O2 In Gas Phase Research Articles

Development of Operando Observation Technique of Electrochemical Reactions at the Solid-Liquid Interface by Fluorescence-yield Wavelength-dispersive Soft X-ray Absorption Spectroscopy

Real-time observation of an electrochemical reaction at a solid–liquid interface was performed using fluorescence-yield wavelength-dispersive soft X-ray absorption spectroscopy (XAS). A dedicated electrochemical cell was prepared for this study. As a model experiment, an electrochemical oxygen evolution reaction (OER) was observed in alkaline solution (0.1 M NaOH) on the surface of a Pt thin film working electrode. With positive potential shift, the XAS signal at K-edge of oxygen, whose peak position is close to that of gas-phase O2, increased in correlation with the electrode potential. This method has potential as a real-time observation method for time-dependent processes in various fields using electrochemical reactions at the solid–liquid interface, especially electrochemical catalytic reactions.

Deactivation, reactivation and super-activation of Fe-N/C oxygen reduction electrocatalysts: Gas sorption, physical and electrochemical investigation using NO and O2

We show that gaseous nitric oxide (NO) and oxygen (O2) are useful molecular probes to uncover complex surface processes in Fe-N/C catalysts. We unravel the difference between using gaseous NO in a temperature programmed desorption experiment and using NO (and progenitors) in an electrochemical experiment. Gas phase O2 adsorption is almost exclusively desorbed as CO2, and continued exposure to oxygen increases the amount of chemisorbed oxygen species on the surface. The oxidation state of the carbon surface is an important activity determining factor, and under normal “electrochemical” conditions many of the active sites are blocked. Only by treatment at 600 °C in Ar can we free those sites for oxygen adsorption, however under atmospheric storage, and especially during the oxygen reduction reaction (ORR), the surface quickly becomes deactivated with chemisorbed oxygen species and water. We demonstrate that the material can be super-activated by reductive electrochemical treatment, both in an electrochemical three electrode cell and in a fuel cell. The energy gained following the treatment is significantly larger than the energetic cost.

Immobilizing ultrafine bimetallic PtAg alloy onto uniform MnO2 microsphere as a highly active catalyst for CO oxidation

Herein, a facile glycol reduction route is successful employed to synthesize bimetallic PtAg alloys with homogeneous distribution of sizes and elements. Experimental studies reveal that the ultrafine PtAg alloys with well-defined sizes from around 3.3 nm to 5.8 nm are immobilized onto MnO2 microsphere, which remarkably enhances the catalytic performances for CO oxidation. Importantly, quasi in-situ X-ray photoelectron spectroscopy (XPS) result reveals that both Mn and Pt ions on the surface of catalysts would realize alternating reduction-oxidation by CO and O2 molecules, and the oxygen vacancy sites could be replenished and excited by gas-phase O2.

Oxidative etching mechanism of the diamond (100) surface

We performed density functional theory calculations with van der Waals corrections to elucidate diamond oxidation mechanism on the atomic-level which could lead to insights that will advance the improvement of nascent nanofabrication technologies. We developed a comprehensive theory of oxidative etching of the diamond (100) surface, from the adsorption of gas phase O2, including details of metastable adsorption states, intersystem crossing, and induced surface dereconstruction, to the desorption of CO and CO2, complete etching of the top surface layer and its subsequent stabilization. Oxygen adsorption induce C-dimer bond breaking through the formation of carbonyl structures at low surface coverage. At high surface coverage, adjacent carbonyls can transform into the more stable ether chain with small energy barrier requirement. A mix of carbonyl and ether will form in surfaces with defects, and several models of possible structures have been presented. CO desorption creates a point defect that serves as nucleation site for the preferred etching direction along [011], perpendicular to the top layer C-dimer bonds. We obtained a wide range of desorption activation energies, which depend on the existence of point defects and reconstruction of surfaces. C-dimer formation and oxygen adsorption stabilize vacancies and single-atomic-layer-deep trough.

