The Effect of Gaseous Diluents on Ozone Generation from Oxygen

Ozone generation in a negative corona discharge has been experimentally investigated using both a pure oxygen and in binary mixtures of oxygen with several gases. The concentration of ozone (O₃) in such mixtures is found to be dependent both on the input energy density η, dissipated in unit volume of gas mixture and on the type and the concentration of the additives. The experimentally measured dependencies of ozone concentration on the input energy density (O₃) = f(η) have been fitted using the Vasiliev–Kobozev–Eremin formula and the specific rate coefficients for ozone formation K_f and ozone decomposition K_d have been calculated. Using Ar, N₂ or CO₂ as admixtures, an increase in the specific rate coefficient for ozone generation was observed for increasing concentrations of added gaseous impurity into oxygen. In contrast, admixtures with SF₆ or CCl₂F₂ caused a substantial reduction of K_f values. The absolute values of ozone concentration at constant input energy density were observed to decrease with decreasing concentrations of oxygen in all mixtures.     

The transient response of ZrO2 oxygen sensors to step changes in gas composition

The transient voltage response of ZrO₂ oxygen sensors was examined following step changes in gas composition. The experiments were performed on a laboratory flow reactor at 600° C. Composition changes between (a) 100% and (b) 1% O₂ in N₂ produced response curves whose symmetry varied between composition steps (a) from low-to-high oxygen and (b) from high-to-low oxygen. This difference is due to the logarithmic dependence of sensor voltage on oxygen partial pressure. Corresponding oxygen partial pressure-time curves, derived from experimental voltage via the Nernst equation, are symmetric with respect to the direction of composition changes. Abrupt transitions are found in voltage-time curves at 600° C following step changes of reactive gases; e.g. from O₂/N₂ mixtures to CO/N₂, H₂/N₂ or D₂/N₂ mixtures. These voltage-steps represent transitions in stoichiometry of the surface boundary layer on the ZrO₂ sensor. Delay times before the transition also reflect reaction stoichiometry. Response times with O₂/CO, O₂/H₂ and O₂/D₂ follow trends predicted by the kinetic theory of gases. A limited number of experiments were performed to examine the relationships between sensor response and sensor catalytic activity. Poorer oxidation catalytic activity parallels slower response characteristics.     

A Comparative Study of the N2O-CO and N2O-H2 Reactions on the Ir(110) Surface with Emphasis on the Oscillatory Behavior

The N₂O-CO reaction has been studied on the Ir(110) surface in the temperature range from 300 up to 800 K and compared with the N₂O-H₂ reaction on the same surface. At temperatures up to 550 K (depending on the N₂O/reducing gas ratio), the reaction with H₂ is faster due to CO inhibition of the reduction of N₂O by CO. Isothermal sustained oscillations in rate were found in the temperature range from 373 to 377 K, for very low CO/N₂O ratios. The reaction products are N₂ and CO₂. Interestingly, the reactant N₂O oscillates in an almost counter-phase with the other reactant, CO. The period is around 60 s; also, highly correlated, synchronous events were found in regular time intervals. A model for the rate oscillations involves periodic transitions between a CO-rich (inactive) and a CO-poor (active) surface. In contrast, the rate oscillations for the N₂O-H₂ reaction are related to periodic transitions between O-rich and O-poor surfaces.     

Adsorption and reactivity during low temperature oxidation of carbon monoxide on Au/TiO2 catalysts studied by rapid response mass spectrometry

