H2O Reaction Research Articles

Comparison of two photolytic calibration methods for nitrous acid

Abstract. Nitrous acid (HONO) plays an important role in tropospheric oxidation chemistry as it is a precursor to the hydroxyl radical (OH). Measurements of HONO have been difficult historically due to instrument interferences and difficulties in sampling and calibration. The traditional calibration method involves generation of HONO by reacting hydrogen chloride vapor with sodium nitrite followed by quantification by various methods (e.g., conversion of HONO to nitric oxide (NO) followed by chemiluminescence detection). Alternatively, HONO can be generated photolytically in the gas phase by reacting NO with OH radicals generated by H2O photolysis. In this work, we describe and compare two photolytic HONO calibration methods that were used to calibrate an iodide adduct chemical ionization mass spectrometer (CIMS). Both methods are based on the water vapor photolysis method commonly used for OH and HO2 (known collectively as HOx) calibrations. The first method is an adaptation of the common chemical actinometry HOx calibration method, in which HONO is calculated based on quantified values for [O3], [H2O], and [O2] and the absorption cross sections for H2O and O2 at 184.9 nm. In the second, novel method HONO is prepared in mostly N2 ([O2]=0.040 %) and is simply quantified by measuring the NO2 formed by the reaction of NO with HO2 generated by H2O photolysis. Both calibration methods were used to prepare a wide range of HONO mixing ratios between ∼400 and 8000 pptv. The uncertainty of the chemical actinometric calibration is 27 % (2σ) and independent of HONO concentration. The uncertainty of the NO2 proxy calibration is concentration-dependent, limited by the uncertainty of the NO2 measurements. The NO2 proxy calibration uncertainties (2σ) presented here range from 4.5 % to 24.4 % (at [HONO] =8000 pptv and [HONO] =630 pptv, respectively) with a 10 % uncertainty associated with a mixing ratio of ∼1600 pptv, typical of values observed in urban areas at night. We also describe the potential application of the NO2 proxy method to calibrating HOx instruments (e.g., LIF, CIMS) at uncertainties below 15 % (2σ).

Fenton process for the treatment of wastewater effluent from the edible oil industry.

The present study intends to investigate the performance of the Fenton reaction as one of the most efficient (AOPs) in a batch mode for treating wastewater effluent from the edible oil industry, as well as the parameters that influence the reaction, such as pH, hydrogen peroxide (H2O2), and ferrous sulfate heptahydrate (FeSO4.7H2O) doses at various reaction times. The response surface methodology (RSM) was applied with a central composite design (CCD) for optimizing the responses of pollutant removals. The obtained results indicated that the authenticated response to the chemical oxygen demand (COD) removal was 93.52%, at optimum values of pH, FeSO4.7H2O dose, H2O2 dose, and reaction time of 3, 1 g/L, 8.38 g/L, and 50 min, respectively. Furthermore, the authenticated response to oil and grease (O&G) removal was 99.8%, at optimum values of pH, FeSO4.7H2O dose, H2O2 dose, and reaction time of 3, 0.71 g/L, 8.7 g/L, and 37.4 min, respectively. Under these conditions, the residual COD and O&G after Fenton oxidation become 155.4 mg/L and 10 mg/L, respectively.

A 6-kW thermally self-sustained two-stage CO2 methanation reactor: design and experimental evaluation of steady-state performance under full-load conditions

