Ronald Vargas

Measurement of phenols dearomatization via electrolysis: the UV-Vis solid phase extraction method.

Dearomatization levels during electrochemical oxidation of p-methoxyphenol (PMP) and p-nitrophenol (PNP) have been determined through UV-Vis spectroscopy using solid phase extraction (UV-Vis/SPE). The results show that the method is satisfactory to determine the ratio between aromatic compounds and aliphatic acids and reaction kinetics parameters during treatment of wastewater, in agreement with results obtained from numerical deconvolution of UV-Vis spectra. Analysis of solutions obtained from electrolysis of substituted phenols on antimony-doped tin oxide (SnO(2)--Sb) showed that an electron acceptor substituting group favored the aromatic ring opening reaction, preventing formation of intermediate quinone during oxidation.

Electrochemical oxygen transfer reactions: electrode materials, surface processes, kinetic models, linear free energy correlations, and perspectives

This work summarizes progresses achieved in electrochemical oxygen transfer reactions from water to organic pollutant molecules on metal oxide electrodes during the past two decades. Fundamental understanding of the dynamics of the electrochemical oxygen transfer reaction is of crucial importance for the development of key concepts of electrocatalytic processes, leading to the implementation of environmental electrochemistry wastewater treatment schemes with rational design of the suitable electrocatalytic systems. We discuss the current knowledge on the electrochemical oxygen transfer reaction, emphasizing the importance of surface processes in order to generalize mechanistically the experimental results obtained on different electrode materials, describing also the practical kinetic models developed and their implications. From the information gathered in this review, it is apparent that explanations for the kinetics of the reactions in relation to the structure of the organic compounds involved is lacking, hence that new information about structure-reactivity relationships is needed. We show in particular that the open circuit decay of the concentration of radical cations, obtained from spectroelectrochemical data, allows correlating the structure of adsorbed states with reactivity during oxygen transfer reactions, pointing as well to research efforts required to understand the catalytic performance of metal oxide electrodes in decomposing organic compounds strongly adsorbed on their surfaces. Finally, some perspectives for future research in this area are briefly commented.

The Photocatalytic Oxidation of 4-Chlorophenol Using Bi 2 WO 6 under Solar Light Irradiation

This report discusses the effects of the initial concentration of 4-chlorophenol (4-CP) on its solar light photoinduced oxidation/mineralization kinetics on Bi2WO6 catalyst. Photocatalytic degradation followed the Langmuir-Hinshelwood (L-H) mechanism. From the kinetic data the Langmuir adsorption equilibrium constant of 4-CP on the Bi2WO6 surface and the L-H maximum reaction rate for 4-CP oxidation have been evaluated. Chromatographic and spectroscopy studies show the presence of p-benzoquinone and maleic acid as the main reaction products; these compounds first increase and then decay until they disappear. Chemical oxygen demand (COD) and produced CO2 measurement show that photocatalytic mineralization of the phenolic compound was readily possible in a wide concentration range.

Photoelectrochemical solar cells based on Bi2WO6

In this study, photoelectrochemical solar cells based on bismuth tungstate electrodes were evaluated. Bi2WO6 was synthesized by a hydrothermal method and characterized by scanning electron microscopy, UV-Vis reflectance spectroscopy, and X-ray powder diffraction. For comparison, solar cells based on TiO2 semiconductor electrodes were evaluated. Photoelectrochemical response of Gratzel-type solar cells based on these semiconductors and their corresponding sensitization with two inexpensive phthalocyanines dyes were determined. Bi2WO6-based solar cells presented higher values of photocurrent and efficiency than those obtained with TiO2 electrodes, even without sensitization. These results portray solar cells based on Bi2WO6 as promising devices for solar energy conversion owing to lower cost of production and ease of acquisition.

