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19 pages, 2812 KiB

Open AccessArticle

Probing Water and CO₂ Interactions at the Surface of Collapsed Titania Nanotubes Using IR Spectroscopy

by Kaustava Bhattacharyya, Weiqiang Wu, Eric Weitz, Baiju K. Vijayan and Kimberly A. Gray

Molecules 2015, 20(9), 15469-15487; https://doi.org/10.3390/molecules200915469 - 26 Aug 2015

Cited by 19 | Viewed by 7660

Abstract

Collapsed titania nanotubes (cTiNT) were synthesized by the calcination of titania nanotubes (TiNT) at 650 °C, which leads to a collapse of their tubular morphology, a substantial reduction in surface area, and a partial transformation of anatase to the rutile phase. There are [...] Read more.

Collapsed titania nanotubes (cTiNT) were synthesized by the calcination of titania nanotubes (TiNT) at 650 °C, which leads to a collapse of their tubular morphology, a substantial reduction in surface area, and a partial transformation of anatase to the rutile phase. There are no significant changes in the position of the XPS responses for Ti and O on oxidation or reduction of the cTiNTs, but the responses are more symmetric than those observed for TiNTs, indicating fewer surface defects and no change in the oxidation state of titanium on oxidative and/or reductive pretreatment. The interaction of H₂O and CO₂ with the cTiNT surface was studied. The region corresponding to OH stretching absorptions extends below 3000 cm⁻¹, and thus is broader than is typically observed for absorptions of the OH stretches of water. The exchange of protons for deuterons on exposure to D₂O leads to a depletion of this extended absorption and the appearance of new absorptions, which are compatible with deuterium exchange. We discuss the source of this extended low frequency OH stretching region and conclude that it is likely due to the hydrogen-bonded OH stretches. Interaction of the reduced cTiNTs with CO₂ leads to a similar but smaller set of adsorbed carbonates and bicarbonates as reported for reduced TiNTs before collapse. Implications of these observations and the presence of proton sources leading to hydrogen bonding are discussed relative to potential chemical and photochemical activity of the TiNTs. These results point to the critical influence of defect structure on CO₂ photoconversion. Full article

(This article belongs to the Special Issue Photocatalysis)

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20 pages, 1514 KiB

Open AccessArticle

Modeling the Photocatalytic Mineralization in Water of Commercial Formulation of Estrogens 17-β Estradiol (E2) and Nomegestrol Acetate in Contraceptive Pills in a Solar Powered Compound Parabolic Collector

by José Colina-Márquez, Fiderman Machuca-Martínez and Gianluca Li Puma

Molecules 2015, 20(7), 13354-13373; https://doi.org/10.3390/molecules200713354 - 22 Jul 2015

Cited by 21 | Viewed by 6734

Abstract

Endocrine disruptors in water are contaminants of emerging concern due to the potential risks they pose to the environment and to the aquatic ecosystems. In this study, a solar photocatalytic treatment process in a pilot-scale compound parabolic collector (CPC) was used to remove [...] Read more.

Endocrine disruptors in water are contaminants of emerging concern due to the potential risks they pose to the environment and to the aquatic ecosystems. In this study, a solar photocatalytic treatment process in a pilot-scale compound parabolic collector (CPC) was used to remove commercial estradiol formulations (17-β estradiol and nomegestrol acetate) from water. Photolysis alone degraded up to 50% of estradiol and removed 11% of the total organic carbon (TOC). In contrast, solar photocatalysis degraded up to 57% of estrogens and the TOC removal was 31%, with 0.6 g/L of catalyst load (TiO₂ Aeroxide P-25) and 213.6 ppm of TOC as initial concentration of the commercial estradiols formulation. The adsorption of estrogens over the catalyst was insignificant and was modeled by the Langmuir isotherm. The TOC removal via photocatalysis in the photoreactor was modeled considering the reactor fluid-dynamics, the radiation field, the estrogens mass balance, and a modified Langmuir–Hinshelwood rate law, that was expressed in terms of the rate of photon adsorption. The optimum removal of the estrogens and TOC was achieved at a catalyst concentration of 0.4 g/L in 29 mm diameter tubular CPC reactors which approached the optimum catalyst concentration and optical thickness determined from the modeling of the absorption of solar radiation in the CPC, by the six-flux absorption-scattering model (SFM). Full article

(This article belongs to the Special Issue Photocatalysis)

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13 pages, 2857 KiB

Open AccessArticle

Theoretical Verification of Photoelectrochemical Water Oxidation Using Nanocrystalline TiO₂ Electrodes

by Shozo Yanagida, Susumu Yanagisawa, Koichi Yamashita, Ryota Jono and Hiroshi Segawa

Molecules 2015, 20(6), 9732-9744; https://doi.org/10.3390/molecules20069732 - 27 May 2015

Cited by 5 | Viewed by 7350

Abstract

Mesoscopic anatase nanocrystalline TiO₂ (nc-TiO₂) electrodes play effective and efficient catalytic roles in photoelectrochemical (PEC) H₂O oxidation under short circuit energy gap excitation conditions. Interfacial molecular orbital structures of (H₂O)₃&OH(TiO₂)₉H [...] Read more.

Mesoscopic anatase nanocrystalline TiO₂ (nc-TiO₂) electrodes play effective and efficient catalytic roles in photoelectrochemical (PEC) H₂O oxidation under short circuit energy gap excitation conditions. Interfacial molecular orbital structures of (H₂O)₃&OH(TiO₂)₉H as a stationary model under neutral conditions and the radical-cation model of [(H₂O)₃&OH(TiO₂)₉H]⁺ as a working nc-TiO₂ model are simulated employing a cluster model OH(TiO₂)₉H (Yamashita/Jono’s model) and a H₂O cluster model of (H₂O)₃ to examine excellent H₂O oxidation on nc-TiO₂ electrodes in PEC cells. The stationary model, (H₂O)₃&OH(TiO₂)₉H reveals that the model surface provides catalytic H₂O binding sites through hydrogen bonding, van der Waals and Coulombic interactions. The working model, [(H₂O)₃&OH(TiO₂)₉H]⁺ discloses to have a very narrow energy gap (0.3 eV) between HOMO and LUMO potentials, proving that PEC nc-TiO₂ electrodes become conductive at photo-irradiated working conditions. DFT-simulation of stepwise oxidation of a hydroxide ion cluster model of OH⁻(H₂O)₃, proves that successive two-electron oxidation leads to hydroxyl radical clusters, which should give hydrogen peroxide as a precursor of oxygen molecules. Under working bias conditions of PEC cells, nc-TiO₂ electrodes are now verified to become conductive by energy gap photo-excitation and the electrode surface provides powerful oxidizing sites for successive H₂O oxidation to oxygen via hydrogen peroxide. Full article

(This article belongs to the Special Issue Photocatalysis)

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14 pages, 1476 KiB

Open AccessArticle

Selective Reduction of Cr(VI) in Chromium, Copper and Arsenic (CCA) Mixed Waste Streams Using UV/TiO₂ Photocatalysis

by Shan Zheng, Wenjun Jiang, Mamun Rashid, Yong Cai, Dionysios D. Dionysiou and Kevin E. O'Shea

Molecules 2015, 20(2), 2622-2635; https://doi.org/10.3390/molecules20022622 - 3 Feb 2015

Cited by 35 | Viewed by 8019

Abstract

The highly toxic Cr(VI) is a critical component in the Chromated Copper Arsenate (CCA) formulations extensively employed as wood preservatives. Remediation of CCA mixed waste and discarded treated wood products is a significant challenge. We demonstrate that UV/TiO₂ photocatalysis effectively reduces Cr(VI) [...] Read more.

