Oxygen-rich Gas Mixture Research Articles

Oxidation of a Platinum–Tin Alloy Surface during Catalytic CO Oxidation

We have investigated the surface composition of a well-ordered Pt3Sn(111) surface during CO oxidation with ambient pressure X-ray photoemission spectroscopy. Oxidation of tin in the surface coincides with the onset of catalytic conversion, and we observe significant differences in the oxidation state and morphology of the oxide formed depending on the gas composition, with an oxygen-rich mixture leading to formation of 2D wetting layers and a CO-rich mixture leading to formation of 3D oxide islands. Spontaneous oscillations in conversion are observed at 300 °C in the oxygen-rich gas mixture and attributed to the combined effects of site blocking by tin oxides and by CO. The results highlight the importance of gas–surface interactions in determining the nature of oxides formed and thus the type and number of interfacial sites under reaction conditions.

Tailoring of TiO2 film crystal texture for higher photocatalysis efficiency

Abstract Nanocrystalline TiO2 thin films were deposited on borosilicate glass substrates using pulsed DC reactive magnetron sputtering in oxygen rich O2 Ar gas mixtures. The influence of sputtering power and argon‑oxygen gas flow ratio over crystallinity, crystal texture and surface morphology of the films was investigated. Interestingly, there was no correlation between the O/Ti concentration ratio in the films and their indirect band gap values. Based on experimentally observed band gap values and other properties of the films the model of the films consisting of nearly stoichiometric TiO2 crystals with a small fraction of weakly coordinated Ti impurities or amorphous low oxidation state titanium suboxide formations is proposed. Photocatalytic activity of the deposited films was analysed using inorganic markers and strong correlation between photocatalytic efficiency of TiO2 films with the (004) Crystalographic Preffered Orientation parameter was observed.

The O2 + NH3 Reaction Over Rh(110): Steady State Kinetics and Oscillatory Behavior

The kinetics of ammonia oxidation with oxygen over a Rh(110) surface were studied in the pressure range 10−5–10−4 mbar. Nitrogen was found to be the preferred product at low partial pressures ratios $$ {\text{p}}_{{{\text{o}}_{ 2} }} :{\text{p}}_{{{\text{NH}}_{ 3} }} $$ , while the NO pathway was favored with oxygen rich gas mixtures and at high temperature. The reactive sticking coefficient of O2 reaches up to 0.05 under steady state conditions. Pronounced hysteresis effects in the reaction rates were found in T-cycling experiments. Sustained oscillations in the reaction rates occurred under isothermal conditions at T = 620 K at a total pressure of 4 × 10−5 mbar.

Doped-vanadium oxides as sensing materials for high temperature operative selective ammonia gas sensors

Abstract Resistive electrochemical sensors based on vanadium oxides equipped with a pair of interdigital Au electrodes can detect NH3 gas selectively at high temperature (500 °C). NH3 addition in a base gas increased the relative conductance (σ/σ0). Addition of less electronegative cation (Ce, Zr, Mg) to V2O5 increased the response and recovery rates, while electronegative cation (Al, Fe, Ni) increased sensor response magnitude. Among the samples tested, Al and Ce co-doped sample (VAlCe) was the most suitable sensor. The VAlCe sensor responded rapidly and linearly to change in concentration of NH3 in the oxygen rich gas mixture and showed high selectivity in the presence of coexisting gases (NO, CO, H2). The presence of water vapor did not markedly decrease the response magnitude but increased the response rate; the 90% response and 50% recovery times were less than 15 s. Based on the in situ UV–vis results, a possible sensing mechanism is proposed; adsorbed NH3 causes reduction of V5+ to V4+, which results in the conductivity increase. Role of surface acidity on the selective detection of NH3 as a basic molecule is also discussed.

