Jean Guillaume Eon

The CO2–CeO2 interaction and its role in the CeO2 reactivity

The interaction between CO2 and CeO2 and its role in the surface reactivity of alumina-supported cerium oxide has been studied by programmed thermodesorption (TPD) of CO2 and FTIR spectroscopy. The performance of Ce/Al2O3 systems was then analyzed for the propane oxidation in presence of CO2. The results have shown that the catalytic activity decreased when carbonate species are formed at the surface of CeO2. This behavior was attributed to the presence of CO2 from three different sources: contamination before use, during the handling of the samples, contamination proceeding from the reactants and from CO2 produced by the reaction itself.

Active sites for ethanol oxidation over SnO2-supported molybdenum oxides

Abstract SnO2-supported molybdenum oxides with varying coverage were synthesized and used for the catalytic oxidation of ethanol. The catalysts were obtained from precipitation of SnCl4 by ammonia in the presence of (NH4)2Mo7O24 (A). Some catalysts were also prepared by impregnation of (NH4)2Mo7O24 on SnO2 (B) for comparison. It was verified that molybdenum oxides inhibited the sintering of SnO2 crystals during calcination for preparation A, resulting in homogeneous systems with high specific areas. The solids were characterized by FTIR, temperature programmed reduction (TPR), DRS-UV, XPS and X-ray diffraction (XRD). The molybdenum coverage was determined by oxygen pulses after reduction at 400°C under hydrogen. The results indicated two structurally different superficial sites. Four-coordinated molybdates were preferentially formed on the surface of co-precipitated catalysts at low molybdenum loading, while six-coordinated polymolybdates were obtained in other cases. Bulk MoO3 oxide was also observed at very high loading. The turnover numbers (TONs) measured for ethanol oxidative dehydrogenation suggested that dispersed, four-coordinated molybdates were the active phase. These species also gave higher selectivity to acetic acid.

The role of water in ethanol oxidation over SnO2-supported molybdenum oxides

The role of water in the oxidation of ethanol to acetic acid on Sn–Mo–O catalysts was studied by catalytic test and FTIR spectroscopy of adsorbed species. The reaction showed a typical behavior of series reactions involving oxidation of ethanol to acetaldehyde and of the latter to acetic acid and CO2. Addition of water to the feed gas decreased the oxidation rate and significantly increased the selectivity to acetic acid, strongly contributing to decreasing the number of secondary products. FTIR analyses showed that water promotes desorption of ethanol and carboxylates, present as bridging and monodentate species. Decreasing catalytic rate values and increasing selectivity to acetic acid in the presence of water follow from site blocking by hydroxyl groups.

A structural analysis of lead hydroxyvanadinite

Hydroxyvanadinite, Pb(10)(VO(4))(6)(OH)(2), was prepared by the co-precipitation method and analyzed by X-ray absorption spectroscopy (XANES, EXAFS), infrared spectroscopy, Raman scattering and X-ray diffraction (XRD). The results showed that the structure is very similar to that of vanadinite, Pb(10)(VO(4))(6)Cl(2), with space group P6(3)/m (176) and cell parameters a = 10.2242(3) A and c = 7.4537(2) A. A Rietveld refinement of the structure was performed using vanadinite as the starting model and fixing the geometry of the vanadate ion as a rigid body. First-principles Density Functional embedded cluster models are developed to analyze electronic structures, bonding, and densities of states. Interaction of Pb with the OH channel anion is examined in detail, as an important structural feature. A periodic band structure approach was used to obtain a further estimate of relaxed atomic coordinates.

Influence of the Precursor on Cerium Distribution over Alumina

The Ce/Al 2 O 3 catalysts were prepared with Ce(NO 3 ) 3 , (NH 4 ) 2 Ce(NO 3 ) 6 and with cerium acethylacetonate precursors over alumina, by impregnation and grafting, respectively. Results have shown that the CeO2 surface area of the catalysts with the nitrate precursors is very similar but much higher on the catalysts with the acethylacetonate after grafting. The reducibility of this catalyst is better than of the catalysts with the nitrate precursor. TPR results indicate a reduction of CeO2, the formation of CeAlO 3 and Ce 2 O 3 . The experimental condition employed here allows to attain saturation over alumina which is probably due to steric effects of the Ce(Acet)3 molecules.

Simultaneous tetralin HDA and dibenzothiophene HDS reactions on NiMo bulk sulphide catalysts obtained from mixed oxides

NiMo bulk sulphide catalysts were obtained from mixed-oxides. The mixed-oxides were obtained by calcining the as-synthesized lamellar precursor with the so-called ϕy structure and (NH4)H2xNi3−x(OH)2(MoO4)2 formula. The corresponding mixed-oxides obtained by calcination were characterized by nitrogen adsorption, XRD, and ICP. Mixtures of α-NiMoO4 and β-NiMoO4 were obtained. A batch reactor was used for CS2/n-hexadecane in situ sulphidation of the mixed-oxides. A mixture of dibenzothiophene and tetralin was used for the liquid phase reaction carried out at 613 K and 70 bar. After the catalytic tests, the bulk sulphide catalysts were characterised by nitrogen physical adsorption, synchrotron light XRD, EXAFS and HR-TEM. The EXAFS simulations are consistent with disordered nickel sulphide particles dispersed in the catalysts. HR-TEM images showed randomly oriented, stacked-layer particles typical of Mo sulphide. The bulk catalysts had larger HDS and HDA activities and selectivities for hydrogenation reactions than alumina supported conventional catalysts containing the same Ni : Mo ratio. A pronounced support effect was observed for both HDS and HDA reactions. The use of the support strongly suppressed both cyclohexylbenzene formation in HDS of DBT and cis-decalin formation in HDA of tetralin. This suggests that similar active sites are involved in the formation of these compounds on the one hand, while another type of site is involved in biphenyl and trans-decalin formation on the other.

On the Nature of Sandwiched Chromium Complexes in Exchanged alpha-Zirconium Phosphate

Layered a-zirconium hydrogenphosphate monohydrate was prepared by direct precipitation, exchanged with n-propylamine and then by chromium aquacomplexes. The solids were characterized by X-ray diffraction, mass spectrometry, infrared and UV-Visible spectroscopy. The results suggest that monomer complexes form extended, hydrogen bonded, sandwiched monolayers in the interlayer space.

Oxidative coupling of methane on Ce/Na/CaO catalysts

Oxidative coupling of methane was investigated on Na/CaO and Ce/Na/CaO catalysts with different sodium and cerium contents. The reaction was carried out in a micro fixed-bed flow reactor operating at atmospheric pressure, at 700 and 750°C and molar ratio feed of CH4 : O2 : N2 = 50 : 10 : 40. Catalysts were compared at isoconversion and were characterized by BET, XRD and XPS. The addition of cerium to Na/CaO catalyst increased the specific activity by a factor of eight. These catalytic and characterization results were related to the presence not only of Na+O- active sites, formed by the introduction of Na+ in the CaO lattice, but also to peroxide active sites. A mechanism is proposed to explain the role of cerium in increasing the rate of the regeneration step of the Os– and Na2O2 sites.