PdAg2 nanoparticles in aqueous solution: Preparation, characterization, and catalytic properties

The reduction of a heterobimetallic complex, Pd(OOCMe)₄Ag₂(HOOCMe)₄, with hydrogen or sodium borohydride in an aqueous solution produces PdAg₂ nanoparticles of an alloy or intermetallic type. It is shown that the catalytic activity of the particles in the reduction of methyl viologen with hydrogen is lower than that of palladium nanoparticles of the same size. Therewith, ??borohydride?? nanoparticles manifest a higher catalytic activity than do ??hydrogen?? ones. Unlike silver nanoparticles, PdAg₂ nanoparticles do not catalyze the decomposition of hydrazine.     

Enhanced visible-light-driven photodegradation of Acid Orange 7 azo dye in aqueous solution using Fe-N co-doped TiO2

FeN -co-doped TiO₂ photocatalysts are prepared by sol–gel method using titanium tetraisopropoxide, urea and iron(II) acetylacetonate as precursors of titania, nitrogen and iron, respectively. The prepared samples are analysed from chemical-physical point of view by X-ray diffraction (XRD), Raman spectroscopy, UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS), specific surface area measurements and scanning electron microscopy (FESEM). UV–Vis DRS spectra evidence that the co-doping of TiO₂ with N and Fe leads to the narrowing of the band gap value (2.7 eV) with respect to Fe-doped TiO₂ (2.8 eV) and N-doped TiO₂ (2.9–3 eV). XRD patterns show that photocatalysts are mainly in anatase phase and Fe and N ions are successfully incorporated into the TiO₂ lattice. The average crystallite size of Fe-N co-doped TiO₂ is slightly lower than the other samples and equal to about 7 nm and the specific surface area of the co-doped sample results to be 117 m² g⁻¹. Photocatalytic performances of all prepared samples are evaluated by analysing the degradation of Acid Orange 7 azo dye under visible light irradiation. Photocatalytic efficiency obtained using FeN co-doped TiO₂ strongly increases compared to undoped TiO₂, N-doped TiO₂ and Fe-doped TiO₂ photocatalysts. In detail, using the co-doped photocatalyst, dye discoloration and mineralization result equal to about 90 and 83% after 60 min of LEDs visible light irradiation, underlining the best performances of the FeN co-doped TiO₂ photocatalyst both in terms of treatment time and electric energy consumption.     

Radiation-induced catalytic reduction of chromium(VI) in aqueous solution containing TiO2, Al2O3 or SiO2 fine particles

Reduction of chromium (VI), Cr(VI) in aqueous neutral or basic solution was promoted by γ-ray irradiation in the presence of oxide particles such as TiO₂, Al₂O₃ or SiO₂. The oxide particles behaved as a catalyst, and the efficiency of the Cr(VI) reduction increased with an increase of the irradiation dose irrespective of the initial Cr(VI) concentration. The insoluble Cr(III) oxide formed through the Cr(VI) reduction also acted as the catalyst.     

Arene ruthenium bis-saccharinato complexes: Synthesis, molecular structure and catalytic oxidation properties in aqueous solution

Arene ruthenium complexes [(η⁶-arene)Ru(sacc)₂(OH₂)] (arene = para-cymeme, benzene) containing an aqua and two saccharinato ligands have been synthesized from [(η⁶-arene)RuCl₂]₂ and sodium saccharinate in a water-ethanol mixture (1:1). The aqua complex [(η⁶-MeC₆H₄Prⁱ)Ru(sacc)₂(OH₂)] reacts with acetonitrile to give the acetonitrile complex [(η⁶-MeC₆H₄Prⁱ)Ru(sacc)₂(NCMe)]. The corresponding benzene derivative [(η⁶-C₆H₆)Ru(sacc)₂(NCMe)] was obtained from [(η⁶-C₆H₆)RuCl₂]₂ and saccNa in an acetonitrile-methanol mixture (1:1). All new complexes show a piano-stool geometry with two mono-hapto nitrogen-bonded saccharinato ligands in addition to a H₂O or MeCN ligand. All complexes of the type [(η⁶-arene)Ru(sacc)₂(OH₂)] and [(η⁶-arene)Ru(sacc)₂(NCMe)] were found to catalyze the oxidation of secondary alcohols with tert-butyl hydroperoxide (Bu^tOOH) to give the corresponding ketones in aqueous solution.     

