The effect of tungsten additive on the surface characteristics of amorphous Ni-P alloy

The variation of surface compositions on amorphous Ni80.4W1.5P18.1 alloy by O-2 oxidation and H-2 reduction treatments have been studied by XPS, UPS and ISS. It shows that addition of tungsten in the amorphous Ni-P alloy leads to dramatic changes of the relating component distributions in the surface layers before and after these treatments. Oxidation of a Ni80.4W1.5P18.1 amorphous alloy in 1 bar of oxygen at 513 K caused a significant segregation of nickel in different oxide states at the surface. The subsequent reduction of the oxidized surface with I bar hydrogen at 553 K resulted in only a small portion of Ni and P being reduced into elemental states, while most of them was found to combine to form a kind of nickel phosphate compound. On the other hand, under the same conditions, the oxidation and reduction of a Ni80P20 alloy gave rise to metallic Ni and elemental P as the predominate species on the alloy surface. The addition of W in the amorphous alloy might act as nuclei for a favorable formation of the phosphate structure which was proposed to be an active species for hydrogen-relating catalytic reactions. (C) 1999 Elsevier Science B.V. All rights reserved.

Different mechanisms for the formation of acetaldehyde and ethanol on the Rh-based catalysts

Different mechanisms for the formation of acetaldehyde and ethanol on the Rh-based catalysts were investigated by the TPR (temperature programmed reaction) method, and the active sites were studied by CO-TPD, TPSR (temperature programmed surface reaction of preadsorbed CO by H-2) and XPS techniques. The TPR results indicated that ethanol and acetaldehyde might be formed through different intermediates, whereas ethanol and methanol might result from the same intermediate. Results of CO-TPD, TPSR, and XPS showed that on the Rh-based catalyst, the structure of the active sites for the formation of C-2-oxygenates is ((RhxRhy+)-Rh-0)-O-Mn+ (M=Mn or Zr, x>>y, 2 less than or equal ton less than or equal to4). The tilt-adsorbed CO species is the main precursor for CO dissociation and the precursor for the formation of ethanol and methanol. Most of the linear and geminal adsorbed CO species desorbed below 500 K. Based on the suggested model of the active sites, detailed mechanisms for the formation of acetaldehyde and ethanol are proposed. Ethanol is formed by direct hydrogenation of the tilt-adsorbed CO molecules, followed by CH2 insertion into the surface CH2-O species and the succeeding hydrogenation step. Acetaldehyde is formed through CO insertion into the surface CH3-Rh species followed by hydrogenation, and the role of the promoters was to stabilize the intermediate of the surface acetyl species. (C) 2000 Academic Press.

The stability of nanosized HZSM-5 zeolite: a high-resolution solid-state NMR study

The thermal and hydrothermal stabilities of HZSM-5 zeolites with crystal sizes less than 100 nm have been studied by multinuclear solid-state NMR, combined with BET and XRD. As evidenced by Al-27 and Si-29 MAS as well as their corresponding cross-polarization/MAS NMR investigations, the thermal stability of nanosized HZSM-5 is not so good as that of microsized HZSM-5. This is due to two processes concerning dealumination and desilicification involved in the calcination of nanosized HZSM-5, while only the dealumination process is conducted in microsized HZSM-5 under the similar calcination process. The hydrothermal stability of nanosized HZSM-5 is, contrary to what was expected, not so bad as that of the microsized HZSM-5 in the course of steam treatment. The actual resistance of the hydrothermal stability to the crystal size of HZSM-5 can be ascribed to an active reconstruction of zeolitic framework through an effective filling of amorphous Si species into nanosized HZSM-5 during hydrothermal treatment. (C) 2001 Published by Elsevier Science B.V.

Imaging of three-dimensional objects in emission electron microscopy

Under investigation by emission electron microscopy, the shape and size of three-dimensional objects are distorted because of the appearance of a characteristic potential relief and a possible contact potential difference between the particles and the substrate. An estimation of these effects for spherical particles is made. It is shown that the apparent size of particles observed in an emission electron microscope (EEM) could be increased as well as decreased depending on the relation between the work functions of the particle and the substrate. The corresponding formulae are given and several possibilities are shown which permit us to determine from the EEM image the real size of particles and their work function relative to the substrate.

On the formation of vertically oriented MCM-22 zeolite crystal films

Growth of MCM-22 zeolite films on glass substrates was studied with the focus on the understanding of the unusual vertical crystal orientation. The films formed were characterized by scanning electron microscopy and X-ray diffraction. Separate thin disk-like MCM-22 crystals were found vertically oriented at the early crystallization stage. With crystallization the crystals grew into thick disks and finally into continuous films. The vertically oriented MCM-22 thin crystals could be developed from the orientation of columnar MCM-22 nuclei, which have larger parameters in their c-directions than those in a and b directions. The preferred orientation of MCM-22 nuclei and the fast growth rate in the layer direction are responsible for the vertical growth of MCM-22 zeolite films. (C) 2001 Elsevier Science B.V. All rights reserved.

Effect of support and acidity of catalyst on the direct oxidation of ethylene to acetic acid

The selective oxidation of ethylene to acetic acid was investigated on Pd-acid/support catalyst system. The catalytic activity is influenced strongly by the acidity of the catalyst. The stronger the catalyst acidity the higher the catalytic activity. The nature of the support also influences the activity of the catalyst substantially. The catalyst has highest activity when it exhibits highest acidity on silica.

