Mingming Fang

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.

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.

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.