NOx Species Research Articles

Insights into the poisoning mechanisms of zinc compounds on V2O5-WO3/TiO2 catalyst for NH3-SCR: Effect of the anion species

V2O5-WO3/TiO2 is commercial catalyst for controlling NOx emissions from municipal solid waste incineration by NH3 Selective Catalytic Reduction (NH3-SCR). However, the flue gas contains a large amount of zinc compounds, which would lead to the deactivation of the catalyst. Therefore, in this work we studied the effect of different zinc species (ZnSO4, ZnO and ZnCl2) on a V2O5-WO3/TiO2 catalyst, investigating the deactivation mechanism. The different compounds negatively affected the activity in the order ZnCl2 >ZnO>ZnSO4, indicating an important influence of the anions. The characterization results points to the evidence that Zn2+ would interact with the active V-OH and OV structure, reducing the content of active Brønsted acid sites and influencing the V5+⇄V4+ redox cycle, as well as inhibiting the activation of NH3. Cl- would interact with V-OH, possibly forming V-Cl and also suppress the formation and mobility of surface adsorbed oxygen, which further exacerbates the negative effect. On the other hand, SO42- would weaken the poisoning effect of Zn2+ by a strong anion/cation interaction and likely form V-O-SO3OH by reaction with V-OH, which is proposed to be an active Brønsted acid site able to active NH3 in the NH3-SCR reaction. Finally, the introduction of Zn2+ increased the adsorption of NOx species to form a stable and inactive nitro species, the formation of which is inhibited by the presence of SO42-.

Influence of the temperature on the reduction mechanism of concentrated nitric acid on a silicon-enriched austenitic stainless steel

Corrosion of silicon-enriched stainless steel in nitric acid strongly depends on the temperature of the electrolyte. Therefore, the reduction reaction of nitric acid on this material was investigated from 40 to 107 °C. For the first time, the autocatalytic reduction mechanism of nitric acid reduction is highlighted by coupling electrochemical measurements (LSW, EIS, Chronoamperometry) with chemical analyses (mass spectrometry, ICP-AES, UV–Vis spectroscopy). This study provides further insights into the nitric acid reduction on an oxidized material compared to that reported earlier on noble metals despite slower kinetics of electrochemical reactions due to the presence of a passivating film. Furthermore, it is shown that HNO2 (or NO+) promotes the autocatalytic reactions in concentrated HNO3 and leads to the formation of NOx species released and detected in the gas phase.

Modular calcination strategy to construct defect-rich nitrogen-doped Nb2O5 for boosting photocatalytic oxidation of cyclohexane to cyclohexanone in solvent-free conditions

The partial oxidation of cyclohexane (CHA) with oxygen to cyclohexanone (CHA-one) is one of the most transformation in the chemical industry in view that CHA-one is an irreplaceable raw material in the production of nylon. Herein, hydrogen, vacuum and air modules combined with ammonia module, respectively, were used to construct defect-rich nitrogen-doped Nb2O5. And, the calcination order and temperature were also considered to establish a modular calcination strategy. A series of modified N-doped Nb2O5 were prepared by the proposed strategy, which were used to catalyze the photo-oxidation of CHA in solvent-free and room temperature conditions. Among these modified N-doped Nb2O5 samples, the HN-Nb2O5 possessed abundant chemical defects of oxygen vacancies and Nb4+, and achieved the best photocatalytic performance. Surprisingly, HN-Nb2O5 achieved an amount of 130.35 μmol for CHA-one, which was approximately 2.2 and 26 times that of the pure Nb2O5 and commercial Nb2O5, respectively, with up to 96.68% selectivity to CHA-one. The outstanding photocatalytic performance of the HN-Nb2O5 can be ascribed to the presence of chemical defect-rich and the NOx species, enhanced light absorption and effective electron-hole separation. DFT calculations explained the promotion of the chemical defects and N doping on Nb2O5 photocatalytic activity in terms of the band electron and band-gap level. The results of EPR analysis and scavenger tests found that e− is the main active species resulted in photocatalytic activity, while O2− is the dominant active species for producing CHA-one in the photo-oxidation of CHA. The present work developed an efficient strategy to construct the defect-rich N-doped metal oxide for the improved photocatalytic oxidation of hydrocarbon.

