activating-molecules-ions-and-solid-particles-with-acoustic-cavitation

Activating Molecules, Ions, and Solid Particles with Acoustic Cavitation

The chemical and physical effects of ultrasound arise not from a direct interaction of molecules with sound waves, but rather from the acoustic ...

UMR 5257 CEA-CNRS-UM2-ENSCM

Herein, we report the sonochemical reactions with MSU-X mesoporous alumina (m-Al(2)O(3)) in aqueous solutions. Sonication (f=20 kHz, I=30 Wcm(-2), W(aq)=0.67 WmL(-1), T=36-38 degrees C, Ar) causes significant acceleration of m-Al(2)O(3) dissolution in the pH range of 4-11. Moreover, power ultrasound has a dramatic effect on the textural properties and phase composition of m-Al(2)O(3). Short-time sonication at pH = 4 leads to the formation of nanorods and nanofibers of boehmite, AlO(OH). Prolonged ultrasonic treatment causes high aspect morphology transformation to aggregated nanosheets in weakly acid solutions or plated nanocrystals in alkaline solutions. Sonochemical products in alkaline medium are composed principally from boehmite and small amounts of bayerite, Al(OH)(3). Silent hydrolysis of m-Al(2)O(3) yields boehmite at pH = 4 and bayerite at pH = 11. The effect of ultrasound on the textural properties of mesoporous alumina as well as on the transformation of nanosized bayerite to boehmite can be consistently attributed to the transient strong heating of the liquid shell surrounding the cavitation bubble which caused the chemical processes similar to those occurred during hydrothermal treatment.

Sonochemical reactions with mesoporous alumina.

Sonochemical disproportionation of carbon monoxide in water: evidence for treanor effect during multibubble cavitation.

This paper describes, for the first time, a simple method for the synthesis of uranyl aluminate (URAL) nanoparticles. URAL was prepared by U(VI) hydrolytic precipitation with ammonia at pH = 11 in the presence of mesoporous alumina MSU-X under 20 kHz of sonication followed by annealing of the obtained solids at 800 degrees C. TEM, XAFS, powder XRD, and (27)Al MAS NMR studies revealed that the speciation of uranium in this system strongly depends on uranium concentration. The sample with 5 wt % of uranium yields air-stable nanoparticles ( approximately 5 nm) of URAL. Presumably, UO(2)(2+) cations in this compound are coordinated with bidentate AlO(2)(-) groups. The increase of uranium concentration to 30 wt % causes mostly formation of U(3)O(8) fine particles ( approximately 50 nm) and small amounts of URAL.

First synthesis of uranyl aluminate nanoparticles.

The kinetics and mechanism of formic acid sonochemical degradation were studied at ultrasonic frequencies of 20, 200, and 607 kHz under argon atmosphere. Total yield of HCOOH sonochemical degradation increases approximately 6-8-fold when the frequency increased from 20 to 200 or to 607 kHz. At low ultrasonic frequencies, HCOOH degradation has been attributed to oxidation with OH(•) radicals from water sonolysis and to the HCOOH decarboxylation occurring at the cavitation bubble-liquid interface. With high-frequency ultrasound, the sonochemical reaction is also influenced by HCOOH dehydration. Whatever the ultrasonic frequency, the sonolysis of HCOOH yielded H(2) and CO(2) in the gas phase as well as trace amounts of oxalic acid and formaldehyde in the liquid phase. However, CO and CH(4) formations were only detected under high-frequency ultrasound. The most striking difference between low-frequency and high-frequency ultrasound is that the sonolysis of HCOOH at high ultrasonic frequencies initiates Fischer-Tropsch hydrogenation of carbon monoxide.

Effect of ultrasonic frequency on the mechanism of formic acid sonolysis.

