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 O₂ and catalyze the oxidation of SO₂ 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 O₂ 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 SO₂ and particulate carbon, and is helpful to the development of more efficient policies to mitigate and control haze pollution

Nanosize Effect of Al₂O₃ in Ag/Al₂O₃ Catalyst for the Selective Catalytic Oxidation of Ammonia

Ammonia (NH₃) 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/Al₂O₃ catalysts with different Al₂O₃ particle sizes (micro-Al₂O₃ and nano-Al₂O₃) were prepared and tested for the selective catalytic oxidation of ammonia (NH₃-SCO). It was shown that Ag/nano-Al₂O₃ was much more active than Ag/micro-Al₂O₃ for NH₃-SCO in the low-temperature range. The results of characterization by BET, TEM, NH₃-TPD, XRD, H₂-TPR, UV–vis, and XAFS revealed that Ag/nano-Al₂O₃ possesses much smaller Ag particles and more metallic Ag species (Ag_NPs) and also contains abundant acid sites, which facilitate the adsorption and dissociation of NH₃, therefore resulting in much higher NH₃-SCO activity. In addition, on the basis of in situ DRIFTS, kinetic measurements, and DFT calculation results, we discovered that the NH₃-SCO reaction over Ag/nano-Al₂O₃ follows a reaction pathway we call the N₂^– mechanism

Unexpected Promotion Effect of H₂O on the Selective Catalytic Reduction of NO_<i>x</i> with NH₃ 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

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

A review on the heterogeneous oxidation of SO₂ 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_<i>x</i> with NH₃

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 [Cu­(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 [Cu­(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 [Cu­(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_<i>x</i> with NH₃

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 [Cu­(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 [Cu­(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 [Cu­(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_<i>x</i> with NH₃

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 [Cu­(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 [Cu­(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 [Cu­(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

AuFe₃@Pd/γ-Fe₂O₃ 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/γ-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/γ-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