Ruohong Sui

A comparison of several nanoscale photocatalysts in the degradation of a common pollutant using LEDs and conventional UV light.

A comparative study on the photocatalytic activities of four different catalysts, P-25 TiO(2), TiO(2) nanofibers, tin-doped TiO(2) nanofibers under UV light irradiation at 350 nm, and coumarin (C-343) coated TiO(2) nanofibers at 436 nm light emitting diodes (LED) is reported. Catalysts performance has been compared based on their reflectance spectrum and activity. A common water contaminant 4-chlorophenol was used as a substrate to compare the activity of the different catalysts under both direct and dye sensitized conditions. Results indicated that amongst the four different catalysts the activity of P-25 was the highest. However the activity of C-343 coated TiO(2) nanofibers in the LED (436 nm) based reactor was competitive. Identification of reaction intermediates implied that the reaction pathways under UV (band gap) and visible (dye sensitized) irradiation were different. Nonetheless, ring opening took place in all reactions with both maleic and dihydroxymaleic have been identified as intermediates. The study indicates that ordered arrays of TiO(2) irradiated by panels of arrays of low cost high intensity LEDs might be used for the design of reactors. The near monochromaticity, long life, and operation under direct currents are advantages of using LEDs.

Sol–gel synthesis of linear Sn-doped TiO2 nanostructures

The sol–gel synthesis of linear Sn-doped TiO2 (TDT) nanostructures with high aspect ratios is reported. These binary metal-oxide nanostructures are readily accessed by treating titanium isopropoxide (Ti(OiPr)4) with appropriate quantities of acetic acid (AcOH) and Sn(OAc)4 in heptanes to generate linear macromolecules that form nanofibers upon calcination. While these linear nanostructures are isolated at low R values (0.05–0.20), where R is defined as the molar ratio of Sn(OAc)4 to Ti(OiPr)4, axially directional growth is not favored at higher R values (0.30–0.50). Scanning and transmission electron microscope (SEM and TEM) imaging of the nanofibers revealed diameters in the 10–20 nm range and lengths in excess of 1 mm. Elemental mapping by STEM-EDS techniques indicates a homogeneous distribution of Sn ions throughout the linear TDT structures. The molecular intermediates that form during the early stages of the reaction were monitored using electrospray-ionization mass spectrometry to confirm the presence of metal-oxide clusters containing both Sn and Ti ions. These intermediates then undergo polycondensation reactions to form the final linear product, thereby indicating the homogeneous incorporation of Sn into the TiO2 lattice and rules out the possibility of independent SnO2 and TiO2 aggregates. Powder X-ray diffraction data indicate that pure TiO2 nanostructures are anatase when calcined at 500 °C, but show a propensity to adopt the rutile phase at progressively higher Sn concentrations.

Solution growth of anatase TiO2 nanowires from transparent conducting glass substrates

Sol–gel reaction conditions that enable the growth of one-dimensional (1D) anatase titanium dioxide (TiO2) nanostructures from fluorine-doped indium tin oxide (FTO) substrates are described. The generation of these linear nanostructures is achieved using acetic acid (HOAc) and titanium isopropoxide (Ti(OiPr)4) in anhydrous heptane in the absence of an external bias or template. The procedure requires the functionalization of base-treated substrates with Ti-oxide nucleation sites, which serve as a foundation for the growth of linear Ti-oxide macromolecules. Calcination of these macromolecules at 450 °C under an ambient atmosphere produce uniform films of randomly oriented anatase TiO2 nanowires. The nucleation and growth processes are both acutely sensitive to the relative molar ratio (R) of HOAc to Ti(OiPr)4. Optimal surface coverage of the nucleation sites is observed when the R value utilized for the nucleation phase (denoted Ri) is equal to 1.3. The highest quality nanowire films were obtained when the R value employed during the gelation phase (denoted Rf) was held between 8.5 and 14. Characterization of the films by electron microscopy revealed a uniform film of disordered anatase TiO2 nanowires with high aspect ratios. The dimensions of the nanostructures correspond to lengths of ca. 1–10 μm and widths of 54 ± 10 nm. High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) techniques demonstrate that the anatase nanowires are a linear arrangement of crystallites ranging in size from 13 to 19 nm. A systematic evaluation of how reaction conditions (e.g., solvent volume, stoichiometry of reagents, substrate base treatment) affect the generation of these TiO2 films is presented.

Preparing titania aerogel monolithic chromatography columns using supercritical carbon dioxide

The search for a method to fabricate monolithic inorganic columns has attracted significant recent attention due to their unique ability in separation applications of various biomolecules. Silica and polymer based monolithic columns have been prepared, but titania and other metal oxide monoliths have been elusive, primarily due to their fragility. This article describes a new approach for preparing nanostructured titania based columns, which offer better performance over conventional particle packed columns for separating a wide variety of biomolecules including phosphopeptides. TiO(2) monolithic aerogels were synthesized in separation columns using in situ sol-gel reactions in supercritical carbon dioxide (scCO(2)) followed by calcination, and compared to those prepared in heptanes. The characterization results show that scCO(2) is a better solvent for the sol-gel reactions, providing lower shrinkage with the anatase TiO(2) monolith composed of nanofibers with very high surface areas. The monolithic columns show the ability to isolate phosphopeptides with little flow resistance compared to conventional titania particle based microcolumns.

