Redox State Dynamics at the Surface of MoVTe(Sb)NbO M1 Phase in Selective Oxidation of Light Alkanes

The influence of Te or Sb on the catalytic properties of the M1 phase of MoVTe(Sb)NbO catalysts in the oxidation of propane or oxidative dehydrogenation of ethane has been investigated. The results show that Te and Sb affect the intrinsic activity of the catalyst for ethane and propane oxidation in the same way and that the Te-containing M1 phase was significantly more active than the Sb-containing phase, even when they contained the same amount of vanadium, which is known to be the active species for the reactions. This feature has been explained by a strong modification of the relative number of available V⁵⁺ sites, which is related to the fact that Sb tend to stabilize vanadium in active site, in a reduced state. Sb and Te were also shown to affect the selectivity in the case of propane oxidation but not in the case of ethane oxidative dehydrogenation. This demonstrated that they should be involved directly in the further transformation of the alkene molecules that are formed as an intermediate and presumably play a role in the α-hydrogen abstraction from these molecules, as proposed earlier. Blocking problems related to the substitution of Te by Sb in view of industrialization are also discussed.     

Polycyanurate networks modified by polyoxytetramethylene glycol

Summary We report results obtained within a collaborative project dealing with the preparation of hybrid polycyanurate/polyoxytetramethylene glycol (PCN/PTMG) networks and the investigation of their structure-property relationships by a variety of experimental techniques. The hybrids were prepared from PCN and PTMG (molar mass 1.000 g/mol) with 10, 20, 30 and 40 wt% PTMG. The degree of incorporation of PTMG into the PCN network was determined by gel fraction measurements. WAXS and SAXS studies had indicated that the materials under investigation are amorphous and exhibit nanostructural heterogeneity, which increases with increasing amount of PTMG. Stress-strain measurements show improvement of the mechanical properties for PTMG contents of 30 and 40%. The focus in this paper is on the detailed investigation of the α relaxation associated with the glass transition in wide ranges of frequency and temperature by dielectric techniques. A single α relaxation was observed in all the compositions shifting systematically to lower temperatures/higher frequencies with increasing PTMG content. The results were systematically analyzed in terms of time scale and relaxation strength of the response and are discussed in terms of plasticization, presence of nanostructural heterogeneities and hybridization. Co-operativity of the α relaxation, quantified in terms of fragility, was found to decrease with increasing amount of PTMG, in correlation with increasing level of nanoheterogenity.     

Divergent pattern of polyisoprenoid alcohols in the tissues of <Emphasis Type="Italic">Coluria geoides</Emphasis>: A new electrospray lonization MS approach

Polyisoprenoid alcohols of the plant Coluria geoides were isolated and analyzed by HPLC with UV detection to determine the nature of the polyprenol and dolichol mixture in the organs studied. In roots, a family of dolichols (Dol-15 to Dol-23, with Dol-16 dominating, where Dol-n is dolichol composed of n isoprene units) was accompanied by traces of polyprenols of similar chain lengths, whereas in hairy roots grown in vitro, identical patterns with a slightly broader chain-length range were found. Conversely, in leaves and seeds polyprenols were the dominant form, and their pattern was shifted toward longer chains (maximal content of Pren-19, where Pren-n is polyprenol composed of n isoprene units). Interestingly, the pattern of dolichols in seeds and leaves (in which Dol-17 dominated) was similar to that found in roots. Structures of the dolichols and polyprenols isolated were confirmed by the application of a new HPLC/electrospray ionization-MS method, which also offers a much higher sensitivity in detection of these compounds compared to a UV detector. The highest sensitivity was obtained when the [M+Na]⁺ ions of polyprenols and dolichols were recorded in the selected ion monitoring mode and a small amount of sodium acetate solution was added post-column to enhance the formation of these ions in an electrospray ion source.     

Preparation of a Novel Cementitious Material from Hydrothermally Synthesized C–S–H Phases

New cementitious materials based on calcium hydrosilicate hydrates were recently developed as potential substitutes for ordinary portland cement, but with a reduced CO₂ footprint. The materials are produced by hydrothermal processing of SiO₂ and Ca(OH)₂, giving rise to calcium silicate hydrates, followed by mechanical activation of the latter via cogrinding with various siliceous materials. Thus, the chemical composition in terms of C/S ratio could be adjusted over a broad range (1–3). In this study the synthesis of a previously unknown cementitious material produced via the combination of mechanical activation in a laboratory mill and thermal treatment of a mixture of quartz and hydrothermally synthesized calcium silicate hydrates: α‐Ca₂[HSiO₄](OH) (α‐C₂SH) and Ca₆[Si₂O₇](OH)₆ (jaffeite) are reported. It forms independently of the type of mill used (eccentric vibrating mill, vibration grinding mill) after thermal treatment of the ground materials at 360°C–420°C. The new material is X‐ray amorphous and possesses a CaO/SiO₂ ratio of 2. A characteristic feature in regards to the silicate anionic structure is the increased silicate polymerization (up to 27% Si₂O₇ dimers) as revealed by the trimethylsilylation method. Infrared (IR) spectra show a very broad absorption band centered at about 935 cm^?1. Another characteristic feature is the presence of ~2.5 wt% H₂O as shown by thermogravimetry (TG) coupled with IR spectroscopy. As this water is bound mostly as hydroxyl to Ca, we refer to this new cementitious material as calcium‐oxide–hydroxide–silicate (C–CH–S). Calorimetric measurements point to a very high hydraulic reactivity which is beyond that for typical C₂S materials. The influence of the type of grinding on the thermal behavior of α‐C₂SH upon its transformation into water‐free Ca₂SiO₄ modifications is discussed.     

