Light switching for product selectivity control in photocatalysis.

Artificial switchable catalysis is a new, rapidly expanding field that offers great potential advantages for both homogeneous and heterogeneous catalytic systems. Light irradiation is widely accepted as the best stimulus to artificial switchable chemical systems. In recent years, tremendous progress has been made in the synthesis and application of photo-switchable catalysts that can control when and where bond formation and dissociation take place in reactant molecules. Photo-switchable catalysis is a niche area in current catalysis, on which systematic analysis and reviews are still lacking in the scientific literature, yet it offers many intriguing and versatile applications, particularly in organic synthesis. This review aims to highlight the recent advances in photo-switchable catalyst systems that can result in two different chemical product outcomes and thus achieve a degree of control over organic synthetic reactions. Furthermore, this review evaluates different approaches that have been employed to achieve dynamic control over both the catalytic function and the selectivity of several different types of synthesis reactions, along with the remaining challenges and potential opportunities. Owing to the great diversity of the types of reactions and conditions adopted, a quantitative comparison of efficiencies between considered systems is not the focus of this review, instead the review showcases how insights from successful adopted strategies can help better harness and channel the power of photoswitchability in this new and promising area of catalysis research.

Cu@CuOx/WO3 with photo-regulated singlet oxygen and oxygen adatoms generation for selective photocatalytic aromatic amines to imines.

Photocatalysts can absorb light and activate molecular O2 under mild conditions, but the generation of unsuitable reactive oxygen species often limits their use in synthesizing fine chemicals. To address this issue, we disperse 1 wt% copper on tungsten trioxide (WO3) support to create an efficient catalyst for selective oxidative coupling of aromatic amines to imines under sunlight irradiation at room temperature. Copper consists of a metallic copper core and an oxide shell. Experimental and density functional theory calculations have confirmed that Cu2O is the primary activation site. Under λ < 475 nm, the light excites electrons of the valence bands in Cu2O and WO3, which activate O2 to superoxide radical •O2-. Then rapidly transforms into oxygen adatoms (•O) and oxygen anion radicals (•O-) species on the surface of Cu2O. Simultaneously, it is captured by holes in the WO3 valence band to generate singlet oxygen (1O2). •O bind to 1O2 promoting the coupling reaction of amines. When λ > 475 nm, intense light absorption due to the localized surface plasmon resonance excites numerous electrons in Cu to promote the oxidative coupling with the adsorbed O2. This study presents a promising approach towards the design of high-performance photocatalysts for solar energy conversion and environmentally-friendly oxidative organic synthesis.

Unraveling the Mechanism of Alkali Metal Fluoride Post-Treatment of SnO2 for Efficient Planar Perovskite Solar Cells.

The facile synthesis and beneficial properties of tin oxide have driven the development of efficient planar perovskite solar cells (PSCs). To increase the PSC performance, alkali salts are used to treat the SnO2 surface to minimize the defect states. However, the underlying mechanism of alkali cations' role in the PSCs needs further exploration. Herein the effect of alkali fluoride salts (KF, RbF, and CsF) on the properties of SnO2 and PSC performance is investigated. The results show different alkali have significant roles depending on their nature. Larger cations Cs+ preferably locate at the SnO2 film surface to passivate surface defects and enhance conductivity, while smaller cations like Rb+ or K+ cations tend to diffuse into the perovskite layer to reduce trap density of the material. The former effect leads to enhanced fill factor while the latter effect increases the open circuit voltage of the device. It is then demonstrated that a dual cation post-treatment of the SnO2 layer with RbF and CsF achieves PSC with a significantly higher power conversion efficiency (PCE) of 21.66% compared to pristine PSC with a PCE of 19.71%. This highlights the significance of defect engineering of SnO2 using selective multiple alkali treatment to improve PSC performance.

High Nitrile Yields of Aerobic Ammoxidation of Alcohols Achieved by Generating •O2- and Br• Radicals over Iron-Modified TiO2 Photocatalysts.

