O.S.G.P. Soares

Nitrate reduction over a Pd-Cu/MWCNT catalyst: application to a polluted groundwater.

The influence of the presence of inorganic and organic matter during the catalytic reduction of nitrate in a local groundwater over a Pd-Cu catalyst supported on carbon nanotubes was investigated. It was observed that the catalyst performance was affected by the groundwater composition. The nitrate conversion attained was higher in the experiment using only deionized water as solvent than in the case of simulated or real groundwater. With exception of sulphate ions, all the other solutes evaluated (chloride and phosphate ions and natural organic matter) had a negative influence on the catalytic activity and selectivity to nitrogen.

Ozonation of Textile Effluents and Dye Solutions in the Presence of Activated Carbon under Continuous Operation

Abstract Ozonation was found to be effective for the decolorization of solutions, but has only a slight effect on TOC removal. On the other hand, adsorption on activated carbon improves the TOC removal, but the progressive uptake of the organic contaminants during the adsorption process decreases its removal efficiency. Decolorization, mineralization, and ozone consumption of colored solutions were evaluated under continuous operation in a column by three different processes: ozonation, adsorption on a fixed activated carbon bed, and ozonation in the presence of the activated carbon bed. The introduction of an ozone flow in a fixed activated carbon bed enhances both the decolorization of the solutions and mainly the mineralization of the organic matter, even when the activated carbon was previously partially saturated. Activated carbon acts both as an adsorbent and as a catalyst in the ozonation of colored solutions. The column configuration plays an important role in the performance of this system. The configurations in series and with activated carbon in the upper part of the column showed the highest colour and TOC removal for dye solutions. The results obtained clearly show that the combination of ozone and activated carbon is a promising technique for the final treatment of colored effluents. Practical applicability of this process was validated by treating two industrial textile effluents, collected after two different biological treatments.

Highly efficient reduction of bromate to bromide over mono and bimetallic ZSM5 catalysts

The reduction of bromate to bromide was successfully catalyzed by mono and bimetallic catalysts based on ZSM5 zeolites. This reaction is important since the presence of bromate in water is potentially carcinogenic to humans. The catalysts were prepared by ion-exchange and incipient wetness methods with different metals (copper, palladium, rhodium and thorium) using ZSM5. Several analytical techniques (N2 adsorption, TPR experiments, NH3-TPD, FTIR, XRD, SEM/EDX and TEM/EDX) were used to characterize the mono and bimetallic catalysts prepared by the two methods. The catalytic tests were carried out in a semi-batch reactor under hydrogen, working at room temperature and pressure. All catalysts prepared are undeniably effective in achieving the complete conversion of bromate into bromide. The most promising among the catalysts tested in this work are the palladium bimetallic catalysts.

Ethyl Acetate Abatement on Copper Catalysts Supported on Ceria Doped with Rare Earth Oxides

Different lanthanide (Ln)-doped cerium oxides (Ce0.5Ln0.5O1.75, where Ln: Gd, La, Pr, Nd, Sm) were loaded with Cu (20 wt. %) and used as catalysts for the oxidation of ethyl acetate (EtOAc), a common volatile organic compound (VOC). For comparison, both Cu-free (Ce-Ln) and supported Cu (Cu/Ce-Ln) samples were characterized by N2 adsorption at −196 °C, scanning/transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and temperature programmed reduction in H2. The following activity sequence, in terms of EtOAc conversion, was found for bare supports: CeO2 ≈ Ce0.5Pr0.5O1.75 > Ce0.5Sm0.5O1.75 > Ce0.5Gd0.5O1.75 > Ce0.5Nd0.5O1.75 > Ce0.5La0.5O1.75. Cu addition improved the catalytic performance, without affecting the activity order. The best catalytic performance was obtained for Cu/CeO2 and Cu/Ce0.5Pr0.5O1.75 samples, both achieving complete EtOAc conversion below ca. 290 °C. A strong correlation was revealed between the catalytic performance and the redox properties of the samples, in terms of reducibility and lattice oxygen availability. Νo particular correlation between the VOC oxidation performance and textural characteristics was found. The obtained results can be explained in terms of a Mars-van Krevelen type redox mechanism involving the participation of weakly bound (easily reduced) lattice oxygen and its consequent replenishment by gas phase oxygen.

