Yuen Wu

Single Cobalt Atoms with Precise N‐Coordination as Superior Oxygen Reduction Reaction Catalysts

A new strategy for achieving stable Co single atoms (SAs) on nitrogen-doped porous carbon with high metal loading over 4 wt % is reported. The strategy is based on a pyrolysis process of predesigned bimetallic Zn/Co metal-organic frameworks, during which Co can be reduced by carbonization of the organic linker and Zn is selectively evaporated away at high temperatures above 800 °C. The spherical aberration correction electron microscopy and extended X-ray absorption fine structure measurements both confirm the atomic dispersion of Co atoms stabilized by as-generated N-doped porous carbon. Surprisingly, the obtained Co-Nx single sites exhibit superior ORR performance with a half-wave potential (0.881 V) that is more positive than commercial Pt/C (0.811 V) and most reported non-precious metal catalysts. Durability tests revealed that the Co single atoms exhibit outstanding chemical stability during electrocatalysis and thermal stability that resists sintering at 900 °C. Our findings open up a new routine for general and practical synthesis of a variety of materials bearing single atoms, which could facilitate new discoveries at the atomic scale in condensed materials.

Syntheses of Water-Soluble Octahedral, Truncated Octahedral, and Cubic Pt–Ni Nanocrystals and Their Structure–Activity Study in Model Hydrogenation Reactions

We developed a facile strategy to synthesize a series of water-soluble Pt, Pt(x)Ni(1-x) (0 < x < 1), and Ni nanocrystals. The octahedral, truncated octahedral, and cubic shapes were uniformly controlled by varying crystal growth inhibition agents such as benzoic acid, aniline, and carbon monoxide. The compositions of the Pt(x)Ni(1-x) nanocrystals were effectively controlled by choice of ratios between the Pt and Ni precursors. In a preliminary study to probe their structure-activity dependence, we found that the shapes, compositions, and capping agents strongly influence the catalyst performances in three model heterogeneous hydrogenation reactions.

Hollow Zn/Co ZIF Particles Derived from Core–Shell ZIF‐67@ZIF‐8 as Selective Catalyst for the Semi‐Hydrogenation of Acetylene

The rational design of metal-organic frameworks (MOFs) with hollow features and tunable porosity at the nanoscale can enhance their intrinsic properties and stimulates increasing attentions. In this Communication, we demonstrate that methanol can affect the coordination mode of ZIF-67 in the presence of Co(2+) and induces a mild phase transformation under solvothermal conditions. By applying this transformation process to the ZIF-67@ZIF-8 core-shell structures, a well-defined hollow Zn/Co ZIF rhombic dodecahedron can be obtained. The manufacturing of hollow MOFs enables us to prepare a noble metal@MOF yolk-shell composite with controlled spatial distribution and morphology. The enhanced gas storage and porous confinement that originate from the hollow interior and coating of ZIF-8 confers this unique catalyst with superior activity and selectivity toward the semi-hydrogenation of acetylene.

Sophisticated Construction of Au Islands on Pt–Ni: An Ideal Trimetallic Nanoframe Catalyst

We have developed a priority-related chemical etching method to transfer the starting Pt-Ni polyhedron to a nanoframe. Utilizing the lower electronegativity of Ni in comparison to Au atoms, in conjunction with the galvanic replacement of catalytically active Au to Ni tops, a unique Au island on a Pt-Ni trimetallic nanoframe is achieved. The design strategy is based on the structural priority mechanism of multimetallic nanocrystals during the synthesis and thus can be generalized to other analogous metal-bimetallic nanocrystal combinations (such as Pd and Cu islands on Pt-Ni nanoframes), which is expected to pave the way for the future development of efficient catalysts.

A Strategy for Designing a Concave Pt–Ni Alloy through Controllable Chemical Etching

This corrosion: Octahedral Pt-Ni alloy nanoparticles (NPs) are converted into concave Pt(3)Ni NPs by a coordination-assisted chemical-etching process. The corroded concave Pt-Ni NPs have a higher density of low-coordinate atoms in steps sites, a decisive property in heterogeneous catalysis.

