Acid In Aqueous Suspensions Research Articles

DEAE/Catechol-Chitosan Conjugates as Bioactive Polymers: Synthesis, Characterization, and Potential Applications.

This work provides the first description of the synthesis and characterization of water-soluble chitosan (Cs) derivatives based on the conjugation of both diethylaminoethyl (DEAE) and catechol groups onto the Cs backbone (Cs-DC) in order to obtain a Cs derivative with antioxidant and antimicrobial properties. The degree of substitution [DS (%)] was 35.46% for DEAE and 2.53% for catechol, determined by spectroscopy. Changes in the molecular packing due to the incorporation of both pendant groups were described by X-ray diffraction and thermogravimetric analysis. For Cs, the crystallinity index was 59.46% and the maximum decomposition rate appeared at 309.3 °C, while for Cs-DC, the values corresponded to 16.98% and 236.4 °C, respectively. The incorporation of DEAE and catechol groups also increases the solubility of the polymer at pH > 7 without harming the antimicrobial activity displayed by the unmodified polymer. The catecholic derivatives increase the radical scavenging activity in terms of the half-maximum effective concentration (EC50). An EC50 of 1.20 μg/mL was found for neat hydrocaffeic acid (HCA) solution, while for chitosan-catechol (Cs-Ca) and Cs-DC solutions, concentrations equivalent to free HCA of 0.33 and 0.41 μg/mL were required, respectively. Cell culture results show that all Cs derivatives have low cytotoxicity, and Cs-DC showed the ability to reduce the activity of reactive oxygen species by 40% at concentrations as low as 4 μg/mL. Polymeric nanoparticles of Cs derivatives with a hydrodynamic diameter (Dh) of around 200 nm, unimodal size distributions, and a negative ζ-potential were obtained by ionotropic gelation and coated with hyaluronic acid in aqueous suspension, providing the multifunctional nanoparticles with higher stability and a narrower size distribution.

TiO2-reduced graphene oxide nanocomposites: Microsecond charge carrier kinetics

In 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.

Reaction Rate Study of the Photocatalytic Degradation of Dichloroacetic Acid in a Black Body Reactor

The light-induced degradation of dichloroacetic acid in aqueous suspensions containing the TiO2 photocatalyst Hombikat UV 100 was investigated. The reactions were performed in a black body reactor in which the rate of conversion, defined as the time derivative of the extent of conversion, is not affected by the light scattering properties of the photocatalysts. At sufficiently high concentrations of both the probe compound and the photocatalyst the rate of conversion was found to be unswayed by the initial concentration of the probe compound, the mass concentration of the photocatalyst, and the suspension volume. Thus, the chosen experimental conditions enable the determination of the rate of conversion and the quantum yield of the light induced degradation of dichloroacetic acid in aqueous photocatalyst suspension with sufficiently good reproducibility. The experimental procedure employed here seems to be generally applicable to determine rates of conversion and quantum yields that possibly allow a comparison of the activities of photocatalysts in aqueous suspensions.

Photocatalytic Reforming of Aqueous Acetic Acid into Molecular Hydrogen and Hydrocarbons over Co-catalyst-Loaded TiO2: Shifting the Product Distribution

Acetic acid in aqueous suspensions of co-catalyst-loaded TiO2 was photocatalytically converted into carbon dioxide, molecular hydrogen, methane, and ethane. The formed amounts of CO2 were found to ...

Fluorine-Doped TiO2 Materials: Photocatalytic Activity vs Time-Resolved Photoluminescence

Aiming at discerning the role of fluorine from that of nitrogen as a dopant in N,F-codoped TiO2, a series of HF-doped TiO2 photocatalysts were investigated in the decomposition of formic and acetic acid in aqueous suspensions, also as a function of the irradiation wavelength (action spectra analysis), in comparison with recent results obtained with an analogous series of NH4F-doped TiO2 photocatalysts. Visible light absorption around 420 nm, which was found to be inactive in acetic acid decomposition, is definitely shown to be associated with nitrogen doping, whereas the enhanced absorption at ca. 365 nm, increasing with increasing calcination temperature, can be unambiguously attributed to structural modifications induced by fluorine doping. Action spectra analysis confirms that this absorption is active in acetic acid decomposition, in both HF- and NH4F-doped TiO2 photocatalysts. From time-resolved photoluminescence (PL) spectroscopy analysis, a clear correlation is outlined between the photoactivity of...