This study designed and evaluated a 6-kW, two-stage CO2 methanation reactor, with the aim of effectively recovering the heat generated by the reaction and condensation of the by-product H2O, securing a high CO2 conversion (≥99%) under low pressure without any external thermal input for catalyst temperature control, i.e., thermally self-sustained. The H2 feed was split, with the majority being fed into the first reactor and the remainder into a second reactor. The heat-carrier flow was also split between the stages and, thereby, the second reactor was cooled by supplying only a fraction of the heat-carrier that had been heated in the first reactor. Reactor inner diameters of 16 and 25 mm, as well as parallel and counter flow heat exchange configurations were compared in the second reactor, using a 1-kW test rig. The results showed that the use of a 16-mm reactor diameter and a parallel flow configuration in the second reactor realize a higher total heat recovery with the 99% CO2 conversion. The steady-state performance of the designed 6-kW two-stage reactor was evaluated at 200 kPa (G) under full load. The reactor consequently recovered more than 70% of the total generated heat at 99% CO2 conversion, increasing the heat-carrier temperature from room temperature (inlet heat-carrier temperature) to 137 °C (outlet heat-carrier temperature).

The Reaction of HO2 and CH3O2: CH3OOH Formed from the Singlet Electronic State Surface

High-level coupled-cluster calculations in combination with two-dimensional master equation simulations were used to study the HO2 + CH3O2 reaction, which plays an important role in the oxidation of methane and hydrocarbons in the Earth’s atmosphere and low-temperature combustion. The main reaction pathways taking place on the lowest-lying triplet and singlet potential energy surfaces (PES) were characterized. Interestingly, methyl hydroperoxide (CH3OOH), the sole product, could be produced from both the triplet and singlet PESs, with a ratio of roughly 9:1. Formaldehyde is not made as a primary product, but can be formed via secondary chemistry. The formation of methyl tetraoxide (MTO) from the singlet PES is unimportant. The calculated reaction rate coefficients were found to be practically pressure-independent for p ≤ 760 Torr and can be given by k(T)=2.75×10−13×e+1.75 kcal mol−1/RT (in cm3/s), an expression useful for kinetics modeling over the range T = 200–800 K. The rate constant has a slight negative Arrhenius energy dependence of about −1.75 kcal mol–1, falling about a factor of 30 from 200 K to 800 K.

Insight into the performance of UV/chlorine/TiO2 on carbamazepine degradation: The crucial role of chlorine oxide radical (ClO•)

The UV/chlorine (UC) system is a homogeneous advanced oxidation process with increasing attention in water decontamination. The addition of TiO2 is a newly found strategy to enhance the generation of hydroxyl radical (HO•) and chlorine radical (Cl•) in the UC system. However, the crucial role of chlorine oxide radical (ClO•, generated by the reactions of HO• and Cl• with chlorine) on pollutant degradation, has not been noticed in UV/chlorine/TiO2 (UCT), the heterogeneous photocatalytic system for chlorine activation. Herein, the role of ClO• in UCT was clarified through quenching experiments combined with model simulations during carbamazepine degradation. Tert-butyl alcohol completely inhibited while bicarbonate only partly suppressed carbamazepine degradation in UCT, indicating the important role of ClO•. The second-order reaction rate constant between ClO• and carbamazepine (kClO•,carbamazepine) was fitted to be (1.21 ± 0.08) × 107 M−1 s−1 by the kinetic model, which avoided the influence of carbonate radical (CO3•−), whose contribution couldn't be excluded during kClO•,carbamazepine determination in commonly used competitive kinetic methods with bicarbonate. With the obtained kClO•,carbamazepine, model simulation suggested that ClO• contributed about 50 % to carbamazepine degradation in UCT, and its concentration was less affected under varied conditions (solution pH, chlorine, bicarbonate, and chloride concentration) to keep an efficient carbamazepine degradation. On the contrary, pollutant degradation dominated by HO• in UCT was largely inhibited with the increase of pH, chlorine, and bicarbonate concentration. In addition to the promotion of degradation efficiency, less disinfection byproducts and lower energy requirement were found in UCT compared with UC. Furthermore, UCT could maintain satisfactory degradation efficiency and energy saving in ground water and surface water samples. Results of this study unraveled the crucial role of ClO• for pollutant degradation in UCT, and showed bright prospects and great potentials of the system in water treatment.