Effect of support and pre-treatment conditions on Pt-Sn catalysts: application to nitrate reduction in water.

The effect of the support (activated carbon or titanium dioxide) on the catalytic activity and selectivity to nitrogen of Pt-Sn catalysts in nitrate reduction was studied. The effects of the preparation conditions and the Pt:Sn atomic ratio were also evaluated. It was observed that the support plays an important role in nitrate reduction and that different preparation conditions lead to different catalytic activities and selectivities. Generally, the catalysts supported on activated carbon were less active but more selective to nitrogen than those supported on titanium dioxide. The monometallic Pt catalyst is active for nitrate reduction only when supported on titanium dioxide, which is explained by the involvement of the support in the reaction mechanism. The catalysts were characterized by different techniques, and significant changes on metal chemical states were observed for the different preparation conditions used. Only metallic Pt and oxidized Sn were observed at low calcination and reduction temperatures, but some metallic Sn was also present when high temperatures were used, being also possible the formation of Pt-Sn alloys.

Effect of cobalt loading on the solid state properties and ethyl acetate oxidation performance of cobalt-cerium mixed oxides

Cobalt-cerium mixed oxides were prepared by the wet impregnation method and evaluated for volatile organic compounds (VOCs) abatement, using ethyl acetate (EtAc) as model molecule. The impact of Co content on the physicochemical characteristics of catalysts and EtAc conversion was investigated. The materials were characterized by various techniques, including N2 adsorption at -196°C, scanning electron microscopy (SEM), X-ray diffraction (XRD), H2-temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) to reveal the structure-activity relationship. The obtained results showed the superiority of mixed oxides compared to bare CeO2 and Co3O4, demonstrating a synergistic effect. The optimum oxidation performance was achieved with the sample containing 20wt.% Co (Co/Ce atomic ratio of ca. 0.75), in which complete conversion of EtAc was attained at 260°C. In contrast, temperatures above 300°C were required to achieve 100% conversion over the single oxides. Notably, a strong relationship between both the: (i) relative population, and (ii) facile reduction of lattice oxygen with the ethyl acetate oxidation activity was found, highlighting the key role of loosely bound oxygen species on VOCs oxidation. A synergistic Co-Ce interaction can be accounted for the enhanced reducibility of mixed oxides, linked with the increased mobility of lattice oxygen.

Naphthopyran-Based Silica Nanoparticles as New High-Performance Photoresponsive Materials

Hybrid nanomaterials based on the covalent grafting of silylated naphthopyrans (NPTs) onto silica nanoparticles (SiO2 NPs) were successfully prepared and studied as new photochromic materials. They were prepared by a two-step protocol consisting of (i) NPTs (derivatives from 2H-naphtho[1,2-b]pyran (2H-NPT) and 3H-naphtho[2,1-b]pyran (3H-NPT)) silylation by a microwave-assisted reaction between hydroxyl-substituted NPTs and 3-(triethoxysilyl)propyl isocyanate, followed by (ii) covalent post-grafting onto SiO2 NPs. In order to study the role of the silylation step, the analogous non-silylated nanomaterials were also prepared by direct adsorption of NPTs. The characterization techniques confirmed the successful NPTs silylation and subsequent grafting to SiO2 NPs. All SiO2-based nanomaterials revealed photoswitching behavior, following a biexponential decay. The SiO2 NPs functionalized with silylated 3H-NPTs (SiO2@S3 and SiO2@S4) presented the most promising photochromic properties, showing fast coloration/decoloration kinetics (coloring in 1 min under UV irradiation and fading in only 2 min) and high values of total color difference (ΔE*ab = 30-50). Also, the 2H-NPTs-based SiO2 NPs (SiO2@S1 and SiO2@S2) presented fast coloration and good color contrasts (ΔE*ab = 54), but slower fading kinetic rates, taking more than 2 h to return to their initial color. In contrast, the SiO2 NPs functionalized with non-silylated NPTs (SiO2@1 and SiO2@3) showed weaker color contrasts (ΔE*ab = 6-10) and slower fading kinetics, proving that the NPT silylation step was crucial to enhance the photochromic behavior of SiO2 NPs based on NPTs. Furthermore, the silylated-based nanomaterials showed good photostability upon prolonged UV light exposure, keeping their photochromic performance unchanged for at least 12 successive UV/dark cycles, anticipating interesting technological applications in several areas.

