Formation Of H2O Research Articles

Quantifying hydroxyl radicals generated by a low-temperature plasma using coumarin: methodology and precautions.

The detection and quantification of hydroxyl radicals (HO˙) generated by low-temperature plasmas (LTPs) are crucial for understanding their role in diverse applications of plasma radiation. In this study, the formation of HO˙ in the irradiated aqueous phase is investigated at various plasma parameters, by probing them indirectly using the coumarin molecule. We propose a quantification methodology for these radicals, combining spectrophotometry to study the coumarin reaction with hydroxyl radicals and fluorimetry to evaluate the formation yield of the hydroxylated product, 7-hydroxycoumarin. Additionally, we thoroughly examine and discuss the impact of pH on this quantification process. This approach enhances our comprehension of HO˙ formation during LTP irradiation, adding valuable insights to plasma's biological applications.

Photocatalytic conversion of CO2 to CO by Ru(II) and Os(II) octahedral complexes: a DFT/TDDFT study.

The reaction mechanisms of the photocatalytic reduction of CO2 to CO catalyzed by [(en)M(CO)3Cl] complexes (M = Ru, Os, en = ethylenediamine) in the presence of triethanolamine (TEOA), R3N (R = -CH2CH2OH), in DCM and DMF solvents, were studied by means of DFT/TDDFT electronic structure calculations. The geometric and free energy reaction profiles for two possible reaction pathways were calculated. Both reaction pathways studied, start with the 17e-, catalytically active intermediate, [(en)M(CO)3]˙+ generated from the first triplet excited state, T1 upon reductive quenching by TEOA which acts as a sacrificial electron donor. In the first possible pathway, TEOA- anion binds to the metal center of the catalytically active intermediate, [(en)M(CO)3]˙+ followed by CO2 insertion into the M-OCH2CH2NR2 bond. The latter upon successive protonations releases a metal 'free' [R2NCH2CH2OC(O)(OH)] intermediate which starts a new and final catalytic cycle, leading to the formation of CO and H2O while regenarating TEOA. In the second possible pathway, the 17e-, catalytically active intermediate, [(en)M(CO)3]˙+ captures CO2 molecule, forming an η1-CO2 complex. Upon 2H+/2e- successive protonations and reductions, CO product is obtained along with regenarating the catalytically active intermediate [(en)M(CO)3]˙+. The nature of the proton donor affects the reaction profiles of both mechanisms. The nature of the solvent does not affect significantly the reaction mechanisms under study. Finally, since photoexcitation and T1 reductive quenching are common to both pathways, we have srutinized the photophysical properties of the [(en)M(CO)3Cl] complexes along with their T1 excited states reduction potentials, . The [(en)M(CO)3Cl] complexes absorb mainly in the UV region while the absolute are in the range 6.4-0.9 eV.

Influence of Acidity, Water and Temperature on the Double Layer Properties of Protic Ionic Liquids for Future Fuel Cell Applications