The highly toxic Cr(VI) is a critical component in the Chromated Copper Arsenate (CCA) formulations extensively employed as wood preservatives. Remediation of CCA mixed waste and discarded treated wood products is a significant challenge. We demonstrate that UV/TiO₂ photocatalysis effectively reduces Cr(VI) to less toxic Cr(III) in the presence of arsenate, As(V), and copper, Cu(II). The rapid conversion of Cr(VI) to Cr(III) during UV/TiO₂ photocatalysis occurs over a range of concentrations, solution pH and at different Cr:As:Cu ratios. The reduction follows pseudo-first order kinetics and increases with decreasing solution pH. Saturation of the reaction solution with argon during UV/TiO₂ photocatalysis had no significant effect on the Cr(VI) reduction demonstrating the reduction of Cr(VI) is independent of dissolved oxygen. Reduction of Cu(II) and As(V) does not occur under the photocatalytic conditions employed herein and the presence of these two in the tertiary mixtures had a minimal effect on Cr(VI) reduction. The Cr(VI) reduction was however, significantly enhanced by the addition of formic acid, which can act as a hole scavenger and enhance the reduction processes initiated by the conduction band electron. Our results demonstrate UV/TiO₂ photocatalysis effectively reduces Cr(VI) in mixed waste streams under a variety of conditions. Full article

(This article belongs to the Special Issue Photocatalysis)

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15 pages, 2655 KiB

Open AccessArticle

Preparation of a Titania/X-Zeolite/Porous Glass Composite Photocatalyst Using Hydrothermal and Drop Coating Processes

by Atsuo Yasumori, Sayaka Yanagida and Jun Sawada

Molecules 2015, 20(2), 2349-2363; https://doi.org/10.3390/molecules20022349 - 30 Jan 2015

Cited by 14 | Viewed by 7741

Abstract

Combinations of TiO₂ photocatalysts and various adsorbents have been widely studied for the adsorption and photocatalytic decomposition of gaseous pollutants such as volatile organic compounds (VOCs). Herein, a TiO₂-zeolite-porous glass composite was prepared using melt-quenching and partial sintering, hydrothermal treatment, [...] Read more.

Combinations of TiO₂ photocatalysts and various adsorbents have been widely studied for the adsorption and photocatalytic decomposition of gaseous pollutants such as volatile organic compounds (VOCs). Herein, a TiO₂-zeolite-porous glass composite was prepared using melt-quenching and partial sintering, hydrothermal treatment, and drop coating for preparation of the porous glass support and X-zeolite and their combination with TiO₂, respectively. The obtained composite comprised anatase phase TiO₂, X-zeolite, and the porous glass support, which were combined at the micro to nanometer scales. The composite had a relatively high specific surface area of approximately 25 m²/g and exhibited a good adsorption capacity for 2-propanol. These data indicated that utilization of this particular phase-separated glass as the support was appropriate for the formation of the bulk photocatalyst-adsorbent composite. Importantly, the photocatalytic decomposition of adsorbed 2-propanol proceeded under UV light irradiation. The 2-propanol was oxidized to acetone and then trapped by the X-zeolite rather than being released to the atmosphere. Consequently, it was demonstrated that the micrometer-scaled combination of TiO₂ and zeolite in the bulk form is very useful for achieving both the removal of gaseous organic pollutants and decreasing the emission of harmful intermediates. Full article

(This article belongs to the Special Issue Photocatalysis)

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14 pages, 2738 KiB

Open AccessArticle

Visible Light Induced Green Transformation of Primary Amines to Imines Using a Silicate Supported Anatase Photocatalyst

by Sifani Zavahir and Huaiyong Zhu

Molecules 2015, 20(2), 1941-1954; https://doi.org/10.3390/molecules20021941 - 26 Jan 2015

Cited by 23 | Viewed by 8248

Abstract

Catalytic oxidation of amine to imine is of intense present interest since imines are important intermediates for the synthesis of fine chemicals, pharmaceuticals, and agricultural chemicals. However, considerable efforts have been made to develop efficient methods for the oxidation of secondary amines to [...] Read more.

Catalytic oxidation of amine to imine is of intense present interest since imines are important intermediates for the synthesis of fine chemicals, pharmaceuticals, and agricultural chemicals. However, considerable efforts have been made to develop efficient methods for the oxidation of secondary amines to imines, while little attention has until recently been given to the oxidation of primary amines, presumably owing to the high reactivity of generated imines of primary amines that are easily dehydrogenated to nitriles. Herein, we report the oxidative coupling of a series of primary benzylic amines into corresponding imines with dioxygen as the benign oxidant over composite catalysts of TiO₂ (anatase)-silicate under visible light irradiation of λ > 460 nm. Visible light response of this system is believed to be as a result of high population of defects and contacts between silicate and anatase crystals in the composite and the strong interaction between benzylic amine and the catalyst. It is found that tuning the intensity and wavelength of the light irradiation and the reaction temperature can remarkably enhance the reaction activity. Water can also act as a green medium for the reaction with an excellent selectivity. This report contributes to the use of readily synthesized, environmentally benign, TiO₂ based composite photocatalyst and solar energy to realize the transformation of primary amines to imine compounds. Full article

(This article belongs to the Special Issue Photocatalysis)

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16 pages, 1112 KiB

Open AccessArticle

Energy and Molecules from Photochemical/Photocatalytic Reactions. An Overview

by Davide Ravelli, Stefano Protti and Angelo Albini

Molecules 2015, 20(1), 1527-1542; https://doi.org/10.3390/molecules20011527 - 16 Jan 2015

Cited by 16 | Viewed by 9252

Abstract

Photocatalytic reactions have been defined as those processes that require both a (not consumed) catalyst and light. A previous definition was whether such reactions brought a system towards or away from the (thermal) equilibrium. This consideration brings in the question whether a part [...] Read more.

Photocatalytic reactions have been defined as those processes that require both a (not consumed) catalyst and light. A previous definition was whether such reactions brought a system towards or away from the (thermal) equilibrium. This consideration brings in the question whether a part of the photon energy is incorporated into the photochemical reaction products. Data are provided for representative organic reactions involving or not molecular catalysts and show that energy storage occurs only when a heavily strained structure is generated, and in that case only a minor part of photon energy is actually stored (ΔG up to 25 kcal·mol⁻¹). The green role of photochemistry/photocatalysis is rather that of forming highly reactive intermediates under mild conditions. Full article

(This article belongs to the Special Issue Photocatalysis)

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13 pages, 4971 KiB

Open AccessArticle

TiO₂ and Fe₂O₃ Films for Photoelectrochemical Water Splitting

by Josef Krysa, Martin Zlamal, Stepan Kment, Michaela Brunclikova and Zdenek Hubicka

Molecules 2015, 20(1), 1046-1058; https://doi.org/10.3390/molecules20011046 - 9 Jan 2015

Cited by 78 | Viewed by 13387

Abstract

Titanium oxide (TiO₂) and iron oxide (α-Fe₂O₃) hematite films have potential applications as photoanodes in electrochemical water splitting. In the present work TiO₂ and α-Fe₂O₃ thin films were prepared by two methods, e.g., [...] Read more.

