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

A Low-Temperature Route Triggered by Water Vapor during the Ethanol-SCR of NO<i>x</i> over Ag/Al₂O₃

A negative temperature dependence was found for the selective catalytic reduction of NO<i>x</i> by ethanol (ethanol-SCR) over Ag/Al₂O₃ in the absence of water vapor. Activation energy measurements for this process confirmed that two reaction routes occurred in different temperature ranges. In situ DRIFTS experiments revealed that these temperature-dependent reactions were closely related to the process of the partial oxidation of ethanol. During the partial oxidation of ethanol at low temperatures below 400 °C, enolic species and acetates were produced, the former of which exhibited much higher activity for NO<i>x</i> reduction than the latter. Therefore, the formation of enolic species and their further transformation to produce N₂ governs the low-temperature route for ethanol-SCR. At temperatures above 400 °C, only acetate appeared during the partial oxidation of ethanol, and its further reaction with NO<i>x</i> accounts for the high-temperature route. More importantly, introduction of water vapor significantly enhanced the deNO<i>x</i> activity of Ag/Al₂O₃ for ethanol-SCR, especially in the low-temperature region. On pure Al₂O₃, however, the ethanol-SCR process was suppressed by the presence of water vapor, indicating that the promotion effect of water vapor is closely related to silver. Within the low-temperature region, water addition promoted the partial oxidation of ethanol to produce enolic species, the occurrence of which also enhanced the formation of NO₂ during the ethanol-SCR over Ag/Al₂O₃. The produced NO₂ in turn accelerated the formation of enolic species and also exhibited a higher reactivity toward enolic species compared with NO. Such synergistic effects of NO₂ and enolic species induced by water vapor addition thus triggered a cyclic reaction pathway for NO<i>x</i> reduction with high efficiency

Heterogeneous Uptake of Amines by Citric Acid and Humic Acid

Heterogeneous uptake of methylamine (MA), dimethylamine (DMA), and trimethylamine (TMA) onto citric acid and humic acid was investigated using a Knudsen cell reactor coupled to a quadrupole mass spectrometer at 298 K. Acid–base reactions between amines and carboxylic acids were confirmed. The observed uptake coefficients of MA, DMA, and TMA on citric acid at 298 K were measured to be 7.31 ± 1.13 × 10^–3, 6.65 ± 0.49 × 10^–3, and 5.82 ± 0.68 × 10^–3, respectively, and showed independence of sample mass. The observed uptake coefficients of MA, DMA, and TMA on humic acid at 298 K increased linearly with sample mass, and the true uptake coefficients of MA, DMA, and TMA were measured to be 1.26 ± 0.07 × 10^–5, 7.33 ± 0.40 × 10^–6, and 4.75 ± 0.15 × 10^–6, respectively. Citric acid, having stronger acidity, showed a higher reactivity than humic acid for a given amine; while the steric effect of amines was found to govern the reactivity between amines and citric acid or humic acid

Silver Valence State Determines the Water Tolerance of Ag/Al₂O₃ for the H₂–C₃H₆–SCR of NO<i>_x</i>

The influence of the silver valence state on Ag/Al₂O₃ on the water tolerance of H₂–C₃H₆–SCR of NO<i>_x</i> was investigated. The valence state of silver species on Ag/Al₂O₃, which was carefully characterized by XPS, UV–vis, and XANES measurements, was adjusted by varying the calcination temperature from 500 to 900 °C. Oxidized silver species were predominant on Ag/Al₂O₃ calcined at temperatures below 600 °C (LT-catalysts), while further increasing the calcination (temperatures above 600 °C, HT-catalysts) promoted the transformation of oxidized silver species into metallic silver clusters. The samples with higher amounts of oxidized silver species exhibited better water tolerance in the H₂–C₃H₆–SCR. Activation energy measurements confirmed that the mechanism of NO<i>_x</i> reduction on these catalysts was the same. In situ DRIFTS studies demonstrated that metallic silver species promoted the formation of active enolic species and the complete oxidation of formate, thus improving the low-temperature activity of HT-catalysts in the absence of water vapor. Water addition eliminated the formate, releasing the active Ag⁺ sites for enolic species formation, and thus promoted the low-temperature activity of LT-catalysts. From a comprehensive point of view, 60% oxidized silver species on Ag/Al₂O₃ catalysts is the optimal percentage for deNO<i>_x</i> performance and water tolerance

Discerning the Role of Ag–O–Al Entities on Ag/γ-Al₂O₃ Surface in NOx Selective Reduction by Ethanol

Alumina-supported silver catalysts (Ag/Al₂O₃) derived from AlOOH, Al­(OH)₃, and Al₂O₃ were investigated for the selective catalytic reduction of NOx by ethanol. In order to discern the role of support Al skeleton in anchoring silver species and reducing NOx, the series of alumina-supported silver catalysts calcined at different temperatures was characterized by means of <i>in situ</i> DRIFTS, XPS, UV–vis DRS, XRD, BET, and NMR. It was found that the NO_<i>x</i> reduction efficiency order as affected by alumina precursors could be generally described as AlOOH > Al₂O₃ ≫ Al­(OH)₃, with the optimum calcination temperature of 600 °C. XPS and UV–vis results indicated that silver ions predominated on the Ag/Al₂O₃ surface. Solid state NMR suggested that the silver ions might be anchored on Al tetrahedral and octahedral sites, forming Ag–O–Al_tetra and Ag–O–Al_octa entities. With the aid of NMR and DFT calculation, Al_octa was found to be the energetically favorable site to support silver ions. However, DFT calculation indicated that the Ag–O–Al_tetra entity can significantly adsorb and activate vital −NCO species rather than the Ag–O–Al_octa entity. A strongly positive correlation between the amount of Al_tetra structures and N₂ production rate confirms the crucial role of Al_tetra in NOx reduction by ethanol

