shell thickness inhibited the accessibility and carburization of internal iron species by CO molecules. Therefore, after the FTS reaction, the content of FexC phase in catalyst reduced while that of Fe3O4 phase obviously increased with the thickening of SiO2 shell (Supplementary Fig. 12). As FexC phase was the active site for FTS reaction, the CO conversion to hydrocarbons decreased obviously from 53.8% of Fe@Mn@0.05Si to 24.3% of Fe@Mn@2.8Si (Fig. 4a). The increase of shell thickness did not inhibit the accessibility of internal iron species by H2 due to the much smaller size of hydrogen molecule (Supplementary Fig. 13), which would lead to a higher H2/CO ratio in the core locality of catalyst and enhance the hydrogenation reaction. As a result, the hydrocarbons distribution shifted towards lower carbon number and the olefins selectivity in hydrocarbons decreased from 66.8% of Fe@Mn@0.05Si to 27.1% of Fe@Mn@2.8Si (Fig. 4c and Supplementary Fig. 14). All the hydrophilic Fe@Mn@xSi catalysts exhibited high CO2 selectivity of about 40%, suggesting that H2O produced by the FTS reaction participated easily in the WGS side reaction (Supplementary Fig. 15).Fig. 4: Effects of shell thickness on the phase structure and catalytic performance.a, b CO conversion of the Fe@Mn@xSi (a) and Fe@Mn@xSi-c (b) catalysts. c, d Hydrocarbons distribution of the Fe@Mn@xSi (c) and Fe@Mn@xSi-c (d) catalysts. e XRD patterns of the spent catalysts. f XPS spectra of Cl 2p on catalysts.As discussed above, although H2O was produced on internal FexC during syngas conversion, the hydrophobic surface could restrict the entry of H2Ooutside, thereby reducing the water concentration in the core vicinity of catalyst (Figs. 2, 3). Therefore, the WGS side reaction was hindered and the oxidation of iron species by H2O was inhibited (Supplementary Fig. 16 and Fig. 1). The increase of SiO2 shell thickness inhibited the accessibility and carburization process of internal iron species by CO molecules, leading to the slight oxidation of iron species before H2Ointside diffused outside the Fe@Mn@0.5Si-c and Fe@Mn@1.0Si-c catalysts (Supplementary Fig. 17). With the gradually thickening of SiO2 shell, the catalytic activity and hydrocarbons distribution of the hydrophobic catalysts presented similar trend with that of the hydrophilic catalysts (Fig. 4b, d and Supplementary Fig. 18). Exceptionally, the Fe@Mn@0.05Si-c catalyst coated with the thinnest SiO2 shell exhibited low CO conversion of < 2% (Fig. 4b), and only Fe3O4 phase was detected in the spent Fe@Mn@0.05Si-c catalyst (Fig. 4e and Supplementary Fig. 19), suggesting that there was other factor that could inhibit the carbonization of iron species.Poisoning effect of chlorine on the phase evolutionWe noticed that chlorine element existed on the Fe@Mn@0.05Si-c catalyst, which was attributed to hydrogen chloride produced during the hydrophobic modification process using chlorotrimethylsilane (Supplementary Fig. 3). Chlorine could not be removed by the washing and vacuum drying procedures of catalyst synthesis and even the reaction at high temperature (Fig. 4f), due to its strong adsorption on the catalyst surface28,29,30. The existence of chlorine on catalyst surface reduced obviously the CO conversion (Supplementary Figs. 20, 21). Sulfur, ammonia and halogen compounds can behave as catalyst poisons and lead to the deactivation of FTS catalyst31,32,33. However, the mechanism of chlorine poisoning of the iron-based catalyst is still unclear. In industry, syngas derived from coal, especially biomass, generally contains chlorine impurity, which presents in the form of hydrogen chloride33,34,35. Thus, the form of chlorine studied in this work is similar to those in real process, and understanding the poisoning mechanism of chlorine on catalyst is also important for the industrial application.To understand the influence of chlorine on the formation of FexC active phase, in situ XRD characterizations on the Fe@Mn@0.05Si and Fe@Mn@0.05Si-c catalysts in the 10%CO/90%N2 atmosphere were conducted. With the increase of