B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
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Diamond Proposal Number(s):
[27121, 24584]
Open Access
Abstract: Palladium and palladium–platinum foils were analysed using temperature-programmed near-ambient pressure X-ray photoelectron spectroscopy (TP-NAP-XPS) under methane oxidation conditions. Oxidation of palladium is inhibited by the presence of water, and in oxygen-poor environments. Pt addition further inhibits oxidation of palladium across all reaction conditions, preserving metallic palladium to higher temperatures. Bimetallic foils underwent significant restructuring under reaction conditions, with platinum preferentially migrating to the bulk under select conditions.
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Apr 2022
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B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
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Alexander
Large
,
Jake
Seymour
,
Wilson
Quevedo Garzon
,
Kanak
Roy
,
Federica
Venturini
,
David C.
Grinter
,
Luca
Artiglia
,
Emily
Brooke
,
Martha
Briceno De Gutierrez
,
Agnes
Raj
,
Kevin
Lovelock
,
Roger A.
Bennett
,
Tugce
Eralp-Erden
,
Georg
Held
Diamond Proposal Number(s):
[19464, 20952, 22702, 24584]
Open Access
Abstract: Near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) was used to study the chemical states of a range of alumina-supported monometallic Pd and bimetallic Pd-Pt nanocatalysts, under methane oxidation conditions. It has been suggested before that for optimal complete methane oxidation, palladium needs to be in an oxidised state. These experiments, combining NAP-XPS with a broad range of characterisation techniques, demonstrate a clear link between Pt presence, Pd oxidation, and catalyst activity under stoichiometric reaction conditions. Under oxygen-rich conditions this behaviour is less clear, as all of the palladium tends to be oxidised, but there are still benefits to the addition of Pt in place of Pd for complete oxidation of methane.
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Jan 2021
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B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
B18-Core EXAFS
I20-EDE-Energy Dispersive EXAFS (EDE)
I20-Scanning-X-ray spectroscopy (XAS/XES)
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Rachel H.
Blackmore
,
Maria Elena
Rivas
,
George F.
Tierney
,
Khaled M. H.
Mohammed
,
Donato
Decarolis
,
Shusaku
Hayama
,
Federica
Venturini
,
Georg
Held
,
Rosa
Arrigo
,
Monica
Amboage
,
Pip
Hellier
,
Evan
Lynch
,
Mahrez
Amri
,
Marianna
Casavola
,
Tugce
Eralp Erden
,
Paul
Collier
,
Peter P.
Wells
Diamond Proposal Number(s):
[20129, 20200, 22063, 15151]
Open Access
Abstract: The use of mechanochemistry to prepare catalytic materials is of significant interest; it offers an environmentally beneficial, solvent-free, route and produces highly complex structures of mixed amorphous and crystalline phases. This study reports on the effect of milling atmosphere, either air or argon, on mechanochemically prepared LaMnO3 and the catalytic performance towards N2O decomposition (deN2O). In this work, high energy resolution fluorescence detection (HERFD), X-ray absorption near edge structure (XANES), X-ray emission, and X-ray photoelectron spectroscopy (XPS) have been used to probe the electronic structural properties of the mechanochemically prepared materials. Moreover, in situ studies using near ambient pressure (NAP)-XPS, to follow the materials during catalysis, and high pressure energy dispersive EXAFS studies, to mimic the preparation conditions, have also been performed. The studies show that there are clear differences between the air and argon milled samples, with the most pronounced changes observed using NAP-XPS. The XPS results find increased levels of active adsorbed oxygen species, linked to the presence of surface oxide vacancies, for the sample prepared in argon. Furthermore, the argon milled LaMnO3 shows improved catalytic activity towards deN2O at lower temperatures compared to the air milled and sol–gel synthesised LaMnO3. Assessing this improved catalytic behaviour during deN2O of argon milled LaMnO3 by in situ NAP-XPS suggests increased interaction of N2O at room temperature within the O 1s region. This study further demonstrates the complexity of mechanochemically prepared materials and through careful choice of characterisation methods how their properties can be understood.
