Showing papers on "Chromium published in 2006"

Investigation of acid sites in a zeotypic giant pores chromium(III) carboxylate.

Abstract: A study of the zeotypic giant pores chromium(III) tricarboxylate CrIII3OFx(OH)1-x(H2O)2·{C6H3−(CO2)3}2·nH2O (MIL-100) has been performed. First, its thermal behavior, studied by X-ray thermodiffractometry and infrared spectroscopy, indicates that the departure of water occurs without any pore contraction and no loss in crystallinity, which confirms the robustness of the framework. In a second step, IR spectroscopy has shown the presence of three distinct types of hydroxy groups depending on the outgassing conditions; first, at high temperatures (573 K), only Cr−OH groups with a medium Bronsted acidity are present; at lower temperatures, two types of Cr−H2O terminal groups are observed; and at room temperature, their relatively high Bronsted acidity allows them to combine with H-bonded water molecules. Finally, a CO sorption study has revealed that at least three Lewis acid sites are present in MIL-100 and that fluorine atoms are located on a terminal position on the trimers of octahedra. A first result of...

Chromium determination and speciation since 2000

Abstract: Chromium is introduced into the environment by effluents in several industries. It is important to control this element since it is both toxic and carcinogenic. As its toxicity depends on its state of oxidation, it is especially interesting to determine its most abundant species, Cr(III) and Cr(VI). In this overview, we describe the requirements for chromium determination and speciation and review the analytical methods that have been used in these studies. Focusing in particular on developments since 2000, we examine the features of detection techniques, pre-treatments and applications of the various methods.

Chromium(III)-selective sensor based on tri-o-thymotide in PVC matrix

Abstract: Tri-o-thymotide (I) has been used as an electroactive material in PVC (poly(vinyl chloride)) matrix for fabrication of chromium(III)-selective sensor. The membrane containing tri-o-thymotide, sodium tetraphenyl borate (NaTPB), dibutyl phthalate (DBP) and PVC in the optimum ratio 5:1:75:100 (w/w) exhibits a working concentration range of 4.0 × 10−6 to 1.0 × 10−1 M with a Nernstian slope of 20.0 ± 0.1 mV/decade of activity in the pH range of 2.8–5.1. The detection limit of this sensor is 2.0 × 10−7 M. The electrode exhibits a fast response time of 15 s, shows good selectivity towards Cr3+ over a number of mono-, bi- and trivalent cations and can also be used in partially non-aqueous medium (up to 15%, v/v) also. The assembly has been successfully used as an indicator electrode in the potentiometric titration of chromium(III) against EDTA and also to determine Cr(III) quantitatively in electroplating industry waste samples.

Chromium Poisoning of Perovskite Cathodes by the ODS Alloy Cr5Fe1Y2O3 and the High Chromium Ferritic Steel Crofer22APU

Abstract: The alloys Cr5Fe1Y 2 O 3 and the ferritic steel Crofer22APU are typical alloys used as solid oxide fuel cell (SOFC) interconnect materials. Alloy Cr5Fe1Y 2 O 3 is an oxide dispersion strengthened (ODS) alloy developed by Plansee, Reutte, Austria, for use at high temperature. A typical material for medium-temperature SOFC, is the high chromium ferritic steel Crofer22APU supplied by Thyssen Krupp VDM, Germany. The two alloys form different oxide scales which affect chromium poisoning. Chromium vaporization as source term and electrochemical degradation of La 1-x Sr x MnO 3 (LSM) and La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) describing the poisoning were studied for the two alloys. The dynamics of the chromium deposition in porous perovskite cathodes was studied by a dc method and impedance spectroscopy. Electrical degradation of the LSM cathode by alloy Cr5Fe1Y 2 O 3 was significantly higher than for Crofer22APU. The microstructure of the cells was studied after measurements by scanning and energy filtering transmission electron microscopy. Significant amounts of chromium were observed at the TPB in the functional layer of cells, with the LSM cathode giving insight into the degradation mechanism. Cells tested with the LSCF cathode clearly show Cr poisoning. Formation of large SrCrO 4 crystals was observed on the surface of the LSCF cathode.

Synthesis of MIL-102, a Chromium Carboxylate Metal−Organic Framework, with Gas Sorption Analysis

Abstract: A new three-dimensional chromium(III) naphthalene tetracarboxylate, CrIII3O(H2O)2F{C10H4(CO2)4}1.5·6H2O (MIL-102), has been synthesized under hydrothermal conditions from an aqueous mixture of Cr(NO3)3·9H2O, naphthalene-1,4,5,8-tetracarboxylic acid, and HF. Its structure, solved ab initio from X-ray powder diffraction data, is built up from the connection of trimers of trivalent chromium octahedra and tetracarboxylate moieties. This creates a three-dimensional structure with an array of small one-dimensional channels filled with free water molecules, which interact through hydrogen bonds with terminal water molecules and oxygen atoms from the carboxylates. Thermogravimetric analysis and X-ray thermodiffractometry indicate that MIL-102 is stable up to ∼300 °C and shows zeolitic behavior. Due to topological frustration effects, MIL-102 remains paramagnetic down to 5 K. Finally, MIL-102 exhibits a hydrogen storage capacity of ∼1.0 wt % at 77 K when loaded at 3.5 MPa (35 bar). The hydrogen uptake is discussed...

