Synthesis of nanocarbon materials by PECVD: challenges to direct synthesis via CO 2 reduction using plasma-soec hybrid reactor

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1 22 nd International Symposium on Plasma Chemistry July 5-10, 2015; Antwerp, Belgium Synthesis of nanocarbon materials by PECVD: challenges to direct synthesis via CO 2 reduction using plasma-soec hybrid reactor S. Mori, N. Matsuura, L.L. Tun and M. Suzuki Department of Chemical Engineering, Tokyo Institute of Technology, Tokyo, Japan Abstract: Synthesis of carbon nanotubes from pure carbon dioxide was studied using hybrid reactor of DBD and Solid Oxide Electrolyser Cell (SOEC). The removal of oxygen from the plasma region suppresses the regeneration of CO 2 from CO and 100% conversion could be obtained. The deactivation of catalyst by oxygen was also suppressed by the hybrid reactor and aligned carbon nanotubes were able to be synthesized from pure CO 2. Keywords: carbon nanotubes, CO 2 reforming, solid oxide electrolyser cell, DBD 1. Introduction So far the reforming of carbon dioxide has been intensively studied by various methods. When we consider utilizing the plasma process for reforming of CO 2, the high cost due to the electricity consumption hinders the feasibility of the process and the reforming process to produce only valuable products can remain as the practical interest. Carbon nanotubes (CNTs) are well recognized high-value added materials and the price is still high. Therefore, CNTs can be a potential target to be reformed from CO 2 using plasma processing and there has been no report to synthesize CNTs from only CO 2 by plasma processing. On the other hand, the synthesis of CNTs from CO has been intensively studied and, among them, the high pressure carbon monoxide process, HiPco, was successfully commercialized [1]. In the process for the synthesis of CNTs from CO, it was reported that the formation of CO 2 deactivates catalyst and terminates the growth of CNTs [2]. Therefore, in order to synthesize CNTs from CO 2, high CO 2 conversion is essential as well as the removal of produced oxygen which is more severely deactivates the catalyst for the CNTs synthesis. However, it is difficult to obtain very high CO 2 conversion by conventional plasma because intensive reverse reaction is unavoidable. In order to suppress the reverse reaction which reproduces CO 2 from CO with reactive oxygen species, it is highly effective to remove oxygen from the plasma reaction region. Therefore, in this study, by the hybridization of oxygen separator and plasma reactor, we try to increase the CO 2 conversion and to synthesize CNTs from CO 2. In this study, solid oxide electrolyzer cell (SOEC) is utilized to separate oxygen. At higher temperature, SOEC can decompose CO 2 electrochemically but disproportionation of CO becomes thermodynamically difficult and requires very high pressure for the substantial reaction rates. In order to develop much milder process in which feed gas is only CO 2 and the temperature is less than 700 C in atmospheric pressure, the hybrid reactor of DBD and SOEC is fabricated in this study. 2. Experimental Fig. 1 shows the schematic diagram of the experimental set-up used in this study. The hybrid reactor of dielectric barrier discharge (DBD) and SOEC was fabricated. SOEC is composed by the yttria-stabilized zirconia (YSZ) tube (Nikatto YSZ-8. o.d. is 15 mm and thickness is 2 mm). Both sides of YSZ tube is coated with lanthanum strontium manganite (LSM) thin films electrodes. LSM thin films are wrapped with chromel wires which work as current collector. LSM electrode on the outer surface of YSZ tube is connected to the ground and inner LSM electrode is connected to the DC power supply (0-15 V). In the hybrid reactor, SOEC is inserted into the quartz tube of 18 mm inner diameter, 22 mm outer diameter, and 500 mm length. The outer surface of the quartz tube was wrapped by the SUS mesh which was connected to the high voltage supply (max. AC voltage: 11 kv, frequency: 18 khz). Carbon dioxide was fed into the space between the quartz tube and SOEC where the dielectric barrier discharge was formed. The plasma input power was measured by the Lissajous curve and was about 11 W in the following conditions. The composition of product gas was analysed by the gas chromatography (Shimadzu GC-8A. column: activated carbon). The inside of SOEC was evacuated by the rotary pump in order to enhance desorption of permeated oxygen. Fig. 1. Schematic diagram of experimental set-up. IN-15 1

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3 3. Reduction of carbon dioxide Fig. 2 shows the carbon dioxide conversion as a function of residence time at 400 C. At this relatively low temperature conditions, the synergistic effect of hybrid reactor is apparent; the conversion of hybrid reactor is much higher than the sum of individual conversions. The CO 2 conversion by single SOEC reactor is not significant but the oxygen separation rate by SOEC would be high enough to suppress the reverse reaction. Fig. 3. CO 2 conversions with residence time at 700 C (SOEC applied voltage: 10 V, SOEC current: ma, CO 2 flow rate: sccm, pressure: 10 kpa). Fig. 2. CO 2 conversions with residence time at 400 C (SOEC applied voltage: 10 V, SOEC current: ma, CO 2 flow rate: sccm, pressure: 10 kpa). Fig. 3 shows the carbon dioxide conversion as a function of residence time at 700 C. This high temperature case, the CO 2 conversion by the single DBD is very low and the synergistic feature of hybrid reactor is weaker. Higher temperature would be preferable to convert CO 2 into CO but the formation of CNT from the CO becomes more difficult. This is because CO is thermodynamically unstable at low temperature but becomes stable at such high temperature. Fig. 4 shows carbon dioxide conversion as a function of reactor temperature at atmospheric pressure with longer residence time of 150 s. In this longer residence time conditions, we obtained 100% conversion by the hybrid reactor at 700 C. With this condition in which we could obtain 100% conversion, we have mainly performed the synthesis of CNTs in the next chapter. 4. Synthesis of CNTs by CO 2 reduction In this study, we used the supported metal catalysts to synthesize CNTs. We have also performed the floating metal catalysts using ferrocene as a metal catalyst source. We could synthesize CNTs by the floating catalyst method using hybrid reactor from pure CO 2. However, the metal catalyst deposited onto the SOEC surface and degraded the SOEC performance. Another problem to Fig. 4. CO 2 conversions as a function of reactor temperature (SOEC applied voltage: 10 V, SOEC current: ma,, CO 2 flow rate 0.6 sccm, pressure: 100 kpa). use the ferrocene for the catalyst source is that the ferrocene can be the carbon source as well because it contains a lot of carbon in its structure (C 10 H 10 Fe). Thus, CNTs can be synthesized by only pure ferrocene without any other carbon sources [3]. In this study, we wanted to show the clear evidence to synthesize CNTs from pure CO 2. Therefore, in this report, we will focus on the process using only the supported catalyst. For the industrial applications, floating catalyst method is still attractive and configuration of hybrid reactor should be modified to avoid the deposition of floating catalyst onto the SOEC surface. IN-15 3