Kinetic Control of Oxygen Interstitial Interaction with TiO2(110) via the Surface Fermi Energy

Atomically clean surfaces of semiconducting oxides efficiently mediate the interconversion of gas-phase O2 and solid-phase oxygen interstitial atoms (Oi). First-principles calculations together with mesoscale microkinetic modeling are employed for TiO2(110) to determine reaction pathways, assess appropriate rate expressions, and obtain corresponding activation energies and pre-exponential factors. The Fermi energy (EF) at the surface influences the rate-determining step for both injection and annihilation of Oi. The barriers range between 0.72-0.82 eV for injection and 0.60-2.34 eV for annihilation and may be manipulated through intentional control of EF. At equilibrium, the microkinetic model and first-principles calculations indicate that interconversion of Oi species in the first and second sublayers limits the rate. The effective pre-exponential factors for injection and annihilation are surprisingly low, probably resulting from the use of simple Langmuir-like rate expressions to describe a complicated kinetic sequence.

Tunable perovskite-based photodetectors in optical sensing

Broad- and narrow-band, tunable perovskite photodetectors (PPDs) with size-dependent fast response times are demonstrated for the first time in optical sensing of analytes, including gas-phase and dissolved oxygen (DO), as well as glucose. The sensors included a LED excitation source and a polystyrene film with embedded oxygen-sensitive dyes, PtOEP or PdOEP. The analyte’s dose-dependent photoluminescence (PL) intensity I and decay time τ were measured. Using the PPDs enabled monitoring gas-phase O2 at levels of 0 %–100 % with a sensitivity comparable to that of a Si photodiode. A broad dynamic range was similarly observed for DO monitoring and the limit of detection for glucose monitoring was ∼0.02 mM at an initial level of ∼0.26 mM DO. Importantly, the size-dependent fast response time of the PPDs enabled analyte monitoring via the preferred measurement of τ, rather than I, over a broad dynamic range, which was unattainable with organic photodetectors. The use of the narrow-band PPDs eliminated the need for optical filters, which leads to more compact device designs.

Promotional removal of oxygenated VOC over manganese-based multi oxides from spent lithium-ions manganate batteries: Modification with Fe, Bi and Ce dopants

In this work, cathode materials of spent lithium-ions manganate batteries are recovered as the precursor of manganese-based oxides catalysts and furthermore, different amount of Fe, Bi, Ce are introduced to modify their properties. A series of MnOx(MS)-X Fe, MnOx(MS)-X Bi and MnOx(MS)-X Ce samples with crystal phase of Mn5O8 are synthesized using combustion method and then the catalytic behavior and physicochemical properties of prepared catalysts are investigated. Compared to binary MnOx-5% Fe, MnOx-15% Bi and MnOx-10% Ce samples, multi MnOx(MS)-5% Fe, MnOx(MS)-15 Bi and MnOx(MS)-10% Ce catalysts display enhanced catalytic performance significantly in the removal of oxygenated VOC, which could be attributed to larger specific surface area, higher concentration of surface active oxygen species and Mn4+ ions and better reducibility at low temperature. In-situ DRIFTS results imply that main oxygen-containing functional groups such as carbonyl (-C=O), carboxyl (-COO), hydroxyl (-OH) can be observed during VOC oxidation and by comparison, it can be found that gas-phase O2 plays a crucial role in facilitating the further oxidation of by-products into CO2. In addition, TD/GC–MS results point out that the main by-products are formaldehyde; 2-propanol, 1-methoxy-; ethanol, 2-methoxy-, acetate; 2-ethoxyethyl acetate; acetic acid during VOC oxidation.

Enhanced catalytic activity of oxygenated VOC deep oxidation on highly active in-situ generated GdMn2O5/GdMnO3 catalysts

In this work, GdMnO3 material is successfully prepared using sol-gel method and GdMn2O5/GdMnO3 materials are in-situ generated by acid treatment. These materials are investigated and applied as catalysts for oxygenated VOC complete oxidation. The evaluation results show that GdMn2O5/GdMnO3-1.00 exhibits a remarkable increase in catalytic activity (T50% = 198 °C and T90% = 225 °C) of 2-ethoxyethanol oxidation when compared with the initial sample GdMnO3 (T50% = 223 °C and T90% = none). Characterization analyses show that acid treatment can result in the significant improvement of specific surface area from 20.502 m2·g−1 to 67.952 m2·g−1, abundant surface Mn4+ content and active oxygen, excellent reducibility at low temperature in GdMn2O5/GdMnO3-1.00 sample. In-situ DRIFTS results point out that the main functional groups such as νas(OCO), νas(COO), νs(CO) are formed in the process of 2-ethoxyethanol oxidation over GdMn2O5/GdMnO3-1.00 sample and some by-products including ethanol, 2-ethoxyethyl acetate, acetic acid, carbonic acid, 2-ethoxyethyl 2-methoxyethyl ester, ethane and 1,1′-oxybis[2-methoxy-] can be produced at a reaction temperature of 200 °C. Additionally, in-situ DRIFTS studies indicate the presence of gas-phase O2 plays a vital role in facilitating 2-ethoxyethanol deep oxidation to final products.

