Applied Catalysis B, Environmental (v.99, #3-4)

Contents (III).

Contents (CO4).

TiO2 photocatalysis-25 years by David Ollis; Pierre Pichat; Nick Serpone (377).

Synthesis, structural characterization and evaluation of sol–gel-based NF-TiO2 films with visible light-photoactivation for the removal of microcystin-LR by Miguel Pelaez; Polycarpos Falaras; Vlassis Likodimos; Athanassios G. Kontos; Armah A. de la Cruz; Kevin O'shea; Dionysios D. Dionysiou (378-387).
Display Omitted▶ Control of the physicochemical properties of TiO2 films using NF doping via sol–gel method. ▶ Optical evidence of localized intra-gap states in TiO2 lattice and formation of brookite phase upon irradiation ≥420 nm. ▶ NF-TiO2 films lead to effective degradation of microcystin-LR under visible light irradiation.This study reports on the synthesis, characterization and environmental application of immobilized nitrogen and fluorine co-doped TiO2 (NF-TiO2) photocatalyst. A fluorosurfactant-based sol–gel approach was employed to enhance the physicochemical properties and photocatalytic activity of NF-TiO2 under visible and UV light for the degradation of the hepatotoxin microcystin-LR (MC-LR). The films were characterized by XRD, environmental scanning electron microscope (ESEM), TEM, AFM, EPR, micro-Raman, X-ray photoelectron spectroscope (XPS), UV–vis spectroscopy and porosimeter analysis. The results revealed that by modifying the molar ratio of the fluorosurfactant, we could effectively control the physicochemical properties and obtain films with high BET surface area and porosity, small crystallite size and narrow pore size distribution. UV–vis spectroscopy showed an increase in the absorption capacity of NF-TiO2 in the visible light range compared to reference films. The existence of interstitial nitrogen and substitutional fluorine in the titanium dioxide (TiO2) lattice was determined by XPS. Comparative EPR measurements between the co-doped and reference samples identified distinct N spin species in NF-TiO2, with a high sensitivity to visible light irradiation. The abundance of these paramagnetic centers verifies the formation of localized intra-gap states in TiO2 and implies synergistic effects between fluorine and nitrogen dopants. Micro-Raman spectroscopy showed the growth of small amounts of brookite concomitantly with the major anatase TiO2 phase, which could promote the system's photocatalytic activity through the formation of anatase/brookite heterojunctions. Analysis of the lower frequency Eg anatase Raman mode indicated the occurrence of size effects reflecting phonon confinement in the anatase nanocrystallites as well as deviations from stoichiometry due to structural defects in the co-doped sample. NF-TiO2 films effectively degraded MC-LR under visible and UV light compared to reference film. Similar MC-LR degradation rates under visible light after three cycles revealed high mechanical stability and no irreversible changes of the film during photocatalysis. This process has the potential of providing environmentally benign routes for drinking water treatment with solar powered photocatalytic systems.
Keywords: NF-TiO2; Fluorosurfactant; Sol–gel method; Dip-coating; Nitrogen doped; Fluorine doped; Microcystin-LR; Photocatalysis; Visible light; TiO2; EPR; Micro-Raman; Cyanotoxins; Water treatment; Sustainability;

