Showing papers by "Xiamen University published in 1998"

Quasi-discrete Hankel transform

Abstract: A quasi-discrete Hankel transform (QDHT) is presented as a new and efficient framework for numerical evaluation of the zero-order Hankel transform A discrete form of Parseval's theorem is obtained for the first time to the authors' knowledge, and the transform matrix is discussed It is shown that the S factor, defined as the products of a truncated radius, is critical to building the QDHT

Nonoxidative dehydrogenation and aromatization of methane over W/HZSM‐5‐based catalysts

Abstract: Highly active and heat‐resisting W/HZSM‐5‐based catalysts for nonoxidative dehydro‐aromatization of methane (DHAM) have been developed and studied. It was found from the experiments that the W−H2SO4/HZSM−5 catalyst prepared from a H2SO4‐acidified solution of ammonium tungstate (with a pH value at 2–3) displayed rather high DHAM activity at 973–1023 K, whereas the W/HZSM‐5 catalyst prepared from an alkaline or neutral solution of (NH4)2WO4 showed very little DHAM activity at the same temperatures. Laser Raman spectra provided evidence for existence of (WO6)n- groups constructing polytungstate ions in the acidified solution of ammonium tungstate. The H2‐TPR results showed that the reduction of precursor of the 3% W–H2SO4/HZSM‐5 catalyst may occur at temperatures below 900 K, producing W species with mixed valence states, W5+ and W4+, whereas the reduction of the 3% W/HZSM‐5 occurred mainly at temperatures above 1023 K, producing only one type of dominant W species, W5+. The results seem to imply that the observed high DHAM activity on the W–H2SO4/HZSM‐5 catalyst was closely correlated with (WO6)n- groups with octahedral coordination as the precursor of catalytically active species. Incorporation of Zn (or La) into the W–H2SO4/HZSM‐5 catalyst has been found to pronouncedly improve the activity and stability of the catalyst for DHAM reaction. Over a 2.5% W–1.5% Zn–H2SO4/HZSM‐5 catalyst and under reaction conditions of 1123 K, 0.1 MPa, and GHSV=1500 ml/(h g−cat.), methane conversion (XCH4) reached 23% with the selectivity to benzene at ∼96% and an amount of coke for 3 h of operation at 0.02% of the catalyst weight used.

Dissociation of methane on different transition metals

Abstract: Dissociation of methane on different transition metals M (M=Ru, Ir, Rh, Ni, Pd, Pt, Cu, Ag, Au) has been investigated using a quasi-relativistic density-functional method. Reaction enthalpies for the steps involved are determined. The activation energies have been estimated using the analytic BOC-MP formula. The transition metals, Ru, Rh, …, Pt are shown to exhibit high activity in the dissociation of methane, whereas the coinage metals (Cu, Ag, Au) are very inactive. The conclusion is in agreement with experimental observations. The total dissociation enthalpy ΔH for the complete dissociation of CH 4 to give surface C and H (CH 4,s →C s +4H s ) can be regarded as a measure for the activity of the metal in methane dissociation. The order of the calculated ΔH's is consistent with the order of methane conversions over the metals. The dissociation of methane is also examined in the presence of adsorbed oxygen. Oxygen at on-top site promotes methane dehydrogenation. Oxygen at hollow site promotes methane dehydrogenation on Pt and the coinage metals, but is not beneficial to that on the other transition metals.

Efficiency bound of a solar-driven Stirling heat engine system

Abstract: A solar-driven Stirling engine is modelled as a combined system which consists of a solar collector and a Stirling engine. The performance of the system is investigated, based on the linearized heat loss model of the solar collector and the irreverisible cycle model of the Stirling engine affected by finite-rate heat transfer and regenerative losses. The maximum efficiency of the system and the optimal operating temperature of the solar collector are determined. Moreover, it is pointed out that the investigation method in the present paper is valid for other heat loss models of the solar collector as well, and the results obtained are also valid for a solar-driven Ericsson engine system using an ideal gas as its engine work substance. © 1998 John Wiley & Sons, Ltd.

Can surface Raman spectroscopy be a general technique for surface science and electrochemistry

Abstract: Surface-enhanced Raman spectroscopy has almost been restricted to the study of only three noble metals of Au, Ag and Cu for two decades. Recently, a new confocal Raman microscope and special surface pretreatments have allowed the acquisition of high-quality surface Raman spectra of organic and inorganic molecules adsorbed on bare Pt, Ni, Co, Fe, Pd, Rh, Ru and Si electrodes over a wide applied potential range for the first time. The present results demonstrate several advantages of in situ surface Raman spectroscopy that could probably make it a general technique widely used in surface science and electrochemistry. © 1998 John Wiley & Sons, Ltd.

