B18-Core EXAFS
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Diego
Gianolio
,
Michael D.
Higham
,
Matthew G.
Quesne
,
Matteo
Aramini
,
Ruoyu
Xu
,
Alex I.
Large
,
Georg
Held
,
Juan-Jesús
Velasco-Vélez
,
Michael
Haevecker
,
Axel
Knop-Gericke
,
Chiara
Genovese
,
Claudio
Ampelli
,
Manfred Erwin
Schuster
,
Siglinda
Perathoner
,
Gabriele
Centi
,
C. Richard A.
Catlow
,
Rosa
Arrigo
Diamond Proposal Number(s):
[24919]
Open Access
Abstract: Operando soft and hard X-ray spectroscopic techniques were used in combination with plane-wave density functional theory (DFT) simulations to rationalize the enhanced activities of Zn-containing Cu nanostructured electrocatalysts in the electrocatalytic CO2 hydrogenation reaction. We show that at a potential for CO2 hydrogenation, Zn is alloyed with Cu in the bulk of the nanoparticles with no metallic Zn segregated; at the interface, low reducible Cu(I)–O species are consumed. Additional spectroscopic features are observed, which are identified as various surface Cu(I) ligated species; these respond to the potential, revealing characteristic interfacial dynamics. Similar behavior was observed for the Fe–Cu system in its active state, confirming the general validity of this mechanism; however, the performance of this system deteriorates after successive applied cathodic potentials, as the hydrogen evolution reaction then becomes the main reaction pathway. In contrast to an active system, Cu(I)–O is now consumed at cathodic potentials and not reversibly reformed when the voltage is allowed to equilibrate at the open-circuit voltage; rather, only the oxidation to Cu(II) is observed. We show that the Cu–Zn system represents the optimal active ensembles with stabilized Cu(I)–O; DFT simulations rationalize this observation by indicating that Cu–Zn–O neighboring atoms are able to activate CO2, whereas Cu–Cu sites provide the supply of H atoms for the hydrogenation reaction. Our results demonstrate an electronic effect exerted by the heterometal, which depends on its intimate distribution within the Cu phase and confirms the general validity of these mechanistic insights for future electrocatalyst design strategies.
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Apr 2023
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B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
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Jack E. N.
Swallow
,
Elizabeth S.
Jones
,
Ashley R.
Head
,
Joshua S.
Gibson
,
Roey
Ben David
,
Michael W.
Fraser
,
Matthijs A.
Van Spronsen
,
Shaojun
Xu
,
Georg
Held
,
Baran
Eren
,
Robert S
Weatherup
Diamond Proposal Number(s):
[25834]
Open Access
Abstract: The reactions of H2, CO2, and CO gas mixtures on the surface of Cu at 200 °C, relevant for industrial methanol synthesis, are investigated using a combination of ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and atmospheric-pressure near edge X-ray absorption fine structure (AtmP-NEXAFS) spectroscopy bridging pressures from 0.1 mbar to 1 bar. We find that the order of gas dosing can critically affect the catalyst chemical state, with the Cu catalyst maintained in a metallic state when H2 is introduced prior to the addition of CO2. Only on increasing the CO2 partial pressure is CuO formation observed that coexists with metallic Cu. When only CO2 is present, the surface oxidizes to Cu2O and CuO, and the subsequent addition of H2 partially reduces the surface to Cu2O without recovering metallic Cu, consistent with a high kinetic barrier to H2 dissociation on Cu2O. The addition of CO to the gas mixture is found to play a key role in removing adsorbed oxygen that otherwise passivates the Cu surface, making metallic Cu surface sites available for CO2 activation and subsequent conversion to CH3OH. These findings are corroborated by mass spectrometry measurements, which show increased H2O formation when H2 is dosed before rather than after CO2. The importance of maintaining metallic Cu sites during the methanol synthesis reaction is thereby highlighted, with the inclusion of CO in the gas feed helping to achieve this even in the absence of ZnO as the catalyst support.
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Mar 2023
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B07-B-Versatile Soft X-ray beamline: High Throughput
E02-JEM ARM 300CF
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Longxiang
Liu
,
Liqun
Kang
,
Arunabhiram
Chutia
,
Jianrui
Feng
,
Martyna
Michalska
,
Pilar
Ferrer
,
David
Grinter
,
Georg
Held
,
Yeshu
Tan
,
Fangjia
Zhao
,
Fei
Guo
,
David
Hopkinson
,
Christopher
Allen
,
Yanbei
Hou
,
Junwen
Gu
,
Ioannis
Papakonstantinou
,
Paul
Shearing
,
Dan
Brett
,
Ivan P.