Hydrogen evolution reactions between small-sized GanM (M = Ga, Al; n = 0, 1, 2, 3) clusters and water molecules

The reaction mechanism of the hydrogen evolution reaction (HER) between small GanM (M = Ga, Al; n = 0, 1, 2, 3) clusters and water molecules has been systematically studied. The ground state structures of GanM clusters and GanM@H2O (M = Ga, Al; n = 0, 1, 2, 3) complexes and the reactions pathway of GanM@H2O are predicted. Our research results show that the Gan + H2O (n = 1, 2, 3, 4) and Al + H2O reactions are all endothermic reactions, while the GanAl + H2O (n = 1, 2, 3) reactions are all exothermic reactions. Ga3Al + H2O reaction has the most excellent performed in GanM + H2O (M = Ga, Al; n = 0, 1, 2, 3) reactions. To further understand the reaction mechanism of Ga3Al + H2O reaction, the electronic structure of reactant and product of the Ga3Al + H2O are analyzed by NBO analysis method, and the DOS diagram of the product is drawn. Our research results provide a deeper molecular understanding of the chemical reaction of Ga, which may help to develop more efficient Ga doped and Ga-based catalysts.

MIL-68 (In)-derived In2O3@TiO2 S-scheme heterojunction with hierarchical hollow structure for selective photoconversion of CO2 to hydrocarbon fuels

The hierarchical hollow structured photocatalysts of TiO2 nanosheet arrays-decorated In2O3 hexagonal nanotubes (core–shell structural In2O3@TiO2) were successfully fabricated by the in-situ hydrothermal method. Hierarchical hollow structure of In2O3@TiO2 photocatalysts with large specific surface area can improve the light harvesting efficiency and the adsorption capability for CO2 and H2O reactants. The internal electric field in In2O3@TiO2 photocatalysts provides the strong driving force for S-scheme electron transfer from conduction band of TiO2 to valence band of In2O3. The In2O3@TiO2 catalysts exhibited super performance for selectively photocatalytic CO2 conversion with H2O into hydrocarbon fuel (CH4) without sacrificial agent. Among the catalysts, In2O3@TiO2-10 catalyst has the largest yield (11.1 μmol g−1h−1) and selectivity (88.1 %) of CH4 product. Based on results of the characterizations, the mechanism of selective CO2 photoreduction is proposed: the surface enrichment of photogenerated electron induced by the S-scheme In2O3-TiO2 heterojunction is key-step for selective CO2 photoreduction into CH4. This work provides new ideas for the design and construction of high-efficient S-scheme photocatalysts.

Insights into carbon monoxide oxidation in supercritical H2O/CO2 mixtures using reactive molecular dynamics simulations

Carbon monoxide is an important intermediate in supercritical water oxidation processes. Carbon monoxide oxidation in supercritical H2O/CO2 mixtures was investigated using reactive molecular dynamics simulations. The results show that CO mainly converts into CO2 through the reaction CO + OH → HOCO → CO2 + H. The intermediate HOCO can also react with OH or O2 to form CO2. The overall activation energies of CO oxidation in the supercritical medium range from 125.3 ± 4.0–159.4 ± 3.6 kJ/mol, which are roughly consistent with the experimental results. Supercritical water not only provides reactive free radicals for CO conversion but also acts as a third body to induce some chemical events. The high concentration of CO2 inhibits the production of OH but highlights the oxidizing action of O and HO2. Oxygen increases the amounts of OH and HO2 through the reaction H2O + O2, thereby promoting the conversion of CO.

