Iskanje po repozitoriju

A+ | A- | | SLO | ENG

Opis: In a bifunctional Ni/ZSM-5 zeolite type catalyst, catalytic properties are usually tuned via varying Al and Ni contents [1]. Here we present a systematic structural study of the Ni/ZSM-5 materials by Ni K-edge XANES and EXAFS analyses, to monitor the changes of local structure and chemical state of Ni species in the catalysts as a function of Al and Ni content. A series of Ni/ZSM-5 type zeolites with different Al to Si and Ni to Si molar ratios were synthesized by a “green”, template free technique [2]. With a combination of XAS, XRD and TEM we resolved the changes in the local environment of Ni species induced by the different Al contents in the Ni/ZSM-5 catalysts. Ni species in Ni/ZSM-5 exist as NiO nanocrystals and as charge compensating Ni2+ cations. The Ni K-edge XANES and EXAFS results enabled the quantification of Ni-containing species. At a low Al to Si ratio (nAl/nSi < 0.04), the NiO nanoparticles predominate in the samples and account for over 65% of Ni phases. However, NiO is outnumbered by Ni2+ cations attached to the zeolite framework in ZSM-5 with a high Al to Si ratio (nAl/nSi = 0.05) due to a higher number of framework negative charges imparted by Al. The obtained results show that the number of highly reducible and active NiO nanocrystals is strongly correlated with the framework Al sites present in Ni/ZSM-5 zeolites.
Ključne besede: Ni EXAFS, XANES Ni/ZSM-5 catalyst
Objavljeno v RUNG: 05.07.2024; Ogledov: 666; Prenosov: 3
Povezava na datoteko
Gradivo ima več datotek! Več...

Opis: Catalyst design is crucial for improving catalytic activity and product selectivity. In a bifunctional Ni/ZSM-5 zeolite type catalyst, catalytic properties are usually tuned via varying Al and Ni contents [1]. While changes in acid properties associated with Al sites are usually closely investigated, Ni phases, however, receive inadequate attention. Herein, we present a systematic structural study of Ni in the Ni/ZSM-5 materials by Ni K-edge XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) analyses, to monitor the local structure and chemical state of Ni species in the catalysts. In combination with XRD and TEM we resolved the changes in the local environment of Ni species induced by the different Al contents of the parent ZSM-5 prepared by a “green”, template free technique [2]. Ni species in Ni/ZSM-5 exist as NiO crystals (3–50 nm) and as charge compensating Ni2+ cations. The Ni Kedge XANES and EXAFS results enabled the quantification of Ni-containing species. At a low Al to Si ratio (nAl/nSi < 0.04), the NiO nanoparticles predominate in the samples and account for over 65% of Ni phases. However, NiO is outnumbered by Ni2+ cations attached to the zeolite framework in ZSM-5 with a high Al to Si ratio (nAl/nSi = 0.05) due to a higher number of framework negative charges imparted by Al. The obtained results show that the number of highly reducible and active NiO crystals is strongly correlated with the framework Al sites present in ZSM-5 zeolites, which depend greatly on the synthesis conditions. Therefore, this kind of study is beneficial for any further investigation of the catalytic activities of Ni/ZSM-5 and other metal-modified bifunctional catalysts.
Ključne besede: Ni/ZSM-5 catalysts, Ni EXAFS, XANES
Objavljeno v RUNG: 19.09.2023; Ogledov: 1543; Prenosov: 5
Povezava na datoteko
Gradivo ima več datotek! Več...

Ključne besede: Magnetic catalyst, Photocatalyst, Water treatment, Sunlight, Contaminants of emerging concern, Photo-Fenton-like systems, Cu, Mn, Fe, XANES, EXAFS
Objavljeno v RUNG: 06.07.2023; Ogledov: 2232; Prenosov: 9
Celotno besedilo (4,32 MB)

