Alejandro Vázquez

Solubilization, dispersion and stabilization of magnetic nanoparticles in water and non-aqueous solvents: recent trends

Recent achievements in the solubilization and stabilization of magnetic nanoparticles (MNPs) are reviewed. The majority of reported MNPs correspond to iron-based {nZVI, superparamagnetic iron oxides (SPIONs), core–shell Fe/Au or FexOy/Au nanoparticles and ferrites} nanoparticles, with a few numbers corresponding to MnO and cobalt nanoparticles. Magnetic nanoparticles can be solubilized in water or non-aqueous solvents for short or long time periods. The main approaches for MNP solubilization are discussed, namely, suitable choice of precursors, pH, surfactants/coating agents, and solvents, as well as functionalizing agents. MNPs are generally solubilized by functionalization with water-soluble compounds/moieties (in particular, sulfonic acid disodium salts, soluble polymers, porphyrins and calixarenes), under conditions of the thermal decomposition of low stability metal complexes, coprecipitation, microwave heating, and by ultrasonication. The polyol strategy is frequently applied to increase MNP solubility. The stabilization of MNPs in solutions could be achieved with the help of inorganic, monomeric and polymeric compounds.

Thermodynamic systems as extremal hypersurfaces

a b s t r a c t We apply variational principles in the context of geometrothermodynamics. The thermodynamic phase space T and the space of equilibrium states E turn out to be described by Riemannian metrics which are invariant with respect to Legendre transformations and satisfy the differential equations following from the variation of a Nambu-Goto-like action. This implies that the volume element of E is an extremal and that E and T are related by an embedding harmonic map. We explore the physical meaning of geodesic curves in E as describing quasi-static processes that connect different equilibrium states. We present a Legendre invariant metric which is flat (curved) in the case of an ideal (van der Waals) gas and satisfies Nambu-Goto equations. The method is used to derive some new solutions which could represent particular thermodynamic systems.

High resolution electron microscopy characterization of small Pt-Pd2SiO particles in oxide-reducing cycles

Abstract In this work we characterized a commercial PtPd alloy (Pt80%Pd20%) supported on amorphous silica using conventional electron microscopy, high resolution electron microscopy and gas chromatography. The samples were maintained on an oxide-reducing cycles, in order to observe the changes in morphology, crystalline structure and chemical phases formed during these treatments. A PtPd alloy wire was evaporated on a SiO2 planar film and reduced in H2 to form the bimetallic particles, which we call a ‘model’ catalyst. The same particles were observed after each chemical treatment. We did not observe segregation of any metal using high resolution electron microscopy (HREM) when the sample was reduced in H2 at 673 K at least at this atomic concentration. We correlated the changes in morphology, and crystalline structure of the sample maintained at different chemical treatments with the catalytic activity. The monometallic ‘real’ catalysts ( Pt 2 SiO and Pd 2 SiO ) prepared by the impregnation method on a comercial silica (Aerosil) were tested in benzene hydrogenation at 373 K and were compared with the bimetallic ‘real’ catalysts also prepared by this method and maintained on an oxide-reducing cycles. We observed differences in catalytic activity for each catalyst maintained at different oxide reducing cycles which we attributed to variations in particles sizes and morphologies and also to the different chemical phases formed in each treatment.

High resolution electron microscopy of MoS2:Ni, MoS2:Co and MoS2:Fe layered crystals

Abstract High resolution electron microscopy (HREM) is a technique which allows one to get information which by other techniques is difficult to obtain, in particular, for diluted systems as for example, catalysts. It provides a direct way to measure the lattice parameters of the chemical phases that are present in the sample and can provide complementary information by means of the surface image profile. In this work we measured the lattice spacings of crystal planes that appear in samples of molybdenum sulfide doped with Co, Ni and Fe, and to observe possible areas of insertion of these atoms between the MoS 2 layers.

