Visible-Light-Induced Photocatalytic Inactivation of Bacteria by Composite Photocatalysts of Palladium Oxide and Nitrogen-Doped Titanium Oxide

Composite photocatalysts of palladium oxide and nitrogen-doped titanium oxide (PdO/TiON) were synthesized by a sol–gel process, as convenient forms of nanopowder or immobilized powder on nanofiber. The PdO/TiON catalysts were tested for visible-light-activated photocatalysis using different bacterial indicators, including gram-negative cells of Escherichia coli and Pseudomonas aeruginosa, and gram-positive cells of Staphylococcus aureus. Disinfection data indicated that PdO/TiON composite photocatalysts have a much better photocatalytic activity than either palladium-doped (PdO/TiO₂) or nitrogen-doped titanium oxide (TiON) under visible-light illumination. The roles of Pd and N were discussed in terms of the production and separation of the charge carriers under visible-light illumination. The photocatalytic activity was thus dependent on dopants and light intensity. Microscopic characterization demonstrated that visible-light photocatalysis on PdO/TiON caused drastic damage on the bacteria cell wall and the cell membrane.     

Quantitative photocatalyzed soot oxidation on titanium dioxide

We report here the titanium dioxide (TiO₂) photocatalyzed oxidation of deposited hurricane lamp soot. Sol–gel derived TiO₂ was coated on quartz crystal microbalance (QCM) elements. Characterization by spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) revealed low surface roughness of 0–17%, and SE showed a linear variation of the TiO₂ thickness versus the number of sol–gel spin coats.Soot was deposited on the calcined TiO₂ film using an analytical rotor passing through a hurricane lamp flame, and subsequently irradiated with near-UV light. Varying the soot mass on the TiO₂-coated QCM crystals revealed behaviors over 20,000 min ranging from total soot destruction of a single pass soot layer to minimal oxidation of an eight pass soot layer, the latter caused by soot screening of the incident UV light. A series/parallel reaction mechanism [P. Chin, G.W. Roberts, D.F. Ollis, Industrial & Engineering Chemistry Research 46 (2007) 7598] developed to describe previous literature data on TiO₂-catalyzed soot photooxidation was successfully employed to capture the longer time changes in presumably graphitic soot mass as a function of UV illumination time from 1000 to 20,000 min and of soot layer thickness. Short time soot mass loss is attributed to oxidation of organic carbons deposited on the graphitic soot components. This kinetic model can be used to predict the rate of TiO₂-catalyzed soot destruction as a function of near-UV illumination time and initial soot layer thickness.     

The investigation of gold/zirconia as a photocatalyst for the direct synthesis of imines from alcohols and aniline

Au/ZrO₂ nanoparticles have been widely used as photocatalysts in various organic syntheses because of their localized surface plasmon resonance (LSPR) effects. In our work, Au/ZrO₂ has been synthesized by a solution method and it was used as a heterogeneous catalyst in the synthesis of imines from alcohols and aniline with irradiation by visible light. The reaction occurred in two steps: step 1 was the aerobic oxidation of the alcohols and step 2 was the nucleophilic addition of aniline. Dimethyl sulfoxide (DMSO) was used as a solvent in the reaction. The selectivity in the synthesis of imines over 3 wt% Au/ZrO₂ (with mean particle size of 5 nm) was high (over 90%) with irradiation by visible light at room temperature, and an obvious difference in the conversion was observed between the reactions with light irradiation and those without light. The intensity and wavelength of the light strongly affected the reaction. The Au/ZrO₂ could be used at least 5 times. A reaction mechanism was proposed based on the experimental results. The results indicate that the reaction of alcohols and aniline using Au/ZrO₂ as the photocatalyst can proceed under mild conditions. Furthermore, this process is environmentally friendly and green.     

Stability studies of PbS sensitised TiO2 nanotube arrays for visible light photocatalytic applications by X-ray photoelectron spectroscopy (XPS)

Titania nanotube array sensitised with lead sulphide (PbS) were synthesised using a successive ionic layer adsorption and reaction (SILAR) method. The PbS sensitation gives visible light sensitivity to the titania nanotube arrays. The PbS sensitised titania nanotube were characterised using X-ray diffraction, UV–visible spectroscopy and scanning electron microscopy. The stability of the PbS sensitised titania nanotube were monitored through the X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of the titania sensitised with PbS were studied by monitoring the degradation of methylene blue under visible light irradiation. The photocatalytic activity and solar cell efficiency is found to be highly depended on the number of SILAR cycle used for the sensitisation of PbS on titania nanotube. The sample coated for 3 SILAR cycle of PbS shows highest solar cell efficiency and photocatalytic activity under visible light. The photocatalytic activity drastically decreased in the samples after the Ist cycle of photocatalytic reaction. The drastic decrease in the photoactivity is due to the dissolution of PbS nanoparticle on the surface of titania nanotube after the first cycle of photocatalytic reaction. XPS was employed as a tool to probe the dissolution of PbS nanoparticles.     