Titanium oxide (TiO₂) and iron oxide (α-Fe₂O₃) hematite films have potential applications as photoanodes in electrochemical water splitting. In the present work TiO₂ and α-Fe₂O₃ thin films were prepared by two methods, e.g., sol-gel and High Power Impulse Magnetron Sputtering (HiPIMS) and judged on the basis of physical properties such as crystalline structure and surface topography and functional properties such as simulated photoelectrochemical (PEC) water splitting conditions. It was revealed that the HiPIMS method already provides crystalline structures of anatase TiO₂ and hematite Fe₂O₃ during the deposition, whereas to finalize the sol-gel route the as-deposited films must always be annealed to obtain the crystalline phase. Regarding the PEC activity, both TiO₂ films show similar photocurrent density, but only when illuminated by UV light. A different situation was observed for hematite films where plasmatic films showed a tenfold enhancement of the stable photocurrent density over the sol-gel hematite films for both UV and visible irradiation. The superior properties of plasmatic films could be explained by ability to address some of the hematite drawbacks by the deposition of very thin films (25 nm) consisting of small densely packed particles and by doping with Sn. Full article

(This article belongs to the Special Issue Photocatalysis)

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16 pages, 783 KiB

Open AccessArticle

A Comparison of the Environmental Impact of Different AOPs: Risk Indexes

by Jaime Giménez, Bernardí Bayarri, Óscar González, Sixto Malato, José Peral and Santiago Esplugas

Molecules 2015, 20(1), 503-518; https://doi.org/10.3390/molecules20010503 - 31 Dec 2014

Cited by 4 | Viewed by 6497

Abstract

Today, environmental impact associated with pollution treatment is a matter of great concern. A method is proposed for evaluating environmental risk associated with Advanced Oxidation Processes (AOPs) applied to wastewater treatment. The method is based on the type of pollution (wastewater, solids, air [...] Read more.

Today, environmental impact associated with pollution treatment is a matter of great concern. A method is proposed for evaluating environmental risk associated with Advanced Oxidation Processes (AOPs) applied to wastewater treatment. The method is based on the type of pollution (wastewater, solids, air or soil) and on materials and energy consumption. An Environmental Risk Index (E), constructed from numerical criteria provided, is presented for environmental comparison of processes and/or operations. The Operation Environmental Risk Index (E_Oi) for each of the unit operations involved in the process and the Aspects Environmental Risk Index (E_Aj) for process conditions were also estimated. Relative indexes were calculated to evaluate the risk of each operation (E/NOP) or aspect (E/NAS) involved in the process, and the percentage of the maximum achievable for each operation and aspect was found. A practical application of the method is presented for two AOPs: photo-Fenton and heterogeneous photocatalysis with suspended TiO₂ in Solarbox. The results report the environmental risks associated with each process, so that AOPs tested and the operations involved with them can be compared. Full article

(This article belongs to the Special Issue Photocatalysis)

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18 pages, 3500 KiB

Open AccessArticle

Photocatalytic Oxidation of Diethyl Sulfide Vapor over TiO₂-Based Composite Photocatalysts

by Dmitry Selishchev and Denis Kozlov

Molecules 2014, 19(12), 21424-21441; https://doi.org/10.3390/molecules191221424 - 19 Dec 2014

Cited by 22 | Viewed by 6899

Abstract

Composite TiO₂/activated carbon (TiO₂/AC) and TiO₂/SiO₂ photocatalysts with TiO₂ contents in the 10 to 80 wt. % range were synthesized by the TiOSO₄ thermal hydrolysis method and characterized by AES, BET, X-ray diffraction and [...] Read more.

Composite TiO₂/activated carbon (TiO₂/AC) and TiO₂/SiO₂ photocatalysts with TiO₂ contents in the 10 to 80 wt. % range were synthesized by the TiOSO₄ thermal hydrolysis method and characterized by AES, BET, X-ray diffraction and FT-IR ATR methods. All TiO₂samples were in the anatase form, with a primary crystallite size of about 11 nm. The photocatalytic activities of the TiO₂/AC and TiO₂/SiO₂ samples were tested in the gas-phase photocatalytic oxidation (PCO) reaction of diethyl sulfide (DES) vapor in a static reactor by the FT-IR in situ method. Acetaldehyde, formic acid, ethylene and SO₂ were registered as the intermediate products which finally were completely oxidized to the final oxidation products – H₂O, CO₂, CO and SO₄²⁻ ions. The influence of the support on the kinetics of DES PCO and on the TiO₂/AC and TiO₂/SiO₂ samples’ stability during three long-term DES PCO cycles was investigated. The highest PCO rate was observed for TiO₂/SiO₂ photocatalysts. To evaluate the activity of photocatalysts the turnover frequency values (TOF) were calculated for three photocatalysts (TiO₂, TiO₂/AC and TiO₂/SiO₂) for the same amount of mineralized DES. It was demonstrated that the TOF value for composite TiO₂/SiO₂ photocatalysts was 3.5 times higher than for pure TiO₂. Full article

(This article belongs to the Special Issue Photocatalysis)

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19 pages, 1255 KiB

Open AccessArticle

Photocatalysis for Renewable Energy Production Using PhotoFuelCells

by Robert Michal, Stavroula Sfaelou and Panagiotis Lianos

Molecules 2014, 19(12), 19732-19750; https://doi.org/10.3390/molecules191219732 - 27 Nov 2014

Cited by 45 | Viewed by 8944

Abstract

The present work is a short review of our recent studies on PhotoFuelCells, that is, photoelectrochemical cells which consume a fuel to produce electricity or hydrogen, and presents some unpublished data concerning both electricity and hydrogen production. PhotoFuelCells have been constructed using nanoparticulate [...] Read more.

The present work is a short review of our recent studies on PhotoFuelCells, that is, photoelectrochemical cells which consume a fuel to produce electricity or hydrogen, and presents some unpublished data concerning both electricity and hydrogen production. PhotoFuelCells have been constructed using nanoparticulate titania photoanodes and various cathode electrodes bearing a few different types of electrocatalyst. In the case where the cell functioned with an aerated cathode, the cathode electrode was made of carbon cloth carrying a carbon paste made of carbon black and dispersed Pt nanoparticles. When the cell was operated in the absence of oxygen, the electrocatalyst was deposited on an FTO slide using a special commercial carbon paste, which was again enriched with Pt nanoparticles. Mixing of Pt with carbon paste decreased the quantity of Pt necessary to act as electrocatalyst. PhotoFuelCells can produce electricity without bias and with relatively high open-circuit voltage when they function in the presence of fuel and with an aerated cathode. In that case, titania can be sensitized in the visible region by CdS quantum dots. In the present work, CdS was deposited by the SILAR method. Other metal chalcogenides are not functional as sensitizers because the combined photoanode in their presence does not have enough oxidative power to oxidize the fuel. Concerning hydrogen production, it was found that it is difficult to produce hydrogen in an alkaline environment even under bias, however, this is still possible if losses are minimized. One way to limit losses is to short-circuit anode and cathode electrode and put them close together. This is achieved in the “photoelectrocatalytic leaf”, which was presently demonstrated capable of producing hydrogen even in a strongly alkaline environment. Full article

(This article belongs to the Special Issue Photocatalysis)

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15 pages, 254 KiB

Open AccessArticle

Formation of Combustible Hydrocarbons and H₂ during Photocatalytic Decomposition of Various Organic Compounds under Aerated and Deaerated Conditions

by Sylwia Mozia, Aleksandra Kułagowska and Antoni W. Morawski

Molecules 2014, 19(12), 19633-19647; https://doi.org/10.3390/molecules191219633 - 26 Nov 2014

Cited by 14 | Viewed by 6230

Abstract

A possibility of photocatalytic production of useful aliphatic hydrocarbons and H₂ from various organic compounds, including acetic acid, methanol, ethanol and glucose, over Fe-modified TiO₂ is discussed. In particular, the influence of the reaction atmosphere (N₂, air) was investigated. [...] Read more.

