Cobalt-molybdenum catalysts promoter atoms

The HDS and HDN activities of a M0S2/AI2O3 catalyst both increase substantially on addition of cobalt or nickel. Several explanations for the promoter function of cobalt and nickel have been proposed (3-6). One model is based on the assumption that the promoter atoms induce a surface reconstruction of the edges of the M0S2 layers, leading to a greater number of exposed molybdenum atoms and, thus, to enhanced activity (24). On the other hand, infrared (19) and EXAFS (16, 20) investigations demonstrated that the molybdenum atoms are covered by the sulfur and the promoter atoms. Thus, if only the molybdenum atoms were active, the activity should decrease upon addition of cobalt or nickel atoms. 

Kushiyama et al (129) reported that phosphorus also promotes the activity of highly divided unsupported molybdenum and cobalt-molybdenum sulfide catalysts for HDS (and HDM) of crude oil, with a maximum at a P/(Co + Mo) atomic ratio of 2. This result warrants attention because it shows that phosphorus may directly modify the active sulfide phase(s) even in the absence of the alumina support. 

The Role of Promoter Atoms in Cobalt-Molybdenum and Nickel-Molybdenum Catalysts... 

TMS catalyst, used in refineries, are mixed sulfides of cobalt-molybdenum (CoMo) or nickel-molybdenum (NiMo) with a promoter atomic ratio Co(Ni)/[Co(Ni) -I- Mo(W)] between 0.2 and 0.4 (2), supported on high surface area materials such as alumina to increase dispersion of the active component of the catalyst. Although CoMo sulfide is the favorite catalyst for HDS reactions, the use of NiMo sulfide is preferred in HDN reactions and hydrogenation processes, to treat feedstock with a high concentration of unsaturated compounds. 

Catalysts based on molybdenum disulfide, M0S2, and cobalt or nickel as promoters are used for the hydrodesulfurization (HDS) and hydrodenitrogenadon (HDN) of heavy oil fractions [48,49]. The catalyst, containing at least five elements (Mo, S, Co or Ni, as well as O and A1 or Si of the support), is rather complex and represents a real challenge for the spectroscopist. Nevertheless, owing largely to research in the last twenty years, the sulfided C0-M0/AI2O3 system is one of the few industrial catalysts for which we know the structure in almost atomic detail [49, 50],... 

In 1959, H. Beuther et al. (8) of Gulf Oil Company published the first systematic study of the HDS activity of CoMo and NiMo supported on alumina as a function of the atomic ratio Co(Ni)/Mo. As a result, they showed what they called a promoter effect of the cobalt (or nickel) on the molybdenum for atomic ratios Co/Mo = 0.3 and Ni/Mo = 0.6. This publication was preceded by several patents proposing similar atomic ratios for cobalt by Union Oil of California (1948) (9) and Shell Oil Company (1954) (10) and for nickel by Union Oil of California (1954)(/7). Figure 1 shows a typical activity curve of NiMo/Al203 catalysts as a function of the value of the atomic ratio Ni/Mo (12). 

The traditional HDS catalysts are CoMo or NiMo based catalysts. Highly dispersed molybdenum sulphide exhibits a low but stable catalytic activity in a sulfur-containing environment. By adding the promoter elements, cobalt or nickel, in an atomic Co/Mo or NiMo ratio of 0.3 to 0.5 a ten-fold increase in activity could be achieved [1], Further improvements were made by addition of chelating agents, phosphate, and fluorine [2]. 

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