B18-Core EXAFS
|
Diamond Proposal Number(s):
[28095]
Open Access
Abstract: Prussian blue is widely utilized as a cathode material in batteries, due to its ability to intercalate alkaline metal ions, including potassium. However, the exact location of potassium or other cations within the complex structure, and how it changes as a function of cycling, is unclear. Herein, we report direct insight into the nature of potassium speciation within Prussian blue during cyclic voltammetry, via operando potassium K-edge X-ray Absorption Near Edge Structure (XANES) analysis. Clear and identifiable spectra are experimentally differentiated for the fully intercalated (fully reduced Fe2+FeII Prussian white), partially intercalated (Prussian blue; Fe3+FeII), and free KNO3(aq) electrolyte. Comparison of the experiment with simulated XANES of theoretical structures indicates that potassium lies within the channels of the Prussian blue structure, but is displaced towards the periphery of the channels by occluded water and/or structural water present resulting from [Fe(CN)6]4− vacancies. The structural composition from the charge carrier perspective was monitored for two samples of differing crystallite size and electrochemical stability. Reproducible potassium XANES spectral sequences were observed for large crystallites (ca. 100 nm) of Prussian blue, in agreement with retention of capacity; in contrast, the capacity of a sample with small crystallites (ca. 14 nm) declined as the potassium became trapped within the partially intercalated Prussian blue. The cause of degradation could be attributed to a significant loss of [Fe(CN)6]–[Fe(NC)6] ordering and the formation of a potassium-free non-conducting ferrihydrite phase. These findings demonstrate the potential of XANES to directly study the nature and evolution of potassium species during an electrochemical process.
|
Jul 2023
|
|
B18-Core EXAFS
|
Ricardo
Navar
,
Giulia
Tarantino
,
Owain T.
Beynon
,
Daniele
Padovan
,
Luca
Botti
,
Emma K.
Gibson
,
Peter P.
Wells
,
Alun
Owens
,
Simon
Kondrat
,
Andrew J.
Logsdail
,
Ceri
Hammond
Diamond Proposal Number(s):
[12597]
Open Access
Abstract: Sn-Beta has emerged as a state-of-the-art catalyst for a range of sustainable chemical transformations. Conventionally prepared by bottom-up hydrothermal synthesis methods, recent research has demonstrated the efficiency of several top-down methods of preparation. One attractive top-down approach is Solid-State Incorporation, where a dealuminated Beta zeolite is physically mixed with a solid Sn precursor, in particular Sn (II) acetate, prior to heat treatment at 550 °C. This procedure is fast and benign, and metal incorporation requires no solvents and hence produces no aqueous Sn-containing waste streams. Although the performances of these catalysts have been well explored in recent years, the mechanism of heteroatom incorporation remains unknown, and hence, opportunities to improve the synthetic procedure via a molecular approach remain. Herein, we utilise a range of in situ spectroscopic techniques, alongside kinetic and computational methods, to elucidate the mechanisms that occur during preparation of the catalyst, and then improve the efficacy of the synthetic protocol. Specifically, we find that successful incorporation of Sn into the lattice occurs in several distinct steps, including i) preliminary coordination of the metal ion to the vacant lattice sites of the zeolite during physical grinding; ii) partial incorporation of the metal ion into the zeolite framework upon selective decomposition of the acetate ligands, which occurs upon heating the physical mixture in an inert gas flow from room temperature to 550 °C; and iii) full isomorphous substitution of Sn into the lattice alongside its simultaneous oxidation to Lewis acidic Sn(IV), when the physically mixed material is exposed to air during a short (<1 h) isotherm period. Long isotherm steps are shown to be unnecessary, and fully oxidised Sn(IV) precursors are shown to be unsuitable for successful incorporation into the lattice. We also find that the formation of extra-framework Sn oxides is primarily dependent on the quantity of Sn present in the initial physical mixture. Based on these findings, we demonstrate a faster synthetic protocol for the preparation of Sn-Beta materials via Solid-State Incorporation, and benchmark their performance of the catalyst for the Meerwein-Ponndorf-Verley transfer hydrogenation reaction and for the isomerisation of glucose to fructose.
|
Sep 2022
|
|
E01-JEM ARM 200CF
|
Naomi
Lawes
,
Isla
Gow
,
Louise R.
Smith
,
Kieran
Aggett
,
James
Hayward
,
Lara
Kabalan
,
Andrew J.
Logsdail
,
Thomas J. A.
Slater
,
Malcolm
Dearg
,
David J.
Morgan
,
Nicholas F.
Dummer
,
Stuart H.
Taylor
,
C. Richard A.
Catlow
,
Michael
Bowker
,
Graham J.
