Synthesis of TiO2 photocatalysts in supercritical CO2 via a non-hydrolytic route

Nanoscaled TiO2 powders with narrow size dispersion were prepared in supercritical carbon dioxide via non-hydrolytic acylation/deacylation of titanium alkoxide precursors with or without tris-fluorination. The microstructures of these powders were characterized by spectroscopic (FTIR, TGA, and XRD), microscopic (SEM or TEM), and surface area (BET) measurements. Photocatalytic oxidation of 1-octanol on these calcined TiO2 powders and on commercial T805 TiO2 suspended in aerated supercritical carbon dioxide revealed relative reactivity controlled by the powder microstructures. Calcined TiO2 prepared from titanium(IV) isopropoxide and trifluoroacetic anhydride was effectively dispersed in aerated supercritical carbon dioxide under stirring and exhibited high photocatalytic oxidation activity.     

Thermal evolution of mesoporous titania prepared by CO2 supercritical extraction

Porous anatase is attractive because of its notable photo-electronic properties. Titania wet gel prepared by hydrolysis of Ti-alkoxide was immersed in the flow of supercritical CO₂ at 60°C and the solvent was extracted (aerogel). Mesoporous TiO₂ consisting of anatase nano-particles, about 5 nm in diameter, have been obtained. Thermal evolution of the microstructure of the aerogel was evaluated by TGA-DTA, N₂ adsorption, TEM and XRD, and discussed in comparison with that of the corresponding xerogel. The diffraction peaks of anatase were found for the as-extracted gel while the xerogel dried at 90°C was amorphous. After calcination at 600°C, the average pore size of the aerogel, about 20 nm in diameter, was 4 times larger than that of the xerogel, and the pore volume, about 0.35 cm³ g⁻¹, and the specific surface area, about 60 m² g⁻¹, were 2 times larger than those of the xerogel. XRD peaks of rutile have been found after calcination at 600°C. The particle sizes of anatase and rutile are about 13 and 25 nm in diameter, respectively. The surface morphology of TiO₂ nano-particles has been discussed in terms of their surface fractal dimensions estimated from the N₂ gas adsorption isotherms.     

Removal of plutonium from carbonate medium using titania microspheres prepared by sol-gel route

Spherical beads of titania were prepared by a sol-gel route. During the preparation a cationic surfactant was added in the feed broth to modify the surface characteristics of the microspheres. The absorption behavior of Pu(IV) from carbonate medium on these microspheres was studied by batch experiments. Loading and elution behavior of Pu(IV) on a bed of titania microspheres was studied and the practical capacity was determined. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation and rheological properties of the supercritical CO2-water pure interface

From the interfacial tension (gamma) measurement, we have analyzed the interfacial organization that occurs between pure H2O and pure CO2 from a kinetical and rheological point of view. This article is the followup to a previous one, where we showed that this equilibrated interface is composed of small H2O-CO2 cluster blocks [Tewes, F.; Boury, F. J. Phys. Chem. B 2004, 108, 2405]. By analyzing the variation of gamma with the square root of time, we found that the organization of the H2O-CO2 interface is, in the initial times, controlled by the diffusion of the CO2 molecules into the water. We compared the frictional coefficient determined from the measured CO2 diffusion coefficient with the frictional coefficient calculated from the Stokes equation (frictional ratio). From that, we concluded that it is a hydrated form of CO2 that diffuses and that the degree of hydration decreases with pressure. Rheological properties of the equilibrated interface vary with CO2 pressure, in the range of 50-90 bar, from a viscoelastic comportment to a purely elastic behavior, showing a change in the interfacial organization. The high equilibrium part of the elasticity (110 mN/m) obtained at 90 bar suggests a highly structured interface. Two phenomena could explain the interfacial rheological behavior: (i) an increase and a growth of the blocks H2O-CO2 cluster with the CO2 pressure or (ii) an increase in the interfacial capacity to form stable clusters under interfacial area compression.     

Formation of porous flat membrane by phase separation with supercritical CO2

Microporous polystyrene membranes were prepared by the phase separation process using the supercritical CO₂ as a nonsolvent for the polymer solution. The thin polymer solution in a laboratory dish was located inside a cell and the supercritical CO₂ was introduced to induce the phase separation. The dry flat microporous membranes were obtained without collapse of the structure after the CO₂ pressure was diminished. Effects of the experimental conditions such as the CO₂ pressure, the polymer concentration and the temperature on the average pore size and membrane porosity were investigated.     

