Kinetics and Mechanism of the Interaction of Alkali Calcium Silicate Glasses with Salt Melts in the KNO3–Pb(NO3)2 System

The interaction of alkali calcium silicate glasses with salt melts in the KNO₃–Pb(NO₃)₂ system is investigated at temperatures of 420–520°C. The chemical composition of crystalline coatings formed upon treatment contains both components of the initial glass (SiO₂, 9–12 wt %; CaO, 0.8–1.2 wt %) and components of the salt melt (PbO, 82–89 wt %). The treatment temperature is the main factor affecting the structure of the modified surface layer. The mechanism of the interaction of alkali calcium silicate glasses with salt melts is analyzed. According to this mechanism, the interaction involves the ion exchange (with the participation of Na⁺, K⁺, Ca²⁺, and Pb²⁺ ions), crystallization of modified surface layers, and incorporation of Pb _x O _y nanoparticles (formed in the salt melt) into the coating structure.     

Ion cyclotron resonance study of CO oxidation in the gas phase in the presence of rhenium cations with carbonyl and oxygen ligands. Comparison with heterogeneous catalysis

Gas-phase oxidation of CO in the presence of rhenium cations with carbonyl and oxygen ligands has been studied by Fourier transform ion cyclotron resonance (FT-ICR) spectrometry. Rhenium cations have been generated by the electron impact of Re₂(CO)₁₀ vapour. Contrary to the unreactive rhenium ions, rhenium monocarbonyi ions have been found to react with O₂ molecules yielding rhenium monoxide ions and CO₂ molecules. ReO⁺ ions are subsequently oxidized with O₂ to di- and trioxide ions. The bond energies in rhenium oxide ions were estimated as D°(Re⁺–O)=104±14, D°(ReO⁺–O)<118, D°ReO ₂ ⁺ –O)=122±4 kcal/mol. Simultaneous addition of CO and O₂ molecules to the reaction volume leads to the gas-phase catalytic oxidation of CO with pairs of rhenium oxide ions ReO ₃ ⁺ /ReO ₂ ⁺ serving as the oxidized and reduced forms of the catalyst. The mechanisms of the above reactions are discussed in connection with that for oxidation of CO over solid oxide catalysts.     

Exploration of molten hydroxide electrochemistry for thermal battery applications

The electrochemistry of molten LiOH–NaOH, LiOH–KOH, and NaOH–KOH was investigated using platinum, palladium, nickel, silver, aluminum and other electrodes. The fast kinetics of the Ag⁺/Ag electrode reaction suggests its use as a reference electrode in molten hydroxides. The key equilibrium reaction in each of these melts is 2 OH^– = H₂O + O^2– where H₂O is the Lux-Flood acid (oxide ion acceptor) and O^2– is the Lux–Flood base. This reaction dictates the minimum H₂O content attainable in the melt. Extensive heating at 500 °C simply converts more of the alkali metal hydroxide into the corresponding oxide, that is, Li₂O, Na₂O or K₂O. Thermodynamic calculations suggest that Li₂O acts as a Lux–Flood acid in molten NaOH–KOH via the dissolution reaction Li₂O_(s) + 2 OH^– = 2 LiO^– + H₂O whereas Na₂O acts as a Lux–Flood base, Na₂O_(s) = 2 Na⁺ + O^2–. The dominant limiting anodic reaction on platinum in all three melts is the oxidation of OH^– to yield oxygen, that is 2 OH^– 1/2 O₂ + H₂O + 2 e^–. The limiting cathodic reaction in these melts is the reduction of water in acidic melts ([H₂O] [O^2–]) and the reduction of Na⁺ or K⁺ in basic melts. The direct reduction of OH^– to hydrogen and O^2– is thermodynamically impossible in molten hydroxides. The electrostability window for thermal battery applications in molten hydroxides at 250–300 °C is 1.5 V in acidic melts and 2.5 V in basic melts. The use of aluminum substrates could possibly extend this window to 3 V or higher. Preliminary tests of the Li–Fe (LAN) anode in molten LiOH–KOH and NaOH–KOH show that this anode is not stable in these melts at acidic conditions. The presence of superoxide ions in these acidic melts likely contributes to this instability of lithium anodes. Thermal battery development using molten hydroxides will likely require less active anode materials such as Li–Al alloys or the use of more basic melts. It is well established that sodium metal is both soluble and stable in basic NaOH–KOH melts and has been used as a reference electrode for this system.     

