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14 pages, 900 KiB

Open AccessArticle

Heterogeneous Catalysis by Tetraethylammonium Tetrachloroferrate of the Photooxidation of Toluene by Visible and Near-UV Light

by Kelsie R. Barnard, Valerie R. Bright, Robert J. Enright, Kira M. Fahy, Adam C. Liu and Patrick E. Hoggard

Catalysts 2018, 8(2), 79; https://doi.org/10.3390/catal8020079 - 13 Feb 2018

Cited by 12 | Viewed by 4210

Abstract

Titanium dioxide is the most extensively used heterogeneous catalyst for the photooxidation of toluene and other hydrocarbons, but it has low utility for the synthesis of benzyl alcohol, of which little is produced, or benzaldehyde, due to further oxidation to benzoic acid and [...] Read more.

Titanium dioxide is the most extensively used heterogeneous catalyst for the photooxidation of toluene and other hydrocarbons, but it has low utility for the synthesis of benzyl alcohol, of which little is produced, or benzaldehyde, due to further oxidation to benzoic acid and cresol, among other oxidation products, and eventually complete mineralization to CO₂. Et₄N[FeCl₄] functions as a photocatalyst through the dissociation of chlorine atoms, which abstract hydrogen from toluene, and the photooxidation of toluene proceeds only as far as benzyl alcohol and benzaldehyde. Unlike TiO₂, which requires ultraviolet (UV) irradiation, Et₄N[FeCl₄] catalyzes the photooxidation of toluene with visible light alone. Even under predominantly UV irradiation, the yield of benzyl alcohol plus benzaldehyde is greater with Et₄N[FeCl₄] than with TiO₂. Et₄N[FeCl₄] photocatalysis yields benzyl chloride as a side product, but it can be minimized by restricting irradiation to wavelengths above 360 nm and by the use of long irradiation times. The photonic efficiency of oxidation in one experiment was found to be 0.042 mol/einstein at 365 nm. The use of sunlight as the irradiation source was explored. Full article

(This article belongs to the Special Issue Photocatalytic materials alternative to TiO₂ for environmental remediation, sustainable chemistry and energy conversion)

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16 pages, 5115 KiB

Open AccessArticle

Relations between Structure, Activity and Stability in C₃N₄ Based Photocatalysts Used for Solar Hydrogen Production

by Ramesh P. Sivasankaran, Nils Rockstroh, Dirk Hollmann, Carsten R. Kreyenschulte, Giovanni Agostini, Henrik Lund, Amitava Acharjya, Jabor Rabeah, Ursula Bentrup, Henrik Junge, Arne Thomas and Angelika Brückner

Catalysts 2018, 8(2), 52; https://doi.org/10.3390/catal8020052 - 29 Jan 2018

Cited by 11 | Viewed by 5561

Abstract

Solar hydrogen production from water could be a sustainable and environmentally friendly alternative to fossil energy carriers, yet so far photocatalysts active and stable enough for large-scale applications are not available, calling for advanced research efforts. In this work, H₂ evolution rates [...] Read more.

Solar hydrogen production from water could be a sustainable and environmentally friendly alternative to fossil energy carriers, yet so far photocatalysts active and stable enough for large-scale applications are not available, calling for advanced research efforts. In this work, H₂ evolution rates of up to 1968 and 5188 μmol h⁻¹ g⁻¹ were obtained from aqueous solutions of triethanolamine (TEOA) and oxalic acid (OA), respectively, by irradiating composites of AgIn₅S₈ (AIS), mesoporous C₃N₄ (CN, surface area >150 m²/g) and ≤2 wt.% in-situ photodeposited Pt nanoparticles (NPs) with UV-vis (≥300 nm) and pure visible light (≥420 nm). Structural properties and electron transport in these materials were analyzed by XRD, STEM-HAADF, XPS, UV-vis-DRS, ATR-IR, photoluminescence and in situ-EPR spectroscopy. Initial H₂ formation rates were highest for Pt/CN, yet with TEOA this catalyst deactivated by inclusion of Pt NPs in the matrix of CN (most pronounced at λ ≥ 300 nm) while it remained active with OA, since in this case Pt NPs were enriched on the outermost surface of CN. In Pt/AIS-CN catalysts, Pt NPs were preferentially deposited on the surface of the AIS phase which prevents them from inclusion in the CN phase but reduces simultaneously the initial H₂ evolution rate. This suggests that AIS hinders transport of separated electrons from the CN conduction band to Pt NPs but retains the latter accessible by protons to produce H₂. Full article

