Adsorption and reaction of CO on (Pd–)Al2O3 and (Pd–)ZrO2: vibrational spectroscopy of carbonate formation

γ-Alumina is widely used as an oxide support in catalysis, and palladium nanoparticles supported by alumina represent one of the most frequently used dispersed metals. The surface sites of the catalysts are often probed via FTIR spectroscopy upon CO adsorption, which may result in the formation of surface carbonate species. We have examined this process in detail utilizing FTIR to monitor carbonate formation on γ-alumina and zirconia upon exposure to isotopically labelled and unlabelled CO and CO2. The same was carried out for well-defined Pd nanoparticles supported on Al2O3 or ZrO2. A water gas shift reaction of CO with surface hydroxyls was detected, which requires surface defect sites and adjacent OH groups. Furthermore, we have studied the effect of Cl synthesis residues, leading to strongly reduced carbonate formation and changes in the OH region (isolated OH groups were partly replaced or were even absent). To corroborate this finding, samples were deliberately poisoned with Cl to an extent comparable to that of synthesis residues, as confirmed by Auger electron spectroscopy. For catalysts prepared from Cl-containing precursors a new CO band at 2164 cm−1 was observed in the carbonyl region, which was ascribed to Pd interacting with Cl. Finally, the FTIR measurements were complemented by quantification of the amount of carbonates formed via chemisorption, which provides a tool to determine the concentration of reactive defect sites on the alumina surface

Electronic structure of aluminum oxide: ab initio simulations of

The electronic structure of γ-Al2O3 and α-Al2O3 has been investigated by means of the density functional theory. A comparison of the calculation results with experimental data for amorphous alumina films is also presented. The electronic structure is described in terms of band structure and density of states. It has been found that γ-Al2O3 have similar electronic structure with α-Al2O3 and amorphous Al2O3. Effective electron masses in γ-Al2O3 as well as in α-Al2O3 equal to $m_e^\ast$ ≈ 0.4 m0 that is in a good agreement with the experimentally found tunnel electron mass in amorphous Al2O3. The heavy holes in both alumina crystals are explained by the valence band top forming by O 2pπ nonbonding orbitals

Activation of the C-O bond on the surface of palladium: An in situ study by X-ray photoelectron spectroscopy and sum frequency generation

An in situ study of the adsorption of CO on atomically smooth and defect Pd(111) surfaces was performed over wide ranges of temperatures (200-400 K) and pressures (10(-6)-1 mbar) by X-ray photoelectron spectroscopy and sum frequency generation. Both of the techniques indicated that CO was adsorbed as threefold hollow, bridging, and terminal species to form well-known ordered structures on the surface. In the course of the in situ experiments, no signs of CO dissociation or of the formation of carbonyl structures (Pd(CO)(n), n > 1) were detected. The mechanism of C-O bond activation in the course of methanol decomposition on the surface of palladium was considered. It was found that the adsorption of methanol on the surface of palladium essentially depends on pressure. Along with the well-known reaction path of methanol dehydrogenation to CO and hydrogen, a slow process of methanol decomposition with C-O bond cleavage was observed at elevated pressures. In this case, the formation of carbon deposits finally resulted in the carbonization and complete deactivation of the surface. A mechanism for C-O bond activation on the surface of palladium was proposed; the geometry of adsorption complexes plays an important role in this mechanism

CO dissociation and CO hydrogenation on smooth and ion-bombarded Pd(111): SFG and XPS spectroscopy at mbar pressures

The CO dissociation probability on transition metals is often invoked to explain the product distribution (selectivity) of catalytic CO hydrogenation. Along these lines, we have investigated CO adsorption and dissociation on smooth and ion-bombarded Pd(111) at pressures up to 1 mbar using vibrational sum frequency generation (SFG) and X-ray photoelectron spectroscopy (XPS). Under high pressure, CO adsorbate structures were observed that were identical to high-coverage structures in UHV. On ion-bombarded surfaces an additional species was detected which was attributed to CO bridge bonded to defect (low-coordinated) sites. On both surfaces, no indications of CO dissociation were found even after hours of 0.1 mbar CO exposure. However, exposing CO/H2 mixtures to ion-bombarded Pd(111) produced carbonaceous deposits suggesting CHxO species as precursors for C–O bond cleavage and that the formation of CHxO is facilitated by surface defects. The relevance of the observations for CO hydrogenation on Pd catalysts is discussed

Epiglottites aigues [i.e.] chez l'adulte (à propos de 5 cas)

Si classiquemerrt, l'épiglottite est une affection de l'enfant, sa fréquence croissante chez l'adulte est loin d'être négligeable. C'est une urgence respiratoire et infectieuse qui impose un diagnostic précoce devant un tableau clinique qui s'articule autour d'une dyspnée inspiratoire d'intensité plus ou moins sévère associée à un état infectieux. Dans notre travail, nous avons relaté cinq cas cliniques d'épiglottites graves de l'adulte en Lorraine entre 2001 et 2005. La bibliographie récente fournie nous a permis d'élaborer deux algorithmes de prise en charge face à une forme d'emblée grave et à une forme initialement mineure. Il en ressort la place prépondérante de la nasolaryngoscopie, suivie ou non d'une intubation nasotrachéale sous fibroscope, et d'une surveillance en milieu spécialisé de toute suspicion d'épiglottite face au risque d'aggravation clinique.NANCY1-SCD Medecine (545472101) / SudocPARIS-BIUM (751062103) / SudocSudocFranceF