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lsf_eclab木虫 (小有名气)
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[交流]
厦门大学孙世刚课题组长篇研究论文登上美国《科学》杂志
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厦门大学化学化工学院孙世刚和美国佐治亚理工学院王中林等科学家采用一种新的电化学方法，首次制备出具有高表面能的二十四面体铂纳米晶粒催化剂，显著提高了铂纳米催化剂的活性和稳定性，在能源、催化、材料、化工等领域具有重大的意义和应用价值。5月4日出版的美国《科学》杂志以三页半篇幅的长篇报道刊登了这项最新成果。这是我校第一篇发表在《科学》杂志上的长篇研究报告。 该研究发展了一种新的电化学方法，能够控制纳米晶体的表面结构和生长，合成具有高表面能的金属纳米晶体，突破了传统化学法只能合成低表面能金属纳米晶体的局限。 提起铂，人们并不陌生，不过，铂的最大用途并不是做成漂亮的白金首饰。铂纳米材料是燃料电池、石油化工、汽车尾气净化等领域中广泛使用的催化剂，全世界用于催化剂的铂每年近100吨，占总产量的一半，价值高达40多亿美元。由于铂的资源匮乏，价格高昂，如何进一步提高铂纳米材料的催化活性、稳定性和利用效率是氢能源和相关领域发展的重大关键问题。 然而，科学家面临的挑战是：传统的化学法只能合成低表面能的金属纳米晶体，一般是立方体、八面体或是截角八面体，其表面为“低指数”晶面结构。科学家认为，“高指数”晶面结构围成的多面体的铂纳米晶粒催化剂能提高催化效率。二十四面体是一种十分罕见的晶体形状，其表面通常由高指数晶面围成。迄今从未见到人工合成二十四面体金属纳米晶体的报道。在自然界中，也仅发现金刚石、萤石和铜矿等极少数矿物能以不完美的二十四面体形式存在。 孙世刚说：“我们在研究中发现，铂族金属单晶的高指数晶面不仅催化活性很高、而且稳定性好，这是因为高指数晶面具有开放的表面结构和高密度的台阶原子,而且处于短程有序环境。因此，制备表面为高指数晶面结构的铂纳米晶粒，是显著提高催化剂活性和稳定性的重要途径。但是，铂晶体在生长过程中沿高指数晶面方向的生长速度远快于沿低指数晶面方向，导致高指数晶面趋于消失，最终生成的铂晶体表面主要由原子排列密集的低能低指数晶面组成。” 孙世刚教授与其博士生田娜和周志有博士，利用高指数晶面在氧化条件下稳定性高的特点，通过方波电位产生的周期性氧化/还原的驱动，调控铂纳米晶体生长过程的表面结构，首次制备出高指数晶面结构的二十四面体铂纳米晶体。同时控制条件，可使铂纳米晶体的尺度在二十到几百纳米内变化。 王中林教授和丁勇博士通过高分辨电子显微镜研究证实，所制备的二十四面体铂纳米晶体由高指数晶面围成，并具有很高的热稳定性，可耐高达800摄氏度的高温。 该中——美团队的电催化研究还证实，二十四面体铂纳米晶体具有很高的催化活性：以单位铂表面积来计算，它对甲酸、乙醇等有机小分子燃料电氧化的催化活性是目前商业铂纳米催化剂的2到4倍，显示其在燃料电池、电催化等领域中的重大应用价值。 孙世刚是国家杰出青年科学基金和国家级教学名师奖获得者，长期从事电化学和电催化研究。王中林是佐治亚理工学院杰出讲座教授和厦门大学客座教授，是著名的电子显微学和纳米材料学专家。上述研究是在孙世刚小组前期工作基础上，两个研究小组一年多紧密合作的结果。 孙世刚深有体会地介绍，在世界顶级的学术期刊《科学》上发表论文是非常不容易的。其实，早在2005年，他与博士生田娜和周志有博士，就已经制备出了这种二十四面体金属纳米晶体，但课题组并未急着发表论文，而是又花了两年功夫静下心来进一步追踪弄清楚这种晶体的生长过程、表征，并不断完善研究方法。论文前后修改了二十多次，博士生田娜还为此申请延期二年毕业。今年一月底，这一中美团队将研究成果投稿到《科学》，三月份《科学》就发来了用稿通知，这其间课题组还与杂志的编辑做了反复沟通，并就编辑所提问题进行了补充说明。 孙世刚和王中林认为该研究所发展的表面结构控制生长的电化学方法可以拓展到其它铂族金属，如钯、铑等，也可以运用到制备其它高指数晶面组成的不同形状的金属纳米晶体，深化了对金属晶体生长规律的认识；该研究不仅开辟了一条通过控制纳米粒子表面原子排列结构提高催化剂性能的崭新途径，也是将模型电催化剂的基础研究推进到实际催化剂设计和研制过程中的一个重大进展。 《科学》杂志的三位评审人也对该工作的原创性和重要性给予高度的评价。认为这一科研成果不仅指明了一种控制纳米粒子生长使高指数晶面暴露在外的新思路和新方法，而且将导致异相催化中的新发现。 孙世刚研究小组得到国家自然科学基金委、教育部、科技部、厦门大学以及固体表面物理化学国家重点实验室的长期支持和资助，还得到厦门大学“优秀博士学位论文”计划的支持。 [ Last edited by 小红豆 on 2007-6-12 at 22:53 ] |
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lsf_eclab
木虫 (小有名气)
- 应助: 0 (幼儿园)
- 金币: 2311
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- 虫号: 88207
- 注册: 2005-08-23
- 性别: GG
- 专业: 电化学
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Fig. 3. Size control of THH Pt NCs and their thermal stability. SEM images of THH Pt NCs grown at (A) 10, (B) 30, (C) 40, and (D) 50 min. The insets in (A) and (B) are the high-magnification SEM images that confirm the shape of THH. Scale bars, 200 nm. (E) Size distributions of THH Pt NCs in (A), B), (C), and (D), respectively, after counting more than 500 particles for each sample. (F) In situ TEM observation on the thermal stability of THH Pt NCs. The images were recorded at various temperatures in TEM at a heating rate of 7°C/min. The NC preserves its shape to 815°C and even higher with a slight truncation