Sulfur, bond strengths

In spite of much theoretical work, we still do not have a complete picture of why the Co and Ni-promoted M0S2 catalyst is so successful. Interpretations range from the promoter-induced weakening of the metal-to-sulfur bond strength to the presence of unique sulfur species bound between molybdenum and the promoter. 

A theoretical foundation for understanding these correlations is found in the calculated bulk electronic structures of the first- and second-row TMS. The electronic environment of the metal surrounded by six sulfur atoms in an octahedral configuration was calculated, using the hypotheses that all the sulfides could be represented by this symmetry as an approximation. There are several electronic factors that appear to be related to catalytic activity the orbital occupation of the HOMO (Highest Occupied Molecular Orbital), the degree of covalency of the metal-sulfur bond, and the metal-sulfur bond strength. These factors were incorporated into an activity parameter (A2), which correlates well with the periodic trends (Fig. 16) (74, 75). This parameter is equal to the product of the number of electrons contained in the... 

In industrial practice, catalytic surfaces are often very complex, not only structurally but also chemically. An example is shown in Fig. 1 from Chianelli et al. [6] for hydrodesulfurization catalysts. The data indicate that maximum dibenzothiophene hydrodesulfurization activity is achieved at intermediate heats of formation of metal sulfides, i.e., at intermediate metal-sulfur bond strengths. Again, while such surface energetic considerations do not have ab initio predictive ability, they are valuable tools for catalyst synthesis and prescreening. 

A catalyst is usually requires in the formation of H2S. Metal sulfides are the most common catalysts used in the laboratory or in large-scale production of H2S. The catalytic activity of metal sulfides is linked to the metal-sulfur bond strength. 

Different fi-om C-tracer studies, these being connected mostly with reaction mechanism problems, sulfur isotopes are connected with apph-cations to the role of sulfur in solid catalysts to a great extent. This is connected with the wide application of sulfide catalysts. Metal-sulfur bond strengths are substantially ( 100kJ/mol) larger than those of the... 

The most general observation is to be made on the basis of the tracer studies that only a part of surface sulfur is mobile or exchangeable on catalysts with sulfides and oxides. Supported Co and Ni contain only mobile sulfur, if sulfided by thiophene and contain both mobile and immobile sulfur, if they are sulfided by H2S/H2.[ 1 This is interpreted with the differences in the metal-sulfur bond strengths, as the edge S-atoms are held more weakly than the top S-atoms of the slab.I This follows from the results of some radiosulfur tracer studies of metal single crystals that different surface S-species are formed at different surface sulfur concentrations, and on different crystal faces of Pt, and... 

According to Berhault et al., the weakening of the metal-sulfur bond strength makes the sulfiir atom shared between Co and Mo more basic and also more labile (81) This result suggests an enhanced quality of the active sites due to the electronic promotion by cobalt. 

Chianelli (85). The resulting effect of this assumption is that the catalysts with the lowest heat of formation are the most active. They also used this model through theoretical calculations to study the promotion effect and fovmd that sulfur removal is favored by Co or Ni (87). However, this model is controversial since too weak sulfur bond strength would probably create anionic vacancies vmable to bond sulfur organic compounds in an efficient way. 

Role of Sulfur Vacancies. The formation of sulfur vacancies is also related to the electronic effect because of the relatively narrow range of metal-sulfur bond strength required for high catalytic activity. Consequently, sulfur vacancies are considered as critical, and, given the high temperature and the reducing... 

Figure 10.4 Periodic trends as observed by reporting the HDS activity results (a) of Pecoraro and Chianelli [121] and (b) ofLedouxet al. [137] versus metal-sulfur bond strength (determined as the total cohesive energy divided by the number of bonds per unit cell, as defined in Refs [138,139]). Adapted from Ref [140], Copyright 2002 with permission from Elsevier.

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