WO3–TiO2vs. TiO2 photocatalysts: effect of the W precursor and amount on the photocatalytic activity of mixed oxides

Aiming at producing TiO2-based photocatalytic materials with reduced charge carriers recombination, WO3–TiO2 mixed oxides were synthesized by a sol–gel method employing either an inorganic salt, Na2WO4, or an organic alkoxide, W(OC2H5)6, as tungsten precursor, with different W/Ti ratios. The so-obtained materials were characterized by XRPD, BET, UV–vis reflectance, XPS and EDX analyses and their photoactivity was tested under UV–visible irradiation in both the mineralization of formic acid in aqueous suspension and the gas phase oxidation of acetaldehyde. Both photoactivity results and photocurrent measurements point to a superior performance of photocatalysts obtained from the organic precursor with an optimal tungsten content (3%). The formation of an intimately mixed oxide, as revealed by XRPD analysis, results in photoactivity higher than that of pure TiO2, and also of benchmark P25 TiO2, consequent to a better charge separation due to the migration of photoproduced holes from WO3 domains to TiO2 and of photopromoted electrons in the opposite direction. The persistence of pure anatase phase in W-containing photocatalysts also after calcination at 700°C and their higher surface area with respect to pure TiO2 also contribute in increasing the photocatalytic activity of the WO3–TiO2 mixed oxides.

Photocatalytic activity of NH4F-doped TiO2 modified by noble metal nanoparticle deposition

The effect of noble metal (Pt and Au) nanoparticle photodeposition on a series of NH4F-doped TiO2 photocatalysts calcined at 700 °C was investigated both in a thermodynamically down-hill reaction, i.e. the degradation of formic acid in aqueous suspension, and in an up-hill reaction, i.e. hydrogen production from methanol-water vapour mixtures. All photocatalysts were characterized by BET, XRD, UV-vis absorption and HRTEM analysis. Intriguing synergistic effects of simultaneous bulk and surface TiO2 modification were evidenced in both photocatalytic reactions, which can be interpreted in relation to the structural features of the materials. On one hand NH4F doping guarantees that the most active TiO2 anatase phase is stabilised up to high calcination temperature, ensuring high crystallinity and good photoinduced charge carriers production, on the other hand noble metal nanoparticles contribute in increasing the separation of photoproduced charge carriers, resulting in enhanced photocatalytic performances of the surface- and bulk-modified photocatalyst systems.

Gold–Titanium(IV) Oxide Plasmonic Photocatalysts Prepared by a Colloid-Photodeposition Method: Correlation Between Physical Properties and Photocatalytic Activities

Colloidal gold (Au) nanoparticles were prepared and successfully loaded on titanium(IV) oxide (TiO(2)) without change in the original particle size using a method of colloid photodeposition operated in the presence of a hole scavenger (CPH). The prepared Au nanoparticles supported on TiO(2) showed strong photoabsorption at around 550 nm due to surface plasmon resonance (SPR) of Au and exhibited a photocatalytic activity in mineralization of formic acid in aqueous suspensions under irradiation of visible light (>ca. 520 nm). A linear correlation between photocatalytic activity and the amount of Au loaded, that is, the number of Au nanoparticles, was observed, indicating that the activity of Au/TiO(2) plasmonic photocatalysts can be controlled simply by the amount of Au loading using the CPH method and that the external surface area of Au nanoparticles is a decisive factor in mineralization of formic acid under visible light irradiation. Very high reaction rates were obtained in samples with 5 wt % Au or more, although the rate tended to be saturated. The CPH method can be widely applied for loading of Au nanoparticles on various TiO(2) supports without change in the original size independent of the TiO(2) phase. The rate of CO(2) formation also increased linearly with increase in the external surface area of Au. Interestingly, the TiO(2) supports showed different slopes of the plots. The slope is important for selection of TiO(2) as a material supporting colloidal Au nanoparticles.

Efficient Decomposition of Perfluorocarboxylic Acids in Aqueous Suspensions of a TiO2 Photocatalyst with Medium-Pressure Ultraviolet Lamp Irradiation under Atmospheric Pressure

Decomposition of environmentally persistent perfluorooctanoic acid (PFOA) in aqueous suspensions of a TiO2 photocatalyst, with use of medium-pressure ultraviolet (MPUV) lamp irradiation alone, was examined under atmospheric pressure. Compared to direct photolysis, TiO2 photocatalysis led to efficient PFOA decomposition and production of CO2 and F–. PFOA decomposition followed pseudo-first-order kinetics, with observed rate constants of 1.3 × 10–2 and 8.6 × 10–2 dm3 h–1 in direct photolysis and TiO2 photocatalysis, respectively. The latter (photocatalytic) rate constant is 5–100 times greater than those obtained in other published research on the photocatalytic decomposition of PFOA. In the proposed decomposition pathway, PFOA molecules adsorb onto the TiO2 surface according to adsorption equilibrium in an aqueous suspension and could be easily decomposed by holes and radicals generated by MPUV lamp irradiation. Under the present reaction conditions, a narrow region of TiO2 concentrations around 1.5 wt % s...