Preparation of Ceria-Zeolite Catalysts by Different Techniques and Its Effect on Selective Catalytic Reduction of NO with NH3 at High Space Velocities

Several zeolite-based catalysts containing Ce³⁺ and/or CeO₂ were prepared by a variety of catalyst preparation techniques like ion exchange, solid-state ion exchange, impregnation and physical mixing and are characterised. Selective catalytic reduction was evaluated using simulated exhaust gas containing NO _x , NH₃, O₂ and H₂O at high space velocities (>180000 h^–1) in the temperature window 150–600 °C. The activity and selectivity in NO _x reduction was found to strongly depend on the charge compensating ions, crystallite size of the zeolite and CeO₂ content in the catalyst. CeO₂ mixed with zeolite having H⁺ or Ce³⁺ co-cations showed benificial effect and increased the NO _x conversion and selectivity. Among the different zeolite materials studied, the structure and the strength and amount of Brønsted acidity did not influence the NO _x conversion.     

Active-oxygen species on non-reducible rare-earth-oxide-based catalysts in oxidative coupling of methane

From supplementary in situ Raman spectroscopic studies of active-oxygen species on non-reducible rare-earth-oxide-based catalysts in the oxidative coupling of methane (OCM) and structural adaptability considerations, further support has been obtained for our proposal that there may be an active and elusive precursor (of O₂ ^– and O₂ ^2– adspecies), most probably O₃ ^2– formed from reversible redox coupling of an O₂ adspecies at an anionic vacancy with a neighboring O^2– in the surface lattice. This active precursor may initiate H abstraction from CH₄ and be itself converted to OH^–+O₂ ^–, or it may abstract an electron from the oxide lattice and be converted to O₂ ^2–+O^–. The prospect of developing this type of OCM catalysts is discussed.     

Thermal and chemical ionization in flames

Conclusions We have determined the rate constants of the potassium ionization process AA⁺+e^– in the flames of 2H₂+O₂+X (Ar, He) mixtures on the temperature interval 1500–2500° K. The activation energy of this process is close to the ionization potential of potassium (100 kcal).In our experiments the rate of ion formation in the front of a hydrogen flame seeded with potassium exceeded the purely thermal ionization rate by 0.5–2 orders. The presumed cause is recombination ionization of the potassium in the flame front, for example, K+O+OK⁺+O₂+e^–. This is confirmed by the intensification of ionization in the reaction zone in the presence of an excess of oxygen in homogeneous H₂-air and H₂–O₂–(He, Ar) mixtures with alkali impurities.At T=1700° K the recombination coefficient for electrons and potassium ions is close to 1·10^–8 cm³·sec^–1. For a more precise determination it is necessary to know the frequency of electron capture by molecules and atoms under the experimental conditions.Experiments on thermal ionization in turbulent flames confirm the earlier conclusion concerning the important role of mass transfer in the chemi-ionization of hydrocarbon flames.Fizika Goreniya i Vzryva, Vol. 6, No. 1, pp. 37–48, 1970     

Formation of no and NH4 + ions in the low-temperature peripheral zone of a flame

It is found that charged nitrogen oxides are formed not only in the front and the high-temperature combustion-product zone of a Bunsen-type flame but also in the relatively cold external region of the flame close to the burner nozzle. The peripheral NO⁺ is formed from atmospheric oxygen drawn in from outside the flame. The concentration of NO⁺ ions increases as the mixture is enriched with fuel. Possible pathways by which the peripheral NO⁺ ions may appear are considered. It is suggested that their precursor is the NH₄ ⁺ ion formed from ammonia.Karaganda. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 3, pp. 111–115, May–June, 1993.     

