I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[18591]
Open Access
Abstract: The adsorption of carboxylic acid molecules at the calcite (104) and the muscovite (001) surface was investigated using surface X-ray diffraction. All four investigated carboxylic acid molecules, hexanoic acid, octanoic acid, lauric acid, and stearic acid, were found to adsorb at the calcite surface. Whereas the shortest two carboxylic acid molecules, hexanoic acid and octanoic acid, showed limited ordering and a flexible, disordered chain, the two longest carboxylic acid molecules form fully ordered monolayers, i.e., these form highly structured self-assembled monolayers. The latter molecules are oriented almost fully upright, with a tilt of up to 10°. The oxygen atoms of the organic molecules are found at similar positions to those of water molecules at the calcite–water interface. This suggests that in both cases, the oxygen atoms compensate for the broken bonds at the calcite surface. Under the same experimental conditions, stearic acid does not adsorb to K+ and Ca2+-functionalized muscovite mica because the neutral molecules do not engage in the ionic bonds typical for the mica interface. These differences in adsorption behavior are characteristic for the differences of the oil–solid interactions in carbonate and sandstone reservoirs.
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May 2022
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E01-JEM ARM 200CF
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Diamond Proposal Number(s):
[20403]
Open Access
Abstract: The composition of the reducing gas in the activation of Co Fischer-Tropsch synthesis catalysts determines the nature of the catalytically active Co species. This study reports on the effect of H2 versus syngas (H2/CO = 2) on the reducibility of Co3O4 nanoparticles supported on hollow carbon spheres, using ex situ and in situ high-resolution aberration-corrected analytical electron microscopy. High-resolution images revealed twinned fcc Co particles encapsulated in carbon from syngas treatment while H2-treated particles were mostly CoO. Moreover, the electron energy loss of the Co-L3,2 and O-K edge fine structures show improved reducibility in syngas than in H2 at 350 °C. The effect of high temperature on the reducibility of the Co3O4 nanoparticles is also explored. Carbon fiber encapsulation of twinned fcc Co particles observed during the syngas treatment provides sinter resistance at high temperatures. Both ex situ and in situ results indicate that syngas activation is efficient for obtaining highly reduced Co nanoparticles at lower temperatures.
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Apr 2022
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I07-Surface & interface diffraction
I09-Surface and Interface Structural Analysis
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Philip J.
Mousley
,
Luke A.
Rochford
,
Paul T. P.
Ryan
,
Philip
Blowey
,
James
Lawrence
,
David A.
Duncan
,
Hadeel
Hussain
,
Billal
Sohail
,
Tien-Lin
Lee
,
Gavin R.
Bell
,
Giovanni
Costantini
,
Reinhard J.
Maurer
,
Christopher
Nicklin
,
D. Phil
Woodruff
Diamond Proposal Number(s):
[14884, 4884]
Open Access
Abstract: While the phenomenon of metal substrate adatom incorporation into molecular overlayers is generally believed to occur in several systems, the experimental evidence for this relies on the interpretation of scanning tunneling microscopy (STM) images, which can be ambiguous and provides no quantitative structural information. We show that surface X-ray diffraction (SXRD) uniquely provides unambiguous identification of these metal adatoms. We present the results of a detailed structural study of the Au(111)-F4TCNQ system, combining surface characterization by STM, low-energy electron diffraction, and soft X-ray photoelectron spectroscopy with quantitative experimental structural information from normal incidence X-ray standing wave (NIXSW) and SXRD, together with dispersion-corrected density functional theory (DFT) calculations. Excellent agreement is found between the NIXSW data and the DFT calculations regarding the height and conformation of the adsorbed molecule, which has a twisted geometry rather than the previously supposed inverted bowl shape. SXRD measurements provide unequivocal evidence for the presence and location of Au adatoms, while the DFT calculations show this reconstruction to be strongly energetically favored.
