Michel-Vital Mathieu

Sulfurization and regeneration of platinum

Abstract Chemisorption of H2S on Al2O3 supported platinum is dissociative and hydrogen is observed in the gaseous phase. Above 100°C, platinum surface atoms are fully covered by sulfur atoms. Hydrogen treatment at 400°C partly restores the catalytic and chemisorptive properties of platinum towards benzene hydrogenation and adsorption of carbon monoxide in the linear form. Adsorptions or reactions requiring polyatomic sites (n-butane hydrogenolysis, chemisorption of CO in bridged form) are still inhibited. Oxygen treatments of the poisoned catalyst, followed by hydrogen reduction under mild conditions, fully restore the properties of the starting material. Sulfur removal from platinum occurs with the formation of sulfate groups bonded to the support. Under more severe conditions of reduction, the reductio the sulfate groups leads again to a poisoning of the metal surface.

Chlorinated alumina. Acidic properties and catalytic activity towards n-butane isomerization

γ-Al2O3 has been chlorinated by CCl4 at 573 K in order to increase its acidity. Different physical methods, including infrared spectroscopy, thermogravimetry and thermodesorption of pyridine, have been used to calculate the actual number of acidic sites, according to their strength. Comparison with samples treated with HCl or AlCl3 leads to the conclusion that stronger Lewis sites are in the form of grafted AlCl3. n-Butane isomerization has been performed on such solids. Reaction occurs without the presence of metal and the higher the acidity the higher the activity and selectivity.

Infra-red determination of the isotherm of hydrogen adsorption on a Pt/Al2O3 catalyst

Of the various forms of hydrogen adsorbed on platinum only hydrogen weakly and reversibly adsorbed is detected by an infra-red band at 2120 cm–1. The preparation of a well dispersed sample of Pt/Al2O3 catalyst containing 10 % platinum allows us to determine accurately the hydrogen adsorption isotherm of this weakly adsorbed species. Only a dissociative Langmuir model fits the experimental results in the pressure range of 10–1 to 700 Torr and temperature range of 27 to 95°C. The heat of adsorption (– 5 × 104 J mol–1) and the isosteric heat of adsorption are in good agreement with previous calorimetric data.

Physicochemical properties and isomerization activity of chlorinated Pt/Al2O3 catalysts

Alumina-supported platinum catalysts with various metal contents have been prepared and chlorinated by CCl4 at 473 and 573 K. Their acidity has been measured by pyridine adsorption and catalytic activity in n-butane isomerization. The state of platinum has been followed by infrared spectroscopy, whereas the modifications of the dispersion were measured by electron microscopy and by H2–O2 titrations.The presence of metal in chlorinated alumina produces only small changes in acidity but a great enhancement in isomerization activity for samples chlorinated at 573 K. After chlorination, platinum is oxidized to platinum chloride, which reacts with AlCl3 to produce the PtCl2·2AlCl3 complex. This complex is mobile on the support and agglomerates in large particles. During the isomerization of n-butane, platinum is reduced in the metallic state leading to very low metal dispersion. Very small platinum clusters are also present, they are able to perform the hydrogenation of butenes and to decrease the coke formation. Thus ageing is limited in comparison with pure alumina. The possible role of HCl leading to superacid species is also discussed.

Inhibition par le cuivre de l'adsorption du monoxyde de carbone sur les alliages nickel-cuivre

Abstract The evolution of the relative concentration of CO bridged species with the NiCu alloys composition allows to estimate the copper efficiency to inhibit this kind of chemisorption. This efficiency decreases as the copper content increases. One copper atom inhibits almost nine nickel atoms for a copper concentration of 5%; this number decreases below two nickel atoms for a copper content greater than 40%.

UV spectroscopic study of oxidized and reduced palladium loaded zeolites

UV and visible reflectance spectra of Pd(II) ion exchanged zeolites have been recorded before and after hydrogen reduction. The reduction was performed either at 25°C (60% reduction) or at 500°C. Reoxidation by oxygen at 400°C restored the initial Pd(II) ions only in the case of the sample reduced at low temperature. With nitric oxide, reoxidation occurred even at 25°C, regardless of the temperature of reduction, The Pd(II) ions were redispersed in the zeolite lattice through the formation of nitrosyl complexes.AbstractСпектры отражения УФ и видимого света цеолитов с ионами Pd(II) были сняты перед и после восстановления водородом. Восстановление осуществляли либо при 25°C (восстановление до 60%) либо при 500°C. Реокисление кислородом при 400°C возрождало исходные ионы Pd(II) лишь в случае образцов, восстановленных при низких температурах. В присутствии окиси азота реокисление протекает даже при 25°C и независимо от температуры восстановления образца; ионы Pd(II) были вновь диспергированы в решетке цеолита через образование нитрозильных комплексов.