Optimizing zirconia-based solid electrolyte cell operated as oxidizing reactor and chromatographic sensor

A zirconia-based (0.9ZrO₂ · 0.1Y₂O₃) high-temperature electrochemical reactor with three-electrode connection circuit was shown promising if used as a chromatographic sensor for quantitative organic gas detection as well as an organic gas sample preparation device for carbon isotopic analysis. The optimized parameters and working mode of the herein proposed solid electrolyte reactor provided complete organic gas oxidation to stoichiometric oxides without oxygen addition to the carrier gas flow at the temperature of 900 to 950°C. The maximum hydrocarbon gas sample amount was calculated for complete oxidation in the designed reactor. Due to its simple and reliable design, the solid electrolyte reactor can be used instead of a standard oxidizing reactor in an isotopic mass spectrometer.     

Zirconium dioxide-based solid electrolyte as a means of oxidation of organic compounds during isotopic assay of carbon

It is demonstrated that a high-temperature electrochemical reactor on the basis of zirconium dioxide (0.9ZrO₂ · 0.1Y₂O₃) with platinum electrodes may be promising as a device for preparing samples of organic gases for isotopic assay of carbon. Owing to a high catalytic activity of the surface of a porous platinum coating, it is possible to realize full oxidation of organic gases to stoichiometric oxides at 900–950°C and an oxygen flux equivalent to an electric current of 100 μA and higher. The reproducibility of the results of isotopic assay is better than that yielded by a standard oxidation reactor. Use of the solid-electrolyte reactor in a new device as a sensor makes it possible to simultaneously measure the concentration of organic gases passing through the reactor on the basis of the charge transported by oxygen ions through the wall of a ceramic tube. Simultaneously one can monitor purity of the carrier gas. Original Russian Text ? V.S. Sevast’yanov, E.M. Galimov, N.E. Babulevich, A.A. Arzhannikov, 2007, published in Elektrokhimiya, 2007, Vol. 43, No. 4, pp. 472–478.     

Solid-state potentiometric gas sensors—a supplement

Potentiometric in situ gas probes with solid electrolytes were described for the first time 50 years ago. It is the intention of the present communication to give some additions on oxygen sensors in reducing gases which have been neglected in a recent review (J Solid State Electrochem, 13:3, 2009). Today, the most frequently used solid electrolyte gas sensors are the lambda probes. In their electrodes is a very quick change between gas phases with excess of oxygen and excess of components of reducing gases. Changes in the interpretation of the mechanism of the interaction between gas and solid electrolyte in the gas sensors are discussed. At low temperatures, mixed potentials appear which have to be expected at oxygen electrodes also with air and technical oxygen, if these gases are not cleaned particularly. Among the CO₂ sensors, the system with the reference electrode SiO₂, Na₂Si₂O₅ on ß-alumina must be mentioned. This system has special advantages and in the commercially available device it reaches excellent long-term stability. A report about long-term measurements of SO₂ traces in air is worth mentioning. For measurements of SO₂ in industrial gases, Westinghouse has offered a complicated system in the 1980s.     

Remarkable Enhancement of O2 Activation on Yttrium‐Stabilized Zirconia Surface in a Dual Catalyst Bed

Yttrium‐stabilized zirconia (YSZ) has been extensively studied as an electrolyte material for solid oxide fuel cells (SOFC) but its performance in heterogeneous catalysis is also the object of a growing number of publications. In both applications, oxygen activation on the YSZ surface remains the step that hinders utilization at moderate temperature. It was demonstrated by oxygen isotope exchange that a dual catalyst bed system consisting of two successive LaMnO₃ and YSZ beds without intimate contact drastically enhances oxygen activation on the YSZ surface at 698 K. It can be concluded that LaMnO₃ activates the triplet ground‐state of molecular oxygen into a low‐lying singlet state, thereby facilitating the activation of the O₂ molecule on the YSZ oxygen vacancy sites. This phenomenon is shown to improve the catalytic activity of the LaMnO₃‐Pd/YSZ system for the partial oxidation of methane.     

