Center for Micro-Engineered Materials and the Department of Chemical and Biological Engineering , The University of New Mexico , Albuquerque , New Mexico 87131 , United States.
Nanotechnology Systems Division , Hitachi High-Technologies America Inc. , 22610 Gateway Center Drive #100 , Clarksburg , Maryland 20871 , United States.
Nano Lett. 2019 Mar 13;19(3):1512-1519. doi: 10.1021/acs.nanolett.8b04121. Epub 2019 Feb 8.
Supported ultrasmall noble metal nanocluster-based (UNMN-based) catalysts are one of the most important classes of solid materials for heterogeneous catalysis. In this work, we present a novel strategy for the controlled synthesis of ligand-free UNMN nanocatalysts based on in situ reduction of a palladium-based (Pd-based) metal-organic cage (MOC) confined within monosized, thiol-modified mesoporous silica nanoparticle (MSN) supports. By taking advantage of the high mutual solubility of MOCs and MSNs in DMSO and the strong interactions between the thiol-modified MSN pore wall and MOC surface, a good dispersion of MOC molecules was achieved throughout the MSN support. The close correspondence of the MSN pore diameter (ca. 5.0 nm) with the diameter of the MOC (ca. 4.0 nm) confines MOC packing to approximately a monolayer. Based on this spatial constraint and electrostatic binding of the MOC to the thiol-modified MSN pore surface, in situ MOC reduction followed by metal atom diffusion, coalescence, and anchoring on the active sites resulted in ligand-free Pd-based UNMNs of approximately 0.9 ± 0.2 nm in diameter decorating the MSN pore surfaces. Control experiments of the reduction of a conventional palladium source or the reduction of free, unconstrained cages in solution under the same conditions only produced large metal nanocrystals (NP, >2 nm), confirming the importance of confined reduction to achieve a highly catalytically active surface. In light of this strategy, two catalytic experiments including the reaction of 4-nitrophenol to 4-aminophenol and the Suzuki C-C coupling reaction show superior catalytic activity of the engineered MSN-supported UNMN nanocatalysts compared to their free form and state of the art commercial catalysts. We believe that our new strategy will open new avenues for artificially designed UNMN-inspired nanoarchitectures for wide applications.
负载型超小金属纳米团簇催化剂是多相催化中最重要的一类固体材料。本工作提出了一种基于钯基金属有机笼（MOC）在单分散、巯基修饰介孔硅纳米颗粒（MSN）载体中原位还原的无配体负载型 UNMN 纳米催化剂的可控合成新策略。利用 MOC 和 MSN 在 DMSO 中的高互溶性以及巯基修饰的 MSN 孔壁与 MOC 表面之间的强相互作用，实现了 MOC 分子在整个 MSN 载体中的良好分散。MSN 孔直径（约 5.0nm）与 MOC 直径（约 4.0nm）的紧密对应限制了 MOC 的堆积约为单层。基于这种空间限制和 MOC 与巯基修饰的 MSN 孔表面的静电结合，MOC 的原位还原随后是金属原子的扩散、聚合并锚定在活性位上，导致约 0.9±0.2nm 直径的无配体钯基 UNMN 负载在 MSN 孔表面上。在相同条件下，对传统钯源的还原或游离、无约束笼在溶液中的还原的控制实验仅产生了大的金属纳米晶体（NP，>2nm），证实了受限还原对于实现高催化活性表面的重要性。根据这一策略，进行了包括 4-硝基苯酚到 4-氨基酚的反应和 Suzuki C-C 偶联反应在内的两个催化实验，与游离形式和现有商业催化剂相比，所设计的负载型 UNMN 纳米催化剂具有优越的催化活性。我们相信，我们的新策略将为广泛应用的人工设计 UNMN 启发的纳米结构开辟新途径。
