Sulfiding, Co-Mo catalysts

Figure 9.19 shows Mossbauer spectra of sulfided Co-Mo catalysts. The Mossbauer spectra of the sulfided Co-Mo/Al203 catalysts contain essentially three contributions, indicated by bar diagrams in Fig. 9.19 ...

A formerly unknown state, labeled Co-Mo-S in Fig. 9.19, which is most evident in sulfided Co-Mo catalysts of low cobalt content. 

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. 

Wang, A.J., Wang, Y., Kabe, T. Chen, Y.Y., Ishihara, A. Qian, W.H. Hydrodesulfurization of dibenzothiophene over siliceous MCM-41-supported catalysts. I. Sulfided Co-Mo catalysts, J. Cata. 2001,199,19. 

Sulfided Mo and Co-Mo catalysts, used in hydrotreating reactions, contain Mo as M0S2. This compound has a layer structure consisting of sandwiches, each of a Mo layer between two S layers. The chemical activity of M0S2 is associated with the edges of the sandwich where the Mo is exposed to the gas phase the basal plane of the anions is... 

Figure 4.23. Infrared spectra of NO probe molecules on sulfided Mo, Co, and Co-Mo hydrodesulfurization catalysts. The peak assignments are supported by the IR spectra of organometallic model compounds. These spectra allow for a quantitative titration of Co and Mo sites in the Co-Mo catalyst.

The XPS spectra of the freshly sulfided Co-Mo/NaY catalysts were measured on an XPS-7000 photoelectron spectrometer (Rigaku, A1 anode 1486.6 eV). The sample mounted on a holder was transferred from a glove bag into a pretreatment chamber attached to the spectrometer as possible as carefully not to be contacted with air. The binding energies (BE) were referenced to the Si2p band at 103.0 eV for the NaY zeolite, which had teen determined by the Cls reference level at 285.0 eV due to adventitious carbon. 

The deuteration pattern of the tetrahydroquinoline (Scheme 53)258 is rather similar to that reported by Laine for the hydrogenation of (110) with the clusters H2Os3(CO)10 and Os3(CO)12,260 as well as with sulfide/Co-Mo/7-Al203 heterogeneous catalysts.248 249 The... 

Cobalt-molybdenum catalysts are in general much more active for HDS than single molybdenum catalysts. Thus, it is essential to investigate the state of cobalt in the sulfided Co-Mo/Al203 catalyst. 

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]).

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... 

The previous EXAFS studies were restricted to supported catalysts. Furthermore, the structural properties determined by MES and EXAFS were mainly related to the HDS activity and not to the other catalytic functions. Presently, we will report EXAFS (both Mo and Co), MES, HDS and hydrogenation activity studies of unsupported Co-Mo catalysts. These catalysts have been prepared by the homogeneous sulfide precipitation method (l8) which permits large amounts of Co to be present as Co-Mo-S. The choice of unsupported catalysts allows one to avoid some of the effects which inherently will be present in alumina supported catalysts, where support interactions may result in both structural and catalytic complexities. 

Sample Preparation. The preparation of the unsupported Co-Mo catalysts has been carried out using the homogeneous sulfide precipitation (HSP) method as described earlier (l8) and only few details will be given here. A hot (335 3 5 K) solution of a mixture of cobalt nitrate and ammonium heptamolybdate with a predetermined Co/Mo ratio is poured into a hot (335 3it5 K) solution of 20 ammonium sulfide under vigorous stirring. The hot slurry formed is continuously stirred until all the water has evaporated and a dry product remains. This product is finally heated in a flow of 2% H2S in H2 at 675 K and kept at this temperature for at least b hr. Catalysts with the following Co/Mo atomic ratios were prepared 0.0, 0.0625, 0.125, 0.25, 0.50, 0.75, and 1.0. 

Hydrogen sulfide in the reaction atmosphere has been reported to accelerate liquefaction directly, in addition to controlling the extent of sulfiding of iron, Ni-Mo, and Co-Mo catalysts (39, 40). 

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