The Activation of a Pd/γ-alumina Catalyst During Methane Combustion: Investigation of the Phenomenon and of Potential Causes

Activation phenomena of a Pd/γ-Al₂O₃ catalyst under methane combustion reaction conditions were studied. The activation of the catalyst with time-on-stream is not due to Cl removal from the surface but seems to be related to sintering of the palladium particles. This could induce structural changes in the palladium particle, which would favor the activation of O₂ on the surface of PdO.     

Origin of transient species present on the surface of a PdO/γ-Al2O3 catalyst during the methane combustion reaction

Operando DRIFTS was applied to the study of the evolution of surface species formed on a Pd (2 wt.%)/γ-Al₂O₃ catalyst in various conditions. No differences were observed as a function of the initial oxidation state of palladium. Formates/carbonates species were identified at low temperature (<400 °C) and disappeared when CO₂ production started. These species come from the Pd-catalyzed interaction of CO with the alumina support, while CO₂ induces hydrogenocarbonates formation at low temperature (<300 °C). Their presence does not explain the inhibiting effect of CO₂ observed in CCM on Pd/γ-Al₂O₃ catalysts.     

Consumer preferences for fresh tomato at the European scale: a common segmentation on taste and firmness

Although tomato flavor has not been a major goal for breeders, nowadays it becomes important as it is a subject of consumer complaint. A better knowledge of tomato consumer preferences, at the European level, should provide the basis for improvement of fruit quality and for market segmentation. In the framework of a large European project, 806 consumers from 3 countries, The Netherlands, France, and Italy, were presented with a set of 16 varieties representing the diversity of fresh tomato offer in order to evaluate their preferences. In parallel, sensory profiles were constructed by expert panels in each country. Preference maps were then constructed in each country revealing the structure of consumer preferences and allowing identification of the most important characteristics. Then a global analysis revealed that preferences were quite homogeneous across countries. This study identified the overall flavor and firmness as the most important traits for improving tomato fruit quality. It showed that consumer preferences from different European countries, with different cultures and food practices, are segmented following similar patterns when projected onto a common referential plan. Moreover, the results clearly showed that diversification of taste and texture is required to satisfy all consumers' expectations as some consumers preferred firm tomatoes, while others preferred melting ones and were more or less demanding in terms of sweetness and flavor intensity. Detailed comparisons also showed the importance of the fruit appearance in consumer preference.     

The oxidizing role of CO2 at mild temperature on ceria-based catalysts

For thermodynamic reasons, CO₂ has always been considered as inert at mild reaction temperatures (300 °C). In this study, we show that CO₂ may be used as a valuable compound for the catalytic combustion of methane (CCM), if ceria-based materials are used as support for the palladium active phase. Adding CO₂ in the feed significantly improves performances of ceria-zirconia supported catalysts. On the contrary, catalytic performances are inhibited on Pd/γ-Al₂O₃. Inhibition can be avoided by mixing the Pd/γ-Al₂O₃ catalyst with some CeO₂ evidencing cooperation phenomena between both catalysts. In situ DRIFTS experiments show that the inhibition of the alumina-supported catalyst is not due to formation of carbonates species. After an in situ reducing pre-treatment, pure CO₂ is able to rapidly oxidize reduced Pd/Ce_0.21Zr_0.79O₂ catalyst at 300 °C. Dissociation of CO₂ on Ce_0.21Zr_0.79O₂ would be responsible for the oxidation process. Thus, CO₂ helps in replenishing the O reservoir (OSC) of the Ce-Zr-O support which is normally consumed by reductants such as CH₄, H₂ or other HC's. XPS experiments show enrichment in oxygen species bound to Ce (Low BE O1s) on the surface of ceria-zirconia when working in the presence of CO₂. Implications of these results on the behavior of ceria-containing catalysts can be important for practical applications, e.g., in automotive exhaust catalysis.     

Kinetic theory and dynamic structure factor of a condensate in the random phase approximation

No Heading We present the microscopic kinetic theory of a homogeneous dilute Bose condensed gas in the generalized random phase approximation (GRPA), which satisfies the following requirements: 1) the mass, momentum and energy conservation laws; 2) the H-theorem; 3) the superfluidity property and 4) the recovery of the Bogoliubov theory at zero temperature ¹. In this approach, the condensate influences the binary collisional process between two normal atoms, in the sense that their interaction force results from the mediation of a Bogoliubov collective excitation traveling throughout the condensate. Furthermore, as long as the Bose gas is stable, no collision happens between condensed and normal atoms. In this paper, we show how the kinetic theory in the GRPA allows to calculate the dynamic structure factor at finite temperature and when the normal and superfluid are in a relative motion. The obtained spectrum for this factor provides a prediction which, compared to the experimental results, allows to validate the GRPA.PACS numbers:03.75.Hh, 03.75.Kk, 05.30.–d     

Laparoscopic approach in acute small bowel obstruction. A review of 68 patients

BACKGROUND/AIMS: Laparoscopic management of acute small bowel obstruction has potential advantages over classical laparotomy. The aims of this study were to assess the feasibility and the safety of this technique, as well as to find any predictive success factors. METHODOLOGY: A laparoscopic approach was undertaken in 68 out of 150 patients admitted between 1991 and 1997 for acute small bowel obstruction. RESULTS: The cause of obstruction was bands or adhesions in 80% of the patients. A correct laparoscopic diagnosis was established in 66% of the cases. A laparoscopic treatment was performed successfully in 31 patients (46%), and was assisted by minilaparotomy in 4 patients (6%), and by open herniorraphy in 2 patients (3%). Thirty-one patients (46%) needed a conversion to laparotomy. There were 6 bowel injuries (9%), all recognized during laparoscopy. There were 2 deaths in converted patients and 2 early reoperations for persisting ileus in patients treated by laparoscopy alone or by assisted laparoscopy. CONCLUSION: Acute small bowel obstruction can be treated by laparoscopy alone, or assisted by minilaparotomy or open herniorraphy with advantages for the patient and few complications despite a high rate of conversion. There were no pre-operative predictive factors for successful laparoscopy, except for an isolated previous scar from an appendectomy. Pre-operative predictive success factors were parietal intestinal adhesions, as the only cause of obstruction. Multiple adhesions will mostly require conversion to laparotomy.