Special Issue "Photocatalysts"

Guest Editor
Prof. Dr. Cooper H. Langford
Chemistry, Science A 109, 731 Campus Place N.W., University of Calgary, 2500 University Dr. NW, Calgary, Alberta, Canada T2N 1N4
Website: http://www.ucalgary.ca/chem/pages/langford
E-Mail: chlangfo@ucalgary.ca
Interests: inorganic photochemistry and element speciation in the natural environment

Guest Editor
Prof. Dr. Bunsho Ohtani
Catalysis Research Center, Hokkaido University, N21W10, Sapporo 001-0021, Japan
Website: http://www.hucc.hokudai.ac.jp/~k15391/index_e.html
E-Mail: ohtani@cat.hokudai.ac.jp
Interests: fundamental studies and applications of heterogeneous photocatalysis; design and development of highly active and selective solid photocatalysts and development of solid particles of anisotropic morphology

Dear Colleagues,

Photocatalysis is a term that combines the basic notion of a catalyst that enhances rate without being consumed with the notion that the reaction is induced and/or accelerated by photons. Focused primarily on heterogeneous processes using “semiconductor” photocatalysts, it has evolved in three phases. The first generation (1975–1985) focused primarily on the studies providing understanding of the illuminated semiconductor/solution interface. The second phase featured work on polycrystalline thin films, doping semiconductors for visible light, dye sensitization, and exploration of the scope of photocatalysis. In the third generation period since 2000, photocatalysis research has continued to expand so that in the 2006–2010 period publications on photocatalysts exceeded those aimed at energy conversion, 16,757 to 12,811. This period has seen two fundamental principles, the special properties of the nanoscale and the power of self-organization, converge to provide a new conceptual platform on which to construct photocatalysts. At this time stimulating work is being done on problems associated with each of these phases, often combining emphases. This special issue invites significant contributions of all of these types plus contributions that show the power of homogeneous systems. It is hoped that it can catalyse communication between researchers in the hybrid area of photocatalysis and the larger catalysis community.

Prof. Dr. Cooper H. Langford
Prof. Dr. Bunsho Ohtani
Guest Editors

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• photocatalyst
• TiO₂
• band gap
• band edge
• hole transfer
• electron capture
• dye sensitization
• photocatalyst doping
• nanoparticles
• self-assembled
• homogeneous photocatalysis

Type of Paper: Review
Title:Conversion of CO₂ via Visible Light Promoted Homogeneous Redox Catalysis
Authors: Richard Reithmeier, Christian Bruckmeier and Bernhard Rieger
Affiliations: Catalysis Research Centre, WACKER-Department of Macromolecular Chemistry, TU-München, Lichtenbergstraße 4, 85748 Garching, Germany; E-Mail: rieger@tum.de
Abstract: This review gives an overview about the principles of light promoted homogeneous redox catalysis in terms of applications in CO₂ conversions. Various chromophors and the advantages of different structures and metal centers as well as optimization strategies are discussed. All aspects of the reduction catalyst site are restricted to CO₂ conversion. An important focus of this review is the question about a replacement of the sacrificial donor which is found in most of the current publications. All this structural requirements of chromophores, oxidation and reduction sites are important for the broadly discussed topic of supramolecular photocatalysts. Furthermore electronic parameters of supramolecular systems are reviewed.
Keywords: CO₂; photoreduction; photocatalysis; photooxidation; sacrificial amines

Type of Paper: Article
Title: Inactivation of E. coli in Water using Photocatalytic, Nanostructured Films Synthesized by Aerosol Routes
Authors: Jinho Park, Eric Kettleson, Woo-Jin An, Yinjie Tang and Pratim Biswas
Affliation: Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, Campus Box 1180, St. Louis, MO 63130, USA; E-Mail: : pbiswas@wustl.edu
Abstract: TiO₂ nanostructured films were synthesized by an aerosol chemical vapor deposition method (ACVD) with different controlled morphologies: columnar, granular and branched structure for the photocatalytic inactivation of Escherichia coli (E. coli) in water. Effects of film morphology and external applied voltage on inactivation rate were investigated. As-prepared films were characterized using SEM, TEM, XRD and UV-VIS spectroscopy. Photochemical and photo-electrochemical inactivation of E. coli in water using as-prepared TiO₂ films were performed under irradiation of UVA light. Inactivation rate constants for each of the films were obtained. The inactivation rate constant for the columnar film was enhanced by 430% with an applied bias voltage on the film, while that for the branched film was increased by 130%. Photochemical inactivation rates of the columnar and the branched films were also compared accounting for the different surface area.
Keywords: E.coli; Inactivation; photocatalysis; TiO₂; Aerosol chemical vapor deposition

