Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 20 de 39
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
Rev Sci Instrum ; 91(10): 105109, 2020 Oct 01.
Artigo em Inglês | MEDLINE | ID: mdl-33138597

RESUMO

We present an approach to determine the absolute thickness profile of flat liquid jets, which takes advantage of the information of thin film interference combined with light absorption, both captured in a single microscopic image. The feasibility of the proposed method is demonstrated on our compact experimental setup used to generate micrometer thin, free-flowing liquid jet sheets upon collision of two identical laminar cylindrical jets. Stable operation was achieved over several hours of the flat jet in vacuum (10-4 mbar), making the system ideally suitable for soft x-ray photon spectroscopy of liquid solutions. We characterize the flat jet size and thickness generated with two solvents, water and ethanol, employing different flow rates and nozzles of variable sizes. Our results show that a gradient of thickness ranging from a minimal thickness of 2 µm to over 10 µm can be found within the jet surface area. This enables the tunability of the sample thickness in situ, allowing the optimization of the transmitted photon flux for the chosen photon energy and sample. We demonstrate the feasibility of x-ray absorption spectroscopy experiments in transmission mode by measuring at the oxygen K-edge of ethanol. Our characterization method and the description of the experimental setup and its reported performance are expected to expand the range of applications and facilitate the use of flat liquid jets for spectroscopy experiments.

2.
Phys Chem Chem Phys ; 2020 Oct 26.
Artigo em Inglês | MEDLINE | ID: mdl-33104144

RESUMO

Calcium looping (CaL) is a CO2 capture technique based on the reversible carbonation/calcination of CaO that is considered promising to reduce anthropogenic CO2 emissions. However, the rapid decay of the CO2 uptake of CaO over repeated cycles of carbonation and calcination due to sintering limits its implementation at the industrial scale. Thus, the development of material design strategies to stabilize the CO2 uptake capacity of CaO is paramount. The addition of alkali metal salts to CaO has been proposed as a strategy to mitigate the rapid loss of its cyclic CO2 uptake capacity. However, there are conflicting results concerning the effect of the addition of alkali metal carbonates on the structure and CO2 capacity of CaO. In this work, we aim at understanding the effect of the addition of Na2CO3 to CaO on the sorbent's structure and its CO2 uptake capacity. We demonstrate that under industrially-relevant conditions the addition of as little as 1 wt% of Na2CO3 reduces severely the CO2 uptake of CaO. Combining TGA, XAS and FIB-SEM analysis allowed us to attribute the performance degradation to the formation of the double salt Na2Ca(CO3)2 that induces strong sintering leading to a significant loss in the sorbent's pore volume. In addition, during the carbonation step the formation of a dense layer of Na2Ca(CO3)2 that covers unreacted CaO prevents its full carbonation to CaCO3.

3.
Phys Chem Chem Phys ; 22(34): 18911-18918, 2020 Sep 14.
Artigo em Inglês | MEDLINE | ID: mdl-32469018

RESUMO

We have made high surface area catalysts for the selective oxidation of methanol to formaldehyde. This is done in two ways - (i) by doping haematite with Al ions, to increase the surface area of the material, but which itself is unselective and (ii) by surface coating with Mo which induces high selectivity. Temperature programmed desorption (TPD) of methanol shows little difference in surface chemistry of the doped haematite from the undoped material, with the main products being CO2 and CO, but shifted to somewhat higher desorption temperature. However, when Mo is dosed onto the haematite surface, the chemistry changes completely to show mainly the selective product, formaldehyde, with no CO2 production, and this is little changed up to 10% Al loading. But at 15 wt% Al, the chemistry changes to indicate the presence of a strongly acidic function at the surface, with additional dimethyl ether and CO/CO2 production characteristic of the presence of alumina. Structurally, X-ray diffraction (XRD) shows little change over the range 0-20% Al doping, except for some small lattice contraction, while the surface area increases from around 20 to 100 m2 g-1. Using X-ray absorption spectroscopy (XAS) it is clear that, at 5% loading, the Al is incorporated into the Fe2O3 corundum lattice, which has the same structure as α-alumina. By 10% loading then it appears that the alumina starts to nano-crystallise within the haematite lattice into the γ form. At higher loadings, there is evidence of phase separation into separate Al-doped haematite and γ-alumina. If we add 1 monolayer equivalent of Mo to the surface there is already high selectivity to formaldehyde, but little change in structure, because that monolayer is isolated at the surface. However, when three monolayers equivalent of Mo is added, we then see aluminium molybdate type signatures in the XANES spectra at 5% Al loading and above. These appear to be in a sub-surface layer with Fe molybdate, which we interpret as due to Al substitution into ferric molybdate layers immediately beneath the topmost surface layer of molybdena. It seems like the separate γ-alumina phase is not covered by molybdena and is responsible for the appearance of the acid function products in the TPD.