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Apr 2022
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[9970]
Open Access
Abstract: The local structural changes associated with the ZIF-8 framework flexibility upon nitrogen gas adsorption have been studied by in situ X-ray absorption spectroscopy (XAS) and high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) spectroscopy. Different thermodynamic conditions (isobar and isotherm) have been used to explore the so-called “gate opening” transition in which the hexagonal pore windows of the sodalite cage open, increasing the accessible volume for gas adsorption. To elucidate the source of the spectral changes in the XANES region of the absorption spectra observed along the gas adsorption and through the transition from the closed to the open pore configuration, ab initio calculations have been performed. Our results demonstrate that the transition from the closed to the open pore configurations involves not only the rotation of the MeIM ligand but also a further bend of the Me group away from the plane defined by the IM ring. Additionally, the contribution of the N2 molecules adsorbed in the center of the 4-ring window has been included in the scattering model to fully reproduce the main features of the X-ray absorption spectra in the open pore configuration.
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Mar 2022
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[17261, 20785]
Abstract: A quantitative structural investigation is reported, aimed at resolving the issue of whether substrate adatoms are incorporated into the monolayers formed by strong molecular electron acceptors deposited onto metallic electrodes. A combination of normal-incidence X-ray standing waves, low-energy electron diffraction, scanning tunnelling microscopy, and X-ray photoelectron spectroscopy measurements demonstrate that the systems TCNQ and F4TCNQ on Ag(100) lie at the boundary between these two possibilities and thus represent ideal model systems with which to study this effect. A room-temperature commensurate phase of adsorbed TCNQ is found not to involve Ag adatoms, but to adopt an inverted bowl configuration, long predicted but not previously identified experimentally. By contrast, a similar phase of adsorbed F4TCNQ does lead to Ag adatom incorporation in the overlayer, the cyano end groups of the molecule being twisted relative to the planar quinoid ring. Density functional theory (DFT) calculations show that this behavior is consistent with the adsorption energetics. Annealing of the commensurate TCNQ overlayer phase leads to an incommensurate phase that does appear to incorporate Ag adatoms. Our results indicate that the inclusion (or exclusion) of metal atoms into the organic monolayers is the result of both thermodynamic and kinetic factors.
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Mar 2022
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Abstract: Niobium oxides have a prominent role in many technological applications, including electrochemical energy storage where they are employed as anodes for lithium- and sodium-ion batteries, hybrid supercapacitors, electrocatalysts in vanadium redox flow batteries, and electrochemical support in fuel cells. Owing to the great complexity of the available coordination environment, they can host other transition metals without phase changes. In this paper, multiple techniques are employed, for the first time, to study the influence of iron cations on the structural and functional properties of monoclinic and orthorhombic FeNb11O29. The element-selectivity of X-ray absorption spectroscopy and Mössbauer spectroscopy demonstrated the disorder of Fe3+ cations over the six octahedral sites of FeNb11O29. Ab-initio Density Functional Theory calculations were used in tandem with spectroscopic techniques to confirm the cationic disorder of the material and link its implications to the conductivity investigated with electrochemical impedance spectroscopy. The paramagnetism given by Fe3+ was confirmed with muon spin relaxation spectroscopy. The differences between FeNb11O29 and other analogous niobium-based compounds were rationalized and discussed, in view of their electrochemical applications.
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Feb 2022
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B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
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Diamond Proposal Number(s):
[26588]
Abstract: Porous boron nitride (BN) has proven promising as a novel class of inorganic materials in the field of separations and particularly adsorption. Owing to its high surface area and thermal stability, porous BN has been researched for CO2 capture and water cleaning, for instance. However, research remains at the laboratory scale due to a lack of understanding of the formation mechanism of porous BN, which is largely a “black box” and prevents scale up. Partial reaction pathways have been unveiled, but they omit critical steps in the formation, including the porosity development, which is key to adsorption. To unlock the potential of porous BN at a larger scale, we have investigated its formation from the perspective of both chemical formation and porosity development. We have characterized reaction intermediates obtained at different temperatures with a range of analytical and spectroscopic tools. Using these analyses, we propose a mechanism highlighting the key stages of BN formation, including intermediates and gaseous species formed in the process. We identified the crucial formation of nonporous carbon nitride to form porous BN with release of porogens, such as CO2. This work paves the way for the use of porous BN at an industrial level for gas and liquid separations.
