B18-Core EXAFS
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Open Access
Abstract: The Fischer–Tropsch (FT) synthesis is traditionally associated with fossil fuel consumption, but recently this technology has emerged as a keystone that enables the conversion of captured CO2 with sustainable hydrogen to energy-dense fuels and chemicals for sectors which are challenging to be electrified. Iron-based FT catalysts are promoted with alkali and transition metals to improve reducibility, activity, and selectivity. Due to their low concentration and the metastable state under reaction conditions, the exact speciation and location of these promoters remain poorly understood. We now show that the selectivity promoters such as potassium and manganese, locked into an oxidic matrix doubling as a catalyst support, surpass conventional promoting effects. La1–xKxAl1–yMnyO3−δ (x = 0 or 0.1; y = 0, 0.2, 0.6, or 1) perovskite supports yield a 60% increase in CO conversion comparable to conventional promotion but show reduced CO2 and overall C1 selectivity. The presented approach to promotion seems to decouple the enhancement of the FT and the water–gas shift reaction. We introduce a general catalyst design principle that can be extended to other key catalytic processes relying on alkali and transition metal promotion.
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May 2023
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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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B18-Core EXAFS
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Madeleine
Han
,
Isabel
Gómez-Recio
,
Daniel Gutiérrez
Martín
,
Nathaly
Ortiz Peña
,
Maria Luisa
Ruiz-González
,
Mohamed
Selmane
,
José M.
González-Calbet
,
Ovidiu
Ersen
,
Andrea
Zitolo
,
Benedikt-Kaiser
Lassalle
,
David
Portehault
,
Christel
Laberty-Robert
Diamond Proposal Number(s):
[21747]
Abstract: Manganese and cobalt perovskite oxides are among the most active precious metal-free electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), respectively. Herein, we question the role of the cationic composition and charge state in manganite, cobaltite, and mixed Mn/Co perovskites in the mechanism of oxygen electrocatalysis for ORR and OER. We synthesize in molten salts a range of perovskite nanoparticles active in ORR (single B-site (LaMn)1−γO3 and (La0.7Sr0.3Mn)1−γO3), in OER (single B-site La0.67Sr0.33CoO3−δ), and in both ORR and OER (mixed B-site (LaMn0.6Co0.4)1−γO3). By using operando X-ray absorption spectroscopy coupled to ex situ electron energy loss spectroscopy, we show that Mn and Co in single B-site perovskites undergo changes in oxidation states at the steady state during electrocatalysis, while their oxidation states remain unchanged in the mixed Mn/Co perovskite during OER and ORR. We relate these distinct behaviors to modifications of the rate-determining steps of both the OER and ORR electrocatalytic cycles, triggered by an increased covalency of B–O bonds in the mixed perovskites. These results highlight how simple cationic substitutions, accompanied by a control of cationic vacancies, offer a pathway to tune oxygen electrocatalysis.
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Apr 2023
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B18-Core EXAFS
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Yunpeng
Zuo
,
Nikolaos
Antonatos
,
Lukáš
Děkanovský
,
Jan
Luxa
,
Joshua D.
Elliott
,
Diego
Gianolio
,
Jiří
Šturala
,
Fabrizio
Guzzetta
,
Stefanos
Mourdikoudis
,
Jakub
Regner
,
Roman
Málek
,
Zdenek
Sofer
Diamond Proposal Number(s):
[31795]
Abstract: As a fascinating innovative class of effective catalysts
for hydrogen evolution reaction (HER), transition-metal tellurides
have emerged as attractive materials, but they are still suffering
from their intrinsic activity for practical applications. Defect
engineering constitutes a promising strategy to optimize the
electronic configuration of the catalyst and further improve the
HER activity. Herein, we present the successful fabrication of
PdTe2-based catalysts with three different types of vacancies (dPdTex), including single Pd, Te defect site, and double Te defect
sites, by using a two-step method. The obtained d-PdTex
demonstrated a remarkable HER activity with an overpotential of
76 mV at 10 mA cm−2 without iR compensation, which is far lower
than that of bulk PdTe2 (259 mV). The procedure followed in this
work may be extended to generate defect sites in a range of
different two-dimensional materials, thus further expanding their potential application fields.
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Feb 2023
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I04-Macromolecular Crystallography
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Amelia K.
Gilio
,
Thomas W.
Thorpe
,
Alex
Heyam
,
Mark R.
Petchey
,
Balázs
Pogrányi
,
Scott P.
France
,
Roger M.
Howard
,
Michael J.
