Single noble metal atoms and ultrafine metal clusters catalysts tend to sinter into aggregated particles at elevated temperatures, driven by the decrease of metal surface free energy. Herein, we report an unexpected phenomenon that noble metal nanoparticles (Pd, Pt, Au-NPs) can be transformed to thermally stable single atoms (Pd, Pt, Au-SAs) above 900 °C in an inert atmosphere. The atomic dispersion of metal single atoms was confirmed by aberration-corrected scanning transmission electron microscopy and X-ray absorption fine structures. The dynamic process was recorded by in situ environmental transmission electron microscopy, which showed competing sintering and atomization processes during NP-to-SA conversion. Further, density functional theory calculations revealed that high-temperature NP-to-SA conversion was driven by the formation of the more thermodynamically stable Pd-N structure when mobile Pd atoms were captured on the defects of nitrogen-doped carbon. The thermally stable single atoms (Pd-SAs) exhibited even better activity and selectivity than nanoparticles (Pd-NPs) for semi-hydrogenation of acetylene.

Non-noble metal isolated single atom site (ISAS) catalysts have attracted much attention due to their low cost, ultimate atom efficiency and outstanding catalytic performance. Herein, atomically dispersed Fe atoms are prepared by a covalent organic framework (COF)-absorption-pyrolysis strategy. The obtained Fe ISASs anchored on COF-derived N-doped carbon nanospheres (Fe-ISAS/CN) served as a multifunctional catalyst in electro-catalysis and organic catalysis, exhibiting better catalytic performance than commercial Pt/C for the ORR with good stability and methanol tolerance. Besides electro-catalysis, the Fe-ISAS/CN also showed outstanding catalytic performance in organic reactions, such as the selective oxidation of ethylbenzene to acetophenone and dehydrogenation of 1,2,3,4-tetrahydroquinoline with excellent reactivity, selectivity, stability and recyclability. Co and Ni ISAS materials can also be prepared by this method, suggesting that it is a general strategy to obtain metal ISAS catalysts. This work will provide new insight into the design of COF-derived metal ISAS multi-functional catalysts for electrocatalysis and organic reactions using rationally designed synthetic routes and the optimized structure of substrates.

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Designing nanocatalysts with synergetic functional component is a desirable strategy to achieve both high activity and selectivity for industrially important hydrogenation reaction. Herein, we fabricated a core-shell hollow Au@Pt NTs@ZIFs (ZIF, zeolitic imidazolate framework; NT, nanotube) nanocomposite as highly efficient catalysts for semi-hydrogenation of acetylene. Hollow Au@Pt NTs were synthesized by epitaxial growth of Pt shell on Au nanorods followed with oxidative etching of Au@Pt nanorod. The obtained hollow Au@Pt NTs were then homogeneously encapsulated within ZIFs through in situ crystallization. By combining the high activity of bimetallic nanotube and gas enrichment property of porous metal-organic frameworks, hollow Au@Pt NT@ZIF catalyst was demonstrated to show superior catalytic performance for the semi-hydrogenation of acetylene, in terms of both selectivity and activity, over those of monometallic Au and solid bimetal nanorod@ZIF counterparts. This catalysts design idea is believed to be inspirable for the development of highly efficient nanocomposite catalysts.

The hydration swelling property of bentonites, which mainly depends on the content and swelling capacity of montmorillonite (MMT), greatly contributes to many engineering applications such as grouting and barrier materials. Much efforts have been made to investigate the process of crystalline swelling by experimental tests, analytical models, and numerical simulations. However, most of the studies are limited to the microscopic structure of the hydrates and the variation of free energy of the system. Via molecular dynamics simulation, this paper discusses the internal forces within the layered structure of Na-MMT hydrates for the mechanical analysis of crystalline swelling. The repulsive force between crystal layers f LL , the repulsive force of interlamellar water molecules to crystal layer f WL , and the attractive force of interlamellar cations to crystal layer f CL in different hydration phases are calculated to illustrate the relationships among the microscopic structure of hydrates, internal forces, and the macroscopic characteristics of swelling. For fully dry Wyoming MMT, f LL is (3.85 ± 0.39) × 10 −9 N/uc (positive for repulsion) and f CL is (−3.93 ± 0.34) × 10 −9 N/uc (negative for attraction). With the increase of water content, the three forces reach relatively stable values, which are 3.43 × 10 −9 , −4.99 × 10 −9 , and 1.25 × 10 −9 N/uc for f LL , f CL , and f WL , respectively. The quantitative analysis shows that the total repulsive force from f LL and f WL is approximately equal to the attractive force f CL during the process of crystalline swelling. The Coulomb electrostatic force term controls the f LL and f CL , while it is comparable to but less than the van der Waals force term for f WL . The Coulomb electrostatic force term of f LL may be regarded as an interaction between "infinite parallel charged plates" when the water content is greater than 6 H 2 O/uc. The Coulomb electrostatic force term of f WL is mainly caused by the polarization effect of interlamellar water molecules. The crystalline swelling can be presented as a stepwise "expansion−filling−saturation" process via the analysis of the change of average density of interlamellar water, whose values are close to the mass density of bulk water at the "saturation" stages.