Parkin
,
Guanjie
He
Diamond Proposal Number(s):
[29340, 32501, 30614, 29809, 32058]
Open Access
Abstract: The electrochemical synthesis of hydrogen peroxide (H2O2) via a two-electron (2e-) oxygen reduction reaction (ORR) process provides a promising alternative to replace the energy-intensive anthraquinone process. However, the development of efficient electrocatalysts is still facing lots of challenges like insufficient understanding of active sites. Herein, we develop a facile template-protected strategy to synthesize a highly active quinone-rich porous carbon catalyst (PCC) for H2O2 electrochemical production. The optimized PCC900 exhibits unprecedented activity and selectivity, of which the onset potential reaches 0.83 V vs. reversible hydrogen electrode in 0.1 M KOH and the H2O2 selectivity is over 95 % in a wide potential range. Comprehensive synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy combined with electrocatalytic characterizations reveals the positive correlation between quinone content and 2e- ORR performance. The effectiveness of chair-form quinone groups as the most efficient active sites is highlighted by the molecule-mimic strategy and theoretical analysis.
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Mar 2023
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B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
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Diamond Proposal Number(s):
[22687]
Abstract: Designing CO2 methanation catalysts that meet industrial requirements is still challenging. We report Ni-Fe hydrotalcite-derived catalysts with a wide range of Ni and Mg loadings showing that an optimised composition with Ni0.4 gives a very high CO2 conversion rate of 0.37 mmol/gcat/s at 300°C. This catalyst is studied by in-situ APXPS and NEXAFS spectroscopies and compared with the other synthesised samples to obtain new mechanistic insights on methanation catalysts active for low-temperature (300°C) methanation, which is an industrial requirement. Under methanation conditions, in-situ investigations revealed the presence of metallic Ni sites and low nuclearity Ni-Fe species at
(Ni loading) = 21.2 mol%. These sites are oxidised on the low Ni-loaded catalyst (
= 9.2 mol%). The best CO2 conversion rate and CH4 selectivity are shown at intermediate
(21.2 mol%), in the presence of Mg. These superior performances are related to the high metallic surface area, dispersion, and optimal density of basic sites. The
(turnover frequency of CO2 conversion) increases exponentially with the fractional density of basic to metallic sites (
) from 1.1 s-1 (
= 29.2 mol%) to 9.1 s-1 (
= 7.6 mol%). It follows the opposite trend of the CO2 conversion rate. In-situ DRIFTS data under methanation conditions evidence that the
at high
is related to the presence of a formate route which is not predominant at low
(high
). A synergistic interplay of basic and metallic sites is present. This contribution provides a rationale for designing industrially competitive CO2 methanation catalysts with high catalytic activity while maintaining low Ni loading.
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Mar 2023
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B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
B18-Core EXAFS
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Huihuang
Fang
,
Simson
Wu
,
Tugce
Ayvali
,
Jianwei
Zheng
,
Joshua
Fellowes
,
Ping-Luen
Ho
,
Kwan Chee
Leung
,
Alexander
Large
,
Georg
Held
,
Ryuichi
Kato
,
Kazu
Suenaga
,
Yves Ira A.
Reyes
,
Ho Viet
Thang
,
Hsin-Yi Tiffany
Chen
,
Shik Chi Edman
Tsang
Open Access
Abstract: Ammonia is regarded as an energy vector for hydrogen storage, transport and utilization, which links to usage of renewable energies. However, efficient catalysts for ammonia decomposition and their underlying mechanism yet remain obscure. Here we report that atomically-dispersed Ru atoms on MgO support on its polar (111) facets {denoted as MgO(111)} show the highest rate of ammonia decomposition, as far as we are aware, than all catalysts reported in literature due to the strong metal-support interaction and efficient surface coupling reaction. We have carefully investigated the loading effect of Ru from atomic form to cluster/nanoparticle on MgO(111). Progressive increase of surface Ru concentration, correlated with increase in specific activity per metal site, clearly indicates synergistic metal sites in close proximity, akin to those bimetallic N2 complexes in solution are required for the stepwise dehydrogenation of ammonia to N2/H2, as also supported by DFT modelling. Whereas, beyond surface doping, the specific activity drops substantially upon the formation of Ru cluster/nanoparticle, which challenges the classical view of allegorically higher activity of coordinated Ru atoms in cluster form (B5 sites) than isolated sites.
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Feb 2023
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B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
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Elan D. R.
Mistry
,
Daphné
Lubert-Perquel
,
Irena
Nevjestic
,
Giuseppe
Mallia
,
Pilar
Ferrer
,
Kanak
Roy
,
Georg
Held
,
Tian
Tian
,
Nicholas M.