Measuring oxygen yields of a thermal conversion/elemental analyzer‐isotope ratio mass spectrometer for organic and inorganic materials through injection of CO

The thermal conversion/elemental analyzer‐isotope ratio mass spectrometer (TC/EA‐IRMS) is widely used to measure the δ¹⁸O value of various substances. A premise for accurate δ¹⁸O measurement is that the oxygen in the sample can be converted into carbon monoxide (CO) quantitatively or at least proportionally. Therefore, a precise method to determine the oxygen yield of TC/EA‐IRMS measurements is needed. Most studies have used the CO peak area obtained from a known amount of a solid reference material (for example, benzoic acid) to calibrate the oxygen yield of the sample. Although it was assumed that the oxygen yield of the solid reference material is 100%, no direct evidence has been provided. As CO is the analyte gas for δ¹⁸O measurement by IRMS, in this study, we use a six‐port valve to inject CO gas into the TC/EA. The CO is carried to the IRMS by the He carrier gas and the CO peak area is measured by the IRMS. The CO peak area thus obtained from a known amount of the injected CO is used to calibrate the oxygen yield of the sample. The oxygen yields of commonly used organic and inorganic reference materials such as benzoic acid (C₆H₅COOH), silver phosphate (Ag₃PO₄), calcium carbonate (CaCO₃) and silicon dioxide (SiO₂) are investigated at different reactor temperatures and sample sizes. We obtained excellent linear correlation between the peak area for the injected CO and its oxygen atom amount. C₆H₅COOH has the highest oxygen yield, followed by Ag₃PO₄, CaCO₃ and SiO₂. The oxygen yields of TC/EA‐IRMS are less than 100% for both organic and inorganic substances, but the yields are relatively stable at the specified reactor temperature and for a given quantity of sample. Copyright © 2014 John Wiley & Sons, Ltd.     

Essential methodological improvements in the oxygen isotope ratio analysis of N‐containing organic compounds

The quantitative conversion of organically bound oxygen into CO, a prerequisite for the ¹⁸O/¹⁶O analysis of organic compounds, is generally performed by high‐temperature conversion in the presence of carbon at ～1450°C. Since this high‐temperature procedure demands complicated and expensive equipment, a lower temperature method that could be utilized on standard elemental analyzers was evaluated. By substituting glassy carbon with carbon black, the conversion temperature could be reduced to 1170°C. However, regardless of the temperature, N‐containing compounds yielded incorrect results, despite quantitative conversion of the bound oxygen into CO. We believe that the problems were partially caused by interfering gases produced by a secondary decomposition of N‐ and C‐containing polymers formed during the decomposition of the analyte. In order to overcome the interference, we replaced the gas chromatographic (GC) separation of CO and N₂ by reversible CO adsorption, yielding the possibility of collecting and purifying the CO more efficiently. After CO collection, the interfering gases were vented by means of a specific stream diverter, thus preventing them from entering the trap and the mass spectrometer. Simultaneously, a make‐up He flow was used to purge the gas‐specific trap before the desorption of the CO and its subsequent mass spectrometric analysis. Furthermore, the formation of interfering gases was reduced by the use of polyethylene as an additive for analytes with a N:O ratio greater than 1. These methodological modifications to the thermal conversion of N‐containing analytes, depending on their structure or O:N ratio, led to satisfactory results and showed that it was possible to optimize the conditions for their individual oxygen isotope ratio analysis, even at 1170°C. With these methodological modifications, correct and precise δ¹⁸O results were obtained on N‐containing analytes even at 1170°C. Differences from the expected standard values were below ±1‰ with standard deviations of the analysis <0.2‰. Copyright © 2010 John Wiley & Sons, Ltd.     

