Effects of reaction conditions on the selective oxidation of propane to acrylic acid on Mo-V-Te-Nb oxides

An effective Mo-1 V(0.3)Te(0.23)Nb(0.12)Ox catalysts for the selective oxidation of propane to acrylic acid was successfully prepared by using rotavap method. The catalyst was characterized by XRD and shown to contain (V0.07Mo0.93)(5)O-14, (Nb0.09Mo0.91)O-2.8,3MoO(2)(.)Nb(2)O(5), Mo5TeO16 and/or TeMo4O13, Te4Nb2O13 and a new TeMO (TeVMoO or TeVNbMoO; M = Mo, V and Nb) crystalline phase as the major phase. Regardless of the intrinsic catalytic characteristics of the catalyst, the external reaction conditions would have strong effects on the catalytic performance for propane oxidation. So in this paper, the effects of reaction conditions were investigated and discussed, including temperature, space velocity, V(air)/V(C3H8) ratio and V(steam)/V(C3H8) ratio. A stability test was also carried out on Mo1V0.3Te0.23Nb0.12Ox catalyst. The experimental run was performed during 100 h under the optimized reaction conditions. During the 100 h of operation, propane conversion and acrylic acid selectivity remained at about 59 and 64%, respectively. (C) 2004 Elsevier B.V. All rights reserved.

Identifying the isolated transition metal ions/oxides in molecular sieves and on oxide supports by UV resonance Raman spectroscopy

Isolated transition metal ions/oxides in molecular sieves and on surfaces are a class of active sites for selective oxidation of hydrocarbons. Identifying the active sites and their coordination structure is vital to understanding their essential role played in catalysis and designing and synthesizing more active and selective catalysts. The isolated transition metal ions in the framework of molecular sieves (e.g., TS-1, Fe-ZSM-5, and V-MCM-41) or on the surface of oxides (e.g., MoO3/Al2O3 and TiO2/SiO2) were successfully identified by UV resonance Raman spectroscopy. The charge transfer transitions between the transition metal ions and the oxygen anions are excited by a UV laser and consequently the UV resonance Raman effect greatly enhances the Raman signals of the isolated transition metal ions. The local coordination of these ions in the rigid framework of molecular sieves or in the relatively flexible structure on the surface can also be differentiated by the shifts of the resonance Raman bands. The relative concentration of the isolated transition metal ion/oxides could be estimated by the intensity ratio of Raman bands. This study demonstrates that the UV resonance Raman spectroscopy is a general technique that can be widely applied to the in-situ characterization of catalyst synthesis and catalytic reactions. (C) 2003 Elsevier Science (USA). All rights reserved.

Flow-through room temperature phosphorescence optosensing for the determination of lead in sea water

The chelates formed between the heavy metal ion Pb(II) and the reagents 8-hydroxy-5-quinolinesulphonic acid, 8-hydroxy-7-quinolinesulphonic acid and 8-hydroxy-7-iodo-5-quinolinesulphonic acid exhibit strong room temperature phosphorescence (RTP) if retained on the surface of anion exchange resin beads. Based on the on-line formation, in a flow-injection system, of such RTP lead chelates and their transient immobilization on an anion exchange resin, three flow-through optosensing systems are investigated for lead in sea water. Optimum experimental conditions and the analytical performance characteristics of the three optosensors are discussed. Relative standard deviations (RSDs) of the order of 3% are typical at 100 ng ml−1 Pb(II) and the active sensing phases can easily be regenerated by passing 500 μl of 6 M hydrochloric acid. A lead(II) detection limit of 0.1 ng ml−1 (3×background SD, for 2 ml sample injection volumes) was achieved for the optosensor based on 8-hydroxy-7-quinolinesulphonic acid. Possible interferences present in sea water, including cations and anions which could affect the sensor response, are discussed in detail. Finally, the selected RTP flow-through optical sensor has been successfully tested for the determination of lead in sea water at a few ng ml−1.

Determination of lead using a poly(vinyl chloride)-based crown ether membrane

A poly(vinyl chloride)(PVC)-based membrane of 15-crown-5 exhibits a good response for lead(II) ions over a wide concentration range. The response time of the sensor is 30 s and the membrane can be used for more than four months without observing any divergence. The selectivity of the sensor is comparable with those reported for other such electrodes. It was possible to determine lead in polluted waters using this electrode assembly.

Specific Adsorption of Trace Amounts of Calcium and Strontium by Hydrous Oxides of Iron and Aluminum1

Freshly prepared Fe and Al hydrous oxide gels and the amorphous product of heating gibbsite selectively adsorbed traces of Ca and Sr from solutions containing a large excess (∼1M) of NaNO3. The fraction of the added Ca (Sr) adsorbed depended principally on the suspension pH, the amount of solid present, and to a lesser extent on the NaNO3 concentration. Significant Ca and Sr adsorption occurred on the Fe and Al gels, and heated gibbsite, at pH values below the points of zero charge (8.1, 9.4, and 8.3±0.1, respectively), indicating specific adsorption. The pH (± 0.10) at which 50% of the Ca was adsorbed (pH50) occurred at pH 7.15 for the Fe gel (0.093M Fe), 8.35 for the Al gel (0.093M Al), and 6.70 for the heated gibbsite (0.181M Al); for Sr, the pH50 values were 7.10, 9.00, and 6.45, respectively. For the Fe gel and heated gibbsite, an empirical model based on the law of mass action described the pH dependence of adsorption reasonably well and suggested that for each Ca or Sr fraction adsorbed, approximately one proton was released. Failure of the Al gel to fit this model may have resulted from its rapid aging.