New data on Ho(α,x) reactions and the aspects of 167Tm and 165Er production for medical use

New data on Ho(α,x) reactions and the aspects of 167Tm and 165Er production for medical use

Seasonal dependency of the atmospheric oxidizing capacity of the marine boundary layer of Bermuda

In this study, we investigate the atmospheric oxidation capacity in the marine boundary layer during two measurement campaigns in spring and summer 2019 at the Tudor Hill Marine Atmospheric Observatory, Bermuda. Measured species included non-methane hydrocarbons (NMHCs), HONO, HNO3, particulate matter, particulate nitrates, NO, NO2, O3, in addition to the meteorological parameters. Measured concentrations of all species were generally higher in spring than in summer. During the spring and summer, OH initiation sources made 61% and 42% of the total OH production rate (POH) during the daytime, respectively. The higher levels of NMHCs and OH production from ozone photolysis lead to a higher oxidation rate of NMHCs in the spring compared to summer. However, the low NO in the spring and summer leads to inefficient recycling of RO2 and HO2 to OH, with HO2 reaction with NO accounting for only 22% and 42% of POH during spring and summer. Sensitivity analysis suggests that OH reactivity in the MBL can be potentially enhanced by an oceanic source rather than a secondary oxidation product. Including halogen monoxides, BrO and IO were found to decrease HO2 by ∼14 and 18% during spring and summer, respectively, with marginal impact on OH due to the higher loss by aerosol uptake. The results demonstrate the complex multiphase HOx-Halogen chemistry and call for a more comprehensive multiphase investigation.

Evaluating the response of nitrogen implantation in ZnO ALD thin films and their photocatalytic assessment

The present work aims to contribute to giving light to the photocatalytic process, where compounds that have been anionically doped, are discussed. The columnar growth of ZnO films produced through the atomic layer deposition technique and their further nitrogen doping using a nitrogen plasma source are discussed. Those films were tested as photocatalysts to degrade the complex lignin molecule. The procedure started manufacturing ZnO thin films through self-limited reactions of diethylzinc and H2O on either glass, or Si (100) substrates using an atomic layer deposition facility. In a further stage, using a hydrothermal process, highly oriented columnar nanostructures, were grown. Those structures were nitrogen-doped through a nitrogen plasma discharge in homemade glass reactor. The films were characterized by XRD, XPS, SEM, Raman, photoluminescence, and UV-Vis spectroscopies. The results suggest that the silicon substrate and the zinc acetate precursor provide samples with the best photocatalytic performance. Besides, nitrogen doping increases the percentage of lignin degradation compared to undoped films. The methodology here presented provides a viable way to produce high-quality N-ZnO thin films with potential application in the field of photocatalysis.

The values of ΔfHo298.15 and So298.15 of the radicals formed by the abstraction of H atom from the p‐Benzylphenol and Dimethyl Phthalate

AbstractIn the present work, the thermochemical properties of the radicals (Yj), formed by abstraction of H atom from Dimethyl Phthalate (o‐Ph(C(O)OCH3)2) and p‐Benzylphenol (p‐PhCH2PhOH), are determined at the B3LYP/6‐311++G(d,p) (j = 1), M06‐2X/6‐311++G(d,p) (j = 2), and RO/CBS‐4 M (j = 3 and 4) levels of theory. Their calculated values of Ho298.15(Yj, CALC) are used for the determination of thermochemistry of atomization (ΔrHo298.15(Yj,CALC)atom) and homodesmotic (ΔrHo298.15((RYi)j,CALC)REAC) reactions of considered radicals. The reported calibration dependencies are used for the correction of calculated values and determination of the most consistent and accurate values of ΔfHo298.15 and ΔfGo298.15. It is found that o‐CH3OC(O)PhC(O)OCH2∙ (1C), p‐PhC∙HPhOH (1D) and p‐PhCH2PhO∙ (3D) are the most thermochemically stable structures. The intersection of reported 3SEj (= 3σj) intervals of ΔfHo298.15(1Cj,CORR)atom of atomization results the value of ΔfHo298.15(1C,CORR)MEAN = −405.1 ± 1.3 kJ/mol (±3SEY = 3σY), while, the most accurate values of ΔfHo298.15(1D and 3D,CORR)REACMEAN = 114.7 ± 0.4 and 135.8 ± 5 kJ/mol (± 3SEYREAC = 3σYREAC) are determined using the values ΔfHo298.15((RYi)j,CORR)REAC of homodesmotic reactions. The values of ΔfGo298.15, determined using the values of ΔfHo298.15 and So298.15 of these radicals, demonstrate that the exothermic reaction (17) HO2∙ + p‐PhCH2PhOH → H2O2 + p‐PhC∙HPhOH (–22.9 kJ/mol) is thermochemically more favorable than the reaction (19) HO2∙ + p‐PhCH2PhOH → H2O2 + p‐PhCH2PhO∙ (–1.8 kJ/mol). However, the value of kT(17) = 1.63 × 10–14(T/298.15)3.6e(‐6282/T), calculated using the VTST, is kinetically more favorable at elevated temperatures, while, the reaction (19) (kT(19) = 1.04×10–16(T/298.15)2.17e(–5207/T)) is more important at T < 200 K. The thermochemical properties of these radicals, as well as the calculated rate constants of their formation in reactions of HO2∙ radicals with p‐Benzylphenol, are reported for the first time.

