Promoters in heterogeneous catalysts may enhance the activity or modify the selectivity either by a structural effect, changing the structure of the active phase (e.g., the exposed crystalline plane, the size or dispersion of the active phase), or by an electronic effect, modifying the electronic characters of the active phase through electron transfer or electronic interaction. For FT synthesis, Ru-based catalysts can work well for the production of heavier hydrocarbons without modifiers, but Fe- and Co-based catalysts generally require promoters such as alkali metal ions, noble metals, or transition metal oxides to attain optimum catalytic performance.11 The elucidation of the effects and the functioning mechanisms of the promoters in FT synthesis is very important for rational design of selective and active FT catalysts.
2.4.1. Promoter effects for Fe-based catalysts
Promoters have played particularly significant roles in Fe-catalyzed FT synthesis. Most of the Fe-based FT catalysts contain alkali metal ions as promoters. The alkali metal ion is expected to function as an electronic promoter to affect the electronic character of Fe, and can modify the activity and selectivity by enhancing the chemisorption of CO and inhibiting that of H2. The effect of various alkali metal ions on the catalytic performance of a precipitated Fe catalyst (Fe/Si=100:4.6) was investigated in a slurry-phase reaction (T=543 K, P=1.3 MPa).65 The use of K+ and Na+ was found to accelerate the activity for both FT synthesis and the WGS reaction, whereas the addition of Li+, Rb+, or Cs+ decreased CO conversion. The alkali metal ion promoters changed the product selectivity. Modification with alkali metal ions could enhance C5+ selectivity. The addition of Na+ particularly favored the C5–C11 selectivity (the gasoline fraction, ca. 35 wt %), whereas the use of K+ and Cs+ provided the highest selectivities to C19+ (ca. 12 wt %) and C12–C18 (ca. 20 wt %), respectively.65 Alkali metal ions increased ethylene selectivity in C2 hydrocarbons. It is proposed that modification with alkali metal ions decreases the hydrogenation ability and thus accelerates the chain-growth probability and the olefin selectivity, possibly as a result of the increased basicity. The effect of K+ content on catalytic performances of a precipitated Mn-modified Fe catalyst operating in a fixed-bed reactor was investigated in detail.66 The CO conversion increased with K+ content and passed through a maximum at a K+ content of 0.7 wt % (Figure 2). The increase of K+ content to about 1.5 wt % decreased the selectivity to CH4 and increased that to C5+ hydrocarbons, especially to C12+ hydrocarbons. The selectivity to oxygenates decreased on addition of K+. The fraction of olefins in C2–C4 hydrocarbons increased significantly at the same time. XRD and Mössbauer spectroscopy studies suggested that the increase in K+ content increased the fraction of iron carbides (χ-Fe5C2 and ε′- Fe2.2C) after the reaction. In a subsequent study for a K+- and Cu-promoted Fe/SiO2 catalyst in slurry-phase reaction (T=523–533 K, P=1.5MPa, H2/CO=0.67), the presence of K+ was found to increase both the activities of the FT synthesis and the WGS reactions and significantly enhance the C5+ selectivity.67 However, too high a K+ content led to rapid catalyst deactivation. Lohitharn and Goodwin68 showed that the FT reaction activity increased by adding an appropriate amount of K+ to Fe or Fe–Mn catalyst, and the optimum activity was observed at a K+ content of 1.5 mol % (relative to Fe). C5+ selectivity and the ratio of olefin to paraffin were also enhanced by the modification with K+. However, activity towards CO2 formation, possibly including the WGS and Boudouard reactions, increased monotonically with K+ content. By measuring coke deposition, it was clarified that the addition of K+ with higher contents significantly promoted the Boudouard reaction [Equation (6)].
Figure 2. Effect of K+ content on catalytic performances of precipitated Mn-promoted Fe catalysts.66 Reaction conditions: T=550–573 K, P=2.5 MPa, H2/CO=2, GHSV=1000 h−1.
