CoMo Catalyst

Figure 9.5. Schematic picture of a sulfided CoMoS catalyst, along with a top view of the M0S2 structure, which is built up from trigonal prisms. Cobalt may be present in three states ...

The Nature of Exposed Co In CoMo Catalysts. Figure 1 shows typical original spectra for NO adsorbed on 2% Co/alumlna. Spectra were recorded after reduction at 600°C, after admission of 8 Torr of NO, and again after 5 min. evacuation at 50°C. With the pure alumina the main NO/Co bands were 10 to 20 wavenumbers higher In frequency than those observed when using alumina with sulfate Impurity. 

Effects of Addition of Co to Mo/Alumlna Shown by NO Bands. Figure 3 shows typical spectra for NO on CoMo catalysts containing from 1 to 6 wt% Co prepared by double Impregnation of Aero 1000, a pure... 

Figure 6 shows spectra of NO adsorbed on a similar series of catalysts having the same concentrations of metals on Filtrol 90 alumina which, as discussed, sulfides the catalyst during subsequent reduction. The effect of sulfide, as on Mo/alumlna catalysts (8,10) and as later seen on CoMo catalysts after more-usual sulfiding, was mainly a marked reduction In the Intensities of the NO bands. These were roughly a fourth as Intense as those on unsulflded catalysts on... 

Figure 9 shows CO adsorbed on CoMo catalysts made on two alumina supports. Most of the CO was weakly-held, being largely removed by evacuation for 5 min. at 50 C. Depending on the extent of sulfiding during reduction, various bands were obtained. In the absence of sulfide, as previously seen on Mo/alumlna catalysts (, only Mo (4+ ) or Co (2+) Ions appear to be exposed after reduction, giving... 

Figure 8. Apparent relative exposure of Co and. Mo Ions on reduced Flltrol 90-supported CoMo catalysts as a function of Co concentration.

Mo/alumina ( ). Exposed Co Ions could also serve as members of active pairs, however. This possibility gains some support from the observation that optimum Co concentrations on CoMo catalysts... 

NiW catalysts are the most active for hydrogenation and are best suited for aromatic saturation and hydrocracking. Accordingly, the poisoning effect of H2S and NH3 is significant in these catalysts. However, their HDS and HDN performance is less attractive than that obtained from NiMo and CoMo catalysts. 

The higher desulfurization activity of the CoMo catalyst makes them more suitable for the treatment of sterically hindered S-compounds, avoiding unnecessary hydrogenation. However, this will only apply for very low N-containing feeds and those where the needs for hydrogenation are also small. These conditions can be found in some FCC cuts and SRGOs. 

Certain catalyst manufacturers claims to have optimized the preparation (CoMo catalysts), the formulation or the promotion (aromatic saturation) of their catalysts to achieve an appropriate balance of the hydrogenation function to desulfurize the sterically hindered compounds and yield the 15 ppm S fuel. However, the actual trend is to use NiMo catalyst for the treatment of the more refractory compounds, below 200 ppm S [22],... 

Peries, J.-P. Jeanlouis, P. E. Schmidt, M., and Vance, P. W., Combining NiMo and CoMo Catalysts for Diesel Hydrotreaters, in NPRA Annual Meeting. 1999, March 21-23. 

The active components of this so-called CoMoS catalyst, cobalt and molybdenum, are only active as sulfides. This implies that sulfur should always be present in the gas to keep the metals in the sulfided state. The operating temperature... 

Combustion, 27 189, 190 reaction, sites for, 33 161-166 reaction scheme, 27 190, 196 Commercial isomerization, 6 197 CoMo catalysts, 40 181 See also Cobalt (nickel)-molybdenum-sulfide catalysts Compact-diffuse layer model, 30 224 Compensation behavior, 26 247-315 active surface, 26 253, 254 Arrhenius parameters, see Arrhenius parameters... 

The results of carbon and hydrogen analysis of the various bpt fractions for the five repeat contacts using CoMo or NiMo as the catalyst are shown in Table IV as H C ratios. In all cases, the H C ratios decrease with the increasing bpt of the fraction, reflecting a gradual increase in the aromatic nature and number of rings of the compounds in the fractions. It is noticeable that the respective values for NiMo and CoMo catalysts are similar and that repeat contact did not influence the results. 

Table V. Gc analysis of products from hydrogenation of Dvrene with oxide and presulphided CoMo catalysts...

By applying an approach similar to the one taken with the unpromoted M0S2 nanoclusters, we recently managed to synthesize a model system for the promoted CoMoS catalyst 119). As a result of co-deposition of molybdenum and cobalt onto the Au(l 1 1) crystal during exposure to an H2S atmosphere and subsequent annealing, crystalline CoMoS clusters formed on the Au(l 1 1) terraces. As shown in Fig. 24, the main new finding is that the CoMoS nanoclusters now adopt a... 

On the basis of the detailed atomic-scale information provided by the STM images, a structural model of the CoMoS nanoclusters is proposed in which cobalt atoms have replaced molybdenum atoms along the sulfur edges of hexagonally truncated nanoclusters. As depicted in the ball model in Fig. 25(a) and (c), a tetrahedral environment of the cobalt atoms is produced if the outermost protrusions are assumed to be sulfur monomers, which agrees well with spectroscopic results reported for supported CoMoS catalysts (93,137-139 and previous DFT investigations (128,140. ... 

The first research group to propose a description of the structure of CoMo catalysts was led by Schuit and Gates (13). This group introduced the so-called monolayer model directly derived from the physical studies of CoMo oxide precursors supported on y-alumina carried out by J. T. Richardson (14) (Richardson first proposed the existence of a special Co/Mo entity.) In this model the upper or first layer contained only sulfur atoms, each bonded to a molybdenum atom of the second layer (below the first one), these molybdenum atoms being bonded to two oxygen atoms also located in this second layer. When a sulfur atom was removed by reduction (H2 flow) of Mo5+ to Mo3+, a vacancy was formed at the surface and became the preferential adsorption site of a sulfur atom in the organic gas phase. The presence of cobalt incorporated into underlying layers of the alumina... 

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. 

Combining the hydrogenation and hydrogenolysis properties of the catalysts, the better selectivity to 4-chloroaniline from 4-chloronitrobenzene is obtained over the CoMo catalyst at low temperature. Indeed, the kinetic... 

Recently we have proposed an HDS catalytic treatment based on sodium-doped CoMo catalysts [Ref. 1-3]. Previous studies concerned essentially alumina-supported catalysts. As carbon was shown to be a good support for sulfided CoMo catalysts [Ref.4], we decided to investigate the performance of carbon-supported catalysts in terpene HDS. 

NiMo and CoMo catalysts, temperatures of laboratory experiments must be kept below 300°C, but on Mo alone, hydrogenation is so slow compared to hydrogenolysis that the thermodynamic limitation can easily be neglected. Under industrial conditions, however, it must never be forgotten in the rate calculation. 

New and improved catalysts and different processing schemes are among the subjects of active research on deep HDS.28 79 81 For example, some recent studies examined carbon-supported CoMo catalysts for deep HDS, 82-85 Binary oxide supports such as Ti02-Al203 have been examined for making improved HDS catalysts.86 89... 

---