Zhi Li

Isolated Single Iron Atoms Anchored on N-Doped Porous Carbon as an Efficient Electrocatalyst for the Oxygen Reduction Reaction

The development of low-cost, efficient, and stable electrocatalysts for the oxygen reduction reaction (ORR) is desirable but remains a great challenge. Herein, we made a highly reactive and stable isolated single-atom Fe/N-doped porous carbon (ISA Fe/CN) catalyst with Fe loading up to 2.16 wt %. The catalyst showed excellent ORR performance with a half-wave potential (E1/2 ) of 0.900 V, which outperformed commercial Pt/C and most non-precious-metal catalysts reported to date. Besides exceptionally high kinetic current density (Jk ) of 37.83 mV cm-2 at 0.85 V, it also had a good methanol tolerance and outstanding stability. Experiments demonstrated that maintaining the Fe as isolated atoms and incorporating nitrogen was essential to deliver the high performance. First principle calculations further attributed the high reactivity to the high efficiency of the single Fe atoms in transporting electrons to the adsorbed OH species.

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 Pm3̅m PdIn with single-atom Pd sites shows high selectivity for semihydrogenation of acetylene, whereas the (111) surface of P4/mmm 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.

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.

Structure Evolution and Associated Catalytic Properties of PtSn Bimetallic Nanoparticles

Bimetallic nanoparticles (NPs) often show new catalytic properties that are different from those of the parent metals. Carefully exploring the structures of bimetallic NPs is a prerequisite for understanding the structure-associated properties. Herein, binary PtSn NPs with tunable composition are prepared in a controllable manner. X-ray characterizations reveal that their structures evolve from SnO2-x -patched PtSn alloys to SnO2-x -patched Pt clusters when more tin is incorporated. An obvious composition-dependent catalytic performance is observed for the hydrogenation of α,β-unsaturated aldehydes: the selectivity to unsaturated alcohol increases substantially at high tin content, whereas the reaction rate follows a volcano shape. Furthermore, Pt sites are responsible for hydrogen dissociation, whereas oxygen vacancy (Ovac ) sites, provided by SnO2-x , drastically enhance the adsorption of carbonyl group.

Formation of Hexagonal-Close Packed (HCP) Rhodium as a Size Effect

Previous studies on the structural and functional properties of rhodium are based on the face-centered-cubic (fcc) structure in the bulk form. Here we report the first discovery of the hexagonal-close packed (hcp) rhodium in the nanoparticle form. The hcp Rh can be directly synthesized by solvothermal reaction or by electron-beam induced decomposition of Rh monolayers. The hcp Rh nanoparticles are stable under electron beam irradiation. Compared with the fcc structure, the hcp Rh nanoparticles show a large lattice expansion (6% larger atomic volume). The first-principles calculations suggest that the lower surface energy of hcp Rh leads to the size effect in the crystal structure.

Fe Isolated Single Atoms on S, N Codoped Carbon by Copolymer Pyrolysis Strategy for Highly Efficient Oxygen Reduction Reaction

Heteroatom-doped Fe-NC catalyst has emerged as one of the most promising candidates to replace noble metal-based catalysts for highly efficient oxygen reduction reaction (ORR). However, delicate controls over their structure parameters to optimize the catalytic efficiency and molecular-level understandings of the catalytic mechanism are still challenging. Herein, a novel pyrrole-thiophene copolymer pyrolysis strategy to synthesize Fe-isolated single atoms on sulfur and nitrogen-codoped carbon (Fe-ISA/SNC) with controllable S, N doping is rationally designed. The catalytic efficiency of Fe-ISA/SNC shows a volcano-type curve with the increase of sulfur doping. The optimized Fe-ISA/SNC exhibits a half-wave potential of 0.896 V (vs reversible hydrogen electrode (RHE)), which is more positive than those of Fe-isolated single atoms on nitrogen codoped carbon (Fe-ISA/NC, 0.839 V), commercial Pt/C (0.841 V), and most reported nonprecious metal catalysts. Fe-ISA/SNC is methanol tolerable and shows negligible activity decay in alkaline condition during 15 000 voltage cycles. X-ray absorption fine structure analysis and density functional theory calculations reveal that the incorporated sulfur engineers the charges on N atoms surrounding the Fe reactive center. The enriched charge facilitates the rate-limiting reductive release of OH* and therefore improved the overall ORR efficiency.

