Taeghwan Hyeon

Acoustic Cavitation and its Chemical Consequences

Acoustic cavitation is responsible for both sonochemistry and sonoluminescence. Bubble collapse in liquids results in an enormous concentration of energy from the conversion of the kinetic energy of liquid motion into heating of the contents of the bubble. The high local temperatures and pressures, combined with extraordinarily rapid cooling, provide a unique means for driving chemical reactions under extreme conditions. A diverse set of applications of ultrasound to enhance chemical reactivity has been explored, with important applications in mixed–phase synthesis, materials chemistry, and biomedical uses. For example, the sonochemical decomposition of volatile organometallic precursors in low–volatility solvents produces nanostructured materials in various forms with high catalytic activities. Nanostructured metals, alloys, carbides and sulphides, nanometre colloids, and nanostructured supported catalysts can all be prepared by this general route. Another important application of sonochemistry to materials chemistry has been the preparation of biomaterials, most notably protein microspheres. Such microspheres have a wide range of biomedical applications, including their use as echo contrast agents for sonography, magnetic resonance imaging contrast enhancement, and oxygen or drug delivery.

Sonochemical synthesis of nanostructured catalysts

Abstract Sonochemistry arises from acoustic cavitation; the formation, growth, and collapse of bubbles in a liquid. The implosive collapse of a bubble generates a localized hot spot; a temperature of ∼5000 K and pressure of ∼1800 atm, with cooling rates that exceed 10 9 K s −1 . Using these extreme conditions, we have developed a new synthetic technique for the synthesis of nanostructured inorganic materials. When irradiated with high intensity ultrasound in low volatility solvents under argon, volatile organometallic precursors produce high surface area solids that consist of agglomerates of nanometer clusters. These sonochemically produced nanostructured solids are active heterogeneous catalysts for hydrocarbon reforming and CO hydrogenation. For Fe and Co, nanostructured metals are formed; for Mo and W, metal carbides (e.g., Mo 2 C) are produced. Using polymeric ligands (e.g. polyvinylpyrrolidone) or oxide supports (alumina or silica), the initially formed nanoscale clusters can be trapped as colloids or supported catalysts, respectively.

Catalytic hydrodenitrogenation of indole over molybdenum nitride and carbides with different structures

The hydrodenitrogenation (HDN) of indole has been studied over Mo2N and Mo2C with different structures. The difference in activity was modest and the structure of the catalysts exerted a more significant effect than the chemical composition. More evident difference was observed in selectivity, and here the composition was the most important factor. Sonochemically prepared Mo2C was an active catalyst for HDN reaction. It formed plate-like aggregates of loosely packed Mo2C (fcc) particles. The crystallinity of sonochemically prepared Mo2C was improved during the reaction, and its behavior in HDN became similar to Mo2C (fcc) prepared by the TPR method.

Applications of Sonochemistry to Materials Synthesis

One of the most important recent applications of sonochemistry has been to the synthesis and modification of inorganic materials [1–5]. In liquids irradiated with high intensity ultrasound, acoustic cavitation drives bubble collapse producing intense local heating, high pressures, and very short lifetimes; these transient, localized hot spots drive high energy chemical reactions [5–11]. As described in detail elsewhere in this monograph, these hot spots have temperatures of roughly 5000°C, pressures of about 1000 atmospheres, and heating and cooling rates above 1010 K/s. Thus, cavitation serves as a means of concentrating the diffuse energy of sound into a unique set of conditions to produce unusual materials from dissolved (and generally volatile) solution precursors.

Structural properties of amorphous bulk Fe, Co and Fe-Co binary alloys

Elastic neutron diffraction experiments on amorphous iron, cobalt and their amorphous binary metallic alloys are presented. The measurements allow, for the first time, to describe the atomic distribution in samples obtained in the form of fine (<30 nm) amorphous elemental particles by sonochemical synthesis. In the case of a-Fe, the structural information from the shape of the radial distribution function is consistent with the Random Packing model (RPD) calculations previously made for films of amorphous iron. Finally the atomic magnetic moment of iron and cobalt in the amorphous bulk phase is also evaluated.

Sonochemical Preparation of Nanostructured Catalysts

The chemical effects of high intensity ultrasound arise from acoustic cavitation : the formation, growth, and implosive collapse of bubbles in a liquid, which generates a transient, localized hot spot. The local conditions reached have temperatures of 5000K, pressure of 1800 atm, but with cooling rates that exceed 10 10 K/s. This paper reports about the use of these extreme conditions to develop a new technique for the synthesis of nanostructured heterogeneous catalysts. These nanostructured solids are active heterogeneous catalysts for hydrocarbon reforming and CO hydrogenation.

Nanostructured Fe-Co catalysts generated by ultrasound

Bimetallic catalysts have been studied intensively because of their unique activity and selectivity. Unsupported alloy catalysts, however, are usually of limited value due to their very small surface areas. We have now developed a sonochemical synthesis of bimetallic alloys that provides both high surface areas and high catalytic activity. We have produced Fe-Co alloys by ultrasonic irradiation of mixed solutions of Fe(CO) 5 and Co(CO) 3 (NO) in hydrocarbon solvents. The alloy composition can be controlled simply by changing the ratio of precursor concentrations. After treatment at 673K under H 2 flow for 2 hours, we obtain nearly pure alloys. BET results show that the surface areas of these alloys are large (10-30 m 2 /g). TEM and SEM show that the alloy particles are porous agglomerates of particles with diameters of 10-20 nm. Sonochemically prepared Fe, Co, and Fe-Co powders have very high catalytic activity for dehydrogenation and hydrogenolysis of cyclohexane. Furthermore, sonochemically prepared Fe-Co alloys show high catalytic selectivity for dehydrogenation of cyclohexane to benzene; the 1:1 ratio alloy has much higher selectivity for dehydrogenation over hydrogenolysis than either pure metal.

Magnetic and structural properties of amorphous transition metals and alloys

Abstract In this paper a neutron scattering study of local order and magnetic properties of the Fe 1− x CO x amorphous system is presented. A wide momentum transfer range has been used for studying local order. The better resolution associated with higher neutron wavelengths provided one with a description of the magnetic contribution measured with and without an applied magnetic field. Preliminary results are also reported on recent SANS measurements, which provide better insight on the magnetic properties and the structure and size of the powder grains.