RESUMEN
Hydroxyapatite [HAP; Ca5(PO4)3(OH)], a biocompatible, osteoconductive material, was perceived, in the present investigation, to mimic a healthy bone mineral. Structural and morphological properties of its bulk and surface were examined versus high-temperature (up to 900 degrees C) thermal treatments in air or wet HCl gas atmosphere, using thermogravimetry, X-ray powder diffractometry, N2 sorptiometry, scanning electron microscopy, X-ray energy dispersive spectroscopy, and ex- and in situ infrared spectroscopy. CO, CDCl3, and methylbutynol were used as infrared probe molecules. Results obtained revealed that, in the absence of HCl, the bulk crystalline structure and the chemical composition of HAP were stable during high-temperature treatments. The surface exposed isolated Lewis acid sites (Ca2+) and reactive base sites (O(x-) and/or OH-) that chemisorbed atmospheric CO2 molecules with the formation of surface carbonate species (CaCO3). It is assumed that surface OH groups may interact with atmospheric oxygen molecules, leading to the formation and incorporation of peroxide (O2(2-) species. In the atmosphere of wet HCl, HAP was shown to suffer loss of chemical integrity, facilitated by its carbonated domains, as well as disintegration (or erosion) of particle aggregates and creation of what appeared to be deep groves.
Asunto(s)
Materiales Biocompatibles/química , Durapatita/química , Ácido Clorhídrico/química , Carbonato de Calcio/química , Dióxido de Carbono/química , Monóxido de Carbono/química , Cloroformo/química , Deuterio/química , Gases , Calor , Microscopía Electrónica , Microscopía Electrónica de Rastreo , Nitrógeno/química , Oxígeno/química , Pentanoles/química , Peróxidos/química , Espectrofotometría Infrarroja , Propiedades de Superficie , Difracción de Rayos XRESUMEN
Single-phase CuMgAl ternary hydrotalcite with (Cu+Mg)/Al atomic ratio of 3.0 and Cu/Mg atomic ratio of 1.0 was synthesized by coprecipitation. Thermoanalytical studies of this sample showed transformations in three stages in the temperature range up to ca. 900 K yielding mainly CuO phase. In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic measurements showed the presence of carbonates even after calcination of the sample at 973 K. The genesis of Cu+ sites during thermal treatment in vacuo at different temperatures for this sample was followed by IR spectroscopy of CO adsorbed at low temperature. Essentially no Cu+ sites are present on a sample calcined at 723 K, consistent with X-ray photoelectron spectroscopic (XPS) data. However, sample subjected to activation (1 h of O2 treatment at 723 K followed by 1 h of evacuation at the same temperature) upon CO adsorption at 85 K unambiguously showed the presence of Cu+ sites. 12CO-13CO coadsorption studies confirmed the presence of dicarbonyls, which are converted to linear Cu+-CO species during evacuation at 85 K. Concentration of the accessible Cu+ sites increased with the increase in activation temperature up to 873 K and decreased with a further temperature rise. The copper sites on the sample are heterogeneously distributed and their distribution depends on the activation temperature. Two routes of reduction of Cu2+ to Cu+ are proposed: (i) autoreduction during evacuation and (ii) reduction by CO.
RESUMEN
1H magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of TiO chi/SiO2 catalysts suggest the interaction of surface TiO chi species with Si-OH groups of the silica. Simultaneously, Ti-OH groups from surface titania species appear. The distribution of TiO chi species over SiO2 is non-uniform, since a considerable part of surface OH groups remains unreacted with supported titania. Supported vanadia species interact both with Si-OH and Ti-OH groups. 51V NMR spectra suggest the interaction of vanadia with supported titania species and show the non-uniform distribution of titania over the SiO2 surface. Deposition of titania as well as vanadia produces strong electron-accepting (Lewis) sites which interact with the terminal N atom of adsorbed N2O molecules, resulting in a downfield shift of the resonance in 15N NMR spectra. The acid strength of electron-accepting sites is similar in both cases. Only about 10% of the total amount of supported titania and vanadia create Lewis sites.