Mass transfer effects on ZrO2 oxygen concentration cells exposed to non-equilibrium H2-O2 mixtures

We report further studies of ZrO₂ oxygen concentration cells exposed to non-equilibrium gas mixtures. Substantial shifts of cell voltage-(H₂/O₂) composition curves are produced by changes in the gas flow rate past the cell. At high flow rates, there is an abrupt voltage step at about 2.2 times the H₂/O₂ stoichiometric ratio. At low flow rates this voltage step occurs at stoichiometry. These findings are understood on the basis of mass transfer to a heterogeneous catalyst/electrode surface.     

Hysteresis effects with ZrO2 oxygen sensors exposed to non-equilibrium oxygen-hydrocarbon mixtures

Hysteresis effects are reported for ZrO₂ oxygen sensors exposed to non-equilibrium oxygen/hydrocarbon gas mixtures. With oxygen/toluene at 800° C, voltage-composition curves differ following composition changes (a) from excess oxygen to excess toluene and (b) from excess toluene to excess oxygen. A catalysis model is developed to account for this behaviour: the Thiele modulus of the catalyst/electrode is presumed to differ under (a) net oxidizing and (b) net reducing conditions. Large scale voltage fluctuations, sometimes exceeding 400 mV, were observed in experiments with oxygen/isobutane mixtures at 600° C. This behaviour is analysed in terms of a kinetic model involving stochastic variations of relative mass transfer coefficients of oxygen and isobutane.     

Influence of H2O,CO2 and various combustible gases on the characteristics of a limiting current-type oxygen sensor

Current-voltage characteristics of limiting current-type oxygen sensors were investigated. The sensor showed a two-stage current plateau in current-voltage characteristics in H₂O–O₂–N₂ and CO₂–O₂–N₂ mixtures. The sensor current in the first stage corresponded to O₂ concentration and was practically independent of H₂O and CO₂ concentration in the gas mixtures. The sensor current in the second stage increased linearly with the H₂O or CO₂ concentration, for a sensor with high electrode activity. The behavior of the sensor suggests that the deoxidization of H₂O or CO₂ occurs at the sensor cathode. For nonequilibrium gas mixtures containing combustible gas and O₂, the sensor current in the first stage decreased linearly with combustible gas concentration. The decrease of the sensor current differed from that corresponding to the O₂ concentration consumed by the reaction of these gases in the ambient gas, depending on the kind of combustible gas. The reduction of the sensor current is explained by a model assuming that the reaction of these gases occurs at the cathode, and the diffusion of the combustible gas in the porous coating is a rate-limiting step.     

Gas sensing properties of asymmetrically reduced Na1/2Bi1/2TiO3–based ceramics

We report the gas sensing properties of a type of new materials, Na_1/2Bi_1/2TiO₃ (NBT)-based ceramics. After the NBT-based ceramics were asymmetrically reduced and coated with Au electrodes, the materials exhibit relatively large electrical responses when exposed to oxygen and some oxidizable gases at a relatively low temperature (≤300 °C). An electric voltage ～60 mV is measured in the mixture of O₂ and N₂ (1% O₂). In oxidizable gases, a negative response can be obtained. The measured voltages are ?45 mV and ?98 mV in the mixtures of H₂/air (1000 ppm H₂) and C₂H₅OH/air (1000 ppm C₂H₅OH), respectively. The electrical responses are proportional to the logarithm of the concentrations of the analyzed gases. Also, the electrical responses to oxygen and oxidizable gases have opposite signs, and the model of mixed-potential is proposed to explain the gas sensing phenomenon. This study provides a new material and a simple design for gas sensors. The proposed gas sensor comprises a reduced NBT-based ceramic wafer with the same electrodes on the opposite surfaces. Additional components in traditional gas sensors, such as sensing or reference electrode, are unnecessary.     

