5 research outputs found

    Tracking Rh Atoms in Zeolite HY: First Steps of Metal Cluster Formation and Influence of Metal Nuclearity on Catalysis of Ethylene Hydrogenation and Ethylene Dimerization

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    The initial steps of rhodium cluster formation from zeolite-supported mononuclear RhĀ­(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub> complexes in H<sub>2</sub> at 373 K and 1 bar were investigated by infrared and extended X-ray absorption fine structure spectroscopies and scanning transmission electron microscopy (STEM). The data show that ethylene ligands on the rhodium react with H<sub>2</sub> to give supported rhodium hydrides and trigger the formation of rhodium clusters. STEM provided the first images of the smallest rhodium clusters (Rh<sub>2</sub>) and their further conversion into larger clusters. The samples were investigated in a plug-flow reactor as catalysts for the conversion of ethylene + H<sub>2</sub> in a molar ratio of 4:1 at 1 bar and 298 K, with the results showing how the changes in catalyst structure affect the activity and selectivity; the rhodium clusters are more active for hydrogenation of ethylene than the single-site complexes, which are more selective for dimerization of ethylene to give butenes

    Structural and Chemical Evolution of Li- and Mn-Rich Layered Cathode Material

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    Lithium (Li)- and manganese-rich (LMR) layered-structure materials are very promising cathodes for high energy density lithium-ion batteries. However, the voltage fading mechanism in these materials as well as its relationships to fundamental structural changes is far from being sufficiently understood. Here we report the detailed phase transformation pathway in the LMR cathode (LiĀ­[Li<sub>0.2</sub>Ni<sub>0.2</sub>Mn<sub>0.6</sub>]Ā­O<sub>2</sub>) during cycling for samples prepared by the hydrothermal assisted (HA) method. It is found that the transformation pathway of the LMR cathode is closely correlated to its initial structure and preparation conditions. The results reveal that the LMR cathode prepared by the HA approach experiences a phase transformation from the layered structure (initial <i>C</i>2/<i>m</i> phase transforms to <i>R</i>3Ģ…<i>m</i> phase after activation) to a LT-LiCoO<sub>2</sub> type defect spinel-like structure (with the <i>Fd</i>3Ģ…<i>m</i> space group) and then to a disordered rock-salt structure (with the <i>Fm</i>3Ģ…<i>m</i> space group). The voltage fade can be well correlated with Li ion insertion into octahedral sites, rather than tetrahedral sites, in both defect spinel-like and disordered rock-salt structures. The reversible Li insertion/removal into/from the disordered rock-salt structure is ascribed to the Li excess environment that permits Li percolation in the disordered rock-salt structure despite the increased kinetic barrier. Meanwhile, because of the presence of a large quantity of oxygen vacancies, a significant decrease in the Mn valence is detected in the cycled particle, which is below that anticipated for a potentially damaging Jahnā€“Teller distortion (+3.5). Clarification of the phase transformation pathway, cation redistribution, oxygen vacancy and Mn valence change provides unique understanding of the voltage fade and capacity degradation mechanisms in the LMR cathode. The results also inspire us to further enhance the reversibility of the LMR cathode via improved surface structural stability

    Iridium Complexes and Clusters in Dealuminated Zeolite HY: Distribution between Crystalline and Impurity Amorphous Regions

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    Dealuminated zeolite HY was used to support IrĀ­(CO)<sub>2</sub> complexes formed from IrĀ­(CO)<sub>2</sub>(C<sub>5</sub>H<sub>7</sub>O<sub>2</sub>). Infrared and X-ray absorption spectra and atomic resolution electron microscopy images identify these complexes, and the images and <sup>27</sup>Al NMR spectra identify impurity amorphous regions in the zeolite where the iridium is more susceptible to aggregation than in the crystalline regions. The results indicate the value of electron microscopy in characterizing the amorphous impurity regions of zeolites and a significant stability limitation of metals in these regions of zeolite catalyst supports

    Mitigating Voltage Fade in Cathode Materials by Improving the Atomic Level Uniformity of Elemental Distribution

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    Lithium- and manganese-rich (LMR) layered-structure materials are very promising cathodes for high energy density lithium-ion batteries. However, their voltage fading mechanism and its relationships with fundamental structural changes are far from being well understood. Here we report for the first time the mitigation of voltage and energy fade of LMR cathodes by improving the atomic level spatial uniformity of the chemical species. The results reveal that LMR cathodes (LiĀ­[Li<sub>0.2</sub>Ni<sub>0.2</sub>M<sub>0.6</sub>]Ā­O<sub>2</sub>) prepared by coprecipitation and solā€“gel methods, which are dominated by a LiMO<sub>2</sub> type <i>R</i>3Ģ…<i>m</i> structure, show significant nonuniform Ni distribution at particle surfaces. In contrast, the LMR cathode prepared by a hydrothermal assisted method is dominated by a Li<sub>2</sub>MO<sub>3</sub> type <i>C</i>2/<i>m</i> structure with minimal Ni-rich surfaces. The samples with uniform atomic level spatial distribution demonstrate much better capacity retention and much smaller voltage fade as compared to those with significant nonuniform Ni distribution. The fundamental findings on the direct correlation between the atomic level spatial distribution of the chemical species and the functional stability of the materials may also guide the design of other energy storage materials with enhanced stabilities

    Demonstration of an Electrochemical Liquid Cell for Operando Transmission Electron Microscopy Observation of the Lithiation/Delithiation Behavior of Si Nanowire Battery Anodes

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    Over the past few years, in situ transmission electron microscopy (TEM) studies of lithium ion batteries using an open-cell configuration have helped us to gain fundamental insights into the structural and chemical evolution of the electrode materials in real time. In the standard open-cell configuration, the electrolyte is either solid lithium oxide or an ionic liquid, which is point-contacted with the electrode. This cell design is inherently different from a real battery, where liquid electrolyte forms conformal contact with electrode materials. The knowledge learnt from open cells can deviate significantly from the real battery, calling for <i>operando</i> TEM technique with conformal liquid electrolyte contact. In this paper, we developed an operando TEM electrochemical liquid cell to meet this need, providing the configuration of a real battery and in a relevant liquid electrolyte. To demonstrate this novel technique, we studied the lithiation/delithiation behavior of single Si nanowires. Some of lithiation/delithation behaviors of Si obtained using the liquid cell are consistent with the results from the open-cell studies. However, we also discovered new insights different from the open cell configurationī—øthe dynamics of the electrolyte and, potentially, a future quantitative characterization of the solid electrolyte interphase layer formation and structural and chemical evolution
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