Showing papers on "Photocatalysis published in 1995"
TL;DR: In this article, the authors focus on interfacial processes and summarize some of the operating principles of heterogeneous photocatalysis systems, including the electron transfer and energy transfer processes in photocatalytic reactions.
Abstract: In 1972, Fujishima and Honda discovered the photocatalytic splitting of water on TiO{sub 2} electrodes. This event marked the beginning of a new era in heterogeneous photocatalysis. Since then, research efforts in understanding the fundamental processes and in enhancing the photocatalytic efficiency of TiO{sub 2} have come from extensive research performed by chemists, physicists, and chemical engineers. Such studies are often related to energy renewal and energy storage. In recent years, applications to environmental cleanup have been one of the most active areas in heterogeneous photocatalysis. This is inspired by the potential application of TiO{sub 2}-based photocatalysts for the total destruction of organic compounds in polluted air and wastewaters. There exists a vast body of literature dealing with the electron transfer and energy transfer processes in photocatalytic reactions. A detailed description of these processes is beyond the scope of this review. Here, the authors tend to focus on interfacial processes and to summarize some of the operating principles of heterogeneous photocatalysis. In section 2, the authors first look at the electronic excitation processes in a molecule and in a semiconductor substrate. The electronic interaction between the adsorbate molecule and the catalyst substrate is discussed in terms of the catalyzed ormore » sensitized photoreactions. In section 3, thermal and photocatalytic studies on TiO{sub 2} are summarized with emphasis on the common characteristics and fundamental principles of the TiO{sub 2}-based photocatalysis systems. In section 4, they address the research effort in the electronic modification of the semiconductor catalysts and its effect on the photocatalytic efficiency. Several representative examples will be presented including the Schottky barrier formation and modification at metal-semiconductor interfaces. Some concluding remarks and future research directions will be given in the final section. 160 refs.« less
10,719 citations
TL;DR: In this paper, the photocatalysed oxidation of phenol, 2-chlorophenol and pentachlorophenol was re-examined under conditions in which TiO2 anatase was sensitized by CdS in air-equilibrated aqueous media; this was to assess whether or not the interparticle electron transfer pathway, first discovered a decade ago, could also be applied to photo-oxidative processes.
Abstract: The photocatalysed oxidation of phenol, 2-chlorophenol and pentachlorophenol was re-examined under conditions in which TiO2 anatase was sensitized by CdS in air-equilibrated aqueous media; this was to assess whether or not the interparticle electron transfer pathway, first discovered a decade ago (N. Serpone, E. Borgarello and M. Gratzel, J. Chem. Soc., Chem. Commun., (1984) 342) and subsequently applied to enhance reductive processes on titania, could also be applied to photo-oxidative processes. The results indicate that combinations such as CdS/TiO2 lead to an enhanced rate of disappearance of the initial substrate by a factor greater than two, consistent with the notion that (irradiated) CdS electrons are vectorially displaced onto the non-illuminated TiO2 particulates. Cadmium sulphide is a poorer photo-oxidation catalyst than titania. Other semiconductor materials have also been examined under a variety of conditions of pH and irradiation wavelength. The data also show that when both semiconductors in a coupled system are illuminated simultaneously and their valence and conduction bands are suitably disposed, both electron and hole transfer occur (as in the CdS/TiO2, ZnO/TiO2, TiO2/Fe2O3 and ZnO/Fe2O3 couples), which will influence the efficiency of photo-oxidation. N2O-saturated aqueous dispersions of TiO2 have no effect on the photo-oxidation of phenol, although it was expected that nitrous oxide would scavenge the photogenerated electrons in a manner similar to chemisorbed molecular oxygen, and enhance the efficiency. It is suggested that the role of oxygen in photo- oxidations may be more than just a simple electron scavenger.
641 citations
TL;DR: An important finding was that competitive adsorption between water and trace (sub-ppmv) contaminants has a significant effect on the oxidation rates of formaldehyde, toluene, and 1,3-butadiene.
