Metal (Hydr)oxides@Polymer Core–Shell Strategy to Metal Single-Atom Materials

Preparing metal single-atom materials is currently attracting tremendous attention and remains a significant challenge. Herein, we report a novel core–shell strategy to synthesize single-atom materials. In this strategy, metal hydroxides or oxides are coated with polymers, followed by high-temperature pyrolysis and acid leaching, metal single atoms are anchored on the inner wall of hollow nitrogen-doped carbon (CN) materials. By changing metal precursors or polymers, we demonstrate the successful synthesis of different metal single atoms dispersed on CN materials (SA-M/CN, M = Fe, Co, Ni, Mn, FeCo, FeNi, etc.). Interestingly, the obtained SA-Fe/CN exhibits much higher catalytic activity for hydroxylation of benzene to phenol than Fe nanoparticles/CN (45% vs 5% benzene conversion). First-principle calculations further reveal that the high reactivity originates from the easier formation of activated oxygen species at the single Fe site. Our methodology provides a convenient route to prepare a variety of metal single-atom materials representing a new class of catalysts

Discovering Partially Charged Single-Atom Pt for Enhanced Anti-Markovnikov Alkene Hydrosilylation

The hydrosilylation reaction is one of the largest-scale application of homogeneous catalysis and is widely used to enable the commercial manufacture of silicon products. However, considerable issues including disposable platinum consumption, undesired side reactions and unacceptable catalyst residues still remain. Here, we synthesize a heterogeneous partially charged single-atom platinum supported on anatase TiO₂ (Pt₁^δ+/TiO₂) catalyst via an electrostatic-induction ion exchange and two-dimensional confinement strategy, which can catalyze hydrosilylation reaction with almost complete conversion and produce exclusive adduct. Density functional theory calculations reveal that unexpected property of Pt₁^δ+/TiO₂ originates from atomic dispersion of active species and unique partially positive charge Pt^δ+ electronic structure that conventional nanocatalysts do not possess. The fabrication of single-atom Pt₁^δ+/TiO₂ catalyst accomplishes a reasonable use of Pt through recycling and maximum atom-utilized efficiency, indicating the potential to achieve a green hydrosilylation industry

Toward a Unified Identification of Ti Location in the MFI Framework of High-Ti-Loaded TS-1: Combined EXAFS, XANES, and DFT Study

Titanium silicalite-1 (TS-1) has been shown to be a heterogeneous catalyst with remarkable efficiency and selectivity; however, the nature of the active Ti site in the MFI framework remains elusive. Here we report combined experimental and theoretical research on Ti distribution in the 12 crystallographically distinct T sites of the MFI framework in high-Ti-loaded TS-1 (2.7 wt % in TiO₂). Using a multishell fit to extended X-ray absorption fine structure, we show that T4 is the most populated site, in marked contrast to the preferential substitution sites and the definitely excluded sites assumed hitherto by diffraction studies. The identification is supported by a good agreement between calculated and experimental X-ray absorption near-edge structure studies and by full periodic density functional theory (DFT) computation. In spite of having the identical most favored site, the preference order for the remaining sites predicted by DFT does not fully match the experimental results. This suggests that Ti distribution in the resulting TS-1 framework is positively correlated with the thermodynamic stability of pure material but can be affected by other factors such as interdefects. These new insights may facilitate the bottom-up design of new zeolites with tailored catalytic performance and studies on mechanisms of various oxidation reactions

Confined Pyrolysis within Metal–Organic Frameworks To Form Uniform Ru₃ Clusters for Efficient Oxidation of Alcohols

Here we report a novel approach to synthesize atomically dispersed uniform clusters via a cage-separated precursor preselection and pyrolysis strategy. To illustrate this strategy, well-defined Ru₃(CO)₁₂ was separated as a precursor by suitable molecular-scale cages of zeolitic imidazolate frameworks (ZIFs). After thermal treatment under confinement in the cages, uniform Ru₃ clusters stabilized by nitrogen species (Ru₃/CN) were obtained. Importantly, we found that Ru₃/CN exhibits excellent catalytic activity (100% conversion), high chemoselectivity (100% for 2-aminobenzaldehyde), and significantly high turnover frequency (TOF) for oxidation of 2-aminobenzyl alcohol. The TOF of Ru₃/CN (4320 h^–1) is about 23 times higher than that of small-sized (ca. 2.5 nm) Ru particles (TOF = 184 h^–1). This striking difference is attributed to a disparity in the interaction between Ru species and adsorbed reactants

Isolated Single-Atom Pd Sites in Intermetallic Nanostructures: High Catalytic Selectivity for Semihydrogenation of Alkynes

Improving the catalytic selectivity of Pd catalysts is of key importance for various industrial processes and remains a challenge so far. Given the unique properties of single-atom catalysts, isolating contiguous Pd atoms into a single-Pd site with another metal to form intermetallic structures is an effective way to endow Pd with high catalytic selectivity and to stabilize the single site with the intermetallic structures. Based on density functional theory modeling, we demonstrate that the (110) surface of <i>Pm</i>3̅<i>m</i> PdIn with single-atom Pd sites shows high selectivity for semihydrogenation of acetylene, whereas the (111) surface of <i>P</i>4/<i>mmm</i> Pd₃In with Pd trimer sites shows low selectivity. This idea has been further validated by experimental results that intermetallic PdIn nanocrystals mainly exposing the (110) surface exhibit much higher selectivity for acetylene hydrogenation than Pd₃In nanocrystals mainly exposing the (111) surface (92% vs 21% ethylene selectivity at 90 °C). This work provides insight for rational design of bimetallic metal catalysts with specific catalytic properties

Uncoordinated Amine Groups of Metal–Organic Frameworks to Anchor Single Ru Sites as Chemoselective Catalysts toward the Hydrogenation of Quinoline

Here we report a precise control of isolated single ruthenium site supported on nitrogen-doped porous carbon (Ru SAs/N–C) through a coordination-assisted strategy. This synthesis is based on the utilization of strong coordination between Ru³⁺ and the free amine groups (−NH₂) at the skeleton of a metal–organic framework, which plays a critical role to access the atomically isolated dispersion of Ru sites. Without the assistance of the amino groups, the Ru precursor is prone to aggregation during the pyrolysis process, resulting in the formation of Ru clusters. The atomic dispersion of Ru on N-doped carbon can be verified by the spherical aberration correction electron microscopy and X-ray absorption fine structure measurements. Most importantly, this single Ru sites with single-mind N coordination can serve as a semihomogeneous catalyst to catalyze effectively chemoselective hydrogenation of functionalized quinolones