The effect of chromium on sulfur resistance of Pd/HY-Al2O3 catalysts for aromatic hydrogenation

Sulfur is a major poison to noble metal catalysts for deep aromatic hydrogenation in the petroleum refining industry. In order to study the sulfur resistance of Pd-based catalysts, a series of Pd, Cr, and PdCr catalysts supported on HY-Al2O3 were studied by NH3-TPD, pyridine-adsorption IR, TPR, IR spectra of adsorbed CO, and toluene hydrogenation in the presence of 3000 ppm sulfur as thiophene under the following conditions: 533-573 K, 4.2 MPa, and WHSV 4.0 h(-1). Cr has no influence on the acidity of the catalysts. TPR patterns and in situ IR spectra of adsorbed CO revealed a strong interaction between Cr and Pd, and the frequency shift of linear bonded CO on Pd indicates that the electron density of Pd decreases with the increase of the Cr/Pd atomic ratio. The catalytic performance of Pd, Cr, and PdCr catalysts shows that the sulfur resistance of Pd is strongly enhanced by Cr, and the activity reaches its maximum when the Cr/Pd atomic ratio equals 8. The active phase model "Pd particles decorated by Cr2O3" is postulated to explain the behavior of PdCr catalysts. (C) 2001 Academic Press.

Strong effect of transitional metals on the sulfur resistance of Pd/HY-Al2O3 catalysts for aromatic hydrogenation

In order to improve the sulfur resistance of noble metal catalysts in the aromatic hydrogenation of diesel fuel, the alloying effect of non-noble metals with Pd was studied. Toluene hydrogenation over Pd and Pd-M bimetallic catalysts (M = Cr, W,La, Mn, Mo, Ag) on a mixed HY-Al2O3 support was investigated in the presence of 3000 ppm sulfur as thiophene in the feedstock. The results showed that the addition of the second metals strongly affected the activity of toluene hydrogenation, which suggests that the sulfur resistibility of Pd-M bimetallic catalysts is much different from single Pd. La, Mn, Mo and Ag decreased the sulfur resistance of the palladium catalysts. For example, the toluene conversion at 553 K was observed to decrease sharply from 39.4 wt.% on Pd to 1.6 wt.% on Pd-Ag, which is by a factor of 25. One of the important findings in this article is that Cr and W increase hydrogenation activity of Pd catalysts. The reactions occurring on these catalysts include hydrogenation, isomerization and hydrocracking, The addition of the second metals has no noticeable effects on the hydrogenation and isomerization selectivity, but it slightly suppresses hydrocracking reactions. The four typical catalysts, Pd-Cr, Pd-W, Pd-Ag and Pd were characterized by infrared (IR) spectroscopy of pyridine and CO. LR spectra of CO revealed the strong interaction between Pd and the second metal as Cr, W and Ag (or their oxide), indicating that the improvement in sulfur resistance originates from electron-deficient Pd with the addition of second metals. (C) 2001 Elsevier Science B.V. All rights reserved.

Sulfur resistance of Pt-W catalysts

The sulfur resistance of low-loaded monometallic Pt catalysts and bimetallic Pt-W catalysts during the partial selective hydrogenation of styrene, a model compound of Pygas streams, was studied. The effect of metal impregnation sequence on the activity and selectivity was also evaluated. Catalysts were characterized by ICP, TPR, XRD, and XPS techniques. Catalytic tests with sulfur-free and sulfur-doped feeds were performed. All catalysts showed high selectivities (>98%) to ethylbenzene. Activity differences between the catalysts were mainly attributed to electronic effects due to the presence of different electron-rich species of Pt0 and electron-deficient species of Ptδ+. Pt0 promotes the cleavage of H2 while Ptδ+ the adsorption of styrene. The catalyst successively impregnated with W and Pt (WPt/Al) was more active and sulfur resistant than the catalyst prepared with an inverse impregnation order (PtW/Al). The higher poison resistance of WPt/Al was attributed to both steric and electronic effects.