Abstract: Persistent indoor air contaminants, those originating from emissions by interior furnishings, occupants, and materials of construction, typically exist in concentrations below 100 parts per billion (ppb) on an individual basis. The total of distinct contaminants may number in the hundreds with an equivalent accumulated concentration of one part per million. This study investigated the effects of humidity and trace (sub-ppmv) contaminant levels on the oxidation rates of formaldehyde, toluene, and 1,3-butadiene. The evaluation also included variations in UV intensity and flow residence time. UV intensities from inexpensive mercury fluorescent lamps, those which are expected to be employed in a practical photocatalytic purifier, are in the mW/cm 2 range. For this reason, the study included UV intensities in that range. The reactor element used in the study was a low pressure drop alumina reticulate, wash-coated with Degussa P25 titania. The data indicated that the reaction was first-order for the three reactants at the subppmv level. An important finding was that competitive adsorption between water and trace (sub-ppmv) contaminants has a significant effect on the oxidation rate. The dependencies of humidity and contaminant concentrations on the oxidation rates are explained as being the results of competitive adsorption on available hydroxyl adsorption sites and of changes in hydroxyl radical population levels
587 citations
TL;DR: The results presented here on the decolorization of the azo dye AO7 represents a major step forward in the development of new advanced oxidation processes for the treatment of such industrial waste.
Abstract: We present here for the first time the results from electrochemically assisted photocatalytic experiments using coupled TiO{sub 2}/SnO{sub 2} semiconductor thin films in the degradation of textile dye effluent. We show that by using such a system the oxidative efficiency of photocatalytic semiconductor systems in degrading a commercial azo dye such as Acid Orange 7 (AO7) can be improved. The results presented here on the decolorization of the azo dye AO7 represents a major step forward in the development of new advanced oxidation processes for the treatment of such industrial waste. The improved charge separation as a result of coupling two semiconductor systems with different energy levels and the applied anodic bias is responsible for the enhancement in the rate of photocatalytic degradation. 20 refs., 6 figs.
552 citations
TL;DR: In this article, the effect of incorporation of Si02 on the behavior of a TiO2-based photocatalyst prepared by a sol-gel technique from organometallic precursors is described.
Abstract: The effect of incorporation of Si02 on the behavior of a TiO2-based photocatalyst prepared by a sol-gel technique from organometallic precursors is described. Application of photocatalysts with different TiOd Si02 ratios to the photodecomposition of rhodamine-6G (R-6G) demonstrates that a ratio of 30/70 produces a catalyst about 3 times more active than Degussa P-25 TiO2. Larger amounts of Si02 decrease the activity. The adsorption of R-6G on the different materials and the photodecomposition of preadsorbed R-6G is also described. The studies suggest that photogenerated intermediates are sufficiently mobile to react with R-6G adsorbed on Si02 sites but that adsorption of R-6G on Ti02 is not a prerequisite for reaction.
474 citations
TL;DR: In this paper, the photocatalytic degradation of malic acid was investigated on several rigid substrates, including glass and stainless steel, and the resulting materials have been characterized by XPS, SEM/EDX, XRD and UV-vis absorption spectroscopy.
Abstract: In the present study TiO 2 has been supported on several rigid substrates. Deposition on glass and quartz was carried out by a dip coating procedure and the deposition on stainless steel by an electrophoretic deposition process. The resulting materials have been characterized by XPS, SEM/EDX, XRD and UV-vis absorption spectroscopy. The materials were then tested for the photocatalytic degradation of malic acid. For this reaction, the sample supported on quartz showed the highest catalytic activity. The photocatalytic activity pattern as a function of the nature of the support (fused silica, glass, stainless steel) followed the decreasing order: TiO 2 /quartz > TiO 2 /steel ≈ TiO 2 /glassaphotolysis. This decline in activity has been correlated with the presence of cationic impurities (Si 4+ , Na + , Cr 3+ , Fe 3+ ) in the layer as a consequence of the necessary thermal treatments to improve the cohesion of the titania layer and its adhesion onto the support.
471 citations
TL;DR: In this paper, an anatase-type TiO2 catalyst with large band gap and numerous surface OH groups showed high efficiency for the formation of CH4 and CH3H.
448 citations
431 citations
TL;DR: In this article, the reaction kinetics of five commercial dyes, namely, acid blue 40, basic yellow 15, Direct Blue 87, direct blue 160, and reactive red 120, were studied in a TiO2/UV slurry reactor.
349 citations
TL;DR: In this paper, the effect of the use of inert supports for TiO2 loading on photocatalyzed decomposition of propionaldehyde in the gas phase was investigated for mordenite support.
