Catalytic Graphitization of Coal-Based Carbon Materials with Light

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Catalytic graphitization of coal-based carbon materials with light rare earth Rongyan Wang, Guimin Lu, Wenming Qiao, and Jianguo Yu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02000 • Publication Date (Web): 02 Aug 2016 Downloaded from http://pubs.acs.org on August 9, 2016

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Catalytic graphitization of coal-based carbon materials with light rare earth Rongyan Wang1, Guimin Lu1,∗, Wenming Qiao2,*, Jianguo Yu1,2 1) National Engineering Research Center for Integrated Utilization of Salt Lake Resources, East China University of Science and Technology, Shanghai, 200237, P. R. China; 2) State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, P. R. China;

KEYWORDS: Light rare earth, catalytic graphitization, graphite, crystallographic orientation, thermal conductivity

ABSTRACT The catalytic graphitization mechanism of coal-based carbon materials with light rare earth was investigated using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, selected area electron diffraction, and high-resolution transmission electron microscopy. The interface between light rare earth and carbon was carefully observed, and two routes of rare earth element catalyzing the carbon materials were found, including dissolution-precipitation and carbide formation-decomposition. These two simultaneous processes certainly accelerate the catalytic graphitization of carbon materials, and light rare earth exhibit 1 ACS Paragon Plus Environment

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significant influence on the microstructure and thermal conductivity of graphite. Moreover, by virtue of praseodymium (Pr), it was interestingly found that highly crystallographic orientation graphite was induced and formed, reasonably being 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 of the orientation of graphite structure.

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 received considerable attention. For Ni, several mechanisms were proposed, for instance, gas-solid interaction to form carbon nano-fiber, solid-state 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 with diffusion, interaction and external force effects1-5. Similarly, Co was found to catalyze amorphous carbon and promote the nucleation and growth of graphene2,6-8. However, there are no studies concerning monolayer or bilayer graphene growth with the aid of Fe due to its high carbon solubility, even though graphitic multilayers can be produced from amorphous carbon materials catalyzed by Fe2,7,9-12. Besides, some atomic-scale reports illustrated that amorphous carbon materials can significantly be catalytic graphitized by other metal catalysts, such as titanium (Ti), tungsten, molybdenum, and chromium7,13. The interface between metal catalyst and carbon was also concerned in the catalytic process. However, the majority of investigations mentioned above have focused on the 2 ACS Paragon Plus Environment

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transformation of amorphous carbon to graphitic order at 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 atomic-scale. In fact, the widely accepted catalytic graphitization process was considered to occur only when carbon materials were treated at 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 being heat treated at 2700 °C by catalyst Ti, whose raw materials were natural graphite flake and mesophase pitch14. Through TEM images and selected area electron diffraction (SAED) analysis, they confirmed that silicon carbide exhibited the catalytic effect with carbide formation-decomposition process, while 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 temperature on atomic-scale, and will attract attention paid for other catalysis. In our previous research 15 , praseodymium oxide (Pr6O11) additive was found to catalyze the graphite anode significantly. The graphite anode with a low electrical resistivity of 5.0 µΩ·m was successfully available by adding 3 wt. % Pr6O11 at 2800 °C. In the present study, based on the excellent performance that Pr6O11 exhibited, three light rare earth (lanthanum (La), cerium (Ce) and praseodymium (Pr)) were selected to be added into the coal-based carbon materials as catalyst respectively to investigate the 3 ACS Paragon Plus Environment

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detailed mechanism of catalytic graphitization induced by light rare earth. The interfaces between light rare earth and carbon materials were investigated, and it should be an important factor of the orientation of graphite structure. As mentioned above, catalysts Ti and Fe were investigated extensively, and they represent different categories because of the differences of their properties, such as the solubility of carbon and molten point. Here, Ti and Fe were also selected for comparison of different mechanisms. Besides, the effects of light rare earth on the crystalline sizes and thermal conductivity of graphite were also investigated.

EXPERIMENTAL SECTION Preparation of materials The feedstock for artificial graphite includes petroleum coke, coal derived pitch coke, needle coke and even natural graphite. Owing to the extensive application, coal-based needle coke and coal tar pitch were used as aggregate and binder, respectively. The detailed sources and basic properties of the raw materials can be found in the reference 15. The relevant additives (Ti, Fe, La, Ce and Pr) were screened with a powder size < 50 µm. Milled coke, pitch (23 wt. %) and metal additive (for instance 10 wt. % Ti) were firstly mixed in a ball-milling machine for 30 min, then moved into a kneading machine to sequentially mix for 30 min at 155 °C. The mixture was compacted to cylindrical body at 135 °C under a pressure of 23.6 MPa. Finally, carbonization and graphitization for the samples were carried out in sequence, with the heating rate of 100 °C/h and 300 °C/h up to 1000 °C and 2800 °C, respectively. The soak time for samples was set to be 0.5 h at 2800 °C. The sample without additive was also prepared via the same process. 4 ACS Paragon Plus Environment