Influence of Adsorbed Water and Oxygen on the Photoluminescence and EPR of Por-TiO2 (Anatase)

The influence of adsorbed water, oxygen, air and vacuum on the photoluminescence (PL) and electron paramagnetic resonance (EPR) has been investigated in situ and quasi in situ for por-TiO₂ (anatase). The broad PL signal in the visible spectral region decreases with increasing partial pressure of oxygen and vanishes at pressures higher than 1 mbar. Adsorption of water leads (i) to a fast quenching and (ii) to a subsequent increase of the PL signal. The concentration of the O₂ ^–, O^–, O₃ ^– anion-radicals depends sensitively on the surface conditioning and on the illumination of the por-TiO₂. Ti₃+ centers could be observed only in vacuum treated samples. The concentrations of the Ti₃+ and oxygen anion radicals are in the range of 10¹⁵ and 10¹⁷ cm^–3, respectively.     

Electrodeposited Cu–ZnO and Mn–Cu–ZnO nanowire/tube catalysts for higher alcohols from syngas

Cu–ZnO and Mn–Cu–ZnO catalysts have been prepared by electrodeposition and tested for the synthesis of higher alcohols via CO hydrogenation. The catalysts were prepared in the form of nanowires and nanotubes using a nanoporous polycarbonate membrane, which served as a template for the electrodeposition of the precursor metals from an aqueous electrolyte solution. Electrodeposition was carried out using variable amounts of Zn(NO₃)₂, Cu(NO₃)₂, Mn(NO₃)₂ and NH₄NO₃ at different galvanostatic conditions. A fixed bed reactor was used to study the reaction of CO and H₂ to produce alcohols at 270 °C, 10–20 bar, H₂/CO = 2/1, and 10,000–33,000 scc/h g_cat. In addition to methane and CO₂, methanol was the main alcohol product. The addition of manganese to the Cu–ZnO catalyst increased the selectivity toward higher alcohols by reducing methane formation; however, CO₂ selectivity remained high. Maximum ethanol selectivity was 5.5%, measured as carbon efficiency.     

Hydrazine Radiolysis by Gamma-Ray in the N2H4–Cu+–HNO3 System

Radiolysis of chemical agents occurs during the decontamination of nuclear power plants. The γ-ray irradiation tests of the N₂H₄–Cu⁺–HNO₃ solution, a decontamination agent, were performed to investigate the effect of Cu⁺ ion and HNO₃ on N₂H₄ decomposition using a Co-60 high-dose irradiator. After the irradiation, the residues of N₂H₄ decomposition were analyzed by Ultraviolet-visible (UV) spectroscopy. NH₄⁺ ions generated from N₂H₄ radiolysis were analyzed by ion chromatography. Based on the results, the decomposition mechanism of N₂H₄ in the N₂H₄–Cu⁺–HNO₃ solution under γ-ray irradiation condition was derived. Cu⁺ ions form Cu⁺N₂H₄ complexes with N₂H₄, and then N₂H₄ is decomposed into intermediates. H⁺ ions and H^● radicals generated from the reaction between H⁺ ion and e_aq⁻ increased the N₂H₄ decomposition reaction. NO₃⁻ ions promoted the N₂H₄ decomposition by providing additional reaction paths: (1) the reaction between NO₃⁻ ions and N₂H₄^●+, and (2) the reaction between NO^● radical, which is the radiolysis product of NO₃⁻ ion, and N₂H₅⁺. Finally, the radiolytic decomposition mechanism of N₂H₄ obtained in the N₂H₄–Cu⁺–HNO₃ was schematically suggested.     