The adsorption and reaction of CO, CO₂ and O₂ on TiO₂ and Au/TiO₂ have been studied using a mass spectrometric method which can detect processes occurring on a time scale of seconds. Adsorption of CO on TiO₂ at 300 K is rapidly reversible and less on reduced samples than oxidised ones indicating that the adsorption sites are oxide ions. The amount adsorbed reversibly on reduced Au/TiO₂ is less still, consistent with enhanced reduction, but additional amounts adsorb irreversibly at a slower rate. The amount of CO₂ adsorbed under similar conditions is also greater on TiO₂ than reduced Au/TiO₂ and approximately one order of magnitude greater than that of CO. However, adsorption of O₂ is undetectable on the time scale of the measurement. Exposure of Au/TiO₂ to mixtures of CO and O₂ results in near instantaneous generation of CO₂ although its appearance is attenuated by adsorption. Adsorption of CO occurs concurrently in a way similar to that seen with CO alone except that the amount of the more slowly adsorbed form seems less. This suggests that it is the form utilised in catalysis. Oxygen uptake beyond that generating CO₂ is appreciable during the initial stages of exposure to reaction mixtures and this capacity is enhanced if one or other reactant is removed and then reintroduced, possibly due to the generation of reducible interface sites. It is concluded that the remarkable activity of Au/TiO₂ for CO oxidation at ambient temperature resides in a very high turnover frequency on sites at the interface between the metal and oxide. This revised version was published online in June 2006 with corrections to the Cover Date.     

On the Mechanisms of Titanium Particle Reactions in O2/N2 and O2/Ar Atmospheres

Combustion of titanium particles in air may potentially be used for the in situ synthesis of nanoscale TiO₂ particles, which can photocatalytically degrade chemical and biological air pollutants. The knowledge of Ti particle reactions in O₂‐containing atmospheres is required to develop this method. In the present work, large (∼3 mm) single Ti particles were heated by a laser in O₂/N₂ and O₂/Ar environments. High‐speed digital video recording, thermocouple measurements and quenching at different stages of the process were used for diagnostics. Analysis of the obtained temperature‐time curves and quenched particles does not show a significant influence of nitrogen on the oxidation of solid Ti. In all experiments, noticeable surface oxidation started at temperatures between ∼850 and ∼950 °C, leading to a sharp temperature rise at ∼1400 °C. During prolonged heating at the Ti melting point (1670 °C), a liquid TiO₂ bead formed and, after an induction period, ejected fragments. It was shown that this phenomenon may result from an excess of oxygen in the liquid bead. Fragment ejection in O₂/N₂ atmospheres was more intense than in O₂/Ar, indicating that N₂ accelerates the oxidation of liquid Ti.     

Some features of isothermal multicomponent mass transfer in the convective instability of gas mixture

Isothermal multicomponent diffusion in the H₂ + Ar–N₂ and CH₄ + Ar–N₂ three-component gas mixtures has been experimentally studied at various pressures and certain concentrations of components in binary mixtures. It has been shown that, in systems where diffusion coefficients differ significantly from each other, convective instability occurs with increasing pressure, which significantly intensifies multicomponent mass transfer. The parameters of transition diffusion mixing to convective can be defined in the framework of stability theory. The comparison carried out between experimental and calculated data shows satisfactory agreement between them.     

Analysis of Plasma Reaction for Decomposition of CHF3 in a Dielectric Barrier Discharge

The decomposition of CHF₃ in a mixture with O₂ and Ar was investigated in a coaxial dielectric barrier discharge at atmospheric pressure. CHF₃ decomposition increased linearly in regard to specific energy input (SEI), whereas energy efficiency decreased. The main product was CO_2, and its selectivity increased with high SEI and the presence of O₂ in the feed, but an increase of O₂ in the feed led to a decrease in decomposition rate. An increase in total flow rate led to an increase of the absolute amount of CHF₃ decomposition and energy efficiency; however, the decomposition of CHF₃ decreased. A complete CHF₃ decomposition occurred under an SEI of 1.54 kJ/L with the selectivity of CO₂ and CO as 89.87% and 7.00%, respectively. Optical emission spectroscopic analysis could explain the available reaction pathways for CHF₃ decomposition in the CHF₃/O₂/Ar atmospheric plasma and show the possibility of F₂ and HF formation.     

