B18-Core EXAFS
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Diamond Proposal Number(s):
[20354]
Abstract: This paper describes the effect of composition on the catalytic activity of carbon-supported Pt-Pb, Pt-Rh, and Pt-Rh-Pb catalysts towards ethanol oxidation in acid media. The catalysts were synthesised by a polyol reduction method and characterised using several experimental techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray absorption near edge structure, and X-ray energy dispersive spectroscopy. The catalytic activity towards ethanol oxidation was evaluated by cyclic voltammetry, chronoamperometry, and in situ Fourier transform infrared spectroscopy (FTIR) experiments. XRD data indicate the presence of Pb in both alloyed and oxide forms. TEM images reveal nanoparticles well-dispersed on the carbon support, with spherical shapes and particle sizes around 2.0–6.5 nm. The Pt3RhPb/C catalyst showed the highest catalytic activity for ethanol oxidation, reaching current densities 6.0 times higher than the commercial Pt/C catalyst. The trimetallic catalyst showed the highest CO2 and acetic acid formation, explaining the higher current densities presented during cyclic voltammetry and chronoamperometry. Additionally, since the oxidation appears to follow a non-selective path, the role of Pb in the trimetallic catalyst is not related to driving the reaction towards the production of CO2. The improvement in catalytic activity occurred due to the synergy between the metals.
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Aug 2022
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B18-Core EXAFS
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Diamond Proposal Number(s):
[21533]
Open Access
Abstract: The electrochemical conversion of carbon dioxide (CO2) to useful chemical fuels is a promising route toward the achievement of carbon neutral and carbon negative energy technologies. Copper (Cu)- and Cu oxide-derived surfaces are known to electrochemically convert CO2 to high-value and energy-dense products. However, the nature and stability of oxidized Cu species under reaction conditions are the subject of much debate in the literature. Herein, we present the synthesis and characterization of copper-titanate nanocatalysts, with discrete Cu–O coordination environments, for the electrochemical CO2 reduction reaction (CO2RR). We employ real-time in situ X-ray absorption spectroscopy (XAS) to monitor Cu species under neutral-pH CO2RR conditions. Combination of voltammetry and on-line electrochemical mass spectrometry with XAS results demonstrates that the titanate motif promotes the retention of oxidized Cu species under reducing conditions for extended periods, without itself possessing any CO2RR activity. Additionally, we demonstrate that the specific nature of the Cu–O environment and the size of the catalyst dictate the long-term stability of the oxidized Cu species and, subsequently, the product selectivity.
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Jan 2022
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B18-Core EXAFS
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Diamond Proposal Number(s):
[15151]
Abstract: Pt-Sn bimetallic catalysts, especially Pt-Sn alloys, are considered highly CO-tolerant and are thus candidates for reformate derived hydrogen oxidation and for direct oxidation of fuel cell molecules. However, it remains unclear if this CO-tolerance originates from Sn in the Pt-Sn alloy or whether SnO 2 , present as a separate phase, also contributes. In this work, a carbon-supported Pt-SnO 2 was carefully synthesized to avoid the formation of Pt-Sn alloy phases. The resulting structure was analysed by scanning transmission electron microscopy (STEM) and detailed X-ray absorption spectroscopy (XAS). CO oxidation voltammograms of the Pt-SnO 2 /C and other SnO 2 -modified Pt surfaces unambiguously suggest that a bifunctional mechanism is indeed operative at such Pt-SnO 2 catalysts for stable CO oxidation at low overpotentials. The results from these studies suggest that the bifunctional mechanism can be attributed to the co-catalysis role of SnO 2 , in which the surface hydroxide of SnO 2 (Sn-OH) reacts with CO adsorbed on Pt surface (Pt-CO ads ) and regenerates via a Sn II /Sn IV reversible redox couple (-0.2–0.3 V vs. reversible hydrogen electrode).
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Jun 2021
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B18-Core EXAFS
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Diamond Proposal Number(s):
[19850]
Abstract: The bifunctional mechanism is well-acknowledged for the promoted CO oxidation on Pt-based bimetallic electrocatalysts. However, the direct identification of the active oxygenated species and the nature and electrochemistry of the second component are still a matter of debate. Herein, Snad-Pt/C catalysts, where Sn ad-atoms are exclusively on the surface of Pt nanoparticles at low coverages ranging from 0.0033 to 0.2 monolayers to avoid sub-surface Sn and alloy formation, were prepared as a model system to resolve these issues using a surface organometallic chemistry approach. Effects of the Sn ad-atoms on CO oxidation were studied by CO stripping voltammograms as a function of Sn coverage. Using in situ XAS measurements, the Sn average oxidation state is estimated to increase from +0.2 to +3.1 as the potential increases from 0 to 0.8 VRHE, with the number of the oxygen neighbours increasing stepwise. Pt4.5-Sn-(OH)1.5 is revealed as the active species responsible for the bifunctional mechanism at low overpotentials and is generated via a redox couple corresponding to Pt4.5-Sn*/Pt4.5-Sn-(OH)1.5.