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This reaction was thought to be the main reason for the rapid catalyst deactivation for the catalysts with higher K+ contents.67, 68 By using a steady-state isotopic transient kinetic analysis (SSITKA) technique, it was demonstrated that the addition of K+ does not significantly affect the intrinsic activity of the Fe site but may increase the number of active surface intermediates leading to hydrocarbon products.68
Copper or noble metals may facilitate the reduction of Fe precursors and thus can increase the activity. The presence of Cu or Ru in a K-Zn-Fe catalyst was found to promote the reduction and carburization rates of Fe species during the catalyst pretreatment in H2/CO.69 The activity per gram of catalyst was increased at the same time, but the TOF was not changed. The selectivities to C5+ and CH4 over the Cu- or Ru-promoted catalyst were similar to those over the unmodified catalyst when the comparison was made at similar CO conversions. The addition of Cu accelerated the reduction of Fe species in Fe–Mn–K/SiO2 catalysts and increased the rate of carburization but did not vary the final degree of carburization at the steady state.70 Accordingly, Cu shortened the induction period but did not exert significant influence on the stead-state activity. For the Fe–Mn–K/SiO2 catalysts, the selectivities to CH4 and lighter hydrocarbons (C2–C11) decreased whereas that to heavier hydrocarbons (C11+) increased on Cu modification. The ratio of olefins to paraffins in C2–C4 also increased after the addition of Cu to Fe–Mn–K/SiO2. These observations are quite different from those for the K–Zn–Fe catalyst.69 The basicity of the Cu-promoted catalyst was also found to be enhanced, possibly due to the synergistic effect between Cu and K+, which might be responsible for the changes in product selectivity.
Some transition metal oxides are also known to promote the Fe-catalyzed FT reactions. Goodwin and co-workers71 investigated the effect of various transition metal oxides (Cr, Mn, Mo, Ta, V, W, and Zr oxides) on catalytic behaviors of a 100 Fe/5 Cu/17 Si catalyst under reaction conditions of T=553 K, P=1.8 MPa, and H2/CO=1, and found that the addition of these metal oxides except for WOx enhanced the activities of both CO hydrogenation and WGS reactions. The enhancing effect was significant using Cr, Zr, and Mn oxide promoters, and the catalyst modified by MnOx was the most stable against catalyst deactivation. However, the hydrocarbon selectivity was not significantly changed by these promoters. Further studies with the Cr-, Mn-, and Zr-modified Fe catalysts suggested that the presence of these promoters increased the dispersion of Fe species but did not significantly change the reducibility or the BET surface area.72 The intrinsic activity estimated by the SSITKA technique was similar for the Fe catalysts with and without these promoters, implying that the enhanced conversions were due to an increased number of active surface intermediates leading to hydrocarbon products.72 Recently, an X-ray absorption near-edge structure (XANES) study was performed for the Mn-promoted Fe catalysts after reaction (with passivation in a 2 % O2/He mixture) to clarify the function of Mn.73 The result suggested that Mn might substitute for octahedral sites in Fe3O4, corresponding to the formation of (Fe1−xMnx)3O4. The formation of this mixed oxide likely decreased the reducibility of Fe. Typically, Fe-based FT catalysts deactivate due to carbon deposition on larger iron carbide (e.g., θ-Fe3C) particles. The lower deactivation rate of this Mn-modified catalyst may indicate that the (Fe1−xMnx)3O4 phase can be transformed to smaller FexC clusters, which are more active for CO hydrogenation and less prone to deactivation.73
Luo and Davis74 studied the influence of alkaline-earth metal ions on the catalytic behavior of Fe-based FT catalysts, and found that Mg and Ca could suppress the WGS reaction and slightly increase the ratio of olefins in C2–C4 hydrocarbons. Gallegos et al. reported that, as compared to the Fe/SiO2 catalyst, the Fe catalyst supported on MgO-covered SiO2 showed a higher TOF for total hydrocarbon productions and a higher ratio of olefin to paraffin in C2–C4 hydrocarbons.75 Moreover, the presence of Mg at an appropriate content (4 wt %) could suppress the formation of CH4. A systematic study on the modifying effect of Mg on a precipitated Fe–Cu–K/SiO2 catalyst showed that there was an optimum content of Mg for obtaining better catalytic performances.76 At an optimized Mg/Fe weight ratio (0.07), the activity of WGS reaction could be suppressed, whereas the FT activity was enhanced. Mg modification could also effectively shift the product distribution to lighter hydrocarbons, especially to gasoline-range hydrocarbons (C5–C11), and could suppress the hydrogenation of light olefins, resulting in higher selectivity to C2–C4 olefins. The characterizations suggested that the presence of an appropriate amount of Mg could promote the reduction and the carburization of Fe species, and over the catalyst with an Mg/Fe ratio of 0.07, the content of χ-Fe5C2 was the highest. This observation further suggests that the iron carbide (likely χ-Fe5C2) is the active phase for hydrocarbon formation, whereas the oxidic iron species is responsible for the WGS reaction.
