SiC: Effective

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Catalytic Hydrotreatment of Kraft Lignin over NiW/SiC: Effective Depolymerization and Catalyst Regeneration Shi Qiu, Mingrui Li, Yong Huang, and Yunming Fang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04803 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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Industrial & Engineering Chemistry Research

Catalytic Hydrotreatment of Kraft Lignin over NiW/SiC: Effective Depolymerization and Catalyst Regeneration Shi Qiu, Mingrui Li, Yong Huang*, Yunming Fang*

College of Chemical Engineering, Beijing University of Chemical Technology, 100029, Beijing, China

ABSTRACT: A key challenge in biomass catalytic conversion, especially in pilot and practical scales, is the stability of catalyst and its support. Depolymerization of Kraft lignin, which is characterized by structural recalcitrant and poison (metal and ash) rich nature, is a good model reaction to demonstrate the above challenge in biomass conversion. In the present study, the potential of SiC based catalyst (commercially available SiC nanofibers supported Ni and W, NiW/SiC) in lignin depolymerization was investigated. The results indicate SiC based catalyst is a milder catalyst than the state of the art activated carbon supported catalyst under the identical conditions and supplies a higher liquid product yield. More importantly, the NiW/SiC catalyst can be easily regenerated by coke combustion and subsequent acid washing, which cannot be achieved by either carbon or metallic oxide supported catalysts. The regenerated catalyst with only 4% Ni input has almost unchanged performance compared with the fresh one. These

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results illustrate that SiC combines the advantages of common supports such as activated carbon and metallic oxides and may be generally applicable as catalyst support in biomass conversion.

1. INTRODUCTION The production of biofuels has drawn worldwide attention since biomass is the only renewable source of organic carbon in the nature and does not contribute to the greenhouse gas emission during life cycle utilization.1,

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One of the most important

factors for the biomass to biofuel conversion is the development of catalyst because biomass has properties much different from hydrocarbon.3 A lot of successful examples have been reported in literatures which realizes high efficiency biofuel production through the development of novel catalytic materials and reaction mechanisms.4,

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However, many challenges from chemical engineering aspect still remain during the practical application of these novel catalysts. A key challenge is the stability of catalyst and its support in biomass reaction. Kraft lignin, obtained through the sulfide pulping process, is an abundant biomass resource with the amount up to 50 million tons per annum.6, 7 However, the Kraft lignin process is highly energetically integrated and the resulting lignin is usually employed as fuel for the process heating. Indeed, the Kraft lignin can potentially serve as a renewable resource for a biorefinery operation because of the abundance of aromatic structures. Compared with protolignin in the parent biomass, the structure of Kraft lignin is


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characterized by a decrease in the number of ether bonds and an increased amount of C-C bonds during the extensive pulping, which results in difficulties in depolymerization of Kraft lignin and thus requires high temperature and high pressure.8, 9 In addition, the sulfide pulping process generally contributes to high ash and sulfur contents in Kraft lignin.10,

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This issue, together with the coke deposition from repolymerization of

primarily depolymerized highly active components, easily leads to catalyst deactivation during the catalytic depolymerization of Kraft lignin.12, 13 Considering the recovery and regeneration, an ideal catalyst for Kraft lignin depolymerization should have combustion resistance to eliminate the coke deposition by calcination and have acid resistance to remove the ash contamination by acid washing during its regeneration.14, 15 To reach this, both support and active component of catalyst should be carefully engineered. Hence depolymerization of Kraft lignin is a good model reaction to study the challenges in biomass conversion. Carbon is reported to be an excellent support in Kraft lignin depolymerization by Narani et al., and the phenolic monomer yield as high as 35 wt% was obtained at 320 °C and 22 MPa for 24 h.16 However, the carbon material supported catalysts cannot be regenerated by combustion after coke deposition. Furthermore, in a practical application, processing (e.g., hydroprocessing) of Kraft lignin with high oxygen content is generally highly exothermic and thus causes adverse effect on the processing.17 Hence, a catalyst support with high thermal conductivity is also very important. SiC exhibits a high thermal stability, chemical inertness, oxidation resistance, thermal conductivity, and mechanical


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strength,18-20 which meets the above criterions for an excellent catalyst support in Kraft lignin conversion.21,