Opis: Selenium (Se) supplied in inorganic form (as selenate or selenite) was shown to decrease mercury (Hg) toxicity by forming HgSe in soils as well as in animal and human tissues, while for plants there is no evidence of Hg-Se complexation. Although Se in not an essential element for plants it was shown to counteract various abiotic stresses when applied at trace amounts. The aim of this work was therefore to study physiological responses and Hg speciation in plant/ fungi-animal food chain. Lettuce (Lactuca sativa) and porcini mushrooms (Boletus edulis) were taken as model plant/ fungal species and Spanish slug (Arion vulgaris) as a model animal species. The plants, fed to the slugs, were grown in HgCl2 contaminated soil or soil from the vicinity of Hg mine in Idrija with traces of HgS and methyl Hg). Physiological parameters of plants and slugs were monitored during the experiment. At the end the biological material was frozen in LN2 and freeze dried. Hg L3-edge (12284 eV) XANES and EXAFS spectra of the biological samples and standards were measured at liquid helium temperature in fluorescence detection mode at the BM30B beamline of the ESRF synchrotron in Grenoble, using the 30-segment germanium solid state detector [1]. The results showed that addition of Se alleviated Hg toxic effects in the food chain started at HgCl2-contaminated soil, while for the soil from Idrija, containing low amounts of highly toxic methyl-Hg, the beneficial effect was less prominent [2]. No Hg-Se complexes were detected in plants, while in mushrooms and slugs the complexation was confirmed. Addition of Se to the plants, however, changed Hg ligand environment in plant tissues from sulphur to nitrogen ligands. Hg and Se both target the -SH functional groups in the plant tissues, so toxic effects of Hg are rather enhanced than alleviated by addition of Se. Nevertheless, the addition of Se to the plants is beneficial for higher trophic levels and lowers Hg toxicity for the primary consumers, the slugs.
Ključne besede: mercury, toxicity, ligand environment, XANES, EXAFS, food chain, plant, slug, fungi
Objavljeno v RUNG: 05.12.2022; Ogledov: 2040; Prenosov: 0
Gradivo ima več datotek! Več...

Opis: The electrolysis efficiency is an important aspect of the Power-to-Solid energy storage technology (EST) based on the iron chloride electrochemical cycle [1]. This cycle employs an aqueous FeCl2 catholyte solution for the electro-reduction of iron. The metal iron deposits on the cathode. The energy is stored as a difference in the redox potential of iron species. Hydrogen, as an energy carrier, is released on demand over a fully controlled hydrogen evolution reaction between metallic Fe0 and HCl (aq) [1]. Due to these characteristics, the cycle is suitable for long-term high-capacity and high-power energy storage. In a previous work [2] we revealed that the electrolyte conductivity linearly increases with temperature. Contrary, the correlation between the electrolyte concentration and efficiency is not so straightforward. Unexpectedly small efficiency variations were found between 1 and 2.5 mol dm-3 FeCl2 (aq) followed by an abrupt efficiency drop at higher concentrations. To explain the behavior of the observed trends and elucidate the role of FeCl2 (aq) complex ionic species we performed in situ X-ray absorption studies. We made a dedicated experimental setup, consisting of a tubular oven and PMMA liquid absorption cell, and performed the measurements at the DESY synchrotron P65 beamline. The XAS investigation covered XANES and EXAFS analyses of FeCl2 (aq) at different concentrations (1 - 4 molL-1) and temperatures (25 - 80 °C). We found that at low temperature and low FeCl2 concentration the octahedral first coordination sphere around Fe is occupied by one Cl ion at a distance of 2.33 (±0.02) Å and five water molecules at a distance of 2.095 (±0.005) Å [3]. The structure of the ionic complex gradually changes with an increase in temperature and/or concentration. The apical water molecule is substituted by a chlorine ion to yield a neutral Fe[Cl2(H2O)4]0. The transition from the single charged Fe[Cl(H2O)5]+ to the neutral Fe[Cl2(H2O)4]0 causes a significant drop in the solution conductivity, which well correlates with the existing conductivity models [3]. [1] M. Valant, “Procedure for electric energy storage in solid matter. United States Patent and Trademark Office. Patent No. US20200308715,” Patent No. US20200308715, 2021. [2] U. Luin and M. Valant, “Electrolysis energy efficiency of highly concentrated FeCl2 solutions for power-to-solid energy storage technology,” J. Solid State Electrochem., vol. 26, no. 4, pp. 929–938, Apr. 2022, doi: 10.1007/S10008-022-05132-Y. [3] U. Luin, I. Arčon, and M. Valant, “Structure and Population of Complex Ionic Species in FeCl2 Aqueous Solution by X-ray Absorption Spectroscopy,” Molecules, vol. 27, no. 3, 2022, doi: 10.3390/molecules27030642.
Ključne besede: Iron chloride electrochemical cycle, Power-to-Solid energy storage, XANES, EXAFS, electrical conductivity, electrolyte complex ionic species structure and population
Objavljeno v RUNG: 26.09.2022; Ogledov: 2490; Prenosov: 0  (1 glas)
Gradivo ima več datotek! Več...