Valence State Of Chromium Research Articles

Chromium ions within zeolites. Part 3.—The influence of the valence state of chromium on the catalytic activity of Cr zeolites in ethylene polymerization

The rate of ethylene polymerization on CrHY and Cr-stabilized Y zeolites has been measured in a static apparatus by the increase in the weight of zeolite with time. The ethylene polymerization depends particularly on the valence state of the Cr ions localized in the cation position of the zeolite. A further necessary condition for ethylene polymerization is a vacant coordination sphere in the isolated Cr ions and their localization in accessible sites. The zeolite catalytic activity increases with increasing electron-acceptor ability of the Cr ions, i.e. from CrII to CrIII and higher valence states. However, the highest catalytic activity of Cr zeolites is not connected with the presence of CrVI but with CrV ions.

X-Ray Photoelectron Spectroscopic Studies of Catalysts—Chromia–Alumina Catalysts—

Abstract The XPS studies of chromia–alumina catalysts were carried out to elucidate the surface of these catalysts. Photoreduction was found to occur during the XPS measurements. Therefore, all spectra were measured rapidly to prevent any photoreduction. It was found that the information about the spin-orbit splitting of the Cr2P level is very useful for determining the valence state of chromium. The surface state of chromium in chromia–alumina catalysts calcined at 500 °C for 5 h is predominantly Cr6+ at low chromia concentration catalysts. However, the Cr3+ phase appears in catalysts of higher chromium content, and the surface concentration of Cr3+ increases with increasing content of supported chromia. The XPS spectra due to Cr5+ were observed after mild reduction of the catalysts. Moreover, it was found that almost all surface chromium is present as Cr5+ at low chromia content catalysts after mild reductions. Qualitatively, the results obtained by the XPS technique, which gives information about the surface of catalysts, agree well with the results obtained by ESR and other techniques, which gives information about the bulk of catalysts, especially low chromium concentration catalysts. These findings indicate a high dispersion of chromium on γ-Al2O3 for low chromium content catalysts.

Study of changes in valence state of chromium in aluminum-chromium catalysts under the influence of aliphatic imino compounds by the EPR method

1. The EPR method was used to study the change in the valence state of chromium in aluminum-chromium catalysts under the influence of hydrogen and aliphatic imino compounds. 2. In the temperature range 20–400° the imines are stronger reducing agents of Cr6+ and Cr5+ compounds than hydrogen.

Adsorption of benzene on aluminum-chromium-potassium catalysts with various valence states of chromium

1. On aluminum-chromium-potassium catalysts with various valence states of chromium: Cr6+, Cr3+, and Cr2+, in the temperature interval 20–453°, there is a physical adsorption of benzene and its chemical interaction with chromium ions. 2. The interaction of benzene with Cr6+ ions is observed at room temperatures, with Cr3+ at temperatures above 100°, and is accompanied by a change in the chemical composition of the surface. 3. The interaction of benzene with Cr2+ occurs at temperatures above 300° and does not change the chemical composition of the surface.

Valence state of chromium in active centers of a potassia-chromia-alumina dehydrogenation catalyst

Ni–Cu/SiO2 xerogel catalysts have been synthesized by cogelation of industrial tetraethoxysilane (Dynasil) and chelates of Ni and Cu with industrial 3-(2-aminoethylamino)propyltrimethoxysilane (Dynasylan DAMO) in industrial ethanol denatured with diethyl phthalate. Despite the use of industrial grade reagents, highly dispersed bimetallic Ni–Cu/SiO2 xerogel catalysts were obtained. These samples are composed of completely accessible Ni–Cu alloy crystallites with sizes of 1.6–3.4 nm located inside silica particles exhibiting a monodisperse microporous distribution. It appears that the bimetallic complex acts as a nucleation agent in the formation of silica particles. The combination of results obtained from the calculation of the metal ratio in catalysts, H2 chemisorption and transmission electron microscopy allowed calculating the surface composition of the nickel–copper particles in Ni–Cu/SiO2 cogelled xerogel catalysts. Values obtained indicate a very pronounced surface enrichment with copper. While 1,2-dichloroethane hydrodechlorination over pure nickel mainly produces ethane, increasing copper content in bimetallic catalysts results in an increase in ethylene selectivity. The specific consumption rate of 1,2-dichloroethane decreases when copper loading increases. The turnover frequency, that is, the number of catalytic cycle per active site (nickel atom and its surrounding copper atoms) and per second, seems to be independent of surface composition of alloy particles.

Activity of chromic oxide catalysts and the chromium valence state

Fourier transform infrared spectra of the inversion-rotation transitions have been measured with 0.010-cm−1 resolution between 40 and 300 cm−1 in the ground state and ν2 excited states of 14NH3. Submillimeterwave spectra of the inversion and inversion-rotation transitions in the ν2 state of 14NH3, including a few Δk = ±3 “perturbation-allowed” transitions, have been measured with microwave accuracy between 540 and 770 GHz. A simultaneous least-squares analysis of these data, the microwave ground-state transition frequencies, and the ν2 infrared-microwave two-photon transition frequencies has been carried out. A theory of the Δk = ±3n interactions in the ground and ν2 excited states of ammonia (S. P. Belov, L. I. Gershtein, A. F. Krupnov, A. V. Maslovskij, Š. Urban, V. Špirko, and D. Papoušek, J. Mol. Spectrosc.84, 288–304 (1980)) has been used in the analysis. A set of the ground- and ν2-state molecular parameters has been obtained which describes the experimental data within the precision of the experiment. The “smoothed” values of transition frequencies can be used for calibration purposes with a precision better than 3 × 10−5 cm−1 in the submillimeterwave region, better than 10−3 cm−1 in the far-infrared region, and better than 1.5 × 10−3 cm−1 in the region 700 – 1200 cm−1.

Reactions in green tatoos. The significance of the valence state of chromium.

Allergic reactions in green tattoos have now been reported in seven cases,1-5of whom four showed a foreign-body and/or sarcoid reaction on histologic examination. The typical case acquires an allergic sensitivity to dichromates and later presents a reaction in areas tattooed with chrome green many years before. The fact that the chrome in dichromates and the chrome used in tattooing differ fundamentally in their chemical, biologic, and allergenic behavior has hitherto been ignored. In the absence of cross sensitivity between chrome in its different states of valence another explanation of this phenomenon must be sought. Report of a Case The patient, a white man, age 35, was first seen in August, 1958. Employed as a bricklayer, he was in frequent contact with wet cement. For one year he has had a persistent eczema of the hands and legs, with occasional generalized dissemination. Patch tests with potassium dichromate were positive