Polymer electrolyte membrane fuel cells (PEMFC) are a viable alternative to combustion engines and rechargeable batteries for automotive applications. However, the operating temperature of PEMFCs using sulfonated fluoropolymers, e.g. NAFION®, is limited below 80 °C (ambient pressure), because the proton conduction relies on the presence of water. A PEMFC operating above 100 °C would allow a much more simplified system setup for water and heat management. This requires novel non-aqueous protic electrolytes. Proton conducting ionic liquids (PIL) are promising candidates. [1,2]. However, the fuel cell relevant electrode reactions—oxygen reduction and hydrogen oxidation reaction (ORR/HOR)—are not as well understood as in aqueous electrolytes.In this study, we employed electrochemical impedance spectroscopy (EIS), cyclovoltammetry (CV), chronoamperomery (CA) and steady state current measurements to elucidate the double layer properties of the platinum electrode/PIL interface, the kinetics and possible mechanism of the ORR. Three PILs with different cation acidities with an Brønsted-acidic cation [HA+][X−] are compared, [Dema][TfO], [1-EIm][TfO] and [2-Sema][TfO].Comparing the PILs with different cation acidity strongly suggest that the first reduction step including the proton transfer to the (catalytic) active sites on the electrode is mainly determining the ORR rate. The presence of residual water, unavoidable also at fuel cell operation >100 °C, is another important parameter. H2O modifies the ordered structure of the electrochemical double layer. Its protolysis equilibrium with an acidic PIL cation results in the formation of H3O+ that serves as a proton donor in the rate determining step and thus influences the ORR kinetics. Highly acidic PIL cations serve as a proton donor as well, particularly at low H2O concentrations, whereas the role of H3O+ as proton donor in the ORR becomes more prominent at higher water concentrations [3]. In low acidic PILs, H3O+ is the predominant proton donor and the ORR rate is significantly smaller resulting in considerably higher overpotentials. Thus, the onset potential of the ORR in a PIL based fuel cell will depend on both the concentration of residual water and the PIL cation acidity.Plots of the potential-dependent data from EIS measurements in the complex capacitance plane (CCP) show that at least two differential double layer capacitances are present, depending on the cell potential U (vs. RHE), water concentration c(H2O) and temperature T. The double layer properties of the highly acidic [2-Sema][TfO] are significantly different compared to the less acidic PILs [1-EIm][TfO] and [Dema][TfO]. The potential dependent capacitance curves were discussed by taking a mean field model, the presence of water and short range correlations of ions into account. [4, 5] The combined electrochemical kinetics and double layer measurements provide a deeper insight into the double layer structure at the Pt electrode/PIL interface to reveal the rate limiting parameters of the ORR and its mechanism.[1] K. Wippermann, J. Giffin, S. Kuhri, W. Lehnert and C. Korte, Phys. Chem. Chem. Phys. 19, 24706 (2017)[2] K. Wippermann, Y. Suo and C. Korte, J. Phys. Chem. C 125(8), 8 (2021)[3] H. Hou, H. M. Schütz, J. Giffin, K. Wippermann, X. Gao, A. Mariani, S. Passerini and C. Korte, ACS Appl. Mater. Interfaces 13, 8370 (2021)[4] Z. A. H. Goodwin, G. Feng and A. A. Kornyshev, Electrochim. Acta 225, 190 (2017)[5] J. Friedl, I. I. E. Markovits, M. Herpich, G. Feng, A. A. Kornyshev and U. Stimming, ChemElectroChem 4, 216 (2017)Figure: ORR rate constant of PILs [HA+][X−] with different cation acidity vs. content of residual water (50 mol% H2O corresponds to 5–6 wt% H2O) Figure 1

Highly selective cathodic H2O2 electrosynthesis by a metal tellurate/carbon black composite material

One sustainable alternative to the traditional anthraquinone technology to produce H2O2 is direct H2O2 electrosynthesis by oxygen reduction reaction (ORR). However, selecting electrocatalysts is a great challenge since most of the materials present selectivity towards the formation of H2O (4-electron pathway). In this work, we present a facile electrocatalyst preparation consisting of Ni3TeO6 (NTO), a metal tellurate, mixed with carbon black (Vulcan) with outstanding selectivity (2.2 transferred electrons) towards the production of H2O2 by ORR. To test the possible real application, the NTO/Vulcan catalyst was tested in a long-term stability test showing excellent faradaic efficiency and 93.5 % of the initial current. TEM and XPS measurements after reaction show that NTO/Vulcan materials are robust and durable electrocatalysts. This work opens the door of novel materials such as metal tellurates based on Ni for their application as ORR electrocatalysts in industrial applications.