at the corners and apexes, as seen in the TEM image. [View Larger Version of this Image (69K GIF file)] Why are the {730} or the {210} type of facets that define the THH shape stable during growth? We have found that ascorbic acid is not the intrinsic reason to produce the THH shape, because THH Pt NCs can still be harvested in ascorbic acid–free solution, although the yield and quality degrade (fig. S9). In the original synthesis procedures (Fig. 1A), the initial stage of treating the nanospheres by square-wave potential, the following processes may occur (25). First, at 1.20 V, the surface Pt atoms on the nanospheres can be oxidized and partially dissolved to form Pt ions. Then, these Pt ions diffuse to the GC surface and are reduced to Pt atoms between –0.20 and –0.10 V. In the square-wave potential procedure, the two processes were repeated periodically at a frequency of 10 Hz, and new Pt NCs grow through nucleation and growth on GC surface with a continuous dissolving of the original Pt nanospheres, because the small nanoparticles grow more rapidly than the larger ones (32). It has been reported that the surface structure of Pt single crystals could be changed by periodic adsorption and desorption of oxygen, depending on their Miller indices (33). At 1.20 V, Pt surface is oxidized and covered by oxygen species (Oad and OHad) originated from the dissociation of H2O in solution. As for the low-energy planes, surface atoms have larger coordination numbers (CNs), such as 9 for atoms on the (111) plane, so oxygen atoms are relatively difficult to adsorb at such surface sites; instead, some oxygen atoms might preferentially diffuse or invade into a Pt surface to form a Pt-O lattice through place-exchange between oxygen and Pt atoms. After reduction between –0.20 and –0.10 V, these displaced Pt atoms cannot always return to their original positions because the synthesis was carried out at room temperature, which then disorders surface structure (33). However, for high-index planes, the CNs of surface atoms are relatively low, only 6 for stepped atoms on the {730} plane. The oxygen atoms preferentially adsorb at such stepped atoms without replacing them, and ordered surfaces are preserved (33, 34). The dynamic oxygen adsorption-desorption mediated by square-wave potential and the different degrees of place-exchange between oxygen and Pt atoms on Pt single-crystal surfaces show that only high-index planes with an open structure, such as the {730} and {210} facets, can survive in the treatment of square-wave potential (fig. S10). The catalytic activity of THH Pt NCs is superior to that of the spherical Pt nanoparticles. The NCs have been applied to promote the electro-oxidation of formic acid and ethanol, which are promising alternative fuels for direct fuel cells. Figure 4A shows a comparison of transient current density of formic acid oxidation at 0.25 V on the THH Pt NCs [ = 81 nm (fig. S11)], the electrodeposited polycrystalline