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6 pages, 222 KiB

Open AccessEditorial

Heterogeneous Photocatalysis: A Solution for a Greener Earth

by Julien G. Mahy and Stéphanie D. Lambert

Catalysts 2022, 12(7), 686; https://doi.org/10.3390/catal12070686 - 23 Jun 2022

Cited by 1 | Viewed by 1426

Abstract

Since the beginning of the industrial era, various human activities have steadily increased, leading to rapid technological developments and high population growth [...] Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

Research

Jump to: Editorial, Review

11 pages, 3374 KiB

Open AccessArticle

Visible Light Induced Nano-Photocatalysis Trimetallic Cu_0.5Zn_0.5-Fe: Synthesis, Characterization and Application as Alcohols Oxidation Catalyst

by Asma Ghazzy, Lina Yousef and Afnan Al-Hunaiti

Catalysts 2022, 12(6), 611; https://doi.org/10.3390/catal12060611 - 2 Jun 2022

Cited by 5 | Viewed by 1892

Abstract

Here, we report a visible light-induced-trimetallic catalyst (Cu_0.5Zn_0.5Fe₂O₄) prepared through green synthesis using Tilia plant extract. These nanomaterials were characterized for structural and morphological studies using powder x-ray diffraction (P-XRD), scanning electron microscopy (SEM) and [...] Read more.

Here, we report a visible light-induced-trimetallic catalyst (Cu_0.5Zn_0.5Fe₂O₄) prepared through green synthesis using Tilia plant extract. These nanomaterials were characterized for structural and morphological studies using powder x-ray diffraction (P-XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The spinel crystalline material was ~34 nm. In benign reaction conditions, the prepared photocatalyst oxidized various benzylic alcohols with excellent yield and selectivity toward aldehyde with 99% and 98%; respectively. Aromatic and aliphatic alcohols (such as furfuryl alcohol and 1-octanol) were photo-catalytically oxidized using Cu_0.5Zn_0.5Fe₂O₄, LED light, H₂O₂ as oxidant, 2 h reaction time and ambient temperature. The advantages of the catalyst were found in terms of reduced catalyst loading, activating catalyst using visible light in mild conditions, high conversion of the starting material and the recyclability up to 5 times without loss of the selectivity. Thus, our study offers a potential pathway for the photocatalytic nanomaterial, which will contribute to the advancement of photocatalysis studies. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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14 pages, 8124 KiB

Open AccessArticle

Natural Clay Modified with ZnO/TiO₂ to Enhance Pollutant Removal from Water

by Julien G. Mahy, Marlène Huguette Tsaffo Mbognou, Clara Léonard, Nathalie Fagel, Emmanuel Djoufac Woumfo and Stéphanie D. Lambert

Catalysts 2022, 12(2), 148; https://doi.org/10.3390/catal12020148 - 25 Jan 2022

Cited by 19 | Viewed by 3307

Abstract

Raw clays, extracted from Bana, west Cameroon, were modified with semiconductors (TiO₂ and ZnO) in order to improve their depollution properties with the addition of photocatalytic properties. Cu²⁺ ions were also added to the clay by ionic exchange to increase the [...] Read more.

Raw clays, extracted from Bana, west Cameroon, were modified with semiconductors (TiO₂ and ZnO) in order to improve their depollution properties with the addition of photocatalytic properties. Cu²⁺ ions were also added to the clay by ionic exchange to increase the specific surface area. This insertion of Cu was confirmed by ICP-AES. The presence of TiO₂ and ZnO was confirmed by the detection of anatase and wurzite, respectively, using X-ray diffraction. The composite clays showed increased specific surface areas. The adsorption property of the raw clays was evaluated on two pollutants, i.e., fluorescein (FL) and p-nitrophenol (PNP). The experiments showed that the raw clays can adsorb FL but are not efficient for PNP. To demonstrate the photocatalytic property given by the added semiconductors, photocatalytic experiments were performed under UVA light on PNP. These experiments showed degradation up to 90% after 8 h of exposure with the best ZnO-modified clay. The proposed treatment of raw clays seems promising to treat pollutants, especially in developing countries. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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13 pages, 1985 KiB

Open AccessArticle

TiO₂-Acetylacetone as an Efficient Source of Superoxide Radicals under Reduced Power Visible Light: Photocatalytic Degradation of Chlorophenol and Tetracycline

by Lucas A. Almeida, Anja Dosen, Juliana Viol and Bojan A. Marinkovic

Catalysts 2022, 12(2), 116; https://doi.org/10.3390/catal12020116 - 19 Jan 2022

Cited by 12 | Viewed by 2836

Abstract

Visible light-sensitive TiO₂-based nanomaterials are widely investigated for photocatalytic applications under high power (≥300 W) UV and visible light. The formation of charge transfer complexes (CTCs) between bidentate ligands and nanocrystalline TiO₂ promotes visible light absorption and constitutes a promising [...] Read more.

