TiO2/β-SiC foam-structured photoreactor for continuous wastewater treatment

Porous Silicon Carbide (SiC): A Chance for Improving Catalysts or Just Another Active-Phase Carrier?

There is an obvious gap between efforts dedicated to the control of chemicophysical and morphological properties of catalyst active phases and the attention paid to the search of new materials to be employed as functional carriers in the upgrading of heterogeneous catalysts. Economic constraints and common habits in preparing heterogeneous catalysts have narrowed the selection of active-phase carriers to a handful of materials: oxide-based ceramics (e.g. Al₂O₃, SiO₂, TiO₂, and aluminosilicates-zeolites) and carbon. However, these carriers occasionally face chemicophysical constraints that limit their application in catalysis. For instance, oxides are easily corroded by acids or bases, and carbon is not resistant to oxidation. Therefore, these carriers cannot be recycled. Moreover, the poor thermal conductivity of metal oxide carriers often translates into permanent alterations of the catalyst active sites (i.e. metal active-phase sintering) that compromise the catalyst performance and its lifetime on run. Therefore, the development of new carriers for the design and synthesis of advanced functional catalytic materials and processes is an urgent priority for the heterogeneous catalysis of the future. Silicon carbide (SiC) is a non-oxide semiconductor with unique chemicophysical properties that make it highly attractive in several branches of catalysis. Accordingly, the past decade has witnessed a large increase of reports dedicated to the design of SiC-based catalysts, also in light of a steadily growing portfolio of porous SiC materials covering a wide range of well-controlled pore structure and surface properties. This review article provides a comprehensive overview on the synthesis and use of macro/mesoporous SiC materials in catalysis, stressing their unique features for the design of efficient, cost-effective, and easy to scale-up heterogeneous catalysts, outlining their success where other and more classical oxide-based supports failed. All applications of SiC in catalysis will be reviewed from the perspective of a given chemical reaction, highlighting all improvements rising from the use of SiC in terms of activity, selectivity, and process sustainability. We feel that the experienced viewpoint of SiC-based catalyst producers and end users (these authors) and their critical presentation of a comprehensive overview on the applications of SiC in catalysis will help the readership to create its own opinion on the central role of SiC for the future of heterogeneous catalysis.

Structured photocatalytic systems: photocatalytic coatings on low-cost structures for treatment of water contaminated with micropollutants-a short review

The persistence of many micropollutants in water and wastewater is of great concern to the contemporary scientific community. Several types of advanced techniques such as heterogeneous photocatalysis are being used for the degradation of micropollutants in waters from domestic, industrial, and agricultural activities. Thus, structured photocatalytic systems are a great alternative in the development of photocatalytic reactors and continuous water treatment systems, as they present good autonomy during the treatment process. Many aspects such as type and geometry of the catalytic structure to be developed must be carefully chosen for the proper functioning of the system, as well as the best routes by which the photocatalysts will be immobilized. In this sense, this work brings the main photocatalytic coating techniques in low-cost structures for the treatment of water and wastewater contaminated with micropollutants. The methodologies and synthesis parameters that can influence the final result of the coating were highlighted, as well as the ability to reuse photocatalysts and methods for pretreating the structural surface. The dip-coating technique was the most reported among the current works due to its simplicity and, predominantly, the pretreatment techniques of the structure are still cleaning the surface with water, soap, and also some alcohols.

Modified-TiO2 Photocatalyst Supported on β-SiC Foams for the Elimination of Gaseous Diethyl Sulfide as an Analog for Chemical Warfare Agent: Towards the Development of a Photoreactor Prototype

In the context of the increase in chemical threat due to warfare agents, the development of efficient methods for destruction of Chemical Warfare Agents (CWAs) are of first importance both for civilian and military purposes. Amongst possible methods for destruction of CWAs, photocatalytic oxidation is an alternative one. The present paper reports on the preparation of Ta and Sn doped TiO2 photocatalysts immobilized on β-SiC foams for the elimination of diethyl sulfide (DES) used as a model molecule mimicking Yperite (Mustard Gas) in gaseous phase. Photo-oxidation efficiency of doped TiO2 catalyst has been compared with TiO2-P25. Here, we demonstrate that the Sn doped-TiO2 with a Polyethylene glycol (PEG)/TiO2 ratio of 7 exhibits the best initial activity (up to 90%) but is deactivates more quickly than Ta doped-TiO2 (40% after 800 min). The activity of the catalysts is strongly influenced by the adsorption properties of the support, as β-SiC foams adsorb DES and other sulfur compounds. This adsorption makes it possible to limit the poisoning of the catalysts and to maintain an acceptable conversion rate even after ten hours under continuous DES flow. Washing with NaOH completely regenerates the catalyst after a firs treatment and even seems to “wash” it by removing impurities initially present on the foams.