Hydrogen sensor based on WO3 subnano-clusters and Pt co-loaded on ZrO2

Abstract Pt and WO3 co-loaded ZrO2 equipped with a pair of interdigital Au electrodes has been tested as a H2 gas sensor. The resistance R estimated from the intersection of the semi-circle with the real axis in a complex-impedance plot was used as a sensing signal. H2 addition in a gas decreased the resistance. Pt–WO3/ZrO2 showed a higher response magnitude to H2 than WO3/ZrO2 and Pt/ZrO2. The sensor was found to respond consistently and rapidly to change in concentration of H2 in the oxygen rich and moist gas mixture at 120–150 °C. The conductivity of the device varied almost linearly with the H2 concentration from 0.25 to 1.25%. The rate constants (k) of Pt–WO3/ZrO2 for the response and recovery transients were 10 and 53 times higher than those of Pt/WO3, respectively. The response magnitude of Pt–WO3/ZrO2 to H2 was not markedly affected by changing the water vapor concentration from 1.0 to 3.4% and O2 concentration from 5 to 20%. The effect of WO3 loading of the Pt–WO3/ZrO2-based sensor combined with STEM and UV–vis results indicates that subnano-sized polytungstate clusters on the zirconia surface play an important role in the conductivity increase by H2 adsorption, which can be due to an increase in the number of acidic Hδ+ species on the polytungstate surface.

Endoscopic Removal of a Bronchial Carcinoma in a Dog Using One‐Lung Ventilation

To describe anesthetic management of endoscopic electrosurgical removal of a bronchial carcinoma, partially blocking the right main stem bronchus in a Cocker Spaniel. Clinical case report. Dog with a bronchial carcinoma. To allow sufficient space for the endoscope and to avoid an oxygen-rich gas mixture in the trachea, which carries the risk of an airway fire when electrocautery is used, a 1 lumen endobronchial tube (EBT) was inserted into the left main stem bronchus. One-lung ventilation (OLV) started with a volume-controlled ventilator was switched to pressure-controlled ventilation in combination with positive end-expiratory pressure (PEEP). Resection of the bronchial carcinoma was successful. The dog was hypercapnic throughout the procedure and a high alveolar-arterial oxygen gradient was measured. An EBT may be a feasible and safe option to provide OLV for bronchoscopic electrocautery with a closed thoracic cavity in dogs. EBT intubation for OLV should be considered as part of the anesthetic management of airway diseases treated with bronchoscopic electrocautery.

Optimization of Mg-based fiber optic hydrogen detectors by alloying the catalyst

Pd-capped gasochromic metal hydrides can be used as sensing layer in fiber optic hydrogen detectors. Using a sensing layer consisting of a 50 nm thick Mg 70Ti 30 film capped with a 30 nm Pd catalytic layer we demonstrate a drop in reflectance by a factor of 10, at hydrogen levels down to 10% of the lower flammability limit. The switching takes place within a few seconds, depending on the actual amount of hydrogen in the vicinity of the detector. We report on the sensitivity of the fiber optic detector for hydrogen, both in argon and oxygen rich environments. We further show that the sensing behavior in oxygen rich gas mixtures and in normal air can be improved by adding silver to the catalytic Pd cap layer.

Impedancemetric gas sensor based on Pt and WO3 co-loaded TiO2 and ZrO2 as total NOx sensing materials

Abstract Pt-loaded metal oxides [WO3/ZrO2, MOx/TiO2 (MOx = WO3, MoO3, V2O5), WO3 and TiO2] equipped with interdigital Au electrodes have been tested as a NOx (NO and NO2) gas sensor at 500 °C. The impedance value at 4 Hz was used as a sensing signal. Among the samples tested, Pt-WO3/TiO2 showed the highest sensor response magnitude to NO. The sensor was found to respond consistently and rapidly to change in concentration of NO and NO2 in the oxygen rich and moist gas mixture at 500 °C. The 90% response and 90% recovery times were as short as less than 5–10 s. The impedance at 4 Hz of the present device was found to vary almost linearly with the logarithm of NOx (NO or NO2) concentration from 10 to 570 ppm. Pt-WO3/TiO2 showed responses to NO and NO2 of the same algebraic sign and nearly the same magnitude, while Pt/WO3 and WO3/TiO2 showed higher response to NO than NO2. The impedance at 4 Hz in the presence of NO for Pt-WO3/TiO2 was almost equal at any O2 concentration examined (1–99%), while in the case of Pt/WO3 and WO3/TiO2 the impedance increased with the oxygen concentration. The features of Pt-WO3/TiO2 are favorable as a NOx sensor that can monitor and control the NOx concentration in automotive exhaust. The effect of WO3 loading of Pt-WO3/ZrO2-based sensor is studied to discuss the role of surface W-OH sites on the NOx sensing.