Experimental H2O formation on carbonaceous dust grains at temperatures up to 85 K

ABSTRACT Water represents the main component of the icy mantles on dust grains, it is of extreme importance for the formation of new species and it represents the main component for life. Water is observed both in the gas-phase and frozen in the interstellar medium (ISM), where the solid-phase formation route has been proven essential to explain abundances in molecular clouds. So far, experiments have focused on very low temperatures (around 10 K). We present the experimental evidence of solid water formation on coronene, PAH-like surface, for a higher range of temperatures. Water is efficiently formed up to 85 K through the interaction of oxygen and hydrogen atomic beams with a carbonaceous grain analogue. The beams are aimed towards the surface connected to a cryostat exploring temperatures from 10 to 100 K. The results are obtained with a QMS and analysed through a temperature-programmed desorption technique. We observe an efficient water formation on coronene from 10 up to 85 K mimicking the temperature conditions from the dense ISM to translucent regions, where the ice mantle onset is supposed to start. The results show the catalytic nature of coronene and the role of chemisorption processes. The formation of the icy mantles could be happening in less dense and warmer environments, helping explaining oxygen depletion in the ISM. The findings have several applications such as the disappearance of PAHs in translucent regions and the snowlines of protoplanetary discs. We stress on how JWST projects characterizing PAHs can be combined with H2O observations to study water formation at warm temperatures.

Synthesis and thermodynamic properties of 4ZnO·B2O3·H2O with three different morphologies

Polycrystalline 4ZnO·B2O3·H2O with flower-like, spherical-like and sheet-like morphologies have been synthesized and characterized by XRD, TG–DSC, SEM and TEM. The molar enthalpies of solution of 4ZnO·B2O3·H2O with different morphologies in 1 mol L−1 HCl(aq) were measured. With the incorporation of the previously determined enthalpy of solution of H3BO3 in 1 mol L−1 HCl(aq), the enthalpy of solution of ZnO in (HCl + H3BO3)(aq) and the standard molar enthalpies of formation of ZnO(s), H3BO3(s) and H2O(l), the standard molar enthalpies of formation of 4ZnO·B2O3·H2O with flower-like, spherical-like, and sheet-like morphologies were calculated to be -(3033.91 ± 2.03), -(3030.91 ± 2.03) and -(3025.94 ± 2.04) kJ·mol−1, respectively.

Plasma-based dry reforming of CH4: Plasma effects vs. thermal conversion

In this work we evaluate the chemical kinetics of dry reforming of methane in warm plasmas (1000–4000 K) using modelling with a newly developed chemistry set, for a broad range of parameters (temperature, power density and CO2/CH4 ratio). We compare the model against thermodynamic equilibrium concentrations, serving as validation of the thermal chemical kinetics. Our model reveals that plasma-specific reactions (i.e., electron impact collisions) accelerate the kinetics compared to thermal conversion, rather than altering the overall kinetics pathways and intermediate products, for gas temperatures below 2000 K. For higher temperatures, the kinetics are dominated by heavy species collisions and are strictly thermal, with negligible influence of the electrons and ions on the overall kinetics. When studying the effects of different gas mixtures on the kinetics, we identify important intermediate species, side reactions and side products. The use of excess CO2 leads to H2O formation, at the expense of H2 formation, and the CO2 conversion itself is limited, only approaching full conversion near 4000 K. In contrast, full conversion of both reactants is only kinetically limited for mixtures with excess CH4, which also gives rise to the formation of C2H2, alongside syngas. Within the given parameter space, our model predicts the 30/70 ratio of CO2/CH4 to be the most optimal for syngas formation with a H2/CO ratio of 2.

Chemical-free vacuum ultraviolet irradiation as ultrafiltration membrane pretreatment technique: Performance, mechanisms and DBPs formation