Abstract: Effects of the use of inert supports for TiO2 loading on photocatalyzed decomposition of propionaldehyde in the gas phase was investigated for mordenite support with various amounts of TiO2 loading and for several kinds of supports such as other zeolites, alumina, silica, and activated carbon. The adsorption constant and the amount of adsorption of propionaldehyde were evaluated for TiO2-loaded supports by obtaining Langmuir adsorption isotherms. By correlating these parameters to the photodecomposition rates of propionaldehyde, the involvement of the support in the photodecomposition reaction is clarified. The photocatalytic activity of TiO2 on mordenite having various amounts of TiO2 loading increases with increase in the amount of loaded TiO2 up to an optimum value (ca. 50 wt %), beyond which a decreasing tendency of the activity appeared. In the region of ascending activity, plenty of adsorbed substrate is available and the activity is controlled by the content of TiO2, while in the region of descending activity, the decrease in the amount of adsorbed substrate due to a decrease in the occupancy of the support by the TiO2 loading is responsible for the activity decrease. The photocatalytic activities are greatly influenced by the kind of inert supports used and show a volcano type dependence on the adsorption constant of the TiO2-loaded supports. In cases where the adsorption constant is low, the decomposition rate is determined by the amount of adsorbed substrate, while if the adsorption constant is very high, plenty of adsorbed substrate is available on the support, but it is not mobile to the loaded TiO2. We conclude that the use of an inert support having a medium adsorption constant is necessary to obtain the highest activity, where a high amount of adsorbed substrate that can be supplied to TiO2 particles is available.
280 citations
TL;DR: In this article, the degradation of methylene blue assisted by TiO2 photocatalysis was investigated in aqueous suspension, and the rate constants for this heterogeneous reaction were evaluated as a function of the concentration of the dye, the amount of TiO 2 and the pH.
Abstract: The degradation (oxidation) of methylene blue assisted by TiO2 photocatalysis (with an irradiation wavelength of 365 nm) was investigated in aqueous suspension. The rate constants for this heterogeneous photocatalysis were evaluated as a function of the concentration of the dye, the amount of TiO2 and the pH. The reaction is found to conform to a Langmuir adsorption isotherm and the rate vs. pH profile exhibits a linear increase with pH in acidic solutions with a maximum at around pH 6.9.
TL;DR: A photochemical reactor system that employs an optical fiber cable as a means of light transmission to solid supported TiO_2 and achieved a maximum quantum efficiency of Φ = 0.011 for the oxidation of 4-chlorophenol was achieved.
Abstract: We have developed, characterized, and utilized a photochemical reactor system that employs an optical fiber cable as a means of light transmission to solid supported TiO_2. Light energy is transmitted to TiO_2 particles, which are chemically anchored onto quartz fiber cores, via radial refraction of light out of the fiber. Operational factors that influence the efficiency of the bundled-array optical fiber reactor are as follows: the uniformity and extent of light propagation down the fiber, the degree of light absorption by the TiO_2 coating of the refracted light, and the ability of the chemical substrates to diffuse into the TiO_2 coating. A TiO_2 coating layer that minimizes the interfacial surface area of the quartz core and TiO_2 particles and operation with incident irradiation angles near 90" enhance light propagation down the fibers. A maximum quantum efficiency of Φ = 0.011 for the oxidation of 4-chlorophenol was achieved. This can be compared to a maximum quantum efficiency of Φ = 0.0065 for 4-chlorophenol oxidation in a TiO_2 slurry reactor operated under similar conditions.
TL;DR: In this paper, the authors used titania and platinized (0.1 wt.-%) titania photocatalysts for the degradation of benzene in oxygen-containing gaseous feed streams.
Abstract: Photocatalytic degradation of benzene in oxygen-containing gaseous feed streams was investigated using titania and platinized (0.1 wt.-%) titania photocatalysts. The titania catalyst was synthesized using sol-gel techniques. Results of this study indicate that, when using this particular photocatalyst, benzene was oxidized to carbon dioxide and water without forming any detectable organic reaction products in the reactor effluent, although only some of the benzene reacted. Both the overall conversion of benzene and its mineralization were improved by platinizing the titania. When the titania catalyst was platinized, both photocatalytic and thermocatalytic reactions were promoted. Rates of photocatalytic reactions were significantly enhanced at reaction temperatures between 70 and 90°C, while at temperatures above 90°C the rates of thermocatalytic oxidation reactions were noticeably increased. It proved possible to obtain the continuous and essentially complete mineralization of benzene by using the platinized titania catalyst and optimizing such parameters as the reaction temperature, space time, and the concentrations of oxygen and water vapor in the feed stream.
TL;DR: Transparent TiO2 thin films with high photocatalytic activity were prepared on glass substrates via the sol-gel method from tetraisopropyl titanium ethanol solution containing polyethylene glycol as mentioned in this paper.
Abstract: Transparent TiO2 thin films with high photocatalytic activity were prepared on glass substrates via the sol-gel method from tetraisopropyl titanium ethanol solution containing polyethylene glycol a...