Type of Paper: Article
Title: Reaction in Photofuel Cells Using Allophane-titania Nanocomposite Electrodes
Author: Hiromasa Nishikiori
Affiliation: Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan
Abstract: Allophane-titania nanocomposite electrodes were prepared from titanium alkoxide sols dispersing the natural clay mineral allophane. Electrochemical measurements indicated that the oxidative degradation of starch in the solutions and suspensions enhanced the generation of electricity during UV irradiation. A photocurrent was observed in the photofuel cell using the allophane-titania nanocomposite electrode adsorbing the starch molecules. Allophane effectively adsorbed the starch molecules and then transported them to the titania, on which their oxidation induced the electrogeneration.

Type of Paper: Article
Title: Microvolume TOC-analysis as Useful Tool in the Evaluation of lab Scale Photocatalytic Processes
Authors: Pegie Cool, Vera Meynen and Monika Kus
Affiliation: Laboratory of Adsorption and Catalysis, Department of Chemistry, University of Antwerpen, Campus Drie Eiken, Universiteitsplein 1, B-2610 Wilrijk, Belgium
Abstract: Analysis methods that require only small microliter volumes of aqueous samples can be of large benefit for several applications when expensive chemicals are involved or available volumes are substantially small and concentrations are low (ppm range). A new method is presented to allow microvolume liquids injection on TOC equipment using a special designed Shimadzu gas injection kit^® in combination with a high precision syringe and Chaney adapter. Next to details on the methodology of micro-volume TOC injections, the technique is shown to be especially beneficial to evaluate the efficiency of photocatalytic dye degradation on titania materials in terms of CO₂ conversion (photomineralization) simultaneously with classic UV-Vis-analysis (photobleaching) measurements within a small, lab scale photocatalytic test setup. The possibility to allow multiple microvolume samplings in short time intervals during several hours in small volume lab scale test setups (< 100 mL) without a substantial decrease in volume/catalyst ratio is of particular value for the evaluation of photocatalysts. By combining both complementary techniques UV-Vis-analysis and µV-TOC-analysis in short time intervals, additional knowledge on the degradation process/mechanism, kinetics and the catalyst efficiency can be obtained in a direct and complete way. It is shown to be an important asset to photocatalytic studies, preventing misinterpretations. Moreover, the developed µV-TOC-analysis can similarly be put into service in a wide range of other small volume setups such as e.g. analytes from high-throughput screening, pharmaceutical applications and other advanced oxidation processes (e.g., Fenton reactions) that formally could not be analyzed due to limited sample volumes.
Keywords: photocatalysis; microvolume analysis; total organic carbon; AOP

Type of Paper: Review
Title: The Influence of Surface Aluminium and Silicon on the Degradation of  Organic Pollutants
Author: Terry Egerton
Affiliation: Newcastle University, UK
Abstract: Practical use of photocatalysis for the degradation of organic pollutants must take into account the influence of other chemicals. Classical colloid studies have shown significant adsorption of aluminium may occur on the surface of titania at the concentrations of dissolved alumina that can occur naturally in freshwater. (Additional aluminium of anthropogenic origin may also be present.) This paper reviews the influence of deliberately deposited silicon and aluminium on the adsorption and photocatalytic behaviour of a  ~140 m² g^-1 rutile TiO₂.  It is shown that some of the nitrogen that is adsorbed on rutile is infrared-active and this activity is attributed to the presence of surface sites associated with a strong electrostatic field.  Surface silica and surface aluminium reduce, and ultimately eliminate, this infrared-active species. They also significantly lessen photocatalytic both oxidation of propan-2-ol to propanone and reduction of diphenyl picryl hydrazine. The reduction was monitored as decrease of the absorption at 520 nm in the spectrum of the diphenyl picryl hydrazine radical. Correspondingly, quantitative measurement of photogreying is used to show that the adsorbed inorganics  also reduce the capture of photogenerated conduction band electrons by Ti⁴⁺ to form  Ti³⁺ .  Finally the influence of adsorbed phosphate on photocatalysis is briefly reviewed. It is concluded that inorganic species naturally occurring in water can significantly reduce the photoactivity that is measured under laboratory conditions in pure water.