4 The supported metal catalysts are prepared by the wet coating with Co and Mo acetate solutions. The solutions were prepared by dissolving Co and Mo acetate into ethanol, individually, to both Co and Mo concentration of 0.3 wt%. Then ml of the Mo solution was dropped onto the substrate and dried in the atmosphere then calcined at 400 C for 5 min in atmosphere. Then ml of the Co solution was dropped onto the substrate and again dried in the atmosphere then calcined at 400 C for 5 min in atmosphere. Then the substrates are inserted into the hybrid reactor and catalyst layer was annealed using an H 2 flow (pressure: 0.4 atm, flow rate: 50 sccm, temperature is same as CVD condition) for 20 minutes before CNTs synthesis, and then a pure CO 2 is fed into the hybrid reactor for the CNTs synthesis. The space inside the hybrid reactor is rather small; therefore, we used the quartz capillary tube (o.d.: 0.5 mm) as substrates for CNTs deposition. The area of the substrate in the hybrid reactor is divided into two parts. Upstream part of the substrate is located in the DBD plasma region and the other part of substrate is in the downstream region of DBD plasma. Thus, upstream DBD part and downstream after DBD part are denoted as Direct-PECVD region and Remote PECVD region, respectively. Figs. 5 and 6 shows the SEM (Hitachi S-4500) images of substrate surface after PECVD at 600 C and 700 C, respectively. At 600 C, aligned CNTs with diameter of nm were synthesized both direct and remote-pecvd region. At 700 C, much thicker CNTs are synthesized in direct-pecvd region; but only very short CNTs are observed in the remote-pecvd area. At 800 C, the CNTs formation was more difficult to be observed and very large particles were found on the surface of the substrate after the synthesis. At such high temperature, the reaction Gibbs energy for the disproportionation reaction of CO becomes positive and this can be the possible reason for the reduction of CNTs growth rate. But from the morphology of the carbon products formed on the substrates, the agglomeration of the catalyst metal particle would be the other main reason for the low growth rate of CNTs at high temperature. Below 500 C, CNTs growth were not observed. In order to obtain high conversion rate, we had to set the flow rate very low. Figs. 7 and 8 show the SEM images of substrate surface after the PECVD with the flow rate of 0.6 and 0.9 sccm, respectively and explain the influence of CO 2 flow rate on the formation of CNTs. With the flow rate of 0.6 sccm, we could still synthesize CNTs; but with 0.9 sccm, only in the remote-pecvd region, formation of CNTs was slightly observed. The difficulty to synthesize CNTs with higher flow rate is attributed to the low ionic current due to the very thick electrolyte. In this study, we use the YSZ with the thickness of 2 mm. So, if we could reduce the thickness of electrolyte of the SOEC, we will obtain higher ionic current and could improve the performance of SOEC and increase the flow rate of CO 2. Our study is now in progress in that direction. Fig. 5. SEM images of carbon deposits after PECVD with the flow rate of 0.45 sccm at 600 C. Direct- Fig. 6. SEM images of carbon deposits after PECVD with the flow rate of 0.45 sccm at 700 C. Direct- 4 IN-15

5 Fig. 9 shows the TEM (Hitachi H-7650) images of carbon deposits with the flow rate of 0.45 sccm at 600 C. Hollow structure of the CNTs are clearly seen in the TEM images and the diameter of CNTs synthesized by Directand Remote-PECVD are both about nm. 50 nm Fig. 7. SEM images of carbon deposits after PECVD with the flow rate of 0.6 sccm at 600 C. Direct- 50 nm Fig. 9. TEM images of carbon deposits after PECVD with the flow rate of 0.45 sccm at 600 C. Direct- The CO 2 flow rate of the present system is very low; however, we could successfully synthesize CNTs directly from the pure CO 2 by the hybridization of DBD and SOEC. 5. Conclusion Hybrid reactor of DBD and SOEC was fabricated. The complete conversion of CO 2 into CO was achieved by the hybrid reactor. Direct synthesis of CNTs from pure CO 2 was also successfully demonstrated by the hybrid reactor. 6. References [1] M.J. Bronikowski, et al. J. Vac. Sci. Technol. A, 19, 1800 (2001) [2] A.G. Nasibulin, et al. Chem. Phys. Lett., 417, 179 (2006) [3] R. Bhatia and V. Prasad. Solid State Comm., 150, 311 (2010) Fig. 8. SEM images of carbon deposits after PECVD with the flow rate of 0.9 sccm at 600 C. Direct- IN-15 5