The kinetics of photocatalytic degradation of multicomponent mixtures of four of the most powerful endocrine disrupting chemicals (estrogens) estrone (E1), 17β-estradiol (E2), 17α-ethynylestradiol (EE2) and estriol (E3) was studied in the presence and in the absence of TiO2 (Degussa P25) suspensions. Experiments were carried out with UVA and UVC radiation in a well characterized annular photoreactor. The results were analysed in terms of a simple first-order kinetic model, but including the explicit effect of photon absorption. This was accomplished by modelling the radiation field under heterogeneous (photocatalysis) conditions and by determining the spatial distribution of the local volumetric rate of photon absorption (LVRPA) in the reactor. The Six-Flux Absorption-Scattering Model (i.e., scattered photons follow the route of the six directions of the Cartesian coordinates) using optical parameters averaged across the spectrum of the incident radiation was used to determine the LVRPA. The intrinsic reaction kinetic constants, of E1, E2, EE2 and E3, independent of reactor geometry and level of radiation absorbed within the reactor were determined under each different oxidation conditions. The quantum yields (moles of estrogens degraded per Einstein of photons absorbed) were estimated. The quantum yields under UVC-TiO2 photocatalysis (2.1 × 10−3 to 3.9 × 10−3) are on average double the yields under UVA-TiO2 photocatalysis (1.2 × 10−3 to 1.8 × 10−3).The established model was found to be appropriate to predict the time-dependent degradation profiles of the estrogens in multicomponent systems. Using this simple approach, intrinsic kinetic data can be obtained.
Keywords: Endocrine disrupting chemical; Estrogens; Wastewater; Titanium dioxide; Degussa P-25; Quantum yield; UV radiation; Photocatalysis; Photolysis; Photoreactor; Suspensions; Radiation absorption; Radiation scattering; Radiation field; Reaction kinetics;

TiO2 for water treatment: Parameters affecting the kinetics and mechanisms of photocatalysis by Donia Friedmann; Cecilia Mendive; Detlef Bahnemann (398-406).
The photocatalytic activity of TiO2 is the result of an interplay between a considerable number of parameters, e.g., phase composition, electronic structure, particle size, exposed surface area, degree of aggregation, mobility of charge carriers, presence of impurities, amount and kind of defects, adsorption of molecules from gas or aqueous phase, lateral interactions between adsorbed species, nature of solvent, etc. Furthermore, these parameters can be broadly subdivided into those that are intrinsic to the photocatalytic material, and those that are extrinsic being influenced by the surrounding environment and conditions. The specific function and influence of a given feature for the photocatalytic performance of a TiO2 sample is difficult to characterize since many of the before-mentioned parameters are strongly coupled. For example, while the degree of aggregation could be inherent to a given material, it is also simultaneously influenced by pH. The degree of aggregation can then influence adsorption of molecules, light scattering and photon adsorption, charge carrier dynamics etc. The plurality of variables driving the nature of the photocatalytic activity, presents a challenge when trying to understand the kinetics and mechanisms underlying photocatalytic processes. It is of primary importance to develop a method to understand and control these properties (or at least some of them). In this paper, we also discuss the relevance of quantum-integrated systems in which the local environment where the molecule is adsorbed is different from the “lonely” photocatalyst or the molecule in solution, and could be treated as a whole.

Oxidation of dichloracetic acid (DCA) by two titanium dioxide catalysts (P25 and a nanoparticulate rutile (NR)) was measured at pH 3 and the results compared with parallel measurements on the same TiO2s modified by deposition of platinum.Although the platinum loading corresponded to less than one platinum crystallite for each TiO2 crystal, platinization significantly increased photocatalytic activity – 30-fold in the case of NR, high area rutile. This has been attributed, by ourselves and others, e.g. , to the increased efficiency of hole-generation as a consequence of electron trapping by the platinum.Each catalyst was then milled, and UV–vis transmission spectra were used to show that milling improved the catalyst dispersion. Milling unplatinized TiO2 caused either insignificant decreases (P25) or increased activity (NR). This contrasts with the previously reported significant decrease in activity for propan-2-ol oxidation. The difference is attributed to a different dependence on UV intensity of DCA degradation and propan-2-ol oxidation. DCA oxidation rate increased linearly with UV intensity, I, whereas propan-2-ol oxidation varies as I 0.5.However, milling did decrease significantly the rate of DCA oxidation by both of the platinized catalysts, especially by the platinized rutile. It is suggested that most of the UV is absorbed by TiO2 crystals that have no platinum, but that in poorly dispersed TiO2, UV-generated electrons may be transported along a chain of TiO2 crystals to a platinized TiO2 at which they are trapped. This is analogous to the antenna model postulated by Bahnemann and co-workers . It is proposed that milling breaks down the antennae. Consequently electron transport is disrupted and the effectiveness of electron trapping by platinum is reduced and the measured photoactivity decreases.
Keywords: Photocatalysis; Titanium dioxide; Rutile; P25; Platinum; Dichloracetic; Antenna; Dispersion;

