Unsupported Co-Mo catalysts

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 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). 

Catalytic and structural information has been obtained for unsupported Co-Mo hydrotreating (HDS) catalysts. 

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... 

AEM studies of unsupported Ni-Mo catalysts with large crystals suggest that the Ni atoms are also located at the M0S2 edges but the results are more qualitative than in the case of the Co-Mo catalysts. 

Figure 3 Magnetic susceptibility (left scale cgs units per gram atom Co) and effective moment (right scale Bohr magnetons) versus temperature for an unsupported Co-Mo HDS catalyst exhibiting Co-Mo-S as the only Co phase (295 data points are shown).

An alternative way for thiophene synthesis is the dehydrogenation of tetrahydrothiophene (THT). This molecule is easily produced, with an excellent yield, by oxygen/sulfor substitution in tetrahydrofuran. THT dehydrogenation was studied by different authors on sulfided Ni, Mo or Co-Mo catalysts. Recently Lacroix et al [5] performed an extensive study of the reaction on unsupported sulfides. 

The activity shown by unsupported Mo sulfide or Co molybdate catalysts W Is not Inconsistent with the nature of the active sites postulated. The essential pair members and Interactions could all exist on unsupported catalysts. Either Co or Mo alone can cause desulfurization. The support serves mainly to Increase the amount of exposed Co and Mo In some desirable configuration. 

Mo EXAFS. In Figure 2a we have shown an X-ray absorption spectrum near the Mo K-edge of the unsupported catalyst with Co/Mo = 0.125. The spectrum has been obtained in situ and at room temperature. After background subtraction, multiplication by k and normalization,... 

Table II. Bond lengths and coordinations numbers obtained by fitting the Fourier filtered Mo EXAFS of the Co-Mo unsupported catalyst recorded in situ at room temperature. ...

It has previously been found (3., 11, 18, 31-3 ) that unsupported catalysts exhibit a HDS activity behavior quite similar to that of supported catalysts. This suggests that although the support is of importance, it does not have an essential role for creation of the active phase. Thus, it is very relevant to study unsupported catalysts, both in their own right and also as models for the more e-lusive supported catalysts. Many different explanations have been proposed to explain the similarity in behavior of unsupported and supported catalysts ( 3, 31-3b). Recently, we have observed that for both types of catalysts the HDS activity behavior can be related to the fraction of cobalt atoms present as Co-Mo-S (9-11 35). 

From the Co EXAFS results alone one cannot conclude whether the Co atoms are located at edges or basal planes but a comparison of the Co EXAFS data with the above Mo EXAFS results indicates that the edge position is the most likely one. This Co location is illustrated in Figure T For the unsupported catalysts, many of these "surface" positions may be present at internal edges (i.e., at the "domain" boundaries). Recently, direct evidence confirming the edge position has been obtained by combining MES results (to ensure that Co is present as Co-Mo-S in the samples studied) with ir spectroscopy (lU) or with analytical electron microscopy (l ) ... 

A comparison of the data for the unsupported and the supported catalysts reveals that the activity per Co atom present as Co-Mo-S is much lower for the former catalyst system. This is probably related to the fact that in these catalysts many of the Co atoms are either inaccessible to the reactants or are subjected to diffusion restrictions. 

---