Showing papers by "Xiamen University published in 1994"

Pulse studies of ch4 interaction with nio/al2o3 catalysts

Abstract: Pulse studies of the interaction of CH4 and NiO/Al2O3 catalysts at 500°C indicate that CH4 adsorption on reduced nickel sites is a key step for CH4 oxidative conversion. On an oxygen-rich surface, CH4 conversion is low and the selectivity of CO2 is higher than that of CO. With the consumption of surface oxygen, CO selectivity increases while the CO2 selectivity falls. The conversion of CH4 is small at 500°C when a pulse of CH4/O2 (CH4∶O2=2∶1) is introduced to the partially reduced catalyst, indicating that CH4 and O2 adsorption are competitive steps and the adsorption of O2 is more favorable than CH4 adsorption

Direct Evidence for β-Cyclodextrin-Induced Aggregation of Ionic Surfactant below Critical Micelle Concentration

Abstract: The fluorescence emission of p-dimethylaminochalcone (DMAC) has been monitored in β-cyclodextrin (β-CD) and cetyltrimethylammonium bromide (CTMAB) or sodium dodecylsulfate (SDS) mixture. In the presence of a fixed amount of β-CD, a peak-shaped variation is observed in the curve of DMAC fluorescence intensity vs. surfactant concentration below critical micelle concentration (cmc). The peak height is dependent on β-CD concentration, and no peak is noted in the absence of β-CD. Such a peak-shaped variation in DMAC fluorescence emission is shown to be direct evidence for β-CD-induced aggregation of the surfactant below cmc in aqueous solution. The 1:1 surfactant/β-CD inclusion complex is the hydrophobic source inducing the aggregation.

Microsecond pulsed glow discharge time-of-flight mass spectrometer

Abstract: A linear time-of-flight mass spectrometer (TOFMS) has been designed, constructed, and coupled with a glow discharge source in microsecond pulsed mode (MSPGD). Orthogonal acceleration, a DC quadrupole and deflecting pulse techniques are used to diminish kinetic distribution and the spatial distribution of ions, and for deflecting Ar+ ions in their flight path. Comparison was made in the same discharge source between MSPGD and DC discharge. The continuous ion current is only 0.2 nA in the DC discharge mode, while the peak ion current reaches over 100 nA in the MSPGD mode. In addition, the ratio of the repelled ions to total ions is much higher in MSPGD than with a DC discharge in TOFMS. The mechanism of MSPGD is discussed. A resolving power of 500 was achieved, which is excellent for elemental analysis. To the authors' knowledge, this is the first time that a MSPGD-TOFMS combination has been described. The system is now being further optimized to improve its performance.

New algorithm for nonorthogonalab initio valence-bond calculations: I. New strategy and basic expressions

Abstract: A new algorithm for nonorthogonalab initio valence bond calculation has been deduced based on the left-coset decomposition of the symmetric groupSN. The strategy in the new approach is that, instead of performing the summation overN! permutations of groupSN, we sum the left cosets, and each coset corresponds to a “positive determinant” of orderN/2 for the evaluation of overlap, or a few positive determinants for Hamiltonian. Therefore, the computation turns into accumulating positive determinants. The expressions for evaluating both overlap and Hamiltonian matrix elements of VB functions are given in detail. Our practice shows that such a positive determinant method is quite attractive and it provides an excellent starting point for developing an even much more efficient algorithm of nonorthogonal VB calculation.

Oxidative dehydrogenation of propane over fluorine promoted rare earth-based catalysts

Abstract: Catalysts based on rare earth complexes such as CeO2/2CeF3, Sm2O3/4CeF3, Nd2O3/ 4CeF3 and Y2O3/4CeF3 were prepared. These catalysts were active for the oxidative dehydrogenation of propane with very high selectivity to propene. At 500°C and 6000 h−1, using CeO2/2CeF3 as the catalyst, the conversion of propane was 41.3%, selectivity to propene reached 81.1%, propene yield was 33.5%. XRD results indicated that F− and O2− were exchanged in the lattices. Raman spectra showed that the O 2 − might be the active oxygen species in propane oxidative dehydrogenation.

Stoichiometric-dependent intramolecular charge transfer flourescence of p-dimethylaminochalcone in β-cyclodextrin host—guest systems

Abstract: Steady state flourescence and absorption spectra of p -dimethylaminochalcone (DMAC) in aqueous β-cyclodextrin (β-CD) solution are reported. The flourescence maximum of DMAC initially shifts to the red and then to the blue on addition of β-CD, accompanied by a progresive increase in the relative flourescence quantum yield. The variation in the absorption maximum of DMAC with β-CD concentration is in the same direction as that of the flourescence maximum, but at a lower rate and the absorbance is nearly constant. A double reciprocal plot indicates that the spectral shifts are due to different stoichiometries in the DMAC—β-CD inclusion complex: the red shift is associated with a 1:1 (guest-host) complex, while the blue shift is due to a 1:2 complex. The effect of β-CD on the intramolecular charge transfer photophysics of DMAC is discussed within the framework of the three excited state (E*, P* and A* or TICT) model; the channel to the non-flourescence state P * is blocked on formation of the DMAC—β-CD inclusion complex.