Hutchings
Diamond Proposal Number(s):
[27530]
Open Access
Abstract: A number of Pd based materials have been synthesised and evaluated as catalysts for the conversion of carbon dioxide and hydrogen to methanol, a useful platform chemical and hydrogen storage molecule. Monometallic Pd catalysts shows poor methanol selectivity, but this is improved through the formation of Pd alloys, with both PdZn and PdGa alloys showing greatly enhanced methanol productivity compared with monometallic Pd/Al2O3 and Pd/TiO2 catalysts. Catalyst characterisation shows that the 1:1 β-PdZn alloy is present in all Zn containing post-reaction samples, including PdZn/Ga2O3, while the Pd2Ga alloy formed for the Pd/Ga2O3 sample. The heats of mixing were calculated for a variety of alloy compositions with high heats of mixing calculated for both PdZn and Pd2Ga alloys, with values of ca. -0.6 eV/atom and ca. -0.8 eV/atom, respectively. However, ZnO is more readily reduced than Ga2O3, providing a possible explanation for the preferential formation of the PdZn alloy, rather than PdGa. when in the presence of Ga2O3.
|
Jun 2022
|
|
E01-JEM ARM 200CF
|
Michael
Bowker
,
Naomi
Lawes
,
Isla
Gow
,
James
Hayward
,
Jonathan
Ruiz Esquius
,
Nia
Richards
,
Louise R.
Smith
,
Thomas J. A.
Slater
,
Thomas E.
Davies
,
Nicholas F.
Dummer
,
Lara
Kabalan
,
Andrew
Logsdail
,
Richard C.
Catlow
,
Stuart
Taylor
,
Graham J
Hutchings
Diamond Proposal Number(s):
[27530]
Open Access
Abstract: The rise in atmospheric CO2 concentration and the concomitant rise in global surface temperature have prompted massive research effort in designing catalytic routes to utilize CO2 as a feedstock. Prime among these is the hydrogenation of CO2 to make methanol, which is a key commodity chemical intermediate, a hydrogen storage molecule, and a possible future fuel for transport sectors that cannot be electrified. Pd/ZnO has been identified as an effective candidate as a catalyst for this reaction, yet there has been no attempt to gain a fundamental understanding of how this catalyst works and more importantly to establish specific design criteria for CO2 hydrogenation catalysts. Here, we show that Pd/ZnO catalysts have the same metal particle composition, irrespective of the different synthesis procedures and types of ZnO used here. We demonstrate that all of these Pd/ZnO catalysts exhibit the same activity trend. In all cases, the β-PdZn 1:1 alloy is produced and dictates the catalysis. This conclusion is further supported by the relationship between conversion and selectivity and their small variation with ZnO surface area in the range 6–80 m2g–1. Without alloying with Zn, Pd is a reverse water-gas shift catalyst and when supported on alumina and silica is much less active for CO2 conversion to methanol than on ZnO. Our approach is applicable to the discovery and design of improved catalysts for CO2 hydrogenation and will aid future catalyst discovery.
|
Apr 2022
|
|
|
Open Access
Abstract: Diamond-based materials have unique properties that are exploited in many electrochemical, optical, thermal, and quantum applications. When grown via chemical vapor deposition (CVD), the growth rate of the (110) face is typically much faster than the other two dominant crystallographic orientations, (111) and (100). As such, achieving sufficiently large-area and high-quality (110)-oriented crystals is challenging and typically requires post-growth processing of the surface. Whilst CVD growth confers hydrogen terminations on the diamond surface, the majority of post-growth processing procedures render the surface oxygen-terminated, which in turn impacts the surface properties of the material. Here, we determine the oxygenation state of the (110) surface using a combination of density functional theory calculations and X-ray photoelectron spectroscopy experiments. We show that in the 0–1000 K temperature range, the phase diagram of the (110) surface is dominated by a highly stable phase of coexisting and adjacent carbonyl and ether groups, while the stability of peroxide groups increases at low temperatures and high pressures. We propose a mechanism for the formation of the hybrid carbonyl-ether phase and rationalize its high stability. We further corroborate our findings by comparing simulated core-level binding energies with experimental X-ray photoelectron spectroscopy data on the highest-quality (110)-oriented diamond crystal surface reported to date.
|
Jan 2022
|
|
I11-High Resolution Powder Diffraction
|
Diamond Proposal Number(s):
[20894]
Abstract: A new polymorph of L‐tryptophan has been prepared by crystallization from the gas phase, with structure determination carried out directly from powder XRD data augmented by periodic DFT‐D calculations. The new polymorph (denoted β) and the previously reported polymorph (denoted α) are both based on alternating hydrophilic and hydrophobic layers, but with substantially different hydrogen‐bonding arrangements. The β polymorph exhibits the energetically favourable L2‐L2 hydrogen‐bonding arrangement, which is unprecedented for amino acids with aromatic side‐chains; the specific molecular conformations adopted in the β polymorph facilitate this hydrogen‐bonding scheme while avoiding steric conflict of the side‐chains.
