Effects of coexisting spin disorder and antiferromagnetism on the magnetic behavior of nanostructured (Fe79Mn21)1xCux alloys

We report a magnetic study on nanostructured (Fe79Mn21)1xCux (0.00x0.30) alloys using static magnetic measurements. The alloys are mainly composed by an antiferromagnetic fcc phase and a disordered region that displays a spin-glass-like behavior. The interplay between the antiferromagnetic and magnetically disordered phases establishes an exchange anisotropy that gives rise to a loop shift at temperatures below the freezing temperature of moments belonging to the disordered region. The loop shift is more noticeable as the Cu content increases, which also enhances the spin-glass-like features. Further, in the x¼0.30 alloy the alignment imposed by applied magnetic fields higher than 4 kOe prevail over the configuration determined by the frustration mechanism that characterizes the spin glass-like phase.Instituto de Físic

Nonmonotonic reversible branch in four model granular beds subjected to vertical vibration

We present results from four independent models of a granular assembly subjected to tapping. We find that the steady-state packing fraction as a function of the tapping intensity is nonmonotonic. In particular, for high tapping intensities, we observe an increase of the packing fraction with tapping strength. This finding challenges the current understanding of compaction of granular media since the steady-state packing fraction is believed to decrease monotonically with increasing tapping intensity. We propose an explanation of our results based on the properties of the arches formed by the particles.Instituto de Física de Líquidos y Sistemas BiológicosGrupo de Aplicaciones Matemáticas y Estadísticas de la Facultad de Ingenierí

Study of the nature and location of silver in Ag-exchanged mordenite catalysts: characterization by spectroscopic techniques

Catalysts based on Na-mordenite (symbolized as M) exchanged with 5, 10, and 15 wt % of Ag were characterized by X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine line structure (EXAFS), X-ray absorption near edge spectroscopy (XANES) and UV-vis diffuse reflectance spectroscopy (DRS) to investigate the effect of different treatments on the chemical state and surface concentration of the silver species. The AgxM catalysts were analyzed in oxidizing (O2) or reducing (H2/Ar) atmospheres and also after being used in the selective catalytic reduction of NOx or in successive cycles of toluene adsorption/desorption. In calcined samples, EXAFS profiles showed two types of AgO spheres of coordination, one due to a dispersed phase of silver oxide and the other due to Ag+ ions in interaction with the oxygen of the zeolite framework. The UV-vis DRS spectra showed the coexistence of isolated Ag+, Agn(delta)+ (n < 10) cationic clusters and AgxO particles. In addition, through the modified Auger parameter (alpha), calculated from XPS measurements, it was possible to identify Ag+ ions at exchange sites (alpha = 722 eV) and AgxO ( alpha = 725 eV) highly dispersed on the surface. Both species constitute stable active centers for the selective catalytic reduction of NOx under severe reaction conditions. However, during the adsorption-desorption of toluene, the reduction of silver oxides produces Ag(0) due to thermal hydrocarbon decomposition.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Nonmonotonic reversible branch in four model granular beds subjected to vertical vibration

We present results from four independent models of a granular assembly subjected to tapping. We find that the steady-state packing fraction as a function of the tapping intensity is nonmonotonic. In particular, for high tapping intensities, we observe an increase of the packing fraction with tapping strength. This finding challenges the current understanding of compaction of granular media since the steady-state packing fraction is believed to decrease monotonically with increasing tapping intensity. We propose an explanation of our results based on the properties of the arches formed by the particles.Instituto de Física de Líquidos y Sistemas BiológicosGrupo de Aplicaciones Matemáticas y Estadísticas de la Facultad de Ingenierí

Inorganically coated colloidal quantum dots in polar solvents using a microemulsion-assisted method

The dielectric nature of organic ligands capping semiconductor colloidal nanocrystals (NCs) makes them incompatible with optoelectronic applications. For this reason, these ligands are regularly substituted through ligand-exchange processes by shorter (even atomic) or inorganic ones. In this work, an alternative path is proposed to obtain inorganically coated NCs. Differently to regular ligand exchange processes, the method reported here produces core–shell NCs and the removal of the original organic shell in a single step. This procedure leads to the formation of connected NCs resembling 1D worm-like networks with improved optical properties and polar solubility, in comparison with the initial CdSe NCs. The nature of the inorganic shell has been elucidated by X-ray Absorption Near Edge Structure (XANES), Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Photoelectron Spectroscopy (XPS). The 1D morphology along with the lack of long insulating organic ligands and the higher solubility in polar media turns these structures very attractive for their further integration into optoelectronic devices.Facultad de Ciencias ExactasInstituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

New Insight on the Hydrogen Absorption Evolution of the Mg–Fe–H System under Equilibrium Conditions