Direct detection of HO2 radicals in the vicinity of TiO2 photocatalytic surfaces using cw-CRDS by Chiheb Bahrini; Alexander Parker; Coralie Schoemaecker; Christa Fittschen (413-419).
Display Omitted▶ Direct detection of HO2 radicals in the gas phase above TiO2 surface. ▶ In situ sensing of the gas phase above surfaces by cw-CRDS. ▶ Quantitative detection of H2O2 by cw-CRDS.This work describes the first ever direct detection of HO2 radicals in the gas phase above photocatalytic surfaces. A glass plate covered with TiO2 has been illuminated in the presence of H2O2 by a 20 W fluorescence lamp centred at 365 nm. The activity of the photocatalytic material has been proven through direct, time resolved observation of the degradation of H2O2 by following its concentration by the very sensitive and selective technique of cw-Cavity Ring Down Spectroscopy (cw-CRDS). An absorption line of H2O2 at 6639.89 cm−1 has been used, permitting a detection limit of [H2O2]min  = 1.3 and 3.6 × 1013  cm−3 for 50 and 200 Torr of synthetic air, respectively. A lower limit of the quantum yield for H2O2 degradation has been estimated to ϕ min  = 0.0024. Under the same conditions, the formation of HO2 radicals has been detected directly and selectively in the gas phase, using the same technique. HO2 radicals have been observed at up to 4 cm above the surface and at total pressures of up to 230 Torr.
Keywords: Photocatalysis; Remote oxidation; HO2 radicals;

Effect of molecular functionality on the photocatalytic oxidation of gas-phase mixtures by Michael E. Zorn; Stephen O. Hay; Marc A. Anderson (420-427).
Developing an effective air purifier for indoor air quality (IAQ) purposes requires knowledge of the reaction rate of target compounds in the presence of other compounds that compete favorably with the target compounds for surface sites. To address this issue, the photocatalytic oxidation (PCO) of five C3 organic compounds on a sol-gel derived TiO2 thin-film catalyst was examined. Photocatalytic degradation of the five compounds was first compared in single component experiments at 50% relative humidity. The photocatalytic oxidation reaction rates proceeded in the following order: 1-propanol > propanal > propanone > propene > propane. The order of reaction rates was partially explained by considering the intermolecular forces of attraction that exist between the gas-phase molecules and the hydrated titania surface, with the lone exception of the ketone. A model that incorporates Henry's law constant and hydroxyl radical reactivity was successful at predicting PCO reactivity. Relative photocatalytic degradation was also studied using multi-component experiments. Multi-component experiments with propanone + propene, propanal + propanone, and ethanol + propanone were conducted. In each case it was observed that compounds with stronger binding energy to the photocatalyst surface displaced compounds with weaker binding energies and inhibited their further reaction until the stronger binding species was oxidized to sufficiently low levels. Further investigation of relative binding energies of compounds of interest for PCO applications should be pursued in the future through combinations of experimental studies and theoretical molecular modeling techniques.
Keywords: Photocatalytic oxidation; Kinetics; Sol–gel processing; Thin-films; Titanium dioxide;