A measurement and modelling investigation of the indoor air chemistry following cooking activities.

Domestic cooking is a source of indoor air pollutants, including volatile organic compounds (VOCs), which can impact on indoor air quality. However, the real-time VOC emissions from cooking are not well characterised, and similarly, the resulting secondary chemistry is poorly understood. Here, selected-ion flow-tube mass spectrometry (SIFT-MS) was used to monitor the real-time VOC emissions during the cooking of a scripted chicken and vegetable stir-fry meal, in a room scale, semi-realistic environment. The VOC emissions were dominated by alcohols (70% of total emission), but also contained a range of aldehydes (14%) and terpenes (5%), largely attributable to the heating of oil and the preparation and heating of spices, respectively. The direct cooking-related VOC emissions were then simulated using the Indoor Chemical Model in Python (INCHEM-Py), to investigate the resulting secondary chemistry. Modelling revealed that VOC concentrations were dominated by direct emissions, with only a small contribution from secondary products, though the secondary species were longer lived than the directly emitted species. Following cooking, hydroxyl radical concentrations reduced by 86%, while organic peroxy radical levels increased by over 700%, later forming secondary organic nitrates, peroxyacylnitrates (PANs) and formaldehyde. Monoterpene emissions were shown to drive the formation of secondary formaldehyde, albeit to produce relatively modest concentrations (average of 60 ppt). Sensitivity analysis of the simulation conditions revealed that increasing the outdoor concentrations of ozone and NOx species (2.9× and 9×, respectively) resulted in the greatest increase in secondary product formation indoors (≈400%, 200% and 600% increase in organic nitrates, PANs and formaldehyde production, respectively). Given the fact that climate change is likely to result in increased ozone concentrations in the future, and that increased window-opening in response to rising temperatures is also likely, higher concentrations of indoor oxidants are likely in homes in the future. This work, therefore, suggests that cooking could be a more important source of secondary pollutants indoors in the future.

Photo-oxidation of nitrite anions in aqueous solution for the benchmarking of nano-TiO2 photocatalytic coatings

Photocatalytic coatings have potential as an air purifying technology for removal of pollutants such as NOx gases, which are converted into and immobilized as nitrate via photo-oxidation. This work is an investigation of the photocatalytic conversion of nitrite into nitrate in the aqueous phase. The study of this reaction is important to understand the mechanism through which the immobilization of the NOx species as nitrates takes place during NOx conversion, especially in humid environments. We investigate the photocatalytic performance of TiO2 nanoparticles (Aeroxide® P25) and the corresponding photocatalytic coatings prepared using acrylic binder (TiO2 NP-acrylic) towards the photo-conversion of nitrite. The combination of a simple colorimetric method (Griess test) for nitrite quantification and UV–vis spectroscopy for nitrite and nitrate analysis is shown to be a useful method for monitoring the nitrite conversion. Photocatalytic tests with suspended TiO2 nanoparticles show that the nitrite conversion benefits from lower pH, higher temperature and higher concentration of the photocatalyst. The advantage of lower pH is attributed to electrostatic attraction between TiO2 with adsorbed H3O+ and NO2−, which facilitates transport of nitrite to the active surface. When the photocatalyst is immobilized in a coating the factor of primary significance is the pigment volume concentration relative to critical pigment volume concentration (λ), as the nitrite conversion increases linearly with λ in the range 0.6–1.2. This is attributed to the creation of a more porous structure that facilitates the access of nitrite and light to the active TiO2 surface sites.

Influence of Solvent on Selective Catalytic Reduction of Nitrogen Oxides with Ammonia over Cu-CHA Zeolite.

