Ultra-Small Face-Centered-Cubic Ru Nanoparticles Confined within a Porous Coordination Cage for Dehydrogenation
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2018 Ruthenium nanoparticles (Ru NPs) with a face-centered-cubic (fcc) structure exhibit high catalytic activity in a number of reactions. In order to obtain highly crystalline and reactive fcc Ru NPs, we applied an anionic porous coordination cage (PCC-2) to encapsulate them and tune their crystal structures. The cage was assembled with six vertex ligands (V) and eight panel ligands (L), creating a truncated octahedral inner cavity 2.5 nm in diameter. As a result of electrostatic attraction, Ru3+ ions were encapsulated within the cavity of PCC-2. By an in situ reduction, metal atoms in the cavity of PCC-2 formed ultra-fine fcc Ru NPs, which were uniform in size. The as-synthesized Ru NPs@PCC-2 composite exhibited record-high catalytic activity in methanolysis of ammonia borane, which is critically important in chemical hydrogen storage. This offers a new way of forming stable metal nanoclusters with ultra-small size and desirable structure within the cavities of a soluble porous material. To overcome environmental issues associated with fossil fuels, hydrogen has been proposed as an alternative renewable energy source. A metal nanoparticle catalyst is commonly used for obtaining hydrogen from chemical hydrogen storage materials. However, because of their high surface energy, metal nanoparticle catalysts often suffer from aggregation, resulting in low reactivity. As a nanoscopic porous support, coordination cages offer a potential platform for obtaining highly reactive nanoparticles via encapsulation. Here, we demonstrate the utility of a coordination cage as a nanoparticle container to encapsulate ruthenium nanoparticles and tune their form to a rare fcc crystalline structure. This nanoparticle-cage composite exhibits record-high catalytic activity toward ammonia borane dehydrogenation. The current work provides a strategy for the encapsulation of metal nanocrystals within a soluble molecular cage to form homogeneous catalysts with unprecedented activity. Porous coordination cages (PCCs) are discrete nanoscopic structures with intrinsic cavities. The solubility and tunable host-guest interactions allow PCCs to form a homogeneous catalytic platform. Through engineering the interactions between the host PCC and the guest nanoparticles, we succeeded in encapsulating ruthenium nanoparticles in PCCs and tuning their crystalline form to a highly reactive face-centered-cubic one. The nanoparticles within the PCC showed extraordinary reactivity toward the dehydrogenation reaction. This approach sheds light on developing high-performance catalysts through interaction engineering.