Photocatalytic degradation of pesticide contaminants in water

Photocatalysis has been proved to be an effective and inexpensive tool for the removal of organic and inorganic pollutants from water. Of particular interest in this context, in recent years, has been the complete photocatalytic mineralisation of a variety of pesticides into harmless products. The technique is now reaching the pre-industrial level, with several pilot plants and prototypes being operational in various countries. This paper reviews the major developments in the area, with special reference to the mechanism of the process involved, nature of the reactive intermediates and final products.     

Optimizing the solar photo-Fenton process in the treatment of contaminated water. Determination of intrinsic kinetic constants for scale-up

The elimination of aromatic compounds present in surface water by photo-Fenton with sunlight as the source of radiation was studied. The concentrations of Fe³⁺ and H₂O₂ are key factors for this process. A solar simulator and a prototype parabolic collector were used as laboratory-scale reactors to find the parameters of those key factors to be used in the CPC (compound parabolic collector) pilot plant reactor. The initial mineralization rate constant (k_obs) was determined and evaluated at different Fe³⁺ and H₂O₂ concentrations to find the best values for maximum efficiency. In all the experiments the mineralization of an aqueous phenol solution was described by assuming a pseudo-first-order reaction. The intrinsic kinetic constants not dependent on the lighting conditions were also estimated for scale-up.     

Photocatalytic performance of particulate semiconductors under natural sunshine—Oxidation of carboxylic acids

With natural sunlight, particulate TiO₂ (anatase), CuO, ZnO, Pb₂O₃, PbO₂ and Bi₂O₃ photomineralize oxalic acid and the mineralization depends linearly on the acid concentration and the surface area of the catalyst bed. Oxygen is essential for mineralization and the metal oxides show sustainable photocatalysis. While oxalic acid is effectively mineralized, the oxalate ion is not so. Pre-sonication fails to enhance the photocatalytic efficiencies and the system does not respond to application of a potential bias for enhancement of photocatalysis. The photocatalytic performance is of the order ZnO>CuOTiO₂Bi₂O₃Pb₂O₃>PbO₂ and the ease of degradation is formic acid>oxalic acid>acetic acid>citric acid.     

Combustion synthesis and characterization of NiO nanoparticles

Nickel oxide nanoparticles (NiO NPs) have been prepared by gel-combustion technique using 1:0.5 (N 105) and 1:1 (N 11) weight ratios of oxidizer, nickel nitrate hexahydrate and fuel, cassava starch, respectively. The X-ray diffraction pattern of the samples revealed cubic phase of nickel oxide and the average crystallite size of 28 and 38 nm for N 105 and N 11, respectively, were noted. In comparison to particle size measured using TEM (35 and 60 nm), the average particle size obtained from DLS (250 and 350 nm) was larger for N 105 and N 11, respectively. NiO NPs were found to have indirect band gap (2.98 and 2.3 eV) as revealed by optical spectroscopy. They were tested for energy storage, antimicrobial activity and photocatalytic applications. Electrochemical studies showed NiO NPs had a reversible capacity of 940 and 785 mA h/g for N 105 and N 11, respectively and they retained a capacity of 59% and 47% upto 50 cycles. The antimicrobial activity was tested against two bacterial strains and a fungal strain. The photocatalytic activity was measured for degradation of methylene blue in ultra-violet as well as sunlight. NiO, obtained by using 1:1 oxidizer to fuel ratio, (N 11) exhibited 94% degradation efficiency in sunlight because of a visible light-active band gap (2.3 eV).     

Modeling of the radiation field in a parabolic trough solar photocatalytic reactor

We calculate the distribution of absorbed radiation inside a solar photocatalytic reactor, by means of radiative transfer theory. The reactor configuration is that of a glass tube illuminated by a parabolic trough collector, where the catalyst consists of titanium dioxide micro-particles suspended in water. The calculations are made within the framework of the P1 approximation, which allows to solve analytically the radiative transfer equations. The obtained solution is used to study the effect of catalyst concentration on the degradation of pollutants, by means of a general kinetic model. The results obtained display the main features which are observed in experiments reported in the literature.     

Solar photoreactors comparison based on oxalic acid photocatalytic degradation

Solar heterogeneous photocatalytic degradation of oxalic acid in water is carried out in four different solar photoreactors: a parabolic trough concentrator (PC), a tubular collector (TC), a compound parabolic collector (CPC), and a V-trough collector (VC). The reactors operate under equal conditions of solar irradiance, collection surface and fluid flow rate to ensure a better comparison between the systems. The effects of TiO₂ catalyst concentration and radiation incidence angle on the degradation are studied. Oxalic acid degrades without appreciable generation of intermediates, and a simple kinetic model is proposed to describe the process. There are differences in the degradation rates depending on the collector geometry. The CPC shows the best overall performance in terms of accumulated energy, followed closely by the VC. Incidence angle affects the total amount of energy collected but does not reduce very much the efficiency of the reactors to use this energy in the photocatalytic process.