Table 1 summarizes the effects of various promoters employed in Fe-based FT catalysts. CO conversions increase with most of these promoters. However, it is unclear whether the intrinsic activity (TOF) of the Fe site can be enhanced. Recent studies appear to indicate that most of the promoters cannot affect the TOF, but are capable of enhancing the reduction and the carburization of Fe species or facilitating the dispersion of active iron carbide species.65–76 The product selectivity can be regulated by choosing appropriate modifiers. To achieve the goals of decreasing the selectivities to CH4 and CO2, and of promoting the C5+ or the light olefin formation, we have to combine different modifiers with different functions in one catalyst. Thus, the interactions among different modifiers should also be considered for the rational design of an efficient Fe-based catalyst. Such information as the synergistic effect between different promoters is still in deficiency.
Table 1. Effects of promoters on catalytic behaviors of Fe-based FT catalysts.
|K+ and alkali metal ions||1) Enhancing both FT and WGS activity. 2) Decreasing selectivity to CH4 and increasing that to C5+ hydrocarbons. 3) Increasing the olefin/ paraffin ratio in C2–C4 hydrocarbons.||65–68|
|Cu or Ru||1) Increasing the activity by facilitating the reduction and carburization of Fe species. 2) Increasing the selectivity to heavier hydrocarbons and the fraction of olefins in lighter hydrocarbons, possibly by enhancing the basicity together with alkali metal ions.||69, 70|
|MnOx||1) Increasing the activity by enhancing the dispersion of Fe species. 2) Decreasing deactivation by forming smaller iron carbide species by the formation of mixed oxides with Fe.||71–73|
|MgO||1) Increasing the FT reaction activity and suppressing the WGS reaction by promoting the reduction and carburization of Fe species. 2) Shifting the hydrocarbon product to lighter range, especially to gasoline range. 3) Increasing the selectivity to olefins.||74–76|
2.4.2. Promoter effects for Co-based catalysts
Typical promoters used for Co-based catalysts are noble metals, transition metal oxides, such as ZrO2 and MnOx, and some rare earth metal oxides.
Many studies have demonstrated that the addition of a noble metal modifier such as Ru or Re can accelerate the reduction of Co precursors into Co0 clusters, which are the active phase for FT synthesis, and may also enhance the dispersion of Co0.10–13 The increase in Co site density on a catalyst surface may not only increase the FT activity but also raise the selectivity to C5+ hydrocarbons and decrease that to CH4 due to the increased probability of readsorption of α-olefins, as proposed by Iglesia.10 Moreover, the presence of a noble metal may increase the steady-state activity by retarding the catalyst deactivation, perhaps through the inhibition of reoxidation or of carbon deposition on Co catalysts during the reaction.10
The promoting effect of reducibility of cobalt precursors by a noble metal was particularly significant for the catalysts containing Co species that are difficult to reduce. Khodakov and co-workers77 found that cobalt species in a calcined Co/SiO2 catalyst, prepared by an impregnation using cobalt acetate as the precursor were mainly barely reducible cobalt silicate, and the addition of Ru by co-impregnation with cobalt acetate could increase the fraction of Co3O4 by influencing the oxidative decomposition of cobalt acetate, and significantly enhanced the degree of reduction after H2 reduction at 673 K. However, for the Co/SiO2 prepared by the impregnation using cobalt nitrate, which was mainly transformed to reducible Co3O4 after calcination, the role of Ru modification was to enhance the dispersion of Co0 instead of the degree of reduction. Because of the stronger interactions with support, it is known that the cobalt species on γ-Al2O3 are difficult to reduce. The addition of small amount of Pt, Ru, or Pd to a Co/γ-Al2O3 was found to enhance the reduction of both the surface Co3O4 and the inactive cobalt oxide species, having strong interactions with the support, and thus increased overall activity.78 The activity increased in the order Co/γ-Al2O3<Pd-Co/γ-Al2O3< Ru-Co/γ-Al2O3<Pt-Co/γ-Al2O3, and the C5+ selectivity was also increased in the presence of noble metal promoters due to the increase in the Co0 site density.
