Cobalt phases

Figure 3.13. Simple relationships between properties and microstriictural geometry (a) hardness of some metals as a function of grain-boundary density (b) coercivity of the cobalt phase in tungsten earbide/coball hard metals as a function of interface density (after Exner 1996).

Figure 9.20 Correlation between the activity of a series of Co-Mo/AI203 catalysts for the HDS reaction, expressed in the reaction rate constant /cT, and the cobalt phases observed in Mossbauer spectra (left) as well as the NO adsorption sites probed with infrared spectra of adsorbed NO (right) (left figure from Wivel et al. [70], right figure adapted from [49] and [74]).

In the present study, MES was used to establish the cobalt phase distribution. In analogy with previous results ( 5, 3, 11,... 

The addition of promoter elements to cobalt-based Fischer-Tropsch catalysts can affect (1) directly the formation and stability of the active cobalt phase structural promotion) by altering the cobalt-support interfacial chemistry, (2) directly affect the elementary steps involved in the turnover of the cobalt active site by altering the electronic properties of the cobalt nanoparticles electronic promotion) and (3) indirectly the behaviour of the active cobalt phase, by changing the local reaction environment of the active site as a result of chemical reactions performed by the promoter element itself synergistic promotion). 

Similar X-ray data were obtained with Co-Zn reaction couples. The intermetallic layer adjacent to the zinc phase was identified as the y2 phase. According to P.J. Brown,280 the monoclinic unit cell of this intermetallic compound, containing 26 zinc atoms and 2 cobalt atoms (space group C2/m), has the parameters =1.3307 nm, =0.7535 nm, c=0.4992 nm, and P=126.8°. The intermetallic layer adjacent to the cobalt phase was mainly the Yi phase with a distorted y-brass structure.142... 

According to our EXAFS and XANES results, the structure of the cobalt phase in sulfided Co/C is in agreement with that in the... 

Co-Mo/C catalyst in both catalysts a high sulfur coordination of the cobalt atoms is present. This is an interesting observation with regard to the activity per cobalt atom in both catalyst systems. Vissers et al. (10) stated that the intrinsic activity of a cobalt site in sulfided Co/C can be close to that in sulfided Co-Mo/C. Hence, our results of the structural resemblance between sulfided Co/C and Co-Mo/C, support the theory of Vissers et al. (10) that the cobalt phase in sulfided Co-Mo catalysts can be the actual active phase. Moreover, on the basis of these results the high HDS activity of a sulfided Co/C catalyst can be understood. 

Figure 2 Iron and cobalt phases detected by XRD, XPS and Mossbauer spectroscopy techniques as a function of the Co/Co+Fe atomic ratio in supported and unsupported catalysts.

The first of these new cobalt catalysts were made in 1986 by coprecipitation techniques using aqueous solutions with ammonium bicarbonate as the precipitant in a similar way to the methods used for methanol synthesis catalysts. The new catalysts were immediately found to be very active and selective catalysts for the conversion of syngas into hydrocarbons. A particularly attractive feature was their low methane make and tolerance of CO2 The CO2 tolerance was ascribed to the interplay between the support and the cobalt phase both in the oxidized and reduced forms. The general belief is that the support stabilizes the cobalt phase such that the catalyst can be operated at the higher temperatures, required to maintain activity despite competitive adsorption by CO2, without any loss in stability. Other investigators e.g. Shell have used similar strategies [2]. 

It is well known that crystalline rare-earth-cobalt phases lend themselves fairly well to permanent magnet applications. Materials related to SmCoj are currently reported to offer energy products higher than 340 kJ m". The manufacturing costs are still appreciable, and have prevented large scale applications. This has led to a search for less expensive materials based on the less expensive Fe rather than on Co. All these attempts have failed thus far since the crystalline phases studied do not have the required type of magnetocrystalline anisotropy and coercive forces are generally low. 

X-ray diffraction pattern for Co powder samples electrodeposited either with 7pd(l) or 7pd(2), from sulfate or chloride electrolytes, is shown in Fig. 2.26. As can be seen the powder consists only of the hexagonal close-packed a-cobalt phase with the lattice parameters of a = 2.5007 A and c = 4.0563 A. No hydroxide or oxide impurities were detected [99]. 

Conditions favorable for Co(ll) reduction seem to be also beneficial to the reduction of Mo(Vl) to Mo(0). One may assume that the contact with metallic cobalt phase is required for full molybdate reduction. 

Commercial ABj alloys have a predominantly CaCuj erystallographic structure. However, within that structure, there are a range of lattice constants brought about by compositional disorder within the material which are important for catalysis, storage capacity and stability to the alkaline environment and embrittlement. These materials also precipitate a nickel-cobalt phase which is important for high rate discharge. ... 

Finally, sintering of cobalt crystallites, the loss of catalytic surface area due to ripening or migration and coalescence of the cobalt phase, is mainly responsible for the loss of activity. 

A positive effect of lanthana was also observed by Bouarab et al [69] for silica-supported cobalt in methane dry reforming. They found a direct relationship between basicity and activity and concluded that lanthana adjusts the acid function of the unpromoted catalyst, which is responsible for coke production. Also, lanthana prevents cobalt phase sintering by avoiding particles coalescence. 

Fischer-Tropsch (FT) synthesis converts natural gas-, coal- and biomass-derived syngas into liquid hydrocarbon fuels which are totally free of sulfur- and nitrogen-containing compounds and have very low aromatic contents [1]. FT synthesis proceeds on cobalt metal sites, the overall niunber of cobalt metal sites on supported catalysts depends on both cobalt dispersion and reducibility. Decomposition of cobalt precursor is an important step in the catalyst preparation, which could significantly influence both cobalt dispersion and cobalt phase composition [2,3]. 

Thermodynamic Parameters Robie and Hemingway (1995) have provided thermodynamic data for the mixed cobalt phase, 00304(5) (CoO-Co203(s)). On the basis of their analysis, they chose the following thermodynamic parameters ... 

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