A possibility of photocatalytic production of useful aliphatic hydrocarbons and H₂ from various organic compounds, including acetic acid, methanol, ethanol and glucose, over Fe-modified TiO₂ is discussed. In particular, the influence of the reaction atmosphere (N₂, air) was investigated. Different gases were identified in the headspace volume of the reactor depending on the substrate. In general, the evolution of the gases was more effective in air compared to a N₂ atmosphere. In the presence of air, the gaseous phase contained CO₂, CH₄ and H₂, regardless of the substrate used. Moreover, formation of C₂H₆ and C₃H₈ in the case of acetic acid and C₂H₆ in the case of ethanol was observed. In case of acetic acid and methanol an increase in H₂ evolution under aerated conditions was observed. It was concluded that the photocatalytic decomposition of organic compounds with simultaneous generation of combustible hydrocarbons and hydrogen could be a promising method of “green energy” production. Full article

(This article belongs to the Special Issue Photocatalysis)

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15 pages, 2758 KiB

Open AccessArticle

Influence of Post-Treatment Operations on Structural Properties and Photocatalytic Activity of Octahedral Anatase Titania Particles Prepared by an Ultrasonication-Hydrothermal Reaction

by Zhishun Wei, Ewa Kowalska and Bunsho Ohtani

Molecules 2014, 19(12), 19573-19587; https://doi.org/10.3390/molecules191219573 - 26 Nov 2014

Cited by 23 | Viewed by 6239

Abstract

The influence of changes in structural and physical properties on the photocatalytic activity of octahedral anatase particles (OAPs), exposing eight equivalent {101} facets, caused by calcination (2 h) in air or grinding (1 h) in an agate mortar was studied with samples prepared [...] Read more.

The influence of changes in structural and physical properties on the photocatalytic activity of octahedral anatase particles (OAPs), exposing eight equivalent {101} facets, caused by calcination (2 h) in air or grinding (1 h) in an agate mortar was studied with samples prepared by ultrasonication (US; 1 h)–hydrothermal reaction (HT; 24 h, 433 K). Calcination in air at temperatures up to 1173 K induced particle shape changes, evaluated by aspect ratio (AR; d₀₀₁/d₁₀₁ = depth vertical to anatase {001} and {101} facets estimated by the Scherrer equation with data obtained from X-ray diffraction (XRD) patterns) and content of OAP and semi-OAP particles, without transformation into rutile. AR and OAP content, as well as specific surface area (SSA), were almost unchanged by calcination at temperatures up to 673 K and were then decreased by elevating the calcination temperature, suggesting that calcination at a higher temperature caused dull-edging and particle sintering, the latter also being supported by the analysis of particle size using XRD patterns and scanning electron microscopic (SEM) images. Time-resolved microwave conductivity (TRMC) showed that the maximum signal intensity (I_max), corresponding to a product of charge-carrier density and mobility, and signal-decay rate, presumably corresponding to reactivity of charge carriers, were increased with increase in AR, suggesting higher photocatalytic activity of OAPs than that of dull-edged particles. Grinding also decreased the AR, indicating the formation of dull-edged particles. The original non-treated samples showed activities in the oxidative decomposition of acetic acid (CO₂ system) and dehydrogenation of methanol (H₂ system) comparable to and lower than those of a commercial anatase titania (Showa Denko Ceramics FP-6), respectively. The activities of calcined and ground samples for the CO₂ system and H₂ system showed almost linear relations with AR and I_max, respectively, suggesting that those activities may depend on different properties. Full article

(This article belongs to the Special Issue Photocatalysis)

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16 pages, 504 KiB

Open AccessArticle

Some Observations on the Development of Superior Photocatalytic Systems for Application to Water Purification by the “Adsorb and Shuttle” or the Interphase Charge Transfer Mechanisms

by Cooper Langford, Maryam Izadifard, Emad Radwan and Gopal Achari

Molecules 2014, 19(12), 19557-19572; https://doi.org/10.3390/molecules191219557 - 26 Nov 2014

Cited by 15 | Viewed by 5880

Abstract

Adsorb and shuttle (A/S) and interfacial charge transfer are the two major strategies for overcoming recombination in photocatalysis in this era of nanoparticle composites. Their relationships are considered here. A review of key literature is accompanied by a presentation of three new experiments [...] Read more.

Adsorb and shuttle (A/S) and interfacial charge transfer are the two major strategies for overcoming recombination in photocatalysis in this era of nanoparticle composites. Their relationships are considered here. A review of key literature is accompanied by a presentation of three new experiments within the overall aim of assessing the relation of these strategies. The cases presented include: A/S by a high silica zeolite/TiO₂ composite, charge transfer (CT) between phases in a TiO₂/WO₃ composite and both A/S and CT by composites of TiO₂ with powered activated carbon (AC) and single-walled carbon nanotubes (SWCNT). The opportunities presented by the two strategies for moving toward photocatalysts that could support applications for the removal of contaminants from drinking water or that lead to a practical adsorbent for organics that could be regenerated photocatalytically link this discussion to ongoing research here. Full article

(This article belongs to the Special Issue Photocatalysis)

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16 pages, 2933 KiB

Open AccessArticle

Tungsten Trioxide as a Visible Light Photocatalyst for Volatile Organic Carbon Removal

by Yossy Wicaksana, Sanly Liu, Jason Scott and Rose Amal

Molecules 2014, 19(11), 17747-17762; https://doi.org/10.3390/molecules191117747 - 31 Oct 2014

Cited by 70 | Viewed by 10909

Abstract

Tungsten trioxide (WO₃) has been demonstrated to possess visible light photoactivity and presents a means of overcoming the UV-light dependence of photocatalysts, such as titanium dioxide. In this study, WO₃ nanostructures have been synthesised by a hydrothermal method using sodium [...] Read more.