Ultrathin Icosahedral Pt-Enriched Nanocage with Excellent Oxygen Reduction Reaction Activity

Cost-efficient utilization of Pt in the oxygen reduction reaction (ORR) is of great importance for the potential industrial scale demand of proton-exchange membrane fuel cells. Designing a hollow structure of a Pt catalyst offers a great opportunity to enhance the electrocatalytic performance and maximize the use of precious Pt. Herein we report a routine to synthesize ultrathin icosahedral Pt-enriched nanocages. In detail, the Pt atoms were conformally deposited on the surface of Pd icosahedral seeds, followed by selective removal of the Pd core by a concentrated HNO3 solution. The icosahedral Pt-enriched nanocage that is a few atomic layers thick includes the merits of abundant twin defects, an ultrahigh surface/volume ratio, and an ORR-favored Pt{111} facet, all of which have been demonstrated to be promoting factors for ORR. With a 10 times higher specific activity and 7 times higher mass activity, this catalyst shows more extraordinary ORR activity than the commercial Pt/C. The ORR activity of icosahedral Pt-enriched nanocages outperforms the cubic and octahedral nanocages reported in the literature, demonstrating the superiority of the icosahedral nanocage structure.

Porous Molybdenum Phosphide Nano-Octahedrons Derived from Confined Phosphorization in UIO-66 for Efficient Hydrogen Evolution

Herein, a series of porous nano-structured carbocatalysts have been fused and decorated by Mo-based composites, such as Mo2 C, MoN, and MoP, to form a hybrid structures. Using the open porosity derived from the pyrolysis of metal-organic frameworks (MOFs), the highly dispersive MoO2 small nanoparticles can be deposited in porous carbon by chemical vapor deposition (CVD). Undergoing different treatments of carbonization, nitridation, and phosphorization, the Mo2 C-, MoN-, and MoP-decorated carbocatalysts can be selectively prepared with un-changed morphology. Among these Mo-based composites, the MoP@Porous carbon (MoP@PC) composites exhibited remarkable catalytic activity for the hydrogen evolution reaction (HER) in 0.5 m H2 SO4 aqueous solution versus MoO2 @PC, Mo2 C@PC, and MoN@PC. This study gives a promising family of multifunctional lab-on-a-particle architectures which shed light on energy conversion and fuel-cell catalysis.

Ionic Exchange of Metal–Organic Frameworks to Access Single Nickel Sites for Efficient Electroreduction of CO2

Single-atom catalysts often exhibit unexpected catalytic activity for many important chemical reactions because of their unique electronic and geometric structures with respect to their bulk counterparts. Herein we adopt metal-organic frameworks (MOFs) to assist the preparation of a catalyst containing single Ni sites for efficient electroreduction of CO2. The synthesis is based on ionic exchange between Zn nodes and adsorbed Ni ions within the cavities of the MOF. This single-atom catalyst exhibited an excellent turnover frequency for electroreduction of CO2 (5273 h-1), with a Faradaic efficiency for CO production of over 71.9% and a current density of 10.48 mA cm-2 at an overpotential of 0.89 V. Our findings present some guidelines for the rational design and accurate modulation of nanostructured catalysts at the atomic scale.

Defect-Dominated Shape Recovery of Nanocrystals: A New Strategy for Trimetallic Catalysts

Here we present a shape recovery phenomenon of Pt-Ni bimetallic nanocrystals that is unequivocally attributed to the defect effects. High-resolution electron microscopy revealed the overall process of conversion from concave octahedral Pt3Ni to regular octahedral Pt3Ni@Ni upon Ni deposition. Further experiments and theoretical investigations indicated that the intrinsic defect-dominated growth mechanism allows the site-selective nucleation of a third metal around the defects to achieve the sophisticated design of trimetallic Pt3Ni@M core-shell structures (M = Au, Ag, Cu, Rh). Consideration of geometrical and electronic effects indicated that trimetallic atomic steps in Pt3Ni@M could serve as reactive sites to significantly improve the catalytic performance, and this was corroborated by several model reactions. The synthesis strategy based on our work paves the way for the atomic-level design of trimetallic catalysts.

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.