(This article belongs to the Special Issue Photocatalytic materials alternative to TiO₂ for environmental remediation, sustainable chemistry and energy conversion)

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14 pages, 1644 KiB

Open AccessArticle

EPR Investigations of G-C₃N₄/TiO₂ Nanocomposites

by Dana Dvoranová, Milan Mazúr, Ilias Papailias, Tatiana Giannakopoulou, Christos Trapalis and Vlasta Brezová

Catalysts 2018, 8(2), 47; https://doi.org/10.3390/catal8020047 - 26 Jan 2018

Cited by 37 | Viewed by 8366

Abstract

The g-C₃N₄/TiO₂ nanopowders prepared by the annealing of melamine and TiO₂ P25 at 550 °C were investigated under dark and upon UV or visible-light photoactivation using X- and Q-band electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra [...] Read more.

The g-C₃N₄/TiO₂ nanopowders prepared by the annealing of melamine and TiO₂ P25 at 550 °C were investigated under dark and upon UV or visible-light photoactivation using X- and Q-band electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra of powders monitored at room temperature and 100 K showed the impact of the initial loading ratio of melamine/TiO₂ on the character of paramagnetic centers observed. For the photocatalysts synthesized using a lower titania content, the paramagnetic signals characteristic for the g-C₃N₄/TiO₂ nanocomposites were already found before exposure. The samples annealed using the higher TiO₂ loading revealed the photoinduced generation of paramagnetic nitrogen bulk centers (g-tensor components g₁ = 2.005, g₂ = 2.004, g₃ = 2.003 and hyperfine couplings from the nitrogen A₁ = 0.23 mT, A₂ = 0.44 mT, A₃ = 3.23 mT) typical for N-doped TiO₂. The ability of photocatalysts to generate reactive oxygen species (ROS) upon in situ UV or visible-light photoexcitation was tested in water or dimethyl sulfoxide by EPR spin trapping using 5,5-dimethyl 1-pyrroline N-oxide. The results obtained reflect the differences in photocatalyst nanostructures caused by the differing initial ratio of melamine/TiO₂; the photocatalyst prepared by the high-temperature treatment of melamine/TiO₂ wt. ratio of 1:3 revealed an adequate photoactivity in both spectral regions. Full article

(This article belongs to the Special Issue Photocatalytic materials alternative to TiO₂ for environmental remediation, sustainable chemistry and energy conversion)

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2742 KiB

Open AccessArticle

Synthesis of Phase Pure Hexagonal YFeO₃ Perovskite as Efficient Visible Light Active Photocatalyst

by Mohammed Ismael, Engy Elhaddad, Dereje H. Taffa and Michael Wark

Catalysts 2017, 7(11), 326; https://doi.org/10.3390/catal7110326 - 03 Nov 2017

Cited by 56 | Viewed by 8022

Abstract

Hexagonal perovskite YFeO₃ was synthesized by a complex-assisted sol-gel technique allowing crystallization at calcination temperatures below 700 °C. As determined by diffuse reflectance spectroscopy (DRS) and Tauc plots, the hexagonal YFeO₃ exhibits a lower optical band gap (1.81 eV) than the [...] Read more.

Hexagonal perovskite YFeO₃ was synthesized by a complex-assisted sol-gel technique allowing crystallization at calcination temperatures below 700 °C. As determined by diffuse reflectance spectroscopy (DRS) and Tauc plots, the hexagonal YFeO₃ exhibits a lower optical band gap (1.81 eV) than the orthorhombic structure (about 2.1 eV or even higher) being typically obtained at elevated temperatures (>700 °C), and thus enables higher visible light photocatalysis activity. Structure and morphology of the synthesized YFeO₃ perovskites were analyzed by powder X-ray diffraction (XRD) and nitrogen adsorption, proving that significantly smaller crystallite sizes and higher surface areas are obtained for YFeO₃ with a hexagonal phase. The photocatalytic activity of the different YFeO₃ phases was deduced via the degradation of the model pollutants methyl orange and 4-chlorophenol. Experiments under illumination with light of different wavelengths, in the presence of different trapping elements, as well as photoelectrochemical tests allow conclusions regarding band positions of YFeO₃ and the photocatalytic degradation mechanism. X-ray photoelectron spectroscopy indicates that a very thin layer of Y₂O₃ might support the photocatalysis by improving the separation of photogenerated charge carriers. Full article