The Effect of Different Oxygen Surface Functionalization of Carbon Nanotubes on the Electrical Resistivity and Strain Sensing Function of Cement Pastes.

Different studies in the literature indicate the effectiveness of CNTs as reinforcing materials in cement–matrix composites due to their high mechanical strength. Nevertheless, their incorporation into cement presents some difficulties due to their tendency to agglomerate, yielding a non-homogeneous dispersion in the paste mix that results in a poor cement–CNTs interaction. This makes the surface modification of the CNTs by introducing functional groups on the surface necessary. In this study, three different treatments for incorporating polar oxygen functional groups onto the surface of carbon nanotubes have been carried out, with the objective of evaluating the influence of the type and oxidation degree on the mechanical and electrical properties and in strain-sensing function of cement pastes containing CNTs. One treatment is in liquid phase (surface oxidation with HNO3/H2SO4), the second is in gas phase (O3 treatment at 25 and 160 °C), and a third is a combination of gas-phase O3 treatment plus NaOH liquid phase. The electrical conductivity of cement pastes increased with O3- and O3-NaOH-treated CNTs with respect to non-treated ones. Furthermore, the oxygen functionalization treatments clearly improve the strain sensing performance of the CNT-cement pastes, particularly in terms of the accuracy of the linear correlation between the resistance and the stress, as well as the increase in the gage factor from 28 to 65. Additionally, the incorporation of either non-functionalized or functionalized CNTs did not produce any significant modification of the mechanical properties of CNTs. Therefore, the functionalization of CNTs favours the de-agglomeration of CNTs in the cement matrix and consequently, the electrical conductivity, without affecting the mechanical behaviour.

Gas-grain modeling of interstellar O2

Molecular oxygen (O2) is essential to human beings on the earth. Although elemental oxygen is rather abundant, O2 is rare in the interstellar medium. It was only detected in two galactic and one extra-galactic region. The inconsistency between observations and theoretical studies is a big challenge for astrochemical models. Here we report a two-phase modeling research of molecular oxygen, using the Nautilus gas-grain code. We apply the isothermal cold dense models in the interstellar medium with two typical sets of initial elemental abundances, as well as the warm-up models with various physical conditions. Under cold dense conditions, we find that the timescales for gas-phase CO, O2 and H2O to reach peak values are dependent on the hydrogen density and are shortened when hydrogen density increases. In warm-up models, O2 abundances are in good agreement with observations at temperatures rising after 105 yr. In both isothermal and warm-up models, the steady-state O2 fractional abundance is independent of the hydrogen density, as long as the temperature is high enough (>30 K), at which O2 is prevented from significant depleting onto grain surface. In addition, low density is preferable for the formation of O2, whether molecular oxygen is under cold conditions or in warm regions.

Continuous Partial Oxidation of Methane to Methanol Catalyzed by Diffusion-Paired Copper Dimers in Copper-Exchanged Zeolites.

Copper-exchanged zeolites can continuously and selectively catalyze the partial oxidation of methane to methanol using only oxygen and water at low temperatures, but the genesis and nature of the active sites are currently unknown. Herein, we demonstrate that this reaction is catalyzed by a [Cu-O-Cu]2+ motif that forms via a hypothesized proton-aided diffusion of hydrated Cu ions within the cages of SSZ-13 zeolites. While various Cu configurations may be present and active for methane oxidation, a dimeric Cu motif is the primary active site for selective partial methane oxidation. Mechanistically, CH4 activation proceeds via rate-determining C-H scission to form a surface-bound C1 intermediate that can either be desorbed as methanol in the presence of H2O/H+ or completely oxidized to CO2 by gas-phase O2. High partial oxidation selectivity can be obtained with (i) high methane and water partial pressures and (ii) maximizing Cu dimer formation by using zeolites with high Al content and low Cu loadings.

