Showing papers on "Chromium published in 2012"

Chromium(III) Terephthalate Metal Organic Framework (MIL-101): HF-Free Synthesis, Structure, Polyoxometalate Composites, and Catalytic Properties

Abstract: Hybrid materials of the metal–organic framework (MOF), chromium(III) terephthalate (MIL-101), and phosphotungstic acid (PTA) were synthesized in aqueous media in the absence of hydrofluoric acid XRD analysis of the MIL101/PTA composites indicates the presence of ordered PTA assemblies residing in both the large cages and small pores of MIL-101, which suggests the formation of previously undocumented structures The MIL101/PTA structure enables a PTA payload 15–2 times higher than previously achieved The catalytic performance of the MIL101/PTA composites was assessed in the Baeyer condensation of benzaldehyde and 2-naphthol, in the three-component condensation of benzaldehyde, 2-naphthol, and acetamide, and in the epoxidation of caryophyllene by hydrogen peroxide The catalytic efficiency was demonstrated by the high (over 80–90%) conversion of the reactants under microwave-assisted heating In four consecutive reaction cycles, the catalyst recovery was in excess of 75%, whereas the product yields were

Efficient catalytic reduction of hexavalent chromium using palladium nanoparticle-immobilized electrospun polymer nanofibers.

Abstract: We report a facile and economic approach to fabricating catalytic active palladium (Pd) nanoparticle (NP)-immobilized electrospun polyethyleneimine (PEI)/polyvinyl alcohol (PVA) nanofibers for catalytic reduction of hexavalent chromium (Cr(VI)) to trivalent chromium (Cr(III)). In this study, PEI/PVA nanofibrous mats were first electrospun from homogeneous mixture solution of PEI and PVA, followed by cross-linking with glutaraldehyde vapor to render the fibers with good water stability. The nanofibrous mats were then alternatively soaked in potassium tetrachloropallidate (K2PdCl4) and sodium borohydride solution, and the PdCl42- anions complexed with the free amine groups of PEI were able to be reduced to form zero-valent Pd NPs. The formed Pd NP-containing PEI/PVA nanofibers were characterized by different techniques. We show that the immobilization of Pd NPs does not significantly change the morphology of the PEI/PVA nanofibers; instead the mechanical durability of the fibers is significantly improved. T...

Green Microwave Synthesis of MIL‐100(Al, Cr, Fe) Nanoparticles for Thin‐Film Elaboration

Abstract: Nanoparticles of mesoporous MIL-100(Al, Cr or Fe) metal–organic frameworks were synthesized by a microwave-assisted solvothermal route with green solvents. The optimization of yield, crystallinity, and particle size was achieved through the control of synthetic parameters such as temperature, heating rate, and dwelling time. The particle size strongly depends on the metallic cation and the metal and linker precursors; nanoparticles smaller than 100 nm were obtained for iron and chromium based MIL-100. Finally, stable colloidal solutions of iron and chromium MIL-100 nanoparticles were used for thin layer dip-coating deposition, which resulted in high optical quality thin films, the porosity of which was investigated by environmental ellipsometric porosimetry.

Preparation of Highly Dispersed Chromium Oxide Catalysts Supported on Mesoporous Silica for the Oxidative Dehydrogenation of Propane Using CO2: Insight into the Nature of Catalytically Active Chromium Sites

Abstract: Highly dispersed chromium oxide catalysts supported on mesoporous silica (Cr-MSU-x) were prepared via a (N0Mn+)I0 pathway with the goal of achieving the high performance oxidative dehydrogenation of propane (ODHP) reaction. The resulting materials exhibited a mesopore structure resembling 3D wormhole-like holes, as characterized by N2 adsorption–desorption isotherms and HR-TEM. Catalytic experimental results revealed that the catalyst with a 0.028 Cr/Si molar ratio showed the highest catalytic activity among the catalysts studied. Two types of chromium species, isolated Cr(VI) and polymeric Cr(VI) species, were observed, as evidenced by H2-temperature-programmed reduction. They were designated as “hard Cr(VI)” and “soft Cr(VI)” sites, respectively. The initial composition of the soft Cr(VI) in the total Cr(VI) is a major determinant factor in the ODHP performance.

Remediation of chromium contaminants using bacteria

Abstract: Chromium contaminants emanating from industrial activities pose a significant threat to human’s well-being. Chromium (III) and Chromium (VI) are the forms in which they are commonly encountered, of which the trivalent form is relatively benign. Hence, biological reduction of hexavalent chromium has been widely explored by researchers, yielding fruitful outcomes, opening up exciting avenues and also throwing up new challenges. This article attempts to review this area of research. Microbes, especially bacteria capable of Chromium (VI) reduction, belonging to a heterogeneous group have been isolated from contaminated sites. They exhibit plasmid-mediated chromate resistance and the reduction is enzymatically mediated. Reduction studies have been carried out with free and immobilized enzymes as well as whole cells. Experiments have been carried out in specifically designed bioreactors operated in batch and continuous modes. Although significant progress has been made, much needs to be done for its successful in situ application as the organism may not withstand the Chromium concentration or may be impeded by the presence of other toxicants. With molecular engineering, it may be possible to derive strains with improved performance even under stressful field conditions.

