Highly dispersed gold on activated carbon fibers for low temperature CO oxidation

Gold nanoparticles of 2–5 nm supported on woven fabrics of activated carbon fibers (ACF) were effective during CO oxidation at room temperature. To obtain a high metal dispersion, Au was deposited on ACF from aqueous solution of ethylenediamine complex [Au(en)2]Cl3 via ion exchange with protons of surface functional groups. The temperature-programmed decomposition method showed the presence of two main types of functional groups on the ACF surface: the first type was associated with carboxylic groups easily decomposing to CO2 and the second one corresponded to more stable phenolic groups decomposing to CO. The concentration and the nature of surface functional groups was controlled using HNO3 pretreatment followed by either calcination in He (300–1273 K) or by iron oxide deposition. The phenolic groups are able to attach Au3+ ions, leading to the formation of small Au nanoparticles (9 nm) Au agglomerates after reduction by H2. These catalysts demonstrated lower activity as compared to the ones containing mostly small Au nanoparticles. Complete removal of surface functional groups rendered an inert support that would not interact with the Au precursor. The oxidation state of gold in the Au/ACF catalysts was controlled by X-ray photoelectron spectroscopy before and after the reduction in H2. The high-temperature reduction in H2 (673–773 K) was necessary to activate the catalyst, indicating that metallic gold nanoparticles are active during catalytic CO oxidation

Phase Analysis of Multilayered, Nanostructured Titanium-Base Alloys by Analytical Electron Microscopy

Microstructure, chemical and phase composition of the hard layer formed on the Ti-6Al-4V alloy after duplex surface treatment were investigated by light microscopy (LM), X-ray diffraction (XRD) and analytical scanning, transmission and scanning transmission electron microscopy (SEM, TEM, STEM), electron diffraction and focused ion beam (FIB). Advanced electron microscopy techniques used for unambiguous identification of phases present in the surface multilayer are critically discussed. The relationship between multilayer micro/ nanostructure containing several phases from the Ni-Ti-P-Al system and improved mechanical and tribological properties is established

Transmission electron microscopy of Ca oxide nano- and microcrystals in alpha-tricalcium phosphate prepared by sintering of beta-tricalcium phosphate

Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to study the porous and non-porous alpha-tricalcium phosphate (alpha-Ca-3(PO4)(2), alpha-TCP) prepared through a sintering procedure at 1200-1400 degrees C of beta-tricalcium phosphate (beta-Ca-3(PO4)(2), beta-TCP). The interpretation of experimental and calculated X-ray and electron diffraction patterns showed that the final product at 1400 degrees C was primarily alpha-TCP but roughly 3.0-8.0 wt.% of the starting beta-TCP phase and up to 8.0 wt.% of CaO were in the final product. TEM images and electron diffraction patterns showed that the CaO phase - formed by decomposition of TCP - exists as micron-sized areas of various oriented nanocrystals embedded into the bulk alpha-TCP material and also as self-standing spherulite particles of a few microns in size. Surprisingly, formation of CaO from TCP decomposition occurred at temperatures below those predicted from the phase diagram of the CaO-P2O5 system. (C) 2009 Elsevier Ltd. All rights reserved

Convergent beam electron diffraction study of Pb₂CoWO₆ at 100 K

A study by convergent beam electron diffraction was performed to clarify the symmetry of the phase III of Pb2CoWO6. Microdiffractions with higher-order Laue zone on monodomain areas have confirmed the cell proposed in a preceding study. In addn., an axial glide plane (c) is normal to the b-axis. In the conditions of observation, the convergent beam electron diffraction patterns lead to an apparent point group mmm. No breakdown of a mirror symmetry could be found. The discrepancy between this result and the existence of a spontaneous polarization is discussed

Structure of precipitates and orientation relationships in Al, Ge, Mo-doped higher manganese silicide crystals

The structure of Al, Ge, Mo-doped Higher Manganese Silicide (HMS) crystals with the general formulas Mn(Si0.99Ge0.01)(1.75), Mn(Si0.995Ge0.005)(1.75) and (Mn0.98Mo0.02)[(Si0.98Ge0.02)(1.75)](0.99)Al-0.01 was investigated by scanning and transmission electron microscopy, electron diffraction and X-ray energy dispersive spectrometry in a wide scale range from a few mm to several angstrom. Several secondary phases were identified in the Mn4Si7 matrix: Ge1-xSix (0.1 < x < 0.9) solid solution precipitates with Ge concentration ranging from 5 at. % up to 93 at.%, MoSi2 platelets, MnSi and Mn5Si3 precipitates. Their morphology, structure and crystallographic relationships with the HMS matrix were determined. Mostly local strains in the matrix and precipitates due to lattice misfits at interfaces derived from crystallographic relationships were found two orders of magnitude higher than deformation induced by thermal expansion mismatch. Only a few exceptions of specific relationships were found when the lattice misfit and thermal mismatch have close values. The largest misfit of about 22% was observed between MnSi and Mn4Si7 what led to big and numerous cracks in crystals. Therefore, doping can improve the material performance (1) by preventing the formation of MnSi precipitates with metallic properties and (2) by reduction of cracking and crack propagation because of larger MnSi/Mn4Si (7) lattice misfit compared to Ge1-xSix/Mn4Si7 or MoSi2/Mn4Si7 misfits

Electron diffraction study of the Pb₂CoWO₆ phases

A study of electron microscopy has been started in order to clarify the crystallographic features of Pb2CoWO6. Diffraction patterns have been obtained on monodomains of the incommensurate phase (II). The average monoclinic symmetry was confirmed and the modulated wave vector determined. For the phase III a new orthorhombic cell is proposed

Chemical and structural changes of sodium molybdate (Na2MoO4) as a methanol dehydrogenation catalyst

Sodium salts such as Na2CO3 and Na molybdate are active and selective MeOH dehydrogenation catalysts for the prodn. of water-free HCHO. Profound chem. and structural changes are experienced by the molybdate when employed under normal reaction conditions (500-700 Deg). The material reacts with the fluid phase and undergoes sepn. into Na2CO3 and Mo2C. The transition occurs relatively slowly and correlates well with start-up behavior of a catalytic fixed-bed reactor showing that an induction period is required before max. HCHO selectivity is achieved. This induction period reflects the transformation to Na2CO3, which is more selective for HCHO than Na molybdate under the conditions employed. TEM shows that the phase sepn. results in the formation of Na2CO3 filaments extending from Mo-rich centers. Hydrogen reduces the molybdate and reaction with C-contg. gases (including CO and CO2) completes the transformation. In addn. to TEM, electron microanal., X-ray powder diffraction, and diffuse reflectance Fourier-transform IR spectroscopy are used for post-reactor catalyst analyses. In-situ transmission IR spectroscopy conforms that the transformation to carbonate takes place during reaction with MeOH >=500 Deg. [on SciFinder (R)