I15-Extreme Conditions
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Paul J.
Smith
,
Simon A.
Kondrat
,
Philip A.
Chater
,
Benjamin R.
Yeo
,
Greg M.
Shaw
,
Li
Lu
,
Jonathan K.
Bartley
,
Stuart H.
Taylor
,
Michael S.
Spencer
,
Christopher J.
Kiely
,
Gordon
Kelly
,
Colin W.
Park
,
Graham J.
Hutchings
Open Access
Abstract: Zincian georgeite, an amorphous copper–zinc hydroxycarbonate, has been prepared by co-precipitation using acetate salts and ammonium carbonate. Incorporation of zinc into the georgeite phase and mild ageing conditions inhibits crystallisation into zincian malachite or aurichalcite. This zincian georgeite precursor was used to prepare a Cu/ZnO catalyst, which exhibits a superior performance to a zincian malachite derived catalyst for methanol synthesis and the low temperature water–gas shift (LTS) reaction. Furthermore, the enhanced LTS activity and stability in comparison to that of a commercial Cu/ZnO/Al2O3 catalyst, indicates that the addition of alumina as a stabiliser may not be required for the zincian georgeite derived Cu/ZnO catalyst. The enhanced performance is partly attributed to the exclusion of alkali metals from the synthesis procedure, which are known to act as catalyst poisons. The effect of residual sodium on the microstructural properties of the catalyst precursor was investigated further from preparations using sodium carbonate.
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Jan 2017
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B18-Core EXAFS
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Grazia
Malta
,
Simon A.
Kondrat
,
Simon J.
Freakley
,
Catherine
Davies
,
Simon
Dawson
,
Xi
Liu
,
Li
Lu
,
Krzysztof
Dymkowski
,
Felix
Fernandez-alonso
,
Sanghamitra
Mukhopadhyay
,
Emma Kate
Gibson
,
Peter P.
Wells
,
Stewart F.
Parker
,
Christopher J.
Kiely
,
Graham J.
Hutchings
Diamond Proposal Number(s):
[10306, 11398, 15214]
Abstract: Single-site Au species supported on carbon have been shown to be the active sites for acetylene hydrochlorination. The evolution of these single-site species has been monitored by Au L3 X-ray Absorption Spectroscopy (XAS). Alternating between a standard reaction mixture of HCl/C2H2 and the single reactants, has provided insights into the reaction mechanism and catalyst deactivation processes. We demonstrate that oxidative addition of HCl across an Au(I) chloride species requires concerted addition with C2H2, in accordance with both the XAS measurements of Au oxidation state and the reaction kinetics being 1st order with respect to each reactant. The addition of excess C2H2 changes the Au speciation and results in the formation of oligomeric acetylene species which were detected by inelastic neutron scattering. Catalyst deactivation at extended reaction times can be correlated with the formation of metallic Au particles. The presence of this Au(0) species generated during the sequential gas experiments or after prolonged reaction times, results in the analysis of the normalised near edge white line intensity of the Au L3 X-ray absorption spectrum alone becoming an unsuitable guide for identifying the active Au species, affecting the strong correlation between normalized white line height and VCM productivity usually observed in the active catalyst. Thus, a combination of scanning transmission electron microscopy and detailed modelling of whole XAS spectrum was required to distinguish active Au(I) and Au(III) species from the spectator Au(0) component.
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Jul 2018
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B18-Core EXAFS
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Ewa
Nowicka
,
Christian
Reece
,
Sultan M.
Althahban
,
Khaled M. H.
Mohammed
,
Simon A.
Kondrat
,
David John
Morgan
,
Qian
He
,
David J.
Willock
,
Stanislaw
Golunski
,
Christopher J.
Kiely
,
Graham J.
Hutchings
Diamond Proposal Number(s):
[8071]
Abstract: A mixed oxide support containing Ce, Zr and Al was synthesized using a physical grinding method and applied in the oxidative dehydrogenation of propane using CO2 as the oxidant. The activity of the support was compared with that of fully-formulated catalysts containing palladium. The Pd/CeZrAlOx material exhibited long-term stability and selectivity to propene (during continuous operation for 140 h) which is not normally associated with dehydrogenation catalysts. From temperature-programmed desorption of NH3 and CO2 it was found that the catalyst possessed both acidic and basic sites. In addition, temperature programmed reduction showed that palladium promoted both the reduction and re-oxidation of the support. When the role of CO2 was investigated in the absence of gas-phase oxidant, using a Temporal Analysis of Products (TAP) reactor, it was found that CO2 dissociates over the reduced catalyst leading to formation of CO and selective oxygen species. It is proposed that CO2 has the dual role of regenerating selective oxygen species, and shifting the equilibrium for alkane dehydrogenation by consuming H2 through the reverse water-gas-shift reaction. These two mechanistic functions have previously been considered to be mutually exclusive.
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Mar 2018
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B18-Core EXAFS
E01-JEM ARM 200CF
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Xi
Sun
,
Simon R.
Dawson
,
Tanja E.
Parmentier
,
Grazia
Malta
,
Thomas E.
Davies
,
Qian
He
,
Li
Lu
,
David J.