Role of Organic Carbon in Heterogeneous Reaction of NO₂ with Soot

A large uncertainty among the reported uptake coefficients of NO₂ on soot highlights the importance of the composition of soot in this reaction. Soot samples with different fractions of organic carbon (OC) were prepared by combusting <i>n</i>-hexane under controlled conditions. The heterogeneous reaction of NO₂ on soot was investigated using a flow tube reactor at ambient pressure. The soot with the highest fuel/oxygen ratio showed the largest uptake coefficient (γ_initial) of NO₂ and yield of HONO (<i>y</i>_HONO). Compared to fresh soot samples, preheated samples exhibited a great decrease in uptake coefficient of NO₂ and HONO yield due to the removal of OC from soot. Ozonized soot also showed a lower reactivity toward NO₂ than fresh soot, which can be ascribed to the consumption of OC with a reduced state (OC_R). A linear dependence of the NO₂ uptake coefficient and yields of HONO and NO on the OC_R content of the soot was established, with γ_initial(NO₂) = (1.54 ± 1.39) × 10^–6 + (1.96 ± 0.35) × 10^–7 × OC_R, <i>y</i>_HONO = (11.6 ± 16.1) + (1.3 ± 0.40) × OC_R, and <i>y</i>_NO = (13.1 ± 1.9) – (0.2 ± 0.05) × OC_R, respectively

Enhanced Activity of Ti-Modified V₂O₅/CeO₂ Catalyst for the Selective Catalytic Reduction of NO_<i>x</i> with NH₃

A novel V₂O₅/CeTiO_<i>x</i> catalyst showed excellent catalytic performance in the selective catalytic reduction (SCR) of NO_<i>x</i> with NH₃. The addition of Ti into V₂O₅/CeO₂ enhanced catalytic activity, N₂ selectivity, and resistance against SO₂ and H₂O. These catalysts were also characterized by N₂ adsorption, XRD, XPS, and H₂-TPR. The lower crystallinity, more reduced species, better dispersion of surface vanadium species, and more acid sites due to the modification of V₂O₅/CeO₂ with TiO₂ all improved the NH₃–SCR activity significantly. Based on <i>in situ</i> DRIFTS, it was concluded that the NH₃–SCR reaction over V₂O₅/CeTiO_<i>x</i> and V₂O₅/CeO₂ mainly followed the Eley–Rideal mechanism

Bacterial diversity of bacteriomes and organs of reproductive, digestive and excretory systems in two cicada species (Hemiptera: Cicadidae)

<div><p>Cicadas form intimate symbioses with bacteria to obtain nutrients that are scarce in the xylem fluid they feed on. The obligate symbionts in cicadas are purportedly confined to specialized bacteriomes, but knowledge of bacterial communities associated with cicadas is limited. Bacterial communities in the bacteriomes and organs of reproductive, digestive and excretory systems of two cicada species (<i>Platypleura kaempferi</i> and <i>Meimuna mongolica</i>) were investigated using different methods, and the bacterial diversity and distribution patterns of dominant bacteria in different tissues were compared. Within each species, the bacterial communities of testes are significantly different from those of bacteriomes and ovaries. The dominant endosymbiont <i>Candidatus</i> Sulcia muelleri is found not only in the bacteriomes and reproductive organs, but also in the “filter chamber + conical segment” of both species. The transmission mode of this endosymbiont in the alimentary canal and its effect on physiological processes merits further study. A novel bacterium of Rhizobiales, showing ~80% similarity to <i>Candidatus</i> Hodgkinia cicadicola, is dominant in the bacteriomes and ovaries of <i>P</i>. <i>kaempferi</i>. Given that the genome of <i>H</i>. <i>cicadicola</i> exhibits rapid sequence evolution, it is possible that this novel bacterium is a related endosymbiont with beneficial trophic functions similar to that of <i>H</i>. <i>cicadicola</i> in some other cicadas. Failure to detect <i>H</i>. <i>cicadicola</i> in <i>M</i>. <i>mongolica</i> suggests that it has been subsequently replaced by another bacterium, a yeast or gut microbiota which compensates for the loss of <i>H</i>. <i>cicadicola</i>. The distribution of this novel Rhizobiales species in other cicadas and its identification require further investigation to help establish the definition of the bacterial genus <i>Candidatus</i> Hodgkinia and to provide more information on sequence divergence of related endosymbionts of cicadas. Our results highlight the complex bacterial communities of cicadas, and are informative for further studies of the interactions and co-evolution of insect-microbial symbioses in Cicadoidea.</p></div