image: Schematic illustration of synergy catalysis of Ru single atom and Ru nanoparticle. The main role of Ru single atom and Ru nanoparticle is to absorb and dissociate hydrogen, and activate quinoline, respectively. H atoms generated at Ru single sites can directly react or migrate to the quinoline bounded Ru nanoparticles to complete hydrogenation process.
Credit: ©Science China Press
In recent years, one classic supported metal catalyst usually consists of active metal species disperse on the surface of high-surface-area support materials (e.g. carbon, zeolites, metal oxides, silica). This kind of heterogeneous catalyst normally have high catalytic activity because of the high exposure of active species, but low stability, especially under harsh reaction conditions, due to the high surface energy of active metal species (nanometer-， subnanometer-， or single atom-scale) as well as the limited interaction between metal and support. Additionally, it is also worth to note that the structural changes (e.g. size, shape) of active metal species during catalysis generally accompanied by a dramatic change in selectivity, thus further affecting the catalytic performance of the classic supported metal catalysts.
To overcome the above challenges, fabrication of catalyst with encapsulated structure is developed and is expected to physically isolate the active metal species and prevent them from migration and coalescence, and thus enhance their catalytic stability. Various inorganic oxides, carbon, and metal-organic frameworks (MOFs) are the most frequently used encapsulating materials to encapsulating metal nanoparticles in nanoshells or nanopores. One of the most remarkable merits of this kind of catalyst, to some extent, has dramatically enhanced stability and recyclability in catalysis in comparison with traditional supported metal catalyst.
However, for the encapsulated catalyst, there is still a long way ahead to always achieve both high activity and stability in chemical synthesis, energy conversion, and environmental protection. For example, the catalyst with oxides or MOFs as nanoshells, they may still suffer from severe catalyst deactivation when applied in some reaction systems containing water and/or acid. Additionally, in most cases, catalyst stability is improved at the cost of activity because a thick nanoshell is beneficial for catalyst stability, but has a negative impact on the accessibility of reactants to active metal species. Last but not least, it is still difficult to conveniently prepare encapsulated metal catalyst with precise control at a large scale and low cost. Hence, the development of metal catalyst with new structure that can enhance catalyst stability while preserving high activity is highly desirable but challenging.
In this study, the researchers develop a new coating-impregnation-pyrolysis-etching strategy to fabricate supported Ru catalyst (Ru-Al2O3@CN-A), which contains both Ru single atoms and highly dispersed Ru nanoparticles, and these Ru species are embedded into N-doped carbon layer. Benefiting from the synergy catalysis of Ru single atoms and Ru nanoparticles, and the semi-embedded structure, the Ru-Al2O3@CN-A catalyst exhibits an excellent catalytic performance for the selective hydrogenation of quinoline.
Combined with experimental, characterization, and density functional theory calculation results, the researchers conclude that that the main role of single Ru atoms and Ru nanoparticles is to absorb and dissociate hydrogen, and activate quinoline, respectively. H atoms generated at Ru single sites can directly react or migrate to the quinoline bounded Ru nanoparticles to complete hydrogenation process. The synergy effect of Ru single atoms and Ru nanoparticles is the main reason for the superior catalytic activity of Ru-Al2O3@CN-A catalyst.
Overall, this study demonstrates the successful fabrication of supported metal catalysts with both high catalytic activity and stability. These findings in this research may provide a new choice for the design of supported metal catalysts with both high catalytic activity and stability.