Effect of titanium dioxide on sintering and stabilization of ZrO2 in zirconium-alumina and spinel-zirconium mixtures

Conclusions The addition of titanium dioxide considerably reduces the sintering point of alumina and zirconia-alumina mixtures. Titanium dioxide does not have this effect on the sintering of the ternary equimolecular mixture ZrO₂MgO Al₂O₃. But the zero porosity was not even attained during firing up to 1700°. When the three-component mixture ZrO₂ + MgO + Al₂O₃ was sintered, spinel formed first, after which, at a higher temperature, there formed a solid solution ZrO₂ and MgO. In the presence of titanium dioxide some of the magnesium oxide apparently forms magnesium titanate with the TiO₂, and this impairs the stabilization of the zirconium dioxide, in view of which it is partially detected in monoclinic form in the fired mixtures. The addition of 2% TiO₂ reduces the temperature of polymorphous transitions of ZrO₂ by approximately 200°.Specimens of the composition 90% Al₂O₃ + 10% ZrO₂ and ZrO₂MgOAl₂O₃=111 show better spalling resistance than those made of alumina and those made of zirconium dioxide stabilized with magnesium or calcium oxide.Pure pre-synthesized spinel does not react with zirconium dioxide when fired up to 1600°, nor with titanium dioxide up to 1500°.The coefficient of thermal linear expansion of the equimolecular mixtures ZrO₂-MgO-Al₂O₃ and ZrO₂-CaO-Al₂O₃ is considerably lower than that of the corresponding mixtures without alumina.When the three component equimolecular mixtures ZrO₂-CaO-Al₂O₃ is sintered, calcium aluminate and the solid solution ZrO₂-CaO are formed.The two-component compositions Al₂O₃-ZrO₂ and three-component MgO-Al₂O₃-ZrO₂ have high refractoriness, satisfactory spalltng resistance, good stability-underload at high temperatures, and can be used as super duty refractories.     

Performance of zirconia membrane oxygen sensors at low temperatures with nonstoichiometric oxide electrodes

Sensor tests on zirconia membrane probes incorporating nonstoichiometric oxides of the general formula (U, M)O_2±x and having the fluorite structure have been described over the temperature range of 300 to 600° C and oxygen concentrations between 100 and 0.1%. The results have been compared with those for porous platinum electrodes. The sensors with (U, M)O_2±x electrodes performed well, down to 350°C, whereas those with porous platinum electrodes showed large deviations from Nernstian behaviour even at 450°C. The variables studied included the effect of dopant concentration and type, surface area of the electrode and heat treatment. The electrolyte used in the sensor tests had at least an order of magnitude lower conductivity compared with the traditionally used electrolytes such as Y₂O₃–ZrO₂ or Sc₂O₃–ZrO₂. Despite this, the better performance of sensors provided with (U, M)O_2±x electrodes suggests that the electrode/electrolyte interface plays a much more dominant role than the electrolyte.     

Characterization of ZrN,ZrO2 and β′–Zr7O11N2 nanoparticles synthesized by pulsed wire discharge

Nanoparticles of ZrN, ZrO₂ and β′–Zr₇O₁₁N₂ were synthesized by pulsed wire discharge using Zr wire in various O₂ and N₂ gas mixtures with different oxygen partial pressures of up to 40 kPa and a total pressure of 100 kPa. The syntheses were carried out at a relative energy ratio (K) of 6.4 which was defined by a charged energy in a capacitor of the synthesis apparatus divided by the evaporation energy of the Zr wire. Morphology and phase analyses were carried out on these nanoparticles by X‐ray diffraction and field‐emission transmission electron microscopy. ZrN and Zr₂N were observed in a sample synthesized in 100% N₂ gas while with increasing the O₂ from 1% of total pressure, formation of β′–Zr₇O₁₁N₂ and ZrO₂ was seen. By bright field image observation, electron energy loss spectroscopy (EELS) and selected area electron‐diffraction analyses, nanoparticles of ZrN, β′–Zr₇O₁₁N₂ and ZrO₂ were separately characterized. In these syntheses, nanoparticles of β′–Zr₇O₁₁N₂ existed in much smaller size and different shape than the ordinary spherical nanoparticles of ZrN and ZrO₂. In gas mixtures where O₂ contents were larger than 22% (dry air composition), ZrN was not detected or detected with just a fractional amount compared to the two major phases of β′‐Zr₇O₁₁N₂ and ZrO₂. EELS data for ZrN and β′–Zr₇O₁₁N₂ were obtained and compared by separated analyses of nanoparticles of these phases. From these data, it was concluded that, in nuclear accidents, small amount of particles can indicate the accident atmosphere around the Zr alloys fuel cladding.     