Experimental Investigation of Plasma Oxidization of Diesel Particulate Matter

Particulate matter (PM) from diesel vehicles is harmful to humans and should be removed from the exhaust gases before its emission into the atmosphere. Plasma PM oxidation is an advanced method to be used for oxidative PM removal. Factors influencing plasma PM oxidation include gas temperature, gas composition, PM amount, the geometry of plasma reactors. The PM oxidation in atmospheric air discharges was carried out using a pulsed dielectric barrier discharge reactor at temperatures of 100, 150, and 200 °C. It was found that PM is oxidized to CO and CO₂. CO₂/CO concentration ratio is a function of PM amount in the discharge space. PM removal efficiency (PM amount oxidized per kWh energy injection) increased with increasing air temperature and PM amount in the discharge space. Water promotes PM oxidation, which suggested that oxygen atoms produced in the discharge space react with water to yield hydroxyl free radicals that are of more reactivity than oxygen atoms. The activation energy of plasma PM oxidation was kinetically calculated to be 15.4 kJ/mol.     

Non-mass-dependent (17) O anomalies generated by a superimposed thermal gradient on a rarefied O(2) gas in a closed system

Cryogenic or heating methods have been widely used in experiments involving gas purification or isolation and in studying phase changes among solids, liquids, or gases for more than a century. Thermal gradients are often present in these routine processes. While stable isotopes of an element are known to fractionate under a thermal gradient, the largely diffusion‐driven fractionation is assumed to be entirely mass‐dependent. We report here, however, that distinct non‐mass‐dependent oxygen isotope fractionation can be generated when subjecting rarefied O₂ gas in a closed system to a simple thermal gradient. The Δ¹⁷O value, a measure of the ¹⁷O anomaly, can be up to ?0.51‰ (standard deviation (s.d.) 1σ = 0.03) in one of the temperature compartments. The magnitude of the ¹⁷O anomalies decreased with increasing initial gas pressures. The authenticity of this phenomenon is substantiated by a series of blank tests and isotope mass‐balance calculations. The observed anomalies are not the result of H₂O contamination in samples or in isotope ratio mass spectrometry. Our finding calls attention to the importance of thermal gradient‐induced isotope fractionation and to its implications in laboratory procedures, stable isotope geochemistry, and the physical chemistry of rarefied gases. Copyright © 2010 John Wiley & Sons, Ltd.     

Selective vapor‐phase oxidation of o‐xylene to phthalic anhydride over Co‐Mn/H3PW12O40@TiO2 using molecular oxygen as a green oxidant

The oxidation of o‐xylene to phthalic anhydride over Co‐Mn/H₃PW₁₂O₄₀@TiO₂ was investigated. The experimental results demonstrated that the prepared catalyst effectively catalyzed the oxidation of o‐xylene to phthalic anhydride. Also, the synergistic effect between three metals plays vital roles in this reaction. From a green chemistry point of view, this method is environmentally friendly due to carrying out the oxidation in a fixed‐bed reactor under solvent‐free condition and using molecular oxygen as a green and cheap oxidizing agent. The resulting solid catalysts were characterized by FT‐IR, XRD, XPS, ICP‐OES, FESEM, TEM, EDX, DR‐UV spectroscopy, BET and thermogravimetric analysis. The oxidation of o‐xylene yields four products: o‐tolualdehyde, phthaldialdehyde, phthalide and finally phthalic anhydride as the main product. The reaction conditions for oxidation of o‐xylene were optimized by varying the temperature, weight hourly space velocity and oxygen flow rate (contact time). The optimum weight percentage of phosphotungstic acid (HPW) and Co/Mn for phthalic anhydride production were 15 wt % and 2 wt%, respectively. The best Co/Mn ratio was found to be 10/1. Oxygen flow rate was very important on the phthalic anhydride formation. The optimum conditions for oxidation of o‐xylene were T = 370 °C, WHSV = 0.5 h^?1 and oxygen flow rate = 10 mL min^?1. Under optimized conditions, a maximum of 88.2% conversion and 75.5% selectivity to phthalic anhydride was achieved with the fresh catalyst. Moreover, reusability of the catalyst was studied and catalytic activity remained unchanged after at least five cycles.     