Catalytic ammoxidation of alcohols into nitriles is an essential reaction in organic synthesis. While highly desirable, conducting the synthesis at room temperature is challenging, using NH3 as the nitrogen source, O2 as the oxidant, and a catalyst without noble metals. Herein, we report robust photocatalysts consisting of Fe(III)-modified titanium dioxide (Fe/TiO2) for ammoxidation reactions at room temperature utilizing oxygen at atmospheric pressure, NH3 as the nitrogen source, and NH4Br as an additive. To the best of our knowledge, this is the first example of catalytic ammoxidation of alcohols over a photocatalyst using such cheap and benign materials. Various (hetero) aromatic nitriles were synthesized at high yields, and aliphatic alcohols could also be transformed into corresponding nitriles at considerable yields. The modification of TiO2 with Fe(III) facilitates the formation of active •O2- radicals and increases the adsorption of NH3 and amino intermediates on the catalyst, accelerating the ammoxidation to yield nitriles. The additive NH4Br impressively improves the catalytic efficiency via the formation of bromine radicals (Br•) from Br-, which works synergistically with •O2- to capture H• from Cα-H, which is present in benzyl alcohol and the intermediate aldimine (RCH═NH), to generate the active carbon-centered radicals. Further, the generation of Br• from the Br- additive consumes the photogenerated holes and OH• radicals to prevent over-oxidation, significantly improving the selectivity toward nitriles. This amalgamation of function and synergy of the Fe(III)-doped TiO2 and NH4Br reveals new opportunities for developing semiconductor-based photocatalytic systems for fine chemical synthesis.

High efficient arsenic removal by In-layer sulphur of layered double hydroxide.

High-risk arsenic contamination found in aqueous system is reported across the world and causing severe environmental issues. In this study, the Mg-Al Layered Double Hydroxide (LDH) modified by sulphur species (LDH-S) was found exhibiting high effectivity and selectivity in As(V) removal owing to the strong interaction between embedded HS- and AsO43-. The LDH-S with Mg to Al ratio 2-1 give the best performance with As(V) adsorption capacity 40.8 mg/g, which is 715% higher than that of pristine LDH (2-1). The adsorbent exhibits a high tolerance to concentrated competitive anions. In the continuous flow test, the adsorbent can reduce the As(V) concentration from 20 ppm to below-ppb-level indicating the potential in industry application. The adsorption mechanism is experimentally investigated and examined by Density Function Theory (DFT) calculation. The result illustrates that, differ from the traditional ion exchange mechanism of LDH, the enhanced removal capacity and selectivity of LDH-S for As(V) is attributed to the strong affinity between H atom from HS- ion (in the interlayer region of LDH) and the O atom from AsO43-.

Improved dark ambient degradation of organic pollutants by cerium strontium cobalt perovskite.

This work investigates the effect of cerium substation into strontium cobalt perovskites (CeSrCoO) for the oxidative degradation of Orange II (OII) in dark ambient conditions without the aid of any external stimulants such as light, heating or chemical additives. The OII degradation rate by CeSrCoO reached 65% in the first hour, whilst for the blank sample without cerium (SrCoO) took over 2 hr to reach the same level of OII degradation. Hence, the cerium substitution improved the catalytic activity of the perovskite material, mainly associated with the Ce0.1Sr0.9CoO3 perovskite phase. Upon contacting CeSrCoO, the -NN- azo bonds of the OII molecules broke down resulting in electron donation and the formation of by-products. The electrons are injected into the CeSrCoO and resulted in a redox pair of Co3+/Co2+, establishing a bridge for the electron transfer between OII and the catalysts. Concomitantly, the electrons also formed reactive species (·OH) responsible for OII degradation as evidenced by radical trapping experiment. Reactive species were formed via the reaction between O2 and donated electrons from OII with the aid of cobalt redox pair. As the prepared materials dispensed with the need for light irradiation and additional oxidants, it opens a window of environmental applications for treating contaminated wastewaters.

Remediation of Cr (VI) by inorganic-organic clay.

Application of inorganic-organic clays (IOCs) for the remediation of hexavalent chromium (Cr (VI)) and the effect of bisphenol-A (BPA) were investigated. IOCs were better adsorbents for the removal of Cr (VI) than organoclay. Solution pH significantly affected the surface charge of IOCs and speciation of Cr in the solution; hence, the adsorption of Cr onto IOCs. Pseudo-second order model and Langmuir model were the best fit for the kinetics and equilibrium data, indicating chemisorption and formation of monolayer on homogenous adsorption sites, respectively. Cr (VI) adsorption by IOCs was driven by electrostatic interactions between positively charged IOC surfaces and HCrO4- anions. The IOC adsorption capacity for Cr (VI) correlates with the amount of loosely held ODTMA on the external surface of the clay. Involvement of an anion-exchange reaction was suggested in which the counter halide ions of the cationic surfactants were replaced by HCrO4- anions. Cr (VI) adsorption was not affected by the presence of BPA in the medium and two mechanisms for the adsorption of BPA and Cr (VI) were suggested. Partial reduction of Cr (VI) to Cr (III) was observed on the surface of IOC and the reducing agent responsible was suggested to be the Fe (II) in montmorillonite.

