Sulfide catalysts activity

The effect of irreversible sulfur on catalyst activity was investigated in the course of cyclohexane dehydrogenation (Table 3). The comparison of relative activities (activity of sulfided catalyst/activity of fresh catalyst) shows that dehydrogenation is more strongly inhibited by sulfur adsorption on bimetallic catalysts. However, the higher the Pt-Re interaction, the higher the remaining activity of sulfided bimetallic catalysts. 

In the second phase, performed at a maximum temperature of about 370°C, the sulfur and a portion of the coke are removed by combustion. The rate and exothermicity are controlled by limiting the flow of combustion gas through the catalyst. Spent base metal catalysts may have sulfur levels of from 6 to 12 wt % in the form of metal sulfides. A high degree of sulfur removal must be achieved in these first two regeneration steps to avoid the formation of sulfate on the support during the final combustion step. Such a formation causes a loss of catalyst activity. 

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

Platinum and rhodium sulfided catalysts are very effective for reductive alkylation. They are more resistant to poisoning than are nonsulfided catalysts, have little tendency to reduce the carbonyl to an alcohol, and are effective for avoidance of dehydrohalogenation in reductive alkylation of chloronitroaromatics and chloroanilines (14,15). Sulfided catalysts are very much less active than nonsulfided and require, for economical use, elevated temperatures and pressures (300-2(KX) psig, 50-l80 C). Most industrial reductive alkylations, regardless of catalyst, are used at elevated temperatures and pressures to maximize space-time yields and for most economical use of catalysts. 

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... 

At least one member of an active pair should probably exist to a greater extent on sulfided than on unsulflded catalysts. Exposed reduced metal sites, either slightly electropositive or uncharged, shown by Infrared to exist appreciably only on reduced sulfided catalysts, thus appear to be likely members of active pair sites. 

The sulfided catalysts were evacuated for 1 h before catalytic reactions. The reactions were carried out under mild conditions by using a circulation system (0.2 dm ) made of glass. The HDS of thiophene was conducted at 623 K and an initial pressure of 20 kPa (Hj/C H, = 36). The thiophene pressure was kept constant (0.54 kPa) during the reaction by holding a small amount of liquid thiophene kept at 273 K in the bottom of a U-tube in the reaction system. The products were analyzed by gas chromatography. The HDS activity was calculated from the amount of H S produced during the reaction. 

It is noteworthy that CoSx/NaY showed a considerably high HDS activity, being comparable with that of MoSx/NaY. In contrast to relatively low HDS activities of the Co sulfide catalysts supported on Al Oj, the Co sulfide species supported on activated carbon have been repotted to show even higher HDS activities Aan Mo sulfide catalysts [14,15]. This is attributed to an extremely high dispersion of the Co sulfide species on activated carbon. The high HDS activity of CoSx/NaY suggests a high dispersion of the Co sulfide species. With the HYD of butadiene, CoSx/NaY showed a much lower activity than MoSx/NaY. 

The HDS activity of CoSx-MoSx/NaY is shown in Fig.3 as a function of the Co/Mo atomic ratio. The Mo content in the catalyst was kept constant at the saturation value for a Mo(CO)j adsorption in NaY, 2.1Mo/SC. The HDS activity increased as the Co/Mo ratio increased up to about unity, followed by an activity decrease at a further addition of Co. The simple sum of the activities of the corresponding CoSx/NaY and MoSx/NaY is presented in Fig.3 for comparison. It is evident that the activity of the Co-Mo binary sulfide catalyst is considerably higher than the simple sum of the activities of the composite sulfides, indicating... 

The IR spectra in Fig.7 indicate the preferential adsorption of NO on the Co sites. It may be conjectured that the Mo sulfide species are physically covered by the Co sulfide species or that Co-Mo mixed sulfide species are formed and the chemical natures of the Co and Mo sulfides are mutually modified. The Mo K-edge EXAFS spectra were measured to examine the formation of mixed sulfide species between Co and Mo sulfides. The Fourier transforms are presented in Fig.8 for MoSx/NaY and CoSx-MoSx/NaY. The structural parameters derived from EXAFS analysis are summarized in Table 1. The structure and dispersion of the Mo sulfides in MoSx/NaY are discussed above. With the Co-Mo binary sulfide catalyst, the Mo-Co bondings are clearly observed at 0.283 nm in addition to the Mo-S and Mo-Mo bondings. The Mo-Co distance is close to that reported by Bouwens et al. [7] for a CoMoS phase supported on activated carbon. Detailed analysis of the EXAFS results for CoSx-MoSx/NaY will be presented elsewhere. It is concluded that the Co-Mo mixed sulfides possessing Co-S-Mo chemical bondings are formed in CoSx-MoSx/NaY. 