Visible light-sensitive TiO₂-based nanomaterials are widely investigated for photocatalytic applications under high power (≥300 W) UV and visible light. The formation of charge transfer complexes (CTCs) between bidentate ligands and nanocrystalline TiO₂ promotes visible light absorption and constitutes a promising alternative for environmental remediation under reduced visible light power. However, the efficiency of photodegradation, the volatilization profile of bidentates, and the role of reactive oxidizing species (ROS) are not fully understood. In this study, thermogravimetric analyses coupled with mass spectroscopy (TGA-MS) were performed on TiO₂-Acetylacetone (ACAC) CTC. TiO₂-ACAC CTC calcined at 300 °C (TiO₂-A300) was applied for the photocatalytic degradation of chlorophenol (4-CP) and tetracycline (TC) under low power visible light (26 W). Furthermore, the ROS scavengers isopropanol and benzoquinone were added for studying the photocatalytic role of •OH and ^•O₂⁻ radicals. The TGA-MS showed the release of ACAC fragments, such as ethyl ions and acetone, in the range between 150 °C and 265 °C, while between 300 °C and 450 °C only CO₂ and H₂O were released during oxidation of ACAC. The photocatalytic abatement of tetracycline (68.6%), performed by TiO₂-A300, was ~two times higher than that observed for chlorophenol (31.3%) after 6 h, indicating a distinct participation of ROS in the degradation of these pollutants. The addition of the ROS scavenger revealed ^•O₂⁻ radicals as primarily responsible for the high efficiency of TiO₂-ACAC CTC under reduced visible light. On the other hand, the •OH radicals are not efficiently generated in the CTC. Therefore, the development of heterostructures based on TiO₂-ACAC CTC can increase the generation of ROS through coupling with semiconductors capable of generating •OH under visible light. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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14 pages, 3981 KiB

Open AccessArticle

Investigation of Photocatalysis by Mesoporous Titanium Dioxide Supported on Glass Fibers as an Integrated Technology for Water Remediation

by Cristina De Ceglie, Sudipto Pal, Sapia Murgolo, Antonio Licciulli and Giuseppe Mascolo

Catalysts 2022, 12(1), 41; https://doi.org/10.3390/catal12010041 - 31 Dec 2021

Cited by 10 | Viewed by 2166

Abstract

The photocatalytic efficiency of an innovative UV-light catalyst consisting of a mesoporous TiO₂ coating on glass fibers was investigated for the degradation of pharmaceuticals (PhACs) in wastewater effluents. Photocatalytic activity of the synthesized material was tested, for the first time, on a [...] Read more.

The photocatalytic efficiency of an innovative UV-light catalyst consisting of a mesoporous TiO₂ coating on glass fibers was investigated for the degradation of pharmaceuticals (PhACs) in wastewater effluents. Photocatalytic activity of the synthesized material was tested, for the first time, on a secondary wastewater effluent spiked with nine PhACs and the results were compared with the photolysis used as a benchmark treatment. Replicate experiments were performed in a flow reactor equipped with a UV radiation source emitting at 254 nm. Interestingly, the novel photocatalyst led to the increase of the degradation of carbamazepine and trimethoprim (about 2.2 times faster than the photolysis). Several transformation products (TPs) resulting from both the spiked PhACs and the compounds naturally occurring in the secondary wastewater effluent were identified through UPLC-QTOF/MS/MS. Some of them, produced mainly from carbamazepine and trimethoprim, were still present at the end of the photolytic treatment, while they were completely or partially removed by the photocatalytic treatment. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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15 pages, 3774 KiB

Open AccessArticle

Phytochemical-Assisted Green Synthesis of Nickel Oxide Nanoparticles for Application as Electrocatalysts in Oxygen Evolution Reaction

by Vidhya Selvanathan, M. Shahinuzzaman, Shankary Selvanathan, Dilip Kumar Sarkar, Norah Algethami, Hend I. Alkhammash, Farah Hannan Anuar, Zalita Zainuddin, Mohammod Aminuzzaman, Huda Abdullah and Md. Akhtaruzzaman

Catalysts 2021, 11(12), 1523; https://doi.org/10.3390/catal11121523 - 15 Dec 2021

Cited by 34 | Viewed by 5714

Abstract

Electrocatalytic water splitting is a promising solution to resolve the global energy crisis. Tuning the morphology and particle size is a crucial aspect in designing a highly efficient nanomaterials-based electrocatalyst for water splitting. Herein, green synthesis of nickel oxide nanoparticles using phytochemicals from [...] Read more.

