15 research outputs found
The Role of CeO<sub>2</sub> as a Gateway for Oxygen Storage over CeO<sub>2</sub>‑Grafted Fe<sub>2</sub>O<sub>3</sub> Composite Materials
The surface grafting of CeO<sub>2</sub> onto Fe<sub>2</sub>O<sub>3</sub> with a 1:5 molar ratio produced
a thermally stable composite
material with greater and faster oxygen storage/release than its separate
constituents. In the composite, CeO<sub>2</sub> and Fe<sub>2</sub>O<sub>3</sub> were intimately contacted by interfacial Ce–O–Fe
bonding, and no solid solutions or mixed Ce and Fe oxides were formed
after heating at 900 °C. The oxygen storage capacity and initial
rate of oxygen release/storage were both increased in the composite
structure by virtue of the Fe<sub>2</sub>O<sub>3</sub> and CeO<sub>2</sub>, respectively. The reduction–oxidation cycles in which
Fe<sub>2</sub>O<sub>3</sub> is reduced via Fe<sub>3</sub>O<sub>4</sub> to Fe metal by CO or H<sub>2</sub> and then reoxidized by O<sub>2</sub> were stabilized by surface-grafting Fe<sub>2</sub>O<sub>3</sub> with CeO<sub>2</sub>. In situ Raman spectra demonstrated that the
surface-grafted CeO<sub>2</sub> acts as an oxygen gateway, activating
the dissociation of O<sub>2</sub> into oxide ions or the recombination
of oxide ions into O<sub>2</sub> and transferring oxide ions to/from
Fe<sub>2</sub>O<sub>3</sub>. Meanwhile, Fe<sub>2</sub>O<sub>3</sub> acts as an oxygen reservoir that expands the O<sub>2</sub> storage
capacity. The composite material was tested in a simulated exhaust
gas stream with lean/rich perturbations (which occur in automotive
three-way catalysts). The synergistic effect of the surface grafting
effectively buffered the system against air-to-fuel ratio fluctuations
Catalytic SO<sub>3</sub> Decomposition Activity and Stability of Pt Supported on Anatase TiO<sub>2</sub> for Solar Thermochemical Water-Splitting Cycles
Pt-loaded anatase
TiO<sub>2</sub> (Pt/TiO<sub>2</sub>-A) was found
to be a highly active and stable catalyst for SO<sub>3</sub> decomposition
at moderate temperatures (∼600 °C), which will prove to
be the key for solar thermochemical water-splitting processes used
to produce H<sub>2</sub>. The catalytic activity of Pt/TiO<sub>2</sub>-A was found to be markedly superior to that of a Pt catalyst supported
on rutile TiO<sub>2</sub> (Pt/TiO<sub>2</sub>-R), which has been extensively
studied at a higher reaction temperature range (≥800 °C);
this superior activity was found despite the two being tested with
similar surface areas and metal dispersions after the catalytic reactions.
The higher activity of Pt on anatase is in accordance with the abundance
of metallic Pt (Pt<sup>0</sup>) found for this catalyst, which favors
the dissociative adsorption of SO<sub>3</sub> and the fast removal
of the products (SO<sub>2</sub> and O<sub>2</sub>) from the surface.