Tuning the surface chemistry of graphene flakes: new strategies for selective oxidation

To accomplish a rational tuning of the surface chemistry of graphene flakes (GF), four different one-step protocols towards the selective oxidation of GF were performed, using different oxidants: nitric acid, potassium permanganate/sulfuric acid, ozone and 3-chloroperbenzoic acid. The characterization of the resulting materials confirmed the successful preparation of oxidized GF with C/O atomic ratios varying in the range of 21.2–4.9, with distinct types of oxygen functionalities. While the oxidation of GF with nitric acid exclusively promotes the introduction of carboxylic groups and carbonyl/quinones, 3-chloroperbenzoic acid is responsible for the introduction of epoxyl groups and carboxylic anhydrides, potassium permanganate favours the introduction of epoxyl and hydroxyl groups and some content of carboxylic anhydrides, and ozone promotes predominantly the introduction in graphene structure of epoxyl groups, carboxylic anhydrides, phenols, quinones and lactones, and in a lesser extension carbonyl groups in α-substituted ketones and aldehydes if the oxidation is performed in the solid phase, or hydroxyl groups and a moderate content of carbonyl groups and aldehydes if GF are in a water dispersion. Furthermore, this work highlighted the possibility of identifying and distinguishing labile groups, namely hydroxyl and epoxyl groups, which are predominant in the structure of GF oxidized with potassium permanganate/sulfuric acid, ozone and 3-chloroperbenzoic acid. This is the first comprehensive study on the fine tuning of the surface of oxidized GF and a major contribution for the rational design of graphene composites since the application of these specific strategies can be useful in the anchoring of other molecules or nanoparticles.

Green synthesis of polypyrrole-supported metal catalysts: application to nitrate removal in water

Pt and Pt/Sn catalysts supported on polypyrrole (PPy) have been prepared using Ar plasma to reduce the metal precursors dispersed on the polymer. The PPy support was synthesized by chemical polymerization of pyrrole with FeCl3·6H2O, this leading to the conducting form of the polymer (conductimetric measurements). The Ar plasma treatment produced a partial reduction of platinum ions, anchored as platinum chloro-complexes to the PPy chain, into metallic platinum. A homogeneous distribution of Pt and Sn nanoparticles was observed by TEM. Activity of the PPy-supported catalysts was evaluated in the reduction of aqueous nitrate with H2 at room temperature. Nitrate concentration in water below the maximum acceptable level of 50 mg L−1 was achieved with all catalysts. However, considering not only efficiency in nitrate reduction, but also minimized concentrations of undesired nitrite and ammonium, the monometallic Pt catalyst seems to be the most promising one.

Sorption of copper, nickel and cadmium on bone char

The sorption capacity of bone char was tested on the removal of copper, nickel and cadmium ions, from aqueous solutions. The Freundlich and Langmuir models were applied to the adsorption isotherms. The equilibrium data fitted well with the Langmuir model and the maximum loading capacities showed the following affinity order: Cu2+ (1.093 mmol g–1 at pHi 3, and 0.884 mmol g–1 at pHi 4) > Cd2+ (0.760 mmol g–1 at pHi 3, and 0.690 mmol g–1 at pHi 4) > Ni2+ (0.453 mmol g–1 at pHi 3, and 0.225 mmol g–1 at pHi 4). The kinetic data follow a diffusion model in the film and inside particles. A sorption mechanism based on ion exchange, attack by protons of the carbonate and hydroxide positions in the apatite lattice, and also on the adsorption of protons on the basic active sites of carbon, is proposed to explain the heavy metals removal and the pH decreasing in solution.