Sonochemical synthesis of platinum nanoparticles (Pt NPs) in formic acid solutions and pure water was investigated using a 20 kHz ultrasonic irradiation. The obtained results gave new insights on the underneath Pt(IV)  reduction mechanism in formic acid media under argon and in pure water under Ar/CO atmosphere. It was shown that in pure water sonochemical reduction of platinum ions occurs by hydrogen issued from homolytic water molecule split. Pt(IV) ion reduction appears to be a very slow process under argon atmosphere in pure water due to formation of oxidizing species like OH radicals and H(2)O(2) leading to reoxidation of intermediate Pt(II)  ions. Sonochemical reduction is accelerated manifold in the presence of formic acid or Ar/CO gas mixture. Solution and gas-phase analyses reveal that both CO and HCOOH act as OH(.) radical scavenger and reducing agent under ultrasonic irradiation. Their ability to reduce platinum ions at room temperature is enhanced due to the local heating in the liquid shell surround the cavitation bubble. An innovative synthesis route for monodispersed Pt NPs in pure water without any templates or capping agents in the presence of Ar/CO gas mixture is then proposed. Obtained Pt NPs within the range of 2-3 nm exhibited a strong stability towards sedimentation in water. Since Ar/CO atmosphere is the only restriction of the process, this procedure can be applied in various media and is also compatible with a large array of experimental conditions.

Mechanism of Pt(IV) sonochemical reduction in formic acid media and pure water.

This study reported the sonochemical deposition of platinum on the surface of polystyrene beads (PSBs) and the transfer of obtained Pt nanoparticles into a porous silica matrix using the PSB as a sacrificial template. Platinum nanoparticle deposition was ensured by the sonochemical reduction of Pt(IV) at room temperature in latex solutions containing polystyrene beads in the presence of formic acid under Ar or under Ar/CO atmosphere without any additives. After ultrasonic treatments for few hours, well dispersed Pt nanoparticles within the range of 3-5 nm deposited on PSB were obtained in both studied conditions. Samples were then mixed with TEOS, dried, and heated at 450°C to ensure the PSB removal from the silica matrix. TEM and SEM results clearly show that final silica pore size is within the same order of magnitude than initial PSB. Finally, platinum decorated silica matrix with chosen pore sizes was successfully prepared.

Sonochemical deposition of platinum nanoparticles on polymer beads and their transfer on the pore surface of a silica matrix.

Response to Comment on "Mechanism of Pt(IV) sonochemical reduction in formic acid media and pure water".

Porous (Ce0.5Zr0.5)O2 solid solutions were prepared by thermolysis (T=285°C) or sonolysis (20kHz, I=32Wcm(-2), Pac=0.46WmL(-1), T=200°C) of Ce(III) and Zr(IV) acetylacetonates in oleylamine or hexadecylamine under argon followed by heat treatment of the precipitates obtained in air at 450°C. Transmission Electron Microscopy images of the samples show nanoparticles of ca. 4-6nm for the two synthetic approaches. The powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray and μ-Raman spectroscopy of solids obtained after heat treatment indicate the formation of (Ce0.5Zr0.5)O2 solid solutions with a metastable tetragonal crystal structure for the two synthetic routes. The specific surface area of the samples varies between 78 and 149m(2)g(-1) depending on synthesis conditions. The use of Barrett-Joyner-Halenda and t-plot methods reveal the formation of mixed oxides with a hybrid morphology that combines mesoporosity and microporosity regardless of the method of preparation. Platinum nanoparticles were deposited on the surface of the mixed oxides by sonochemical reduction of Pt(IV). It was found that the materials prepared by sonochemistry exhibit better resistance to dissolution during the deposition process of platinum. X-ray photoelectron spectroscopy analysis shows the presence of Pt(0) and Pt(II) on the surface of mixed oxides. Porous (Ce0.5Zr0.5)O2 mixed oxides loaded with 1.5%wt. platinum exhibit high activity in catalytic wet air oxidation of formic acid at 40°C.

Thermal and sonochemical synthesis of porous (Ce,Zr)O2 mixed oxides from metal β-diketonate precursors and their catalytic activity in wet air oxidation process of formic acid.

Tony Chave

Marcoule Institute for Separative Chemistry

UMR 5257 CEA-CNRS-UM2-ENSCM

Activating Molecules, Ions, and Solid Particles with Acoustic Cavitation