Influence of sulfidation on the morphology and hydrodesulfurization activity of palladium particles on silica

Abstract The size and morphology of palladium particles evaporated onto a SiO 2 film and deposited by impregnation on a porous SiO 2 support were examined by transmission electron microscopy (TEM) following sequential treatment on a gas mixture of 80% H 2 , 20% H 2 S at 523, 623 and 723 K. In model catalysts, progressive sulfidation of palladium from the surface to the bulk took place. By increasing the temperature in this process, coalescence and spreading of PdS particles was observed. In actual supported catalysts the same phenomenon was observed, but at higher temperature several particles appeared faceted with cubic shapes. The palladium real catalyst annealed in hydrogen at 673 K was found to be more active for the hydrodesulfurization of thiophene than those showing the sulfide phase. Also resistance to deactivation was unexpectedly high.

Curvature as a Measure of the Thermodynamic Interaction

We present a systematic and consistent construction of geometrothermodynamics by using Riemannian contact geometry for the phase manifold and harmonic maps for the equilibrium manifold. We present several metrics for the phase manifold that are invariant with respect to Legendre transformations and induce thermodynamic metrics on the equilibrium manifold. We review all the known examples in which the curvature of the thermodynamic metrics can be used as a measure of the thermodynamic interaction.

Structural study of small AuPd/SiO2 particles maintained at a sequential sulfidation treatment

Abstract AuPd/SiO 2 particles maintained at a sequential sulfidation treatment in a H 2 /20%H 2 S atmosphere were studied in order to observe the crystalline structure and chemical phases presented during the treatment at 250 and 350°C. To avoid mistakes in the measurements image processing techniques were used to measure the lattice parameters obtained by high-resolution electron microscopy (HREM) of the different phases present in the treated supported particles.

Cadmium Sulfide and Zinc Sulfide Nanostructures Formed by Electrophoretic Deposition

Cadmium sulfide (CdS) and zinc sulfide (ZnS) nanostructures were formed by means of electrophoretic deposition of nanoparticles with mean diameter of 6 nm and 20 nm, respectively. Nanoparticles were prepared by a microwave assisted synthesis in aqueous dispersion and electrophoretically deposited on aluminum plates. CdS thin films and ZnS one-dimensional nanostructures were grown on the negative electrodes after 24 hours of electrophoretic deposition at direct current voltage. CdS and ZnS nanostructures were characterized by means of scanning electron (SEM) and atomic force (AFM) microscopies analysis. CdS thin films homogeneity can be tunable varying the strength of the applied electric field. Deposition at low electric field produces thin films with particles aggregates, whereas deposition at relative high electric field produces smoothed thin films. The one-dimensional nanostructure size can be also controlled by the electric field strength. Two different mechanisms are considered in order to describe the formation of the nanostructures: lyosphere distortion and thinning and subsequent dipole-dipole interactions phenomena are proposed as a possible mechanism of the one-dimensional nanostructures, and a mechanism considering pre-deposition interactions of the CdS nanoparticles is proposed for the CdS thin films formation.

Influence of Precursor and Power Irradiation on the Microwave-Assisted Synthesis of ZnS Nanoparticles

Results on the synthesis of ZnS nanoparticles from ZnSO 4 and Zn(CH 3 COO) 2 as precursors under microwave (MW) heating conditions are reported. An MW oven (1650 W) operating at 60 and 100% of the nominal power in periods for 60 s was employed. The obtained dispersions were analyzed by UV-Vis spectroscopy, X-ray diffraction, photoluminescence, and transmission electron microscopy. The results show that the dispersion concentration depends mainly on the applied power, whereas the luminescent properties are related with the employed precursors, because the sulfate ions promote the electronic transition in the ZnS nanoparticles, synthesized with a mean radius of 7.1 nm and possessing a hexagonal morphology.

Thermally induced modifications on the crystal surface of molybdate-type catalysts

Abstract The alternate appearance of the (100) and (001) forbidden reflections (group P bnm ) in MoO 3 crystals was followed as a function of temperature under partial reduction conditions. It is demonstrated that a surface layer reconstruction takes place from 250°C (H 2 ), but above 350°C new surface arrays on the molybdate crystallites are detected and finally at 450°C a crystal fragmentation occurs. These modifications are potentially important for the catalytic reactions on molybdate-type catalysts.