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Jun 2021
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B18-Core EXAFS
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Diamond Proposal Number(s):
[10306, 19850]
Open Access
Abstract: Understanding the surface structure of bimetallic nanoparticles is crucial for heterogeneous catalysis. Although surface contraction has been established in monometallic systems, less is known for bimetallic systems, especially of nanoparticles. In this work, the bond length contraction on the surface of bimetallic nanoparticles is revealed by XAS in H2 at room temperature on dealloyed Pt–Sn nanoparticles, where most Sn atoms were oxidized and segregated to the surface when measured in air. The average Sn–Pt bond length is found to be ∼0.09 Å shorter than observed in the bulk. To ascertain the effect of the Sn location on the decrease of the average bond length, Pt–Sn samples with lower surface-to-bulk Sn ratios than the dealloyed Pt–Sn were studied. The structural information specifically from the surface was extracted from the averaged XAS results using an improved fitting model combining the data measured in H2 and in air. Two samples prepared so as to ensure the absence of Sn in the bulk were also studied in the same fashion. The bond length of surface Sn–Pt and the corresponding coordination number obtained in this study show a nearly linear correlation, the origin of which is discussed and attributed to the poor overlap between the Sn 5p orbitals and the available orbitals of the Pt surface atoms.
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May 2021
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I20-EDE-Energy Dispersive EXAFS (EDE)
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Andrew S.
Leach
,
Jennifer
Hack
,
Monica
Amboage
,
Sofia
Diaz-Moreno
,
Haoliang
Huang
,
Patrick L.
Cullen
,
Martin
Wilding
,
Emanuele
Magliocca
,
Thomas
Miller
,
Christopher
Howard
,
Daniel
Brett
,
Paul
Shearing
,
Paul F.
Mcmillan
,
Andrea E.
Russell
,
Rhodri
Jervis
Diamond Proposal Number(s):
[22008, 15650]
Open Access
Abstract: A polymer electrolyte fuel cell (PEFC) has been designed to allow operando X-ray absorption spectroscopy (XAS) measurements of catalysts. The cell has been developed to operate under standard fuel cell conditions, with elevated temperatures and humidification of the gas-phase reactants, both of which greatly impact the catalyst utilisation. X-ray windows in the endplates of the cell facilitate collection of XAS spectra during fuel cell operation while maintaining good compression in the area of measurement. Results of polarisation curves and cyclic voltammograms (CVs) showed that the operando cell performs well as a fuel cell, while also providing XAS data of suitable quality for robust XANES analysis. The cell has produced comparable XAS results when performing a cyclic voltammogram to an established in situ cell when measuring the Pt LIII edge. Similar trends of Pt oxidation, and reduction of the formed Pt oxide, have been presented with a time resolution of 5 seconds for each spectrum, paving the way for time-resolved spectral measurements of fuel cell catalysts in a fully-operating fuel cell.
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May 2021
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[16479]
Open Access
Abstract: In situ X-ray absorption and emission spectroscopies (XAS and XES) are used to provide details regarding the role of the accessibility and extent of redox activity of the Mn ions in determining the oxygen reduction activity of LaMnO3 and CaMnO3, with X-ray absorption near-edge structure (XANES) providing the average oxidation state, extended X-ray absorption fine structure (EXAFS) providing the local coordination environment, and XES providing the population ratios of the Mn2+, Mn3+, and Mn4+ sites as a function of the applied potential. For LaMnO3, XANES and XES show that Mn3+ is formed, but Mn4+ ions are retained, which leads to the 4e– reduction between 0.85 and 0.6 V. At more negative potentials, down to 0.2 V, EXAFS confirms an increase in oxygen vacancies as evidenced by changes in the Mn–O coordination distance and number, while XES shows that the Mn3+ to Mn4+ ratio increases. For CaMnO3, XANES and XES show the formation of both Mn3+ and Mn2+ as the potential is made more negative, with little retention of Mn4+ at 0.2 V. The EXAFS for CaMnO3 also indicates the formation of oxygen vacancies, but in contrast to LaMnO3, this is accompanied by loss of the perovskite structure leading to structural collapse. The results presented have implications in terms of understanding of both the pseudocapacitive response of Mn oxide electrocatalysts and the processes behind degradation of the activity of the materials.
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May 2021
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B18-Core EXAFS
I20-Scanning-X-ray spectroscopy (XAS/XES)
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V.
Celorrio
,
D. J.
Fermin
,
L.
Calvillo
,
A.
Leach
,
H.
Huang
,
G.
Granozzi
,
J. A.
Alonso
,
A.
Aguadero
,
R. M.
Pinacca
,
A. E.
Russell
,
D.