Characterization of an LED based photoreactor to degrade 4-chlorophenol in an aqueous medium using coumarin (C-343) sensitized TiO2

A detailed performance evaluation of a simple high intensity LED based photoreactor exploiting a narrow wavelength range of the LED to match the spectrum of a dye in a photocatalysis system is reported. A dye sensitized (coumarin-343, lambda max = 446 nm) TiO 2 photocatalyst was used for the degradation of 4-chlorophenol (4-CP) in an aqueous medium using the 436 nm LED based photoreactor. The LED reactor performed competitively with a conventional multilamp reactor and sunlight in the degradation of 4-CP. Light intensities entering the reaction vessel were measured by conventional ferrioxalate actinometry. The results can be fitted by approximate first order kinetic behavior in this system. Hydroxyl radicals were detected by spin trapping EPR, and effects of OH radical quenchers on kinetics suggest that the reaction is initiated by these radicals or their equivalents. LEDs operating at competitive intensities offer a number of advantages to the photochemist or the environmental engineer via long life, efficient current to light conversion, narrow bandwidth, forward directed output, and direct current power for remote operation. Matching light source spectrum to chromophore is a key.     

Enhanced adsorption of atrazine from aqueous solution by molecularly imprinted TiO2 film

TiO₂ film imprinted by atrazine molecule at the surface of quartz crystal was prepared using molecular imprinting and surface sol-gel process. The molecularly imprinted TiO₂ film was characterized by scanning electron microscopy and cyclic voltammetry, and the atrazine adsorption was investigated by quartz crystal microbalance (QCM) technique. In comparison with non-imprinted TiO₂ film, the molecularly imprinted TiO₂ film exhibits high selectivity for atrazine, better reversibility and a much higher adsorption capacity for the target molecule, the adsorption equilibrium constant estimated from the in situ frequency measurement is about 6.7 × 10⁴ M⁻¹, which is thirteen times higher than that obtained on non-imprinted TiO₂ film.     

Palladium nanoparticles in aqueous solution: Preparation,properties, and effect of their size on catalytic activity

A method has been developed for the preparation of palladium nanoparticles with different sizes of up to 7 nm via the reduction of Pd(II) ions with hydrogen in an aqueous solution on seed metal nanoparticles (2.5 nm). The effect of the size of nanoparticles on their catalytic activity in methyl viologen reduction with molecular hydrogen in an alkaline medium has been studied. It has been found that the specific catalytic activity of palladium nanoparticles is independent of their size.     

Electroluminescent method for determining hydrogen peroxide and peroxydisulphate ions in aqueous solution using TiO(2) film electrodes

An electrochemical optical sensor system with luminescence response was proposed for the continuous determination of hydrogen peroxide or peroxydisulphate concentration in aqueous solutions. The electroluminescence (EL) of TiO(2) film electrodes, which arises under conditions of the cathodic polarization as a result of the hole injection into the TiO(2) from high-energy OH or SO(4)(2-) radicals produced by the electroreduction of H(2)O(2) or S(2)O(8)(2-) ions on the electrode surface, was used as the analytical signal. The EL response is linearly related, in a logarithmic scale, to the hydrogen peroxide or peroxydisulphate concentration ranging from 10(-3) to 10(-1) M H(2)O(2) and from 5 x 10(-4) to 1 M Na(2)S(2)O(8). It was shown that a substantial increase in the quantum efficiency of the EL and, as a consequence, in the sensitivity of the sensor system can be achieved by doping TiO(2) films with chromium. The potential dependence of the EL spectrum for TiO(2) electrodes in S(2)O(8)(2-) solutions differs essentially from that in H(2)O(2) solutions which allows measurement of the concentration of S(2)O(8)(2-) ions when they coexist with H(2)O(2) in solution.     