Polytungstate clusters on zirconia as a sensing material for a selective ammonia gas sensor

Tungstated-zirconia has been tested as a functional material for a selective ammonia gas sensor. The sensor has been found to respond consistently and rapidly to changes in concentration of ammonia in the oxygen rich and moist gas mixture at 673 K. The sensor showed negligible cross-sensitivity to propene, CO, and NO. From characterizations of tungstated-zirconia by XRD, Raman, UV–vis, high-resolution TEM, and ammonia-TPD, it was found that subnano-sized polytungstate clusters are uniformly supported on the zirconia surface, and the strong acid sites derived from the polytungstate clusters are suggested to effectively work as selective adsorption sites for ammonia. The fabricated ammonia sensor utilizing tungstated-zirconia is expected to be useful for controlling the injection amount of ammonia in urea-SCR systems.

NO2 Sensor Using Sc2O3-ZrO2 Solid Electrolyte and CuO + CuCr2O4 Sensing Electrode

Solid-state planar NO2 sensor using scandia-stabilised zirconia (8mol%Sc2O3-ZrO2) solid electrolyte and seven different oxide sensing electrodes have been tested in NO2 containing gaseous atmosphere in the range of 10 ppm to 500 ppm NO2 between 520 oC to 660 oC. Among the sensors incorporating seven different kinds of oxide sensing electrode, the sensor with CuO + CuCr2O4 mixed-oxide electrode has been found to respond consistently and rapidly to change in concentration of NO2 in the oxygen rich gas mixture at 660 ºC. A logarithmic relationship between the measured electromotive force (EMF) and NO2 concentration from 100 to 500 ppm has been observed at 660 ºC whereas a linear relationship between EMF and NO2 concentration has been observed below 30 ppm at the same temperature of measurement. The sensor exhibited negligible cross-sensitivity to CO and CH4 in the gas stream at 660 oC. A possible sensing mechanism has been discussed.

Amplification and scintillation properties of oxygen-rich gas mixtures for optical-TPC applications

We studied electron amplification and light emission fromavalanches in oxygen-containing gas mixtures. The mixturesinvestigated in this work included, among others, CO2 and N2Omixed with Triethylamine (TEA) or N2. Double-Step Parallel Gap(DSPG) multipliers and THick Gas Electron Multipliers (THGEM) wereinvestigated. High light yields were measured from CO2 + N2 andCO2 + TEA, though with different emission spectra. We observed thecharacteristic wave-length emission of N2 and of TEA and used apolymer wave-length shifter to convert TEA UV-light into the visiblespectrum. The results of these measurements indicate the applicabilityof optical recording of ionizing tracks in a TPC target-detectordesigned to study the cross-sections of the16O(γ, α)12C reaction, a central problem in nuclearastrophysics.

High-selectivity mixed-potential NO 2 sensor incorporating Au and CuO + CuCr 2O 4 electrode couple

A solid-state planar sensor consisting of scandia-stabilised zirconia (8 mol% Sc 2O 3–ZrO 2) solid electrolyte sandwiched between CuO + CuCr 2O 4 mixed-oxide and Au foil electrodes has been tested in NO 2 containing gaseous atmosphere in the range of 100–500 ppm NO 2 at 611 and 658 °C. The sensor has been found to respond consistently to change in concentration of NO 2 in the oxygen rich gas mixture at 611 and 658 °C within 8 s. The sensor showed negligible cross-sensitivity to O 2, CO and CH 4 in the gas stream at 611 °C and 658 °C.

Novel high-selectivity NO 2 sensor incorporating mixed-oxide electrode

Solid-state planar sensor using scandia-stabilised zirconia (8 mol% Sc 2O 3–ZrO 2) solid electrolyte and CuO + CuCr 2O 4 mixed-oxide sensing electrode has been tested in NO 2 containing gaseous atmosphere in the range of 10–500 ppm NO 2 between 518 and 659 °C. The sensor has been found to respond consistently and rapidly to change in concentration of NO 2 in the oxygen rich gas mixture at 659 °C. At 659 °C, a logarithmic relationship between the measured electromotive force (EMF) and NO 2 concentration from 100 to 500 ppm has been observed whereas a linear relationship between EMF and NO 2 concentration has been observed below 30 ppm. The sensor showed negligible cross-sensitivity to O 2, CO and CH 4 in the gas stream at 659 °C. A possible sensing mechanism has been discussed.

