Metal sulfide active materials

Table II. Properties of Metal Sulfide Active Materials...

The chapter Fundamental Studies of Transition-Metal Sulfide Catalytic Materials by Chianelli, Daage, and Ledoux reviews current understanding of the relationship between structural and other properties of these catalysts and their catalytic activity and selectivity in hydrodesulfurization. In view of increasing environmental demands, this field has been heavily researched. The authors show how systematic studies and applications of novel methods can provide considerable understanding of these important catalysts. 

Extensive testing of metal sulfide electrode materials was performed in cells of the type shown in Figure 1. The metal sulfide electrode case and current take-off rod were made of dense graphite to avoid any contact and possible contamination of the active material by metallic constituents. The active material (metal sulfide powder in a porous graphite matrix or a mixture of metal sulfide and graphite powder) was contained in the cavity (2.5 cm in diameter X 0.6 cm deep) of the dense graphite electrode case. A porous ceramic separator was placed over the active material and secured with high purity alumina pins. 

CatalyticaHy Active Species. The most common catalyticaHy active materials are metals, metal oxides, and metal sulfides. OccasionaHy, these are used in pure form examples are Raney nickel, used for fat hydrogenation, and y-Al O, used for ethanol dehydration. More often the catalyticaHy active component is highly dispersed on the surface of a support and may constitute no more than about 1% of the total catalyst. The main reason for dispersing the catalytic species is the expense. The expensive material must be accessible to reactants, and this requires that most of the catalytic material be present at a surface. This is possible only if the material is dispersed as minute particles, as smaH as 1 nm in diameter and even less. It is not practical to use minute... 

The performance of a supported metal or metal sulfide catalyst depends on the details of its preparation and pretreatraent. For petroleum refining applications, these catalysts are activated by reduction and/or sulfidation of an oxide precursor. The amount of the catalytic component converted to the active ase cind the dispersion of the active component are important factors in determining the catalytic performance of these materials. This investigation examines the process of reduction and sulfidation on unsupported 00 04 and silica-supported CO3O4 catalysts with different C03O4 dispersions. The C03O4 particle sizes were determined with electron microscopy. X-ray diffraction (XRD), emd... 

Carbon, activated Chlorates Calcium hypochlorite, all oxidizing agents, unsaturated oils Ammonium salts, acids, metal powders, sulfur, finely divided organic or combustible materials, cyanides, metal sulfides, manganese dioxide, sulfur dioxide, organic acids... 

While catalytic HDM results in a desirable, nearly metal-free product, the catalyst in the reactor is laden with metal sulfide deposits that eventually result in deactivation. Loss of catalyst activity is attributed to both the physical obstruction of the catalyst pellets pores by deposits and to the chemical contamination of the active catalytic sites by deposits. The radial metal deposit distribution in catalyst pellets is easily observed and understood in terms of the classic theory of diffusion and reaction in porous media. Application of the theory for the design and development of HDM and HDS catalysts has proved useful. Novel concepts and approaches to upgrading metal-laden heavy residua will require more information. However, detailed examination of the chemical and physical structure of the metal deposits is not possible because of current analytical limitations for microscopically complex and heterogeneous materials. Similarly, experimental methods that reveal the complexities of the fine structure of porous materials and theoretical methods to describe them are not yet... 

Many of the catalysts for the hydrodesulfurization process are produced by combining (Table 5-5) a transition metal (or its salt) with a solid support. The metal constituent is the active catalyst. The most commonly used materials for supports are alumina, silica, silica-alumina, kieselguhr, magnesia (and other metal oxides), as well as the zeolites. The support can be manufactured in a variety of shapes or may even be crushed to particles of the desired size. The metal constituent can then be added by contact of the support with an aqueous solution of the metal salt. The whole is then subjected to further treatment that will dictate the final form of the metal on the support (i.e., the metal oxide, the metal sulfide, or even the metal itself). 

Prins summarizes advances in understanding of the reactions in catalytic hydrodenitrogenation (HDN), which is important in hydroprocessing of fossil fuels. Hydroprocessing is the largest application in industrial catalysis based on the amount of material processed. The chapter addresses the structures of the oxide precursors and the active sulfided forms of catalysts such as Ni-promoted Mo or W on alumina as well as the catalytically active sites. Reaction networks, kinetics, and mechanisms (particularly of C-N bond rupture) in HDN of aliphatic, aromatic, and polycyclic compounds are considered, with an evaluation of the effects of competitive adsorption in mixtures. Phosphate and fluorine promotion enhance the HDN activity of catalysts explanations for the effect of phosphate are summarized, but the function of fluorine remains to be understood. An account of HDN on various metal sulfides and on metals, metal carbides, and metal nitrides concludes this chapter. 

Catalysts were some of the first nanostructured materials applied in industry, and many of the most important catalysts used today are nanomaterials. These are usually dispersed on the surfaces of supports (carriers), which are often nearly inert platforms for the catalytically active structures. These structures include metal complexes as well as clusters, particles, or layers of metal, metal oxide, or metal sulfide. The solid supports usually incorporate nanopores and a large number of catalytic nanoparticles per unit volume on a high-area internal surface (typically hundreds of square meters per cubic centimeter). A benefit of the high dispersion of a catalyst is that it is used effectively, because a large part of it is at a surface and accessible to reactants. There are other potential benefits of high dispersion as well— nanostructured catalysts have properties different from those of the bulk material, possibly including unique catalytic activities and selectivities. 

Although the hydrogenation activity of metal sulfides is lower by several orders of magnitude than that of metal catalysts, sulfides allow operations under conditions that are impractical for metals. They are generally used as highly dispersed materials on a high surface area support, such as y-alumina, in fixed bed operation. Most important is catalyst design to minimize deactivation due to the deposition of metals (V, Ni) in the feed and of coke at the mouths of the pores. Metal sulfides can also be used as finely dispersed phases in continuous slurry reactors to reduce the mass transport limitations of heavy oils. 

For all metal reactors, the yields of ethylene and total coke were often found to vary significantly with the past history or pretreatment of the reactor. Pretreatments used were with either oxygen, steam, or hydrogen sulfide. Total coke is defined here as the sum of the CO, C02, and net coke it is postulated that CO and C02 were formed by oxidation of part of the coke which formed and that net coke is the amount of coke left on the reactor walls at the end of the run. The results for runs M01 and D44 are one example of the large differences in yields that can be obtained in reactors of the same material of construction (see Table I). Run M01 was made after a stainless steel 304 reactor had been treated with hydrogen sulfide. Hydrogen sulfide results in the formation of metal sulfides (8) and acts in most cases to passivate the surface so that coke formation is minimized. D44 was made using a new stainless steel 304 reactor, whose surface became very active when it was treated with steam. 

As industrial catalysts, activated carbon,195-202 metal sulfides and oxides,193,195-197 and zeolites203,204 are preferred. To diminish side products these catalysts can contain inert materials (graphite, quartz, china),198 which also extend the duration of catalytic activity. 

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