Mixed Matrix Carbon Molecular Sieve and Alumina (CMS-Al2O3) Membranes.

This work shows mixed matrix inorganic membranes prepared by the vacuum-assisted impregnation method, where phenolic resin precursors filled the pore of α-alumina substrates. Upon carbonisation, the phenolic resin decomposed into several fragments derived from the backbone of the resin matrix. The final stages of decomposition (>650 °C) led to a formation of carbon molecular sieve (CMS) structures, reaching the lowest average pore sizes of ~5 Å at carbonisation temperatures of 700 °C. The combination of vacuum-assisted impregnation and carbonisation led to the formation of mixed matrix of CMS and α-alumina particles (CMS-Al2O3) in a single membrane. These membranes were tested for pervaporative desalination and gave very high water fluxes of up to 25 kg m(-2) h(-1) for seawater (NaCl 3.5 wt%) at 75 °C. Salt rejection was also very high varying between 93-99% depending on temperature and feed salt concentration. Interestingly, the water fluxes remained almost constant and were not affected as feed salt concentration increased from 0.3, 1 and 3.5 wt%.

Development of a hybrid pollution index for heavy metals in marine and estuarine sediments.

Heavy metal pollution of sediments is a growing concern in most parts of the world, and numerous studies focussed on identifying contaminated sediments by using a range of digestion methods and pollution indices to estimate sediment contamination have been described in the literature. The current work provides a critical review of the more commonly used sediment digestion methods and identifies that weak acid digestion is more likely to provide guidance on elements that are likely to be bioavailable than other traditional methods of digestion. This work also reviews common pollution indices and identifies the Nemerow Pollution Index as the most appropriate method for establishing overall sediment quality. Consequently, a modified Pollution Index that can lead to a more reliable understanding of whole sediment quality is proposed. This modified pollution index is then tested against a number of existing studies and demonstrated to give a reliable and rapid estimate of sediment contamination and quality.

Weak acid extractable metals in Bramble Bay, Queensland, Australia: temporal behaviour, enrichment and source apportionment.

Sediment samples were taken from six sampling sites in Bramble Bay, Queensland, Australia between February and November in 2012. They were analysed for a range of heavy metals including Al, Fe, Mn, Ti, Ce, Th, U, V, Cr, Co, Ni, Cu, Zn, As, Cd, Sb, Te, Hg, Tl and Pb. Fraction analysis, Enrichment Factors and Principal Component Analysis-Absolute Principal Component Scores (PCA-APCS) were carried out in order to assess metal pollution, potential bioavailability and source apportionment. Cr and Ni exceeded the Australian Interim Sediment Quality Guidelines at some sampling sites, while Hg was found to be the most enriched metal. Fraction analysis identified increased weak acid soluble Hg and Cd during the sampling period. Source apportionment via PCA-APCS found four sources of metals pollution, namely, marine sediments, shipping, antifouling coatings and a mixed source. These sources need to be considered in any metal pollution control measure within Bramble Bay.

Temporal trends and bioavailability assessment of heavy metals in the sediments of Deception Bay, Queensland, Australia.

Thirteen sites in Deception Bay, Queensland, Australia were sampled three times over a period of 7 months and assessed for contamination by a range of heavy metals, primarily As, Cd, Cr, Cu, Pb and Hg. Fraction analysis, enrichment factors and Principal Components Analysis-Absolute Principal Component Scores (PCA-APCS) analysis were conducted in order to identify the potential bioavailability of these elements of concern and their sources. Hg and Te were identified as the elements of highest enrichment in Deception Bay while marine sediments, shipping and antifouling agents were identified as the sources of the Weak Acid Extractable Metals (WE-M), with antifouling agents showing long residence time for mercury contamination. This has significant implications for the future of monitoring and regulation of heavy metal contamination within Deception Bay.

Fabrication of macro-mesoporous titania/alumina core-shell materials in oil/water interface.