Tsubaki et al.79 made a detailed comparison of promoting effects of different noble metals added into a Co/SiO2, and found that the CO hydrogenation rate increased in the order Co/SiO2<Pt–Co/SiO2<Pd–Co/SiO2<Ru–Co/SiO2. The addition of a small amount of Ru increased the degree of reduction of Co and the cobalt–time yield (moles of CO converted per total moles of Co per unit time) remarkably. The TOF based on surface Co evaluated by H2 uptake increased to some extent by Ru modification. The introduction of Ru led to the appearance of bridge-adsorbed CO to a larger extent, indicating the increase in CO dissociation ability. In contrast, Pd and Pt only slightly affected the degree of reduction but enhanced the cobalt dispersion, and decreased the TOF.79 CH4 selectivity was significantly raised over the Pt- and Pd-modified catalysts. The addition of a small amount of Pt (0.1 wt %) into 15 wt % Co/Al2O3 was found not to change the size of Co3O4 particles but to accelerate the reduction of smaller Co3O4 particles, resulting in smaller average size of supported Co0 particles in the reduced catalysts.80 The addition of Pt resulted in a significant increase in the FT reaction rate (cobalt–time yield), but some decreases in C5+ selectivity and increases in CH4 selectivity were detected.80
Rhenium is another widely used promoter for Co catalysts. Early studies suggested that Re increased the Co dispersion on TiO2 by preventing agglomeration of CoOx particles during calcination treatments and/or oxidative regenerations.10 Re/TiO2 itself was approximately two orders of magnitude less active than Co/TiO2 for FT synthesis under the same reaction conditions (T=494 K, P=0.56 MPa, H2/CO=2).57 Whereas some groups reported no significant effect of Re on the activity and selectivity of Co catalysts,57 Storsæter et al.81 observed that modification of Co/Al2O3, Co/SiO2 and Co/TiO2 by Re could significantly increase the hydrocarbon formation rate per gram of catalyst and slightly enhance the selectivity to C5+ hydrocarbons, in turn, decreasing the selectivity to CH4. Martínez et al.82 showed that the introduction of roughly 1 wt % Re into Co/SBA-15 enhanced the reducibility of Co species and increased the activity, but did not alter the intrinsic activity (TOF) of Co sites. The C5+ and especially C10+ selectivity was enhanced by the presence of Re, and CH4 selectivity was decreased at the same time. Another study also argued that the presence of Re could not raise the TOF.83
Besides noble metal promoters, many metal oxides are known to modify the structure and the catalytic behavior of supported Co catalysts.10, 13 ZrO2 was demonstrated to be a good promoter to improve the CO conversion activity and C5+ selectivity of a Co/SiO2 catalyst.84 The promotion of a 20 wt % Co/SiO2 with ZrO2 increased the FT reaction rate, and the sequentially impregnated Zr–Co/SiO2 catalyst (Zr/Co=0.02–0.28) was the most active.85 The TOF based on H2 uptake also increased for the series of Zr–Co/SiO2 catalysts. It was suggested that ZrO2 may create an active interface with Co, which may be responsible to some extent for the enhancement in Co activity by facilitating CO dissociation. A detailed study further confirmed that the increase of Zr content in Zr–Co/SiO2 catalysts reduced the interaction between Co and SiO2 and caused a somewhat weaker Co–ZrO2 interaction, leading to a rise in the degree of reduction of the catalysts after H2 reduction at 673 K.86 The modification by Zr increased the size of Co crystallites but the agglomeration of Co species became less serious, leading to the homogeneous distribution of Co particles in the modified catalysts. The TOF based on the metal dispersion of the freshly reduced catalysts increased on ZrO2 modification, but this appeared to be related to the improved catalyst stability of the larger Co particles.86 The C5+ selectivity increased significantly at a lower Zr content (Zr/Co=0.015), and a further increase in Zr content decreased the C5+ selectivity again. Similar enhancing effects of ZrO2 were also found for a 10 wt % Co/SiO2 catalyst prepared by a sol–gel method.87
