Tandem nanocatalyst design: putting two step-reaction sites into one location towards enhanced hydrogen transfer reactions

串联反应纳米催化剂的设计:基于分步反应活性位点协调统一的氢转移反应活性提升（英文）

Efficient tandem reactions on a single catalytic nanostructure would be beneficial to improving chemical transformation efficiency and reducing safety implications. It is imperative to identify the active sites for each single step reaction so that the entire reaction process can be optimized by designing and integrating the sites. Herein, hydrogen transfer reaction is taken as a proof-of-concept demonstration to show that the spatial integration of active sites is important to the catalytic efficiency of the entire process in tandem reactions. We identified specific active sites (i.e., various sites at faces versus corners and edges) for formic acid decomposition and alkene/nitrobenzene hydrogenation-the two steps in hydrogen transfer reactions, by employing three different shapes of Pd nanocrystals in tunable sizes. The investigation reveals that the decomposition of formic acid occurs preferentially at the edge sites of cubic nanocrystal and the plane sites of octahedral/ tetrahedral nanocrystals, while the hydrogenation takes place mainly at the edge sites of both cubic and octahedral/ tetrahedral nanocrystals. The consistency of active edge sites during different step reactions enables cubic nanocrystals to exhibit a higher activity than octahedral nanocrystals in hydrogen transfer reactions, although octahedrons offer comparable activities to cubes in formic acid decomposition and hydrogenation reactions. Guided by these findings, we further improved the overall performance of tandem catalysis by specifically promoting the limiting step through nanocatalyst design. This work provides insights into the rational design of heterogeneous nanocatalysts in tandem reactions.

在一种催化纳米结构上实现高效的多步串联反应有利于提高化学转化的效率并且减少安全隐患.在确定每一个分步反应中的活性位点之后,才能够优化整个串联反应的活性.因此,确认分步反应中的活性位点是在催化纳米结构上实现高效多步串联反应的关键要素之一.在本文中,我们以氢转移反应为模型体系,展示了串联反应中分步反应的活性位点协调统一是催化剂设计中不容忽视的关键要素.我们合成了3种结构的钯纳米晶体(包括纳米立方体、八面体、四面体),得到了不同类型的催化位点(面位点及棱角位点),并在氢转移反应的分步反应(甲酸分解和加氢反应)中探究了不同催化位点的活性贡献程度.结果表明甲酸分解反应比较倾向于在纳米立方体的棱上和纳米八面体/四面体的面上发生,而加氢反应倾向于在3种纳米晶体的棱上发生.纳米立方体的棱位点在两个分步反应中都贡献了较大的活性,这导致了其在氢转移反应中明显具有更高的活性.即使纳米八面体/四面体在分步反应中具有相近的活性,由于串联反应的活性位点不统一,导致其在氢转移反应中的活性明显低于纳米立方体.基于以上发现,我们进一步通过提高限速步的反应速率,从而提高整个氢转移反应的活性.本工作为合理设计串联反应中的异相纳米催化剂提供了新的思路.