Highly efficient oxidation of alcohols by the system "hydrogen peroxide-[lmn(o)3mnl](pf6)2 (l = 1,4,7-trimethyl-1,4,7-triazacyclononane)-oxalic acid"

Summary The dinuclear manganese(IV) complex [LMn(O)₃MnL](PF₆)₂ (1, L = 1,4,7-trimethyl-1,4,7-triazacyclononane) catalyzes the extremely efficient oxidation of alcohols with hydrogen peroxide at room temperature. Oxalic acid is an obligatory co-catalyst. The oxidation of isopropanol, for example, yields acetone with turnover numbers up to 40000 after 5-10 h in the absence of a solvent. 2-Cyanoethanol was oxidized by this system with somewhat lower efficiency (conversion 70%). The catalytically active cation from salt 1 was obtained in an insoluble form containing a heteropoly anion [Mn₂O₃(TMTACN)₂]₂[SiW₁₂O₄₀]. Oxidation of 2-cyanoethanol using this heterogenized catalyst and oxalic acid gave the oxo-products with the 54% total yield.     

Sorption of europium on TiO2 and cement at high Ph in the presence of organic ligands

Summary Organic substances present in radioactive waste lower the sorption of metal ions at the high pH in cement matrices and, hence, enhance their possible migration. The aim of this study was to develop a method to compare organic substances or their degradation products with respect to what extent they affect metal sorption. Batch sorption studies were performed with cement or TiO₂ as solid phase and Eu(III) as a model element for trivalent lanthanides and actinides at pH 12.5 (representative of a cement waste matrix during the first approximately 100,000 years). Different kinds of ligands were studied in a broad concentration range, e.g., organic acids, cement additives, cleaning agents and degradation products from ion-exchange resin.     

Seasonal variations of 222Rn and SGD fluxes to Ubatuba embayments, São Paulo

Summary We describe here an application of excess ²²²Rn to estimate submarine groundwater discharge in a series of small embayments of Ubatuba, S?o Paulo State, Brazil. Excess ²²²Rn inventories obtained in 11 vertical profiles varied from (3.3±1.1) ^. 10³ to (19±5) ^. 10³ dpm ^. m^-2. The estimated total fluxes required to support the inventories varied from (0.6±0.2) ^. 10³ to (3.4±0.9) ^. 10³ dpm ^. m^{-2 .} d^-1. Considering these results, the submarine groundwater discharge advective rates necessary to balance the sub-pycnocline fluxes calculated in Ubatuba embayments ranged from 0.06 to 1.9 cm ^. d^-1. During the period of this investigation (from March/03 to May/2004), the highest ²²²Rn in excess inventories were observed late in the summer season (March). Taking into account all vertical profiles established, the relative variability was 67%. Although, if we consider only the fluxes determined in both Flamengo and Fortaleza embayments, the relative variation was 21%.     

Water – A key parameter in the stability of anion exchange membrane fuel cells

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The role of the isotonizing algorithm in Stein’s covariance matrix estimator

Covariance matrix estimation is central to many applications in statistics and allied fields. A useful estimator in this context was proposed by Stein which regularizes the sample covariance matrix by shrinking its eigenvalues together. This estimator can sometimes yield estimates of the eigenvalues that are negative or differ in order from the observed eigenvalues. In order to rectify this problem, Stein also proposed an ad hoc “isotonizing” procedure which pools together eigenvalue estimates in such a way that the original ordering and positivity of the estimates are enforced. From numerical studies, Stein’s “isotonized” estimator is known to have good risk properties in comparison with the maximum likelihood estimator. However, it remains unclear what role is played by the isotonizing procedure in the remarkable risk reductions achieved by Stein’s estimator. Through two distinct lines of investigations, it is established that Stein’s estimator without the isotonizing algorithm gives only modest risk reductions. In cases where the isotonizing algorithm is frequently used, however, Stein’s estimator can lead to significant risk reductions for certain domains of the parameter. In other cases, Stein’s estimator can even yield risk increases, such as when (1) the theoretical eigenvalues are well separated, and/or (2) when the sample size is moderate to large, leading to over-shrinkage.     

Localization of dominant eigenpairs and planted communities by means of Frobenius inner products

We propose a new localization result for the leading eigenvalue and eigenvector of a symmetric matrix A. The result exploits the Frobenius inner product between A and a given rank-one landmark matrix X. Different choices for X may be used, depending on the problem under investigation. In particular, we show that the choice where X is the all-ones matrix allows to estimate the signature of the leading eigenvector of A, generalizing previous results on Perron-Frobenius properties of matrices with some negative entries. As another application we consider the problem of community detection in graphs and networks. The problem is solved by means of modularity-based spectral techniques, following the ideas pioneered by Miroslav Fiedler in mid-’70s.     

A Pd/C‐CeO2 Anode Catalyst for High‐Performance Platinum‐Free Anion Exchange Membrane Fuel Cells

One of the biggest obstacles to the dissemination of fuel cells is their cost, a large part of which is due to platinum (Pt) electrocatalysts. Complete removal of Pt is a difficult if not impossible task for proton exchange membrane fuel cells (PEM‐FCs). The anion exchange membrane fuel cell (AEM‐FC) has long been proposed as a solution as non‐Pt metals may be employed. Despite this, few examples of Pt‐free AEM‐FCs have been demonstrated with modest power output. The main obstacle preventing the realization of a high power density Pt‐free AEM‐FC is sluggish hydrogen oxidation (HOR) kinetics of the anode catalyst. Here we describe a Pt‐free AEM‐FC that employs a mixed carbon‐CeO₂ supported palladium (Pd) anode catalyst that exhibits enhanced kinetics for the HOR. AEM‐FC tests run on dry H₂ and pure air show peak power densities of more than 500 mW cm^?2.