DNAzyme-based Colorimetric Sensing of Lead (Pb2+) Using Unmodified Gold Nanoparticle Probes

Novel functional oligonucleotides, especially DNAzymes with RNA-cleavage activity, have been intensively studied due to their potential applications in therapeutics and sensors. Taking advantage of the high specificity of 17E DNAzyme for Pb2+, highly sensitive and selective fluorescent, electrochemical and colorimetric sensors have been developed for Pb2+. In this work, we report a simple, sensitive and label-free 17E DNAzyme-based sensor for Pb2+ detection using unmodified gold nanoparticles (GNPs) based on the fact that unfolded single-stranded DNA could be adsorbed on the citrate protected GNPs while double-stranded DNA could not. By our method the substrate cleavage by the 17E DNAzyme in the presence of Pb2+ could be monitored by color change of GNPs, thereby Pb2+ detection was realized.

Perovskite-Like Mixed Oxides (LaSrMn1-x Ni (x) O4+delta, 0 a parts per thousand currency sign x a parts per thousand currency sign 1) as Catalyst for Catalytic NO Decomposition: TPD and TPR Studies

Catalytic NO decomposition on LaSrMn1-x Ni (x) O4+delta (0 a parts per thousand currency sign x a parts per thousand currency sign 1) is investigated. The activity of NO decomposition increases dramatically after the substitution of Ni for Mn, but decreases when Mn is completely replaced by Ni (x = 1.0). The optimum value is at x = 0.8. These indicate that the catalytic performance of the samples is contributed by the synergistic effect of Mn and Ni. O-2-TPD and H-2-TPR experiments are carried out to explain the change of activity. The former indicates that only when oxygen vacancy is created, could the catalyst show enhanced activity for NO decomposition; the latter suggests that the best activity is obtained from catalyst with the most matched redox potentials (in this work, the biggest Delta T and Delta E values).

A method for quantitative analysis of nitrogen oxides and N2 by means of mass spectrometry with the assistance of a catalytic technique for the reduction of NO with CO or hydrocarbons

Mass spectrometry is not able to differentiate NOx and N2 from other interferences (e.g. CO and C2H4) in the deNOx reactions. In the present study, a quantitative method for analysis of NOx and N2 simultaneously in these reactions with an assisted converter operated at higher temperature under O2-rich condition, which eliminates the interferences, is developed. The NOx conversion from this method is comparable to the one from an Automotive Emission Analyser equipped with NOx electrochemical sensor. Two types of deNOx reactions are tested in terms of selectivity of N2 production. The application of this method is discussed.

Potassium-Lead-Switched G-Quadruplexes: A New Class of DNA Logic Gates

A cation-driven allosteric G-quadruplex DNAzyme (PW17) was utilized to devise a conceptually new class of DNA logic gate based on cation-tuned ligand binding and release. K+ favors the binding of hemin to parallel-stranded PW17, thereby promoting the DNAzyme activity, whereas Pb2+ induces PW17 to undergo a parallel-to-antiparallel conformation transition and thus drives hemin to release from the G-quadruplex, deactivating the DNAzyme. Such a K+-Pb2+ switched G-quadruplex, in fact, functions as a two-input INHIBIT logic gate. With the introduction of another input EDTA, this G-quadruplex can be further utilized to construct a reversibly operated IMPLICATION gate.

High-sensitivity determination of lead and cadmium based on the Nafion-graphene composite film

Graphene nanosheets, dispersed in Nafion (Nafion-G) solution, were used in combination with in situ plated bismuth film electrode for fabricating the enhanced electrochemical sensing platform to determine the lead (Pb2+) and cadmium (Cd2+) by differential pulse anodic stripping voltammetry (DPASV). The electrochemical properties of the composite film modified glassy carbon electrode were investigated. It is found that the prepared Nafion-G composite film not only exhibited improved sensitivity for the metal ion detections, but also alleviated the interferences due to the synergistic effect of graphene nanosheets and Nafion. The linear calibration curves ranged from 0.5 mu g L-1 to 50 mu g L-1 for Pb2+ and 1.5 mu g L-1 to 30 mu g L-1 for Cd2+. respectively. The detection limits (S/N = 3) were estimated to be around 0.02 mu g L-1 for Pb2+ and Cd2+. The practical application of the proposed method was verified in the water sample determination.

Highly efficient inverted top-emitting organic light-emitting diodes using a lead monoxide electron injection layer

By introducing an effective electron injection layer (EIL) material, i.e., lead monoxide (PbO), combined with the optical design in device structure, a high efficiency inverted top-emitting organic light-emitting diode (ITOLED) with saturated and quite stable colors for different viewing angles is demonstrated. The green ITOLED based on 10-(2-benzothiazolyl)-1, 1, 7, 7-tetramethyl-2, 3, 6, 7-tetrahydro-1H, 5H, 11H-[1] benzopyrano [6, 7, 8-ij] quinolizin-11-one exhibits a maximum current efficiency of 33.8 cd/A and a maximum power efficiency of 16.6 lm/W, accompanied by a nearly Lambertian distribution as well as hardly detectable color variation in the 140 forward viewing cone. A detailed analysis on the role mechanism of PbO in electron injection demonstrates that the insertion of the PbO EIL significantly reduces operational voltage, thus greatly improving the device efficiency.