A computational study of physical and chemical inhibition in a perfectly stirred reactor

This paper reports a set of modeling studies that were undertaken to acquire a more detailed knowledge of combustion inhibition mechanisms. Mixtures of H₂/O₂/Ar reacting in the idealized perfectly stirred reactor were investigated. Three H₂/O₂ kinetic mechanisms were considered, differing from one another by the number of HO₂ reactions included. Two physical inhibitors, Ar and N₂, and one chemical inhibitor, HBr, were investigated. Additional parameters considered were pressure, equivalence ratio, inhibitor concentration and rate coefficient variation. The most effective inhibitor was HBr which acted chemically and caused substantial reduction in radical concentrations in the mixtures considered. The molecules Ar and N₂ acted as physical diluents with N₂, the more effective of the two due to its larger heat capacity.     

Sorption studies of CO<Subscript>2</Subscript>, CH<Subscript>4</Subscript>, N<Subscript>2</Subscript>, CO,O<Subscript>2</Subscript> and Ar on nanoporous aluminum terephthalate [MIL-53(Al)]

Aluminum terephthalate, MIL-53(Al), metal–organic framework synthesized hydrothermally and purified by solvent extraction method was used as an adsorbent for gas adsorption studies. The synthesized MIL-53(Al) was characterized by powder X-Ray diffraction analysis, surface area measurement using N₂ adsorption–desorption at 77 K, FTIR spectroscopy and thermo gravimetric analysis. Adsorption isotherms of CO₂, CH₄, CO, N₂, O₂ and Ar were measured at 288 and 303 K. The absolute adsorption capacity was found in the order CO₂>CH₄>CO>N₂>Ar>O₂. Henry’s constants, heat of adsorption in the low pressure region and adsorption selectivities for the adsorbate gases were calculated from their adsorption isotherms. The high selectivity and low heat of adsorption for CO₂ suggests that MIL-53(Al) is a potential adsorbent material for the separation of CO₂ from gas mixtures. The high selectivity for CH₄ over O₂ and its low heat of adsorption suggests that MIL-53(Al) could also be a compatible adsorbent for the separation of methane from methane–oxygen gas mixtures.     

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.     

Detailed Mechanism for Reduction of N2O over Rhodium by CO in Automotive Exhaust

Elementary-steps based mechanisms of CO–O₂ and CO–N₂O over rhodium catalyst were proposed and utilized to simulate experimental data from literature. The results showed that the mechanisms possess good prediction capability. It was found that the dissociation of adsorbed N₂O is the rate limiting step of N₂O reduction under conditions characterized by high CO coverages. The rather high light-off temperature (50 % conversion) of CO–N₂O (638 K) compared to that of CO–O₂ (453 K) is explained by the high temperature to initiate N₂O dissociation to offer surface oxygen needed for CO oxidation. Removing CO out of the reaction system, the oxygen generated via the dissociation of adsorbed N₂O accumulates on the surface of Rh, and finally leads to a poisoned catalyst and termination of the N₂O reduction process. However, increasing the inlet CO concentration inhibits the adsorption of N₂O to some extent, thus the reduction rate of N₂O is lowered on the contrary. Analysis of kinetic parameters showed that facilitating CO desorption or the decomposition of adsorbed N₂O leads to higher conversion of N₂O, with the latter having larger influence.     

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.     

An overview: Comparative kinetic behaviour of Pt, Rh and Pd in the NO + CO and NO + H2 reactions

This paper reports a comparative kinetic investigation of the overall reduction of NO in the presence of CO or H₂ over supported Pt-, Rh- and Pd-based catalysts. Different activity sequences have been established for the NO+H₂ reaction Pt/Al₂O₃>Pd/Al₂O₃>Rh/Al₂O₃ and for the NO+CO reaction Rh/Al₂O₃>Pd/Al₂O₃> Pt/Al₂O₃. It was found that both reactions differ from the rate determining step usually ascribed to the dissociation of chemisorbed NO molecules. The rate enhancement observed for the NO+H₂ reaction has been mainly related to the involvement of a dissociation step of chemisorbed NO molecules assisted by adjacent chemisorbed H atoms. The calculation of the kinetic and thermodynamic constants from steady-state rate measurements and subsequent comparisons show that Pd and Rh are predominantly covered by chemisorbed NO molecules in our operating conditions which could explain either changes in activity or in selectivity with the lack of ammonia formation on Rh/Al₂O₃ during the NO+H₂ reaction. Interestingly, Pd and Rh exhibit similar selectivity behaviour towards the production of nitrous oxide (N₂O) irrespective of the nature of the reducing agent (CO or H₂). A weak partial pressure dependency of the selectivity is observed which can be related to the predominant formation of N₂ via a reaction between chemisorbed NO molecules and N atoms, while over Pt-based catalysts the associative desorption of two adjacent N atoms would occur simultaneously. Such tendencies are still observed under lean conditions in the presence of an excess of oxygen. However, a detrimental effect is observed on the selectivity with an enhancement of the competitive H₂+O₂ reaction, and on the activity behaviour with a strong oxygen inhibiting effect on the rate of NO conversion, particularly on Rh.     