Membrane fouling induced by natural organic matter (NOM) has seriously affected the further extensive application of ultrafiltration (UF). Herein, a simple, green and robust vacuum ultraviolet (VUV) technology was adopted as pretreatment before UF and ultraviolet (UV) technology was used for comparison. The results showed that control effect of VUV pretreatment on membrane fouling was better than that of UV pretreatment, as evidenced by the increase of normalized flux from 0.27 to 0.38 and 0.73 after 30 min UV or VUV pretreatment, respectively. This is related to the fact that VUV pretreatment exhibited stronger NOM degradation ability than UV pretreatment owing to the formation of HO•. The steady-state concentration of HO• was calculated as 3.04 × 10−13 M and the cumulative exposure of HO• reached 5.52 × 10−10 M s after 30 min of VUV irradiation. And the second-order rate constant between NOM and HO• was determined as 1.36 × 104 L mg−1 s−1. Furthermore, fluorescence EEM could be applied to predict membrane fouling induced by humic-enriched water. Standard blocking and cake filtration were major fouling mechanisms. Moreover, extension of UV pretreatment time increased the disinfection by-products (DBPs) formation, the DBPs concentration was enhanced from 322.36 to 1187.80 μg/L after 210 min pretreatment. However, VUV pretreatment for 150 min reduced DBPs content to 282.57 μg/L, and DBPs content continued to decrease with the extension of pretreatment time, revealing that VUV pretreatment achieved effective control of DBPs. The variation trend of cytotoxicity and health risk of DBPs was similar to that of DBPs concentration. In summary, VUV pretreatment exhibited excellent effect on membrane fouling alleviation, NOM degradation and DBPs control under a certain pretreatment time.

Improving the electrocatalytic performances of Pt-based catalysts for oxygen reduction reaction via strong interactions with single-CoN4-rich carbon support

Developing platinum-group-metal (PGM) catalysts possessing strong metal-support interaction and controllable PGM size is urgent for the sluggish oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells. Herein, we propose an in-situ self-assembled reduction strategy to successfully induce highly-dispersed sub-3 nm platinum nanoparticles (Pt NPs) to attach on resin-derived atomic Co coordinated by N-doped carbon substrate (Pt/CoSA-N-C) for ORR. To be specific, the interfacial electron interaction effect, along with a highly robust CoSA-N-C support endow the as-fabricated Pt/CoSA-N-C catalyst with significantly enhanced catalytic properties, i.e., a mass activity (MA) of 0.719 A/mgPt at 0.9 ViR‑free and a reduction of 24.2% in MA after a 20,000-cycles test. Density functional theory (DFT) calculations demonstrate that the enhanced electron interaction between Pt and CoSA-N-C support decreases the d-band center of Pt, which is in favor of lowering the desorption energy of *OH on Pt/CoSA-N-C surface and accelerating the formation of H2O, thus enhance the instinct activity of ORR. Furthermore, the higher binding energy between Pt and CoSA-N-C compared to Pt and C indicates that the migration of Pt has been suppressed, which theoretically explains the improved durability of Pt/CoSA-N-C. Our work offers an enlightenment on constructing composite Pt-based catalysts with multiple active sites.

Maintenance of cell integrity during hydroxyl radical rapid inactivation of Pseudanabaena sp. and simultaneous mineralization of odor compound 2-methylisoborneol

Pseudanabaena sp. and the odor compound it produces, 2-methylisoborneol (2-MIB), has been reportedly responsible for off-flavor pollution worldwide, leading to substandard drinking water sensory indicators and serious water supply crises. In this paper, the hydroxyl radical (•OH) produced by the synergistic effect of strong ionization discharge and hydrodynamic cavitation rapidly inactivated Pseudanabaena sp. and simultaneously mineralized 2-MIB to a concentration of 2.57 ng/L, which is below the odor threshold of 10 ng/L for a total reactive oxidants (TRO) concentration of 1.2 mg/L within 12 s. Crucially, the intracellular 2-MIB level was maintained in approximately 155.26– 162.29 ng/L range, indicating that 2-MIB was not released from the cells. Based on the scanning electron microscopy (SEM) results, the integrity of Pseudanabaena sp. cells was maintained with intact membranes and no intracellular organic matters (IOM) released during •OH inactivation. In contrast, ClO2 caused severe membrane rupture and massive IOM release. Based on the gas chromatograph/mass spectrometer (GC/MS) analyses and mass spectral database, the chromatogram fitted the baseline with a TRO concentration of 4 mg/L and no peaks corresponding to intermediates were detected. Moreover, •OH could mineralize 2-MIB by opening the ring structures of 1,2,3,3-tetramethyl-4-cyclopentenone, neomenthol, and 2-methylcyclohexene-1-aldehyde to produce small-molecule compounds, finally leading to CO2 and H2O formation via three reaction pathways. Therefore, the •OH not only maintained the cell integrity of Pseudanabaena sp. during inactivation but also mineralized 2-MIB simultaneously.