(This article belongs to the Special Issue Photocatalytic materials alternative to TiO₂ for environmental remediation, sustainable chemistry and energy conversion)

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8983 KiB

Open AccessArticle

The Preparation of a Highly Efficient Ag₃PO₄/Ag/Bi₂O₂CO₃ Photo-Catalyst and the Study of Its Photo-Catalytic Organic Synthesis Reaction Driven by Visible Light

by Zhi Guo, Hui Xin, Jingjing Ma, Meifen Bai, Yan Wang and Jingyi Li

Catalysts 2017, 7(9), 276; https://doi.org/10.3390/catal7090276 - 17 Sep 2017

Cited by 10 | Viewed by 5412

Abstract

Ag₃PO₄/Ag/Bi₂O₂CO₃ composites were prepared by a hydrothermal and precipitation method. The morphology, structure, and valence state of the photo-catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Scanning [...] Read more.

Ag₃PO₄/Ag/Bi₂O₂CO₃ composites were prepared by a hydrothermal and precipitation method. The morphology, structure, and valence state of the photo-catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) specific surface areas, and UV-vis diffuse reflectance spectra (UV-vis DRS). They were applied as heterogeneous catalysts in the synthesis of esters from aldehydes (or alcohols) and alcohols and the synthesis of imines from alcohols and amines under visible light irradiation. The photo-catalytic activities of the esterification reactions of aldehydes and alcohols were heavily dependent on the loading of Ag₃PO₄/Ag/Bi₂O₂CO₃ as well as the intensity and wavelength of the visible light. Furthermore, their conversion under visible light irradiation was superior to that in the dark. Herein a reaction mechanism from aldehydes and alcohols to esters was proposed, and the Ag₃PO₄/Ag/Bi₂O₂CO₃ catalysts could be used six times without a significant decrease in activity. Using these catalysts under visible light could motivate future studies to develop efficient recyclable photo-catalysts and facilitate many synthetic organic reactions. Full article

(This article belongs to the Special Issue Photocatalytic materials alternative to TiO₂ for environmental remediation, sustainable chemistry and energy conversion)

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6796 KiB

Open AccessArticle

Structure-Dependent Photocatalytic Performance of BiOBr_xI_1−x Nanoplate Solid Solutions

by Huan-Yan Xu, Xu Han, Qu Tan, Xiu-Lan He and Shu-Yan Qi

Catalysts 2017, 7(5), 153; https://doi.org/10.3390/catal7050153 - 13 May 2017

Cited by 18 | Viewed by 6725

Abstract

BiOX_xY_1−x (X, Y = Cl, Br, and I) solid solutions have been regarded as promising photocatalysts attributed to their unique layered structure, tunable band structure, and chemical and optical stability. In this study, BiOBr_xI_1−x nanoplate [...] Read more.

BiOX_xY_1−x (X, Y = Cl, Br, and I) solid solutions have been regarded as promising photocatalysts attributed to their unique layered structure, tunable band structure, and chemical and optical stability. In this study, BiOBr_xI_1−x nanoplate solid solutions with a high exposure of {001} crystal facets were prepared by a facile alcoholysis method at room temperature and atmospheric pressure. X-ray diffraction (XRD) peaks exhibited a slight shift to lower diffraction angle with the increase of I content in BiOBr_xI_1−x samples, which resulted in a gradual increase in their cell parameters. Field emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM) images revealed that BiOBr_xI_1−x samples exhibited 2D plate-like structure with the in-plane wrinkles. The regular changes in optical absorption threshold and E_g value seen in UV-vis diffuse reflectance spectra (UV-vis DRS) indicated that the optical absorption property and band structure could be modulated by the formation of BiOBr_xI_1−x solid solutions. The photocatalytic degradation of active dye Rhodamine B (RhB) over BiOBr_xI_1−x solid solutions showed that BiOBr_0.75I_0.25 had the best photocatalytic activity. The RhB photodegradation processes followed a pseudo-first-order kinetic model. The synergistic effect of structural factors (including amount of exposed {001} facets, interlayer spacing of (001) plane, and energy-level position of the valence band) determined the photocatalytic performance of BiOBr_xI_1−x solid solutions. Full article