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To compare the effects of light rare earth on the microstructures and thermal property of graphite, a series of graphite samples prepared with light rare earth were prepared via the typical process of artificial graphite production. As described in our previous publication15, needle coke was used with the size distribution as follows: 1-2 mm 12.5 wt. %, 0.5-1 mm 22.5 wt. %, 0.15-0.5 mm 30 wt. %, 0.076-0.15 mm 25 wt. %, and < 0.076 mm 10 wt. %. It was mixed with coal tar pitch (23 wt. %) and light rare earth additive (0-5 wt. %) at 155 °C. The mixture was hot-molded at 135 °C under a pressure of 23.6 MPa. Then, the samples with the size of 120 × 60 × 20 mm were treated as the following the procedure,

including

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/Max-2550VB+/PC diffraction using Cu Kα1 (wavelength: 0.154056 nm) radiation. The morphology and elemental component were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), respectively. The elemental distributions of the samples were mapped. TEM images and SAED analysis were performed on a JEOL JEM 2100 microscopy, 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 ultra-sonication, dropping onto carbon coated copper grids (300 mesh), and finally being dried. Silicon was used as the internal standard to calibrate the diffraction peaks in XRD 5 ACS Paragon Plus Environment

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examination 16 . The interlayer spacing (d002), degree of graphitization (g), and crystalline sizes (La and Lc) of the graphite samples were calculated with the data using the appropriate equations (Bragg, Mering-Maire, and Alexander equations)17. Preferred orientation (R) of representative samples was investigated by XRD and obtained using R=

360∑ ωi

(1)

360

where ωi is the ratio of integral intensity to correspondent intensity for the peak18. Samples were cut into cubic sheet with a thickness of 1.5 mm for measurement. The detector was fixed in the position, which corresponded to the (002) plane of graphite. Then the sample was rotated along the azimuthal angle with 15°/min. Raman measurements were performed with an excitation wavelength of 445 nm and an incident power of ~10 mW using a Thermo DXR spectrograph. The D, G, and 2D peaks were fitted with Lorentz functions. The ID/IG ratio was calculated using the height of the D and G peaks intensities. Twenty spectra were collected for each sample. The crystalline size along the a-axis (La) was calculated using the Cançado equation17-19. Thermal diffusivity of the graphite was measured using the laser flash technique (Linseis LFA447)20. Samples were cut into circle sheet with a diameter of 12.67 mm and a thickness of 2 mm for measurement. Three parallel measurements were carried out in vacuum to obtain the mean value and standard deviation. In the laser flash technique, one side of the sample is irradiated by an infrared laser pulse, while the temperature of the other side of the sample is measured using an InSb infrared detector. The thermal diffusivity α is calculated following ωL2

α = 2∙t

(2)

1⁄2

where L is the thickness of the sample, ω is a constant that accounts for thermal losses, and t1⁄2 is the time it takes that the temperature of the side detected reaches the half 6 ACS Paragon Plus Environment

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of the temperature in opposite side. Once the diffusivity is calculated, the thermal conductivity can be calculated using k T= α T ∙ CP T·ρ

(3)

where ρ is the density of the sample, and CP is the specific heat capacity of the material at constant pressure.

RESULTS AND DISCUSSION The graphitization of amorphous carbon with the aid of transition metal catalysts was discussed in several literatures2-9,11,13-14. There are mainly two kinds of mechanism: one is the dissolution of carbon into molten metal, and then metal and graphite crystals precipitate from the saturation; another is the formation of an intermediate product — metal carbide, which then decomposes and recrystallizes to metal and graphite crystals21-22. Ti and Fe were proved to follow different paths to affect carbon materials owing to their different interactions with carbon. Previously, Ti was confirmed to catalyze the graphitization of natural graphite flake followed by both catalytic mechanisms, while Fe just followed by the former mechanism 2, 14. Based on this, the catalytic effects of light rare earth (La, Ce and Pr) were investigated accompanied with those of Ti and Fe for comparison.

XRD investigation Figure 1 showed that the sharp peaks of hexagonal graphite are observed in all of the samples. For the graphitized sample with Ti, there are five extra peaks corresponding to TiC (face-centered cubic, a=4.316 Å) aside from the graphite-2H phase. Whereas there were no additional peaks existing in Figure 1 (c), indicating the absence of Fe or iron carbide. Fe phase was not observed in the XRD pattern maybe due to the dusting 7 ACS Paragon Plus Environment

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process, and iron carbide due to its low decomposition temperature (1252 °C) although it probably had formed8-9,23. Both the observations of graphitized samples with Ti and Fe are consistent with previous publications7-9,14,24-25. For the samples prepared with light rare earth catalysts, peaks of carbides are not so obvious unless the intensity value of the XRD pattern was performed on a squared scale. From the point view of their binary phase diagrams, rare earth carbides (REC2) indeed existed when carbon materials were heat treated at or above relevant temperatures. The residual carbides indicated here at least demonstrated that the reaction between metal and carbon involves the formation of carbide.