Adsorption of sulfur dioxide on chemically modified natural clinoptilolite. Acid modification

Natural Bulgarian clinoptilolite from the south-eastern Rhodopes mountain was modified through treatment with hydrochloric acid with various normality, both single and repeatedly, as well as through a charring of a preliminary obtained NH4-form. The parameters concerning the uptake of the ion-exchangeable cations (Ca2+, Na+ and K+), as well as the uptake of aluminium from the natural material were calculated on the basis of the chemical contents. The highest extent of cations removal was attained in the case of the treatment with NH4Cl solution, while the highest aluminium deficiency was established in the samples treated by hydrochloric acid solutions with increasing concentration. Sulfur dioxide adsorption on the obtained decationised and dealuminised samples was studied according to the frontal-dynamic method. The parameters of the breakthrough curves, namely breakthrough time, saturation time and some of the statistical moments of the curve distribution, were determined. The dynamic adsorption capacities were also specified. Comparing the momentum values it was established that as a result of the natural zeolite treatment with NH4Cl and with low concentrated acid, the diffusion resistance decreases because of the dominant exchange of the presenting exchangeable cations in the samples with the smaller size protons and because of enlargement of the pores opening. Intensified dealuminisation was observed when more concentrated acid solutions are used. The capacity is enhanced, probably due to an increase in the total pore volume.

Selenium and sulfur: mitigation in plant stresses

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Selenium and sulfur: mitigation in plant stresses

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

CE-ESI-MS metabolic fingerprinting of Leishmania resistance to antimony treatment

Metabolomics has become an invaluable tool to unveil biology of pathogens, with immediate application to chemotherapy. It is currently accepted that there is not one single technique capable of obtaining the whole metabolic fingerprint of a biological system either due to their different physical-chemical properties or concentrations. In this work, we have explored the capability of capillary electrophoresis mass spectrometry with a sheathless interface with electrospray ionization (CE-ESI-TOF-MS) to separate metabolites in order to be used as a complementary technique to LC. As proof of concept, we have compared the metabolome of Leishmania infantum promastigotes BCN 150 (Sb (III) IC50 = 20.9 mu M) and its variation when treated with 120 mu M of Sb(III) potassium tartrate for 12 h, as well as with its Sb(III) resistant counterpart obtained by growth of the parasites under increasing Sb(III) in a step-wise manner up to 180 mu M. The number of metabolites compared were of 264 for BCN150 Sb(III) treated versus nontreated and of 195 for Sb(III) resistant versus susceptible parasites. After successive data filtering, differences in seven metabolites identified in databases for Leishmania pathways, showed the highest significant differences, corresponding mainly to amino acids or their metabolite surrogates. Most of them were assigned to sulfur containing amino acids and polyamine biosynthetic pathways, of special relevance considering the deterioration of the thiol-dependent redox metabolism in Leishmania by Sb(III). Given the low concentrations typical for most of these metabolites, the assay can be considered a success that should be explored for new biological questions.

CoO nanoparticles embedded in three-dimensional nitrogen/sulfur co-doped carbon nanofiber networks as a bifunctional catalyst for oxygen reduction/evolution reactions

High-performance and low-cost bifunctional electrocatalysts play crucial roles in oxygen reduction and evolution reactions. Herein, a novel three-dimensional (3D) bifunctional electrocatalyst was prepared by embedding CoO nanoparticles into nitrogen and sulfur co-doped carbon nanofiber networks (denoted as CoO@N/S-CNF) through a facile approach. The carbon nanofiber networks were derived from a nanostructured biological material which provided abundant functional groups to nucleate and anchor nanoparticles while retaining its interconnected 3D porous structure. The composite possesses a high specific surface area and graphitization degree, which favors both mass transport and charge transfer for electrochemical reaction. The CoO@N/S-CNF not only exhibits highly efficient catalytic activity towards oxygen reduction reaction (ORR) in alkaline media with an onset potential of about 0.84 V, but also shows better stability and stronger resistance to methanol than Pt/C. Furthermore, it only needs an overpotential of 1.55 V to achieve a current density of 10 mA cm^-2, suggesting that it is an efficient electrocatalyst for oxygen evolution reaction (OER). The ΔE value (oxygen electrode activity parameter) of CoO@N/S-CNF is calculated to be 0.828 V, which demonstrates that the composite could be a promising bifunctional electrocatalyst for both ORR and OER.

Smoothing a Discrete Hazard Function for the Number of Patients Colonized with Methicillin-Resistance Staphylococcus Aureus in an Intensive Care Unit

A new method for estimating the time to colonization of Methicillin-resistant Staphylococcus Aureus (MRSA) patients is developed in this paper. The time to colonization of MRSA is modelled using a Bayesian smoothing approach for the hazard function. There are two prior models discussed in this paper: the first difference prior and the second difference prior. The second difference prior model gives smoother estimates of the hazard functions and, when applied to data from an intensive care unit (ICU), clearly shows increasing hazard up to day 13, then a decreasing hazard. The results clearly demonstrate that the hazard is not constant and provide a useful quantification of the effect of length of stay on the risk of MRSA colonization which provides useful insight.