Transient analysis of oxygen storage capacity of Pt/CeO2–ZrO2 materials with millisecond- and second-time resolution

A simple method for determining the speed of oxygen release from Pt/CeO₂–ZrO₂ materials is proposed. We determined the speed of oxygen release from three differently prepared Pt(1 wt%)/CeO–ZrO₂ catalysts by two methods: the method employing temporal analysis of products (TAP) reactor with CO, O₂, and CO₂ pulsed gases, and a conventional method with CO and O₂ switch gases. Strong CO₂ adsorption precluded correct analysis of oxygen release speed, when it was determined from the amount of CO₂ formed. Therefore, instead of the amount of CO₂ formed, we suggested the t_max value, defined as the position of maximal intensity of CO₂ transient responses obtained upon low-intensity (10¹⁵–10¹⁶ molecules) CO pulsing in the TAP reactor. Shorter the t_max value, higher the rate of oxygen release, because the temperature dependence of the t_max determined in the present study resembles the previous results of oxygen release analysis [T. Tanabe, A. Suda, C. Descorme, D. Duprez, H. Shinjoh, M. Sugiura, Stud. Surf. Sci. Catal. 138 (2001) 135].     

Ceramics with eutectic microstructure in the ZrO2–PrOx system

Praseodymium oxides present redox properties analogous to those of Ce-based systems and have been proposed for catalytic applications in combination with CeO₂, ZrO₂, or both. However, uncertainties remain concerning the nature and redox behavior of Pr-rich mixtures, especially with ZrO₂. Here we study the eutectic composites of the ZrO₂–PrO_x system, focusing on the sensitivity of their microstructure, phase symmetry, and composition to variations of the processing atmosphere from oxidizing to reducing. Mixed oxides have been produced by a laser-assisted directional solidification technique in O₂, air, N₂, or 5%H₂(Ar) environment, and the resulting materials have been analyzed by scanning electron microscopy/energy-dispersive X-ray spectroscopy, X-ray diffraction, Raman spectroscopy, and magnetic susceptibility. In air, N₂, or 5%H₂(Ar) atmosphere, a lamellar, eutectic-like microstructure forms, the major phase being the one with less Pr content. Both the Pr concentration in each phase as the PrO_x molar percentage of the eutectic composites decrease as the atmosphere becomes more reducing. Both eutectic phases are fluorite-like when processing in air, whereas in N₂ or 5%H₂(Ar), the phase with high Pr content is of the A-R₂O₃ type, and the phase with low Pr content can be described as a fluorite phase containing C-R₂O₃-like short-range-ordered regions. The results obtained for samples processed in O₂ suggest that for high enough pO₂ no eutectic forms, in analogy with the ZrO₂–CeO₂ system. The evolution of the phase composition and symmetry is discussed in terms of the limited stability of the phases found in the ZrO₂–Pr₂O₃ system, namely, A- or C-R₂O₃-like, beyond a certain Pr oxidation degree and oxygen content.     