Characterization of trivalent chromium process coating on AA2024-T3

Abstract: Trivalent Chromium Process (TCP) coatings, which have been used recently as an alternative to chromate conversion coatings (CCCs), have been characterized. The TCP coating was found to be a dense layer consisting of particles, hundreds of nm in size on the coating surface, which is similar to CCCs. The TCP coating has a two layered structure, with zirconium-chromium mixed oxide in the outer layer and aluminum oxide or oxyfluoride at the metal/coating interface. No Cr(VI) was found, which supports its use as an environmentally friendly replacement for CCC. The TCP thickness was in the range of 40–120 nm, considerably thicker than the zirconium based coating without chromium species. The high vacuum condition in the SEM dehydrates the coating quickly resulting in the formation of mud-crack artifacts. The TCP coating provides corrosion protection to the AA2024-T3 through suppressing the oxygen reduction reaction on aluminum alloy surfaces by acting as a protective barrier layer. Two pretreatments were investigated but found to result in little difference in the resulting TCP formed on AA2024-T3.

Effects of Different Levels of Organic and Inorganic Chromium on Growth Performance and Immunocompetence of Broilers under Heat Stress

Abstract: This experiment was carried out to investigate the effects of different levels of organic and inorganic chromium on the performance, immune function and some serum mineral concentrations of broilers under heat stress condition (23.9-37°C cycling). A total of 150 one-day-old broiler chicks according to a completely randomized design were assigned into five treatment groups. Each treatment consisted of three replicates and each replicate contained ten chicks. Chicks were fed on corn-soybean meal basal diets with added different concentrations of chromium (0, 600 and 1,200 μg kg(-1) chromium chloride or 600 and 1,200 μg/kg chromium L-methionine) from 1 to 49 days of age. Humoral immunity was assessed by intravenous injection of 7% sheep red blood cell (SRBC) followed by evaluation of serum for antibody titers in primary and secondary responses. Cell-mediated immunity was assessed by the cutaneous basophil hypersensitivity (CBH) test to phytohemagglutinin (PHA)-P at day 32 and PHA-M at day 48. Heterophil/lymphocyte (H/L) ratio was also measured as a reliable indicator of stress. The body mass, feed intake and conversion ratio were not influenced by dietary chromium (P > 0.05). Dietary supplementation of both organic and inorganic chromium significantly increased primary and secondary antibody responses (P < 0.01), and also improved H/L ratio (P < 0.05), CBH response (P < 0.01) as well as relative weights of thymus (P < 0.05) and spleen (P < 0.01). Both dietary organic and inorganic chromium caused an increase in serum concentrations of Cr and Zn (P < 0.01), but decreased the serum concentration of Cu (P < 0.01). These results suggest that supplemental chromium especially in organic form offers a good management practice to reduce heat stress-related depression in immunocompetence of broiler chicks.

Toward inexpensive superhard materials: tungsten tetraboride-based solid solutions.

Abstract: To enhance the hardness of tungsten tetraboride (WB4), a notable lower cost member of the late transition-metal borides, we have synthesized and characterized solid solutions of this material with tantalum (Ta), manganese (Mn), and chromium (Cr). Various concentrations of these transition-metal elements, ranging from 0.0 to 50.0 at. %, on a metals basis, were made. Arc melting was used to synthesize these refractory compounds from the pure elements. Elemental and phase purity of the samples were examined using energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), and microindentation was utilized to measure the Vickers hardness under applied loads of 0.49–4.9 N. XRD results indicate that the solubility limit is below 10 at. % for Cr and below 20 at. % for Mn, while Ta is soluble in WB4 above 20 at. %. Optimized Vickers hardness values of 52.8 ± 2.2, 53.7 ± 1.8, and 53.5 ± 1.9 GPa were achieved, under an applied load of 0.49 N, when ∼2.0, 4.0, and 10.0 at. % Ta, Mn, and Cr were added to W...

Alkali Induced High Temperature Corrosion of Stainless Steel: The Influence of NaCl, KCl and CaCl2

Abstract: The influence of KCl, NaCl and CaCl2 on the oxidation of 304-type (Fe18Cr10Ni) stainless steel at 600 A degrees C in 5 %O-2 + 40 %H2O was investigated. Prior to exposure, a small amount of the preferred salt (cation equivalent: 1.35 mu mol/cm(2)) was deposited on the samples. Exposure time was 1-168 h. The oxidized samples were analyzed by SEM/EDX, XRD, FIB and IC. The presence of KCl and NaCl strongly accelerates high temperature corrosion of 304L. Corrosion attack is initiated by the formation of alkali chromate through the reaction of alkali with the protective oxide. Chromate formation is a sink for chromium in the oxide and leads to a loss of its protective properties. Subsequently a rapidly growing scale forms, consisting of an outer hematite layer with chromate particles on top and an inner spinel oxide layer. In contrast to NaCl and KCl, CaCl2 is not very corrosive. At temperature, CaCl2 is rapidly converted to CaO. Small amounts of CaCrO4 form where CaO is in direct contact with the scale. CaO also reacts with the scale to form Ca2Fe2O5.