Sulfide catalysts crystal structure

Hariita et al. (14) prepared spherical particles of molybdenum sulfide and cobalt sulfide with a narrow size distribution by reaction of dilute ammonium orthomolybdate or cobalt(II) acetate with sulfide ions liberated from thioacetamide as a reservoir of S2- ions in weakly acidic media. The compositions of these metal sulfides were estimated to be Mo S 0 = 1.0 1.7 3.0 and Co S 0 = 1,0 4.5 6.4 by chemical analysis. Figure 3.1.4 shows an SEM of a thus prepared uniform molybdenum sulfide particles sample. These sulfide particles were of no distinct crystal structure as shown by x-ray diffractometry. The isoelectric points of the Mo sulfide and Co sulfide particles in terms of pH were 1.9 and 3.1, respectively. Both of them are useful as hydrodesulfurization catalysts. 

In the preceding part of this paper the predominance of the "periodic effect on HDS by sulfide catalysts was described. Because periodicity dominates, crystal structure is of secondary importance. However, in this section we briefly examine the effect of crystal structure on the catalytic properties of the transition metal sulfides. In the case of catalysts such as M0S2 and WS2, the most industrially important catalysts, the effect of crystal structure is quite pronounced. An understanding of the effect of crystal structure in these catalysts is essential to optimizing their catalytic properties for a given application. 

The effect of crystal structure may be investigated by preparing catalysts, as described above, at various temperatures which assures a set of catalysts having variable surface areas, pore size distributions, and crystallinity. Measuring the catalytic activity as a function of these physical properties will help to define the role of crystal structure for the particular transition metal sulfide. In general, the HDS is poorly correlated to N2 BET surface area. This non-correlation can be most easily seen by preparing a... 

Crystal structure plays a secondary role in catalysis by the Transition Metal Sulfides. As the periodic trends for HDS of the binary sulfides shows the dominant effect is which transition metal is present in the reaction, this transition metal takes on the structure and stoichiometry of the phase which is most stable in the sulfur containing catalytic environment. The unsupported promoted catalyst systems can be grouped into "synergic" pairs of sulfides. Because these pairs are related to the basic periodic trends of the binary Transition Metal Sulfides through average heats of formation. 

In comparison with the information about geometrical factors for metallic catalysts, little parallel progress has been made with oxides or sulfides. This is largely due to the uncertainty which exists concerning the precise chemical composition of these catalysts in their active state, and partly to the lack gf reliable data for the crystal structures of some of the lower oxides and sulfides. 

Role of Crystal Structure on Reaction Selectivity. The question of selectivity of the HDS reaction and its control by the sulfide catalyst has been little studied. However, the existence of two active sites on M0S2 is well established throughout much of the literature, particularly in the work of Voorhoeve (16). One site is usually described as a direct sulfur removal site and the other as a hydrogenation site. 

We begin with the structure of a noble metal catalyst. The emphasis is on the preparation of rhodium on aluminum oxide and the nature of the metal-support interaction. Next we focus on a promoted surface in a review of potassium on noble metals. This section illustrates how single crystal techniques have been applied to investigate to what extent promoters perturb the surface of a catalyst. The third study deals with the sulfidic cobalt-molybdenum catalysts used in hydrotreating reactions. Here we are concerned with the composition and structure of the catalytically active... 

The catal3dic activity of TMS catalysts is related to surface defects in their crystal lattice. However, the properties and stability of these defects are determined by the bulk atomic and electronic structure. According to this point of view, the stability of these TMS phases in a catal3dic environment has to be known to understand the fundamental origins of the catal3dic effects in the metal sulfides. 

Metal oxides belong to a class of widely used catalysts. They exhibit acidic or basic properties, which make them appropriate systems to be used as supports for highly dispersed metal catalysts or as precursors of a metal phase or sulfide, chloride, etc. Simple metal oxides range from essentially ionic compounds with the electropositive elements to covalent compounds with the nonmetals. However, taking into account the large variety of metal oxides, the principal objective of this book is to examine only metal oxides that are more attractive from the catalytic point of view, and most specifically transition metal oxides (TMO). In particular, TMO usually exhibit nonstoichiometry as a consequence of the presence of defective structures. The interaction of TMO with surfaces of the appropriate carriers develop monolayer structures of these oxides. The crystal and electronic structure, stoichiometry and composition, redox properties, acid-base character and cation valence sates are major ingredients of the chemistry investigated in the first part of the book. New approaches to the preparation of ordered TMO with extended structure of texturally well defined systems are also included. 

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