Detecting hierarchical and overlapping community structures in networks

2014-2015 > Academic research: refereed > Refereed conference paperVersion of RecordPublishe

Surface steps dominate the water formation on Pd(111) surfaces

Water formation is relevant in many technological processes and is also an important model reaction. Although water formation over Pd surfaces is widely studied, questions regarding the active site and the main reaction path (OH* + OH*) or (OH* + H*) are still open. Combining first-principles density functional theory calculations and kinetic Monte Carlo simulations, we find that the reaction rate is dominated by surface steps and point defects over a wide range of conditions. The main reaction path is found to be temperature dependent where the OH* + OH* path dominates at low temperatures, whereas the OH* + H* path is the main path at high temperatures. Steps facilitate the OH* formation, which is the rate limiting step under all conditions. OH* is formed via O* + H* association or OOH* splitting at low temperatures, whereas OH* is exclusively formed via O* + H* association at high temperatures. The results of the first-principles-based kinetic model are in excellent agreement with experimental observations at high and low temperatures as well as different gas-phase compositions

Mitigation of low-frequency current ripple in fuel-cell inverter systems through waveform control

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Decentralized adaptive pinning control for cluster synchronization of complex dynamical networks

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On the Reaction Mechanism of Direct H2O2Formation over Pd Catalysts

Hydrogen peroxide (H2O2) is an effective green oxidant, which is used in many industrial processes. Here, the reaction mechanism for direct formation of H2O2 from H2 and O2 over Pd catalysts is studied using density functional theory calculations and mean-field kinetic modeling. The state of the catalyst as a function of reaction conditions is determined from ab initio thermodynamics. It is found that Pd is in a hydride phase during typical reaction conditions. Reaction landscapes are constructed for the reaction over PdH(111) and PdH(211). Formation of H2O2 instead of H2O requires that O2 adsorbs and that the surface intermediates O2, OOH, and H2O2 do not dissociate. We find that these requirements are fulfilled on the stepped PdH(211) surface. Surface steps are needed for O2 chemisorption as the adsorption on PdH(111) is endothermic. The high H coverage on the surface of the hydride is important to slow down the unwanted scission of the O-O bond and promote the desorption of the products. Comparative calculations for the Pd(111) surface show that this surface is inactive for both H2O2 and H2O formation below room temperature for typical reaction mixtures. Our findings demonstrate the importance of surface steps and high hydrogen coverage for direct synthesis of H2O2 from H2 and O2 over Pd catalysts. The results imply that the selectivity of the reaction toward H2O2 is enhanced by a high partial pressure of H2, which is in agreement with experimental observations

Dirac Leptogenesis with a Non-anomalous U(1)′U(1)^{\prime} Family Symmetry

We propose a model for Dirac leptogenesis based on a non-anomalous $U(1)^{\prime}$ gauged family symmetry. The anomaly cancellation conditions are satisfied with no new chiral fermions other than the three right-handed neutrinos, giving rise to stringent constraints among the charges. Realistic masses and mixing angles are obtained for all fermions. The model predicts neutrinos of the Dirac type with naturally suppressed masses. Dirac leptogenesis is achieved through the decay of the flavon fields. The cascade decays of the vector-like heavy fermions in the Froggatt-Nielsen mechanism play a crucial role in the separation of the primodial lepton numbers. We find that a large region of parameter space of the model gives rise to a sufficient cosmological baryon number asymmetry through Dirac leptogenesis.Comment: 8 pages, 8 figures, version to appear in JHE

Multiple Roles of Alkanethiolate-Ligands in Direct Formation of H2O2 over Pd Nanoparticles

