Complete oxidation of methane at low temperature over Pt and Pd catalysts for the abatement of lean-burn natural gas fuelled vehicles emissions: influence of water and sulphur containing compounds

The catalytic activity of fresh Pd and Pt catalysts supported on γ-alumina in the complete oxidation of CH₄ traces under lean-burn conditions was studied in the presence or the absence of water or H₂S. Steam-aged catalysts were also studied in order to simulate long-term ageing in real lean-burn natural gas fuelled vehicles (NGVs) exhaust conditions. Without water or H₂S added to the feed, Pd catalysts exhibit a superior catalytic activity in methane oxidation compared to Pt ones, whatever the catalysts were fresh or aged. The addition of 10 vol.% water vapour to the feed strongly affects the activity of the fresh Pd catalyst, thus being only slightly more efficient than the fresh Pt one. H₂S has a strong poisoning effect on the catalytic activity of Pd catalysts, while Pt catalysts are more resistant. The fresh H₂S-poisoned Pd/Al₂O₃ catalyst was studied by TPD in O₂/He. Poisoning species decompose above 873 K as SO₂ and O₂ in relative concentrations consistent with the decomposition of surface sulphate species. However, a treatment in O₂/He at temperatures as high as 923 K does not allow the complete regeneration of the catalytic activity of H₂S-poisoned Pd/Al₂O₃. A mechanism involving the poisoning of PdO by sulphate species is proposed. Different diffusion processes by which these sulphate species can migrate back and forth between PdO and the support, depending on the experimental conditions, are suggested.     

Effects of ZrO2 precursors on the synthesis of V-ZrSiO4 solid solutions by the sol-gel method

The preparation of V-ZrSiO₄ solid solutions starting from different ZrO₂ precursors by using sol-gel methods is reported. The starting materials were hydrolysed and the dried gels were fired at a temperature between 500 and 900 °C with soaking times of 12h. The organic character of zirconia precursors was stronger, i.e. the starting material had more carbon atoms, a higher temperature was necessary to make the first crystalline phase appear (ZrO₂(tetragonal)) and the temperature range for the whole phase transformation was narrower. In all dried gel samples the presence of infrared bands which might be associated with either Si-O-Zr or Si-O-V was not observed. On the other hand, some bands could be attributed to a silica network and ZrO₈ groups. The main steps in V-ZrSiO₄ solid solutions were confirmed. ZrO₂(tetragonal) is crystallized on heating from an amorphous sample. The ZrO₂(tetragonal) ZrO₂(monoclinic) phase transformation then occurs and immediately afterwards the zircon formation begins. Finer textures in samples were obtained from polymeric gels rather than for colloidal gel samples, as seen from the scanning electron micrographs.     

Complete Oxidation of Methane at Low Temperature over Pt Catalysts Supported on High Surface Area SnO2

The catalytic activity of Pt catalysts supported on high surface area tin(IV) oxide in the complete oxidation of CH₄ traces under lean conditions at low temperature was studied in the absence and in the presence of water (10 vol.%) or H₂S (100 vol.ppm). Their catalytic properties were compared to those of Pd/Al₂O₃ and Pt/Al₂O₃. In the absence of H₂S in the feed, Pt/SnO₂appears as a very promising catalyst for CH₄ oxidation, being even significantly more active under wet conditions than the best reference catalyst, Pd/Al₂O₃. Catalysts steamed-aged at 873 K were also studied in order to simulate long term ageing in real lean-burn NGV exhaust conditions. To this respect, Pt/SnO₂ is slightly less resistant than Pd/Al₂O₃. In the presence of H₂S, Pt/SnO₂catalysts are rapidly and almost completely poisoned, comparably to Pd/Al₂O₃and the catalytic activity is hardly restored upon oxidising treatment below 773 K. A synergetic effect between Pt and specific surface SnO₂sites active in CH₄oxidation is proposed to explain the superior catalytic behaviour of Pt/SnO₂.     

