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A significant amount of work has been previously dedicated to the understanding of methylene selectivity parameter. The conventional theory applied for this understanding was mostly based on the assumption that the difference in the Gibbs free energy of transfer from the mobile phase to the stationary phase is a constant for any two compounds in a homologous series that differ by a CH2 group. In the present study, it is shown based on solvophobic theory that this assumption is indeed correct, but it provides a theoretical justification for it. Exemplification of the results of theory was obtained using the values for methylene selectivity (α(CH2)) measured experimentally for seven different C18 chromatographic columns including two core–shell columns and using water and either methanol or acetonitrile as an organic component. Four different homologous series of compounds were used for evaluation. The study proved the theoretical prediction that the values for α(CH2) obtained using different homologous series of compounds are only slightly different from those obtained using the toluene–butylbenzene series. Even using different homologous series, the same type of information regarding the columns comparison, and the changes in log α(CH2) with the solvent composition was obtained. 相似文献
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Determining how electrode structure governs the performance of an electrocatalyst requires techniques capable of probing structure at the atomic scale, often in situ and operando. In recent years, there have been numerous advances in the main experimental techniques for determining the structure of the electrochemical interface. In situ/operando synchrotron surface x-ray diffraction measurements are key to investigate the atomic structure of the electrode surfaces as well as understand the structure-reactivity relations in electrocatalysis. Here we discuss some recent improvements that have taken place in surface x-ray diffraction and how we expect them to lead to an enhanced understanding of electrocatalysis. 相似文献
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A novel tunable and reconfigurable microwave photonic filter based on two cascaded modulators and a dispersive medium is proposed, theoretically discussed and experimentally demonstrated. A single-wavelength LD and a modulator (either a Mach-Zehnder modulator or a phase modulator which can avoid the dc bias drift problem) are used to obtain a multi-wavelength optical source. By adjusting the modulation frequency and the bias voltage on the modulators, the wavelength spacing and the relative amplitude of the optical tones change, thus making the filter tunable and reconfigurable. The experimental results show that the proposed filters have good tunability and reconfigurability and the mainlobe-to-sidelobe ratio of around 25 dB is achieved. 相似文献
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Balmes O Resta A Wermeille D Felici R Messing ME Deppert K Liu Z Grass ME Bluhm H van Rijn R Frenken JW Westerström R Blomberg S Gustafson J Andersen JN Lundgren E 《Physical chemistry chemical physics : PCCP》2012,14(14):4796-4801
The structure and chemical composition of Pd nanoparticles exposed to pure CO and mixtures of CO and O(2) at elevated temperatures have been studied in situ by a combination of X-ray Diffraction and X-ray Photoelectron Spectroscopy in pressures ranging from ultra high vacuum to 10 mbar and from room temperature to a few hundred degrees celsius. Our investigation shows that under CO exposure, above a certain temperature, carbon dissolves into the Pd particles forming a carbide phase. Upon exposure to CO and O(2) mixtures, the carbide phase forms and disappears reversibly, switching at the stoichiometric ratio for CO oxidation. This finding opens new scenarios for the understanding of catalytic oxidation of C-based molecules. 相似文献
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Yi-Lin Yu Edvin Skaljo Hoa Le Minh Zabih Ghassemlooy 《Fiber and Integrated Optics》2013,32(5-6):383-394
AbstractThis article presents a novel bidirectional wavelength reconfigurable optical network utilizing a remotely pumped erbium-doped fiber amplifier and tunable fiber Bragg gratings. The system is experimentally demonstrated at a 10-Gb/s per channel over 20-km fiber span that verifies the metro-network range system performance. The achieved power penalty is less than 1 dB when compared to the back-to-back transmission link. An example of practical application where the proposed module is used as an add/drop multiplexer and a remote node in the bidirectional wavelength division multiplexing passive optical network system is described. 相似文献
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Fernando Garcia-Martinez Dr. Carlos García-Fernández Dr. Juan Pablo Simonovis Adrian Hunt Dr. Andrew Walter Dr. Iradwikanari Waluyo Dr. Florian Bertram Dr. Lindsay R. Merte Dr. Mikhail Shipilin Sebastian Pfaff Dr. Sara Blomberg Dr. Johan Zetterberg Dr. Johan Gustafson Prof. Edvin Lundgren Prof. Daniel Sánchez-Portal Dr. Frederik Schiller Prof. J. Enrique Ortega 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(45):20212-20218
The catalytic oxidation of CO on transition metals, such as Pt, is commonly viewed as a sharp transition from the CO-inhibited surface to the active metal, covered with O. However, we find that minor amounts of O are present in the CO-poisoned layer that explain why, surprisingly, CO desorbs at stepped and flat Pt crystal planes at once, regardless of the reaction conditions. Using near-ambient pressure X-ray photoemission and a curved Pt(111) crystal we probe the chemical composition at surfaces with variable step density during the CO oxidation reaction. Analysis of C and O core levels across the curved crystal reveals that, right before light-off, subsurface O builds up within (111) terraces. This is key to trigger the simultaneous ignition of the catalytic reaction at different Pt surfaces: a CO-Pt-O complex is formed that equals the CO chemisorption energy at terraces and steps, leading to the abrupt desorption of poisoning CO from all crystal facets at the same temperature. 相似文献
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Lisa Rullik Jonas Evertsson Niclas Johansson Florian Bertram Jan-Olov Nilsson Alexei A. Zakharov Anders Mikkelsen Edvin Lundgren 《Surface and interface analysis : SIA》2019,51(12):1214-1224
We report on the influence of oxygen partial pressure for the development of surface oxides covering the industrial aluminum alloy standard 6063 at temperatures ranging from room temperature to 500° C. Using an array of synchrotron-based techniques, we followed the change in oxide thickness, chemical composition, and the lateral distribution of alloying elements. The impact of the oxygen chemical potential is most visible at high temperatures where the oxide composition changes from mostly Al based to mostly Mg based. This is in stark contrast to the ultra-high vacuum (UHV) conditions where only a partial compositional transition is observed. The microscopy data demonstrate that in the UHV case, Mg segregation onto the surface occurs firstly at grain boundaries at 300° C and secondly at sites over the entire surface at 400° C. Further, the initial oxide thickness is 45 Å, as determined by XPS and XRR, decreases in all observed cases after heating to 300° C. At higher temperatures, however, the oxygen partial pressure highly influences the resulting oxide thickness as evident from our X-ray reflectivity data. 相似文献