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In terms of the catalyst active component, nickel promoted

tungsten oxides (NiO-WO3) were also proved to be a powerful catalyst for depolymerization of Kraft lignin. More importantly, the main part of this catalyst active component (i.e., WO3) has a high resistance to calcination and acid washing,23 which enables NiO-WO3 to be a potentially regenerable catalyst in Kraft lignin conversion. In this paper, NiW/SiC catalyst was synthesized, characterized and tested in the depolymerization of Kraft lignin, and the results obtained were compared with those from carbon supported catalyst. Furthermore, the regeneration of NiW/SiC was carried out by calcination and acid washing to prove the potential in practical application of this catalyst in Kraft lignin depolymerization and other biomass catalytic reactions. 2. EXPERIMENTAL SECTION 2.1. Materials Kraft Lignin was purchased from Tokyo Chemical Industry (TCI), and its properties are shown in Table 1. Commercial activated carbon (AC) was purchased from Sigma-Aldrich. Ni(NO3)2·6H2O (99%), (NH4)6H2W12O40·H2O (99%) were purchased from Alfa-Aesar. SiC nanofibers prepared by chemical vapor deposition with a diameter of 0.1–0.6 µm and a length of 50–100 µm were provided by Sinet Advance Materials Co., Ltd. Methanol, acetone, and nitric acid were provided by J&K Scientific. All the chemicals were used without further purification. 2.2. Synthesis of catalysts


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The SiC support was sieved to 40–60 mesh. The NiW/SiC catalyst with 5 wt% of NiO and 15 wt% of WO3 was prepared by the incipient wetness impregnation method. The aqueous solution of Ni(NO3)2·6H2O and (NH4)6H2W12O40·H2O were added to the support slowly with continuous stirring. The resulting solid was dried at 100 °C overnight and calcined at 450 °C for 5 h in air. NiW catalyst was also prepared with activated carbon as support. During the preparation, the atmosphere for calcination was changed to N2. 2.3. Characterization methods X-ray diffraction (XRD) experiments were carried out on a Bruker diffractometer with Cu Kα radiation (40 kV, 120 mA), data were obtained in the 2θ range of 10–90°. The angular step size was 0.05° and the counting time was 8 s per step. Transmission electron microscopy (TEM) was obtained with Philips Tecnai G2 microscope and operated at 200 kV. The samples dispersed in ethanol were deposited on copper grid before measurement. Scanning electron microscopy (SEM) was determined by Hitachi S-4800 microscope. The samples were prepared by dispersing the catalyst particle onto double sided carbon tape with a copper stub. Then the samples were sputter coated with a thin gold film resisting charging effects. Inductively Coupled Plasma (ICP) test was carried out on ICP-AES (Thermo Fisher iCAP 6000 series) and ICP-MS (Agilent 7700 Serials) to analyze the content of metal. Before the ICP analysis, 0.05–0.1 g of sample was put in 2 mL concentrated HNO3 (65–68 wt%) to dissolve the metal at least 24h, and solution was transferred into a 50 mL volumetric flask diluted with deionized water subsequently. Nitrogen adsorption-desorption isotherms were determined on a Micromeritics ASAP


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2020 HD 88 surface area and porosity analyzer. Prior to the adsorption measurements, 0.1–0.2 g samples were degassed at 623 K in a vacuum of 1.33×10-3 Pa for 10 h firstly and then converted to the analysis station for adsorption–desorption analysis at liquid nitrogen temperature. The pore volume was calculated from the maximum amount of nitrogen adsorbed in SiC or C samples. X-ray photoelectron spectroscopy (XPS) spectra were obtained by ESCALAB 250 spectrometer equipped with an Al Kα source. Deconvolutions analysis were delivered by XPS Peak 4.1 software. The liquid product (methanol soluble matter, MSM) was analyzed by GC-MS (Agilent 7890A/5975C system) equipped with a HP-5 column (30 m × 0.25 mm). The temperature program of the column was starting at 50 °C and holding for 1 min, then ramping to 300 °C at a rate of 5 °C/min, and holding at 300 °C for 4 min. The analysis of the mass spectra was mainly based on an automatic library search (NIST11, version 2.0). Elemental analysis was determined using a Vario EL III elemental analyzer. 2.4. Catalytic conversion of Kraft lignin The catalytic hydrotreatment of Kraft lignin was carried out in a 1 L high pressure autoclave with an overhead stirrer. Typically, the autoclave was charged with 2.5 g catalyst, 10 g of lignin, and 300 mL of methanol. The reactor was pressurized with 3 MPa H2 at room temperature. The reactor was heated to 320 °C for 8 h and stirred at 500 rpm with about 22 MPa working pressure. After the reactor cooled to room temperature, the resulting mixture was filtered to MSM and solid residue. The solid residue was dried in an oven and the MSM was quantified by removal of the solvent.


