José M. Ramallo-López

Synthesis and characterization of gold at gold(i)-thiomalate core at shell nanoparticles.

In this paper, the synthesis of gold at gold(I)-thiolate core at shell nanoparticles is described for the first time. The chemical nature and structure of these nanoparticles were characterized by a multi-technique approach. The prepared particles consist of gold metallic cores, about 1 nm in size, surrounded by stable gold(I)-thiomalate shells (Au at Au(I)-TM). These nanoparticles could be useful in medicine due to the interesting properties that gold(I)-thiomalate has against rheumatoid arthritis. Furthermore, the described results give new insights in the synthesis and characterization of metallic and core at shell nanoparticles.

EXAFS, TDPAC and TPR characterization of PtInFerrierite: The role of surface species in the SCR of NOx with CH4

Abstract Ferrierite exchanged with In and with Pt, In (InFerrierite and PtInFerrierite) is characterized by extended X-ray absorption fine structure (EXAFS), time differential perturbed angular correlation (TDPAC) and temperature-programmed reduction (TPR). The presence of different In and Pt species is correlated with activity and selectivity during the NO selective reduction (SCR) with CH4 in the presence of excess oxygen. Both catalysts are active and selective for the said reaction. When a dry feed is used, both monometallic and bimetallic catalysts behave similarly. However, when water is present in the feed, while InFerrierite loses activity in the whole temperature range, PtInFerrierite increases its activity at temperatures above 450°C. EXAFS characterization indicates that in PtInFerrierite only a fraction of Pt is at exchange positions, the remaining fractions adopting the form of PtO2 crystals located in the outer-surface of the zeolitic structure. When this bimetallic catalyst is treated under wet reaction conditions, the formation of small metal clusters due to the reduction of PtO2 crystals occurs. Thus, the increase in activity could be due to a catalytic effect of Pt metal clusters on the NO oxidation reaction (NO+1/2O2→NO2). TDPAC characterization indicates the presence of two In species both in the monometallic and in the bimetallic catalysts: In2O3 and (InO)+Z− (In at exchange positions), the latter being the active In species for the reaction under study. TPR results also suggest the presence of highly dispersed non-crystalline In oxide species not bonded to the zeolite matrix. Neither EXAFS results nor TDPAC characterization show the presence of intermetallic Pt–In species. The results of this work prove that EXAFS and TDPAC techniques, combined with TPR and activity measurements, are powerful tools for the characterization of bimetallic catalysts.

Spontaneous oxidation of disordered fcc FePt nanoparticles

In this work we present new results on spontaneous oxidation of disordered fcc FePt nanoparticles. The “as-made” oleic acid and oleylamine coated FePt nanoparticles of average size 4 nm synthesized by a high-boiling coordinating solvent method were exposed to air over a period of days and characterized structurally and magnetically by means of different techniques such as XANES, XPS, EXAFS, and SQUID magnetometry. The “as-made” FePt nanoparticles stabilize in the disordered fcc structure and have a very low magnetic saturation (Ms=11 emu/g) and a huge coercive field (Hc=1800 Oe) compared to the low temperature bulk values of the disordered fcc FePt. We observed that the coercive field and the magnetic saturation change with the time the sample is exposed to air and these changes are associated with the oxidation or passivation of the nanoparticle surface that gives place to a core-shell structure. Indeed, the study on the electronic properties of the nanoparticles confirms the magnetic results and indicat...

Shape Changes of Pt Nanoparticles Induced by Deposition on Mesoporous Silica

Polyvinylpyrollidone (PVP)-capped platinum nanoparticles (NPs) are found to change shape from spherical to flat when deposited on mesoporous silica substrates (SBA-15). Transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and extended X-ray absorption fine structure (EXAFS) analyses are used in these studies. The SAXS results indicate that, after deposition, the 2 nm NPs have an average gyration radius 22% larger than in solution, while the EXAFS measurements indicate a decrease in first neighbor co-ordination number from 9.3 to 7.4. The deformation of these small capped NPs is attributed to interactions with the surface of the SBA-15 support, as evidenced by X-ray absorption near-edge structure (XANES).

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 (13)C and (29)Si 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 Pb(2+) compared to Cd(2+).

Structure of extremely nanosized and confined In-O species in ordered porous materials

Perturbed-angular correlation, x-ray absorption, and small-angle x-ray scattering spectroscopies were suitably combined to elucidate the local structure of highly diluted and dispersed InOx species confined in the porous of the ZSM5 zeolite. This novel approach allow us to determined the structure of extremely nanosized In-O species exchanged inside the 10-atom-ring channel of the zeolite, and to quantify the amount of In2O3 crystallites deposited onto the external zeolite surface.