Study of the sol-gel reaction mechanism in supercritical CO2 for the formation of SiO2 nanocomposites.

Direct sol-gel reactions in supercritical CO2 (scCO2) have attracted significant interest for synthesizing nanomaterials by reacting alkoxides with a carboxylic acid. In this study, the hydrolysis of silicon alkoxides (TEOS or TMOS) was carried out using scCO2 as the solvent to generate silica nanoparticles within the matrix of polyethylene for the synthesis of polymeric nanocomposites. This methodology provides advantages of combining the sol-gel reactions and drying into a one-step process for producing polymer nanocomposites. The synthesized polymer silica composites were characterized by SEM, FTIR, and XPS. When the TEOS loading was below 10 wt % Si content, nanometer-sized silica particles were formed that were well dispersed within the polyethylene matrix. The mechanism of the silicon alkoxides reacting with acetic acid in scCO2 was further studied using online GC-MS and offline 13C NMR. Oligomer structures with a bridging methoxyl group between the two silicon atoms and the acetate monodentate were observed. This study suggests a new sol-gel pathway in scCO2 that is different from the hydrolysis-condensation reactions using the conventional sol-gel process.

Selective Adsorption of Thiols Using Gold Nanoparticles Supported on Metal Oxides

Selective capture of thiols from a synthetic hydrogen sulfide containing mixture using supported nanogold materials has been explored for the potential removal of thiols from sour gas production fluids. In this research, TiO2-, Al2O3-, SiO2-, and ZnO-supported gold nanoparticles have been studied for their usage as regeneratable adsorbents to capture CH3SH, C2H5SH, and i-C3H7SH. Au/TiO2 and Au/Al2O3 showed promising properties for removing the thiols efficiently from a gas-phase mixture; however, Au/Al2O3 did catalyze some undesirable side reactions, e.g., carbonyl sulfide formation. It was found that a mild temperature of T = 200 °C was sufficient for regeneration of either Au/TiO2 or Au/Al2O3 adsorbent. The metal oxide mesopores played an important role for accommodating gold particles and chemisorption of the thiols, where smaller pore sizes were found to inhibit the agglomeration/growth of gold particles. The nature of thiol adsorption and the impact of multiple adsorption-desorption cycles on the adsorbents have been studied using electron microscopy, XPS, XRD, GC, and physi/chemiadsorption analyses.

Capture of sulfur dioxide from Claus tail gas using fiber-like alumina-based adsorbents

This paper describes the preparation of alumina fibers doped with CaO, MgO and La2O3 and reports their use as SO2 adsorbents. These materials were characterized using electron microscopy, powder X-ray diffraction and infrared analysis and were examined for their ability to capture SO2 selectively from gas mixtures containing large quantities of H2O, CO2 and some O2 at temperatures in excess of 353 K. Overall, it was found that these adsorbents could remove SO2 selectively and that they could be regenerated by treatment with an H2S-containing gas at approximately 600 K. Adsorption capacity was retained over several cycles. Fourier transform infrared analysis showed that SO2 was adsorbed as free SO2 and also in a combined form as sulfite and sulfate species. In the regeneration step, the adsorbed sulfur species were reduced to elemental sulfur and H2S or were desorbed as SO2. It is proposed that the chemistry described here could be applied to design of a process for capture of all sulfur species typically found in a Claus-based sulfur recovery system.

Organosulfur adsorbents by self-assembly of titania based ternary metal oxide nanofibers

By doping with secondary and tertiary species, the electron configuration of titanium oxide can be tuned for the selective adsorption of natural gas contaminants such as thiols. In this study, we attempted to co-incorporate copper group metals/oxides and lanthanum oxide within titania nanofibers via linear poly-condensations of multiple metal acetate complexes. In all cases, a sol–gel synthesis in heptane allowed the nanofibers to randomly pack, forming 3 dimensional network bundles. The resulting nanostructures were characterized using electron microscopy, mass spectrometry, X-ray diffraction, X-ray photoelectron spectroscopy, N2 physisorption and Raman spectroscopy. Multicomponent breakthrough studies with three thiols, H2S, CO2 and CH4 show that doping a TiO2 matrix with copper group metals/oxides and La2O3 increased the thermal stability of anatase crystallites and nanostructures. We note that Au and Ag2O accumulated on the surfaces of the doped materials, where the La2O3 doping contributed more to the materials thermal stability. The Cu and La doped material was found to be the best adsorbent for thiols with remarkably high selectivity, demonstrating potential applications in industrial gas treatment. In addition, xerogel adsorbents through the random packing of linear structures provide the advantage of a macro-porous bulk material, which is less susceptible to fouling.