Chloride‐induced migration of supported platinum and palladium across phase boundaries

Previous work showed that calcination in O₂ of physical mixtures of Fe₂O₃ and supported Pt leads to a strong reduction enhancement of the Fe₂O₃, but that a much smaller effect was observed with supported Pd. The present results show that a strong reduction enhancement could be achieved by pretreating Pt/Al₂O₃ or Pd/Al₂O₃ with NH₄Cl and then decomposing NH₄Cl, before mixing the solid with Fe₂O₃. Such pretreatment with NH₄Cl has no effect on SiO₂ or zeolite‐supported metals, because only Al₂O₃ retains chloride ions at its surface. In the physical mixtures, chlorides migrate from Al₂O₃ to Fe₂O₃ at elevated temperature and form a volatile compound, presumably FeCl₃. Layered‐bed experiments show that this FeCl₃ sublimes, and that its chemical interaction with Pt or Pd on any support results in the formation of mobile Pt– or Pd–chloro complexes that reach Fe₂O₃ particles by surface migration. After exposure to an H₂ flow, the complexes are reduced, and Pt or Pd particles are formed on the Fe₂O₃, enhancing its reduction by H spillover. These metal particles on the Fe₂O₃ have been identified by TEM and X‐ray energy dispersive spectroscopy (EDS). Abundant formation of PdFe alloys upon reduction is verified by TPR/TPD, indicating that almost all Pd has interacted with the volatile Fe chloride. In the absence of a transition metal, chloride ions retard the reduction of Fe₂O₃. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation and characterisation of selenium-enriched mushroom aqueous enzymatic extracts (MAEE) obtained from the white button mushroom (Agaricus bisporus)

Selenium-enriched mushroom aqueous enzymatic extracts (MAEE) were obtained from the white button mushroom (Agaricus bisporus) by a procedure based on enzyme and membrane technology. MAEE, with a selenium concentration of 51.8 ± 5.58 μg/g is a product suitable for achieving the recommended daily dose (RDD) of 55 μg with a small amount of around 1 g of product, which can be incorporated into any type of solid or liquid food without modifying its organoleptic properties. Chemical characterisation and selenium speciation are also reported; more than 86% of the selenium-containing products are organic in nature. The utilisation of this product would help in the treatment and/or prevention of diseases associated with low selenium concentrations, such as ageing, and neurodegenerative, cardiovascular and immunological diseases, while avoiding the risk of reaching high plasma selenium concentrations which has recently been associated with deleterious effects.     

Comparison of the performance and durability of PEM fuel cells with different Pt-activated microporous layers

We report on a comparative study of the performance level of H₂/O₂ PEM fuel cells in which the catalytic layers containing Pt nanoparticles were deposited on the microporous layer side of gas diffusion electrodes, using three different deposition techniques: (i) by magnetron sputtering, (ii) by impregnation followed by chemical reduction (using either ethylene glycol or hydrogen or sodium borohydride as reducing agent), and (iii) by spraying a catalytic ink (containing either Pt/C or bulk Pt particles). The microstructure and chemical composition of the different catalytic layers has been determined by SEM, TEM, XRD and XPS analysis. Their electrochemical surface areas have been determined by cyclic voltammetry. The i-V curves have been measured and compared. A durability stress test based on cycles of potential was used to assess the degradation rate of the different catalytic layers and to rank performance.     

A new hydrogen storage system based on efficient reversible catalytic hydrogenation/dehydrogenation of terphenyl

Noble metal catalysts on mesoporous SiO₂ and modified carbon supports were found to enhance the activities of terphenyl (TPh) hydrogenation and tercyclohexane (TCH) dehydrogenation without side reactions, such as cracking, hydrogenolysis, ring opening and/or coke formation. The noble metal catalysts could be used for a reversible hydrogen storage system. Five percent Pt/SiO₂ catalyst was highly active in TCH dehydrogenation without stirring, due to an easier diffusion of organic molecules to the small catalyst particles during dehydrogenation.     

Effects of High-Intensity Pulsed Electric Fields Processing Parameters on the Chlorophyll Content and Its Degradation Compounds in Broccoli Juice

The influence of high-intensity pulsed electric fields (HIPEF) parameters including electric field strength (15–35 kV/cm), treatment time (500–2,000 μs), and polarity (monopolar or bipolar mode) on the content of chlorophylls (Chls), pheophytin (Phe), chlorophyllide (Chlide), and pheophorbide (Phb) and chlorophyllase activity (Chlase) in broccoli juice were assessed. A significant effect of HIPEF parameters on Chlase, Chls, and Chls degradation compounds was observed through a response surface methodology design. However, polarity did not exert influence neither on Chl a nor on Chl b. The optimum HIPEF treatment was found to be 35 kV/cm for 1,980 μs in bipolar mode, where the highest content of Chls was kept, the lowest Chlase residual activity was reached, and the minimal quantities of Chls degradation compounds content were formed. Additionally, at these HIPEF conditions, broccoli juice exhibited greater content of Chls than thermally treated or untreated juice. These outcomes demonstrated that HIPEF processing could be a suitable technology to maintain the Chls content in broccoli juice.