Electrochemical and gas-phase photocatalytic performance of nanostructured TiO2(B) prepared by novel synthetic route

Abstract Nanostructured phase pure TiO2(B) with microfibrous morphology was synthesized by newly developed protocol employing amorphous TiO2 as a precursor. Compared to traditional syntheses from K2Ti4O9, the new product exhibited better electrochemical performance and stability. Cyclic voltammetry of Li-insertion into the TiO2(B) evidences a pseudocapacitive faradaic process of Li accommodation which is basically different from the diffusion-controlled lithium storage in anatase or rutile. The presence of two pairs of peaks in cyclic voltammogram with formal potentials of ca. 1.5 and 1.6 V is specific for TiO2(B). This enables to use cyclic voltammetry for identification of this phase in a broad palette of TiO2 materials of various origin. The photocatalytic activity of TiO2(B) in a gas phase was evaluated using the total oxidation of propane with oxygen and the photocatalytic reduction of NO to N2 in an oxygen rich gas mixture. For the total oxidation of propane in the gas mixture containing 300 ppm propane and 20% oxygen, the reaction rates per 1 m2/g of the BET surface area of the catalyst for TiO2(B) prepared by our protocol and Hombifine N (anatase, SBET = 300 m2/g) are comparable. For the photocatalytic NO reduction to N2 in an atmosphere containing 20% oxygen the ratio of quantum yields for TiO2(B) and Hombifine N was found to be 0.08, which is roughly equivalent to the ratio of their BET surface areas (0.09), despite different phase composition of both materials. In comparison with the standard catalyst our material exhibited higher selectivity in the reduction of NO to N2.

On the Mechanism of NO Selective Catalytic Reduction by Hydrocarbons over Cu-ZSM-5 via X-ray Absorption Spectroscopic Study

An understanding of the catalytic mechanism of NOx reduction is critical for the development of next-generation high-fuel efficiency, low-emission vehicles. This paper compiles our investigations in recent years on the mechanism of NO selective catalytic reduction (SCR) by hydrocarbon over Cu-ZSM-5. The studies were focused on the oxidation state and coordination chemistry of the exchanged Cu as the active site during the catalytic reaction using X-ray absorption spectroscopic (XAS) techniques, mainly XANES and EXAFS. Our experiment demonstrated the existence of a redox mechanism which involves cyclic switching of the oxidation states between Cu(II) and Cu(I) in an oxygen-rich gas mixture under elevated temperature. We also observed the coordination structural change of copper ion in ZSM-5 accompanying the change of oxidation state. A correlation between cuprous ion concentration and catalytic activity was found in NO SCR by propene. The impact of another two hydrocarbons, propane and methane, on the copper redox behavior also appears to correlate to catalytic activities in the respective mixtures. Discussions on the nature of the active sites and the mechanism of SCR are presented based on the XAS data analysis. The similarity and difference of the physical properties of copper ion between NO catalytic decomposition and NO SCR are also discussed.

In situ characterization of Cu-ZSM-5 by X-ray absorption spectroscopy: XANES study of the copper oxidation state during selective catalytic reduction of nitric oxide by hydrocarbons

Reported here is our recent investigation of the mechanism of the selective nitric oxide reduction by hydrocarbons on Cu-ZSM-5 catalysts in an oxygen-rich gas mixture. We studied the copper oxidation state change during the catalytic reaction using the X-ray Absorption Near Edge Structure (XANES) method. We observe that even under strongly net oxidizing conditions, a significant fraction of the copper ions in ZSM-5 is reduced to Cu I at elevated temperature, when propene is present in the reactant stream. XANES spectra show that the Cu I 1s → 4p transition intensity, which is proportional to cuprous ion concentration, changes with the reaction temperture in a pattern similar to the NO conversion activity. For comparison purposes, we also studied the Cu I concentration change using a gas mixture in which propene was replaced by a stoichiometrically equivalent concentration of methane. Unlike propene, methane provides no NO selective reduction pathway over Cu-ZSM-5. No window of enhanced Cu I concentration was observed using methane as the reductant. Our study indicates that, even in a strongly oxidizing environment, cupric ion can be partially reduced by propene to form Cu I, possibly by way of allylic intermediate, which may be a crucial step for effective NO conversion through a redox mechanism.