This overview of photocatalytic purification/deodorization of indoor air using TiO2 includes four parts. First, considering that TiO2 exposed to indoor air – which contains several thousands ppmv of water – is covered by layers of water molecules, it is suggested that: (i) the approach to the surface of pollutants at ppbv concentrations is hindered, (ii) the pollutants are not dissociated by mere adsorption, (iii) •OH radicals are formed by mechanisms identical to those proposed for TiO2 exposed to liquid water, and (iv) •OH radicals are mobile within the adsorbed water layers. Second, the formation and role of superoxide, singlet molecular oxygen and labile surface oxygen atoms of TiO2 are reviewed. Third, the competition between pollutants at ppbv levels for elimination under usual conditions of purifiers is discussed. Fourth, the question of the effect of indoor air photocatalytic treatment on the average concentration of low-molecular-mass carbonyls is addressed.
Keywords: Photocatalysis; TiO2; Indoor air treatment; Oxygen species; Pollutants;

The deactivation of photocatalytic based air purifiers by ambient siloxanes by Stephen O. Hay; Timothy N. Obee; Catherine Thibaud-Erkey (435-441).
One of the more promising applications of ultraviolet photocatalytic oxidation (UVPCO) technology is air purification for improved indoor air quality (IAQ). Successful implementation has been hindered by lack of understanding of the behavior of UVPCO systems in real, constantly evolving ambient air environments. The focus of most published research is on the demonstration of the rate of removal of various single compounds in a laboratory experiment, often at high concentrations relative to those encountered in ambient air. UVPCO products have been released worldwide with wildly varying claims of effectiveness and lifetime, but limited data is available relative to the long term effectiveness of this promising technology.This study presents results from laboratory photocatalyst deactivation studies, and field testing of prototype reactors in two office building locations in CT, USA. The goal was both to investigate deactivation by silicon-containing volatile and semi-volatile organic compounds (VOCs and SVOCs) and to determine the durability of the photocatalyst in situ in an office environment. Ambient air quality was monitored by total VOC sensors, and relative humidity and temperature were documented. Limited air analysis was performed. Photocatalysts employed in the testing were either Degussa P25 or a 3% WO3 coated P25. The results indicate that rapid catalyst deactivation is observed in photocatalytic air purifiers deployed in ambient office air.Display Omitted▶ Photocatlyitic air purifiers deactivate rapidly in ambient office air. ▶ Siloxanes in ambient air are the probable deactivating agent. ▶ Siloxane prefiltering (removal) extends catalyst life.
Keywords: Photocatalytic oxidation; Deactivation; Air purification; Siloxanes; Titanium dioxide;

The environmental burden associated with some functionalization reactions of nitrogen-heteroaromatics via TiO2 (solar) photocatalysis is compared with the same reactions under thermal conditions. Two methods, LCA (Life Cycle Assessment) and EATOS (Environmental Assessment Tool for Organic Synthesis), both based on a detailed analysis of the experiments, are used for the assessment. The thermal processes are consistently evaluated to give a better environmental performance. Photocatalysis is disfavored by the required management of a large amount of excess reagent and solvent involved. Still the good yield and simplicity of the photocatalytic method are promising (and are positively evaluated by a third assessment, EcoScale). The precautions for making photocatalytic syntheses environmentally viable are discussed.
Keywords: Photocatalysis; Titanium dioxide; Life Cycle Assessment; Environment; EATOS Assessment;

A review of patents on the application of titanium dioxide photocatalysis for air treatment is presented. A comparison between water treatment and air treatment reveals that the number of scientific publications dedicated to photocatalytic air treatment is significantly lower than the number of scientific manuscripts dedicated to photocatalytic water treatment, yet the situation is reversed upon comparing relevant patents. This indicates a growing interest in the implementation of photocatalysis for air treatment purposes, which surpasses that of water treatment.This manuscript analyzes the various patents in the area of air treatment, while differentiating between indoor air treatment and outdoor air treatment. Specific efforts were made to characterize the main challenges and achievements en-route for successful implementation, which were categorized according to mass transport, adsorption of contaminants, quantum efficiency, deactivation, and, no less important, the adherence and the long term stability of the photocatalyst.
Keywords: Titanium dioxide; Air treatment; Patents; Photocatalysis; Review;