Supramolecular synthesis of coumarin derivatives catalyzed by a coordination-assembled cage in aqueous solution

A self-assembled Pd_4L_2 cage is employed as a water-soluble molecular flask for the synthesis of functionalized coumarins from a series of salicylaldehyde derivatives and cyanoacetates/malononitrile. The catalytic reaction features mild aqueous conditions and broad substrate scope. Crystal structures of the host-guest complexes for two substrates and one analogous intermediate have been obtained, shedding light on the supramolecular reaction mechanism. Michaelis-Menten kinetic studies were performed in one typical case, revealing that the rate of product formation has been enhanced by over 23-fold in contrast to the background reaction without cage. Moreover, the same reaction catalyzed by a smaller Pd_6L_4 cage gives a mixture of products and much lower yields, suggesting that fine-tuning on the size and symmetry of the cages' cavity is crucial for their applications in supramolecular catalysis.     

Light-excited superhydrophilicity of amorphous TiO2 thin films deposited in an aqueous peroxotitanate solution

We report on the photoinduced superhydrophilicity of the surface of amorphous TiO2. Amorphous TiO2 thin films were prepared on self-assembled monolayers by the peroxotitanate-complex deposition (PCD) and liquid-phase deposition (LPD) methods. The surface morphology and topography were characterized in detail. The contact angles were 34 degrees and 66 degrees for the as-deposited thin films through the PCD and LPD methods, respectively, which slowly increased to about 70 degrees and 73 degrees after being stored in air. After irradiation by UV light, the contact angle vanished and the surface exhibited superhydrophilicity. The superhydrophilicity and hydrophobicity could be switched by alternatively exposing the surface to UV light and drying in an atmosphere filled with organic gases. Although the oxidation of the contamination on the surface has effects on the increase in hydrophilicity, the X-ray photoelectron spectroscopy results suggested that the superhydrophilicity was also related to the transformation of the Ti-OH groups to groups that have dangling bonds. This paper indicates that an amorphous TiO2 thin film does not need to be heated to obtain superhydrophilicity; such a self-cleaning surface can be achieved at room temperature by our newly developed environmentally friendly method.     

Microstructure and catalytic properties of Rh and Ni dispersed on TiO2-SiO2 aerogels

The influence of the preparation method on the microstructure and catalytic behavior of Rh and Ni dispersed on TiO₂-SiO₂ aerogels is investigated.The autoclave method has been followed to prepare titania-silica aerogels with TiO₂ contents ranging between 0 and 10 mole %. These aerogels have been used as matrices to disperse catalytically active metals: Rh and Ni. The metals can be deposited by impregnation of aerogels, or alternatively, can be added into the hydrolysis water used in the synthesis of gels. The resulting catalysts present surface areas higher than 550 m²·g^–1.The percentage of titania, the method followed for the introduction of the metal, and the nature of the metal itself affect both the activities and selectivities of the catalysts in the hydrogenolysis of n-butane. Thus, the presence of titania in Rh catalysts increases the activity values, and the samples prepared by impregnation present selectivities towards ethane higher than 80%. Whereas, the rhodium catalysts in which the metal has been introduced before gelling, do not orientate the reaction in favor of a definite product. For the case of Ni, it is quite frequent to obtain high selectivities towards the breakdown of the C-C terminal bonds. In summary, the preparation methods allow to modulate into very broad limits the catalytic behavior of the samples.     

