Segregation Of Cobalt Research Articles

Mixed impregnated thiosalt decomposition catalysts characterized by X-ray diffraction

We used the impregnated thiosalt decomposition method (ITD) to prepare catalysts of molybdenum sulfide promoted with cobalt in atomic ratios (r = Co/(Co + Mo)) ranging from 0.0 to 1.0. Measurements obtained by X-ray diffraction (XRD) show the presence of the MoS2-2H phase in all mixed samples, and segregation of cobalt in two phases: Co9S8, forr ≥ 0.3, and CoS1.035, for 0.3 ≤r ≤ 0.5.

Surface segregation studies in iron-cobalt alloys by Auger electron spectroscopy

Auger electron spectroscopy has been used to study the surface segregation in five binary iron-cobalt alloys, the concentration of cobalt varying from 5.4 to 88.6 at%. Enrichment of cobalt at the surface was observed in all the alloys and the extent of enrichment was maximum in the alloy containing the minimum concentration of cobalt. These results were in agreement with those of Bevolo who has reported the segregation of cobalt in iron-cobalt alloy but were in contradiction to those reported by Morán-López and Wise who have observed an enrichment of iron at the surface of Fe-50%Co alloy. The observations made in the present work were consistent with the trend predicted by some current theoretical models.

Homogeneous sulfide precipitation catalysts characterized by X-ray diffraction

Samples from molybdenum sulfide to cobalt sulfide were prepared with mixtures in atomic ratios r= Co (Co + Mo) of 0.0,0.2, 0.3, 0.5, 0.7 and 1.0. We used the common method of homogeneous sulfide precipitation (HSP). Measurements obtained by X-ray diffraction (XRD) show the presence of the MoS 2-2H phase and segregation of cobalt in two phases, Co 9S 8 and CoS 1.035.

Comparative study of the surface oxidation behaviour of amorphous and crystallized Fe67Co18B14Si1 metallic glass

The oxidation of amorphous and crystallized specimens of the metallic glass Fe67Co18B14Si1 were studied under controlled conditions using Auger electron spectroscopy and X-ray photo-electron spectroscopy. The different oxides were characterized as boron and iron oxide forming in that sequence for both the amorphous and crystalline specimens. No cobalt oxide was detected. A higher oxygen uptake was measured for the amorphous than for the crystalline specimens. Comparison with the literature shows that the difference in oxygen uptake is heavily composition dependent. The oxidation of the amorphous specimens constitutes a driving force for boron to segregate to the surface and causes a boron enriched layer. Segregation of boron and cobalt occurs during annealing and the sequence of the enriched layers is influenced by environmental conditions.

X‐Ray Photoelectron Spectroscopy Studies on the Oxidation Behavior of CoNi Thin Films

The oxidation behavior of films of varying composition was studied using x‐ray photoelectron spectroscopy (XPS). At 500°C and under of oxygen, preferential oxidation of cobalt over nickel was observed in the alloys, although pure cobalt and pure nickel showed similar oxidation kinetics. The cobalt oxide, identified as , grew parabolically at a rate dependent on the alloy composition. Oxidation‐induced segregation of cobalt at the surface was also observed in the alloys.

Oxidation behavior of obliquely deposited CoNi magnetic thin films

The oxidation behavior of Co80Ni20 thin films used for magnetic recording was studied using Auger and X-ray photoelectron spectroscopy. At 500 °C and under an oxygen pressure of 1 × 10−5 Torr cobalt oxidized to CoO but nickel remained in the metallic state. The growth of CoO followed a parabolic dependence on the oxygen exposure time, suggesting diffusion-controlled kinetics for oxidation. The [Co]/[Ni] ratio at the surface increased with oxidation indicating an oxygen-induced segregation of cobalt in the alloy.

The photoelectrochemical properties of MoS 2 single crystals grown in the presence of cobalt

MoS 2 single crystals have been grown in the presence of cobalt. Photoresponse has been measured and compared with that from undoped MoS 2. The results are consistent with the segregation of cobalt on the surface edges of the doped crystals.

Elemental segregation and mechanism of sputtering in rf-sputtered Co-Cr films

RF sputtered Co-Cr films have been analyzed for elemental segregation at grain boundaries. These segregations are detected by an electron nanoprobe (50-100 Å diameter) in a transmission electron microscope. The results indicate slight segregation of cobalt and chromium at low substrate temperatures; however, the films homogenize upon heating at 400°C.

Edge plane segregation in cobalt-doped MoS 2 crystals

Single crystals of cobalt-doped MoS 2 have been examined by Auger spectroscopy. Segregation of cobalt to growth steps (edge planes) of the faceted crystals has been detected. Edge planes are preferred sites for oxygen adsorption. Some evidence has been obtained that oxygen adsorption enhances cobalt segregation. These findings are consistent with the pseudo-intercalation model of segregation in MoS 2 crystals.