Positional effect of vortex generators on the combustion efficiency of the micro combustor fuelled with hydrogen/air mixture

The focus of this study is to explore the effect of vortex generator placement in a planar hydrogen combustor. The optimization of vortex generator placement is crucial for improving combustion efficiency and reducing pollutant emissions in the exhaust by achieving higher combustion. A numerical simulation model was developed using ANSYS- Reynolds-averaged Navier Stokes to examine the effect of vortex generators. Vortex generators were placed at four different locations away from the inlet at distances of 0.25 m, 0.30 m, 0.40 m, and 0.60 m such as 0.25S, 0.3S, 0.4S, and 0.6S models, respectively. Parameters such as combustion temperature, mass fraction of H2, mass fraction of H2O, turbulence intensity, and heat of reactions (HOR) were determined. The flow was assumed to be turbulent and steady, with hydrogen and air used as the fuel mixture. Hydrogen was released from the nozzle at a pressure of 300 bar and the velocity of air was 10 m/s. The strategic placement of vortex generators near the air inlet has been shown to have a significant impact on combustion temperatures and mixing, resulting in notable improvements in H2O formation and peak HOR. Furthermore, the utilization of vortex generators has been observed to enhance velocity magnitude and HOR, with the 0.25S model demonstrating the highest HOR values. 0.25S model also exhibited elevated temperatures and turbulent kinetic energy, indicating its potential for achieving optimal combustion efficiency. However, it is crucial to optimize the design and placement of vortex generators to mitigate disturbances in fuel spray patterns and attain the desired performance levels. Overall, these findings strongly suggest that vortex generators represent a promising technology for enhancing combustion efficiency and reducing emissions in planar combustors.

Effects of oxidant molecules on the thermal decomposition of 4,7‐diaminopyridazino[4,5‐c]furanoxane crystal: Ab initio molecular dynamics simulations

AbstractInserting oxidant molecules into the crystal voids to form oxidant cocrystal can greatly enhance the detonation properties of energetic materials. Therefore, oxidant cocrystals are regarded as one type of potential candidates of high energy density materials. The effects of the oxidants (HNO3 and HClO4) on the thermal decomposition mechanisms of two oxidant cocrystals (DPF(4,7‐diaminopyridazino[4,5‐c] furanoxane):HNO3 and DPF:HClO4) were investigated by ab initio molecular dynamics (AIMD). AIMD simulations are performed using a NVT ensemble. The temperature was controlled by a Nosé thermostat. First, we performed 5 ps simulations at 298.15 K to get an equilibrium system. Then, the simulations were performed at 3000 K with a time step of 1 fs and a total time of 15 ps. The initial degradation pathways of the two cocrystals have the same and involve the cleavage of the N–H and N–O bonds. Five different decay pathways were found in the subsequent reactions: intermolecular H transfer, N–O bond cleavage, N–N bond cleavage, and C–C bond formation and cleavage. Both HNO3 and HClO4 can promote the formation of N2, H2O and CO2, but only have a certain effect. Different oxidant molecules hardly affect the initial and subsequent decomposition paths of DPFH.

Roaming Dynamics in Hydroxymethyl Hydroperoxide Decomposition Revealed by the Full-Dimensional Potential Energy Surface of the CH2OO + H2O Reaction.