The modifying effects of ZrO2 on the catalytic behavior of Co/Al2O3 were also investigated. Goodwin and co-workers88 demonstrated that the addition of ZrO2 to γ-Al2O3 could enhance the reducibility of Co species (20 wt % Co) by preventing the formation of Co surface aluminate, which could not be observed by XRD but was suggested from Raman spectra. Li and co-workers89 observed the formation of CoAl2O4 crystallites from XRD for a 15 wt % Co/γ-Al2O3 catalyst, and found that the addition of ZrO2 could inhibit the formation of crystalline CoAl2O4, enhancing the reduction of cobalt species. These studies all observed the increases in both CO conversion and C5+ selectivity with increasing Zr content.88, 89 The increase in Zr content was found to raise the ratio of olefin to paraffin in C2–C17,89 indicating the decrease in hydrogenation ability for the ZrO2-modified catalysts. The intrinsic site rate estimated by the SSITKA technique was found to be unchanged after ZrO2 modification.88 The higher surface Co site density and the increased coverage of the reaction intermediates were proposed to contribute to the increase in the reaction rate and the C5+ selectivity.88, 89 The addition of ZrO2 was also found to promote CO conversion and C5+ selectivity (especially C5–C20 selectivity) over a Co/AC catalyst.90 After FT reactions, Co crystallites with both hcp and fcc structures were detected for the 15 wt % Co/AC, and the addition of ZrO2 suppressed the formation of hcp Co. Over the AC support, the size of Co crystallites was found to decrease from roughly 20 to 10 nm on ZrO2 modification. Temperature-programmed surface reaction (TPSR) experiments suggested that the CO dissociation ability was enhanced due to the presence of ZrO2.90
Manganese oxide, an efficient promoter for Fe-based catalysts,71–73 has also been studied as a promoter for Co-based catalysts. Weckhuysen and co-workers performed a series of studies to elucidate the functions of Mn in the Mn-promoted Co/TiO2 catalyst. The addition of 2 wt % Mn to a 7.5 wt % Co/TiO2 catalyst increased the CO conversions and the cobalt–time yields at different pressures (0.1–1.8 MPa).91 The presence of MnOx also led to a rise in C5+ selectivity and a decrease in CH4 selectivity. The preparation procedure was claimed to be key to obtaining such enhancing effects.92 The preparation of Co/TiO2 by a homogeneous deposition–precipitation (HDP) method followed by incipient wetness impregnation (IWI) to load Mn could produce catalysts with associated Co and Mn. STEM-EELS, combined with XPS and EXAFS characterization, suggested a clear Co–Mn interaction in the calcined catalyst, in which a solid-solution spinel compound (MnxCo3−xO4) may have formed.91, 92 This interaction suppressed the reduction of cobalt species, as indicated by in situ XAS studies.93 After reduction by H2, this solid solution was partially transformed to Co0, and MnO migrated to TiO2, but a certain Co–Mn interaction still remained, since MnO clusters were detected in the vicinity of the Co0 particles. This interaction was proposed to affect the electronic state of cobalt, forming Coδ+ and leading to the lowered hydrogenation ability, and thus decreasing the selectivity to CH4. It was also suggested that the MnO in the vicinity of Co0 may act as new sites for the insertion of CO into the grown alkyl chains, forming CxHyOz intermediates, which were subsequently hydrogenated over the neighboring Co0.91 The ratio of olefin to paraffin in C2–C8 also increased gradually with increasing Mn content in Mn–Co/TiO2 catalysts, further confirming that the addition of MnO decreased the hydrogenation ability of Co species.94 The decrease in electron density on Co particles via direct electronic interaction between Co and MnO was confirmed from diffuse reflectance infrared spectroscopic studies.94 By adopting a strong electrostatic adsorption (SEA) technique, Weckhuysen and co-workers95 succeeded in preparing Mn-promoted Co/TiO2 catalysts with stronger Co–Mn interactions. It was proposed that MnO4− anions were selectively deposited onto the supported Co3O4 [point of zero charge (PZC)=8] but not onto the TiO2 (PZC=3.7) by adjusting the solution of pH. The addition of a small amount of Mn (0.03 wt %) onto 10 wt % Co/TiO2 by this technique could enhance the reduction of cobalt species, resulting in a higher cobalt–time yield. However, higher loadings of Mn hindered the reduction of cobalt species and the uptake of H2 because Mn species may have preferentially covered the Co surfaces. In spite of the decreased activity, a higher content of Mn increased the selectivity to C5+ and decreased that to CH4 (Figure 3). The ratio of olefin to paraffin also increased significantly with Mn contents.