Electrocatalytic water splitting is a promising solution to resolve the global energy crisis. Tuning the morphology and particle size is a crucial aspect in designing a highly efficient nanomaterials-based electrocatalyst for water splitting. Herein, green synthesis of nickel oxide nanoparticles using phytochemicals from three different sources was employed to synthesize nickel oxide nanoparticles (NiO_x NPs). Nickel (II) acetate tetrahydrate was reacted in presence of aloe vera leaves extract, papaya peel extract and dragon fruit peel extract, respectively, and the physicochemical properties of the biosynthesized NPs were compared to sodium hydroxide (NaOH)-mediated NiO_x. Based on the average particle size calculation from Scherrer’s equation, using X-ray diffractograms and field-emission scanning electron microscope analysis revealed that all three biosynthesized NiO_x NPs have smaller particle size than that synthesized using the base. Aloe-vera-mediated NiO_x NPs exhibited the best electrocatalytic performance with an overpotential of 413 mV at 10 mA cm⁻² and a Tafel slope of 95 mV dec⁻¹. Electrochemical surface area (ECSA) measurement and electrochemical impedance spectroscopic analysis verified that the high surface area, efficient charge-transfer kinetics and higher conductivity of aloe-vera-mediated NiO_x NPs contribute to its low overpotential values. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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12 pages, 3217 KiB

Open AccessArticle

Ceramized Fabrics and Their Integration in a Semi-Pilot Plant for the Photodegradation of Water Pollutants

by Lara Faccani, Simona Ortelli, Magda Blosi and Anna Luisa Costa

Catalysts 2021, 11(11), 1418; https://doi.org/10.3390/catal11111418 - 22 Nov 2021

Cited by 6 | Viewed by 2256

Abstract

The use of nano-photocatalysts for the water/wastewater purifications, particularly in developing regions, offers promising advantages over conventional technologies. TiO₂-based photocatalysts deposited on fabrics represent an efficient solution for obtaining heterogeneous photocatalysts, which are easily adaptable in the already installed water treatment [...] Read more.

The use of nano-photocatalysts for the water/wastewater purifications, particularly in developing regions, offers promising advantages over conventional technologies. TiO₂-based photocatalysts deposited on fabrics represent an efficient solution for obtaining heterogeneous photocatalysts, which are easily adaptable in the already installed water treatment plants or air purification systems. Despite the huge effort spent to develop and characterize novel nano-photocatalysts, which are especially active under solar light, knowledge gaps still persist for their full-scale application, starting from the reactor design and scale-up and the evaluation of the photocatalytic efficiency in pre-pilot scenarios. In this study, we offered easily scalable solutions for adapting TiO₂-based photocatalysts, which are deposited on different kinds of fabrics and implemented in a 6 L semi-pilot plant, using the photodegradation of Rhodamine B (RhB) as a model of water pollution. We took advantage of a multi-variable optimization approach to identify the best design options in terms of photodegradation efficiency and turnover frequency (TOF). Surprisingly, in the condition of use, the irradiation with a light-emitting diode (LED) visible lamp appeared as a valid alternative to the use of UV LED. The identification of the best design options in the semi-pilot plant allowed scaling up the technology in a 100 L pilot plant suitable for the treatment of industrial wastewater. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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19 pages, 47335 KiB

Open AccessFeature PaperArticle

Innovative Ag–TiO₂ Nanofibers with Excellent Photocatalytic and Antibacterial Actions

by Petronela Pascariu, Corneliu Cojocaru, Anton Airinei, Niculae Olaru, Irina Rosca, Emmanouel Koudoumas and Mirela Petruta Suchea

Catalysts 2021, 11(10), 1234; https://doi.org/10.3390/catal11101234 - 13 Oct 2021

Cited by 22 | Viewed by 3341

Abstract

Ag–TiO₂ nanostructures were prepared by electrospinning, followed by calcination at 400 °C, and their photocatalytic and antibacterial actions were studied. Morphological characterization revealed the presence of one-dimensional uniform Ag–TiO₂ nanostructured nanofibers, with a diameter from 65 to 100 nm, depending on [...] Read more.

Ag–TiO₂ nanostructures were prepared by electrospinning, followed by calcination at 400 °C, and their photocatalytic and antibacterial actions were studied. Morphological characterization revealed the presence of one-dimensional uniform Ag–TiO₂ nanostructured nanofibers, with a diameter from 65 to 100 nm, depending on the Ag loading, composed of small crystals interconnected with each other. Structural characterization indicated that Ag was successfully integrated as small nanocrystals without affecting much of the TiO₂ crystal lattice. Moreover, the presence of nano Ag was found to contribute to reducing the band gap energy, which enables the activation by the absorption of visible light, while, at the same time, it delays the electron–hole recombination. Tests of their photocatalytic activity in methylene blue, amaranth, Congo red and orange II degradation revealed an increase by more than 20% in color removal efficiency at an almost double rate for the case of 0.1% Ag–TiO₂ nanofibers with respect to pure TiO₂. Moreover, the minimum inhibitory concentration was found as low as 2.5 mg/mL for E. coli and 5 mg/mL against S. aureus for the 5% Ag–TiO₂ nanofibers. In general, the Ag–TiO₂ nanostructured nanofibers were found to exhibit excellent structure and physical properties and to be suitable for efficient photocatalytic and antibacterial uses. Therefore, these can be suitable for further integration in various important applications. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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13 pages, 1745 KiB

