氨基酸调控自组装多级Pd基纳米晶的可控合成及其对甲酸的电催化氧化

The electrocatalytic activity of Pd on formic acid oxidation drop rapidly over time, which hinders its practical application in direct formic acid fuel cells (DFAFCs). In this project, the proposed multi-disciplinary program of research is to design and fabricate three-dimensional (3D) interconnected Pd-based nanoassemblies (Pd-3DNAs) in different morphology or hierarchical microstructure with specific surface chemistry for solving the problem, probably holding much promise to meet the performance goals necessary for the application of DFAFCs. Amino acid is a natural–origin biomolecule with diversely different R group and variable protonation state depending on the pH values. Due to its relatively low cost, environmental friendliness, desirable metal-binding affinity, tunable surface charge and configuration, and robust assembly capability, the amphiphilic amino acid, containing a range of functional groups (e.g., -COOH, -OH, -NH2, -SH), is selected as the directing and assembling agent to make desirable Pd-3DNAs. The amino acid is to provide dynamic control of the nucleation and crystal growth, and spatial control of nanoassemblies. The 3D hierarchical Pd-based nanoassemblies with pores inside are expected to effectively reduce crystalline migration, dissolution, aggregation and Ostwald ripening in fuel cell environments, meanwhile, having promoted mass transfer and efficiency of electronic transmission. More control by introduction of a transition metal as a second component, allows further adjustment of the intrinsic nanocrystal properties. These all contribute to significant improvements on activity and durability of the Pd-3DNAs. Apart from the over-arching aim of this project mentioned above, there are a number of other outcomes that will be achieved as summarized in the following: 1) Systematically investigate how the pH, temperature, type of reductants, nature of the amino acid, and its concentration affect final product composition, structure and morphology, to gain insight into the mechanism and kinetics, establishing scientific basis for the rational design and synthesis of hierarchical nanostructures; 2) Outline the interactions, including electrostatic interaction, hydrogen bonding, stacking, hydrophobic interactions, among the amino acids on the crystal lattice structure in aqueous solution, to present an in-depth look on the dynamic assembly process mediated by the amino acid, for control of nanoassemblies; 3) Investigate the influence of the hierarchical structure, chemical composition on the activity and stability of Pd-3DNAs, elucidating the underlying relationship and interactive mechanism to guide the synthesis of proper catalysts with suitable microstructure. Meanwhile, assess the prospect of the fabricated Pd-3DNAs as anode for DFAFCs. The proposed program of research will pave the way to develop economically competitive electrocatalysts for fuel cells with improved power output and prolonged net life time.

Pd对甲酸氧化反应(FAOR)的电催化活性随时间急剧下降，已成为限制其应用的关键问题。本项目拟以富含多种功能基团、荷电和空间取向可控、具有有趣的自组装特性的氨基酸为成核和晶体生长的调控剂，通过第二体金属的遴选与掺杂，在环境友好的水基体系中，设计并制备出化学组成可变，尺寸和形貌可控，具有三维内联结构的Pd基纳米晶超结构体，有效降低Pd纳米晶的溶解、迁移、聚集及Ostwald熟化，优化电子传输效率，从而大幅提高Pd催化剂的活性和稳定性；系统研究氨基酸种类、溶液pH值、反应温度以及还原剂类型等与纳米晶组成、结构和形貌的相互关系，探索作用机制和形成机理，最终实现三维多级结构的可控自组装；利用光谱和电化学的方法研究Pd基超结构体对FAOR的活性及稳定性，阐明影响规律及作用机理。本项目的研究对高活性、高稳定性电催化剂的设计和制备，对发展概念简单、环境友好的可控合成方法具有理论和实践指导意义。

Pd催化甲酸直接氧化为CO2存在的主要问题是经过首次高效催化后，活性迅速下降。通过调控Pd基催化剂的组成、形貌、尺寸及表界面电子结构，不仅可以有效提高催化剂的活性和稳定性，而且可以以此为基础探讨Pd基催化剂衰减机理。本项目在氨基酸水基体系中以谷氨酸、脯氨酸、精氨酸、组氨酸、类氨基酸等为成核和晶体生长的调控剂，合成了一系列具有不同形貌和表界面结构的多维度一元/多元Pd基纳米晶自组装体，其中代表性的包括：PtPd@Pt核壳卫星结构纳米晶、自支撑Pd纳米链纳米片、Pd-Cu四足纳米晶、空心多孔Pd纳米笼、CuPd@Pd凹四面体、PdIr凹四面体，PdIr纳米线、Pd3M(M=Co,Ni)纳米自组装体，PdRh纳米链，Pd@Pd17Se15超薄片层网络等；探讨了氨基酸分子的柔性链或刚性环、聚集态、可变的侧链R基团(疏水碳链有长有短，其中的功能基团也各不相同)的配位性、结构导向性及对Pd-M纳米晶组成和结构的影响，在此基础上研究了结构和形貌与催化性能的关系。研究发现：1) 廉价金属离子通过氨基酸功能基团的络合，可以实现在Pd等贵金属表面的欠电位共沉积，形成合金结构；原位生成的位于表面的较活泼金属还会发生Galvanic置换反应，形成多级纳米孔洞结构，界面富含台阶、边缘、扭结、位错等配位不饱和的表面原子或原子团，亦或产生孪晶界面，是高的催化活性位。2) 第二体金属或非金属的掺杂，形成合金、核壳结构和独特的内联结构及吸附的适量氨基酸对催化剂表面的功能化等不仅有效降低了运行过程中Pd的溶解，重组、Ostwald熟化和聚集及甲酸直接在Pd表面的催化自分解等，提高了催化剂的稳定性；还有效地改善了催化剂表面电子结构，通过调节d带中心、与催化小分子的结合能、协同作用等进一步提高了催化剂的活性。3) 通过引入较活泼的第二体金属，在形成一定形貌的纳米合金结构后，利用化学刻蚀法，借助于条件控制可以实现第二体金属溶解的同时，纳米结构的骨架基本维持，形成各种形貌的多孔框架结构。氨基酸绿色、无毒，利用氨基酸在水相中制备特定形貌和结构的纳米材料符合绿色环保理念。本项目共完成SCI论文19篇，申请专利5项，其中授权4项，1项转让专利权至企业。发表会议论文4篇，其中一篇为在美国洛杉矶举行的国际会议论文。有一位获江苏省优秀硕士论文奖，一位获校级优秀博士论文奖。圆满完成了项目预期的研究目标和研究任务。