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
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
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 JahnāTeller 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
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
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 solāgel 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
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 configurationīøthe dynamics of
the electrolyte and, potentially, a future quantitative characterization
of the solid electrolyte interphase layer formation and structural and chemical evolution