Periodic solutions for perturbed differential equations

A variant of the Alekseev variation of constants integral equation is obtained relating the solutions of systems of the form $\text{x}\text{?}\text{= f(t, x, λ)}$ and $\text{y}\text{?}\text{= f(t, y, ψ(t, y)) + g(t, y)}$. For the case when f, g, and ψ have period P in t several theorems are given for the existence of periodic solutions extending known results when f is linear in x and does not depend on the parameter m-vector λ. Comparison with an older technique gives hypotheses where the method above is advantageous for establishing periodic solutions. An example is given for constructing limit cycles of autonomous second-order systems.     

Development of anodes for direct electrocatalytic oxidation of methane in solid oxide fuel cells

Gold-doped nickel/zirconia and nickel/ceria cermet anodes incorporating different levels of gold have been prepared and studied as potential anodes for the direct electrocatalytic oxidation of methane in solid oxide fuel cells. The methane conversion activity and selectivity towards synthesis gas products of these anodes have been determined over a range of SOFC operating temperatures and methane/oxidant ratios, and compared with the corresponding undoped nickel cermet. The amount of carbon deposition has been determined using post-reaction temperature programmed oxidation. The influence of gold loading on the methane conversion activity, product selectivity and amount of carbon deposition has been determined. The addition of small amounts of gold to nickel cermets results in a dramatically increased tolerance to carbon deposition and a reduction in the activity of the anode and the selectivity towards partial oxidation compared to the corresponding undoped nickel cermet. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Pattern formation during the oxidation of CO on Pt{100}: a mesoscopic model

Constantly changing irregular patterns of carbon monoxide (CO) and oxygen are seen during CO oxidation on platinum crystals in the [100] orientation. Ours is the first reaction-diffusion model to reproduce this pattern formation on physically feasible length and time scales, faithfully incorporating the available experimental data. Numerical simulations show patterns made up of CO and oxygen fronts moving at similar speeds to those seen in experiments.     

EXAFS characterization of Ti/Al2O3 supports and Co/Ti/Al2O3 catalysts

The structure of Ti/Al₂O₃ supports (0–14 wt% Ti) and Co/Ti/Al₂O₃ catalysts (3 wt% Co) was examined by EXAFS. The results indicated that the Ti was present primarily as a highly dispersed surface phase. The Ti EXAFS results indicated that the Ti species were octahedrally coordinated. Evidence of Ti—Ti interactions was found for all loadings (2–14 wt% Ti) suggesting that the Ti surface species are present as small clusters of TiO₂.The Co EXAFS results showed evidence for several structurally different Co surface phases as a function of Ti loading. Evidence of a Co species interacting with the Ti surface phase was observed for the 3% Co/2% Ti-3%Co/6%Ti catalysts. At the highest loadings studied, 3%Co/8%Ti and 3%Co/14%Ti, evidence was found for a CoTiO₃-like phase.     

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) of polyisoprenes

Polyisoprenes (PIPs) with average molecular weights from 650 to 800,000 Da have been studied by time-of-flight secondary ion mass Spectrometry (TOF-SIMS) in the static mode. Polymer samples were bombarded by argon primary ions, and positive SIMS spectra were collected. Effects of branching and unsaturation in the polymer structure on ion formation were studied. The pendant methyl group showed little tendency to fracture as a cation. In the low mass region, C_nH _2n–1 ⁺ appeared to be more intense than C_nH _2n+1 ⁺ , attributed to the double bond structure of polyisoprene. Additionally, ion formation varied as a function of polymer molecular weight. Cationized intact oligomers and fragments dominate the high mass region. Oligomer distributions were used to calculate average molecular weights for polyisoprenes. A statistical chain scission mechanism was used to qualitatively explain the formation of five clusters within a unique fragmentation pattern. Detailed studies of the cluster structure pointed out that each cluster contained several species having varied degrees of unsaturation. It is believed that double bond rearrangements occur.     

The structures of [C5H5O]+ ions formed from isomers of nitrophenol in the gaseous phase

Translational energy release measurments on metastable ions are used in the comparison of the structures of isomeric ions. Metastable ions, m₂⁺, formed from m₁⁺ ions as the result of a high energy process in the ion source are compared with isomeric metastable ions formed as daughters from fragmentation of metastable m₁⁺ ions in a field. In the case of o-, m- and p-nitrophenol the structure of the [C₅H₅O]⁺ ions formed from [C₆H₅O]⁺ ions by these two independent methods is different as verified by comparison of the behaviour of [C₅H₅O]⁺ ions formed from several other compounds.     

Insights into the synthesis of lipopolysaccharide and antibiotics through the structures of two retaining glycosyltransferases from family GT4

Glycosyltransferases (GTs) catalyze the synthesis of the myriad glycoconjugates that are central to life. One of the largest families is GT4, which contains several enzymes of therapeutic significance, exemplified by WaaG and AviGT4. WaaG catalyses a key step in lipopolysaccharide synthesis, while AviGT4, produced by Streptomyces viridochromogenes, contributes to the synthesis of the antibiotic avilamycin A. Here we present the crystal structure of both WaaG and AviGT4. The two enzymes contain two "Rossmann-like" (beta/alpha/beta) domains characteristic of the GT-B fold. Both recognition of the donor substrate and the catalytic machinery is similar to other retaining GTs that display the GT-B fold. Structural information is discussed with respect to the evolution of GTs and the therapeutic significance of the two enzymes.