Lorenzo Stievano

Better Cycling Performances of Bulk Sb in Na-Ion Batteries Compared to Li-Ion Systems: An Unexpected Electrochemical Mechanism

Pure micrometric antimony can be successfully used as negative electrode material in Na-ion batteries, sustaining a capacity close to 600 mAh g(-1) at a high rate with a Coulombic efficiency of 99 over 160 cycles, an extremely high capacity compared to any other compound tested against both Li and Na. The reaction mechanism with Na does not simply go through the alloying mechanism observed for Li where the intermediate species are those expected from the phase diagram. In the case of Na, the intermediate phases are mostly amorphous and could not be precisely identified. Surprisingly, we evidenced that a competition takes place at the end of the discharge of the Sb/Na cell between the formation of the hexagonal and the cubic polymorphs of Na(3)Sb, the last being described in the literature as unstable at atmospheric pressure and only synthesized under high pressure (1-9 GPa). In addition, fluoroethylene carbonate added to the electrolyte combined with an appropriate electrode formulation based on carboxymethyl cellulose, carbon black, and vapor ground carbon fibers seems to be determinant in the excellent performances of this material.

Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials

While platinum has hitherto been the element of choice for catalysing oxygen electroreduction in acidic polymer fuel cells, tremendous progress has been reported for pyrolysed Fe-N-C materials. However, the structure of their active sites has remained elusive, delaying further advance. Here, we synthesized Fe-N-C materials quasi-free of crystallographic iron structures after argon or ammonia pyrolysis. These materials exhibit nearly identical Mössbauer spectra and identical X-ray absorption near-edge spectroscopy (XANES) spectra, revealing the same Fe-centred moieties. However, the much higher activity and basicity of NH3-pyrolysed Fe-N-C materials demonstrates that the turnover frequency of Fe-centred moieties depends on the physico-chemical properties of the support. Following a thorough XANES analysis, the detailed structures of two FeN4 porphyrinic architectures with different O2 adsorption modes were then identified. These porphyrinic moieties are not easily integrated in graphene sheets, in contrast with Fe-centred moieties assumed hitherto for pyrolysed Fe-N-C materials. These new insights open the path to bottom-up synthesis approaches and studies on site-support interactions.

Before striking gold in gold-ruby glass

The red colour of gold-ruby glass is caused by small particles of metallic gold that form when the gold-containing colourless glass is annealed. But the chemical state of the gold before striking red has been a mystery — is it dissolved in the glass as individual neutral gold atoms, like a frozen-in metal vapour that precipitates on annealing, or as gold cations that must be reduced before the metallic clusters can grow and impart the lustrous colour? Here we use Mössbauer spectroscopy to show that this gold is monovalent in the colourless glass, forming linear bonds to two neighbouring oxygen atoms.

X-ray Absorption Near-Edge Structure and Nuclear Magnetic Resonance Study of the Lithium–Sulfur Battery and its Components

Understanding the mechanism(s) of polysulfide formation and knowledge about the interactions of sulfur and polysulfides with a host matrix and electrolyte are essential for the development of long-cycle-life lithium-sulfur (Li-S) batteries. To achieve this goal, new analytical tools need to be developed. Herein, sulfur K-edge X-ray absorption near-edge structure (XANES) and (6,7) Li magic-angle spinning (MAS) NMR studies on a Li-S battery and its sulfur components are reported. The characterization of different stoichiometric mixtures of sulfur and lithium compounds (polysulfides), synthesized through a chemical route with all-sulfur-based components in the Li-S battery (sulfur and electrolyte), enables the understanding of changes in the batteries measured in postmortem mode and in operando mode. A detailed XANES analysis is performed on different battery components (cathode composite and separator). The relative amounts of each sulfur compound in the cathode and separator are determined precisely, according to the linear combination fit of the XANES spectra, by using reference compounds. Complementary information about the lithium species within the cathode are obtained by using (7) Li MAS NMR spectroscopy. The setup for the in operando XANES measurements can be viewed as a valuable analytical tool that can aid the understanding of the sulfur environment in Li-S batteries.