The copper-exchanged zeolite Cu-CHA has received considerable attention in recent years, owing to its application in the selective catalytic reduction (SCR) of NOx species. Here, we study the NH3-SCR reaction mechanism on Cu-CHA using the hybrid quantum mechanical/molecular mechanical (QM/MM) technique and investigate the effects of solvent on the reactivity of active Cu species. To this end, a comparison is made between water- and ammonia-solvated and bare Cu species. The results show the promoting effect of solvent on the oxidation component of the NH3-SCR cycle since the formation of important nitrate species is found to be energetically more favorable on the solvated Cu sites than in the absence of solvent molecules. Conversely, both solvent molecules are predicted to inhibit the reduction component of the NH3-SCR cycle. Diffuse reflectance infrared fourier-transform spectroscopy (DRIFTS) experiments exploiting (concentration) modulation excitation spectroscopy (MES) and phase-sensitive detection (PSD) identified spectroscopic signatures of Cu-nitrate and Cu-nitrosamine (H2NNO), important species which had not been previously observed experimentally. This is further supported by the QM/MM-calculated harmonic vibrational analysis. Additional insights are provided into the reactivity of solvated active sites and the formation of key intermediates including their formation energies and vibrational spectroscopic signatures, allowing the development of a detailed understanding of the reaction mechanism. We demonstrate the role of solvated active sites and their influence on the energetics of important species that must be explicitly considered for an accurate understanding of NH3-SCR kinetics.

The mechanism for the oxidation of benzoquinoline: A theoretical study

The relationship between the coal nitrogen chemistry and the emission of NOx is a topic attracting wide-ranging interest due to its adverse impact on the environment. In this work, a comprehensive reaction network is proposed based on the DFT calculations to show the possible oxidation mechanisms of benzoquinoline that may result in the generation of NOx and/or other nitrogen-containing species. The kinetics of the reaction network is evaluated by Chemkin using the rate parameters computed from the transition state theory (TST). The first important contribution is the consideration of the role of O2 in the oxidation reaction, with the indication that the addition of O2 to N atom and neighboring CN bond that can activate the C(N) species will be more competitive than the addition of O2 to neighboring CC towards the benzoquinoline oxidation. Furthermore, benzoquinoline will be directly oxidated to NO and indirectly to HCN with the help of H radical. The H radical will promote the rupture of the CN bond, and thus accelerate the generation of HCN. The second important contribution is the obtained transition of the reaction mechanism over the temperature range of 1173–1273 K. The increase of the temperature will shift back the low-temperature formed NO to some important intermediates that can lead to the release of HCN. The mechanisms reported are capable of reproducing some general observations that previously did not have a fundamental explanation, and will be important for the future design of the low-NOx combustion technology.

Promotional mechanism of activity of CeEuMnOx ternary oxide for low temperature SCR of NOx

Due to strong synergistic effect of the elements, a series of XEuMnOx ternary oxides (X = Ce, Ni, Co, Sb, Sn, Mo) were synthesized by one-pot co-precipitation method, and composite components were identified and optimized to maintain high activity and superior SO2 and H2O endurance in selective catalytic reduction of NOx with NH3 (NH3-SCR). NOx conversion of CeEuMnOx ternary oxide catalysts attains more than 90% at 100–250 °C, and finally achieves 74% under existence of 50 × 10−6 SO2 and 10 vol% H2O at 230 °C. The facile electron transfer through redox cycle of Mn3+ + Ce4+ ↔ Mn4+ + Ce3+ and enhanced oxygen mobility can promote formation of more Mn species in high oxidation state and chemisorbed oxygen, accelerating oxidation of NO and the adsorbed NO2 formed can facilitate “fast SCR” reaction to improve low-temperature activity. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study reveals that addition of Ce to EuMnOx catalyst boosts adsorption of NH3 and NOx species. NH3 species are activated as crucial intermediate (NH2) to promote NH3-SCR reaction. This research provides a novel material for practical deNOx application of stationary source combustion flue gas in the future.

Promoting effect of basic metal additives on DeNOx reactions over Pt-based three-way catalysts

Pt has been economically favored over Rh and Pd. However, Pt-based three-way catalysts (TWCs) have been studied less than Pd- and Rh-based TWCs; therefore, their properties have not yet been sufficiently evaluated, especially under conditions relevant to modern three-way catalysis operation, wherein deviations from stoichiometric conditions are often encountered. In this study, we examined the promotional effect of basic metal additives in Pt/M/Al2O3 (M = Ba, La, Sr) on the efficiency of three-way catalytic reactions. The catalytic activities of the Pt/M/Al2O3 catalysts for NO, CO, and total hydrocarbons were superior to those of Pt/Al2O3; moreover, Ba enhanced the catalytic activity of Pt-based catalysts more efficiently than La and Sr. The best-performing catalyst, Pt/Ba/Al2O3, was subjected to kinetic studies and various in situ/operando spectroscopic experiments, namely, X-ray absorption spectroscopy, infrared spectroscopy, and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), to investigate the role of Ba in the NO reduction reaction. The Pt0 species loaded on Ba/Al2O3 were more electron-rich than those loaded on Al2O3, thereby promoting NO dissociation into N and O atoms, as revealed by the AP-XPS results. Moreover, the efficient formation of intermediate surface NOx species, including nitrites and nitrates, and their reactivities toward reductant gases, such as H2 or CO, were critical for promoting the effect of Ba on the NO reduction reaction.