Tungsten trioxide (WO₃) has been demonstrated to possess visible light photoactivity and presents a means of overcoming the UV-light dependence of photocatalysts, such as titanium dioxide. In this study, WO₃ nanostructures have been synthesised by a hydrothermal method using sodium tungstate (Na₂WO₄·2H₂O), sulphate precursors and pH as structure-directing agents and parameters, respectively. By altering the concentration of the sulphate precursors and pH, it was shown that different morphologies and phases of WO₃ can be achieved. The effect of the morphology of the final WO₃ product on the visible light photoactivity of ethylene degradation in the gas phase was investigated. In addition, platinum (Pt) was photodeposited on the WO₃ structures with various morphologies to enhance the photocatalytic properties. It was found that the photocatalytic properties of the WO₃ samples greatly depend on their morphology, chemical composition and surface modification. WO₃ with a cuboid morphology exhibited the highest visible light photoactivity compared to other morphologies, while adding Pt to the surface improved the performance of certain WO₃ structures. Full article

(This article belongs to the Special Issue Photocatalysis)

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11 pages, 618 KiB

Open AccessArticle

Field Performance Test of an Air-Cleaner with Photocatalysis-Plasma Synergistic Reactors for Practical and Long-Term Use

by Tsuyoshi Ochiai, Erina Ichihashi, Naoki Nishida, Tadashi Machida, Yoshitsugu Uchida, Yuji Hayashi, Yuko Morito and Akira Fujishima

Molecules 2014, 19(11), 17424-17434; https://doi.org/10.3390/molecules191117424 - 29 Oct 2014

Cited by 12 | Viewed by 8048

Abstract

A practical and long-term usable air-cleaner based on the synergy of photocatalysis and plasma treatments has been developed. A field test of the air-cleaner was carried out in an office smoking room. The results were compared to previously reported laboratory test results. Even [...] Read more.

A practical and long-term usable air-cleaner based on the synergy of photocatalysis and plasma treatments has been developed. A field test of the air-cleaner was carried out in an office smoking room. The results were compared to previously reported laboratory test results. Even after a treatment of 12,000 cigarettes-worth of tobacco smoke, the air-cleaner maintained high-level air-purification activity (98.9% ± 0.1% and 88% ± 1% removal of the total suspended particulate (TSP) and total volatile organic compound (TVOC) concentrations, respectively) at single-pass conditions. Although the removal ratio of TSP concentrations was 98.6% ± 0.2%, the ratio of TVOC concentrations was 43.8% after a treatment of 21,900 cigarettes-worth of tobacco smoke in the field test. These results indicate the importance of suitable maintenance of the reactors in the air-cleaner during field use. Full article

(This article belongs to the Special Issue Photocatalysis)

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26 pages, 779 KiB

Open AccessArticle

Radical Intermediates in Photoinduced Reactions on TiO₂ (An EPR Spin Trapping Study)

by Dana Dvoranová, Zuzana Barbieriková and Vlasta Brezová

Molecules 2014, 19(11), 17279-17304; https://doi.org/10.3390/molecules191117279 - 28 Oct 2014

Cited by 236 | Viewed by 17505

Abstract

The radical intermediates formed upon UVA irradiation of titanium dioxide suspensions in aqueous and non-aqueous environments were investigated applying the EPR spin trapping technique. The results showed that the generation of reactive species and their consecutive reactions are influenced by the solvent properties [...] Read more.

The radical intermediates formed upon UVA irradiation of titanium dioxide suspensions in aqueous and non-aqueous environments were investigated applying the EPR spin trapping technique. The results showed that the generation of reactive species and their consecutive reactions are influenced by the solvent properties (e.g., polarity, solubility of molecular oxygen, rate constant for the reaction of hydroxyl radicals with the solvent). The formation of hydroxyl radicals, evidenced as the corresponding spin-adducts, dominated in the irradiated TiO₂ aqueous suspensions. The addition of ¹⁷O-enriched water caused changes in the EPR spectra reflecting the interaction of an unpaired electron with the ¹⁷O nucleus. The photoexcitation of TiO₂ in non-aqueous solvents (dimethylsulfoxide, acetonitrile, methanol and ethanol) in the presence of 5,5-dimethyl-1-pyrroline N-oxide spin trap displayed a stabilization of the superoxide radical anions generated via electron transfer reaction to molecular oxygen, and various oxygen- and carbon-centered radicals from the solvents were generated. The character and origin of the carbon-centered spin-adducts was confirmed using nitroso spin trapping agents. Full article

(This article belongs to the Special Issue Photocatalysis)

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16 pages, 3030 KiB

Open AccessArticle

Photocatalytic Solar Tower Reactor for the Elimination of a Low Concentration of VOCs

by Nobuaki Negishi and Taizo Sano

Molecules 2014, 19(10), 16624-16639; https://doi.org/10.3390/molecules191016624 - 15 Oct 2014

Cited by 6 | Viewed by 7637

Abstract

We developed a photocatalytic solar tower reactor for the elimination of low concentrations of volatile organic compounds (VOCs) typically emitted from small industrial establishments. The photocatalytic system can be installed in a narrow space, as the reactor is cylindrical-shaped. The photocatalytic reactor was [...] Read more.

We developed a photocatalytic solar tower reactor for the elimination of low concentrations of volatile organic compounds (VOCs) typically emitted from small industrial establishments. The photocatalytic system can be installed in a narrow space, as the reactor is cylindrical-shaped. The photocatalytic reactor was placed vertically in the center of a cylindrical scattering mirror, and this vertical reactor was irradiated with scattered sunlight generated by the scattering mirror. About 5 ppm toluene vapor, used as representative VOC, was continuously photodegraded and converted to CO₂ almost stoichiometrically under sunny conditions. Toluene removal depended only on the intensity of sunlight. The performance of the solar tower reactor did not decrease with half a year of operation, and the average toluene removal was 36% within this period. Full article

(This article belongs to the Special Issue Photocatalysis)

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12 pages, 379 KiB

Open AccessArticle

Design of Composite Photocatalyst of TiO₂ and Y-Zeolite for Degradation of 2-Propanol in the Gas Phase under UV and Visible Light Irradiation

by Takashi Kamegawa, Yasushi Ishiguro, Ryota Kido and Hiromi Yamashita

Molecules 2014, 19(10), 16477-16488; https://doi.org/10.3390/molecules191016477 - 13 Oct 2014

Cited by 25 | Viewed by 8053

Abstract

Hydrophobic Y-zeolite (SiO₂/Al₂O₃ = 810) and TiO₂ composite photocatalysts were designed by using two different types of TiO₂ precursors, i.e., titanium ammonium oxalate and ammonium hexafluorotitanate. The porous structure, surface property and state of TiO [...] Read more.

Hydrophobic Y-zeolite (SiO₂/Al₂O₃ = 810) and TiO₂ composite photocatalysts were designed by using two different types of TiO₂ precursors, i.e., titanium ammonium oxalate and ammonium hexafluorotitanate. The porous structure, surface property and state of TiO₂ were investigated by various characterization techniques. By using an ammonium hexafluorotitanate as a precursor, hydrophobic modification of the Y-zeolite surface and realizing visible light sensitivity was successfully achieved at the same time after calcination at 773 K in the air. The prepared sample still maintained the porous structure of Y-zeolite and a large surface area. Highly crystalline anatase TiO₂ was also formed on the Y-zeolite surface by the role of fluorine in the precursor. The usages of ammonium hexafluorotitanate were effective for the improvement of the photocatalytic performance of the composite in the degradation of 2-propanol in the gas phase under UV and visible light (λ > 420 nm) irradiation. Full article

(This article belongs to the Special Issue Photocatalysis)

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22 pages, 4683 KiB

Open AccessArticle

Surface Properties and Photocatalytic Activity of KTaO₃, CdS, MoS₂ Semiconductors and Their Binary and Ternary Semiconductor Composites

by Beata Bajorowicz, Anna Cybula, Michał J. Winiarski, Tomasz Klimczuk and Adriana Zaleska

Molecules 2014, 19(9), 15339-15360; https://doi.org/10.3390/molecules190915339 - 24 Sep 2014

Cited by 34 | Viewed by 10997

Abstract

Single semiconductors such as KTaO₃, CdS MoS₂ or their precursor solutions were combined to form novel binary and ternary semiconductor nanocomposites by the calcination or by the hydro/solvothermal mixed solutions methods, respectively. The aim of this work was to study [...] Read more.