Open AccessFeature PaperArticle

Photocatalytic Degradation of Thiacloprid Using Tri-Doped TiO₂ Photocatalysts: A Preliminary Comparative Study

by Antonietta Mancuso, Wanda Navarra, Olga Sacco, Stefania Pragliola, Vincenzo Vaiano and Vincenzo Venditto

Catalysts 2021, 11(8), 927; https://doi.org/10.3390/catal11080927 - 30 Jul 2021

Cited by 15 | Viewed by 2772

Abstract

Different tri-doped TiO₂ photocatalysts (Fe-N-P/TiO₂, Fe-N-S/TiO₂, Fe-Pr-N/TiO₂, Pr-N-S/TiO₂, and P-N-S/TiO₂) were successfully prepared and tested in the photocatalytic removal of thiacloprid (THI) under UV-A, visible, and direct solar light irradiation. The physical-chemical [...] Read more.

Different tri-doped TiO₂ photocatalysts (Fe-N-P/TiO₂, Fe-N-S/TiO₂, Fe-Pr-N/TiO₂, Pr-N-S/TiO₂, and P-N-S/TiO₂) were successfully prepared and tested in the photocatalytic removal of thiacloprid (THI) under UV-A, visible, and direct solar light irradiation. The physical-chemical properties of the prepared catalysts were analyzed by different characterization techniques, revealing that dopants are effectively incorporated into the anatase TiO₂ lattice, resulting in a decrease of the energy band gap. The reduction of photoluminescence intensity indicates a lower combination rate and longer lifespan of photogenerated carriers of all doped samples in comparison with the un-doped TiO₂. The doped photocatalysts not only significantly promote the photodegradation under UV-A light irradiation but also extend the optical response of TiO₂ to visible light region, and consequently improve the visible light degradation of THI. Fe-N-P tri-doped TiO₂ sample exhibits the highest THI photodegradation degree (64% under UV-A light, 29% under visible light and 73% under solar light). Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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13 pages, 2239 KiB

Open AccessArticle

Cytostatic Drug 6-Mercaptopurine Degradation on Pilot Scale Reactors by Advanced Oxidation Processes: UV-C/H₂O₂ and UV-C/TiO₂/H₂O₂ Kinetics

by Luis A. González-Burciaga, Juan C. García-Prieto, Manuel García-Roig, Ismael Lares-Asef, Cynthia M. Núñez-Núñez and José B. Proal-Nájera

Catalysts 2021, 11(5), 567; https://doi.org/10.3390/catal11050567 - 29 Apr 2021

Cited by 9 | Viewed by 2575

Abstract

6-Mercaptopurine (6-MP) is a commonly used cytostatic agent, which represents a particular hazard for the environment because of its low biodegradability. In order to degrade 6-MP, four processes were applied: Photolysis (UV-C), photocatalysis (UV-C/TiO₂), and their combination with H₂O [...] Read more.

6-Mercaptopurine (6-MP) is a commonly used cytostatic agent, which represents a particular hazard for the environment because of its low biodegradability. In order to degrade 6-MP, four processes were applied: Photolysis (UV-C), photocatalysis (UV-C/TiO₂), and their combination with H₂O₂, by adding 3 mM H₂O₂/L (UV-C/H₂O₂ and UV-C/TiO₂/H₂O₂ processes). Each process was performed with variable initial pH (3.5, 7.0, and 9.5). Pilot scale reactors were used, using UV-C lamps as radiation source. Kinetic calculations for the first 20 min of reaction show that H₂O₂ addition is of great importance: in UV-C experiments, highest k was reached under pH 3.5, k = 0.0094 min⁻¹, while under UV-C/H₂O₂, k = 0.1071 min⁻¹ was reached under the same initial pH; similar behavior was observed for photocatalysis, as k values of 0.0335 and 0.1387 min⁻¹ were calculated for UV-C/TiO₂ and UV-C/TiO₂/H₂O₂ processes, respectively, also under acidic conditions. Degradation percentages here reported for UV-C/H₂O₂ and UV-C/TiO₂/H₂O₂ processes are above 90% for all tested pH values. Ecotoxicity analysis of samples taken at 60 min in the photolysis and photocatalysis processes, suggests that contaminant degradation by-products present higher toxicity than the original compound. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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15 pages, 4162 KiB

