Ru particle size effect in Ru/CNT-catalyzed Fischer-Tropsch synthesis

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Abstract

Carbon nanotube (CNT)-supported Ru nanoparticles with mean sizes ranging from 2.3 to 9.2 nm were prepared by different post-treatments and studied for Fischer-Tropsch (FT) synthesis. The effects of Ru particle size on catalytic behaviors were investigated at both shorter and longer contact times. At shorter contact time, where the secondary reactions were insignificant, the turnover frequency (TOF) for CO conversion was dependent on the mean size of Ru particles; TOF increased with the mean size of Ru particles from 2.3 to 6.3 nm and then decreased slightly. At the same time, the selectivities to C5+ hydrocarbons increased gradually with the mean size of Ru particles up to 6.3 nm and then kept almost unchanged with a further increase in Ru particle size. At longer contact time, C10–C20 selectivity increased significantly at the expense of C21+ selectivity, suggesting the occurrence of the selective hydrocracking of C21 + to C10–C20 hydrocarbons.

Introduction

As a key step for the transformation of various non-petroleum carbon resources such as coal, natural gas and biomass via synthesis gas (also known as syngas, H2+CO), Fischer-Tropsch (FT) synthesis has attracted much renewed interest in recent years because of the diminishing crude oil and the rapidly growing global demand for liquid fuels. The liquid fuels obtained from FT synthesis can be sulfur- and nitrogen-free, and thus may easily meet the increasingly stringent environmental regulations [1]. Typically, the products of FT synthesis follow Anderson-Schulz-Flory (ASF) distribution because of the polymerization mechanism [2]. However, such a distribution is unselective for the middle-distillate products, which are usually the target products. For examples, the maximum selectivity to gasoline-range (C5–C11) hydrocarbons is ∼45% and that to diesel-range (C10-C20) hydrocarbons is ∼35% [3]. The development of selective FT catalysts, which can tune the selectivities to desired products, is one of the most challenging targets in FT synthesis.

Many recent studies have been devoted to clarifying the key factors influencing the catalytic behaviors, developing new catalysts or uncovering the reaction mechanism for FT synthesis [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. The size of the active phase is one of the most important factors determining the catalytic behaviors of a heterogeneous catalyst [16]. Some recent studies for Co-based catalysts have suggested that FT synthesis is a structure-sensitive reaction and the turnover frequency (TOF) is dependent on the mean size of Co particles [17, 18, 19, 20, 21, 22]. For example, de Jong and co-workers demonstrated that the TOF for CO conversion over carbon nanofiber (CNF)-supported Co catalysts increased with the mean size of Co particles up to 6–8 nm and then remained almost unchanged [17]. The selectivity to C5+ hydrocarbons increased and that to CH4 decreased with Co particle size up to 6–8 nm and then changed insignificantly. The phenomenon that the TOF decreases with the particle size below a critical point (typically 6–10 nm) has also been observed in other systems such as Co/ITQ-2 [19], Co/SiO2 [20], Co/CNT (CNT = carbon nanotube) and Co/CS (CS = carbon sphere) [21], and Co/γ-Al2O3 [22]. A very recent study showed that Fe-catalyzed FT synthesis exhibited a quite different particle-size dependence; TOF for CH4 formation increased sharply on decreasing the size of Fe carbide particles from 4 to 2 nm, whereas that for C2+ hydrocarbon formation was almost independent of Fe carbide particle size [23]. However, there are also a few publications pointing out that FT synthesis is intrinsically structure-insensitive [24, 25, 26, 27]. For example, through studies of model Co/SiO2 catalysts with different mean sizes of Co particles, Goodman and co-workers recently attributed the lower TOF and the higher CH4 selectivity observed for Co particles with sizes <3.5 nm to the easier oxidation of the smaller Co0 particles to CoO by the formed H2O under reaction conditions [27].