Glycine and lysine adsorption and reactivity on the surface of amorphous silica

In order to better control the adsorption of proteins on mineral surfaces, it is useful to gain a thorough understanding of the behaviour of their constituting monomers, namely, amino acids. We have therefore investigated the adsorption of glycine, lysine, and other small biomolecules on well-characterised surfaces of silica. While similar systems have been studied before, these studies usually concentrated on evidencing the effect of surfaces on peptide bond formation in a phenomenological, mostly macroscopic approach. In contrast, we have considered the adsorption process from the view-point of the surface, using knowledge previously gained on the molecular identification of surface functional groups to better characterise their interaction with amino acids, both from the aqueous phase and from the vapour phase. We combined macroscopic level information such as adsorbed amounts, pH dependence, or TGA, with molecular characterisation by vibrational spectroscopies and 13 C NMR of the adsorbed molecules. In parallel, we have carried out molecular modelling of candidate clusters containing the amino acid and the adsorption site by DFT. The structures of lowest energy were also those that best reproduced the observed spectroscopic properties. Different adsorption mechanisms can be postulated, corresponding to different spectroscopic signatures of the amino acid/adsorption site complexes. On silica surfaces, this implies cooperative hydrogen bonding in a kind of molecular recognition. The existence of molecular recognition is also evidenced by experiments on the coadsorption of different amino acids, where strong selectivity effects may be observed. Such phenomena may have much practical significance in the fields of biofilms and prebiotic chemistry.

Structure and catalytic activity of palladium-platinum aggregates obtained by laser vaporisation of bulk alloys

Abstract The European emission legislation concerning the pollution abatement of the exhaust gas from light cars imposes a reduction of the concentration of aromatics in gasoline, down to 2%. The hydrogenation of these hydrocarbons can be performed on Pd or Pt catalysts. However, these catalysts are poisoned by traces of sulphur — 100–300 ppm — remaining in the fuels. It is claimed in the literature that bimetallic Pd–Pt catalysts are less affected by this poisoning than the pure metals. To verify this point, both Pd–Pt and pure metal aggregates were synthesised by laser vaporisation of bulk alloys and deposited on alumina. Analytical microscopy and EXAFS have shown that these clusters have a very narrow composition distribution and form alloys. The study of their activity in the hydrogenation of Tetralin, a model molecule, in the presence of variable amounts of H2S, has shown that Pt is more active than Pd at low sulphur concentration, whereas Pd becomes more active for the highest H2S content. Contrary to what is claimed in the literature, no synergetic effect has been found found by alloying these two metals; actually their activity is the simple additivity of that of the two metals.

A comparative study of the catalysis of peptide bond formation by oxide surfaces

It is well-known that amino acids deposited on some inorganic oxides undergo peptidic condensation. It is seldom realised however that a large diversity of behaviours can be observed in such systems. Here we use the apparently simple case of glycine-non-porous silica as a reference system, in which glycine (Gly) dimerisation to diketopiperazine (DKP) is easy to evidence, especially when using TG in combination with NMR. We then proceed to compare it with other AA deposited on the same support on the one hand, with Gly deposited on other mineral surfaces on the other hand. In a final section, we provide more detailed mechanistic information on the glycine condensation process on silica, including kinetic data and a (13)C solid-state NMR follow up of the species at various stages of thermal condensation. The best mechanism to rationalise these data involves a crucial step of isomerisation from zwitterion to neutral glycine, and the participation of several distinct types of surface sites probably consisting of silanol ensembles.