The CH2OO + H2O reaction is an important atmospheric process that leads to the formation of formic acid (HCOOH) and water via the intermediate hydroxymethyl hydroperoxide (HOCH2OOH, HMHP). We investigated the intricacies of this process by employing quasiclassical trajectory calculations on an accurate, full-dimensional ab initio potential energy surface (PES). In addition to the direct mechanism via the transition state (TS), an interesting roaming mechanism was found to play the predominant role in producing H2O and HCOOH. This roaming pathway is featured as the near direct dissociation of HMHP into OH and hydroxymethoxy radical, followed by the retraction of OH and abstraction of the H atom, culminating in the formation of H2O. Due to the longer interaction time of the roaming mechanism, less product translational energy was released, but more internal energies of HCOOH were obtained, as compared with the direct TS mechanism. The enhanced yield of H2O and formic acid achieved through roaming dynamics underscores the significance of dynamics simulations based on an accurate full-dimensional PES. This work provides new insights into the dynamics of the CH2OO + H2O reaction and its implications for atmospheric chemistry.

Study on the mechanism of nitrogen conversion in NH3-doped methane premixed flame based on multi-spectral analysis methods

It is essential to obtain the in-situ key radicals and components in the combustion process of NH3 oxidation to reveal the volatile nitrogen conversion mechanism. In this work, multi-spectral analysis methods, including image spectroscopy, Tunable diode laser absorption spectroscopy (TDLAS), and plane laser-induced fluorescence (PLIF) were used to obtain the spatial distribution characteristics of the main excited radicals (C2*/CH*/CN*/OH*), H2O, and NO in NH3-doped methane flames, respectively. Furthermore, the kinetic simulation was combined to obtain the key reactions and main pathways of nitrogen conversion. The experimental results showed that when the concentration of NH3 was low, the NO generated in lean flame was less than that in rich flame, which was more than that in rich flame when the concentration of NH3 was high. NH3 also promoted the formation of H2O in flames. The image spectral intensity of CN* had a significant linear relationship with the CN concentration obtained by simulation, indicating that CN* was mainly generated by CN thermal excitation. The results of sensitivity and path analysis indicated that rich flame contributed to the reaction of N-species with H and that high NH3 concentration promoted the conversion of NH2 to HNO. With increasing NH3 concentration and equivalence ratio, the proportion of CH3 conversion to C2H6 was enhanced, while the proportion of CH3 oxidized decreased. This work reveals the mechanism of volatile nitrogen (NH3) conversion and the influence of equivalence ratio and NH3 concentration on it, which provides theoretical guidance for NOx emission reduction.

Numerical investigation of sonochemical production in a single bubble under dual-frequency acoustic excitation

A numerical computation study about sonochemical activity of an oxygen bubble oscillations in a dual-frequency acoustic field is presented in this paper. The variation of excitation frequency causes the change of magnitude and duration of pressure acting on the bubble, affecting the instantaneous bubble radius, internal bubble temperature as well as chemical reactions. The numerical results indicate the formation of O2, HO2 •, •OH, H•, O, H2 and H2O2 in an O2 bubble during oscillations. The main oxidant is •OH radical. The influence of several parameters (e.g. couples of frequencies, the total pressure amplitude and the ratio of pressure amplitudes of the two waves) on the molar yields of free radicals is analyzed. There is an optimum bubble collapse temperature (around 5648 K) for the •OH production. Moreover, the corresponding frequency intervals of enhanced effect and weakened effect under different coupled frequencies (i.e. 40, 140, 213, 355 and 515 kHz) are also investigated.

The link between lithospheric scale deformations and deep fluid emanations: Inferences from the Southeastern Carpathians, Romania

