12 research outputs found
Capacitive-Controlled Prussian White with a Nickel Iron Hexacyanoferrate Composite Cathode for Rapid Sodium Diffusion
Prussian
blue analogues receive tremendous attention owing to their
spacious three-dimensional skeleton, high theoretical specific capacity,
facile synthesis procedure, and high cost-effectiveness as among the
most promising candidates for cathode materials in sodium-ion batteries
(SIBs). Nonetheless, the practical specific capacity, especially under
high current, is particularly frail due to the sluggish ion diffusion.
In this study, the strategy of Ni substitution and formation of water-coordinated
Fe is applied to lower the crystal field energy and elevate the active
low-spin (LS) Fe content, which leads to a capacitive sodium storage
mechanism, resulting in a substantial specific capacity under high
current density. The delivered specific capacity of PW-325@2NiFe-55
is 95 mAh g–1 at 50 C, which is 72.5% capacity retention
of the one at 0.5 C. Also, it maintains 80.2% of its initial specific
capacity after 500 cycles at 5 C. Furthermore, a hypothesis of a joint
diffusion-controlled and capacitive mechanism for high-spin (HS) Fe
and a mere capacitive mechanism for LS Fe is put forward and verified
through potentiastatic tests, operando 57Fe Mössbauer spectroscopy, and ex situ XRD,
which provides a new horizon to enhance the electrochemical performance
for SIBs
High Activity of Au/γ-Fe<sub>2</sub>O<sub>3</sub> for CO Oxidation: Effect of Support Crystal Phase in Catalyst Design
Au/γ-Fe<sub>2</sub>O<sub>3</sub> and Au/α-Fe<sub>2</sub>O<sub>3</sub> catalysts with identical
size of Au nanoparticles,
chemical state of Au species, and amount of surface OH<sup>–</sup> group were prepared. The Au/γ-Fe<sub>2</sub>O<sub>3</sub> catalyst
exhibited exceptionally high activity, regardless of the heat treatments.
The CO-TPR, sequential pulse reaction, and in situ Raman spectra demonstrate
that the much higher activity of Au/γ-Fe<sub>2</sub>O<sub>3</sub> originated from its higher redox property at low temperature. Systematic
study shows that this higher-redox-property-based higher activity
could be extended to γ-Fe<sub>2</sub>O<sub>3</sub>-supported
Pt-group metals and to other reactions that follow Mars–Van
Krevelen mechanism. This finding may provide a new avenue for catalyst
improvement or development by choosing the suitable crystal phase
of the oxide support
Iodine Ions Mediated Formation of Monomorphic Single-Crystalline Platinum Nanoflowers
Well-defined and strikingly monomorphic single-crystalline
Pt nanoflowers
were successfully synthesized through the addition of a large amount
of iodine ions into polyol process (5 mM H<sub>2</sub>PtCl<sub>6</sub>, 30 mM KI, and 50 mM PVP in ethylene glycol solution at 160 °C).
The detailed structures of the Pt nanoflowers were studied with high-resolution
TEM, indicating that high-quality production of the Pt nanoflowers
could be obtained when the KI concentration was increased to six times
of H<sub>2</sub>PtCl<sub>6</sub>. The size of Pt nanoflowers could
be tuned by changing the concentration of H<sub>2</sub>PtCl<sub>6</sub> with the constant Pt/I ratio (1:6). The formation process of the
nanoflowers was investigated by the UV–vis and EXAFS spectroscopic
studies, demonstrating that the iodine ions played a key role in inducing
the formation of the single-crystalline Pt nanoflowers. After the
addition of iodine ions into the polyol synthesis, the Pt–I
complex was formed and reduced by different kinetics compared with
that of H<sub>2</sub>PtCl<sub>6</sub> to induce the overgrowth of
Pt nanocrystals. Additionally, a small portion of iodine element was
found to be strongly adsorbed on the surfaces of Pt nanoflowers, which
probably also favored the anisotropic overgrowth of Pt nanocrystals
resulting in the single-crystalline Pt nanoflowers. A comprehensive
set of systematic studies on the synthesis factors (the concentrations
of Pt precursor, iodine ions and PVP, reaction temperature, different
kinds of Pt precursors and reaction atmosphere) was also reported
Characterization of α‑Fe<sub>2</sub>O<sub>3</sub>/γ-Al<sub>2</sub>O<sub>3</sub> Catalysts for Catalytic Wet Peroxide Oxidation of <i>m</i>‑Cresol
Fe/<b>γ-</b>Al<sub>2</sub>O<sub>3</sub> catalysts calcined
at different temperatures were evaluated in the catalytic wet peroxide
oxidation (CWPO) of <i>m</i>-cresol. BET, XRD, and <sup>57</sup>Fe Mössbauer spectroscopy (MS) were used to characterize
the catalysts and showed that the calcination temperature changed
the crystal particles of α-Fe<sub>2</sub>O<sub>3</sub> and the
