M. del Arco

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.

Characterisation of K, Na, and Li birnessites prepared by oxidation with H2O2 in a basic medium. Ion exchange properties and study of the calcined products

Birnessites containing Na, K or Li in the interlayer have been prepared by oxidation of Mn(II) cations with H2O2 in a basic medium with different alkaline cation/Mn molar ratios. The solids prepared have been characterised by elemental chemical analysis, powder X-ray diffraction, thermal analyses (differential thermal analysis and thermogravimetric analysis), FT-IR spectroscopy and surface texture assessment by adsorption of N2 at −196°C. Crystalline birnessites are obtained for A/Mn ratios (A = K, Li) larger than 3.4, but MnO(OH) has been also identificed when such a ratio is smaller than 3.4. Ion exchange is topotactic, but is not complete for exchanging Na, K, or Mg for pre-existing Li. The solids are stable up to 400°C, and formation of spinels and solids with tunnel structures is observed at this temperature. Li-containing birnessites are transformed to LiMn2O4 spinel at 400°C, and co-crystallization of bixbyte (Mn2O3) is observed at higher temperatures. Bixbyte and cryptomelane are formed at 500°C for the K-containing birnessites.

Solubility and release of fenamates intercalated in layered double hydroxides

Abstract The effect of a Mg- and Al-containing layered double hydroxide (LDH), used as an additive or as a matrix, on the dissolution and solubility of two non-steroidal anti-inflammatory drugs (NSAIDs), namely, mefenamic and meclofenamic acids, is reported. The use of MgAl-LDH enhances the solubility of mefenamic acid at the three pH values tested, and the value is larger when the acid is intercalated as opposed to simply being mixed with the layered material. For pure mefenamic acid, the most unfavourable pH is 1.2, while for the physical mixture and the sample with the intercalated drug, the most unfavourable pH is 4.5, for which the minimum increase in the amount of dissolved acid is recorded. The (dissolution) release-rate for both drugs when they are intercalated between the layers is much slower than for the physical mixture or for the dissolution of the pure drug.

Surface and textural properties of hydrotalcite-like materials and their decomposition products

Abstract Layered double hydroxides (LDHs) with the hydrotalcite-like (HT) structure, and the formula [Mg 1-x Al x (OH) 2 ] (CO 3 ) x/2 .nH 2 O, have been synthetised following the constant pH method. Samples have been submitted to hydrothermal treatment (samples I) at 400 K or left under room conditions (samples 0) for 7 days. X-ray diffraction, thermal analysis, FT-IR spectroscopy and transmission electron microscopy results confirm the HT-like structure. Surface texture assessment by nitrogen adsorption at 77 K indicates type II nitrogen-adsorption isotherms with a narrow hystheresis cycle, due to pores open on both ends, but is not recorded in the isotherms of the solids obtained upon calcination of the samples at increasing temperatures. This treatment increases the specific surface area up to ca. 85 m 2 /g when the samples are calcined at 1100 K, where X-ray diffraction data indicate formation of MgAl 2 O 4 and MgO. Such a change in the specific surface area (specially upon calcining at moderate temperatures) is even marked in the hydrothermally-treated sample.

FTIR study of isopropanol reactivity on calcined layered double hydroxides

A FTIR spectroscopy study on the surface reactivity of calcined layered double hydroxides (LDH), with different cations in the interlayer space (namely, carbonate, nitrate, silicate, and borate), for isopropanol oxidation is reported. The LDHs were previously calcined at 600°C and then their surface acidity was quantitatively determined ia volumetric determination of ammonia adsorption. Surface Lewis and/or Bronsted acid and basic sites were qualitatively studied by FTIR monitoring of the adsorption of pyridine and boric acid trimethyl ester. Carbonate and nitrate samples show surface acidity and basicity and hence the maximum activity for acetone formation through dehydrogenation of isopropanol. In contrast, although the silicate sample displays maximum surface acidity, the lack of an appreciable amount of surface basic sites gives rise to a lower activity in the named reaction, of the same order as that shown by the borate sample.

