Catherine Louis

Catalytic properties of silica-supported molybdenum catalysts in methanol oxidation: the influence of molybdenum dispersion

Two types of MoSiO2 catalysts prepared by classical impregnation or by the grafting method have been tested in the methanol oxidation reaction, which is known to be structure sensitive. It is shown that there is a dependence of the formation of formaldehyde and methyl formate, the principal products, on the molybdenum content for grafted catalysts with a higher selectivity for methyl formate. In contrast, the dependence is not clear for the impregnated catalysts, and this appears to be due to the lack of reproducibility in their preparation, and to their Mo dispersion being lower than that of the grafted MoSiO2 catalysts. The study of the formation of methyl formate using different reactive mixtures and kinetic calculations led us to propose a mechanism which involves the formation of formaldehyde from methanol on Mo sites, followed by its migration on silica, where it further reacts with methoxy groups to form methyl formate via a hemiacetal intermediate.

New generation of titanium dioxide support for hydrodesulfurization

Mesoporous titanium oxide with a high specific surface area of 120 m 2 /g prepared by a novel method developed by Chiyoda was used for supporting molybdenum sulfide. In order to examine the influence of the surface area on the properties of the molybdenum sulfide phase, two different samples of titanium oxide were studied, a commercial one with a surface area of 72 m 2 /g and that prepared by Chiyoda. Molybdenum was deposited on the TiO2 supports by incipient wetness impregnation with ammonium heptamolybdate in one or two steps depending on the Mo loading. Some samples were also prepared by impregnation of ammonium heptamolybdate basified by ammonia. Raman spectroscopy and XPS were used to examine the nature of the molybdate phase and its dispersion in the oxidic state. HREM and XPS were used for studying the sulfided state. As expected, the maximum amount of well-dispersed molybdenum is higher on the Chiyoda support than on the reference support with a lower surface area. The catalytic properties of the catalysts were studied in dibenzothiophene conversion. For the Chiyoda support, the catalytic activity varied linearly with the Mo loading up to 6–7 Mo/nm 2 then became nearly constant for the higher loadings. Much higher activities (six times, expressed per gram of catalyst) were obtained compared to molybdenum sulfide supported on alumina.

Surface Segregation of Pd from TiO2‐Supported AuPd Nanoalloys under CO Oxidation Conditions Observed In situ by ETEM and DRIFTS

A TiO2‐supported AuPd bimetallic catalyst with an Au/Pd atomic ratio of 8 was prepared by deposition‐precipitation with urea, and its activity in CO oxidation at room temperature was compared to the one of a monometallic Au/TiO2 catalyst. X‐ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses suggest that Au‐Pd/TiO2 contains bimetallic nanoparticles after reduction under H2 at 500u2009°C although the presence of monometallic Au particles cannot be totally excluded. The evolution of the AuPd nanoparticles surface composition during exposure to O2 and CO/O2 was studied inu2005situ by environmental high resolution electron microscopy (ETEM) and DRIFTS. Pd segregation at the surface of the bimetallic nanoparticles was evidenced by DRIFTS and directly observed by ETEM under O2 and CO/O2 with the formation of Aucore‐Pdshell structure. The changes in the surface composition of the Au‐Pd nanoparticles under CO/O2 was paralleled with the higher rate of deactivation in the first reaction stages observed for Au‐Pd/TiO2 compared to Au/TiO2, which could be related to the possible replacement of Au in low coordination sites, at the origin of the high activity in CO oxidation, by Pd atoms. These results noticeably underline the modifications induced by the reactant that can undergo a bimetallic AuPd catalyst.

Influence of MoSiO2 catalyst preparation on molybdenum coordination and catalytic properties

Abstract Moue5f8SiO 2 catalysts were prepared by grafting, i.e., by reacting MoCl 5 with the hydroxyl groups of silica. The analysis of the Mo coordination sphere during grafting by reflectance spectroscopy and EPR indicates that grafting occurs probably through the following mechanism: MoCl 5 + SiOH → SiOMoCl 4 + HCl. Grafted Moue5f8SiO 2 catalysts differ from Moue5f8SiO 2 catalysts prepared by impregnation by the presence of a tetracoordinated Mo 5+ species (identified by EPR after thermal activation), a better molybdenum dispersion and stronger Moue5f8silica interactions. The methanol oxidation reaction was found to be sensitive to molybdenum dispersion and this might be an appropriate way to evaluate the dispersion of supported oxides.

EPR study of the stability and the role of the O2− species on La2O3 in the oxidative coupling of methane

Abstract The paramagnetic superoxide ion detectable by EPR spectroscopy, can be generated on fully decarbonated La2O3 by oxygen adsorption. After pretreatment in vacuo at 650°C, oxygen may be admitted at two different temperatures: i) at room temperature: all the O2− ions are located on the La2O3 surface; ii) at 650 °C: 85% of the O2− species are on the surface and 15% are between the (LaO)22+ layers of La2O3. The O2− radicals are no longer observed by EPR after the oxidative coupling of methane reaction. The study of their stability in the presence of the reactant gases and oxidation products of the reaction, shows that the main reason for the disappearance of the O2− species is that they are unstable under the low oxygen partial pressure of the gas feed (15 Torr). It appears unlikely that the O2− species are the active sites for the activation of methane.