Type of Paper: A Review
Title: Photocatalysis: Recent Developments and Applications
Authors: A.O. Ibhadon¹ and Paul Fitzpatrick²
Affiliations: ¹ Centre for Environmental and Marine Sciences, Faculty of Science, University of Hull, Scarborough Campus, Scarborough, YO11 3AZ, UK
² CTech Innovation, Capenhurst Technology Park, Capenhurst, Chester, CH1 6EH England, UK
Abstract: Semiconductor photocatalysis, the subject of this review, is a versatile, low-cost and environmentally benign treatment technology for a host of pollutants of biological, organic and inorganic origin in water and air containing low concentrations of these pollutants. The efficient and successful application of photocatalysis demands that the pollutant, the catalyst and source of illumination are in close proximity or contact with each other. The ability of advanced oxidation technology to remove low levels of persistent organic pollutants and as well as microorganisms in water has been widely demonstrated and, progressively, the technology is being commercialized in many areas of the world including developing nations. This short review considers recent developments in the research and application of semiconductor photocatalysis for the treatment of low level concentrations of pollutants in water and air using titanium dioxide as a ‘model’ semiconductor. The review considers charge transport characteristics on the semiconductor surface, photocatalyst reactor design and organic degradation mechanistic pathways. The effects of photoreactor operating parameters on the photocatalytic process are discussed in addition to mineralization and disinfection kinetics.
Keywords: semiconductor; photocatalysis; band gap; illumination; reactor; catalyst; mineralization; degradation

Article type: Review
Title: Photocatalytic Water Treatment by Titanium dioxide: Recent Updates
Authors: Manoj A. Lazar ^1,2,*, Shaji Varghese ³ and Santhosh S. Nair ^1,2
1 School of Applied Sciences and Engineering, Monash University, Churchill VIC, 3842, Australia. E-Mails: manoj.ainikalkannath@monash.edu (M.A.L); santhosh.nair@monash.edu (S.S.N.)
² School of Chemistry, Monash University, Clayton VIC, 3800, Australia
³ Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, Complesso Universitario Monserrato, CA, Italy. E-Mail: shajivarg@gmail.com
* Author to whom correspondence should be addressed; E-Mail: manoj.ainikalkannath@monash.edu, manojlazar2005@gmail.com(MAL); Tel.: +61 3 990 26411; Fax: +61 3 990 26738.
Abstract: Photocatalytic water treatment using nanocrystalline titanium dioxide (NTO) is a well-known advance oxidation process (AOP) for environmental remediation. With the in situ generation of hydroxyl radicals, upon light irradiance, NTO can mineralize a wide range of organic compounds into harmless end products such as carbon dioxide, water and other inorganic ions. Photocatalytic degradation of pollutants by NTO follows Langmuir-Hinshelwood kinetics, especially at low catalyst concentration. Different NTO morphologies or surface treatments on NTO can increase the photocatalytic efficiency in degradation reactions. Wisely designed photocatalytic reactors can decrease the energy consumption or can avoid post-separation stage in photocatalytic water treatment processes. Doping NTO with metals or non-metals can reduce the band gap of the doped catalyst, causing the light absorption from the visible region. Coupling NTO photocatalysis with other water treatment technologies can be more beneficial especially in large scale treatments. This review describes recent developments in the field of photocatalytic water treatment using NTO.
Keywords: Titanium dioxide; advanced oxidation process, photocatalysis; water treatment; degradation; Langmuir-Hinshelwood kinetics; photocatalytic reactor; doping