Conductive protection layers on oxidation resistant alloys for SOFC interconnect applications

Abstract: Conductive oxide coatings are used as protection layers on metallic interconnects in SOFCs to improve their surface stability and electrical performance, as well as to mitigate or prevent chromium poisoning to cells. This paper discusses materials requirements for this particular application and summarizes our systematic study on varied conductive oxides as potential candidate materials for protection layers on stainless steel substrates. Overall, it appeared that chromites such as (La,Sr)CrO 3 improved surface stability, but might not be good candidates for protection layer applications due to chromium vaporization, albeit at a lower rate than Cr 2 O 3 , from these oxides at high temperatures in air or moist air. The application of non-chromite perovskite (La,Sr)FeO 3 (LSF) protection layers resulted in improved oxidation resistance and electrical performance. It is doubtful, however, that LSF can be an effective barrier to prevent chromium release during long term SOFC stack operation due to chromium diffusion through the LSF coatings. With a high oxygen ion conductivity, the coatings of Sn-doped In 2 O 3 failed to provide protection to the metal substrate and are thus not suitable for the protection layer applications. The best performance was achieved using thermally-grown (Mn,Co) 3 O 4 spinel protection layers that substantially improved the surface stability of the metal substrates, and prevented chromium outward migration.

A chromium-free conversion coating of magnesium alloy by a phosphate–permanganate solution

Abstract: A chromium-free conversion coating for AZ91D magnesium alloy has been obtained by using a phosphate–permanganate solution, which has been developed out without acid pickling. Examinations have been carried out on the conversion coatings for morphology, composition, adhesion force, and corrosion resistance. Results show that the conversion coatings are relatively uniform and continuous, with thickness from 7 μm to 10 μm. They demonstrate good adhesion to matrix and have some non-penetrating tiny holes on the surface. The main elements of the conversion coating of AZ91D alloy are Mg, O, P, K, Al and Mn by EPMA analysis. Results of corrosion resistance test indicate that the corrosion resistance of the conversion coating by phosphate–permanganate solution is matched to that of the conversion coating by traditional chromate solution, but for the corrosion resistance after painting, the former is better than the later.

Overview of Chromium(VI) in the Environment: Background and History

Abstract: CONTENTS 1.1 Purpose 2 1.2 Introduction to the Chromium(VI) Problem 3 1.2.1 Exposure Pathways 3 1.2.2 Physical and Chemical Characteristics 4 1.2.3 Analytical Methods 5 1.2.4 Remediation Overview 6 1.2.5 Regulatory Concentrations 6 1.2.6 Health 7 1.3 Historical Perspective 7 1.4 Origin and Properties 14 1.5 Production and Use of Chromium 14 1.5.1 Chromium Production Methods 14 1.5.2 World Production 15 1.5.3 Resources 15 1.5.4 Consumption 15 1.5.5 Economics 16 1.5.6 Chromium Substitutes 16 1.5.7 Uses 16 1.5.7.1 Paint 17 1.5.7.2 Stainless Steel 17 1.5.7.3 Furnace Linings 17 1.5.7.4 Tanning and Dying Processes 17 1.5.7.5 Photography 17 1.5.7.6 Specialized Steels 18 1.5.8 Chromium Processing and Alloys 18 1.5.9 Chromium Isolation 19 L1608_C01.fm Page 1 Monday, October 25, 2004 10:18 AM

Method for plasma etching a chromium layer suitable for photomask fabrication

Abstract: A method for etching a chromium layer is provided herein. In one embodiment, a method for etching a chromium layer includes providing a filmstack in an etching chamber, the filmstack having a chromium layer partially exposed through a patterned layer, providing at least one halogen containing process gas to a processing chamber, biasing the layer disposed on a substrate support in the processing chamber with a plurality of power pulses less than 600 Watts, and etching the chromium layer through a patterned mask. The method for plasma etching a chromium layer described herein is particularly suitable for fabricating photomasks.

Development of chromium-free iron-based catalysts for high-temperature water-gas shift reaction

Abstract: Chromium-free iron-based catalysts were prepared and studied in regard to their performance in the high-temperature water-gas shift reaction (HTS). The effects of various catalyst preparation variables (i.e., Fe/promoter ratio, pH of precipitation medium, calcination and reduction temperatures) and preparation methods were investigated. Aluminum is a potential chromium replacement in HTS catalysts. Further improvement in WGS activity of Fe–Al catalysts can be achieved by the addition of small amounts of copper or cobalt. Catalysts were characterized using BET surface area measurements, temperature-programmed reduction (TPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). As a textural promoter, aluminum and chromium prevent the sintering of iron oxides and stabilize magnetite phase by retarding its further reduction to FeO and metallic Fe. The promotional effect of Cu is found to be strongly dependent on the preparation method.