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Instruments / Technical Area	Publication Details	Published
I20-EDE-Energy Dispersive EXAFS (EDE)	Dynamics of palladium single-atoms on graphitic carbon nitride during ethylene hydrogenation Maurice Vennewald , Nina Michelle Sackers , Andree Iemhoff , Isabella Kappel , Claudia Weidenthaler , Ansgar Meise , Marc Heggen , Rafal E. Dunin-Borkowski , Luke Keenan , Regina Palkovits Journal Of Catalysis 118 DOI: 10.1016/j.jcat.2023.03.011 Diamond Proposal Number(s): [29667] Show Abstract ▼ Abstract: Single-atoms on carbon-nitrogen supports are considered catalysts for a multitude of reactions. However, doubts remain whether really these species or subnanometer clusters formed under reaction conditions are the active species. In this work, we investigate the dynamics of palladium single-atoms on graphitic carbon nitride during ethylene hydrogenation and H2-D2 exchange. By employing aberration-corrected scanning transmission electron microscopy, x-ray photoelectron spectroscopy and x-ray absorption spectroscopy, we will show that palladium, originally present as single-atoms, agglomerates to clusters at 100 °C in a gas atmosphere that contains both ethylene and hydrogen. This agglomeration goes in hand with the emergence of catalytic activity in both ethylene hydrogenation and H2-D2 exchange, suggesting that clusters, rather than single-atoms, are the active species. The results presented herein highlight the potential of analytics over the course of reaction to identify the active species and provide new insights into the influence of gas atmosphere on metal speciation.	Mar 2023
I20-Scanning-X-ray spectroscopy (XAS/XES)	Unraveling the molecular details of the “gate opening” phenomenon in ZIF-8 with X-ray absorption spectroscopy Roberto Boada , Jesus Chaboy , Shusaku Hayama , Luke Keenan , Adam A. Freeman , Monica Amboage , Sofia Diaz-Moreno The Journal Of Physical Chemistry C DOI: 10.1021/acs.jpcc.2c00373 Diamond Proposal Number(s): [9970] Open Access Show Abstract ▼ 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.	Mar 2022
I20-EDE-Energy Dispersive EXAFS (EDE)	Important role of open copper sites on adsorption of various molecules in Cu-BTC metal organic framework Iuliia Mikulska , Roberto Boada , Shusaku Hayama , Matteo Aramini , Luke Keenan , Monica Amboage , Sofia Diaz-Moreno XAFS2021 Show Abstract ▼ Abstract: Cu-BTC metal-organic framework (MOF) has been found to be a promising candidate for removing hazardous substances from air via adsorption1. To understand the role that the copper sites in Cu-BTC play in the adsorption process, we have performed a detailed X-ray absorption spectroscopy (XAS) study. Conventional XAS and high energy resolution fluorescence detection XAS (HERFD-XAS) were collected on degassed Cu-BTC, and after being exposed to CO2, water and benzene. The EXAFS analysis reveals that, although the local environment around the copper centers in all samples is similar, differences can be found in the first and second coordination shells. We have found that the Cu-O distance in the first coordination shell is slightly larger when the sample is immersed in water and benzene than in the degassed sample, while it does not change for the sample exposed to CO2. Small differences are also observed in the Cu-Cu distance, gradually increasing from 2.49 Å in the degassed sample, to 2.52 Å, 2.58 Å and 2.63 Å upon adsorption of CO2, benzene and water, respectively. HERFD-XAS has been used to obtain information about the electronic and geometric structure around the copper metal centers, as the use of high energy resolution enhances the features in the XANES spectrum enabling the detection of subtle changes2. We have observed differences in the intensity and the energy position of some of the XANES spectral features upon adsorption of different adsorbates that can be attributed to changes in the local environment around the copper centers, as detected in the EXAFS analysis. In this study we show that XAS is a powerful and very sensitive tool for studying host-guest interactions in MOFs, providing atomic-level insights into adsorption mechanisms.	Jul 2021

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