Inés Rodríguez Martín

A FTIR spectroscopic study of surface acidity and basicity of mixed Mg, Al-oxides obtained by thermal decomposition of hydrotalcite

Abstract A hydrotalcite with a Mg:Al ratio of 2.5:1 has been synthesized and characterized. The solids obtained upon calcination of this material at 770 and 1023 K have been also characterized, and have been found to be a mixture of MgAl 2 O 4 and MgO. Surface acidity and basicity of the solid calcined at 1023 K have been assessed by FTIR monitoring of pyridine and formic acid adsorption. It has been found that the surface is not acid, although a residual acidity, larger than that in pure MgO, exists. Adsorption of formic acid takes place dissociatively on surface, isolated hydroxyl groups, contrary to previous results with MgO, where dissociative adsorption took place through interaction with exposed oxide ions.

A FTIR spectroscopy study of isopropanol reactivity on alkali-metal-doped MoO3/TiO2 catalysts

Abstract The transformation of isopropanol on MoO 3 /TiO 2 catalysts doped with alkali-metal cations has been studied by the pulse technique. The FTIR studies have provided evidence of the dissociative adsorption of isopropanol and the formation of isopropoxide species which decompose at higher temperatures to acetone. Catalytic measurements have shown that the addition of alkali-metal cations leads to a drastic decrease in the yield of propene, owing to the elimination of Bronsted centres originally existing on the catalyst surface, whereas the rate of dehydrogenation to acetone is affected only slightly. The extent of the changes observed depends on the nature of the doping alkali-metal cation.

Surface dispersion of molybdena supported on silica, alumina and titania

Surface species formed upon impregnation of silica, alumina and titania with aqueous solutions of ammonium molybdate and calcination at 770 K have been studied by X-ray diffraction, specific surface area assessment, scanning electron microscopy, laser Raman spectroscopy and temperature-programmed reduction. Dispersion decreases in the order TiO2> Al2O3SiO2. Polymolybdate species are formed for low loadings, that lead to bulk MoO3 for loadings of 0.4 monolayer on SiO2, but persist for higher loadings on titania and alumina. On this last support, isolated molybdate species have been also found.

Probing the surface acidity of lithium aluminium and magnesium aluminium layered double hydroxides

The surface acidity of a range of lithium aluminium layered double hydroxides intercalated with carbonate, chloride, nitrate and decavanadate anions has been measured using Hammett indicators. In addition, the adsorption of pyridine and isopropanol has been monitored using infrared spectroscopy. The properties of the materials have been compared to their magnesium aluminium analogues. Surface measurements are compared with the observed catalytic activity for acetaldehyde self-condensation and 2-methylbut-3-yn-2-ol conversion. The MgAl matrix was found to be more stable than the LiAl matrix following calcination at the elevated temperatures which yielded the most active catalysts (ca. 450C). The order of acidity of the materials varied with the nature of the anion such that: carbonatenitratechloridedecavanadate. Thus it was concluded that the nature of the interlayer anion had a large effect on the acidity of the materials, although the acidity was largely unaffected by the identity of the host matrix itself.

Fourier-transform infrared study of the oxidation of ethene on MoO3/TiO2 catalysts doped with alkali metals

An FTIR spectroscopy study of the adsorption and oxidation of ethene on MoO3/TiO2 catalysts (undoped or doped with alkali-metal cations, Li, Na, K and Rb) has been carried out. The undoped catalyst is less active, although it shows a larger selectivity towards acetaldehyde formation. Reactive adsorption is observed, even at room temperature, on the catalyst containing lithium, leading to formation of acetate.

A FTIR assessment of surface acidity and dispersion of surface species in titania and alumina-supported molybdena

Abstract Adsorption of pyridine on the MoO3/TiO2 and MoO3/Al2O3 systems has been studied by FTIR spectroscopy, in order to identify surface acid sites existing in samples with different molybdena loadings. The results show that both Lewis and Bronsted surface acid sites exist, whatever the molybdena loading. The percentage of Bronsted sites is larger for loadings below the theoretical monolayer, and should correspond to bidimensional molybdenum oxides species, while for loadings above the monolayer these sites are associated with bulk MoO3.