temperature, the phase composition of the Fe@Mn@0.05Si catalyst gradually evolved (Fig. 5b). The Fe2O3 phase was converted into Fe3O4 phase at 350 °C, which was transformed rapidly into FeO phase at 400 °C. Subsequently, Fe5C2 phase appeared at 500 °C. These results revealed the carbonization path of Fe2O3 → Fe3O4 → FeO→Fe5C2 of iron species, and this phase evolution process proceeded easily in the Fe@Mn@0.05Si catalyst with an initial transformation temperature of 350 °C. However, as for the Fe@Mn@0.05Si-c catalyst, the Fe2O3 phase was only converted slightly into Fe3O4 phase without the formation of FexC phase even at high temperature of 500 °C (Fig. 5c). The above results suggested that CO could hardly influence the phase composition of the Fe@Mn@0.05Si-c catalyst, thus no FexC active phase was formed during syngas conversion and the CO conversion was less than 2% (Fig. 5a).Fig. 5: Effect of chlorine on the phase evolution and CO adsorption behavior.a CO conversion of catalysts. b, c In situ XRD patterns of the Fe@Mn@0.05Si (b) and Fe@Mn@0.05Si-c (c) catalysts in the 10%CO/90%N2 atmosphere. d, e, g, h In situ DRIFTS spectra of CO adsorption on the Fe@Mn@0.05Si (d, e) and Fe@Mn@0.05Si-c (g, h) catalysts. The catalysts were exposed to the CO atmosphere at 320 °C for 120 min and then swept in the Ar atmosphere at 320 °C for another 120 min. f, i Normalized intensities of the surface species on the Fe@Mn@0.05Si (f) and Fe@Mn@0.05Si-c (i) catalysts as a function of time.To shed light on the reason of different phase evolution processes between the Fe@Mn@0.05Si and Fe@Mn@0.05Si-c catalysts, we further explored the CO adsorption behavior on the two catalysts by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Several bands at 2172, 2117, 1606, 1498, 1415, and 1364 cm−1 appeared on the Fe@Mn@0.05Si catalyst after CO adsorption (Fig. 5d). The doublet bands at 2172 and 2117 cm−1 was attributed to the vibrations of R-branch and P-branch of the gaseous CO on catalyst36, which diminished fast upon sweeping in the Ar atmosphere (Fig. 5e). The bands located in the region of 1700 ~ 1200 cm−1 were assigned to carbonate species formed on the metal oxides37,38,39. The intensity of the bands at 1498 and 1364 cm−1 approached the maximum value within only 20 min after exposing to the CO atmosphere and then gradually decreased (Fig. 5f). Simultaneously, new bands at 1606 and 1415 cm−1 appeared and their intensity continuously increased, implying the formation of new sites for CO adsorption. This was attributed to that the adsorbed CO on catalyst surface had strong redox ability and led to the reduction of metal oxides, which was consistent with the results of in situ XRD. Besides, the intensity of these carbonate species still remained above 50% of its maximum value after sweeping in the Ar atmosphere for 120 min, suggesting that the adsorption ability of CO on the Fe@Mn@0.05Si catalyst was strong. In comparison, quite different CO adsorption behavior was observed on the Fe@Mn@0.05Si-c catalyst. Only gaseous CO and no carbonate species was detected (Fig. 5g–i), implying that CO could hardly adsorb on the catalyst containing chlorine.Summarizing the information of in situ XRD and in situ DRIFTS characterizations, it could be deduced that chlorine could hinder the formation of FexC active phase via inhibiting the adsorption of CO molecules. Moreover, chlorine also suppressed notably the reduction process of iron species by H2 molecules (Supplementary Fig. 22). Thus, even exposing to the syngas atmosphere at reaction temperature, only Fe3O4 without FexC existed in the catalyst containing chlorine, thereby leading to the deactivation of FTS catalyst. No chlorine element was detected on the Fe@Mn@0.2Si-c catalyst (Fig. 4f) and Fe2O3 converted easily into iron carbide during reaction (Fig. 1), suggesting that appropriate thickness of SiO2 shell could prevent the contact of hydrogen chloride with internal metal species during the hydrophobic modification procedure of catalyst synthesis and thus protect the catalyst from chlorine poisoning.