Figure 1 XRD patterns of the samples prepared (a) without additive; (b-f) with Ti, Fe, La, Ce, and Pr after heat-treatment at 2800 °C, respectively. The dotted lines represent the peaks of hexagonal graphite phase. Additional peaks represent the carbides

SEM investigation Combined with the XRD results and EDS analyses, it was found that a large 8 ACS Paragon Plus Environment

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number of TiC remained in the graphitized sample with Ti, while the carbon microsphere formed after the evaporation of Fe in the sample with Fe. The details for the graphitized samples with Ti and Fe were shown in Supporting Information. The well-acknowledged dissolution-precipitation mechanism could explain these two different pathways for catalytic graphitization obviously. However, the TiC formation-decomposition mechanism was experimentally validated in the reference 14 not in this paper. For the samples prepared with light rare earth catalysts, the amount of residual catalyst in the prepared samples after heat treatment at 2800 °C reduced, compared with those at 1000 °C. As indicated in Figure 2, part of the REC2 decomposed, and then rare earth escaped from the graphitized samples. The morphology of graphitized samples with light rare earth was similar to the sample without additive. However, XRD patterns and SEM images only provide the statistical and apparent information of the graphitized samples, and detailed exploration by TEM is needed for the investigation of the microstructure and catalytic graphitization mechanism.

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Figure 2 SEM images of the sample prepared with Pr after heat-treatment at (a) 1000 °C and (b) 2800 °C, respectively; Elemental results (c, d) obtained by EDS mapping of images in (a, b), respectively. The elemental distribution in SEM image was displayed by different colors (yellow: Pr)

TEM investigation Existence of residual metal particles Based on the elemental analysis, the main component in all the samples was carbon, accompanied with small amount of metal. TEM images showed that the samples prepared with catalysts consist of dark grains (metal-rich phase) embedded in a light matrix (carbon-rich phase), as shown in Figures 3 (b-f). It demonstrated that a strong graphite-metal interface where catalytic graphitization occurred existed. This interface becomes the key point to reveal the effect of catalyst on the graphitization process of 10 ACS Paragon Plus Environment

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carbon materials, which will be discussed below.

Figure 3 TEM images of the samples prepared (a) without additive; (b-f) with Ti, Fe, La, Ce, and Pr after heat-treatment at 2800 °C, respectively

The dark nanoparticles with size of 50–80 nm were the residual of metal catalysts in the graphitized samples after heat treatment at 2800 °C (except for the graphitized sample with Ti). The large grains (with the size of ~200 nm) in the graphitized sample with Ti indicated that most TiC would remain stable in the sample after catalytic graphitization. The elongation of TiC nanoparticle resulted from its melt and flow, similar to the catalytic behaviors of amorphous carbon materials with Fe and Ni revealed by in situ technical investigation1,11. There is a difference between the TEM observation and XRD patterns providing the information concerning the existence of Fe. Therefore, the atomic microstructure of residual Fe in the graphitized sample was studied further by HRTEM and SAED instruments, as shown in Figure 4. From the SAED analysis, it was demonstrated that the graphite coexisted with Fe (body-centered cubic structure). Three most important 11 ACS Paragon Plus Environment

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crystal planes of graphite were observed in the SAED pattern, corresponding to the strongest diffraction peaks in the XRD pattern. However, the most areas of the specimen with Fe were observed only possessing graphite sheets, being similar to the image of Figure 3 (a), indicating that the Fe had escaped.

Figure 4 (a) TEM image of the sample prepared with Fe after heat-treatment at 2800 °C; (b) SAED pattern surrounding the dark area marked with a white cycle in (a); (c and d) HRTEM images of the area marked with a white rectangle in (a) and in Figure 3 (c), respectively. Lattice spacing of 0.335 nm corresponds to graphite, while 0.203 nm could be assigned to Fe

In fact, the difference of these results from the techniques can be reasonably explained from the different mechanisms of these detection methods26. It is ubiquitous that the amounts of residual metal particles were small, resulting in the weak peaks or even no peak observed in the XRD patterns. Just because of this, the other methods (such as SAED and HRTEM) were essential to assist XRD in revealing the real 12 ACS Paragon Plus Environment

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mechanism. Since the Fe had evaporated at such high temperature, it cannot be detected by XRD. The catalytic graphitization of carbon materials induced by Fe is followed the dissolution-precipitation mechanism, being consistent with the previous investigations, although the existence of Fe particle was observed in the TEM image27.