4.
Sci Total Environ ; 715: 136895, 2020 May 01.
Artigo em Inglês | MEDLINE | ID: mdl-32007883

RESUMO

Phosphorus (P) resource availability is declining and the efficiency of applied nutrients in agricultural soils is becoming increasingly important. This is especially true for P fertilizers from recycled materials, which often have low plant availability. Specific co-fertilization with ammonium can enhance P plant availability in soils amended with these P fertilizers, and thus the yield of plants. To investigate this effect, we performed a pot experiment with maize in slightly acidic soil (pH 6.9) with one water-soluble (triple superphosphate [TSP]) and two water-insoluble (sewage sludge-based and hyperphosphate [Hyp]) P fertilizers and an ammonium sulfate nitrate with or without a nitrification inhibitor (NI). The dry matter yield of maize was significantly increased by the NI with the Hyp (from 14.7 to 21.5 g/pot) and TSP (from 40.0 to 45.4 g/pot) treatments. Furthermore, P uptake was slightly increased in all three P treatments with the NI, but not significantly. Olsen-P extraction and P K-edge micro-X-ray absorption near-edge structure (XANES) spectroscopy showed that apatite-P of the water-insoluble P fertilizers mobilized during the plant growth period. In addition, novel nitrogen (N) K-edge micro-XANES spectroscopy and the Mogilevkina method showed that the application of an NI increased the fixation of ammonium in detectable hot spots in the soil. Thus, the delay in the nitrification process by the NI and the possible slow-release of temporarily fixed ammonium in the soil resulted in a high amount of plant available ammonium in the soil solution. This development probably decreases the rhizosphere pH due to release of H+ by plants during ammonium uptake, which mobilizes phosphorus in the amended soil and increases the dry matter yield of maize. This is especially important for water-insoluble apatite-based P fertilizers (conventional and recycled), which tend to have poor plant availability.


Assuntos
Solo , Fertilizantes , Nitrificação , Nitrogênio , Fósforo , Zea mays
5.
Phys Chem Chem Phys ; 22(19): 10641-10652, 2020 May 21.
Artigo em Inglês | MEDLINE | ID: mdl-31894785

RESUMO

The ability to reproduce the experimental structure of water around the sodium and potassium ions is a key test of the quality of interaction potentials due to the central importance of these ions in a wide range of important phenomena. Here, we simulate the Na+ and K+ ions in bulk water using three density functional theory functionals: (1) the generalized gradient approximation (GGA) based dispersion corrected revised Perdew, Burke, and Ernzerhof functional (revPBE-D3) (2) the recently developed strongly constrained and appropriately normed (SCAN) functional (3) the random phase approximation (RPA) functional for potassium. We compare with experimental X-ray diffraction (XRD) and X-ray absorption fine structure (EXAFS) measurements to demonstrate that SCAN accurately reproduces key structural details of the hydration structure around the sodium and potassium cations, whereas revPBE-D3 fails to do so. However, we show that SCAN provides a worse description of pure water in comparison with revPBE-D3. RPA also shows an improvement for K+, but slow convergence prevents rigorous comparison. Finally, we analyse cluster energetics to show SCAN and RPA have smaller fluctuations of the mean error of ion-water cluster binding energies compared with revPBE-D3.

