Reactivity of lithium and barium carbonates with ZrO2:Y2O3 and Nasicon in solid state electrochemical gas sensors

The mutual reactivity in mixtures containing Nasicon (Na₃Zr₂Si₂PO₁₂) or YSZ (ZrO₂:Y₂O₃) solid electrolytes with Li₂CO₃ or Li₂CO₃:BaCO₃ sensing electrode materials was investigated using simultaneous DTA and TG and ex situ XRD techniques. The uncontrolled chemical reaction is suspected to be responsible for the instability of electrochemical gas sensors constructed from these materials. DTA and TG results obtained for Nasicon-carbonate mixtures indicate the possibility of reaction in the temperature range from about 470 to 650°C, which overlaps the sensor operating temperature range (300–525°C). The results obtained for YSZ-carbonate mixtures indicate that reaction between carbonate and the ZrO₂ takes place at higher temperatures and cannot explain the instability drift of investigated sensors. The mechanism of observed reactions in systems studied is also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Silicon Nitride Capacitive Chemical Sensor for Phosphate Ion Detection Based on Copper Phthalocyanine – Acrylate‐polymer

In this work, we report the development of a highly sensitive capacitance chemical sensor based on a copper C,C,C,C‐ tetra‐carboxylic phthalocyanine‐acrylate polymer adduct (Cu(II)TCPc‐PAA) for phosphate ions detection. A capacitance silicon nitride substrate based Al−Cu/Si‐p/SiO₂/Si₃N₄ structure was used as transducer. These materials have provided good stability of electrochemical measurements. The functionalized silicon‐based transducers with a Cu(II)Pc‐PAA membrane were characterized by using Mott‐Schottky technique measurements at different frequency ranges and for different phosphate concentrations. The morphological surface of the Cu(II)Pc‐PAA modified silicon‐nitride based transducer was characterized by contact angle measurements and atomic force microscopy. The pH effect was also investigated by the Mott‐Schottcky technique for different Tris‐HCl buffer solutions. The sensitivity of silicon nitride was studied at different pH of Tris‐HCl buffer solutions. This pH test has provided a sensitivity value of 51 mV/decade. The developed chemical sensor showed a good performance for phosphate ions detection within the range of 10⁻¹⁰ to 10⁻⁵ M with a Nernstian sensitivity of 27.7 mV/decade. The limit of detection of phosphate ions was determined at 1 nM. This chemical sensor was highly specific for phosphate ions when compared to other interfering ions as chloride, sulfate, carbonate and perchlorate. The present capacitive chemical sensor is thus very promising for sensitive and rapid detection of phosphate in environmental applications.     

Emission-based optical carbon dioxide sensing with HPTS in green chemistry reagents: room-temperature ionic liquids

We describe the characterization of a new optical CO₂ sensor based on the change in the fluorescence signal intensity of 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) in green chemistry reagents—room-temperature ionic liquids (RTILs). As far as we are aware, this is the first time RTILs, 1-methyl-3-butylimidazolium tetrafluoroborate (RTIL-I) and 1-methyl-3-butylimidazolium bromide (RTIL-II), have been used as matrix materials with HPTS in an optical CO₂ sensor. It should be noted that the solubility of CO₂ in water-miscible ionic liquids is approximately 10 to 20 times that in conventional solvents, polymer matrices, or water. The response of the sensor to gaseous and dissolved CO₂ has been evaluated. The luminescence intensity of HPTS at 519 and 521 nm decreased with the increasing concentrations of CO₂ by 90 and 75% in RTIL-I and RTIL-II, respectively. The response times of the sensing reagents were in the range 1–2 min for switching from nitrogen to CO₂, and 7–10 min for switching from CO₂ to nitrogen. The signal changes were fully reversible and no significant hysteresis was observed during the measurements. The stability of HPTS in RTILs was excellent and when stored in the ambient air of the laboratory there was no significant drift in signal intensity after 7 months. Our stability tests are still in progress.      

