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Special Issue "Photocatalysis"

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A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Organic Synthesis".

Deadline for manuscript submissions: closed (15 August 2014)

Special Issue Editor

Guest Editor
Dr. Pierre Pichat

Photocatalyse et Environnement, CNRS/ Ecole Centrale de Lyon (STMS), 69134 Ecully CEDEX, France
E-Mail
Interests: heterogeneous photocatalysis, advanced oxidation processes, environmental chemistry, catalysis, photochemistry

Special Issue Information

Dear Colleagues,

The field of heterogeneous photocatalysis has given rise to thousands of papers. Most often, these papers are dispersed in many journals. Consequently, from time to time, there is a need for a Special Issue to provide a broad survey and address the various aspects of this field. For the readers, this Special Issue will give an attractive opportunity to obtain, more easily, information concerning the different facets of heterogeneous photocatalysis. For the authors, it will be an appropriate occasion to make their results and analyses more visible, and to show that they are active members of the scientific community in heterogeneous photocatalysis. This Special Issue will contain contributions discussing all the aspects broadly indicated by the keywords. Reviews articles by experts in the field will also be welcome.

Dr. Pierre Pichat
Guest Editor

Submission

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. Papers will be published continuously (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

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Keywords

  • fundamentals
  • UV and visible-light sensitive photocatalysts/supports/composites
  • reactors and modelling
  • organic synthesis
  • environmental potentialities and applications
  • hydrogen and other fuels production
  • testing methods
  • comparisons with other processes

Published Papers (31 papers)