Cracking study of pentakis(dimethylamino)tantalum vapors by Knudsen cell mass spectrometry

Organometallic molecules are commonly used as gaseous precursors in Atomic Layer Deposition/Chemical Vapor Deposition (ALD/CVD) processes. However, the use of these molecules, which are generally thermally unstable at temperatures close to the deposition temperature, requires an understanding of their gas‐phase chemical behavior. The thermal cracking of the gaseous precursor, pentakis(dimethylamino) tantalum (PDMAT), generally adopted in the ALD/CVD TaN deposition processes, has been studied in the temperature range from 343 to 723K using a specific reactor coupled with a high‐temperature mass spectrometer. This reactor – built as tandem Knudsen cells – consists of two superimposed cells. The first stage reactor – an evaporation cell – provides an input saturated vapor flow operating from room temperature to 333K. The second stage cell, named the cracking cell, operated from 333 to 723K in the present study. Experiments showed the appearance of many gaseous species when the cracking temperature increased and, in particular, dimethylamine, corresponding to the saturated organic branches of PDMAT. Decomposition products of the HNC₂H₆ branch were observed at relatively high temperature, namely above 633K. This gas‐phase study – as for the preceding saturated one – shows the presence of oxygen‐containing molecules in PDMAT cracked vapor. Thus, it explains the systematic presence of oxygen contamination in the deposited TaN films observed in ALD/CVD industrial processes. Copyright © 2010 John Wiley & Sons, Ltd.     

Electrochemical characteristics of platinum-based binary catalysts for middle-temperature hydrogen-air fuel cells with phosphoric acid electrolyte

The high-temperature synthesis based on commercial catalyst E-TEK (40% Pt) using cobalt, chromium, and iron organic precursors as well as d-metal salts yielded PtM (1:1) catalysts (PtCo, PtCr, PtMn, PtNi, PtFe, and PtV) for electroreduction of molecular oxygen in concentrated H₃PO₄ at the temperature of 160°C. The phase composition of the synthesized catalysts was studied by powder diffraction. The electrochemical measurements were carried out in 15 M H₃PO₄ at 20 and 160°C using a model gas diffusion electrode. An assumption was made that close charging curves recorded for synthesized PtM catalysts in both hydrogen and oxygen adsorption ranges were due to formation of the core-shell structure: alloy core and surface layers enriched with platinum. The Tafel curves of molecular oxygen reduction in 15 M H₃PO₄ at 160°C were characterized with the sole slope of 0.10 to 0.11 V. The catalytic activity in the range of potentials from 0.8 to 0.9 V (RHE) was shown approximately twice as that of pure platinum catalyst. The highest activity was recorded for PtCo and PtCr binary catalysts. Their use in middle-temperature hydrogen-air fuel cells with solid polymeric electrolyte based on polybenzimidazole doped with phosphoric acid enabled 2- to 3-fold decrease of the platinum share in the cathode.     

Photocatalytic oxidation of gaseous trichloroethylene by UV/TiO2 process

Summary The objective of this study was to investigate the reaction behavior of the photocatalytic oxidation of gaseous trichloroethylene (TCE) using titanium dioxide at room temperature. The experiments were carried out under various humidity levels and oxygen contents of carrier gas in an annular photoreactor. Experimental results indicated that the factors affecting the photocatalysis of TCE by UV/TiO₂ process are carrier gases and humidity. It was found that increasing the relative humidity in inlet gas flow under low humidity could improve the decomposition of trichloroethylene and the mineralization of organic intermediates at an irradiation UV light intensity of 2.82 W m^-2 by UV/TiO₂ process. The photocatalytic kinetics of gaseous trichloroethylene can be described by the Langmuir-Hinshelwood rate equation.     