Graphitization mechanism of graphitized samples with light rare earth The XRD results of graphitized samples with light rare earth, which present the REC2 phases with graphite phase, revealed the formation of carbide. As shown in Figures 5 (a-c), SAED patterns of graphitized samples with light rare earth indicated that the graphite phase was accompanied with the phase of light rare earth and the corresponding carbide phase, which is different with the XRD results. Detailed information of SAED patterns were provided in Supporting Information Table S1. SAED method appears to provide more information about the interaction between graphite and metal because of its selected area feature. It is notable that the diffraction ring corresponding to the (002) plane of graphite is obscure (only several bright spots), showed in Figure 5 (b). SAED results can be further confirmed through HRTEM images. Graphitic layers were observed with lattice spacing of 0.335 nm, corresponding to (002) plane of graphite. In Figure 5 (e), the dark phase was identified as pure Pr phase concerning its lattice spacing (0.298 nm corresponds to (004) plane of the standard XRD pattern of Pr). It is remarkable that graphitic layers would curve along the flux of metal particle, as shown in the boundary of graphite and metal with black dotted line in Figure 5 (e).


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Figure 5 (a-c) SAED patterns surrounding the dark area in Figures 3 (d-f), respectively; (d, e) HRTEM images of the area marked with a white rectangle in Figures 3 (e, f), respectively. Lattice spacing of 0.335 nm and 0.336 nm correspond to graphite, while 0.298 nm could be assigned to Pr

Carbide, which connects the graphite and metal, plays its role not only as the product but also as the intermediate. This hypothesis was proposed based on the graphitized carbon matrix with carbide or pure metal crystals embedded in it. This avails the investigations of graphitization process and catalytic mechanism. The transformation from amorphous carbon to graphite is driven by the energy decreasing of system, and the reaction barrier can be further reduced by active metal. The metal crystals were saturated with carbon before the precipitation, during which the bulk diffusion of carbon in the metal occurred, and finally benefited the nucleation and growth of graphene2. Once the amorphous carbon layer disappears from the interface, a graphitic layer forms on the same surface of metal crystal, but the growth occurs on the other side. Then the graphitic multilayers accumulated and finally become graphite. As shown in 14 ACS Paragon Plus Environment

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Figures 5 (d) and (e), the graphitic layers were observed to distribute along the edges of metal particle orderly and smooth interaction between the metal particle and carbon supported this viewpoint. The dissolution-precipitation mechanism can be confirmed during the graphitization process of carbon materials with light rare earth. Besides, the process of carbide formation-decomposition also occurs. The reaction of metal and carbon begins to occur when temperature reaches the definite temperature, at which the active energy is enough to drive to form relative carbide. With increasing temperature, the carbide becomes unstable and then decomposes into graphite and metal. In Figure 5 (e), Pr particle was encapsulated in graphitic layers, and part of it flowed away in several directions, resulting in angular boundaries. The results of the amorphous carbon induced by Fe also presented this tendency, and Anton proposed that the spread of metal only occurs after the encapsulation and reaction to carbide11. The residual carbide was not only detected from the XRD and SAED patterns, but also observed in the HRTEM images. This also demonstrated our viewpoint of carbide acting as product. However, the existence of carbide detected from different instruments represents different stages of it during the graphitization. For instance, the intensity of REC2 peaks in the XRD patterns was considered as the final residue, which was considered to form in the close pores of carbon materials and finally keep undecomposed. However, most of the carbide decomposed into metal and carbon from the evident results of EDS mapping analysis. Coincidentally, the decomposition was captured at different stages using SAED and HRTEM techniques. Therefore, the results from them were considered as local residual found accidentally. This indicated that most REC2 decomposed. One of the products, pure metal, was expelled from graphite during the graphitization process. 15 ACS Paragon Plus Environment

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At cooling stage of the heat treatment, the recrystallization of molten metal occurs gradually. It firstly occurs in the inner of materials, and then spreads towards the boundaries. Dark area in Figure 5 (d), which may be considered as Ce phase, showed the possible evolution of crystal lattice from metal side to graphite. Both the obvious crystal spacing of graphite and metal were presented, whereas the chaotic interaction should be considered as the carbide or the mixture of carbon and metal. Therefore, light rare earth assists the transformation of coal-based carbon material to graphite by two routes, including dissolution-precipitation and carbide formation-decomposition.

Effect of Pr on the crystallographic orientations of graphite In Figure 6 (b), sharp diffraction spots were presented in addition to the diffuse rings. These spots correspond to the (101) and (201) crystal planes of graphite-2H phase. Correspondingly, Figures 6 (d) and (f) exhibit the SAED patterns of graphite at [001] zone axis taken from the area marked with white cycle in Figures 6 (c) and (e), respectively. Part of the graphitized sample with Pr can be even remarkably observed to possess highly ordered graphitic structure, which is similar to single crystal structure of graphite. Pr appears to be in favor of the crystallographic orientation of graphite, and it reasonably plays an important role on the rearrangement of graphitic layers, although the process was not visible here.