Conversely, Pt was easily oxidized to the much less active PtO<sub>2</sub> (Pt<sup>4+</sup>), with the strong interactions between the
oxide and rutile TiO<sub>2</sub> forming a fully coherent interface
that limited the active sites. A long-term stability test of Pt/TiO<sub>2</sub>-A conducted for 1000 h at 600 °C demonstrated that there
was no indication of noticeable deactivation (activity loss ≤
4%) over the time period; this was because the phase transformation
from anatase to rutile was completely prevented. The small amount
of deactivation that occurred was due to the sintering of Pt and TiO<sub>2</sub> and the loss of Pt under the harsh reaction atmosphere
Macroporous Supported Cu–V Oxide as a Promising Substitute of the Pt Catalyst for Sulfuric Acid Decomposition in Solar Thermochemical Hydrogen Production
The macroporous supported Cu–V oxide prepared
by a novel
dissolution–reprecipitation process was found to be the first
example of a promising substitute of Pt catalysts for sulfuric acid
decomposition at moderate temperatures (∼600 °C), which
is required in solar thermochemical hydrogen production. Stepwise
impregnation of CuÂ(NO<sub>3</sub>)<sub>2</sub> and NH<sub>4</sub>VO<sub>3</sub> onto 3-D ordered mesoporous SiO<sub>2</sub>, and subsequent
heating at 650 °C yielded the deposition of copper pyrovanadate
(Cu<sub>2</sub>V<sub>2</sub>O<sub>7</sub>, melting point: 780 °C)
not only in mesopores but also on the external surface. Thermal aging
at 800 °C caused the congruent melting of Cu<sub>2</sub>V<sub>2</sub>O<sub>7</sub> followed by smooth penetration of the melt into
mesopores and homogeneous covering of cavity walls. Because of the
solubility of SiO<sub>2</sub> into the molten vanadate, dissolution–reprecipitation
should be equilibrated to allow substantial structural conversion
from mesoporous to macroporous SiO<sub>2</sub> frameworks. The resulting
macroporous catalyst consisting of highly dispersed thin layers of
active Cu<sub>2</sub>V<sub>2</sub>O<sub>7</sub> is considered efficient
for catalytic reactions and the mass transfer of reactants and products
in the presence of high-concentration vapors
Platinum Supported on Ta<sub>2</sub>O<sub>5</sub> as a Stable SO<sub>3</sub> Decomposition Catalyst for Solar Thermochemical Water Splitting Cycles
Platinum
supported on Ta<sub>2</sub>O<sub>5</sub> was found to be a very active
and stable catalyst for SO<sub>3</sub> decomposition, which is a key
reaction in solar thermochemical water splitting processes. During
continuous reaction testing at 600 °C for 1,800 h, the Pt/Ta<sub>2</sub>O<sub>5</sub> catalyst showed no noticeable deactivation (activity
loss ≤ 1.5% per 1,000 h). This observed stability is superior
to that of the Pt catalyst supported on anatase TiO<sub>2</sub> developed
in our previous study and to those of Pt catalysts supported on other
SO<sub>3</sub>-resistant metal oxides Nb<sub>2</sub>O<sub>5</sub> and
WO<sub>3</sub>. The higher stability of Pt/Ta<sub>2</sub>O<sub>5</sub> is due to the abundance of metallic Pt (Pt<sup>0</sup>), which favors
the dissociative adsorption of SO<sub>3</sub> and the smooth desorption
of the products (SO<sub>2</sub> and O<sub>2</sub>). This feature is
in accordance with a lower activation energy and a less negative partial
order with respect to O<sub>2</sub>. Pt sintering under the harsh
reaction environment was also suppressed to a significant extent compared
to that observed with the use of other support materials. Although
a small fraction of the Pt particles were observed to have grown to
more than several tens of nanometers in size, nanoparticles smaller
than 5 nm were largely preserved and were found to play a key role
in stable SO<sub>3</sub> decomposition
Local Structures and Catalytic Ammonia Combustion Properties of Copper Oxides and Silver Supported on Aluminum Oxides
The local structures and catalytic
NH<sub>3</sub> combustion properties
of copper oxides (CuO<sub><i>x</i></sub>) and silver (Ag)
catalysts supported on aluminum oxides (Al<sub>2</sub>O<sub>3</sub>) were studied. In order to achieve high catalytic NH<sub>3</sub> combustion activity and high N<sub>2</sub> (low N<sub>2</sub>O/NO)
selectivity, the preparation conditions for impregnated binary catalysts
were optimized. In comparison with the single CuO<sub><i>x</i></sub>/Al<sub>2</sub>O<sub>3</sub> and Ag/Al<sub>2</sub>O<sub>3</sub>, binary CuO<sub><i>x</i></sub>–Ag supported on
Al<sub>2</sub>O<sub>3</sub> showed high performance for catalytic
NH<sub>3</sub> combustion. Among the binary catalysts, sequentially
impregnated CuO<sub><i>x</i></sub>/Ag/Al<sub>2</sub>O<sub>3</sub> exhibited the highest activity and N<sub>2</sub> selectivity.