Open AccessArticle

Comparison of Advanced Oxidation Processes for the Degradation of Maprotiline in Water—Kinetics, Degradation Products and Potential Ecotoxicity

by Nuno P. F Gonçalves, Zsuzsanna Varga, Edith Nicol, Paola Calza and Stéphane Bouchonnet

Catalysts 2021, 11(2), 240; https://doi.org/10.3390/catal11020240 - 11 Feb 2021

Cited by 9 | Viewed by 2560

Abstract

The impact of different oxidation processes on the maprotiline degradation pathways was investigated by liquid chromatography-high resolution mass spectrometry (LC/HRMS) experiments. The in-house SPIX software was used to process HRMS data allowing to ensure the potential singular species formed. Semiconductors photocatalysts, namely Fe-ZnO, [...] Read more.

The impact of different oxidation processes on the maprotiline degradation pathways was investigated by liquid chromatography-high resolution mass spectrometry (LC/HRMS) experiments. The in-house SPIX software was used to process HRMS data allowing to ensure the potential singular species formed. Semiconductors photocatalysts, namely Fe-ZnO, Ce-ZnO and TiO₂, proved to be more efficient than heterogeneous photo-Fenton processes in the presence of hydrogen peroxide and persulfate. No significant differences were observed in the degradation pathways in the presence of photocatalysis, while the SO₄⁻ mediated process promote the formation of different transformation products (TPs). Species resulting from ring-openings were observed with higher persistence in the presence of SO₄⁻. In-silico tests on mutagenicity, developmental/reproductive toxicity, Fathead minnow LC₅₀, D. magna LC₅₀, fish acute LC₅₀ were carried out to estimate the toxicity of the identified transformation products. Low toxicant properties were estimated for TPs resulting from hydroxylation onto bridge rather than onto aromatic rings, as well as those resulting from the ring-opening. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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21 pages, 5539 KiB

Open AccessFeature PaperArticle

Green Synthesis of N/Zr Co-Doped TiO₂ for Photocatalytic Degradation of p-Nitrophenol in Wastewater

by Hayette Benkhennouche-Bouchene, Julien G. Mahy, Cédric Wolfs, Bénédicte Vertruyen, Dirk Poelman, Pierre Eloy, Sophie Hermans, Mekki Bouhali, Abdelhafid Souici, Saliha Bourouina-Bacha and Stéphanie D. Lambert

Catalysts 2021, 11(2), 235; https://doi.org/10.3390/catal11020235 - 10 Feb 2021

Cited by 13 | Viewed by 3535

Abstract

TiO₂ prepared by a green aqueous sol–gel peptization process is co-doped with nitrogen and zirconium to improve and extend its photoactivity to the visible region. Two nitrogen precursors are used: urea and triethylamine; zirconium (IV) tert-butoxide is added as a source of [...] Read more.

TiO₂ prepared by a green aqueous sol–gel peptization process is co-doped with nitrogen and zirconium to improve and extend its photoactivity to the visible region. Two nitrogen precursors are used: urea and triethylamine; zirconium (IV) tert-butoxide is added as a source of zirconia. The N/Ti molar ratio is fixed regardless of the chosen nitrogen precursor while the quantity of zirconia is set to 0.7, 1.4, 2, or 2.8 mol%. The performance and physico-chemical properties of these materials are compared with the commercial Evonik P25 photocatalyst. For all doped and co-doped samples, TiO₂ nanoparticles of 4 to 8 nm of size are formed of anatase-brookite phases, with a specific surface area between 125 and 280 m² g⁻¹ vs. 50 m² g⁻¹ for the commercial P25 photocatalyst. X-ray photoelectron (XPS) measurements show that nitrogen is incorporated into the TiO₂ materials through Ti-O-N bonds allowing light absorption in the visible region. The XPS spectra of the Zr-(co)doped powders show the presence of TiO₂-ZrO₂ mixed oxide materials. Under visible light, the best co-doped sample gives a degradation of p-nitrophenol (PNP) equal to 70% instead of 25% with pure TiO₂ and 10% with P25 under the same conditions. Similarly, the photocatalytic activity improved under UV/visible reaching 95% with the best sample compared to 50% with pure TiO₂. This study suggests that N/Zr co-doped TiO₂ nanoparticles can be produced in a safe and energy-efficient way while being markedly more active than state-of-the-art photocatalytic materials under visible light. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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17 pages, 1268 KiB

Open AccessArticle

Assessing an Integral Treatment for Landfill Leachate Reverse Osmosis Concentrate

by Javier Tejera, Daphne Hermosilla, Ruben Miranda, Antonio Gascó, Víctor Alonso, Carlos Negro and Ángeles Blanco

Catalysts 2020, 10(12), 1389; https://doi.org/10.3390/catal10121389 - 28 Nov 2020

Cited by 15 | Viewed by 2577

Abstract

An integral treatment process for landfill leachate reverse osmosis concentrate (LLROC) is herein designed and assessed aiming to reduce organic matter content and conductivity, as well as to increase its biodegradability. The process consists of three steps. The first one is a coagulation/flocculation [...] Read more.