Pt nanospheres [ = 115 nm (fig. S12)], and the commercial 3.2-nm Pt/C catalyst from E-TEK, Inc. (Somerset, New Jersey, USA) (fig. S13) at room temperature. The oxidation current has been normalized to electroactive Pt surface area so that the current density (j) can be directly used to compare the catalytic activity of different samples (25). The oxidation current density on THH Pt NCs is nearly double that on Pt nanospheres or Pt/C catalyst. The potential dependence of the steady-state current density recorded at 60 s is shown in Fig. 4B. The current density of formic acid oxidation on THH Pt NCs is higher than that on the Pt nanospheres or the Pt/C catalyst, and the enhancement factor R, which is defined as the ratio of the current density measured on THH Pt NCs versus that acquired on Pt nanospheres or Pt/C catalyst, varies from 160% to 400% for Pt nanospheres and from 200% to 310% for Pt/C catalyst, depending on electrode potential. The THH Pt NC showed no appreciable morphological change after the reaction, as indicated by an SEM image of a THH Pt NC inset in Fig. 4B, which still preserves the THH shape, showing its chemical stability. Fig. 4. Comparison of catalytic activity per unit Pt surface area among THH Pt NCs, polycrystalline Pt nanospheres, and 3.2-nm Pt/C catalyst. (A) Transient current density curves of formic acid oxidation at 0.25 V. (B) Potential-dependent steady-state current density (left, recorded at 60 s) of formic acid oxidation on THH Pt Ncs, Pt nanospheres, and commercial Pt/C catalyst, and the ratios R (right) between that of THH with the latter two, respectively. The inset in (B) is an SEM image of a THH Pt NC after reaction, indicating the preservation of shape. (C) Transient current density curves of ethanol oxidation at 0.30 V for the THH, Pt nanospheres, and Pt/C catalyst. (D) Potential-dependent steady-state current density (left) of ethanol oxidation on THH Pt NCs, Pt nanospheres, and commercial Pt/C catalyst, and the ratios R (right). The current density j was normalized in reference to the electrochemical active surface area for each sample, so that the current density j directly corresponds to the catalytic activity of unit Pt surface area of the sample. Thus, it can be directly compared for three types of particles. The results clearly demonstrate the largely enhanced catalytic activities of the THH Pt NCs per unit surface area. [View Larger Version of this Image (39K GIF file)] For ethanol oxidation, the transient current density on THH Pt NCs at 0.30 V is enhanced to 230% of that on the nanospheres and 330% of that on the Pt/C catalyst (Fig. 4C). A comparison of the steady-state current densities obtained on THH Pt NCs, Pt nanospheres, and 3.2-nm commercial Pt/C catalyst is shown in Fig. 4D, in which the enhancement factor R varies between 200% and 430% for Pt nanospheres and between 250% and 460% for Pt/C catalyst in the potential region of 0.20 to 0.55 V. In addition, we have measured