José Peral

Heterogeneous photocatalytic oxidation of gas-phase organics for air purification: Acetone, 1-butanol, butyraldehyde, formaldehyde, and m-xylene oxidation

Photocatalyzed degradations of trace levels of various oxygenates and an aromatic in air were carried out using near-UV-illuminated titanium dioxide (anatase) powder. The initial rates of degradation for acetone, 1-butanol, formaldehyde, and m-xylene were well described by Langmuir-Hinshelwood rate forms. No reaction intermediates were detected for acetone oxidation at conversions of 5–20%. Butyraldehyde was the main product of 1-butanol oxidation for conversions of 20–30%. The influence of 5% water (simulating partial humidification) in the feedstream varied strongly: water vapor inhibited acetone oxidation, but had no influence on the 1-butanol conversion rate. m-Xylene conversion was enhanced by trace water addition, but inhibited at higher water levels. Some catalyst deactivation was detected between 1-butanol runs; the activity could be easily recovered by illuminating the catalyst in fresh air. Formaldehyde was also successfully oxidized. These results, taken together with earlier literature citations for photocatalyzed total oxidation of methane, ethane, trichloroethylene (but see (27)), toluene, and a very recent report for oxidation of odor compounds, indicate a favorable technical potential for photocatalyzed treatment of air in order to degrade and remove all major classes of oxidizable air contaminants.

Heterogeneous Photocatalysis for Purification, Decontamination and Deodorization of Air

A research review of gas–solid heterogeneous photocatalysis is presented, ranging from details of pioneering works, which dealt with basic phenomena like oxygen and water vapor adsorption, to recent applications to pollutant removal in contaminated atmospheres. Special interest is taken in describing the different reactor configurations studied so far in this emerging and promising field.

Aniline mineralization by AOP's: anodic oxidation, photocatalysis, electro-Fenton and photoelectro-Fenton processes

Abstract The aniline degradation in acidic medium of pH≈3 under photocatalytic and electrochemical conditions has been investigated. The efficiency for substrate mineralization in each process has been comparatively analysed by the decrease in TOC of aniline solutions. Particular emphasis has been made on the role of Fe(II) ions and H 2 O 2 . The electrochemical experiments performed in the presence of both species (electro-Fenton conditions) leads to a fast aniline mineralization, which is notably increased by UVA irradiation (photoelectro-Fenton process). In photocatalysis with TiO 2 suspensions, the presence of H 2 O 2 and Fe(II) ions in solution notably increases the aniline degradation rate at the initial stages of the process, whereas the opposite effect occurs at long irradiation times. Benzoquinone, hydroquinone, nitrobenzene, phenol and 1,2,4-benzenetriol were detected as intermediates by HPLC in both, electrochemical and photocatalytic experiments. Short chain aliphatic acids, such as maleic and fumaric acids, were only found in the electrochemical experiments. Ammonium ions (75–80% of initial nitrogen) were generated in all solutions tested. A general reaction pathway that accounts for aniline mineralization to CO 2 involving those products is proposed.

Fenton and photo-Fenton oxidation of textile effluents

The simultaneous use of Fenton reagent and irradiation for the treatment of textile wastewaters generated during a hydrogen peroxide bleaching process is investigated. The experimental conditions tested during this study provide the simultaneous occurrence of Fenton, Fenton-like and photo-Fenton reactions. The batch experimental results are assessed in terms of total organic carbon reduction. Identification of some of the chemical constituents of the effluent was performed by means of GC-MS. Other pollution related features of the initial effluent-like COD and color were also measured. The main parameters that govern the complex reactive system, i.e., light intensity, temperature, pH, Fe(II) and H2O2 initial concentrations have been studied. Concentrations of Fe(II) between 0 and 400 ppm, and H2O2 between 0 and 10,000 ppm were used. Temperatures above 25 degrees C and up to 70 degrees C show a beneficial effect on organic load reduction. A set of experiments was conducted under different light sources with the aim to ensure the efficiency of using solar light irradiation. The combination of Fenton, Fenton-like and photon-Fenton reactions has been proved to be highly effective for the treatment of such a type of wastewaters, and several advantages for the technique application arise from the study.

Aniline degradation by combined photocatalysis and ozonation

Abstract The combination of TiO 2 -assisted photocatalysis and ozonation in the degradation of aniline in aqueous solution is investigated. From the experimental results obtained it is observed that the ozonation pretreatment followed by photocatalysis strongly increases the yield of TOC removal in comparison to either ozonation or photocatalysis carried out separately. The opposite sequence (photocatalysis pretreatment followed by ozonation) does not enhance the efficiency of aniline degradation. Nevertheless, the highest TOC removal was achieved by simultaneous ozonation and photocatalysis. A mechanism involving the formation of an ozonide anion radical previous to the generation of OH radicals is suggested to explain the synergic effect between ozone and TiO 2 under illumination.

