B18-Core EXAFS
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Diamond Proposal Number(s):
[10306]
Open Access
Abstract: The interaction between Pd and TiO2 for promoting photocatalytic activity was investigated by tailoring the size of Pd nanoparticles and monitoring the photocatalytic activity of methanol photo-reforming reaction for hydrogen gas production. We show that at 0.6 % wt. Pd loading, catalyst with highly dispersed nanoparticles obtained at 1 oC temperature exhibits superior photocatalytic activity for hydrogen gas production. At different weight of Pd loading, tailoring two sets of catalysts with different structural properties provide correlation between the changes of Pd local structures with the rate of hydrogen production. The impact of controlling the structural properties of metal nanoparticles in influencing H2 production outweighs the effect of metal loading variation. The differences of Pd/TiO2 activity at the variation of metal loading were correlated with the changes in Pd local structure consequently affecting electronic transfer and photocatalytic efficiency.
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Jun 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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I20-Scanning-X-ray spectroscopy (XAS/XES)
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George F.
Tierney
,
Donato
Decarolis
,
Norli
Abdullah
,
Scott M.
Rogers
,
Shusaku
Hayama
,
Martha
Briceno De Gutierrez
,
Alberto
Villa
,
C. Richard A.
Catlow
,
Paul
Collier
,
Nikolaos
Dimitratos
,
Peter
Wells
Diamond Proposal Number(s):
[17283]
Open Access
Abstract: Sol-immobilization is increasingly used to achieve supported metal nanoparticles (NPs) with controllable size and shape; it affords a high degree of control of the metal particle size and yields a narrow particle size distribution. Using state-of-the-art beamlines, we demonstrate how X-ray absorption fine structure (XAFS) techniques are now able to provide accurate structural information on nano-sized colloidal Au solutions at μM concentrations. This study demonstrates: (i) the size of Au colloids can be accurately tuned by adjusting the temperature of reduction, (ii) Au concentration, from 50 μM to 1000 μM, has little influence on the average size of colloidal Au NPs in solution and (iii) the immobilization step is responsible for significant growth in Au particle size, which is further exacerbated at increased Au concentrations. The work presented demonstrates that an increased understanding of the primary steps in sol-immobilization allows improved optimization of materials for catalytic applications.
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May 2019
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B18-Core EXAFS
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Caomhan
Stewart
,
Emma K.
Gibson
,
Kevin
Morgan
,
Giannantonio
Cibin
,
Andrew J.
Dent
,
Christopher
Hardacre
,
Evgenii V.
Kondratenko
,
Vita A.
Kondratenko
,
Colin
Mcmanus
,
Scott
Rogers
,
Cristina E.
Stere
,
Sarayute
Chansai
,
Yichi
Wang
,
Sarah J.
Haigh
,
Peter P.
Wells
,
Alexandre
Goguet
Diamond Proposal Number(s):
[11747]
Open Access
Abstract: The promotional effect of H2 on the oxidation of CO is of topical interest and there is debate over whether this promotion is due to either thermal or chemical effects. As yet there is no definitive consensus in the literature. Combining spatially resolved mass spectrometry and X-ray absorption spectroscopy (XAS) we observe a specific environment of the active catalyst during CO oxidation, having the same specific local coordination of the Pd in both the absence and presence of H2. In combination with Temporal Analysis of Products (TAP), performed under isothermal conditions, a mechanistic insight into the promotional effect of H2 was found, providing clear evidence of non-thermal effects in the hydrogen promoted of oxidation of carbon monoxide. We have identified that H2 promotes the Langmuir Hinshelwood mechanism and we propose this is linked to the increased interaction of O with the Pd surface in the presence of H2. This combination of spatially resolved MS and XAS and TAP studies has provided previously unobserved insights into the nature of this promotional effect.