A laser Raman spectroscopy study of molybdenum oxide supported on alumina and titania

Abstract Molybdenum oxide supported on alumina and titania prepared by three different methods (impregnation, calcination and hydrothermal treatment of mechanical mixtures), have been studied by laser Raman spectroscopy. Different oxomolybdenum species have been detected depending on the method of preparation and on the calcination conditions. Heptamolybdate and polymolybdate species were found in the impregnated and in the uncalcined, hydrothermally treated samples supported on alumina and titania, respectively, whereas polymolybdate species were formed on the surface of alumina when the mechanical mixtures and the hydrothermally treated samples were calcined in the presence of water vapour. However, two kinds of species, polymolybdate and mono-oxomolybdenum, were found in the case of the mechanical mixtures and the hydrothermally treated samples calcined in the presence of water vapour when supported on titania.

The effect of the preparation method on the nature and dispersion of surface species formed upon reaction of molybdenum trioxide with alumina and titania

Titania- and alumina-supported molybdenum-containing systems have been prepared from mechanical mixtures of the support and molybdenum trioxide. The systems have been characterized by X-ray diffraction, temperature-programmed reduction and scanning electron microscopy, and Fourier transform-infrared monitoring of pyridine adsorption has been used to determine surface acidity. Dispersion of the supported phase, and its transformation to well-dispersed species has been attained by mechanical grinding and calcination in air, or by hydrothermal treatment of such mixtures and calcination. In the latter case, a higher dispersion is reached, despite the shorter time of calcination. Also hydrothermally treated samples show a larger concentration of surface Brönsted acid sites.

Electronic Spectra of Titania-Supported First Transition Series Cations

Abstract Titanium dioxide is one of the most throughly investigated chemicals for photochemical conversion of solar energy through splitting of the water molecule. However, its photoresponse is mainly located in the near u. v. region, and so dopants are generally used to extend its photoresponse towards the visible range of the solar spectrum1–4.

Photoactivity of nanostructured TiO2 catalysts in aqueous system and their surface acid-base, bulk and textural properties

This work reports on the characterisation and determintion of the photocatalytic activity of some TiO2 catalysts prepared using TiCl4 as precursor. The samples, consisting of suspensions, dispersions or powders of nanonstructured anatase or rutile phases, were obtained in very mild conditions, i.e., by boiling the solution obtained after hydrolysis of TiCl4 at atmospheric pressure: it is worth noting that no calcination was necessary to obtain photoactive phases. The samples were characterised by X-ray diffraction (XRD), determination of the BET specific surface area (SSA), scanning electron microscopy (SEM) observations and diffuse reflectance spectroscopy (DRS). FT-IR spectroscopy was used to study the surface acidity of the solids. Photodegradation of 4-nitrophenol and gemfibrozil (5-(2,5-dimethylphenoxy)-2,2-dimethyl-pentanoic acid) was used as probe reactions to evaluate the photoactivity and the obtained results showed that the home prepared samples are effective to degrade both substrates. The photoreactivity results were compared to those obtained using commercial photocatalysts.

Characterization of MoO3-P2O5-ZrO2 catalysts: an oxide-supported mixed oxide

Abstract P2O5/ZrO2, MoO3/ZrO2 and MoO3-P2O5/ZrO2 samples have been prepared by impregnation of a commercial zirconia support with ammonium phosphate and/or heptamolybdate aqueous solutions. The structural and morphological properties have been studied by XRD diffraction, nitrogen adsorption—desorption experiments and skeletal FT-IR and FT-Raman spectra. Original zirconia is in monoclinic phase with traces of tetragonal phase. Only Mo and P amount higher than the theoretical monolayer molybdite is observed. The incorporation of P and/or Mo causes a slight decrease in the value of the specific surface area with respect to pure support. All samples are mesoporous, whose texture can be described by tubular pores with widened parts. Surface MexOy (where Me = P or Mo) groups have been detected. Fourier transform infrared spectra of adsorbed ammonia and carbon dioxide show that the addition of Mo and/or P on zirconia causes a relevant increase of the surface acidity and destroys the surface nucleophilic oxygen present on the pure support.