Periodic Control for Improved Low-Temperature Catalytic Activity

The influence of transient changes in the gas composition on the low-temperature activity of a commercial three-way catalyst and a Pt/Al₂O₃ model catalyst has been studied. By introducing well-controlled periodic O₂ pulses to simple gas mixtures of CO or C₃H₆ (in N₂), a substantial improvement of the low temperature oxidation activity was observed for both catalysts. The reason for low activity at low temperatures is normally attributed to self-poisoning by CO or hydrocarbons. The improved catalytic performance observed here is suggested to origin from the transients causing a surface reactant composition that is favourable for the reaction rate.     

N2O Decomposition and Formation of NO x Species on Fe–ferrierite. Effect of NO and CO Addition on the Decomposition and the Role of Surface Species

The adsorption of CO and CO₂ on Pt supported on ZrO₂ and Ce/La-promoted ZrO₂ was studied using DRIFTS. The presence of both La and Ce resulted in a decrease in the adsorption of CO at room temperature after reduction at 350 °C. The reduction in the CO adsorption is ascribed to an increase in the support reducibility when La and Ce are both present. Reduction at 350 °C leads to the formation of oxygen defects in the dual promoted support which have been probed using DRIFTS to monitor CO₂ dissociation. Hydrogen assisted dissociation is demonstrated on the ZrO₂, CeZrO₂, and LaZrO₂ supports. In the absence of hydrogen, the presence of oxygen vacancies is shown to be necessary for CO₂ dissociation.     

Phase transformations in stabilized ZrO2-Al2O3 compositions and properties of zirconium-corundum refractories

Solid-phase reactions in mixtures of cubic ZrO₂ with different compositions synthesized by induction melting and Al₂O₃ with different thermal prehistories are investigated at a temperature of 1750°C. Data on the change in the principal engineering properties of zirconium-corundum refractories as a function of their chemical composition are obtained.Translated from Ogneupory, No. 12, pp. 5–8, December, 1995.     

Poisoning of the adsorption of CO, O2 and D2 on Pt(111) by preadsorbed H2O

We have measured the influence of adsorbed H₂O on the sticking coefficients and saturation coverages of CO, O₂ and D₂ on Pt(111) at ∼100 K. Strong poisoning is observed for all three gases. For O₂ and D₂, the surface is essentially totally poisoned at 1 monolayer (ML) water coverage. For CO, the effect is weaker, with some CO adsorption still occurring at 2–3 ML H₂O. The influence of these results on the kinetics of the CO and H₂ oxidation reactions are discussed briefly. It is concluded that the influence of water must be included in kinetics simulations, at least at low temperatures, when significant humidity levels are present in inlet gas mixtures, or produced by the reactions themselves. This revised version was published online in July 2006 with corrections to the Cover Date.     

Investigation of the effect of ZrO2 and ZrO2/Al2O3 additions on the hot-pressing and properties of equimolecular mixtures of α- and β-Si3N4

In this work, hot-pressing of equimolecular mixtures of α- and β-Si₃N₄ was performed with addition of different amounts of sintering additives selected in the ZrO₂–Al₂O₃ system. Phase composition and microstructure of the hot-pressed samples was investigated. Densification behavior, mechanical and thermal properties were studied and explained based on the microstructure and phase composition. The optimum mixture from the ZrO₂–Al₂O₃ system for hot-pressing of silicon nitride to give high density materials was determined. Near fully dense silicon nitride materials were obtained only with the additions of zirconia and alumina. The liquid phase formed in the zirconia and alumina mixtures is important for effective hot-pressing. Based on these results, we conclude that pure zirconia is not an effective sintering additive. Selected mechanical and thermal properties of these materials are also presented. Hot-pressed Si₃N₄ ceramics, using mixtures from of ZrO₂/Al₂O₃ as additives, gave fracture toughness, K_IC, in the range of 3.7–6.2 MPa m^1/2 and Vicker hardness values in the range of 6–12 GPa. These properties compare well with currently available high performance silicon nitride ceramics. We also report on interesting thermal expansion behavior of these materials including negative thermal expansion coefficients for a few compositions.     