that, at a given oxidation current density of technical interest in fuel cell application, as indicated by dashed lines in Fig. 4, B and D, the corresponding potential on THH Pt NCs is much lower than that on Pt nanospheres or Pt/C catalyst. In the case of formic acid oxidation, the potential on THH Pt NCs is shifted negatively by 60 mV as compared with Pt nanospheres at the same current density of 0.5 mA cm–2, whereas for ethanol oxidation, the negative shift is 80 mV at a current density of 0.2 mA cm–2. The above results show that THH Pt NCs exhibit much enhanced catalytic activity per unit surface area for the oxidation of small organic molecules. This may be attributed to the high density of stepped atoms on the surfaces of THH Pt NCs. However, for catalytic activity per unit weight of Pt, our estimation indicates that, when the transformation is complete, the overall activity of these larger THH NCs is less than that of the 3-nm commercial Pt nanoparticles (10% as active). Future research will be needed to improve the synthesis technique so that smaller-size THH NCs are made in high yield and still consume almost all of the starting Pt nanoparticles. References and Notes 1. J. Zhang, K. Sasaki, E. Sutter, R. R. Adzic, Science 315, 220 (2007).[Abstract/Free Full Text] 2. V. R. Stamenkovic et al., Science 315, 493 (2007).[Abstract/Free Full Text] 3. R. Narayanan, M. A. El-Sayed, Nano Lett. 4, 1343 (2004). [CrossRef] [ISI] 4. F. J. Vidal-Iglesias et al., Electrochem. Commun. 6, 1080 (2004). [CrossRef] 5. G. A. Somorjai, D. W. Blakely, Nature 258, 580 (1975). [CrossRef] 6. G. A. Somorjai, Chemistry in Two Dimensions: Surfaces (Cornell University Press, Ithaca, NY, 1981). 7. S. G. Sun, A. C. Chen, T. S. Huang, J. B. Li, Z. W. Tian, J. Electroanal. Chem. 340, 213 (1992). [CrossRef] 8. N. Hoshi, S. Kawatani, M. Kudo, Y. Hori, J. Electroanal. Chem. 467, 67 (1999). [CrossRef] 9. S. G. Sun, J. Clavilier, Chem. J. Chin. Univ. 11, 998 (1990). 10. W. F. Banholzer, R. I. Masel, J. Catal. 85, 127 (1984). [CrossRef] 11. H. E. Buckley, Crystal Growth (Wiley, New York, 1951). 12. T. S. Ahmadi, Z. L. Wang, T. G. Green, A. Henglein, M. A. El-Sayed, Science 272, 1924 (1996).[Abstract] 13. F. Kim, S. Connor, H. Song, T. Kuykendall, P. D. Yang, Angew. Chem. Int. Ed. 43, 3673 (2004). [CrossRef] 14. Y. J. Xiong, J. M. McLellan, Y. D. Yin, Y. N. Xia, Angew. Chem. Int. Ed. 46, 790 (2007). [CrossRef] 15. Y. Sun, Y. Xia, Science 298, 2176 (2002).[Abstract/Free Full Text] 16. H. Song, F. Kim, S. Connor, G. A. Somorjai, P. D. Yang, J. Phys. Chem. B 109, 188 (2005). [Medline] 17. X. G. Liu, N. Q. Wu, B. H. Wunsch, R. J. Barsotti, F. Stellacci, Small 2, 1046 (2006). [CrossRef] [ISI] [Medline] 18. A. Wieckowski, E. Savinova, C. G. Vayenas (Eds.), Catalysis and Electrocatalysis at Nanoparticle Surfaces (Marcel Dekker, Inc., New York, 2003). 19. R. M. Heck, R. J. Farrauto, Appl. Catal. Gen. 221, 443 (2001). [CrossRef] 20. J. Larminie, A. Dicks, Fuel Cell Systems Explained, 2nd Edition (Wiley, Chichester, West Sussex, 2003). 