9 research outputs found
Role of Carbonaceous Aerosols in Catalyzing Sulfate Formation
The persistent and
fast formation of sulfate is a primary factor
driving the explosive growth of fine particles and exacerbating China’s
severe haze development. However, the underlying mechanism for the
persistent production of sulfate remains highly uncertain. Here, we
demonstrate that soot is not only a major component of the particulate
matter but also a natural carbocatalyst to activate molecular O<sub>2</sub> and catalyze the oxidation of SO<sub>2</sub> to sulfate under
ambient conditions. Moreover, high relative humidity, typically occurring
in severe haze events, can greatly accelerate the catalytic cycle
by reducing the reaction barriers, leading to faster sulfate production.
The formation pathway of sulfate catalyzed by carbonaceous soot aerosols
uses the ubiquitous O<sub>2</sub> as the ultimate oxidant and can
proceed at night when photochemistry is reduced. The high relative
humidity during haze episodes can further promote the soot-catalyzed
sulfate-producing process. Therefore, this study reveals a missing
and widespread source for the persistent sulfate haze formation in
the open atmosphere, particularly under highly polluted conditions
characterized by high concentrations of both SO<sub>2</sub> and particulate
carbon, and is helpful to the development of more efficient policies
to mitigate and control haze pollution
Nanosize Effect of Al<sub>2</sub>O<sub>3</sub> in Ag/Al<sub>2</sub>O<sub>3</sub> Catalyst for the Selective Catalytic Oxidation of Ammonia
Ammonia
(NH<sub>3</sub>) has potentially harmful effects on human
health and has recently been found to be an important factor in the
formation of haze; thus, its emission control is urgent, especially
during haze pollution periods. In this work, two kinds of Ag/Al<sub>2</sub>O<sub>3</sub> catalysts with different Al<sub>2</sub>O<sub>3</sub> particle sizes (micro-Al<sub>2</sub>O<sub>3</sub> and nano-Al<sub>2</sub>O<sub>3</sub>) were prepared and tested for the selective
catalytic oxidation of ammonia (NH<sub>3</sub>-SCO). It was shown
that Ag/nano-Al<sub>2</sub>O<sub>3</sub> was much more active than
Ag/micro-Al<sub>2</sub>O<sub>3</sub> for NH<sub>3</sub>-SCO in the
low-temperature range. The results of characterization by BET, TEM,
NH<sub>3</sub>-TPD, XRD, H<sub>2</sub>-TPR, UV–vis, and XAFS
revealed that Ag/nano-Al<sub>2</sub>O<sub>3</sub> possesses much smaller
Ag particles and more metallic Ag species (Ag<sub>NPs</sub>) and also
contains abundant acid sites, which facilitate the adsorption and
dissociation of NH<sub>3</sub>, therefore resulting in much higher
NH<sub>3</sub>-SCO activity. In addition, on the basis of in situ
DRIFTS, kinetic measurements, and DFT calculation results, we discovered
that the NH<sub>3</sub>-SCO reaction over Ag/nano-Al<sub>2</sub>O<sub>3</sub> follows a reaction pathway we call the N<sub>2</sub><sup>–</sup> mechanism
Promoting Effect of Nitride as Support for Pd Hydrodechlorination Catalyst
Pd-catalyzed reductive decontamination is considerably
promising
in the safe handling of various pollutants, and previous studies on
heterogeneous Pd catalysts have demonstrated the key role of support
in determining their catalysis performance. In this work, metal nitrides
were studied as supports for Pd as a hydrodechlorination (HDC) catalyst.
Density functional theory study showed that a transition metal nitride
(TMN) support could effectively modulate the valence-band state of
Pd. The upward shift of the d-band center reduced the energy barrier
for water desorption from the Pd site to accommodate H2/4-chlorophenol and increased the total energy released during HDC.
The theoretical results were experimentally verified by synthesizing
Pd catalysts onto different metal oxides and the corresponding nitrides.
