José J. Calvino

Transforming Nonselective into Chemoselective Metal Catalysts for the Hydrogenation of Substituted Nitroaromatics

It is generally accepted that good hydrogenation noble and nonnoble metal catalysts such as Pt, Ru, or Ni are not chemoselective for hydrogenation of nitro groups in substituted aromatic molecules. We have found that it is possible to transform nonchemoselective into highly chemoselective metal catalysts by controlling the coordination of metal surface atoms while introducing a cooperative effect between the metal and a properly selected support. Thus, highly chemoselective and general hydrogenation Pt, Ru, and Ni catalysts can be prepared by generating nanosized crystals of the metals on the surface of a TiO 2 support and decorating the exposed (111) and (100) crystal faces by means of a simple catalyst activation procedure. By doing this, it has been possible to change the relative rate for hydrogenating competitive groups present in the molecule by almost 2 orders of magnitude, increasing the chemoselectivity from less than 1% to more than 95%.

Some contributions of electron microscopy to the characterisation of the strong metal–support interaction effect

Abstract The contribution of electron microscopy techniques to establishing the existence and actual nature of the metal–support interaction effects occurring in a variety of supported metal catalysts is reviewed. Data pertaining to systems based on both classic reducible supports like TiO 2 , CeO 2 or some ceria-based mixed oxides, and several others generally considered as non-reducible oxides, like La 2 O 3 or SiO 2 , are presented and discussed. The specific temperature and chemical conditions under which strong metal–support interaction phenomena are onset or reverted in each case are also analysed. The whole set of data presented and discussed here clearly shows that the electron microscopy techniques have made an outstanding contribution to the characterisation of the strong metal/support interaction effects exhibited by different metal/oxide systems. Likewise, it is demonstrated that this powerful family of techniques has very much helped to discriminating between true SMSI-like phenomena, as originally defined by Tauster et al., and several other apparent effects which, though at a first sight show some of the chemical, nano-structural and/or catalytic characteristics of the SMSI effect, have a neatly different origin.

Hydrogen chemisorption on ceria: influence of the oxide surface area and degree of reduction

The chemisorption of hydrogen on two ceria samples (CeO2-BS, 4 m2 g–1; CeO2-SM, 56 m2 g–1) reduced at temperatures ranging from 623 to 1173 K has been studied by Fourier-transform infrared (FTIR) spectroscopy and temperature-programmed desorption followed by thermal conductivity (TPD-TC) and mass spectrometry (TPD-MS). The concentration of the oxygen vacancies created by the reduction treatments was determined by using a combination of O2 pulses and temperature-programmed oxidation. According to our TPD-MS study, hydrogen can be desorbed from ceria as both H2(reversible adsorption) and H2O (irreversible adsorption), the relative contribution of these two forms depending on the reduction temperature. For samples reduced at 773 K or higher temperatures, H2 was the only desorption product. From this observation, some earlier TPD-TC and TPR-TC results could be better understood. Upon reduction at 773 K, the amount of H2 chemisorbed per mole of CeO2 was ten times larger for CeO2-SM than for CeO2-BS. Likewise, the molar chemisorptive capability of CeO2-SM strongly decreased (45 times) with the reduction temperature. No simple relationship could be observed between the amount of chemisorbed hydrogen and the total concentration of oxygen vacancies in the oxide. In contrast to earlier results on the contribution of a massive bronze-like phase when chemisorbing H2 at 195–500 K, the results reported here show that the hydrogen chemisorption on reduced ceria is a surface-related process. Furthermore, the highest value for the hydrogen chemisorption we have obtained, 7.1 H atom nm–2(BET), suggests a pure surface process.

Comparative Structural and Chemical Studies of Ferritin Cores with Gradual Removal of their Iron Contents

Transmission Electron Microscopy (TEM), X-ray Absorption Near Edge Spectroscopy (XANES), Electron Energy-Loss Spectroscopy (EELS), Small-Angle X-ray Scattering (SAXS), and SQUID magnetic studies were performed in a batch of horse spleen ferritins from which iron had been gradually removed, yielding samples containing 2200, 1200, 500, and 200 iron atoms. Taken together, findings obtained demonstrate that the ferritin iron core consists of a polyphasic structure (ferrihydrite, magnetite, hematite) and that the proportion of phases is modified by iron removal. Thus, the relative amount of magnetite in ferritin containing 2200 to 200 iron atoms rose steadily from approximately 20% to approximately 70% whereas the percentage of ferrihydrite fell from approximately 60% to approximately 20%. These results indicate a ferrihydrite-magnetite core-shell structure. It was also found that the magnetite in the ferritin iron core is not a source of free toxic ferrous iron, as previously believed. Therefore, the presence of magnetite in the ferritin cores of patients with Alzheimers disease is not a cause of their increased brain iron(II) concentration.