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Jun 2020
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B18-Core EXAFS
E01-JEM ARM 200CF
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Diamond Proposal Number(s):
[15151]
Open Access
Abstract: Mechanochemistry offers a solventless, ‘waste free’ route to preparing metal oxide catalysts, however, there is limited information on the chemical steps involved. In this work, the perovskite LaMnO3 has been successfully synthesized via mechanochemistry from metal oxide powders, La2O3 and Mn2O3, at room temperature, using a planetary ball mill. Separate ex situ ‘time slices’ were taken during the milling procedure to provide insights into the underlying chemistry. The crystalline material was assessed using XRD, which identified 100% perovskite phase after 3 h of milling. Conversely, characterization by X-ray absorption spectroscopy (XAS) at both the Mn K-edge and La L3-edge provides a very different picture. The XAS data shows that there are significant structural alterations as early as 30 min of milling, with the La precursor dispersed over Mn2O3. Increasing milling time then allows for mechanical activation of both precursors and the formation of powdered LaMnO3, with no calcination step required. The XAS highlights that there is a significant amount of amorphous, oxygen deficient, content even when XRD has identified 100% perovskite phase. The samples were tested for the decomposition of the environmental pollutant N2O; at a milling time of 3 h, the LaMnO3 catalyst displays a much early onset production of N2 compared to a traditional sol–gel synthesized LaMnO3, resulting from increased oxygen deficiency at the surface, confirmed by XPS and STEM-EELS. This is an encouraging sign that mechanochemical routes can be harnessed to provide a sustainable route to preparing mixed metal oxide catalysts with enhanced catalytic performance.
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Dec 2019
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B18-Core EXAFS
I20-EDE-Energy Dispersive EXAFS (EDE)
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Ellie K.
Dann
,
Emma K.
Gibson
,
C. Richard A.
Catlow
,
Veronica
Celorrio
,
Paul
Collier
,
Tugce
Eralp
,
Monica
Amboage
,
Christopher
Hardacre
,
Cristina
Stere
,
Anna
Kroner
,
Agnes
Raj
,
Scott
Rogers
,
Alexandre
Goguet
,
Peter P.
Wells
Diamond Proposal Number(s):
[17340, 17725]
Open Access
Abstract: Spatially resolved, combined energy dispersive EXAFS (EDE) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements have been performed over a fixed catalyst bed of Pd/γ-Al2O3 during kinetic oscillations of CO oxidation. The kinetic oscillations of CO oxidation over Pd (or for that matter Pt or Rh) catalysts are a complicated phenomenon that require characterisation techniques with high time resolution and spatial resolution in order to make links between catalyst structure and surface reactivity. By measuring the extent of Pd oxidation at the nanoparticle surface, from Pd K-edge EDE, and matching this with the CO coverage, from DRIFTS spectra, at multiple positions of the fixed bed reactor it is found that the majority of the catalyst undergoes a sharp transition from the CO poisoned catalyst to the highly active, oxidised Pd surface. This transition occurs initially at the end of the catalyst bed, nearest the outlet, and propagates upstream with increasing temperature of the reactor. The oscillations in Pd surface oxide formation and CO coverage are observed only in the first ∼1 mm of the bed, which gives rise to oscillations in CO2 and O2 concentrations observed by end-pipe mass spectrometry after the light-off temperature. The catalyst initially exists as less active, CO poisoned metallic Pd nanoparticles before light-off which transition to a highly active state after light-off when the Pd nanoparticle surface becomes dominated by chemisorbed oxygen. Kinetic oscillations only occur at the front of the catalyst bed where there is sufficient concentration of CO in the gas phase to compete with O2 for adsorption sites at the catalyst surface. We demonstrate the complex nature of the evolving catalyst structure and surface reactivity during catalytic operation and the need for spatially resolved operando methods for understanding and optimising catalyst technologies.
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May 2019
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B18-Core EXAFS
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Ellie K.
Dann
,
Emma K.
Gibson
,
Richard A.
Catlow
,
Paul
Collier
,
Tugce
Eralp Erden
,
Diego
Gianolio
,
Christopher
Hardacre
,
Anna
Kroner
,
Agnes
Raj
,
Alexandre
Goguet
,
Peter
Wells
Diamond Proposal Number(s):
[10306]
Abstract: The rational design of catalysts is of great industrial significance, yet there is a fundamental lack of knowledge in some of the most well-established processes e.g. formation of supported nanoparticle structures through impregnation. Here, the choice of precursor has a significant influence on the resulting catalytic properties of the end material, yet the chemistry that governs the transformation from defined molecular systems to dispersed nanoparticles is often over-looked. A spectroscopic method for advanced in situ characterization is employed to capture the formation of PdO nanoparticles supported on γ-Al2O3 from two alternative molecular precursors; Pd(NO3)2 and Pd(NH3)4(OH)2. Time resolved DRIFTS is able to identify the temperature assisted pathway for ligand decomposition, showing that NH3 lig-ands are oxidised to N2O and NO- species, whereas, NO3- ligands assist in joining Pd centres via bidentate bridging co-ordination. Combining with simultaneous XAFS, the resulting nucleation and growth mechanism of the precious metal oxide nanoparticles are resolved. The bridging ability of palladium nitrate aids formation and growth of larger PdO nanoparticles at lower onset temperature (<250°C). Conversely, impregnation from [Pd(NH3)4]2+ results in well isolated Pd centres, anchored to the support, which require higher temperature (>360°C) for migration to form observ-able Pd-Pd distances of PdO nanoparticles. These smaller nanoparticles have improved dispersion and an increased number of step and edge sites compared to those formed from the conventional Pd(NO3)2 salt, favouring a lower light off temperature for complete methane oxidation.
