Sandra C. de Castro

Stability and surface acidity of niobium(V) oxide grafted on a silica gel surface

Silica gel with a specific surface area, SBET, of 322 m2 g–1, and an average pore diameter of 6 nm, has been chemically modified with NbV oxide by a grafting reaction. The quantity of attached atoms was 8.9 × 10–2 mol g–1 with a surface density of 1.7 atoms nm–2. The XPS technique showed that the atomic Nb/Si ratio of ca. 0.03 remained unchanged when the samples were heated from 423 to 1573 K. At high calcination temperatures, i.e. T 1473 K, the specific surface area of Nb2O5/SiO2 was nearly reduced to zero as consequence of a crystallization process. Spectroscopic and X-ray diffraction data showed that Nb2O5 and SiO2 crystallized as separated phases at this temperature. Heating the material up to 973 K did not affect its transition-metal exchange capacity. Lewis and Bronsted acidities were also studied by the pyridine sorption technique. The Lewis acid sites were stable in samples precalcined up to 973 K and the Bronsted acid sites could be detected for samples precalcined up to 773 K.

Lithium insertion and electrochromism in polycrystalline molybdenum oxide films

In this work, molybdenum oxide thin films were deposited by r.f. reactive sputtering of metallic molybdenum target in an Ar + O 2 atmosphere. Thin films with different compositions and crystal structures were obtained by varying the oxygen flow (o). All samples, with the exception of the MoO 2 film, showed an electrochromic behavior, due to the reversible Li + /e insertion or extraction process. The transmittance change (λ = 632.8 nm), for the best sample, was ∼ 70%. The film having the best optical behavior is mostly formed by the β-MoO 3 phase. The composition, optical and mechanical properties were determined and analyzed in as-grown and in Li intercalated films. The local order structure was analyzed by X-ray absorption spectroscopy measurements at the Mo L 2,3 -edges. The valence band was studied by Photoelectron Spectroscopy.

Antimony(v) oxide grafted onto a silica gel surface: acidic properties and thermal stability

Antimony(v) oxide grafted onto a silica gel surface was obtained in highly dispersed form, with antimony density of 0.65 atoms nm–2. The material presents Bronsted and Lewis acid properties different from pyrochlore, Sb2O5. These sites disappear upon thermal treatment at 500 °C owing to reaction of antimony oxide with the silica surface.

Adsorption of plasma proteins to DMAA hydrogels obtained by ionizing radiation and its relationship with blood compatibility

The interaction of plasma proteins such as albumin, gamma-globulin, and fibrinogen with the surface of graft copolymers DMAA-G-PTFE, DMAA-G-PETFE, and DMAA-G-PE obtained by radiation graft polymerization was studied. The adsorption of serum proteins was affected by the hydrophilicity of the graft copolymers. Increased albumin adsorption and decreased fibrinogen and gamma-globulin adsorption with increasing grafting levels was shown. A certain range of degrees of grafting showed an improved blood compatibility of the polymeric surfaces due to the existence of a hydrophilic/hydrophobic balance on the polymers. The results suggest that the DMAA-G-PTFE, DMAA-G-PETFE, and DMAA-G-PE graft copolymers can be used as biomaterials for long-term use in cardiovascular systems.

Structure and electrochemical property of the cobalt(II) hexacyanoferrate complex immobilized on Sn(IV) oxide coated on silica gel surface

Abstract The complex cobalt hexacyanoferrate was immobilized onto a silica gel surface, chemically modified with tin(IV) oxide. The formal potential of the attached electroactive species was dependent on the supporting electrolyte used: NaNO 3 = 0.39 V, KNO 3 = 0.50 V and NH 4 NO 3 = 0.53 V. A Nernstian behavior was observed for KNO 3 and NH 4 NO 3 and a quasi-Nernstian one for NaNO 3 when they were used as the supporting electrolyte. The LiNO 3 did not generate a reversible current peak since the hydrated ionic radius is larger than the zeolite-type cavity of the surface mixed valence complex.

