Erico Teixeira-Neto

Volcano-like Behavior of Au-Pd Core-shell Nanoparticles in the Selective Oxidation of Alcohols

Gold-palladium (AuPd) nanoparticles have shown significantly enhanced activity relative to monometallic Au and Pd catalysts. Knowledge of composition and metal domain distributions is crucial to understanding activity and selectivity, but these parameters are difficult to ascertain in catalytic experiments that have primarily been devoted to equimolar nanoparticles. Here, we report AuPd nanoparticles of varying Au:Pd molar ratios that were prepared by a seed growth method. The selective oxidation of benzyl alcohol was used as a model reaction to study catalytic activity and selectivity changes that occurred after varying the composition of Pd in bimetallic catalysts. We observed a remarkable increase in catalytic conversion when using a 10:1 Au:Pd molar ratio. This composition corresponds to the amount of Pd necessary to cover the existing Au cores with a monolayer of Pd as a full-shell cluster. The key to increased catalytic activity derives from the balance between the number of active sites and the ease of product desorption. According to density functional theory calculations, both parameters are extremely sensitive to the Pd content resulting in the volcano-like activity observed.

Oxidation of benzyl alcohol catalyzed by gold nanoparticles under alkaline conditions: weak vs. strong bases

Gold nanoparticles have shown excellent catalytic properties in selective oxidation of alcohols in the presence of base; however, the influence of the nature and concentration of the base on gold catalyst activity and selectivity is not completely understood. We here present a study of the effect of using weak and strong bases on the conversion and selectivity of PVA-stabilized gold nanoparticles supported on titania (AuPVA/TiO2) for benzyl alcohol oxidation. The increase in the concentration of base had little effect on conversion when a weak base was used (K2CO3, Na2B4O7 and Na(CH3COO)), due to the buffer effect, but strongly affected selectivity. The bases with higher pKa values provided higher conversions and increased production of benzoic acid. For the strong base NaOH, benzoic acid was always the major product, although conversion decreases in excess of base. The formation of benzoic acid is avoided by using K2CO3 in non-aqueous media; benzaldehyde is the main product in cyclohexane whereas benzyl benzoate is also formed in significant amounts in solvent-free conditions. The promotion effect observed in the presence of base was discussed in terms of reaction mechanism.

Layered double hydroxide and sulindac coiled and scrolled nanoassemblies for storage and drug release

Sulindac, a non-steroidal anti-inflammatory of the indene acetic acid class, was immobilized inside layered double hydroxide Mg2Al and Zn2Al (LDH) nanovessels through a one pot reaction. LDH–drug materials were characterized by chemical elemental analysis, X-ray diffraction (XRD) (one dimensional electron distribution along the c-stacking axis, pair distribution function analysis), scanning and transmission electron microscopies, mass coupled thermal analyses, vibrational infrared and Raman spectroscopies, and solid state 13C NMR. Density Functional Theory (DFT) calculations were performed for sulindac (protonated and deprotonated forms) with the aim to assign the LDH–drug spectroscopic results. All converge towards a spatial organization of interleaved sulindac molecules close to that reported for the pristine polymorph II crystal structure. Of relevance for human treatment because of its biocompatibility and non-immunogenic effect, in vitro sulindac release experiments were performed in a phosphate buffer mimicking biological fluids and release profiles were refined using kinetic models.

Hybrid composites of xanthan and magnetic nanoparticles for cellular uptake

We describe a fast and simple method to prepare composite films of magnetite nanoparticles and xanthan networks. The particles are distributed close to hybrid film surface, generating a coercivity of 27 ± 2 Oe at 300 K. The proliferation of fibroblast cells on the hybrid composites was successful, particularly when an external magnetic field was applied.

Surface composition and structural changes on titanium oxide-supported AuPd nanoparticles during CO oxidation

To investigate the influence of morphological changes on the performance of bimetallic catalyst nanoparticles, two AuPd/TiO2 catalysts with the same starting composition were synthesized via thermal treatments in reducing (H2) or oxidizing (O2) atmospheres, resulting in different morphologies. The surface composition of the bimetallic particles evolved under a CO oxidation reaction environment, thereby changing the performance of both catalysts in repeated heat ramps. In the oxidized material, composed mainly of large AuPd particles and PdO particles, the CO oxidation reaction led to a broadening in the size distribution caused by the appearance of smaller particles, together with an increase in the surface Pd concentration. In the reduced material, the CO oxidation reaction led to particle aggregation, Pd oxidation, and surface Pd enrichment in Pd-rich particles. During the CO oxidation experiments, the oxidized material was activated (decrease of Tlight-off), while the reduced catalyst suffered a deactivation process (increase of Tlight-off). Our results showed that the catalysts converge to a common surface composition and performance, and this convergence seemed to be independent of the initial particle morphology.