6.
New Phytol ; 225(4): 1476-1490, 2020 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-31591727

RESUMO

Rhizosphere soil has distinct physical and chemical properties from bulk soil. However, besides root-induced physical changes, chemical changes have not been extensively measured in situ on the pore scale. In this study, we couple structural information, previously obtained using synchrotron X-ray computed tomography (XCT), with synchrotron X-ray fluorescence microscopy (XRF) and X-ray absorption near-edge structure (XANES) to unravel chemical changes induced by plant roots. Our results suggest that iron (Fe) and sulfur (S) increase notably in the direct vicinity of the root via solubilization and microbial activity. XANES further shows that Fe is slightly reduced, S is increasingly transformed into sulfate (SO4 2- ) and phosphorus (P) is increasingly adsorbed to humic substances in this enrichment zone. In addition, the ferrihydrite fraction decreases drastically, suggesting the preferential dissolution and the formation of more stable Fe oxides. Additionally, the increased transformation of organic S to sulfate indicates that the microbial activity in this zone is increased. These changes in soil chemistry correspond to the soil compaction zone as previously measured via XCT. The fact that these changes are colocated near the root and the compaction zone suggests that decreased permeability as a result of soil structural changes acts as a barrier creating a zone with increased rhizosphere chemical interactions via surface-mediated processes, microbial activity and acidification.

7.
Chemistry ; 25(69): 15879-15886, 2019 Dec 10.
Artigo em Inglês | MEDLINE | ID: mdl-31553090

RESUMO

Hollow ZSM-5 zeolites of size below one micrometer can be produced by desilication of crystals with aluminium zoning. The parent crystals have a core-shell structure: the core part has nearly no aluminium, whereas the aluminium content in the shell increases when extending to exterior surface. Transmission electron microscopy confirmed the preservation of the crystalline shell after base leaching, but could not identify its subtle change. An increase of the Si/Al ratio of the surface was detected upon leaching the parent material to form the hollow zeolite by using ambient pressure X-ray photoelectron spectroscopy and infrared spectroscopy of substituted alkylpyridines. 27 Al MAS NMR showed that base leaching results in a reduced percentage of distorted tetrahedrally coordinated aluminium. The reprecipitation of dissolved species occurs and tetrahedrally coordinated tin atoms can thus be introduced to the shell framework. Overall, the formation of hollow ZSM-5 zeolites by desilication involves not only the removal of silicon-rich core, but also a reduced percentage of exterior aluminium-related acid sites, which should be considered while using hollow zeolites in acid-catalyzed reactions.

8.
Nanoscale ; 11(39): 18142-18149, 2019 Oct 10.
Artigo em Inglês | MEDLINE | ID: mdl-31555787

RESUMO

Understanding the complex chemistry of functional nanomaterials is of fundamental importance. Controlled synthesis and characterization at the atomic level is essential to gain deeper insight into the unique chemical reactivity exhibited by many nanomaterials. Cerium oxide nanoparticles have many industrial and commercial applications, resulting from very strong catalytic, pro- and anti-oxidant activity. However, the identity of the active species and the chemical mechanisms imparted by nanoceria remain elusive, impeding the further development of new applications. Here, we explore the behavior of cerium oxide nanoparticles of different sizes at different temperatures and trace the electronic structure changes by state-of-the-art soft and hard X-ray experiments combined with computational methods. We confirm the absence of the Ce(iii) oxidation state at the surface of CeO2 nanoparticles, even for particles as small as 2 nm. Synchrotron X-ray absorption experiments at Ce L3 and M5 edges, combined with X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and small angle X-ray scattering (SAXS) and theoretical calculations demonstrate that in addition to the nanoceria charge stability, the formation of hydroxyl groups at the surface profoundly affects the chemical performance of these nanomaterials.

9.
Chem Commun (Camb) ; 55(72): 10725-10728, 2019 Sep 16.
Artigo em Inglês | MEDLINE | ID: mdl-31429426

RESUMO

A facile aerosol-based method for the synthesis of pure and stable amorphous calcium carbonate (ACC) is presented. The method relies on the instantaneous carbonation of calcium hydroxide aerosols with carbon dioxide followed by rapid drying of the freshly formed ACC. The ACC display extended stability against humidity induced crystallization.