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Research

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Open AccessArticle Probing Water and CO2 Interactions at the Surface of Collapsed Titania Nanotubes Using IR Spectroscopy
Molecules 2015, 20(9), 15469-15487; doi:10.3390/molecules200915469
Received: 24 April 2015 / Revised: 12 August 2015 / Accepted: 12 August 2015 / Published: 26 August 2015
Cited by 3 | PDF Full-text (2812 KB) | HTML Full-text | XML Full-text
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) Print Edition available
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
Molecules 2015, 20(7), 13354-13373; doi:10.3390/molecules200713354
Received: 16 April 2015 / Revised: 13 June 2015 / Accepted: 6 July 2015 / Published: 22 July 2015
Cited by 1 | PDF Full-text (1514 KB) | HTML Full-text | XML Full-text
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) Print Edition available
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Open AccessArticle Theoretical Verification of Photoelectrochemical Water Oxidation Using Nanocrystalline TiO2 Electrodes
Molecules 2015, 20(6), 9732-9744; doi:10.3390/molecules20069732
Received: 31 December 2014 / Accepted: 12 May 2015 / Published: 27 May 2015
PDF Full-text (2857 KB) | HTML Full-text | XML Full-text | Supplementary Files
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) Print Edition available
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Open AccessArticle Selective Reduction of Cr(VI) in Chromium, Copper and Arsenic (CCA) Mixed Waste Streams Using UV/TiO2 Photocatalysis
Molecules 2015, 20(2), 2622-2635; doi:10.3390/molecules20022622
Received: 15 December 2014 / Revised: 8 January 2015 / Accepted: 22 January 2015 / Published: 3 February 2015
Cited by 8 | PDF Full-text (1476 KB) | HTML Full-text | XML Full-text
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) Print Edition available
Open AccessArticle Preparation of a Titania/X-Zeolite/Porous Glass Composite Photocatalyst Using Hydrothermal and Drop Coating Processes
Molecules 2015, 20(2), 2349-2363; doi:10.3390/molecules20022349
Received: 28 December 2014 / Revised: 15 January 2015 / Accepted: 22 January 2015 / Published: 30 January 2015
Cited by 1 | PDF Full-text (2655 KB) | HTML Full-text | XML Full-text
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) Print Edition available
Open AccessArticle Visible Light Induced Green Transformation of Primary Amines to Imines Using a Silicate Supported Anatase Photocatalyst
Molecules 2015, 20(2), 1941-1954; doi:10.3390/molecules20021941
Received: 30 October 2014 / Accepted: 22 January 2015 / Published: 26 January 2015
Cited by 2 | PDF Full-text (2738 KB) | HTML Full-text | XML Full-text | Supplementary Files
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) Print Edition available
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Open AccessArticle Energy and Molecules from Photochemical/Photocatalytic Reactions. An Overview
Molecules 2015, 20(1), 1527-1542; doi:10.3390/molecules20011527
Received: 5 November 2014 / Accepted: 7 January 2015 / Published: 16 January 2015
Cited by 4 | PDF Full-text (1112 KB) | HTML Full-text | XML Full-text | Supplementary Files
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) Print Edition available
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Open AccessArticle TiO2 and Fe2O3 Films for Photoelectrochemical Water Splitting
Molecules 2015, 20(1), 1046-1058; doi:10.3390/molecules20011046
Received: 5 November 2014 / Accepted: 9 December 2014 / Published: 9 January 2015
Cited by 10 | PDF Full-text (4971 KB) | HTML Full-text | XML Full-text
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) Print Edition available
Open AccessArticle A Comparison of the Environmental Impact of Different AOPs: Risk Indexes
Molecules 2015, 20(1), 503-518; doi:10.3390/molecules20010503
Received: 17 September 2014 / Accepted: 24 December 2014 / Published: 31 December 2014
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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 (EOi) for each of the unit operations involved in the process and the Aspects Environmental Risk Index (EAj) 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) Print Edition available
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Open AccessArticle Photocatalytic Oxidation of Diethyl Sulfide Vapor over TiO2-Based Composite Photocatalysts
Molecules 2014, 19(12), 21424-21441; doi:10.3390/molecules191221424
Received: 31 August 2014 / Revised: 4 December 2014 / Accepted: 12 December 2014 / Published: 19 December 2014
Cited by 3 | PDF Full-text (3500 KB) | HTML Full-text | XML Full-text | Supplementary Files
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 TiO2 samples 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) Print Edition available
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Open AccessArticle Photocatalysis for Renewable Energy Production Using PhotoFuelCells
Molecules 2014, 19(12), 19732-19750; doi:10.3390/molecules191219732
Received: 6 October 2014 / Revised: 14 November 2014 / Accepted: 24 November 2014 / Published: 27 November 2014