Preparation and properties of antimony (V) oxide adsorbed on silica–titania mixed oxide

Abstract SiO 2 /TiO 2 binary oxides were initially prepared by the sol–gel process and further reacted with antimony (V) in aqueous solution. Calcined and uncalcined SiO 2 /TiO 2 /Sb 2 O 5 composites with different compositions were characterized by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy techniques. Large ion exchange capacities were determined for alkaline metal ions: Li + , between 0.91 and 1.15 mmol g −1 ; Na + , between 1.32 and 1.70 mmol g −1 ; K + , between 1.65 and 1.95 mmol g −1 . The proton exchange capacities increased for samples with larger amount of Sb 2 O 5 in the composites. The thermal treatment of the samples, i.e., comparing the exchange capacities of uncalcined samples with that of the calcined ones (773 K), did not show significant change.

X-ray absorption and XPS study of titanium mixed oxides synthesized by the sol–gel method

Abstract The mixed oxides SiO 2 /TiO 2 and SiO 2 /TiO 2 /Sb 2 O 5 were prepared with different Ti and Sb amounts and investigated by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) at the Ti K edge. There is no evidence for structural modification of titanium environment in the samples having between 5.6 and 20.3 wt% of TiO 2 . All the experimental evidence indicates that titanium has a local environment close to that of TiO 2 anatase. XAS show that this similarity is extends up to the next neighbouring Ti shells. XPS analysis shows that Ti and Sb are present as TiO 2 and Sb 2 O 5 .

Preparation and characterization of glass-ceramic materials in the BaO-Li2O-ZrO2-SiO2 system and their dependence on treatment temperatures

The preparation and characterization of transparent glass-ceramics in the composition of 30Li2O:5ZrO2:xBaO:(100−x) SiO2 with x = 0, 5, 10, 15, and 20 mol% are described. Glasses were melted in a platinum crucible at 1100°C for 2 h and then heat-treated at 900°C for 3 h. The characterizations were performed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman and infrared spectroscopy, and scanning electron microscopy (SEM). The experimental results indicate that there was a structural change in the glass-ceramics as the BaO concentration in the mixture increased. In the XRD patterns of samples without heat treatment, only the halo was observed. After heat treatment, the appearance of the materials was verified by X-ray diffraction peaks. The reorganization of the amorphous solid was confirmed by Raman and IR spectroscopy along with XPS and SEM, with a more homogeneous phase formation being observed.

X-Ray photoelectron spectroscopy and mössbauer spectroscopy study of iron(III) and antimony(V) oxides grafted onto a silica gel surface

Pronounced antimony surface enrichment on a silica gel surface grafted with Sb2O5 is evident after adsorption of Fe3–, and the surface new phase, presumably FeSbO4, is thermally very stable.

Surface characterization by UHV techniques and cyclic voltammetry of thermal IrO2-based electrocatalysts containing TiO2 and CeO2

Electrodes of nominal composition Ir 0.3Ti 0.7- xCe xO 2 (0? x?0.7) have been prepared by thermal decomposition of mixtures of the chloride precursors (450°C). A systematic study of the morphology, chemical composition and surface properties of these electrodes was performed using both exsitu (SEM, EDX, XPS) and insitu (cyclic voltammetry) techniques. A spectrophotometric study of the precursor salt mixtures was done. SEM analysis showed the introduction of CeO 2 into the binary system to provoke a significant increase in the active surface area of these electrode materials, which is corroborated by the CV data (anodic voltammetric charge, qa*, double layer capacitance, Cdl, and roughness factor). Apparent Ti-enrichment of the more external region of the coatings, as evidenced by EDX, is related to the fact this technique analyses a thicker part of the oxide bulk (estimated as ca. 1/3 of the layer thickness). No real surface Ti-enrichment occurs as supported by close to nominal Ir/Ti ratios obtained by XPS (a real surface technique). XPS measurements also support Ir, Ti and Ce are present in freshly prepared oxide layers in two oxidation states, which is in agreement with the spectrophotometric results.