The effect of the 3-trifluoromethyl substituent in polypyrazolylborato complexes on the iron(II) spin state; X-ray diffraction and absorption and Mössbauer studies ☆

Abstract A series of bis(polypyrazolylborato)iron(II) complexes (with Tp 3CF 3 , Tp 4Me , Tp 4Br , and pzTp 4Me ) has been prepared and characterised by 57 Fe Mossbauer spectroscopy. In addition, the Tp 3CF 3 has been studied and compared with Tp 3CH 3 by X-ray absorption investigation. The X-ray crystal structure of iron(II)bis(3-trifluoromethyl-1 H -pyrazol-1-yl)borato has been resolved. In Fe(Tp 3CF 3 ) 2 the coordination geometry around the Fe centre is distorted octahedral, with the FeN bond distances in the range 2.219(3)–2.251(3) A that are slightly longer than typical distances for high-spin iron(II) complexes. A close comparison of the Tp 3CF 3 and Tp 3CH 3 has revealed small but nonetheless significant differences as evidenced from low-temperature Mossbauer studies and from X-ray absorption near-edge structure approach. At variance with Fe(Tp 3CHF 3 ) 2 , the high spin state for Fe(Tp 3CF 3 ) 2 is to be taken as the most stable one.

Bulk and surface structure and composition of V–Sb mixed-oxide catalysts for the ammoxidation of propane

Abstract Vanadium–antimony mixed oxides, which are active and selective catalysts for the ammoxidation of propane to acrylonitrile, were obtained via different preparation routes and studied with a number of bulk and surface-sensitive techniques to elucidate the bulk composition of these complex materials and the character of their exposed surfaces. The V–Sb oxides were prepared via a redox reaction between NH 4 VO 3 and Sb 2 O 3 in an aqueous slurry, with subsequent calcination, or via a solid-state reaction between Sb 2 O 3 and V 2 O 5 . The characterisation techniques employed were X-ray diffraction (XRD), N 2 physisorption, electron microscopy, potentiometric titration, Mossbauer, EPR, X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), ultraviolet photoelectron spectroscopy (UPS), and IR spectroscopy. It was found that samples obtained by the solid-state reaction were more homogeneous than those prepared via the slurry route. The former consisted of (non-stoichiometric) VSbO 4 for Sb/V=1, or a physical mixture of it with defective Sb 2 O 4 for Sb/V=2. Their bulk Sb/V ratio was found also for the surface region, the outmost part of which was enriched in vanadium to a small extent. Materials prepared via the slurry route consisted of (non-stoichiometric) VSbO 4 , Sb 2 O 4 , and V 2 O 5 if Sb/V=1; the latter was missing for Sb/V=2 and Sb/V=5. While the character of the mixed surfaces was not determined by the V 2 O 5 present, amorphous V oxide structures (highly dispersed species and aggregates) were supported on the Sb 2 O 4 surface, which caused a significant vanadium excess in the overall surface composition in samples of Sb/V>1. The average oxidation degree of the surface V species was higher than 4+. Use of these catalysts in the propane ammoxidation reaction caused the surface V oxidation degree to approach 4+ and diminished the degree of surface enrichment in vanadium. This was due both to a disappearance of the dispersed V entities and a decreased detectability of V oxide aggregates (aggregate growth or solid-state reaction with supporting Sb 2 O 4 ). No evidence is available for surface spreading of Sb species as discussed in the literature.

Organotin(IV) polypyrazolylborates. XII. Hydridotris(4-bromo-1H-pyrazol-1-yl) borates: characterization, Mössbauer study and X-ray crystal structure of MeCl2Sn(4-BrPz)3BH

A series of tin(IV) and organotin(IV) hydridotris(4-bromo-1H-pyrazol-1-yl)borates has been prepared. The compounds of general formula RnSnCl4−n−1[HB(4-Br-pz)3] (where R = Me or Ph, n = 0, 1, 2, or 3) are rigid in solution and contain a six-coordinate tin(IV) site with a tridentate ligand. They were characterised by IR, 1H, 13C, 119Sn NMR, and 119Sn Mossbauer spectroscopy. Their stability towards self-decomposition decreases with increasing number of Sn-bonded aryl or alkyl groups. The compound MeCl2Sn[HB(4-Br-pz)3] crystallises in the space group (no. 148), trigonal, with .