Nature and photoreactivity of of TiO2-rGO nanocomposites in aqueous suspensions under UV-A irradiation

This Accepted Manuscript will be available for reuse under a CC BY-NC-ND licence after 24 months of embargo periodDifferent studies to unravel the nature of the activity of TiO2 photocatalysts modified with rGO were carried out. The band edge potentials and the band gap energy of the nanocomposites were determined by performing electrochemical impedance and UV–vis diffuse reflectance measurements, respectively. However, no changes were observed when TiO2 is modified with rGO. Nevertheless, the presence of rGO in the hybrid composite led to a low charge transfer resistance across the electrode-electrolyte interface, observing even a tenfold increase in the photocurrent values in methanol photo-oxidation for P25-rGO 1%. Moreover, a higher oxygen reduction current was found when increasing the rGO concentration that could lead to a higher ROS formation. In order to analyze the beneficial properties of the hybrid materials, the influence of rGO doping ratio on oxalic acid photocatalytic degradation and on oxalic acid adsorption onto the nanocomposites surface was studied, both of them under UV-A light irradiation. In addition, the photoactivity of the conduction band electrons and the valence band holes was investigated by performing EPR and transient absorption spectroscopy measurements under UV-A illumination in O2 or N2 atmospheres. It was demonstrated that rGO behaved as an electron acceptor. Finally, TAS results under O2 and N2 atmospheres proved that the role played by rGO was not as crucial in excess of dissolved O2 as in N2 atmosphere. These findings agree with the observed photocatalytic activity and EPR measurements. Nevertheless, generation of HO[rad] in N2 saturated suspensions was highly increased with the addition of rGOThis work has been supported by the Spanish Plan Nacional de I+D+i through the project CTM2015-64895-R and CTM2016-76454-R. Alvaro Tolosana-Moranchel thanks to Ministerio de Educación, Cultura y Deporte for his FPU grant (FPU14/01605

Photocatalytic conversion of acetate into molecular hydrogen and hydrocarbons over Pt/TiO2: pH dependent formation of Kolbe and Hofer-Moest products

The photocatalytic generation of H2 and hydrocarbons from aqueous acetic acid has been studied employing TiO2 P25-based photocatalyst particles. The effect of pH on the distribution of reaction products and their formation rates during the photochemical as well as the photoelectrochemical oxidation of acetic acid has been investigated. The photocatalytic formation rates for H2 and hydrocarbons were found to be higher for Pt-loaded TiO2 than for bare TiO2. The major reaction products resulting from the photocatalytic decomposition of aqueous acetic acid, as determined quantitatively in the gas phase, were found to be H2, CO2, and CH4. Furthermore, traces of C2H6, C3H8, CO, CH3CHO, HCHO, CH3OH, C2H5OH, and HCOOH were also detected. After 15 h of illumination, the average formation rates of H2, CO2, CH4, and C2H6 evolved at pH 2 were found to be 22, 65, 35, and 2 ?mol/h, respectively. The ratio of H2 to hydrocarbons strongly depends on the pH values, i.e., at acidic pH the ratio of H2 to CH4 formation was found to be 0.6. On the contrary, at neutral and basic pH values negligible amounts of hydrocarbons were formed and H2 was found to be the main product with formation rates of 12 and 5 ?mol/h at pH 7 and 11, respectively. It is therefore assumed that the hydroxide ion has a significant effect on the reaction pathways. Due to the fact that methanol and ethanol are formed as reaction products, water or hydroxide ions are apparently required for the formation of the major oxidizing agent, that is the hydroxyl radical. Herein, a detailed mechanism for the photocatalytic decomposition of acetic acid at different pH values is presented. ? 2017 Elsevier Inc.119sciescopu

معالجة المياه الصناعية الملوثة باستخدام الحفازات المنشطة ضوئياً

A novel Plexiglas double-skin sheet reactor ( DSSR ) able to use diffuse as well as direct sunlight for the detoxification of industrial wastewater was used. The efficiency of the photocatalytic treatment of a highly polluted industrial wastewater was studied. Two different sets of experiments were performed. In the first set of experiments the original wastewater was teated photocatalytically without any pre-treatment. The TOC decrased from 605 mg/L to 373 mg/L within 57.4 hours of illumination. In the second set of experiments the wastewater was pretreated by sedimentation after the addition of a flocculent followed by the photocatalytic oxidation process. The TOC was reduced from 880 ppm before treatment to only 19.7 ppm after treatment in 16.7 hours.أستخدم مفاعل ذي مزايا نادرة إِذ أن له القدرة على استخدام أشعة الشمس أو أشعة اصطناعية لمعالجة المياه الصناعية شديدة التلوث . استخدمت في هذا البحث طريقتين . في الطريقة الأولى أستخدم المحفز المنشط ضوئياً مع الماء الملوث وكانت النتيجة انخفاض كمية الملوثات من 605 إلى 373 جزء بالمليون وذلك في 57.4 ساعة من التعرض للضوء . في الطريقة الثانية تم معالجة المياه كيميائياً ثم استخدمت الطريقة الضوئية فانخفضت الملوثات من 880 وحتى 19.7 جزء بالمليون في 16.7 ساعة

