Synthesis, structure characterization and photocatalytic properties of two new uranyl naphthalene-dicarboxylate coordination polymer compounds

Two new uranyl-organic coordination compounds, UO₂(NDC)[(CH₃)₂SO)]₂ (1) and [UO₂(NDC)(CH₂OH)₂] (2) (H₂NDC = 1,4-naphthalene dicarboxylate), have been synthesized from the solutions of dimethyl sulfoxide (DMSO) and ethylene glycol (EG), respectively, through a volatilization method. Single-crystal X-ray diffraction analyses show that compound 1 consists of novel straight chains modified by pairs of DMSO molecules, whereas compound 2 features one-dimensional zigzag chains which are cross-linked by non-coordinating solvent molecules through hydrogen bonding interactions, forming a two-dimensional network. Both 1 and 2 decompose rhodamine B (RhB) molecules efficiently under the irradiation of UV and visible light.     

Integration of photocatalysis and membrane distillation for removal of mono- and poly-azo dyes from water

The paper reports investigations on the application of anatase-phase TiO₂ for the removal of azo dyes in a hybrid system coupling photocatalysis with direct contact membrane distillation (DCMD, MD). The process was conducted in a laboratory-scale installation equipped with a PP capillary module. The influence of reaction temperature and initial concentration of azo dyes on the effectiveness of their photodegradation was especially investigated. Two mono-azo dyes: Acid Red 18 (AR18) and Acid Yellow 36 (AY36) and one poly-azo dye, Direct Green 99 (DG99) were applied as model compounds. The increase of the reaction temperature from 313 to 333 K resulted in an improvement of the efficiency of photodecomposition of the dyes, as was found on the basis of changes of their masses in the feed solution. The comparison of the results obtained during photocatalysis alone and hybrid photocatalysis-MD process revealed that the reduction of feed volume in MD did not affect the photodegradation rate of the azo dyes. An improvement of the effectiveness of the degradation of dyes was obtained by an application of solutions with lower initial concentration (10 instead of 30 mg/dm³). Regardless of the process parameters applied, the product (distillate) was almost pure water with conductivity lower than 0.3 mS/m and pH above 5.2.     

A comparative study of photocatalytically active nanocrystalline tetragonal zyrcon-type and monoclinic scheelite-type bismuth vanadate

Monoclinic scheelite-type BiVO₄ is currently considered as one of the most promising non-titania photocatalysts, wheras tetragonal zircon-type BiVO₄ is still poorly understood. Herein, a new and simple synthetic approach was applied and nanostructured single-phase zircon-type BiVO₄ was successfully prepared by a controllable ethylene-glycol colloidal route. In addition, nanostructured monoclinic scheelite-type BiVO₄ powders were also fabricated through annealing of the as-prepared samples. A comparative study of the two BiVO₄ polymorphs was conducted and it turned out that the novel synthetic approach had a significant impact on porosity and photocatalytic performance of zircon-structured BiVO₄. All the prepared materials, as-prepared and annealed, were mesoporous, while measured values of specific surface area of some zircon-structured samples (～34?m²/g) were ～7 times higher than those reported thus far for this phase. Interestingly, for the first time, zircon-type BiVO₄, previously considered to be a poor photocatalyst, exhibited a better overall performance in degradation of methyl orange compared to monoclinic scheelite-type BiVO₄. Hence, it could be expected that the here-presented synthesis and observations will both arouse interest in scarcely studied tetragonal zircon-type BiVO₄ and facilitate as well as speed up further research of its properties.     

Nanocomposites of SnO2 and g-C3N4: Preparation,characterization and photocatalysis under visible LED irradiation

Tin dioxide nanoparticles were prepared in the presence of graphitized carbon nitride (g-C₃N₄) forming nanocomposites with different contents of SnO₂ up to 40 %. G-C₃N₄ was synthetized by heating of melamine at 550 °C in the open air and Sn²⁺ ions were precipitated by sodium hydroxide in g-C₃N₄ aqueous dispersions. Resulting mixtures were dried by freezing at ?20 °C and calcined at 450 °C to obtain SnO₂/g-C₃N₄ nanocomposites.The nanocomposites were characterized by common characterization methods in solid state and in their aqueous dispersions using dynamic light scattering (DLS) analysis and photocatalysis. SnO₂ nanoparticles in the nanocomposites were found to have an average size of 4 nm, however, those precipitated without g-C₃N₄ had an average size of 14 nm. Separation of photoinduced electron and holes via heterojunction between SnO₂ and g-C₃N₄ was demonstrated by photocatalytic decomposition of Rhodamine B (RhB) under LED visible irradiation (416 nm) and photocurrent measurements. The most photocatalytically active nanocomposite contained 10 % of SnO₂. Graphitized carbon nitride was assumed to serve as a template structure for the preparation of SnO₂ nanoparticles with a narrow size distribution without using any stabilizing additives.     