Morgan
,
Nicholas
Carthey
,
Peter
Johnston
,
Simon A.
Kondrat
,
Simon J.
Freakley
,
Christopher J.
Kiely
,
Graham J.
Hutchings
Diamond Proposal Number(s):
[22766, 20643, 19580]
Abstract: Single-site catalysts can demonstrate high activity and selectivity in many catalytic reactions. The synthesis of these materials by impregnation from strongly oxidizing aqueous solutions or pH-controlled deposition often leads to low metal loadings or a range of metal species. Here, we demonstrate that simple impregnation of the metal precursors onto activated carbon from a low-boiling-point, low-polarity solvent, such as acetone, results in catalysts with an atomic dispersion of cationic metal species. We show the generality of this method by producing single-site Au, Pd, Ru and Pt catalysts supported on carbon in a facile manner. Single-site Au/C catalysts have previously been validated commercially to produce vinyl chloride, and here we show that this facile synthesis method can produce effective catalysts for acetylene hydrochlorination in the absence of the highly oxidizing acidic solvents previously used.
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Apr 2020
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B18-Core EXAFS
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Diamond Proposal Number(s):
[15151]
Abstract: Coupling reactions to form new C-C bonds are extensively used in industrial synthetic processes. Gold has been shown to be an active catalyst for such reactions however, conflicting reports exist as to whether cationic Au or metallic Au is acting as the active species. We prepared a heterogeneous catalyst consisting of atomically dispersed Au-Clx supported on carbon and showed this to be active in the homocoupling of phenylboronic acid to biphenyl. However; characterisation of the catalyst materials, even after just a short exposure time to the reactants, revealed rapid reduction and sintering of the Au species into larger metallic nanoparticles which we propose to be the true active species in this instance. This study suggests that if cationic Au is an active catalyst, it must be stabilised against reduction and agglomeration by either forming complexes which are more stable than common chlorides or by strongly anchoring them firmly onto alternative support materials; as in this case the carbon supported Au-Cl species were easily reduced.
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Dec 2017
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B18-Core EXAFS
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Grazia
Malta
,
Simon A.
Kondrat
,
Simon J.
Freakley
,
Catherine J.
Davies
,
Li
Lu
,
Simon
Dawson
,
Adam
Thetford
,
Emma K.
Gibson
,
David J.
Morgan
,
Wilm
Jones
,
Peter
Wells
,
Peter
Johnston
,
C. Richard A.
Catlow
,
Christopher J.
Kiely
,
Graham J.
Hutchings
Diamond Proposal Number(s):
[10306, 11398, 15214]
Abstract: There remains considerable debate over the active form of gold under operating conditions of a recently validated gold catalyst for acetylene hydrochlorination. We have performed an in situ x-ray absorption fine structure study of gold/carbon (Au/C) catalysts under acetylene hydrochlorination reaction conditions and show that highly active catalysts comprise single-site cationic Au entities whose activity correlates with the ratio of Au(I):Au(III) present. We demonstrate that these Au/C catalysts are supported analogs of single-site homogeneous Au catalysts and propose a mechanism, supported by computational modeling, based on a redox couple of Au(I)-Au(III) species.
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Mar 2017
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B18-Core EXAFS
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Grazia
Malta
,
Simon A.
Kondrat
,
Simon J.
Freakley
,
David J.
Morgan
,
Emma K.
Gibson
,
Peter P.
Wells
,
Matteo
Aramini
,
Diego
Gianolio
,
Paul B. J.
Thompson
,
Peter
Johnston
,
Graham J.
Hutchings
Diamond Proposal Number(s):
[15214]
Open Access
Abstract: The replacement of HgCl2/C with Au/C as a catalyst for acetylene hydrochlorination represents a significant reduction in the environmental impact of this industrial process. Under reaction conditions atomically dispersed cationic Au species are the catalytic active site, representing a large-scale application of heterogeneous single-site catalysts. While the metal nuclearity and oxidation state under operating conditions has been investigated in catalysts prepared from aqua regia and thiosulphate, limited studies have focused on the ligand environment surrounding the metal centre. We now report K-edge soft X-ray absorption spectroscopy of the Cl and S ligand species used to stabilise these isolated cationic Au centres in the harsh reaction conditions. We demonstrate the presence of three distinct Cl species in the materials; inorganic Cl−, Au–Cl, and C–Cl and how these species evolve during reaction. Direct evidence of Au–S interactions is confirmed in catalysts prepared using thiosulfate precursors which show high stability towards reduction to inactive metal nanoparticles. This stability was clear during gas switching experiments, where exposure to C2H2 alone did not dramatically alter the Au electronic structure and consequently did not deactivate the thiosulfate catalyst.
