Sulfided noble metals

A general explanation cannot be given, which is valid for different catalysts and reactions, even if these are limited to HDS. One of the characteristic examples is the essential difference in the mechanism of catal3dic effect on sulfided noble metals in comparison with that of molybdenum and tungsten-oxide based catalyst, due to large differences in the energy of their sulfide formation. This difference in HDS mechanism is also well seen from the rather different effect of sulfidation on precious metal promoted MoOx, in comparison with CoMoOx, in the initial period of sulfidation. 

In general, platinum, with or without modifiers, makes the best catalyst for minimizing dehalogenalion, combined with a fast rate of reduction of the nilro function. Excellent results have been obtained by use of supported noble-metal sulfides (4/). These catalysts [manufactured by Engelhard Industries, Newark, New Jersey (5/)] have a high intrinsic selectivity for this type of reduction and have given excellent results under a wide range of conditions. Elevated temperatures and pressures are necessary to achieve reasonable rates (33,34). 

Silica, the silicates and, in general, crystals containing oxygen or fluorine and metal atoms which can form noble-gas type cations are found to satisfy the electrostatic valence rule and the other rules for ionic crystals, which indicates that the ionic character of these substances is of greatest importance. The sulfides of metals other than the alkalies and alkaline earths, on the other hand, do not satisfy these rules magnetic evidence further shows these substances not to be ionic. 

On sulfided metallic phases the hydrotreatment reactions also takes place. Noble metal catalysts usually include a zeolitic support. They are particularly used for fulfilling two different objectives, in the case of a gasoline oriented HCK their cracking and isomerization activity is the most important (increasing high octane and conversion yield). In a diesel HCK unit, the noble metal catalyst is mainly oriented to aromatic saturation and cetane improvement. However, in this latter case, also sulfided metal catalysts are used, especially NiW. 

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 acid component of a hydrocracking catalyst can be an amorphous oxide, e.g., a silica-alumina ora zeolite, eg., USY. This component usually serves as a support for the metal compound responsible for the hydrogenation function. The metal compound can be a noble metal, e.g., Pt or Pd, or a mixture of sulfides, e.g., of Ni/Mo, NiAV, or Co/Mo. The relative amounts of the respective compounds have to be thoroughly balanced to achieve an optimum performance. 

In order to get a catalytic cycle it is necessary that the metal sulfide intermediate can react with hydrogen to form the reduced metal complex (or compound) and H2S. For highly electropositive metals (non-noble metals) this is not possible for thermodynamic reasons. The co-ordination chemistry and the oxidative addition reactions that were reported mainly involved metals such as ruthenium, iridium, platinum, and rhodium. 

Divalent sulfur is a poison for most noble metal catalysts so that catalytic hydrogenation of sulfur-containing compounds poses serious problems (p. 10). However, allyl phenyl sulfide was hydrogenated over tris trisphenyl-phosphine)rhodium chloride in benzene to give 93% yield of phenyl propyl sulfide [674. ... 

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 second metal, for example, the promoter, may also be added by subsequent impregnation of binary sulfide. When a nonreactive promoter precursor, for example, metal nitrate, is used it is necessary to resulfide the impregnated sulfide in order to decompose the precursor. Another variation of this method consists in using reactive promoter precursors that will react with the surface of the binary sulfide. In this case, further treatment of the catalyst may not be required. Good precursors include metal carbonyls and metal alkyls (32, 33). The precursor decomposition approach been most widely applied to the MoS2-based systems. However, it has also been extended to the mixed noble-metal sulfides by Breysse and co-workers (34) at Lyon following the work of Passaretti et al (35). 

Careful studies of the physical chemistry of the growth process so as to understand the trade offs between growth rate, pressure, temperature and quality were essential in finding economically successful conditions. In order to understand the kinetics, solubility(10) and p-v-t(77) studies were necessary. The solubility in pure water was found to be too small for crystal growth (0.1 - 0.3 wgt %) but the solubility could be markedly increased by the addition of (OH) which acts as a mineralizer. We have studied mineralizers and their reactions for complexing various refractory oxides and sulfides.(72-76) A variety of complexers are known including (OH) , Cl-, F, Br , r and acid media for the crystallization of Au and other noble metals. Frequently the ratio (solubility/mineralizer concentration) is constant and independent of mineralizer concentration over wide ranges and sometimes it is a small rational number or fraction. 

Dovell and Greenfield have shown that base metal sulfides as well as noble metal sulfides are excellent catalysts for the preparation of secondary amines by the reductive alkylation of primary amines (or their nitro precursors) with ketones.36,37 There was little or no hydrogenation of aromatic rings for arylamines, an important side re-... 

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