Single semiconductors such as KTaO₃, CdS MoS₂ or their precursor solutions were combined to form novel binary and ternary semiconductor nanocomposites by the calcination or by the hydro/solvothermal mixed solutions methods, respectively. The aim of this work was to study the influence of preparation method as well as type and amount of the composite components on the surface properties and photocatalytic activity of the new semiconducting photoactive materials. We presented different binary and ternary combinations of the above semiconductors for phenol and toluene photocatalytic degradation and characterized by X-ray powder diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) specific surface area and porosity. The results showed that loading MoS₂ onto CdS as well as loading CdS onto KTaO₃ significantly enhanced absorption properties as compared with single semiconductors. The highest photocatalytic activity in phenol degradation reaction under both UV-Vis and visible light irradiation and very good stability in toluene removal was observed for ternary hybrid obtained by calcination of KTaO₃, CdS, MoS₂ powders at the 10:5:1 molar ratio. Enhanced photoactivity could be related to the two-photon excitation in KTaO₃-CdS-MoS₂ composite under UV-Vis and/or to additional presence of CdMoO₄ working as co-catalyst. Full article

(This article belongs to the Special Issue Photocatalysis)

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Review

Jump to: Research

55 pages, 5870 KiB

Open AccessReview

Advances and Recent Trends in Heterogeneous Photo(Electro)-Catalysis for Solar Fuels and Chemicals

by James Highfield

Molecules 2015, 20(4), 6739-6793; https://doi.org/10.3390/molecules20046739 - 15 Apr 2015

Cited by 66 | Viewed by 21920

Abstract

In the context of a future renewable energy system based on hydrogen storage as energy-dense liquid alcohols co-synthesized from recycled CO₂, this article reviews advances in photocatalysis and photoelectrocatalysis that exploit solar (photonic) primary energy in relevant endergonic processes, viz., H [...] Read more.

In the context of a future renewable energy system based on hydrogen storage as energy-dense liquid alcohols co-synthesized from recycled CO₂, this article reviews advances in photocatalysis and photoelectrocatalysis that exploit solar (photonic) primary energy in relevant endergonic processes, viz., H₂ generation by water splitting, bio-oxygenate photoreforming, and artificial photosynthesis (CO₂ reduction). Attainment of the efficiency (>10%) mandated for viable techno-economics (USD 2.00–4.00 per kg H₂) and implementation on a global scale hinges on the development of photo(electro)catalysts and co-catalysts composed of earth-abundant elements offering visible-light-driven charge separation and surface redox chemistry in high quantum yield, while retaining the chemical and photo-stability typical of titanium dioxide, a ubiquitous oxide semiconductor and performance “benchmark”. The dye-sensitized TiO₂ solar cell and multi-junction Si are key “voltage-biasing” components in hybrid photovoltaic/photoelectrochemical (PV/PEC) devices that currently lead the field in performance. Prospects and limitations of visible-absorbing particulates, e.g., nanotextured crystalline α-Fe₂O₃, g-C₃N₄, and TiO₂ sensitized by C/N-based dopants, multilayer composites, and plasmonic metals, are also considered. An interesting trend in water splitting is towards hydrogen peroxide as a solar fuel and value-added green reagent. Fundamental and technical hurdles impeding the advance towards pre-commercial solar fuels demonstration units are considered. Full article

(This article belongs to the Special Issue Photocatalysis)

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42 pages, 2981 KiB

Open AccessReview

A Review of Heterogeneous Photocatalysis for Water and Surface Disinfection

by John Anthony Byrne, Patrick Stuart Morris Dunlop, Jeremy William John Hamilton, Pilar Fernández-Ibáñez, Inmaculada Polo-López, Preetam Kumar Sharma and Ashlene Sarah Margaret Vennard

Molecules 2015, 20(4), 5574-5615; https://doi.org/10.3390/molecules20045574 - 30 Mar 2015

Cited by 204 | Viewed by 16709

Abstract

Photo-excitation of certain semiconductors can lead to the production of reactive oxygen species that can inactivate microorganisms. The mechanisms involved are reviewed, along with two important applications. The first is the use of photocatalysis to enhance the solar disinfection of water. It is [...] Read more.

Photo-excitation of certain semiconductors can lead to the production of reactive oxygen species that can inactivate microorganisms. The mechanisms involved are reviewed, along with two important applications. The first is the use of photocatalysis to enhance the solar disinfection of water. It is estimated that 750 million people do not have accessed to an improved source for drinking and many more rely on sources that are not safe. If one can utilize photocatalysis to enhance the solar disinfection of water and provide an inexpensive, simple method of water disinfection, then it could help reduce the risk of waterborne disease. The second application is the use of photocatalytic coatings to combat healthcare associated infections. Two challenges are considered, i.e., the use of photocatalytic coatings to give “self-disinfecting” surfaces to reduce the risk of transmission of infection via environmental surfaces, and the use of photocatalytic coatings for the decontamination and disinfection of medical devices. In the final section, the development of novel photocatalytic materials for use in disinfection applications is reviewed, taking account of materials, developed for other photocatalytic applications, but which may be transferable for disinfection purposes. Full article

(This article belongs to the Special Issue Photocatalysis)

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38 pages, 2458 KiB

Open AccessReview

The Viability of Photocatalysis for Air Purification

by Stephen O. Hay, Timothy Obee, Zhu Luo, Ting Jiang, Yongtao Meng, Junkai He, Steven C. Murphy and Steven Suib

Molecules 2015, 20(1), 1319-1356; https://doi.org/10.3390/molecules20011319 - 14 Jan 2015

Cited by 78 | Viewed by 15512

Abstract

Photocatalytic oxidation (PCO) air purification technology is reviewed based on the decades of research conducted by the United Technologies Research Center (UTRC) and their external colleagues. UTRC conducted basic research on the reaction rates of various volatile organic compounds (VOCs). The knowledge gained [...] Read more.