X‐ray photoelectron spectroscopy study of the surface composition of CoO‐MgO solid solutions

AbstractThe surface composition of CoO–MgO solid solutions has been investigated by XPS. The samples (prepared at 1200°C in air) were checked to be homogeneous by X‐ray diffraction analysis and by lattice parameter measurements. The study utilized peaks at different kinetic energies: Co 2p, O 1s, Co 3s, Mg 2s, Co 3p, Mg 2p and the Auger transition Mg KLL. It was shown that the specimens have a surface composition slightly different from the bulk, because of the formation of surface magnesium hydroxide. No segregation of cobalt was found. The results have a bearing on the catalytic activity of oxide solid solutions.

Influence of cobalt on the textural, redox and catalytic properties of stoichiometric vanadium phosphate

The influence of coprecipitated cobalt on the catalytic, redox and textural properties of βVPO 5 is reported. Four catalysts were prepared in which the Co/V ratio was varied between 0 and 0.05 and the P/V ratio was held at unity. Cobalt strongly reduced the surface area of βVPO 5 and diminished reactivity toward hydrogen was observed as a monotonous feature as the cobalt content increased. By contrast this material showed complex kinetic behaviour during reoxidation from reduced states. When reoxidation was confined to surface layers a maximum in rate was observed for Co/V ratios of ca. 0.02. Bulk reoxidation was strongly sensitive to small amounts of cobalt but it was independent of cobalt content above Co/V = 0.01. Results of catalytic testing for n-butane partial oxidation to maleic anhydride indicate that conversion was at a maximum for the undoped material but selectivity showed an optimum value for Co/V of ca. 0.02. Above this value a sharp drop in selectivity was observed. A strong increase in areal reoxidisability was observed for Co/V above 0.02. X.P.S. data indicated that some surface segregation of cobalt had occured for Co/V = 0.05. Two domains of influence by cobalt on the catalytic activity of βVPO 5 were identified; one in which it influenced the solid state properties and increased selectivity, the other in which a surface enrichment in cobalt provoked total oxidation of n-butane.

High-temperature effects on a CoMoAl 2O 3 catalyst

The effects of high-temperature firing in air on a hydrotreating catalyst were investigated by X-ray diffraction, X-ray fluorescence, X-ray photoelectron spectroscopy, and scanning electron microscopy. The firing process did not cause any loss of cobalt or molybdenum from the catalyst. However, changes in the distribution of cobalt, molybdenum, silicon, and calcium were observed in the outer and fractured surfaces of the catalyst pellets. Also, the chemical states of cobalt and molybdenum were altered by the firing process. The crystal structure of the alumina support was found to be in the η form for temperatures up to 800 °C. Between 800 ° and 900 °C a transformation to the α form occurred with concurrent segregation of cobalt, molybdenum, calcium, and silicon into isolated inclusions.

X-ray photoelectron spectroscopy study of cobalt-molybdenum binary oxide catalysts

The surface compositions of cobalt-molybdenum binary oxide catalysts were studied with X-ray photoelectron spectroscopic techniques for the calcined catalysts and for the catalysts exposed to various reactive gases relating to the hydrodesulfurization of thiophene. In the cases of the cobalt-molybdenum catalysts prepared by calcining the mixtures of cobalt nitrate and ammonium paramolybdate, a surface segregation of molybdenum was observed over a wide range of the bulk composition of the catalyst, while a cobalt enrichment was observed in the catalyst with a small cobalt content (< 6 at.%). However, the catalysts prepared by the calcination of the mixed composite oxides, CoO and MoO 3, showed no enrichment of cobalt or molybdenum in the catalyst surface, except for the catalyst containing a small amount of CoO. It was found that the surface composition of the catalyst was changed considerably by treating at 400 °C with 10 Torr of various reactive gases. Hydrogen reduction caused a surface enrichment of molybdenum, accompanied by the reduction of both oxides to MoO 2 and Co metal. Thiophene/H 2 also resulted in a surface segregation of molybdenum and the sulfidation of MoO 2 to MoS 2. H 2S H 2 treatments, on the other hand, produced a drastic segregation of cobalt in the catalyst surface and sulfided the catalyst completely to form MoS 2 and CoS and/or more likely Co 9S 8, together with excess sulfur. A similar behavior was observed for a zinc-molybdenum binary oxide catalyst. It is concluded that the surface compositions of the cobalt-molybdenum catalysts depend strongly on the preparation procedures and on the kinds of gases in contact with the catalysts.