Rolf Jürgen Behm教授课题组关于甲醇阳极氧化的研究

Behm教授课题组致力于电催化领域二十年,对甲醇、乙醇的阳极氧化反应的催化剂优选、反应终端产物分析,中间产物分析、工艺条件参数影响都做了全面细致的研究。Behm教授课题组充分利用原位红外技术和原位质谱检测技术结合循环伏安技术对甲醇乙醇在Pt基催化剂上的催化反应做了深入研究。

在甲醇电催化氧化方面,他们使用溶胶凝胶法制备了PtRu比例约为1:1的合金纳米颗粒,以质量分数为20%金属负载量分布在活性碳上。他们获得了与E-TEK 公司商业化的20%的$Pt_{0.5}Ru_{0.5}$/Vulcan carbon催化及相媲美的质量电流密度,并提出他们可能会优化PtRu的比例以提高电流密度【1999,Electrochem. Comm】。接下来他们在未负载的PtRu催化剂上添加第三组分(W, Mo,V), 发现第三组分以氧化物的形式存在。通过CO吸附氧化方法来测定活性比表面积,发现由于氧化物的覆盖而将低了活性比表面积。电流通过比表面归一,比表面电流密度的顺序PtRuVOx > PtRuMox > PtRu > PtRuWOx = PtRu(ETEK),他们认为氧化物的存在会加速甲醇氧化【2002,J. Power Sources】。随后他们在电解质流动的电化学反应池中发现,当施加0.6Vvs. RHE恒外电压时,随着Pt/C的单位面积的装填量(7-35 µg Pt $cm^{-2}$)的改变,甲醇氧化的效率,产物分布,以及各个目标产物(甲醛,甲酸和$CO_2$)的转化频率都会受到很大的影响。增加Pt/C的装填量,甲醛的选择性会降低,而$CO_2$的选择性会升高【2003,Langmuir】。接着在另外一篇文章探讨了电解质流速对反应产物$CO_2$影响,结果显示流速越小,$CO_2$的产率越高,$CO_2$的法拉第电流效率越高。这一结果与Seidel提出的“脱附-再吸附-反应”模型【2008,Faraday Discuss】相吻合:即具有反应活性的未被完全氧化的中间产物脱附后,又被催化剂吸附继续发生进一步的氧化反应。对于甲醇电氧化体系,中间产物甲醛和甲酸再低流速的情况下会被再吸附、进而被深度氧化成$CO_2$,增加催化剂装填的厚度,因部分氧化而生成的产物则需要更长的时间扩散到电解质中,因此增加了反应时间,而提高了$CO_2$选择性。深刻理解传质过程对反应路径的影响有助于直接甲醇燃料电池的研发,最大效率的利用催化剂,防止非完全氧化产物生成【2009,Langmuir】。随后他们研究了温度对甲醇阳极氧化反应的影响并测得了在不同外加电压下的(0.45-0.70V vs. RHE)总反应和生成$CO_2$反应的表观活化能。该研究发现发现提高温度会增加电流密度且提高$CO_2$的产率,且在给定电压下生成$CO_2$的表观活化能要高于总反应的表观活化能【2010,J. Phys. Chem. C】。

Behm 教授课题组还研究了甲醇电氧化中间产物对反应的影响。在甲醇电氧化过程中的中间产物主要有甲酸、甲醛、和被吸附的CO,通过添加微量的甲酸和甲醛来测试其中间产物的影响,为了确定其产物的来源,他们使用了同位素标定法。结果显示,含有微量甲醛的甲醇溶液,很快就有被吸附的CO分子,并阻碍了$CO_2$的生成。但是,对于含有微量甲酸的甲醇溶液,COad累积速度要比还有甲醛的要慢,可能是因为甲酸需要断裂C-O键形成COad的步骤慢于甲醛直接脱氢。 即使有COad在催化剂表面生成,但是并不影响$CO_2$的生成,说明甲酸可以在被CO吸附的催化剂表面继续发生氧化反应。然而,在有甲酸的条件下,COad生成速度快于不含有甲酸的甲醇溶液,说明甲酸的吸附阻碍了甲醇的分解。在纯的甲醇条件下,来自于甲醇的COad开始出现于0.2V,而当混有甲酸和甲醛时,来自于甲醇的CO在0.4V之后才生成。 所以,从0.2到0.3V,甲醇的解离吸附被来自于甲酸或甲醛分解的COad所抑制。(2013, ChemPhysChem;2014,Phys. Chem. Chem. Phys.)

Reference

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  7. H. Wang, Z. Jusys, and R.J. Behm. Ethanol electrooxidation on carbon-supported Pt, PtRu and Pt3Sn catalysts - a quantitative DEMS study. J. Power Sources 154, 351-359 (2006).

  8. L. Colmenares, H. Wang, Z. Jusys, L. Jiang, S. Yan, G.Q. Sun, and R.J. Behm. Ethanol oxidation on novel, carbon supported Pt alloy catalysts - model studies under defined diffusion conditions. Electrochim. Acta. 52, 221-233 (2006).

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  11. S. Sun, M. Chojak Halseid, M. Heinen, Z. Jusys, and R.J. Behm. Ethanol electrooxidation on a carbon-supported Pt catalyst at elevated temperature and pressure: a high-temperature / high-pressure DEMS study. J. Power Sources, 190, 2-13 (2009).

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  13. W. Zhao, Z. Jusys, and R.J. Behm. Quantitative on-line analysis of liquid phase products of methanol oxidation in aqueous sulfuric acid solutions using electrospray ionization mass spectrometry. Anal. Chem., 82, 2472-2479 (2010)

  14. M. Heinen, Z. Jusys, and R.J. Behm. Ethanol, acetaldehyde and acetic acid adsorption/electrooxidation on a Pt thin film electrode under continuous electrolyte flow: An in situ ATR-FTIRS flow cell study. J. Phys. Chem. C, 114, 9850–9864 (2010)

  15. M. Chojak-Halseid, Z. Jusys, and R.J. Behm. Methanol oxidation over a Pt/C catalyst at high temperatures and pressure: An on-line electrochemical mass spectrometry study. J. Phys. Chem. C, 114, 22573–22581 (2010)

  16. W. Zhao, Z. Jusys, and R.J. Behm. Quantitative on-line Detection of Volatile and Non-volatile Methanol Electrooxidation Products by Combined Electron Impact Mass Spectrometry and Electrospray Ionization Mass Spectrometry. ECS Transaction, 35, 9-19 (2011)

  17. Wei Zhao, Z. Jusys, and R.J Behm. Complete Quantitative online Analysis of Methanol Electrooxidation Products via Electron Impact and Electrospray Ionization Mass Spectrometry. Anal. Chem., 84, 5479–5483 (2012)

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  20. R. Reichert, J. Schnaidt, Z. Jusys, and R.J. Behm. The influence of reactive side products on the electrooxidation of methanol - A combined in situ infrared spectroscopy and online mass spectrometry study. Phys. Chem. Chem. Phys., 16, 13780-13799 (2014)

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