XRD and XPS studies on the ultra-uniform Raney-Ni catalyst prepared from the melt-quenching alloy

We present a novel method for preparing an ultra-uniform Raney-Ni catalyst, which includes melt-quenching, hydrogen treatment and leaching in an alkali solution. The resultant catalyst shows superior activity in the reaction of cyclohexanone hydrogenation. X-ray diffraction (XRD) and XPS have been employed to characterize the catalysts. As demonstrated, the pretreatment with hydrogen caused a distinct phase transfer of the Ni-Al alloys, forming more of the Ni2Al3 component. In the subsequent leaching process, the Ni2Al3 component shows high activity and the resultant catalyst exhibits high surface areas and small pores. Moreover, metallic Al in the hydrogen-pretreated alloy appeared to be leached more easily and thus the aluminium species remaining on the catalyst surface is aluminium oxide predominantly, which serves as a matrix to stabilize active Ni species on the surface. Copyright (C) 2001 John Wiley & Sons, Ltd.

Xenon probe for detecting the microporous structure of nanosized HZSM-5 zeolite

The secondary pores in the nanosized HZSM-5 zeolite have been observed for the first time via Xe-129 NMR spectroscopy using xenon as a probe; the location of non-framework Al can also be identified.

Titanium species in titanium silicalite TS-1 prepared by hydrothermal method

The titanium species existing in titanium silicalite TS-1, which is prepared by hydrothermal method, were investigated using chemical analysis, XRD, FT-IR, Si-29 MAS NMR, UV-VIS, ESR. It has been observed that several kinds of titanium species may exist in titanium silicalite. The form that titanium atoms incorporate into the framework of titanium silicalite synthesized using tetrapropylammonium bromide (TPABr) as template differs from that using the classical method. But, the symmetry of titanium silicalite, changes from monoclinic to orthorhombic with the increase of titanium content in both methods. The Ti-O-2(-) originated from framework titanium and H2O2 has the moderate stability and may be active site in oxidation reaction. TS-1 synthesized using TPABr as template does not contain anatase, but contains a kind of partly condensed titanium species with six-fold coordination. The titanium species may correspond to 270-280 mn band in UV-VIS spectra and also can form Ti-O-2(-). But, this kind of Ti-O-2(-) is very stable and cannot be catalytic active site. So, the six-fold coordination titanium species may be inactive in both the oxidation reaction and the decomposition of H2O2. The hypothesis has been further proved by the phenomena that the titanium species is easily washed off using acid, and acid treating will not influence the catalytic performance of TS-1. (C) 2001 Elsevier Science B.V. All rights reserved.

The study of Ti doped ZSM-5 particles and cavities inside them

The TEM study of titanium-containing ZSM-5 zeolite before and after hydrothermal treatment was performed. The use of different TEM techniques, such as conventional TEM, HRTEM and EDX-line scans provides important information about the microscopic structure of the zeolite catalyst consisting from several phases. The hydrothermal treatment of zeolite powder leads to strong changes in the morphology of the constituting particles. They are characterized by a homogeneous structure before hydrothermal treatment while the occurrence of holes after thermal treatment was observed, These changes lead to the enrichment of zeolite with titanium which obviously enhance its catalytic activity. Some of the titanium surplus precipitates as TiO2 anatase nanoparticles within the holes. (C) 2001 Elsevier Science B.V. All rights reserved.

Structure and properties of samarium overlayer and Sm/Rh surface alloy on Rh(100)

The structure and properties of Sm overlayer and Sm/Rh surface alloy have been investigated with Auger electron spectroscopy (AES), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption spectroscopy (TDS). The growth of Sm on Rh(100) at room temperature (RT) appears following the Stranski-Krastanov growth mode and only the trivalent state Sm is observed from XPS results. Thermal treatment of the Sm film at 900 K leads to the formation of ordered surface alloy which shows the c(5 root2 x root2)R45 degrees and c(2 x 2) LEED patterns. Annealing the Sm film at temperature above 400 K makes the binding energy (B.E.) of Sm 3d(5/2) shift to higher energy by 0.7 eV, which indicates charge transfer from Sm to Rh(100) substrate, causing the increase of CO desorption temperature.

Controlled growth of uniform silver clusters on HOPG

We have investigated growth of silver clusters on three different, i.e. normally cleaved, thermally oxidized and Ar+ ion sputtered highly oriented pyrolytic graphite (HOPG), surfaces. Scanning tunneling microscopy (STM) observations reveal that uniformly sized and spaced Ag clusters only form on the sputtered surface. Ar+ sputtering introduces relatively uniform surface defects compared to other methods. These defects are found to serve as preferential sites for Ag cluster nucleation, which leads to the formation of uniform clusters. (c) 2005 Elsevier B.V. All rights reserved.

Combined single-pass conversion of methane via oxidative coupling and dehydro-aromatization

A single-pass process with the combination of oxidative coupling (OCM) and dehydro-aromatization (MDA) for the direct conversion of methane is carried out. With the assistance of the OCM reaction over the SrO-La2O3/CaO catalyst loaded on top of the catalyst bed, the duration of the dehydro-aromatization reaction catalyzed by a 6Mo/HMCM-49 catalyst shows a significant improvement, and. the initial deactivation rate constant of the overall process revealed about 1.5 x 10(-6) s(-1). Up to 72 h on stream, the yield of aromatics was still maintained at 5.0% with a methane conversion of 9.6%, which is obviously higher than that reported for the conventional MDA process with single catalyst. Upon the TPR results, this wonderful enhancement would be attributed to an in-situ formation of CO2 and H2O through the OCM reaction, which serves as a scavenger for actively removing the coke formed during the MDA reaction via a reverse Boudouard reaction and the water gas reaction as well.