Isotopic Labeling Reveals Active Reaction Interfaces for Electrochemical Oxidation of Lithium Peroxide

The unresolved debate on the active reaction interface of electrochemical oxidation of lithium peroxide (Li₂O₂) prevents rational electrode and catalyst design for lithium‐oxygen (Li‐O₂) batteries. The reaction interface is studied by using isotope‐labeling techniques combined with time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) and on‐line electrochemical mass spectroscopy (OEMS) under practical cell operation conditions. Isotopically labelled microsized Li₂O₂ particles with an Li₂¹⁶O₂/electrode interface and an Li₂¹⁸O₂/electrolyte interface were fabricated. Upon oxidation, ¹⁸O₂ was evolved for the first quarter of the charge capacity followed by ¹⁶O₂. These observations unambiguously demonstrate that oxygen loss starts from the Li₂O₂/electrolyte interface instead of the Li₂O₂/electrode interface. The Li₂O₂ particles are in continuous contact with the catalyst/electrode, explaining why the solid catalyst is effective in oxidizing solid Li₂O₂ without losing contact.     

Gas–liquid mass transfer in the presence of ionic surfactant: effect of counter‐ions and interfacial turbulence

This work investigated the effect of counter‐ions and interfacial turbulence on oxygen transfer from gas to liquid phase containing ionic surfactant, and experiments were performed in a mechanically stirred reactor with flat gas–liquid interface. Counter‐ions in terms of hydration ability and polarizability influence the interfacial coverage of ionic surfactants (i.e. cetytrimethylammonium bromide (CTAB) and cetytrimethylammonium chloride) with the same hydrocarbon chain length, producing hindrance but in different extent on oxygen transfer. The addition of electrolyte (NH₄Br) substantially reduced the interfacial tension and surface charge of micelles (zeta potential) in CTAB system, and this salt effect greatly compressed interfacial double layer leading to gas transfer inhibition. The surface charge, aggregation number as well as stability of micelles formed above the critical micelle concentration could also alter interfacial configuration of surfactant layer reflected by gas absorption rate. Liquid turbulence was analyzed to decide the role of surfactant present in water on gas–liquid mass transfer, since Marangoni instability effect playing positive role should be taken into consideration under moderate liquid flow, while in turbulent system, contribution of Marangoni effect became overshadowed and consequently surfactant pose ‘barrier’ effect on gas transfer due to its surface active nature. Copyright © 2013 John Wiley & Sons, Ltd.     

Thermal and structure analysis of FeSO4·H2O-BaO2 mixtures

Investigations on the thermal decomposition of FeSO₄·H₂O-BaO₂ mixtures were carried out under isothermal conditions by using simultaneously solid electrolyte cell (EMF-method). Evoked interactions producing oxygen in the temperature range 553-673 K were established by means of a solid electrolyte oxygen analyzer. Based on Mössbauer spectroscopy data and X-ray analysis it was proved that these reactions were associated with the release of oxygen from barium peroxide, oxidation of ferrosulphate-monohydrate to FeOHSO₄, and formation of barium ferrites such as BaFe₂O₄ and BaFe₁₂O₁₉.     

Two‐Dimensional Spatial Resolution of Concentration Profiles in Catalytic Reactors by Planar Laser‐Induced Fluorescence: NO Reduction over Diesel Oxidation Catalysts

Planar laser‐induced fluorescence (PLIF) enables noninvasive in situ investigations of catalytic flow reactors. The method is based on the selective detection of two‐dimensional absolute concentration maps of conversion‐relevant species in the surrounding gas phase inside a catalytic channel. Exemplarily, the catalytic reduction of NO with hydrogen (2 NO+5 H₂→2 H₂O+2 NH₃) is investigated over a Pt/Al₂O₃ coated diesel oxidation catalyst by NO PLIF inside an optically accessible channel reactor. Quenching‐corrected 2D concentration maps of the NO fluorescence above the catalytic surface are obtained under both, nonreactive and reactive conditions. The impact of varying feed concentration, temperature, and flow velocities on NO concentration profiles are investigated in steady state. The technique presented has a high potential for a better understanding of interactions of mass transfer and surface kinetics in heterogeneously catalyzed gas‐phase reactions.     