Fluorinated surfactants in supercritical CO2

Liquid or supercritical carbon dioxide has important environmental and economic advantages over petrochemical solvents currently used for industrial processes. However, low solubility in CO₂, particularly of polar compounds, is a hurdle to its implementation as an acceptable alternative. These solubility problems have been overcome by employing specialised fluorinated surfactants to stabilise water nano-droplets as water-in-supercritical/liquid CO₂ microemulsions. Such novel microemulsions can now facilitate innovative ‘green-and-clean’ applications of carbon dioxide technology.     

Hemoglobin/colloidal silver nanoparticles immobilized in titania sol-gel film on glassy carbon electrode: direct electrochemistry and electrocatalysis

By vapor deposition method, both hemoglobin (Hb) and colloidal silver nanoparticles (CSNs) were entrapped in a titania sol-gel matrix on the surface of a glassy carbon electrode (GCE). CSNs could greatly enhance the electron transfer reactivity of Hb and its catalytic ability toward nitrite. Direct fast electron transfer between Hb and the GCE was achieved, and a pair of well-defined, quasi-reversible redox peaks was observed. The anodic and cathodic peak potentials are located at -0.298 V and -0.364 V (vs. Ag/AgCl), respectively. The dependence of the formal potential on solution pH indicated that the direct electron transfer reaction of Hb was a one-electron transfer coupled with a one-proton transfer reaction process. Meanwhile, the catalytic ability of Hb toward the reduction of NO2- was also studied. Accordingly, a NO2- biosensor was prepared, with a linear range from 0.2 mM to 6.0 mM and a detection limit of 34.0 microM. The apparent Michaelis-Menten constant was calculated to be 7.48 mM. Moreover, the biosensor had good long-term stability.     

Formation of polyaniline nanofibers: a morphological study

Polyaniline (PANI) powders were prepared by solution precipitation, rapid mixing polymerization, and interfacial polymerization to find the key factors that influence the formation and growth of PANI nanofibers. In chemical oxidative polymerization of aniline, the morphology of the product is mainly determined by aniline concentration. In the case of lower aniline concentration, PANI nanofibers were formed and can be preserved and collected as final product, while in the case of higher aniline concentration, larger sized PANI particles or agglomerates were obtained owing to the growth of the nanofibers. Without participation of the oxidizing step, solid PANI samples with compact structures and dissimilar morphologies were achieved by random accumulation of PANI molecules.     

Efficient cooling in supersonic jet expansions of supercritical fluids: CO and CO2

Pulsed, supersonic beams of pure carbon monoxide and carbon dioxide at stagnation conditions above their critical point have been investigated by time-of-flight measurements as a function of pressure and temperature. Although both molecules form clusters readily in adiabatic expansions, surprisingly large speed ratios (above 100) indicative of very low translational temperatures (below 0.1 K) have been achieved. In particular, the supersonic expansion of CO(2) at stagnation temperatures slightly above the phase transition to the supercritical state results in unprecedented cold beams. This efficient cooling is attributed to the large values of the heat capacity ratio of supercritical fluids in close vicinity of their critical point.     

The promotion effect of titania nanoparticles on the direct electrochemistry of lactate dehydrogenase sol-gel modified gold electrode

The promotion effect of titania nanoparticles (nano-TiO(2)) on the direct electron transfer between lactate dehydrogenase (LDH) and the silica sol-gel modified gold electrode was investigated by adding nano-TiO(2) (50 nm) in the modification process. This nano-TiO(2)-LDH electrode showed a pair of quasi-reversible cyclic voltammetry peaks with the formal potential of 70 mV (vs. SCE). Compared to the previous result of LDH modified electrode with only an irreversible cathodic peak, an anodic peak appeared and the cathodic peak potential shifted to the positive direction on this nano-TiO(2)-LDH electrode, which demonstrated that the direct electrochemistry of LDH was enhanced by nano-TiO(2). We supposed that the direct electrochemistry of LDH may be due to the redox reaction of some electroactive amino acids in the LDH molecule. The surface morphologies of electrodes characterized by SEM indicated that LDH was successfully immobilized on the sol-gel matrix and also had some interactions with nano-TiO(2). This electrode can be used as a biosensor for the determination of lactic acid. The calibration range of lactic acid was from 1.0 to 20 micromolL(-1) and the detection limit was 0.4 micromolL(-1). Meanwhile, the small K(m)(app) value (2.2 micromolL(-1)) suggested that LDH possessed high enzymatic activity and good affinity to lactic acid owing to the promotion effect of nano-TiO(2).     

