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Ultrasmall and Highly Dispersed Pt Entities Deposited on Mesoporous N?doped Carbon Nanospheres by Pulsed CVD for Improved HER
No full textVapor based deposition techniques are emerging approaches for the design of carbon supported metal powder electrocatalysts with tailored catalyst entities, sizes, and dispersions. Herein, a pulsed CVD Pt pCVD approach is employed to deposit different Pt entities on mesoporous N doped carbon MPNC nanospheres to design high performance hydrogen evolution reaction HER electrocatalysts. The influence of consecutive precursor pulse number 50 250 and deposition temperature 225 300 C are investigated. The Pt pCVD process results in highly dispersed ultrasmall Pt clusters amp; 8776;1 nm in size and Pt single atoms, while under certain conditions few larger Pt nanoparticles are formed. The best MPNC Pt pCVD electrocatalyst prepared in this work 250 pulses, 250 C reveals a Pt HER mass activity of 22.2 1.2 A mg amp; 8722;1Pt at 50 mV versus the reversible hydrogen electrode RHE , thereby outperforming a commercially available Pt C electrocatalyst by 40 as a result of the increased Pt utilization. Remarkably, after optimization of the Pt electrode loading, an ultrahigh Pt mass activity of 56 2 A mg amp; 8722;1Pt at 50 mV versus RHE is found, which is among the highest Pt mass activities of Pt single atom and cluster based electrocatalysts reported so fa
Cooperative Sulfur Transformations at a Dinickel Site A Metal Bridging Sulfur Radical and Its H Atom Abstraction Thermochemistry
No full textStarting from the dinickel II dihydride complex [ML Ni H 2] 1M , where L3 is a bis tridentate pyrazolate bridged bis amp; 946; diketiminato ligand and M is Na or K , a series of complexes [KLNi2 S2 ] 2K , [MLNi2S] 3M , [LNi2 SMe ] 4 , and [LNi2 SH ] 5 has been prepared. The amp; 956; sulfido complexes 3M can be reversibly oxidized at E1 2 amp; 8722;1.17 V in THF; vs Fc Fc to give [LNi2 S ] 6 featuring a bridging S radical. 6 has been comprehensively characterized, including by X ray diffraction, SQUID magnetometry, EPR and XAS XES spectroscopies, and DFT calculations. The pKa of the amp; 956; hydrosulfido complex 5 in THF is 30.8 0.4, which defines a S H bond dissociation free energy BDFE of 75.1 1.0 kcal mol 1. 6 reacts with H atom donors such as TEMPO H and xanthene to give 5, while 5 reacts with 2,4,6 tri tert butyl phenoxy radical in a reverse H atom transfer to generate 6. These findings provide the first full characterization of a genuine M amp; 956; S M complex and provide insights into its proton coupled electron transfer PCET reactivity, which is of interest in view of the prominence of M amp; 956; SH amp; 956; S M units in biological systems and heterogeneous catalysi
Chemical bonding effects in Sc compounds studied using X ray absorption and X ray photoelectron spectroscopies
No full textAdvances on understanding the nature of the chemical bonding and electron correlation effects during the X ray absorption process in ionic covalent metal complexes has been achieved for most of the transition elements, except for scandium, due to the lack of a systematic series of spectroscopic reference spectra and the shortage of standard crystallographic data on scandium compounds. To close the gap, the chemical bonding effects in eight Sc compounds are studied using X ray absorption spectroscopy XAS at Sc K and L2,3 absorption edges and X ray photoelectron spectroscopy XPS . Indeed, the fine structure of the XAS Sc K edge reflects the chemical sp3 like bond formed between scandium and the ligand while the L2,3 edge and the pre edge features of the K edge provide a direct insight into the crystal field parameters at the Sc site in the coordination compound. The XPS data provide the information on binding energies of the core electrons involved in the electron transitions caused by the absorption of high energy X rays. XAS and XPS complement each other by accessing the information on Sc structure on bulk and the surface. Herein, comprehensive information on the electronic structure of well known crystalline materials based on Sc is given with spectroscopic fingerprints X ray data. This will help to predict the formation of chemical bonds in the unknown components via the systematic evaluation of the available spectroscopic fingerprint
The Tetrapyrollic Motif in Nitrogen Doped Carbons and M N C Electrocatalysts as Active Site in the Outer Sphere Mechanism of the Alkaline Oxygen Reduction Reaction