Influence of the reactant concentration in selective hydrogenation of 1,3-butadiene over supported gold catalysts under alkene rich conditions: A consideration of reaction mechanism

This article is about the reaction of selective hydrogenation of 1,3-butadiene performed in an excess of alkenes (propene/butadiene ratio = 100) over supported gold catalysts, so as to mimic the conditions of purification of light alkenes (C4 cuts) from the presence of impurities (highly unsaturated compounds, 1–3%). Gold was found highly selective (close to 100%, no propane or butane formation) for the hydrogenation of butadiene to butenes at 100% conversion of butadiene. The influence of the concentrations of the various reactants, i.e. hydrogen, butadiene and propene, on the rates of butadiene and propene hydrogenation was investigated, and the apparent reaction orders were deduced from the dependence of rate on partial pressure in the gas phase. The reason for the high selectivity of gold catalysts is that the reaction of hydrogenation of alkenes occurs at much higher temperature (0.1% conversion at 300°C) than that of selective hydrogenation of butadiene (100% at T <170°C). Moreover, the presence of propene has only a limited influence on the reaction of butadiene hydrogenation, while butadiene has no influence on propene hydrogenation; the two reactions of hydrogenation hardly influence each other. Hydrogen dissociation is the rate limiting step of the reaction (partial order of 0.7 and 1 with respect to H2 for butadiene and propene hydrogenation, respectively), responsible for the low activity of gold. The catalytic properties of gold were compared to those of palladium-based catalysts, and reasons for their different behaviour were discussed.

Coordination chemistry involving oxide catalysts

Abstract The application of various spectroscopic techniques to investigate the coordination of transition metal ions is reviewed using examples which include the preparation of highly dispersed catalysts, the investigation of the coordination sphere of supported transition metal ions on oxide surfaces, the coordination sphere of catalytically active sites and the migration of transition metal ions into the bulk of oxides during catalyst pretreatment. The features of coordination chemistry for supported transition metal ions appear to be very similar to their solution analogues and the same concepts can thus be applied.

HRTEM and STEM-HAADF characterisation of Au–TiO2 and Au–Al2O3 catalysts for a better understanding of the parameters influencing their properties in CO oxidation

Gold catalysts supported on titania (Au-TiO(2)) and alumina (Au-Al(2)O(3)) were prepared by deposition-precipitation with urea and then activated before characterisation and reaction in CO oxidation, either by calcination in air at 500 °C or reduction under H(2) at 300 °C. Gold nanoparticles with average size in the range 2-4 nm were obtained, with calcination leading to larger gold nanoparticles than reduction. For Au-TiO(2), high activity was observed in CO oxidation at room temperature, independent of the activation treatment. This high activity could not be correlated to the presence of sub-nanometer gold clusters as reported in the literature, since they could not be detected by atomic-resolution high-angle annular dark-field scanning-transmission electron microscopy (HAADF-STEM). In the case of Au-Al(2)O(3), the performance in CO oxidation was found to strongly depend on the water content in the reaction gas feed and on the activation conditions, with calcination resulting in a poorly active catalyst whereas reduction gave activity of the same order as Au-TiO(2). A comparative study of Au-TiO(2) and Au-Al(2)O(3) by electron microscopy did not reveal distinct differences in the shapes of the Au nanoparticles, which are mostly flattened through interaction with the substrate in both samples, with side profile shapes varying from rounded hemispherical to well faceted truncated cubo-octahedra. More faceting is found for the samples calcined at 500 °C than reduced at 300 °C. Various possible parameters affecting the catalytic properties of gold in CO oxidation are discussed in the context of the relevant literature.

Gold catalysts supported on titanium oxide for catalytic wet air oxidation of succinic acid

Abstract Catalytic wet air oxidation of a representative organic compound (aqueous solution of 5 g l −1 succinic acid at 190 °C and 50 bar total air pressure) was investigated over gold on titania prepared from the deposition–precipitation method (with urea or NaOH) and compared to experiments performed over a Ru/TiO 2 catalyst. These preliminary results demonstrate that gold catalysts are efficient for the degradation of this organic acid. The catalytic activity is strongly dependent on the gold particle size characterized by transmission electron microscopy (TEM) with smaller particles producing higher turnover frequencies. Modification of metal dispersion occurs during reaction, leading to minor activity.

EPR study of the stability of the O−2 species on La2O3 and of their role in the oxidative coupling of methane.

Abstract The O−2 superoxide species can be generated on fully decarbonated La2O3 in two different ways. They can be obtained by oxygen adsorption at room temperature after vacuum treatment at 650°C, in which case all the O−2 species are located on the La2O3 surface. They can also be formed by oxygen treatment at 650°C and in that case 15% of the O−2 species are located between the (LaO)2n+2n sheets of La2O3. The adsorption sites are probably vacancies in the coordination sphere of La3+. The O−2 radicals are not observed by EPR after the reaction of oxidative coupling of methane. The study of O−2 stability in the presence of the different reactant gases and products of the reaction shows that the main reason for the non-observation of the O−2 species is that they are unstable under the low oxygen partial pressure of the reactant gas feed (15 Torr). In consequence, the O−2 species are probably not the sites directly responsible for the hydrogen abstraction of methane in this reaction. The possible nature of the active sites is discussed.