Synthesis and characterization Of SiO2-Nb2O5 systems prepared by the sol-gel method: structural stability studies

X-Ray diffraction (XRD), N-2 absorption (BET specific surface area), transmission electron microscopy (TEM), Raman and Fourier infrared (FT-IR) spectroscopic techniques have been applied to characterize the texture, structure and niobia-silica interaction of SiO2-Nb2O5 systems prepared by the sol-gel method containing different amounts of Nb2O5 and calcined at temperatures between 393 and 1473 K. For a small loading of niobia, amorphous niobium species were well dispersed in the silica LIP to 1273 K. Crystallization is observed only after thermal treatment at 1473 K, with the T-phase predominating as observed by XRD and TEM analyses and confirmed by Raman spectra. For samples with a higher amount of niobia. the formation of a T-Nb2O5 phase was observed at 1273 K, and the H-Nb2O5 phase predominates at 1473 K. FT-IR results indicate the presence of Si-O-Nb linkages at the silica-niobia interface, formed during preparation, which are responsible for the higher structural stability of the SiO2-Nb2O5 system. The stability of the SiO2-Nb2O5 system is confirmed by obtaining high specific surface areas even after the high temperature of calcination.1282552255

Local order structure and surface acidity properties of a Nb2O5/SiO2 mixed oxide prepared by the sol-gel processing method

The sol-gel processing method was used as an alternative route to obtain Nb2O5 phase homogenously dispersed in the SiO2 matrix, improving the thermal stability of the Bronsted acid sites, Nb-OH and Nb-OH-Si groups. The local niobium structure and the influence of the amount of niobia, on the surface of the Nb2O5/SiO2 system were studied by XAS and XPS, respectively. For the samples calcined at 423 and 873 K, the 3d(5/2) BE values are at ca. 208.2 eV, indicating an ionic character for Nb(V) species in the SiO2 matrix, probably associated to Si-O-Nb linkages. The features of Nb K-edge XANES spectra of samples show the absence of Nb=O species. The Nb K-edge EXAFS oscillations exhibit a shoulder at ca. 5.6 Angstrom(-1), which probably arises from Nb-O-Si. This fact corroborates the EXAFS simulation data of the second coordination shell, whose best fitting is achieved with three distances, two Nb-Nb lengths and one Nb-Si. (C) 2004 Elsevier Inc. All rights reserved.17772432243

Carbon paste electrodes of the mixed oxide SiO2/Nb2O5 prepared by the sol-gel method: dissolved dioxygen sensor

A carbon paste of SiO2/Nb2O5 material was used as the electrode in the development of a dissolved dioxygen sensor in 1.0 mol1(-1) KCl solution at pH 6.2. The material was prepared by the sol-gel method. In the investigation of its electrochemical properties, linear and cyclic voltammetric and chronoamperometric techniques were employed. Dioxygen reduction, which was diffusion controlled, occurred at -280 mV vs. SCE by a two electron mechanism, producing peroxide. A linear response between the cathodic peak current intensity and the dissolved 02 concentration was obtained for the region between 1.0 and 13.6 mg1(-1). The stability proved to be very good over successive voltammetric cycles. The electrode response time was about 5 s. The electron transfer reactions were explained as being to an n-type semiconductor of niobia dispersed in the silica surface. (C) 2004 Elsevier B.V. All rights reserved.574229129

Synthesis and acidic properties of the SiO2/SnO2 mixed oxides obtained by the sol-gel process. Evaluation of immobilized copper hexacyanoferrate as an electrochemical probe

SiO2/SnO2 mixed oxides were prepared by the sol-gel processing method using SnI4 as the tin oxide precursor reagent. Solids with Sri compositions (in wt.%) of 4.1, 12.9 and 18.4 and presenting specific surface areas (detennined by the BET method) of 492, 658 and 712 m 2 (-l) 9, respectively, were obtained. Transmission microscopic images showed nanosized SnO2 particles with average dimensions of (5.3 +/- 0.5) mu for samples having 12.9 wt.% Sri and (7.0 +/- 0.7) nm for samples having 18.4 wt.% Sri. For the sample presenting 4 wt.% of Sn the crystallites were poorly defined, barely being observed. The amorphous SnO2 particles started to crystallize at different temperatures, i.e., 1273, 1173 and 1073 K for samples with 4.1, 12.9 and 18.4 wt.% of Sri, respectively. The X-ray diffraction patterns showed that only cassiterite crystallites were present in every case and, even at a temperature of 1473 K, the SiO2, remained as an amorphous matrix. The Lewis and Bronsted acid sites were thermally stable up to a temperature of 523 K for all the compositions, as probed using pyridine molecules. The infrared spectra showed that Si-O-Sn bonds are formed at the interface between SiO2 and SnO2 particles. These bonds are the ones mainly responsible for the low mobility of the oxide particles, avoiding crystallization of SnO2. Copper hexacyanoferrate was prepared in situ on the SiO2/SnO2 surface and cyclic voltammetry tests were carried out by sweeping the potential between 0.2 and 1.0 V. The midpoint potential corresponding to the redo process: equivalent to SnOH2+/{KCu potential between 0.2 and 1.0 V. The midpoint potential corresponding to the redox process: SnOH2+/KCu[Fe(CN)(6)}(-) Cu[Fe(CN)(6)}(_)+K++e(-) was observed at about 0.7 V. The electrochemical impedance spectroscopic data showed a charge transfer resistance of 17.8 Omega. This low value favors the oxidation-reduction process in the pores of the material. (C) 2004 Elsevier B.V All rights reserved.1674167116517