All studied TMNs, including TiN, Mo2N, and CoN, showed
satisfactorily stabilized Pd and render Pd with high dispersity. In
line with theoretical prediction, TiN most effectively modulated the
electronic states of the Pd sites and enhanced their HDC performance,
with mass activity much higher than those of counterpart catalysts
on other supports. The combined theoretical and experimental results
shows that TMNs, especially TiN, are new and potentially important
support for the highly efficient Pd HDC catalysts
Unexpected Promotion Effect of H<sub>2</sub>O on the Selective Catalytic Reduction of NO<sub><i>x</i></sub> with NH<sub>3</sub> over Cu-SSZ-39 Catalysts
Water molecules commonly inhibit the selective catalytic
reduction
(SCR) of NOx with NH3 on most
catalysts, and water resistance is a long-standing challenge for SCR
technology. Herein, by combining experimental measurements and density
functional theory (DFT) calculations, we found that water molecules
do not inhibit and even promote the NOx conversion to some extent over the Cu-SSZ-39 zeolites, a promising
SCR catalyst. Water acting as a ligand on active Cu sites and as a
reactant in the SCR reaction significantly improves the O2 activation performance and reduces the overall energy barrier of
the catalytic cycle. This work unveils the mechanism of the unexpected
promotion effect of water on the NH3–SCR reaction
over Cu-SSZ-39 and provides fundamental insight into the development
of zeolite-based SCR catalysts with excellent activity and water resistance
A review on the heterogeneous oxidation of SO<sub>2</sub> on solid atmospheric particles: Implications for sulfate formation in haze chemistry
The oxidation of sulfur dioxide (SO2) to sulfate in the atmosphere is an important concern in regional air quality, global climate change, and human health. While gas-phase and liquid-phase oxidation of SO2 are widely regarded as important sources of sulfate, the contribution of the heterogeneous oxidation process on particle surfaces is controversial. Recently, this heterogeneous chemistry has been considered to be an important mechanism that is missing in current models to explain sulfate concentrations observed in haze episodes in East Asia. Therefore, the heterogeneous oxidation of SO2 on particles under the conditions of complex air pollution needs to be reassessed. This review summarizes the fundamental understanding of the heterogeneous reactions of SO2 on solid particles such as mineral dust, black carbon, sea salts, organic aerosol, and so on. The factors affecting the mechanism and kinetics of the heterogeneous reactions of SO2, including coexisting components (O3, NO2, H2O2, NH3, and VOCs), reactive sites, surface properties, relative humidity, and illumination, are reviewed. Reactive oxygen species involved in the heterogeneous oxidation of SO2 on particles are discussed. To our knowledge, while previous reviews have appeared on the oxidation of SO2 in the aqueous-phase, this is the first review on the atmospheric heterogeneous reactions of SO2 on the surface of solid particles, which can be of help in understanding the sulfur cycle in the atmosphere and its environmental impacts. A number of recommendations for future research are also presented.</p
Spatial Distribution of Brønsted Acid Sites Determines the Mobility of Reactive Cu Ions in the Cu-SSZ-13 Catalyst during the Selective Catalytic Reduction of NO<sub><i>x</i></sub> with NH<sub>3</sub>
The formation of dimer-Cu species, which serve as the
active sites
of the low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR), relies
on the mobility of CuI species in the channels of the Cu-SSZ-13
catalysts. Herein, the key role of framework Brønsted acid sites
in the mobility of reactive Cu ions was elucidated via a combination
of density functional theory calculations, in situ impedance spectroscopy, and in situ diffuse reflectance
ultraviolet–visible spectroscopy. When the number of framework
Al sites decreases, the Brønsted acid sites decrease, leading
to a systematic increase in the diffusion barrier for [CuÂ(NH3)2]+ and less formation of highly reactive
dimer-Cu species, which inhibits the low-temperature NH3-SCR reactivity and vice versa. When the spatial distribution of
Al sites is uneven, the [CuÂ(NH3)2]+ complexes tend to migrate from an Al-poor cage to an Al-rich cage
(e.g., cage with paired Al sites), which effectively accelerates the
formation of dimer-Cu species and hence promotes the SCR reaction.