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Dec 2021
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[22322]
Abstract: Small pore zeolites that show framework flexibility, such as merlinoite (topology type MER), possess a high potential for the selective adsorption of small gas molecules including CO2. The CO2 adsorption properties of Na-, K-, and Cs-exchanged forms of a merlinoite zeolite with Si/Al = 4.2 have been measured at 298 K, and in situ PXRD was used to follow their structural response to dehydration and CO2 uptake. The Na- and Cs-forms convert from a wide-pore to a narrow-pore form upon dehydration, while the K-form remains in the wide-pore form. The Na- and Cs-forms exhibit stepped CO2 adsorption isotherms, consistent with breathing behavior and expansion from narrow- to wide-pore phases, while K6.2-MER remains in the wide-pore structure throughout. Synchrotron PXRD of the K- and Cs-forms reveals the effects of CO2 adsorption on the cation site distributions and the framework configuration. All cation forms of MER (4.2) show enhanced adsorption kinetics for Ar compared to those with lower Si/Al, and the wide-pore structure of K6.2-MER (4.2) shows particularly rapid sorption for both Ar and CO2. Breakthrough curves over K6.2-MER (4.2) demonstrate good separation of CO2 from CH4 in flowing CO2/CH4 mixtures, even in pelletized form with an alumina binder.
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Dec 2021
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I05-ARPES
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Diamond Proposal Number(s):
[4906, 5008]
Abstract: The electronic spectral function of BaNi2As2 is investigated using both angle-resolved photoemission spectroscopy (ARPES) and a combined computational scheme of local density approximation and dynamical mean-field theory (LDA + DMFT). In contrast to the well-studied isostructural iron arsenide high-temperature superconductors, BaNi2As2 demonstrates weak correlation effects, although Ni 3d electrons have an even larger on-site interaction than Fe 3d electrons. The LDA + DMFT effective mass enhancement for bands crossing the Fermi level is found to be only about 1.2, which agrees well with ARPES data. This reduction of the correlation manifestation with respect to iron pnictides comes from the increase of 3d-orbital filling when going from Fe to Ni. The electron correlations cause a remarkable reconstruction of the bare BaNi2As2 LDA band structure below −0.8 eV due to a self-energy effect. A simplified toy model to understand the weakness of correlation effects in BaNi2As2 and to describe the LDA + DMFT self-energy shape is discussed. For a more realistic comparison of LDA + DMFT spectral function maps with ARPES data, we take into account several experimental features: the photoemission cross section, the experimental energy and angular resolutions, and the photo-hole lifetime effects. Thus, the LDA + DMFT calculations presented here with experimental features provide a nearly qualitative agreement with ARPES data and assure the observation of an apparent dramatic decrease of the correlation strength compared to the Fe compounds.
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Dec 2021
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B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
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Jordan
Cole
,
Zoe
Henderson
,
Andrew G.
Thomas
,
Claudia L.
Compeán-González
,
Adam
Greer
,
Christopher
Hardacre
,
Federica
Venturini
,
Wilson
Quevedo Garzon
,
Pilar
Ferrer
,
David C.
Grinter
,
Georg
Held
,
Karen L.
Syres
Diamond Proposal Number(s):
[20532]
Abstract: In situ photoemission and near-edge X-ray absorption fine structure (NEXAFS) techniques have been used to study the interaction of CO2 with an ionic liquid thin film. A thin film of the superbasic ionic liquid (SBIL) trihexyltetradecylphosphonium benzimidazolide ([P66614][benzim]) was prepared on a rutile TiO2 (110) surface and exposed to CO2 at near-ambient pressures. NEXAFS measurements combined with density functional theory calculations indicate a realignment of [benzim]− anions from 27° from the surface normal to 54° upon exposure to CO2. Angle-resolved X-ray photoelectron spectroscopy (AR-XPS) shows evidence of irreversible CO2 absorption in thin films of [P66614][benzim] and a greater concentration of CO2-reacted anions in the deeper layers. These results give a new perspective on CO2 uptake in ionic liquids and fundamental interactions at the liquid–gas interface. Understanding this interfacial behavior is important for developing ILs for gas capture applications and may influence the performance of other IL-based technologies.
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Oct 2021
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