Hydrophobic sulfur core–shell layered metallic iron for nitrate reduction with nearly 100% dinitrogen selectivity: Mechanism and field studies

We prepared hydrophobic sulfur (S) core–shell-layered nano-zero-valent iron (Fe) (S-nZVI) via a post-sulfidation method with varying Fe/S mass ratios for NO3− reduction. Notably, S0.125ZVI (Fe/S = 0.125) showed good NO cleavage properties owing to its high electron (e−) transfer efficiency and low surface passivation. As a result, the S0.125ZVI exhibited higher selectivity of NO3− reduction toward N2 than sole nZVI in synthetic and actual NO3− groundwater in batch experiments. Density functional theory (DFT) calculations showed that H2 evolution over S-nZVI was suppressed by the S atom in the hollow site of the Fe(1 1 0) surface, resulting in nearly 100 % denitrification selectivity. Quenching tests revealed that e− transfer through the S atom toward the surface bounded by NOx species is the dominant denitrification mechanism of S-nZVI. Up-flow column tests using actual groundwater were conducted for 127 d, and S0.125ZVI demonstrated a removal capacity of up to 1907 mg-N/g NO3−. Field experiments using S0.125ZVI for NO3−-contaminated groundwater remediation were conducted over four months, confirming that S-nZVI may be an alternative to nZVI for in situ groundwater remediation.

The role of Pt valence state and La doping on titanium supported Pt-La/TiO2 catalyst for selective catalytic reduction with H2

As a promising technology for the removal of NOx at low-temperature, selective catalytic reduction of NOx by hydrogen (H2-SCR) has received attention. Although the H2-SCR removed of NOx more low temperature than the conventional SCR technologies, however, it provides relatively low N2 selectivity. In this study, La were used as a promoter to facilitate NOx to N2 conversion by providing more available NOx species in the Pt based catalysts. The results revealed that the catalyst having high metallic Pt ratio exhibited outstanding NOx conversion. The superior NOx to N2 conversion of La doped over Pt/TiO2 was ascribed to the offering available NOx species. The mechanism of Pt based catalysts were proposed that (1) the formation of NH3 as a intermediate then (2) subsequent selective catalytic reduction or (3) oxidation of ammonia to N2 or N2O. Overall, the proposed La doped Pt/TiO2 catalyst offers significant catalytic performance for H2-SCR.

Mesoporous–optimized BaMnO3 perovskite small-scale nanocrystals in SSZ-13 to enhance activity for the selective catalytic reduction reaction of NO

As one of the cores of ammonia-selective catalytic reduction technology (NH3-SCR), conventional microporous Cu-SSZ-13 has developed hierarchical pores in recent years. The reasonable use of newly mesoporous channels to improve catalytic activity has become a new challenge. We designed novel hierarchical Cu-SSZ-13 catalyst supported with BaMnO3 perovskite, which exhibited excellent low-temperature SCR activity. A series of characterizations illustrated that BaMnO3 was introduced into mesopores located in hierarchical Cu-SSZ-13 and form small-scale nanocrystals of 5–10 nm, which significantly improves the ability to oxidize NO to NO2. In addition, the synergistic effect caused by introduced BaMnO3 and Cu species makes BaMnO3-Cu-SSZ-13 have higher Mn4+ proportion and stronger redox ability. In situ DRIFTS experiments revealed that the introduced BaMnO3 enhanced the reactivity of ammonia and NOx species. Different from the monodentate nitrate observed on Cu-SSZ-13, the reaction intermediate bidentate nitrate and ads-NO2 appeared on BaMnO3-Cu-SSZ-13 in the reaction process, which is also a crucial reason for the higher low-temperature activity.