interactions between α-Fe<sub>2</sub>O<sub>3</sub> and the <b>γ-</b>Al<sub>2</sub>O<sub>3</sub> support. The reaction-condition
experiments showed that changing the reaction conditions to higher
reaction temperature and acidic pH promoted the degradation of <i>m</i>-cresol. <i>m</i>-Cresol was degraded completely,
and TOC removal reached 51.0% after CWPO for 2 h at an initial pH
of 4 and a temperature of 60 °C over an Fe/<b>γ-</b>Al<sub>2</sub>O<sub>3</sub> catalyst calcined at 350 °C (Fe/<b>γ-</b>Al<sub>2</sub>O<sub>3</sub>-350). Both reactions for
10 consecutive reaction cycles and for 120 continuous hours in a fixed-bed
reactor showed that the Fe/<b>γ-</b>Al<sub>2</sub>O<sub>3</sub>-350 catalyst was stable and a promising catalyst for heterogeneous
CWPO. The degradation intermediates were identified by GC-MS, and
the possible degradation pathway of <i>m</i>-cresol was
investigated
Synthesis, Characterization, and Catalytic Applications of Core–Shell Magnetic Carbonaceous Nanocomposites
Magnetic
resorcinol/formaldehyde resin (RF) and silica/RF nanocomposites
with well-defined core–shell architecture and tunable structural
parameters had been prepared via the extended Stöber method.
The carbon counterparts with similar structure and morphology can
be obtained by thermal-treating the polymer precursors under N<sub>2</sub> atmosphere. The prepared materials were characterized by
X-ray diffraction, transmission electron microscopy, scanning electron
microscopy, Mössbauer spectroscopy, N<sub>2</sub> physical
adsorption/desorption, and thermogravimetric analysis. The catalytic
applications of the synthesized magnetic nanocomposites were also
explored. It has been shown that the magnetite core was oxidized to
γ-Fe<sub>2</sub>O<sub>3</sub> during polymer coating process
and further reduced to original Fe<sub>3</sub>O<sub>4</sub> phase
during carbonization. In addition, the iron oxide core can react with
the shell when carbonization temperature reaches 700 °C. The
structural stability of magnetic silica/RF is superior to magnetic
RF because of the existence of an inner silica shell. Through a simple
deposition–precipitate method, active platinum nanoparticles
can be loaded in high dispersity onto the surface of the nanocomposites.
The constructed magnetic catalysts are very active in hydrogenation
of nitroarenes to corresponding amines and can be separated facilely
with an external magnetic field
Selectivity-Switchable Conversion of Cellulose to Glycols over Ni–Sn Catalysts
The
direct hydrogenolysis of cellulose represents an attractive
and promising route for green polyol production. Designing a catalyst
system that could control the selectivity of polyols of this process
is highly desirable. In this work, we realized the selectivity-switchable
production of ethylene glycol (EG) and 1,2-propylene glycol (1,2-PG)
by using Sn species with different valences in combination with Ni
catalysts. The combination of Ni/AC and metallic Sn powders exhibited
a superior activity toward EG (57.6%) with up to 86.6% total polyol
yield, while the combination of Ni/AC and SnO favored the formation
of 1,2-PG (32.2%) with a 22.9% yield of EG. The Sn species in NiSn
alloy in situ formed from metallic Ni and Sn powders was found to
be the active sites for the high selectivity of EG as evidenced by
control experiments and characterizations including X-ray diffraction,
X-ray photoelectron spectroscopy, high-resolution transmission electron
microscopy, energy dispersive X-ray mapping, and <sup>119</sup>Sn
Mössbauer spectroscopy. The effects of Sn loading, reaction
temperature, reaction time, and the concentration of cellulose were
investigated for Ni/AC + Sn powders. Because of the formation of NiSn
alloy, the Ni–Sn catalyst showed good stability during repeated
use. Experimental results disclosed that the Sn species with different
valence possessed distinct catalytic functions. Both SnO and the alloyed
Sn species could catalyze the retro-aldol condensation of glucose
to glycolaldehyde, and meanwhile, SnO was also active for the isomerization
of glucose to fructose. Therefore, controlling the glycol products
distribution could be realized using SnO or the alloyed Sn species
as catalysts
<sup>119</sup>Sn Mössbauer and Ferromagnetic Studies on Hierarchical Tin- and Nitrogen-Codoped TiO<sub>2</sub> Microspheres with Efficient Photocatalytic Performance
Hierarchical tin-doped
titania (ST) and tin- and nitrogen-codoped
titania (SNT) microspheres were prepared by a hydrothermal method
and nitriding treatment, in particular, SNT-enhanced visible light
photocatalytic activity for the degradation of rhodamine B in aqueous
