物理化学学报

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PVP封端剂对Pd纳米晶电催化氧化甲醇和乙醇性能的影响

段会梅1, 王惠娟2, 黄伟新1   

  1. 1 中国科学技术大学化学与材料科学学院, 合肥 230026;
    2 中国科学技术大学工程与材料科学实验中心, 合肥 230026
  • 收稿日期:2020-03-02 修回日期:2020-04-02 录用日期:2020-04-16 发布日期:2020-04-20
  • 通讯作者: 段会梅, 黄伟新 E-mail:duanhm@mail.ustc.edu.cn;huangwx@ustc.edu.cn
  • 基金资助:
    国家自然科学基金(21525313)和教育部长江学者奖励计划资助项目

Influence of Polyvinylpyrrolidone Capping Ligands on Electrocatalytic Oxidation of Methanol and Ethanol over Palladium Nanocrystal Electrocatalysts

Huimei Duan1, Huijuan Wang2, Weixin Huang1   

  1. 1 School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, P. R. China;
    2 Experimental Center of Engineering and Material Science, University of Science and Technology of China, Hefei 230026, P. R
  • Received:2020-03-02 Revised:2020-04-02 Accepted:2020-04-16 Published:2020-04-20
  • Supported by:
    The project was supported by National Natural Science Foundation of China (21525313) and the Changjiang Scholars Program of the Chinese Ministry of Education.

摘要: 钯基电催化剂广泛应用于直接醇类燃料电池碱性介质电催化剂。理解Pd电催化剂在醇类电催化氧化反应中的结构效应具有重要意义。本文中我们制备了PVP封端的不同形貌和尺寸的Pd纳米晶并对比研究了其在碱性介质中甲醇和乙醇的电催化氧化活性。实验结果表明具有相近尺寸(7–8 nm)的Pd纳米立方体和纳米八面体在甲醇和乙醇电氧化反应中表现出相近的本征活性,且Pd纳米立方体电催化甲醇和乙醇氧化本征性能随尺寸增大而增加。不同Pd纳米晶的电催化甲醇和乙醇氧化反应行为可归因于Pd纳米晶暴露晶面和受尺寸依赖的Pd纳米晶表面PVP封端剂覆盖度和PVP向Pd纳米晶电荷转移作用影响的Pd纳米晶电子结构。本文结果展示了金属纳米晶表面封端剂对其表面结构和催化性能不可忽略的影响。

关键词: PVP, 表面结构, 表面位, 形貌, 尺寸

Abstract: Direct alcohol fuel cells (DAFCs) have attracted considerable research interest because of their potential application as alternative power sources for automotive systems and portable electronics. Pd-based catalysts represent one of the most popular catalysts for DAFCs due to their excellent electrocatalytic activities in alkaline electrolytes. Thus, it is of great importance to understand the structure-activity relationship of Pd electrocatalysts for alcohol electrocatalysis. Recently, size- and shape- controlled Pd nanocrystals have been successfully synthesized and subsequently used to study the size and shape effects of Pd electrocatalysts on alcohol electrocatalysis, in which the Pd (100) facet exhibited higher electrocatalytic oxidation activity for small alcohol molecules than the Pd (111) and (110) facets. Although it is well known that capping ligands, which are widely used in wet chemistry for the size- and shape-controlled synthesis of metal nanocrystals, likely chemisorb onto the surfaces of the resulting metal nanocrystals and influence their surface structure and surface-mediated properties, such as catalysis, this issue was not considered in previous studies of Pd nanocrystal electrocatalysts for electrocatalytic oxidation of small alcohol molecules. In this study, we prepared polyvinylpyrrolidone (PVP)-capped Pd nanocrystals with different morphologies and sizes and comparatively studied their electrocatalytic activities for methanol and ethanol oxidation in alkaline solutions. The chemisorbed PVP molecules transferred charge to the Pd nanocrystals, and the finer Pd nanocrystals had a higher coverage of chemisorbed PVP, and thus exposed fewer accessible surface sites, experienced more extensive PVP-to-Pd charge transfer, and were more negatively charged. The intrinsic electrocatalytic activity, represented by the electrochemical surface area (ECSA)-normalized electrocatalytic activity, of Pd nanocubes with exposed (100) facets increases with the particle size, indicating that the more negatively-charged Pd surface is less electrocatalytically active. The Pd nanocubes with average sizes between 12 and 19 nm are intrinsically more electrocatalytically active than commercial Pd black electrocatalysts, while the activity of Pd nanocubes with an averages size of 8 nm is less. This suggests that the enhancement effect of the exposed (100) facets surpasses the deteriorative effect of the negatively charged Pd surface for the Pd nanocubes with average sizes between 12 and 19 nm, whereas the deteriorative effect of the negatively charged Pd surface surpasses the enhancement effect of the exposed (100) facets for the Pd nanocubes with average sizes of 8 nm due to the extensive PVP-to-Pd charge transfer. Moreover, the Pd nanocubes with average sizes of 8 nm exhibit similar intrinsic electrocatalytic activity to the Pd nanooctahedra with (111) facets exposed and average sizes of 7 nm, indicating that the electronic structure of Pd electrocatalysts plays a more important role in influencing the electrocatalytic activity than the exposed facet. Since the chemisorbed PVP molecules block the surface sites on Pd nanocrystals that are accessible to the reactants, all Pd nanocrystals exhibit lower mass-normalized electrocatalytic activity than the Pd black electrocatalysts, and the mass-normalized electrocatalytic activity increases with the ECSA. These results clearly demonstrate that the size- and shape-dependent electrocatalytic activity of Pd nanocrystals capped with PVP for methanol and ethanol oxidation should be attributed to both the exposed facets of the Pd nanocrystals and the size-dependent electronic structures of the Pd nanocrystals resulting from the size-dependent PVP coverage and PVP-to-Pd charge transfer. Therefore, capping ligands on capped metal nanocrystals inevitably influence their surface structures and surface-mediated properties, which must be considered for a comprehensive understanding of the structure-activity relationship of capped metal nanocrystals.

Key words: PVP, Surface structure, Surface site, Morphology, Size

MSC2000: 

  • O646