Article pubs.acs.org/Langmuir
Catalytic Graphitization of Coal-Based Carbon Materials with Light Rare Earth Elements Rongyan Wang,† Guimin Lu,*,† Wenming Qiao,*,‡ and Jianguo Yu†,‡ †
National Engineering Research Center for Integrated Utilization of Salt Lake Resources and ‡State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China
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ABSTRACT: The catalytic graphitization mechanism of coalbased carbon materials with light rare earth elements was investigated using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, selectedarea electron diffraction, and high-resolution transmission electron microscopy. The interface between light rare earth elements and carbon materials was carefully observed, and two routes of rare earth elements catalyzing the carbon materials were found: dissolution−precipitation and carbide formation− decomposition. These two simultaneous processes certainly accelerate the catalytic graphitization of carbon materials, and light rare earth elements exert significant influence on the microstructure and thermal conductivity of graphite. Moreover, by virtue of praseodymium (Pr), it was found that a highly crystallographic orientation of graphite was induced and formed, which was reasonably attributed to the similar arrangements of the planes perpendicular to (001) in both graphite and Pr crystals. The interface between Pr and carbon was found to be an important factor for the orientation of graphite structure.
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INTRODUCTION Expounding the explicit mechanism of catalytic graphitization has been a long-standing challenge owing to various carbon precursors and catalysts. Among those catalysts, iron (Fe), cobalt (Co), and nickel (Ni) have attracted considerable attention. For Ni, several mechanisms were proposed, for instance, gas−solid interaction to form carbon nanofibers, solidstate transformation for amorphous carbon to form graphene by diffusion, Ni-induced crystallization of amorphous carbon via dissolution−precipitation, and the modified dissolution− precipitation mechanism combining diffusion, interaction, and external force effects.1−5 Similarly, Co was found to catalyze amorphous carbon and promote the nucleation and growth of graphene.2,6−8 However, there are no studies concerning monolayer or bilayer graphene growth with the aid of Fe because of its high carbon solubility, even though graphitic multilayers can be produced from amorphous carbon materials catalyzed by Fe.2,7,9−12 Besides, some atomic-scale reports illustrated that amorphous carbon materials can significantly be catalytically graphitized by other metal catalysts such as titanium (Ti), tungsten, molybdenum, and chromium.7,13 The interface between the metal catalyst and carbon attracted attention in the catalytic process. However, the majority of investigations mentioned above have focused on the transformation of amorphous carbon to graphitic order at a relatively low temperature (≤1000 °C), under which condition in situ transmission electron microscope (TEM) studies can be carried out to reveal the catalytic mechanisms of carbon materials on an atomic scale. In fact, the widely accepted catalytic graphitization process was considered © 2016 American Chemical Society
to occur only when carbon materials were treated at a high temperature (2400−3000 °C). In a narrow sense, high temperature becomes the limitation of investigation with in situ techniques. Recently, Lin et al. studied the catalytic graphitization process of graphite blocks that were heat-treated at 2700 °C with Ti as the catalyst, whose raw materials were natural graphite flake and mesophase pitch.14 Through TEM images and selected-area electron diffraction (SAED) analysis, they confirmed that silicon carbide exhibited the catalytic effect with carbide formation−decomposition process, whereas both carbide formation−decomposition and dissolution−precipitation were involved in the formation process of graphite doped with Ti. Titanium carbide (TiC) was even found to be influential on the crystallographic orientations of the basal plane stacking sequence of graphite owing to its crystalline structure. The different interfaces between TiC and amorphous carbon resulted in the different crystallographic orientations of the basal plane stacking sequence of graphite. This investigation gains deeper insight into the catalytic graphitization at high temperatures on atomic scale and will attract attention to other catalysts. In our previous research,15 praseodymium oxide (Pr6O11) additive was found to catalyze graphite anodes significantly. A graphite anode with a low electrical resistivity of 5.0 μΩ·m was successfully obtained by adding 3 wt % Pr6O11 at 2800 °C. In the present study, based on the excellent performance that Received: June 3, 2016 Revised: July 29, 2016 Published: August 2, 2016 8583
DOI: 10.1021/acs.langmuir.6b02000 Langmuir 2016, 32, 8583−8592
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Langmuir
Figure 1. XRD patterns of the samples prepared (a) without an additive and (b−f) with Ti, Fe, La, Ce, and Pr after heat treatment at 2800 °C. The dotted lines represent the peaks of the hexagonal graphite phase. Additional peaks represent the carbides. the following procedure: baking, two cycles of coal tar pitch impregnation/carbonization, and finally graphitization at 2800 °C for 1 h. The samples were labeled as G−XY, where G is graphite, X is the additive, and Y is the amount of the additive. Characterization. The phases of graphitized samples were characterized by X-ray diffraction (XRD) with a RIGAKU D/Max2550VB+/PC diffraction using Cu Kα1 (wavelength: 0.154056 nm) radiation. The morphology and elemental component were characterized using scanning electron microscopy (SEM) and energydispersive X-ray spectroscopy (EDS), respectively. The elemental distributions of the samples were mapped. TEM images and SAED analysis were performed using a JEOL JEM 2100 microscope, operated at 200 kV, with a point resolution of 0.20 nm. TEM specimens were prepared following the process of grinding, dispersion in ethanol by ultrasonication, dropping onto carbon-coated copper grids (300 mesh), and finally drying. Silicon was used as the internal standard to calibrate the diffraction peaks in XRD examination.16 The interlayer spacing (d002), degree of graphitization (g), and crystalline sizes (La and Lc) of the graphite samples were calculated using appropriate equations (Bragg, Mering− Maire, and Alexander equations).17 Preferred orientation (R) of representative samples was investigated by XRD and obtained using
Pr6O11 exhibited, three light rare earth elements [lanthanum (La), cerium (Ce), and praseodymium (Pr)] were selected to be added to the coal-based carbon materials as catalyst to investigate the detailed mechanism of catalytic graphitization induced by light rare earth elements. The interface between the light rare earth elements and carbon materials was investigated, and it should be an important factor for the orientation of graphite structure. As mentioned above, catalysts Ti and Fe were investigated extensively, and they represent different categories because of the differences in their properties, such as the solubility of carbon and melting point. Here, Ti and Fe were also selected for the comparison of different mechanisms. Besides, the effects of light rare earth elements on the crystalline size and thermal conductivity of graphite were also investigated.
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EXPERIMENTAL SECTION
Preparation of Materials. The feedstock for artificial graphite includes petroleum coke, coal-derived pitch coke, needle coke, and natural graphite. Owing to the extensive application, coal-based needle coke and coal tar pitch were used as an aggregate and a binder, respectively. Detailed information on the sources and basic properties of the raw materials can be found in ref 15. The relevant additives (Ti, Fe, La, Ce, and Pr) were screened with a powder size of