Karmilowicz
,
Russell
Lewis
,
Nicholas
Turner
,
Gideon
Grogan
Diamond Proposal Number(s):
[18598]
Open Access
Abstract: Imine reductases (IREDs) catalyze the asymmetric reduction of cyclic imines, but also in some cases the coupling of ketones and amines to form secondary amine products in an enzyme-catalyzed reductive amination (RedAm) reaction. Enzymatic RedAm reactions have typically used small hydrophobic amines, but many interesting pharmaceutical targets require that larger amines be used in these coupling reactions. Following the identification of IR77 from Ensifer adhaerens as a promising biocatalyst for the reductive amination of cyclohexanone with pyrrolidine, we have characterized the ability of this enzyme to catalyze couplings with larger bicyclic amines such as isoindoline and octahydrocyclopenta(c)pyrrole. By comparing the activity of IR77 with reductions using sodium cyanoborohydride in water, it was shown that, while the coupling of cyclohexanone and pyrrolidine involved at least some element of reductive amination, the amination with the larger amines likely occurred ex situ, with the imine recruited from solution for enzyme reduction. The structure of IR77 was determined, and using this as a basis, structure-guided mutagenesis, coupled with point mutations selecting improving amino acid sites suggested by other groups, permitted the identification of a mutant A208N with improved activity for amine product formation. Improvements in conversion were attributed to greater enzyme stability as revealed by X-ray crystallography and nano differential scanning fluorimetry. The mutant IR77-A208N was applied to the preparative scale amination of cyclohexanone at 50 mM concentration, with 1.2 equiv of three larger amines, in isolated yields of up to 93%.
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Jan 2023
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B18-Core EXAFS
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Diamond Proposal Number(s):
[22225]
Open Access
Abstract: The most studied catalysts for methane dehydroaromatization (MDA)─Mo/ZSM-5─are not commercialized yet due to the rapid deactivation and insufficient activity. Catalytic systems based on Fe and Re are potential alternatives to Mo-containing zeolites. Here, we compare the catalytic performance of these catalysts as a function of metal type and its loading in ZSM-5 zeolite. The results show that the catalytic activity decreases in the order of Re/ZSM-5 > Mo/ZSM-5 > Fe/ZSM-5, while the catalyst stability decreases in the opposite order: Fe/ZSM-5 > Mo/ZSM-5 > Re/ZSM-5. The active metal species in the working catalysts were determined by operando X-ray absorption near-edge structure spectroscopy combined with mass spectrometry. We found that Re0 and Fe2+ species are the most likely active species for the catalytic dehydroaromatization of CH4 to aromatics in respective catalysts. Combining the pulse reaction technique with operando thermogravimetry analysis–mass spectrometry experiments, we demonstrate that the length of the induction period strongly correlates to the activity of the catalyst. The longer induction period of the Fe/ZSM-5 catalyst indicates the slow growth of hydrocarbon pool intermediates inside the zeolite pores and thus explains its poor catalytic performance. Finally, both the formation of hydrocarbon pool species and the activity of Fe/ZSM-5 can be improved by increasing the Fe loading, reaction pressure, and space velocity.
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Dec 2022
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I04-Macromolecular Crystallography
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Zhen
Zhang
,
Mochen
Dong
,
Rémi
Zallot
,
George Michael
Blackburn
,
Nini
Wang
,
Chengjian
Wang
,
Long
Chen
,
Patrick
Baumann
,
Zuyan
Wu
,
Zhongfu
Wang
,
Haiming
Fan
,
Christian
Roth
,
Yi
Jin
,
Yuan
He
Diamond Proposal Number(s):
[18812]
Abstract: Glycan sulfation is an important modification supporting the functionalities of many proteins in biology. Exo-acting 6S-GlcNAcases from human microbiota are glycosidases that participate in the removal of 6-sulfo-GlcNAc from host glycans and thereby play an important role in human health and disease. Nonetheless, mechanisms underlying their ability to recognize the sulfate group remain poorly understood. Using structural and kinetic analyses, we here reveal the catalytically important amino acids directly involved in the recognition and cleavage of 6S-GlcNAc, but not of 6-phospho-GlcNAc, in BbhII from Bifidobacterium bifidum, Bt4394 from Bacteroides thetaiotaomicron, and SGL from Prevotella spp. The defining features of their sulfate recognition motifs underpin a genomic enzymological exploration of 6S-GlcNAcases to identify a wider range of human health-associated bacterial species having 6S-GlcNAcase activity. Our data provide significant insights into distinct molecular mechanisms of sulfated sugar recognition employed by 6S-GlcNAcases from both Gram-positive and Gram-negative bacteria along with valuable information for the exploration of extensive interactions between microbiota and their host glycans.