Theoretical evaluation of SiO2/Nb2O5 interface using density function theory

Interaction between SiO2/Nb2O5 system was studied with theoretical methods. Different modes of interaction were calculated using DFT (Density Functional Theory) with deMon2002 computational packcage. Both calculated energetic and structural parameters were compared with XANES (X-ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorttion Fine Structure) experimental data. The best over all agreement between theory and experiment can be observed for the bb (bidentate ligand binuclear complex) mode of interaction. (c) 2005 Elsevier B.V. All rights reserved.72441699151

Co (II) porphyrin adsorbed on SiO2/SnO2/phosphate prepared by the sol-gel method

This paper describes the immobilization procedure of 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21-H,23-H-porphyrin ion on SiO2/SnO2/phosphate, obtained by the sol-gel processing method. P 2p X-ray photoelectron and the P-31 MAS NMR spectra revealed that dihydrogen phosphate is the species present on the surface. The porphyrin was adsorbed on the surface of the modified material and furthermore metallized in situ with Co (H) ion. The porphyrin metallation process was followed with UV-vis spectroscopy by inspecting the Q bands of the free and metallated porphyrin. The free porphyrin presented four Q bands associated to a D-2h local symmetry and the metallated one, two bands related to a D-4h local symmetry. The amount of electroactive species adsorbed on the material was estimated by integrating the area under the peak of Co (II) -> Co (I) reduction by using the pulse differential voltammetric technique. The amount of the metallated porphyrin was 2.3 x 10(-10) Mol cm(-2). A carbon paste electrode of the modified material containing metallated porphyrin was used to study the electrocatalytic reduction of dissolved dioxygen by means of cyclic voltammetry, chronoamperometry and linear sweeping voltammetry. The modified electrode was very stable and exhibited the electrocatalytic reduction of dissolved dioxygen at -180mV versus SCE by a two-electron mechanism, producing hydrogen peroxide at pH 5.4. The electroactive species was strongly retained on the material surface, presumably inside the pores of the material, since in a test of various oxidation-reduction cycles no significant decrease of the current densities was detected, indicating that it was not leached off during the experiment. (c) 2005 Elsevier Ltd. All rights reserved.50224378438

Surface modification with phosphoric acid of SiO2/Nb2O5 prepared by the sol-gel method: Structural-textural and acid sites studies and an ion exchange model

In this work, the structural and textural properties of the SiO2/Nb2O5 system prepared by the sol-gel method and then modified by phosphoric acid were studied. The different materials were prepared, with three different mol % Nb2O5 (2.5, 5.0, and 7.5 mol %), and calcined in the temperature range of 423-1273 K. BET specific surface area determinations, scanning electron microscopy connected to a X-ray emission analyzer, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS) were used for the investigation. For the lowest temperature of calcination (423 K), the mesopores and micropores of the modified material were blocked, resulting in a decrease of the specific surface area compared to the S-BET values obtained for the SiNb matrix. Under intermediate temperatures of calcination (423-873 K), the modified material acquired textural stability. By XPS analysis, the presence of the dihydrogenphosphate species was identified, the P/Nb atomic ratios being independent of the thermal treatment. P-31 magic angle spinning NMR confirmed the XPS data and also showed that the chemical shift of the (H2PO4)(-) ions strongly depended on the crystallization degree of the Nb2O5. Structural thermal stability was also shown by the presence of Bronsted acid sites in the modified material calcined at high temperature (1273 K). The thermal stability is directly associated with obtainment of the same value for K+ exchange capacity (0.74 mmol g(-1), average value) for the modified materials calcined at 423 and 1273 K. The chemical analyses of phosphorus for the modified materials were made by using the inductively coupled plasma. The value was 0.36 mmol g(-1), corroborating the presence of (H2PO4)(-) ions. The ion exchange isotherms presented an S-shaped form characteristic of energetically heterogeneous ion exchangers, permitting application of a model of fixed polydentate centers, in which ion exchange took place.20208707871

X-ray photoelectron spectroscopy, x-ray absorption spectroscopy, and x-ray diffraction characterization of CuO-TiO2-CeO2 catalyst system

X-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), and x-ray absorption spectroscopy (XAS) techniques have been applied to characterize the surface composition and structure of a series of CuO-TiO2-CeO2 catalysts. For a small loading of cerium, ceria was mainly dispersed on the titania surface and a minor amount of CeO2 crystallite appeared. At higher loading of cerium, the CeO2 phase increased and the atomic Ce/Ti ratio values were smaller than the nominal composition, as a consequence of cerium agglomeration. This result suggests that only a fraction of cerium can be spread on the titania surface. For titanium-based mixed oxide, we observed that cerium is found as Ce3+ uniquely on the surface. The atomic Cu/(Ce+Ti) ratio values showed no influence from cerium concentration on the dispersion of copper, although the copper on the surface was shown to be dependent on the cerium species. For samples with a high amount of cerium, XPS analysis indicated the raise of second titanium species due cerium with spin-orbit components at higher binding energies than those presented by Ti4+ in a tetragonal structure. The structural results obtained by XAS are consistent with those obtained by XRD and XPS. (C) 2001 American Vacuum Society