Oxidation of carbon monoxide and deuterium on a Pd(100) film

For Pd/MgO(100) pre-exposed to oxygen, the catalytic oxidation of CO and D₂, respectively, has been studied in the temperature range 100–300°C. At temperatures 200°C, the CO₂ desorption rate is independent of oxygen coverage, _O, and the reactive sticking coefficient for CO is close to unity. The D₂O desorption rate is strongly dependent on _O. D₂ adsorption is blocked by adsorbed oxygen and the maximum D₂O desorption rate is reached when almost all oxygen has been consumed (<0.03). The formation of an oxygen c(2×2) structure, coexisting with the initial p(2×2) phase, is reflected in the oxidation rates.     

Adsorption and separation of CH4/CO2/N2/H2/CO mixtures in hexagonally ordered carbon nanopipes CMK-5

Adsorption and separation of N₂, CH₄, CO₂, H₂ and CO mixtures in CMK-5 material at room temperature have been extensively investigated by a hybrid method of grand canonical Monte Carlo (GCMC) simulation and adsorption theory. The GCMC simulations show that the excess uptakes of pure CH₄ and CO₂ at 6.0 MPa and 298 K can reach 13.18 and 37.56 mmol/g, respectively. The dual-site Langmuir–Freundlich (DSLF) model was also utilized to fit the absolute adsorption isotherms of pure gases from molecular simulations. By using the fitted DSLF model parameters and ideal adsorption solution theory (IAST), we further predicted the adsorption separation of N₂–CH₄, CH₄–CO₂, N₂–CO₂, H₂–CO, H₂–CH₄ and H₂–CO₂ binary mixtures. The effect of the bulk gas composition on the selectivity of these gases is also studied. To improve the storage and separation performance, we finally tailor the structural parameters of CMK-5 material by using the hybrid method. It is found that the uptakes of pure gases, especially for CO_2, can be enhanced with the increase of pore diameter D_i, while the separation efficiency is apparently favored in the CMK-5 material with a smaller D_i. The selectivity at D_i=3.0 nm and 6.0 MPa gives the greatest value of 8.91, 7.28 and 27.52 for S_CO2/N2, S_CH4/H2 and S_CO2/H2, respectively. Our study shows that CMK-5 material is not only a promising candidate for gas storage, but also suitable for gas separation.     

Evaluation of CO2 permeation through functional assembled mono-layers: Relationships between structure and transport

CO₂ transport through functional assembled mono-layers was evaluated in relation to H₂O and nonpolar gases such as CH₄, O₂, N₂. Membranes based on Pebax^®2533 were functionalised by incorporating chemical compounds containing free hydroxyl, N-alkyl sulphonamide, bulky benzoate groups. The effects of both the chemical nature and concentration of the modifier on the gas transport were reported, respectively. The permeability coefficients of different penetrating chemical species were compared, evidencing the higher affinity of the layers to water vapour and carbon dioxide, due to favourable interactions between polar moieties and penetrant. The condensability of the penetrant directed the permeability of the species considered and was responsible for the high solubility selectivity between H₂O and CO₂ (i.e. , DH₂O/D₂CO=0.6, SH₂O/S₂CO=11.4 at 25 °C for Pebax/KET 50/50 w/w). An increase in polar moieties resulted in enhanced permeability and selectivity with respect to the pure polymer. In contrast, the functionalised polymer was not capable to discriminate between the smallest penetrants such as O₂ and N₂, with consequent decrease in the ideal selectivity (P₂CO/O₂, P₂CO/N₂). The functional layers exhibited permeability and selectivity covering broad ranges of values.     