Ti-Modified LaFeO3/β-SiC Alveolar Foams as Immobilized Dual Catalysts with Combined Photo-Fenton and Photocatalytic Activity

Ti-modified LaFeO₃/β-SiC alveolar foams were used as immobilized, highly robust dual catalysts with combined photocatalytic wet peroxide oxidation and photocatalytic activity under UV-A light. They were prepared by incipient wetness impregnation of a β-SiC foam support, by implementing a sol-gel Pechini synthesis at the foam surface in the presence of dried amorphous sol-gel titania as a titanium source. The physicochemical and catalytic features suggest the stabilization at the foam surface of a substituted La_1-xTi_xFeO₃ catalyst analogous to its powdery counterpart. Taking 4-chlorophenol removal in water as a model reaction, its dual nature enables both high reaction rates and full total organic carbon (TOC) conversion because of a synergy effect, while its macroscopic structure overcomes the drawback of working with powdery catalysts. Further, it yields photonic efficiencies for degradation and mineralization of ca. 9.4 and 38%, respectively, that strongly outperform those obtained with a reference TiO₂ P25/β-SiC foam photocatalyst. The enhancement of the catalyst robustness upon Ti modification prevents any Fe leaching to the solution, and therefore, the optimized macroscopic foam catalyst with 10 wt % catalyst loading operates through pure heterogeneous surface reactions, without any activity loss during reusability test cycles.

Facile synthesis of Cu-doped ZnO nanoparticle in triethyleneglycol: photocatalytic activities and aquatic ecotoxicity

A new synthetisis method of Cu-doped ZnO nanoparticles is presented in this work, this novel approach allow one to produce Zinc oxide nanocristal owing to a modified Polyol process that makes use of triethyleneglycol (TREG) as a solvent. The structure and morphology of the nanoparticles were characterized by high-resolution transmission electron microscopy (HRTEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), N₂ adsorption study, UV-Vis diffuse reflectance spectroscopy, inductively coupled plasma optical emission spectroscopy and Raman spectroscopy. The lightly doped Zn_1-xCu_xO photocatalysts consisted in a novel nanorods structure of Zn_0.9990Cu_0.0010O nanoparticles. Taking the photocatalytic degradation of diuron under solar light as liquid phase test reaction, the lightly doped Zn_0.9990Cu_0.0010O nanorods photocatalysts showed strongly enhanced photocatalytic activity when compared to the bare ZnO counterpart. The apparent rate constant value of Zn_0.9990Cu_0.0010O was 22 times higher than that of pure ZnO. In order to study the environmental risk of Cu-ZnO, clams Ruditapes decussatus were exposed to Cu-ZnOC1 = 0.5 mg/L, Cu-ZnOC2 = 1 mg/L and Cu-ZnO C3 = 5 mg/L. Catalase (CAT) activities, malondialdehyde (MDA) content and acetylcholinesterase (AChE) activity were determined in gills and digestive gland of treated and untreated clams. Thus, no significant effects were detected in the gills of exposed clams after 7 days compared to control. Thus, MDA level and CAT activity showed significant differences in digestive glands of groups treated by the highest concentration of Cu-ZnO NPs compared to the control. No adverse effects on AChE activity was detected after Cu-ZnO NPs exposure. These results demonstrated that, although Cu-ZnO NPs is not acutely toxic to Ruditapes decussatus, it does exert oxidative stress on clams. These results are encouraging for the Cu-ZnO NPs use in variety of applications due to its high photocatalytic and low environmental toxicity.

Kinetics and mechanism of Paraquat's degradation: UV-C photolysis vs UV-C photocatalysis with TiO2/SiC foams

In this study, the photolytic and photocatalytic removal of the herbicide paraquat is investigated under UV-C (254 nm). For photocatalytic experiments, SiC foams were used with P25-TiO₂ nanoparticles deposited by dip-coating. The foams were characterized by scanning electron microscopy and paraquat's degradation under UV-C photolysis or photocatalysis, followed by UV-vis spectroscopy, total organic carbon analyzer, LC-MS and ion chromatography. After 3 h of reactions by photolysis and photocatalysis, 4% and 91% of TOC removal were observed. An analysis of degradation by-products showed a similar degradation pathway with pyridinium ions observed by LC/MS and carboxylic acids (succinate, acetate, oxalate and formate) detected by ion chromatography. In conclusion, these two different photo-degradation processes are able to remove paraquat and produce similar by-products. However, the kinetics of degradation is rather slow during photolysis and it is recommended to combine the UV-C lightning with a TiO₂ photocatalyst to improve the mineralization rate.