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Jul 2017
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Open Access
Abstract: Near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) is used to study the chemical state of methane oxidation catalysts in-situ. Al2O3–supported Pd catalysts are prepared with different particle sizes ranging from 4 to 10 nm. These catalysts were exposed to conditions similar to those used in the partial oxidation of methane (POM) to syn-gas and simultaneously monitored by NAP-XPS and mass spectrometry. NAP-XPS data show changes in the oxidation state of the palladium as the temperature increases, from metallic Pd0 to PdO, and back to Pd0. Mass spectrometry shows an increase in CO production whilst the Pd is in the oxide phase, and the metal is reduced back under presence of newly formed H2. A particle size effect is observed, such that CH4 conversion starts at lower temperatures with larger sized particles from 6 to 10 nm. We find that all nanoparticles begin CH4 conversion at lower temperatures than polycrystalline Pd foil.
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Mar 2016
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I06-Nanoscience (XPEEM)
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Diamond Proposal Number(s):
[75]
Abstract: In this paper, we report the surprising formation of square-based facetted islands with linear dimension of the order of 500 nm upon dewetting of a Cr multilayer on W(100). We show that these square islands are composed of inclined facets surrounding a depressed center such that the facet slopes inward with the outer edges of the islands thicker than the centers. The islands’ shapes do not represent traditional equilibrium crystal shapes as expected for a Wulf construction. In situ UV and x-ray photoelectron emission microscopy allied to spatially resolved spectroscopy throws considerable light on the nature of the dewetting and shows that the metal surface between the islands remains covered by a thin pseudomorphic wetting layer of ∼1 ML. Low-energy electron diffraction and scanning tunneling and atomic force microscopies allow quantification of facet slopes, and we identify a predominance of tilted Cr(100) facets ±5° off of the substrate normal bound by (210) planes at ∼26°. The epitaxial Cr islands adopt the bulk Cr lattice constant but are tilted with respect to the surface normal. We suggest that the Cr crystallite tilting creates a vicinal-like interface structure that determines the island morphology.
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Jul 2012
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Abstract: The alignment of model amyloid peptide YYKLVFFC is investigated in bulk and at a solid surface using a range of spectroscopic methods employing polarized radiation. The peptide is based on a core sequence of the amyloid beta (A beta) peptide, KLVFF. The attached tyrosine and cysteine units are exploited to yield information on alignment and possible formation of disulfide or dityrosine links. Polarized Raman spectroscopy on aligned stalks provides information on tyrosine orientation, which complements data from linear dichroism (LD) on aqueous solutions subjected to shear in a Couette cell. LD provides a detailed picture of alignment of peptide strands and aromatic residues and was also used to probe the kinetics of self-assembly. This suggests initial association of phenylalanine residues, followed by subsequent registry of strands and orientation of tyrosine residues. X-ray diffraction (XRD) data from aligned stalks is used to extract orientational order parameters from the 0.48 nm reflection in the cross-beta pattern, from which an orientational distribution function is obtained. X-ray diffraction on solutions subject to capillary flow confirmed orientation in situ at the level of the cross-beta pattern. The information on fibril and tyrosine orientation from polarized Raman spectroscopy is compared with results from NEXAFS experiments on samples prepared as films on silicon. This indicates fibrils are aligned parallel to the surface, with phenyl ring normals perpendicular to the surface. Possible disulfide bridging leading to peptide dimer formation was excluded by Raman spectroscopy, whereas dityrosine formation was probed by fluorescence experiments and was found not to occur except under alkaline conditions. Congo red binding was found not to influence the cross-beta XRD pattern.
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May 2010
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I06-Nanoscience (XPEEM)
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Abstract: Low-energy and photoemission electron microscopy enables the determination of facet planes of polycrystalline surfaces and the study of their chemical composition at the sub-μm scale. Using these techniques the early oxidation stages of nickel were studied. After exposing the surface to 20 L of oxygen at 373 K a uniform layer of chemisorbed oxygen was found on all facets. After oxygen exposure at 473-673 K, small NiO crystallites are formed on all facets but not in the vicinity of all grain boundaries. The crystallites are separated by areas of bare Ni without significant oxygen coverage.
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Feb 2010
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