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The photodegradation of methyl orange in water by the catalyst TiO2 or the oxidants (H2O2, KIO4) or their combination (TiO2 + H2O2, TiO2 + KIO4) under UV light illumination is studied. Duri...

The photodegradation of methyl orange in water by the catalyst TiO

Doping of TiO2 with element Na from NaCl and immobilization of TiO2 on raschig rings are carried out simultaneously by calcination at 500 °C for 5 h. XPS survey spectrum of the Na(0.1% mol)–TiO2 (500 °C–5 h) shows that the element Na can be penetrated into TiO2 lattice. The Na+ in the Na(0.1% mol)–TiO2 (500 °C–5 h) captures the photogenerated electron and Na+ is reduced to Na, the electron and the hole can be separated effectively, which causes Na(0.1% mol)–TiO2 (500 °C–5 h) to show stronger photocatalytic activity than pure TiO2 (500 °C–5 h). Repetitive photocatalytic tests show that the Na(0.1% mol)–TiO2/RR (500 °C–5 h) is stable enough in water. Based on the XPS data analysis, we point out the presence of Ti3+ and Ti4+ and the conversion between them on the catalyst surface, the formation of Ti3+ is beneficial to the photocatalysis.

In this work, we have studied the decolouration of 0.10 mM methyl orange in the presence of (1) H 2 O 2 , (2) oxone, (3) the combination of Fe 2+ /H 2 O 2 , (4) the combination of Co 2+ /oxone, (5) the combination of Co 2+ /H 2 O 2 , (6) the combination of Fe 2+ /oxone. Under the experimental conditions, the data show that H 2 O 2 itself cannot oxidize methyl orange, oxone itself can oxidize methyl orange; all combinations can oxidize methyl orange. The acceleration sequence is: Co 2+ /oxone > oxone > Fe 2+ /oxone > Fe 2+ /H 2 O 2 > Co 2+ /H 2 O 2 > H 2 O 2 (no effect). The roles of Fe 2+ and Co 2+ are catalysts, through recycling–catalysis of M 2+ (M = Fe, Co) in the combination of M 2+ /oxidant (M = Fe, Co; oxidant = H 2 O 2 , oxone), a great quantity of HO are generated, active HO can attack methyl orange molecules and accelerate the decolouration of methyl orange. The accelerations from the combinations of oxone and different Co 2+ -salts (CoSO 4 , CoCl 2 , Co(NO 3 ) 2 , CoAc 2 ) are almost the same for the decolouration of the azo dye methyl orange (with one azo bond) or congo red (with two azo bonds).

This work presents the preparation and photocatalytic activity of BiOX (X = Cl, Br), it is feasible to prepare BiOX by the hydrolysis of BiX 3 (X = Cl, Br). Both BiOCl and BiOBr show photocatalytic activity for the photodegradation of azo dyes (methyl orange and congo red) under the simulated sunlight. We have measured the HO generated in the above process by terephthalic acid indirectly. Both BiOCl and BiOBr have been characterized by XPS, DRS, SEM, TEM, XRD and ξ potential. XPS data indicate that the surfaces of the BiOX samples are covered by low amounts of adsorbed azo dye molecules or/and intermediates produced during the dye photodecomposition.

Parylene films loaded with TiO2 are reported as hotocatalysts in azo dye discoloration processes. The TiO2 loading of the parylene film was 0.32% (w/w) and the amount of TiO2 on the film was about two orders of magnitude below the TiO2 added in suspension to discolore the same solution of Methyl Orange used as a probe. The parylene/TiO2 films showed a similar activity in the presence of O2 or H2O2 during the discoloration of dyes. This shows the efficient role of O2 as $e_{cb}^-$ scavenger. The photonic efficiency of the parylene/TiO2 film during the Methyl Orange discoloration was 0.04. Based on X-ray photoelectron spectroscopy (XPS) data, the TiO2 particles loaded on the parylene film were shown to be at first encapsulated in the polymer. After the encapsulation is broken, the TiO2 particles are fully exposed to the dye solution. The lack of surface intermediates like C-residues, N and S-species after the photocatalytic process implies an efficient decomposition of the dye at the catalyst interface. During the dye degradation carbonates and carboxylates were detected by XPS and Fourier transform infrared spectroscopy (FTIR) disappearing at the end of the discoloration process. Evidence is presented during the photocatalysis for the formation of a composite parylene/TiO2 film. The formation of this composite involves surface modification of parylene (partial lost of chlorine) in the outermost surface layer with concomitant densification of the TiO2 particles on the parylene film. The parylene film presented a side with high rugosity and one with low rugosity attaching different amounts of TiO2 in each case as observed by transmission electron microscopy (TEM).