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Figure 6 (a, c and e) TEM images of the sample prepared with Pr after heat-treatment at 2800 °C; (b, d and f) SAED patterns of the area marked with a white cycle in (a, c and e), respectively

It was stated that there is no influence of catalytic metal on the crystallographic orientation of graphitic structures in some literatures1,11. The highly orientation structure was only achieved in the graphitized sample catalyzed with Pr, and this phenomenon appears to be not accidental. As summarized above, the catalytic route of 17 ACS Paragon Plus Environment

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Pr

affecting

graphitization

process

included

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two

distinct

mechanisms,

dissolution-precipitation and carbide formation-decomposition. Something interesting can be pointed out by the comparison among the SAED patterns of the graphitized sample with Pr. The promotion of the graphitic structure to be large and less-defect sheets became obvious after the departure of Pr, no matter for further catalysis or elimination. It was reflected that the less the proportion of Pr in the selected area of SAED investigation is, the more highly orientation structure the graphite achieved. To explore the effect of Pr on the crystallographic orientation of graphite, the crystalline structures of pure metal Pr and graphite were investigated. The atomic arrangements in (001) planes of both graphite and Pr crystals are in the hexagonal configuration. They possess the same space group (P63/mmc), but their lattice parameters are different. As illustrated in Figure 7 (a), the unit cells of graphite and Pr crystal exhibit slightly different. Pr atoms in unit cell can be divided into two forms, which were distinguished by color hue. According to this, it can be found that atomic arrangement of pure Pr crystal is similar to the ABAB··· stacking sequence of graphite. Moreover, they almost parallel to each other, and the interlayer spacing of Pr crystal is twice that of graphite (0.5914 nm and 0.3354 nm, respectively). In a 2×2×1 super cell, the maximum value of difference between their planes was 0.159 nm, as revealed in Figure 7 (b). During the catalytic graphitization process, Pr acts its role as bridge to connect the graphitic sheets, and helps them to arrange regularly via creating as many active sites as possible, as illustrated in Figure 7 (c). Therefore, Pr affects the orientation structure of graphite when Pr moves away from graphite. This was supported by the different results from the SAED patterns of the graphitized sample with Pr. Almost all of the Pr atoms participate in this catalytic process, utilizing the similar lattice structure features. The resulting graphite sheets will possess larger and 18 ACS Paragon Plus Environment

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higher orientation structure. With the aid of Pr, highly crystallographic orientation structure of graphite can be obtained, which was illustrated in Figure 7 (c). Our viewpoint can be supported by the research in the reference 14, where Lin found that the basal plane stacking sequence of precipitated graphite is significantly influenced by the orientation of (111)TiC planes relative to the interface.

Figure 7 (a) Unit cells of graphite and pure metal Pr, respectively; (b) Views of graphite and Pr crystals planes perpendicular to (001), respectively; (c) Schematic image of the action of Pr with graphitic layers, revealing the effect of Pr on the crystallographic orientation of graphite consequently


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Comparison of the catalytic effects of light rare earth Effects of light rare earth on the microstructure of graphite In our previous study, the interaction between Pr and carbon atoms was found to benefit the dissociation of Pr from carbon attraction, and only small amount (1 wt. %) of Pr6O11 significantly promoted the degree of graphitization and increased the crystalline sizes of graphite anode15. To investigate the effects of light rare earth as well as to confirm the effect of Pr on the crystallographic orientation of graphite, the graphite was prepared by the catalysis of light rare earth with different amounts. The data from XRD, which were collected by a step-scanning method, were calculated strictly according to the standard procedures. Statistical analysis of the Raman spectra was performed because the peak positions in the spectrum are scattered by charged impurities16,28. Figure 8 showed the results of representative graphite without and with Pr. The G bands of graphite without and with Pr (5.0 wt. %) were corresponded to 1582 cm-1 and 1580 cm-1, respectively. The catalyst Pr is interpreted to lead to the slightly blue shift. This phenomenon was also observed in the Raman spectra of graphite with the other rare earth, as shown in Supporting Information Figure S2. Moreover, La and Ce resulted in the D band’s broadening, while this was not observed in the Raman spectra of graphite with Pr. The 2D peak is the second order of D peak, and light rare earth appear to have no influence on the position and full width at half maximum (FWHM) of 2D peak.


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Figure 8 (a) XRD patterns for representative graphite without (black curve) and with Pr (green curve); (b) Raman spectra for representative graphite without and with Pr; Distribution of (c) position of G peak; (d) position of D peak, (e) ID/IG, (f) I2D/IG, and (g) FWHM of G peak; (h) ID/IG ratios as a function of FWHM(G). 21 ACS Paragon Plus Environment

