Jongbok Choi

Mesoporous TiO2 encapsulating a visible-light responsive upconversion agent for enhanced sonocatalytic degradation of bisphenol-A

Herein, we report the integration of a visible-light active upconversion agent (Er:Y2O3) with mesoporous TiO2 via a modified two-step sol–gel coating method. The resultant material possesses a well-defined core–shell structure with a good upconversion property and exhibits uniform worm-like mesopores (∼3.8 nm), a high BET surface area (∼151.5 m2 g−1) and a large pore volume (∼0.23 cm3 g−1). The composite was demonstrated as an advanced sonocatalyst, showing a superior degradation performance for bisphenol-A (BPA). The effect of the erbium dopant content on the upconversion property and sonocatalytic performance was evaluated for the production of the best sonocatalyst. We found that the maximum pseudo first-order reaction rate constant in the presence of the composite with an Er3+ content of 3 wt% in Y2O3 nanocrystals is calculated to be 0.155 min−1, which is 2.9 and 2 times larger than that of US alone (0.054 min−1) and hollow structured mesoporous TiO2 (0.077 min−1), respectively. More importantly, the degradation rate is much higher than that of the sonocatalysts reported previously for treatment of BPA (0.09–0.14 min−1). The superior catalytic activity can be attributed to an intensified cavitation reaction zone and an enhanced amount of photo-generated charges. Moreover, the recycling test shows that a constant catalytic activity is retained even after 4 cycles. This study paves a promising way for the development of a multi-functional catalyst for sonochemical processes.

Synthesis, characterization and sonocatalytic applications of nano-structured carbon based TiO2 catalysts

In order to enhance sonocatalytic oxidation of a recalcitrant organic pollutant, rhodamine B (RhB), it is necessary to study the fundamental aspects of sonocatalysis. In this study, TiO2-incorporated nano-structured carbon (i.e., carbon nanotubes (CNTs) or graphene (GR)) composites were synthesized by coating TiO2 on CNTs or GR of different mass percentages (0.5, 1, 5, and 10 wt%) by a facile hydrothermal method. The sonocatalytic degradation rates of RhB were examined for the effect of ultrasound (US) frequency and calcination temperature by using the prepared TiO2-NSC composites. Since US frequency affected the sonoluminescence (SL) intensities, it was proposed that there exists a correlation between the surface area or band-gap of the sonocatalysts and the degradation kinetic constants of RhB. In addition, the reusability of TiO2-GR composites was also investigated. Overall, the performance of TiO2-GRs prepared by the hydrothermal method was better than that of calcined TiO2-CNTs. Among TiO2-GRs, 5% GR incorporated media (TiO2-GR-5) showed the best performance. Interestingly, the kinetic constants of sonocatalysts prepared under hydrothermal conditions had a negative linear relationship with the band-gap energy for the corresponding media. Furthermore, the strongest SL intensity and highest degradation rates of RhB for both carbonaceous composites were observed at 500 kHz. The kinetic constants of calcined media decreased linearly as the specific area of the media decreased, while the band-gap energy could not be correlated with the kinetic constants. The GR combined TiO2 composite might be a good sonocatalyst in wastewater treatment using ultrasound-based oxidation because of its high stability.

A review on heterogeneous sonocatalyst for treatment of organic pollutants in aqueous phase based on catalytic mechanism

Heterogeneous sonocatalysis, as an emerging advanced oxidation process (AOP), has shown immense potential in water treatment and been widely demonstrated to remove persistent organic compounds in the past decade. The present article aims to provide a comprehensive review on the development of a heterogeneous catalyst for enhancing the ultrasonic degradation rate of organic pollutants from a viewpoint of sonocatalytic mechanism. The rational design and fundamentals for preparing sonocatalysts are presented in the context of facilitating the heterogeneous nucleation and photo-thermal-catalytic effects as well as considering the mechanical stability and separation capacity of the heterogeneous catalyst. In addition, some new trends, ongoing challenges and possible methods to overcome these challenges are also highlighted and proposed.

Degradation of polychlorinated dibenzo-p-dioxins and dibenzofurans in real-field soil by an integrated visible-light photocatalysis and solvent migration system with p-n heterojunction BiVO4/Bi2O3

Degradation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in real-field soil was conducted using an integrated photocatalysis-solvent migration system of BiVO4/Bi2O3 and n-hexane. The photocatalyst BiVO4/Bi2O3 was synthesized, and its performance was found to be affected by the BiVO4 content, with 20wt% BiVO4 showing the best performance owing to its p-n heterojunction being well formed. Migration was affected by the amount of n-hexane, with 15% n-hexane giving the most effective transportation of PCDD/Fs. 37.2% of 17 PCDD/Fs was removed in 60h by the integrated photocatalysis-solvent migration system, although the reaction zone covered 8.5% of the volume of the soil. The result showed that migration via n-hexane fulfilled the aim of carrying contaminants from inside of the soil to the surface. Electron-scavenging experiments with BiVO4/Bi2O3 showed an 18.4% of performance in removal compared to no-scavenging condition, which showed that the main reactions driving BiVO4/Bi2O3 visible-light photocatalysis for aryl-chloride were found to be reduction-based. Owing to the hindering effect of Cl atoms, degradation by hydroxyl radical could proceed after initial dechlorination. This study establishes the applicability of integrated photocatalysis-solvent migration systems in real-field settings, and is the first report of a visible-light photocatalyst, BiVO4/Bi2O3, for the degradation of PCDD/Fs in soil.

Sonochemical Degradation of Reactive Black 5 with a Composite Catalyst of TiO2/Single-Walled Carbon Nanotubes

In the sonocatalytic process, composites of TiO2-carbon were used because carbon provides more adsorption sites and acts like an electron sink to prevent the recombination of an electron/hole. Therefore, in the present study, the characteristics of a TiO2/single-walled carbon nanotubes catalyst (TiO2/SWCNTs) have been investigated, and the optimal weight ratio of SWCNTs and the dose for degradation of reactive black 5 (RB5) were also evaluated. TiO2/SWCNT composite was characterized using Brunauer–Emmett–Teller analysis, scanning electron microscopy, energy-dispersive X-ray diffraction microanalysis and spectra, and X-ray diffraction patterns. The degradation rate constants of RB5 with the ratio of SWCNTs were found to depend on the adsorption phenomenon of a surface catalyst, light absorbance, and the recombination of electrons and holes. As a result, the optimal ratio of carbon in the sono-TiO2/SWCNTs process for degradation of RB5 was TiO2:SWCNTs= 200:1. Additionally, the optimal dose of the catalyst was 0.5 g/L.

Enhancement of sonochemical oxidation reactions using air sparging in a 36 kHz sonoreactor

The effect of air sparging on sonochemical oxidation reactions was investigated using a relatively large reactor equipped with a 36 kHz transducer module at the bottom. KI dosimetry and luminol techniques were used for quantitative and qualitative analysis of the reactions. The cavitation yield increased and then varied minimally as the liquid height increased from 1λ (42 mm) to 8λ (333 mm) with no air sparging. The flow rate of the air used for sparging and the position of the sparger significantly affected the extent of the sonochemical oxidation reactions. A significant enhancement in the sonochemical oxidation by air sparging was observed for higher liquid height and higher flow rate conditions at a constant input power. This enhancement is attributed to the violent mixing effect and the significant change in the sound field and cavitation-active zone in the liquid. Higher sonochemical activity was obtained when air sparging was applied closer to the transducer module at a higher flow rate. Imaging the motion of the liquid surface and sonochemiluminescence revealed that the instability of the liquid body was directly related to the sonochemical activity.