21 research outputs found
Enhanced Activity of Ti-Modified V<sub>2</sub>O<sub>5</sub>/CeO<sub>2</sub> Catalyst for the Selective Catalytic Reduction of NO<sub><i>x</i></sub> with NH<sub>3</sub>
A novel V<sub>2</sub>O<sub>5</sub>/CeTiO<sub><i>x</i></sub> catalyst showed excellent catalytic
performance in the selective
catalytic reduction (SCR) of NO<sub><i>x</i></sub> with
NH<sub>3</sub>. The addition of Ti into V<sub>2</sub>O<sub>5</sub>/CeO<sub>2</sub> enhanced catalytic activity, N<sub>2</sub> selectivity,
and resistance against SO<sub>2</sub> and H<sub>2</sub>O. These catalysts
were also characterized by N<sub>2</sub> adsorption, XRD, XPS, and
H<sub>2</sub>-TPR. The lower crystallinity, more reduced species,
better dispersion of surface vanadium species, and more acid sites
due to the modification of V<sub>2</sub>O<sub>5</sub>/CeO<sub>2</sub> with TiO<sub>2</sub> all improved the NH<sub>3</sub>–SCR
activity significantly. Based on <i>in situ</i> DRIFTS,
it was concluded that the NH<sub>3</sub>–SCR reaction over
V<sub>2</sub>O<sub>5</sub>/CeTiO<sub><i>x</i></sub> and
V<sub>2</sub>O<sub>5</sub>/CeO<sub>2</sub> mainly followed the Eley–Rideal
mechanism
Improvement of Nb Doping on SO<sub>2</sub> Resistance of VO<sub><i>x</i></sub>/CeO<sub>2</sub> Catalyst for the Selective Catalytic Reduction of NO<sub><i>x</i></sub> with NH<sub>3</sub>
The influence of sulfation treatment
on Nb–VO<sub><i>x</i></sub>/CeO<sub>2</sub> and VO<sub><i>x</i></sub>/CeO<sub>2</sub> catalysts for the selective
catalytic reduction
(SCR) of NO<sub><i>x</i></sub> with NH<sub>3</sub> was fully
investigated. The Nb–VO<sub><i>x</i></sub>/CeO<sub>2</sub> catalyst showed higher catalytic activity and stronger resistance
to SO<sub>2</sub> than VO<sub><i>x</i></sub>/CeO<sub>2</sub>. The formation of sulfates, small specific surface area, and reduction
in the number of active sites were all responsible for the low catalytic
activity over VO<sub><i>x</i></sub>/CeO<sub>2</sub> after
sulfation under SCR conditions. On the contrary, Nb–VO<sub><i>x</i></sub>/CeO<sub>2</sub> adsorbed much more nitrate
than sulfate when sulfated under SCR conditions and showed much higher
NH<sub>3</sub>-SCR activity than VO<sub><i>x</i></sub>/CeO<sub>2</sub> after the same treatment. After sulfation by SO<sub>2</sub> + O<sub>2</sub> only, instead of sulfation under SCR conditions,
both of the samples exhibited decreased NH<sub>3</sub>-SCR activity,
mainly due to the formation of sulfates and the blockage of the Langmuir–Hinshelwood
reaction pathway
Respective Role of Fe and Mn Oxide Contents for Arsenic Sorption in Iron and Manganese Binary Oxide: An X‑ray Absorption Spectroscopy Investigation
In
our previous studies, a synthesized Fe–Mn binary oxide was
found to be very effective for both AsÂ(V) and AsÂ(III) removal in aqueous
phase, because AsÂ(III) could be easily oxidized to AsÂ(V). AsÂ(III)
oxidation and AsÂ(V) sorption by the Fe–Mn binary oxide may
also play an important role in the natural cycling of As, because
of its common occurrence in the environment. In the present study,
the respective role of Fe and Mn contents present in the Fe–Mn
binary oxide on AsÂ(III) removal was investigated via a direct <i>in situ</i> determination of arsenic speciation using X-ray
absorption spectroscopy. X-ray absorption near edge structure results
indicate that Mn atoms exist in a mixed valence state of +3 and +4
and further confirm that MnO<sub><i>x</i></sub> (1.5 < <i>x</i> < 2) content is mainly responsible for oxidizing AsÂ(III)
to AsÂ(V) through a two-step pathway [reduction of MnÂ(IV) to MnÂ(III)
and subsequent MnÂ(III) to MnÂ(II)] and FeOOH content is dominant for
adsorbing the formed AsÂ(V). No significant AsÂ(III) oxidation by pure
FeOOH had been observed during its sorption, when the system was exposed
to air. The extended X-ray absorption fine structure results reveal
that the As surface complex on both the AsÂ(V)- and AsÂ(III)-treated
sample surfaces is an inner-sphere bidentate binuclear corner-sharing
complex with an As–M (M = Fe or Mn) interatomic distance of
3.22–3.24 Å. In addition, the MnO<sub><i>x</i></sub> and FeOOH contents exist only as a mixture, and no solid solution
is formed. Because of its high effectiveness, low cost, and environmental
friendliness, the Fe–Mn binary oxide would play a beneficial
role as both an efficient oxidant of AsÂ(III) and a sorbent for AsÂ(V)
in drinking water treatment and environmental remediation
Supplementary_Material - Inflammatory Responses are Sex Specific in Chronic Hypoxic–Ischemic Encephalopathy
<p>Supplementary_Material for Inflammatory Responses are Sex Specific in Chronic Hypoxic–Ischemic Encephalopathy by Abdullah Al Mamun, Haifu Yu, Sharmeen Romana, and Fudong Liu in Cell Transplantation</p
The Effects of Mn<sup>2+</sup> Precursors on the Structure and Ozone Decomposition Activity of Cryptomelane-Type Manganese Oxide (OMS-2) Catalysts
The effects of Mn<sup>2+</sup> precursors on the structure and
ozone decomposition activity of cryptomelane-type manganese oxide
(OMS-2) catalysts were investigated under high-humidity conditions.