Type of Paper: Article
Title: Photocatalytic Activity of Vis-responsive Ag-nanoparticles/TiO2 Composite Thin Films Fabricated by the Molecular Precursor Method (MPM)
Authors: Likius S. Daniel ¹, Hiroki Nagai ², Naoya Yoshida ³, and Mitsunobu Sato ^{2, 4}*
Affiliation: ¹ Department of Applied Chemistry and Chemical Engineering, Graduate School, Kogakuin University, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan
² Research Institute of Science and Technology, Kogakuin University, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan
³ Department of Environmental and Energy Chemistry, Graduate School, Kogakuin University, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan
⁴ Coordination Engineering Laboratory, Division of Liberal Arts, Kogakuin University, 2665-1 Nakano, Hachioji City, Tokyo 192-0015, Japan
Abstract: The Ag-nanoparticles (Ag-NP)/TiO2 composite thin films of various Ag contents (10–80 mol%) were examined as a potential photocatalyst by decoloration reaction of methylene blue (MB) in an aqueous solution. These composite thin films of ca. 100 nm thickness were fabricated by the MPM at 600ºC in air. The decoloration rates monitored by the absorption intensity of the MB solution indicated that the composite thin films of Ag content less than 40 mol% are not effective under Vis-irradiation, though they can work as a photocatalyst under UV-irradiation. Further, the UV-sensitivity of the composite thin films gradually decreased to almost half level of that of the TiO2 thin film fabricated under the identical conditions, when the Ag content increased from 10 to 40 mol%. Contrarily, the composite thin films of Ag content larger than 50 mol% showed the Vis-responsive activity, whose level was slightly lower than the decreased UV-sensitivity. Diffuse reflectance spectra suggested that the Vis-responsive activity of the composite thin films is dominantly due to the localized surface plasmon resonance of Ag-NP. It was also elucidated that the Vis-responsive level of the composite thin films corresponds to their electrical conductivity that depends on the Ag content.

Type of Paper: Article
Title: A Comparative Investigation of Selective Photooxidation of Alcohols on N-doped and Commercial TiO₂
Authors: R. Amadelli ¹, L. Samiolo ¹ and P. Pichat ²
Affiliations: ¹ ISOF-CNR, c/o Department of Chemistry, University of Ferrara, via L. Borsari 46, I-44121 Ferrara, Italy; E-Mail: amr@unife.it
² CNRS, Ecole Centale de Lyon (STMS), Photocatalyse et Environnement, 69134 Ecully CEDEX, France
Abstract: We report on the selective photooxidation of alcohols in the liquid phase using nitrogen-doped TiO₂ (N-TiO₂) as photocatalyst with visible light response. An impressive number of publications has appeared in the last decade; however, the photo-electrocatalytic activity of this doped oxide remains controversial. We found, for example, that photo-oxidation of phenylmethanol (benzyl alcohol) in an aqueous medium does not proceed; conversely, in dried acetonitrile formation of aldehydes takes place with 100% selectivity. Herein results obtained with N-TiO₂ are compared with those recorded using commercially available TiO₂ samples. Recently, in fact, a number of articles appeared which report on the occurrence of visible light chemo-selective oxidation of phenylmethanol to phenylmethanal (benzaldehyde) on unmodified TiO₂. Photoactivity in the visible is attributed to formation of a charge transfer complex by interaction of the alcohol with the surface. Our comparative study shows that the photooxidation quantum yield of N-TiO₂ is almost an order of magnitude higher than that of the best commercial catalyst. In this case, the reason is not to be found in a higher adsorption of the alcohol on the modified photocatalyst. Rather, results of an EPR investigation seem to underline the role of oxygen vacancies and superoxide. The latter is detected using the spin-trapping technique. In contrast to the case of benzyl alcohol, the photooxidation of 4-pyridynemethanol (4-pyridylcarbinol) occurs in water and yields the corresponding pyridinecarboxyaldehyde with good selectivity, and the process can proceed under ambient light illumination. The highest performance is again observed with N-TiO₂ and, from FTIR measurements, we established that the visible light photoactivity is attributable to a relatively high number of Lewis acid sites that characterizes the doped TiO₂ catalyst.