Promotive effects of Ba addition on lean NOx reduction by CO over IrRu/Al2O3 catalyst

A series of IrRu/Al2O3 based catalysts, where Ba promoter was added in differing amounts were applied for the lean reduction of NOx using CO (CO-SCR), and the promotive effects of Ba addition were thoroughly analyzed. Below 225 °C, IrRu/Al2O3 catalyst exhibits high de-NOx activity, but at high temperature above 250 °C, suffers from drastic decline of NOx conversion. Promotion of Ba enhances the catalytic activity by suppressing NO2 production in the high temperature region, which expands the operation window. Ba species is capable of trapping the NOx species, following its traditional role as a NOx storage compartment. However, when compared to the Ba/IrRu/Al2O3 catalyst, physically mixing Ba compartment with IrRu/Al2O3 catalyst did not lead to substantial promotion. Thus, participation of Ba species during the reaction is not limited to NOx storage, where it is proposed that a continuous regeneration of NOx-trapping Ba sites takes place by direct reaction with the CO reductant catalyzed by IrRu species. It is also revealed that promotion of Ba led to fundamental alterations in IrRu species, by maintaining their active metallic states under oxidizing reaction conditions and modifying their intrinsic activity towards NO bond cleavage, thereby leading to enhanced activity at elevated temperatures. The utilization of Ba/IrRu/Al2O3 catalysts in lean NOx reduction using CO as a reductant is anticipated to enable the facile reduction of NOx over a wide temperature range with high efficiency and stability, while also eliminating the need to employ conventional urea-based reductant, even under oxidizing conditions.

Chronic High-Altitude Hypoxia Alters Iron and Nitric Oxide Homeostasis in Fetal and Maternal Sheep Blood and Aorta.

The mammalian fetus thrives at oxygen tensions much lower than those of adults. Gestation at high altitude superimposes hypoxic stresses on the fetus resulting in increased erythropoiesis. We hypothesized that chronic hypoxia at high altitude alters the homeostasis of iron and bioactive nitric oxide metabolites (NOx) in gestation. To test for this, electron paramagnetic resonance was used to provide unique measurements of iron, metalloproteins, and free radicals in the blood and aorta of fetal and maternal sheep from either high or low altitudes (3801 or 300 m). Using ozone-based chemiluminescence with selectivity for various NOx species, we determined the NOx levels in these samples immediately after collection. These experiments demonstrated a systemic redistribution of iron in high altitude fetuses as manifested by a decrease in both chelatable and total iron in the aorta and an increase in non-transferrin bound iron and total iron in plasma. Likewise, high altitude altered the redox status diversely in fetal blood and aorta. This study also found significant increases in blood and aortic tissue NOx in fetuses and mothers at high altitude. In addition, gradients in NOx concentrations observed between fetus and mother, umbilical artery and vein, and plasma and RBCs demonstrated complex dynamic homeostasis of NOx among these circulatory compartments, such as placental generation and efflux as well as fetal consumption of iron-nitrosyls in RBCs, probably HbNO. In conclusion, these results may suggest the utilization of iron from non-hematopoietic tissues iron for erythropoiesis in the fetus and increased NO bioavailability in response to chronic hypoxic stress at high altitude during gestation.

Electro-reduction of NOx-species in alkaline medium at modified carbon-supported palladium nanoparticles with variable concentration of C-sp2: An in-situ mass-spectrometry approach

Electrochemical denitrification is a promising technology for removing NOx species due to its environmental impact. In this work, nanostructured palladium electrocatalysts (3.2–3.7 nm) stabilized with different compounds namely lignin, oleylamine, octylamine, and 2-methyl-2butanol supported on Vulcan carbon were synthesized from organometallic precursors for the electrocatalytic reduction of nitrated species. For this purpose, electrochemical techniques such as cyclic voltammetry at different concentrations of NaNO2 and NaNO3 (saturated with NO2 gas, synthesized in situ) in the alkaline medium were carried out. The current versus potential characteristic (i-E) showed that the as-synthesized Pd/lignin/C catalyst exhibited the highest activity in the reduction of NO2− and NO3−, with hydrogen production at more negative potentials, as demonstrated by differential electrochemical mass spectroscopy (DEMS). Additionally, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) indicate the presence of metallic palladium and other PdyOx species well distributed on the carbon support matrix promoting different interfacial redox processes depending on the stabilizer in turn and the carbon hybridization namely, D-parameter (D = sp2/sp3).