The OMS-2 catalysts were synthesized using a hydrothermal approach.
Characterization of OMS-2 was carried out using X-ray diffraction
(XRD), scanning electron microscopy (SEM), N<sub>2</sub> physical
adsorption, Raman spectroscopy, X-ray absorption fine structure (XAFS),
H<sub>2</sub> temperature-programmed reduction (H<sub>2</sub>-TPR),
and inductively coupled plasma (ICP) spectroscopy. The OMS-2-Ac synthesized
using MnAc<sub>2</sub> as a Mn<sup>2+</sup> precursor showed the best
catalytic activity for ozone decomposition (∼80%) under RH
= 90% and space velocity of 600000 h<sup>–1</sup> and is a
promising catalyst for purifying waste gases containing ozone under
high-humidity conditions. Acetate groups could prevent the aggregation
of manganese oxide particles, which may introduce more crystalline
defects. On the basis of the characterization results, it is supposed
that the greater surface area and higher amount of Mn<sup>3+</sup> are the main factors that contribute to the excellent performance
of OMS-2-Ac. This study can improve our understanding of ozone decomposition
on OMS-2 catalysts and serve as a guide in using OMS-2 for ozone removal
NH<sub>3</sub>‑SCR Performance of Fresh and Hydrothermally Aged Fe-ZSM‑5 in Standard and Fast Selective Catalytic Reduction Reactions
Hydrothermal stability
is one of the challenges for the practical
application of Fe-ZSM-5 catalysts in the selective catalytic reduction
(SCR) of NO with NH<sub>3</sub> (NH<sub>3</sub>-SCR) for diesel engines.
The presence of NO<sub>2</sub> in the exhaust gases can enhance the
deNOx activity because of the fast SCR reaction. In this work, a Fe-ZSM-5
catalyst was prepared by a solid-state ion-exchange method and was
hydrothermally deactivated at 800 °C in the presence of 10% H<sub>2</sub>O. The activity of fresh and hydrothermal aged Fe-ZSM-5 catalysts
was investigated in standard SCR (NO<sub>2</sub>/NO<i>x</i> = 0) and in fast SCR with NO<sub>2</sub>/NO<i>x</i> =
0.3 and 0.5. In standard SCR, hydrothermal aging of Fe-ZSM-5 resulted
in a significant decrease of low-temperature activity and a slight
increase in high-temperature activity. In fast SCR, NO<i>x</i> conversion over aged Fe-ZSM-5 was significantly increased but was
still lower than that over fresh catalyst. Additionally, production
of N<sub>2</sub>O in fast SCR was much more apparent over aged Fe-ZSM-5
than over fresh catalyst. We propose that, in fast SCR, the rate of
key reactions related to NO is slower over aged Fe-ZSM-5 than over
fresh catalyst, thus increasing the probabilities of side reactions
involving the formation of N<sub>2</sub>O
Nature of Ag Species on Ag/γ-Al<sub>2</sub>O<sub>3</sub>: A Combined Experimental and Theoretical Study
The nature of silver species on Ag/Al<sub>2</sub>O<sub>3</sub> catalysts
with different silver loadings was studied by photoelectron spectroscopy
(XPS) and X-ray absorption near-edge spectroscopy (XANES) and extended
X-ray absorption fine structure spectroscopy (EXAFS) combined with
theoretical calculation (DFT). On the basis of selective catalytic
reduction of NO<sub><i>x</i></sub> by ethanol experiments,
it was found that the optimum silver content varies from 1 wt % to
2 wt %. The supported silver species are predominated by +1 oxidation
state ions attached to surface oxygen atoms (Ag–O) under low
silver loading of 2 wt %, which play a crucial role during the HC-SCR
process. An Ag–Ag shell emerged clearly in analysis of EXAFS
data when silver loading was increased to 2 wt %, which was beneficial
for low-temperature activity. The theoretical models for Ag<sub>n</sub><sup>δ+</sup> species (1 ≤ <i>n</i> ≤