solution and room-temperature ferromagnetism (RTFM). We initiated
the photovoltaic study of ST and SNT microspheres to confirm the photovoltaic
properties of perovskite solar cell (PSC) applications. The SNT microspheres
exhibited relatively large saturation magnetization and high photocatalytic
activity. To clarify the enhancement of the photoactivity, the prepared
samples were characterized by <sup>119</sup>Sn Mössbauer spectrometry
under external magnetic fields. The broadened peaks of ST and SNT
microspheres were decomposed into a doublet and a sextet under an
external magnetic field because the samples showed weak ferromagnetism,
although these samples are composed of nonmagnetic components. It
is noteworthy that ST and SNT microspheres exhibits oxygen vacancies
and defect-induced magnetism. The results signify the clear details
of the RTFM and the photocatalytic activity of hierarchical ST and
SNT microspheres
Highly Active and Sintering-Resistant Pt Clusters Supported on FeO<sub><i>x</i></sub>–Hydroxyapatite Achieved by Tailoring Strong Metal–Support Interactions
The catalytic performance of supported metal catalysts
is closely
related to their structure. While Pt-based catalysts are widely used
in many catalytic reactions because of their exceptional intrinsic
activity, they tend to deactivate in high-temperature reactions, requiring
a tedious and expensive regeneration process. The strong metal–support
interaction (SMSI) is a promising strategy to improve the stability
of supported metal nanoparticles, but often at the price of the activity
due to either the coverage of the active sites by support overlay
and/or the too-strong metal–support bonding. Herein, we newly
constructed a supported Pt cluster catalyst by introducing FeOx into hydroxyapatite (HAP) support to fine-tune
the SMSIs. The catalyst exhibited not only high catalytic activity
but also sintering resistance, without deactivation in a 100 h test
for catalytic CO oxidation. Detailed characterizations reveal that
FeOx introduced into HAP weaken the strong
covalent metal–support interaction (CMSI) between Pt and FeOx while simultaneously inhibiting the oxidative
strong metal–support interaction (OMSI) between Pt and HAP,
giving rise to both high activity and thermal stability of the supported
Pt clusters
Strong Metal–Support Interactions between Gold Nanoparticles and Nonoxides
The
strong metal–support interaction (SMSI) is of great
importance for supported catalysts in heterogeneous catalysis. We
report the first example of SMSI between Au nanoparticles (NPs) and
hydroxyapatite (HAP), a nonoxide. The reversible encapsulation of
Au NPs by HAP support, electron transfer, and changes in CO adsorption
are identical to the classic SMSI except that the SMSI of Au/HAP occurred
under oxidative condition; the opposite condition for the classical
SMSI. The SMSI of Au/HAP not only enhanced the sintering resistance
of Au NPs upon calcination but also improved their selectivity and
reusability in liquid-phase reaction. It was found that the SMSI between
Au and HAP is general and could be extended to other phosphate-supported
Au systems such as Au/LaPO<sub>4</sub>. This new discovery may open
a new way to design and develop highly stable supported Au catalysts
with controllable activity and selectivity
Decoration of Gold and Platinum Nanoparticle Catalysts by 1 nm Thick Metal Oxide Overlayer and Its Effect on the CO Oxidation Activity
Exfoliated M–Al layered double hydroxide (M–Al
LDH;
M = Mg, Co, Ni, and Zn) nanosheets were adsorbed on Au/SiO2 and calcined to transform LDH into mixed metal oxides (MMOs) and
yield Au/SiO2 coated with a thin MMO overlayer. These catalysts
showed a higher catalytic activity than pristine Au/SiO2. In particular, the 50% CO conversion temperature decreased by more
than 250 °C for Co–Al MMO-coated Au/SiO2. In
contrast, the deposition of CoAlOx on
Au/SiO2 by impregnation or the deposition of Au on Co–Al
MMO-coated SiO2 resulted in a worse catalytic activity.
Moreover, the presence of a thick MMO overlayer decreased the catalytic
activity, suggesting that the control of the overlayer thickness to
less than 1 nm is a requisite for obtaining a high catalytic activity.
Moreover, the thin Co–Al MMO overlayer on Au/SiO2 possessed abundant oxygen vacancies, which would play an important
role in O2 activation, resulting in a highly active interface
between Au and the defect-rich MMO on the Au NP surface. Finally,
this can be applied to Pt/SiO2, and the obtained Co–Al
MMO-coated Pt/SiO2 also exhibited a much improved catalytic
activity for CO oxidation