Mg3Y2Ge3O12:Bi3+ UV fluorescent phosphor as the TiO2 “sensitizer” for enhancing the heavy oil viscosity reduction

Nowadays, visible fluorescent materials based on rare earth (RE) and non-RE ions doping have been extensively explored for white LEDs. As for the UV fluorescent materials, it is well known that they are not suitable for the lighting applications. As a result, when compared to the visible fluorescent materials, previous works paid little attention to the UV fluorescent materials. In this work, we report a type of Mg₃Y₂Ge₃O₁₂:Bi³⁺ UV fluorescent phosphor. To understand the crystal structural information and photoluminescence (PL) properties of samples, we have used the X-ray diffraction (XRD), scanning electronic microscope (SEM), UV–visible diffuse reflectance and PL spectra to characterize them. The structural results reveal that the Bi³⁺ doped sample show their particle size at about 30 μm. The PL results show that the Bi³⁺ doped sample upon excitation at 230 nm can show a broad emission band that can almost cover the whole UV spectral region from 290 nm to 410 nm. Since this UV fluorescent band is exactly in agreement with the UV absorption region of TiO₂ semiconductor, we have fabricated several Mg₃Y₂Ge₃O₁₂:Bi³⁺/TiO₂-based ceramic plates and proposed used them to serve as an efficient UV irradiation source for photocatalytic application. As a result, we find that the TiO₂ can exhibit the significantly enhanced photocatalytic property for the heavy oil viscosity reduction after adding the Mg₃Y₂Ge₃O₁₂:Bi³⁺ UV fluorescent phosphor.     

Solar light-driven reduction of crystal violet by a composite of g-C3N4, β-Ag2Se, γ-Fe2O3 and graphite

Abstract

In this work, a new g-C₃N₄-based Z-scheme with γ-Fe₂O₃ and β-Ag₂Se both n-type semiconductors, and graphite to favor electron exchange is presented. The composite material was studied by XRD, FTIR, UV-Vis, TEM, XPS, TGA, DSC and TOF-SIMS, and the ability of this photocatalytic system to act as a photo-reductant was assessed using crystal violet (CV⁺) dye. Solar light driven photo-reduction of CV⁺ in the presence of tri-sodium citrate evidenced a synergistic enhancement of the activity of the composite toward reduction, with ～20 times higher conversion rates per unit of surface area than those of g-C₃N₄. Photo-oxidation experiments under Xe lamp irradiation in the presence of H₂O₂ also showed that the AgFeCN composite featured a higher activity (～8×) than g-C₃N₄. This Z-scheme may deserve further study as a photo-reductant to obtain hydrogen or hydrogenated compounds. Moreover, the use of CV⁺ may represent a facile procedure that can aid in the selection of new photocatalysts to be used in hydrogen production.     

Hydrogen production by photoreforming of formic acid in aqueous copper/TiO2 suspensions under UV-simulated solar radiation at room temperature

A new photocatalytic system consisting of two subsystems – Cu⁰/Cl⁻/H⁺/UV_ssr and Cu^II/H⁺/TiO₂/formic acid/UV_ssr – is proposed as a tool to produce hydrogen by reforming an organic species. Formic acid is used as hole scavenger during the experimental runs. An experimental campaign is carried out to demonstrate that the systems can generate hydrogen and to assess how the generation rate depends upon experimental conditions such as copper and TiO₂ loads, chloride and proton ions, and formic acid concentrations. A strong dependence of the Cu⁰/Cl⁻/H⁺/UV_ssr subsystem reactivity upon copper load, chloride concentration and pH is observed. The investigation on the complete system, starting from zero-valent copper, indicates that the addition of TiO₂ and formic acid to the Cu⁰/Cl⁻/H⁺/UV_ssr subsystem does not result into any gain in terms of hydrogen produced and, renders the system totally unreactive under some conditions. On the other hand, when a complete system, starting from cupric ions, is adopted, hydrogen production is observed also for prolonged reaction times with a surplus of hydrogen production with respect to that generated by the Cu⁰/Cl⁻/H⁺/UV_ssr subsystem with the same Cu⁰ starting load and in absence of TiO₂ and sacrificial agent.     

Heterogeneous photocatalytic hydrogen generation in a solar pilot plant

Few studies have been published about large scale heterogeneous photocatalysis hydrogen generation with simultaneous removal of organic pollutants. The purpose of the present work was to study the simultaneous photocatalytic hydrogen production and organic pollutant removal under direct solar irradiation at pilot-plant scale. The experiments were performed in a Compound Parabolic Collector (CPC) at the Plataforma Solar de Almería (PSA). The efficiencies of two different photocatalytic systems, one based on a nitrogen doped and platinized TiO₂, and the other using a platinized CdS–ZnS composite were evaluated. Formic acid and glycerol were used as sacrificial electron donors. Also, experiments using real municipal wastewaters were carried out showing simultaneous hydrogen generation and partial water pollutant removal. The largest amounts of hydrogen were obtained with aqueous solutions of formic acid, although the experiments with real wastewater gave moderate amounts of hydrogen, pointing towards the possible future use of such waters for photocatalytic hydrogen generation.     

Hydrogen production from water splitting under visible light irradiation using sensitized mesoporous-assembled TiO2–SiO2 mixed oxide photocatalysts

This work focused on hydrogen production from the photocatalytic water splitting under visible light irradiation using Eosin Y-sensitized mesoporous-assembled TiO₂–SiO₂ mixed oxide photocatalysts, of which the mesoporous-assembled TiO₂–SiO₂ mixed oxides with various TiO₂-to-SiO₂ molar ratios were synthesized by a sol–gel process with the aid of a structure-directing surfactant. The effects of SiO₂ content, calcination temperature, and phase composition of the mixed oxide photocatalysts were investigated. The experimental results showed that the TiO₂–SiO₂ mixed oxide photocatalyst with the TiO₂-to-SiO₂ molar ratio of 97:3 and calcined at 500 °C provided the maximum photocatalytic hydrogen production activity. The characterization results supported that the 0.97TiO₂–0.03SiO₂ mixed oxide photocatalyst (with the suitable SiO₂ content of 3 mol%) possessed superior physicochemical properties for the photocatalytic reaction as compared to the pure TiO₂, particularly higher specific surface area, lower mean mesopore diameter, higher total pore volume, and lower crystallite size, which led to an enhanced photocatalytic activity.