Further data on solid solutions in ZrO2-TiO2 system

Conclusions It was confirmed that the compound ZrTiO₄ and two types of solid solutions are formed in the system ZrO₂-TiO₂. The beginning of the interaction between ZrO₂ and TiO₂ is noticed at 1200° after 30 hours.The solid solution of the distorted fluorite type is marked by a reduction in the monoclinic zirconium dioxide lattice parameters. The formation of a solution does not hamper the polymorphous transitions of the zirconium dioxide. The solid solution of the rutile type in the TiO₂ region is marked by an increase in the parameters of the titanium dioxide elementary cells. No polymorphous transitions of the ZrO₂ were noticed in these solid solutions.The addition of titanium dioxide shifts the polymorphous transition from 1050° in the case of pure zirconium dioxide to 600° in the specimen with an addition of 40–50 mole% TiO₂.The solid solutions of the rutile series as well as the compound ZrTiO₄ show equal thermal expansion, and despite the high porosity which they maintain up to 1700° may be put to practical use.The solid solutions of the zirconium series, containing up to 15% TiO₂, show sharp volumetric changes during the polymorphous transition and cannot be used for producing parts on account of this.     

Effect of gaseous medium on ZrO2 and CeO2 reactions and the fired material properties

Conclusions Cerium dioxide mixed with ZrO₂ and in the form of tetragonal solid solution with ZrO₂, preliminarily synthesized in an oxidizing atmosphere, is reduced in vacuum and in a reducing atmosphere to Ce₂O₃, and in this state forms solid solutions with ZrO₂; CeO₂ in vacuum and in a reducing atmosphere is partially reduced to the metal.At 1750–2000°C and with concentrations of 5.55–16.65 mol% Ce₂O₃ forms with ZrO₂ cubic solid solutions with a crystal lattice of the fluorite type which during rapid cooling to room temperature preserves its structure, and with sufficiently slow cooling decomposes into a solid solution of Ce₂Zr₂O₇-ZrO₂ of the pyrochlore type, and monoclinic solid solution ZrO₂-Ce₂O₃.During combined stabilization and sintering the composition ZrO₂-Ce₂O₃ sinters badly; and furthermore the density and strength of the fired products diminish with increase in the content of stabilizing oxides. Dense and strong articles of composition ZrO₂-Ce₂O₃ are obtained from presynthesized solid solutions ZrO₂-CeO₂ by intermediate fine grinding.Solid solutions ZrO₂-Ce₂O₃ with an increase in temperature are oxidized in air, which leads to destruction of dense articles. The high (15% or more) porosity of the specimens contributes to the compensation of the volume changes in the changes during oxidation, so there are no essential changes in the strength of the articles.Translated from Ogneupory, No.1, pp. 40–46, January, 1970.     

Electrochemical decomposition of CO2 and CO gases using porous yttria-stabilized zirconia cell

Electrochemical decomposition of CO₂ and CO gases using a porous cell of Ru-8 mol% yttria-stabilized zirconia (YSZ) anode/porous YSZ electrolyte/Ni–YSZ cathode system at 400–800 °C was studied by analyzing the flow rate and composition of outlet gas, current density, and phases and elementary distribution of the electrodes and electrolyte. A part of CO₂ gas supplied at 50 ml/min was decomposed to solid carbon and O₂ gas through the cell at the electric field strengths of 0.9–1.0 V/cm. The outlet gas at a flow rate of 3 ml/min included 61–63% CO₂ and 37–39% O₂ at 700–800 °C and the outlet gas at a flow rate of 50 ml/min included 73–96% (average 85%) CO₂ and 4–27% (average 15%) O₂ at 800 °C. On the other hand, the supplied CO gas was also decomposed to solid carbon, O₂ and CO₂ gases at 800 °C. The fraction of outlet gas at a flow rate of 50 ml/min during the CO decomposition at 800 °C for 5 h was 11–36% CO, 59–81% O₂ and 2–9% CO₂. The detailed decomposition mechanisms of CO₂ and CO gases are discussed. Both Ni metal in the cathode and porous YSZ grains under the DC electric field have the ability to decompose CO gas into solid carbon and O²⁻ ions or O₂ gas.