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Dec 2022
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B18-Core EXAFS
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Samuel
Pattisson
,
Simon R.
Dawson
,
Grazia
Malta
,
Nicholas F.
Dummer
,
Louise R.
Smith
,
Anna
Lazaridou
,
David J.
Morgan
,
Simon J.
Freakley
,
Simon A.
Kondrat
,
Joost J.
Smit
,
Peter
Johnston
,
Graham J.
Hutchings
Diamond Proposal Number(s):
[15214, 15151]
Abstract: The commercialization of gold for acetylene hydrochlorination represents a major scientific landmark. The development of second-generation gold catalysts continues with a focus on derivatives and drop-in replacements with higher activity and stability. Here, we show the influence that the support surface oxygen has on the activity of carbon supported gold catalysts. Variation in the surface oxygen content of carbon is achieved through careful modification of the Hummers chemical oxidation method prior to the deposition of gold. All oxidized carbon-based catalysts resulted in a marked increase in activity at 200 °C when compared to the standard nontreated carbon, with an optimum oxygen content of ca. 18 at % being observed. Increasing oxygen and relative concentration of C–O functionality yields catalysts with light-off temperatures 30–50 °C below the standard catalyst. This understanding opens a promising avenue to produce high activity acetylene hydrochlorination catalysts that can operate at lower temperatures.
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Nov 2022
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Abstract: The surface chemistry of alcohol synthesis from CO2 hydrogenation has been investigated using kinetic testing, ambient pressure X-ray photoelectron spectroscopy (AP-XPS), and DFT calculations over a multicomponent system, where Pt and ceria nanoparticles coexisted on a titania template, Pt/CeOx/TiO2(110). Due to its high ability to bind and activate CO2, not seen for typical Cu–ZnO catalysts, the Pt–CeOx–TiO2 interface is excellent for the hydrogenation of CO2 to methanol, with some ethanol also being produced (21% selectivity). The results of AP-XPS and DFT calculations indicate that the active state involves a mixture of Ti4+/Ti3+, Ce3+, and Pt0/Pt+. A fast pathway for the formation of CH3O species is only plausible when Ce3+ and Pt are present. The addition of water to the reaction feed facilitates the first hydrogenation of CO2 and substantially enhances the surface coverage of C-containing species (CH3O, HCOO, CO3, CHx), facilitating the formation of C–C bonds and the production of ethanol (38% selectivity).
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Nov 2022
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E01-JEM ARM 200CF
I20-EDE-Energy Dispersive EXAFS (EDE)
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Hui
Luo
,
Victor Y.
Yukuhiro
,
Pablo S.
Fernández
,
Jingyu
Feng
,
Paul
Thompson
,
Reshma R.
Rao
,
Rongsheng
Cai
,
Silvia
Favero
,
Sarah J.
Haigh
,
James R.
Durrant
,
Ifan E. L.
Stephens
,
Maria-Magdalena
Titirici
Diamond Proposal Number(s):
[28663, 25476]
Open Access
Abstract: Pt-based bimetallic electrocatalysts are promising candidates to convert surplus glycerol from the biodiesel industry to value-added chemicals and coproduce hydrogen. It is expected that the nature and content of the elements in the bimetallic catalyst can not only affect the reaction kinetics but also influence the product selectivity, providing a way to increase the yield of the desired products. Hence, in this work, we investigate the electrochemical oxidation of glycerol on a series of PtNi nanoparticles with increasing Ni content using a combination of physicochemical structural analysis, electrochemical measurements, operando spectroscopic techniques, and advanced product characterizations. With a moderate Ni content and a homogenously alloyed bimetallic Pt–Ni structure, the PtNi2 catalyst displayed the highest reaction activity among all materials studied in this work. In situ FTIR data show that PtNi2 can activate the glycerol molecule at a more negative potential (0.4 VRHE) than the other PtNi catalysts. In addition, its surface can effectively catalyze the complete C–C bond cleavage, resulting in lower CO poisoning and higher stability. Operando X-ray absorption spectroscopy and UV–vis spectroscopy suggest that glycerol adsorbs strongly onto surface Ni(OH)x sites, preventing their oxidation and activation of oxygen or hydroxyl from water. As such, we propose that the role of Ni in PtNi toward glycerol oxidation is to tailor the electronic structure of the pure Pt sites rather than a bifunctional mechanism. Our experiments provide guidance for the development of bimetallic catalysts toward highly efficient, selective, and stable glycerol oxidation reactions.
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Nov 2022
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