Synthesis of Cr x Ti1−2x Nb x O2O20≤x≤0.5 rutile solid solutions from alkoxides

In this study, Cr _x Ti_1–2x Nb _x O₂ (0 x 0.5) rutile solid solutions have been synthesized from gels built from hydrolysis-condensation of Cr (III) acetylacetonate, NbCl₅ and Ti (IV) isopropoxide mixture (polymeric gel). Characterization of these solid solutions was carried out by X-ray diffraction, ultraviolet-visible and infrared spectroscopy, differential thermal and thermogravimetric analysis and CIELAB (Commission Internationale del'Eclairage L^*a^*b^*) parameter measurements. The results obtained by the polymeric gel method were compared with those obtained by traditional ceramic synthesis. This comparison reveals some differences with regard to synthesis temperatures and reaction mechanisms. The formation of Cr _x Ti_1–2x Nb _x O₂ (0 x 0.5) rutile solid solutions by the ceramic method requires temperatures of about 1200°C and soaking times of several days. These solid solutions are synthesized at 1000°C in 24 h by the polymeric gel method. In ceramic synthesis, the CrNbO₄ compound with rutile structure appears as an intermediate compound in the formation of rutile solid solutions. In polymeric gel synthesis, however, the CrNbO₄ rutile compound was not detected in the samples.     

Study of phase-transfer-catalyst influence in CaO-SiO2 Gel preparation

Gels in a CaO-SiO₂ (2080 wt%) system were prepared in a heterogeneous phase. The following phase-transfer catalysts were tested: 18-Crown-6, 15-Crown-5 and tetrabutylammonium hydrogenosulphate. The influences of the polarity of two non-classical solvents (toluene and dichloromethane) were also studied. The degrees of polymerization and of oxide contents were analysed by infrared spectroscopy and thermal analysis. Finally, the morphology and structure of the dry and fired gels were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD).     

Structural and electrical conductivity studies on rutile solid solutions [FexTi1-2xMxO2 (M=Nb, Ta)]

In this study, FexTi1-xMxO2 (M=Nb, Ta; 0x0.5) solid solutions have been synthesized from both ceramic and gel methods, and electrical and structural characterization hasbeen carried out. The solid solutions show rutile structure except for the case of FeNbO4, which forms an orthorhombic structure. The electrical behaviour of the system studied corresponds to extrinsic p-type semiconduction and it is related to the presence of Fe(II) ions. Conductivity is enhanced by local distortions of the M–O lengths associated with ordering oxidation states such as those in Fe3O4. The distortions are more frequent when the asymmetry is greater (M=Nb). No electrical changes have been detected when the synthesis method is different. © 1998 Kluwer Academic Publishers     

Bathymetric study of Lake Hayq,Ethiopia

The generation of scientific information for improved understating of the physical dynamics of a lake is fundamental for guiding lake stakeholders and managers at the local level to implement best management practices and help design effective management strategies and policies at higher levels. Multitemporal bathymetric information on lakes is very important in hydrology and sediment studies to more clearly indicate environmental changes and to understand the effects of land processes on the hydrology of lakes. Accordingly, the purpose of this study was to map bathymetric charts of Lake Hayq ¹ in Ethiopia and to derive morphometric parameters, including depth, volume, area, width and length, and to plot curves illustrating the relationships between these parameters. The bathymetric survey was carried out using a combination of a SonarLite Portable Echo Sounder and Global Positioning System (GPS) to generate three‐dimensional (XYZ) hydrographic data. Surfer 8.01 and ArcGIS 9.3 software program were used for surface, gridding and morphometric analyses. Comparison of the results of this study with a previous study conducted in 1941 indicated the lake has experienced changes in depth and surface area. To reduce the negative impacts of human‐induced activities on the ecohydrology of the lake, and to maintain its ecological integrity, appropriate and integrated lake management practices must be adopted. This will necessitate policy formulation, active lake basin stakeholder involvement and implementation of basin‐wide lake management to ensure sustainable use of the lake and its basin resources.     

Study of hydroxycinnamic acids and malvidin 3-monoglucoside derivatives using capillary zone electrophoresis and ultra-performance liquid chromatography

Model solutions (pH = 3.5, 12% ethanol) of malvidin 3-O-glucoside (Mv3glc), the most common free anthocyanin in grapes and red wines from Vitis vinifera, and three free hydroxycinnamic acids present in wines (caffeic, ferulic and p-coumaric acids) were studied.     

Human infection with Hymenolepis diminuta: case report from Spain

We report a case of Hymenolepis diminuta infection in a human. The patient was a 5-year-old girl referred to us through the onset of a cyanotic attack. Treatment with a single dose (10 mg/kg of body weight) of praziquantel was ineffective, but the parasite was eradicated after three treatment cycles with the same drug at dosages of 25 mg/kg/day for 5 days.