Promotion effects of Pd on tungsten carbide catalysts: physiochemical properties and cellulose conversion performance

A multi-functional catalyst, which is able to perform both retro-aldol reactions followed by hydrogenation, is required to convert cellulose into value-added ethylene glycol (EG) in a one-pot reaction. Herein, we show the results of a catalyst comprising Pd-promoted tungsten carbide supported on activated carbon. Pd acts to hinder carbon deposition onto the catalyst surface during the carburization process, through the inhibition of methane decomposition species on the catalyst surface. XPS and TEM analysis show a cleaner surface which improves the W/C ratio. The enhanced interaction between Pd and W2C, results in higher cellulose conversions, and higher ethylene glycol (EG) yields when compared to non-promoted or Ni promoted tungsten carbide catalysts reported elsewhere. XAS, HR-TEM and SI-XEDS analysis showed Pd nanoparticles are well dispersed on the surface and in contact with W2C. Despite higher carburization temperatures, Pd-promoted tungsten carbide catalysts enhance the catalytic properties of the one-pot cellulose conversion reaction towards EG production when compared to Ni-promoted catalysts under the same catalytic reaction conditions.

Syntheses and structural understanding of a Ti–Ta alloy-based nanotubular oxide photocatalyst

Herein, we have synthesized Ti–Ta based mixed oxide vertically oriented and highly ordered freestanding nanotubes having an average length of 60 μm by anodic oxidation of a homemade Ti–Ta (50–50 at%) alloy. The effect of heat treatment on the morphology, phase transformation to TiTa2O7 nanostructures, crystalline structures, optical and surface properties has been investigated by a variety of characterization techniques. The samples remained mainly amorphous at temperatures up to 700 °C, and according to the Rietveld refinement analyses the bulk formation of crystalline Ta2O5 and TiO2-anatase phases occurred only at 800 °C in the tubular matrix followed by a slight solid to solid transformation of TiTa2O7 and TiO2-rutile phases starting from 850 °C. At 1000 °C, the content of TiTa2O7 increased followed by a decrease in TiO2 and Ta2O5 content; however, the nanotubes appeared to be interconnected particles imitating the verticality of the nanotubes. Compared to the literature (∼1350 °C), in this work the transformation of TiTa2O7 occurred at a lower temperature which is related to the amorphous nature of the starting nanotubular samples. The HAADF-STEM images and localized EDS mapping showed the distribution of Ti and Ta elements along the tubular matrix demonstrating that the as-anodized nanotubes consist of a uniform mixture of TiO2 and Ta2O5 whereas for the calcined samples random distribution of Ti was observed indicating the migration of Ti inside the tubular matrix for the formation of mixed oxide and TiTa2O7 phases. X-ray photoelectron spectroscopy (XPS) analysis identified the dislocation in the binding energies of Ti 2p and Ta 4f regions for the heat treated samples as compared to the as-anodized nanotubes; related to the formation of TiTa2O7. The optical band gap of TiTa2O7 was found to be 3.08 eV. These oxide mixtures were applied in photocatalytic H2 generation under solar irradiation; where the sample heat treated at 1000 °C has resulted in a production rate of 41.58 μmol h−1 g−1 which was higher than all the other samples synthesized in this work. For continuous 48 h irradiation the photocatalytic activity of the sample remained highly stable. This result is related to the higher content and lower bandgap energy of TiTa2O7 in the mixed oxide matrix indicating that TiTa2O7 is a promising photocatalyst.

Hybrid Organic-Inorganic Anatase as a Bifunctional Catalyst for Enhanced Production of 5-Hydroxymethylfurfural from Glucose in Water

Hybrid organic-inorganic anatase (hybrid-TiO2 ) is prepared by a facile hydrothermal synthesis method employing citric acid. The synthetic approach results in a high surface-area nanocrystalline anatase polymorph of TiO2 . The uncalcined hybrid-TiO2 is directly studied as a catalyst for the conversion of glucose into 5-hydroxymethylfurfural (HMF). In the presence of the hybrid-TiO2 , HMF yields up to 45 % at glucose conversions up to 75 % were achieved in water at 130 °C in a monophasic batch reactor. As identified by Ti K-edge XANES, hybrid-TiO2 contains a large fraction of fivefold coordinatively unsaturated TiIV sites, which act as the Lewis acid catalyst for the conversion of glucose into fructose. As citric acid is anchored in the structure of hybrid-TiO2 , carboxylate groups seem to catalyze the sequential conversion of fructose into HMF. The fate of citric acid bound to anatase and the TiIV Lewis acid sites throughout recycling experiments is also investigated. In a broader context, this contribution outlines the importance of hydrothermal synthesis for the creation of water-resistant Lewis acid sites for the conversion of sugars. Importantly, the use of the hybrid-TiO2 with no calcination step contributes to dramatically decreasing the energy consumption in the catalyst preparation.