Enhancement of catalytic activity by UV-light irradiation in CeO2 nanocrystals

Ultraviolet (UV) light irradiation on CeO2 nanocrystals catalysts has been observed to largely increase the material’s catalytic activity and reactive surface area. As revealed by x-ray absorption near edge structure (XANES) analysis, the concentration of subvalent Ce3+ ions in the irradiated ceria samples progressively increases with the UV-light exposure time. The increase of Ce3+ concentration as a result of UV irradiation was also confirmed by the UV-vis diffuse reflectance and photoluminescence spectra that indicate substantially increased concentration of oxygen vacancy defects in irradiated samples. First-principle formation-energy calculation for oxygen vacancy defects revealed a valence-hole-dominated mechanism for the irradiation-induced reduction of CeO2 consistent with the experimental results. Based on a Mars-van Krevelen mechanism for ceria catalyzed oxidation processes, as the Ce3+ concentration is increased by UV-light irradiation, an increased number of reactive oxygen atoms will be captured from gas-phase O2 by the surface Ce3+ ions, and therefore leads to the observed catalytic activity enhancement. The unique annealing-free defect engineering method using UV-light irradiation provides an ultraconvenient approach for activity improvement in nanocrystal ceria for a wide variety of catalytic applications.

Interface reaction activity of recyclable and regenerable Cu-Mn spinel-type sorbent for Hg0 capture from flue gas

To capture elemental mercury from flue gas, a series of novel Cu-Mn spinel-type sorbents were synthesized using the proposed low-temperature sol-gel auto-combustion synthesis method. CuMn2O4 sorbent exhibited excellent mercury removal performance in a wide temperature window of 50–350 °C, which is closely related to the mobile-electron environment caused by the Jahn-Teller distortion. The adsorbed mercury species exist in the forms of Cu-Hg amalgam and HgO on the sorbent surface. The chemisorption mechanism is responsible for Hg0 adsorption on CuMn2O4 surface. Mercury removal by CuMn2O4 sorbent is controlled by mass transfer process when the superficial velocity of flue gas is less than 16.9 cm/s. The surface reaction kinetics dominates mercury removal process when the superficial velocity is larger than 16.9 cm/s. The results of repeated adsorption-regeneration experiments indicate that CuMn2O4 sorbent exhibits excellent regenerability and recyclability for mercury removal from flue gas. During the regeneration process, the gas-phase O2 in air can decompose over sorbent surface to regenerate the consumed chemisorbed oxygen and replenish the oxygen vacancy. Mercury desorption from spent sorbents is a first-order desorption reaction process. The pre-exponential factor and activation energy for Hg0 desorption from used sorbent surface are 1.06 × 104 s−1 and 61.43 kJ/mol, respectively.

Adsorption and oxidation of elemental mercury from coal-fired flue gas over activated coke loaded with Mn-Ni oxides.

A series of Mn-Ni/AC (AC, activated coke) catalysts were synthesized by the impregnation method for the removal of elemental mercury (Hg0) from simulated flue gas. The samples were characterized by BET, ICP-OES, SEM, XRD, XPS, H2-TPR, FT-IR, and TGA. Mn6Ni0.75/AC exhibited optimal removal efficiency of 96.6% in the condition of 6% O2 and balanced in N2 at 150°C. The experimental results showed that both O2 and NO facilitated Hg0 removal. SO2 could restrain the Hg0 removal in the absence of O2, while the inhibitory effect of SO2 was weakened with the aid of 6% O2. In addition, H2O exhibited a slightly negative influence on Hg0 removal. The characterization of the samples indicated that Mn6Ni0.75/AC possessed larger specific surface area, higher dispersion of metal oxides, and stronger redox ability. In the meantime, the results of XPS and FT-IR demonstrated that the lattice oxygen and chemisorbed oxygen made contributions to Hg0 removal and the consumed oxygen could be compensated by the redox cycle of metal oxides and gas-phase O2. Meanwhile, the mechanisms of Hg0 removal were proposed based on the above studies.