An integral treatment process for landfill leachate reverse osmosis concentrate (LLROC) is herein designed and assessed aiming to reduce organic matter content and conductivity, as well as to increase its biodegradability. The process consists of three steps. The first one is a coagulation/flocculation treatment, which best results were obtained using a dosage of 5 g L⁻¹ of ferric chloride at an initial pH = 6 (removal of the 76% chemical oxygen demand (COD), 57% specific ultraviolet absorption (SUVA), and 92% color). The second step is a photo-Fenton process, which resulted in an enhanced biodegradability (i.e., the ratio between the biochemical oxygen demand (BOD₅) and the COD increased from 0.06 to 0.4), and an extra 43% of the COD was removed at the best trialed reaction conditions of [H₂O₂]/COD = 1.06, pH = 4 and [H₂O₂]/[Fe]_mol = 45. An ultra violet-A light emitting diode (UVA-LED) lamp was tested and compared to conventional high-pressure mercury vapor lamps, achieving a 16% power consumption reduction. Finally, an optimized 30 g L⁻¹ lime treatment was implemented, which reduced conductivity by a 43%, and the contents of sulfate, total nitrogen, chloride, and metals by 90%. Overall, the integral treatment of LLROC achieved the removal of 99.9% color, 90% COD, 90% sulfate, 90% nitrogen, 86% Al, 77% Zn, 84% Mn, 99% Mg, and 98% Si; and significantly increased biodegradability up to BOD₅/COD = 0.4. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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11 pages, 2759 KiB

Open AccessArticle

Synergistic Effect on Photocatalytic Activity of Co-Doped NiTiO₃/g-C₃N₄ Composites under Visible Light Irradiation

by Duc Quang Dao, Thi Kim Anh Nguyen, Thanh-Truc Pham and Eun Woo Shin

Catalysts 2020, 10(11), 1332; https://doi.org/10.3390/catal10111332 - 16 Nov 2020

Cited by 11 | Viewed by 3942

Abstract

Co-doped NiTiO₃/g-C₃N₄ composite photocatalysts were prepared by a modified Pechini method to improve their photocatalytic activity toward methylene blue photodegradation under visible light irradiation. The combination of Co-doped NiTiO₃ and g-C₃N₄ and Co-doping into [...] Read more.

Co-doped NiTiO₃/g-C₃N₄ composite photocatalysts were prepared by a modified Pechini method to improve their photocatalytic activity toward methylene blue photodegradation under visible light irradiation. The combination of Co-doped NiTiO₃ and g-C₃N₄ and Co-doping into the NiTiO₃ lattice synergistically enhanced the photocatalytic performance of the composite photocatalysts. X-ray photoelectron spectroscopy results for the Co-doped NiTiO₃/g-C₃N₄ composite photocatalysts confirmed Ti-N linkages between the Co-doped NiTiO₃ and g-C₃N₄. In addition, characteristic X-ray diffraction peaks for the NiTiO₃ lattice structure clearly indicated substitution of Co into the NiTiO₃ lattice structure. The composite structure and Co-doping of the C-x composite photocatalysts (x wt % Co-doped NiTiO₃/g-C₃N₄) not only decreased the emission intensity of the photoluminescence spectra but also the semicircle radius of the Nyquist plot in electrochemical impedance spectroscopy, giving the highest k_app value (7.15 × 10⁻³ min⁻¹) for the C-1 composite photocatalyst. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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15 pages, 6345 KiB

Open AccessArticle

Photocatalytic Reactivity of Carbon–Nitrogen–Sulfur-Doped TiO₂ Upconversion Phosphor Composites

by Seong-Rak Eun, Shielah Mavengere, Bumrae Cho and Jung-Sik Kim

Catalysts 2020, 10(10), 1188; https://doi.org/10.3390/catal10101188 - 15 Oct 2020

Cited by 6 | Viewed by 2567

Abstract

Sol–gel synthesized N-doped and carbon–nitrogen–sulfur (CNS)-doped TiO₂ solutions were deposited on upconversion phosphor using a dip coating method. Scanning electron microscopy (SEM) imaging showed that there was a change in the morphology of TiO₂ coated on NaYF₄:Yb,Er from spherical [...] Read more.