Photocatalytic oxidation (PCO) air purification technology is reviewed based on the decades of research conducted by the United Technologies Research Center (UTRC) and their external colleagues. UTRC conducted basic research on the reaction rates of various volatile organic compounds (VOCs). The knowledge gained allowed validation of 1D and 3D prototype reactor models that guided further purifier development. Colleagues worldwide validated purifier prototypes in simulated realistic indoor environments. Prototype products were deployed in office environments both in the United States and France. As a result of these validation studies, it was discovered that both catalyst lifetime and byproduct formation are barriers to implementing this technology. Research is ongoing at the University of Connecticut that is applicable to extending catalyst lifetime, increasing catalyst efficiency and extending activation wavelength from the ultraviolet to the visible wavelengths. It is critical that catalyst lifetime is extended to realize cost effective implementation of PCO air purification. Full article

(This article belongs to the Special Issue Photocatalysis)

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23 pages, 1579 KiB

Open AccessReview

Using Dyes for Evaluating Photocatalytic Properties: A Critical Review

by Malka Rochkind, Sagi Pasternak and Yaron Paz

Molecules 2015, 20(1), 88-110; https://doi.org/10.3390/molecules20010088 - 23 Dec 2014

Cited by 280 | Viewed by 14331

Abstract

This brief review aims at analyzing the use of dyestuffs for evaluating the photocatalytic properties of novel photocatalysts. It is shown that the use of dyes as predictors for photocatalytic activity has its roots in the pre visible-light activity era, when the aim [...] Read more.

This brief review aims at analyzing the use of dyestuffs for evaluating the photocatalytic properties of novel photocatalysts. It is shown that the use of dyes as predictors for photocatalytic activity has its roots in the pre visible-light activity era, when the aim was to treat effluents streams containing hazardous dyes. The main conclusion of this review is that, in general, dyes are inappropriate as model compounds for the evaluation of photocatalytic activity of novel photocatalysts claimed to operate under visible light. Their main advantage, the ability to use UV-Vis spectroscopy, is severely limited by a variety of factors, most of which are related to the presence of other species. The presence of a second mechanism, sensitization, diminishes the generality required from a model contaminant used for testing a novel photocatalyst. While it is recommended not to use dyes for general testing of novel photocatalysts, it is still understandable that a model system consisting of a dye and a semiconductor can be of large importance if the degradation of a specific dye is the main aim of the research, or, alternatively, if the abilities of a specific dye to induce the degradation of a different type of contaminant are under study. Full article

(This article belongs to the Special Issue Photocatalysis)

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28 pages, 1373 KiB

Open AccessReview

A Review on Visible Light Active Perovskite-Based Photocatalysts

by Pushkar Kanhere and Zhong Chen

Molecules 2014, 19(12), 19995-20022; https://doi.org/10.3390/molecules191219995 - 1 Dec 2014

Cited by 513 | Viewed by 29056

Abstract

Perovskite-based photocatalysts are of significant interest in the field of photocatalysis. To date, several perovskite material systems have been developed and their applications in visible light photocatalysis studied. This article provides a review of the visible light (λ > 400 nm) active perovskite-based [...] Read more.

Perovskite-based photocatalysts are of significant interest in the field of photocatalysis. To date, several perovskite material systems have been developed and their applications in visible light photocatalysis studied. This article provides a review of the visible light (λ > 400 nm) active perovskite-based photocatalyst systems. The materials systems are classified by the B site cations and their crystal structure, optical properties, electronic structure, and photocatalytic performance are reviewed in detail. Titanates, tantalates, niobates, vanadates, and ferrites form important photocatalysts which show promise in visible light-driven photoreactions. Along with simple perovskite (ABO₃) structures, development of double/complex perovskites that are active under visible light is also reviewed. Various strategies employed for enhancing the photocatalytic performance have been discussed, emphasizing the specific advantages and challenges offered by perovskite-based photocatalysts. This review provides a broad overview of the perovskite photocatalysts, summarizing the current state of the work and offering useful insights for their future development. Full article

(This article belongs to the Special Issue Photocatalysis)

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20 pages, 3081 KiB

Open AccessReview

Spectroscopic Investigation of the Mechanism of Photocatalysis

by Yoshio Nosaka, Masami Nishikawa and Atsuko Y. Nosaka

Molecules 2014, 19(11), 18248-18267; https://doi.org/10.3390/molecules191118248 - 7 Nov 2014

Cited by 46 | Viewed by 11884

Abstract

Reaction mechanisms of various kinds of photocatalysts have been reviewed based on the recent reports, in which various spectroscopic techniques including luminol chemiluminescence photometry, fluorescence probe method, electron spin resonance (ESR), and nuclear magnetic resonance (NMR) spectroscopy were applied. The reaction mechanisms elucidated [...] Read more.

Reaction mechanisms of various kinds of photocatalysts have been reviewed based on the recent reports, in which various spectroscopic techniques including luminol chemiluminescence photometry, fluorescence probe method, electron spin resonance (ESR), and nuclear magnetic resonance (NMR) spectroscopy were applied. The reaction mechanisms elucidated for bare and modified TiO₂ were described individually. The modified visible light responsive TiO₂ photocatalysts, i.e., Fe(III)-deposited metal-doped TiO₂ and platinum complex-deposited TiO₂, were studied by detecting paramagnetic species with ESR, •O₂⁻ (or H₂O₂) with chemiluminescence photometry, and OH radicals with a fluorescence probe method. For bare TiO₂, the difference in the oxidation mechanism for the different crystalline form was investigated by the fluorescence probe method, while the adsorption and decomposition behaviors of several amino acids and peptides were investigated by ¹H-NMR spectroscopy. Full article

(This article belongs to the Special Issue Photocatalysis)

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23 pages, 1342 KiB

Open AccessReview

UV-Absorption—The Primary Process in Photocatalysis and Some Practical Consequences

by Terry A. Egerton

Molecules 2014, 19(11), 18192-18214; https://doi.org/10.3390/molecules191118192 - 6 Nov 2014

Cited by 66 | Viewed by 14265

Abstract

TiO₂ photochemistry studies generally address reactions of photogenerated charge-carriers at the oxide surface or the recombination reactions which control the proportion of charge carriers that reach the surface. By contrast, this review focuses on UV absorption, the first photochemical step in semiconductor [...] Read more.

TiO₂ photochemistry studies generally address reactions of photogenerated charge-carriers at the oxide surface or the recombination reactions which control the proportion of charge carriers that reach the surface. By contrast, this review focuses on UV absorption, the first photochemical step in semiconductor photocatalysis. The influence of particle size on absorption and scattering of light by small TiO₂ particles is summarized and the importance of considering, the particle size in the application, not the BET or X-ray line broadening size, is emphasized. Three different consequences of UV absorption are then considered. First, two commercially important systems, pigmented polymer films and paints, are used to show that TiO₂ can protect from direct photochemical degradation. Then the effect of UV absorption on the measured photocatalytic degradation of aqueous solutions of organics is considered for two separate cases. Firstly, the consequences of UV absorption by TiO₂ on the generation of hydroxyl radicals from H₂O₂ are considered in the context of the claimed synergy between H₂O₂ and TiO₂. Secondly, the effect of altered UV absorption, caused by changed effective particle size of the catalyst, is demonstrated for photocatalysis of propan-2-ol oxidation and salicylic acid degradation. Full article

(This article belongs to the Special Issue Photocatalysis)

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27 pages, 6146 KiB

Open AccessReview

Coupled Microwave/Photoassisted Methods for Environmental Remediation

by Satoshi Horikoshi and Nick Serpone

Molecules 2014, 19(11), 18102-18128; https://doi.org/10.3390/molecules191118102 - 5 Nov 2014

Cited by 24 | Viewed by 7988

Abstract

The microwave-induced acceleration of photocatalytic reactions was discovered serendipitously in the late 1990s. The activity of photocatalysts is enhanced significantly by both microwave radiation and UV light. Particularly relevant, other than as a heat source, was the enigmatic phenomenon of the non-thermal effect(s) [...] Read more.

