氧化铁对铂在碱性介质中催化氧化甲醇的促进作用

doi:10.1016/S1872-2067(17)62773-6

摘要/Abstract

摘要：

Pt催化剂是电催化领域用途最为广泛的贵金属催化剂.Pt资源稀缺，价格昂贵，同时它的物理化学特性又决定了其在多种催化反应中难以被替代.在质子交换膜燃料电池的小分子醇类电氧化过程中，难免存在Pt的毒化现象，其催化性能有待进一步提升.因此，围绕着Pt催化剂纳米结构的设计、抗毒性及反应机理的探索一直是电催化研究面临的重要课题.
目前，已被广泛认可的提高Pt催化性能的方法之一是引入第二种金属，通过金属间协同效应（双功能机理）、张力效应或电子效应等对Pt的催化行为进行改性.对于由双/多金属组成的纳米结构催化剂，无论是协同效应还是电子效应，催化活性的提高都需要金属间有丰富的接触界面和恰当的邻近状态.通过调变两组元的种类、原子比和接触状态等可以实现对金属-金属界面的调控，进而调变催化剂性能.除金属助剂外，金属氧化物对Pt催化剂的助催化作用也引起广泛关注.由于金属氧化物与Pt之间的密切接触作用，氧化物的形貌特点对Pt的催化性能可产生重要影响.
到目前为止，有关催化剂形貌效应的研究主要集中于贵金属纳米颗粒上（Pt，Au，Pd等），但关于金属氧化物载体/助剂的形貌对贵金属催化性能影响的研究尚不多.具有明确形貌的金属氧化物载体/助剂，暴露的晶面不同，表面原子的配位状态也不同，从而造成与之密切接触的Pt的性质发生改变.因此，金属氧化物的表面性质以及Pt-金属氧化物的界面性质将对电催化性能产生重要影响，深入阐释贵金属-金属氧化物的表/界面性质以及建立有效的构效关系，对设计和制备高效电催化剂具有一定的指导意义.
为了提高Pt基催化剂活性、抗CO中毒能力以及稳定性，本文采用共沉淀法和水热法分别制备了纳米棒和六边形纳米片状的Fe₂O₃作为Pt催化剂的助剂，考察了助剂形貌对Pt催化剂在碱性介质中催化氧化甲醇的促进作用.通过X射线衍射、扫描电子显微镜、透射电子显微镜、X射线光电子能谱以及电化学技术对催化剂进行了表征.结果显示，Fe₂O₃的存在能显著提高Pt催化剂在碱性介质中对甲醇氧化的电催化性能，而且以Fe₂O₃纳米棒为助剂制备的Pt-Fe₂O₃/C-R催化剂催化活性以及稳定性比Fe₂O₃纳米片为助剂制备的Pt-Fe₂O₃/C-P催化剂更高.这种促进效应可能与助剂Fe₂O₃的形态有关.Pt-Fe₂O₃/C-R催化剂中Pt的质量比活性为5.32 A/mg_Pt，本征活性为162.7 A/m_Pt²，分别是Pt-Fe₂O₃/C-P催化剂的1.67和2.04倍，是商业PtRu/C样品的4.19和6.16倍.协同效应和电子效应是Pt催化性能提升的主要原因.此外，Pt-Fe₂O₃/C-R样品中高价态Pt的含量较高，可能也是加速甲醇氧化反应动力学的原因之一.高价态的Pt可能会增强甲醇分子在Pt表面的吸附强度，促进Pt上甲醇氧化反应初始步.这些发现不仅可对甲醇电催化氧化机理有了更深的理解，而且对设计和制备高性能甲醇氧化电催化剂也具有一定的指导意义.

关键词: 铂, 氧化铁, 助催化作用, 甲醇电氧化反应, 碱性电解质

Abstract:

Fe₂O₃ nanorods and hexagonal nanoplates were synthesized and used as the promoters for Pt electrocatalysts toward the methanol oxidation reaction (MOR) in an alkaline electrolyte. The catalysts were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, cyclic voltammetry and chronoamperometry. The results show that the presence of Fe₂O₃ in the electrocatalysts can promote the kinetic processes of MOR on Pt, and this promoting effect is related to the morphology of the Fe₂O₃ promoter. The catalyst with Fe₂O₃nanorods as the promoter (Pt-Fe₂O₃/C-R) exhibits much higher catalytic activity and stability than that with Fe₂O₃ nanoplates as the promoter (Pt-Fe₂O₃/C-P). The mass activity and specific activity of Pt in a Pt-Fe₂O₃/C-R catalyst are 5.32 A/mg_Pt and 162.7 A/m_Pt², respectively, which are approximately 1.67 and 2.04 times those of the Pt-Fe₂O₃/C-P catalyst, and 4.19 and 6.16 times those of a commercial PtRu/C catalyst, respectively. Synergistic effects between Fe₂O₃ and Pt and the high content of Pt oxides in the catalysts are responsible for the improvement. These findings contribute not only to our understanding of the MOR mechanism but also to the development of advanced electrocatalysts with high catalytic properties for direct methanol fuel cells.

Key words: Platinum, Ferric oxide, Promoting effect, Methanol electrooxidation reaction, Alkaline electrolyte

宋桂花, 杨海芳, 孙雅飞, 王静怡, 曲卫东, 张强, 马令娟, 冯媛媛. 氧化铁对铂在碱性介质中催化氧化甲醇的促进作用[J]. 催化学报, 2017, 38(3): 554-563.

Guihua Song, Haifang Yang, Yafei Sun, Jingyi Wang, Weidong Qu, Qiang Zhang, Lingjuan Ma, Yuanyuan Feng. Promoting effects of Fe₂O₃ to Pt electrocatalysts toward methanol oxidation reaction in alkaline electrolyte[J]. Chinese Journal of Catalysis, 2017, 38(3): 554-563.

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