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2.5. Regeneration of catalyst support After the depolymerization of the Kraft lignin (high ash content), the dried solid residue containing NiW/SiC was calcined at 550 °C with a rate of 5 °C/min for 1 h. The calcined catalyst was further treated by nitric acid solution (pH ≈ 4.0) for at least 24 h to remove ash. The resulting solid was washed by deionized water until the filtrate was neutral. After 4 wt% Ni was redeposited on the solid, the regenerated catalyst was used for next cycle. 3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of catalyst In order to demonstrate the general applicability of SiC support, a commercially available SiC was used as support and a common incipient wetness impregnation method was used for metal introduction. The catalyst was comprehensively characterized and compared with the reference activated carbon based catalyst. As shown in Figure 1a, all of the diffraction peaks in the XRD pattern of the commercial SiC support can be assigned to β-SiC, indicating the high purity of SiC materials. Nitrogen adsorption isotherm of SiC support shown in Figure 1b exhibits a type II isotherm. The textural properties of SiC support are shown in Table 2. The adsorption volume and surface area of SiC are 23.2 cm3·g-1 and 10.1 m2·g-1, respectively. It should be noted that the surface area of SiC support is far less than that of carbon (775.69 m2·g-1), which may influence the effective dispersion of metal on it.24 The morphology and internal structure of SiC support was investigated by SEM and TEM,


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respectively. From Figure 1c, the SiC support is typical nanofibers with high aspect ratio. Figure 1d further reveals that the nanofibers are constructed by the sequential stacking of SiC nanoparticles.25-27 After supporting NiW catalyst, the SiC structure and textural properties are almost unchanged based on the powder XRD patterns (Figure 2a) and low temperature nitrogen adsorption (Figure 2b and Table 2). No obviously detectable XRD diffraction peak assigned to NiO was found for both NiW/SiC and NiW/C due to a good dispersion and small size of the metal.28 However, the characteristic peaks corresponding to WO3 are evident in both catalysts (Figure 2a).29 The low intensity of peaks and large peak widths suggested that W species had crystallites domain with small size. The TEM image of NiW/SiC in Figure 2c indicates the high dispersion of NiW metal on the support, as the NiW/C catalyst does in Figure 2d. The main peaks of XPS spectra in Figure 2e and 2f are at binding energy positions of 856.9 and 856.3 eV, respectively, which are ascribed to the spin-splitting Ni 2p3/2. Two peaks of W 4f5/2 and W 4f7/2 spectra are at binding energy positions of 37.7 and 35.6 eV in Figure 2g (37.9 and 35.7 eV in Figure 2h), respectively, which are assigned to W6+ species.30, 31 In summary, commercially available SiC nanofibers with surface area being 10 m2/g are an effective support for Ni-W dispersion. Though the dispersion of Ni-W over SiC is lower than that over activated carbon due to the low surface area, the Ni-W metal still keeps a high dispersion state. Hence, the supported catalyst was tested in depolymerization of Kraft lignin, a typical biomass catalytic reaction.


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3.2. Depolymerization of Kraft lignin over NiW/SiC Kraft lignin is available in large quantity from paper industry and an interesting feedstock for biofuel production. Its effective conversion is still a challenge task in biorefinery. Recently, Narani et al. reported an effective depolymerization of Kraft lignin in supercritical methanol over supported NiW catalysts on different supports, and activated carbon was found to be one of the most effective catalyst supports.16 Here the NiW/SiC catalyst was tested in Kraft lignin depolymerization reaction. MSM yield was used as a main indicator for the lignin depolymerization degree according to the literature.16 The MSM yield from Kraft lignin without any catalyst in methanol was 45.7 wt% (entry 1, Table 3). In comparison to the case of the presence of SiC support, the MSM yield increased to 51.9 wt% (entry 2, Table 3), which indicates that SiC support itself have some positive effect on lignin depolymerization. The load of NiW catalyst on SiC delivered an enormous change in MSM yield, which is as high as 74.1 wt% (entries 4, Table 3). The use of carbon support instead of SiC under the same conditions only supplied 61.5 wt% of MSM yield (entry 3, Table 3). Assuming that the missing percentages in Table 3 are gaseous products, NiW/C and NiW/SiC produce 23.1 and 12.2 wt% gases, respectively. It suggests that NiW/SiC is a milder catalyst than NiW/C and thus less liquid products are converted to gases products. Quantification of all GC-MS detectable phenolic monomers in the MSM was performed by comparison with authentic samples acquired from commercial purchase or independent synthesis,