No full textDevelopment and fundamental understanding of precious group metal free electrocatalysts is hampered by limitations in the quantification of the intrinsic activity of different catalytic sites and understanding the different reaction mechanisms. Comparing isomorphic nitrogen doped carbons, Zn N Cs and Fe N Cs with the common tetrapyrrolic motif, a catalyst independent outer sphere rate determining step in the alkaline oxygen reduction reaction is observed. Density functional theory DFT simulations on tetrapyrrolic model structures indicate the highest occupied molecular orbital HOMO level as a good descriptor for the catalytic activity. Contour plots suggest that the electron transfer occurs directly from the tetrapyrrolic coordination site, rather than from the metal center. Metal free tetrapyrrolic N4 sites are discovered to be highly active oxygen reduction reaction ORR active sites in alkaline that reach turnover frequencies TOF of 0.33 and 1.84 s amp; 8722;1 at 0.80 and 0.75 VRHE in the order of magnitude of tetrapyrrolic Fe N4 sites in the acidic ORR. While Zn coordination lowers the HOMO level and therefore the catalytic activity, Fe coordination lifts the HOMO level resulting in TOF values of 0.4 and 4 s amp; 8722;1 for tetrapyrrolic Fe N4 sites at 0.90 and 0.85 VRHE, respectively. At higher mass activities, the peroxide reduction becomes rate limiting, where highest peroxide production rates are observed for the nitrogen doped carbo
Bacteria employ lysine acetylation of transcriptional regulators to adapt gene expression to cellular metabolism
No full textThe Escherichia coli TetR related transcriptional regulator RutR is involved in the coordination of pyrimidine and purine metabolism. Here we report that lysine acetylation modulates RutR function. Applying the genetic code expansion concept, we produced site specifically lysine acetylated RutR proteins. The crystal structure of lysine acetylated RutR reveals how acetylation switches off RutR DNA binding. We apply the genetic code expansion concept in E. coli in vivo revealing the consequences of RutR acetylation on the transcriptional level. We propose a model in which RutR acetylation follows different kinetic profiles either reacting non enzymatically with acetyl phosphate or enzymatically catalysed by the lysine acetyltransferases PatZ YfiQ and YiaC. The NAD dependent sirtuin deacetylase CobB reverses enzymatic and non enzymatic acetylation of RutR playing a dual regulatory and detoxifying role. By detecting cellular acetyl CoA, NAD and acetyl phosphate, bacteria apply lysine acetylation of transcriptional regulators to sense the cellular metabolic state directly adjusting gene expression to changing environmental condition
Growth of a Tessellation Geometric rules for the Development of Stingray Skeletal Patterns
No full textThe skeletons of sharks and rays, fashioned from cartilage, and armored by a veneer of mineralized tiles tesserae present a mathematical challenge How can the continuous covering be maintained as the skeleton expands? This study, using microCT and custom visual data analyses of growing stingray skeletons, systematically examines tessellation patterns and morphologies of the many thousand interacting tesserae covering the hyomandibula a skeletal element critical to feeding , over a two fold developmental change in hyomandibula length. The number of tesserae remains surprisingly constant, even as the hyomandibula expands isometrically, with all hyomandibulae displaying self similar distributions of tesserae shapes sizes. Although the distribution of tesserae geometries largely agrees with the rules for polyhedra tiling of complex surfaces dominated by hexagons and a minor fraction of pentagons and heptagons, but very few other polygons the agreement with Euler s classic mathematical laws is not perfect. Contrary to the assumed uniform growth rate which is shown would create geometric incompatibilities , larger tesserae grow faster to accommodate skeletal expansion. It is hypothesized that this local regulation of global system complexity is driven by tension from cartilaginous core expansion in the fibers connecting tesserae, with strain responsive cells orchestrating local mineral appositio
Unveiling the Electro Chemo Mechanical Failure Mechanism of Sodium Metal Anodes in Sodium Oxygen Batteries by Synchrotron X Ray Computed Tomography
No full textRechargeable sodium oxygen batteries NaOBs are receiving extensive research interests because of their advantages such as ultrahigh energy density and cost efficiency. However, the severe failure of Na metal anodes has impeded the commercial development of NaOBs. Herein, combining in situ synchrotron X ray computed tomography SXCT and other complementary characterizations, a novel electro chemo mechanical failure mechanism of sodium metal anode in NaOBs is elucidated. It is visually showcased that the Na metal anodes involve a three stage decay evolution of a porous Na reactive interphase layer NRIL from the initially dot shaped voids evolved into the spindle shaped voids and the eventually developed ruptured cracks. The initiation of this three stage evolution begins with chemical resting and is exacerbated by further electrochemical cycling. From corrosion science and fracture mechanics, theoretical simulations suggest that the evolution of porous NRIL is driven by the concentrated stress at crack tips. The findings illustrate the importance of preventing electro chemo mechanical degradation of Na anodes in practically rechargeable NaOB
