In Situ Anomalous Small-Angle X-ray Scattering Studies of Platinum Nanoparticle Fuel Cell Electrocatalyst Degradation

Polymer electrolyte fuel cells (PEFCs) are a promising high-efficiency energy conversion technology, but their cost-effective implementation, especially for automotive power, has been hindered by degradation of the electrochemically active surface area (ECA) of the Pt nanoparticle electrocatalysts. While numerous studies using ex situ post-mortem techniques have provided insight into the effect of operating conditions on ECA loss, the governing mechanisms and underlying processes are not fully understood. Toward the goal of elucidating the electrocatalyst degradation mechanisms, we have followed Pt nanoparticle growth during potential cycling of the electrocatalyst in an aqueous acidic environment using in situ anomalous small-angle X-ray scattering (ASAXS). ASAXS patterns were analyzed to obtain particle size distributions (PSDs) of the Pt nanoparticle electrocatalysts at periodic intervals during the potential cycling. Oxide coverages reached under the applied potential cycling protocols were both calculated and determined experimentally. Changes in the PSD, mean diameter, and geometric surface area identify the mechanism behind Pt nanoparticle coarsening in an aqueous environment. Over the first 80 potential cycles, the dominant Pt surface area loss mechanism when cycling to 1.0-1.1 V was found to be preferential dissolution or loss of the smallest particles with varying extents of reprecipitation of the dissolved species onto existing particles, resulting in particle growth, depending on potential profile. Correlation of ASAXS-determined particle growth with both calculated and voltammetrically determined oxide coverages demonstrates that the oxide coverage is playing a key role in the dissolution process and in the corresponding growth of the mean Pt nanoparticle size and loss of ECA. This understanding potentially reduces the complex changes in PSD and ECA resulting from various voltage profiles to a response dependent on oxide coverage.     

Thermal Characterization of Gliclazide/β-Cyclodextrin Inclusion Complexes

Gliclazide/β-CD systems were prepared by various techniques: kneading, co-grinding, spray-drying, coprecipitation and neutralization. The thermal behaviour of these systems was studied by using DSC and TG, in order to find occurrence of the occurrence of solid-state inclusion complexation. The use of a complementary technique, Fourier transform infrared spectroscopy (FTIR), was intended to resolve possible ambiguous conclusions from the thermal study. Both studies demonstrated the existence of inclusion complexation in all cases except the kneading technique, which does not lead to a true inclusion complex.     

Uptake and effect of mercury on amino acid losses from the gills of the bivalve mollusks Mytilus californianus and Anodonta californiensis

Inorganic mercury (Hg(2+)) and herbicides are important contaminants of world water systems with effects on aquatic organisms and humans. The uptake of Hg(2+) and glycine by the gills of the bivalve mollusks Mytilus californianus and Anodonta californiensis was determined. Additionally, the effects of glycine, 2,4-dichlorophenoxyacetic acid (2,4-D), and 2,4-dinitrophenol (DNP) on the uptake of Hg(2+) were also determined. The loss of primary amines from the excised gills of both species was measured in the presence and absence of Hg(2+) or MeHg(+). The results indicate that (1) the uptake of Hg(2+) is approximately equivalent in both species; (2) comparison of the uptakes with that of inulin, which occupies only extracellular space, shows that Hg(2+) is taken up; (3) the uptake of Hg(2+) is slightly altered by the presence of glycine and herbicides such as 2,4-D and DNP; (4) the rate of loss of primary amines was highly increased relative to the control by the presence of Hg(2+) and to a lesser extent MeHg(+) for both species. These results showed that both inorganic and MeHg(+) are effective in disrupting the permeability of cell membranes, causing leakage of essential amino acids from the cell. This could result in discharge of potential gradients, reduced efficiency of energy coupling, and consequently cell death.     