One-pot synthesis of lightly doped Zn1-x Cu x O and Au-Zn1-x Cu x O with solar light photocatalytic activity in liquid phase

We report on the facile and low-temperature one-pot chemical synthesis of lightly doped Zn_1-x Cu _x O and hybrid Au-Zn_1-x Cu _x O photocatalysts with low Cu molar content (0 < x < 0.7%) using 1,3-propanediol polyol simultaneously as solvent, reducing and a stabilizing agent, without any final thermal treatment. The photocatalysts have been characterized by X-ray diffraction, N₂ adsorption study, UV-vis diffuse reflectance spectroscopy, inductively coupled plasma optical emission spectroscopy, and transmission electron microscopy. The lightly doped hybrid Au-Zn_1-x Cu _x O photocatalysts consisted in faceted quasi-spherical large-size Au nanoparticle cores surrounded by closely packed small-size Zn_1-x Cu _x O nanoparticles. Taking the photocatalytic degradation of Diuron under solar light as liquid-phase test reaction, the lightly doped Au-Zn_1-x Cu _x O hybrid photocatalysts with optimized x = 0.09% Cu content showed strongly enhanced photocatalytic activity when compared to the bare ZnO counterpart. The observed 16-fold higher degradation rate constant resulted jointly from the light doping of ZnO with Cu to form Zn_1-x Cu _x O photocatalyst and further from the addition of gold nanoparticles allowing interfacial oxide-to-metal electron transfer within the hybrid Au-Zn_1-x Cu _x O photocatalyst.

TiO2 hybrid photocatalytic systems: impact of adsorption and photocatalytic performance

For the past 40 years, titanium dioxide (TiO2) nanomaterials have attracted immense attention because of their potential applications in the photodegradation of organic pollutants, photocatalytic water splitting for H2 generation, and dye-sensitized solar cells. Despite the fact that the potential applications of TiO2 nanoparticles are ubiquitous, they are not problem free, examples include a large interfacial area that causes slow charge carrier transport, a wide optical band gap that leads to limited applications using solar light, and single-phase and nanoscales features that induce fast recombination of photo-reduced carriers. Therefore, this review highlights the development associated with the adsorption photocatalysis hybrid system for treating wastewater. The immobilization of TiO2 photocatalysts in metal oxide, carbon, and ceramic materials to form TiO2 hybrid systems could prevent the problem of particle recovery, adsorption capacity, and the separation process. Such hybrid systems require significant effort of optimizing the specific surface area-to-volume ratio of the supported photocatalysts with its photocatalytic activities.

A highly sensitive electrochemical sensor for simultaneous determination of hydroquinone and bisphenol A based on the ultrafine Pd nanoparticle@TiO2 functionalized SiC

A titanium dioxide-silicon carbide nanohybrid (TiO2-SiC) with enhanced electrochemical performance was successfully prepared through a facile generic in situ growth strategy. Monodispersed ultrafine palladium nanoparticles (Pd NPs) with a uniform size of ∼2.3 nm were successfully obtained on the TiO2-SiC surface via a chemical reduction method. The Pd-loaded TiO2-SiC nanohybrid (Pd@TiO2-SiC) was characterized by transmission electron microscopy and X-ray diffractometry. A method for the simultaneous electrochemical determination of hydroquinone (HQ) and bisphenol A (BPA) using a Pd@TiO2-SiC nanocomposite-modified glassy carbon electrode was established. Utilizing the favorable properties of Pd NPs, the Pd@TiO2-SiC nanohybrid-modified glassy carbon electrode exhibited electrochemical performance superior to those of TiO2-SiC and SiC. Differential pulse voltammetry was successfully used to simultaneously quantify HQ and BPA within the concentration range of 0.01-200 μM under optimal conditions. The detection limits (S/N=3) of the Pd@TiO2-SiC nanohybrid electrode for HQ and BPA were 5.5 and 4.3 nM, respectively. The selectivity of the electrochemical sensor was improved by introducing 10% ethanol to the buffer medium. The practical application of the modified electrode was demonstrated by the simultaneous detection of HQ and BPA in tap water and wastewater samples. The simple and straightforward strategy presented in this paper are important for the facile fabrication of ultrafine metal NPs@metal oxide-SiC hybrids with high electrochemical performance and catalytic activity.