Tiwari
Diamond Proposal Number(s):
[10306, 15151, 16479]
Abstract: Oxygen electrocatalysis at transition metal oxides is one of the key challenges underpinning electrochemical energy conversion systems, involving a delicate interplay of the bulk electronic structure and surface coordination of the active sites. In this work, we investigate for the first time the structure–activity relationship of A2RuMnO7 (A = Dy3+, Ho3+, and Er3+) nanoparticles, demonstrating how orbital mixing of Ru, Mn, and O promotes high density of states at the appropriate energy range for oxygen electrocatalysis. The bulk structure and surface composition of these multicomponent pyrochlores are investigated by high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption spectroscopy, X-ray emission spectroscopy (XES), and X-ray photoemission spectroscopy (XPS). The materials exhibit high phase purity (cubic fcc with a space group Fd3̅m) in which variations in M–O bonds length are less than 1% upon replacing the A-site lanthanide. XES and XPS show that the mean oxidation state at the Mn-site as well as the nanoparticle surface composition was slightly affected by the lanthanide. The pyrochlore nanoparticles are significantly more active than the binary RuO2 and MnO2 toward the 4-electron oxygen reduction reaction in alkaline solutions. Interestingly, normalization of kinetic parameters by the number density of electroactive sites concludes that Dy2RuMnO7 shows twice higher activity than benchmark materials such as LaMnO3. Analysis of the electrochemical profiles supported by density functional theory calculations reveals that the origin of the enhanced catalytic activity is linked to the mixing of Ru and Mn d-orbitals and O p-orbitals at the conduction band which strongly overlap with the formal redox energy of O2 in solution. The activity enhancement strongly manifests in the case of Dy2RuMnO7 where the Ru/Mn ratio is closer to 1 in comparison with the Ho3+ and Er3+ analogs. These electronic effects are discussed in the context of the Gerischer formalism for electron transfer at the semiconductor/electrolyte junctions.
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Jan 2021
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B18-Core EXAFS
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Diamond Proposal Number(s):
[15151]
Abstract: The electrochemical oxidation of ethanol results in the formation of strongly adsorbed intermediates. Pt–Rh catalysts are proposed as alternatives since they easy the C–C bond breaking. However, the effect of the Pt–Rh structure on the catalytic activity and selectivity to CO2 is not well understood. Here, we synthesised Pt/C and two different Pt–Rh/C catalyst architectures, an alloy (Pt3Rh/C) and a bimetallic mixture (Pt3–Rh/C) to study the effect of catalyst structure on its catalytic activity and on the products formed during the ethanol oxidation in acid media. The nanoparticles were prepared by a modified polyol reduction method using ethylene glycol as a co-reducing agent and Pb as a material of sacrifice, to obtain very small and well-dispersed nanoparticles on the carbon support. Fourier transform infrared spectroscopy and derivative voltammetry was used to give insights about the ethanol oxidation mechanism occurring at the developed catalysts. The samples characterised by X-ray diffraction analysis showed distortions in the Pt lattice parameters for the Pt-Rh alloy structure due to the presence of Rh in the catalyst’s composition. Transmission electron microscopy analyses indicate that nanoparticles were well-dispersed on a carbon support, with spherical shapes and small particle sizes (2–3 nm). In situ X-ray absorption spectroscopy data evidence that Pt–Rh interactions produce changes in the Pt 5d band vacancy. The electronic effect is maximized when Pt forms an alloy with Rh, resulting in the highest d-band vacancy of the Pt3Rh/C. The Pt3Rh/C catalyst showed the highest activity towards ethanol oxidation, presenting current densities in a quasi-steady-state condition (measured at 600 mV) around 5.2 times higher than the commercial Pt/C (Alfa Aesar). Moreover, the onset potential for ethanol oxidation shifts to more negative potentials (110 mV lower taken at 1 mA cm–2) was also observed. In situ FTIR data revealed that Pt/C catalyst favours the formation of acetic acid. The synergistic effect between Rh and the alloy structure results in an easier C–C bond breaking for Pt3Rh/C, in comparison to Pt3–Rh mixture, thus favouring CO2 formation at lower potentials.
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Apr 2019
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B18-Core EXAFS
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Diamond Proposal Number(s):
[10306]
Open Access
Abstract: Comprehensive identification of the phases and atomic configurations of bimetallic nanoparticle catalysts are critical in understanding structure-properties relationships in catalysis. However, control of the structure, whilst retaining the same composition, is challenging. Here, the same carbon supported Pt3Sn catalyst is annealed under air, Ar and H2 resulting in variation of the extent of alloying of the two components. The atmosphere-induced extent of alloying is characterised using a variety of methods including TEM, XRD, XPS, XANES, and EXAFS and is defined as the fraction of Sn present as Sn0 (XPS and XANES) or the ratio of the calculated composition of the bimetallic particle to the nominal composition according to the stoichiometric ratio of the preparation (TEM, XRD, and EXAFS) . The values obtained depend on the structural method used, but the trend air < Ar < H2 annealed samples is consistent. These results are then used to provide insights regarding the electrocatalytic activity of Pt3Sn catalysts for CO, methanol, ethanol, and 1-butanol oxidation and the roles of alloyed Sn and SnO2.
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Apr 2018
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