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Jul 2018
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B18-Core EXAFS
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Diamond Proposal Number(s):
[10306]
Abstract: The use of sol-immobilisation to prepare supported metal nanoparticles is an area of growing importance in heterogeneous catalysis; it affords greater control of nanoparticle properties compared to conventional catalytic routes e.g. impregnation. This work, and other recent studies, demonstrate how the properties of the resultant supported metal nanoparticles can be tailored by adjusting the conditions of colloidal synthesis i.e. temperature and solvent. We further demonstrate the applicability of these methods to the hydrogenation of nitrophenols using a series of tailored Pd/TiO2 catalysts, with low Pd loading 0.2 wt. %. Here, the temperature of colloidal synthesis is directly related to the mean particle diameter and the catalytic activity. Smaller Pd particles (2.2 nm, k = 0.632 min-1, TOF = 560 h-1) perform better than their larger counterparts (2.6 nm, k = 0.350 min-1, TOF = 370 h-1) for the hydrogenation of p-nitrophenol, with the catalyst containing smaller NPs found to have increased stability during recyclability studies, with high activity (> 90% conversion after 5 minutes) maintained across 5 catalytic cycles.
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Mar 2018
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B18-Core EXAFS
I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[11446, 10306, 8071]
Open Access
Abstract: The sol-immobilisation method, in which metal nanoparticles are ‘preformed’
(stabilised by the polymer, polyvinyl alcohol) before they are anchored to a support
material, was adapted in order to prepare monometallic Au/TiO2 and Pd/TiO2
catalysts, with tailored properties. Varied temperature and solvent environments (H2O,
mixed H2O:EtOH and EtOH) were employed during colloidal metal formation,
generating metal particles with distinct characteristics (metal particle diameter and
available metal sites). The metal nanoparticle properties in the resulting catalysts were
fully characterised using a range of spectroscopic (XAFS, IR and UV-Vis) and
imaging techniques (TEM and HAADF STEM). It was determined that the preparation
of metal nanoparticles at −30°C, in a mixed H2O:EtOH solvent afforded the smallest
average particle diameter, regardless of the choice of metal (2.0 nm for Au, 1.4 nm for
Pd). However, when prepared at 1°C in H2O, a higher population of small Au (< 5
atoms) or Pd clusters (< 20 atoms) existed, compared with any other environment.
The performance of the catalysts were tested in three different reactions; Au/TiO2 for
the oxidation of glycerol, and Pd/TiO2 for the hydrogenation of furfural and pnitrophenol.
For the two former reactions, it was established that metal particle size is
not the only factor influencing performance; the highly active isolated metal clusters,
as well as the solvent-PVA-metal interaction, are considered very important factors,
and are discussed.
Understanding colloidal metal formation, including nucleation and growth
phenomena, is vital in the future design of metal nanoparticle properties, and was
investigated by means of in situ XAFS. A continuous flow method of nanoparticle
synthesis was first explored and developed, before a synchrotron based experiment
was performed to monitor the nanoparticle generation (colloidal reduction) in a range
of reactors fabricated from different materials (silicon/glass, PTFE and PEEK).
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Sep 2017
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B18-Core EXAFS
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Scott M.
Rogers
,
C. Richard A.
Catlow
,
Carine E.
Chan-Thaw
,
Arunabhiram
Chutia
,
Nan
Jian
,
Richard E.
Palmer
,
Michal
Perdjon
,
Adam
Thetford
,
Nikolaos
Dimitratos
,
Alberto
Villa
,
Peter
Wells
Diamond Proposal Number(s):
[10306]
Open Access
Abstract: The conversion of biomass to useful chemical products requires precise catalytic properties to achieve the required activity, selectivity and durability. Here we show, through optimized colloidal synthesis, the tandem control of Pd size and site availability for the directed hydrogenation of the bio-derived intermediate, furfural. Adjusting the temperature of colloidal reduction dictates the size of Pd nanoparticles; in some instances ultra small clusters <20 atoms are achieved. Whereas, changing the solvent system, affects the PVA-Pd interaction and relative proportion of available surface sites (corners, edges, planes), allowing us to control the selectivity to the valuable hydrogenation products of furfuryl alcohol and tetrahydrofurfuryl alcohol. We demonstrate, through combined experimental and computational studies, that Pd nanoparticle planes are more prone to deactivation through the formation of Pd carbide, and the resulting reduced efficacy of furfural binding. This approach to nanoparticle optimization is an important strategy for producing long lasting, high performance catalysts for emerging sustainable technologies.
