Photodegradation of Pollutants in Air: Enhanced Properties of Nano-TiO2Prepared by Ultrasound

Nanocrystalline TiO2samples were prepared by promoting the growth of a sol–gel precursor, in the presence of water, under continuous (CW), or pulsed (PW) ultrasound. All the samples turned out to be made of both anatase and brookite polymorphs. Pulsed US treatments determine an increase in the sample surface area and a decrease of the crystallite size, that is also accompanied by a more ordered crystalline structure and the samples appear to be more regular and can be considered to contain a relatively low concentration of lattice defects. These features result in a lower recombination rate between electrons and holes and, therefore, in a good photocatalytic performance toward the degradation of NOxin air. The continuous mode induces, instead, the formation of surface defects (two components are present in XPS Ti 2p3/2region) and consequently yields the best photocatalyst. The analysis of all the characterization data seems to suggest that the relevant parameter imposing the final features of the oxides is the ultrasound total energypervolume (Etot/V) and not the acoustic intensity or the pulsed/continuous mode

Effect of power ultrasound on the electrochemical platinum HCOOH interface

International audienceThe effect of ultrasound (f=20 kHz) on the electrochemical behavior of a platinum electrode was investigated in aqueous media. A study of HCOOH oxidation on platinum showed that low-intensity ultrasound (on the order of Ius=2 W cm−2) increases the potential oxidation zone and the current intensity. The catalytic activity of freshly reduced platinum was also observed to increase significantly under the effect of ultrasound, depending on the upper potential investigated during platinum oxidation. Monitoring the potential during electrolysis of formic acid at an imposed current with a platinum electrode under ultrasound showed oscillating reactions characteristic of passivation/depassivation phenomena on the platinum electrod

Sonolysis of 4-chlorophenol in aqueous solution: Effects of substrate concentration, aqueous temperature and ultrasonic frequency

The sonolysis of 4-chlorophenol (4-CP) in O2-saturated aqueous solutions is investigated for a variety of operating conditions with the loss of 4-CP from solution following pseudo-first-order reaction kinetics. Hydroquinone (HQ) and 4-chlorocatechol (4-CC) are the predominant intermediates which are degraded on extended ultrasonic irradiation. The final products are identified as Cl-, CO2, CO, and HCO2H. The rate of 4-CP degradation is dependent on the initial 4-CP concentration with an essentially linear increase in degradation rate at low initial 4-CP concentrations but with a plateauing in the rate increase observed at high reactant concentrations. The results obtained indicate that degradation takes place in the solution bulk at low reactant concentrations while at higher concentrations degradation occurs predominantly at the gas bubble-liquid interface. The aqueous temperature has a significant effect on the reaction rate. At low frequency (20 kHz) a lower liquid temperature favours the sonochemical degradation of 4-CP while at high frequency (500 kHz) the rate of 4-CP degradation is minimally perturbed with a slight optimum at around 40 °C. The rate of 4-CP degradation is frequency dependant with maximum rate of degradation occurring (of the frequencies studied) at 200 kHz. © 2005 Elsevier B.V. All rights reserved