Use of overtone bands to monitor the state of the catalyst active phases during infrared studies of adsorption and catalytic reactions

Overtone bands are observable in the i.r. spectra of mixed and supported oxide catalysts, and may be informative on the state of the active phases and their interactions with absorbates. In the cases of supported V2O5/TiO2, MoO3/Al2O3 and WO3/Al2O3 the behaviour of such bands indicates that single uncoupled MO bonds are exposed on the surface. In the case of bulk VPO catalysts they are informative on the presence of oxidized phases. \ua9 1986

Fourier transform I.R. evidence of the formation of dioxymethylene species from formaldehyde adsorption on anatase and thoria

HCHO adsorption on activated thoria produces, at 240 K, a species characterized as dioxymethylene; by heating at room temperature, it leads to formate and methoxy groups: the three species are observed by adsorption of HCHO on anatase at room temperature

FT-IR characterization of the surface acidity of different titanium dioxide anatase preparations

The surface acidity of four different anatase preparations has been studied by FT-IR spectroscopy of adsorbed benzene, ammonia, pyridine, carbon monoxide and dioxide. Our results indicate that surface chemistry of anatase, in particular acidity, greatly depends on the preparation of the sample, both through the resulting morphology and the presence of surface impurities. Different morphologies may result in different types of Lewis acid sites. If pure, anatase does not show Br\uf8nsted acidity. The presence of silica impurities is responsible for an enhancement of Br\uf8nsted acidity on several samples, and may be evaluated by the intensity of an IR band situated near 3730 cm-1. Very strong Br\uf8nsted acidity is generated by the presence of sulfate impurities. \ua9 1985

In situ characterization of transition metal sulfide catalysts by IR probe molecules adsorption and model reactions

This work reports a detailed characterization of reduced states of RuS2/SiO2 catalyst by combining catalytic activity measurements and IR probe molecule adsorption. Depending on the solid composition monitored by a progressive reduction these surfaces gradually moves from an acid-base character to a metallic one. Both Lewis and Brønsted acidic sites are created in mild reduction conditions and the Lewis acidic sites play an important role in the activation of sulfur containing molecules and subsequently on their transformations. The hydrogenation properties are related to Ru sites with a low sulfur coordination

Modifications of the AgMo3P2O14 Catalyst in the Oxidation of Propane

The catalytic performances in the oxidation of propane of the AgMo3P2O14 catalyst are quite good, leading to 70% of propene selectivity at 7% of propane conversion. We have checked that, before reaction, results of XANES and EXAFS obtained at the Mo K-edge (19999 eV) confirm those obtained by X-ray spectroscopy (oxidation state and Mo number of oxygen neighbour of 5.66, Mo=O bond length=1.72Å). After reaction, we have shown that the catalyst is partially reduced [5.66 → 5.2], the number of oxygen atoms increases to 6 and the mean molybdenyl bond length decreases (1.68Å) which is in agreement with the variation of this bond length with the oxidation state of the molybdenum in molybdenum phosphates. XPS measurements also confirmed a partial reduction of molybdenum species and showed a migration of the silver cation to the surface, certainly participating to the surface stabilization. Moreover, a correlation between the catalytic activity and the modification of the catalyst is made showing that an equilibrium between MoV and MoVI species in the catalyst leading to the avrerage oxidation state of molybdenum of 5.2 seems to be very important

Microcalorimetric and Fourier transform infrared spectroscopic studies of methanol adsorption on Al2O3

Adsorption microcalorimetry and infrared measurements using CH3OH, CD3OH, and CHD2OH show the existence of at least three steps during the adsorption of methanol on alumina activated at 773 K. At very low coverages, molecules coordinated on the strongest Lewis sites are evident, which easily transform into bridged methoxide species. With increasing coverage, another form, irreversibly adsorbed at room temperature but desorbed by evacuation at 373-473 K, becomes predominant. It is identified as undissociated methanol strongly hydrogen bonded on a cation-anion couple having a strong basic character. Finally, at high coverages a reversible form hydrogen bonded to basic sites is detected. \ua9 1985 American Chemical Society