2,4-Dichlorophenoxyacetic acid degradation by catalyzed ozonation: TiO2/UVA/O3 and Fe(II)/UVA/O3 systems

The oxidative degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) by means of two different advanced oxidation technologies based on ozone activation has been studied. Ozonation of 2,4-D in presence of UVA light and TiO2, and in presence of UVA and Fe(II), has been carried out. Both treatment methodologies have been compared in terms of the dependence on several experimental parameters like reaction time, pH, light intensity and catalyst concentration on reaction efficiency. It is shown that the initial solution pH practically has no influence on the efficiency of the processes, due to the fact that the pH rapidly decreases during the first minutes of reaction, always attaining a final value close to pH 3. A linear variation of TOC removal with square root of light intensity has been observed. Some dependence of the amount of catalyst (TiO2 or Fe(II)) in solution on TOC degradation has been detected. The rapidness of substrate dehalogenation has also been examined by following the time course of chloride concentration in solution.

Experimental design of Fenton and photo-Fenton reactions for the treatment of cellulose bleaching effluents

Multivariate experimental design was applied to the treatment of a cellulose conventional bleaching effluent in order to evaluate the use of the Fenton reagent under solar light irradiation. The effluent was characterised by the general parameters total organic carbon (TOC), chemical oxygen demand and color, and it was analysed for chlorinated low molecular weight compounds using GC-MS. The main parameters that govern the complex reactive system: Fe(II) and H(2)O(2) initial concentration, and temperature were simultaneously studied. Factorial experimental design allowed to assign the weight of each variable in the TOC removal after 15 min of reaction. Temperature had an important effect in the organic matter degradation, especially when the ratio of Fenton reagents was not properly chosen. Fenton reagent under solar irradiation proved to be highly effective for these types of wastewaters. A 90% TOC reduction was achieved in only 15 min of treatment. In addition, the GC-MS analysis showed the elimination of the chlorinated organic compounds initially detected in the studied bleaching effluents.

TiO2 deactivation during gas-phase photocatalytic oxidation of ethanol

The deactivation of TiO2 Degussa P25 during the gas-phase photocatalytic oxidation of ethanol has been studied. Water vapor plays a clear competitive role for surface sites adsorption, thus hampering the ethanol photo-oxidation. Dark adsorption of ethanol on a fresh catalyst shows a Langmuirian behavior with the formation of a monolayer of adsorbate. Dark adsorption in a TiO2 surface that has been used in consecutive photocatalytic experiments of ethanol degradation gives non-Langmuirian isotherms, indicating the existence of noticeable changes of the catalyst surface structure. After several irradiations the catalyst activity decreases. Such deactivation has been investigated, observing that the rate constant of ethanol and acetaldehyde (its main degradation product) oxidation decreases with irradiation time. Several surface treatments have been studied in order to find suitable procedures for catalytic activity recovery, but regular decay of activity is always observed after every treatment.

Photocatalytic degradation of short-chain organic diacids

Abstract The heterogeneous photocatalytic oxidation of fumaric, maleic and oxalic acids over TiO 2 has been investigated. For aqueous suspensions at pH lower than the point of zero charge (pzc) of TiO 2 , the photocatalytic degradation of the three studied diacids follows the Langmuir–Hinshelwood kinetic model, with the rate constant of the process decreasing in the order oxalic acid > maleic acid ≅ fumaric acid. At these low pH media, the adsorption of the organic diacids onto TiO 2 particles is a key feature for their degradation, which is initiated by a photo-Kolbe process. For fumaric and maleic acids, a cis – trans isomerisation induced by the interaction between adsorbed molecule and semiconductor surface occurs. At pH’s higher than the pzc of TiO 2 the rate of oxalic acid oxidation decreases noticeably, while fumaric and maleic acids are both efficiently degraded in homogeneous phase by reacting with OH radicals photochemically generated on the TiO 2 surface, giving rise to a significant increment of both isomers degradation rate with increasing pH. At these pH’s higher than the pzc of the TiO 2 , the three studied diacids show a very low degree of adsorption onto the semiconductor surface and no evidence of cis – trans isomerisation for both maleic and fumaric acids is detected. In accordance with the observed pH effects on degradation rate and over detected intermediates, a different mineralisation pathway is proposed as function of initial pH.

TiO2 photocatalyst deactivation by gas-phase oxidation of heteroatom organics

TiO2 deactivation is studied during the gas-phase photocatalytic oxidation of organics containing three different heteroatoms: Si, N and S. Auger electron spectroscopy is used to characterize the catalyst poisoning species. A kinetic equation is presented which account for the activity vs. time observed data. Finally, an estimation is made of the equivalent monolayers of reactant converted prior total activity disappearance.