The fate of O2 in photocatalytic CO2 reduction on TiO2 under conditions of highest purity.

Although the photocatalytic reduction of CO2 to CH4 by using H2O as the oxidant presupposes the formation of O2, it is often not included in the product analysis of most of the studies dealing with photocatalytic CO2 reduction or it is reported to be not formed at all. The present study aims to clarify the absence of O2 in the photocatalytic gas phase CO2 reduction on TiO2. By modifying P25-TiO2 with IrOx co-catalysts it was possible to observe photocatalytic water splitting, i.e. the formation of gaseous O2 and H2 in almost stoichiometric amounts, without the use of sacrificial agents, while bare P25-TiO2 showed no activity in H2 and O2 formation under similar reaction conditions. Investigating the effect of improved H2O oxidation properties on the photocatalytic CO2 reduction revealed that the CH4 formation on P25 from CO2 was completely inhibited as long as the H2O splitting reaction proceeded. Furthermore, we found that a certain amount of O2 is consumed under conditions of photocatalytic water oxidation. A quantification showed it to be in the same order of magnitude as the oxygen which is missing as a byproduct from photocatalytic CO2 conversion. A detailed interpretation of the results in the context of the general understanding of the photocatalytic CO2 reduction with H2O on TiO2 allows the hypothesis that P25-TiO2 undergoes a stoichiometric reaction, meaning that the CH4 formation is not based on a true catalytic cycle and runs only as long as TiO2 can consume oxygen.

Production of 160 mg/L ozone water using circulating water electrolysis system

A circulating water electrolysis system was developed to produce highly concentrated O3-dissolved water (ozone water). It enabled the production of 160 and 112 mg/L ozone water in batch and continuous-withdrawal operation, respectively. The ratios of the actual gas-phase O3 concentrations to those calculated from the dissolved O3 concentration using Henry's law at 1 atm were about 0.5, suggesting the supersaturation of ozone water. Once the O3 gas dissolved in the electrolysis cell operated at 0.15 MPa/G, it should have taken time for the ozone water to reach the gas-liquid equilibrium. The electric conductivity of the ozone water increased to 1.2 mS/cm and the formation of ClO2 was observed at the end of the run. The electrophoretic migration of Cl− ions from the catholyte to the ozone water is considered to be the cause of this impurity. The time dependence of the dissolved O3 concentration and the maximum dissolved O3 concentration obtained in the experiment were well reproduced by a circulating water electrolysis simulator.

Cobalt doped ceria for abundant storage of surface active oxygen and efficient elemental mercury oxidation in coal combustion flue gas

Cobalt doped CeO2 (Co-CeO2) prepared by a single-step hydrothermal method was used for Hg0 catalytic oxidation. The catalysts were characterized by scanning electron microscope, transmission electron microscope, X-ray diffraction, Raman spectroscopy, electron paramagnetic resonance, X-ray photoelectron spectroscopy, H2-temperature programmed reduction test, and thermogravimetric measurement. The results show that the exposed planes for the flake-shaped Co-CeO2 mainly were (110) and (100) with low oxygen vacancy formation energies. Abundant oxygen vacancy defects were identified on the Co-CeO2, which further induced plentiful chemisorbed oxygen on the surface. The oxygen storage capacity for the Co-CeO2 was up to 1.43 μmol O2 m−2 at 200 °C, and a large portion of this capacity was for chemisorbed oxygen. Without the introduction of gas-phase O2, Hg0 oxidation probably occurred through a Mars-Maessen mechanism, where active chemisorbed oxygen originated from oxygen vacancy defects reacts with adsorbed Hg0 to form HgO. Active chlorine species was generated from the interactions of chemisorbed oxygen with trace amount gaseous HCl. The active chlorine compensated the slight inhibitive effects of SO2 and H2O, leading to above 90% Hg0 oxidation under simulated low-rank coal burning flue gas atmosphere at a gas hourly space velocity of 160,000 h−1. The abundant active chemisorbed oxygen played important role in and guaranteed an efficient Hg0 oxidation in extremely adverse application environment. These results reveal the role of doping metals on structure-catalytic property relations for CeO2, which opened a new strategy for controlling mercury emission from coal-fired flue gas using CeO2 based catalysts by tuning the morphology and exposed planes.