Cited by 9 | PDF Full-text (1255 KB) | HTML Full-text | XML Full-text
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) Print Edition available
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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
Molecules 2014, 19(12), 19557-19572; doi:10.3390/molecules191219557
Received: 21 August 2014 / Revised: 31 October 2014 / Accepted: 5 November 2014 / Published: 26 November 2014
Cited by 2 | PDF Full-text (504 KB) | HTML Full-text | XML Full-text
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) Print Edition available
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Open AccessArticle Influence of Post-Treatment Operations on Structural Properties and Photocatalytic Activity of Octahedral Anatase Titania Particles Prepared by an Ultrasonication-Hydrothermal Reaction
Molecules 2014, 19(12), 19573-19587; doi:10.3390/molecules191219573
Received: 25 September 2014 / Revised: 13 November 2014 / Accepted: 17 November 2014 / Published: 26 November 2014
Cited by 6 | PDF Full-text (2758 KB) | HTML Full-text | XML Full-text
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 (Imax), 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 Imax, respectively, suggesting that those activities may depend on different properties. Full article
(This article belongs to the Special Issue Photocatalysis) Print Edition available
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Open AccessArticle Formation of Combustible Hydrocarbons and H2 during Photocatalytic Decomposition of Various Organic Compounds under Aerated and Deaerated Conditions
Molecules 2014, 19(12), 19633-19647; doi:10.3390/molecules191219633
Received: 1 September 2014 / Revised: 4 November 2014 / Accepted: 11 November 2014 / Published: 26 November 2014
Cited by 1 | PDF Full-text (254 KB) | HTML Full-text | XML Full-text
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) Print Edition available
Open AccessArticle Tungsten Trioxide as a Visible Light Photocatalyst for Volatile Organic Carbon Removal
Molecules 2014, 19(11), 17747-17762; doi:10.3390/molecules191117747
Received: 27 August 2014 / Revised: 22 October 2014 / Accepted: 27 October 2014 / Published: 31 October 2014
Cited by 8 | PDF Full-text (2933 KB) | HTML Full-text | XML Full-text | Supplementary Files
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) Print Edition available
Open AccessArticle Field Performance Test of an Air-Cleaner with Photocatalysis-Plasma Synergistic Reactors for Practical and Long-Term Use
Molecules 2014, 19(11), 17424-17434; doi:10.3390/molecules191117424
Received: 30 August 2014 / Revised: 6 October 2014 / Accepted: 22 October 2014 / Published: 29 October 2014
Cited by 2 | PDF Full-text (618 KB) | HTML Full-text | XML Full-text
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
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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) Print Edition available
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Open AccessArticle Radical Intermediates in Photoinduced Reactions on TiO2 (An EPR Spin Trapping Study)
Molecules 2014, 19(11), 17279-17304; doi:10.3390/molecules191117279
Received: 13 August 2014 / Revised: 29 September 2014 / Accepted: 22 October 2014 / Published: 28 October 2014
Cited by 13 | PDF Full-text (779 KB) | HTML Full-text | XML Full-text | Supplementary Files
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
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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) Print Edition available
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Open AccessArticle Photocatalytic Solar Tower Reactor for the Elimination of a Low Concentration of VOCs
Molecules 2014, 19(10), 16624-16639; doi:10.3390/molecules191016624
Received: 28 August 2014 / Revised: 29 September 2014 / Accepted: 8 October 2014 / Published: 15 October 2014
PDF Full-text (3030 KB) | HTML Full-text | XML Full-text
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
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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) Print Edition available
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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
Molecules 2014, 19(10), 16477-16488; doi:10.3390/molecules191016477
Received: 4 August 2014 / Revised: 22 September 2014 / Accepted: 30 September 2014 / Published: 13 October 2014
Cited by 5 | PDF Full-text (379 KB) | HTML Full-text | XML Full-text
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) Print Edition available
Open AccessArticle Surface Properties and Photocatalytic Activity of KTaO3, CdS, MoS2 Semiconductors and Their Binary and Ternary Semiconductor Composites
Molecules 2014, 19(9), 15339-15360; doi:10.3390/molecules190915339
Received: 30 July 2014 / Revised: 27 August 2014 / Accepted: 15 September 2014 / Published: 24 September 2014
Cited by 9 | PDF Full-text (4683 KB) | HTML Full-text | XML Full-text
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) Print Edition available
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Review