Figure 3. Effect of manganese content on A) product selectivity and B) the molar ratio of olefin to paraffin for C2, C4 and C6 hydrocarbons over Mn- promoted Co/TiO2 catalysts (Co loading=10 wt %).95 Reaction conditions: T=493 K, P=0.1 MPa, H2/CO=2, Wcat=50 mg, F=12 mL min−1.
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Rare earth oxides have also been employed as promoters for Co-based catalysts. The addition of La2O3 to 20 wt % Co/SiO2 (La/Co=0–0.75) was found to increase the cobalt–time yield and decrease the selectivities to CH4 and C2–C4 hydrocarbons under the reaction conditions of T=493 K, P=0.1 MPa, and H2/CO=2.96 With increasing La/Co ratio from 0 to 0.1, the α value increased remarkably, from 0.57 to 0.70, and the ratio of olefin to paraffin in C2–C5 hydrocarbons increased from roughly 1 to 5 at steady states. The SSITKA studies suggested that the presence of La2O3 did not change the intrinsic activity of the Co species, but likely increased the concentration of active sites or active intermediates. Characterizations showed that the presence of La3+ in the aqueous solution for catalyst preparation appeared to moderate the strong Co–support interactions, leading to better reducible cobalt oxides and to a larger number of exposed Co0 atoms.97 The impregnation procedure was found to be important for La2O3-promoted Co/γ-Al2O3 catalysts.98 The impregnation of Co precursor, first onto γ-Al2O3 followed by La impregnation (La/Al=0.013–0.078) had little effect on the catalytic activity and selectivity, and there was little evidence of La–Co interactions in this series of catalysts. For the catalysts prepared by impregnation of La precursors onto γ-Al2O3 followed by Co impregnation, the selectivity to higher hydrocarbons and olefins was increased with increasing La content in the region of La/Al≤0.026. However, a high ratio of La/Al caused significant amount of amorphous La–Co mixed oxides, which were difficult to reduce, and decreased the activity significantly. The enhancement in the selectivity to higher hydrocarbons owing to the modification of Co/γ-Al2O3 by La2O3 was also observed by Vada et al.99 A small amounts of La2O3 (0.7–1.7 wt %) could increase the CO conversion and C5+ selectivity of a 15 wt % Co/AC under reaction conditions of T=503 K, P=2.5MPa and H2/CO=2.100 However, too high a La2O3 loading rather decreased the C5+ selectivity and increased the CH4 selectivity. The addition of La2O3 was found to decrease the reducibility of cobalt species but to enhance their dispersion. The strength of CO chemisorption was increased by La2O3 modification.
The introduction of CeO2 (4.5–38.2 wt %) into 25 wt % Co/SiO2 was found to change the product selectivity, although the CO conversion and TOF were not significantly changed.101 The distribution of hydrocarbons in the C5+ fraction was modified after the addition of CeO2; the selectivity to C5–C13 became higher (ca. 33 % at its highest) whereas that to C22+ (wax) decreased. However, the selectivity to CH4 and C2–C4 hydrocarbons increased significantly in the presence of CeO2 modifier. It was proposed that the presence of CeO2 accelerated the hydrogenation ability. Moreover, the hydrogenation of the formate species formed on partially reduced CeO2−x may also have contributed to the increased CH4 selectivity. Shen and co-workers102, 103 reported quite different modifying effects of CeO2 in CeOx–Co/SiO2 (10.5–15 wt % Co; Ce/Co=0.2–0.6) catalysts prepared by a co-impregnation method. The addition of CeO2 to Co/SiO2 was found to increase CO conversion and the selectivity to heavier hydrocarbons. The presence of CeO2 decreased the percentage of gasoline (C5–C10) fraction and increased those to middle distillates (C11–C16) and C16+ (Figure 4).102 The presence of CeO2 slightly inhibited the reducibility of cobalt species but enhanced Co dispersion. The TPSR studies of H2 with the chemisorbed CO suggest that the modification by CeO2 accelerates the dissociation of CO on catalyst surface and increases the concentration of active intermediates for chain growth.103
Figure 4. Change of distribution of C5+ hydrocarbons after the addition of CeOx into a Co/SiO2 catalyst. Reaction conditions: T=483 K, P=1.2 MPa, H2/CO=2.