Photocatalytic Mineralization of Acetic Acid in Aqueous Suspension of Metal-loaded Cerium(IV) Oxide under Irradiation of Visible Light

Abstract Photocatalytic activity of cerium(IV) oxide (CeO2) particles in decomposition of acetic acid in aqueous suspension was examined under irradiation of visible light. Acetic acid was decomposed to carbon dioxide (CO2) and the rate of CO2 formation was increased by loading of 0.1 wt % cocatalysts (Pt, Au, Rh, Ru, Pd, Ag, Cu, and Ir) on CeO2 by photodeposition. Cocatalysts showed almost the same levels of enhancement of CO2 evolution, i.e., Ag/CeO2 and Cu/CeO2 exhibited rates similar to that of Pt/CeO2. The action spectrum revealed that decomposition of acetic acid was induced by photoabsorption of CeO2.

Photocatalytic activity of S- and F-doped TiO2 in formic acid mineralization

Two series of doped titanium dioxide samples (S-TiO(2) and F-TiO(2)) were prepared by the sol-gel method in the presence of different amounts of dopant source (thiourea and NH(4)F, respectively), followed by calcination at 500, 600 or 700 °C, and characterised by BET, UV-vis absorption, XPS, HRTEM, XRD and EPR analyses. Reference undoped materials were prepared by the same synthetic procedure. Their photocatalytic activity under visible light was investigated employing the photocatalytic degradation of formic acid in aqueous suspension as test reaction. S-doped TiO(2) showed a photocatalytic activity quite similar to that of undoped materials. In this regard, the insertion of S, characterised by a relatively large ionic radius, into the TiO(2) crystalline structure appears rather difficult, as confirmed by XPS analysis. On the contrary, moderate F doping was beneficial in increasing the rate of formic acid photocatalytic degradation, especially for photocatalysts calcined at high temperature, consisting of highly crystalline pure anatase, in which the rate of detrimental charge carrier recombination was reduced. For both series of doped materials, high doping levels appear to limit the semiconductor photoactivity, probably due to the formation of a progressively increasing number of charge recombination centres. The EPR characterisation of the investigated doped TiO(2) samples evidenced the presence of nitrogen containing species (nitric oxide radical encapsulated in micro-void, with no photoactivity, and N(b)˙ species, active in visible light sensitisation) and of titanium reduced centres Ti(3+), due to charge imbalance consequent to dopant introduction in the TiO(2) lattice either in anionic (F(-)) or in cationic form (S(6+)).

Structural and Electrical Characterization of Three-Dimensional Superlattices of Organic Monolayer Protected Gold Nanocrystals

Micrometer-sized three-dimensional superlattices were fabricated by assembling Au nanocrystals modified with mercaptosuccinic acid in aqueous suspensions. Each superlattice formed a single crystal arrangement with a hexagonal close-packed structure where a=5.1 nm. The superlattices were densely deposited on a glass substrate and their electrical properties were investigated. The superlattice-assembled film showed ohmic behavior in the temperature range from 20 to 285 K. The absolute value of the resistivity was in the semiconductor region, but the resistivity slightly decreased as the temperature decreased. The temperature coefficient of the film was one order of magnitude smaller than that of bulk Au.

Absorption and action spectra analysis of ammonium fluoride-doped titania photocatalysts

The photocatalytic behaviour of a series of ammonium fluoride (NH(4)F)-doped titania (TiO(2)) photocatalysts was investigated in the decomposition of acetic acid in aqueous suspensions and in the gas phase mineralization of acetaldehyde. Very similar photocatalytic activity trends, usually increasing with increasing the calcination temperature for a given nominal dopant amount, were obtained for the two test reactions. Moderately doped TiO(2) calcined at 700 °C, consisting of pure anatase, was the best performing photocatalyst in both reactions. The photocatalytic oxidation of acetic acid was investigated systematically as a function of irradiation wavelength, by collecting so-called action spectra. By comparing the shapes of the action spectra with those of the absorption spectra of the investigated photocatalysts a model is proposed, based on spectral features deconvolution, which allows a clear distinction between inactive light absorption and effective photoactivity in acetic acid decomposition.

Refractive Index Regulation of Gold Nanocrystal Superlattices by Varying the Nanocrystal Size

Micrometer-sized three-dimensional superlattices with different lattice constants were fabricated by assembling Au nanocrystals modified with mercaptosuccinic acid in aqueous suspensions. The refractive index of the superlattices was varied between 2.3 and 3.3 by altering the size of the component nanocrystals between 3.11 and 4.68 nm. As a result, the metal content of the superlattices varied between 23 and 36%. The correlation between nanocrystal size and the optical permittivity estimated from the refractive index was explained using the Maxwell–Garnett theory.