Elucidating the Complex Oxidation Behavior of Aqueous H3PO3 on Pt Electrodes via In Situ Tender X ray Absorption Near Edge Structure Spectroscopy at the P K Edge
Get PDFIn situ tender X ray absorption near edge structure XANES spectroscopy at the P K edge was utilized to investigate the oxidation mechanism of aqueous H3PO3 on Pt electrodes under various conditions relevant to high temperature polymer electrolyte membrane fuel cell HT PEMFC applications. XANES and electrochemical analysis were conducted under different tender X ray irradiation doses, revealing that intense radiation induces the oxidation of aqueous H3PO3 via H2O yielding H3PO4 and H2. A broadly applicable experimental procedure was successfully developed to suppress these undesirable radiation induced effects, enabling a more accurate determination of the aqueous H3PO3 oxidation mechanism. In situ XANES studies of aqueous 5 mol dm 3 H3PO3 on electrodes with varying Pt availability and surface roughness reveal that Pt catalyzes the oxidation of aqueous H3PO3 to H3PO4. This oxidation is enhanced upon applying a positive potential to the Pt electrode or raising the electrolyte temperature, the latter being corroborated by complementary ion exchange chromatography measurements. Notably, all of these oxidation processes involve reactions with H2O, as further supported by XANES measurements of aqueous H3PO3 of different concentrations, showing a more pronounced oxidation in electrolytes with a higher H2O content. The significant role of water in the oxidation of H3PO3 to H3PO4 supports the reaction mechanisms proposed for various chemical processes observed in this work and provides valuable insights into potential strategies to mitigate Pt catalyst poisoning by H3PO3 during HT PEMFC operatio
Disorder induced band gap lowering in kesterite type Cu2ZnSnSe4 and Ag2ZnSnSe4 A first principles and special quasirandom structures investigations
Get PDFQuaternary chalcogenides, i.e. Cu2ZnSnS4, crystallising in the kesterite crystal structure have already been demonstrated as potential building blocks of thin film solar cells, containing only abundant elements and exhibiting power conversion efficiencies of about 13 so far. However, a correct determination of their ground state crystal structure has been hindered in the past by two structurally similar polymorphs, namely the kesterite and the stannite crystal structure. Additional complications arose from the later identified Cu Zn disorder, present in virtually all thin film samples. Subsequently, is has been shown experimentally that this unavoidable Cu Zn disorder leads to a band gap lowering in the respective samples. Additional theoretical investigations, mostly based on Monte Carlo methods, tried to understand the atomistic origin of this disorder induced band gap lowering. Here, we present theoretical results from first principles calculations based on density functional theory for the disorder induced band gap lowering in kesterite Cu2ZnSnSe4 and Ag2ZnSnSe4, where the Cu Zn disorder is modelled via a supercell approach and special quasirandom structures. Results of subsequent analyses of structural, electronic, and optical properties will be discussed with respect to available experimental results, and will provide additional insight and knowledge towards the atomistic origin of the observed disorder induced band gap lowering in kesterite type material
Orientational anisotropy due to molecular field splitting in sulfur 2p photoemission from CS2 and SF6 theoretical treatment and application to photoelectron recoil
No full textPhotoelectron recoil strongly modifies the high kinetic energy photoemission spectra from atoms and molecules as well as from surface structures. In most cases studied so far, photoemission from atomic like inner shell or core orbitals has been assumed to be isotropic in the molecular frame of reference. However, in the presence of molecular field splitting of p or d orbitals, this assumption is not justified per se. We present a general theoretical treatment, linking the orientational distribution of gas phase molecules to the electron emission and detection in a certain direction in the laboratory frame. The approach is then applied to the S 2p photoemission from a linear molecule such as CS2 and we investigate, how the predicted orientational anisotropies due to molecular field splitting affect the photoelectron recoil excitations. Lastly, experimental S 2p high kinetic energy photoelectron spectra of SF6 and CS2 are analyzed using the modeled recoil lineshapes representing the anisotropy affected recoil effect