Microstructure of supercritical CO2-in-water microemulsions: a systematic contrast variation study

Microemulsions of the type H(2)O-scCO(2)-surfactant are potential candidates for novel solvent mixtures in the field of green chemistry. Furthermore, scCO(2)-microemulsions are highly interesting from a fundamental point of view since their properties such as the bending elastic constants can be strongly influenced solely by varying the pressure without changing the components. With this motivation we studied the phase behavior and the microstructure of water-rich scCO(2)-microemulsions. Such microemulsions were formulated using the technical grade non-ionic surfactants Zonyl FSO 100 and Zonyl FSN 100. At elevated pressures the temperature dependent phase behavior of these systems follows the general patterns of non-ionic microemulsions. Small angle neutron scattering experiments were conducted to determine the length scales and the topology of the microstructure of these systems. Having determined the exact scattering length densities and the composition of the respective sub-phases by a systematic contrast variation we could show that these systems consist of CO(2)-swollen microemulsion droplets that are dispersed in a continuous aqueous-phase. The scattering data were analyzed using a newly derived form factor for polydisperse, spherical core/shell particles with diffuse interfaces. The underlying analytical density profiles could be confirmed applying the model-free Generalized Indirect Fourier Transformation (GIFT) to the scattering data. Following the general patterns of non-ionic microemulsions the radius of the microemulsion droplets is found to increase almost linearly upon the addition of CO(2).     

C2H+H2CO: a new route for formaldehyde removal

The title unknown reaction is theoretically studied at various levels to probe the interaction mechanism between the ethynyl radical (HC triple bond C) and formaldehyde (H(2)C double bond O). The most feasible pathway is a barrier-free direct H-abstraction process leading to acetylene and formyl radical (C(2)H(2)+HCO) via a weakly bound complex, and then the product can take secondary dissociation to the final product C(2)H(2)+CO+H. The C-addition channel leading to propynal plus H-atom (HCCCHO+H) has the barrier of only 3.6, 2.9, and 2.1 kcal/mol at the CCSD(T)/6-311+G(3df,2p)MP2//6-311G(d,p)+ZPVE, CCSD(T)/6-311+G(3df,2p)//QCISD/6-311G(d,p)+ZPVE, and G3//MP2 levels, respectively [CCSD(T)--coupled cluster with single, double, and triple excitations; ZPVE--zero-point vibrational energy; QCISD--quadratic configuration interaction with single and double excitations; G3//MP2-Gaussian-3 based on Moller-Plesset geometry]. The O addition also leading to propynal plus H atom needs to overcome a higher barrier of 5.3, 8.7, and 3.0 kcalmol at the three corresponding levels. The title no-barrier reaction presents a new efficient route to remove the pollutant H(2)CO, and should be included in the combustion models of hydrocarbons. It may also represent the fastest radical-H(2)CO reaction among the available theoretical data. Moreover, it could play an important role in the interstellar chemistry where the zero- or minute-barrier reactions are generally favored. Discussions are also made on the possible formation of the intriguing propynal in space via the title reaction on ice surface.     

Effect of phosphate ions in the properties of titania sol-gel

Sol-gel titania was phosphated in two different ways: i) “in situ” phosphation using phosphoric acid as hydrolysis catalyst in the titanium butoxide gelling system, and ii) gelling with nitric acid and impregnation with ammonium phosphate solution. In calcined samples at 600°C a positive effect on the specific surface area for the “in situ” phosphated sample was found. X-ray diffraction patterns showed that the presence of phosphates ions stabilized the anatase phase. FTIR-pyridine adsorption identified only Lewis acid sites in phosphated samples. In the isopropanol decomposition for “in situ” phosphated titania, the activity was six times higher than that obtained for titania phosphated by impregnation, showing strong acidity for the in situ phosphated TiO₂ catalyst.     

Mesoporous TiO2 nanostructures: a route to minimize Pt loading on titania photocatalysts for hydrogen production

Mesostructured TiO(2) nanocrystals have been prepared using Pluronic F127 as the structure-directing agent. Platinum nanoparticles at different contents (0.1-1.0 wt%) have been photochemically deposited onto the mesoporous TiO(2). TEM investigation of 0.2 wt% Pt/TiO(2) calcined at 450 °C reveals that the TiO(2) particles are quite uniform in size and shape with the particle sizes of TiO(2) and Pt being 10 and 3 nm, respectively. The photocatalytic activities of the Pt loaded TiO(2) have been assessed and compared with those of nonporous commercial Pt/TiO(2)-P25 by determining the rates and the photonic efficiencies of molecular hydrogen production from aqueous methanol solutions. The results show that the amount of hydrogen evolved on Pt/TiO(2)-450 at low Pt loading (0.2 wt%) is three times higher than that evolved on Pt/TiO(2)-P25 and twelve times higher than that evolved on Pt/TiO(2)-350. Despite the BET surface area of the TiO(2)-450 photocatalyst being 3.5 times higher than that of TiO(2)-P25, a 60% smaller amount of the Pt co-catalyst is required to obtain the optimum photocatalytic hydrogen production activity. The reduced Pt loading on the mesoporous TiO(2) will be important both from a commercial and an ecological point of view.     