TL;DR: In this article, transient reaction techniques were used to study the room-temperature, photocatalytic oxidation (PCO) of 2-propanol on a thin catalyst film of titania (Degussa P25) in an annular reactor.
TL;DR: In this paper, the reaction rate of trichloroethylene in solution with air is oxidized rapidly in the presence of irradiated titanium dioxide, and an expression for the prediction of rate as a function of reactant partial pressure is provided.
TL;DR: The photocatalytic degradation of 2,4-dichlorophenoxyacetic acid in UV-illuminated aqueous TiO 2 suspension was studied at pH 1-12 and the radical mechanism predominates for DCP transformation, whereas the hole mechanism predominate for carboxyl-bearing byproducts formed during late stages of mineralization.
Abstract: The photocatalytic degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) in UV-illuminated aqueous TiO 2 suspension was studied at pH 1-12. At pH ∼3, the initial step is chiefly direct hole (h + ) oxidation, while below and (especially) above pH 3, it shifts progressively to a hydroxyl radical (HO . )-like reaction following rate-limiting h + oxidation of surface hydroxyls. The hole pathway gives products expected from one-electron oxidation of the carboxyl group-near stoichiometric yield of 14 CO 2 from [carboxy- 14 C]-2,4-D and high yields of 2,4-dichlorophenol (DCP), 2,4-dichlorophenol formate (DCPF), and formaldehyde (HCHO)-and is little affected by 0.1 M methanol or tert-butanol. The weak competition by alcohols is proof of a surface reaction since the alcohols scavenge free HO . . The radical pathway results in low yields of 14 CO 2 , DCP, DCPF, and HCHO, indicating that attack shifts to the aromatic rings and is strongly inhibited by the alcohols. Solvent kinetic and product D isotope effects at pH 2 and pH 12 are consistent with the dual mechanism. Shift to the radical mechanism at low pH may result from a lower oxidation potential and/or coordinating ability of the R-CO 2H , while the shift at high pH is due to enhanced oxidation potential of the increasingly charged surface, charge repulsion of carboxylate, and/or OH- competition with carboxylate for coordination sites. At pH ∼3, judging from scavenger effects, the radical mechanism predominates for DCP transformation, whereas the hole mechanism predominates for carboxyl-bearing byproducts formed during late stages of mineralization.
TL;DR: In this article, the problems raised by the immobilization of a photocatalyst, namely TiO2, on a support are reviewed, from the standpoints of the procedure itself and of the structure, adherence, morphology and photocatalytic activity of the deposit.
Abstract: The problems raised by the immobilization (or anchoring) of a photocatalyst, namely TiO2, on a support are reviewed, from the standpoints of the procedure itself and of the structure, adherence, morphology and photocatalytic activity of the deposit. Some new experimental findings are also reported. It is concluded that the immobilized photocatalyst absorbs all the actinic light that it can, but the immobilization treatment generally lowers its activity. It would therefore be advisable to tailor a treatment which would at least maintain this activity, and even increase it, possibly by size quantization.
TL;DR: In this article, a Taylor vortex reactor was designed, constructed and tested as a novel photocatalytic reactor that incorporated illumination and dark recovery times via fluid mixing, and the photoefficiency in the reactor increased by nearly a factor of three with the optimal conditions of an inner cylinder rotation rate of 300 rpm and an unusually high weight loading of 10 g/l of TiO 2 photocatalyst.
TL;DR: The characterization results suggest that the photocatalyst consists of individual single crystal particles of the rutile and anatase phases of titania; no amorphous titania particles could be identified conclusively.
TL;DR: In this paper, it was found that TiO2 can be readily supported on polythene films without inhibiting the photocatalytic activity of phenol and showed that it is an excellent photocatalyst for complete mineralization of organic contaminants.
Abstract: TiO2 is known to be an excellent photocatalyst for complete mineralization of organic contaminants in water. However, the separation of TiO2 particles creates problems in practical application of the process. It is found that TiO2 can be readily supported on polythene films without inhibiting the photocatalytic activity. As an example, data on photocatalytic degradation of phenol by TiO2 supported on polythene films is presented.
TL;DR: In this article, the influence of the direction of ultraviolet irradiation upon the yields of the photocatalytic decomposition of pure water has been investigated in a Pt/TiO2 aqueous suspension system.
Abstract: The influence of the direction of ultraviolet irradiation upon the yields of the photocatalytic decomposition of pure water has been investigated in Pt/TiO2 aqueous suspension system. The yields of the photocatalytic decomposition irradiated from the top of the reaction cell are about 103 times higher than those irradiated from the bottom. The difference seems due to the reverse reaction of the formed H2 and O2 in the suspension.