10.
J Synchrotron Radiat ; 26(Pt 4): 1226-1237, 2019 Jul 01.
Artigo em Inglês | MEDLINE | ID: mdl-31274448

RESUMO

Recent advances in segmented low-gain avalanche detectors (LGADs) make them promising for the position-sensitive detection of low-energy X-ray photons thanks to their internal gain. LGAD microstrip sensors fabricated by Fondazione Bruno Kessler have been investigated using X-rays with both charge-integrating and single-photon-counting readout chips developed at the Paul Scherrer Institut. In this work it is shown that the charge multiplication occurring in the sensor allows the detection of X-rays with improved signal-to-noise ratio in comparison with standard silicon sensors. The application in the tender X-ray energy range is demonstrated by the detection of the sulfur Kα and Kß lines (2.3 and 2.46 keV) in an energy-dispersive fluorescence spectrometer at the Swiss Light Source. Although further improvements in the segmentation and in the quantum efficiency at low energy are still necessary, this work paves the way for the development of single-photon-counting detectors in the soft X-ray energy range.

11.
J Am Chem Soc ; 140(43): 14289-14299, 2018 10 31.
Artigo em Inglês | MEDLINE | ID: mdl-30265527

RESUMO

Calcium carbonate (CaCO3) is one of the most abundant biominerals that is prevalent in rocks and often used as a structural material in marine animals. Many of these natural CaCO3-based materials display excellent mechanical properties that are difficult to reproduce by man-made counterparts. This difficulty arises from the incomplete understanding of the influence of processing conditions on the structure and composition of CaCO3. To gain a better understanding of the evolution of the structure and composition of amorphous CaCO3 (ACC) particles during early stages, we introduce a new, organic solvent-free method that quenches this process with a high temporal resolution. We produce ACC particles inside small airborne drops that are formed with a microfluidic spray-dryer. These drops dry within 100 ms to 10 s and thereby arrest the formation of CaCO3 particles on that time scale. Using the microfluidic spray-dryer, we demonstrate that the amount of mobile water contained in ACC particles increases with increasing formation time and hence with increasing particle size. As a result of the higher concentration of mobile water, larger particles are less stable against temperature-induced solid-state crystallization and electron beam-induced decomposition than smaller counterparts. The amount of mobile water contained in ACC can be substantially reduced, and hence their kinetic stability against solid-state transformations increased, if certain organic additives, such as poly(acrylic acid) (PAA), are incorporated. These insights might open up new opportunities to fabricate biomimetic CaCO3-based materials with tunable structures and hence with properties that can be adapted to the needs of specific applications.

12.
Phys Chem Chem Phys ; 20(37): 24408-24417, 2018 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-30221299

RESUMO

Interactions between trace gases and ice are important in environmental chemistry and for Earth's climate. In particular, the adsorption of trace gases to ice surfaces at temperatures approaching the melting point has raised interest in the past, because of the prevailing pre-melting. Here, we present Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy data at ambient partial pressure of water to better define the onset temperature of pre-melting at the interfacial region of ice. Further, this study directly compares the interaction between an organic acid common in the atmosphere, formic acid, and that of an aliphatic carbon with ice at 253 K. It makes use of X-ray Photoelectron Spectroscopy (XPS) with its inherent narrow probing depth covering both the surface and near-surface bulk region when detecting electrons. We use the tender X-ray range for excitation to locate the organic species within the interfacial region with an extended probing depth compared to published XPS work. Electron kinetic energy dependent C1s photoemission data indicate that, at low coverage of a few 1014 molecules cm-2, the presence of formic acid is restricted to the upper ice layers of the interfacial region. Increasing the dosage, formic acid penetrates 6-7 nm into the air-ice interface. The presence of the more hydrophobic aliphatic carbon is restricted to the upper ice monolayers. This direct comparison of an organic acid with an aliphatic compound confirms the emerging picture where solutes enter the interfacial region of ice at a depth related to their specific tendency to form solvation shells.