21. F. Favier, E. C. Walter, M. P. Zach, T. Benter, R. M. Penner, Science 293, 2227 (2001).[Abstract/Free Full Text] 22. H. Gong, S. G. Sun, Y. J. Chen, S. P. Chen, J. Phys. Chem. B 108, 11575 (2004). 23. A. Visintin, J. C. Canullo, W. E. Triaca, A. J. Arvia, J. Electroanal. Chem. 239, 67 (1988). [CrossRef] 24. W. A. Egli, A. Visintin, W. E. Triaca, A. J. Arvia, Appl. Surf. Sci. 68, 583 (1993). 25. Materials and methods are available as supporting material on Science Online. 26. G. H. Gilmer, H. Huang, C. Roland, Comput. Mater. Sci. 12, 354 (1998). [CrossRef] 27. A. E. Seaman Mineral Museum of Michigan Technological University (available at www.museum.mtu.edu/Gallery/copper.html). 28. A. A. Proussevitch, D. L. Sahagian, Comput. Geosci. 27, 441 (2001). [CrossRef] 29. S. G. Sun, D. F. Yang, S. J. Wu, J. Ociepa, J. Lipkowski, J. Electroanal. Chem. 349, 211 (1993). [CrossRef] 30. D. W. Blakely, G. A. Somorjai, Surf. Sci. 65, 419 (1977). 31. P. J. Knight, S. M. Driver, D. P. Woodruff, Chem. Phys. Lett. 259, 503 (1996). [CrossRef] [ISI] 32. H. Reiss, J. Chem. Phys. 19, 482 (1951). [CrossRef] 33. N. Furuya, M. Shibata, J. Electroanal. Chem. 467, 85 (1999). [CrossRef] 34. A. V. Tripkovic, R. R. Adzic, J. Electroanal. Chem. 205, 335 (1986). [CrossRef] 35. This work was supported by grants from the Natural Science Foundation of China (20433060, 20503023, and 20673091) and Special Funds for Major State Basic Research Project of China (2002CB211804). Y.D. and Z.L.W. were supported by U.S. NSF grant DMR 9733160 and by the Georgia Institute of Technology. N.T. thanks the Ph.D. program of Xiamen University for support. |
6楼2007-05-11 19:21:50
lsf_eclab
木虫 (小有名气)
- 应助: 0 (幼儿园)
- 金币: 2311
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- 帖子: 167
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- 虫号: 88207
- 注册: 2005-08-23
- 性别: GG
- 专业: 电化学
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近日，国际化学界再次将目光聚焦到厦门大学。5月4日出版的美国《科学》杂志以长篇报告的形式发表了厦门大学化学化工学院、固体表面物理化学国家重点实验室孙世刚教授等关于铂纳米催化剂的最新成果。同期杂志还配发了美国科罗拉多大学化学和生物化学系纳米材料专家丹 L. 费尔德门教授的题为“催化剂的新面孔”的评论，他指出，未来的能源安全很大程度上取决于化学家和材料科学家能够针对生产和应用替代燃料，发现更有效、更安全和更经济的催化剂。美国能源部将此确定为重大挑战，而深入认识催化剂结构和化学性能的关系，开发先进的纳米尺度材料合成技术是关键。他认为孙世刚等人的最新成果是纳米尺度催化剂合成的一个重大突破。 论文发表后，在国际学术界迅速引起重大反响，美国《化学工程新闻》周刊、《每日科学》、英国的《新科学家》和英国皇家化学会的《化学世界》等国际主流科技媒体都在第一时间，分别以“多面体催化剂－铂多面体的催化性能胜过其球形表兄”，“铂纳米晶体提升了其在燃料氧化、氢能源中的催化活性”，“纳米晶体‘珍宝’是高性能化学催化剂”和“铂的多面”等为题，报道和评述了这项重要研究成果。我国科学网和《科学时报》在第一时间和头版头条发表了题为“科学家首次制备二十四面体铂纳米晶体：显著提高了铂纳米催化剂的活性和稳定性”的报道，5月9日的《科技日报》也在第一版显要位置刊发了消息。人民网等门户网站也做了报道。此外，一些重要学术机构，如美国科学促进会、美国材料研究学会、麻省理工学院和著名的国际催化剂制备公司以及中国科学院和国家自然科学基金委等的网站也在第一时间转载和评述了这项成果。 孙世刚是1995年度国家杰出青年科学基金和2006年度国家级教学名师奖获得者。他在电化学和电催化研究所取得的原创性研究成果为国内外同行广泛认可。2007年，孙世刚当选国际电化学会特别会员。这项荣誉专门授予为电化学科学做出杰出贡献的科学家，至今全世界仅有24人获此称号，孙世刚是其中唯一的中国学者。 [ Last edited by lsf_eclab on 2007-5-11 at 13:59 ] |
2楼2007-05-11 13:54:00
3楼2007-05-11 16:32:30
lsf_eclab
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4楼2007-05-11 19:18:31