Sol–gel synthesized N-doped and carbon–nitrogen–sulfur (CNS)-doped TiO₂ solutions were deposited on upconversion phosphor using a dip coating method. Scanning electron microscopy (SEM) imaging showed that there was a change in the morphology of TiO₂ coated on NaYF₄:Yb,Er from spherical to nanorods caused by additional urea and thiourea doping reagents. Fourier transform infrared (FTIR) spectroscopy further verified the existence of nitrate–hyponitrite, carboxylate, and SO₄²⁻ because of the doping effect. NaYF₄:Yb,Er composites coated with N- and CNS-doped TiO₂ exhibited a slight shift of UV-Vis spectra towards the visible light region. Photodecomposition of methylene blue (MB) was evaluated under 254 nm germicidal lamps and a 300 W Xe lamp with UV/Vis cut off filters. The photodegradation of toluene was evaluated on TiO₂/NaYF₄:Yb,Er and CNS-doped TiO₂/NaYF₄:Yb,Er samples under UV light illumination. The photocatalytic reactivity with CNS-doped TiO₂/NaYF₄:Yb,Er surpassed that of the undoped TiO₂/NaYF₄:Yb,Er for the MB solution and toluene. Photocatalytic activity is increased by CNS doping of TiO₂, which improves light sensitization as a result of band gap narrowing due to impurity sites. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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15 pages, 3153 KiB

Open AccessFeature PaperArticle

Activation Treatments and SiO₂/Pd Modification of Sol–Gel TiO₂ Photocatalysts for Enhanced Photoactivity under UV Radiation

by Julien G. Mahy, Valériane Sotrez, Ludivine Tasseroul, Sophie Hermans and Stéphanie D. Lambert

Catalysts 2020, 10(10), 1184; https://doi.org/10.3390/catal10101184 - 14 Oct 2020

Cited by 5 | Viewed by 2162

Abstract

The objective of this work is to improve the efficiency of TiO₂ photocatalysts by activation treatments and by modification with palladium nanoparticles and doping with SiO₂. The influence of the additive loading was explored, and two activation treatments were performed: [...] Read more.

The objective of this work is to improve the efficiency of TiO₂ photocatalysts by activation treatments and by modification with palladium nanoparticles and doping with SiO₂. The influence of the additive loading was explored, and two activation treatments were performed: UV exposition and H₂ reduction. TiO₂/SiO₂/Pd photocatalysts were synthesized by an original cogelation method: a modified silicon alkoxide, i.e., [3-(2-aminoethyl)aminopropyl]trimethoxysilane (EDAS), was used to complex the palladium ions, thanks to the ethylenediamine group, while the alkoxide groups reacted with TiO₂ precursors. Pure TiO₂ was also synthesized by the sol–gel process for comparison. X-ray diffraction evidenced that the crystallographic structure of TiO₂ was anatase and that Pd was present, either in its oxidized form after calcination, or in its reduced form after reduction. The specific surface area of the samples varied from 5 to 145 m² g^-1. Transmission electron microscopy allowed us to observe the homogeneous dispersion and nanometric size of Pd particles in the reduced samples. The width of the band gap for pure TiO₂ sample, measured by UV/Visible diffuse reflectance spectroscopy at approximately 3.2 eV, corresponded to that of anatase. The band gap for the TiO₂/SiO₂/Pd composite samples could not be calculated, due to their high absorption in visible range. The photocatalytic activity of the various catalysts was evaluated by the degradation of a methylene blue solution under UV radiation. The results showed that the photocatalytic activity of the catalysts was inversely proportional to the content of silica present in the matrix. A small amount of silica improved the photocatalytic activity, as compared to the pure TiO₂ sample. By contrast, a high amount of silica delayed the crystallization of TiO₂ in its anatase form. The activation treatment under UV had little influence on photocatalytic efficiency. The introduction of Pd species increased the photocatalytic activity of the samples because it allowed for a decrease in the rate of electron–hole recombinations in TiO₂. The reduction treatment improved the activity of photocatalysts, whatever the palladium content, thanks to the reduction of Ti⁴⁺ into Ti³⁺, and the formation of defects in the crystallographic structure of anatase. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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19 pages, 20468 KiB

Open AccessArticle

In-Depth Structural and Optical Analysis of Ce-modified ZnO Nanopowders with Enhanced Photocatalytic Activity Prepared by Microwave-Assisted Hydrothermal Method

by Otman Bazta, Ana Urbieta, Susana Trasobares, Javier Piqueras, Paloma Fernández, Mohammed Addou, Jose Juan Calvino and Ana Belén Hungría

Catalysts 2020, 10(5), 551; https://doi.org/10.3390/catal10050551 - 15 May 2020

Cited by 15 | Viewed by 2922

Abstract

Pure and Ce-modified ZnO nanosheet-like polycrystalline samples were successfully synthesized by a simple and fast microwave-based process and tested as photocatalytic materials in environmental remediation processes. In an attempt to clarify the actual relationships between functionality and atomic scale structure, an in-depth characterization [...] Read more.