13.
ACS Appl Mater Interfaces ; 10(36): 30495-30505, 2018 Sep 12.
Artigo em Inglês | MEDLINE | ID: mdl-30110151

RESUMO

Al2O3 on Si is known to form an ultrathin interfacial SiO2 during deposition and subsequent annealing, which creates a negative fixed charge ( Qfix) that enables field-effect passivation and low surface recombination velocities in Si solar cells. Various concepts were suggested to explain the origin of this negative Qfix. In this study, we investigate Al-O monolayers (MLs) from atomic layer deposition (ALD) sandwiched between deliberately grown/deposited SiO2 films. We show that the Al atoms have an ultralow diffusion coefficient (∼4 × 10-18 cm2/s at 1000 °C), are deposited at a constant rate of ∼5 × 1014 Al atoms/(cm2 cycle) from the first ALD cycle, and are tetrahedral O-coordinated because the adjacent SiO2 imprints its tetrahedral near-order and bond length into the Al-O MLs. By variation in the tunnel-SiO2 thickness and the number of Al-O MLs, we demonstrate that the tetrahedral coordination alone is not sufficient for the formation of Qfix but that a SiO2/Al2O3 interface within a tunneling distance from the substrate must be present. The Al-induced acceptor states at these interfaces have energy levels slightly below the Si valence band edge and require charging by electrons from either the Si substrate or from Si/SiO2 dangling bonds to create a negative Qfix. Hence, tunneling imposes limitations for the SiO2 and Al2O3 layer thicknesses. In addition, Coulomb repulsion between the charged acceptor states results in an optimum number of Al-O MLs, i.e., separation of both interfaces. We achieve maximum negative Qfix of ∼5 × 1012 cm-2 (comparable to thick ALD-Al2O3 on Si) with ∼1.7 nm tunnel-SiO2 and just seven ALD-Al2O3 cycles (∼8 Å) after optimized annealing at 850 °C for 30 s. The findings are discussed in the context of a passivating, hole-selective tunnel contact for high-efficiency Si solar cells.

14.
Chem Commun (Camb) ; 54(39): 4939-4942, 2018 May 10.
Artigo em Inglês | MEDLINE | ID: mdl-29700510

RESUMO

When cycling diphosphonate-based organic-inorganic hybrid materials as negative battery electrodes, specific charges exceeding the maximum for a metal redox reaction are recorded. Classical explanations are electrolyte reduction and solid electrolyte interphase (SEI) oxidation. Using operando X-ray absorption spectroscopy (XAS) at the P K-edge experiment, we demonstrate an additional contribution of reversible ligand co-cycling based on the P-atoms of the diphosphonate ligands, upon delithiation occurring during the first potential plateau, which matches perfectly to previous investigations using Fe K-edge XAS. We thus investigate experimentally an anionic redox process in situ. Thereby we could demonstrate a possible methodology to measure P K-edge XANES on Li-ion battery active materials under operando conditions, which could be applied to verify the predicted P co-cycling of other conversion-type negative electrode materials.

15.
Sci Adv ; 4(1): eaao6283, 2018 01.
Artigo em Inglês | MEDLINE | ID: mdl-29387793

RESUMO

Mechanisms of CaCO3 nucleation from solutions that depend on multistage pathways and the existence of species far more complex than simple ions or ion pairs have recently been proposed. Herein, we provide a tightly coupled theoretical and experimental study on the pathways that precede the initial stages of CaCO3 nucleation. Starting from molecular simulations, we succeed in correctly predicting bulk thermodynamic quantities and experimental data, including equilibrium constants, titration curves, and detailed x-ray absorption spectra taken from the supersaturated CaCO3 solutions. The picture that emerges is in complete agreement with classical views of cluster populations in which ions and ion pairs dominate, with the concomitant free energy landscapes following classical nucleation theory.