Numerical study on the effects of reaction conditions on single-particle char combustion in O2/H2O atmospheres

The distributions of temperature, reaction rate, and component concentration in char are significant for the stable char-O2/H2O combustion. Based on the single-particle char combustion numerical model in O2/H2O atmospheres, the present work quantitatively studied the effects of reaction conditions on distributions of temperature, reaction rate, and component concentration in char under O2/H2O conditions. The simulation results show that the char surface temperature is always higher than the center temperature in the heating stage. The H2O and O2 concentrations have little effect on the char heating time and heating rate. The char center temperature gradually exceeds the surface temperature after the heating stage, and this gap narrows with the rise in H2O concentration and the decrease in O2 concentration. The C-O2 and C-H2O reaction rates first increase and then decrease along the char radius, and the ratios of the C-H2O reaction rate gradually reduce along the radius. At 1373 K with 5 % O2/20 % H2O, the ratios are below 20 %. The ratios increase significantly with the rise of temperature to 1673 K, especially in the char center, more than 70 %. The H2O and O2 concentrations in char are higher on the surface than in the center. However, the CO and H2 concentrations show the opposite characteristics, and the distribution of CO2 is highly uniform in char. The char-O2/H2O reaction gradually shifts to the diffusion controlled regime with the rise in temperature and O2 concentration, while it slowly turns to the chemical kinetic controlled regime with the increase in H2O concentration.

Understanding of Etching Mechanism of Si3N4 Film in H3PO4 Solution For The Fabrication of 3D NAND Devices

