Renato V. Gonçalves

Insights into the active surface species formed on Ta2O5 nanotubes in the catalytic oxidation of CO

Freestanding Ta2O5 nanotubes were prepared by an anodizing method. As-anodized amorphous nanotubes were calcined at high temperature to obtain a crystalline phase. All materials were studied by means of BET analysis, XRD, TEM, SEM, XPS, and FTIR and were evaluated in the catalytic oxidation of CO. An XPS study confirmed the formation of different tantalum surface species after high temperature treatment of amorphous Ta2O5 nanotubes. Calcination at 800 °C generated Ta(4+) while higher temperature (1000 °C) treatment led to the formation of Ta(3+) species. These materials also showed significant differences in catalytic activity. Higher activity was observed for samples calcined at 800 °C than at 1000 °C, suggesting that Ta(4+) species are active sites for CO oxidation.

Easy Access to Metallic Copper Nanoparticles with High Activity and Stability for CO Oxidation.

Copper catalysts are very promising, affordable alternatives for noble metals in CO oxidation; however, the nature of the active species remains unclear and differs throughout previous reports. Here, we report the preparation of 8 nm copper nanoparticles (Cu NPs), with high metallic content, directly deposited onto the surface of silica nanopowders by magnetron sputtering deposition. The as-prepared Cu/SiO2 contains 85% Cu0 and 15% Cu2+ and was enriched in the Cu0 phase by H2 soft pretreatment (96% Cu0 and 4% Cu2+) or further oxidized after treatment with O2 (33% Cu0 and 67% Cu2+). These catalysts were studied in the catalytic oxidation of CO under dry and humid conditions. Higher activity was observed for the sample previously reduced with H2, suggesting that the presence of Cu-metal species enhances CO oxidation performance. Inversely, a poorer performance was observed for the sample previously oxidized with O2. The presence of water vapor caused only a small increase in the temperature require for the reaction to reach 100% conversion. Under dry conditions, the Cu NP catalyst was able to maintain full conversion for up to 45 h at 350 °C, but it deactivated with time on stream in the presence of water vapor.

On the crystallization of Ta2O5 nanotubes: structural and local atomic properties investigated by EXAFS and XRD

Metal oxide nanotubes (NTs) semiconductors prepared by anodization are promising materials due to their expected unique optical and electric properties. However, most of the work reported to date did not find photoelectrochemical devices with higher efficiency than those assembled with nanoparticles. Moreover, this behavior is due to the difficulty of having non-defective crystalline structures and the disruption of the tubular shape during thermal treatment while trying to reduce oxygen vacancies. This work describes in detail the local atomic configuration, surface area and morphology properties of Ta2O5 NTs prepared by anodization as a function of the temperature and the crystallization time by using X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). The crystallization process of adhered and freestanding Ta2O5 NTs is discussed. Adhered NTs crystallized at 550 °C due to the oxidation of the Ta metal during annealing in the air atmosphere and not the NT array itself. Freestanding Ta2O5 NTs crystallized after annealing at 800 °C. Rietveld refinements were performed to investigate the effects of the temperature and the annealing time on the grain size and microstrain and obtain information about Ta–O interatomic distances. The local structure of amorphous and crystalline Ta2O5 NTs was investigated with EXAFS. Low coordination numbers were found in the as-anodized samples as well as the samples annealed for 30 min at 800 °C. The coordination number increased when annealing was performed above 800 °C or when the annealing time was longer than 30 min. Moreover, the decrease of defects was followed by an increase in the crystal size and collapse of the tubular shape due to the increase in internal stress generated by the increase in the crystallinity of the tubes and the orthorhombic Ta2O5 crystal size.

Magnetic nanocatalysts: supported metal nanoparticles for catalytic applications

Abstract This review is focused on metal nanoparticles (NPs) supported on magnetic responsive solids and their recent applications as magnetically recoverable nanocatalysts. Magnetic separation is a powerful tool for the fast separation of catalysts from reaction medium and an alternative to time-, solvent-, and energy-consuming separation procedures. Metal NPs attached to a magnetic solid can be easily carried and recovered by magnetic separation. Some examples of magnetically recoverable metal NPs used in hydrogenation, oxidation, C-C coupling reactions, photocatalysis, and other organic reactions will be given.

Gold nanoparticles supported on magnesium ferrite and magnesium oxide for the selective oxidation of benzyl alcohol

Au nanoparticles (Au NPs) have gained significant attention as catalysts for the selective oxidation of alcohols; however, the catalytic activity is highly dependent on the presence of a base. Alternatives to the use of strong bases, such as NaOH, are still needed. Here, we explored the basicity of magnesium ferrite/oxide supports to study the catalytic behaviour of supported Au NPs for the oxidation of benzyl alcohol. The presence of Mg2+ ions in the ferrite structure improved the catalytic activity of supported Au NPs to ca. 35% conversion in the absence of an additional base. After modifying the support with MgO, the catalytic activity of supported Au NPs was further improved to ca. 50% conversion, but the catalyst deactivated in successive recycling tests. When the catalysts were tested in the presence of a sub-stoichiometric amount of K2CO3, they became more active and remained stable upon recycling with no loss of activity and selectivity for the preferential production of benzoic acid.

Polymorphic phase study on nitrogen-doped TiO2 nanoparticles: effect on oxygen site occupancy, dye sensitized solar cells efficiency and hydrogen production

In this work we show that phase formation and oxygen substitution can be controlled by the source of nitrogen used during the synthesis of TiO2 nanoparticles. By performing a thorough study on the structure of the nanoparticles, the use of NH4+ or NO3− was found to influence not only the N-doping level but also the formation of the polymorphic phase. Structural and microstructural refinement obtained by XRD pattern and data processing performed by the Rietveld refinement revealed that TiO2 obtained with HNO3 presents ca. 98% of anatase and ca. 2% for rutile. Meanwhile TiO2 nanoparticles synthesized with NH4F and NH4Cl presents a single anatase phase with ca. 7.0% and 4.4% of nitrogen substitutional oxygen sites, respectively. The local structure of N-doped TiO2 around the Ti atoms was investigated by X-ray absorption spectroscopy. The XANES spectra show that N-doped TiO2 possesses a characteristic pre-edge of a single anatase structure. The coordination number decreased and the shrinking Ti–O bond distances are due to the N-doping in the TiO2 structure. The most efficient dye sensitized solar cell and the higher hydrogen production was obtained from the TiO2/NH4Cl, which was obtained as a single anatase phase with intermediary concentration of N substitutional oxygen sites.

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.

Photocatalytic Water Splitting by Suspended Semiconductor Particles

Starting and emphasizing the importance for future demand on Earth energy feed, this chapter discusses the state-of-the-art of photocatalytic water splitting (PWS) using suspended particles for hydrogen evolution. Herein, the thermodynamics requirements for photocatalyst semiconductors for water splitting for efficient hydrogen and oxygen production, to date overview on important photocatalysts and related fine-tuning processes adopted to meet the desired properties for PWS with improved photocatalytic activities, a profound discussion and review about tandem photocatalyst systems are covered. A descriptive discussion on efficiency determination including solar-to-hydrogen efficiency and apparent quantum efficiency to compare the photocatalytic performance of the photocatalysts is presented.

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.