Cobalt species and cobalt-support interaction in glow discharge plasma-assisted Fischer–Tropsch catalysts
Graphical abstract
Pretreatment of silica-supported cobalt catalysts with glow discharge plasma leads to smaller cobalt particles without any significant increase in the concentration of barely reducible cobalt silicate. Due to the combination of higher cobalt dispersion and optimized reducibility, ruthenium promoted plasma-assisted cobalt catalyst exhibited an enhanced activity in Fischer–Tropsch synthesis.
Introduction
Fischer–Tropsch (FT) synthesis converts natural gas-, coal- and biomass-derived syngas into liquid fuels which are totally free of sulfur- and nitrogen-containing compounds and have very low aromatic contents. The interest in FT synthesis has been due to the shortage of petroleum crude oil reserves and environmental constraints [1], [2], [3]. Cobalt-based catalysts could be the catalysts of choice for low-temperature FT synthesis [1], [2], [3]: they have high activity, high selectivity to linear C5+ hydrocarbons, low activities for water–gas shift reaction, and lower price compared to noble metals.
FT synthesis proceeds on cobalt metal sites. The overall number of cobalt sites on supported catalysts depends on both cobalt dispersion and reducibility. In monometallic cobalt catalysts, however, an increase in cobalt dispersion often coincides with a drop in cobalt reducibility [2], [3], [4], [5], [6]. Indeed, smaller cobalt particles are usually more difficult to reduce than larger ones. In addition, very small cobalt particles in highly dispersed catalysts could strongly interact with support. This leads to the formation of mixed oxides (i.e., cobalt silicates in the case of Co/SiO2 catalysts), which are difficult to reduce and which are inactive for FT synthesis [6]. Use of catalyst promoters such as noble metals can break this dispersion-reducibility dependence. In addition to the overall number of cobalt surface sites, the catalytic performance of cobalt-based FT catalysts can be affected by cobalt particle size [7], [8], [9], [10], [11], [12], [13], [14]. Cobalt particles with the optimal size (6–10 nm), higher reducibility and higher stability are required for the design of efficient cobalt FT catalysts [2], [3], [8], [11].
Plasma is an ionized gas that can be generated by a number of methods, including electric discharges (glow, microwave, plasma jet, radio frequency). Glow discharge plasma is a kind of non-thermal plasma, which is characterized by high electron temperature (10,000–100,000 K) and relatively low gas temperature [15]. Plasma intensity is determined by the applied power in terms of electric voltage and gap between two electrodes. The energetic species (electrons, ions and radicals) in the plasma could modify the catalyst surface, particle size and morphology of active phase, as well as active phase-support interactions in the catalysts [16]. Catalyst pretreatment with plasma could lead to some specific catalytic properties [15], [16], [17], [18], [19], [20], [21], [22]. Various catalysts such as Pd/HZSM [17], [19], Pt/NaZSM-5 [21], Ni-Fe/Al2O3 [20], Pt/TiO2 [22] and Pd/Al2O3 [23] were treated by glow discharge plasma and tested in different catalytic processes, including methane combustion [17], [19], NO reduction by methane [21], partial oxidation of methane [20], [24] and others.
The plasma technique has not been well explored for the design of cobalt catalysts for FT synthesis. Chu et al. [25] have recently investigated the effects of pretreatment with glow discharge plasma on cobalt dispersion, reducibility and catalytic performance in alumina-supported catalysts in FT synthesis and gained some preliminary results. No information about the influence of glow discharge plasma on metal dispersion and catalytic performance of silica-supported cobalt FT catalysts has been available in the literature.
In this work, a series of silica-supported cobalt-based catalysts were prepared by glow discharge plasma with different plasma power voltages (which correspond to different plasma intensities). The effects of pretreatment with glow discharge plasma on cobalt dispersion, reducibility and structure of monometallic and bimetallic silica-supported cobalt FT catalysts were investigated using a wide range of methods, including X-ray diffraction (XRD), temperature-programmed reduction (TPR), in situ and ex situ X-ray absorption spectroscopy (XANES and EXAFS), propene chemisorption and in situ magnetic method. The catalytic performance in FT synthesis was evaluated in a fixed-bed microreactor.
Section snippets
Catalyst preparation
The silica-supported cobalt catalysts were prepared by conventional incipient wetness impregnation of silica gel using aqueous solutions of cobalt nitrate; for the catalysts promoted with Ru, the impregnating solution also contained ruthenium nitrosyl nitrate. A commercial silica gel (Qingdao Haiyang Chemical Co., Ltd., China), with SBET = 355 m2/g, pore diameter of 7.8 nm, and total pore volume of 0.90 cm3/g, was used as a catalytic support. After impregnation, the catalysts were dried at 363 K in
Cobalt precursor decomposition
Cobalt and ruthenium precursors in the impregnated and dried silica-supported catalysts were decomposed either by calcination or by glow discharge plasma. The catalyst calcination temperature was chosen on the basis of TGA data. The TGA curves of bulk cobalt nitrate and monometallic impregnated and dried Co/SiO2 catalyst in air flow are shown in Fig. 2. Two weight losses were observed at around 361 K (∼11.7%) and 459 K (∼13.8%) during the heating of Co/SiO2 sample (Fig. 2a), which were attributed
Discussion
High cobalt dispersion along with adequate cobalt reducibility seems to be key parameters in the design of active catalysts for FT synthesis. Cobalt dispersion in FT catalysts can be enhanced by several different techniques: optimization of support texture [12], [41], [42], [43], decomposition of cobalt precursors and catalyst calcination at mild conditions [32], [44], [45], [46] or in the presence of NO/He [47], [48], addition of organic compounds during impregnation [30], [49], [50], [51],
Conclusion
It was found that pretreatment of impregnated and dried silica-supported monometallic and ruthenium-promoted cobalt catalysts with glow discharge plasma yielded smaller cobalt particles than conventional calcination. Cobalt dispersion was influenced primarily by the plasma intensity; higher plasma intensity led to higher cobalt dispersion. The concentration of cobalt silicate in plasma-assisted samples was low. Smaller cobalt particles in the plasma-assisted monometallic cobalt catalysts
Acknowledgments
J.H. thanks the Embassy of France in China for a Ph.D. Fellowship. The supports from the Natural Science Foundation of China (NSFC 205903603) and the 985 Project and 211 Project of Sichuan University are also acknowledged. The authors thank L. Burylo, O. Gardoll, L. Olivi, A. Cognigni and O. Safonova for the help with conventional X-ray diffraction, TPR measurements and synchrotron experiments respectively. The authors would like to thank Prof. E. Payen, Prof. X.Y. Dai, the colleagues in the
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