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according to the method reported from Xiao et al.32 It shows the yield of all phenolic monomers obtained from NiW/SiC being 35.1 wt%, which is higher than that from NiW/C (31.1 wt%). The influence of the reaction time was further examined with NiW/SiC catalyst. A shorter time (6 h) and longer time (10 h) enabled the yields of MSM to be 58.5 wt% and 70.0 wt%, respectively (entries 5 and 6, Table 3), both of which were less than the MSM yield at 8 h due to the insufficient conversion of lignin at 6 h and the repolymerization of the lignin fragments at 10 h, respectively. Reducing the reaction temperature to 280 °C and 300 °C also resulted in a decrease in the MSM yield because of the weak activity of catalyst under these conditions (entries 7 and 8, Table 3). In addition, the MSM yield decreased to 67.3 wt% with a less dosage of catalyst, while there was no significate change in MSM yield with increasing the dosage of catalyst (entries 9 and 10, Table 3). The MSM obtained under the optimum conditions was further analyzed by GC-MS. The total ion chromatograms (TICs) of the product mixture from the depolymerization of Kraft lignin with NiW/SiC and NiW/C catalysts are shown in Figure 3. The guaiacol lignin subunits (G1–G6) were the main products in both cases of NiW/SiC and NiW/C catalysts. However, some mono-oxygen-containing phenols such as ethylphenols (P1), propylphenols (P2), methylated propylphenols (P3), and dimethylated propylphenols (P4) were relatively abundant in the NiW/C derived MSM. These phenols (P1–P4) can be obtained through transmethylation reaction from the G type products. Based on the above results, SiC is a promising support in Kraft lignin conversion, and the products from NiW/SiC maintain the integrality of guaiacol subunits containing methoxyl


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group. This is in good agreement with the result that NiW/SiC is a milder catalyst than NiW/C. 3.3. Regeneration of NiW/SiC Regeneration is a critical factor for catalyst application and this point is even more important in biomass conversion since biomass has a higher coking tendency when compared to hydrocarbon feedstock.33 In this section, the regeneration of NiW/SiC from Kraft lignin depolymerization was investigated. The deposition of coke and ash on catalyst active sites were found and considered to be the main reason for catalyst deactivation. The coke deposited on SiC supported catalyst can be easily removed by calcination while remaining the ash with used catalyst. As shown in Table 1, the ash content of Kraft lignin is as high as 12.2 wt%. The effect of such high ash content on catalyst performance was then tested by recycling the catalyst without removal of the ash. As shown in Figure 4, the MSM yield from depolymerization of Kraft lignin continuously reduced from 74.1 wt% to 56.5 wt% along with the two recycling of NiW/SiC, resulting from the high yield of ash in Kraft lignin. To further understand the effect of the ash, the contents of some typical metals (K, Ca, Na, Mg, and Al) deposited on the regenerated catalysts were subjected to ICP analysis. As listed in Table 4, the content of Na deposited on NiW/SiC is increasingly high along with the catalyst recycling number. This result is in good agreement with the changes in the MSM yield from NiW/SiC catalytic system. The mixture of ash and used catalyst was also analyzed by XRD. The diffraction peaks of Na2WO4 (formed during the lignin


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depolymerization, Figure 5b), Na2SO4 (originally presented in the lignin sample, Figure 5a), SiO2 (formed from the lignin combustion, Figure S1 in Supporting Information), NiWO4, WO3, and SiC are found, as shown in Figure 5c, which suggests that the active components of the catalyst (i.e., Ni and W) are present as WO3, NiWO4, and Na2WO4 after calcination. It should be noted that WO3 and NiWO4 are stable under acidic condition (pH > 3.21).23 Acid washing was then performed for the calcined NiW/SiC to remove the ash. Most of the XRD diffraction peaks from ash are disappeared, while WO3, NiWO4, and SiC are still present (Figure 5d), indicating the high potential of NiW/SiC as a regenerable catalyst. This was also confirmed by the XPS analysis of the acid washed catalyst, which shows that the states of Ni and W are the same as the flesh one (Figure 6). However, Ni species are easily dissolved in acidic solution, and a portion of W species (mainly Na2WO4) can be removed by washing. The loss of Ni and W by acid washing thus requires their subsequent supplements in catalyst recycling run. ICP analysis of the resulting solution after the catalyst acid washing shows around 4 wt% Ni and 2 wt% W species were lost during the regeneration processes. Since only 1 wt% of Ni and main portion of W (13 wt%) were left in the regenerated catalyst, 4 wt% Ni input for each run is necessary to maintain the synergistic effect from Ni and W. The performance of the regenerated catalyst with 4% Ni input and no W input was tested in two sequential recycling runs, and the MSM yield can be stabilized at around 70 wt% (Figure 4), which is very close to the initial yield (74.1 wt%). The