Catalytic partial oxidation of CH4 over bimetallic Ni‐Re/Al2O3: Kinetic determination for application in microreactor

The activity of a novel Ni‐Re/Al₂O₃ catalyst toward partial oxidation of methane was investigated in comparison with that of a precious‐metal Rh/Al₂O₃ catalyst. Reactions involving CH₄/O₂/Ar, CH₄/H₂O/Ar, CH₄/CO₂/Ar, CO/O₂/Ar, and H₂/O₂/Ar were performed to determine the kinetic expressions based on indirect partial oxidation scheme. A mathematical model comprising of Ergun equation as well as mass and energy balances with lumped indirect partial oxidation network was applied to obtain the kinetic parameters and then used to predict the reactant and product concentrations as well as temperature profiles within a fixed‐bed microreactor. H₂ and CO production as well as H₂/CO₂ and CO/CO₂ ratios from the reaction over Ni‐Re/Al₂O₃ catalyst were higher than those over Rh/Al₂O₃ catalyst. Simulation revealed that much smoother temperature profiles along the microreactor length were obtained when using Ni‐Re/Al₂O₃ catalyst. Steep hot‐spot temperature gradients, particularly at the entrance of the reactor, were, conversely, noted when using Rh/Al₂O₃ catalyst. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1691–1701, 2018     

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.     

Reduction and oxidation properties of Fe2O3/ZrO2 oxygen carrier for hydrogen production

Fe₂O₃ is a promising oxygen carrier for hydrogen production in the chemical-looping process. A set of kinetic studies on reduction with CH₄, CO and H₂ respectively, oxidation with water and oxygen containing Ar for chemical-looping hydrogen production was conducted. Fe₂O₃ (20 wt.%)/ZrO₂ was prepared by a co-precipitation method. The main variables in the TGA (thermogravimetric analyzer) experiment were temperatures and gas concentrations. The reaction kinetics parameters were estimated based on the experimental data. In the reduction by CH₄, CO and H₂, the reaction rate changed near FeO. Changes in the reaction rate due to phase transformation were observed at low temperature and low gas concentration during the reduction by CH₄, but the phenomenon was not remarkable for the reduction by CO and H₂. The reduction rate achieved using CO and H₂ was relatively faster than achieved using CH₄. The Hancock and Sharp method of comparing the kinetics of isothermal solid-state reactions was applied. A phase boundary controlled model (contacting sphere) was applied to the reduction of Fe₂O₃ to FeO by CH₄, and a different phase boundary controlled model (contacting infinite slab) was fit well to the reduction of FeO to Fe by CH₄. The reduction of Fe₂O₃ to Fe by CO and H₂ can be described by the former phase boundary controlled model (contacting sphere). This phase boundary controlled model (contacting sphere) also fit well for the oxidation of Fe to Fe₃O₄ by water and FeO to Fe₂O₃ by oxygen containing Ar. These kinetics data could be used to design chemical-looping hydrogen production systems.     