Photocatalysis has presently become a major discipline owing to two factors: (i) the intuition of the pioneers of last 20th century and (ii) the mutual enrichment of scientists arising from different fields: photochemistry, electrochemistry, analytical chemistry, radiochemistry, material chemistry, surface science, electronics, and hopefully catalysis. Since heterogeneous photocatalysis belongs to catalysis, all the bases of this discipline must be respected: (i) proportionality of the reaction rate to the mass of catalyst (below the plateau due to a full absorption of photons); (ii) implication of the Langmuir–Hinshelwood mechanism of kinetics with the initial rate being proportional to the coverages θ in reactants;(iii) conversions obtained above the stoichiometric threshold defined as the maximum number of potential active sites initially present at the surface of a mass m of titania used in the reaction. In addition, one should respect photonics, with the photocatalytic activity, i.e. the reaction rate being (i) parallel to the absorbance of the photocatalyst and (ii) proportional to the radiant flux Φ. In every study, one should determine the quantum yield (QY) (or efficiency), which, although dimensionless, is a “doubly kinetic” magnitude defined as the ratio of the reaction rate r (in molecules converted/second) to the efficient photonic flux (in photons/second) received by the solid. This is an instantaneous magnitude directly linked to the parameters mentioned above, in particular to the concentration. It can vary from a maximum value of ca. 40% in pure liquid phase to very low values (10−2%) in diluted media (pollutants trace eliminations). To establish true photocatalytic normalized tests, the above recommendations must be observed with a real catalytic activity independent of non-catalytic side-reaction. In particular, dye decolorization, especially in the visible, provides an apparent “disappearance” of the dye, due to a limited stoichiometric electron transfer from the photo-excited dye molecule to titania, subsequently compensated by an additional ionosorption of molecular oxygen.The energetics of photocatalysis on TiO2, being based on the energy E of the photons, i.e. E  ≥ 3.2 eV, enables one to produce OH• radicals, the second best oxidizing agent. The decrease of energy E to the visible may be thermodynamically detrimental for the generation of such highly cracking and degrading species. Concerning solid state chemistry, it is now finally admitted that cationic doping is detrimental for photocatalysis. In conclusion, all these recommendations have to be addressed and experiments have to be operated in suitable conditions before claiming that one deals with a true photocatalytic reaction.
Keywords: Photocatalysis; Fundamentals; Right protocol; Misconceptions;

The visible light activity of TiO2 particles was improved by Fe3+-doping. In order to characterize and describe the effect of Fe3+-doping on the electronic and structural properties of TiO2, a combination of experimental structural methods and density functional theory (DFT) calculations was used. A series of Fe3+-doped photocatalysts with different Fe3+ contents were prepared by an incipient wet impregnation method, in order to prevent penetration of the dopant cations into the bulk of TiO2. An obvious decrease in the band-gap and a red shift of the absorption threshold were observed by UV-DRS. The Fe3+-doped photocatalysts were characterized by FT-IR, XRD, Raman and XPS. The morphological structure of the photocatalysts was examined by SEM. Energy-dispersive X-ray analysis (EDX) in the SEM was also taken for the chemical analysis of the doped samples. The results indicate substitutional Fe3+-doping of TiO2. In the computational part of the study, a neutral, stoichiometric cluster Ti3O8H4 cut from the anatase bulk structure and three new models for the substitutional Fe3+-doped TiO2 were developed. The DFT calculations were carried out by the hybrid B3LYP functional, by using double-zeta, LanL2DZ basis set. A higher photocatalytic activity for the degradation of 4-nitrophenol was obtained for the Fe3+-doped TiO2 compared to the undoped TiO2. The results of the DFT calculations indicate that the origin of the visible light activity of the Fe3+-doped TiO2 is due to the introduction of additional electronic states within the band-gap.
Keywords: TiO2; DFT calculations; Fe3+-doping; 4-Nitrophenol; Heterogeneous photocatalysis;