Silicon nitride (Si3N4) has been widely used as an insulating or etch stop layer in the semiconductor devices [1]. For example, in a 3D V-NAND flash memory manufacturing process, Si3N4 is alternately stacked with silicon oxide (SiO2) to form a NAND structure. Here, it is necessary to selectively etch Si3N4 avoiding etching of SiO2 in the repeated Si3N4/SiO2 multi-stack layers. In general, phosphoric acid (H3PO4) is used to selectively etch Si3N4 against SiO2 [2]. However, since the concentration of H3PO4 solution changes with the evaporation of H2O, it is not easy to determine the concentration dependence of Si3N4 etching rate in H3PO4 solution. In order to properly control the shape of the 3D V-NAND structure, it is necessary to extensively understand the effect of H3PO4 concentration on the Si3N4 etching. The study of Si3N4 etching has been mainly focused on the effect of additives in H3PO4 on the behavior of Si3N4 etching to solve the issue such as oxide regrowth so far [3, 4]. Fundamental study of the Si3N4 etching reaction mechanism in H3PO4 is lacking. In this study, the Si3N4 etching mechanism in H3PO4 solution is elucidated systematically.For the investigation of Si3N4 etching reaction mechanism, LPCVD Si3N4 blanket wafer and Si3N4/SiO2 multi-stack layered trench wafer were used. Etching experiments of of Si3N4 were conducted in 10-95 wt% H3PO4 solutions at 160 °C. Spectroscopic ellipsometry and field emission scanning electron microscopy were used to measure the etching rate of Si3N4 after etching process.First, the changes in etching rate of Si3N4 in the various concentrations of H3PO4 solution were investigated. As shown in Fig. 1, according to our kinetic calculation and experiments, etching rate of Si3N4 increased as the H3PO4 concentration increased until the concentration of H3PO4 reaches a certain concentration that produced the highest etching rate. However, etching rate of Si3N4 decreased with the concentration of H3PO4 beyond the critical H3PO4 concentration. According to the results, there may be two concentrations of H3PO4 solution which produce an identical etching rate of Si3N4. It is also suggested that not only H3PO4 but also H2O plays an important role in the Si3N4 etching kinetics.To investigate the role of H2O and H3PO4 in the etching reaction of Si3N4, kinetic isotope effect (KIE) of Si3N4 etching reaction in H3PO4 solution was used. KIE provides information about which reactant determines the rate-limiting step. In this study, each reactant of Si3N4 etching reaction, H2O or H3PO4, was substituted with D2O and D3PO4, respectively. Then, etching of Si3N4 was conducted in four different solutions. As shown in Fig. 2, when H3PO4 was replaced by D3PO4, the Si3N4 etching rate was not significantly changed. However, when H2O was replaced by D2O, the Si3N4 etching rate decreased by 30 %. Therefore, it is thought that rate-limiting step of the Si3N4 etching reaction is determined by an elementary reaction related to H2O rather than H3PO4.Based on the above results, Si3N4 etching reaction mechanism was suggested. First, the Si3N4 surface termination (-NH2) is substituted with H2PO4 -, which is a weak nucleophile, by an SN1-like reaction. When H2PO4 - binds to Si, the backbone of Si3N4 is weakened because the electronegativity of O (3.44) is greater than N (3.04). Next, the Si-N of the weakened backbone of Si3N4 is substituted with Si-OH by an SN2-like reaction of H2O. This SN2-like reaction is considered as a rate-limiting step. It is believed that understanding of etching mechanism of Si3N4 in H3PO4 solution will improve selective etching process of Si3N4 for the integration of 3D V-NAND.

The role of trace N-Oxyl compounds as redox mediator in enhancing antiviral ribavirin elimination in UV/Chlorine process

Ribavirin (RBV) is an antiviral drug used for treating COVID-19 infection. Its release into natural waters would threaten the health of aquatic ecosystem. This study reports an effective approach to degrade RBV by the trace N-oxyl compounds (2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and N-Hydroxyphthalimide (NHPI)) enhanced UV activated free chlorine (UV/Chlorine) process. The results indicated that TEMPO and NHPI at low concentrations (0.1 μM and 1 μM, respectively) could strongly enhance RBV degradation in both deionized water with different pHs and practical surface water. The enhancement was verified to be attributed to the transformation of TEMPO and NHPI into their reactive forms (i.e., TEMPO+ and PINO), which generations deeply relied on radicals. The two N-oxyl compounds inhibit ClO• yield by hindering the reaction of free chlorine vs. HO• and Cl•. The analyses on acute toxicities of RBV degradation products indicate that UV/Chlorine/N-oxyl compounds process can detoxify RBV more efficiently than UV/Chlorine process.

