•In situ XAFS investigation of a Pd catalyst for the direct synthesis of H2O2.•Reduction of PdO during the catalytic run in the presence of bromide ions.•Appreciable leaching of Pd during the ...catalytic run in the presence of bromide ions.•Clear evidence of restructuring of the metal clusters under reaction conditions.
The direct synthesis of hydrogen peroxide is catalyzed by palladium catalysts supported over different solids. With the aid of a suitable plug-flow reaction cell, we carried out a preliminary X-ray absorption fine structure spectroscopy (EXAFS) characterization of a palladium catalyst supported on the commercial resin K2621 under reaction conditions (in situ). Whereas the catalyst, which in the dry catalyst presents metal Pd and PdO when fresh, is practically unaffected by the reaction medium (methanol) or by the reaction mixture (CO2/H2/O2, 86/10/4, v/v) it undergoes an apparent reduction of part of PdO to metal Pd and some metal leaching during the reaction in the presence of bromide ions. These findings suggest the role of bromide ions as enhancers of the selectivity of palladium catalysts in the direct synthesis of hydrogen peroxide could not be limited to the selective blocking of the sites responsible for the undesired formation of water, but could also entail phase modifications of the active metal.
Abstract
Ultrafast optical reflectivity measurements of silicon, germanium, and gallium arsenide have been carried out using an advanced set-up providing intense subpicosecond pulses (35 fs FWHM,
...$$\lambda $$
λ
= 400 nm) as a pump and broadband 340–780 nm ultrafast pulses as a white supercontinuum probe. Measurements have been performed for selected pump fluence conditions below the damage thresholds, that were carefully characterized. The obtained fluence damage thresholds are 30, 20.8, 9.6 mJ/
$$\hbox {cm}^2$$
cm
2
for Si, Ge and GaAs respectively. Ultrafast reflectivity patterns show clear differences in the Si, Ge, and GaAs trends both for the wavelength and time dependences. Important changes were observed near the wavelength regions corresponding to the
$$E_1$$
E
1
,
$$E_1+\Delta $$
E
1
+
Δ
singularities in the joint density of states, so related to the peculiar band structure of the three systems. For Ge, ultrafast reflectivity spectra were also collected at low temperature (down to 80 K) showing a shift of the characteristic doublet peak around 2.23 eV and a reduction of the recovery times.
LISA is the acronym of Linea Italiana per la Spettroscopia di Assorbimento di raggi X (Italian beamline for X-ray Absorption Spectroscopy) and is the upgrade of the former GILDA beamline installed on ...the BM08 bending magnet port of European Synchrotron Radiation Facility (ESRF). Within this contribution a full description of the project is provided.
Carbon-coated ZnFe2O4 (ZFO-C) spinel ferrite nanoparticles can be used in electrodes for Li-ion batteries and are known to show capacities larger than those calculated for an ideal spinel structure. ...In this work, the local structure evolution and reordering of this material upon lithium insertion are studied using K-edge and L-edge x-ray absorption near edge spectroscopy (XANES). XANES simulations corresponding to different lithiation stages are performed using full multiple scattering (Fe, Zn K-edge, Zn L-edge) and ligand field multiplet (LFM) calculations (Fe L-edge). XANES simulations are compared with experimental spectra obtained on ZFO-C nanoparticles previously characterized by electrochemical measurements. It is shown that a satisfactory agreement for the XANES Fe and Zn K-edges of pristine ZFO-C bulk nanoparticles can be obtained introducing a mixed spinel structure with Fe and Zn partially occupying tetrahedral and octahedral sites. Upon lithiation, changes in the XANES spectra are interpreted introducing displacements of the cations as an effect of occupation of Li into empty lattice sites. In particular, comparison of the simulations with the XANES data indicates that reversible Li insertion is accompanied by a migration of the Zn and Fe atoms from tetrahedral to octahedral sites. Furthermore, by studying L-edge XANES spectra, we show that the relocation and valence change of metal ions occur at earlier lithiation stages at the surface of the active material, gradually extending to the bulk for larger Li uptakes.
•Comparison of experimental XAS of Zn and Fe edges with simulations shows presence of mixed spinel structure.•The XANES spectra upon lithiation is interpreted introducing displacements of the cations upon Li insertion.•Migration of Zn ions upon lithiation is mainly due to the repulsive interaction while for Fe a reduction from Fe3+ to Fe2+ is observed.•Spinel inversion plays an important role in the increased lithium uptake capacity.•The relocations of metal ions occurs at earlier stages at the surface and gradually extend to the bulk.
