Jaclyn Teo

Porous platelike hematite mesocrystals: synthesis, catalytic and gas-sensing applications

Porous platelike α-Fe2O3 mesocrystals, composed of single crystalline nanoparticles, were prepared via a controlled solvent evaporation process in the presence of an ionic liquid [Bmim]Cl, which acted as both the solvent and the templating reagent. Compared to the Au/α-Fe2O3 (Fluka) catalyst, a significantly enhanced CO oxidation activity on the Au/porous platelike α-Fe2O3 mesocrystal catalyst was observed, which may be attributed to its more highly exposed (110) facet in the porous platelike α-Fe2O3 mesocrystals. Moreover, good repeatability and fast response and recovery times were obtained in an acetone sensitivity test conducted on the α-Fe2O3 nanoplates, produced via annealing of the porous platelike α-Fe2O3 mesocrystals at 400 °C. It is expected that this method is applicable to the synthesis of other inorganic functional oxides with unique structures and properties.

Correlation of the catalytic activity for oxidation taking place on various TiO2 surfaces with surface OH groups and surface oxygen vacancies

Three catalytic oxidation reactions have been studied: The ultraviolet (UV) light induced photocatalytic decomposition of the synthetic dye sulforhodamine B (SRB) in the presence of TiO(2) nanostructures in water, together with two reactions employing Au/TiO(2) nanostructure catalysts, namely, CO oxidation in air and the decomposition of formaldehyde under visible light irradiation. Four kinds of TiO(2) nanotubes and nanorods with different phases and compositions were prepared for this study, and gold nanoparticle (Au-NP) catalysts were supported on some of these TiO(2) nanostructures (to form Au/TiO(2) catalysts). FTIR emission spectroscopy (IES) measurements provided evidence that the order of the surface OH regeneration ability of the four types of TiO(2) nanostructures studied gave the same trend as the catalytic activities of the TiO(2) nanostructures or their respective Au/TiO(2) catalysts for the three oxidation reactions. Both IES and X-ray photoelectron spectroscopy (XPS) proved that anatase TiO(2) had the strongest OH regeneration ability among the four types of TiO(2) phases or compositions. Based on these results, a model for the surface OH group generation, absorption, and activation of molecular oxygen has been proposed: The oxygen vacancies at the bridging O(2-) sites on TiO(2) surfaces dissociatively absorb water molecules to form OH groups that facilitate adsorption and activation of O(2) molecules in nearby oxygen vacancies by lowering the absorption energy of molecular O(2). A new mechanism for the photocatalytic formaldehyde decomposition with the Au/TiO(2) catalysts is also proposed, based on the photocatalytic activity of the Au-NPs under visible light. The Au-NPs absorb the light owing to the surface plasmon resonance effect and mediate the electron transfers that the reaction needs.

Ultrahigh sensitivity of Au/1D α-Fe2O3 to acetone and the sensing mechanism.

Hematite (α-Fe(2)O(3)) is a nontoxic, stable, versatile material that is widely used in catalysis and sensors. Its functionality in sensing organic molecules such as acetone is of great interest because it can result in potential medical applications. In this report, microwave irradiation is applied in the preparation of one-dimensional (1D) α-FeOOH, thereby simplifying our previous hydrothermal method and reducing the reaction time to just a few minutes. Upon calcination, the sample was converted to porous α-Fe(2)O(3) nanorods, which were then decorated homogeneously by fine Au particles, yielding Au/1D α-Fe(2)O(3) at nominally 3 wt % Au. After calcination, the sample was tested as a potential sensor for acetone in the parts per million range and compared to a similarly loaded Pt sample and the pure 1D α-Fe(2)O(3) support. Gold addition results in a much enhanced response whereas Pt confers little or no improvement. From tests on acetone in the 1-100 ppm range in humid air, Au/1D α-Fe(2)O(3) has a fast response, short recovery time, and an almost linear response to the acetone concentration. The optimum working temperature was found to be 270 °C, which was judged to be a compromise between the thermal activation of lattice oxygen in hematite and the propensity for acetone adsorption. The surface reaction was investigated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and a possible sensing mechanism is proposed. The presence of Au nanoparticles is believed to promote the dissociation of molecular oxygen better in replenishing O vacancies, thereby increasing the instantaneous supply of lattice oxygen to the oxidation of acetone (to H(2)O and CO(2)), which proceeds through an adsorbed acetate intermediate. This work contributes to the development of next-generation sensors, which offer ultrahigh detection capabilities for organic molecules.