Ru is known to be the most active metal for CO hydrogenation and the supported Ru catalysts are capable of working efficiently for the production of long-chain hydrocarbons without any promoters [3]. Moreover, Ru0 can be sustained even under high partial pressures of H2O [28, 29]. Therefore, in spite of the higher price of Ru as compared with Fe and Co, Ru-based catalysts are suitable for fundamental studies to gain clear-cut insights into some crucial issues such as the size effect of the active phase. To date, only limited studies have been devoted to clarifying the effect of Ru particle size on catalytic behaviors for FT synthesis [25, 30, 31, 32, 33, 34, 35]. An early study showed that the TOFs for CO conversion over Ru catalysts loaded on several metal oxides were almost independent of the dispersion of Ru in a range of 0.0009–0.60 [25]. However, the results from other groups suggested structure-sensitive features for Ru nanoparticle-catalyzed CO hydrogenation [30, 31, 32, 33, 34]. For example, Kou and co-workers reported that Ru nanoparticles stabilized by poly(N-vinyl-2-pyrrolidone)(PVP) could catalyze FT synthesis in aqueous phase at 423 K, and Ru nanoclusters with a mean size of2 nm exhibited the highest TOF for CO conversion [32]. Recently, Carballo et al. found that the TOF over Ru/γ-Al2O3 catalysts increased with Ru particle size from 4 nm, reaching a plateau when the mean size of Ru particles exceeded 10 nm [33]. These discrepancies undoubtedly request more fundamental research. Furthermore, most of these studies have not discussed the effect of Ru particle size on the product selectivity, which is also a key issue in designing highly efficient FT catalysts.

In a short communication [34], we demonstrated that the exploitation of a CNT pretreated by concentrated HNO3 as the support of Ru instead of the conventional metal oxides, activated carbon or zeolites could provide a significantly higher selectivity to C10–C20 hydrocarbons, which were the diesel-fuel fraction products. The spillover H species and the acidic functional groups on CNT surfaces were proposed to accelerate the secondary reaction, i.e., the hydrocracking of C21+ hydrocarbons, favoring the selective formation of C10–C20. We also briefly showed that the size of Ru particles affected the catalysis of Ru/CNT catalyst [34]. Herein, we report our studies in detail on the effect of Ru particle size on catalytic behaviors of Ru/CNT catalysts.

Section snippets

Catalyst preparation

CNTs, which were provided by the group of Prof. Hongbin Zhang at Xiamen University, were typically pretreated in concentrated HNO3 (68 wt%) at 413 K under refluxing conditions to remove the remaining Ni catalyst and to create oxygenate-containing function groups. Ru/CNT catalysts with different mean sizes of Ru particles were prepared by an impregnation method combined with different post-treatments. Typically, CNTs after pretreatment were added into an aqueous solution of RuCl3, and then

Characterizations of Ru/CNT catalysts with different Ru particle sizes

To gain information about the possible state of Ru in Ru/CNT catalysts prepared by the impregnation followed by different post-treatments, we carried out XRD studies. Figure 1 shows the XRD patterns for these samples. The diffraction peaks at 2θ of 26.0°, 43.0° and 53.5° can be ascribed to (002), (100) and (004) reflections of CNTs [37], and the peak of 2θ = 26.0° indicates that the spacing between the sp2-C layers in our CNTs was 0.343 nm. No diffraction peaks assignable to Ru could be

Discussion

The understanding of the size effect is fundamentally important to deeply understand the true active sites and is also helpful for the rational design of more efficient FT catalyst. For the Ru-catalyzed FT synthesis, it was once reported that the TOF did not vary significantly with the dispersion of Ru loaded on SiO2, Al2O3 and TiO2 [25]. However, Kellner and Bell [30] reported a rise of TOF on increasing the dispersion of Ru from 0.3 to 0.75 over Ru/Al2O3 catalysts, corresponding to a decrease

Conclusions

CNT-supported Ru nanoparticles with mean sizes ranging from 2.3 to 9.2 nm could be prepared by an impregnation of CNT with RuCl3 aqueous solution followed by different post-treatments. While the direct reduction of the impregnated sample caused the formation of smaller Ru particles, the introduction of calcination step before H2 reduction increased the mean size of Ru particles. Our studies revealed that Ru/CNT-catalyzed FT synthesis was a structure-sensitive reaction. At the shorter contact

Acknowledgements

This work was supported by the National Basic Research Program of China (No. 2013CB933100), the National Natural Science Foundation of China (21173174, 21161130522, 21033006 and 20923004), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1036), and the Research Fund for the Doctoral Program of Higher Education (No. 20090121110007). We acknowledge Profs. H. B. Zhang and G. D. Lin of Xiamen University for providing CNTs.

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    This work was supported by the National Basic Research Program of China (No. 2013CB933100), the National Natural Science Foundation of China (21173174, 21161130522, 21033006 and 20923004), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1036), and the Research Fund for the Doctoral Program of Higher Education (No. 20090121110007).

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