This study presents the synthesis and activity of ZnSO4–TiO2 coated Raschig rings (RR). The activity of the Zn-doped catalyst (from now on Zn–TiO2/RR) was found to be higher than of the pure Degussa P25 TiO2 during the photodegradation of organic compounds. The most suitable doping level of TiO2 for Zn was 4 mol% noted as Zn (4% mol)–TiO2/RR during the degradation of azo-dye Orange II and of the transparent 2-propanol solutions. The degradation kinetics of the probe molecules was systematically investigated as a function of the catalyst make-up and some of the solution parameters. XPS measurements show Zn 2p3/2 peak positions for Zn–TiO2 sample in the range 1021.58–1022.00 eV. This energy range was observed to be lower than 1022.54 eV recorded for the reference ZnSO4 sample. This shift indicates that the Zn-atoms act as electron acceptors in the Zn-doped TiO2 and may be responsible for the higher catalytic activity observed for the Zn (4% mol)–TiO2/RR compared to the TiO2/RR photocatalyst during Orange II degradation. Insight is given onto the complex oxidation/reduction processes takes place at the interface of Zn–TiO2. The changes occurring in the Zn, TiO2 and S catalyst components are discussed as a function of the time of reaction. The Zn–TiO2 catalyst was characterized by elemental analysis (EA), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), confocal microscopy, X-ray photoelectron microscopy (XPS) and X-ray diffraction (XRD).

The accelerated discoloration of Orange II by an innovative Co3O4/Raschig ring photocatalyst (from now on Co3O4/RR) is feasible and proceeds to completion using oxone as an oxidant within the surprisingly short time of ∼5 min. The preparation of Co3O4 small clusters (2–10 nm in size) on RR is reported. The discoloration/mineralization of the azo-dye Orange II was carried out in a concentric coaxial photo-reactor and was a function of the Orange II and oxone concentrations, the solution pH and the recirculation rate. At bio-compatible pH-values, the concentration of Co-ions in solution after photocatalysis (15 min) was found to be between 0.5 and 2 ppm, within the limits allowed for treated waters. The generation of peroxide was observed as long as Orange II was still available in solution. By elemental analysis (EA), the amount of Co of the Raschig rings was determined to be ∼65% (w/w) before and after the photocatalysis. This confirms the stability observed during long-term operation of the Co3O4/RR catalyst. The sizes of the Co3O4 clusters on the RR surface were determined by transmission electron spectroscopy (TEM). A non-uniform distribution of Co3O4 particles on RR with sizes between 2 and 10 nm was found. The presence of Co-clusters on the RR-surface was confirmed by electron dispersive spectroscopy (EDS) showing 12.6% surface Co-enrichment before the photocatalysis and 18.8% surface enrichment after the photocatalysis. By confocal microscopy the irregularly thick shaped Co3O4 on the Raschig rings was analyzed. The most striking observation is very large shift of Co2p3/2 line from 779.6 eV at time zero to 782.2 eV within 10 min after due to the photocatalysis taking place. This indicates a strong reduction of electron density on the cobalt atoms of Co3O4/RR and providing the evidence for the strong oxidation properties of this catalyst.