Insights into the Oxidation and Decomposition of CO on Au/α-Fe2O3 and on α-Fe2O3 by Coupled TG-FTIR

CO oxidation and decomposition behaviors over nanosized 3% Au/alpha-Fe2O3 catalyst and over the alpha-Fe2O3 support were studied in situ via thermogravimetry coupled to on-line FTIR spectroscopy (TG-FTIR), which was used to obtain temperature-programmed reduction (TPR) curves and evolved gas analysis. The catalyst was prepared by a sonication-assisted Au colloid based method and had a Au particle size in the range of 2-5 nm. Carburization studies of H 2-prereduced samples were also made in CO gas. According to gravimetry, for the 3% Au/alpha-Fe2O3 catalyst, there were three distinct stages of CO interaction with the Au catalyst but only two stages for the catalyst support. At low temperatures (<or=100 degrees C), only the Au catalyst had a rapid weight loss, which confirmed that CO reacted with highly active absorbed oxygen species and/or OH species which were associated with and promoted by the Au nanoparticles. Around 300 degrees C, both the catalyst and support samples experienced the reduction of Fe2O3 to Fe3O4, while above 400 degrees C further reduction to FeO and Fe metal took place. Au played no part in the kinetics of Fe3O4 formation because lattice O mobility was rate-limiting. At higher temperature where Fe3O4 was further reduced to FeO and Fe 0, the initially formed metallic Fe 0 nuclei could decompose CO molecules and release O species. Both this coproduced O species and the lattice oxygen could react with CO molecules. Thus, the CO oxidation was not limited by the mobility of lattice oxygen, and the catalytic function of Au was revealed again. Carburization of metallic Fe, created by prereduction in H 2, revealed a distinct weight gain at 350 degrees C corresponding to Fe 3C formation, as subsequently confirmed by X-ray diffraction (XRD). Sustained carbon deposition ensued at 450 degrees C. In the cases of the 3% Au/gamma-Al 2O 3 and Au/ZrO 2 catalysts prepared by the same method, however, after exposure to CO in the same temperature range, no carbon deposit was observed, indicating that although Au nanoparticles could activate the absorbed oxygen molecules at low temperatures, they were not able to activate the lattice oxygen in the three catalyst supports or to dissociate the CO molecules directly.

Robust Gold-Decorated Silica−Titania Pebbles for Low-Temperature CO Catalytic Oxidation

A deposition-precipitation (DP) process was used to prepare silica-titania core-shell pebbles decorated with nanocrystalline gold suitable for low-temperature catalytic oxidation of carbon monoxide (CO). The microstructure, phase content, crystallography, and catalytic activity were correlated with the pH (3-8), aging time (15, 30, 60 min), and heat treatment employed for gold crystallization (200-400 degrees C). A homogeneous metal distribution, high gold loading (3.7-4.4 wt %), and superior interfacial adhesion between gold and titania were obtained when the support pebbles were prepared at 600 degrees C, a temperature lower than that required for the anatase-to-rutile transformation. Nucleation and growth of {111} faceted gold was favored at mid-pH (6.4-8), while smaller crystals (<7.5 nm) were obtained at short aging times (<or=60 min) and low growth temperatures (<or=300 degrees C). Catalytic activity was optimized by homogeneously dispersing gold nanocrystals (3 nm) using pH 6.4 and an aging time of 30 min. These robust materials may offer superior activity and lifetimes when deployed in fluidized bed catalytic cracking units.

D-glucose-derived polymer intermediates as templates for the synthesis of ultrastable and redispersible gold colloids.

A two-stage hydrothermal process was developed for the synthesis of highly dispersed Au colloids. In the first stage, a novel glucose-derived polymer template was prepared by the hydrothermal treatment of glucose at 160 degrees C. This template was then further used in the next step to synthesize highly dispersed gold (Au) colloids by hydrothermal treatment with HAuCl(4.) The templates treated at 160 degrees C with changing reaction times had different templating effects toward Au species. The 3-h treated template was able tightly adhere to the Au colloids. As a result, an unusual stability was observed for the prepared Au particles that could be repeatedly precipitated and redispersed with the template in H(2)O and were also stable against heating (below 160 degrees C) and aging. Meanwhile, the 5-h and 7-h treated templates had much poorer templating effects to Au species, leading to severe aggregation of the Au colloids immobilized on them. The various templating effects were correlated to the different structural features of the templates. Compared to the 5- or 7-h treated templates that were deeply carbonized, the 3-h treated template was only slightly carbonized, thus possessing a lot of functional and hydrophilic O-containing groups that could bind to Au species. These differences in templating ability were also observed in the Au samples prepared by the sonication-assisted method. The highly dispersed Au colloids immobilized on the 3-h treated template were tested for CO oxidation, and a good catalytic activity and stability for CO oxidation was observed.