AKD-Modification of bacterial cellulose aerogels in supercritical CO2

Different approaches towards hydrophobic modification of bacterial cellulose aerogels with the alkyl ketene dimer (AKD) reagent are presented. If AKD modification was performed in supercritical CO₂, an unexpectedly high degree of loading was observed. About 15 % of the AKD was bound covalently to the cellulose matrix, while the other part consisted of re-extractable AKD-carbonate oligomers, which are novel chemical structures described for the first time. These oligomers contain up to six AKD and CO₂ moieties linked by enolcarbonate structures. The humidity uptake from environments with different relative humidity by samples equipped with up to 30 % AKD is strongly reduced, as expected due to the hydrophobization effect. Samples above 30 % AKD, and especially at very high loading between 100 and 250 %, showed the peculiar effect of increased humidity uptake which even exceeded the value of unmodified bacterial cellulose aerogels.     

Spectroscopic investigation of the composition of electrospun titania nanofibers

We investigate the structure and chemical nature of titania nanofibers synthesized by electrospinning techniques. Fourier transform infrared (FTIR) spectroscopy is used to identify CO₂ clathrates trapped within the nanofiber structures. These molecular species are formed during pyrolysis of the guide polymer. In addition, X‐ray photoelectron spectroscopy (XPS) identifies silicon within the nonwoven sheets. This led us to discover that impurities can be inadvertently incorporated during the electrospinning process from something as simple as a short piece of silicone tubing on a syringe pump. These findings should help advance the field of electrospinning by demonstrating the importance of spectroscopic characterization of materials synthesized by this technique. Copyright © 2006 John Wiley & Sons, Ltd.     

Interaction of supercritical CO2 with alkyl-ammonium organoclays: changes in morphology

This paper investigates the influence of supercritical carbon dioxide on the morphology and surface chemistry of three organic modified montmorillonite species. Alkyl based quaternary ammonium surfactants with differing numbers of chains attached, were chosen to vary the degree of CO(2)-philicity exhibited by the organoclay. In a high pressure batch vessel, the different organoclays were suspended in the supercritical solvent at temperatures of 50 and 200 degrees C and pressures of 7.6 and 9.6 MPa and then removed after de-pressurization at 0.2 or 4.8 MPa/s. The structures of these treated clays were characterized by X-ray diffraction (XRD), differential scanning microscopy (DSC), and thermogravimetric analysis (TGA), and their chemical properties were analyzed by various methods including atomic absorption spectroscopy, and water uptake measurement. Solute-solvent interactions plasticized the organic medium while suspended in the supercritical fluid, which resulted in greater chain mobility and further cation exchange. The results indicate that intercalated surfactants exhibiting a paraffin complex arrangement were most likely to experience significant basal expansion, provided the tilt angle was not already close to being perpendicular to the silicate surface. At the lower processing temperature condition, the chemistry of the clay surface was notably altered by the CO(2) associations with the Lewis acid/base sites, which significantly reduced the moisture adsorption capacity of the material. For those organoclays demonstrating basal expansion, it was noted that the resulting particle size was increased due to enhanced porosity.     

Interaction of pharmaceutical hydrates with supercritical CO2

The aim of this work was to study the solubility in supercritical CO₂ of the hydrated phase of three model drugs, namely theophylline, carbamazepine, and diclofenac sodium, in comparison with the respective anhydrous form. Possible solid-state modifications, stemming from the interaction with supercritical CO₂, were investigated by differential scanning calorimetry, thermogravimetric analysis, hot stage microscopy, Fourier Transform infrared spectroscopy and Karl-Fischer titrimetry. It was found that all three pharmaceutical hydrates exhibited higher solubility in supercritical CO₂ than the relevant anhydrous phases. In the case of theophylline monohydrate, the instability of the crystal phase at the experimental temperature adopted has been evidenced. Diclofenac sodium tetrahydrate represents a peculiar case of chemical interaction with the acid supercritical fluid, mediated by crystal water. This revised version was published online in July 2006 with corrections to the Cover Date.