16.
J Synchrotron Radiat ; 25(Pt 1): 16-19, 2018 Jan 01.
Artigo em Inglês | MEDLINE | ID: mdl-29271745

RESUMO

One of the remaining challenges for accurate photon diagnostics at X-ray free-electron lasers (FELs) is the shot-to-shot, non-destructive, high-resolution characterization of the FEL pulse spectrum at photon energies between 2 keV and 4 keV, the so-called tender X-ray range. Here, a spectrometer setup is reported, based on the von Hamos geometry and using elastic scattering as a fingerprint of the FEL-generated spectrum. It is capable of pulse-to-pulse measurement of the spectrum with an energy resolution (ΔE/E) of 10-4, within a bandwidth of 2%. The Tender X-ray Single-Shot Spectrometer (TXS) will grant to experimental scientists the freedom to measure the spectrum in a single-shot measurement, keeping the transmitted beam undisturbed. It will enable single-shot reconstructions for easier and faster data analysis.

17.
Chimia (Aarau) ; 71(11): 768-772, 2017 11 29.
Artigo em Inglês | MEDLINE | ID: mdl-29179801

RESUMO

We review our recent results on the implementation of picosecond (ps) X-ray absorption spectroscopy to probe the electronic and geometric structure of centres formed by photoexcitation of solar materials such as TiO2 polymorphs and inorganic Cs-based perovskites. The results show electron localization at Ti defects in TiO2 anatase and rutile and small hole polaron formation in the valence band of CsPbBr3, all within 80 ps. This method is promising for the study of the ultrafast time scales of such processes, especially with the advent of the Swiss X-ray Free Electron Laser (SwissFEL).

18.
Scanning ; 2017: 6346212, 2017.
Artigo em Inglês | MEDLINE | ID: mdl-29109824

RESUMO

Many handmade ancient and recent oriental wool carpets show outstanding brilliance and persistence of colour that is not achieved by common industrial dyeing procedures. Anthropologists have suggested the influence of wool fermentation prior to dyeing as key technique to achieve the high dyeing quality. By means of µ-XRF elemental mapping of mordant metals we corroborate this view and show a deep and homogenous penetration of colourants into fermented wool fibres. Furthermore we are able to apply this technique and prove that the fermentation process for ancient specimens cannot be investigated by standard methods due to the lack of intact cuticle layers. This finding suggests a broad range of further investigations that will contribute to a deeper understanding of the development of traditional dyeing techniques. Spectroscopic studies add information on the oxidation states of the metal ions within the respective mordant-dye-complexes and suggest a partial charge transfer as basis for a significant colour change when Fe mordants are used.

19.
Angew Chem Int Ed Engl ; 56(45): 14031-14035, 2017 11 06.
Artigo em Inglês | MEDLINE | ID: mdl-28981203

RESUMO

Fluid catalytic cracking is a chemical conversion process of industrial scale. This process, utilizing porous catalysts composed of clay and zeolite, converts heavy crude-oil fractions into transportation fuel and petrochemical feedstocks. Among other factors iron-rich reactor and feedstream impurities cause these catalyst particles to permanently deactivate. Herein, we report tomographic X-ray absorption spectroscopy measurements that reveal the presence of dissimilar iron impurities of specific localization within a single deactivated particle. Whereas the iron natural to clay in the composite seems to be unaffected by operation, exterior-facing and feedstream-introduced iron was found in two forms. Those being minute quantities of ferrous oxide, located near regions of increased porosity, and impurities rich in Fe3+ , preferentially located in the outer dense part of the particle and suggested to contribute to the formation of an isolating amorphous silica alumina envelope.

20.
J Phys Chem Lett ; 8(19): 4757-4762, 2017 Oct 05.
Artigo em Inglês | MEDLINE | ID: mdl-28902513

RESUMO

The interfacial ionization of strong acids is an essential factor of multiphase and heterogeneous chemistry in environmental science, cryospheric science, catalysis research and material science. Using near ambient pressure core level X-ray photoelectron spectroscopy, we directly detected a low surface coverage of adsorbed HCl at 253 K in both molecular and dissociated states. Depth profiles derived from XPS data indicate the results as physisorbed molecular HCl at the outermost ice surface and dissociation occurring upon solvation deeper in the interfacial region. Complementary X-ray absorption measurements confirm that the presence of Cl- ions induces significant changes to the hydrogen bonding network in the interfacial region. This study gives clear evidence for nonuniformity across the air-ice interface and questions the use of acid-base concepts in interfacial processes.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA