Cobalt species

A model similar to that of the iron complex 31 was proposed for the cobalt species synthesized as a result of co-condensation of cobalt vapors with pyrrole in vacuum. A frozen matrix formed is subsequently warmed to room temperature (89JA3881). An oligomer or a polymer results, in which a- and ir-donor functions are realized simultaneously. The model proposed differs from that for the iron pyrrolyl complex by inclusion of the Co—Co bonds to attain the 18-electron configuration. 

Jones (refs. 11,12) subsequently investigated the relative reactivities of the various cobalt(III) species with Br, Mn and H2O2. The active p-oxodimer, Co was two to four orders of magnitude more reactive than Co which was four to five times more reactive than (Fig. 13). Furthermore, it should be noted that the rate of conversion of Co to Co is much higher than the rate of reaction of Co with ArCH3. In other words, in the absence of Br or Mn the cobalt species that reacts with ArCH3 cannot be Co. ... 

Results of the cyclohexane oxidation tests are shown in Table 41.4. Mono-oxygenated products are cyclohexanone, cyclohexanol and cyclohexyl hydroperoxide. Cu and Cr were very active, but subsequent tests showed considerable leaching for both metals, whereas Co-Si-TUD-1 did not show ai r leaching. Tests with different Co loadings indicate that the lowest Co concentration has the best conversion and ketone selectivity. Isolated cobalt species are most efficient for the conversion of cyclohexane, as agglomeration of Co reduces... 

Khodakov, A.Y., Lynch, J., Bazin, D., Rebours, B., Zanier, N., Moisson, B., and Chaumette, P. 1997. Reducibility of cobalt species in silica-supported Fischer-Tropsch catalysts. J. Catal. 168 16-25. 

Borg, Q., Ronning,M., Storsaeter, S., van Beek, W., and Holmen, A. 2007. Identification of cobalt species during temperature programmed reduction of Fischer-Tropsch catalysts. Stud Surf. Sci. Catal. 163 255-72. 

Fig. 5. Flash photolysis of (PNP)CoCO leading to a triplet three-coordinate cobalt species, with slow rebinding of carbon monoxide (96).

Most hydroformylation investigations reported since 1960 have involved trialkyl or triarylphosphine complexes of cobalt and, more recently, of rhodium. Infrared studies of phosphine complex catalysts under reaction conditions as well as simple metal carbonyl systems have provided substantial information about the postulated mechanisms. Spectra of a cobalt 1-octene system at 250 atm pressure and 150°C (21) contained absorptions characteristic for the acyl intermediate C8H17COCo(CO)4 (2103 and 2002 cm-1) and Co2(CO)8. The amount of acyl species present under these steady-state conditions increased with a change in the CO/ H2 ratio in the order 3/1 > 1/1 > 1/3. This suggests that for this system under these conditions, hydrogenolysis of the acyl cobalt species is a rate-determining step. 

In the case of iron-containing small molecule analogs of Mb and Hb a much rockier road to successful model compounds was encountered. Even though the syntheses of iron porphyrin complexes were carried out in analogous manner to the cobalt species described above, their irreversible oxidation to the p-oxo dimer upon... 

What is the structure of this Co-Mo-S phase A model system, prepared by impregnating a MoS2 crystal with a dilute solution of cobalt ions, such that the model contains ppms of cobalt only, appears to have the same Mossbauer spectrum as the Co-Mo-S phase. It has the same isomer shift (characteristic of the oxidation state), recoilfree fraction (characteristic of lattice vibrations) and almost the same quadrupole splitting (characteristic of symmetry) at all temperatures between 4 and 600 K [71]. Thus, the cobalt species in the ppm Co/MoS2 system provides a convenient model for the active site in a Co-Mo hydrodesulfurization catalyst. 

The EXAFS signal from the Co K edge gives information on the surroundings of cobalt. As an active sulfided Co-Mo/AI2O3 catalyst contains at least two cobalt species, namely ions inside the A1203 lattice and in the Co-Mo-S phase, it is better to investigate the Co-Mo-S phase in carbon-supported catalysts. The latter can be... 

Catalysts were prepared by incipient wetness impregnation of commercial supports using cobalt nitrate as a precursor. Metallic cobalt species were active centers in the ethanol steam reforming. Over 90% EtOH conversion achieved. Nature of support influences the type of byproduct formation. Ethylene, methane and CO are formed over Co supported on A1203, Si02 and MgO, respectively... 

H2, 1% H2S/H2) catalysts for determining the identity, concentration, and reducibility of the cobalt species. At least three different types of cobalt species were present on the calcined... 

C0/K-AI2O3. The determination of the types of cobalt species present on each support and their reduction properties were used to explain the catalysts CO hydrogenation activities. 

We undertook this investigation in order to examine the relationship of physical structure and composition of cobalt catalysts to catalytic activity. Several different cobalt species have been detected on supported cobalt catalysts ( lj 7 ) the type, amount, and reactivity of the cobalt species varied with support, metal loading, and preparation procedures. For this investigation, the supports were varied and the other parameters were held constant. Si02> Ti02, AI2O3, and K-AI2O3 were used as... 

The total surface areas determined by the N2 BET method for the calcined, supported catalysts are listed in Table II. The X-ray diffraction (XRD) results showed diffraction peaks from a cubic lattice with a unit cell distance of 6.1 A were present on all of the calcined catalysts. Both C03O4 and C0AI2O4 have structures consistent with that lattice spacing, making assignment of the type of crystalline cobalt species present on the alumina supports difficult. 

Thus on the calcined C0/AI2O3 there were at least three cobalt species one that reduced at 480°C in H2, one that suldifed at... 

The catalysts we examined in this study contained several cobalt species with different reduction properties. In this section, we will employ those findings to explain the observed CO hydrogenation activity ranking. The differences observed in catalyst selectivity will not be addressed in this paper. 

CO hydrogenation activity was found to depend on both the amount of available surface cobalt metal and the environment of the reduced cobalt. Factors influencing these cobalt properties included the type of support, the type of cobalt species present on a given support, and the presence of additives. Due to the interrelationship of these properties, a detailed examination of the activity and structure of each catalyst is needed in order to determine the importance of each factor for CO hydrogenation activity. 

The amount, type, and reducibility of the cobalt species detected on each catalyst was important in determining its CO hydrogenation activity. 

---