Adsorption technology for direct recovery of compressed,pure CO<Subscript>2</Subscript> from a flue gas without pre-compression or pre-drying

The effects of the sorption and the regeneration temperatures on the performance of a novel rapid thermal swing chemisorption (RTSC) process (Lee and Sircar in AIChE J. 54:2293–2302, 2008) for removal and recovery of CO₂ from an industrial flue gas without pre-compression, pre-drying, or pre-cooling of the gas were mathematically simulated. The process directly produced a nearly pure, compressed CO₂ by-product stream which will facilitate its subsequent sequestration. Na₂O promoted alumina was used as the CO₂ selective chemisorbent, and the preferred temperatures were found to be, respectively, 150 and 450 °C for the sorption and regeneration steps of the process. The specific cyclic CO₂ production capacity of the process and the pressure of the by-product CO₂ gas were substantially increased over those previously achieved by using the sorption and regeneration temperature of, respectively, 200 and 500 °C (Lee and Sircar in AIChE J. 54:2293–2302, 2008). The net compressed CO₂ recovery from the flue gas (∼92%) did not change. However, substantially different amounts of high and low pressure steam purges were necessary for comparable degree of desorption of CO₂. A first pass estimation of the capital and the operating costs of the RTSC process was carried out for a relatively moderate size application (flue gas clean up and CO₂ recovery from a ∼80 MW coal fired power plant). Both costs were substantially lower than those for a conventional absorption process using MEA as the CO₂ solvent (Desideri and Paolucci in Energy Convers. Manag. 40:1899–1915, 1999).     

Thermal behavior of Mg–Al–CO3 layered double hydroxide characterized by emanation thermal analysis

Emanation thermal analysis (ETA) was used to characterize microstructure changes during heating of Mg–Al–CO₃ layered double hydroxide (LDH) in the temperature range of 293–1473 K. It was confirmed by ETA that the formation of an intermediate phase with grafted CO₃^2– anions in the hydroxide layers took place in the temperature range of 508–523 K and the formation of Mg–Al mixed oxide (MO) occurred in the range 623–773 K. The small peak of the emanation rate at 603 K indicated the degradation of the layered structure and the broad peak in the range of 1073–1273 K characterized the onset of the separation of the decomposition products of MO into MgO and Mg₂Al₄O₇. The ETA results revealed that dehydration of the product with grafted CO₃^2– anions occurred at lower temperatures than that of the initial Mg–Al–CO₃ LDH.     

Carbon nanotube networks as gas sensors for NO2 detection

Networks of different carbon nanotube (CNT) materials were investigated as resistive gas sensors for NO₂ detection. Sensor films were fabricated by airbrushing dispersions of double-walled and multi-walled CNTs (DWNTs and MWNTs, respectively) on alumina substrates. Sensors were characterized by resistance measurements from 25 to 250 °C in air atmosphere in order to find the optimum detection temperature. Our results indicate that CNT networks were sensitive to NO₂ concentrations as low as 0.1 ppm. All tested sensors provided significantly lower response to interfering gases such as H₂, NH₃, toluene and octane. We demonstrate that the measured sensitivity upon exposure to NO₂ strongly depends on the employed CNT material. The highest sensitivity values were obtained at temperatures ranging between 100 and 200 °C. The best sensor performance, in terms of recovery time, was however achieved at 250 °C. Issues related to the gas detection mechanisms, as well as to CNT network thermal stability in detection experiments performed in air at high operation temperatures are also discussed.     

Non-catalytic pyrolysis of ethane to ethylene in the presence of CO2 with or without limited O2

Influence of the presence of CO₂, which is a mild oxidant, on the performance of the thermal cracking of ethane to ethylene in the absence or presence of limited O₂ at different temperatures (750–900‡C), space velocities (1500–9000 h^-1) and CO₂/C₂H₆ and O₂/C₂H₆ mole ratios (0–2.0 and 0–0.3 respectively) has been investigated. In both the presence and absence of limited O₂, ethane conversion increases markedly because of the presence of CO₂, indicating its beneficial effect on the ethane to ethylene cracking. The increased ethane conversion is, however, not due to the oxidation of ethane to ethylene by CO₂; the formation of carbon monoxide in the presence of CO₂ is found to be very small. It is most probably due to the activation of ethane in the presence of CO₂.     