Unraveling the role of EGR olefins at advanced combustion conditions in the presence of nitric oxide: Ethylene, propene and isobutene

The role of EGR (exhaust gas recirculation) olefinic constituents at advanced combustion conditions in the presence of nitric oxide is unraveled in this study through experimental and modeling efforts using a twin-piston rapid compression machine operating at a stochiometric fuel loading with 20% EGR by mass, pressures of 20 and 40 bar, and temperatures from 680 to 950 K. Five different levels of olefin addition, focusing on ethylene, propene and isobutene, with a fixed amount of NO at 70 ppm are doped into test mixtures of PACE-20, a multi-component gasoline surrogate, where olefin addition effects are characterized through changes in ignition times and heat release rates. Experiments indicate that all three EGR olefins inhibit autoignition reactivity and low-temperature heat release at Tc < 850 K, with isobutene exhibiting the greatest impact, while at Tc > 850 K, low ethylene and propene additions promote reactivity. A recently updated chemical kinetic model, with detailed gasoline/NOx interacting and olefin/NOx interacting chemistry incorporated, is adopted to simulate the experiments. Simulation results are somewhat inconsistent with the experiments, where the model captures the inhibiting effects of all olefins on first-stage ignition reactivity, while consistently predicting a promoting effect on main ignition reactivity. Sensitivity and rate of production analyses reveal that adding olefins greatly alters the role of the consuming pathways for the olefins and their primary derivatives at the initial stage of the oxidation process, particularly with the presence of NO, where the olefins and their derivatives interact with both NOx species such as NO2 and other species such as OH and HO2. The olefin/NOx interactions are particularly pronounced with propene and isobutene addition, where these lead to increased ignition reactivity by facilitating NO production and are mostly responsible for the disagreement between the model and experiments. Further investigations of olefin interacting chemistry, particularly those with NOx species, are needed for chemistry models to accurately predict the complicated effects of EGR.

Unchanged nitrate and nitrite isotope fractionation during heterotrophic and Fe(II)-mixotrophic denitrification suggest a non-enzymatic link between denitrification and Fe(II) oxidation

Natural-abundance measurements of nitrate and nitrite (NOx) isotope ratios (δ15N and δ18O) can be a valuable tool to study the biogeochemical fate of NOx species in the environment. A prerequisite for using NOx isotopes in this regard is an understanding of the mechanistic details of isotope fractionation (15ε, 18ε) associated with the biotic and abiotic NOx transformation processes involved (e.g., denitrification). However, possible impacts on isotope fractionation resulting from changing growth conditions during denitrification, different carbon substrates, or simply the presence of compounds that may be involved in NOx reduction as co-substrates [e.g., Fe(II)] remain uncertain. Here we investigated whether the type of organic substrate, i.e., short-chained organic acids, and the presence/absence of Fe(II) (mixotrophic vs. heterotrophic growth conditions) affect N and O isotope fractionation dynamics during nitrate (NO3–) and nitrite (NO2–) reduction in laboratory experiments with three strains of putative nitrate-dependent Fe(II)-oxidizing bacteria and one canonical denitrifier. Our results revealed that 15ε and 18ε values obtained for heterotrophic (15ε-NO3–: 17.6 ± 2.8‰, 18ε-NO3–:18.1 ± 2.5‰; 15ε-NO2–: 14.4 ± 3.2‰) vs. mixotrophic (15ε-NO3–: 20.2 ± 1.4‰, 18ε-NO3–: 19.5 ± 1.5‰; 15ε-NO2–: 16.1 ± 1.4‰) growth conditions are very similar and fall within the range previously reported for classical heterotrophic denitrification. Moreover, availability of different short-chain organic acids (succinate vs. acetate), while slightly affecting the NOx reduction dynamics, did not produce distinct differences in N and O isotope effects. N isotope fractionation in abiotic controls, although exhibiting fluctuating results, even expressed transient inverse isotope dynamics (15ε-NO2–: –12.4 ± 1.3 ‰). These findings imply that neither the mechanisms ordaining cellular uptake of short-chain organic acids nor the presence of Fe(II) seem to systematically impact the overall N and O isotope effect during NOx reduction. The similar isotope effects detected during mixotrophic and heterotrophic NOx reduction, as well as the results obtained from the abiotic controls, may not only imply that the enzymatic control of NOx reduction in putative NDFeOx bacteria is decoupled from Fe(II) oxidation, but also that Fe(II) oxidation is indirectly driven by biologically (i.e., via organic compounds) or abiotically (catalysis via reactive surfaces) mediated processes co-occurring during heterotrophic denitrification.