Anodic stability of propylene carbonate electrolytes at potentials above 4V against lithium: An on-line MS andin situ FTIR study

On-line mass spectroscopy (DEMS) andin situ FTIR spectroscopy provide a valuable extension to classical cyclic voltammetry since they enable the identification of reaction products as a function of applied potential. We have compared the CO₂ evolution during the electrooxidation of 0.5 M LiAsF₆, 0.5 M LiBF₄ and 0.5 M LiClO₄/propylene carbonate on platinum combining these techniques. The highest CO₂ formation rate was measured for 0.5 M LiClO₄/PC and the lowest for 0.5 M LiAsF₆, both with an on-set potential at 4.0 V against Li/Li⁺. On-line MS results in 0.5 M LiBF₄/PC show strong evidence for the formation (above 4.7 V) of carbonyl fluoride and other fluorinated species parallel to the CO₂ evolution. This indicates an anodic decomposition of BF ₄ ^– anions interacting with oxidized PC species. The role of the OH^–ions on the film formation on platinum at 2.0 V against Li/Li⁺ was also investigated within situ FTIR for the three electrolytes.This paper is dedicated to Professor Dr Fritz Beck on the occasion of his 60th birthday.     

Light‐Addressable Potentiometric Sensor with Nitrogen‐Incorporated Ceramic Sm2O3 Membrane for Chloride Ions Detection

A light‐addressable potentiometric sensor (LAPS) with ceramic samarium oxide (Sm₂O₃)‐sensing membrane treated by nitrogen plasma immersion ion implantation (PIII) has been proposed for chloride ion detection. For the pure Sm₂O₃‐LAPS, a potassium ion sensitivity of 39.21 mV/pK is obtained. With the nitrogen PIII treatment on Sm₂O₃‐sensing membrane, the N–O peak is observed by X‐ray photoelectron spectroscopy, implying the formation of positive charges, (N‐O)⁺ and (N‐O‐N)⁺, within the Sm₂O₃ film. The positive charges can attract the chloride ions to react with the surface sites of OH₂⁺, achieving a superior chloride ion sensitivity of 36.17 mV/pCl for the Sm₂O₃‐LAPS with nitrogen PIII treatment. The LAPS structure with ceramic Sm₂O₃‐sensing membrane can be used for future biosensing applications, especially for the potassium and chloride ions detection in serum.     

Corrosion of nickel in sodium sulphate-sodium chloride melts. Part I

The corrosion behaviour of Ni in molten Na₂SO₄, NaCl, and in mixtures of these two salts, at 900° C, in laboratory air and under O₂+SO₂/SO₃ atmospheres has been examined by electrochemical curves and topochemical analysis of corrosion products.Ni passivity in pure Na₂SO₄ was observed under potentiodynamic and potentiometric conditions, the passive film corresponding to NiO. Passivity was not so easy to achieve in chloride melts as in sulphate alone, but once a thick oxide film forms on the specimen, the Cl^– addition is accompanied by an increase in the film stability. The inhibiting role of NaCl on Ni in the passive-transpassive area was also evidenced. In opposition, halide additions (especially those up to 25%) increased the dissolution current densities of Ni in the active region. These higher dissolution rates are represented by the equation Ni₃S₂+4NaCl+1/2 O₂ = 2NiCl₂+2Na₂S + NiO which is also suggested as a critical factor in the Ni passivation.The passive capability found for Ni in Na₂SO₄/NaCl melts, in air, is destroyed by SO₃ atmospheres. This corrosion-stimulation is due to the SO₃ role in promoting reactions such as NiS + 3O^2– = Ni²⁺+SO₃+8e which would be potential-determining at the Ni surface until Ni²⁺ precipitates or the conjugate oxygen cathodic reduction process takes place. Microprobe analysis also evidenced S penetration which might be the reason for the Ni embrittlement.The polarization curves for Ni in pure NaCl showed the lack of a passive region; occurrence of extensive intergranular attack was also indicated by metallographic observation. The observed dissolution must occur at the expense of the Ni interactions with the species which intervene in the reaction equilibrium between O₂ and molten NaCl (O₂, Cl^–, Cl₂ and O^2–) as well as with the Na⁺ cations, as has been discussed elsewhere. Its self-sustaining nature is enhanced by the continuous reduction of the nickel ion content of the melt by NiCl₂ evaporation.     