Understanding the formation, migration and emanation of deep CO2, H2O and noble gases (He–Ne) in deep-seated deformation settings is crucial to know the complex relationship between deep-originated fluids and lithospheric deformation. To gain a better insight into these phenomena, we studied the origin of H2O, CO2 and noble gases of gas-rich springs found in the Târgu Secuiesc Basin located in the southeasternmost part of the Carpathian-Pannonian region of Europe. This study area is one of the best natural examples to understand the connection between the deep sources of gas emanations and deep-seated deformation zones, providing an excellent analogue for regions with similar tectonic settings and fluid emanation properties. We studied the δ2H and δ18O stable isotopic ratios of the spring waters, and the δ13C, He and Ne stable isotopic ratio of the emanating CO2-rich gases dissolved in the mineral spring waters in Covasna town and its vicinity. Based on the δ2H, δ13C, δ18O stable isotopic ratios, the spring waters and the majority of the gases are released through two consecutive fluid infiltration events. The preservation of the metamorphic signal of the upwelling H2O is linked to the local groundwater flow and fault abundancy. Furthermore, the noble gas isotopic ratios show a high degree of atmospheric contamination in the dissolved water gasses that is most likely related to the local hydrogeology. Nevertheless, the elevated corrected helium stable isotopic ratios (Rc/Ra) of our filtered data suggest that part of the emanating gases have a potential upper mantle source component. Beneath the Southeastern Carpathians, mantle fluids can have multiple origin including the dehydration of the sinking slab hosting the Vrancea seismogenic zone, the local asthenospheric upwelling and the lithospheric mantle. The flux of the mantle fluids is enhanced by lithospheric scale deformation zones that also support the fluid inflow from the upper mantle into the lower crust. The upwelling CO2–H2O mantle fluids may induce the release of crustal fluids by shifting X(CO2) composition of the pore fluid and, consequently, initiating decarbonisation and devolatilization metamorphic reactions as a result of carbonate and hydrous mineral destabilisation in the crust. Based on the p-T-X(CO2) conditions of calc-silicates and the local low geotherm, we emphasise the importance of the upwelling fluids in the release and upward migration of further H2O and CO2 in the shallower lower and upper crust. We infer that migration of deep fluids may also play an important role in addition to temperature control on the generation of crustal fluids in deep-seated deformation zones.

Enhanced formaldehyde oxidation over MnO2 and doped manganese-based catalysts: Experimental and theoretical Insights into mechanism and performance

Thermal catalytic degradation of formaldehyde (HCHO) over manganese-based catalysts is garnering significant attention. In this study, both theoretical simulations and experimental methods were employed to elucidate the primary reaction pathways of HCHO on the MnO2(110) surface. Specifically, the effects of doping MnO2 with elements such as Fe, Ce, Ni, Co, and Cu on the HCHO oxidation properties were evaluated. Advanced characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), and X-ray photoelectron spectroscopy (XPS), were employed to discern the physical properties and chemical states of the active components on the catalyst surface. The comprehensive oxidation pathway of HCHO on the MnO2(110) surface includes O2 adsorption and dissociation, HCHO adsorption and dehydrogenation, CO2 desorption, H2O formation and desorption, oxygen vacancy supplementation, and other elementary reactions. The pivotal rate-determining step was identified as the hydrogen migration process, characterized by an energy barrier of 234.19 kJ mol−1. Notably, HCHOO and *CHOO emerged as crucial intermediates during the reaction. Among the doped catalysts, Fe-doped MnO2 outperformed its counterparts doped with Ce, Ni, Co, and Cu. The optimal degradation rate and selectivity were achieved at a molar ratio of Fe: Mn = 0.1. The superior performance of the Fe-doped MnO2 can be ascribed to its large specific surface area, conducive pore structure for HCHO molecular transport, rich surface-adsorbed oxygen species, and a significant presence of oxygen vacancies.

Reaction Mechanism of Nitrogen-Containing Heterocyclic Compounds Affecting Coal Spontaneous Combustion.