Two-dimensional SnO 2 /graphene heterostructures for highly reversible electrochemical lithium storage

The ever-growing market demands for lithium ion batteries have stimulated numerous research efforts aiming at the exploration of novel electrode materials with higher capacity and long-term cycling stability. Two-dimensional (2D) nanomaterials and their heterostructures are an intense area of study and promise great potential in electrochemical lithium storage owing to their unique properties that result from structural planar confinement. Here we report a microwave chemistry strategy to integrate ultrathin SnO2 nanosheets into graphene layer to construct surface-to-surface 2D heterostructured architectures, which can provide unique structural planar confinement for highly reversible electrochemical lithium storage. The as-synthesized 2D SnO2/graphene heterostructures can exhibit high reversible capacity of 688.5 mA h g−1 over 500 cycles with excellent long-term cycling stability and good rate capability when used as anode materials for lithium ion batteries. The present work definitely reveals the advantages of 2D heterostructures featured with a surface-to-surface stack between two different nanosheets in energy storage and conversion devices.摘要对锂离子电池日益增长的市场需求已经引起巨大的研究热情来开发具有更高容量和超长循环性的新型电极材料. 而由于具有独特 的平面结构限域性能, 二维材料及其异质结构目前成为材料研究领域的热点, 并有望在电化学能量储存方面发挥巨大的潜力. 因此, 本文 借助于微波化学策略把超薄SnO2纳米片与石墨烯薄层耦合在一起构筑一种面对面型二维异质结构, 利用其独特的平面结构限域性能来获 得高可逆性的电化学储锂性能. 实验结果表明, 所制备的二维SnO2/石墨烯异质结构表现出良好的电化学性能, 连续500次循环之后其可逆 容量仍能保持在688.5 mA h g−1, 同时也具有很好的倍率性能. 本工作揭示了具有面对面型二维异质结构在能量储存与转化设备应用方面 的明显优势, 为新型电极材料的制备提供了更多选择.

Direct observation of noble metal nanoparticles transforming to thermally stable single atoms

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-N4 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.Pd, Pt and Au nanoparticles are encapsulated in ZIF-8. The progressive atomic dispersion of these PGM atoms as single sites in the N-doped carbon derived from ZIF-8 pyrolysis is observed.

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 (Pt1δ+/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 Pt1δ+/TiO2 originates from atomic dispersion of active species and unique partially positive charge Ptδ+ electronic structure that conventional nanocatalysts do not possess. The fabrication of single-atom Pt1δ+/TiO2 catalyst accomplishes a reasonable use of Pt through recycling and maximum atom-utilized efficiency, indicating the potential to achieve a green hydrosilylation industry.

Constructing NiCo/Fe3O4 Heteroparticles within MOF-74 for Efficient Oxygen Evolution Reactions

Metal-organic frameworks (MOF) have recently emerged as versatile precursors to fabricate functional MOF derivatives for oxygen evolution reactions (OER). Herein, we developed a controlled partial pyrolysis strategy to construct robust NiCo/Fe3O4 heteroparticles within MOF-74 for efficient OER using trimetallic NiCoFe-MOF-74 as precursor. The partial pyrolysis method preserves the framework structure of MOF for effective substrates diffusion while producing highly active nanoparticles. The as-prepared NiCo/Fe3O4/MOF-74 delivered remarkably stable OER current with an overpotential as low as 238 mV at 10.0 mA cm-2 and an Tafel slop of 29 mV/dec, outperforming those of pristine NiCoFe-MOF-74, totally decomposed MOF derivatives, and most reported non-noble metal based electrocatalysts. The key for the formation of NiCo/Fe3O4/MOF-74 nanostructures is that the metals can be decomposed from NiCoFe-MOF-74 in the order of Ni, Co, and Fe under controlled heat treatment. Density functional theory calculations reveals that the underlying NiCo promotes the OER activity of Fe3O4 through exchange stabilization of active oxygen species.