The microwave-induced acceleration of photocatalytic reactions was discovered serendipitously in the late 1990s. The activity of photocatalysts is enhanced significantly by both microwave radiation and UV light. Particularly relevant, other than as a heat source, was the enigmatic phenomenon of the non-thermal effect(s) of the microwave radiation that facilitated photocatalyzed reactions, as evidenced when examining various model contaminants in aqueous media. Results led to an examination of the possible mechanism(s) of the microwave effect(s). In the present article we contend that the microwaves’ non-thermal effect(s) is an important factor in the enhancement of TiO₂-photoassisted reactions involving the decomposition of organic pollutants in model wastewaters by an integrated (coupled) microwave-/UV-illumination method (UV/MW). Moreover, such coupling of no less than two irradiation methods led to the fabrication and ultimate investigation of microwave discharged electrodeless lamps (MDELs) as optimal light sources; their use is also described. The review focuses on the enhanced activity of photocatalytic reactions when subjected to microwave radiation and concentrates on the authors’ research of the past few years. Full article

(This article belongs to the Special Issue Photocatalysis)

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22 pages, 3357 KiB

Open AccessReview

Structural Formation and Photocatalytic Activity of Magnetron Sputtered Titania and Doped-Titania Coatings

by Peter J. Kelly, Glen T. West, Marina Ratova, Leanne Fisher, Soheyla Ostovarpour and Joanna Verran

Molecules 2014, 19(10), 16327-16348; https://doi.org/10.3390/molecules191016327 - 13 Oct 2014

Cited by 35 | Viewed by 7413

Abstract

Titania and doped-titania coatings can be deposited by a wide range of techniques; this paper will concentrate on magnetron sputtering techniques, including “conventional” reactive co-sputtering from multiple metal targets and the recently introduced high power impulse magnetron sputtering (HiPIMS). The latter has been [...] Read more.

Titania and doped-titania coatings can be deposited by a wide range of techniques; this paper will concentrate on magnetron sputtering techniques, including “conventional” reactive co-sputtering from multiple metal targets and the recently introduced high power impulse magnetron sputtering (HiPIMS). The latter has been shown to deliver a relatively low thermal flux to the substrate, whilst still allowing the direct deposition of crystalline titania coatings and, therefore, offers the potential to deposit photocatalytically active titania coatings directly onto thermally sensitive substrates. The deposition of coatings via these techniques will be discussed, as will the characterisation of the coatings by XRD, SEM, EDX, optical spectroscopy, etc. The assessment of photocatalytic activity and photoactivity through the decomposition of an organic dye (methylene blue), the inactivation of E. coli microorganisms and the measurement of water contact angles will be described. The impact of different deposition technologies, doping and co-doping strategies on coating structure and activity will be also considered. Full article

(This article belongs to the Special Issue Photocatalysis)

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21 pages, 590 KiB

Open AccessReview

Unraveling the Photocatalytic Mechanisms on TiO₂ Surfaces Using the Oxygen-18 Isotopic Label Technique

by Xibin Pang, Chuncheng Chen, Hongwei Ji, Yanke Che, Wanhong Ma and Jincai Zhao

Molecules 2014, 19(10), 16291-16311; https://doi.org/10.3390/molecules191016291 - 10 Oct 2014

Cited by 44 | Viewed by 10382

Abstract

During the last several decades TiO₂ photocatalytic oxidation using the molecular oxygen in air has emerged as a promising method for the degradation of recalcitrant organic pollutants and selective transformations of valuable organic chemicals. Despite extensive studies, the mechanisms of these photocatalytic [...] Read more.

During the last several decades TiO₂ photocatalytic oxidation using the molecular oxygen in air has emerged as a promising method for the degradation of recalcitrant organic pollutants and selective transformations of valuable organic chemicals. Despite extensive studies, the mechanisms of these photocatalytic reactions are still poorly understood due to their complexity. In this review, we will highlight how the oxygen-18 isotope labeling technique can be a powerful tool to elucidate complicated photocatalytic mechanisms taking place on the TiO₂ surface. To this end, the application of the oxygen-18 isotopic-labeling method to three representative photocatalytic reactions is discussed: (1) the photocatalytic hydroxylation of aromatics; (2) oxidative cleavage of aryl rings on the TiO₂ surface; and (3) photocatalytic decarboxylation of saturated carboxylic acids. The results show that the oxygen atoms of molecular oxygen can incorporate into the corresponding products in aqueous solution in all three of these reactions, but the detailed incorporation pathways are completely different in each case. For the hydroxylation process, the O atom in O₂ is shown to be incorporated through activation of O₂ by conduction band electrons. In the cleavage of aryl rings, O atoms are inserted into the aryl ring through the site-dependent coordination of reactants on the TiO₂ surface. A new pathway for the decarboxylation of saturated carboxylic acids with pyruvic acid as an intermediate is identified, and the O₂ is incorporated into the products through the further oxidation of pyruvic acid by active species from the activation of O₂ by conduction band electrons. Full article

(This article belongs to the Special Issue Photocatalysis)

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13 pages, 2242 KiB

Open AccessReview

Are TiO₂ Nanotubes Worth Using in Photocatalytic Purification of Air and Water?

by Pierre Pichat

Molecules 2014, 19(9), 15075-15087; https://doi.org/10.3390/molecules190915075 - 19 Sep 2014

Cited by 28 | Viewed by 8123

Abstract

Titanium dioxide nanotubes (TNT) have mainly been used in dye sensitized solar cells, essentially because of a higher transport rate of electrons from the adsorbed photo-excited dye to the Ti electrode onto which TNT instead of TiO₂ nanoparticles (TNP) are attached. The [...] Read more.

Titanium dioxide nanotubes (TNT) have mainly been used in dye sensitized solar cells, essentially because of a higher transport rate of electrons from the adsorbed photo-excited dye to the Ti electrode onto which TNT instead of TiO₂ nanoparticles (TNP) are attached. The dimension ranges and the two main synthesis methods of TNT are briefly indicated here. Not surprisingly, the particular and regular texture of TNT was also expected to improve the photocatalytic efficacy for pollutant removal in air and water with respect to TNP. In this short review, the validity of this expectation is checked using the regrettably small number of literature comparisons between TNT and commercialized TNP referring to films of similar thickness and layers or slurries containing an equal TiO₂ mass. Although the irradiated geometrical area differed for each study, it was identical for each comparison considered here. For the removal of toluene (methylbenzene) or acetaldehyde (ethanal) in air, the average ratio of the efficacy of TNT over that of TiO₂ P25 was about 1.5, and for the removal of dyes in water, it was around 1. This lack of major improvement with TNT compared to TNP could partially be due to TNT texture disorders as seems to be suggested by the better average performance of anodic oxidation-prepared TNT. It could also come from the fact that the properties influencing the efficacy are more numerous, their interrelations more complex and their effects more important for pollutant removal than for dye sensitized solar cells and photoelectrocatalysis where the electron transport rate is the crucial parameter. Full article

(This article belongs to the Special Issue Photocatalysis)

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