TL;DR: In this article, an analysis of the photodegradation of TNT in a TiO2 slurry reactor is presented, and the rates and extent of TNT transformation and mineralization are compared for photocatalytic and direct photolytic reactions under conditions of varying light energies and in the presence and absence of oxygen.
TL;DR: In this paper, the feasibility of photocatalytic degradation of organophosphorus pesticides using thin films of TiO 2 was studied under illumination with a medium pressure mercury lamp of 375 W. The results showed that 0.65 x 10 -4 mol dm -3 of dichlorvos, monocrotophos, phorate, parathion can be completely photocatally degraded into PO 4 3- within a short time under illumination.
Abstract: This paper studies the feasibility of photocatalytic degradation of organophosphorus pesticides using thin films of TiO 2 . The results show that 0.65 x 10 -4 mol dm -3 of dichlorvos, monocrotophos, phorate, parathion can be completely photocatalytically degraded into PO 4 3- within a short time under illumination with a medium pressure mercury lamp of 375 W. The effects of parameters such as the amount of TiO 2 attached, initial concentration of organophosphorus pesticides, initial pH, amount of air flow, concentrations of H 2 O 2 and Fe 3+ on the photocatalytic degradation are studied. The possible mechanisms of photocatalytic degradation are discussed. It is our assumption that the degradation reactions occur on the surface of TiO 2 via primary produced oxidizing species such as the OH radical and O 2 2- . After 80 h illumination, there is no significant loss of the photocatalytic activity of TiO 2 .
TL;DR: In this paper, the authors studied the photocatalytic oxidation of 2,4-dichlorophenol by CdS in the presence and absence of thioacetamide as a function of pH.
TL;DR: In this paper, a model for the kinetics of photocatalytic processes on spherical TiO2 particles is developed, and the quantum yield for the photooxidation of organic molecules by oxygen in aqueous solution is derived for two cases.
TL;DR: In this paper, the photocatalytic degradation of phenol was investigated in the oxygen-saturated titanium dioxide suspensions in the presence of dissolved metal ions Ca 2+, Mg 2+, Zn 2+ and Ni 2+ in the concentration range 0.28-1.1 mM.
Abstract: The photocatalytic degradation of phenol was investigated in the oxygen-saturated titanium dioxide suspensions in the presence of dissolved metal ions Ca 2+ , Mg 2+ , Zn 2+ , Ni 2+ , Mn 2+ , Co 2+ , Cu 2+ , Cr 3+ and Fe 3+ in the concentration range 0.28–1.1 mM. The most photoactive systems for phenol degradation were found in the presence of ferric ions, while the addition of Cr 3+ strongly inhibited the photocatalytic decomposition of phenol. The role of dissolved metal ions in the photocatalytic processes on TiO 2 surface is discussed.
TL;DR: In this paper, the photocatalytic efficiency of the TiO2-containing papers was investigated by measuring the decomposition of gaseous acetaldehyde under illumination from a weak UV light source such as a conventional white fluorescent light bulb.
Abstract: TiO2-containing papers with highly photocatalytic activity have been prepared. Their photocatalytic activity was investigated by measuring the decomposition of gaseous acetaldehyde under illumination from a weak UV light source such as a conventional white fluorescent light bulb. It was found that the photocatalytic efficiency of the TiO2-containing papers was higher than that of Degussa P-25, one of the most efficient commercial TiO2 powders, indicating that paper pulp serves as a good matrix for highly efficient TiO2 photocatalysts.
TL;DR: In this article, the photocatalyzed degradation of all six isomeric forms of xylenol and the mineralization of 3,4-dimethylphenol were examined in air-equilibrated and oxygenated, illuminated titania dispersions.
Abstract: Dimethylphenols (xylenols) represent an important component of the phenolic substances in coal tar creosote, a mixture used widely as a wood preservative. The photocatalyzed degradation of all six isomeric forms of xylenol and the mineralization of 3,4-dimethylphenol were examined in air-equilibrated and oxygenated, illuminated titania dispersions. Some of the hydroxylated intermediates were identified by high performance liquid chromatography (HPLC). The effects of varying the pH, concentration of TiO2 and xylenol, temperature and light intensity on the process kinetics were also examined. Kinetic considerations emphasize the notion that the oxidative mineralization process follows natural saturation-type kinetics for which the Langmuir-Hinshelwood model, often invoked in photocatalysis, is a special case; however, it is emphasized that this model is not an a priori requirement in heterogeneous photocatalysis.