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Other metal oxide promoters, such as MgO, Al2O3, and TiO2, are known to enhance the catalytic performances of Co/SiO2 catalysts.104–106 The addition of MgO to Co/SiO2 was found to increase the CO conversion to some extent and decrease the selectivity of CO2 significantly.104 The addition of Al2O3 or TiO2 to Co/SiO2 could enhance the dispersion of Co species although the reducibility was somewhat decreased.105, 106 CO hydrogenation activity in the slurry-phase reactions was enhanced by the addition of Al2O3 or TiO2 to Co/SiO2. In the case of Al2O3 modification, CH4 selectivity also decreased slightly. Interestingly, a reversible promoting effect also existed; the addition of a small amount of SiO2 (5–20 wt %) to Co/Al2O3 also increased CO conversion and decreased CH4 selectivity.107 The reducibility of Co species was enhanced in this case. These observations further confirm that the balance between reducibility and dispersion of Co species is crucial for FT synthesis.
The effects of promoters typically used for Co-based FT catalysts are summarized in Table 2. Besides the identity of these promoters, many studies have shown that the introduction method or procedure of the promoters (preparation technique) is a key for obtaining the optimum promoting effect.92, 95, 98, 101, 102 The technique capable of creating direct interactions between the promoter and the cobalt species has been proven to be crucial for MnOx-modified Co/TiO2 catalysts.91–95 Appropriate location of the promoters over the supported Co catalysts should be considered in the future design of effective catalysts.
Table 2. Effects of typical promoters on catalytic behaviors of Co-based FT catalysts.
|Noble metals such as Ru and Re||1) Increasing the CO conversion activity by either enhancing the reduction of Co precursors or facilitating the dispersion of Co species by preventing the aggregation of CoOx or Co0 during calcination or reduction. 2) For Ru or Re, increasing the selectivity to C5+ and decreasing that to CH4, owing to the increased Co site density; for Pt and Pd (with higher hydrogenation ability) decreasing C5+ selectivity. 3) Increasing the bridge-chemisorbed CO and thus enhancing the CO dissociation ability. 4) Retarding deactivation by inhibiting the reoxidation of Co0 or carbon deposition.||10–13, 57, 77–83|
|ZrO2||1) Increasing the CO conversion activity by enhancing the Co reducibility through replacing stronger metal–support interactions with the weaker Co-ZrO2 interaction. 2) Enhancing the C5+ selectivity because of the increased Co site density or the lowered hydrogenation ability. 3) Increasing the olefin/paraffin ratio due to the decreased hydrogenation ability. 4) Increasing Co0 size over SiO2- or Al2O3-supported catalysts but decreasing Co0 size over AC-supported catalysts; Enhancing CO dissociation ability over the AC-supported catalyst. (5) Increasing the concentration of active intermediates but not changing the intrinsic Co activity in many cases.||84–90|
|MnOx||1) Increasing the selectivity to C5+ and decreasing that to CH4, possibly by decreasing the hydrogenation ability through electronic modification of Co species to form some Coδ+; the existence of Co–Mn interactions (e.g., forming MnxCo3−xO4 solid solution in catalyst precursor) is essential. 2) Increasing the ratio of olefin to paraffin because of the decreased hydrogenation ability. 3) Possibly increasing the CO conversion activity if the Mn content is low.||91–95|
|Rare earth oxides such as La2O3 and CeO2||1) Increasing the C5+ selectivity and the olefin/paraffin ratio of in the case of La2O3 by modifying the Co–support interactions; 2) The effect of CeO2 on product selectivity is complicated and depends on catalyst preparation technique. An appropriate preparation method may result in higher selectivity to middle distillates. 3) Raising CO conversion activity with a appropriate content and preparation technique.||96–103|