Development of coking-resistant Ni-based catalyst for partial oxidation and CO2-reforming of methane to syngas

Abstract: Addition of small amount of trivalent-metal oxides, Cr 2 O 3 and La 2 O 3 , to a Ni Mg O (Ni/Mg=1/1, mol/mol) catalyst for partial oxidation of methane (POM) and CO 2 -reforming of methane (MCR) reactions has been found to improve the performance of the catalyst for coking-resistance. The POM operation at 1053 K for 50 h, or the MCR operation at 1100 K for 6 h, did not leave any detectable amount of carbon deposit on the surface of the catalyst. Studies of XRD, XPS, and H 2 -TPR spectroscopies showed that the doping of small amounts of Cr 3+ and La 3+ to the Ni Mg O system led to the formation of a host-dopant-type Ni Mg Cr La O solid solution, with a considerable number of Schottky defects in the form of cationic vacancies. An increase in the degree of disorder in the solid solution due to Cr 2 O 3 and La 2 O 3 dissolved in Ni x Mg 1− x O lattice would be expected to enhance the mobility of the lattice oxygen anions. This would be in favor of speeding up the reaction between the carbon-containing species and reactive oxygen species via migration of the lattice O 2− so as to inhibit the deposition of carbon on the surface of the catalyst. On the other hand, part of the Schottky defects in the form of cationic vacancies may diffuse to the surface, where Ni + -species can be well accommodated and stabilized, thus, forming a rich-in-Ni (with mixed valence states) surface layer. As a result, the proportion of the reducible Ni-species was pronouncedly increased, but the temperature for their reduction was considerably raised, so that the surface Ni-species were maintained with higher possibility in positive valence states under POM and MCR reaction conditions. This would, to some extent, lead to the reduction of the rate of deep dehydrogenation of methane to carbon, therefore tending to reduce, if not avoid, coking caused by an excess of carbon on the surface.

Nanomodification of polypyrrole and polyaniline on highly oriented pyrolytic graphite electrodes by atomic force microscopy

Abstract: This paper describes the first observation of localized electropolymerization of pyrrole and aniline on highly oriented pyrolytic graphite (HOPG) substrates under atomic force microscopy (AFM) tip−...

Xerocomus badius - Picea abies, an ectomycorrhiza of high activity and element storage capacity in acidic soil.

Abstract: Mycorrhizas were collected from three Norway spruce (Picea abies) stands in southwest Germany, sorted on the morphotype level and analysed by fluorescein diacetate vital fluorescence staining and the accumulation of elements using inductively coupled plasma-atomic emission spectrometry (ICP-AES) and electron energy-loss spectroscopy (EELS). Xerocomus badius – Picea abies mycorrhizas showed a higher frequency of active hyphal sheaths and a higher potential to store nitrogen, phosphorus, potassium, magnesium, iron and zinc than other mycorrhizal types. Phosphorus and nitrogen were localized by EELS in vacuolar bodies which occurred consistently in the sheath of X. badius mycorrhizas. The results indicate that X. badius is well adapted to acidic stands and that its mycorrhizas are very efficient in uptake and storage of macronutrients.

Efficient algorithm for the spin-free valence bond theory. I. New strategy and primary expressions

Abstract: A new algorithm for nonorthogonal valence bond (VB) method is presented by using symmetric group approach. In the present algorithm, a new function, called paired-permanent-determinant (PPD), is defined, which is an algebrant and has the same symmetry of a corresponding VB structure. The evaluation of a PPD is carried out by using a recursion formula similar to the Laplace expansion method for determinants. An overlap matrix element in the spin-free VB method may be obtained by evaluating a corresponding PPD, while the Hamiltonian matrix element is expressed in terms of the products of electronic integrals and sub-PPDs. In the present work, some important properties of PPDs are discussed, and the primary procedure for the evaluation of PPD is deduced. Furthermore, the expressions for evaluating both the overlap and Hamiltonian matrix elements are also given in details, which are essential to develop an efficient algorithm for nonorthogonal VB calculations. In the present study, some further effective technical considerations will be adopted, and a new ab initio VB program will be introduced. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 287–297, 1998

Chemical Bonding in XeF2, XeF4, KrF2, KrF4, RnF2, XeCl2, and XeBr2: From the Gas Phase to the Solid State

Abstract: This paper presents a systematic investigation of chemical bonding in a series of noble-gas halides in both the gas phase and the solid state. The crystalline environment was simulated by a cutoff type Madelung potential. Geometries, dissociation energies, force constants, and enthalpies of formation and of sublimation were determined. The calculated properties are in good agreement with available experimental data. The crystal field model is capable of reproducing all significant differences observed between the gas phase and the solid state. KrF4, XeCl2, and XeBr2 are predicted to be rather unstable against molecular dissociation. The stabilities of the dihalides follow the order KrF2 ≪ XeF2 < RnF2 and XeF2 ≫ XeCl2 ≈ XeBr2. The calculated trends account for the fact that only the heavier noble gases form compounds and that the electronegativity of the ligand has to be large. The outer polarization orbitals play an important role in the bonding. Relativistic effects on the molecular properties are neglig...