These findings unveil the mechanism by which framework Brønsted
acid sites influence the intercage diffusion and reactivity of [CuÂ(NH3)2]+ complexes in Cu-SSZ-13 catalysts
and provide new insights for the development of zeolite-based catalysts
with excellent SCR activity by regulating the microscopic spatial
distribution of framework Brønsted acid sites
Spatial Distribution of Brønsted Acid Sites Determines the Mobility of Reactive Cu Ions in the Cu-SSZ-13 Catalyst during the Selective Catalytic Reduction of NO<sub><i>x</i></sub> with NH<sub>3</sub>
The formation of dimer-Cu species, which serve as the
active sites
of the low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR), relies
on the mobility of CuI species in the channels of the Cu-SSZ-13
catalysts. Herein, the key role of framework Brønsted acid sites
in the mobility of reactive Cu ions was elucidated via a combination
of density functional theory calculations, in situ impedance spectroscopy, and in situ diffuse reflectance
ultraviolet–visible spectroscopy. When the number of framework
Al sites decreases, the Brønsted acid sites decrease, leading
to a systematic increase in the diffusion barrier for [CuÂ(NH3)2]+ and less formation of highly reactive
dimer-Cu species, which inhibits the low-temperature NH3-SCR reactivity and vice versa. When the spatial distribution of
Al sites is uneven, the [CuÂ(NH3)2]+ complexes tend to migrate from an Al-poor cage to an Al-rich cage
(e.g., cage with paired Al sites), which effectively accelerates the
formation of dimer-Cu species and hence promotes the SCR reaction.
These findings unveil the mechanism by which framework Brønsted
acid sites influence the intercage diffusion and reactivity of [CuÂ(NH3)2]+ complexes in Cu-SSZ-13 catalysts
and provide new insights for the development of zeolite-based catalysts
with excellent SCR activity by regulating the microscopic spatial
distribution of framework Brønsted acid sites
Spatial Distribution of Brønsted Acid Sites Determines the Mobility of Reactive Cu Ions in the Cu-SSZ-13 Catalyst during the Selective Catalytic Reduction of NO<sub><i>x</i></sub> with NH<sub>3</sub>
The formation of dimer-Cu species, which serve as the
active sites
of the low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR), relies
on the mobility of CuI species in the channels of the Cu-SSZ-13
catalysts. Herein, the key role of framework Brønsted acid sites
in the mobility of reactive Cu ions was elucidated via a combination
of density functional theory calculations, in situ impedance spectroscopy, and in situ diffuse reflectance
ultraviolet–visible spectroscopy. When the number of framework
Al sites decreases, the Brønsted acid sites decrease, leading
to a systematic increase in the diffusion barrier for [CuÂ(NH3)2]+ and less formation of highly reactive
dimer-Cu species, which inhibits the low-temperature NH3-SCR reactivity and vice versa. When the spatial distribution of
Al sites is uneven, the [CuÂ(NH3)2]+ complexes tend to migrate from an Al-poor cage to an Al-rich cage
(e.g., cage with paired Al sites), which effectively accelerates the
formation of dimer-Cu species and hence promotes the SCR reaction.
These findings unveil the mechanism by which framework Brønsted
acid sites influence the intercage diffusion and reactivity of [CuÂ(NH3)2]+ complexes in Cu-SSZ-13 catalysts
and provide new insights for the development of zeolite-based catalysts
with excellent SCR activity by regulating the microscopic spatial
distribution of framework Brønsted acid sites
AuFe<sub>3</sub>@Pd/Îł-Fe<sub>2</sub>O<sub>3</sub> Nanosheets as an In Situ Regenerable and Highly Efficient Hydrogenation Catalyst
Heterogenous Pd catalysts play a
pivotal role in the
chemical industry;
however, it is plagued by S2– or other strong adsorbates
inducing surface poisoning long term. Herein, we report the development
of AuFe3@Pd/Îł-Fe2O3 nanosheets
(NSs) as an in situ regenerable and highly active
hydrogenation catalyst. Upon poisoning, the Pd monolayer sites could
be fully and oxidatively regenerated under ambient conditions, which
is initiated by •OH radicals from surface defect/FeTetra vacancy-rich γ-Fe2O3 NSs via the Fenton-like
pathway. Both experimental and theoretical analyses demonstrate that
for the electronic and geometric effect, the 2–3 nm AuFe3 intermetallic nanocluster core promotes the adsorption of
reactant onto Pd sites; in addition, it lowers Pd’s affinity
for •OH radicals to enhance their stability during oxidative
regeneration. When packed into a quartz sand fixed-bed catalyst column,
the AuFe3@Pd/Îł-Fe2O3 NSs are
highly active in hydrogenating the carbon–halogen bond, which
comprises a crucial step for the removal of micropollutants in drinking
water and recovery of resources from heavily polluted wastewater,
and withstand ten rounds of regeneration. By maximizing the use of
ultrathin metal oxide NSs and intermetallic nanocluster and monolayer
Pd, the current study demonstrates a comprehensive strategy for developing
sustainable Pd catalysts for liquid catalysis