To uncover the reaction mechanism of nitrogen-containing heterocyclic compounds affecting coal self-heating, quantum chemical calculations and X-ray photoelectron spectroscopy (XPS) experiments were applied to elucidate the reaction pathways and thermodynamic characteristics of pyrrole, pyridine, indole, quinoline, and carbazole. Results show that in pyrrole, pyridine, indole, quinoline, and carbazole, the reaction with O2 captures the H atom and leads to the formation of ·OOH and pyrrolyl, pyridinyl, indolyl, quinolinyl, and carbazolyl radicals, respectively. The activation energies are 118.15, 86.642, 34.132, 21.004, and 47.259 kJ/mol, respectively. ROO· formed by spontaneous adsorption of O2 by nitrogen-containing radicals undergoes self-reaction, and the O-O bond is broken and dehydrogenated to generate ·OH. Subsequently, at room temperature, ·OH reacts with pyrrole, pyridine, indole, quinoline, and carbazole, resulting in the formation of H2O and pyrrolyl, pyridinyl, indolyl, quinolinyl, and carbazolyl radicals, respectively, thereby forming a cyclic chain reaction. The XPS analysis yielded the following findings: (i) when the temperature rises to 70 °C, the N-5 and N-6 content decrease, which is attributed to the activation energy; (ii) when the temperature reaches 200 °C, the N-5 content decreases, which can be attributed to the activation energy required for the oxidation of pyrrole (118.5 kJ/mol).

Synergistic effect of HCl, SO2 and O2 on mercury removal by Mn0.5Cu0.5Fe2O4 adsorbent

Ferrite-structured adsorbents have demonstrated high potential for adsorbing zero-valent mercury (Hg0), but their performance in complex flue gas is not well understood. This study investigated the effects of HCl, O2, and SO2 on Hg0 oxidation capacity using Mn0.5Cu0.5Fe2O4 adsorbent. The morphology and microstructure of Mn0.5Cu0.5Fe2O4 were examined, and mass transfer theoretical analysis and X-ray photoelectron spectroscopy experiments were conducted to explore the synergistic effect of HCl, SO2, and O2 on mercury removal. The results indicated that the adsorbent achieved a high Hg0 removal efficiency of approximately 98% when HCl, SO2, and O2 coexisted in the flue gas at 120 °C. The reaction can be separated into three steps. In the first step, HCl dominates the reaction, resulting in the formation of HgCl2 and H2O. In the second step, oxygen defect sites accelerate the breakdown of O2 into chemisorbed oxygen. In the third step, the chemisorbed oxygen combines with Hg0 and SO2 to generate HgO and HgSO4. This study has important implications for the development of ferrite-structured adsorbents in complex flue gas.

Mid-infrared absorption spectroscopy measurements and model improvements during the oxidation of ammonia/ethanol and ammonia/diethyl ether blends in a shock tube

Blending ammonia with highly active fuels (hydrogen, hydrocarbons, oxygenated hydrocarbons, etc.) has been identified as an efficient way to improve the mixture's reactivity. In this study, the temperature, CO, CO2, H2O, NH3, and NO time-histories were measured simultaneously during the oxidation of ammonia/ethanol and ammonia/diethyl ether (DEE) blends (equivalence ratio ϕ ∼ 1.2) using mid-infrared laser absorption spectroscopy behind reflected shock waves. Various models (ten for ammonia/ethanol and three for ammonia/DEE) were selected to perform the simulation and compared with the measurements. The predictions are very different for both mixtures, among which the CRECK model was selected to perform discussion and kinetics analyses for the ammonia/ethanol mixture and the Shrestha 2022 model was selected for the ammonia/DEE mixture. The CRECK model performs well in predicting the temperature rise/plateau, H2O formation rate/plateau, CO consumption rate/plateau, and CO2 formation rate. However, it underpredicts the CO formation rate and slightly overpredicts the CO2 plateau and NH3 consumption rate; for the NO time-history, the CRECK model well captures the rapid NO formation but underpredicts the near-equilibrium NO concentration and gives an unnormal initial NO production. The Shrestha 2022 model accurately predicts the plateaus of temperature and H2O, but significantly underpredicts the reactivity of the ammonia/DEE mixture and overpredicts the near-equilibrium NO concentration. According to the kinetics analyses, key reactions affecting the species' time-histories were selected. Fourteen reactions in the CRECK model and eight reactions in the Shrestha 2022 model were updated by the reported data or estimations to improve the predictions. However, discrepancies still exist, which are mainly located in the CO formation and NH3 consumption rates. Further effort should mainly focus on these two aspects, as well as the interactions between oxygenated hydrocarbons and ammonia at high temperatures.