Jump to: Research

Open AccessReview Advances and Recent Trends in Heterogeneous Photo(Electro)-Catalysis for Solar Fuels and Chemicals
Molecules 2015, 20(4), 6739-6793; doi:10.3390/molecules20046739
Received: 20 January 2015 / Revised: 20 March 2015 / Accepted: 20 March 2015 / Published: 15 April 2015
Cited by 10 | PDF Full-text (5870 KB) | HTML Full-text | XML Full-text
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
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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) Print Edition available
Open AccessReview A Review of Heterogeneous Photocatalysis for Water and Surface Disinfection
Molecules 2015, 20(4), 5574-5615; doi:10.3390/molecules20045574
Received: 29 December 2014 / Revised: 16 March 2015 / Accepted: 18 March 2015 / Published: 30 March 2015
Cited by 24 | PDF Full-text (2981 KB) | HTML Full-text | XML Full-text
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
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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) Print Edition available
Open AccessReview The Viability of Photocatalysis for Air Purification
Molecules 2015, 20(1), 1319-1356; doi:10.3390/molecules20011319
Received: 10 September 2014 / Accepted: 16 December 2014 / Published: 14 January 2015
Cited by 10 | PDF Full-text (2458 KB) | HTML Full-text | XML Full-text
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
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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) Print Edition available
Open AccessReview Using Dyes for Evaluating Photocatalytic Properties: A Critical Review
Molecules 2015, 20(1), 88-110; doi:10.3390/molecules20010088
Received: 11 November 2014 / Accepted: 12 December 2014 / Published: 23 December 2014
Cited by 20 | PDF Full-text (1579 KB) | HTML Full-text | XML Full-text
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
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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) Print Edition available
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Open AccessReview A Review on Visible Light Active Perovskite-Based Photocatalysts
Molecules 2014, 19(12), 19995-20022; doi:10.3390/molecules191219995
Received: 25 September 2014 / Revised: 13 November 2014 / Accepted: 16 November 2014 / Published: 1 December 2014
Cited by 33 | PDF Full-text (1373 KB) | HTML Full-text | XML Full-text
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
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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) Print Edition available
Open AccessReview Spectroscopic Investigation of the Mechanism of Photocatalysis
Molecules 2014, 19(11), 18248-18267; doi:10.3390/molecules191118248
Received: 1 September 2014 / Revised: 21 October 2014 / Accepted: 27 October 2014 / Published: 7 November 2014
Cited by 7 | PDF Full-text (3081 KB) | HTML Full-text | XML Full-text
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
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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) Print Edition available
Open AccessReview UV-Absorption—The Primary Process in Photocatalysis and Some Practical Consequences
Molecules 2014, 19(11), 18192-18214; doi:10.3390/molecules191118192
Received: 27 August 2014 / Revised: 20 October 2014 / Accepted: 23 October 2014 / Published: 6 November 2014
Cited by 7 | PDF Full-text (1342 KB) | HTML Full-text | XML Full-text
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
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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) Print Edition available
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Open AccessReview Coupled Microwave/Photoassisted Methods for Environmental Remediation
Molecules 2014, 19(11), 18102-18128; doi:10.3390/molecules191118102
Received: 31 August 2014 / Revised: 27 October 2014 / Accepted: 27 October 2014 / Published: 5 November 2014
Cited by 4 | PDF Full-text (6146 KB) | HTML Full-text | XML Full-text
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)
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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) Print Edition available
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Open AccessReview Structural Formation and Photocatalytic Activity of Magnetron Sputtered Titania and Doped-Titania Coatings
Molecules 2014, 19(10), 16327-16348; doi:10.3390/molecules191016327
Received: 27 August 2014 / Revised: 25 September 2014 / Accepted: 2 October 2014 / Published: 13 October 2014
Cited by 2 | PDF Full-text (3357 KB) | HTML Full-text | XML Full-text
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) Print Edition available
Open AccessReview Unraveling the Photocatalytic Mechanisms on TiO2 Surfaces Using the Oxygen-18 Isotopic Label Technique
Molecules 2014, 19(10), 16291-16311; doi:10.3390/molecules191016291
Received: 5 August 2014 / Revised: 24 September 2014 / Accepted: 6 October 2014 / Published: 10 October 2014
Cited by 4 | PDF Full-text (590 KB) | HTML Full-text | XML Full-text
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) Print Edition available
Open AccessReview Are TiO2 Nanotubes Worth Using in Photocatalytic Purification of Air and Water?
Molecules 2014, 19(9), 15075-15087; doi:10.3390/molecules190915075
Received: 24 July 2014 / Revised: 29 August 2014 / Accepted: 9 September 2014 / Published: 19 September 2014
Cited by 4 | PDF Full-text (2242 KB) | HTML Full-text | XML Full-text
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) Print Edition available
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Planned Papers

The below list represents only planned manuscripts. Some of these manuscripts have not been received by the Editorial Office yet. Papers submitted to MDPI journals are subject to peer-review.

Feature Papers List

Dr. Angelo Albini Photocatalysis and (green)synthesis
Dr. Stephen O. Hay The Viability of Photocatalytic Air Purification
Dr. Alexei Emeline Photophysics events in heterogeneous photocatalysis
Dr. James Highfield Recent advances in photocatalysis and mechanism elucidation by in-situ spectroscopies
Dr. Gianluca Li-Puma
Dr. Dionysios D. Dionysiou and Dr. Kevin O'Shea

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