Tuning the work function of ultrathin oxide films on metals by adsorption of alkali atoms

We report a theoretical investigation of the adsorption of alkali metal atoms deposited on ultrathin oxide films. The properties of Li, Na, and K atoms adsorbed on SiO(2)/Mo(112) and of K on MgO / Ag(100) and TiO(2)/Pt(111) have been analyzed with particular attention to the induced changes in the work function of the system, Phi. On the nonreducible SiO(2) and MgO oxide films there is a net transfer of the outer ns electron of the alkali atom to the metal substrate conduction band; the resulting surface dipole substantially lowers Phi. The change in Phi depends (a) on the adsorption site (above the oxide film or at the interface) and (b) on the alkali metal coverage. Deposition of K on reducible TiO(2) oxide films results in adsorbed K(+) ions and in the formation of Ti(3+) ions. No charge transfer to the metal substrate is observed but also in this case the surface dipole resulting from the K-TiO(2) charge transfer has the effect to considerably reduce the work function of the system.     

Characterization of green copper phase pigments in Egyptian artifacts with X-ray absorption spectroscopy and principal components analysis

The characterization of materials in historical artifacts can contribute significantly to their preservation and understanding; however, sampling and characterization are ideally performed using non-destructive approaches. The analysis of green pigments from Egyptian artifacts presents a further challenge as responses to laboratory based techniques have proven unsuccessful in many cases. An alternative approach is the use of non-destructive X-ray absorption near-edge structure spectroscopy, which was performed on a reference set of copper-containing green minerals and other compounds. Data projection using principal component analysis was used to explore the spectral data structures and to illustrate the relationship between the spectra and copper speciation, resulting in a calibration or training set of the reference materials used. Data from the training set were compared with samples from Egyptian artifacts. The combination of X-ray absorption spectroscopy with principal components analysis provides a novel approach in archeometry and the characterization of objects of cultural heritage.     

Solid-state NMR study of the charge-transfer complex between ubiquinone-8 and disulfide bond generating membrane protein DsbB

Ubiquinone (Coenzyme Q) plays an important role in the mitochondrial respiratory chain and also acts as an antioxidant in its reduced form, protecting cellular membranes from peroxidation. De novo disulfide bond generation in the E. coli periplasm involves a transient complex consisting of DsbA, DsbB, and ubiquinone (UQ). It is hypothesized that a charge-transfer complex intermediate is formed between the UQ ring and the DsbB-C44 thiolate during the reoxidation of DsbA, which gives a distinctive ~500 nm absorbance band. No enzymological precedent exists for an UQ-protein thiolate charge-transfer complex, and definitive evidence of this unique reaction pathway for DsbB has not been fully demonstrated. In order to study the UQ-8-DsbB complex in the presence of native lipids, we have prepared isotopically labeled samples of precipitated DsbB (WT and C41S) with endogenous UQ-8 and lipids, and we have applied advanced multidimensional solid-state NMR methods. Double-quantum filter and dipolar dephasing experiments facilitated assignments of UQ isoprenoid chain resonances not previously observed and headgroup sites important for the characterization of the UQ redox states: methyls (~20 ppm), methoxys (~60 ppm), olefin carbons (120-140 ppm), and carbonyls (150-160 ppm). Upon increasing the DsbB(C41S) pH from 5.5 to 8.0, we observed a 10.8 ppm upfield shift for the UQ C1 and C4 carbonyls indicating an increase of electron density on the carbonyls. This observation is consistent with the deprotonation of the DsbB-C44 thiolate at pH 8.0 and provides direct evidence of the charge-transfer complex formation. A similar trend was noted for the UQ chemical shifts of the DsbA(C33S)-DsbB(WT) heterodimer, confirming that the charge-transfer complex is unperturbed by the DsbB(C41S) mutant used to mimic the intermediate state of the disulfide bond generating reaction pathway.     