Mg₂FeH₆ is regarded as potential hydrogen and thermochemical storage medium due to its high volumetric hydrogen (150 kg/m³) and energy (0.49 kWh/L) densities. In this work, the mechanism of formation of Mg₂FeH₆ under equilibrium conditions is thoroughly investigated applying volumetric measurements, X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and the combination of scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy (EDS) and high-resolution transmission electron microscopy (HR-TEM). Starting from a 2Mg:Fe stoichiometric powder ratio, thorough characterizations of samples taken at different states upon hydrogenation under equilibrium conditions confirm that the formation mechanism of Mg₂FeH6 occurs from elemental Mg and Fe by columnar nucleation of the complex hydride at boundaries of the Fe seeds. The formation of MgH₂ is enhanced by the presence of Fe. However, MgH₂ does not take part as intermediate for the formation of Mg₂FeH₆ and acts as solid-solid diffusion barrier which hinders the complete formation of Mg₂FeH₆. This work provides novel insight about the formation mechanism of Mg₂FeH₆.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

EXAFS and DFT study of the cadmium and lead adsorption on modified silica nanoparticles

Silica nanoparticles of 7 nm diameter were modified with (3-aminopropyl) triethoxysilane (APTES) and characterized by CP-MAS 13C and 29Si NMR, FTIR, zeta potential measurements, and thermogravimetry. The particles were shown to sorb successfully divalent lead and cadmium ions from aqueous solution. Lead complexation with these silica nanoparticles was clearly confirmed by EXAFS (Extended X-ray Absorption Fine Structure) with synchrotron light measurements. Predicted Pb–N and Pb–C distances obtained from quantum-chemical calculations are in very good agreement with the EXAFS determinations. The calculations also support the higher APTES affinity for Pb2+ compared to Cd2+.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasCentro de Investigaciones ÓpticasCentro de Investigación y Desarrollo en Fermentaciones Industriale

Improvement of the catalytic activity of Ni/SiO<SUB>2</SUB>-C by the modification of the support and Zn addition: Bio-propylene glycol from glycerol

In previous studies, we prepared Ni catalysts supported on a silica-carbon composite which is selective for obtaining 1,2-propylene glycol (1,2-PG). In order to improve the activity levels of this catalyst, in the present work we propose to modify the catalyst formulation by means of two strategies: on the one hand, to modify the acidity of the catalyst through the functionalization of the carbon support and, on the other hand, to modify the metallic phase by the addition of Zn from a controlled preparation technique (Surface Organometallic Chemistry on Metals), which allows the selective addition of small amounts of modifier on the metallic particles. The functionalization of the SC support at 80 °C employing HNO3 at 60 wt% as oxidizing agent allowed increasing the number of surface acid sites that provide Lewis-type acidity without loss of specific surface area. The addition of 1.1–1.8 wt% of Zn (which corresponds to catalysts NiZn0.2/SC and NiZn0.32/SC) generates the formation of an active site composed of an α-NiZn alloy responsible for the increase in activity. When the addition of Zn is 2.8 wt% (which corresponds to catalyst NiZn0.5/SC), the generation of a new tetragonal phase of β1-NiZn would cause the decrease of catalytic activity. These results indicate that the Zn addition has a more significant effect upon the activity and selectivity on the CeO bond cleavage reactions than the effect of the support acidity upon the dehydration activity.Facultad de Ciencias Exacta

Improvement of the catalytic activity of Ni/SiO<SUB>2</SUB>-C by the modification of the support and Zn addition: Bio-propylene glycol from glycerol

In previous studies, we prepared Ni catalysts supported on a silica-carbon composite which is selective for obtaining 1,2-propylene glycol (1,2-PG). In order to improve the activity levels of this catalyst, in the present work we propose to modify the catalyst formulation by means of two strategies: on the one hand, to modify the acidity of the catalyst through the functionalization of the carbon support and, on the other hand, to modify the metallic phase by the addition of Zn from a controlled preparation technique (Surface Organometallic Chemistry on Metals), which allows the selective addition of small amounts of modifier on the metallic particles. The functionalization of the SC support at 80 °C employing HNO3 at 60 wt% as oxidizing agent allowed increasing the number of surface acid sites that provide Lewis-type acidity without loss of specific surface area. The addition of 1.1–1.8 wt% of Zn (which corresponds to catalysts NiZn0.2/SC and NiZn0.32/SC) generates the formation of an active site composed of an α-NiZn alloy responsible for the increase in activity. When the addition of Zn is 2.8 wt% (which corresponds to catalyst NiZn0.5/SC), the generation of a new tetragonal phase of β1-NiZn would cause the decrease of catalytic activity. These results indicate that the Zn addition has a more significant effect upon the activity and selectivity on the CeO bond cleavage reactions than the effect of the support acidity upon the dehydration activity.Facultad de Ciencias Exacta