Influence of mineral matter in coal on decomposition of NO over coal chars and emission of NO during char combustion

NO-char reaction and char combustion in the presence and absence of mineral matter were studied in a quartz fixed bed reactor. Eight chars were prepared in a fluidized bed at 950 °C from four Chinese coals that were directly carbonized without pretreatment or were first deashed before carbonization. The decomposition of NO over these coal-derived chars was studied in Ar, CO/Ar and O₂/Ar atmospheres, respectively. The results show that NO is more easily reduced on chars from the raw coals than on their corresponding deashed coal chars. Mineral matter affects the enhancement both of CO and O₂ on the reduction of NO over coal chars. Alkali metal Na in mineral matter remarkably catalyzes NO-char reaction, while Fe promotes NO reduction with CO significantly. The effect of mineral matter on the emission of NO during char combustion was also investigated. The results show that the mineral constituents with catalytic activities for NO-char reaction result in the decrease of NO emission, whereas mineral constituents without catalytic activities lead to the increase of NO emission. Correlation between the effects of mineral matter on NO-char reaction and NO emission during char combustion was also discussed.     

Oxidation of ß-SiC at high temperature in Ar/O2, Ar/CO2, Ar/H2O gas mixtures: Active/passive transition

An experimental study of the active/passive transition in the oxidation of ß-SiC was carried out between 1650?°C and 1800?°C in Ar/O₂, Ar/H₂O and Ar/CO₂ gas mixtures or mixtures including two among these oxidant species. For that purpose an experimental device based on a Joule-heated SCS-6 fiber permitted determination of the oxidation regime from changes in electric current. The observed transitions were compared to results from other authors. Two predictive models of the active/passive transition were studied by means of a 3-D simulation of the experimental device in order to have an estimation of the volatilization rate of SiO₂. These models were compared against experimental results.     

Reactions of dinitrogen pentoxide and nitrogen dioxide with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine

The reactions of gaseous dinitrogen pentoxide (N₂O₅) and nitrogen dioxide (NO₂) with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) coated on the inside surface of a glass reaction cell were studied at 298 K. Unsaturated phosphatidylcholines are significant components of pulmonary surfactant in the alveolar region of the lung and hence serve as a simple model to examine reactions of pulmonary surfactant with these oxidant air pollutants. Using high-performance liquid chromatography (HPLC), Fourier transform infrared and fast atom bombardment mass spectroscopy, the major products of reactions of POPC with N₂O₅ and NO₂ were separated and identified. In the POPC-N₂O₅ reaction using either air or helium as a buffer gas, the nitronitrate, vinyl nitro and allylic nitro derivatives, as well as a small amount of thetrans-isomer of the starting material, were obtained. The nature of the products obtained from the POPC-NO₂ reaction depends on the concentration of NO₂ as well as whether air is present. At low NO₂ concentrations (P_{NO 2}/_{N 2}O₄≤3.8 Torr) in air or in helium, thetrans-isomer of POPC was formed almost exclusively. At higher NO₂ concentrations (P_{NO 2}/N₂O₄≥20 Torr) in helium, the dinitro, vinyl nitro and nitro alcohol derivatives were formed. In the presence of air (or 24%¹⁸O₂ in helium), a nitronitrate and a dinitronitrate were additional products. Mechanisms for the formation of the observed products and implications for the inhalation of oxides of nitrogen are discussed.     

Comparisons of pulverized coal combustion in air and in mixtures of O2/CO2

Pulverized coal combustion in air and the mixtures of O₂/CO₂ has been experimentally investigated in a 20 kW down-fired combustor (190 mm id×3 m). Detailed comparisons of gas temperature profiles, gas composition profiles, char burnouts, conversions of coal–N to NO_x and coal–S to SO₂ and CO emissions have been made between coal combustion in air and coal combustion in various O₂/CO₂ mixtures. The effectiveness of air/oxidant staging on reducing NO_x emissions has also been investigated for coal combustion in air and O₂/CO₂ mixtures. The results show that simply replacing the N₂ in the combustion air with CO₂ will result in a significant decrease of combustion gas temperatures. However, coal combustion in 30% O₂/70% CO₂ can produce matching gas temperature profiles to those of coal combustion in air while having a lower coal–N to NO_x conversion, a better char burnout and a lower CO emission. The results also confirm that air/oxidant staging is very effective in reducing NO_x emissions for coal combustion in both air and a 30% O₂/70% CO₂ mixture. SO₂ emissions are proved to be almost independent of the combustion media investigated.