Sonochemical synthesis of nanostructured catalysts

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 10⁹ 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., Mo₂C) 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.