Characterization of Supported Gold Catalysts with 197Au Mössbauer Effect Spectroscopy

We report on a ¹⁹⁷Au Mössbauer study of several types of supported gold catalysts. Differences in particle size show up in the Mössbauer spectra by a change in the relative weight of the spectral contribution of the surface atoms. The presence of ionic gold in active gold catalysts is not observed. The spectra can be interpreted in terms of bulk-like contributions from the inner-core atoms plus contributions from the outermost atoms at the surface of the particles.     

An investigation of the promotional effect of propylene or propane traces on the activity of sulfatedPt/g-Al2O3andPt-Sn/g-Al2O3inCH4combustion

Summary A strong promoting effect of the presence of C₃H₈or C₃H₆was determined for the combustion of CH₄in excess oxygen, over pre-sulfated 1%Pt/g-Al₂O₃and pre-sulfated 1%Pt-2%Sn/g-Al₂O₃catalysts.</o:p>     

Raman scattering in low-dimensional HfTe5

Nine long-wavelength phonons have been observed by Raman scattering in the quasi-one-dimensional crystal HfTe₅. The assignment of these phonon modes is made on the basis of symmetry arguments using the correlation method. The similarity between the HfTe₅ and the ZrTe₃ chain-like crystal structures instigates the attribution of the most characteristic modes like the high-frequency chalcogen diatomic-mode and the low frequency rigid-chain mode. Although no new line appears even at 14 K, the irregular temperature dependence of the spectra suggests the possibility of a structural instability connected with a large peak in the electrical resistivity and with the X-ray diffraction anomalies.     

Using of Volatile Release Measurements to Understand Polysaccharide Molecular Interactions: An Example with Guar Gum Study

It is well establishment that polysaccharides are commonly used in many fields of formulation such as food and cosmetic industries as viscosity modifiers or gelling agents. Among the different impacts on the mixture properties, these materials are also known to affect the aroma release or retention thus its sensory perception; the effect depends whether polysaccharide structure induces specific retention or release phenomena from the complex media. Present study deals with studying aroma compounds behaviour from dilute and semi-dilute aqueous polysaccharide solutions consisting of different guar gum samples. Results allow stating that the whole polymer characteristics as determined by means of structural analysis, protein content determination, rheological and surface tension measurements play a major role on the polysaccharide/aroma compound interaction mechanisms. In particular, results evidence a strong Mannose to Galactose (M/G) ratio effect, thus allowing stating the occurrence of hydrophobic intra and intermolecular mechanism. In addition to that, proteins are demonstrated to only slightly affect the volatile retention. More generally, such an original study allows new highlights concerning macromolecular organisation and interactions in solution.     

The electron-hole system in GaSe at high densities

Absorption spectra in the limit of extremely high optical intensity in the region of the direct threshold present an induced transparency effect which is attributed to the disappearance of the bound excitonic states. This occurs at levels of excitation which decrease with increasing classical volume of each state. The effect is attributed to the evolution to a plasma state which appears to occur well above the Mott critical density for the metal-insulator transition.     

Assessment of the concentrations of U and Th in PM2.5 from Mexico City and their potential human health risk

Journal of Radioanalytical and Nuclear Chemistry - This is one of the first studies in small particulate matter (PM2.5) by inductively coupled plasma sector field mass spectrometry to measure the...     

Anomalous ratios of radioisotopes in PM10 as tracer of global fallout impact in the centre of Mexico

The monitoring and evaluation of radioactive content in samples of PM₁₀ aerosols have been investigated. The specific radioactivity concentrations (SRC) of ²³⁴U, ²³⁵U, ²³⁸U and ²³²Th were determined using inductively coupled plasma-sector field mass spectrometry in 13 samples collected in Mexico City and 8 samples collected in Cuernavaca in the centre of Mexico. The SRC of the radioisotopes analysed in PM₁₀ were larger than those reported in PM_2.5. The enrichment factor was greater than 5, indicating anthropogenic influences in both sites. The activity ratios of these isotopes in the samples were determined. The ²³⁵U/²³⁸U ratio showed variations with respect to the natural value, while the ²³⁴U/²³⁸U and ²³²Th/²³⁸U ratios did not show any secular equilibrium in all sites, corroborating that the increase of uranium is not influenced by natural sources. The annual dose results obtained have no impact on health.     

Roughness and fibre reinforcement effect onto wettability of composite surfaces

Wettability of glass/epoxy and carbon/epoxy composites materials has been determined via sessile drop technique. Good-Van Oss approach has been used to evaluate surface free energy parameters of smooth and rough surfaces. Results obtained point out the influence of fibre reinforcement on surface free energy of composite materials. In addition, the interest of surface treatment to increase surface roughness has been discussed in terms of wettability. To sum up, results obtained clearly demonstrate the necessity of considering properties of a given composite surface not only as a polymer but a fibre/polymer couple. The drawn conclusions are of great interest as it may have numerous consequences in applications such as adhesion.     

A study of Pt–Pd/γ-Al2O3 catalysts for methane oxidation resistant to deactivation by sulfur poisoning

The catalytic oxidation of methane was studied over calcined and reduced Pt–Pd/γ-Al₂O₃ catalysts, in the presence and the absence of SO₂ in the CH₄–O₂ reaction feed. The effect of sulfation (SO₂ + O₂ for 4 h at 500 °C) was also studied on the catalyst resistance to deactivation by sulfur poisoning. Sulfating the calcined Pt–Pd/γ-Al₂O₃ catalysts resulted in a strong deactivation for the CH₄–O₂ reaction. However, the catalytic activity of the reduced-sulfated Pt–Pd/γ-Al₂O₃ catalyst for CH₄–O₂ reaction remained rather unaffected in the presence and in the absence of SO₂ in the reaction feed. XPS analysis revealed, over reduced-sulfated Pt–Pd/γ-Al₂O₃ catalysts, the presence of Pt(0) metallic surface species on which SO₂ interactions may be faster related to Pd surface species. The presence of Pt(0) may be necessary to prevent the interactions between SO₂ and Pd surface species. Long time catalytic tests showed that the activity of a reduced Pt–Pd/γ-Al₂O₃ catalysts for CH₄–O₂ reactions remained rather unaffected despite the presence of SO₂ in the reaction feed.     

Raman scattering in quasi-one-dimensional ZrTe5

Raman scattering experiments on the compound ZrTe₅ isostructural with HfTe₅ have been performed at temperatures between 300–313 K. The Raman spectra of ZrTe₅ resemble those of HfTe₅ excepting two high frequency modes which should include the most characteristic diatomic mode. In spite of structural similarity with the transition-metal trichalcogenides, the Raman scattering activity cannot be directly compared. These unexpected results and the unusual temperature dependence of the Raman intensities suggest the possibility of a structural instability in HfTe₅ and/or ZrTe₅ in relation with the anomalies in the transport properties and in the X-ray forbidden reflections.