Nanostructured catalytic and magnetic materials: Sonochemical synthesis and characterization.

摘要

Sonochemistry arises from acoustic cavitation; the formation, growth, and implosive collapse of bubbles in a liquid. The implosive collapse of bubble generates localized hot spots, with temperatures of {dollar}sim{dollar}5000 K, pressures of {dollar}sim{dollar}1800 atm, and cooling rates that exceed {dollar}10sp{lcub}10{rcub}{dollar} K/s. Using these extreme conditions, we have developed a new synthetic technique for the synthesis of nanostructured materials.; Sonochemial decomposition of molybdenum hexacarbonyl produced nanostructured metal carbide. After thermal treatment, pure molybdenum carbide without oxygen or carbon contamination was produced. The material consists of highly porous aggregates of 2-3 nm sized particles with very a high total surface area (130 m{dollar}sp2{dollar}/g). These carbides are excellent dehydrogenation catalysts with selectivity and activity comparable to Pt.; Nanostructured metals (Fc and Co) and alloys (Fe-Co) have been synthesized by sonochemical decomposition of volatile organometallic precursors (Fe(CO){dollar}sb5{dollar} and Co(CO){dollar}sb3{dollar}(NO)). The sonochemically prepared Fe, Co, and Fe-Co powders have high catalytic activity for the dehydrogenation and hydrogenolysis of cyclohexane. Compared to the conventionally prepared Fe-Co alloys, the sonochemically prepared Fe-Co alloys have much higher catalytic selectivities for the dehydrogenation of cyclohexane to benzene, with 1:1 ratio Fe-Co alloys having selectivities as high as 100%. A carbonaceous deposit on the surfaces of sonochemically prepared catalysts is at least one of the reasons causing the higher dehydrogenation selectivity.; Nanometer iron and cobalt particles dispersed in polyvinylpyrrolidone (PVP) matrix has been synthesized by sonochemical decomposition of Fe(CO){dollar}sb5{dollar} and Co(CO){dollar}sb3{dollar}(NO). Transmission electron micrographs show that these particles range in size from 3 to 8 nm. Electron microdiffraction revealed that the particles are amorphous, and after in situ crystallization these particles become bcc iron and fcc Co. Magnetic measurements revealed that these nanometer iron particles are superparamagnetic with a saturation mgnetization of 100 emu/g at 290K. Cobalt colloid stabilized by PVP is weakly paramagnetic but after refluxing it becomes extremely soft ferromagnetic with 51 saturation magnetization of 160 emu/g.; The sonochemical decomposition of molybdenum hexacarbonyl in the presence of sulfur generated nanostructured molybdenum sulfide. The materials consist of {dollar}<{dollar}50 nm sized particles with a surface area in excess of 150 m{dollar}sp2{dollar}/g.