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Jun 2020
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B18-Core EXAFS
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Diamond Proposal Number(s):
[19850]
Open Access
Abstract: The chemical and structural nature of potassium compounds involved in catalytic soot oxidation have been studied by a combination of temperature programmed oxidation and operando potassium K-edge X-ray absorption spectroscopy experiments. These experiments are the first known operando studies using tender X-rays (∼3.6 keV) under high temperature oxidation reaction conditions. X-ray absorption near edge structure analysis of K2CO3/Al2O3 catalysts during heating shows that, at temperatures between 100 and 200 °C, potassium species undergo a structural change from an initial hydrated K2CO3·xH2O and KHCO3 mixture to well-defined K2CO3. As the catalyst is heated from 200 °C to 600 °C, a feature associated with multiple scattering shifts to lower energy, indicating increased K2CO3 dispersion, due to its mobility at high reaction temperature. This shift was noted to be greater in samples containing soot than in control experiments without soot and can be attributed to enhanced mobility of the K2CO3, due to the interaction between soot and potassium species. No potassium species except K2CO3 could be defined during reactions, which excludes a potential reaction mechanism in which carbonate ions are the active soot-oxidising species. Simulations of K-edge absorption near edge structures were performed to rationalise the observed changes seen. Findings showed that cluster size, unit cell distortions and variation in the distribution of potassium crystallographic sites influenced the simulated spectra of K2CO3. While further simulation studies are required for a more complete understanding, the current results support the hypothesis that changes in the local structure on dispersion can influence the observed spectra. Ex situ characterisation was carried out on the fresh and used catalyst, by X-ray diffraction and X-ray photoelectron spectroscopy, which indicated changes to the carbonate species, in line with the X-ray absorption spectroscopy experiments.
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Jul 2020
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B18-Core EXAFS
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Sarwat
Iqbal
,
Simon
Kondrat
,
Daniel R.
Jones
,
Daniel C.
Schoenmakers
,
Jennifer K.
Edwards
,
Li
Lu
,
Benjamin R.
Yeo
,
Peter
Wells
,
Emma
Gibson
,
David J.
Morgan
,
Christopher J.
Kiely
,
Graham J.
Hutchings
Diamond Proposal Number(s):
[8071]
Open Access
Abstract: The hydrogenation of lactic acid to form 1,2-propanediol has been investigated using Ru nanoparticles supported on carbon as a catalyst. Two series of catalysts which were prepared by wet impregnation and sol-immobilization were investigated. Their activity was contrasted with that of a standard commercial Ru/C catalyst (all catalysts comprise 5 wt % Ru). The catalyst prepared using sol-immobilization was found to be more active than the wet impregnation materials. In addition, the catalyst made by sol-immobilization was initially more active than the standard commercial catalyst. However, when reacted for an extended time or with successive reuse cycles, the sol-immobilized catalyst became less active, whereas the standard commercial catalyst became steadily more active. Furthermore, both catalysts exhibited an induction period during the first 1000 s of reaction. Detailed scanning transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray absorption fine structure analysis data, when correlated with the catalytic performance results, showed that the high activity can be ascribed to highly dispersed Ru nanoparticles. Although the sol-immobilization method achieved these optimal discrete Ru nanoparticles immediately, as can be expected from this preparation methodology, the materials were unstable upon reuse. In addition, surface lactide species were detected on these particles using X-ray photoelectron spectroscopy, which could contribute to their deactivation. The commercial Ru/C catalysts, on the other hand, required treatment under reaction conditions to change from raft-like morphologies to the desired small nanoparticle morphology, during which time the catalytic performance progressively improved.
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Sep 2015
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B18-Core EXAFS
I15-Extreme Conditions
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Simon
Kondrat
,
Paul J.
Smith
,
Peter
Wells
,
Philip
Chater
,
James H.
Carter
,
David J
Morgan
,
Elisabetta M.
Fiordaliso
,
Jakob B.
Wagner
,
Tom
Davies
,
Li
Lu
,
Jonathan K.
Bartley
,
Stuart H.
Taylor
,
Michael S.
Spencer
,
Christopher J.
Kiely
,
Gordon J.
Kelly
,
Colin W.
Park
,
Matthew
Rosseinsky
,
Graham J.
Hutchings
Diamond Proposal Number(s):
[8071]
Abstract: Copper and zinc form an important group of hydroxycarbonate minerals that include zincian malachite, aurichalcite, rosasite and the exceptionally rare and unstable—and hence little known and largely ignored1—georgeite. The first three of these minerals are widely used as catalyst precursors2, 3, 4 for the industrially important methanol-synthesis and low-temperature water–gas shift (LTS) reactions5, 6, 7, with the choice of precursor phase strongly influencing the activity of the final catalyst. The preferred phase2, 3, 8, 9, 10 is usually zincian malachite. This is prepared by a co-precipitation method that involves the transient formation of georgeite11; with few exceptions12 it uses sodium carbonate as the carbonate source, but this also introduces sodium ions—a potential catalyst poison. Here we show that supercritical antisolvent (SAS) precipitation using carbon dioxide (refs 13, 14), a process that exploits the high diffusion rates and solvation power of supercritical carbon dioxide to rapidly expand and supersaturate solutions, can be used to prepare copper/zinc hydroxycarbonate precursors with low sodium content. These include stable georgeite, which we find to be a precursor to highly active methanol-synthesis and superior LTS catalysts. Our findings highlight the value of advanced synthesis methods in accessing unusual mineral phases, and show that there is room for exploring improvements to established industrial catalysts.
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Feb 2016
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