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It is notable that the amount of catalyst has no influence on the position of peaks in Raman spectra. However, the amount of catalyst affects the degree of graphitization of graphite. The effects of different catalysts on the degree of graphitization, preferred orientation and crystalline sizes of graphite were listed in Table 1. Lc values were calculated from the expression proposed by Alexander, while the calculation of La values from it was slight difficult owing to the weakly intensity of (110) peak in the XRD pattern. Considering this, several equations were proposed to calculate La values from Raman spectra19,29-31. Specially, the estimation of La value calculated from the Cançado equation was more suitable when the ID/IG ratio of sample was above 0.117. Therefore, from Table 1, it was indicated that the catalysts (La, Ce and Pr) promote the degree of graphitization and crystalline sizes of graphite. When compared with the other graphite added by other catalysts with the same amount, all the graphite with catalyst Pr exhibit the highest degree of graphitization and the larger crystalline sizes (both La and Lc). Besides, the graphite with 1 wt. % Pr exhibited the highest R value, which further confirmed the effect of Pr on the crystallographic orientation of graphite. Table 1 Crystal parameters of the graphites without/with light rare earth catalysts determined from the XRD patterns and Raman spectra Samples

d002 (nm)

g (%)

R (%)

G–0

0.3366

86.05

61.2

G–La1.0

0.3361

92.21

68.7

G–La3.0

0.3361

G–La5.0

ID/IG

Lc (nm)

La (nm) Alexander

Cançado

Alexander

0.171

77.5

55.0

66.2

0.148

135

63.6

87.9

91.54

0.166

-

56.7

76.4

0.3362

90.38

0.183

-

51.4

78.7

G–Ce1.0

0.3362

90.57

0.142

128

66.3

90.1

G–Ce3.0

0.3362

90.37

0.146

-

64.5

77.2

G–Ce5.0

0.3362

91.27

0.162

-

58.1

92.3

G–Pr1.0

0.3358

95.06

0.125

141

75.3

91.7

G–Pr3.0

0.3360

92.16

0.138

117

68.2

111

G–Pr5.0

0.3362

91.54

0.142

112

66.3

71.9

67.2

83.0


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Effects of light rare earth on thermal conductivity of graphite Thermal conductivity could be attributed to three concurrent phenomena: conduction by phonons (the most important), conduction by electrons (occupied 1 % by applying the Wiedemann-Franz law) and radiative transfer (could be negligible)32. There is a relationship between the degree of graphitization and transport properties in carbon materials33. An increase of crystalline sizes would also enhance the transport properties by reducing phonon scattering at the boundaries and defects of materials. From Figure 9, the effect of catalyst on the thermal conductivity of graphite was significant, even outdistanced the effect of bulk density. However, the effect of catalyst amount appears to be negligible. The highest thermal conductivity of 257.24 W/(m·K) was obtained in the graphite with catalyst Pr, and it was comparable to or higher than the thermal conductivity of some typical metals. Besides, there is a relationship between the thermal conductivity and electrical conductivity of carbon materials33. The graphite anode with a low electrical resistivity of 5.0 µΩ·m was successfully available by adding 3 wt. % Pr6O11 at 2800 °C15. Therefore, the enhancement of the thermal conductivity and electrical conductivity of graphite demonstrated again that it became more likely to promote the graphite to be larger and less-defect sheets with the aid of Pr, and highly crystallographic orientation structure of graphite could be obtained.


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Figure 9 Effect of light rare earth additives (La, Ce and Pr) on the thermal conductivity of graphite with different bulk densities. The number beside the data represents the amount of catalyst in the graphite (wt. %)

CONCLUSIONS There are two different types of graphitization mechanism of light rare earth to catalyze the carbon materials. The dissolution-precipitation leaving behind the rare earth in a regular crystallite morphology was directly observed by HRTEM. The carbide formation-decomposition causes the residual rare earth carbides detected by XRD and SAED. These two simultaneous mechanisms certainly accelerated the graphitization of carbon materials and improved the degree of graphitization. Significant effect of Pr on the crystallographic orientation of graphite was found, reasonably due to the same space group between Pr and graphite, and the similar hexagonal configuration of arrangements in the planes perpendicular to (001). The interface between Pr and carbon was found to be an important factor of the orientation of graphite structure. The effect of Pr on the crystallographic orientation of graphite was confirmed by the 24 ACS Paragon Plus Environment

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comparison of the microstructure and thermal conductivity of graphite added with different light rare earth. The degree of graphitization and the crystalline sizes of graphite with Pr were higher and larger than those with the others rare earth additives. The thermal conductivity of graphite added with Pr was twice that of the graphite without additive, 257.24 W/(m·K) and 133.84 W/(m·K), respectively. Pr was proved to be the most efficient catalyst for the carbon materials, even affected the crystallographic orientation of graphite.

ASSOCIATED CONTENT Supporting Information SEM images and elemental mapping of the prepared samples treated at different temperature. Detailed information for SAED patterns of graphitized samples. Statistical analysis results of Raman spectra of graphite with light rare earth. This material is available free of charge on the ACS publications website via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (G.M. Lu) *E-mail: [email protected] (W.M. Qiao)

Notes The authors declare no competing financial interest.

Funding Sources 25 ACS Paragon Plus Environment

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This study is supported by National Natural Science Foundation of China (No. U1407202).