Optical sensor for carbon dioxide combining colorimetric change of a pH indicator and a reference luminescent dye

A new optical CO₂ sensor based on the luminescence intensity change of the europium(III) complex tris(thenoyltrifluoroacetonato) europium(III) dihydrate ([Eu(tta)₃]) caused by the absorption change of various pH indicators—thymol blue, phenol red, or cresol red—with CO₂ was developed and its CO₂ sensing properties were investigated. For all the CO₂ sensors using pH indicators the observed luminescence intensity from [Eu(tta)₃] at 613 nm increased with increasing CO₂ concentration. The linear calibration method based on the plot of (I₁₀₀–I₀)/(I–I₀) versus the inverse of CO₂ concentration was suggested, where I₀ and I₁₀₀ were luminescence intensities at 613 nm of the CO₂ sensor film in 100% nitrogen and 100% gaseous CO₂. In all cases the plots showed good linearity and the correlation factors of the plots, r², were 0.991 for thymol blue, 0.990 for phenol red, and 0.998 for cresol red. The slopes of the plots (A/B) for thymol blue, phenol red, and cresol red were 2.2, 5.2, and 9.0%, respectively. The response times of the CO₂ sensor film were 4.0 s for thymol blue, 4.4 s for phenol red, and 8.8 s for cresol red for switching from nitrogen to CO₂, and the recovery times of films were 36 s for thymol blue, 39.2 s for phenol red, and 56.6 s for cresol red for switching from CO₂ to nitrogen. The signal changes were fully reversible and hysteresis was not observed during the measurements. The highly sensitive CO₂ sensor was developed using thymol blue as an indicator for the CO₂-sensing probe.     

Fiber-optic fluorescence sensor for dissolved oxygen detection based on fluorinated xerogel immobilized with ruthenium (II) complex

A fiber-optic sensor based on fluorescence quenching was designed for dissolved oxygen (DO) detection. The fluorinated xerogel-based sensing film of the present sensor was prepared from 3, 3, 3-trifluoropropyltrimethoxysilane (TFP–TriMOS). Oxygen-sensitive fluorophores of tris (2, 2′- bipyridine) ruthenium (II) (Ru(bpy)₃²⁺) were immobilized in the sensing film and the emission fluorescence was quenched by dissolved oxygen. In the sensor fabrication, a two-fiber probe was employed to obtain the best fluorescence collection efficiency and the sensing film was attached to the probe end. Scanning electron microscope (SEM), UV–Vis absorption spectroscopy (UV–Vis) and fourier transform infrared spectroscopy (FTIR) measurements have been used to characterize the sensing film. The sensor sensitivity is quantified by I _deoxy/I _oxy, where I _deoxy and I _oxy represented the detected fluorescence intensities in fully deoxygenated and fully oxygenated environments, respectively. Compared with tetramethoxysilane (TMOS) and methyltriethoxysilane (MTMS)-derived sensing films, TFP–TriMOS-based sensor exhibited excellent performances in dissolved oxygen detection with short response time of 4 s, low limit of detection (LOD) of 0.04 ppm (R.S.D. = 2.5%), linear Stern–Volmer calibration plot from 0 to 40 ppm and long-term stability during the past 10 months. The reasons for the preferable performances of TFP–TriMOS-based sensing film were discussed.     

The kinetics of gasification of char derived from sewage sludge

Gasification of char derived from sewage sludge was studied under different oxidizing atmospheres containing CO₂, O₂ or H₂O. The gasification tests were carried out in thermobalance at different temperatures and oxidizing reagent concentrations. The most efficient were the gaseous mixtures containing oxygen. The reaction took place at temperature 400–500 °C, whilst in the case of CO₂ and steam much higher temperatures (700–900 °C) were necessary to complete the conversion. Two rate models for gas–solid reaction were applied to describe the effect of char conversion on reaction rate. The shrinking core model for reaction-controlled regime was found to be the best for predicting the rate of char gasification in CO₂ and O₂ atmosphere. The experimental data for steam gasification of the char were fitted best by the first-order kinetics. The kinetic parameters estimated from the experimental data are in accordance with the literature for lignocellulosic char gasification and are the first published for sewage sludge char gasification.     

Sensitivity to hydrogen of sensor materials based on SnO2 promoted with 3d metals

The sensitivity to hydrogen and the catalytic activity in the oxidation of hydrogen of sensor materials based on tin dioxide and doped with cobalt, nickel, iron, and copper have been studied. The sensitivity of the sensors and the degree of conversion of hydrogen pass symbatically through a maximum with increasing quantity of each of the dopants. The results are explained by the influence of grain boundaries between tin dioxide and the dopants applied during the course of the oxidation reaction of hydrogen on the sensor material and on the sensitivity of the sensor to H₂. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 2, pp. 121–125, March–April, 2008.     