Binary flux-promoted formation of trigonal ZnIn2S4 layered crystals using ZnS-containing industrial waste and their photocatalytic performance for H2 production

The accumulation of solid waste due to rapid industrialization has a negative impact on the environment. In this study, ZnS-containing waste from the mining-metallurgy industry was utilized for the synthesis of trigonal ZnIn2S4 layered crystals by a flux method using various binary fluxes: CaCl2 : InCl3, SrCl2 : InCl3, BaCl2 : InCl3, NaCl : InCl3, KCl : InCl3, and CsCl : InCl3. Among the binary fluxes used, KCl : InCl3 was found to be the most favorable for the synthesis of phase-pure trigonal ZnIn2S4 layered crystals. The XRD and SEM results revealed that the flux-grown ZnIn2S4 crystals have a trigonal structure and a morphology composed of large stacked layers. Unexpectedly, the UV-vis diffuse reflectance spectrum exhibited the onset of the absorption edge at approximately 700 nm for trigonal ZnIn2S4 crystals. The photocatalytic activities for H2 production of the Pt-photodeposited ZnIn2S4 samples grown using CaCl2 : InCl3, NaCl : InCl3, and KCl : InCl3 fluxes were evaluated. Trigonal ZnIn2S4 crystals grown using the KCl : InCl3 flux in this study exhibited higher photocatalytic activity for H2 evolution (132 μmol h−1) than previously reported hexagonal ZnIn2S4 synthesized by a hydrothermal method due to the decreased defect density and higher crystallinity achieved by the binary flux method. The presence of secondary crystalline phases (ZnS and In2S3) in ZnIn2S4 crystals grown using NaCl : InCl3 and CaCl2 : InCl3 fluxes positively impacted the photocatalytic activity and exhibited photocatalytic H2 evolution rates of 188 and 232 μmol h−1, respectively, because of the efficient separation and transfer of photogenerated charge carriers. The decay of the transient absorption of electrons in three samples at 2000 cm−1 was monitored by transient absorption spectroscopy, confirming that the lifetime of free electrons becomes longer depending on the binary flux used: KCl : InCl3 < NaCl : InCl3 < CaCl2 : InCl3. The binary flux method applied in this study demonstrates that accumulated solid industrial wastes can be turned into beneficial photocatalytic materials.

MECHANISTIC ASPECTS OF PHOTOCATALYTIC ACTIVITY OF METALLOPORPHYRIN-TITANIUM MIXTURES IN MICROEMULSIONS

Photocatalytic activity of titanium citrate and its water-in-oil microemulsions mix with zinc (II) tetraphenylporphyrin have been studied by means of adding 4-chlorophenol and following its degradation using artificial ultraviolet and visible light. 4-Chlorophenol is degraded in 71 and 86%, respectively, in 30 min with the generation of various intermediaries, whose possible formation mechanisms have been proposed based in the results of this research. The generation of benzene derivatives, different to those usually reported for this type of treatment, suggests the possibility of controlling the generation of intermediaries by simply changing the type of surfactant. In the case discussed here, we have derived a simple method for isomer generation of trimethylbenzene using a model organohalogenated compound as water contaminant and cetyltrimethylammonium bromide as surfactant.

ELECTROCHEMICAL OXIDATION OF LAMBDACYHALOTRIN ON PBO2-BI ELECTRODES

The electrochemical oxidation of lambdacyhalotrin in a triton X-100 water solution on a PbO2-Bi electrode has been studied. It was discovered that electrocatalytic degradation proceeded through the Langmuir-Hinshelwood (L-H) mechanism. The Langmuir adsorption equilibrium constant of the organic compound on the PbO2-Bi surface (0.67 (±0.02) mg-1L) and the L-H maximum reaction rate for lambdacyhalotrin oxidation (0.040 (±0.002) mg L-1 min-1) was also determined on the basis of kinetic data. Oxidation/mineralization was tested at electrode potential higher than 2.3 V vs. Ag/AgCl, in this conditions the higher degradation percent of 85 (±4) % has been obtained.