Ni-Mo -S phase

Fig. 3. Three forms of nickel present in a sulfided Ni-MoA.1203 catalyst as active sites on the M0S2 edges (the so-called Ni-Mo-S phase), as segregated M3S2, and as Ni2+ ions in the support lattice.

Fig. 4. Structure involving the nickel atoms in the Ni-Mo-S phase as determined by EXAFS spectroscopy(20) solid circle, nickel open circles, sulfur and shaded circles, molybdenum atoms.

The interaction between oxidic nickel or cobalt and the phosphated support becomes weaker (114,115). Less nickel or cobalt is lost to the support, and more is available for the formation of the active Ni-Mo-S phase on the catalyst surface. 

A third model therefore attributed the promotion effect to cobalt present in the Co-Mo-S phase, with cobalt atoms located at the M0S2 surface a significant contribution of separate Co9S8 was excluded (29). This so-called Co-Mo-S model (or Ni-Mo-S model for Ni-Mo catalysts) is currently the one most widely accepted. 

Commercial heterogeneous HDS catalysts for refinery use consist, almost without exception, of nickel- and/or cobalt-promoted molybdenum oxide located on a high surface area (approx. 300 m g ) alumina or silica-alumina support. Cobalt and nickel promoters increase the catalytic activity, particularly towards thiophenes whether Co or Ni is used as a promoter depends on the specific function for which the catalyst should be optimal. The catalyst material is shaped into porous pellets, a few millimeters in size, and these pellets are loaded into the reactor, forming a catalyst bed of 30-200 m volume. During start-up of a freshly loaded reactor, the catalyst bed, which is in the oxidic form, is sulfided, typically by treatment with an oil feed which has been spiked with a reactive sulfur compound that readily generates H2S in situ. The oxidic precursor phases (non-stoichiometric CoMo or NiMo surface oxides) are thereby converted into sulfidic phases termed Co-Mo-S and Ni-Mo-S. The conversion from the oxidic phase to the sulfidic is accompanied by a reduction in Mo oxidation state from +6 to +4. 

Ni-W-S phase in the NiW/AC catalysts. In this case, the WS2 particle growth in the c direction was observed on the addition of Ni. The NiW/AC catalyst was more active than the CoW/AC catalyst. Although Co-Mo-S phase was detected, this catalyst was prone to the formation of CogSg. For the same amounts of active metals, presence of the Ni-W-S phase in the NiW/AC catalyst was more evident than the Co-W-S phase in the CoW/AC catalyst. A similar observation was also made for the CoMo/AC catalyst. ... 

The progress of sulfiding of the NiW/AC and NiW/Al203 catalysts could be monitored by XRD, MOS and EXAFS techniques. These techniques confirmed the coexistence of the Ni-W-S and NiS-WO S phases. The presenee of the latter phase was more evident on the y-Al203 support than on AC. This resulted from the stronger interaetion of the W oxide with the former support. In addition to these phases, a separate Ni sulfide was also identified. The study also showed that the conversion of the W oxide to sulfide phase inereased when sulfidation was conducted at a high pressure. In every case, it was easier to sulfide Mo oxide than W oxide. 

Calcined and sulfided Ni-Mo catalysts supported on ultrastable Y zeolite, NaY, mordenite and ZSM-5 were studied by high-resolution TEM [271]. In USY zeolite, Ni-Mo-S clusters were found in the supercages of the zeolite, whereas, on other zeolites, the sulfide phase was predominantly on the external surface. Bendezu et al. [272] also studied the dispersion and location of Ni-S, W-S and a Ni-W-S phase in US zeolite. 

Catalytic studies of the hydrodesulfurization of thiophene performed at atmospheric pressure in a single-pass microreactor has been studied with Ni-Mo-S/Al203 catalysts derived from the decomposition of (N2H5)Ni (N2H3C00)3 H20 (Figure 6.13). This hydrazinium precursor based catalyst performs better than industrial catalysts [34]. Scanning transmission electron microscopy (STEM) spot analysis shows that Mo is not phase segregated from the promoter metals (Co, Ni, and Co-Ni) on the... 

Studies of fresh ash produced by coal combustion have shown that many trace elements (As, B, Bi, Cd, Cr, Cu, Ge, Hg, Mo, Pb, Ni, Se, Sr, Tl, V, W, Zn) are enriched in the fly ash compared to the bottom ash (Hansen Fisher 1980 Eary et al. 1990 Mukhopadhyay et al. 1996 Karayigit et al. 2001). For example, Mukhopadhyay et al. (1996) reported 10-20 times enrichment of most trace elements in the fly ash compared to the feed coal and association of As with crystalline Fe-0 and Fe-S phases in the bottom ash from a power plant in Nova Scotia fed by eastern Canadian coal. Elements enriched in fly ash are typically those more easily volatilized. Because fly ash particles also have smaller sizes and therefore greater reactivity than bottom ash, the probability of metal leaching is correspondingly greater. Ainsworth Rai (1987) and Rai et al. (1988) found that most of the Cu, Mo, Se, Sr, and V in fly ash was readily soluble. 

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