4, both ions and oxidized silver clusters) on alumina were consistent
with the coordination structure analysis by EXAFS. The predominant
silver ions are most likely stabilized at isolated tetrahedral Al
sites (Ag–O–Al<sub>IVb</sub>) on the γ-Al<sub>2</sub>O<sub>3</sub> (110) surface. However, the most reactive silver
ion seems to be anchored on a tricoordinate Al<sub>III</sub> site
(Ag–O–Al<sub>III</sub>). Density of states analysis
revealed that the Ag–O–Al<sub>III</sub> entity might
be a very active silver species in terms of the hybridization of Ag,
O, and Al orbitals to promote its catalytic activity
Excellent Performance of One-Pot Synthesized Cu-SSZ-13 Catalyst for the Selective Catalytic Reduction of NO<sub><i>x</i></sub> with NH<sub>3</sub>
Cu-SSZ-13
samples prepared by a novel one-pot synthesis method
achieved excellent NH<sub>3</sub>–SCR performance and high
N<sub>2</sub> selectivity from 150 to 550 °C after ion exchange
treatments. The selected Cu<sub>3.8</sub>-SSZ-13 catalyst was highly
resistant to large space velocity (800 000 h<sup>–1</sup>) and also maintained high NO<sub><i>x</i></sub> conversion
in the presence of CO<sub>2</sub>, H<sub>2</sub>O, and C<sub>3</sub>H<sub>6</sub> in the simulated diesel exhaust. Isolated Cu<sup>2+</sup> ions located in three different sites were responsible for its excellent
NH<sub>3</sub>–SCR activity. Primary results suggest that the
one-pot synthesized Cu-SSZ-13 catalyst is a promising candidate as
an NH<sub>3</sub>–SCR catalyst for the NO<sub><i>x</i></sub> abatement from diesel vehicles
Significant Promotion Effect of Mo Additive on a Novel Ce–Zr Mixed Oxide Catalyst for the Selective Catalytic Reduction of NO<sub><i>x</i></sub> with NH<sub>3</sub>
A novel Mo-promoted Ce–Zr
mixed oxide catalyst prepared by a homogeneous precipitation method
was used for the selective catalytic reduction (SCR) of NO<sub><i>x</i></sub> with NH<sub>3</sub>. The optimal catalyst showed
high NH<sub>3</sub>-SCR activity, SO<sub>2</sub>/H<sub>2</sub>O durability,
and thermal stability under test conditions. The addition of Mo inhibited
growth of the CeO<sub>2</sub> particle size, improved the redox ability,
and increased the amount of surface acidity, especially the Lewis
acidity, all of which were favorable for the excellent NH<sub>3</sub>-SCR performance. It is believed that the catalyst is promising for
the removal of NO<sub><i>x</i></sub> from diesel engine
exhaust
Accelerating Catalytic Oxyanion Reduction with Inert Metal Hydroxides
Adding CrIII or AlIII salts into
the water
suspension of platinum group metal (PGM) catalysts accelerated oxyanion
pollutant reduction by up to 600%. Our initial attempts of adding
K2CrVIO4, K2CrVI2O7, or KCrIII(SO4)2 into Pd/C enhanced BrO3– reduction
with 1 atm H2 by 6-fold. Instrument characterizations and
kinetic explorations collectively confirmed the immobilization of
reduced CrVI as CrIII(OH)3 on the
catalyst surface. This process altered the ζ-potentials from
negative to positive, thus substantially enhancing the Langmuir–Hinshelwood
adsorption equilibrium constant for BrO3– onto Pd/C by 37-fold. Adding AlIII(OH)3 from
alum at pH 7 achieved similar enhancements. The Cr–Pd/C and
Al–Pd/C showed top-tier efficiency of catalytic performance
(normalized with Pd dosage) among all the reported Pd catalysts on
conventional and nanostructured support materials. The strategy of
adding inert metal hydroxides works for diverse PGMs (palladium and
rhodium), substrates (BrO3– and ClO3–), and support materials (carbon, alumina,
and silica). This work shows a simple, inexpensive, and effective
example of enhancing catalyst activity and saving PGMs for environmental
applications