Fabiano Bernardi

Structural characterization of the Co2FeZ (Z=Al, Si, Ga, and Ge) Heusler compounds by x-ray diffraction and extended x-ray absorption fine structure spectroscopy

This work reports on the structure of Fe containing, Co2-based Heusler compounds that are suitable for magnetoelectronic applications. The compounds Co2FeZ (where Z=Al, Si, Ga, and Ge) were investigated using the x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) techniques. Using XRD, it was shown conclusively that Co2FeAl crystallizes in the B2 structure whereas Co2FeSi crystallizes in the L21 structure. For compounds containing Ga or Ge, the XRD technique cannot be used to easily distinguish between the two structures. For this reason, the EXAFS technique was used to elucidate the structure of these two compounds. Analysis of the EXAFS data indicated that both compounds crystallize in the L21 structure.

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.

Challenging thermodynamics: hydrogenation of benzene to 1,3- cyclohexadiene by Ru@Pt nanoparticles

Since the earliest reports on catalytic benzene hydrogenation, 1,3‐cyclohexadiene and cyclohexene have been proposed as key intermediates. However, the former has never been obtained with remarkable selectivity. Herein, we report the first partial hydrogenation of benzene towards 1,3‐cyclohexadiene under mild conditions in a catalytic biphasic system consisting of Ru@Pt nanoparticles (NPs) in ionic liquid (IL). The tandem reduction of [Ru(COD)(2‐methylallyl)2] (COD=1,5‐cyclooctadiene) followed by decomposition of [Pt2(dba)3] (dba=dibenzylideneacetone) in 1‐n‐butyl‐3‐methylimidazolium hexafluorophosphate (BMI⋅PF6) IL under hydrogen affords core–shell Ru@Pt NPs of 2.9±0.2 nm. The hydrogenation of benzene (60 °C, 6 bar of H2) dissolved in n‐heptane by these bimetallic NPs in BMI⋅PF6 affords 1,3‐cyclohexadiene with an unprecedented 21 % selectivity at 5 % benzene conversion. Conversely, almost no 1,3‐cyclohexadiene was observed when using monometallic Pt0 or Ru0 NPs under the same reaction conditions and benzene conversions. This study reveals that the selectivity is related to synergetic effects of the bimetallic composition of the catalyst material as well as to the performance under biphasic reaction conditions. It is proposed that colloidal metal catalysts in ILs and under multiphase conditions (“dynamic asymmetric mixtures”) can operate far from the thermodynamic equilibrium akin to chemically active membranes.

Artificial cerium-based proenzymes confined in lyotropic liquid crystals: synthetic strategy and on-demand activation

Inorganic nanoparticles that mimic the activity of enzymes are promising systems for biomedical applications. However, they cannot distinguish between healthy and damaged tissues, which could cause undesired effects. Natural enzymes avoid this drawback via activation triggered by specific biochemical events in the body. Inspired by this strategy, we proposed an artificial cerium-based proenzyme system that could be activated to a superoxide dismutase-like form using H2O2 as the trigger. To achieve this goal, an innovative and easy strategy to synthesize Ce(OH)3 nanoparticles as artificial proenzymes was developed using a lyotropic liquid crystal composed of phytantriol, which was essential to maintain their stability at physiological pH. The transmission electron microscopy measurements showed that the Ce(OH)3 nanoparticles were as small as 2 nm. The nanoparticles were fitted into the tiny aqueous channels of the liquid crystal matrix, which presented a Pn3m space group. X-ray absorption near edge structure measurements were used to determine the Ce(iii) fraction of the proenzyme-like nanoparticles, which was around 85%. The Ce(iii) fraction dramatically dropped to around 5% after contact with H2O2 because of the conversion of Ce(OH)3 to CeO(2-x) nanoparticles. The CeO(2-x) nanoparticles showed superoxide dismutase-like activity in contrast to the inactive Ce(OH)3 form. The proof of concept presented in this work opens up new possibilities for using nanoparticles as artificial proenzymes that are activated by a biochemical trigger in vivo.

Cycloaddition of carbon dioxide to epoxides catalysed by supported ionic liquids

Very simple and economical SiO2 supported ionic liquid phase (SILP) materials are efficient catalysts for the addition of CO2 to epoxides, producing cyclic carbonates in high yields (up to 99%) and selectivities (up to 99%). A range of ionic liquid (IL) concentrations (5–100 wt%), SiO2-supported 1-n-butyl-3-methylimidazolium halides (SBMIm·X: X = Cl, Br and I) (1–7) and 1-ethyl-3-(3-(trimethoxysilyl)propyl)-imidazolium halides (SEPIm·X) (8, 9), were prepared and fully characterised. These hybrid materials are very active catalysts under mild reaction conditions (low temperature and atmospheric pressure or adsorbed CO2). Under the optimal reaction conditions ([S] = 3.34 mmol, [cat]/[S] = 0.50, PCO2 = 5 bar, T = 80 °C), the best SILP system yields maximum conversion in just 30 min and can be reused at least five times without a noticeable decrease in activity and selectivity. The catalytic system is also active when using a CO2 gas mixture from an industrial exhaust in both batch and continuous flow systems. A detailed structural and electronic analysis indicates that increasing the IL and water concentrations induces a solvation effect through the contact ion pair of the IL that drives the anions (Cl, Br and I) to the deeper regions of the confined space of SiO2. The catalytic performance is directly related to the presence of the nucleophilic Br anion on the outermost exposed layer of the hybrid material.

In situ XAS studies of PtxPd1−x nanoparticles under thermal annealing

In this work, we have studied PtxPd1−x (x = 1, 0.7 or 0.5) nanoparticles subjected to H2 reduction and sulfidation under H2S atmosphere, both at 300 °C. The system was studied by in‐situ x‐ray absorption spectroscopy (in‐situ XAS). We observed that the efficiency of sulfidation is directly proportional to the quantity of Pd atoms in the nanoparticle, provided the reduction process has been achieved.