The repetitive discoloration kinetics of the azo-dye Methyl Orange (taken as a model organic compound) was followed under solar simulated radiation (90 mW/cm2) to assess the performance of the TiO2/Tedlar® composite photocatalyst. The influence of solution parameters on the photo-discoloration process: pH, dye concentration, applied light intensity and concentration of H2O2 were systematically investigated. During the photocatalysis a modification occurs in the TiO2/Tedlar® composite due to the TiO2 interaction with the Tedlar® film. Physical insight is given for the stabilization mechanism of the TiO2 particles in the Tedlar matrix based on the data obtained by X-ray photoelectron spectroscopy (XPS). The F 1s peak of the Tedlar film indicates that the TiO2 is loaded on the Tedlar fluoro-groups. The loading of TiO2 on the 75 μm thick Tedlar® film was 0.9% (w/w) as determined by atomic absorption spectrophotometry (AAS). Attenuated total reflection infrared spectroscopy (ATRIR) shows no formation of additional bands within the photodiscoloration reaction. This shows that an efficient catalysis taking place on the TiO2/Tedlar® surface. The rugosity (mean square roughness, rms) of the TiO2/Tedlar® film was determined by atomic force microscopy (AFM) to be 19.7 nm. This value remained constant during long-term operation. Transmission electron microscopy (TEM) reports the thickness and coverage of TiO2 Degussa P-25 on the Tedlar® surface before and after photocatalysis.

Accelerated bleaching and photobleaching of diluted solutions of Methyl Orange and other dyes occur only when Co2+-ions are present in solution mediating oxone (2KHSO(5)center dot KHSO4 center dot K2SO4) decomposition. The bleaching of Methyl Orange, Orange II and Methylene Blue dyes in dilute solutions (0.01 mM) proceeds within a few minutes and occurs at biocompatible pH leading to a decrease in the toxicity of the initial solution under simulated daylight radiation. A reduction in the toxicity of 35% was observed at biocompatible pH-values when a solution Orange II (0.01 mM) was irradiated in the presence of oxone (0.06 mM)/Co2+ (0.004 mM). Only traces of Co2+ were necessary to accelerate the decomposition of the dyes in the presence of oxone in the dark and even more under daylight irradiation. The photobleaching proceeds with a photonic efficiency of similar to 0.24. The solution parameters were optimized for the photobleaching of azo-dyes by the oxone/Co2+ reagent. H2O2 generation was observed to be possible only as long as Orange II was present in the solution. The decomposition kinetics of H2O2 was followed under solar radiation. The dye decomposition was also investigated as a function of the applied light intensity. No saturation effects were observed when simulated solar light with 90% AM1 was applied. The photobleaching reaction proceeded with acceptable kinetics with light intensities 5-10 times lower than AM1. This makes the photocatalytic treatment suitable under diffuse daylight. (c) 2006 Elsevier B.V. All rights reserved.

An innovative way to fix preformed nanocrystalline TiO(2) on low-density polyethylene film (LDPE-TiO(2)) is presented. The LDPE-TiO(2) film was able to mediate the complete photodiscoloration of Orange II using about seven times less catalyst than a TiO(2) suspension and proceeded with a photonic efficiency of approximately 0.02. The catalyst shows photostability over long operational periods during the photodiscoloration of the azo dye Orange II. The LDPE-TiO(2) catalyst leads to full dye discoloration under simulated solar light but only to a 30% TOC reduction since long-lived intermediates generated in solution seem to preclude full mineralization of the dye. Physical insight is provided into the mechanism of stabilization of the LDPE-TiO(2) composite during the photocatalytic process by X-ray photoelectron spectroscopy (XPS). The adherence of TiO(2) on LDPE is investigated by electron microscopy (EM) and atomic force microscopy (AFM). The thickness of the TiO(2) film is seen to vary between 1.25 and 1.69 microm for an unused LDPE-TiO(2) film and between 1.31 and 1.50 microm for a sample irradiated 10h during Orange II discoloration pointing out to a higher compactness of the TiO(2) film after the photocatalysis.