ACKNOWLEDGMENTS The authors acknowledge financial support of National Science Foundation of China under award number U1407202. R. Wang thanks Dongfan Liu and Haiou Ni (National Engineering Research Center for Integrated Utilization of Salt Lake Resources, China) for the help. Also thanks Lihui Zhou (Analytical Instrumentation Center, East China University of Science and Technology, China) for the valuable discussions. REFERENCES ( 1 ) Helveg, S.; López-Cartes, C.; Sehested, J.; Hansen, P. L.; Clausen, B. S.; Rostrup-Nielsen, J. R.; Abild-Pedersen, F.; Nørskov, J. K. Atomic-scale Imaging of Carbon Nanofiber Growth. Nature 2004, 427 (6973), 426-429. (2 ) Rodríguez-Manzo, J. A.; Pham-Huu, C.; Banhart, F. Graphene Growth by a Metal-Catalyzed Solid-State Transformation of Amorphous Carbon. ACS Nano 2011, 5 (2), 1529-1534. (3) Saenger, K. L.; Tsang, J. C.; Bol, A. A.; Chu, J. O.; Grill, A.; Lavoie, C. In Situ X-ray Diffraction Study of Graphitic Carbon Formed during Heating and Cooling of Amorphous-C/Ni Bilayers. Appl. Phys. Lett. 2010, 96 (15), 153105. (4) Liu, Y. C.; Liu, Q. L.; Gu, J. J.; Kang, D. M.; Zhou, F. Y.; Zhang, W.; Wu, Y.; Zhang, D. Highly Porous Graphitic Materials Prepared by Catalytic Graphitization. Carbon 2013, 64, 132-140. (5) Aikawa, S.; Kizu, T.; Nishikawa, E. Catalytic Graphitization of an Amorphous Carbon Film under Focused Electron Beam Irradiation due to the Presence of Sputtered Nickel Metal Particles. Carbon 2010, 48 (10), 2997-2999. 26 ACS Paragon Plus Environment

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(6) Kim, J.; Lee, J.; Choi, Y.; Jo, C. Synthesis of Hierarchical Linearly Assembled Graphitic Carbon Nanoparticles via Catalytic Graphitization in SBA-15. Carbon 2014, 75, 95-103. (7) Maldonado-Hódar, F. J.; Moreno-Castilla, C.; Rivera-Utrilla, J.; Hanzawa, Y.; Yamada, Y. Catalytic Graphitization of Carbon Aerogels by Transition Metals. Langmuir 2000, 16 (9), 4367-4373. (8) Bacsa, R. R.; Cameán, I.; Ramos, A.; Garcia, A. B.; Tishkova, V.; Bacsa, W. S.; Gallagher, J. R.; Miller, J. T.; Navas, H.; Jourdain, V.; Girleanu, M.; Ersen, O.; Serp, P. Few Layer Graphene Synthesis on Transition Metal Ferrite Catalysts. Carbon 2015, 89, 350-360. (9) Sajitha, E. P.; Prasad, V.; Subramanyam, S. V.; Eto, S.; Takai, K.; Enoki, T. Synthesis and Characteristics of Iron Nanoparticles in a Carbon Matrix along with the Catalytic Graphitization of Amorphous Carbon. Carbon 2004, 42 (14), 2815-2820. (10) Matveev, V. N.; Levashov, V. I.; Kononenko, O. V.; Matveev, D. V.; Kasumov, Y. A.; Khodos, I. I.; Volkov, V. T. Hall Effect Sensors on the Basis of Carbon Material. Mater. Lett. 2015, 158, 384-387. (11) Anton, R. In Situ TEM Investigations of Reactions of Ni, Fe and Fe–Ni Alloy Particles and Their Oxides with Amorphous Carbon. Carbon 2009, 47 (3), 856-865. ( 12 ) Bokhonov, B.; Korchagin, M. Formation of Graphite Encapsulated Metal Nanoparticles during Mechanical Activation and Annealing of Soot with Iron and Nickel. J. Alloy. Compd. 2002, 333 (1–2), 308-320. (13) Tang, J.; Wang, T.; Sun, X.; Guo, Y. X.; Xue, H. R.; Guo, H.; Liu, M. Z.; Zhang, X. X.; He, J. P. Effect of Transition Metal on Catalytic Graphitization of Ordered Mesoporous Carbon and Pt/Metal Oxide Synergistic Electrocatalytic Performance. Micropor. Mesopor. Mat. 2013, 177, 105-112. 27 ACS Paragon Plus Environment