Role of propene in the selective reduction of nitrogen monoxide in copper-exchanged zeolites

Gas switching experiments, examining the first few minutes of reaction, have been used to study the selective reduction of nitrogen monoxide by propene on a Cu-ZSM-5 catalyst in an oxygen-rich gas mixture. It has been found that the conversion of nitrogen monoxide to nitrogen reaches a steady-state activity in a very short period of time. It is concluded that carbon deposition is not responsible for the conversion of nitrogen monoxide into nitrogen. It is proposed instead that the hydrocarbon and oxygen act to maintain the active copper sites in an oxidation state suitable for direct nitrogen monoxide decomposition.

Reactive ion etching of poly(tetrafluoroethylene) in O2–CF4 plasmas

Films of poly(tetrafluoroethylene) (PTFE) were etched in O2–CF4 rf plasmas. The dependence of the etching rate of PTFE on the relative concentrations of these two gases was compared with data from optical emission spectra and measurements of peak-to-peak rf voltages Vpp indicative of relative sheath potentials. At a constant rf power (Vpp varied), etching rates were the greatest when using pure O2 and decreased with the addition of CF4. For Vpp held fixed (rf power varied), rates were invariant over a wide range of gas compositions. Ion bombardment-induced polymer radical generation is proposed as the initiating step in the PTFE etching sequence. Increases in etching rates with load size, contrary to conventional loading effect behavior, is attributed to a corresponding increase in Vpp for larger loads. Surface roughening induced by etching is more pronounced in oxygen-rich gas mixtures.

Treatment of Low Arterial Oxygen Tension in Anesthetized Horses with Clenbuterol

Clenbuterol (0.8 microgram/kg intravenously) was administered to 10 anesthetized horses with an abnormally low PaO2 (less than 90 mm Hg) despite controlled ventilation with an oxygen-rich gas mixture. Results were compared with those from 10 controls to which no clenbuterol was given and in which conventional methods to increase PaO2 were ongoing. Horses treated with clenbuterol had higher PaO2 values for at least 90 minutes. Clenbuterol was associated with increased heart rate and profuse sweating. Clenbuterol can be administered intravenously to increase the PaO2 of mechanically ventilated horses that have low arterial oxygen tension while under inhalation anesthesia. Further studies are warranted to define more precisely the circumstances under which clenbuterol may be used safely.

Competitive reactions of fluorine and oxygen with W, WSi2, and Si surfaces in reactive ion etching using CF4/O2

We have studied CF4/O2 based plasma and reactive ion etching of dc magnetron-sputtered tungsten, chemical vapor deposited tungsten silicide and single-crystal silicon. Etch rates and optical emission intensities of atomic fluorine and oxygen in the gas phase were determined as a function of gas composition. Etched surfaces were characterized using in situ x-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectrometry. All three materials exhibit an etch rate maximum as a function of oxygen addition. For silicon (7% O2) and WSi2 (10% O2) it occurs for a smaller percentage of added oxygen than the optical emission maximum of atomic fluorine (15% O2), whereas for W films (35% O2) it occurs at a greater percentage of added oxygen. In postplasma XPS analysis it is shown that for all materials the etch rate decrease coincides with the onset of significant oxidation. From the analysis of the degree of fluorination of the top surface, the oxygen/fluorine ratio of the reaction layer, and the oxygen percentage of the etch rate maximum relative to the fluorine emission maximum, we conclude that the etching of Si and WSi2 is limited by a reduction of fluorine adsorption because of competitive oxygen adsorption. For tungsten, which shows a very low degree of fluorination of the top surface, both the drop in etch rate and the formation of tungsten oxide can be explained by a reduced arrival rate of fluorine atoms to the tungsten surface when etching in oxygen-rich gas mixtures.