Anionic Clays as Potential Slow-Release Fertilizers: Nitrate Ion Exchange

Several nitrate containing anionic clays were synthesized at different temperatures and the kinetics of NO₃ ^– release were determined to test their suitability as slow-release N fertilizers. A sample (Mg:Al = 2:1) synthesized at 60°C with smaller particle size released 75, 86 and 100% of its NO₃ ^– in 1, 3 and 7 days, respectively when equilibrated with a simulated soil solution. On the other hand, the 175°C/2 hrs sample with larger particle size released 65, 77 and 84% of its nitrate in 1, 3 and 7 days, respectively. Another anionic clay (synthesized at 175°C/24 hrs) of higher charge density (Mg:Al = 2:1) containing NO₃ ^– was equilibrated with a 0.012 N NaCl or Na₂CO₃ to test the role of different anions in releasing the NO₃ ^– anion from the interlayers. The results showed that Cl^– released more NO₃ ^– than did CO₃ ^2– from this anionic clay after all the treatment times probably as a result of the CO₃ ^2– anion blocking the release of NO₃ ^– from the interior of the crystals. When a lower charge density (Mg:Al = 3:1) sample (synthesized at 175°C/48 hrs) was equilibrated with 0.02N solution of anions the release of nitrate was as follows: Cl^– < F^– < SO₄ ⁼ CO₃ ^2–. These results suggest that the divalent SO₄ ⁼ and CO₃ ^2– anions are more effective in the release of NO₃ ^– from this lower charge density anionic clay. Time-resolved structural analysis of NO₃ ^– exchange with CO₃ ^2– in the above anionic clay using synchrotron x-ray diffraction showed that ion exchange is rapid because of small crystal size and lower charge density. Thus the release of NO₃ ^– from anionic clays is an interplay among the type of anions present in soil solution, their concentration, pH of soil solution, the charge density and crystal size of anionic clay etc.     

Experimental and modeling of the electrodialytic and dialytic treatment of a fly ash containing Cd,Cu and Pb

A one-dimensional model is developed for simulating the electrodialytic and dialytic treatment of a fly ash containing cadmium, copper and lead. Two experimental systems have been used, a column of ash and a stirred ash suspension. The movement of Cd, Cu and Pb has been modeled taking into account the diffusion transport resulting from the concentration gradients of their compounds through the membranes and boundary layers and the electromigration of their ionic, simple and complex species during the operation. The model also includes the electromigration of the non-contaminant most important principal ionic species in the system, H⁺ and OH⁻, proceeding of the electrolysis at the electrodes, Ca²⁺, CO₃ ⁼, SO₄ ⁼, etc. proceeding from the ash and Na⁺ and NO₃ ⁻, or citrate and ammonium ions incorporated as electrolyte solutions and/or as agent solution during the ash treatment. The simulation also takes into account that OH⁻ generated on the cathode, during the electrodialytic remediation, is periodically neutralized by the addition of nitric acid in the cathode compartment. The anion and cation-exchange membranes are simply represented as ionic filters that preclude the transport of the cations and anions, respectively, with the exception of H⁺ which is retarded but pass through the anion-exchange membrane.     

Transport of nitric acid through the anion-exchange membrane NEOSEPTA-AFN

The paper presents the determination of mobility of H₃O⁺ and NO₃⁻ ions and diffusivity of the non-dissociated form of acid on the basis of experiments with nitric acid carried out in a batch mixed cell with an anion-exchange membrane NEOSEPTA-AFN, which have been completed by measurements of membrane conductivity. The dependencies of mobility of H₃O⁺ and NO₃⁻ ions upon the acid concentration in the membrane were approximated by the second degree polynomial, whose coefficients were determined by the numerical integration of the differential equation describing the time dependence of nitric acid in the cell followed by an optimizing procedure. Diffusivity of the non-dissociated form was calculated from mobilities of H₃O⁺ and NO₃⁻ ions. The model used is based on the Nernst-Planck electro-diffusion equation and takes into account ionic equilibrium between the species. Using all the experimental data obtained at various acid concentrations and rotational speeds ofthe stirrers, it was found that the transport characteristics mentioned above (except for mobility of H₃O⁺ ions) were affected by the nitric acid concentration in the membrane.     