A combined in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) experiment was carried out to monitor hydrogen desorption from NbH0.9 nanoclusters embedded into MgH2. Just after ...the MgH2 → Mg transition, a NbH x bcc nanophase is detected, whose lattice parameter measured by XRD is significantly longer than the one inferred from XAS. This difference is explained considering the broad niobium hydride cluster size distribution and in particular the fact that the XRD signal, differently from the XAS one, is dominated by larger NbH x crystalline structures. The results indicate that, during the hydride to metal phase transformation of the matrix, the NbH x cluster composition depends on the cluster size. It is shown for the first time for embedded nanoparticles that faster (and complete) Nb dehydrogenation is favored for small (1.5–4 nm) clusters with respect to larger (∼20 nm) ones. The role of the matrix and of the annealing atmosphere in the stability of the Nb-related nanophases is discussed.
Spinel metal oxide nanoparticles are promising alternative anode materials for Li-ion batteries showing enhanced cycling performances and specific theoretical capacity resulting from a combination of ...alloying and conversion reactions during dis-/charge. In this work, we study the effect of the initial lithium insertion into carbon-coated ZnFe2O4 anodes by X-ray absorption spectroscopy. We are able to closely monitor the structural changes and the electronic structure in-situ and in-operando. For low lithium uptake (up to 0.3 Li+ per formula unit) the initial crystalline structure is not significantly modified. Further lithium insertion causes the migration of Zn atoms from tetrahedral 8a sites into vacant octahedral 16c sites, and Fe is gradually reduced from Fe3+ to Fe2+ upon lithium insertion. In the early stage of lithiation (up to 2 Li+ per formula unit) we find no evidence of Zn reduction. Also, a gradual damping of EXAFS signals, linked to the increasing structural disorder, is observed.
•Carbon-coated zinc ferrite nanoparticles is proposed as advanced anode material.•Initial spinel structure is modified upon the first lithium insertion.•At the initial lithiation iron is reduced and Li + hosted in the vacant sites.•Repulsion of Li + displaces zinc atoms from tetrahedral to octahedral sites.
The evolution of the solid electrolyte interphase (SEI) during the first Li uptake in advanced Li-ion electrodes is studied by X-ray absorption spectroscopy (XAS). The As atoms present in the ...electrolyte solution were used as a local probe for monitoring the SEI growth on different electrodes. High-quality As K-edge spectra were collected in fluorescence mode for a set of graphite and carbon-coated ZnFe2O4 electrodes. XAS measurements have been preceded and corroborated by electrochemical characterizations. SEI phase evolution was analyzed by distinct As valence states in the subsequent stages of SEI formation, while X-ray fluorescence (XRF) was used to estimate the As content. Detailed structural results are presented for different Li contents in different electrodes including the estimated thickness of the SEI layer, contribution of different As oxidation states, and As local structure. The formation of AsF N complexes with different local coordination N is clearly observed and measured at various SEI evolution stages. Evidence of a partially reversed redox process, taking place within the SEI by charge–discharge cycling, was also obtained.
An amorphous Zn biomineralization (“white mud”), occurring at Naracauli stream, Sardinia, in association with cyanobacteria Leptolyngbya frigida and diatoms, was investigated by electron microscopy ...and X-ray absorption spectroscopy. Preliminary diffraction analysis shows that the precipitate sampled on Naracauli stream bed is mainly amorphous, with some peaks ascribable to quartz and phyllosilicates, plus few minor unattributed peaks. Scanning electron microscopy analysis shows that the white mud, precipitated in association with a seasonal biofilm, is made of sheaths rich in Zn, Si, and O, plus filaments likely made of organic matter. Transmission electron microscopy analysis shows that the sheaths are made of smaller units having a size in the range between 100 and 200 nm. X-ray absorption near-edge structure and extended X-ray absorption fine structure data collected at the Zn K-edge indicate that the biomineral has a local structure similar to hemimorphite, a zinc sorosilicate. The differences of this biomineral with respect to the hydrozincite biomineralization documented about 3 km upstream in the same Naracauli stream may be related to either variations in the physicochemical parameters and/or different metabolic behavior of the involved biota.
A Thallium (Tl) L3-edge x-ray absorption experiment was performed on a NaI(Tl) scintillation crystal at room temperature using fluorescence detection. The data analysis provides clear evidence for a ...≃5 % average bond length expansion of the first shell I atoms surrounding the Tl impurities with respect to the Na-I crystallographic bond-length. Possible evidence for a slight expansion of the second and third shells is obtained. The data are compatible with a previous experiment performed at liquid nitrogen temperature considering thermal expansion. A critical review of existing theoretical predictions and density functional theory supercell calculations support present findings and suggest the opportunity to perform a high quality temperature dependent experiment.
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