A. Aboukaïs

Effect of incorporation of copper or nickel on hydrogen storage in ceria. Mechanism of reduction

Interaction of hydrogen with a series of mixed oxides CeMx(M = Cu, Ni; 0 < x⩽ 5) precursors of catalysts for hydrogenation reactions has been studied in the 300–1073 K temperature range. In situ XRD, XPS, EPR and thermogravimetric techniques have been used to characterize the processes occurring during the hydrogen treatment, and the amounts of hydrogen occluded in the solids when treated at different temperatures under flowing H2 have also been determined. The most interesting phenomena have been found to occur in the activation temperature range. They involve a non-negligible expansion of the ceria-based lattice which has been attributed to the high amount of hydrogen stored in the host oxide and the reduction of Ce4+ to Ce3+. The M2+ reduction is slowed down, depending on the content of M2+ in the ceria lattice: redox processes between Ce4+, Ce3+, M and M+ or M2+ have been clearly demonstrated. Finally, a reduction mechanism, partly based on heterolytic dissociation of H2, is proposed. This may be extended to bulk mixed oxides with other transition metals, and to a certain extent to metals of Group VIII (Rh, Pd, Ir, Pt) deposited on ceria.

Redox behaviour of copper(II) species on CuCe oxide catalysts: electron paramagnetic resonance (EPR) study

Abstract Electron paramagnetic resonance (EPR) has been used to study the effect of heat treatment under air or hydrogen on the behaviour of CuCe oxide catalysts. Monomers, dimers and clusters of Cu2+ ions have been detected in CeO2 after different calcination temperatures. Monomers and dimers are located in specific and similar sites of the ceria lattice in place of Ce4+ ions. When two Cu2+ ions are situated at a distance of 3.6 ± 0.3 A and bridged by an oxygen, the dimer signal is formed. The reduction of dimers by hydrogen gives monomers in a first step and these transform into Cu− or Cu0 species in a second step.

ESR study of deep-bed calcined NH4Y and aluminum deficient zeolites

Abstract Electron spin resonance and irradiation by γ-rays at 77 °K or at room temperature were used to study the removal of alumina from the Y type zeolite framework by heat treatment in a deep bed geometry. X and Q band ESR spectra were recorded at 77 °K and showed respectively 12 and 16 lines corresponding to an isotropic hyperfine interaction with two aluminum atoms (I = 5 2 ) (c ∥∼- c ⊥ ∼- a iso Al = 10.0 Oe ) , the eleven hyperfine lines being split by a uniaxial symmetry of the g tensor ( g ∥ = 2.0125, g ⊥ = 2.0030, and g iso = 2.0062). The paramagnetic center was identified with a V center, namely a hole of an electron trapped on an oxygen atom bonding two nearby aluminum atoms. This center is localized on alumina which has been removed from the zeolite framework. The removal of alumina occurred only when the samples were heated above 400 °C in the presence of both water vapor and ammonia. It is suggested that the mechanism of migration involves the formation of aluminic acid in a dimer form and a reorganization of the lattice leaving no aluminum atom vacancies. A special emphasis is placed on the heat treatment conditions.

Oxidative dehydrogenation of isobutyric acid on H4PVMo11O40, NaxH4-xPVMo11O40 and CUyH4-2yPVMo11O40 heteropolyacid catalysts supported on silica

Abstract The oxidative dehydrogenation of isobutyric acid (IBA) to methacrylic acid (MAA) was studied over the heteropolyacid H4VMo11O40 and the acidic salts NaxH4-xPVMo11O40 and CuyH4-2yPVMo11O4. supported on silica, as catalysts. The presence of copper as counter-cations in the heteropoly compounds increases the activity and the selectivity to MAA. This increase is probably due to the simultaneous presence in the solids of Bronsted acidity and oxygen vacancies with an adequate ratio. These latter sites can more easily be obtained in the presence of Cu2+ and V4+ than with Na+ counter-ions in the catalysts.

Effect of copper addition on the activity and selectivity of oxide catalysts in the combustion of carbon particulate

Abstract Different stable oxides Al 2 O 3 , ZrO 2 , TiO 2 and CeO 2 pure and supported copper (2 wt% CuO) systems have been studied in the catalytic combustion of a carbon black. Two oxidation phenomena were evidenced: the first one was related to a tight carbon particulate—catalyst contact and the second one, occurring at higher temperatures, was assigned to a loose contact. The copper introduction was found to favour the carbon black—oxide contact and a redox mechanism was involved with copper based catalyst during the combustion reaction. The presence of small CuO particles, besides isolated copper species on oxide carriers, appeared as a determining parameter versus the activity and the selectivity.

Spectroscopic characterization of CuTh oxide catalysts in decomposition of isopropyl alcohol

Copper–thorium oxide catalysts prepared with different atomic ratios Cu : Th by coprecipitation of hydroxides and calcination in dry air at 1073 K, have been characterized by electron paramagnetic resonance (EPR) and UV–VIS spectroscopies before and after being tested as catalysts for the decomposition of isopropyl alcohol. The catalysts are selective for dehydrogenation to form acetone only. The simultaneous presence of Cu2+, Cu+ and/or Cu0 in the thoria with a ratio of (Cu0+ Cu+)/Cu2+≈ 2 is required for activity toward isopropyl alcohol dehydrogenation.

Reactional mechanism of various isomeric C6 alkenes over reduced mixed CuCeAl oxide catalysts

Abstract Hydrogenation and isomerization of many C6 alkenes are studied over reduced Cuue5f8Ceue5f8Al oxide catalysts. Activity and selectivity are strongly dependent on the steric and electronic effects characteristic of the reagent molecule. The less crowded the double bond of the alkene, the more important the activity is. On the other hand, the more polar the reagents are, the more favoured the hydrogenation selectivity is. The proposed reaction mechanism can be considered as a balance between the passage by an intermediary σ-alkyl leading mainly to hydrogenation product, and a concerted mechanism where the resultant is a double bond migration product. The equilibrium of this balance depends essentially on the polarity of the used reagent.