A new approach to determine soil microbial biomass by calorimetry

A method to determine soil microbial biomass (SMB) by isothermal calorimetry is reported. Soil samples ranging in pH from 6.2 to 9.4 and different textures were used to develop the method. Soil at 60% of its field capacity humidity was amended with a previously determined amount of glucose as to give the maximal response of CO₂ evolution. Then, an aliquot was weighed in the calorimeter ampoule and specific thermal power (p)–time (t) curves were obtained at 25 °C. After 1–2 h, a vial containing a 0.5 M NaOH solution was introduced to determine the specific thermal power due to CO₂ evolution, pCO₂ during 1–2 h. Then, the vial was removed and the experiment continued for 1–2 h. Specific thermal power due to CO₂ evolution was converted to rate (CO₂-C/mm³ g⁻¹ h⁻¹) by using the heat of reaction of CO₂ with NaOH and the molar volume. This value was further converted into SMB/μg g⁻¹ by using a conversion factor of 32.4. A guide to perform the calculations is given. Values of log SMB were linearly related with values of log p giving a similar relation to a previously reported where SMB was determined by conventional methods.     

A structural influence on the electrical double-layer characteristics of Al4C3-derived carbon

Several carbon materials were produced by reacting aluminum carbide with chlorine gas at different temperatures (400–900 °C). Chlorination temperature and porosity values showed the inversely related trends whereby the graphitization degree rises with the chlorination temperature. Electrochemical measurements performed in three-electrode test cells with 1.0-M Et₃MeNBF₄ electrolyte revealed that the changes in porosity parameters and the degree of graphitization are in good correlation with specific capacitance values. Capacitance depends on the structure of carbon and varies in studied chlorination range from 109 to 60 F g⁻¹ and from 114 to 64 F g⁻¹ for the negatively and positively charged electrode materials, respectively. An exceptionally low capacitance was observed for the material produced at 700 °C that was explained by the multiwall carbon nanobarrels and the highly ordered curved graphitic flakes, which have low specific surface and possess the relatively low specific surface-related capacitance.     

Effect of pore expansion and amine functionalization of mesoporous silica on CO2 adsorption over a wide range of conditions

Adsorption of CO₂ was investigated over a wide range of conditions on a series of mesoporous silica adsorbents comprised of conventional MCM-41, pore-expanded MCM-41 silica (PE-MCM-41) and triamine surface-modified PE-MCM-41 (TRI-PE-MCM-41). The isosteric heat of adsorption, calculated from adsorption isotherms at different temperatures (298–328 K), showed a significant increase in CO₂–adsorbent interaction after amine functionnalization of PE-MCM-41, consistent with the high CO₂ uptake in the very low range of CO₂ concentration. The CO₂ adsorption isotherm and kinetics data showed the high potential of TRI-PE-MCM-41 material for CO₂ removal in gas purification and separation applications. With TRI-PE-MCM-41, the CO₂ selectivity over N₂ was drastically improved over a wide range of conditions compared to pure mesoporous silica. Moreover, the adsorption was reversible and fast, and the adsorbent was thermally stable and tolerant to moisture.     

A gas sensor based on Prussian blue film for the detection of chlorobenzene vapor

A new resistance-type sensor based on Prussian blue film has been fabricated for the detection of chlorobenzene vapor. The effect of Prussian blue preparation conditions on the response of sensor was studied. The sensor exhibited good response and selectivity to chlorobenzene vapor. The sensor prepared with Fe₂(SO₄)₃ at 298 K has response 8.5 at operating voltage of 10 V. The selectivity of the sensor to chlorobenzene against all other tested gases is exceeding almost by 5.6 times. The sensor showed linear response to chlorobenzene vapor in the concentration range of 24–169 ppm at room temperature and at a 10 V operating voltage. The response and recovery time of the sensor was about 18 and 12 s, respectively. Sensor stability test indicated the sensor had a good stability. Furthermore, seven real samples of chlorobenzene vapor was measured using the sensor. The relative error was in the range of about ±1.3%.     

An ab initio and density functional study of microsolvation of carbon dioxide in water clusters and formation of carbonic acid

Geometry of the CO₂–H₂O complex and reaction barriers leading to the formation of H₂CO₃were studied at the RHF/6-311++G**, MP2/6-311++G**, B3LYP/AUG-cc-pVDZ, B3LYP/AUG-cc-pVTZ, MP2/AUG-cc-pVDZ and CCD/AUG-cc-pVDZ levels of theory. The rotational barrier of the CO₂–H₂O complex and the reaction barrier leading to the formation of H₂CO₃–H₂O from CO₂–(H₂O)₂ were studied using the first three of the above-mentioned methods. Microsolvation of CO₂ in water clusters having upto eight water molecules was studied using the B3LYP/AUG-cc-pVDZ method. Various methods except MP2/AUG-cc-pVDZ predict the equilibrium structure of the CO₂–H₂O complex to be symmetric while the MP2/AUG-cc-pVDZ method predicts it to be unsymmetric. Formation of H₂CO₃ from CO₂–H₂O is strongly catalyzed by the presence of a second water molecule. Atomic orbitals are strongly rehybridized in going from the equilibrium structures of the CO₂–H₂O and CO₂–(H₂O)₂ complexes to the transition states involved in the formation of H₂CO₃ and H₂CO₃–H₂O, respectively, as shown by hybridization displacement charges.     