|
Oct 2019
|
|
Theoretical Physics
|
Open Access
Abstract: The source of n-type conductivity in undoped transparent conducting oxides has been a topic of debate for several decades. The point defect of most interest in this respect is the oxygen vacancy, but there are many conflicting reports on the shallow versus deep nature of its related electronic states. Here, using a hybrid quantum mechanical/molecular mechanical embedded cluster approach, we have computed formation and ionization energies of oxygen vacancies in three representative transparent conducting oxides: In2O3,SnO2, and ZnO. We find that, in all three systems, oxygen vacancies form well-localized, compact donors. We demonstrate, however, that such compactness does not preclude the possibility of these states being shallow in nature, by considering the energetic balance between the vacancy binding electrons that are in localized orbitals or in effective-mass-like diffuse orbitals. Our results show that, thermodynamically, oxygen vacancies in bulk In2O3 introduce states above the conduction band minimum that contribute significantly to the observed conductivity properties of undoped samples. For ZnO and SnO2, the states are deep, and our calculated ionization energies agree well with thermochemical and optical experiments. Our computed equilibrium defect and carrier concentrations, however, demonstrate that these deep states may nevertheless lead to significant intrinsic n-type conductivity under reducing conditions at elevated temperatures. Our study indicates the importance of oxygen vacancies in relation to intrinsic carrier concentrations not only in In2O3, but also in SnO2 and ZnO.
|
May 2018
|
|
Theoretical Physics
|
Open Access
Abstract: Density functional theory calculations have been performed to investigate the use of CO as a probe molecule for the determination of the structure and composition of Au, Ag and AuAg nanoparticles. For very small nanoclusters (x = 1 − 5), the CO vibrational frequencies can be directly correlated to CO adsorption strength, whereas larger 147-atom nanoparticles show a strong energetic preference for CO adsorption at a vertex position but the highest wavenumbers are for the bridge positions. We also studied CO adsorption on Au and Ag (100) and (111) surfaces, for a 1 monolayer coverage, which proves to be energetically favourable on atop only and bridge positions for Au (100) and atop for Ag (100); vibrational frequencies of the CO molecules red-shift to lower wavenumbers as a result of increased metal coordination. We conclude that CO vibrational frequencies cannot be solely relied upon in order to obtain accurate compositional analysis, but we do propose that elemental rearrangement in the core@shell nanoclusters, from Ag@Au (or Au@Ag) to an alloy, would result in a shift in the CO vibrational frequencies that indicate changes in the surface composition.
|
Feb 2018
|
|
|
Open Access
Abstract: We perform a systematic investigation of (100) surfaces for rocksalt-structured group 2 metal oxides, namely MgO, CaO, SrO and BaO, using GGA and Hybrid-DFT exchange-correlation functionals. We examine the structural, energetic and electronic properties of the surfaces, with a specific focus on the surface ionisation potential and band bending; the latter of which we quantify by examining the density of states as a function of depth from the system surface. We report structural and energetic results in-line with previous experimental work when we use the Hybrid-DFT method, and for the electronic structure we find inequivalent band bending for the valence and conduction bands, which results in reduced ionisation potentials and the closure of the band gap at the surface when compared to bulk systems. We also report downward bending of the conduction band for MgO that brings it below the vacuum potential, unlike previous theoretical investigations, and thus indicates an origin of the positive electron affinity found in the experiment.
|
Dec 2015
|
|
|
Abstract: High calcination temperatures are often required in nanoparticle synthesis and an understanding of how heating affects the structure and chemical arrangement of bimetallic nanoclusters is essential to design efficient fabrication processes. We have investigated the kinetic stability of 147-atom Au–Ag nanoalloys with varying composition and chemical ordering using ensemble molecular dynamics simulations to replicate these high temperature conditions. Ag-rich mixed alloys undergo a diffusion-less “martensitic” structural transition from cuboctahedral to icosahedral; the melting temperature (Tm) of the subsequent icosahedra is dependent on the Au:Ag stoichiometry. Core@shell chemical arrangements do not behave in a similar manner: Tm strongly depends on the shell component; additionally Au55@Ag92 exhibits increased stability as a result of its icosahedral Au core. We also report a novel dependence of nanocluster phase transitions on the chemical arrangement, as shown by low temperature nonmartensitic atomic diffusion of Ag atoms to the surface for Ag55@Au92. This finding establishes why Ag@Au chemical arrangements, in particular, are difficult to maintain experimentally. Overall, the kinetic phenomena observed help to explain why particular morphologies and chemical arrangements are more abundant in experimental synthesis and how postprocessing affects structure and chemical arrangement.
|
Oct 2015
|
|