Transition-metal sulfide catalysts catalytic materials

Many minerals or their synthetic equivalents are of industrial importance, and the possibilities of greater understanding of their behavior and properties through application of the methods described in this book is a major stimulus to research. Two examples are chosen for discussion here The first is the zeolites, a group of framework alumino-silicates (with interstitial Na+, Ca +, and HjO), characterized by very open frameworks with large interconnecting spaces or channels the second is the transition-metal sulfide catalysts, materials already generally discussed in Chapter 6, but considered here specifically in relation to their catalytic properties. 

Catalytic hydrodesulfurization (HDS), the removal of sulfide in the form of H2S from petroleum, is a critical step in the industrial refinement process and one of increasing importance as the cleaner world supplies of petroleum feedstocks dwindle and the poorer quality feedstocks have to be used. The removal of sulfur (and certain other impurities such as nitrogen in hydrodenitrogenation) is undertaken using transition-metal sulfide catalysts (Weisser and Landa, 1973). The most widely used materials... 

The main classes of materials employed as catalysts are metals (generally transition and noble metals), oxides (including transition-metal oxides), transition-metal sulfides and zeolites. In the following sections, we discuss some of the more common structures and chemistry exhibited by catalytic systems. 

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. 

The presence of V3S4 crystals can only be attributed either to an autocatalytic mechanism of this type or the migration of the deposited metals. It is known that deposited Ni and V sulfides possess some catalytic activity (see Section IV). Slurry processes have been proposed which utilize Ni and V deposited from the oil onto a slurry material (Bearden and Aldridge, 1981). Studies have appeared in the literature demonstrating that nearly all of the transition metals are catalytically active for HDS reactions and presumably for HDM (Harris and Chianelli, 1984). Rankel and Rollmann (1983) impregnated an alumina catalyst base with Ni and V and concluded that these sulfides display an order of magnitude lower activity than the standard Co-Mo sulfide catalyst for HDS reactions, but exhibited similar activity for HDM reactions. 

Transition Metal Salts and Oxides on Alumina. Transition metal salts, particularly chlorides and nitrates, are frequently used as starting materials for the preparation of supported transition metal oxides or supported precursors for supported metal catalysts. Also, many catalytic materials, particularly supported molybdenum and tungsten oxide and sulfide catalysts, contain transition metal ions, namely Co, Ni , and Fe " as promoters. Thus, it is interesting to study the spreading and wetting behavior of salts of these transition metals and of their oxides. This is of particular importance for promoted catalyst materials, since in practice the incorporation of the active phase and the promoter should be possible in one step for economic reasons. 

As shown by Eqs. 1 and 5, the ORRs in the aqueous alkaline electrolyte and in the non-aqueous electrolyte share the same two-electron reduction. This means that all catalysts showing the catalytic activity towards the ORR in alkaline fuel cells and metal-air batteries are theoretically suitable for the non-aqueous electrolyte Li-air batteries. Base on the chemical composition of the materials, these catalysts can be briefly classified into the following categories (1) porous carbon and doped carbon materials, (2) transition metal oxides, nitrides, and sulfides, (3) marcocyclic transition metal complexes, (4) non-precious metals and alloys, (5) precious metals and alloys, (6) organic redox mediators. 

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