Perturbed Angular Correlation Characterization of Indium Species on In/H-ZSM5 in the Presence of Water and Catalytic Deactivation Studies During the SCR of NOx with Methane

An In(4 wt%)-impregnated H-ZSM5 catalyst was characterized under wet and dry conditions by time-differential perturbed angular correlation (PAC) and by temperature-programmed reduction (TPR). Different indium species were quantified, correlating their structure with the catalyst deactivation due to the presence of water during the NO selective catalytic reduction (SCR) with methane. The fresh sample contains ∼60% of indium oxide and 40% of (InO)+ species at exchange sites, the latter being the active species for the reaction under study. Under wet atmosphere, hyperfine interactions determined by PAC indicate the formation of two types of In hydroxide species and the decrease of both (InO)+ and In2O3. TPR and PAC characterizations also show that deactivation is due to the decrease of (InO)+ at exchange sites to form, after water is removed, different non-active In oxide species.

Synthesis of water-soluble gold clusters in nanosomes displaying robust photoluminescence with very large Stokes shift

This paper reports a novel procedure using nanosomes, made of bola-hydroxyl and mercapto-palmitic acids, for the production of gold clusters with robust luminescent emissions and very large Stokes shifts. It shows that these results cannot be explained by the currently accepted mechanism based on ligand-to-metal charge transfer absorptions involving electron-rich ligands attached to the cluster core. Exhaustive characterization of the cluster samples using Mass Spectrometry, HR-TEM/STEM, XPS, EXAFS, and steady-state and time-resolved luminescence allows to deduce that a mixture of two cluster sizes, having non-closed shell electronic configurations, are firstly generated inside the nanosome compartments due to the difference in bonding strength of the two types of terminal groups in the fatty acids. This initial bimodal cluster size distribution slowly evolves into very stable, closed-shell Au cluster complexes (Au6-Au16 and Au5-Au14) responsible for the observed luminescent properties. The very small (≈1.2 nm) synthesized cluster complexes are water soluble and suitable to be used for the conjugation of biomolecules (through the terminal COO(-) groups) making these systems very attractive as biomarkers and offering, at the same time, a novel general strategy of fabricating stable atom-level quantum dots with large Stokes shifts of great importance in many sensor applications.

Synthesis of ultra-small cysteine-capped gold nanoparticles by pH switching of the Au(I)-cysteine polymer.

We report a synthetic approach for the production of ultra-small (0.6 nm) gold nanoparticles soluble in water with a precise control of the nanoparticle size. Our synthetic approach utilizes a pH-depending Au-cysteine polymer as a quencher for the AuNPs grown. The method extends the synthetic capabilities of nanoparticles with sizes down to 1 nm. In addition to the strict pH control, the existence of free -SH groups present in the mixture of reaction has been observed as a key requirement for the synthesis of small nanoparticles in mild conditions. UV-Vis, SAXS, XANES, EXAFS and HR-TEM, has been used to determinate the particle size, characterization of the gold precursor and gold-cysteine interaction.

A novel catalytic route for hydrogenation–dehydrogenation of 2LiH + MgB2via in situ formed core–shell LixTiO2 nanoparticles

Aiming to improve the hydrogen storage properties of 2LiH + MgB2 (Li-RHC), the effect of TiO2 addition to Li-RHC is investigated. The presence of TiO2 leads to the in situ formation of core–shell LixTiO2 nanoparticles during milling and upon heating. These nanoparticles markedly enhance the hydrogen storage properties of Li-RHC. Throughout hydrogenation–dehydrogenation cycling at 400 °C a 1 mol% TiO2 doped Li-RHC material shows sustainable hydrogen capacity of ∼10 wt% and short hydrogenation and dehydrogenation times of just 25 and 50 minutes, respectively. The in situ formed core–shell LixTiO2 nanoparticles confer proper microstructural refinement to the Li-RHC, thus preventing the materials agglomeration upon cycling. An analysis of the kinetic mechanisms shows that the presence of the core–shell LixTiO2 nanoparticles accelerates the one-dimensional interface-controlled mechanism during hydrogenation owing to the high Li+ mobility through the LixTiO2 lattice. Upon dehydrogenation, the in situ formed core–shell LixTiO2 nanoparticles do not modify the dehydrogenation thermodynamic properties of the Li-RHC itself. A new approach by the combination of two kinetic models evidences that the activation energy of both MgH2 decomposition and MgB2 formation is reduced. These improvements are due to a novel catalytic mechanism via Li+ source/sink reversible reactions.