Tuning the photocatalytic selectivity of TiO2 anatase nanoplates by altering the exposed crystal facets content

TiO2 anatase nanoplates were fabricated by a solvothermal method using titanium isopropoxide as a titanium precursor and HF as a capping agent in order to enhance the formation of the {001} crystal facets of the anatase crystal. Two different surface modification procedures were applied in order to remove the adsorbed fluoride anions on the {001} crystal facets of the nanoplates. The first procedure was by calcining the as-prepared TiO2 anatase nanoplates up to 600°C and the second one was by washing them with a NaOH aqueous solution. Importantly, the surface modification procedure leads to the formation of two different morphologies of the TiO2 anatase nanoplates which exhibited tunable photocatalytic selectivity in air pollutants purification. The calcined nanoplates became larger and their {101} crystal facets expanded by shrinking the {001} crystal facets. In contrast the washed nanoplates maintained their shape which was formed by the solvothermal method. All samples that were calcined or washed, exhibited high photonic efficiency for air pollutants oxidation. The calcined TiO2 anatase nanoplates exhibited the best photocatalytic activity in oxidizing the NO gas to NO2 and NO3- whereas the washed TiO2 anatase nanoplates, preserving the initial morphology, exhibited the best photocatalytic activity in decomposing acetaldehyde. The dominant exposed {101} or {001} crystal facets of the TiO2 anatase nanoplates is the key factor in tuning the adsorption selectivity of the air pollutants. © 2013 Elsevier B.V

Nitrogen(II) Oxide Charge Transfer Complexes on TiO2: A New Source for Visible-Light Activity

A combined experimental and computational study of the photocatalytic activity of titanium dioxide (TiO2) for the degradation of nitrogen(II) oxide (NO) and acetaldehyde is reported. We employ five different TiO2 photocatalysts including pure anatase and rutile samples. The experimental photonic efficiencies indicate that, under visible irradiation, the samples containing pure TiO2 are active for the decomposition of NO but inactive for the acetaldehyde degradation. This is in accordance with our theoretical predictions, which reveal the presence of weak absorption bands in the visible region of the absorption spectra of the TiO2–NO complexes. We demonstrate that these bands originate from charge-transfer excitations between the pollutant and the substrate. Although a ligand-to-metal charge-transfer process is expected to predominate, we find a competing mechanism in which one electron is promoted from the valence band of the semiconductor to the virtual π* states of NO. Both experimental and theoretical results show an enhanced vis-activity of anatase TiO2 compared to rutile. We observe from the theoretical simulations the formation of reactive monocoordinated oxygen atoms at the anatase (001) surface for moderately low concentrations of the contaminant. Based on these findings, a new mechanism for the photo-oxidation of NO is proposed

TiO2-reduced graphene oxide nanocomposites: Microsecond charge carrier kinetics

This Accepted Manuscript will be available for reuse under a CC BY-NC-ND licence after 24 months of embargo periodIn this work, transient absorption spectroscopy studies in the microsecond time scale were carried out to investigate the dynamics of photogenerated electron-hole pairs in TiO2-rGO nanocomposites, prepared by a hydrothermal method, under different atmospheres: N2, O2, and N2 saturated in CH3OH. Under N2 atmosphere, the transient absorption signal detected in the region between 450 and 700 nm dropped as the rGO mass concentration in the composite was raised. The electron transfer from TiO2 to rGO was confirmed by using a model based on fractal surfaces which describes the decay kinetics. In the presence of methanol as hole acceptor, P25-rGO 0.5% and 1% were able to reach the maximum transient absorption faster than the other studied nanocomposites. However, after 10 μs, the P25-rGO 0.1% nanocomposite yielded the highest transient absorption signal and the best conversion and initial reaction rate in the photocatalytic degradation of dichloroacetic acid in aqueous suspensions. The effect of rGO on free electrons was investigated by detecting the transient signal at 980 nm under N2 saturated in CH3OH, for the different samples. It was found that the measured signals followed the same response than at 660 nm further evincing the electron transfer process. No sensitization effect of rGO was observed when the samples were excited at 450 nm.This work has been supported by the Spanish Plan Nacional de I+D+i through the project CTM2015-64895-R. Alvaro Tolosana-Moranchel thanks to Ministerio de Educación, Cultura y Deporte for his FPU grant (FPU14/01605) and (EST16/00065). The authors are also grateful to Technical Research Support Unit of Instituto de Catálisis y Petroleoquímica (CSIC