Fluorescence Spectroscopic Investigation of the Effect of α-Cyclodextrin on the Twisted Intramolecular Charge Transfer of p-Dimethylaminobenzoic Acid in Aqueous Media:

Abstract: The effect of α-cyclodextrin (α-CD) on the twisted intramolecular charge transfer (TICT) of p-dimethylaminobenzoic acid (DMABOA) in aqueous media is investigated with the use of TICT-typical dual fluorescence. With the increase of α-CD concentration in aqueous DMABOA solution, the LE fluorescence is slightly quenched and is not shifted in position, while the TICT fluorescence is strongly enhanced and shifted to blue. The correlation of the intensity ratio of the TICT band to the normal band, Ia/Ib, of DMABOA with medium polarity in aqueous α-CD media is opposite to that in pure organic solvent, which is explained in terms of the different TICT behavior in aqueous solution. The pKa of DMABOA is not affected by the presence of α-CD, and the orientation of the DMABOA molecule in the DMABOA/α-CD inclusion complex is determined to be with the dimethylamino group in the cavity. The stoichiometry and association constant of the DMABOA/α-CD inclusion complex are investigated by monitoring the Ia/Ib of DMABOA as a function of α-CD concentration. The Ia/Ib value of DMABOA in α-CD media is generally much lower than that of DMABN in the same media, which is interpreted by the difference in the energy barrier from the locally excited (LE) to the TICT state and in the orientation of the guest molecules in the α-CD cavity. The effect of the cyclodextrin cavity is clearly demonstrated to be a micro-environmental effect.

Ph-dependence of the twisted intramolecular charge-transfer (tict) of p-n, n-dimethylaminobenzoic acid in aqueous-solution

Abstract: The pH dependence of the twisted intramolecular charge transfer (TICT) of p - N , N -dimethylaminobenzoic acid (DMABOA) in aqueous solution was examined using its typical dual fluorescence. Fluorometric pH titration indicates that the excited state proton exchange is much slower than fluorescence in the acid or conjugate base form of DMABOA, and that the conjugate base form has a higher relative population of the TICT state and a stronger excitation wavelength dependence of TICT fluorescence than the acid form.

The oxidative coupling of methane and the activation of molecular o2 on ceo2/baf2

Abstract: CeO2/BaF2 was used as the catalyst for the oxidative coupling of methane (OCM). At 800°C and CH4∶O2=2.7∶1,CH4 conversion of 34% with C2 hydrocarbon selectivity of 54.3% was obtained. XRD measurement showed that partial anion (O2−,F−) and/or cation (Ce4+,Ba2+) exchange between CeO2 and BaF2 lattices occurred. ESR study showed that O− species existed on degassed catalyst. XPS study revealed that, when BaF2 was added to CeO2, the binding energy of Be 3d5/2 was 2.2 eV lower than that in CeO2, and the “electron-enriched lattice oxygen” species was detected. XPS, ESR and Raman study showed that, under O2 adsorbing conditions, O22− and O−2 species were detected on CeO2/BaF2.

Valence-band offsets and band tailoring in compound strained-layer superlattices

Abstract: A numerical method, in which the average bond energy is considered as a reference level for determining the valence-band offset (VBO) in strained-layer superlattices (SLS's), is suggested and tested by performing ab initio electronic-structure calculations for the SLS's (InP${)}_{\mathit{n}}$/(InAs${)}_{\mathit{n}}$(001), (n=1,3,5), (GaP${)}_{1}$/(GaAs${)}_{1}$(001), (AlP${)}_{1}$/(AlAs${)}_{1}$(001), and (AlP${)}_{1}$/(InP${)}_{1}$(001). Based on this method, the VBO's of the SLS's composed of any two of the six compounds InP, InAs, GaP, GaAs, AlAs, and AlP under arbitrary elastic strain are determined systematically, and their strain-induced effects are discussed. Good agreement is found between the present results and the available experimental data.

Liquid chromatography series dual‐electrode amperometric detection for aromatic nitro compounds

Abstract: A new design of dual-electrode electrochemical detector for liquid chromatography is described utilizing two series generator/detector electrodes. The detector developed is based on the larger electrode area design compared with commercial ones and has higher electrolytic efficiency, which lead to a higher detection sensitivity when in a reductive-oxidative series detection mode. Analytes are first reduced at the upstream electrode and the reaction products are then detected by oxidation at the downstream electrode. So, the influence of dissolved oxygen and trace amounts of heavy metals in the mobile phase and sample is eliminated. Thus, exhaustive removal of dissolved oxygen before analysis is not required, and the method can be easily automated. The technique is tested on seven aromatic nitro compounds. Under the optimized conditions, the assay is linear in a concentration range of 2.0 × 10−7 to 4.0 × 10−5 g/mL and has low detection limits of 1.0 to 2.0 ng using a 10 μL sample. The largest standard deviation was 5.2%. The recovery of the detection of water samples was 93 to 107%.

The beneficial effect of alkali metal salt on supported aqueous-phase catalysts for olefin hydroformylation

Abstract: Alkali metal salt modified SAP (supported aqueous-phase) rhodium catalysts prepared by coimpregnation method using alkali metal chloride were found to be active and selective for olefin hydroformylation. The salt addition promoted the formation of aldehydes with high selectivity, the aldehyde yield being increased more than 2.5 times at a proper salt/Rh ratio. Changes in stretching frequency of the carbonyl species were detected during ethene hydroformylation, which appeared at ca. 1625 cm−1 on the non-modified SAP catalyst sample, while at ca. 1586 cm−1 on the KCl-modified one, as shown by in situ IR spectroscopy. The results of a deuterium isotope effect experiment showed that the hydroformylation rate for aldehyde formation on SAP rhodium catalyst under atmospheric pressure of CO/D2 was about 1.3 times faster than that under CO/H2, implying that the rate-determining step involved in aldehyde formation is most probably a step related with hydrogen. The role of the alkali metal salt is discussed in relation with the reaction mechanism.