(This article belongs to the Special Issue Photocatalytic materials alternative to TiO₂ for environmental remediation, sustainable chemistry and energy conversion)

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4869 KiB

Open AccessArticle

Treatment of Aqueous Bromate by Superparamagnetic BiOCl-Mediated Advanced Reduction Process

by Xiaowei Liu, Lili Wang, Zhe Sun, Yu Shao and Tingchao Yu

Catalysts 2017, 7(5), 131; https://doi.org/10.3390/catal7050131 - 01 May 2017

Cited by 5 | Viewed by 4439

Abstract

Bromate (

{BrO}_{3}^{-}

) contamination in drinking water is a growing concern. Advanced reduction processes (ARPs) are reportedly promising in relieving this concern. In this work, UV/superparamagnetic BiOCl (BiOCl loaded onto superparamagnetic hydroxyapatite) assisted with small molecule carboxylic acid (formate, citrate, [...] Read more.

Bromate (

{BrO}_{3}^{-}

) contamination in drinking water is a growing concern. Advanced reduction processes (ARPs) are reportedly promising in relieving this concern. In this work, UV/superparamagnetic BiOCl (BiOCl loaded onto superparamagnetic hydroxyapatite) assisted with small molecule carboxylic acid (formate, citrate, and acetate), a carboxyl anion radical (

{CO}_{2}^{• -}

)-based ARP, was proposed to eliminate aqueous

{BrO}_{3}^{-}

. Formate and citrate were found to be ideal

{CO}_{2}^{• -}

precursor, and the latter was found to be safe for practical use.

{BrO}_{3}^{-}

(10 μg·L⁻¹, WHO guideline for drinking water) can be completely degraded within 3 min under oxygen-free conditions. In this process,

{BrO}_{3}^{-}

degradation was realized by the reduction of

{CO}_{2}^{• -}

(major role) and formyloxyl radical (minor role) in bulk solution. The formation mechanism of radicals and the transformation pathway of

{BrO}_{3}^{-}

were proposed based on data on electron paramagnetic resonance monitoring, competitive kinetics, and degradation product analysis. The process provided a sustainable decontamination performance (<5% deterioration for 10 cycles) and appeared to be more resistant to common electron acceptors (O₂,

{NO}_{3}^{-}

, and Fe³⁺) than hydrated electron based-ARPs. Phosphate based-superparamagnetic hydroxyapatite, used to support BiOCl in this work, was believed to be applicable for resolving the recycling problem of other metal-containing catalyst. Full article

(This article belongs to the Special Issue Photocatalytic materials alternative to TiO₂ for environmental remediation, sustainable chemistry and energy conversion)

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Review

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47 pages, 13106 KiB

Open AccessReview

g-C₃N₄-Based Nanomaterials for Visible Light-Driven Photocatalysis

by Santosh Kumar, Sekar Karthikeyan and Adam F. Lee

Catalysts 2018, 8(2), 74; https://doi.org/10.3390/catal8020074 - 09 Feb 2018

Cited by 194 | Viewed by 27324

Abstract

Graphitic carbon nitride (g-C₃N₄) is a promising material for photocatalytic applications such as solar fuels production through CO₂ reduction and water splitting, and environmental remediation through the degradation of organic pollutants. This promise reflects the advantageous photophysical properties [...] Read more.

Graphitic carbon nitride (g-C₃N₄) is a promising material for photocatalytic applications such as solar fuels production through CO₂ reduction and water splitting, and environmental remediation through the degradation of organic pollutants. This promise reflects the advantageous photophysical properties of g-C₃N₄ nanostructures, notably high surface area, quantum efficiency, interfacial charge separation and transport, and ease of modification through either composite formation or the incorporation of desirable surface functionalities. Here, we review recent progress in the synthesis and photocatalytic applications of diverse g-C₃N₄ nanostructured materials, and highlight the physical basis underpinning their performance for each application. Potential new architectures, such as hierarchical or composite g-C₃N₄ nanostructures, that may offer further performance enhancements in solar energy harvesting and conversion are also outlined. Full article

(This article belongs to the Special Issue Photocatalytic materials alternative to TiO₂ for environmental remediation, sustainable chemistry and energy conversion)

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