Synthesis of novel proton‐conducting highly sulfonated polybenzimidazoles for PEMFC and the effect of the type of bisphenyl bridge on polymer and membrane properties

Six series of novel highly sulfonated polybenzimidazoles (sPBIs) with high molecular weight were prepared by direct polycondensation between 3,3′‐diaminobenzidine and original multisulfonated dinuclear dicarboxylic acids containing bridging ether, sulfone, and hexafluoroisopropylidene moieties. All reactions were carried out in polyphosphoric acid, which acts as both solvent and catalyst. The degree of sulfonation was modulated in the final products by varying the proportion of sulfonated to nonsulfonated dicarboxylic acids used in the synthesis. The high purity of the disulfonated and tetrasulfonated monomers allows wholly sulfonated homopolymers to be obtained. Confirmation of the chemical structure and the degree of sulfonation were derived from ¹H nuclear magnetic resonance spectroscopy. Inherent viscosity was estimated as between 0.70 and 5.33 dL g^?1 for sPBIs with ion exchange capacity in the range 0.87–4.68 mequiv g^?1. Dynamic thermogravimetric analysis in air showed no weight loss below 350 °C (heating rate 5 °C min^?1). The nature of the bisphenyl bridge has clear influence on the water uptake and proton conduction properties of the resulting sPBI membranes, with hexafluoroisopropylidene links providing materials of highest conductivity as well as favoring film‐forming characteristics. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

X-Ray absorption spectroscopy investigation of 1-alkyl-3-methylimidazolium bromide salts

X-ray absorption spectroscopy (XAS) has been used to unveil the bromide ion local coordination structure in 1-alkyl-3-methylimidazolium bromide [C(n)mim]Br ionic liquids (ILs) with different alkyl chains. The XAS spectrum of 1-ethyl-3-methylimidazolium bromide has been found to be different from those of the other members of the series, from the butyl to the decyl derivatives, that have all identical XAS spectra. This result indicates that starting from 1-buthyl-3-methylimidazolium bromide the local molecular arrangement around the bromide anion is the same independently from the length of the alkyl chain, and that the imidazolium head groups in the liquid ILs with long alkyl chains assume locally the same orientation as in the [C(4)mim]Br crystal. With this study we show that the XAS technique is an effective direct tool for unveiling the local structural arrangements around selected atoms in ILs.     

Full groups of Cantor minimal systems

We associate different types of full groups to Cantor minimal systems. We show how these various groups (as abstract groups) are complete invariants for orbit equivalence, strong orbit equivalence and flip conjugacy, respectively. Furthermore, we introduce a group homomorphism, the socalled mod map, from the normalizers of the various full groups to the automorphism groups of the (ordered)K ⁰-groups, which are associated to the Cantor minimal systems. We show how this in turn is related to the automorphisms of the associatedC ^*-crossed products. Our results are analogues in the topological dynamical setting of results obtained by Dye, Connes-Krieger and Hamachi-Osikawa in measurable dynamics. Research supported in part by operating grants from NSERC (Canada). Research supported in part by the Norwegian Research Council for Science and Humanities.     

Elucidating the Glucokinase Activating Potentials of Naturally Occurring Prenylated Flavonoids: An Explicit Computational Approach

Glucokinase activators are considered as new therapeutic arsenals that bind to the allosteric activator sites of glucokinase enzymes, thereby maximizing its catalytic rate and increasing its affinity to glucose. This study was designed to identify potent glucokinase activators from prenylated flavonoids isolated from medicinal plants using molecular docking, molecular dynamics simulation, density functional theory, and ADMET analysis. Virtual screening was carried out on glucokinase enzymes using 221 naturally occurring prenylated flavonoids, followed by molecular dynamics simulation (100 ns), density functional theory (B3LYP model), and ADMET (admeSar 2 online server) studies. The result obtained from the virtual screening with the glucokinase revealed arcommunol B (−10.1 kcal/mol), kuwanon S (−9.6 kcal/mol), manuifolin H (−9.5 kcal/mol), and kuwanon F (−9.4 kcal/mol) as the top-ranked molecules. Additionally, the molecular dynamics simulation and MM/GBSA calculations showed that the hit molecules were stable at the active site of the glucokinase enzyme. Furthermore, the DFT and ADMET studies revealed the hit molecules as potential glucokinase activators and drug-like candidates. Our findings suggested further evaluation of the top-ranked prenylated flavonoids for their in vitro and in vivo glucokinase activating potentials.