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composition of the MSMs from the two regenerated runs are also similar with the initial one, mainly containing compounds G1–G6 and P1–P4, as shown in Figure 7. The stable MSM yield and insignificant change in MSM composition from the recycling runs with only 4% Ni input demonstrated that NiW/SiC is a potentially regenerable catalyst in Kraft lignin depolymerization. Although 4% Ni input is necessary, the regenerable property still makes sense in a practical operation in terms of cost saving. Future study will focus on more recycling tests or even continuous operation of the catalyst. 3.4. Perspective of SiC based catalysts in biomass catalytic conversion According to the above study, the SiC nanofibers are an effective support for the NiW metal. The NiW/SiC catalyst is milder than the state of the art activated carbon based catalyst and supplies higher liquid product yield. It should be noted that the reaction conditions used in the current study are almost the same as that used in the literature and relevant industry. It would be interesting to see the kinetic difference between SiC and activated carbon supported catalysts. More importantly, the SiC based catalysts are easily regenerated through simple combustion (removing coke) and acid washing (removing ash) according to the different deactivation mechanisms. Our result clearly proved that the SiC support combines the advantages of activated carbon and metallic oxide supports. The results presented here point out that the SiC based catalysts is very promising for lignin or even biomass catalytic conversion in applied nature. 4. CONCLUSIONS


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Commercially available SiC nanofibers were used as support for NiW and used in Kraft lignin depolymerization. The following conclusion can be drawn based on the present study: 1) SiC is an effective support for the metals, such as NiW. The SiC supported catalyst is milder than the state of the art activated carbon supported catalyst under the identical conditions. 2) The SiC based catalyst could be regenerated by coke combustion and acid washing. The recovered NiW/SiC catalyst with only 4% Ni input has almost unchanged performance compared with the fresh NiW/SiC. 3) SiC support combines the advantages of common supports such as activated carbon and metallic oxides and is generally applicable as a catalyst support in biomass conversion. AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.H.) *E-mail: [email protected] (Y.F.)

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Grants U1663227 and 21506008)


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SUPPORTING INFORMATION AVAILABLE XRD pattern of the ash from the lignin sample. This material is available free of charge via the Internet at http://pubs.acs.org.

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Page 20 of 31

Table 1. Properties of feedstocks used in the present study Elemental analysisa (wt%)

Sample Kraft lignin a

b

C

H

52.4

4.1

Ob

N

S

37.8

0.1

5.6

Ashc (wt%) 12.2

c

On a dry and ash-free basis. Calculated by difference. On a dry basis.


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Table 2. Textural properties of the supports and the catalysts Samples

SBET/m2· g-1

Pore volume/cm3· g-1

Average diameter/nm

SiC AC NiW/SiC NiW/C

10.10 775.69 12.97 567.23

0.03 0.52 0.03 0.38

13.03 2.68 10.28 2.66


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Page 22 of 31

Table 3. Depolymerization of Kraft lignin under varying reaction conditionsa Entry

Catalyst (g)

Temperature (°C)

T (h)

Residue (wt%)

MSM (wt%)

Monomer (wt%)

1 2b 3c 4 5 6 7d 8e 9 10

2.5 2.5 2.5 2.5 2.5 2.5 2.5 1.5 3.5

320 320 320 320 320 320 280 300 320 320

8 8 8 8 6 10 8 8 8 8

37.8±0.5 32.8±0.3 15.4±0.6 13.7±0.3 17.2±0.5 13.9±0.7 30.7±0.3 22.3±0.2 23.0±0.5 18.3±0.3

45.7±0.4 51.9±0.3 61.5±0.2 74.1±0.5 58.5±0.4 70.0±0.6 45.2±0.3 63.1±0.4 67.3±0.6 71.1±0.5

31.1±0.5 35.1±0.7 34.1±0.3 33.1±0.6

a

Reaction conditions: 10 g of Kraft lignin, 2.5 g of NiW/SiC, 300 mL of methanol, 3 MPa H2 at room temperature, and

8 h. Reaction pressure is approximately 22 MPa. Yields were on a sulfur- and ash-free basis, assuming that all sulfur and ash remained in the solid product. bReaction with SiC support. cReaction with NiW/C catalyst. dReaction pressure is 18 MPa. eReaction pressure is 21 MPa.


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Table 4. ICP analysis of typical metal content in raw materials and recovered catalysts by calcination. Sample

Metal Content (wt%)

Kraft lignin Recovered catalyst from the 1st run Recovered catalyst from the 2nd run

Na

K

Mg

Ca

Al

1.55 1.95 3.83

0.03 0.04 0.06

0.05 0.02 0.05

0.14 0.18 0.15

SiC: Effective

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