The nature of colloidal platinum in aqueous solution: features of catalytic reactions

The specific features of the catalytic reduction of methylviologen by dihydrogen in water in the presence of platinum colloids synthesized by various methods are studied. The colloids prepared by the radiation-chemical reduction of PtCl₄ ^2– in the presence of polyacrylate or polyphosphate as stabilizers and colloids prepared by the reduction with dihydrogen efficiently catalyze the reaction. The citrate colloids synthesized by the reduction of PtCl₆ ^2– with citric acid are characterized by a prolonged induction period after which these colloids also gain the catalytic activity. We assume that the citrate platinum colloids are giant clusters with the close-packed metal core containing the magic number of platinum atoms (Pt₅₆₁), which are coordinated by the ligand molecules blocking the surface of the metal particle. In the presence of H₂, the ligand molecules are decomposed or removed from the surface, which is accompanied by the appearance of the catalytic activity.     

Influence of solution composition and ultrasonic treatment on optical spectra of TiO2 aqueous suspensions

Investigation of TiO(2) aqueous suspensions has shown that their optical spectra can be unstable, with instability not related to precipitation or adherence of TiO(2) particles to the vessel walls. Increase of ionic strength of the suspension as well as neutralization of charged TiO(2) particles via pH adjustment accelerates the optical density drop. Vice versa, increasing the charge of TiO(2) particles via shifting pH in acidic or basic directions stabilizes the suspension's optical spectra, and ultrasonic treatment promotes optical density recovery. The observed behavior is attributed to alteration in the size of the suspension aggregates.     

Stability of aqueous TiO2 suspensions: influence of ethanol

Dispersion of titania particles in water has been achieved thanks to the addition of two different compounds: (C(2)H(5))(4)NOH and Tiron. To compact particles using an electrophoretic deposition process, ethanol, up to 20 vol%, was added as cosolvent to prevent water electrolysis. Alcohol addition strongly affects zeta potential values of particles and increases suspension viscosity but without influence the surface charge. The more realistic interpretation is that integration of ethanol molecules into the water network moves the slipping plane away from the surface, reducing the repulsive potential between particles that leads to suspension destabilization.     

Preparation by high-energy milling, characterization, and catalytic properties of nanocrystalline TiO2

Titanium dioxide (TiO2) is widely used for applications in heterogeneous photocatalysis. We prepared nanocrystalline powders of the anatase as well as the rutile modification by high-energy ball milling of the coarse grained source materials for up to 4 h. The resulting average grain size was about 20 nm. The morphology of the powders was investigated with transmission electron microscopy, X-ray powder diffraction, and BET surface area determination. Measurements of the catalytic activity reveal a maximum as a function of the milling time at about 40 min. This maximum could be explained by a superposition of two counteracting effects. The first one is the increase of the specific surface area resulting in an increase of the catalytic activity, and the second one is a change of the electronic structure at the surface of the TiO2 particles corresponding to a reduction of the surface. The latter one was confirmed by light absorption experiments, X-ray photoelectron spectroscopy, and electron paramagnetic resonance spectroscopy.     

双十八烷基二甲基氯化铵水溶液CMC值的测定

采用电导法和荧光法测定了阳离子表面活性剂-双十八烷基二甲基氯化铵(DODAC)的临界胶束浓度(CMC), 在温度为14 ℃时, 电导法测得DODAC溶液的临界胶束浓度为5.24×10-5 mol/L, 荧光法测得DODAC溶液的临界胶束浓度为9.67×10-5 mol/L.     

Photodegradation of Congo red in aqueous solution on ZnO as an alternative catalyst to TiO2

In this research, the efficiency of ZnO and TiO₂ was compared by photocatalytic degradation of Congo red azo dye. The effects of some parameters such as pH, dye concentration and irradiation time on the degradation rate of dye solution were also examined. In addition, the aggregation feature of Congo red was studied by using UV-Vis spectroscopy techniques. The recovery of the ZnO catalyst was also investigated.