A series of macro-mesoporous TiO2/Al2O3 nanocomposites with different morphologies were synthesized. The materials were calcined at 723 K and were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscope (TEM), N2 adsorption/desorption, Infrared Emission Spectroscopy (IES), X-ray photoelectron spectroscopy (XPS) and UV-visible spectroscopy (UV-visible). A modified approach was proposed for the synthesis of 1D (fibrous) nanocomposite with higher Ti/Al molar ratio (2:1) at lower temperature (<100°C), which makes it possible to synthesize such materials on industrial scale. The performance-morphology relationship of as-synthesized TiO2/Al2O3 nanocomposites was investigated by the photocatalytic degradation of a model organic pollutant under UV irradiation. The samples with 1D (fibrous) morphology exhibited superior catalytic performance than the samples without, such as titania microspheres.

Enrichment, distribution and sources of heavy metals in the sediments of Deception Bay, Queensland, Australia.

Sediment samples from 13 sampling sites in Deception Bay, Australia were analysed for the presence of heavy metals. Enrichment factors, modified contamination indices and Nemerow pollution indices were calculated for each sampling site to determine sediment quality. The results indicate significant pollution of most sites by lead (average enrichment factor (EF) of 13), but there is also enrichment of arsenic (average EF 2.3), zinc (average EF 2.7) and other heavy metals. The modified degree of contamination indices (average 1.0) suggests that there is little contamination. By contrast, the Nemerow pollution index (average 5.8) suggests that Deception Bay is heavily contaminated. Cluster analysis was undertaken to identify groups of elements. Strong correlation between some elements and two distinct clusters of sampling sites based on sediment type was evident. These results have implications for pollution in complex marine environments where there is significant influx of sand and sediment into an estuarine environment.

Decoration of titania nanofibres with anatase nanoparticles as efficient photocatalysts for decomposing pesticides and phenols.

Using a series of partial phase transitions, an effective photocatalyst with fibril morphology was prepared. The catalytic activities of these materials were tested against phenol and herbicide in water. Both H-titanate and TiO(2)-(B) fibres decorated with anatase nanocrystals were studied. It was found that anatase coated TiO(2)-(B) fibres prepared by a 45 h hydrothermal treatment followed by calcination were not only superior photocatalysts but could also be readily separated from the slurry after photocatalytic reactions due to its fibril morphology.

Surface modification of alumina nanofibres for the selective adsorption of alachlor and imazaquin herbicides.

The effective removal of pollutants using a thermally and chemically stable substrate that has controllable absorption properties is a goal of water treatment. In this study, the surfaces of thin alumina (γ-Al(2)O(3)) nanofibres were modified by the grafting either of two organosilane agents, 3-chloro-propyl-triethoxysilane (CPTES) and octyl-triethoxysilane (OTES). These modified materials were then trialed as absorbents for the removal of two herbicides, alachlor and imazaquin from water. The formation of organic groups during the functionalisation process established super hydrophobic sites on the surfaces of the nanofibres. This super hydrophobic group is a kind of protruding adsorption site which facilitates the intimate contact with the pollutants. OTES grafted substrate were shown to be more selective for alachlor while imazaquin selectivity is shown by the CPTES grafted substrate. Kinetics studies revealed that imazaquin was rapidly adsorbed on CPTES-modified surfaces. However, the adsorption of alachlor by OTES grafted surface was achieved more slowly.

Sodium niobate adsorbents doped with tantalum (TaV) for the removal of bivalent radioactive ions in waste waters.

Sodium niobates doped with different amounts of tantalum (Ta(V)) were prepared via a thermal reaction process. It was found that pure nanofibrils and bar like solids can be obtained when tantalum is introduced into the reaction system. For the well crystallized fibril solids, the Na(+) ions are difficult to exchange, and the radioactive ions such as Sr(2+) and Ra(2+) just deposit on the surface of the fibers during the sorption process, resulting in lower sorption capacity and distribution coefficients (K(d)). However, the bar like solids are poorly crystallized and have many exchangeable Na(+) ions. They are able to remove highly hazardous bivalent radioactive isotopes such as Sr(2+) and Ra(2+) ions. Even in the presence of many Na(+) ions, they also have higher K(d). More importantly, such sorption finally intelligently triggers considerable collapse of the structure, resulting in permanent entrapment of the toxic bivalent cations in the solids, so that they can be safely disposed of. This study highlights new opportunities for the preparation of Nb-based adsorbents to efficiently remove toxic radioactive ions from contaminated water.

Application of infrared emission spectroscopy to the thermal transition of indium hydroxide to indium oxide nanocubes.