Carbon dioxide adsorption on carbon nanomaterials

The adsorption of CO₂ on a number of activated carbons, thermal carbon black, and oxide materials at 195 K was studied using static and dynamic techniques. The landing surface areas ω(CO₂) ≈ 0.19 nm² on thermal carbon black and the absolute values of sorption for P/P ₀ < 0.4 were determined. The density of adsorbed CO₂ in the micropore volume was estimated at ρ(CO₂) = 0.91 g/cm³. It was demonstrated that the previously found effect of a weakening of the sorption interaction of nitrogen molecules with thin-walled materials (which manifested itself in an analysis of sorption isotherms by a comparative method) was pronounced to a lesser degree for the sorption of CO₂. At the same time, the presence of supermicropores in activated carbon samples resulted in overestimated values of surface areas. A dynamic method was proposed to measure the spectra of CO₂ desorption at 195–260 K using a SORBI-MS system for evaluating the binding energy of sorbate molecules with the surface.     

Effect of SnO2 particle size on the hydrogen sensitivity of adsorption–semiconductor sensors with CoxOy/SnO2 active coating

The effect of cobalt additions on the electrical resistance of adsorption–semiconductor sensors based on nanosized SnO₂ in air, their sensitivity to hydrogen, and the catalytic activity of the corresponding sensor materials in the oxidation of H₂ were studied. The extremal nature of the obtained relationships is explained by morphological features of the investigated systems based on nanosized SnO₂.     

Preparation and structural characterization of nanosized BiFe<Subscript>0.6</Subscript>Mn<Subscript>0.4</Subscript>O<Subscript>3</Subscript> as a novel material with high sensitivity towards LPG

Nanocrystalline BiFe_0.6Mn_0.4O₃ powders were synthesized by sol–gel citrate method and studied for gas sensing behavior to reducing gases such as LPG, CO, CH₄ and NH₃. The composition and the structure of the powders have been investigated by means of XRD and TEM. The result shows that the BiFe_0.6Mn_0.4O₃ powders have a rhombohedral distorted perovskite structure with an average crystallite size of 35–40 nm. The BiFe_0.6Mn_0.4O₃-based LPG sensor shows better sensitivity at an operating temperature of 250 °C. The dispersion of Pd on BiFe_0.6Mn_0.4O₃ in the ratio of 0.8 wt.% improved the sensitivity, selectivity and response time. In addition, it reduced the operating temperature from 250 to 210 °C for LPG sensor. The response time for LPG was less than 1 min.     

Effect of pyrolysis temperature and operating temperature on the performance of nanoporous carbon membranes

Technology designed to capture and store carbon dioxide (CO₂) will play a significant role in the near-term reduction of CO₂ emissions and is considered necessary to slow global warming. Nanoporous carbon (NPC) membranes show promise as a new generation of gas separation membranes suitable for CO₂ capture.We have made supported NPC membranes from polyfurfuryl alcohol (PFA) at various pyrolysis temperatures. Positron annihilation lifetime spectrometry (PALS) and wide angle X-ray diffraction (WAXD) results indicate that the pore size decreases whilst the porosity increases with increasing pyrolysis temperature. The membrane performance results support these findings with a significant increase in permeance being seen with increasing pyrolysis temperature, which relates to the increase in porosity.Mixed gas performance measurements also show an increase in CH₄ permeance as the operating temperature is increased from 35 to 200 °C, which can be related to an increase in the rate of diffusion. However, the selectivity decreases with increasing operating temperature due to the smaller changes in the CO₂ permeance. These smaller changes in CO₂ permeance can be related to the stronger adsorption of this gas on the carbon surface at lower operating temperatures. Interestingly, regardless of the original pyrolysis temperature, the selectivity at higher operating temperatures is similar, whereas the permeance remains related to this pyrolysis temperature.