Co-Mo-S phase

The overall degradation of (103) assisted by the cluster [(Cp )2 M o2Co2S3(CO)4] (Cp = CH3C5H4) is the model reaction that best resembles the heterogeneous counterparts, particularly those classified as Co/Mo/S phase,158 in terms of both structural motif and HDS activity.229 Morever, the Co/Mo/S cluster has successfully been employed to show that the C—S bond scission in the desulfurization of aromatic and aliphatic thiols occurs in homolytic fashion at 35 °C and that thiolate and sulfido groups can move over the face of the cluster as they are supposed to do over the surface of heterogeneous catalysts.230... 

Figure 9.19 In situ Mossbauer emission spectra of 57Co in (left) a series of sulfided Co-Mo/A1203 catalysts and (right) MoS2 particles doped with different amounts of cobalt, corresponding to Co/Mo ratios of a) about 3 parts per million, b) 0.05 and c) 0.25. The Co-Mo-S phase, active in the HDS reaction, has a spectrum unlike that of any bulk cobalt sulfide and is most clearly observed in the spectra of Co-Mo/Al203 catalysts of low Co content, and in the MoS2 particles doped with ppms of cobalt (from Wivel et al. [70] and Topspe et al. [71]).

The interesting point about this Co-Mo-S phase is that its presence correlates with the catalytic activity for the desulfurization reaction, as shown in Fig. 9.20. Thus this Co-Mo-S phase is either active itself or at least closely associated with the active site. Figure 9.20 represents one of the few examples in the literature where... 

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. 

As the chemical reactivity of MoS2 is associated with edges, we would expect this Co-Mo-S phase to be found at or near the edges. Scanning Auger spectroscopy and electron microscopy provide evidence that this is indeed the case. 

In order to draw a crystallographic picture of the Co-Mo-S phase, we need precise data on the location of the cobalt atom with respect to the molybdenum and the sulfur atoms. For this, EXAFS is the indicated technique. 

Figure 9.22 Left Mo K-edge EXAFS Fourier transforms of MoS2 and sulfided, carbon supported Mo and Co-Mo catalysts, showing the reduced S and Mo coordination in the first shells around molybdenum in the catalyst (from Bouwens et at. [68]). Right Co K-edge Fourier transforms of the same catalysts and of a Co9S8 reference. Note the presence of a contribution from Mo neighbors in the Fourier transform of the Co-Mo-S phase (from Bouwens et at. [76]).

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... 

Table 9.3 Structural parameters from EXAFS of a carbon-supported Co-Mo-S phase [67,76].

The data analysis in Table 9.3 summarizes the crystallographic information of the Co-Mo-S phase active for hydrodesulfurization. The Co-S distance in Co-Mo-S is 0.22 nm, with a high sulfur coordination of 6.2 1.3. Each cobalt has on average 1.7 0.35 molybdenum neighbors at a distance of 0.28 nm. Based on these distances and coordination numbers one can test structure models for the CoMoS phase. The data are in full agreement with a structure in which cobalt is on the edge of a MoS2 particle, in the same plane as molybdenum. 

Irrespective of the exact configuration around the promoter atom, we have a detailed picture of the Co-Mo-S phase on the atomic scale. Figure 9.23 summarizes schematically what a working Co-Mo/A1203 hydrodesulfurization catalyst looks like. It contains MoS2 particles with dimensions of a few nanometers, decorated with cobalt to form the catalytically highly active Co-Mo-S phase. It also contains cobalt ions firmly bound to the lattice of the alumina support, and it may contain crystallites of the stable bulk sulfide Co9S8, which has a low activity for the HDS reaction [49]. 

Mossbauer Spectroscopy. Figure 1 shows room temperature Mossbauer emission spectra of two of the unsupported Co-Mo catalysts which we have studied in the present investigation. It is observed that the MES spectra of the two catalysts are quite different. For the catalyst with the low Co/Mo ratio (0.0625), a quadrupole doublet with an isomer shift of 6=0.33 mm/s and a quadrupole splitting of AE =1.12 mm/s are observed (spectrum a). These parameters are very similar to those observed previously for the Co-Mo-S phase in other catalysts (6-9). Furthermore, the spectrum of an unsupported catalyst with Co/Mo = 0.15 is found to be essentially identical to spectrum (a). The MES spectrum (b) of the catalyst with Co/Mo =... 

They also applied many other physical characterization techniques to the analysis of this system to confirm the existence of the Co/Mo/S phase 02 and NO chemisorption, XPS, and transmission electron microscopy (TEM), with marginal results because of the difficulty caused by the intrinsic disorder present in the MoS2, as discussed below. This disorder makes... 

Fig. 9.18 M0S2 has a layer structure each slab of M0S2 is a sandwich of Mo4+ ions between two layers of S2 ions. Two defect sites, exposing one and two Mo ions are indicated. The bottom structure shows the Co-Mo-S phase, with cobalt atoms at the edges of a MoS2 particle.

In order to investigate the precise structure of the Co-Mo-S phase, techniques are required which provide information on the atomic scale. The infrared spectra... 

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