Because the combustion activity is closely associated with the Ag–Ag
coordination number estimated from Ag K-edge XAFS, highly dispersed
Ag nanoparticles supported on Al<sub>2</sub>O<sub>3</sub> are considered
to play a key role in the low-temperature light-off of NH<sub>3</sub>. CuO<sub><i>x</i></sub>/Ag/Al<sub>2</sub>O<sub>3</sub> also showed higher N<sub>2</sub> (lower NO) selectivity for temperatures
at which NH<sub>3</sub> conversion reached approximately 100%, indicating
that the N<sub>2</sub> is directly produced from the NH<sub>3</sub> combustion reaction over CuO<sub><i>x</i></sub>/Ag/Al<sub>2</sub>O<sub>3</sub>. Based on several analyses, a reaction mechanism
for catalytic NH<sub>3</sub> combustion over CuO<sub><i>x</i></sub>/Ag/Al<sub>2</sub>O<sub>3</sub> was finally suggested
Structures and Catalytic Properties of Cr–Cu Embedded CeO<sub>2</sub> Surfaces with Different Cr/Cu Ratios
The
effects of the Cr/Cu ratios of Cr–Cu-embedded CeO<sub>2</sub> surfaces (0.065 wt % Cu loading) on their local structures
and catalytic activities were studied using experimental and theoretical
approaches. The sample with a weight ratio of Cr/Cu = 1, which was
prepared by wet impregnation followed by subsequent thermal aging
at 900 °C for 25 h, showed catalytic activity higher than that
of the Cu/CeO<sub>2</sub> sample in both CO–O<sub>2</sub> and
CO–NO reactions. The activity of the catalyst was enhanced
by increasing the Cr/Cu ratio. The highest activity occurred for a
Cr/Cu ratio of around 3, and after it had been thermally aged, its
activity was superior to that of Rh/CeO<sub>2</sub>. Having more Cr
than Cu increases the surface concentration of the Cu<sup>+</sup> sites,
which promotes CO adsorption and its reaction with surface O atoms.
As-formed surface O vacancies are filled by the dissociative adsorption
of O<sub>2</sub> and/or NO. At the optimum composition, almost all
of the Ce sites on the outermost layer are replaced by Cr and Cu,
and oxidative chemisorption of CO and NO as carbonate and nitrate/nitrite,
respectively, on the CeO<sub>2</sub> surface becomes difficult. This
situation enables more efficient dissociation of adsorbed NO and faster
desorption of CO<sub>2</sub>, thereby leading to higher catalytic
activity. Isocyanate species (NCO) that form on the Cu<sup>+</sup> sites are a possible reaction intermediate for the CO–NO
reaction
Copper Oxides Supported on Aluminum Oxide Borates for Catalytic Ammonia Combustion
The
catalytic NH<sub>3</sub> combustion properties and local structures
of copper oxides (CuO<sub><i>x</i></sub>) supported on aluminum
oxide borates (Al<sub>20</sub>B<sub>4</sub>O<sub>36</sub>, 10Al<sub>2</sub>O<sub>3</sub>·2B<sub>2</sub>O<sub>3</sub>: 10A2B) were
studied by means of high-angle annular dark-field scanning transmission
electron microscopy, energy dispersive X-ray mapping, X-ray absorption
fine structure, X-ray photoelectron spectroscopy, gas adsorption techniques,
etc. Among the CuO<sub><i>x</i></sub> supported on various
metal oxide materials, CuO<sub><i>x</i></sub>/10A2B exhibited
high catalytic NH<sub>3</sub> combustion activity, highest N<sub>2</sub> (lowest N<sub>2</sub>O·NO) selectivity, and high thermal stability.
Because the combustion activity is closely associated with the reducibility
and dispersion of CuO<sub><i>x</i></sub>, highly dispersed
CuO<sub><i>x</i></sub> nanoparticles on supports are considered
to play a key role in the low temperature light-off of NH<sub>3</sub>. For NO and N<sub>2</sub>O selectivities, the oxidation state of
CuO<sub><i>x</i></sub> and the dissociative species of adsorbed
NH<sub>3</sub> are suggested to be important catalytic combustion
properties, respectively. On the basis of these discussions, the reaction
mechanism of catalytic NH<sub>3</sub> combustion over CuO<sub><i>x</i></sub>/10A2B is described
Redox Dynamics of Rh Supported on ZrP<sub>2</sub>O<sub>7</sub> and ZrO<sub>2</sub> Analyzed by Time-Resolved In Situ Optical Spectroscopy
In situ time-resolved
diffuse reflectance spectroscopy was first
applied to supported Rh catalysts (0.4 wt % Rh/ZrO<sub>2</sub> and
Rh/ZrP<sub>2</sub>O<sub>7</sub>) under dynamic three-way catalysis
conditions fluctuating between fuel-lean and fuel-rich gas atmospheres.
