Sulfide catalysts CoMoS phase

The IR spectra in Fig.7 indicate the preferential adsorption of NO on the Co sites. It may be conjectured that the Mo sulfide species are physically covered by the Co sulfide species or that Co-Mo mixed sulfide species are formed and the chemical natures of the Co and Mo sulfides are mutually modified. The Mo K-edge EXAFS spectra were measured to examine the formation of mixed sulfide species between Co and Mo sulfides. The Fourier transforms are presented in Fig.8 for MoSx/NaY and CoSx-MoSx/NaY. The structural parameters derived from EXAFS analysis are summarized in Table 1. The structure and dispersion of the Mo sulfides in MoSx/NaY are discussed above. With the Co-Mo binary sulfide catalyst, the Mo-Co bondings are clearly observed at 0.283 nm in addition to the Mo-S and Mo-Mo bondings. The Mo-Co distance is close to that reported by Bouwens et al. [7] for a CoMoS phase supported on activated carbon. Detailed analysis of the EXAFS results for CoSx-MoSx/NaY will be presented elsewhere. It is concluded that the Co-Mo mixed sulfides possessing Co-S-Mo chemical bondings are formed in CoSx-MoSx/NaY. 

Impregnation of cobalt and molybdenum (without sodium) increases largely the isomerizing activity of the catalyst the /3-pinene is then completely converted. The catalysts prepared with sodium molybdate and sodium hydroxide (Co-Mo-Na and Na-Co-Mo-Na) have lower isomerizing activities while their HDS activities are significantly increased. As in the case of alumina supported catalysts the sulfided CoMo phase protected by a double layer of alkaline ions on the carbon support gives the best results in HDS of /3-pinene. The behaviour of this catalyst was examined in desulfurization of the turpentine oil (40% a-pinene, 25% /3-pinene, 25% A -carene and 10% camphene + dipentene + myrcene, 1500 ppm S). The results are recorded in Table 6. 

Kinetic data also suggest that, despite the massive buildup of deposits, some of the catalytic sites maintain some degree of integrity. The data of Takeuchi et al. (1985) in Fig. 42 suggest that much of the Co Mo remains exposed in order for the catalyst to exhibit HDS activity above that of the deposited metal sulfides. The working catalyst s specific activity probably results from the contributions of both that portion of the high-activity CoMo phases that remain uncovered and the lower activity Ni and V sulfides which coat the catalyst surface. Furthermore, coke deposits cannot... 

Studies on reduced and sulfided catalysts were predominatly done on the CoMo/Al catalyst. In a detailed study of catalysts reduced at 600°, Grimblot and Bonnelle (70) concluded that a Mo4Co phase was present, which they denoted as... 

These tetrahedral distorted cobalt atoms can be observed by NMR as a pure phase on carbon supports in the absence of molybdenum and are thus stable these probably correspond to the Co sites observed by Topspe s group using Mossbauer spectroscopy because Craje et al. (93) found a similar Mossbauer doublet for both cobalt in CoMo catalysts and pure cobalt sulfide on carbon support. They are also active for HDS and confirm the findings of Prins and co-workers (94) and Ledoux (96). These different structures are in full agreement with the XANES experiments performed by Prins and co-workers (95) and Ledoux (96). These structures also led Ledoux et al. to an incorrect interpretation of the synergy effect (64). On poorly dispersed catalysts supported on silica or in bulk form, their presence and activity are large enough to explain the increase in activity when cobalt is added to molybdenum, but on well-dispersed catalysts i.e., on alumina or carbon support this interpretation is shown to be incorrect if the activity is carefully measured. 

The most obvious choice to determine phases that may be present in the molybdena catalyst is XRD. Matching of diffraction lines obtained for the catalyst with those of pure bulk compounds gives unequivocal identification of phases present. This is one of the few techniques that yields positive results. The absence of matching diffraction lines, however, is not proof that the phase in question is not present in the catalyst. The XRD technique is limited to particle sizes of above approximately 40 A for oxides or sulfides, lower sized particles giving no discernible pattern over that of the broad alumina pattern. Thus, the presence of a highly dispersed phase, either as small crystallites or as a surface compound of several layers thickness will not be detected. Also, if the phase is highly disordered (amorphous), a sharp pattern will not be obtained, although some broad structure above that of the alumina may be detected. It is a moot point as to whether such a case is considered as a separate phase or a perturbation of the alumina structure. Ratnasamy et al. (11) have examined their CoMo/Al catalyst from the latter point of view, with particular emphasis on the effect of calcination temperature. 

One of the earliest studies on the CoMo/Al catalyst was done by Richardson (59) employing magnetic measurements. He characterized the oxidized catalyst in terms of bulk compounds, which is clearly incorrect in view of later work. On this basis, and the known sulfidibility of the compounds, he deduced the active catalyst consisted of an MoS2 phase containing Co of an unknown stoichiometry. 

Tops0e et al. (98) found from the IR spectra of adsorbed NO that in sulfided CoMo/Al or NiMo/Al catalysts the phosphorus addition favors a less sulfided environment of the CoMoS or NiMoS phases. Iwamoto and Grimblot (67) determined that phosphorus decreases the sulfidabUity of molybdenum in NiMo/Al as in the case of Mo/Al, whereas the addition of phosphorus has a less positive effect on nickel sulfidability because the nickel species in NiMo/Al are predominantly associated with molybdenum species rather than with alumina. 

---