Synthesis and photocatalytic activity of boron-doped TiO2 in aqueous suspensions under UV-A irradiation

Boron-doped TiO(2) photocatalysts were synthesized employing a sol-gel method. Boric acid was used as the boron source and titanium tetra-isopropoxide as the TiO(2) precursor, both dissolved in isopropanol. Nominal boron to titanium atomic ratios were in the range 0 to 4%. After the hydrolysis step, two different procedures for the recovery of TiO(2) were followed, based on either centrifugation of the resulting reaction mixture or evaporation of the solvent under reduced pressure, both followed by a subsequent calcination step performed at 400 or 500 degrees C. The photocatalytic efficiency of the synthesized photocatalysts was assessed by measuring the photocatalytic mineralization of dichloroacetic acid in aqueous suspensions under UV-A irradiation and it was compared to the corresponding efficiency of the commercial Degussa P 25 TiO(2). Photocatalytic efficiency of the synthesized catalysts was higher for the boron-doped TiO(2) synthesized at 2% boron to titanium nominal atomic ratio, centrifuged after the hydrolysis step followed by calcinations at 400 degrees C. However, all photocatalysts synthesized in this work showed lower photocatalytic activity than Degussa P 25 TiO(2), thus highlighting the need of further improvements of the proposed method.

Facile Hydrothermal Preparation and Photocatalytic Activity of Bismuth Tungstate Polycrystalline Flake-ball Particles

Abstract Micrometer-sized spherical particles of bismuth tungstate (Bi2WO6) with hierarchical architecture were prepared by facile hydrothermal reaction without using any surfactants and polymers as structure-directing agents. The particles were assemblies of polycrystalline flakes composed of rectangular platelets. The hierarchical polycrystalline particles of “flake-ball” shape exhibited relatively high photocatalytic activity for oxidative decomposition of acetic acid in aqueous suspensions.

Heterogeneous photocatalysed degradation of 4-chlorophenoxyacetic acid in aqueous suspensions

The photocatalysed degradation of 4-chlorophenoxyacetic acid (4-CPA, 1) has been investigated in aqueous suspensions of titanium dioxide under a variety of conditions. The degradation was studied by monitoring the change in substrate concentration employing UV spectroscopic analysis technique and depletion in total organic carbon (TOC) content as a function of irradiation time. The influence of various parameters such as, different types of titanium dioxide (TiO 2) powders, pH, catalyst and substrate concentrations, and in the presence of electron acceptor such as hydrogen peroxide (H 2O 2) besides molecular oxygen has been investigated. The effects of these parameters on the degradation rates were found to be significant. The volatile degradation product 4-chlorophenol was analyzed by GC–MS technique and probable pathways for the formation of product has been proposed.

Comparative photocatalytic study of two selected pesticide derivatives, indole-3-acetic acid and indole-3-butyric acid in aqueous suspensions of titanium dioxide

Heterogeneous photocatalysed degradation of two selected pesticide derivatives such as indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) has been investigated in aqueous suspensions of titanium dioxide by monitoring the change in substrate concentration employing UV spectroscopic analysis technique and depletion in total organic carbon (TOC) content as a function of irradiation time. The degradation kinetics was studied under different conditions such as pH, types of TiO 2, substrate and catalyst concentration, and in the presence of electron acceptor such as hydrogen peroxide (H 2O 2) besides molecular oxygen. The degradation rates were found to be strongly influenced by all the above parameters. The photocatalyst Degussa P25 showed comparatively highest photocatalytics. The pesticide derivative, indole-3-acetic acid was found to degrade slightly faster than indole-3-butyric acid.

Photodegradation of a herbicide derivative, 2,4-dichlorophenoxy acetic acid in aqueous suspensions of titanium dioxide

The photocatalytic degradation of a herbicide derivative, 2,4-dichlorophenoxy acetic acid (2,4-D, 1), has been investigated in aqueous suspensions of titanium dioxide. The degradation was studied by monitoring the change in substrate concentration employing UV spectroscopic analysis and decrease in Total Organic Carbon (TOC) content as a function of irradiation time in the presence of UV light source. The degradation kinetics was investigated under a variety of conditions, such as different types of TiO2, pH, catalyst and substrate concentrations. Higher photonic efficiencies were observed with Degussa P25 as compared with other photocatalysts. The degradation products were analysed by GC-MS and probable pathways for the formation of different products were proposed.

Photocatalytic degradation of a cyanuric acid, a recalcitrant species

Degradation of cyanuric acid in aqueous suspensions of Degussa P25 TiO 2 has been achieved by the addition of fluoride ion at low pH. Consistent with the work of Minero and Pelizzetti [Langmuir 16 (2000) 2632, Langmuir 16 (2000) 8964], it is suggested that this is due to the formation of homogeneous phase hydroxyl radicals. Support for this hypothesis is brought from successful degradations using other hydroxyl-generating conditions and the successful degradation of 4- t-butylpyridine, another organic compound previously shown to be resistant to TiO 2-mediated photocatalytic degradation.