The optical absorption at 650 nm was found to decrease upon lean-to-rich
switching of the gas feed, which led to the reduction of Rh oxide
(Rh<sup>3+</sup>) to metallic Rh (Rh<sup>0</sup>), followed by a reversible
increase upon back switching rich-to-lean. The kinetic analysis suggested
that the reduction of Rh<sup>3+</sup> to Rh<sup>0</sup> was faster
than the reoxidation over Rh/ZrP<sub>2</sub>O<sub>7</sub>, whereas
the reduction was comparable with or slower than the reoxidation over
Rh/ZrO<sub>2</sub>. The activation energy of Rh/ZrP<sub>2</sub>O<sub>7</sub> for the reduction, 13.6 kJ mol<sup>–1</sup>, was smaller
than that for the oxidation, 48.7 kJ mol<sup>–1</sup>, which
contrasted with those of Rh/ZrO<sub>2</sub> (21.4 and 34.1 kJ mol<sup>–1</sup>, respectively). These results were closely associated
with the higher NO reduction activity of Rh/ZrP<sub>2</sub>O<sub>7</sub> than Rh/ZrO<sub>2</sub> under a lean-gas atmosphere because Rh was
more active in the metallic state than in the oxide state. Applying
fast lean–rich perturbation of the gas feed with 1 s intervals
led to an immediate and significant drop of the optical absorption
intensity, suggesting that the reduction of Rh substantially penetrated
to deeper layers under the surface. This study provided the first
in situ evidence for the formation of active metallic Rh species under
high-frequency lean–rich oscillations
Unusual Redox Behavior of Rh/AlPO<sub>4</sub> and Its Impact on Three-Way Catalysis
The
influence of the redox behavior of Rh/AlPO<sub>4</sub> on automotive
three-way catalysis (TWC) was studied to correlate catalytic activity
with thermal stability and metal–support interactions. Compared
with a reference Rh/Al<sub>2</sub>O<sub>3</sub> catalyst, Rh/AlPO<sub>4</sub> exhibited a much higher stability against thermal aging under
an oxidizing atmosphere; further deactivation was induced by a high-temperature
reduction treatment. In situ X-ray absorption fine structure experiments
revealed a higher reducibility of Rh oxide (RhO<sub><i>x</i></sub>) to Rh, and the metal showed a higher tolerance to reoxidation
when supported on AlPO<sub>4</sub> compared with Al<sub>2</sub>O<sub>3</sub>. This unusual redox behavior is associated with an Rh–O–P
interfacial linkage, which is preserved under oxidizing and reducing
atmospheres. Another effect of the Rh–O–P interfacial
linkage was observed for the metallic Rh with an electron-deficient
character. This leads to the decreasing back-donation from Rh <i>d</i>-orbitals to the antibonding π* orbital of chemisorbed
CO or NO, which is a possible reason for the deactivation by high-temperature
reduction treatments. On the other hand, surface acid sites on AlPO<sub>4</sub> promoted oxidative adsorption of C<sub>3</sub>H<sub>6</sub> as aldehyde, which showed a higher reactivity toward O<sub>2</sub>, as well as NO, compared with carboxylate adsorbed on Al<sub>2</sub>O<sub>3</sub>. A precise control of the acid–base character
of the metal phosphate supports is therefore a key to enhance the
catalytic performance of supported Rh catalysts for TWC applications
Rhodium Nanoparticle Anchoring on AlPO<sub>4</sub> for Efficient Catalyst Sintering Suppression
Rhodium
catalysts exhibited higher dispersion with tridymite-type
AlPO<sub>4</sub> supports than with Al<sub>2</sub>O<sub>3</sub> during
thermal aging at 900 °C under an oxidizing atmosphere. The local
structural analysis via X-ray photoelectron spectroscopy, transmission
electron microscopy, X-ray absorption fine structure, and infrared
spectroscopy suggested that the sintering of AlPO<sub>4</sub>-supported
Rh nanoparticles was significantly suppressed because of anchoring
via a Rh–O–P linkage at the interface between the metal
and support. Most of the AlPO<sub>4</sub> surface was terminated by
phosphate P–OH groups, which were converted into a Rh–O–P
linkage when Rh oxide (RhO<sub><i>x</i></sub>) was loaded.
This interaction enables the thin planar RhO<sub><i>x</i></sub> nanoparticles to establish close and stable contact with the
AlPO<sub>4</sub> surface. It differs from Rh–O–Al bonding
in the oxide-supported catalyst Rh/Al<sub>2</sub>O<sub>3</sub>, which
causes undesired solid reactions that yield deactivated phases. The
Rh–O–P interfacial linkage was preserved under oxidizing
and reducing atmospheres, which contrasts with conventional metal
oxide supports that only present the anchoring effect under an oxidizing
atmosphere. These experimental results agree with a density functional
theory optimized coherent interface RhO<sub><i>x</i></sub>/AlPO<sub>4</sub> model