Harrison
,
Sandrine
Heutz
,
Camille
Petit
Diamond Proposal Number(s):
[26511]
Open Access
Abstract: A family of boron nitride (BN)-based photocatalysts for solar fuel syntheses have recently emerged. Studies have shown that oxygen doping, leading to boron oxynitride (BNO), can extend light absorption to the visible range. However, the fundamental question surrounding the origin of enhanced light harvesting and the role of specific chemical states of oxygen in BNO photochemistry remains unanswered. Here, using an integrated experimental and first-principles-based computational approach, we demonstrate that paramagnetic isolated OB3 states are paramount to inducing prominent red-shifted light absorption. Conversely, we highlight the diamagnetic nature of O–B–O states, which are shown to cause undesired larger band gaps and impaired photochemistry. This study elucidates the importance of paramagnetism in BNO semiconductors and provides fundamental insight into its photophysics. The work herein paves the way for tailoring of its optoelectronic and photochemical properties for solar fuel synthesis.
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Feb 2023
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B07-B-Versatile Soft X-ray beamline: High Throughput
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Abstract: We present a new beamline for Versatile Soft X-ray Spectroscopy at Diamond Light Source, VerSoX B07-B, with a medium X-rays flux in the photon energy range 45-2200 eV. B07-B has two endstations permitting studies of a wide range of interfaces and materials. ES-2 enables high-throughput NEXAFS (Near-Edge X-ray Absorption Fine Spectroscopy) under ambient-pressure conditions, ES-1 is dedicated to high-throughput X-ray Photoelectron Spectroscopy (XPS) and in ultra-high vacuum (UHV). ES-1 is fully motorised and automated; it is equipped with fast entry lock, rotary distribution, sample storage and two sample preparation chambers for standard UHV sample preparation and characterisation.
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Jan 2023
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B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
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Diamond Proposal Number(s):
[28470]
Abstract: The chemical structures of aluminosilicate hydrates presented in alkali-activated geopolymer materials underpin their performances. Mg-substituted sodium aluminosilicate hydrates (N(-M)-A-S-H) are likely to be present in alkali-activated geopolymer materials prepared using MgO-containing precursors, however, their atomic-level structures remain unclear. The lack of such knowledge made it challenging to identify and distinguish N(-M)-A-S-H from complex alkali-activated geopolymer systems (i.e., alkali-activated slag, alkali-activated Mg-rich minerals), and therefore brought challenges in understanding and predicting their durability. This study characterised for the first time the atomic structures of the synthetic N(-M)-A-S-H gels, prepared through ion-exchange or co-synthesis, using X-ray absorption near-edge spectroscopy (XANES) at Si, Al and Mg K-edge. The results suggest that the substitution of Mg in the extra-framework locations of the alkali aluminosilicate hydrates (N-A-S-H) leads to negligible changes in the coordination environments of the aluminosilicate framework. However, the Mg coordination environment is distinguishably different from other Mg-containing phases in the systems, e.g., hydrotalcite. The Mg K-edge XANES of N(-M)-A-S-H shows a 0.8–1.2 eV shift compared with hydrotalcite. The results presented in this study can be used as the fingerprint to probe the presence of N(-M)-A-S-H in alkali-activated geopolymer materials containing Mg element.
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Nov 2022
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B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
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Open Access
Abstract: Understanding how the microstructure of the active Cu0 component in the commercially applicable Cu/ZnO/Al2O3(−Cs2O) low-temperature water-gas shift catalyst evolves under various H2 partial pressures in the presence/absence of a Cs promoter during thermal activation has been investigated. Time-resolved XRD and spatially-resolved XRD-CT data were measured as a function of H2 concentration along a packed bed reactor to elucidate the importance of the zincite support and the effect of the promoter on Cu sintering mechanisms, dislocation character and stacking fault probability. The rate of Cu reduction showed a dependency on [Cs], [H2] and bed height; lower [Cs] and higher [H2] led to a greater rate of metallic copper nanoparticle formation. A deeper analysis of the XRD line profiles allowed for determining a greater edge character to the dislocations and subsequent stacking fault probability was also observed to depend on higher [H2], smaller Cu0 (and ZnO) crystallite sizes, increased [ZnO] (30 wt.%, sCZA) and lower temperature. The intrinsic activity of Cu/ZnO/Al2O3 methanol synthesis catalysts has been intimately linked to the anisotropic behaviour of copper, and thus the presence of lattice defects; to the best knowledge of the authors, this study is the first instance in which this type of analysis has been applied to LT-WGS catalysts.
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Aug 2022
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B07-B-Versatile Soft X-ray beamline: High Throughput
B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
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Open Access
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Jul 2022
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