TG measurement of reactivity of candidate oxygen carrier materials

This study examined several candidate raw materials for use as the reactive agents in developing new oxygen carriers for chemical looping combustion. A thermogravimetric analyzer, Mettler TGA/DSC1, was used to measure oxygen capacity and relative reaction rates during oxidation and reduction cycles. The reactive gases used were 4 % hydrogen in inert gas for the reduction cycle and air for the oxidation cycle, with a nitrogen purge between reduction and oxidation cycles. Samples were typically tested for at least ten cycles to study any change in reactivity or oxygen capacity. Reaction temperatures tested ranged from 700 to 900 °C. Materials tested included an iron oxide ore, iron-based tailings from a metals extraction process, a nickel oxide supported on nickel aluminate and a copper oxide plus inert material system. The materials varied in their oxygen capacity, reactivity and the change in properties with repeat cycles. Of the samples tested, the NiO–NiAl₂O₄ oxygen carrier demonstrated the fastest reaction in reduction and oxidation and had stable properties over ten cycles. The iron oxide ore sample performance declined significantly with repeat cycles. The performance of the iron-based tailings declined slightly over the ten cycles. The addition of inert second phase materials to CuO improved the performance by inhibiting sintering of the oxide at the operating temperature. Although the reactivity of the tailings and iron hydroxide samples was not as high as the NiO based oxygen carrier, they are promising carrier materials due to their low cost and lower toxicity relative to nickel. Future experiments will look at CO and CH₄ reduction reactions using the TG, surface characterization using SEM, XRD, and cyclic testing in a batch fluidized bed reactor.     

Mixed Conduction in Tb2O3 Doped BaCeO3

BaxCe0.8Tb0.2O3-a （x=0.98-1.03） solid electrolytes were synthesized and characterized by using X-ray diffraction （XRD）. By using AC impedance spectroscopy and gas concentration cell electromotive force （EMF） measurements, the electrical conduction behavior of the specimens was investigated in different gases during 500-1000 ℃ The influence of nonstoichiometry in the specimens with x ≠ 1 on conduction properties was studied and compared with that in the specimen with x = 1. The results show that the specimens are all of perovskite-type orthorhombic structure. In 500-1000 ℃, electronic hole conduction is dominant in dry and wet oxygen, air or nitrogen. Protonic conduction is dominant in wet hydrogen and it is about two orders of magnitude higher than that in hydrogen-free atmospheres （oxygen, air and nitrogen）. The electrical conductivity of the same specimen in water vapor-saturated oxygen, air or nitrogen is slightly higher than that in corresponding gas without water vapor. The electrical conductivities of the nonstoichiometric specimens are higher than those of the stoichiometric one.     

The reaction kinetics of dimethyl ether. I: High‐temperature pyrolysis and oxidation in flow reactors

Dimethyl ether reaction kinetics at high temperature were studied in two different flow reactors under highly dilute conditions. Pyrolysis of dimethyl ether was studied in a variable‐pressure flow reactor at 2.5 atm and 1118 K. Studies were also conducted in an atmospheric pressure flow reactor at about 1085 K. These experiments included trace‐oxygen‐assisted pyrolysis, as well as full oxidation experiments, with the equivalence ratio (ϕ) varying from 0.32 ≤ ϕ ≤ 3.4. On‐line, continuous, extractive sampling in conjunction with Fourier Transform Infra‐Red, Non‐Dispersive Infra‐Red (for CO and CO₂) and electrochemical (for O₂) analyses were performed to quantify species at specific locations along the axis of the turbulent flow reactors. Species concentrations were correlated against residence time in the reactor and species evolution profiles were compared to the predictions of a previously published detailed kinetic mechanism. Some changes were made to the model in order to improve agreement with the present experimental data. However, the revised model continues to reproduce previously reported high‐temperature jet‐stirred reactor and shock tube results. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet: 32: 713–740, 2000