Single-Step Process To Prepare CeO2 Nanotubes with Improved Catalytic Activity

CeO(2) nanotubes have been grown electrochemically using a porous alumina membrane as a template. The resulting material has been characterized by means of scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy, high-angle annular dark-field scanning transmission electron microscopy tomography, high-resolution electron microscopy (HREM), and electron energy loss spectroscopy. According to SEM, the outer diameter of the nanotubes corresponds to the pore size (200 nm) of the alumina membrane, and their length ranges between 30 and 40 microm. HREM images have revealed that the width of the nanotube walls is about 6 nm. The catalytic activity of these novel materials for the CO oxidation reaction is compared to that of a polycrystalline powder CeO(2) sample prepared by a conventional route. The activity of the CeO(2) nanotubes is shown to be in the order of 400 times higher per gram of oxide at 200 degrees C (77.2 x 10(-2) cm(3) CO(2) (STP)/(gxs) for the nanotube-shaped CeO(2) and 0.16 x 10(-2) cm(3) CO(2) (STP)/(gxs) for the powder CeO(2)).

3 D Characterization of Gold Nanoparticles Supported on Heavy Metal Oxide Catalysts by HAADF‐STEM Electron Tomography

Living on the edge: Three-dimensional reconstructions from electron tomography data recorded from Au/Ce(0.50)Tb(0.12)Zr(0.38)O(2-x) catalysts show that gold nanoparticles (see picture; yellow) are preferentially located on stepped facets and nanocrystal boundaries. An epitaxial relationship between the metal and support plays a key role in the structural stabilization of the gold nanoparticles.

Magnetic Nanoparticles-Templated Assembly of Protein Subunits: A New Platform for Carbohydrate-Based MRI Nanoprobes

A new approach for the preparation of carbohydrate-coated magnetic nanoparticles is reported. In a first step, we show that the pH-driven assembly-disassembly natural process that occurs in apoferritin protein is effective for the encapsulation of maghemite nanoparticles of different sizes: 4 and 6 nm. In a second step, we demonstrate that the presence of functional amine groups in the outer shell of apoferritin allows functionalization with two carbohydrates, N-acetyl-D-glucosamine and d-mannose. High-resolution electron microscopy (HREM), high angle annular dark field scanning electron microscopy (HAADF-STEM), electron energy loss spectroscopy (EELS), X-ray diffraction (XRD), and SQUID technique have been used to characterize the magnetic samples, termed herein Apomaghemites. The in vivo magnetic resonance imaging (MRI) studies showed the efficiency in contrasting images for these samples; that is, the r(2) NMR relaxivities are comparable with Endorem (a commercial superparamagnetic MRI contrast agent). The r(2) relaxivity values as well as the pre-contrast and post-contrast T(2)*-weighted images suggested that our systems could be used as perspective superparamagnetic contrast agents for magnetic resonance imaging (MRI). The carbohydrate-functionalized Apomaghemite nanoparticles retained their recognition abilities, as demonstrated by the strong affinity with their corresponding carbohydrate-binding lectins.

High-resolution electron microscopy investigation of metal–support interactions in Rh/TiO2

High-resolution electron microscopy (HREM), combined with digital image processing and computer simulation techniques, has been used to carry out a microstructural characterization of some Rh/TiO2 catalysts reduced at 473, 773 or 823 K. A sample resulting from the reduction of the catalyst at 773 K, reoxidized with flowing O2 at 673 K and finally reduced at 473 K was also investigated. In contrast to earlier literature reports, the HREM images recorded here suggest the existence of some definite structural relationships between both the rutile and anatase titania microcrystals and the rhodium particles grown on them. Though some micrographs have been interpreted in terms of slightly reduced Magneli phases, our HREM study suggests that the major microstructural feature of the high temperature (773 and 823 K) reduced samples is the formation of an amorphous reduced titania phase, associated with which metal decoration occurs. Some metal sintering is also observed. The reoxidation treatment, although inducing reversal of the decoration process leads to larger metal particles than those of the starting low-temperature reduced catalysts. Note, also, that twinned metal particles are more often observed in the catalyst resulting from the mild reduction of the reoxidized sample.

Lanthanide salts as alternative corrosion inhibitors

Abstract Lanthanum and samarium nitrates and chlorides have been investigated as corrosion inhibitors of AISI 434 SS in sodium chloride solutions, at room temperature. Electrochemical techniques allowed to evaluate the degree of protection and the cathodic nature of the inhibitors. Scanning electron microscopy and energy dispersive spectrometry were used to analyze the composition of the protective films formed after full-immersion tests.

Structural characterisation of a VMgO catalyst used in the oxidative dehydrogenation of propane

AbstractA VMgO catalyst (containing 14 wt% vanadium) that is used in the oxidative dehydrogenation of propane (ODHP) reaction has been examined in detail by in situ EXAFS, in situ XRD and HREM. These characterisation techniques have revealed that, as prepared, the catalyst is in effect a three-component system comprising discrete magnesium orthovanadate (Mg3V2O8) particles, magnesium oxide and a disordered vanadium-containing overlayer supported on the MgO. When the catalyst is exposed to typical ODHP reaction conditions at