Display Omitted▶ Photocatalyzed removal of organic, sulfur, and soot layers is modeled. ▶ All true reaction kinetics are zero order, but physical phenomena (light absorption, surface diffusion) occurring simultaneously can disguise the apparent kinetic order or rate constant value. ▶ Comparison of models to literature data is good for all cases presented.In each case, we consider a simple film-catalyst configuration, propose a corresponding one-dimensional physical model for reaction, and compare model results with literature data to evaluate the correspondence between model and experiment. These examples cover both direct and lateral oxidation by photocatalysis. The respective physical and chemical phenomena which determine these rates of film removal include intrinsic catalyst kinetics (1), simultaneous reaction and light attenuation (2), reaction with light absorption by non-transparent organic film (3), and oxidant lateral transport (surface diffusion) (4). In each case, a simple model suffices to represent the key kinetic phenomena. In all cases, the true kinetic order is zero, but the apparent order may be influenced by light absorption (case 2). The apparent rate constant may be influenced by catalyst light absorption (case 2) or overlayer (case 3), or by catalyst–reactant separation (case 4).
Keywords: Photocatalysis; Film removal; Self-cleaning surfaces; Direct; Lateral oxidation;

The TiO2-photoassisted degradations of the anionic dodecylbenzenesulfonate (DBS) and cationic hexadecyltrimethylammonium (HTAB) surfactants have been examined in aqueous media along with the cationic/anionic 1:1 binary complexes of DBS/HTAB. The processes were monitored by surface tension measurements, total organic carbon (TOC) assays and carbon dioxide evolution. The degradation of the anionic surfactant was significantly involved in prior adsorption of the species on the positively charged metal-oxide particle surface. By contrast, the degradation of the cationic surfactant was rather limited owing to the lack of adsorption on the TiO2 surface under the same conditions. An adsorption model is proposed for the binary complexes on the metal-oxide surface.
Keywords: Cationic surfactant; Anionic surfactant; DBS; HTAB; Photoassisted degradation; Anionic–cationic surfactant complexes; TiO2;

Characterization of microwave effects on metal-oxide materials: Zinc oxide and titanium dioxide by Satoshi Horikoshi; Akihiro Matsubara; Sadatsugu Takayama; Motoyasu Sato; Futoshi Sakai; Masatsugu Kajitani; Masahiko Abe; Nick Serpone (490-495).
Microwave specific effects originating from either the magnetic and/or electric field of the microwaves that may impact the photoactivity of TiO2 and ZnO have been examined in the photodegradation of the 4-chlorophenol toxin.The microwave specific effect(s) that can impact a microwave-assisted and photo-assisted reaction occurring on the surface of ZnO or TiO2 (P-25) particles was (were) examined by comparing the process occurring under rich magnetic field conditions and under magnetic/electric field conditions. The features of the photo-assisted process in the presence of microwaves rich in a magnetic field (H) and an electric field (E) are described on the basis of (i) the degradation dynamics of 4-chlorophenol (4-CP) at ambient temperatures, (ii) the number of •OH radicals produced, and (iii) the dielectric properties of the metal oxides (in pellet form). For ZnO, the photoactivity is enhanced by a microwave specific non-thermal (i.e. non-caloric) effect originating from the microwaves’ magnetic field, but decreased by the thermal (i.e. caloric) factor originating from the microwaves’ electric field. Contrary to ZnO, the photoactivity of TiO2 (P-25) was enhanced by the synergistic effect between the magnetic and electric fields of the microwave radiation. Photocorrosion of ZnO in the aqueous dispersions was negligibly small (<0.05%) under UV, MW-EH, and UV/MW-EH irradiation conditions.
Keywords: Microwaves; Photocatalyst; Magnetic field; Electric field; 4-Chlorophenol; ZnO; TiO2;