Pure and Ce-modified ZnO nanosheet-like polycrystalline samples were successfully synthesized by a simple and fast microwave-based process and tested as photocatalytic materials in environmental remediation processes. In an attempt to clarify the actual relationships between functionality and atomic scale structure, an in-depth characterization study of these materials using a battery of complementary techniques was performed. X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-Ray spectroscopy-scanning transmission electron microscopy (STEM-XEDS), photoluminescence spectroscopy (PL) and UV–Visible absorption spectroscopy were used to evaluate the effect of Ce ions on the structural, morphological, optical and photocatalytic properties of the prepared ZnO nanostructures. The XRD results showed that the obtained photocatalysts were composed of hexagonal, wurtzite type crystallites in the 34–44 nm size range. The SEM and TEM showed nanosheet-shaped crystallites, a significant fraction of them in contact with bundles of randomly oriented and much smaller nanoparticles of a mixed cerium–zinc phase with a composition close to Ce_0.68Zn_0.32O_x. Importantly, in clear contrast to the prevailing proposals regarding this type of materials, the STEM-XEDS characterization of the photocatalyst samples revealed that Ce did not incorporate into the ZnO crystal lattice as a dopant but that a heterojunction formed between the ZnO nanosheets and the Ce–Zn mixed oxide phase nanoparticles instead. These two relevant compositional features could in fact be established thanks to the particular morphology obtained by the use of the microwave-assisted hydrothermal synthesis. The optical study revealed that in the ZnO:Ce samples optical band gap was found to decrease to 3.17 eV in the samples with the highest Ce content. It was also found that the ZnO:Ce (2 at.%) sample exhibited the highest photocatalytic activity for the degradation of methylene blue (MB), when compared to both the pure ZnO and commercial TiO₂-P25 under simulated sunlight irradiation. The kinetics of MB photodegradation in the presence of the different photocatalysts could be properly described using a Langmuir–Hinshelwood (LH) model, for which the ZnO:Ce (2 at.%) sample exhibited the highest value of effective kinetic constant. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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29 pages, 5906 KiB

Open AccessFeature PaperReview

Eco-Friendly Colloidal Aqueous Sol-Gel Process for TiO₂ Synthesis: The Peptization Method to Obtain Crystalline and Photoactive Materials at Low Temperature

by Julien G. Mahy, Louise Lejeune, Tommy Haynes, Stéphanie D. Lambert, Raphael Henrique Marques Marcilli, Charles-André Fustin and Sophie Hermans

Catalysts 2021, 11(7), 768; https://doi.org/10.3390/catal11070768 - 24 Jun 2021

Cited by 23 | Viewed by 5312

Abstract

This work reviews an eco-friendly process for producing TiO₂ via colloidal aqueous sol–gel synthesis, resulting in crystalline materials without a calcination step. Three types of colloidal aqueous TiO₂ are reviewed: the as-synthesized type obtained directly after synthesis, without any specific treatment; [...] Read more.

This work reviews an eco-friendly process for producing TiO₂ via colloidal aqueous sol–gel synthesis, resulting in crystalline materials without a calcination step. Three types of colloidal aqueous TiO₂ are reviewed: the as-synthesized type obtained directly after synthesis, without any specific treatment; the calcined, obtained after a subsequent calcination step; and the hydrothermal, obtained after a specific autoclave treatment. This eco-friendly process is based on the hydrolysis of a Ti precursor in excess of water, followed by the peptization of the precipitated TiO₂. Compared to classical TiO₂ synthesis, this method results in crystalline TiO₂ nanoparticles without any thermal treatment and uses only small amounts of organic chemicals. Depending on the synthesis parameters, the three crystalline phases of TiO₂ (anatase, brookite, and rutile) can be obtained. The morphology of the nanoparticles can also be tailored by the synthesis parameters. The most important parameter is the peptizing agent. Indeed, depending on its acidic or basic character and also on its amount, it can modulate the crystallinity and morphology of TiO₂. Colloidal aqueous TiO₂ photocatalysts are mainly being used in various photocatalytic reactions for organic pollutant degradation. The as-synthesized materials seem to have equivalent photocatalytic efficiency to the photocatalysts post-treated with thermal treatments and the commercial Evonik Aeroxide P25, which is produced by a high-temperature process. Indeed, as-prepared, the TiO₂ photocatalysts present a high specific surface area and crystalline phases. Emerging applications are also referenced, such as elaborating catalysts for fuel cells, nanocomposite drug delivery systems, or the inkjet printing of microstructures. Only a few works have explored these new properties, giving a lot of potential avenues for studying this eco-friendly TiO₂ synthesis method for innovative implementations. Full article

(This article belongs to the Special Issue Heterogeneous Photocatalysis: A Solution for a Greener Earth)

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