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.     

Thermal stability of palladium on ceria-doped alumina

Several palladium on alumina and ceria/alumina catalysts were prepared and oxidized in air between 400 and 1000°C. The metal dispersion was determined by hydrogen titration of adsorbed oxygen. Dispersions above 50% were maintained on 0.2% Pd/Al₂O₃ up to 900°C. Adding 5.0% ceria, or increasing the metal loading to 2.5%, greatly reduces the thermal stability of the palladium, such that the dispersion falls rapidly at 600°C. The rates of methane oxidation (moles of CO₂/g Pd h) at 250°C and 5% excess oxygen are nearly equal on 0.22–2.50% Pd/3.5–5.2% CeO₂/Al₂O₃, dispersion 14–42%, and 0.20–0.46% Pd/Al₂O₃, dispersion 59–86%, but are 10 to 20 times lower than the rate on 2.3% Pd/Al₂O₃, dispersion 11%. The lower rate of methane oxidation on ceria-promoted and highly dispersed palladium on alumina might be due to the conversion of the palladium into less active palladium oxide during reaction.     

An In Situ ESR Study of Pd/H-ZSM-5 Interaction with Different Adsorbents

In situ ESR at 120–473 K permits to monitor formation of transient paramagnetic ions/complexes (isolated Pd⁺ sites; Pd⁺/H₂O; Pd⁺/C₆H₆) upon interaction of isolated Pd²⁺ cations stabilized by the H-ZSM-5 matrix with different organic compounds and gas mixtures (NO, O₂, H₂O, H₂, propene, benzene). The in situ study provides insight into the elementary steps of redox processes on isolated Pd species in H-ZSM-5 zeolite under realistic conditions. Adsorbed water stabilizes the transient paramagnetic complex and decreases the rate of Pd²⁺ to Pd⁰ reduction by H₂. Strong bonding of NO _x ligands to Pd²⁺ species suppresses the reduction of Pd(II) ions. Sorption of benzene on preoxidized Pd²⁺/HZSM-5 is accompanied by an easy formation of organic cation-radicals and of a Pd⁺/benzene complex, the paramagnetic Pd⁺/benzene structure indicating a surprisingly high resistance to further reduction to Pd⁰. Illumination of the Pd/HZSM-5 by UV-visible light causes no measurable change in the redox properties of the catalyst.     

In situ DRIFTS study of NO reduction with CH4 over PtCo-ferrierite catalysts

The effect of platinum incorporated into Co-ferrierite catalyst on the selective catalytic reduction of NO_x with CH₄ was studied by means of in situ DRIFTS technique. NO adsorption on Co- and PtCo-ferrierite catalysts gave dinitrosyl and mononitrosyl species on Co²⁺ ions. The adsorption of NO + O₂ on Co-ferrierite yielded NO₂ (NO ₂ ⁺) species and also nitrites and nitrates. Similar species were observed on PtCo-ferrierite, although chemisorbed NO₂ was much more stable since it persisted at reaction temperatures as high as 723 K. The spectra of the pre-reduced bimetallic PtCo-ferrierite catalyst exposed to the CH₄ + NO + O₂ reaction mixture showed bands at 2200–2100 cm^-1, which were similar to results for a Pt-free sample but slightly more intense. In addition, strong bands of nitrate, almost unchanged with temperature, were observed. A very stable Co²⁺–NO₂ intermediate species was developed upon incorporation of Pt into the base Co-ferrierite catalyst.