Balancing acid and redox sites of phosphorylated CeO2 catalysts for NOx reduction: The promoting and inhibiting mechanism of phosphorus

The role of phosphorus in metal oxide catalysts is still controversial. The precise tuning of the acidic and redox properties of metal oxide catalysts for the selective catalytic reduction in NOx using NH3 is also a great challenge. Herein, CeO2 catalysts with different degrees of phosphorylation were used to study the balance between the acidity and redox property by promoting and inhibiting effects of phosphorus. CeO2 catalysts phosphorylated with lower phosphorus content (5 wt%) exhibited superior NOx reduction performance with above 90% NOx conversion during 240–420 °C due to the balanced acidity and reducibility derived from the highest content of Brønsted acid sites on PO43− to adsorb NH3 and surface adsorbed oxygen species. Plenty of PO3− over CeO2 catalysts phosphorylated with the higher phosphorus content (≥ 10 wt%) significantly disrupted the balance between the acidity and the redox property due to the reduced acid/redox sites, which resulted in the less active NOx species. The mechanism of different structural phosphorus species (PO43− and PO3−) in promoting or inhibiting the NOx reduction over CeO2 catalysts was revealed. This work provides a novel method for qualitative and quantitative study of the relationship between acidity/redox property and activity of catalysts for NOx reduction.

Interrogation of the Plasma-Catalyst Interface via In Situ/Operando Transmission Infrared Spectroscopy.

Plasma-surface coupling has emerged as a promising approach to perform chemical transformations under mild conditions that are otherwise difficult or impossible thermally. However, a few examples of inexpensive and accessible in situ/operando techniques exist for observing plasma-solid interactions, which has prevented a thorough understanding of underlying surface mechanisms. Here, we provide a simple and adaptable design for a dielectric barrier discharge (DBD) plasma cell capable of interfacing with Fourier transform infrared spectroscopy (FTIR), optical emission spectroscopy (OES), and mass spectrometry (MS) to simultaneously characterize the surface, the plasma phase, and the gas phase, respectively. The system was demonstrated using two example applications: (1) plasma oxidation of primary amine functionalized SBA-15 and (2) catalytic low temperature nitrogen oxidation. The results from application (1) provided direct evidence of a 1% O2/He plasma interacting with the aminosilica surface by selective oxidation of the amino groups to nitro groups without altering the alkyl tether. Application (2) was used to detect the evolution of NOX species bound to both platinum and silica surfaces under plasma stimulation. Together, the experimental results showcase the breadth of possible applications for this device and confirm its potential as an essential tool for conducting research on plasma-surface coupling.

Effect of La/Ce modification over Cu based Y zeolite catalysts on high temperature selectivity for selective catalytic reduction with ammonia

CuY zeolite catalyst is considered to be one of most promising catalysts for selective catalytic reduction with ammonia (NH3-SCR), but its main byproduct is the greenhouse gas N2O. To increase N2 selectivity, the modification mode and promotion mechanism of La/Ce doping with two-step ion exchange method were investigated. Impressively, CuLaY achieved above 90% efficiency from 140 °C to 380 °C and less N2O production especially at the temperature above 250 °C. The characterization results showed La/Ce doping increased the content of surface Cu2+ species and improved the oxidation ability of Cu ions in catalysts. However, the formation of La/Ce oxides on the zeolite surface had an adverse impact on the SCR activity, while Ce oxides were more easily formed than La oxides. In addition, more abundant adsorbed NOx species and stronger NH3 adsorption on the surfaces enhanced L-H mechanism for CuLaY with high N2 selectivity above 250 °C.