Catalysts used for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of heavy oil fractions are largely based on alumina-supported molybdenum or tungsten to which cobalt or nickel is added as a promoter [11]. As the catalysts are active in the sulfided state, activation is carried out by treating the oxidic catalyst precursor in a mixture of H2S and H2 (or by exposing the catalyst to the sulfur-containing feed). The function of hydrogen is to prevent the decomposition of the relatively unstable H2S to elemental sulfur, which would otherwise accumulate on the surface of the... 

Another SIMS study on model systems concerns molybdenum sulfide catalysts. The removal of sulfur from heavy oil fractions is carried out over molybdenum catalysts promoted with cobalt or nickel, in processes called hydrodesulfurization (HDS) [17]. Catalysts are prepared in the oxidic state but have to be sulfided in a mixture of H2S and H2 in order to be active. SIMS sensitively reveals the conversion of Mo03 into MoSi, in model systems of MoCf supported on a thin layer of Si02 [21]. 

Combustion, 27 189, 190 reaction, sites for, 33 161-166 reaction scheme, 27 190, 196 Commercial isomerization, 6 197 CoMo catalysts, 40 181 See also Cobalt (nickel)-molybdenum-sulfide catalysts Compact-diffuse layer model, 30 224 Compensation behavior, 26 247-315 active surface, 26 253, 254 Arrhenius parameters, see Arrhenius parameters... 

Hydrogen sulfide At low levels, hydrogen sulfide can inhibit aromatic ring saturation. This results in higher-octane gasoline and low-smoke-point jet fuel. At high concentrations, cracking catalyst activity is adversely affected. 

The use of nitrides, along with sulfides and carbides, as catalysts for hydroprocessing has recently been extensively reviewed by Furimsky and will not be discussed in detail here. Subsequently, Al-Megren et have published a comparison of the activities of bulk CoMo carbide, oxide, nitride and sulfide catalysts for pyridine hydrodenitrogenation. Of these, the sulfide catalysts were reported to possess more stable activity, with the carbide being next, followed... 

Pyridine (1.0 gm) added as a polymerization inhibitor. b S(CH2CH2OH)2 (6.0 gm) added as a catalyst. c Activated carbon with silver oxide or Ni-W sulfide catalyst added. a Anion-exchange resin (40-100 gm) added see U.S. Pat. 2,614,099 (1952). Absolute ethanol (62 gm) and 1 gm of FeCl3 or TiCl3 added. 

Unfortunately, the initial promise of this approach was followed by disappointment, as the ternary sulfide catalyst rapidly lost activity with extended use. XPS analyses of used catalysts showed that Ru catalyzed the reduction of Mo, which led eventually to crystal growth of the MoS2, which resulted in loss of activity. Perhaps other supports having higher surface areas or... 

Liquefaction catalysts, such as sulfides, lose their catalytic activity, especially hydrogenation activity when they are transformed into sulfate or oxide. Even reduction of the extent of sulfiding leads to a significant loss in catalyst activity. The crystalline form of the catalyst may also influence the catalytic activity. Thus, the level of sulfur during coal liquefaction is critical. This can be controlled by the addition of sulfur additives. 

TMS catalysts fell into a special category due to their exceptional resistance to poisons. In fact, the presence of sulfur compounds, the most common poison of metallic and oxide catalysts, does not decrease their catalytic activity, but is needed to maintain high activity. Sulfide catalysts are also very resistant to carbon deposition, which is illustrated by their use for converting residual oils. Arsenic, as well as nickel and vanadium contained in heavy petroleum fractions, are some of the few substances that cause significant deactivation, and this only occurs by physical blockage of pore structure in supported catalysts. 

Massoth, when discussing the oxidation state of the TMS catalysts, concentrated on the typical commercial supported catalyst (7). Because of this, the article reflected a very confused picture with heavy emphasis on the supported and reduced state of the oxide catalyst. The emphasis was placed here because it was still believed at the time that the support was fundamentally crucial to the activity of the catalyst. Today we know that the role of the support is to disperse the catalyst and that the sulfided state of the catalyst is responsible for the stable activity. Massoth reported that at that time the state of the sulfided catalyst was very unclear. By the time Prins et al.(4) wrote their article, it was clear that the stable operating states of Mo/Co and related systems were as the sulfides. It is therefore essential to understand the oxidation state of the bulk sulfide and how this affects the oxidation state of the surface defects. 

Again (as mentioned in Section V,C) sulfur compounds perform better than CO, as can be seen in Fig. 20, because they are better dehydrating agents. When Cr/silica is reduced by COS or CS2 a black chromium sulfide forms. Reoxidation then converts it back to the hexavalent oxide. The catalyst retains no sulfur, but it often takes on a new reddish hue and the activity is greatly improved. This is probably an extension of the trend already observed in Fig. 10, which shows both activity and termination to increase as the catalyst is dehydrated. Perhaps the color change from yellow to orange, and finally to red for sulfided catalysts, indicates a transition from chromate to dichromate, or maybe just less coordination to hydroxyls. Adding water vapor to a sulfided catalyst completely reverses the benefit. 

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