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51.
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In the paper we solve the problem of D
ℋ-optimal design on a discrete experimental domain, which is formally equivalent to maximizing determinant on the convex hull
of a finite set of positive semidefinite matrices. The problem of D
ℋ-optimality covers many special design settings, e.g., the D-optimal experimental design for multivariate regression models. For D
ℋ-optimal designs we prove several theorems generalizing known properties of standard D-optimality. Moreover, we show that D
ℋ-optimal designs can be numerically computed using a multiplicative algorithm, for which we give a proof of convergence. We
illustrate the results on the problem of D-optimal augmentation of independent regression trials for the quadratic model on a rectangular grid of points in the plane. 相似文献
53.
54.
Radoslav Fulek 《Discrete Mathematics》2009,309(6):1736-2410
In this note by saying that a 0-1 matrix A avoids a pattern P given as a 0-1 matrix we mean that no submatrix of A either equals P or can be transformed into P by replacing some 1 entries with 0 entries. We present a new method for estimating the maximal number of the 1 entries in a matrix that avoids a certain pattern. Applying this method we give a linear bound on the maximal number of the 1 entries in an n by n matrix avoiding pattern L1 and thereby we answer the question that was asked by Gábor Tardos. Furthermore, we use our approach on patterns related to L1. 相似文献
55.
Radoslav Halko Simona Procházková Lenka Okenicová Milan Hutta Peter Božek 《Chromatographia》2014,77(15-16):1019-1025
A selective sensitive RP-LC–UV/VIS method with pre-column derivatization was developed for the determination of copper in human urine at a trace level. This method is based on the selective reaction of 2,9-dimethyl-1,10-phenanthroline (neocuproine) with copper(I) to produce a yellow-orange hydrophobic complex in a neutral or slightly acidic buffer solution (adjusted to pH 5.9). Copper(II) was reduced to copper(I) ions by ascorbic acid as a weak reducer, which was added both to urine sample and mobile phase, respectively. A hydrophobic copper(I)–neocuproine chelate was determined by RP-LC–UV/VIS using a monolithic column Chromolith Performance RP-8e (100 × 4 mm I.D.) at 30.0 ± 0.1 °C with a methanol: aqueous buffer (pH 5.9, ammonium acetate and ascorbic acid 2.8 mmol L?1) mobile phase at flow rate of 2.00 mL min?1. Sample injection volume was 20 μL and detection was done at 453 nm. The method was validated over a concentration range of 0.09–11.50 μmol L?1. The LOD of copper in human urine was found to be 0.07 μmol L?1 concentration level, suitable for clinical analysis. The precision of the results, reported as the RSD, was below 4.6 % for copper concentration within range 0.5–5.0 μmol L?1 in the spiked human urine samples. 相似文献
56.
Kuanping Gong Jinseong Park Dong Su Radoslav R. Adzic 《Journal of Solid State Electrochemistry》2014,18(5):1171-1179
We describe an electrostatically induced self-assembly method to prepare ultrathin Pd nanowires (NWs) surrounding individual multiwalled carbon nanotubes, i.e., PdNW/MWNTs, that are noticeable for improving performance in the oxygen reduction reaction (ORR) of their supported PtML electrocatalyst. The carbonaceous by-products in MWNTs, rather than the nanotubes themselves, are modified with the oxygenated terminals to allow the negatively charged and hydrophilic surface while retaining the intrinsic nature of the MWNTs. Encompassing the nanotubes' length are 2-nm-thick Pd NWs that are closely packed and homogeneously dispersed due to the unique processes for preparing PdNW/MWNTs and its components. Although the crystal lattice of the Pd NWs expands somewhat, which should cause an unfavorable interaction with supported PtML, this adverse effect is counterweighed by the shape-determined features of Pd NWs, including their high specific surface area, excellent contiguousness, and low-energy atomic configuration. Consequently, these distinct chemical and physical properties substantially expedite the desorption of the intermediates to refresh the active centers during the reduction of oxygen with the PtML electrocatalyst while ensuring a desirable electron transfer rate, so improving the overall ORR kinetics. Indeed, PtML/PdNW/MWNTs exhibits the Pt mass and specific activities of 1.45 A/mgPt and 0.65 mA/cm2 Pt, respectively, each of which are several times those of the Pt/C and even higher than those of the PtML supported on Pd nanoparticles. These high activities remained over a long-term stability test using the latest US Department of Energy-established protocol. 相似文献
57.
In proton exchange membrane fuel cells, platinum (Pt) has been the dominant choice for both the cathode and the anode catalysts. The high Pt content and high associated costs particularly at the cathode, and sluggish oxygen reduction reaction (ORR) kinetics and poor stability, remain a challenge. Pt monolayer (ML) catalysts offer a distinctively reduced Pt content while providing considerable possibilities for enhancing their catalytic activity and stability for the ORR. In this opinion, we first review the achievement in active and stable Pt ML on palladium (Pd) nanoparticle catalysts for the ORR. We then describe the mechanisms that rationalize their high activity and durability. Recently, we developed several novel nanostructured cores to further improve the ORR activity and stability by optimizing their surface orientation, composition, and morphology. The results from the Pt ML catalysts significantly impact the research of electrocatalysis and fuel-cell technology, as they demonstrate an exceptionally effective way of design and syntheses of catalysts. 相似文献
58.
Švec Jiří Šiler Pavel Másilko Jiří Novotný Radoslav Koplík Jan Janča Martin Hajzler Jan Matějka Lukáš Opravil Tomáš Kolářová Iva 《Journal of Thermal Analysis and Calorimetry》2020,142(5):1749-1758
Journal of Thermal Analysis and Calorimetry - With the increasing use of secondary raw materials and alternative fuels in the cement production industry, the amount of present trash... 相似文献
59.
Koenigsmann C Santulli AC Gong K Vukmirovic MB Zhou WP Sutter E Wong SS Adzic RR 《Journal of the American Chemical Society》2011,133(25):9783-9795
We report on the synthesis, characterization, and electrochemical performance of novel, ultrathin Pt monolayer shell-Pd nanowire core catalysts. Initially, ultrathin Pd nanowires with diameters of 2.0 ± 0.5 nm were generated, and a method has been developed to achieve highly uniform distributions of these catalysts onto the Vulcan XC-72 carbon support. As-prepared wires are activated by the use of two distinctive treatment protocols followed by selective CO adsorption in order to selectively remove undesirable organic residues. Subsequently, the desired nanowire core-Pt monolayer shell motif was reliably achieved by Cu underpotential deposition followed by galvanic displacement of the Cu adatoms. The surface area and mass activity of the acid and ozone-treated nanowires were assessed, and the ozone-treated nanowires were found to maintain outstanding area and mass specific activities of 0.77 mA/cm(2) and 1.83 A/mg(Pt), respectively, which were significantly enhanced as compared with conventional commercial Pt nanoparticles, core-shell nanoparticles, and acid-treated nanowires. The ozone-treated nanowires also maintained excellent electrochemical durability under accelerated half-cell testing, and it was found that the area-specific activity increased by ~1.5 fold after a simulated catalyst lifetime. 相似文献
60.