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Jan 2017
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B18-Core EXAFS
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Diamond Proposal Number(s):
[8071]
Open Access
Abstract: Multicomponent oxide shell@core catalysts have been prepared, affording overlayers of MoOx on Fe2O3. This design approach allows bulk characterization techniques, such as X-ray Absorption Fine Structure (XAFS), to provide surface sensitive information. Coupling this approach with in situ methodologies provides insights during crucial catalytic processes. Calcination studies were followed by a combination of XAFS and Raman, and demonstrate that amorphous multi-layers of MoOx are first converted to MoO3 before formation of Fe2(MoO4)3. However, a single overlayer of Oh Mo units remains at the surface at all times. In situ catalysis studies during formaldehyde production identified that Mo6+ was present throughout, confirming that gas phase oxygen transfer to molybdenum is rapid under reaction conditions. Reduction studies in the presence of MeOH resulted in the formation of reduced Mo-Mo clusters with a bonding distance of 2.6 Å. It is proposed that the presence of the clusters indicates that the selective conversion of MeOH to formaldehyde requires multiple Mo sites.
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Aug 2015
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B18-Core EXAFS
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Scott
Rogers
,
C. Richard A.
Catlow
,
Carine E.
Chan-Thaw
,
Diego
Gianolio
,
Emma
Gibson
,
Anna
Gould
,
Nan
Jian
,
Andrew J.
Logsdail
,
Richard E.
Palmer
,
Laura
Prati
,
Nikolaos
Dimitratos
,
Alberto
Villa
,
Peter
Wells
Diamond Proposal Number(s):
[8071]
Open Access
Abstract: Poly(vinyl alcohol) (PVA)-stabilized Au nanoparticles (NPs) were synthesized by colloidal methods in which temperature variations (−75 to 75 °C) and mixed H2O/EtOH solvent ratios (0, 50, and 100 vol/vol) were used. The resulting Au NPs were immobilized on TiO2 (P25), and their catalytic performance was investigated for the liquid phase oxidation of glycerol. For each unique solvent system, there was a systematic increase in the average Au particle diameter as the temperature of the colloidal preparation increased. Generation of the Au NPs in H2O at 1 °C resulted in a high observed activity compared with current Au/TiO2 catalysts (turnover frequency = 915 h–1). Interestingly, Au catalysts with similar average particle sizes but prepared under different conditions had contrasting catalytic performance. For the most active catalyst, aberration-corrected high angle annular dark field scanning transmission electron microscopy analysis identified the presence of isolated Au clusters (from 1 to 5 atoms) for the first time using a modified colloidal method, which was supported by experimental and computational CO adsorption studies. It is proposed that the variations in the populations of these species, in combination with other solvent/PVA effects, is responsible for the contrasting catalytic properties.
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Jul 2015
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B18-Core EXAFS
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Emma
Gibson
,
Andrew
Beale
,
C. Richard A.
Catlow
,
Arunabhiram
Chutia
,
Diego
Gianolio
,
Anna
Gould
,
Anna
Kroner
,
Khaled M. H.
Mohammed
,
Michal
Perdjon
,
Scott
Rogers
,
Peter P.
Wells
Diamond Proposal Number(s):
[8071]
Open Access
Abstract: The use of AuPd nanoparticles in catalysis is widespread, with the activity being attributed to their precise structural properties. We demonstrate the restructuring of AuPd nanoparticles under CO oxidation conditions using a combined XAFS/DRIFTS approach. The fresh catalyst exhibits PdO islands at the surface of the nanoparticles, which are reduced under reaction conditions, a process observed via both DRIFTS and Pd K-edge XAFS measurements. From the EXAFS analysis alone the nanoparticles were observed to have a Au rich core with an outer region of intimately mixed Au and Pd atoms. This structure was found to remain mostly unaltered throughout reaction. However, the DRIFTS spectra showed that although Au was present on the surface during the initial stages of reaction the surface rearranged just before light-off, and contained only Pd atoms thereafter. This study highlights the advantage of this combined approach, where both the surface structure and local environment of the constituent metals can be probed simultaneously, allowing a complete picture of the restructuring of these bimetallic particles to be obtained under reaction conditions.
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May 2015
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