Coadsorbed organic species including thiolates can promote direct synthesis of hydrogen peroxide from H2 and O2 over Pd particles. Here, density functional theory based kinetic modeling, augmented with activity measurements and vibrational spectroscopy are used to provide atomistic understanding of direct H2O2 formation over alkylthiolate(RS) Pd. We find that the RS species are oxidized during reaction conditions yielding RSO2 as the effective ligand. The RSO2 ligand shows superior ability for proton transfer to the intermediate surface species OOH, which accelerates the formation of H2O2. The ligands promote the selectivity also by blocking sites for unselective water formation and by modifying the electronic structure of Pd. The work rationalizes observations of enhanced selectivity of direct H2O2 formation over ligand-funtionalized Pd nanoparticles and shows that engineering of organic surface modifiers can be used to promote desired hydrogen transfer routes

Identifying variants that contribute to linkage for dichotomous and quantitative traits in extended pedigrees

Compared to genome-wide association analysis, linkage analysis is less influenced by allelic heterogeneity. The use of linkage information in large families should provide a great opportunity to identify less frequent variants. We perform a linkage scan for both dichotomous and quantitative traits in eight extended families. For the dichotomous trait, we identified one linkage region on chromosome 4q. For quantitative traits, we identified two regions on chromosomes 4q and 6p for Q1 and one region on chromosome 6q for Q2. To identify variants that contribute to these linkage signals, we performed standard association analysis in genomic regions of interest. We also screened less frequent variants in the linkage region based on the risk ratio and phenotypic distribution among carriers. Two rare variants at VEGFC and one common variant on chromosome 4q conferred the greatest risk for the dichotomous trait. We identified two rare variants on chromosomes 4q (VEGFC) and 6p (VEGFA) that explain 12.4% of the total phenotypic variance of trait Q1. We also identified four variants (including one at VNN3) on chromosome 6q that are able to drop the linkage LOD from 3.7 to 1.0. These results suggest that the use of classical linkage and association methods in large families can provide a useful approach to identifying variants that are responsible for diseases and complex traits in families

A note on Makeev's conjectures

A counterexample is given for the Knaster-like conjecture of Makeev for functions on $S^2$. Some particular cases of another conjecture of Makeev, on inscribing a quadrangle into a smooth simple closed curve, are solved positively

Multiple Ca2+ signaling pathways regulate intracellular Ca 2+ activity in human cardiac fibroblasts

Ca2+ signaling pathways are well studied in cardiac myocytes, but not in cardiac fibroblasts. The aim of the present study is to characterize Ca2+ signaling pathways in cultured human cardiac fibroblasts using confocal scanning microscope and RT-PCR techniques. It was found that spontaneous intracellular Ca2+ (Cai 2+) oscillations were present in about 29% of human cardiac fibroblasts, and the number of cells with Cai 2+ oscillations was increased to 57.3% by application of 3% fetal bovine serum. Cai 2+ oscillations were dependent on Ca2+ entry. Cai2+ oscillations were abolished by the store-operated Ca2+ (SOC) entry channel blocker La3+, the phospholipase C inhibitor U-73122, and the inositol trisphosphate receptors (IP3Rs) inhibitor 2-aminoethoxydiphenyl borate, but not by ryanodine. The IP3R agonist thimerosal enhanced Ca2+ i oscillations. Inhibition of plasma membrane Ca2+ pump (PMCA) and Na+-Ca2+ exchanger (NCX) also suppressed Ca i 2+ oscillations. In addition, the frequency of Ca i 2+ oscillations was reduced by nifedipine, and increased by Bay K8644 in cells with spontaneous Cai 2+ oscillations. RT-PCR revealed that mRNAs for IP3R1-3, SERCA1-3, CaV1.2, NCX3, PMCA1,3,4, TRPC1,3,4,6, STIM1, and Orai1-3, were readily detectable, but not RyRs. Our results demonstrate for the first time that spontaneous Cai 2+ oscillations are present in cultured human cardiac fibroblasts and are regulated by multiple Ca2+ pathways, which are not identical to those of the well-studied contractile cardiomyocytes. This study provides a base for future investigations into how Ca2+ signals regulate biological activity in human cardiac fibroblasts and cardiac remodeling under pathological conditions. © 2009 Wiley-Liss, Inc.postprin