2006 Fiscal Year Final Research Report Summary
Synthesis of carbon nanotubes and catalytic functions for adsorption on carbon nanotube
| Project/Area Number |
16360397
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Catalyst/Resource chemical process
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| Research Institution | University of Tsukuba |
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Principal Investigator |
NAKAMURA Junji University of Tsukuba, Graduate School of Pure and Applied Sciences, Professor, 大学院数理物質科学研究科, 教授 (40227905)
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| Co-Investigator(Kenkyū-buntansha) |
KIJIMA Masashi University of Tsukuba, Graduate School of Pure and Applied Sciences, Associate Professor, 大学院数理物質科学研究科, 助教授 (70177822)
MATSUISHI Kiyoto University of Tsukuba, Graduate School of Pure and Applied Sciences, Associate Professor, 大学院数理物質科学研究科, 助教授 (10202318)
SUZUKI Shugo University of Tsukuba, Graduate School of Pure and Applied Sciences, Associate Professor, 大学院数理物質科学研究科, 助教授 (90241794)
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| Project Period (FY) |
2004 – 2006
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| Keywords | carbon nanotube / hydrogen storage / fuel cell / surface chemistry / scanning tunneling microscope / graphite / catalysis / electrode catalyst |
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| Research Abstract |
A reduction in Pt usage is one of the key requirements for the commercialization of fuel cells for use in everyday life, because of its high price and limited availability, and the difficulty of finding suitable substitutes. The cheaper MO_2C is known to possess similar catalytic activities and electronic structures to Pt. Carbon black (CB) is widely used as the support for Pt nanoparticles. However, we found that when carbon nanotubes (CNTs) rather than CB are used as the support, the performance is improved, especially below 600 mA/cm^2. We found that a combination of MO_2C catalyst and carbon nanotubes in the anode provides performance as high as half that of the current PEFCs with Pt catalysts below 600 mA/cm^2.The PtRu catalysts supported on CNT were found to be CO tolerant very much. In order to clarify the reason for the advantage of CNT, we have carried out surface science studies using model systems of metal catalysts/HOPG (Highly oriented pyrolytic graphite). The shape of Pt
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particles is flat with one or two atomic heights. This suggests that electronic structures of Pt should be different between CNT support and CB support. In fact, XPS measurements show that Pt 4f core level is shifted to higher energy with decreasing the size of Pt particles on HOPG. This is currently ascribed to charge transfer from Pt to carbon by hybridization of wave functions between the small Pt particles and the graphite surface. The reduction of the particle size leads to a decrease in adsorption energy of hydrogen on Pt, which is shown by TPD of H_2 as well as H_2-D_2 exchange reaction at high pressures. The decrease in the adsorption energy can be explained by lowering d band center induced by electron transfer from Pt to carbon upon reduction of particles size. That is, support effect of carbon, which explains the results of real catalysts of CNT supported catalysts. Concerning CNT synthesis, the kinetics of carbon nanotube (CNT) synthesis by decomposition of CH_4 over Mo/Co/MgO and Co/MgO catalysts was studied to clarify the role of catalyst component. In the absence of the Mo component, Co/MgO catalysts are active in the synthesis of thick CNT (outer diameter of 7-27 nm) at lower reaction temperatures, 823-923 K, but no CNTs of thin outer diameter are produced. Co/MgO catalysts are significantly deactivated by carbon deposition at temperatures above 923 K. For Mo-including catalysts (Mo/Co/MgO), thin CNT (2-5 walls) formation starts at above 1000 K without deactivation. The significant effects of the addition of Mo are ascribed to the reduction in catalytic activity for dissociation of CH_4, as well as to the formation of Mo_2C during CNT synthesis at high temperatures. On both Co/MgO and Mo/Co/MgO catalysts, the rate of CNT synthesis is proportional to the CH_4 pressure, indicating that the dissociation of CH_4 is the rate-determining step for a catalyst working without deactivation. The deactivation of catalysts by carbon deposition takes place kinetically when the formation of the grapheme network lags carbon formation by deposition of CH_4. Less
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