Heterogeneous reaction mechanism of elemental mercury oxidation by oxygen species over MnO2 catalyst

MnO2-based catalysts have attracted great attention in the field of elemental mercury (Hg0) catalytic oxidation because of their superior catalytic performance and wide temperature window. Quantum chemistry calculations based on density functional theory (DFT) combined with periodic slab models were carried out to investigate the heterogeneous mechanism of Hg0 oxidation by oxygen species (gas-phase O2, chemisorbed oxygen, and lattice oxygen) on MnO2 surface. The results indicate that Hg0 and HgO are chemically adsorbed on MnO2 surface with the adsorption energies of −69.50 and −226.48 kJ/mol, respectively. The adsorption of O2 on MnO2 surface belongs to chemisorption. O2 can decompose on MnO2 surface with an energy barrier of 97.46 kJ/mol to produce two atomic adsorbed oxygen. The perpendicular adsorbed O2 and dissociative adsorbed O2 are more favorable for Hg0 catalytic oxidation than lattice oxygen, and perpendicular adsorbed O2 is the most active oxygen for Hg0 oxidation. The reaction pathway of Hg0 oxidation by perpendicular adsorbed O2 includes three reaction steps: Hg0 → Hg(ads) → HgO(ads) → HgO. The third step (HgO(ads) → HgO) is endothermic by 168.17 kJ/mol with an energy barrier of 179.48 kJ/mol, and it is the rate-limiting step of the whole Hg0 oxidation reaction.

A DFT Study on the Selective Oxidation of Ethane Over Pure SBA-15 and SBA-15-supported Vanadium Oxide

The selective oxidation of ethane over pure SBA-15 and V/SBA-15 were theoretically studied by density functional theory. The cluster models of pure SBA-15 and V/SBA-15 were proposed. The structure properties of these two models were calculated and were found to be in good agreement with experimental values. The catalytic reaction pathways for the ethane oxidation to acetaldehyde and ethylene were determined. Our results show that the hydroxyl groups on pure SBA-15 can activate the gas-phase O2 to form a peroxide species, which acts as the active site for the selective oxidation of ethane. The formation of ethylene is much more preferred than that of acetaldehyde over pure SBA-15. For V/SBA-15, the peroxide species also acts as the active center. The energy barrier of C–H bond activation over V/SBA-15 is by 14.63 kJ/mol lower than that over pure SBA-15. The formation of acetaldehyde is preferred than that of ethylene over V/SBA-15. On the basis of our results, the reaction mechanisms of ethane selective oxidation over pure SBA-15 and V/SBA-15 were systematically compared and discussed. The theoretical results in this study are in good agreement with our previous experimental results. They can reasonably explain the catalytic nature of pure SBA-15 and the effect of vanadium, opening new perspectives in the understanding of the chemistry of SBA-15.

NH3 inhibits mercury oxidation over low-temperature MnOx/TiO2 SCR catalyst

Manganese oxide/titanium dioxide (MnOx/TiO2, or MnTi) catalyst prepared by an ultrasound-assisted impregnation method was employed for simultaneous nitrogen oxide (NO) reduction and elemental mercury (Hg0) oxidation under a simulated selective catalytic reduction (SCR) atmosphere. An NO reduction rate of 83% and Hg0 oxidation rate of 96% were simultaneously achieved at a relatively low temperature of 200 °C. Ammonia (NH3) as a reducing agent in the NO-SCR process was systematically investigated for its effect on Hg0 oxidation. The results showed that NH3 inhibited Hg0 oxidation over the low-temperature MnTi catalysis process. In the presence of NH3, competitive adsorption between NH3 and Hg0 on the MnTi catalyst was partly responsible for the weakening Hg0 oxidation. NH3 consumed surface oxygen (O2) on the MnTi catalyst, which also resulted in lower Hg0 oxidation rate, especially in the absence of gas phase O2 in the flue gas. More importantly, oxidized mercury was reduced by NH3 to form Hg0 with the aid of NO, i.e., the combined presence of NO and NH3 induced the reduction of oxidized mercury, and hence partially offset Hg0 oxidation. The effects of individual SCR gases, such as O2, NO, and the combinations, on Hg0 oxidation were also elaborated.