Novel CeOx-modified In2O3 with stabilized Ce3+ states as a highly efficient photocatalyst for photoreduction of CO2 with CH4 or H2O

In this work, CeOx/In2O3 containing mass ratios of 0.1, 0.2 and 0.3 CeOx were prepared via a simple wet impregnation method and their photoactivity was evaluated for CO2 photoreduction with CH4 or H2O. HRTEM verified the formation of an oxide-oxide interface in CeOx/In2O3. XPS analysis revealed the presence of trivalent Ce3+ as the predominant species in CeOx/In2O3 that were formed due to the stabilization of Ce3+ states by In2O3. The presence of a higher content of Ce3+ reflected a strong interfacial contact between CeOx and In2O3. The optimized CeOx/In2O3 showed 4–7 times enhancement as compared to the unmodified In2O3 for CO2 photoreduction with CH4 or H2O reactions. The selectivity of CO and H2 were dependent on the reductant used. The selectivity of CO and H2 for CO2 photoreduction with CH4 remained at 33 % and 66 % after CeOx addition. Contrarily, the selectivity of H2 showed a gradual drop from 61 % to 18 % with an increase in CeOx loading when H2O was used as reductant. The improved activity of CeOx/In2O3 was because of (i) promoted charge transfer and separation due to strong interfacial contact and Type II band alignment, (ii) enhanced light absorption, and (iii) increased basic sites for CO2 adsorption.

Characterization of OH species in kHz He/H2O atmospheric pressure dielectric barrier discharges

This work investigates experimentally and numerically the chemical kinetics of OH species generated in kHz helium atmospheric-pressure dielectric barrier discharges with different H2O concentrations. The OH densities of cases are determined from measurements of the ultraviolet absorption spectroscopy (UVAS) system with transition parameters evaluated by LIFBASE. The 1D plasma fluid model (PFM) with compiled chemistry covering chemical kinetics of helium, H2O, and interaction reactions of reactive species including the formation of charged cluster ions is employed to capture the discharge dynamics for analyzing mechanisms of OH species properly. The simulated current densities of cases with 0.8% and 2.0% H2O concentrations increase from 29 to 51 A m−2, agreeing with experimental measurements. The simulated OH density increases from 1.7 × 1019 m−3 to 2.4 × 1019 m−3 as the H2O concentration increases from 0.4% to 2.0%, agreeing with OH densities interpreted from the UVAS system for cases with various H2O concentrations. In general, the 1D PFM developed captures the discharge behavior and predicts the OH densities of cases with different H2O concentrations. The simulated results reveal that the electron and H2O+ are essential species for OH production through dissociative reactions (e + H2O → e + OH + H) and (H2O+ + H2O → H+(H2O) + OH) contributing to 61% and 23% of OH production, respectively. It is observed that the OH self-recombination reaction (OH + OH + He → H2O2 + He) contributes to 32% of OH consumption as the dominant reaction. Moreover, the stepwise recombination reactions (OH + H2O2 → H2O + HO2 and OH + HO2 → O2 + H2O) initiated by OH species contribute to the overall 27% of OH consumption as another dominant mechanism. The Penning ionization reactions involving water molecules are the dominant reactions for electron production in the present discharge with H2O up to 0.8%, then the electron-impact ionization becomes the dominant reaction for electron production in cases with higher H2O concentrations. As the essential species contributing to the OH consumption, H species is produced majorly from the dissociation reaction (e + H2O → e + OH + H). As one of the dominant OH production reactions, the dissociative attachment (e + H2O → H− + OH) is the dominant electron consumption reaction, resulting in the reactive H− species as the source of negative cluster ions. The high average reaction rates of dissociative attachment reactions consume most of the electrons after the breakdown, which leads to the variation of discharge current density. The saturation of OH species, which is observed experimentally and numerically, is attributed to the transition of electron chemical kinetics and the recombination reactions of OH species.

