Effective rate constant for nanostructured heterogeneous catalysts.

There is a great deal of interest in the use of nanostructured heterogeneous catalysts, particularly those based on expensive precious metals, in order to maximise the surface to volume ratio of the catalyst, potentially reducing the cost without sacrificing performance. When there is an abundance of reactants available, the effective reactivity will depend on the surface density of the catalytically active sites. However, under diffusion-limited conditions, catalytically active sites may compete for reactants, potentially leading to diminishing returns from the use of nanostructures. In this paper we apply a mathematical homogenization approach to investigate the effect of scale and patterning on the effective activity of catalytic sites on a heterogeneous catalyst operating under diffusion-limited conditions. We test these theoretical results numerically using Monte Carlo simulations, and show that in the continuum limit the theory works well. In particular, in the limit where the mean free path is much less than the scale of patterning of catalytically active sites, the effective rate constant is found to be equal to the area-weighted harmonic mean of the rate constants on the surface. However, as the length scale of the patterns becomes comparable to the mean free path length, the simulations show that the effective activity of the system can exceed the theoretical limit suggested by the continuum theory.

Frequency of Group B Streptococcal Colonization in Pregnant Women Aged 35- 37 Weeks in Clinical Centers of Shahed University, Tehran, Iran.

BACKGROUND & OBJECTIVES: One of the important infectious factors in pregnant mothers and newborns is Group B Streptococcus (GBS). There is no perfect report about prevalence of GBS in Iran and in the case of preterm rupture of amniotic membrane or preterm labor all patients are treated by antibiotics without culture so this has led to adverse taking antibiotics and drug resistance. The present study is intended to determine the frequency of colonization of GBS in the pregnant mother (35-37 weeks), referred to medical centers of Shahed University. METHODS: Overall, 210 pregnant women (35-37 weeks), referred to medical center of Shahed University, Tehran, Iran were selected as sample group and after filling out the questionnaires about demographic data and midwifery status and the related information of post- partum, the rectovaginal culture was done for them. RESULTS: Among 210 samples, 7 (3.3%) included positive culture in terms of GBS colonization while all these cases were sensitive to penicillin, ampicillin, cephalothin, nitrofurantoin, and all of the samples were resistant against tetracycline and contrimoxazole. There was no relationship among age, job, education, number of pregnancy, blood pressure background, diabetes and preterm childbirth with positive culture. Similarly, because of limited number of positive cases it was not possible to examine the relationship among GBS colonization and infection in mother and newborn. CONCLUSION: There was a low frequency in GBS colonization in the studied hospitals and the study inside the country also confirms this finding.

Waiting time distributions for the transport through a quantum-dot tunnel coupled to one normal and one superconducting lead.

We have studied the waiting time distributions (WTDs) for subgap transport through a single-level quantum-dot tunnel coupled to one normal and one superconducting lead. The WTDs reveal the internal dynamics of the system, in particular, the coherent transfer of Cooper pairs between the dot and the superconductor. The WTDs exhibit oscillations that can be directly associated to the coherent oscillation between the empty and doubly occupied dot. The oscillation frequency is equal to the energy splitting between the Andreev bound states. These effects are more pronounced when the empty state and double-occupied state are in resonance.