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(14) Lin, Q. Y.; Feng, Z. H.; Liu, Z. J.; Guo, Q. G.; Hu, Z. J.; He, L. L.; Ye, H. Q. Atomic Scale Investigations of Catalyst and Catalytic Graphitization in a Silicon and Titanium Doped Graphite. Carbon 2015, 88, 252-261. (15) Wang, R. Y.; Lu, G. M.; Qiao, W. M.; Sun, Z.; Zhuang, H. Z.; Yu, J. G. Catalytic Effect of Praseodymium Oxide Additive on the Microstructure and Electrical Property of Graphite Anode. Carbon 2015, 95, 940-948. (16) Iwashita, N.; Park, C. R.; Fujimoto, H.; Shiraishi, M.; Inagaki, M. Specification for a Standard Procedure of X-ray Diffraction Measurements on Carbon Materials. Carbon 2004, 42 (4), 701-714. (17) Badenhorst, H. Microstructure of Natural Graphite Flakes Revealed by Oxidation: Limitations of XRD and Raman Techniques for Crystallinity Estimates. Carbon 2014, 66, 674-690. (18) Qin, X. Y.; Lu, Y. G.; Xiao, H.; Hao, Y. C.; Pan, D. Improving Preferred Orientation and Mechanical Properties of PAN-Based Carbon Fibers by Pretreating Precursor Fibers in Nitrogen. Carbon 2011, 49, 4595-4607. (19) Cançado, L. G.; Takai, K.; Enoki, T.; Endo, M.; Kim, Y. A.; Mizusaki, H.; Jorio, A.; Coelho, L. N.; Magalhães-Paniago, R.; Pimenta, M. A. General Equation for the Determination of the Crystallite Size La of Nanographite by Raman Spectroscopy. Appl. Phys. Lett. 2006, 88 (16), 163106. (20 ) Parker, W. J.; Jenkins, R. J.; Butler, C. P.; Abbott G. L. Flash Method of Determining Thermal Diffusivity, Heat Capacity, and Thermal Conductivity. J. Appl. Phys. 1961, 32, 1679. (21) Fitzer, E.; Kegel, B. Reactions of Carbon Saturated Vanadium Carbide with Disordered Carbon (Effects on Catalytic Graphitization). Carbon 1968, 6 (4), 433-446. (22) Murty, H. N.; Biederman, D. L.; Heintz, E. A. Catalytic Graphitization of Model 28 ACS Paragon Plus Environment

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Orlova, T. S. Thermal Conductivity of Fe Graphitized Wood Derived Carbon. Mater. Design 2016, 99, 528-534.

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Figure 1 XRD patterns of the samples prepared (a) without additive; (b-f) with Ti, Fe, La, Ce, and Pr after heat-treatment at 2800 °C, respectively. The dotted lines represent the peaks of hexagonal graphite phase. Additional peaks represent the carbides 562x377mm (300 x 300 DPI)

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Figure 2 SEM images of the sample prepared with Pr after heat-treatment at (a) 1000 °C and (b) 2800 °C, respectively; Elemental results (c, d) obtained by EDS mapping of images in (a, b), respectively. The elemental distribution in SEM image was displayed by different colors (yellow: Pr) 799x599mm (200 x 200 DPI)


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Figure 3 TEM images of the samples prepared (a) without additive; (b-f) with Ti, Fe, La, Ce, and Pr after heat-treatment at 2800 °C, respectively 182x125mm (96 x 96 DPI)


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Figure 4 (a) TEM image of the sample prepared with Fe after heat-treatment at 2800 °C; (b) SAED pattern surrounding the dark area marked with a white cycle in (a); (c and d) HRTEM images of the area marked with a white rectangle in (a) and in Figure 3 (c), respectively. Lattice spacing of 0.335 nm corresponds to graphite, while 0.203 nm could be assigned to Fe 233x171mm (96 x 96 DPI)


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Figure 5 (a-c) SAED patterns surrounding the dark area in Figures 3 (d-f), respectively; (d, e) HRTEM images of the area marked with a white rectangle in Figures 3 (e, f), respectively. Lattice spacing of 0.335 nm and 0.336 nm correspond to graphite, while 0.298 nm could be assigned to Pr 1500x899mm (72 x 72 DPI)


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Figure 6 (a, c and e) TEM images of the sample prepared with Pr after heat-treatment at 2800 °C; (b, d and f) SAED patterns of the area marked with a white cycle in (a, c and e), respectively 207x261mm (96 x 96 DPI)


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Figure 7 (a) Unit cells of graphite and pure metal Pr, respectively; (b) Views of graphite and Pr crystals planes perpendicular to (001), respectively; (c) Schematic image of the action of Pr with graphitic layers, revealing the effect of Pr on the crystallographic orientation of graphite consequently 220x238mm (96 x 96 DPI)


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Figure 8 (a) XRD patterns for representative graphite without (black curve) and with Pr (green curve); (b) Raman spectra for representative graphite without and with Pr; Distribution of (c) position of G peak; (d) position of D peak, (e) ID/IG, (f) I2D/IG, and (g) FWHM of G peak; (h) ID/IG ratios as a function of FWHM(G). 338x500mm (96 x 96 DPI)


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Figure 9 Effect of light rare earth additives (La, Ce and Pr) on the thermal conductivity of graphite with different bulk densities. The number beside the data represents the amount of catalyst in the graphite (wt. %) 287x220mm (300 x 300 DPI)


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Catalytic Graphitization of Coal-Based Carbon Materials with Light

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