The reaction characteristics and mechanism of pine sawdust chemical-looping gasification based on CoFe2O4 oxygen carrier

Biomass chemical-looping gasification (BCLG) is a promising technology for high quality syngas production. CoFe2O4 with good redox stability and synergistically enhanced performance is considered as a better oxygen carrier. Herein, the gasification characteristic and mechanism between pine sawdust and CoFe2O4 were studied by using the thermodynamic calculation, thermogravimetric analysis (TGA) and fixed bed reactor experiments. Thermodynamic calculation reveals that the oxygen carrier to pine sawdust ratio, H2O volume fraction and reaction temperature of 0.08, 50%, and 900 °C, respectively, could be optimal for obtaining syngas with high quality and biomass conversion efficiency. CoFe2O4 can provide lattice oxygen for pine sawdust gasification and accelerate the carbon conversion rate of pine sawdust. The existence of Co in CoFe2O4 can synergistically improve the activity of Fe3+ and promote the further reduction of Fe3+ to Fe0. Experimental results verify that pine sawdust gasification process with an oxygen carrier to pine sawdust ratio of 0.08, reaction temperature of 900 °C, and H2O volume fraction of 50% has the highest syngas quality and biomass conversion. After five redox cycles, CoFe2O4 still maintains good reactivity and redox stability. These results can offer useful information for the utilization of biomass energy and can be used to improve the BCLG process.

A potential source of tropospheric secondary organic aerosol precursors: The hydrolysis of N2O5 in water dimer and small clusters of sulfuric acid

The hydrolysis of dinitrogen pentoxide (N2O5) is not only a dominant sink for N2O5 under atmospheric conditions but also a critical route in the formation of acid rains and aerosol clusters. Herein, the hydrolysis of N2O5 assisted by (H2O)2, H2SO4, H2SO4⋯ H2O, and (H2SO4)2 has been investigated theoretically. The calculations show that, as compared with catalysts of H2O and NH3, the addition of (H2O)2, H2SO4, H2SO4⋯ H2O, and (H2SO4)2 in the hydrolysis of N2O5 not only reduces another energy barrier of 8.7–18.2 kcal mol−1, but also plays a more crucial catalytic role in increasing the reaction rate by at least 2 orders of magnitude within the 213–300 K. Pseudo-first-order rate coefficient (kt′) show that (H2SO4)2-assisted reaction is the favorable hydrolysis of N2O5 assisted by small clusters of sulfuric acid (H2SO4 in 108 molecules ⋅ cm−3 and H2O in 100% RH) with its kt′ respectively larger by 2–6 and 1–5 orders of magnitude. Moreover, (H2O)2–assisted N2O5 + H2O reaction dominates over the reaction without and with (H2SO4)2, H2O (100% RH) and NH3 (2900 ppbv) within the 280–300 K and 0 km altitude, whereas (H2SO4)2–assisted reaction can make an important contribution to the hydrolysis of N2O5 under atmospheric conditions of 213–259 K and 5–15 km. This work helps one understand the process of HNO3 formation from the hydrolysis of N2O5 promoted by water clusters and small clusters of sulfuric acid.

Evaluation of Hydroxyl Radical Reactivity by Thioether Group Proximity in Model Peptide Backbone: Methionine versus S-Methyl-Cysteine

Hydroxyl radicals (HO•) have long been regarded as a major source of cellular damage. The reaction of HO• with methionine residues (Met) in peptides and proteins is a complex multistep process. Although the reaction mechanism has been intensively studied, some essential parts remain unsolved. In the present study we examined the reaction of HO• generated by ionizing radiation in aqueous solutions under anoxic conditions with two compounds representing the simplest model peptide backbone CH3C(O)NHCHXC(O)NHCH3, where X = CH2CH2SCH3 or CH2SCH3, i.e., the Met derivative in comparison with the cysteine-methylated derivative. We performed the identification and quantification of transient species by pulse radiolysis and final products by LC–MS and high-resolution MS/MS after γ-radiolysis. The results allowed us to draw for each compound a mechanistic scheme. The fate of the initial one-electron oxidation at the sulfur atom depends on its distance from the peptide backbone and involves transient species of five-membered and/or six-membered ring formations with different heteroatoms present in the backbone as well as quite different rates of deprotonation in forming α-(alkylthio)alkyl radicals.