Electrogeneration of hydrogen peroxide (H 2 O 2) has potential application in advanced oxidation processes. Amorphous carbon is well known as catalyst for oxygen reduction reaction (ORR) through two-electron pathway. However, modification of the carbon can improve its selectivity for the H 2 O 2 electrogeneration. In the present study, we investigated the properties of ZrO 2 nanoparticles supported on carbon black (Printex L6) as electrocatalyst for H 2 O 2 production in acidic medium. The catalytic activity of ZrO 2 /C for oxygen reduction to H 2 O 2 is higher than the catalytic activity of treated carbon black. The highest selectivity of the ZrO 2 /C catalyst for H 2 O 2 production is attributable to the presence of oxygenated functional groups on its surface and consequently increase of the surface hydrophilicity in comparison with treated carbon black. This surface effect leads to highest H 2 O 2 electrogeneration, which is shown as a high current efficiency (I(H 2 O 2)%). In fact, increased H 2 O 2 yields from 74.5 to 84.2% were observed for the treated carbon black and ZrO 2 /C catalysts, respectively, whereas the I(H 2 O 2)% for the unmodified carbon black was 65.3%. Furthermore, the modification of carbon by ZrO 2 nanoparticles shifted the ORR half-wave potential towards ca. 137 mV, indicating lower energy consumption for producing H 2 O 2. Thus, the ZrO 2 /C nanoparticles are shown to be promising electrocatalysts for environmental applications.
The flow-by electrochemical reactor is generally used in batch mode with extended process time in order to achieve high efficiency in the removal of organic contaminant from aqueous solution. Moreover, the batch mode system is inappropriate for the treatment of wastewater in high volume or as a continuous stream. This paper presents a new operational mode for the electrochemical reactor in which the electrolyte passes over the electrodes just once at a reduced flow rate and with high hydraulic retention time. The concentration of hydrogen peroxide generated in the reactor running in the single-pass flow mode attained 10.2 mg L −1 cm −2 in acid medium, a value similar to that obtained previously with the same type of reactor operating in batch mode. In the single-pass mode, the electrochemical reactor showed high rates of removal of the antibiotic levofloxacin (initial 50 mg L −1) and organic load (initial 130 mg L −1) up to 99 and 86%, respectively, when operated under electro-or photo-Fenton degradation conditions (acid medium with Fe 2+ ions; absence or presence of UV irradiation). Degradation of levofloxacin generated high levels of nitrate (up to 4.5 mg L −1) and various byproducts that could be identified by liquid chromatography-mass spectrometry.
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