Co-Mo catalysts

Because hydrocarbon feeds for steam reforming should be free of sulfur, feed desulfurization is required ahead of the steam reformer (see Sulfur REMOVAL AND RECOVERY). As seen in Figure 1, the first desulfurization step usually consists of passing the sulfur-containing hydrocarbon feed at about 300—400°C over a Co—Mo catalyst in the presence of 2—5% H2 to convert organic sulfur compounds to H2S. As much as 25% H2 may be used if olefins... 

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 most direct information on the state of cobalt has come from Mossbauer spectroscopy, applied in the emission mode. As explained in Chapter 5, such experiments are done with catalysts that contain the radioactive isotope 57Co as the source and a moving single-line absorber. Great advantages of this method are that the Co-Mo catalyst can be investigated under in situ conditions and the spectrum of cobalt can be correlated to the activity of the catalyst. One needs to be careful, however, because the Mossbauer spectrum one obtains is strictly speaking not that of cobalt, but that of its decay product, iron. The safest way to go is therefore to compare the spectra of the Co-Mo catalysts with those of model compounds for which the state of cobalt is known. This was the approach taken... 

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. 

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 MES investigations showed that part of the promoter atoms in Co-Mo catalysts is generally present in a structure also containing molybdenum and sulfur atoms (6j. This structure was termed the Co-Mo-S structure ( 3) and since the promotion of the HDS activity was found to be associated with this structure (9., 11) much work has been initiated in order to characterize further the properties of this Co-Mo-S structure. 

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. 

Mossbauer Measurements. Co-Mo catalysts cannot be studied directly in absorption experiments since neither cobalt nor molybdenum has suitable Mossbauer isotopes. However, by doping with 57Co the catalysts can be studied by carrying out Mossbauer emission spectroscopy (MES) experiments. In this case information about the cobalt atoms is obtained by studying the 57Fe atoms produced by the decay of 57Co. The possibilities and limitations on the use of the MES technique for the study of Co-Mo catalysts have recently been discussed (8., 25.). 

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 =... 

Figure 1. Examples of in situ Mossbauer emission spectra of unsupported Co-Mo catalysts, a) Co/Mo = 0.0625 b) Co/Mo = 0.50.

Figure 2. a) X-ray absorption spectrum near the Mo K-edge of the Co/Mo = 0.125 unsupported Co-Mo catalyst recorded in situ at room temperature b) normalized Mo EXAFS spectrum c) absolute magnitude of the Fourier transform d) fit of the first shell e) fit of the second shell. The solid line in d) and e) is the filtered EXAFS, and the dashed line is the least squares fit. 

Figure 5 (A) Relative selectivities for hydrogenation of butenes for the unsupported Co-Mo catalysts (B) first-order rate parameters for hydrogenation (k ) and HDS...

The EXAFS data recorded after exposure to air of the unsupported Co-Mo catalysts with different cobalt content allow one to examine the effect of cobalt. In spite of a great uncertainty in the coordination numbers, the promoted catalysts seem to have a somewhat smaller domain size than the unpromoted catalyst as indicated both by the smaller second shell coordination numbers and by the larger effect of air exposure (i.e., reduced sulfur coordination number in first shell). This influence of cobalt on the domain size may be related to the possibility that cobalt atoms located at edges of M0S2 stabilize the domains towards growth in the basal plane direction. Recent results on C0-M0/AI2O3 catalysts indicate that Co may also have a similar stabilizing effect in supported catalysts (36). 

Figure 5A shows that the selectivity towards butane formation (i.e. the rate of formation of butane relative to that of the butenes) decreases as the Co/Mo ratio increases in the unsupported catalysts. Similar results have previously been reported for alumina supported Co-Mo catalysts (37, 38) and this behavior does therefore appear to be a quite general feature of Co-Mo catalysts. The large change in the selectivity is observed (Figure 5B) to be related to a greater promotion of the HDS reaction rate compared... 

Ab initio methods, 147-49 Acetate ion, decomposition, 135 Acetylene, interaction with palladium, tunneling spectroscopy, 435,437f Acid-dealuminated Y zeolites catalytical properties, 183 sorption, 175-78 Acid sites, on zeolites, 254 acidification effects, 266 Acoustic ringing, in NMR, elimination, 386 Active sites, nature, 104 Activity measurements, Co-Mo catalysts, 74 Adsorbed molecules,... 

In commercial catalysts, not all of the promoter added to the formulation results in the formation of the unique Co(Ni)-Mo-S species. Figure 17a illustrates the various materials that have been identified in commercial Co-Mo catalysts. The figure also shows how these various forms of cobalt... 

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). 

Heteroatoms, principally O, S, and N, in aromatic rings require Ni-Mo or Co-Mo catalysts for their extensive removal to the levels experienced in petroleum refinement, where these constitutents are associated with aliphatic moieties that are easily removed thermally as well as catalytically under hydrogen pressure. 

Iron and chloride catalysts are basically disposable because they are considered to be rather cheap and difficult to recover from residual products, while Ni-Mo and Co-Mo catalysts are too expensive to be considered disposable (82). Recovery of very fine particles of MoS2 by hydroclone separation has been shown to be promising (83). Disposable catalysts added at levels similar to that of ash mineral contents significantly reduce the potential recovery of oil in both distillation and extraction. This is problematic because equal volumes of oil adhere to solid particles after separation. Slurry transportation of residues suffers from the same problem. Even if the cost of the disposable catalysts is affordable, adding 1 to 5% of the catalyst to the... 

Carbon-supported Co-Mo catalysts used in terpene hydrodesulfurization (SM = sodium molybdate, AHM = ammonium heptamolybdate, CN = cobalt nitrate). All the catalysts contain 7 wt.-% CoO and 4.4 wt.-% M0O1 corresponding to a Co-to-Co+Mo atomic ratio of 0.75. J... 

Unwanted sulfur-containing components may be removed from petroleum oils by hydrodesulfurization over a Co-Mo catalyst at high temperatures under pressure ( unifining or hydrofining ).38 However, benzo[6]thiophene is hydrodesulfurized with difficulty over a molybdenum catalyst89 40 and it is difficult to remove completely from petroleum oils by hydrofining.41... 

Supported Mo and Co-Mo catalysts have largely been studied in comparison, because of their importance in relation to their structure begins to arise. However, these catalysts are not particularly attractive for HDN, although a short survey of the literature dealing with them could be of value in order to introduce future studies devoted to new Mo-based catalysts for hydrotreatment other than HDS. 

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