Influence of liming and acidification on the activity of the mycorrhizal communities in a Picea abies (L.) Karst, stand

Abstract: A study of mycorrhizal activity was conducted in a mature Norway spruce ((Picea abies) [L.] Karst.) stand subjected to soil treatments of liming and acidification for six years (Hoglwald research project). Samples were collected five times during one growing season using a soil corer. All the turgescent, not shriveled mycorrhizal tips were sorted out and identified on the fungal species level as far as yet possible. The proportion of each mycorrhizal type on the plots was calculated. The results revealed a shift in the mycorrhizal communities caused by both acid rain and liming. Data are in agreement with the findings of the more comprehensive study on the mycorrhizal communities carried out by another research group on the same plots (Taylor et al., submitted) The activity of the predominant types of mycorrhizas, Piceirhiza gelatinosa, Piceirhiza nigra, Russula ochroleuca-P. abies, Tuber puberulum-P. abies, Tylospora sp.-P. abies, Xerocomus badius-P. abies, was investigated by staining hand sectioned tips with FDA and their fluorescence. Different FDA-hydrolysing activities of the mycorrihizal types had been found in a previous-year study on the same plots and were confirmed during the second year. The proportion of the different stages of activity of the mycorrhizal tissues was calculated on the type level and in connection to the soil treatments. X. badius-P. abies and R. ochroleuca-P. abies displayed the most active fungal tissues and proportion increased on the acidic plots while Tuber puberulum-P. abies and Piceirhiza nigra were the most active types and occurred in higher proportion after liming. Thus, the overall activities of the mycorrhizas were only slightly changed by the treatments. In addition to the mycorrhizal effect acidification reduced while liming enhanced the meristematic activity of the short root tips. The same tendency was found by studying root production on the same plots (Hahn and Marschner, 1998). Although nearly 3000 mycorrhizal tips were studied, the data are still limited, allowing no statistical validation. This is, however, the first investigation connecting overall activity of the mycorrihizal tissues with the proportion of the mycorrhizal types as influenced both by alterations of the forest soil caused by acid rain and liming. The results are interesting and reasonable but further investments are necessary to validate the general conclusions.

Deriving the integral representation of a fractional Hankel transform from a fractional Fourier transform.

Abstract: We derive the integral representation of a fractional Hankel transform (FRHT) from a fractional Fourier transform. Some basic properties of the FRHT such as Parseval's theorem and its optical implementation are discussed qualitatively.

SERS studies of electrode/electrolyte interfacial water part II—librations of water correlated to hydrogen evolution reaction

Abstract: Electrode/electrolyte interfacial water is one of the most interesting systems studied by surface-enhanced Raman spectroscopy (SERS) in the past two decades, and is briefly reviewed. It has been shown that the SERS characteristics of water are very different from those of other surface species. Strong libration bands and the abnormal enhancement of the bending mode of water on Cu and Ag electrodes have been observed in the hydrogen evolution reaction (HER) potential region (<-1.0 V vs. SCE). The observations may be attributed to the special interfacial structure of water molecules, which is in favor of the deprotonation of water and mass transfer of the proton and the reaction product OH- from the surface to the bulk through the hydrogen bonding net during HER. A new enhancement mechanism for SERS involving charge transfer (CT) from metal to water is suggested by the correlation of HER current density and SERS band intensity. The generality of this CT enhancement is briefly discussed. © 1998 John Wiley & Sons Ltd.

Thermodynamic properties of an ideal Fermi gas in an external potential with U=br(t) in any dimensional space

Abstract: The properties of an ideal Fermi gas in an external potential in any dimensional space are studied, based on the semiclassical (Thomas-Fermi) approximation. The general analytical expressions of the total particle number $N,$ density of states $D(\ensuremath{\varepsilon}),$ Fermi energy ${E}_{F},$ total energy $E,$ and heat capacity $C$ have been derived, where $N,$ $E,$ and $C$ are expressed by the Fermi integration. Moreover, the analytical expressions of the total energy $E,$ chemical potential \ensuremath{\mu}, and heat capacity $C$ in the high- and low-temperature approximations are given. From these results, how the characteristics of the Fermi gas depend on an external potential and the dimension of space is discussed.