Cubic indium hydroxide nanomaterials were obtained by a low-temperature soft-chemical method without any surfactants. The transition of nano-cubic indium hydroxide to cubic indium oxide during dehydroxylation has been studied by infrared emission spectroscopy. The spectra are related to the structure of the materials and the changes in the structure upon thermal treatment. The infrared absorption spectrum of In(OH)(3) is characterized by an intense OH deformation band at 1150 cm(-1) and two O-H stretching bands at 3107 and 3221 cm(-1). In the infrared emission spectra, the hydroxyl-stretching and hydroxyl-bending bands diminish dramatically upon heating, and no intensity remains after 200 °C. However, new low intensity bands are found in the OH deformation region at 915 cm(-1) and in the OH stretching region at 3437 cm(-1). These bands are attributed to the vibrations of newly formed InOH bonds because of the release and transfer of protons during calcination of the nanomaterial. The use of infrared emission spectroscopy enables the low-temperature phase transition brought about through dehydration of In(OH)(3) nanocubes to be studied.

Smectite flocculation structure modified by Al13 macro-molecules--as revealed by the transmission X-ray microscopy (TXM).

The aggregate structure which occurs in aqueous smectitic suspensions is responsible for poor water clarification, difficulties in sludge dewatering and the unusual rheological behaviour of smectite rich soils. These macroscopic properties are dictated by the 3D structural arrangement of smectite finest fraction within flocculated aggregates. Here, we report results from a relatively new technique, transmission X-ray microscopy (TXM), which makes it possible to investigate the internal structure and 3D tomographic reconstruction of the smectite clay aggregates modified by Al(13) Keggin macro-molecule [Al(13)(O)(4)(OH)(24)(H(2)O)(12)](7+). Three different treatment methods were shown resulted in three different micro-structural environments of the resulting flocculation. In case of smectite sample prepared in Methods 1 and 3 particles fall into the primary minimum where Van der Waals forces act between FF oriented smectite flakes and aggregates become approach irreversible flocculation. In case of sample prepared using Method 2, particles contacting by edges (EE) and edge to face (EF) orientation fell into secondary minimum and weak flocculation resulted in severe gelation and formation of the micelle-like texture in fringe superstructure, which was first time observed in smectite based gel.

Absorption of the selenite anion from aqueous solutions by thermally activated layered double hydroxide.

The presence of selenite or selenate in potable water is a health hazard especially when consumed over a long period of time. Its removal from potable water is of importance. This paper reports technology for the removal of selenite from water through the use of thermally activated layered double hydroxides. Mg/Al hydrotalcites with selenite in the interlayer were prepared at different times from 0.5 to 20 h through ion exchange. X-ray diffraction of the MgAlSeO3 hydrotalcites indicates that the selenite anion entered the interlayer spacing of Mg/Al hydrotalcite and MgAlSeO3 hydrotalcite was formed. Raman spectra proved the presence of selenite anion in the hydrotalcite interlayer as the counter anion. The band intensity and width of MgAlSeO3 hydrotalcite in the region of 3800-3000 cm(-1) increase with the adsorption of selenite by the Mg/Al hydrotalcite. The characteristic bands of free selenite anions in the MgAlSeO3 hydrotalcites are located between the region between 850 and 800 cm(-1). The Raman spectra of the lower wave number region of 550-500 cm(-1) show a shift toward higher wave numbers with adsorption of the selenite. An estimation of the amount of selenite anion removed by the thermally activated layered double hydroxide was obtained through the measurement of the intensity of the selenite Raman bands at 814 and 835 cm(-1) resulting from the amount of selenite anion remaining in solution. Thermally activated LDHs provide a mechanism for removing selenite anions from aqueous solutions.

Synthesis, characterization of mono, di and tri alkyl surfactant intercalated Wyoming montmorillonite for the removal of phenol from aqueous systems.

Organoclays were synthesized by the ion exchange of cationic surfactants containing single, double and triple alkyl chains for sodium ions in an aqueous suspension of Wyoming Na-montmorillonite. The characterization of organoclays with and without adsorbed phenol was determined by X-ray diffraction, TEM and thermal analysis. Differences in the surfaces and in the interlayer of the mono, di and tri alkyl chain organoclays resulted in differences in the adsorption efficiency for phenol with tri > di > mono >> Na-Mt. The results prove that organoclays can be effective for the removal of phenol from an aqueous solution and this removal is a function of the surfactant molecule and its concentration. In general, the higher the concentration as measured by the CEC value and the greater the number of alkyl chains in the surfactant molecule, the greater the percentage of the phenol that is removed.